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LEARNINGCENTRE
WELDING PROCESSES
AND EQUIPMENT
MODULE 4
Copyright © 2007 by The CWB Group· Industry Services
Revised March 2007
All rights reserved.
Although due care has been taken in the preparation of this module neither the
CWB Learning Centre nor any contributing author can accept any liability arising
from the use or misuse of any information contained herein or for any errors that
may be contained in the module. Information is presented for educational purposes
and should not be used for design, material selection, procedure selection or similar
purposes without independent verification . Where reference to other documents,
such as codes and standards, is made readers are encouraged to consult the
original sources in detail.
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Module 4
TABLE OF CONTENTS
LESSON OBJECTIVES . ....... ....... . .. . .... .. . .. .. . ...... . . .. . .. .. . . .. .. .. .4
1.
1.1
1.2
1.3
1.4
1.5
INTRODUCTION TO WELDING PROCESSES
Historical Background ............. . .. .. ....... . . • . .. .. . . .. . . . . ..... . . ... 6
Welding Terms and Definitions ........ .. • .. ..... . . .. . .. . .. .. .. ... . ... . . .. .8
Grouping of Welding Processes . . . .... .. ..... . . . .•••.........•...... . . .. .15
The Welding Arc . . ................. . ...... . . .•.• ..... . ..... . .. . . . ..... .16
Health and Safety . . . .................. ......... . . .. . . .. . .. • .... : . . .... 27
2.
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
GAS TUNGSTEN ARC WELDING (GTAW)
Principles of Operation .... . ... . . .. . .. . .. ... . . .. .. . ... .. ... . . . . . . . ... . . .. 30
Current and Equipment . .. .. .. . . . .. . . . . ............... . . .. . .•.. . ...•. . .. 31
Welding Consumables ... . ..... ..... .. . . . .... . ..•. • ... . . . .. • •......•.. . .37
Applications and Limitations . .. . . . .. ...... .. . . . .••.... .. . . ..• • . .. .. . .... .42
Joint Design ... .. .. . ... .. . . . . . . . ..... .. . .. ........ .. . . .. •• . . . . . . ... . . .43
Arc Ignition (Starting the Arc) . . . . .. . . . .. . . . . . ... . . • .. . . . . . . ... . .. .. . ..... .44
Welder Technique .. .......... . . ... •.... .. .. . • .... .. .. .. .. ..... . . •.... .45
Weld Quality ....... ..... . ........... . . .... .. ........ ...........• • .... .47
3.
3.1
3.2
3. 3
3.4
3.5
3. 6
SHIELDED METAL ARC WELDING (SMAW)
Principles of Operation .................... .. . ..... . .... . . . • ...... . •..... 50
Power Sources .. . . . .... ....... . . • • . . .. . ... . ...... ..... . • .......• . ... .. 51
Types of Electrodes . . .. .. ....... . ....... .... . .... .... ... •. ... .. . ... . ... 55
Classification of Electrodes . .. . ...... ........... . . .. ..... . •. . . ....• . . . ... 56
Applications and Limitations of the SMAW Process . . . ... . ..... . .. .. .. . .. . . . . .59
Shielded Metal Arc Welding of Carbon and Low Alloy Steels ... .. •. .. . . .. .... ... 61
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4.
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
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GAS METAL ARC WELDING (GMAW)
Principles of Operation .......... .. ......•. ,' ........••... . ...... • ...... 74
Equipment . .... . ......... .. ...... ..... . .................... . ....... 76
Metal Transfer Across the Arc in GMAW .... . ................ . .. . ......... 80
Shielding Gases ........... . ...... . .............. ..• ......... .. ...... 90
Advantages and Limitations of the GMAW Process .... .. . . •....... . . . •..... 94
Electrode Wires for Gas Metal Arc Welding . ......................... •..... 96
Application of Gas Metal Arc Welding Process .. . . .. . . .. . ..•.. .. . . .. . .. . . . .97
Effect of Arc Welding Variables in Arc Welding Processes .... • . . ... . . . ... . .. 100
5.
FLUX CORED ARC WELDING (FCAW)
5.1 Principles of Operation . . . .. . . .. . ..... . ................ . ..... . ... . . . .. .110
5.2 Equipment ........................... . ............. .. ......... . •.... 112
5.3 Advantages and Applications of the Cored Wire Processes . ... . . •• ...... . .. ... 113
5.4 Classification of Cored Wires .. . .. . ............ . .... . . . .• ••....... . ..... 114
5.5 Shielding Gases for Gas Shielded Tubular Electrodes ....•.. .. .. ..... •....... 121
5.6 Gas Shielded Flux Cored Arc Welding of Carbon
and Low Alloy Steels . . ... .. . .... . . ...... ... .. . ................ • ... . ... 122
6.
6.1
6.2
6.3
6.4
6.5
SUBMERGED ARC WELDING (SAW)
Principles of Operation ... . .............. .. . . ........ .. ...... . .•. . ..... 128 ·
Current Type and Equipment . ... . ... . . . . . ... . ..... . .. . . . .. . . ..... . . .. .. 129
Advantages and Applications of Submerged Arc Welding ....... . . ... . . . . . .... 130
Multiple Wire Submerged Arc Welding ................ .... .. .. .. . .•. . ... .. 133
Wires and Fluxes for Submerged Arc Welding of Carbon and Low
Alloy Steels .. . ...... . . ............. . ....... . ............. . ... ....... 134
6.6 Submerged Arc Welding of Carbon and Low Alloy Steels ..... . ... . .. .• ....... 138
7.
7.1
7.2
7.3
RESISTANCE WELDING (RW)
Principles of Operation ........ . .............. • ... . . . ..... . ..•....... .150
Process Parameters ... . .. .. ... .. ...... . .. . . . . .... ......... . .... . .... 151
Types of Resistant Welding Processes ....•...... • ......... . .... ........ 152
8.
8.1
8.2
8.3
8.4
8.5
PLASMA ARC WELDING (PAW)
Principles of Operation ........... . . . . ... .. .. .•. • ... . .. ... ..... . .... . .162
Process Variables ...... . ... .. . ... .... . . .. . . ... .... ...... . .. . . ... .... 163
Equipment for PAW ..... ... ... .. . . .. .... . .. . . .. . ... .. ... . . . . . . . ...... 164
Advantages and Disadvantages .. . ........................ . . .. ....... . .166
Applications ......... . ... .. . ........... . . . . • ...... .. .. ... . . ......... 166
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9.
9.1
9.2
9.3
9.4
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ELECTRON BEAM WELDING (EBW)
Principles of Operation ..................... .... .•............•....... 168
Equipment .... . . . ...... . ... .. . ... . .. . . . . . .. ........... . ....•....... 169
Advantages and Disadvantages .. ... ....•. .. . ... ...• .... . .. . . .. • ....... 170
Applications ..... . ................. . •• ..... . . . .••...... .. ........... 171
10.
LASER BEAM WElDING (LBW)
10.1 Principles of Operation ............. .. ... ...... ...... .. .......• . . .. ... 174
10.2 Laser Types . .. .. . ...... . ... .. . . . . . . ... . . . . .... . .. . ................. 174
10.3 Advantages and Disadvantages .................. . ........ . .... ........ 177
11.
ELECTROSLAG WELDING (ESW)
11.1 Principles of Operation .......................••...... . . ......•....... 180
11.2 Equipment ............ . ......... .. .•...................... •.... .... 181
11.3 Applications ... . . . .... . ........... . ... . ....................••....... 182
12.
12.1
12.2
12.3
12.4
DIFFUSION WELDING (DFW)
Principles of Operation ........ . .. . .... . ..
Diffusion Welding Mechanisms .. . . ..... .. ..
Bonding Variables .............. .. . ...• . .
Applications ....... . ........... .. . .. ... .
13.
13.1
13.2
13.3
13.4
EXPLOSION WELDING (EXW)
Principles of Operation ..... . . . ........... ... ...... ..... . ............. 188
Explosion Welding Parameters and Variables ........•.........•.......... 188
Mechanisms of EXW ... ..................... ...• ........••• • •........ 189
Applications ..................... . ...... . .....•........•• •• ......... 189
14.
14.1
14.2
14.3
14.4
FRICTION WELDING (FRW)
Principles of Operation ....... . ... . ..... . . . .. . ... . ........•••.. .. ..... 192
Process Variables ......... . . .... .. ..•........••.. ... ... •••.. . ....... 193
Metallurgical Parameters ..... .. .. .. .......... . ..... . ......•.••. . . ..... 193
Applications ............. . . . .. . .. ... ... . ... .. .. ..... . . . •• ••. . .... .. .194
. .... .. ... .. .... . .. .•. . .. . .. 184
.. . .. ... . .. .... . .. ..•.. . .... 184
. ... ...• • . .... . .....•.. .. ... 185
.... . ..... . .. ......• •. . ..... 186
15. THERMITE WELDING (TW)
15.1 Principles of Operation ..............• . .... . .•..........••... . ........ 196
15.2 Applications . ................. . ...• • ....... ... .......••............. 197
GUIDES AND EXERCISES ............ . ...........•....... . .. •.... ......... 199
TEST AND ANSWER SHEETS ......... ...... .. . . .. .... . ............ ..... ... 210
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WELDING PROCESSES AND EQUIPMENT
LESSON OBJECTIVES
The objective of this module is to provide a general description of the major welding processes
ava ilable, with emphasis on the arc welding processes. The intent is to provide the reader
with a broad understanding of the principles of operation of the process, general process
characteristics and equ ipment used. The processes covered in this module along with their
letter designations are as follows:
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Gas Tungsten Arc Welding (GTAW)
Shielded Metal Arc Welding (SMAW)
Gas Metal Arc Welding (GMAW)
Flux Cored Arc Welding (FCAW)
Submerged Arc Welding (SAW)
Other welding processes include:
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Resistance Welding (RW)
Plasma Arc Welding (PAW)
Electron Beam (EBW) and Laser Beam Welding (LBW)
Electroslag (ESW)
Diffusion Welding (DFW)
Explosion Welding (EXW)
Friction Welding (FRW)
Thermit Welding (TW)
Important Notice:
This module has been updated to reflect the new electrode designations listed in CSA Standard W48. The
reader wi ll notice these changes throughout the module and should be aware of them during their study of the
material. Examples of these changes are:
Old Designation:
SMAW
E41010
E48018
GMAW
ER480S-2fER49S-2
FCAW
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E480H-5-XX
SAW
E480A5-EM12K
Designation in W48-06:
E4310
E4918
ISO 14341-B-G 49A 3C G2
E49H-5XMJ
E49A5-EM12K
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Chapter 1
INTRODUCTION TO WELDING PROCESSES
1.1
1.2
1.3
1.4
1.5
Historical Background .. . ....... . . . •... . ......... . ... . . • . ••..... . .... .. 6
Welding Terms and Definitions ......•.... . ...... . . . ........ . .. . ........ .8
Grouping of Welding Processes . . .... . . . ....•.. . . ... . . . ..• . ...... . . .. .. 15
The Welding Arc . . .. ................... ..•..... . . . ..•• ••............ 16
Health and Safety ... ; .• . ... . ... : .. . . ..........•........... . .••.... . .27
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1. INTRODUCTION TO WELDING PROCESSES
1.1
Historical Background
Until very recently in the history of mankind, the only method available to join metals was by
which requires two pieces of metal to be heated and then pressed or hammered
together to develop a metallurgical bond between the two. Modern welding technology can
trace its origins to the first half of the 19th century when advances in electrical technology
such as the production of an arc between two carbon electrodes and invention of the electric
generator took place. By the e,!1d of the 19th centy-ry, these advances had led to the
development of three new welding processes; arc weldin ,resistance welding and oxyacetylene wel9iDR. The arc welding process in its numerous variations IS now lie most
Important and widely used welding process.
f~rge...Y!'elding
The first major patent for arc welding was awarded in the UK to two Russians, ,bIikolas d~
Benardos and Stanislav Olszewski in 1885, who employed a carbon electrode as the positive
pole to obtain an arc with the workpiece (negative pole). The arc heated the workpiece
comprising two adjoining pieces of lead or iron so that they locally melted and fused with each
other. Soon thereafter, in 1889, Lavianoff in Russia and Charles Coffin in the USA were able to
substitute a metal electrode for the carbonelectrode.
- - - - - - - - - -. -----A significant advancement in welding came with the use of a metal electrode. Carbon
electrodes previously in use could not provide filler metal. Further advances and applications
of the metal arc welding process depended on the development of improved metal electrodes
for greater arc stability and means to shield the molten weld pool from contamination from the
air surrounding the arc which embrittled the weld metal.
The earliest effort in this regard was the application of coating or covering to the metal
electrode. Oscar Kjellberg of Sweden applied the coating by dipping short lengths of iron wires
in a thick mixture of carbonates and silicates, and then letting them dry. The British were the
first to attempt application of the arc welding technology on a significant scale as a substitute
for rivetting in the fabrication of ships. In the USA, around the same time at the start of World '
War I, German ships interned in New York harbour and scuttled by their crews were rapidly
brought back into service by effecting repairs using arc welding. The first all welded ship, the
Fulagar was launched by the British in 1920.
During the 1920's, arc welding was applied for fabrication of heavy wall pressure vessels and
buildings. In Canada, a 500 foot long, three span bridge having an all welded construction
was erected in Toronto in 1923. However, widespread use of arc welding had to wait until
1927 when an extrusion process to economically apply covering to the electrode was
developed . Electrodes for welding stainless steel, coverings that reduced the amount of
hydrogen in the weld metal or that contained more easily ionized ingredients for arc
stabilization were developed soon after.
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In 1930, Robinoff was awarded a patent for submerged arc welding (welding under powder or
flux, continuous wire without any covering) of longitudinal seams in pipes. Being a highly
productive, mechanized process, it is still very popular for welding of thick steels .
Use of externally applied gases instead of slag and gases formed from the electrode covering
to shield the weld pool in arc welding had also been investigated. During the 1920s, Hobart
and Devers in the USA experimented with argon and helium as shielding gases and this was a
precursor to the development of the gas tungsten arc welding process used for welding of
magnesium, aluminum and stainless steel , during World War II. Their work also demonstrated
the use of continuous wire being fed through a nozzle for arc welding with external inert
shielding gases and this was later developed into the gas metal arc welding process in 1948
at Battelle Memorial Institute. Availability of smaller diameter wires and constant voltage power
sources made this process more popular for joining of non-ferrous metals and alloys.
Application of gas metal arc welding to steels had to await the introduction of carbon dioxide
as a shielding gas in 1953, and since then there have been numerous developments in
regards to use of gas mixtures containing argon, helium, oxygen and carbon dioxide for gas
metal arc welding of steels.
Another significant innovation in the mid-1950's was the development of tubular wires that
contained fluxing agents on the inside. The gases generated by the decomposition of the
fluxing agents as well as an externally applied gas are used for shielding the pool from
atmospheric contamination. Initially known as the Dualshield process, it is known as flux
cored arc welding today. Other variations of tubular wires that have a significant usage today
include self shielded flux cored arc welding wires (i.e., without using the external gas shield)
and metal cored arc welding wires (only metal powder and some arc stabilizing materials
inside and used with external shielding gas).
Welding processes other than those using an arc heat for welding have also been developed
over the years since the last part of the 19th century. Briefly, these can be summarized as
follows:
resistance welding and its variations (spot welding, seam welding, projection welding,
flash butt welding) over the period 1885 to 1900;
thermit welding for joining rails in 1903;
electroslag welding during the 1950's;
electrogas welding in 1961;
plasma arc welding in 1957;
electron beam welding in late 1950's.
More recent developments in welding processes include friction welding and laser welding .
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1.2
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Welding Terms and Definitions
Some of the terms frequently used in this module are briefly described below to assist the
reader in better understanding the contents of this module.
active flux
(submerged arc welding)
A flux formulated to produce a weld metal composition that is
dependent on the welding parameters. especially arc voltage.
alloy
A substance with metallic properties and composed of two or more
chemical elements of which at least one is a metal.
al/oy flux
A flux containing ingredients that react with the filler metal to
establish a desired alloy content in the weld metal.
(submerged arc welding)
alternating current
Current flow in an electrical circuit where its direction (and
therefore, direction of electron flow) continually reverses itself,
usually at a pre-determined frequency.
arc blow
The deflection of an arc from its normal path because of magnetic
forces.
arc force
The axial force developed by an arc plasma.
arc length
The distance from the tip of the welding electrode to the adjacent
surface of the weld pool.
arc plasma
A gas that has been heated by an arc to at least a partially ionized
condition, enabling it to conduct an electric current.
arc voltage
The electrical potential between the electrode and the workpiece.
arc welding electrode
A component of the welding circuit through which current is
conducted and which terminates at the arc.
arc welding gun
A device used to transfer current to a continuously fed consumable
electrode , guide the electrode and direct the shielding gas and the
arc.
arc welding torch
A device used to transfer current to a fixed electrode, position the
electrode and direct the shielding gas (if used in the process) and
the arc.
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arm, resistance welding
A projecting beam extending from the frame of a resistance welding
machine that transmits the electrode force and may conduct the
welding current.
autogeneous weld
A fusion weld made without addition of filler metal.
bare electrode
A filler metal electrode that has been produced as a wire, strip or
bar with no coating or covering other than that which is incidental
to its manufacture or preservation.
base metal
The metal or alloy that is welded, brazed, soldered or cut .
bevel
An angular edge shape.
bonded flux
A granular flux produced by baking a pelletized mixture of
powdered ingredients and bonding agents at a temperature below
its melting point , but high enough to create a chemical bond,
followed by processing to produce the desired partical size.
(submerged arc welding)
buildup
A surfacing variation in which surfacing material is deposited to .
achieve the required dimensions.
cold crack
A crack which develops after solidification is complete.
consumable electrode
An electrode that provides filler metal.
covered electrode
A composite filler metal electrode consisting of a core of a metal
rod to which a covering sufficient to provide a slag layer on the
weld bead has been applied. The covering may contain materials
providing materials providing such functions as shielding from the
ambient air, cleansing the weld metal, arc stabilization, source of
metallic addition to the weld.
crater
A depression in the weld face at the termination of a weld bead.
deposition rate
The weight of material deposited in a unit of time.
deposition efficiency
The ratio of the weight of filler metal deposited in the weld metal to
the weight of the filler metal melted, expressed in percent.
direct current electrode
negative (dcen)
An arrangement of direct current arc welding leads in which the
electrode is the negative pole and workpiece is the positive pole of
the welding arc.
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direct current electrode
positive (deep)
An arrangement of direct current arc welding leads where the
electrode is the positive pole and workpiece is the negative pole of
the welding arc.
drag angle
The travel angle when the electrode is pointing in a direction
opposite to the progression of welding . This angle can also be used
to partially define the position of guns, torches, rods and beams .
duty cycle
The percentage of time during a specified test period that a power
source can be operated at the rated output without overheating .
electrode
A component of the electrical circuit that terminates at the arc,
molten conductive slag , or base metal.
electrode extension
The length of the electrode extending beyond the end of the
contact tip .
(flux cored arc welding,
electrogas welding,
gas metal arc welding,
submerged arc welding)
electrode extension
(tungsten arc welding,
plasma arc welding)
electrode force
(resistance welding)
The length of the tungsten electrode extending beyond the end of
the collet.
The force applied by the electrodes to the workpieces in making
spot, seam or projection welds .
electrode setback
orifice
The distance the electrode is recessed behind the constricting
of the plasma arc torch, measured from the outer face of the
nozzle.
electron beam gun
A device for producing and accelerating ele ctrons (suitable for
as a source of heat for welding ).
filler metal
The metal or alloy to be added in making a brazed , soldered or
welded joint.
fiat welding position
The welding position used to weld from the upperside of the joint at
a point where the weld axis is approximately horizontal, and the
weld face lies in an approximately horizontal plane .
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flux
A material used to hinder or prevent the formation of oxides and
other undesirable substances in molten metal and on solid metal
surfaces , and to dissolve or otherwise facilitate the removal of such
substances ,
friction speed
The relative velocity of the workpieces at the time of initial contact.
(friction welding)
friction upset distance
The decrease in length of work pieces during the time of friction
welding force application,
fused flux
A granular flux produced by mixing the ingredients followed by
melting, cooling to the solid state and processing to produce the
desired particle size ,
(submerged arc welding)
fusion (fusion welding)
The melting together of filler metal and base metal, or base metal
only, to produce a weld,
heat-affected zone
The portion of the base metal whose mechanical properties or
microstructure have been altered by the heat of welding, brazing,
soldering or thermal cutting,
(HAZ)
horizontal welding
position
(fillet weld)
horizontal welding
position
(groove weld)
The welding position in which the weld is on the upper side of an
approximately horizontal surface and against an approximately
vertical surface,
The welding position in which the weld face lies in an
approximately vertical plane and the weld axis at the point of
welding is approximately horizontal.
incomplete fusion (IF)
A weld discontinuity in which fusion did not occur between the weld
metal and fusion faces or adjoining weld beads,
inert gas
A gas that normally does not combine chemically with materials,
joint design
The shape, dimensions and configuration of the joint.
joint geometry
The shape, dimensions and configuration of the joint prior to
welding,
joint penetration
The distance the weld metal extends from the weld face into a jOint,
exclusive of the weld reinforcement.
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keyhole welding
A technique in which a concentrated heat source penetrates
partially or completely through a workpiece, forming a hole
(keyhole) at the leading edge of the weld pool. As the heat source
progresses, the molten metal fills in behind the hole to form the
weld bead.
laser
A device that produces a concentrated coherant light beam by
stimulated electronic or molecular transitions to lower energy levels.
Laser is an acronym for light amplification by stimulated emmission
of radiation.
manual welding
Welding with the torch, gun or electrode holder held and
manipulated by hand.
mechanized welding
Welding with equipment that requires manual adjustment of the
equipment controls in response to visual observation of the
welding, with the torch, gun or electrode holder held by a
mechanized device.
melting rate.
The weight or length of electrode, wire, rod or powder melted in a
unit of time.
neutral flux
A flux formulated to produce a weld metal composition that is not
dependent on the weld ing parameters, especially arc voltage .
(submerged arc welding)
nonconsumable
electrode
An electrode that does not provide filler metal.
nontransferred arc
An arc established between the electrode and the constricting
nozzle of the plasma arc torch or thermal spaying gun . The
workpiece is not in the electrical circuit.
open circuit voltage
The voltage between the output terminals of the power source
when no current is flowing to the torch or gun.
orifice gas
The gas that is directed into the plasma arc torch or thermal
spraying gun to surround the electrode. It becomes ionized in the
arc to form the arc plasma and issues from the constricting orifice
of the nozzle as a plasma jet.
overhead welding
position
The welding position in which welding is performed from the
underside of the joint.
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partial joint
penetration weld
A groove weld in which incomplete joint penetration exists.
platen
A member with a substantially flat surface to which dies, fixtures,
backups or electrode holders are attached and that transmit the
electrode force or upset force. One platen usually is fixed and the
other moveable.
(res is lance welding)
porosity
Cavity-type discontinuities formed by gas entrapment during
solidification or in a thermal spray deposit.
root face
The portion of the groove face within the joint root.
semi-automatic welding
Manual welding with equipment that automatically controls one or
more of the welding conditions.
shielding gas
Protective gas used to prevent or reduce atmospheric
contamination.
slag
A nonmetallic product resulting from the mutual dissolution of flux
and nonmetallic impurities in some welding and brazing processes.
slope
Quantitative measure of the incline of the power source voltampere cu rve .
surface tension
Force at the surface of liquid that tries to reduce its surface area
and prevents area and prevents it from wetting the solid that it is in
contact with.
surfacing
The application by welding, brazing or thermal spraying, of a layer
or layers of material, to a surface to obtain desirable properties or
dimensions, as opposed to making a joint.
thermit crucible
A container or a vessel in which the thermite reaction takes place.
thermit mixture
A mixture of metal oxide and finely divided aluminum with the
addition of alloying metals as required .
thermit mold
A mold formed around the workpieces to receive molten metal.
transferred arc
A plasma arc established between the electrode of the plasma arc
torch and the workpiece.
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travel angle
The angle less than 90· between the electrode axis and a line
perpendicular to the weld axis, in a plane determined by the
electrode axis and the weld axis.
undercut
A groove melted into the base metal adjacent to the weld toe or
weld root and left unfilled by weld metal.
volt-ampere curve
A graphical representation of the voltage - current relationship for a
given power source when a steady load is placed on it.
welding head
The part of a welding machine in which a welding gun or torch is
incorporated .
welding leads
The workpiece lead and electrode lead of an arc welding circuit.
welding procedure
The detailed methods and practices involved in the production of a
weldment.
welding procedure
qualification record
A record of welding variables used to produ ce an acceptable test
weldment and the results of tests conducted on the weldment to
qualify a welding procedure specification.
(WPQR)
weld metal
Metal in a fusion weld consisting of that portion of the base metal
and filler metal melted during welding.
weld pool
The localized volume of molten metal in a weld prior to its
solidification as weld metal.
weld reinforcement
Weld metal in excess of the quantity required to fill a joint.
weld root
The points shown in cross section, at which the weld metal
intersects the base metal and extends furthest into the weld joint.
wefting
The phenomenon wherby a liquid filler metal or flux spreads and
adheres in a thin continuous layer on a solid base metal.
wire feed speed
The rate at which wire is consumed in arc cutting, thermal spraying
or welding.
work angle
The angle less than 90· between a line perpendicular to the major
workpiece and the weld axis. In a T-joint or corner joint, the line is
perpendicular to the nona butting member.
workpiece
The part that is welded, brazed soldered, thermal cut or thermal
sprayed.
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Grouping of Welding Processes
There are many ways in which welding processes may be grouped and different countries
have adopted various ways to classify them based on the application of heat, with or without
external pressure, type of energy involved (mechanical, electrothermic, thermochemical) etc.
The American Welding Society (AWS) groups welding processes based primarily on the mode
of energy transfer, and secondarily on the influence of capillary action in effecting distribution
of filler metal in the joint (as in brazing and soldering).
AWS defines welding as, "a joining process that produces coalescence (i.e., growing together)
of materials by heating them to the welding temperature, with or without the application of
pressure or by the application of pressure alone, and with or without the use of filler material".
In the AWS approach, welding processes are grouped into the following major categories:
Arc welding (AW) ,.."oS f G 0,.,. rv--" "'Solid state welding (SSW)
Resistance welding (RW)
Oxyfuel gas welding (OFW)
Soldering (S)
Brazing (B)
Other welding
Allied processes (such as cutting, thermal spraying)
Arc welding processes are, by far, the most commonly used in the welding industry and are,
therefore, the main focus in this module. However, arc welding involves melting and most
metals, when melted in air, become contaminated with oxides and nitrides through contact
with the oxygen and nitrogen in the air. This contamination may result in a poor quality weld.
Most arc welding processes have some means of shielding (protecting) the molten metal from
the air or some other means of removing the harmful effects of oxygen and nitrogen. The two
main methods of arc shielding are:
Flux shielding
Gas shielding
Most of the arc welding processes are distinguished principally by the method of shielding or
the way in which it is applied.
The exact selection of an arc welding process for a particular application involves several
considerations including:
Is the process suitable for welding the metal or alloy involved? In the required thickness
and position?
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Would the welded joint have the required quality and physical (mechanical, corrosion)
properties?
Is it the most economical of the available choices?
Are the equipment and skilled welders available for the chosen process?
1.4
The Welding Arc
Most metals and alloys conduct electricity at room temperature due to the presence of free
electrons. A considerable amount of heat can be produced due to the flow of the current in a
circuit. Typical examples of this heating effect, also called resistance heating, are tungsten
filament bulbs and heating coils in ovens. If sufficient energy is applied to a gas it can become
conductive. When sufficient voltage is applied to a gas it can be ionized - changed into .
positively charged ions and negatively charged electrons. The electrons move in response to
the applied voltage to produce a current flow and this movement of electrons allows the
initiation of an arc. In comparison, gases like oxygen, nitrogen, carbon dioxide, etc. do not
conduct any electricity at room temperature. The current flow causes resistance heating in the
gas which promotes its further ionization and increasing current flow. As long as the voltage
source is able to supply the necessary voltage and the current needed by the arc, it can be
sustained in a stable manner and used for welding applications.
Based on the above prinCiple, a conventional arc is formed between two non-consumable
electrodes in a gas or gaseous medium when an appropriate voltage depending on the
electrode material and gas phase is applied to the electrodes. As seen in Figure 1.1 , one of
the two electrodes forms a positive terminal of the electrica l circuit and is called the anode; the
negative terminal of the circuit is ca lled the cathode. When an arc is created, electrons are
emitted from the cathode and transferred to the anode through the ionized gas in between.
Flow of electrons is the same thing as flow of current or electricity.
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I·
Potential Drop
/7
- to
()O'h .
_
+++++++++
~
~
+
,---1
--::::::--;::::
Anode
-;::::....-
~-
-
~
- ",
Cathode
Electron
Flow
~ ~ ~ I~
+
-
01------------'
Power Supply
Current
Figure 1.1: An Arc between Two Electrodes
A welding arc is formed when a fairly high current (10 to 2000 A) is forced to flow across a gap
between two electrodes at relatively low voltage (10 to 50 V). A welding arc is intensely hot
with temperatures exceeding 30000°C (see Figure 1.2) and forms a concentrated heat source
suitable for melting most metals rapidly. The intense heat of the welding arc causes the filler
metal to melt and when added to the locally hot melted workpiece, it forms the weld fusion
zone. Its subsequent freezing (solidification) produces the bond (weld) between the
workpieces. Arc welding processes do not require application of pressure to cause fusion.
Tungsten
Cathode
200 A
12.1 V
l-----
2420 W
18x 10'C
'V~I-I-u.:~---- 16 x 10' C
5 mm (0.2 in)
-L-I~~~.........-
15x 10'C
14x10' C
13x 10'C
12x10'C
11x10'C
10x 10' C
Figure 1.2: Temperature Distribution in a 200 A Arc in Argon.
(from AWS Welding Handbook, Vol. I)
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In welding, the arc may be established between an electrode and th e workpiece, or between
two electrodes. In the latter case (for example, series arc submerged arc welding), the arc
would be positioned close to the work pieces being welded, but the electric current would not
be passing through the workpiece.
When the workpiece is one of the electrodes of the electrical circuit, the other electrode may
be consumable or nonconsumable. A consumable electrode is designed to melt and add filler
material to the weld joint (SMAW, FCAW, GMAW, SAW processes). The tungsten electrode in
gas tungsten arc welding is a nonconsumable electrode. When no additional filler metal is
added during the welding operation, as may be the case in gas tungsten arc or electron beam
welding, the weld produced is called an autogenous weld.
:::I~---I 3 Phase6000V~:O:~lIfornnerl~----~
Tr
Welding Power
250A 29V
Figure 1.3: Transforming Electrical Power
The electric current for welding is provided by a "power source" which draws high voltage
electric power from the main transformer and converts it into higher current and lower voltage
values suitable for welding (Figure 1.3). Power sources are broadly classified as constant
current or constant voltage type, and the static volt-ampere output characteristics for these two
types of power sources are shown in Figure 1.4.
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____ Open Circuit
80 ~~
Voltage (OCV)
~"",
..... .....
.... ' ......... ....
\''''''
\
" '" ,/constant Current (CC)
, " " ....
\.
\\
Voltage
(Volts)
....
" .... " "
" "
Open Circuit ",
Voltage (OCV) \
'"
"
Constant
Voltage
',(CV)
'
~~~~~~\~--~'~~'~~~~~-50 ~
' \ '
..
Welding Current (Amps)
Figure 1.4: Characteristic Volt-Ampere Curve
for Welding Power Sources
Arc Efficiency
The welding arc provides the intense heat needed to locally melt the workpiece and the filler
metal. In fact, all the electrical energy supplied by the power source is converted into heat
(current x voltage). Some energy is lost in the electrical leads, and therefore the energy
available for welding is the product of the current (I) and voltage drop between the electrode
where the current enters it and the weld pool (V). For example, with 400 A current and 25 V
drop from the contact tip to the weld pool , the arc energy is 10,000 Joules/second. This arc
energy is partly used up in heating the electrode, melting the consumable electrode or the
separately added filler metal in nonconsumable electrode process, and heating and locally
melting the workpiece. The rest of the heat is lost by conduction, convection, radiation, spatter
etc .. The proportion of the energy that is available to melt the electrode/filler metal and the
workpiece is termed the ~rc efficiency,-
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The arc efficiency for some of the commonly used arc welding processes is:
Submerged arc welding
Shielded metal arc, gas metal and flux cored arc welding:
Gas tungsten arc welding
90 to 95%
65 to 85%
20 to 45%
As will be seen later, the arc in submerged arc welding is under a flux cover and therefore
heat loss to the surrounding atmosphere is minimized. In gas tungsten arc welding, some
energy is used up in heating the nonconsumable electrode and does not contribute to melting
of base metal or filler metal. Other open arc processes (SMAW, GMAW, FCAW) have arc
efficiency in the middle range with values for GMAW being at the high end of the range and
that for SMAW in the lower end of the range. For a given process, factors like welding in a
deep groove, arc length, etc. also influence the arc efficiency. Higher arc efficiency usually
means that for a given arc energy, a greater amount of weld metal is deposited and the
workpiece cools at a comparatively slower rate.
Voltage Distribution along the Arc
In any welding set up, there is a continuous drop in voltage from the lower most point of
contact between the contact tip and the wire, to the molten weld pool or the workpiece. Figure
1.5 schematically shows that this voltage drop occurs in four steps.
contact~
Tube
'"
I
Electrode Extension
____________l ___ _
-----rl ::hOde Drop Zone
Cathode
Spot
__.I
TAnode Drop Zone
Anode _
Spot
_ _J
Figure 1.5: Voltage Drop in the Region of the Welding Arc
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First, there is a drop in voltage over the electrode extension, that is the length of electrode
between the point of electrical contact with the contact tip, and its melting tip, also called
cathode spot for the current flow direction shown in the sketch. The magnitude of this voltage
drop depends on the electrode extension and the wire diameter as well as the current; longer
electrode extension, smaller wire diameter and higher current, all increase the voltage drop
over the electrode extension length.
The voltage drop over the arc length, that is the distance between the cathode spot and the
anode spot (molten weld pool surface in Figure 1.5) takes place in three steps. Right next to
the anode and cathode spots are small thin, gaseous regions called the anode drop zone and
cathode drop zone, respectively, and over these zones there can be a significant drop in
voltage, in the range 1 to 12 V depending on the electrode material.
Direction of
Electron Flow
Magnetic
Field
\
\
.". ~~~.::: "....
.. ......
.................... - .....
: :~:.~ ~: :,
_
............... .
..........
..... .
... .
....
.... -, ' " ....... ................... .
::::
.. .......
. .........
..... -.... - , .... --_.- .......... .
::::
........
.... ........ ..
. .... .
, ,_
........ -
.- ..... ~~::::)
Electrical
Conductor
Figure 1.6: Magnetic Field Surrounding a
Current Carrying Conductor
In between the two drop zones, there is the arc column with a relatively small drop in voltage,
of the order of 1 to 2 V per centimetre length of the arc column. There is a jet like flow of the
ionized gases in the arc column which gives it some stiffness and force (resistance to
deflection). Th is enables the welder to manipulate the gun and direct the molten metal to be
deposited at the desired location in the weld joint. Shorter arcs have greater stiffness than
longer arcs.
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\ o~er ""\-<- c<.'c- - "': 0:."'.ex
~~ ..:Jo\"'''5e.-- 11M'? ' ~oe s de ...J(\
Arc length is a critical and controllable parameter, which is directly related to the arc voltage .
Arc voltage depends on the space between electrodes as well as electrode composition ,
diameter and extension , shielding gas composition , base metal thickness, joint design, welding
position , etc. The voltage, measured at the power supply, is greater than the arc voltage.
Output voltage represents the sum of arc voltage and the voltage drop in the remaining part of
the electrical circuit. The longer the electrical cables the greater will be the difference between
the voltage read at the power supply gauge and the actual arc voltage.
Magnetic Field Associated with a Welding Arc
When an electric current passes through a conductor a magnetic field is created which
surrounds the conductor (Figure 1.6). Unless this magnetic field is balanced in all directions,
the welding arc will tend to be deflected from its normal axial orientation in line with the
electrode. This phenomenon is called arc blow. It is more likely to be present during welding
of magnetic materials (steels) and can cause incomplete fusion types of flaws in welds.
Some degree of imbalance in the magnetic field is always present. The path of the magnetic
flux in the workpiece is continuous behind the arc and discontinuous ahead , due to the change
in the direction of the current as it goes from workpiece to electrode (Figure 1.7). Since a
shorter arc is more stiff, it is also less susceptible to arc blow.
Magnetic "'Field
'"
Work Piece Lead
Unwelded
Joint
Path of
Current
Figure 1,7: Imbalance in the Magnetic Field due to
Change in the Direction of Current and Part Unwelded Joint
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The magnetic field introduced by the cu rrent flowing in the electrode also plays a role in metal
transfer. When the tip of the electrode melts, there are several forces that act at the molten
tip . These include surface tension, gravity, plasma jet and electromagnetic pinch force.
Surface tension tends to prevent the detachment of the liquid drop at the electrode tip,
irrespective of the welding position . Gravity supports droplet detachment when welding in the
flat (down hand) position and attempts to prevent it in the overhead position. The plasma jet in
most situations tries to detach and propel the molten drop across the arc column to the work
piece.
The electromagnetic pinch force helps in the process of detaching the molten metal drops
from the electrode tip . Generally, when there is some necking between the molten tip and the
unmelted electrode, the magnetic field introduces a pinch force acting in both directions away
from the neck (Figure 1.8). This helps to separate the drop from the electrode. Since th is
pinch force increases as the square of the current, smaller and sma ller drops are detached as
the curre nt increases.
Magnetic
Field'\....
........
.
I-
Anode(+)
~L.J....~ _~~...J
X
(
~
,
\
''\
M
'-'
I Cathode H <... _-- .... >
) Pinch force Aiding
•.... .
................
.
.... ...............
.......
.......
.
... ::~ ........ ..;::::.....
..... .
........ .................
.. ::~ ........ ..;::::-
Drop Detachment
/
Pinch
Effect
Figure 1.8: Detachment of Molten Metal Drop Due to Pinch Force
Effect of Polarity
The electric current used in a welding arc may be either direct current (DC) or alternating
current (AC). Direct current flows constantly in one direction. Alternating current is continually
changing direction. When direct current is used for welding, the welding electrode
(consumable or nonconsumable) can be the positive pole or negative pole in the electrical
circuit. The workpiece will have the opposite polarity. These two arrangements for current flow
are called DC electrode positive (DCEP) and DC electrode negative (DCEN), respectively
(Figure 1.9). The type of current selected and its polarity can have a significant influence on
the shape of the weld bead.
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Anode (+)
Eleclrode~
1/
+
Calhode (-)
Eleclrode~
Contact
Tube
Contact
Tube
""""'I
"
Electron
Flow
Power
Source
Power
Source
+
Anode (+)
DC EN
Figure 1.9: DCEP and DCEN Arrangements for Electrical Leads
For example, in gas tungsten arc welding, a nonconsumable electrode welding process, direct
current electrode negative (DCEN) is the polarity used most often. Electrons are easily
emitted from the tungsten electrode (cathode) . When the electrons travel through the arc they
accelerate to very high speed. About 70% of the arc heat is released at the workpiece (anode
or positive pole) due to electrons striking the surface at high speed. This produces a weld
bead with greater penetration. When the polarity is reversed (DCEP) the workpiece becomes
the cathode . The weld pool cannot easily emit electrons because the molten pool is at a much
lower temperature than the tungsten and will resist the release of electrons. While DeEP is
helpful in cleaning the weld pool by removing the oxides, about 70% of the arc heat is now
generated at the electrode (anode). This reduces the life of the tungsten electrode and the
weld bead has reduced penetration. The use of alternating current provides arc characteristics
that are average of those for DCEN and DCEP (Figure 1.10).
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Current Type
Electrode Polarity
DC EN
DCEP
AC (batanced
Negative
Positive
Work End: 70%
Electrode End: 30%
Work End: 30%
Electrode End: 70%
Electrode End: 50%
Deep, Narrow
Shallow, Wide
Medium
Electron and
Ion Flow
Penetration
Characteristics
Heat Balance in
The Arc (approx.)
Penetration
Work End: 50%
II/
"o.+e~!cOx
Figure 1.10: Effect of Current Type and Polarity in GTA Welding
(from AWS Handbook, Vol. 2)
fj ..... I'=-. c(e.",-\'e..
The heat balance in consumable electrode processes differs from that in tungsten arcs. Thus,
a greater amount of heat is generated at the cathode rather than the anode.
When using the gas metal arc process, direct current electrode positive is the polarity of
choice as it leads to greater heat generation at the workpiece (cathode) and therefore greater
penelration. Conversely, DCEN polarity produces more heat at the electrode (cathode), and
therefore increases the electrode melt-off rate and reduces penetration.
Effect of Electrode Extension (Stickout)
When an electric current flows through a conductor, a certain amount of heat is generated due
to the current having to overcome the electrical resistance of the cond uctor. This is called
resistance heating and it is proportional to I'R where I is the current and R is conductor
resistance. The resistance, R, increases with the length of the conductor and decreases as
the diameter increases.
In continuous wire consumable electrode welding, the electrode extension represents an
electrical conductor through which a fairly high welding current passes. When the electrode
extension is increased, its resistance increases and therefore the magnitude of resistance
heating also increases. As a result, for the same welding current, Ihe consumable wire me lis
at a faster rate and thus increases the deposition rate for the same arc energy. However, this
heating effect means that less heat is available to heat and mell the workpiece. Consequently,
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penetration is reduced and the risk of incomplete fusion type of flaws is increased . Also, due
to an increase in voltage drop over a longer electro'de extension, a higher voltage setting is
usually needed to maintain a constant arc length as with the shorter electrode extension.
The effect of electrode extension for individual arc welding processes is addressed later.
Hydrogen in Weld Metals
Invariably, there is some amount of hydrogen present in the solidified and cooled weld zone.
This hydrogen is introduced by the arc heat breaking down the moisture present in and around
the welding arc. Possible sources of this moisture include the electrode covering, flux,
shielding gas, atmospheric humidity and condensation on the work pieces.
When welding steels, absorbed hydrogen can cause cracking . Therefore, in welding carbon
and low alloy steels, martensitic stainless steels, etc., an important consideration in selecting
the welding process and filler metals is the amount of hydrogen that might be introduced in the
weld zone . Non-ferrous materials react differently to hydrogen.
Figure 1.11 shows the typical range for weld metal hydrogen content for different processes
and filler metals. The gas metal arc and gas tungsten arc welding processes generally
introduce the least amount of hydrogen since welding grade shielding gases have a low dew
point « -40oC) and therefore little moisture. For the remaining processes, the amount of
hydrogen introduced depends on the manufacturing process details and type of flux or
electrode covering.
Very low
hydrogen
Low
hydrogen
Non-hydrogen controlled
Non-hydrogen
controlled SMAW
!~
•
5
10
15
20
25
Weld metal hydrogen content, mll1 00 9 of deposited metal
Figure 1.11: Range of Weld Metal Hydrogen Levels
for a variety of Welding Processes (after Coe)
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Health and Safety
Like most manufacturing and fabrication processes, the welding operation presents various
hazards to the health and safety of the welder and personnel working near a welding
operation. These hazards include:
~
~
~
~
~
Electrical shock
Arc radiation
Smoke and fumes
Compressed gases
Other hazards related to specific processes, locations, etc.
These hazards are well recognized and when proper precautions are taken, welding is a safe
operation. It is therefore extremely important that before performing any welding operation,
the operator be fully aware of these precautions as well as be knowledgeable about the
equipment to be used and its operation. The reader is referred to Module 1 for detailed
guidance on the health and safety aspects of welding, and it is strongly urged to have the
knowledge therein before performing any welding.
With the above introduction to welding processes, the following sections describe arc welding
and other selected welding processes in greater detail.
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Chapter 2
GAS TUNGSTEN ARC WELDING (GTAW)
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
Principles of Operation .... . . ... . . ... .. ...... .. ......... . ........ . ..... 30
Current and Equipment .... .... . . . . .... .. . .....• •. ...... . . . . ......... .. 31
Welding Consumables .. ... ... . . . .. . .. .. . . . .. . . ••...... . . ............. 37
Applications and Limitations ... . •.........•. . .. . . ... . ..... . . ........... .42
Joint Design ........ . . . .. . .......... .. .. ... .. .. . ....... .•• .... . . ... .43
Arc Ignition (Starting the Arc) .. . ... ..... . . ..... .. •... . ..... ..•. .... . ... .44
Welder Technique . ...... ... . ................ . .. ... . . . .. .. . ...... . ... .45
Weld Quality .... . .. . ...... . . ... .. ... . . ....... . ... . .. ... ..• . . . ...... .47
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2. GAS TUNGSTEN ARC WELDING (GTAW)
2.1
Principles of Operation
Gas tu ngsten arc welding is a 'non-consumable electrode' arc welding process. As seen in
the schematic sketch in Figure 2.1, the electric arc is established between the workpiece and
a tungsten electrode which does not melt during the welding operation . The liquid weld pool,
surrounding area and the tungsten itself are protected from the atmosphere by a blanket of
inert gas. Inert gases, such as helium and argon do not engage in chemical reactions in the
arc or with the molten weld metal. The most common inert gas used is argon. Active gases
such as hydrogen are occasionally added to shielding gases used in the GTAW process.
If no filler metal is added, the weld formed is called an autogenous weld and is common when
thin materials are welded. For thicker materials, it becomes necessary to add filler material,
however, it does not form a part of the main welding circuit. The molten filler metal is not
transferred across the arc, rather, the arc heat simply melts and adds it to the weld pool.
0000
r
Travel
-
Current Conductor
Shielding Gas In
Gas Nozzle
Tungsten Electrode
--
?~----l7-+:
I
~:~::fied
Metal
Figure 2.1: Schematic Sketch of the GTAW Process
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Current and Equipment
Current and Power Sources
The welding circuit for GTA welding is shown in Figure 2.2. and its two main components are
the power source with appropriate controls and a torch. The power sources used for GTAW
are invariably constant current (see Figure 1.4). Transformer-rectifier type power sources are
the most common. With recent advances in power source technology, inverters are now
commonly used for GTAW. The 'constant current' characteristic of the power source helps to
maintain a nearly constant welding current even if the voltage (or arc length) changes in the
manual application of the process.
Cooling
Water
Supply
(may be deleted if
torch cooled by gas)
Inert Gas
Supply
GTAW Torch"
00 0
Filler Metal ~
Base Metal
0:0
"~--=:~,,,,-~/jtR~~==='in
00 Power
00
1-+++,,-;
Foot Pedal
(optional)
Water
Drain
Source
Gas
orHeatt~==§EI~e~ct~ro~d~e~L~e~ad~==~
Exchanger
Work Lead
Figure 2.2: Circuit Diagram for GTAW
Both alternating current and direct current of either polarity can be used depending on the
application (see Figure 1.9). When direct current electrode positive polarity is used, a cathodic
cleaning action takes place at the workpiece. This is most important in the welding of
aluminum and magnesium alloys. The cleaning action removes the refractory aluminum or
magnesium oxides formed at the workpiece surface. The heat produced at the electrode is
very high, (see Figure 1.10) reducing the tungsten electrode life or requiring the use of a larger
diameter electrode. Also, the penetration is relatively limited and therefore DeEP is used
principally for welding of thin aluminum only, and for surfacing applications to limit melting of
the base metal.
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Direct current electrode negative polarity is chosen most often for welding materials such as
steels, titanium, copper, beryllium, etc. and also thick section aluminum since th is arrangemen t
produces more heat at the workpiece and therefore provides deeper weld penetration (see
Figure 1.10).
Jf'Figure 2.3: Transformer/Rectifier Power Sources for GTAW
f
-f Fig 2.4: Inverter Type Power Source for Direct Current GTAW
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Pulsing the dc current, as in Figure 2.5, between a peak value and a background current at
0.5 to 20 Hz considerably expands the application of the process. Some of the advantages of
pulsed DCEN welding include reduced heat input and lower distortion while achieving good
penetration and fusion, and the ability to (i) weld materials of different thickness, (ii) weld in all
positions and (iii) weld over gaps.
Using a higher frequency (approx. 20 kHz) in pulsed GTAW gives a very stiff and stable arc
even if the average current is quite low. High frequency GTA welding is useful in automatic
applications or applications requiring a high degree of precision. However, the power sources
can be quite noisy depending on the exact frequency range.
PUISeTime
r
r
'
11 4 H
Cycle Time
[
Current, A
EI
D
r-
I--
r-
L....-
.:..
L--
Time
Peak Pulse Current
,....
r
Background
Current
L...-
DDO¢
Figure 2.5: CurrentlTime Relationship for Pulsed Current GTAW
Alternating current is used when the advantages of DC EN polarity as well as the cleaning
action of DCEP polarity are needed. As indicated in Figure 1.10, alternating current provides
penetration and cleaning action that are a compromise between DCEP and DCEN. There are
problems in the use of normal 60 Hz alternating current. Essentially, it can be visualized as
switching between DCEP and DCEN, 120 times a second and going through an arc
extinction/re-ignition step each time the polarity changes and current goes through zero as
shown in Figure 2.6(a). When the polarity changes from DCEP to DCEN, the tungsten
electrode is able to immediately emit electrons and reignite the arc. However, when the
polarity changes next from DCEN to DCEP, the weld pool is not able to emit electrons as
effectively due to its temperature and the presence of oxides . This leads to a delay in reignition of the arc which creates an unstable arc. This phenomenon is called partial
rectification.
33
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DCEP
Current
.'
.'.'
-
DCEN
Time
Detay
Figure 2.6(a): Partial Rectification in AC Welding
r
Unbalanced Wave
DCEP
Current
DCEN
Figure 2.6(b): Inherent Rectification in AC Welding
This problem was initially addressed by adding a high frequency generator to maintain a
conductive (ionized) path while current flow changes direction. The next generation of power
supplies were designed to supply a square wave output. Electronic switching is used to
change very rapidly from the electrode negative to the electrode positive half of the cycle so
that the time at low power is even shorter. The newest generation of power sources use an
additional power supply within the main power source enclosure to deliver an accurately timed
second output that is opposite in polarity to the main output, thereby virtually eliminating the
time at zero power.
While the above described approaches increase arc stability, the welding current is higher in
the DCEN half cycle because the tungsten electrode, due to its high temperature, emits
electrons at a very high rate as shown in Figure 2.6(b). In the DCEP half cycle, as noted
earlier, the electrons are not emitted as effectively and therefore the current is lower. The
resulting imbalance in the current amplitude wave is called inherent rectification. Modern day
electronic power sources are able to correct and control the imbalance in the wave since in
certain applications, one can emphasize the cleaning action or the penetration by setting the
imbalance at a specific acceptable level as shown in Figure 2.7.
34
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Balanced Wave
AC Balance
2;®,a
o
More Heat Into Workpiece
-
Electrode
-
35%
Positive
Electrode
Negative
65%
Greatest Cleaning Action
Ele ctrode
Positive
----
o
10
Max. Penetration
.....
55%
Elect(ode
Negative
2:@.a
-
....
10
Balanced
23~7a
1~9
45%
o
10
Max. Cleaning
Figure 2.7: AC Balance Control for Desired Weld Features
GTAWTorch
The GTAW torch conducts the electric current from the power source to the tungsten electrode
and the arc, and also provides the shielding gas to protect the weld zone from atmospheric
contamination. The torch may be gas cooled if its current carrying capacity is typically less
than 200 A, or water cooled for higher currents . A cross section of a water cooled torch is
shown in Figure 2.8.
In a conventional torch , the flow of the shielding gas at a short distance after exiting the nozzle
can become quite turbulent leading to ineffective gas shielding of the weld zone. Some
nozzles are specially designed (internal streamlining, flared ends, elongated trailing section) to
counter thi s, however, the use of a gas lens to provide laminar gas flow is the most effective.
A gas lens is made up of multiple layers of fine mesh screening and is designed to fit around
the tungsten electrode or the collet as seen in Figure 2.9. By providing a longer region of
uniform gas flow, the gas lens allows the welder to use a larger nozzle-to-work distance and
thus have a better view of the weld pool. Gas lenses are most appropriate for welding of
reactive metals and materials for service in severe corrosion environments.
35
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r Shlelding·Gas Outlet
, {Low Vetoerty)
J
/
f
I
,-
,~Co~et
/
,-
I
edlet
Hdder
r- Collet Nut
I
:"
,r - T()(ch
{
Cap
/
' .r"" !
~"
-- ... ---L
J- 0;£Eb:>-- -'
--"
~ ~;~ ~ -- '" ="-"-:J"
,':=:::;J
T.:::~:.::::.:
-',
.~.
.
~~-~-L- .~ ,
,I
-; '·L,WA
\
1.
',--- Nozzle
\. ~;
f
---~~~ /. C'L... ~
j 'I III . _I. .-+ t', I,~\
"'"
"
".'\\ \ ~
\, \ \
\
' - - Tungsten EIBCtrode
\~ y",'
(
\
Handle
r ....
\3>: ':~" _~\' ','," '~
" :..,
'"'
,'_
-
Water- Discharge
Hose
_
Cooling-WatH
,,
~(~~
:~
';~~
~'~~'(
'.- :~
\, ~,~
.. ,
\
~\\
'. ,-,,\~'., ~....
\
":""\
.
';':."' /11 '< ,
'Cooling Water Inlel. - -
"-'lIcl
PONcrCable
I
Shieldng-Gas Inlel - '
Figure 2.8: Schematic Cross-section of a Water Cooled GTAW Torch
Collet
Gas Streaming Patterns
Collet Body
Insulator
Gas Cup
~
Gas Lens
Integral with
Collet Body
/
--
Torch with Gas Lens
Electrode
Figure 2.9: A Torch With a Gas Lens
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Welding Consumables
Tungsten Electrodes
Tungsten electrodes are available with diameters ranging from 0.5 to 6.4 mm, and AWS
Standard A5.12 makes provision for seven types of tungsten alloys for use as electrodes in
the GTAW process. These alloys along with their AWS classification and application are
shown in Table 2.1 .
Table 2.1: Tungsten alloys for GTAW
Alloy
Pure tungsten (W)
W - 1% thorium oxide
W - 2% thorium oxide
W - 0.25% zirconium oxide
W - 2% cerium oxide
W - 1% lanthanum oxide
94 .5% W, remainder specified
by manufacturer
Application
AWS Classification
AC & DC welding
DC welding
DC weldinQ
AC welding
AC & DC welding
DC welding
Specified by
manufacturer
EWP
EWTh-1
EWTh-2
EWZr-1
EWCe-2
EWLa-1
EWG
COlour
Code
Green
Yellow
Red
Brown
Orange
Black
Grey
In general, alloyed tungsten electrodes have a higher current carrying capacity. At a given
current density, alloy electrodes operate at a lower temperature than do pure tungsten
eleclrodes . Also, the alloyed electrodes provide greater arc stability, easier arc starting and
less weld metal contamination due to erosion of the tungsten electrode.
The choice of the type and size of tungsten electrode for an application depends on the
operating current and current type as shown in Table 2.2.
37
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Table 2.2: Typical Current Ranges for Tungsten Electrodes of Different Diameters
Electrode
Dia. (mm)
1.6
2.4
3.2
4.0
5.0
6.0
DCEN
(EWX-X)
70 - 150
150 - 250
250 - 400
400 - 500
500 - 750
750 - 1000
DCEP
(E'M<-X)
10 - 20
15 - 30
25 - 40
40 - 55
55 - 80
80 - 125
AC Unbalanced W<Ne
AC Balanced W<Ne
EWP
EWX-X
EWP
EWX-X
50 - 100
100-160
150 - 200
200 - 275
250 - 350
325 - 450
70 - 150
140 - 235
225 - 325
300 - 400
400 - 500
500 - 630
30 - 80
60 - 130
100-180
160 - 240
190 - 300
250 - 400
60 - 120
100 - 180
160 - 250
200 - 320
290 - 390
340 - 525
Excessive current causes electrode erosion, ("spitting" tungsten droplets across the arc)
causing contamination of the weld metal. At very low current the arc will wander erratically at
the electrode tip and will be difficult to control. An electrode diameter should be chosen to
operate near the higher end of its current carrying capacity, without overheating.
Aluminum and magnesium are usually welded with alternating current. A variety of electrodes
can be used . Pure tungsten electrodes are quite common . The ends of pure tungsten
electrodes always melt, forming a ball equal to or even larger than the original tungsten
diameter. It is common to melt the end into a ball before use. This can be achieved by
striking an arc on a piece of scrap or a copper block using AC or DCEP polarity. Large balls
have a defocussing effect making it more difficult to make sound fillet welds in aluminum. To
solve this, alloyed electrodes, which can be pointed, are increasingly in use.
A few of the alloyed electrodes are
used for AC welding. Electrodes
with thoria (EWTh-1 or EWTh-2)
additions can split when used with
AC at high current levels and
therefore are usually avoided. Since
relatively high heat is developed at
the electrode with AC, pointed
electrodes can melt easily. The size
of the hemisphere for alloyed
electrodes should not exceed the
electrode diameter as it may detach.
The retention of a smaller ball is
promoted through the selection of a
larger diameter electrode and
pOinting it. See Figure 2.10. The
result is a smaller ball can be formed.
2,4mm
Diameter
'Balled'
Tungsten
Figure 2.10: Tungsten Preparation for Aluminum Welding
(Balled Tungsten)
38
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Gas tungsten arc welding power sources are available with 'AC Balance ' control enabling the
operator to adjust the heat released at the electrode during the electrode positive half of the
cycle. This adjustment allows the use of pointed alloy tungsten electrodes without forming a
"balled" end. Figure 2.10 describes the method of preparing a tungsten for welding aluminium
and other metals which form a high temperature oxide , when using a conventional
transformer/rectifier power supply.
Inverter based power supplies are available which allow the use of a sharpened alloyed
tungsten without a balled end. See Figure 2.11 for details of blunted end geometry used with
inverters. Transistorized switching to create alternating current from direct current provides
control over AC frequency. As the welding current frequency rises above 60 cycles per
second, the arc begins to focus on the end of the electrode increasing penetration and control
of the arc. Alloyed tungsten electrodes EWCe-2 and EWLa-1 are most commonly
recommended . (See Module 5 for details of power supply operation .)
Gas tungsten arc welding of steels and nickel alloys is performed using direct current,
electrode negative polarity. The tip of the electrode is ground to the shape of a cone, with a
smaller vertex angle for thin workpieces and larger vertex angle for thicker workpieces, and
Figure 2.11 shows the recommended vertex angles for manual welding. The vertex angle can
have a significant effect on the weld bead shape at high currents as shown in Figure 2.12, and
therefore for mechanized applications involving high currents, close attention should be paid to
the vertex angle for consistent results.
The ground, tapered ends of tungsten electrodes can become contaminated with embedded
metal particles if grinding stones or disks have been used for other purposes. To prevent this
from occurring , grinders should be designated "Tungsten Electrodes Only". Figure 2.13 shows
the recommended technique for grinding tungsten electrodes.
End Blunted After Grinding to Reduce
the Possibility of Tungsten Inclusion
2" d for Thinner
Materials (22°)
1/'2 d for Heavier
Materials (56°)
?
22°
_ _--,1/32
t
Vertex Angle
Figure 2.11: Recommended Tungsten Electrode Tip Geometry for Manual Welding
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v
v
Weld Widlh
(W)
Penetration
(P)
100 Amps
100 Am s
9
0'
0'
9
1200
Figure 2.12 : Effect of Tungsten Electrode Vertex Angl e (9) on Weld Width and Penetration
a) hold lightly
agai nst wheel
b)grinding marks " ......\'<>.\
should be alo ng le ngth
of tungsten
d) WRONG
c) blunt end slightly
to complete prepa ration
e) WRONG
Figure 2.13: Preparing a tungsten by grinding.
40
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Shielding Gases
Shielding gases used for GTA welding are completely inert since the presence of oxygen or
carbon dioxide will oxidize the tungsten electrode and limit it's life. Inert gases commonly
used are argon, helium and argon-helium mixtures at 99.9% purity or above. Gases do not
normally conduct electricity and must be forced to do so. This is called "ionizing". Each pure
gas requires a different amount of energy to cause it to become conductive, called "ionization
potential", measured in electron volts. The ionization potential of some pure gases are:
Table 2.3: Electron Voltages
ArQon
Hvdroaen
Helium
NitroQen
Oxvaen
15.8eV
13.6 eV
24.9 eV
14.5 eV
13.6 eV
Argon is normally the shielding gas of choice because of its:
smoother and quieter arc;
lower arc voltage for a given current and arc length , making it easier to weld thin materials
without burn-through;
greater cleaning action in welding aluminum and magnesium using alternating current;
easier arc starting;
lower flow rates for good shielding and better resistance to wind drafts;
lower cost and greater availability.
Helium, for the same arc length,
requires greater voltage than
argon leading to higher heat
inputs as demonstrated in Figure
2.14. Pure helium is therefore
used advantageously for shielding
when welding materials with high
thermal conductivity, for example
copper or th ick alum inum. Helium
is however lighter than air, and
not used alone when welding
other materials. More often, it is
added in varying amounts to
argon to achieve a combination of
advantages gases of both.
35 r-----------------------~
30
25
20
Arc Voltage,
V
15
10
Helium
~ArgOn
'-....._ - - - - -
5
00
100
200
300
400
500
Arc Current, A
Figure 2.14: Effect of Shielding Gas
on Gas Tungsten Arc Voltage
41
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Argon-hydrogen mixtures are also used but mainly for stainless steels , nickel-copper, and
nickel base alloys. These alloys are not prone to hydrogen cracking , however, the amount of
hydrogen in the mixture needs to be controlled to avoid porosity. The optimum amount of
hydrogen depends on the workpiece thickness and the joint type, and typically varies from 0.5
to 5%. Argon-hydrogen mixtures cannot be used for welding carbon steels.
Filler Metals
Filler meta ls when required are procured to the same specifications as those for gas metal arc
welding con sum abies. For GTAW, these can be in th e form of continuous wire to be fed
automatically, or individual rods for manual feeding into the weld pool.
2.4
Applications and Limitations
Two main advantages of the GTAW process are :
a high quality weld deposit which for carbon and low alloy steels is also low in hydrogen
content;
it can be used to weld virtually all weldable metals and alloys though it is not used
extenSively for joining cast or wrought iron.
The weld produced is smooth and attractive , requiring virtually no post weld cleaning such as
removal of slag. Autogenous welds as well as welds with filler metal addition can be made in
all welding positions, using manual, semi-automatic and completely mechanized modes of
application. Travel speeds for automatic, autogenous appli cations can be very high . Also,
heat input and filler material additions can be independently and reliably controlled.
GTAW is the process of choice for welding all thin materials when weld quality requirements
are demanding. As a result, it is used extensively in the production of aircraft and space
vehicles. In the latter, applications include the shell structure, tanks, and tubing in rocket
engines.
Two of the common applications of GTAW are orbital welding of small diameter tu bes and
pipes, and tube to tube sheet welds. In the former case, the entire process is automated and
the welding head traverses circumferential butt jOints between two fixed pipes. The we lding
position, 5G, continuously changes from flat to overhead (1 G to 4G), and vertical (3G) welding
is done in both downwards and upwards progression (3 O'clock and 9 O'clock positions,
respectively) . The weld can be thought of as a multi-layer but Single pass weld since after
starting the arc in the root, there shou ld be no stops until the jOint is completely filled. For tube
to tube sheet joints, the process is again automated, and the main differences with respect to
pipe butt joints are in the different joint design and a constant welding position. Also, these
automated applications generally use pulsed dc current that can be pre-programmed to
change in a prescribed fa sh ion as the welding arc moves along the circumference and the
welding position changes continuously.
42
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The main disadvantages of the GTAW
process are the special equipment needed for
arc ignition and its low deposition rate in
manual applications. As a result, it is seldom
used to weld materials more than 10 mm
thick. However, it is used to deposit root
passes for circumferential welds in thick pipes
because of control over underbead profile
and weld soundness. Subsequent passes
may then be deposited by other welding
processes.
When the GTAW process is automated, it is
highly controllable and automated filler metal
feed tremendously increases the deposition
rate. Further increase in deposition rate may
be achieved by preheating the filler metal
wire (by an electric current) before it is fed
into the weld pool. This variation of the
process is called hot-wire GTAW, where
deposition rates can exceed 5 kg/hr.
Fig 2.15: Orbital GTAW
GTAW is also used extensively for repair and maintenance of components made from tool and
die steels, aluminum and magnesium alloys , or other critical components.
2.5
Joint Design
The jOint design for GTAW depends on several factors including the base material to be
welded and its thickness, absence or addition of filler metal, weld penetration requirements,
etc., and therefore optimum joint designs should be confirmed by performing procedure
qualification tests.
43
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2.6 Arc Ignition (Starting the Arc)
Initiating an arc across a small gap wilhout the tungsten electrode touching the workpiece
requires a high voltage, of the order of 300 V, which can be a safety hazard. Therefore, three
other methods have been developed to help initiate the arc. These are:
(i) Touch start: In this method, the torch is lowered to allow a momentary contact between the
electrode and the workpiece which sends a surge of current and heats the electrode. Next,
when the electrode is quickly pulled back and withdrawn, an arc is established as shown in
Figure 2.16. Though the method is simple, there is a possibility of the electrode sticking to the
workpiece, and causing electrode and weld zone contamination.
(ii) High Frequency Start: Suitable for both ac and dc power sources, this approach adds a
high frequency generator in the circuit which superimposes a high voltage ac output at radio
frequency on top of the normal power source output shown in Figure 2.17. This high voltage
enables the arc to be initiated but because of its nature, is not hazardous to the welder.
However, the generator output can interfere with radio, electronic and computer equipment,
and therefore such equipment should be used with due care following the manufacturers
recommendations.
High Surg e of Current --t~.
+
Figure 2.16: Touch Start Electrode Method for Arc Ignition
44
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(iii) Electronic Touch Starting: This approach involves passing a low current through the
electrode in contact with the workpiece. This low current preheats the electrode so that when
it is lifted, an electronic control signal sends a surge of current for arc ignition. Within a
fraction of a second the current is returned to a value set for welding. The short current surge
is relatively low since the electrode tip is already preheated. While it eliminates the problem of
tungsten electrode and weld zone contamination, the approach is suitable for dc welding only.
2.7 Welder Technique
In manual application of the GTAW process, the welder technique is critical in obtaining sound,
clean and porosity free welds. The main features of correct welder technique for manul GTA
welding are:
High Frequency
Generator
Power Source
•
Work
Piece
+
Figure 2.17: High Frequency Generator for Arc Starting
45
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Once the arc has been initiated, develop the weld pool by holding the electrode vertical
and applying a small circular or side to side motion. This should be possible within 3 to
5 seconds. If it takes longer, the arc is too cold and current needs to be increased.
Similarly, if it takes less than 3 seconds, the arc is too hot and current needs to be
decreased.
To start welding, tilt back the torch slightly so that a travel angle of 5 to 15 degrees is
maintained (Figure 2.18(a)). Excessive angle can cause air entrainment which could
lead to porosity or other problems. A forehand/push technique is followed so that the
electrode points in the direction of electrode travel. (See Section 1.3 for travel angle
definition.)
Filler rod, if used, is added by rapidly dabbing it in and out of the leading edge of the
weld pool (Figure 2.18(b)) often rocking the torch towards the back of the puddle in time
with filler metal additions, ensuring all the time that end of the rod always stays within
the gas shield so that it does not oxidize, especially in the case of stainless steels,
nickel or titanium alloys .
...---:,,
,,
,,
,,
,/~
,,,
,,
Tilt Torch
Back Slightly
Add Filler
~DO Travel
.-------'"'<0..,..+"'·----,
~DO Travel
15'
,-------,'-,<,
'A-.'· - - - ,
r------I'---_'-./_-'
(a)
(b)
Figure 2.18: Manual GTAW Technique
To maintain adequate gas shielding, extension of the electrode beyond the gas cup
should be minimized. Recommended values are 3 to 5 mm for thick materials and 8 to
10 mm for thin materials. A short arc length should be maintained and this is helped by
keeping the standoff distance to less than the diameter of the gas cup (Figure 2.19).
46
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Minimize Extension of
I
Electrode Beyond the ________
Gas Cup
~ ________
Standoff Distance Should
Not Exceed Diameter of Gas Cup
Use a Short Arc
Figure 2.19: Electrode Extension and Standoff Distance
to Maintain Proper Gas Shielding
When welding is to be stopped, the current is slowly decreased while the filler metal is still
added. Otherwise, there will be an unfilled crater, and in susceptible materials like some
aluminum alloys, crater cracking can result.
2.B Weld Quality
As mentioned earlier, GTAW generally provides a high quality deposit. Nonetheless, weld
metal flaws do occur in GTA welds. Some of the common ones encountered and their likely
causes are:
Porosity : Is usually caused by inadequate gas shielding, contaminated workpieces or
contaminated filler material. Inadequate gas shielding may be due to inadequate gas delivery
due to clogging, leaking in the gas system, or too high (causing turbulence and mixing with air)
or due to wind and drafts.
Tungsten inclusions : Particles of tungsten that are introduced into the weld metal from the
tungsten electrode. Common causes of this are: the tungsten electrode touching the weld
pool, or the filler metal, electrode size and type that is incompatible with the current level, and
excessive extension of the electrode beyond the collet leading to overheating of the electrode.
Solidification Cracking : These cracks commonly form along the centerline of the weld bead ,
especially in the craters. To minimize its incidence, it should be ensured that the correct filler
metal is used, and that, in manual welding, it is added uniformly and not intermittently. Proper
crater filling te chn iques and use of run-on, run-off tabs help minimizing this flaw.
47
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Electrode contamination: This may also be caused by inadequate gas shielding, excessive
electrode extension or excessive torch angte.
Dirty weld bead: Dirty weld beads are invariabty caused by contamination. Sources of
contamination can be (inadequate) gas shielding, air and water leaks in the gas system and
torch, inadequately cleaned workpiece or filler material, and electrode contamination.
48
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Chapter 3
SHIELDED METAL ARC WELDING
3.1
3.2
3.3
3.4
3.5
3.6
Principles of Operation ... . . .. . . . .• •.. . .. . •........ .. ...... .. .. •...... .50
Power Sources ............ . .. . . .. . . . ....... .. . . . ... . ...... .• •. . .. .. .51
Types of Electrodes .... . ......... . .... .... .... .. . .. . • ...... . . ....... .55
Classification of Electrodes ............ .. . . . .. ... . . ......... . .. .•• . . , .. 56
Applications and Limitations of the SMAW Process .. . . . ...... . . . . .. .. . ..... .59
Shielded Metal Arc Welding of Carbon and Low Alloy Steels . . .. . . ..... . ...... 61
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SHIELDED METAL ARC WELDING
Shielded metal arc welding (SMAW) or manually operated metal arc welding with covered
electrodes is still a commonly used welding method. It allows the greatest amount of flexibility
in terms of the range of materials and thicknesses that can be joined in all welding positions.
However, there are other arc welding processes that provide higher depOSition rate, and for
large thicknesses. the shield metal arc process becomes uneconomical.
The knowledge of joint design, arc action, heat control and metal reaction gained from
shielded metal arc welding has been of great value in developing all other variations of the arc
welding process.
3.1
Principles of Operation
In shielded metal arc welding, an arc is established between the end of a covered metal
electrode and the workpiece to be welded. The heat of the arc melts the surfaces of the joint
as well as the metal electrode . The filler metal is carried across the arc into the weld joint and
mixes with the molten base metal. As the arc is moved at a suitable travel speed along the
joint, the progressive melting of the metal electrode and the base metal provides a moving
pool of molten metal which cools and solidifies behind the arc (Figure 3.1).
Electrode Coating
1--
- --
- Electrode Wire
Arc
Protective Gas From
Electrode Coating
Molten Metal
Slag
___
Solidified Weld
Metal
Metal Droplets
--;-~-:~-------~:::;;;,';:::k~.2....T
Base
Metal
Figure 3.1: Schematic Sketch of the Shielded Metal Arc Process
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The electrical circuit for shielded metal arc welding is relatively simple and is shown in Figure
3.2. It comprises a power source with electrical leads connected to the workpiece and the
electrode holder. The arc characteristics, weld bead shape and weld metal soundness and
properties depend on the selection of the type of power source, electrode, joint design, as well
as welding parameters and welder skill.
Electrode Holder
Electrode Lead
Electrode
Base Metal
-~+
Figure 3.2: Electrical Circuit for SMAW
3.2
Power Sources
The power source used for SMAW is a constant current type, i.e., it has a drooping voltampere curve (see Figure 1.4). With such power sources, the welder sets the required current
at the power source and the voltage is controlled by the arc length that the welder maintains
during welding . The drooping power sou rce is preferred because there is a sma ll but continual
variation in the arc length due to the manual nature of the welding process. This is reflected in
a continual change in the arc voltage but due to the drooping characteristics of the voltam pere curve, the accompanying changes in the arc current, and therefore the electrode melt
off and deposition rates are smal l. Figure 3.3 shows typical SMAW power sources.
~",,(......oR.r U>M-S~ ~ c..\.4,fT~Y \,}e~S 0" ~S.
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.. ...... II----
\r ~,
Different power sources however
. .... ..
.
--.- ,
.
.
may have different slope (or
.
.
.
.
.
incline) for the volt-ampere
curve, and some machines are
designed to enable some
adjustment of the slope. Figure
3.4 shows that when the voltampere curve is flatter, there is a
greater change in current for a
given change in voltage. The
adjustment of the slope of the
volt-ampere curve enables the
welder to maintain better control
of the weld pool and penetration
in certain situations such as out
of position welding (vertical or
overhead positions) or
depositing a root pass in a pipe
over a varying gap. For
example, by adjusting the voltampere curve to be flatter; an
intentional increase in the arc
length caused by a welder
pulling the electrode away
increases the arc voltage and
decreases the current sufficiently
to reduce penetration or risk of
burn-through .
Conversely, electrode sticking is
Figure 3.3: Typical SMAW Power Sources
also prevented when the rod is
in near contact with the base
material, the arc length and therefore voltage are reduced, and the current increases
sufficiently to increase the burn off rate to prevent sticking . With a pure drooping (or vertical)
volt-ampere curve, there would be no change in the current due to change in voltage or arc
length. The welder would have no control over the electrode burn-off rate in this case.
.. . .. .. .. . .. .. . .. .. .. .. ... . . . ..
'-." ,'
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100
~
Maximum OCV
Lower slope gives a greater
change in welding clJrrent for a
given change ill arc voltage.
Voltage
50
32
27
22
Long Arc
}
Normal Arc Length _+--I"~~___ Arc
Short Arc
Voltage
Current, A 100J 15LL
t;1
200
Figure 3.4: Change in Current Due to Change in Voltage
in a Constant Current Power Source
Power sources are available to provide direct current (DC), alternating current (AC), or both.
Transformer or an alternator type of source is used for AC welding, and transformer-rectifier or
motor generator type for DC welding. Some power sources (single phase transformer-rectifier
or alternator rectifier type) can be used for AC and DC welding. Inverters are becoming'
popular due to their portability and smooth operating characteristics. Figure 3.5 shows a
typical inverter power supply.
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~I
I
!
!
Figure 3.5: Portable Inverter Type Constant Current Power Source
In addition to the type of power sources selected, adequate size in su lated electrode holders
must be used together with suitable size cables to prevent excessive power loss during
welding. A common type of electrode holder is shown in Figure 3.6, and recommended cable
sizes are shown in Table 3.1.
~:=:lj[~~ii~'~I!'~"~~'~'~~~~~~
l
Hing e Pin
/)
Return
""
,-
1----...
,/, ,
Spring--</u1
Cable
Clamp -
-
--11-1
Handle
Electrode --~II
Figure 3.6: Example of an Electrode Holder
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Table 3.1 : Recommended cable sizes for SMAW (from The Procedure Handbook of Arc
Welding published by The Lincoln Electric Company)
cy~~~: %
in
Copper cable sizes for Cv
~
~
400
500
600
650
,
.
20
20
30
50
60
20
30
60
60
60
60
60
(in ",ele,,) of
0'00"
vud
and
Qound cable
UP to 15
100
180
180
200
,ul" o v I~ng!hs
#B
#5
114
1/3
#2
114
J I_3
1/1/0
1/2/0
#2/0
#3/0
#3/0
15 - 30
#4
#4
#4
113
1/2
113
113
30-45
#3
#3
#3
#2
#2
#2
1/2
111/0
1/1/0
#2/0
#3/0
#3/0
11410
11210
- 11210 #3/0
#3/0
45·60
112
#2
#2
111
#1
1/1
1/1
1/210
1/3/0
1/3/0
#4/0
•
60 -75
111
'. 111
#1
#1/0
11110
1/1/0
1/1/0
113/0
11410
1/4/0
....
• Use double strand of #2/0." Use double strand of 1/3/0.
3.3 Types of Electrodes
Electrodes for shielded metal arc welding generally comprise a coated, solid electrode wire
(core) of limited length. Occasionally, the solid electrode can be repla ced by a metallic sheath
containing metal powders with the objective of adding specific alloying elements to the weld
metal.
The covering on the electrodes can be applied either by an extrusion process or by dipping,
though extrusion is far more com mon . The covering itself contains several ingredients
depending on the type of electrode. The function of these ingredients is generally one of the
following :
55
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provide a gas shield to prevent contamination of the weld metal by atmospheric gases;
provide a slag cover to protect the hot weld metal from atmospheric contamination ; and
controlling the bead shape;
scavenge some of the impurities in the weld metal;
stabilize the arc by promoting electrical conduction across the arc; this is especially
important for AC welding where the arc effectively goes out and needs to be reestablished
after each current reversal;
provide a means to add alloying elements to enhance mechanical/corrosion properties, and
iron powder to increase deposition rate.
Each electrode classification produces different amounts of gases and slag to shield the weld
metal. Electrodes that rely on slag to protect the metal can carry higher current and provide a
higher deposition rate. Conversely, electrodes producing a smaller amount of slag and relying
on the gas shield are stable to operate at lower currents and therefore are more suitable for
out of position welding.
All electrodes can be used with direct current though some are designed for use with AC also.
Use of AC reduces arc blow and voltage drop in welding cables.
Direct current (DC) power has certain advantages:
~
~
~
~
easier arc initiation
better arc stability
good wetting action
ability to maintain a short arc
Direct current is especially useful for applications requiring small diameter electrodes and low
currents, e.g., out of position welding, welding of thin materials, etc. When direct current is
used for SMAW, DCEP polarity provides deeper penetration and DCEN polarity provides a
higher electrode melting rate.
3.4
Classification of Electrodes
Shielded metal arc welding electrodes are available for welding of carbon and low alloy steels,
stainless steels , cast irons, and aluminum, copper and nickel and their alloys. However,
electrodes for welding carbon and low alloy steels and for stainless steels are of greatest
commercial significance, and systems for their classification as described in CSA Standard
W48 are summarized here. For more details, see Module 6 - Electrodes and Consumables of
the MLS Series.
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Carbon and low alloy steel electrodes
The electrode designation comprises the letter E (for electrode) followed by 4 or more digits,
e.g ., E4918 for metric designation (E7018 is used for imperial). The first two digits (metric)
after E indicate the minimum tensile strength of the weld metal in increments of 10 MPa when
used to make a welded joint in a prescribed manner. The first two digits detail tensile in ksi
imperial designations. In complete penetration groove welds, the minimum tensile strength of
the weld metal should normally be equal to, or slightly greater than the minimum specified
tensile strength of the steel being welded. This is not essential for partial penetration groove
welds or fillet welds as long as weld sizes have been determined in light of the service loads in
accordance with CSA Standard W59 or other equivalent standards.
The third digit indicates the welding position for which the electrode is designed with 1
meaning suitable for all welding positions (flat, horizontal, vertical and overhead), 2 suitable for
horizontal fillet and flat positions only and 4 meaning suitable for vertical, downwards
progression only. The fourth digit (metric) indicates the usability characteristics of the
electrodes (type of coating, welding current type, etc.). Thus, 0 and 1 at the fifth digit position
indicate cellulosic covering, 2 and 3 indicate covering containing rutile, 8 indicates low
hydrogen, iron powder containing covering, etc. The first five digits may be followed by
additional letters and digits which are usually indicators of weld metal toughness or alloy
content. Further details about digits in the fourth position, suitable current type and polarity for
each, etc. can be found in CSA Standard W48 or from electrode manufacturers.
E 49 1
'
L'=
~-X
-'1
Arc welding _ _ _ _ _ _ _
electrode
Minimum tensile strength of
deposited weld metal in
megapascals (Mpa)
Optional designator for improved
notch toughness
Electrode covering type
and type of current
Welding positions of useability - - - - - '
The usability characteristics of some of the more commonly used electrode types can be
summarized as follows:
EXX10 :
high cellulose, sodium compounds for arc stability, dc electrode
positive polarity; deeply penetrating arc; suitable for all welding
positions; may be used for welding from one side with adequate back
bead profile; 5 mm or smaller diameter electrodes used for all position welding;
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EXX11 :
high cellulose, potassium compounds for arc stability, AC or DC
electrode positive polarity; otherwise similar to EXX 10 electrodes;
EXX12:
high titania with sodium compounds, ac or dc electrode negative
polarity; medium penetrating, quiet arc; most often used for single
pass, high speed, high current, horizontal fillet welds;
EXX13:
high titania with potassium compounds, similar to EXX12 type; used
for sheet metal work for vertical down welding; provides better
radiographic quality in mUltipass welds than EXX12 electrodes;
EXX14:
high titania and iron powder covering; AC or DC either polarity; similar
to EXX12 or 13 but with iron powder providing a higher deposition
rate;
EXX15 :
basic covering with sodium compounds; dc electrode positive polarity;
limestone and other basic ingredients in the covering provide weld
metal with good toughness and low hydrogen content; also suitable for
welding high sulfur steels; usually 4 mm or smaller diameters are used
for all position welding;
EXX16:
basic covering with potassium compounds; ac or dc electrode positive
polarity; otherwise similar to EXX15;
EXX18:
basic, iron powder covering; similar to EXX15 or 16 but with iron
powder in the covering thus providing higher deposition rates; most
structural steels are welded with EXX18 type of electrodes;
EXX22:
iron oxide covering; ac or dc either polarity; used for single pass, high
speed, high current flat and horizontal lap and fillet welds in sheet
metal;
EXX24:
titania, high iron powder covering; AC or DC either polarity; similar to
EXX14 electrodes but restricted to welding in flat and horizontal
positions; used mostly for fillet welds;
EXX28:
basic, high iron powder covering; AC or dc electrode positive polarity;
similar to EXX18 but with higher iron powder content; suitable for
welding horizontal fillets and flat position welds only;
EXX48:
basic, iron powder covering; AC or dc either polarity; also similar to
EXX18 but designed for welding in the vertical position with
downwards progression.
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)JO e..-lec.-\-cde..s o\\. 6x..0...""Stainless Steel Electrodes
Requirements for covered electrodes for welding stainless steels are included in CSA
Standard W48. These electrodes are classified based on the chemical composition of the
undiluted weld metal, the welding position and the type of welding current for which the
electrode is designed. A typical designation can be represented as EXXXxx-XX where E
represents electrode, and the next three digits and any letters immediately thereafter (e.g.,
309L, E310M) indicate the weld metal composition . The last two digits are usually 15, 16, 17
or 26 where digit 1 indicates suitability for all position welding for electrode diameters up to 4
mm. Conversely, digit 2 indicates suitability for flat and horizontal positions only. The number 5
indicates that the covering contains calcium carbonate (limestone) and sodium silicate, and
that the electrode is suitable for welding using dc electrode positive polarity. The letter 6
indicates the presence of titania and potassium silicate in addition to the calcium carbonate.
The presence of potassium compounds makes the electrode suitable for AC welding. The 7
signifies an acid flux with a significant amount of silica which makes the slag more fluid .
E 308 ·1~
"E" electrode
~
IL
L
alloy type - - - - - - - '
flux type
positions of usability
The EXXXxx-15 electrodes provide a more penetrating arc, and a convex and coarsely rippled
bead. These electrodes are preferred for out of position welding since the slag solidifies
quickly. The EXXXxx-16 electrodes provide a smoother arc, less spatter, and a finely rippled
bead . These electrodes are less popular for out of position work because the slag is quite
fluid. For more details, see Module 6 - "Electrodes and Consumables" of the MLS series.
Handling and Storage of Electrodes
The electrodes should be handled with care to ensure that the electrode covering does not
chip off. Unopened boxes should be stored at 30C ± 1DoC with relative humidity being less
than 50%. Cellulosic electrodes (EXX10, EXX11) are manufactured with a certain amount of
moisture in the coating (3%-7%). The performance of these electrodes could be adversely
affected if they are exposed to excessive amounts of moisture or have been dried out in an
electrode oven.
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Electrodes with basic (low hydrogen) coatings (containing calcium carbonate) are prone to
moisture absorption from the atmosphere and therefore should be packaged in hermetically
sealed containers. Once a box is opened, the electrodes should be removed from their
packaging and stored in a holding oven at a temperature of about 120·C. Also, if the basic
electrodes for welding carbon and low alloy steel have been exposed at ambient temperature
for 4 hours or more, or if their packaging has been damaged, they are to be rebaked at a
temperature (370° to 430°C) and for a time (1 to 2 hours) recommended by the electrode
manufacturer. Cellulosic electrodes however should not be placed in holding ovens or
rebaked.
3.5
Applications and Limitations of the SMAW Process
The shielded metal arc welding process can be used to weld most metals and alloys of
engineering significance. It has been extensively used to weld all types of steels (carbon and
low alloy steels, stainless steels, etc.) in the fabrication of pressure vessels, oil and natural
gas pipelines, field storage tanks, bridges, buildings, ships and offshore structures, railway
cars, trucks and automobiles, nuclear power stations, and numerous other miscellaneous
products including those made from cast iron. Amongst the non-ferrous alloys, the shielded
metal arc welding process is used for welding nickel and nickel-based alloys and to some
extent copper alloys, such as bronzes. Though electrodes are available, it is not popular for
welding aluminum alloys. The process is also used for hardsurfacing various components
exposed to wear, impact, corrosion and heat.
The shielded metal arc welding process is usually the most appropriate for repair and
maintenance welding since each job is usually a one-time-only situation, the amount of
welding required is relatively small and in-situ locations are most suitable for the shielded
metal arc process only. The process is also frequently the only one in shops where welding
constitutes only a small portion of the complete manufacturing process. The shielded metal
arc welding process is also generally the easiest to use in the field due to the simplicity of the
equipment and its tolerance to the normal outdoor environment. Nonetheless, it is advisable
to install protective enclosures when welding in the field in order to get protection from rain,
wind, etc.
The advantages of the shielded metal arc welding process thus include its applications to a
variety of materials, and the ability to weld in all positions (vertical and overhead as well as flat
and horizontal) and at most locations. As well the equipment required is easily portable and
relatively inexpensive. The main limitation of the SMAW process is the necessity of frequent
breaks as each electrode is consumed to about 50 mm of its original length and a new one
used to re-initiate the welding operation. This frequent change of electrode along with the
need to chip off the slag means that duty cycle (percentage of time that an arc is maintained
for the purposes of welding) is less than 20% and the deposition rate is low. Also, the
unusable electrode stubs add to waste and cost of the filler material.
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Shielded Metal Arc Welding of Carbon and Low Alloy Steels
Joint Oesign
For base metal thickness up to about 6 mm, a square groove with suitable root opening may
be employed for a complete penetration groove weld provided that welding is performed from
both sides and in the flat position. At the low end, a skilled welder can weld base metal as
thin as 1.6 mm. For larger thicknesses, the base metal edges must be beveled, and in very
thick sections, J- and U-grooves become more economical by reducing the weld metal volume
required . The root gap for groove welds is typically equal to the electrode diameter in order to
achieve complete penetration and the groove angle should be large enough to achieve side
wall fusion and minimize slag entrapment. In assembling a joint for welding , the fit-up should
be good enough to maintain the groove geometry within acceptable tolerances. Thus, too
small a root gap or misalignment between the two members to be joined can locally lead to
incomplete joint penetration. Fit-up tolerances and workmanship and some prequalified joint
geometries given in CSA Standard W 59 "Welded Steel Construction" are shown in Table 3.2
and Figure 3.7, respectively.
Table 3.2: Fit-up and Workmanship Tolerances for SMAW Groove Welds
Root Not Gouged
Root Gouged
1. Root Face of Joint
:+:2mm
Not limited
2. Root Opening of Joints:
Without Steel Backing
:+: 2 mm
+ 2 mm
- 3 mm
+ 6 mm, -2 mm
Not applicable
+ 10" , _5 "
+10 " ,_5 "
With Steel Backing
3. Groove Angle of Joint
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/
G
~----~
I/
G
~~____~ ____~
1
J>
IT
L...--TT-\::::
' ===7-T~---l----.!.
--
G
T
Backing Strip
m "
m"
=T
= 10
/T(T[[I
8
G
~e
IT
4
Backing Strip
e
G
Positions
20·
30·
12
10
F. a only
45·
6
Fva
60·
5
F. v.a
--+ G ~
Figure 3.7 - Typical Pre qualified Complete Joint Penetration Groove Welds for the Shielded
Metal Arc Welding Process (SMAW)
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Welding Positions
Though it is always preferable to perform welding in the flat position since less welder skill is
required and higher deposition rates are possible, it is sometimes necessary to perform
welding in vertical or overhead positions. In such situations, the SMAW process has the
flexibility to be used for out of position welding since the force of the arc will propel the molten
metal in a spray of globules in any direction required. However, a more skilled welder is
generally required and the joint design may be somewhat different from that for welding in the
flat position.
Selection of Electrode Diameter and Current
The classification and size of electrode, and the welding current for a given application are
chosen in light of the thickness of the material to be welded, groove geometry and welding
position. Generally, larger diameter electrodes are used for welding thick materials and in the
flat position so that higher deposition rates can be achieved. Smaller diameter electrodes are
generally needed for welding the root passes in V -grooves and for out of position welds so
that the welder can have better control of the weld pool and the bead shape.
For prequalified joints, CSA Standard W 59 "Welded Steel Construction" limits the maximum
electrode size to 4 mm for welding in the vertical position (fillet and groove welds), and to 5
mm for groove welds in horizontal and overhead positions, and fillet welds in the overhead
positions. Larger diameter electrodes are used for welding in the horizontal and flat positions
only.
Table 3.3 shows typical current ranges for satisfactory electrode burn off and stable arc
conditions using steel electrodes of various diameters. However, the complete range of
current may not be suitable for all situations. When welding on thinner material, the lower end
of the range might be applicable. This would also apply when welding in the vertical or
overhead positions. For example, 3.2 mm diameter E4310 electrode, according to Table 3.3
has a usable current range of 75 to 125 A. For joining heavy material in the flat position, it
would be logical to use the upper part of the range, 100 to 125 A. But if welding is to be done
in the vertical up position, the range might be 90 to 110 A. In practice, an attempt is made to
position the work piece such that welding is performed in the flat position , where ever
practicable, so as to permit the use of larger diameter electrodes and higher currents, thus
providing a higher deposition rate.
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Table 3.3: Typical Current Ranges in Amperes for Electrodes of Different Diameters (from
CSA Standard W48)
Electrode
E4XOO
E4X 10
E4 X 11
Diameter,
Mm
1.6
2.0
2.5
3.2
4.0
5.0
6.0
8.0
Electrode
diameter,
.
45 - 85
75 -125
110 - 170
155 -235
190 - 290
275 - 425
E4915
E4916
E4 X 12
E4 X 13
20-40
25-60
40 - 90
80 -140
110 - 190
155 - 265
225 - 360
300 -500
E4918
20-40
25 - 60
50 - 90
80 -130
105 - 180
165-250
225 - 315
320-430
E4924
E4928
80 -110
115-165
150 - 220
220 - 350
285 - 360
375 - 470
110-160'
140 - 190
180 - 250
250 -335
300 - 390
400 - 525'
E4X22
E4 X27
E4914
.
.
.
.
-
90 - 135
110 - 160
150-210
220 - 300
295 - 375
390 - 500
110-160
140 - 190
200 - 410
380 - 520
-
125 160 230 270 375 -
185
240
330
380
475
E4948
mm
2.5
3.2
4.0
5.0
6.0
8.0
70 - 120
110 -150
140 - 220
200 -280
270 - 350
375 -475
80 - 140
150 - 220
210 - 270
-
• These values do not apply to the E4928 classificalion.
Deposition Rate
Deposition rate for any arc welding process is the amount of weld metal deposited in a given
period of time . For the shielded metal arc welding process, it equals the amount of electrodes
used up in a given period of time less stub losses, and losses due to spatter and formation of
slag. The melting rate of electrodes will depend on the current, size of electrode and type of
coating. Some electrodes contain iron powder in the flux which increases the deposi tion rate
for a given current. Arc Voltage has little effect on the melting rate as seen in Figure 3.8, but
the melting rate increases approximately in proportion to the current.
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----I ~_
I-
Electrode
Melting Rate
Grams/Sec
1.20
1.00
0.8
0.6
-
Variation of Melting Rille
l'IiUI Vol/age
_ _ Variation of Melting Rate
with Current
200 Amperes
--------
0.4
0.2
0.0
a
10
20
30
40
Voltage
a
40
I
I
80
120
160
200
240
260
Current
Figure 3.8: Effect of Current and Voltage on Electrode Melting Rate
The deposition rates achieved with va rious electrodes for joining welda ble structural and
pressure vessel steels are shown in Figure 3.9.
14
12
10
Depos ition Rate
Il b/hr)
B
6
4
2
100
150
200
250
300
350
400
Welding Current AC (amp)
Figure 3.9: Deposition Rates for Various Mild·Steel Electrodes
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Welder Technique
A welder has control over three aspects of the welding technique that influence weld quality
and incidence of flaws such as arc strikes, undercut, porosity, slag inclusions, etc. These are:
proper striking of the arc, and maintaining the correct work and travel angles (see Section 1.3
for definitions). Arc may be initially struck either by tapping the electrode on the workpiece
along the joint or scratching the electrode along the workpiece (Figure 3.10) and quickly
withdrawing it to be about 3 mm from the work surface. Once an arc is struck, the electrode
should be held at the starting point of the weld until the weld pool begins to form and is about
twice the diameter of the electrode. Then the electrode is moved along the joint at a speed
that keeps the weld pool at a uniform size.
Scratching
Tapping
~
- - Electrode
-
Electrode
~
I
I
I
I
, *0
r-=:------~~
Plate
Figure 3.10: Striking an Arc
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The travel angle for shielded metal arc welding is typically 5 to 10 degrees dragging though for
electrodes with heavy iron coatings, it can be in the 10 to 30 degree range. Except for vertical
up welding , the electrode should point in a direction opposite to the welding as seen in Figure
3.11 (drag or backhand technique); in the vertical up position, the electrode points in th e
direction of we lding as shown in Figure 3.12 (push or forehand techn ique).
Travel Angle - -
5° - 10°
Travel
I~
- - - Electrode
D DOc)
Figure 3.11: Electrode Travel Angle (backhand technique)
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Travel
{[
o
o
o
-------------------------7--- -~~-~-_______
10 0 - 20 0
--~--~
~'~~----------
Electrode
Figure 3.12: Electrode Travel Angle in Vertical Up Welding (forehand technique)
The work angle is typically 90 degrees for groove welds and 35 to 55 degrees for fillet welds
depending on the welding position (see Figure 3.13).
90 0
Fillet Weld
Groove Weld
Figure 3.13: Work Angle in Groove and Fillet Welds in the Overhead Position
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Examples of SMA W procedures
CSA Standard W59 on "Welded Steel Construction", limits layer thickness in multipass groove
welds to 6 mm for the root pass and 5 mm for subsequent layers. Similarly, the maximum
prequalified single pass fillet sizes are 10 mm, 8 mm and 12 mm in flat, horizontal or
overhead, and vertical positions, respectively. However, it may not always be desirable to
employ a welding procedure that maximizes the layer thickness or the fillet weld size because
the accompanying increase in current and heat input may adversely affect mechanical
properties of the welded joint, especially its notch toughness.
Typical welding procedure schedules for a butt joint in 40 mm thick G40.21 Gr 350WT steel in
the flat position and for fillet welds in the horizontal position are shown Figure 3.14.
Side 1
~
25
3
Sid82
~
Sida1
Beads 1 & 2
Side 1
Side 2
Beads 3 & 4
Beads 5 10 Fill
Side 1
Fill
4mm
5mm
4mm
5mm
E4918
E4918
E4918
E4918
(E70181
(E7018)
(E70181
(E7018)
165A
200A
2QOA
200A
(a) Groove Weld
Fillel Size
5mm
6mm
8mm
Electrode
E4918 (E7018) 4mm
E4918 (E7018) 4mm
E4918 (E7018) 4mm
A
emlmln
210
25
210
15
210
9.5
(b) Fillet Weld
3.14: Typical Welding Procedures for a Complete Penetration Groove Weld and Fillet Welds
in Carbon Steel
Weld Soundness
The soundness of a shielded metal arc weld depends on the correct selection of the electrode
type, electrode size and welding current, keeping in mind the welding position, workpiece
thickness, joint design, etc. Out of position welding and thinner workpieces require lower
currents to control the weld pool and avoid burn-through.
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Welder technique and skill is another important factor. Maintaining an appropriate travel
speed and arc length, correct travel and work angles and suitable electrode manipulation are
some of the factors in this category. Manual metal arc welding requires experienced welding
operators. Simple welding operations may be taught in a few weeks, but an operator capable
of making satisfactory welds in all positions, on a variety of metals, is a highly skilled artisan,
usually of long experience.
Some of the weld metal flaws that can be present in the weld are:
Porosity: Holding a proper arc length and using proper current can help minimize
porosity.
Slag inclusions: Slag inclusions can be prevented by proper contouring of the weld
bead surface before depositing the next pass, by proper welder technique that ensures
that the arc stays at the leading edge of the pool and that slag does not run ahead, and
by exercising care in interpass slag removal.
Incomplete fusion! penetration: Depending on the exact situation, one or more of the
following can help eliminate this flaw: smaller root face, larger root gap, larger groove
angle, smaller diameter electrode, higher current, slower travel speed, smooth and
cleaned surface before depositing the weld.
Undercut: Lower current and shorter arc can help reduce undercut. Welder technique
(electrode position, travel speed) are other factors influencing the occurence of
undercut.
Cracks: Cracking can occur at high temperatures (hot cracking or solidification
cracking) during the welding operation, or at low temperature (cold cracking),
sometimes several hours after weld completion. Preventing solidification cracking may
require a change in filler metal composition, and change in preheat and interpass
temperature, and possibly a reduction in the welding current. Cold cracking is avoided
by ensuring that properly conditioned, low hydrogen electrodes are used and that the
specified welding procedure (heat input, preheat and interpass temperature) are
followed. See Module 10 - "Weld Faults and Causes" of the MLS Series for more
information on weld defects.
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Special Welding Techniques
There are two other special shielded metal arc welding techniques that the reader should be
aware of. These are:
Gravity welding: Gravity welding is special "automatic" welding version of the shielded
metal arc welding process where, using special feeders, and to 700 mm long and up
to 6 mm diameter, E4327 or E4924 or E4928 type electrodes are automatically fed to
the joint to be welded. The process is used for groove welding in the flat position and
for horizontal fillet welds only. Higher welding current possible with the larger diameter
and longer arc time possible due to the electrode length enable considerably higher
deposition rates to be achieved. Also, a single operator can look after several gravity
feeders at the same time thus further increasing productivity. The process was
developed for welding of ships where the weld length to be deposited can be in
kilometers.
Vertical down welding : When performing welding in the vertical position, the weld
progression is normally upwards. However, in certain applications, such as large
diameter natural gas pipeline girth welds, the welding progression is downwards. In
this orientation, gravity pulls on the molten weld pool and slag, and the welder must
perform welding at a high travel speed so that the arc is at the leading edge of the pool
and flaws like trapped slag inclusions or lack of interpass fusion are avoided. Cellulosic
electrodes are commonly used for pipe welding though recently low hydrogen
electrodes suitable for vertical down welding have been developed.
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Chapter 4
GAS METAL ARC WELDING (GMAW)
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
Principles of Operation .... . ............ . . . ...... ...• ..... . ........ 74
Equipment
................... ... . . . . . . . . ..... . . . •....... 76
Metal Transfer Across the Arc in GMAW .. ..... ...... .. ...... . ...... .. 80
Shielding Gases
...... . . . ....... .. ............ .. ..... . .. ...... 90
Advantages and Limitations of the GMAW Process .. . . . . . . . • .... . ....... 94
Electrode Wires for Gas Metal Arc Welding .......... . ... . . .. ... . ...... 96
Application of Gas Metal Arc Welding Process ..... . ..... . .. . . .. . .... . .97
Effect of Arc Welding Variables in Arc Welding Processes ..... ..... . .. .. .100
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GAS METAL ARC WELDING (GMAW)
4.1 Principles of Operation
The gas metal arc welding process is shown schematically in Figure 4.1. Compared to the
shielded metal arc welding process the metal electrode is bare (without any covering). The
coiled wire electrode is fed continuously through the welding gun. The continuous wire feed
improves the productivity of the process by allowing longer welds to be made without stopping.
In SMAW, the length of weld that can be deposited is limited by the length of the electrode.
The protection of the weld zone from atmospheric contamination is provided by a continuous
stream of a gas or gas mixture.
Solid Wire Electrode ~
Current Conductor
Travel
~ ~
Shielding Gas In
DDO¢
Wire Guide and Contact Tube
Shield ing Gas ~
Molten Weld
Metal
Solidified _
Metal
_
Gas Nozzle
.'
:.": :
r====~~~~~~"-----l
~
_
Base _ __
Metal
~ ~
L -_ _ _ __ __ _ _ __ _ __
-'
Figure 4. 1: Schematic Representation of the GMAW Process
An arc is struck between a continuously fed bare consumable wire electrode and the
workpiece . The heat generated by the arc melts the end of the electrode and part of the base
metal in the weld area. The arc transfers the molten metal from the tip of the melting
electrode to the workpiece where it combines with the melted base metal to form the weld
deposit.
The process was first applied to the welding of aluminum using inert gases for shielding the
arc and the weld pool. The term MIG (Metal Inert Gas) welding has been a popular name for
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the process. However, for joining of steels, it is common to have carbon dioxide and/or
oxygen present in the sh ielding gas mix. These two gases are not inert. Changing the
proportions of carbon dioxide and oxygen in the shielding medium can influence the chemical
composition and therefore the properties of the weld metal. The process therefore is
sometimes referred to as Metal Active Gas (MAG) welding in Europe. In North America, a
more generic description "Gas Metal Arc Welding (GMAW) has been adopted.
Th e equipment arrangement for the GMAW process is shown schematically in Figure 4.2. It
comprises of a power source, electrode wire feeder and control system, the welding gun and a
supply of shielding gas.
• Flowmeter
• Wire Feed Speed
Control
• Voltage/Amperage
Output Adjustment
1....----+- . Output Selector (Ae.
ee. eV)
Gun
• Shielding Gas
Work Lead
• = Va riables that must be selected for GMAW
Figure 4.2 Equipment Arrangement for GMAW
A constant potential (i.e., a constant voltage) power source and a constant speed wire feeder
are generally used for GMAW welding, and current type is almost exclusively direct current
with electrode positive (DCEP). In such an arrangement, the amperage controls the electrode
melting rate (wire feed speed) and the power source tries to maintain a constant voltage at its
present value by adjusting current output. Voltage is closely related to arc length . Should
there be a change in the arc length (for example, due to welding over a tack weld or moving
the gun towards or away from the workpiece), the power source responds by changing current
output. Since current effects wire melt off rate, controlling current to keep the wire burning off
at the same distance from the puddle will maintain an essentially constant arc length -
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constant arc voltage. The power source is constantly responding to the changing demands of
the arc, and fluctuating input power. In order for the process to operate in a stable manner the
power source must be capable of responding the correct amount. The following describes a
typical power source response.
In Figure 4.3(a), the preset welding parameters are 400 A and 34 V, and let us assume that
the corresponding wire feed speed is 400 inches per minute (one inch per minute per ampere
welding current). When arc length increases, the power source responds by reducing current
output and thereby slowing wire melt off rate . Figure 4.3(a) shows, that of arc voltage would
increase to 37 V, the operating current and wire melt off rate would be 325 inches per minute .
However, the wire feeder will still keep on feeding wire at 400 inches per minute. Since the
wire feed rate rate is greater than the wire melt off rate, the electrode extension will increase
and the arc length will progressively decrease, and the operating point will again move
towards the initial setting. If the arc length were to shorten inadvertently, then the adjustment
would be just reversed (see Figure 4.3(b».
56
52
48
44
40
Voltage 36
(Volts) 32 28
24
20
16 12 0
56r--r--~~--~~--~
52
48
44
40
36
Voltage
(Volts) 32
28
24
- - - - - - - -. - _________
I
t
_
1(31,475)
I New
20 Operating
16
I Point
12 I
I
I
I
I
I
I
I
I
I
I
I
100
200
300
I
I,
o
400
500 600
Welding Current (Amps)
100
200
300 400
500 600
Welding Current (Amps)
Figure 4.3(a) Shift in Operating Point
Due to an Increase in Arc Length
4.2
-
Figure 4.3(b): Shift in Operating Point
Due to a Decrease in Arc Length
Equipment
The GMAW process is most often used in the semi-automatic mode, that is, a welder holding
the gun moves and guides it along the weld seam while depositing the weld metal. This also
gives him the flexibility to manipulate the gun to maintain appropriate weld pool shape and
wetting and fusion along the side walls. A typical gun is 250 to 375 mm in length and provides
the means to provide current, continuously feed the wire and supply the shielding gas to
protect the arc and the molten weld pool (see Figure 4.4). Some guns rated for higher
currents or higher duty cycles may also have provision for water cooling .
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'Water Hoses
Shielding Gas Hose
Steel liner
Gas Diffuser
Protective
Sheath
Gas Nozzle
Copper
Contact Tip
Gun
Tri,nm>r_~
Figure 4.4: Schematic Sketch of GMAW Gun
The direct current constant potential power sources used for gas metal arc welding can be
engine driven generators, transformer rectifiers or inverters. The latter two types are more
common since the generator type supplies respond slowly to changing arc conditions.
It should be noted that though the power source recommended is a constant voltage type, the
volt-ampere curve does have some slope instead of being a flat horizontal line. Also, the
electric circuit in the power source has some inductance, a characteristic that controls the rate
at which the current increases in the case of a short circuit. Slope and inductance together
determine the dynamic characteristics of the power source and are key factors affecting the
performance of a GMAW power source for semi-automatic applications. As a result of
different slopes and inductance values, one power source may operate more smoothly for a
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given set of welding conditions than another. Some power sources are available with
adjustable slope and inductance allowing them to provide smooth operation for a range of wire
types and diameters. More information about these features can be found in Module 5
"Power Sources for Welding".
In a conventional semi-automatic equipment set up, an analog constant speed wire feeder is
used in conjunction with the constant voltage power source. The wire feeder's main
components are drive rolls, guide tubes, gear box, variable speed motor, wire support and
controls and metres. Figure 4_5 shows a four drive roll system which is more dependable than
a two roll system. A grooved roll is usually combined with a flat roll for feeding solid wires.
The groove is usually V shaped for carbon and stainless steel, and U shaped for softer
aluminum wires.
With the basic analog wire feeder and standard power source set up, the wire feed is adjusted
by using an incremental dial on the feeder and checking the wire feed speed with a hand held
metre. Alternatively, the dial may be adjusted to obtain a desired current reading. Although
connected , the wire feeder and power source do not communicate with each other in this set
up; the power source simply supplies the necessary power to burn off the wire as fast as it is
fed into the arc. As a result, for the same dial setting , the actual wire feed speed can vary
depending on the actual line voltage, slippage, etc. Therefore, unless the wire feed speed is
verified on a regular basis, there will be variations in welding current and arc characteristics for
no 'apparent' reason.
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r
Pressure Adjusting
Screws
l
Inlet Guide
Tube
Centre Guide
Tube
Wire Feed and
Power Cable
Consumable
Inlet Guide
Assembly
Outlet Guide
- Tube
Idler Roll
Drive Gear
Figure 4.5: Four Drive Roll Wire Feed System
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Digital wire feeders having wire feed speed modulation capabilities, the target wire feed speed
can be set directly. The feeder provides an accurate set-to-actual speed relationship through
the use of better speed control on the feed motors, feedback of actual speed from tachogenerators, etc. Otherwise, there is no special communication between the feeder and power
source. Digital wire feeders, compared to analog ones, result in better repeatability of
procedures which positively affects the quality and economy of welding operations.
The more recent GMAW equipment packages, some or all of the following features may be
offered: synergic or "one knob" control of the welding output, pulsing capabilities, programming
of weld schedules, programming of custom pulse programs, etc. In addition, the feeder
usually must be matched to the power source in order that they may communicate properly.
There is not yet an industry standard with regards to control methods or terminology for these
types of machines. In each case, the relationship between the feeder and the power source
and the actual arc control methods varies. These packages can be generally grouped by
which part contains the control circuitry and interface, i.e., the wire feeder, the power source,
or a combination between the two. Essentially, it is not important which part of the machine is
the "brains" of the operations or how a given machine controls the arc, provided the desired
arc characteristics and features can be provided.
4,3
Metal Transfer Across the Arc in GMAW
The GMAW process is identified with a number of different modes of metal transfer depending
on the welding parameters (current and electrode size), shielding gas composition, electrode
chemistry and the type of power source.
It was mentioned in Chapter 1 that once the tip of the electrode melts into a globule of molten
metal due to the heat of the arc, one of the forces acting to detach it and propel it across the
arc to weld pool is the electromagnetic pinch effect. The strength of the magnetic field, and
therefore the pinch effect depends most strong ly on the current density (welding current
divided by the cross sectional area of the electrode). Consequently, the rate and mode of
droplet detachment also depends on the current density. The principal droplet transfer modes
of interest in GMAW are: short circuiting, globular, spray, and pulsed where pulsed transfer is
a form of spray transfer.
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Short Circuiting Transfer
In the short circuiting mode (Figure 4.6), the current density, i.e., the amperage used in
relation to the wire size, is relatively low. The wire therefore melts at the electrode tip but th e
pinch force is not enoug h to detach it. However, the wire feeder keeps on feeding the wire
and therefore the molten electrode tip comes into contact with the weld pool. When this
happens, the constant voltage power source increases the amperage which in turn increases
electrode heating and the magnetic pinch effect acting at the electrode tip. The magnetic
forces pinch off the droplet which is then drawn into the weld pool by surface tension forces.
The gap between the electrode and the weld pool is then recreated and the arc is reestablished. This process repeats itself very quickly, typically more than 100 times per
second, so that the human eye does not notice the short circuits and the arc seems
continuous.
-
I-
r-
Drop
L
--.......
".---Arc
[
\
New Arc
'2r No Arc
\;::;V
_ _r----~;r----~_~
Before
Transfer
During
Short
Circuit
After
Transfer
Figure 4.6: Short Circuit Transfer
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In the short circu iting mode of metal transfer, the amount and speed at which the current
increases during the short circuit must be controlled for the process to operate smoothly. The
amount by which the current increases is determined by the slope of the volt-ampere curve
(Figure 4.7(a)). When the slope is too small, the current increases to a higher value and when
the slope is higher, the short circuit current has a smaller value. Similarly, if the inductance is
too low (Figure 4.7(b)), the current increases very rapidly and overshoots the optimum value
for drop detachment whereas if there is more inductance in the electrical circuit, the current
increases in a gradual fashion. When the short-circuiting current is too high (small slope, low
inductance), the current (density) can be so high that the molten drop explodes causing
spatter.
The welding parameters (current and voltage) for short circuit welding, also called short arc or
dip transfer welding, are relatively low and therefore it is best suited for welding of thin ferrous
materials in all welding positions, and root passes of thicker steels. The short circuiting mode
of metal transfer can be difficult to apply successfully to thicker materials because of the
smaller diameters wires (1.2mm or smaller) and low currents (less than 200 A for 0.9mm
diameter wire) and the resulting low heat input which can cause fusion problems (cold
welding). Consequently, welding specifications for critical structural applications such as
pressure vessels, bridges, naval vessels, etc, prohibit short circuit transfer mode if GMAW
process is to be used. The shielding gases used for carbon-manganese steels are normally
carbon dioxide (C0 2 ) or 75% argon - 25% CO 2 •
Excessive Current
High Spatter
Curve A (Higher Slope)
Curve B . Very Low Inductance
Short
Circuit
Current
Operating Point
Vol lage.
V
Current,
A
~
Desired CUrrent
For Good Stability
And Low Spatter
Time,
S
Current, I
Figure 4.7(a): Effect of Volt-Ampere Curve
Slope on Short Circuiting Current
Figure 4.7(b): Effect of Inductance
on Short Circuiting Current
The short circuiting mode of metal transfer can not be applied to non-ferrous metals and
alloys. Cast irons are mostly welded in the short circuiting mode.
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"-
"'"
~,-,..r
"
--"'"
'"
L
,
I
-
,
-
~
~
J
~~\
[
~
'i:
( U
,
-
t--
-
~
J
(}/~V:J
Figure 4.8: Globular Transfer
Globular Transfer
Globular transfer occurs as the current and voltage increase beyond those for short circuiting
transfer In this transfer mode (Figure 4.8), the molten drop of metal at the electrode tip can
reach a diameter 1,5 to 3 times the wire diameter, This large drop of metal detaches from the
electrode tip due to the force of gravity, It has an irregular shape, may have a rotational
motion and takes an irregular path across the arc, The glob of molten metal splashes into the
weld pool causing expulsion of some liquid metal (spatter), Globular transfer in GMAW tends
to splatter and is usually avoided,
Carbon dioxide as well as argon rich gas mixtures containing CO, or oxygen can provide
globular transfer Very good penetration characteristics can be produced at higher current
levels, The main drawbacks of globular transfer are spatter formation, irregular bead shapes
and formation of numerous slag islands.
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Spray Transfer
Spray transfer (Figure 4.9). also called axial spray, occurs at current and voltage levels above
those for globular transfer, and when an argon rich (85% minimum) shielding gas mixture is
used. The molten metal is transferred across the arc in a continuous stream of fine droplets,
and the droplet diameter is typically less than the wire diameter. The arc is quite stiff so that
the drops travel directly along the centerline of the electrode and into the weld pool, and
therefore can be easily directed without affecting the arc behaviour.
Figure 4.9: Spray Transfer
The transition current (current at which the mode of transfer changes) for the change from
globular to spray transfer (Figure 4.10) depends on the wire diameter, shielding gas
composition, electrode composition and the electrical extension. At very high currents , above
the range for axial spray, the line of metal drops begins to rotate about the electrode axis, and
there is an increase in spatter.
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Short
Circuiting
Transfer
Voltage
Globular
Transfer
1
· ·. . . ·f·. . . . . . . . . .
\.•••
~........
l (~\
I
Transition
Current - --.j
I
Spray
Transfer
Current (amperage)
Figure 4.10: Transition Current for Globular to Spray Transfer
Spray transfer is characterized by:
minimal spatter;
a relatively quiet and smooth arc;
weld beads with good penetration and nice appearance.
However. because of the high current and voltage levels. the weld pool is rather large and
difficult to control for out of position welds . Spray transfer is therefore suitable for welding in
the flat and horizontal positions. and welding of thick materials.
Pulsed Transfer (GMAW-P)
Pulsed transfer is a form of spray transfer. Its primary benefits are;
All position capability for ferrous and non-ferrous metals
More productive for thin material than GTAW
No spatter even for difficult filler metals
Larger diameter electrodes can be used
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Pulsed spray transfer involves the use of a specially designed power source whose cu rrent
output changes or "pulses" between a peak value and a background value at a rapid but
controllable rate (Figure 4.11 (a)). Peak current surges to above the transition value for spray
transfer then drops to a background level so that in each pulse a drop of metal is detached
and transferred across the arc. Background current is sufficient to maintain the arc and keep
th e electrode tip hot and ready to detach the next droplet during the next pulse.
The average current is generally in the range for globular transfer. well below the spray
transition value but the bead appea rance resembles that obta in ed with spra y tra nsfer. Also.
the lower average current implies a smaller weld pool and lower heat input. thus enabling out
of position welding and welding of thin materials.
Pulse
Peak J
Current
tp
I ASpike
@)
,
0
Spray Transfer
Current Range
Current. ---~----------~--~---------~
A
Globular
Transfer
~
o
0
Background
Current -
~C u rrent
Range
i
Time
Pulsed-Spray Welding Current Characteristics
Drop Formation
Figure 4.11 (a): Pulsed Spray Transfer
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Aluminum and other reactive metals are welded with pulsed spray transfer. Larger diameter
electrodes improve feeding and reduce weld pool contamination to significant reduction in wire
surface incorporated into the deposit. The length of electrode per kilogram(pound) greatly
reduces as the electrode diameter increases. (Eg. 1 kg of 0.9mm diameter aluminum wire is
592m long. By comparison the same wire at 1.1 mm diameter is only 358m long. A reduction
of about 40%.)
An electronically controlled wire feeder with real-time wire feed regulation is used ensure wire
feed speed always remains close to the set speed.
Effects of Pulse Parameters
Electronically controlled pulsed power supplies allow adjustment of a number of pulsing
parameters;
Pulse rate
Pulse width
Peak amperage
Background amperage
Pulse Rate
Changes in wire feed speed are accompanied by changes in pulse frequency. As wire speed
increases, the pulse frequency and therefore average current must increase so that wire feed
speed and burn-off rate are continually matched. The increase in average current causes an
increase in heat input. Pulse frequency can readily be used to control arc length .
Pulse Width
Pulse width is the time at peak amperage. Average amperage and heat input are directly
effected by pulse width- both increase with increasing pulse width. Increasing pulse width
also has some effect on increasing droplet size and widens the arc cone (bead width
increases).
Peak Amperage
Peak current must be high enough to be above the spray transfer transition. Peak current
detaches droplets and propels them across the arc. Peak current directly affects arc length arc length increases with increasing peak current. Some power sources produce a spike to
promote droplet detachment from the electrode tip.
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Background Amperage
Co ntrol of current rise and fali during the pulse cycle is used to control droplet shape and to
shape the electrode end in anticipation of the next droplet detachment.
ltNV\J\J\ •
!truuL •
ltlVV\Jl•
JtflllJUL•
ltN\fVl •
ltrvvvL •
ltrvvvL •
lfNVVl •
Increasing Pulse Rate:
Increase Pulse Rate (pulses per second)
- increases arc length
- increases heat inpu t
Deuease Pulse Rato (pul ses per second)
Increasing Pulse Width:
-
Increase Pulse Width (pul se peak time)
increases
increases
increases
increases
arc length
heat input
penetration
bead width
Ocacase Pulse Rate (pulses per second)
Increasing Peak Amperage:
lnacase Peak Amperago
- increases BUrn off rate
- increases arc length
- decreases droplet size
Dcucase Peal<. Amperage
Increasing Background
Amperage :
-
increases
increases
increases
increases
arc length
heat input
penetration
wetti ng action
Oecrease B<lckground Amperag e
Figure 4. 11(b): Summary of Effects of Pulse Parameters
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Modern power sources allow peak current (Ip), background current (Ib) and the pulse width
(duration or frequency) to be pre-programmed for a given application (i.e., shielding gas, wire
type and diameter) and changes in wire feed speed are accompanied by changes in pulse
frequency. As wire speed increases, the frequency and thus the average current increases so
that wire feed speed and burn-off rate are continually matched. Some power sources produce
a spike to facilitate the droplet detachment from the electrode tip .
With modern GMAW-P equipment there is a wide variation from one manufacturer to another
in arc control methods and pulse programming. As a result, care must be taken in selecting
appropriate equipment. Procedures which were successful with one equipment package may
not be duplicated successfully on a different package and a certain amount of procedure
development may be required for each case.
Synergic power sources are electronically controlled power sources that can provide a variable
pulse frequency that is proportional to wire feed speed. Synergic control is a "one knob"
system which changes a number of interrelated variables at one time - simplifying operator
control. Synergic power sources are commonly pre-programmed for 0.9, 1.2 and 1.6 mm
diameter mild steel, stainless steel and aluminum wires . The systems are designed allow reprogramming or "fine tuning" of the pre-packaged programs .
The pulsed spray mode of metal transfer can be substituted for any of the three transfer
modes discussed in the preceding paragraphs. When developed and applied correctly, the
pulsed spray transfer mode enables welding in all positions, and helps reduce heat input,
distortion and spatter.
The effect of metal transfer made on weld bead shapes is shown in Figure 4.12(a) and 4.12(b)
which present cross sections of six bead on plate welds.
Figure 4.12(a): Short Circuit and Globular Transfer
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Figure 4.12(b): Globular and Spray Transfer
4.4
Shielding Gas
The following shielding gas or gas mixtures are normally used for welding of carbon and alloy
steels:
Carbon dioxide
Argon - carbon dioxide
Argon - oxygen
Carbon dioxide is the least expensive of the shielding gases used for gas metal arc welding.
Once ionized, carbon dioxide has a high thermal conductivity which helps to keep the arc
plasma as a small, dense column under the electrode and the metal is transferred in either the
short circuiting or globular mode. The arc is less stable and spatters. The deposited weld
bead has a rough surface but deep and round penetration as carbon dioxide transfers the
greatest amount of heat to the weld pool (Figure 4.13).
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Rough Bead
Appearanc
Cold, Peaked Bead
From High Surface
Tension
Excessive
Spatter
Steel
Finger-Like Penetrali
From Axial Spray
Round , Deep
Penetration from Non-axial Transfer
co,
Argon
Helium
Figure 4.13: Effect of Shielding Gas on Weld Bead Shape
Carbon dioxide is an active gas in the sense that at the arc temperature , it dissociates to
produce carbon and oxygen, and the latter can oxidize the weld metal. The GMAW wires for
use with carbon dioxide shielding gas therefore have sufficient level of deoxidizers like silicon,
manganese, etc. to tie up the oxygen. As a result, the manganese and silicon contents. of the
weld metal tend to be lower than those in the wire. Conversely, in the case of stainless steels,
the weld metal can pick up some carbon which can make stainless welds more prone to
corrosion.
Argon has a low ionization potential which means that arc voltage and therefore the arc length
can be smaller. Also, in the ionized form, argon has a low thermal conductivity. This causes
the arc column to expand and extend upwards above the end of the electrode as the welding
current is increased. The electrons hitting the electrode above the tip cause local heating and
tapering of the electrode. This increases the local current density and the pinch force, causing
small droplets to be easily detached and propelled at a high velocity to the weld pool in the
form of a spray. However, the arc tends to be cold and unstable, and the weld bead formed is
peaky with undercut and finger shape penetration (Figure 4.13). As a result, pure argon is not
used for welding of steels. With higher conductivity gases such as carbon dioxide, the plasma
column does not expand as much and therefore the electrons are restricted to striking the end
of the electrode only. Therefore, no preheating of the wire end occurs and globular or short
circuit transfer is promoted.
Small additions of active gases like carbon dioxide or oxygen to argon lead to the formation of
a small amount of iron oxide on the surface of the weld pool. The oxide is able to increase arc
stability as it is a better electron emitter, and it also reduces the weld pool surface tension .
Lower surface tension promotes weld pool fluid motion and helps to reduce the tendency
toward lack of fusion type flaws. Carbon dioxide also transfers more heat to the base material
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nd promotes rounded rather than finger penetration. A popular argon-carbon dioxide mixture
contains 75% argon and 25% carbon dioxide, common ly referred to as C-25 gas. This mixture
provides better bead appearance and less spatter than straight CO, (Figure 4.14). It is
gnerally used on mild and low alloy steels with short-circuiting or globular transfer.
a
98% Argon
2% Oxygen (spray)
95% Argon
5% CO, (spray)
{o
75% Argon
25% CO, (globular)
91 % Argon
5% CO,
4% Oxygen
Figure 4.14: Effect of Argon Rich Shielding Gas Mixture on Weld Bead Shape
Reducing the carbon dioxide content decreases the transition current for spray transfer, and
therefore mixtures containing 15% or less carbon dioxide are more conveniently used for
spray transfer. When the argon content is 85 to 92%, good penetration and smooth bead
appearance are obtained. The current required for spray transfer is still reasonably high and
the resulting higher arc energy and good penetration makes this gas composition range
suitable for welding thicker materials.
With further increase in the argon content of the gas mixture to 95%, stable spray transfer can
be maintained at a lower voltage. As a result, the arc energy is somewhat lower and therefore
these higher argon containing gases are more suitable for welding thinner material in the flat
and horizontal positions though they can be used for thicker materials as well.
Since argon is an inert gas, it does not influence the weld metal composition . Therefore as
the amount of carbon dioxide, an active gas, is reduced in the argon-carbon dioxide mixture, a
greater proportion of manganese and silicon present in the wire will be retained in the weld
metal. As well, the weld metal will have a lower oxygen content and this can help to improve
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the notch toughness of the weld metal. Thus, argon - 5% carbon dioxide is a commonly used
gas mixture for welding of high performance naval steels where high notch toughness is very
desirable.
Argon with 1 to 5% oxygen also improves the bead contour and penetration profile (reducing
the tendency to finger penetration) compared to that obtained with pure argon . Oxygen
contents more than 5% adversely affect the weld metal toughness and also cause undercut.
The addition of oxygen to argon reduces the transition current, and therefore argon-oxygen
mixtures are used for welding carbon and stainless steels with the spray mode of metal
transfer.
Proprietary argon based gas mixtures are also available that have additions of both oxygen
and carbon dioxide but with in the above ranges. For example, Ar-5% CO 2-4% O 2 and Ar-8%
CO 2-2% 0 2 ' These mixtures offer the advantages of individual additions of carbon dioxide and
oxygen, and in addition, allow spray transfer at low amperages which is important for pulsed
current welding. Also, these mixtures may enable faster welding speeds and less spatter and
fumes compared to argon-carbon dioxide mixtures.
In some gas mixtures, a part of the argon can be replaced by helium, another inert gas. Pure
helium has a high ionization potential. The greater arc voltage required therefore leads to a
hotter arc and longer arc length . Like carbon dioxide, it has high thermal conductivity and
therefore promoted globular transfer as opposed to spray transfer with pure argon . However,
helium is much lighter than air and therefore to offer adequate protection of the arc and the
pool from atmospheric contamination , higher gas flow rate is necessary with helium. For
welding of carbon and low alloy steels, only small amounts of argon are replaced by helium
(up to 30% helium is allowed as per CSA W48). Helium rich gas mixtures are more common
for welding of stainless steels (90% He - 7.5% Ar - 2.5% CO 2 ),
Hydrogen can also be added in small amounts to gas mixtures. Its influence on arc
characteristics is similar to that of helium. Hydrogen is a reducing agent. It easily takes away
and combines with oxygen present in oxides, and therefore leads to the formation of cleiln,
un oxidized weld beads. However, hydrogen containing gas mixtures, for example, 96.25% Ar
- 2.75% CO 2 - 1% H2 , may be used only for austenitic stainless steels since the hydrogen will
cause embrittlement and cracking in ferritic and martensitic steels.
For welding of metals and alloys other than steels and stainless steels, argon-helium gas
mixtures are frequently employed, i.e., there are no additions of oxygen or carbon dioxide.
Increasing the helium content to 50 to 75% range makes the arc hotter by virtue of the higher
arc voltage and therefore is useful for welding heavy thicknesses of aluminum , magnesium,
and copper. Argon with up to 5% hydrogen is used for welding nickel and nickel alloys. Also,
argon-25% hydrogen mix is recommended for welding of thick section copper. Hydrogen also
increases the arc heat due to the higher arc voltage needed and is therefore useful due to the
high heat conductivity of copper.
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Safety with Gas Cylinders
The shielding gases and gas mixtures are normally supplied as compressed gases in
cylinders. Whether in use or in storage, the cylinders must be secured and handled carefully
since knocks or falls may damage the cylinder or the valve, and may cause a leak or an
accident. Following precautions should be taken in the use of gas cylinders:
Always properly secure the cylinders;
While standing to one side, momentarily open the valve to clear any dirt present before
connecting a regulator;
After connecting the regulator, release the pressure adjusting screw and then slowly open
the cylinder valve to prevent a high pressure gas surge in the regulator;
The cylinder valve should be shut off and adjusting screw backed off when the cylinder is
not in use.
The cautions given above apply to all shielding gases, whether for use with the GMAW
process or other gas shielded processes (FCAW, MCAW, GTAW) discussed later. For more
details on Welding Safety, reference CSA Standard W117.2 "Safety in Welding, Cutting and
Allied Processes" or Module 1 "Welding Health and Safety" of the MLS series.
4.5
Advantages and Limitations of the GMAW Process
The main advantages of the GMAW process are its application to a wide variety of materials,
higher deposition rate and productivity compared to the shielded metal arc welding process
(Figure 4,15) and the better quality of the deposited weld metal. As well, with the recently
developed advanced welding power sources and the availability of smaller diameter wires (0.9
and 1.1 mm diameter), welding procedures can be developed to apply the process in all
welding positions, both in semi-automatic and automatic modes.
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35r------------------------------------,
30
Typical for GMAW
_1 .1 mm wire;
stickout 19 mm
25
Deposition 20
Rate,
15
Iblh
Typical SMA W
Range of Sizes
and Types
10
5
o
o
200
400
600
Welding Current, A
Figure 4.15: Comparative Deposition Rates of GMAW and SMAW
The higher deposition rate results from the absence of electrode covering and higher current
density (same current but smaller diameter wire). The higher productivity of this process
results from:
a higher duty cycle
the time saved in not having to clean slag or flux from the deposited metal
higher utilization of the filler metal
The weld metal deposited using the GMAW process is generally cleaner (fewer non-metallic
inclusions) and in the case of high strength structural steels, weld metal with superior
toughness can be obtained with proper selection of shielding gas (Ar-5 to 15%). Such
applications include girth welding of large diameter natural gas and oil pipelines, submarine
hulls, etc. The "low hydrogen" nature of the process is an additional important characteristic,
especially for welding of high strength steels.
One of the main limitations of the gas metal arc welding process is its sensitivity to the welding
parameters. Seemingly small changes in voltage, electrode extension, etc. can have a
significant influence on the bead shape and penetration , and thus on the incidence of weld
flaws such as incomplete fusion . Faithful reproduction of qualified welding procedures is
therefore critical to obtain sound production welds . In this regard , matching wire feed speed
between the qualification procedure and production situation is a good indicator that the
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correct procedure has been implemented. One must also be aware that air drafts can reduce
the effectiveness of the shielding medium causing porosity in the weld metal.
4.6
Electrode Wires for Gas Metal Arc Welding
Wire electrodes and deposits for GMAW of non-alloyed and fine-grained steels are classified
and certified under CSA Standard W48 using the designations and classification requirements
specified in CAN/CSA-ISO 14341 .
A typical gas metal arc welding electrode designation is:
ISO 14341 B-G 49A 3 C G2 (formerly ER49S-2)
where :
B
G
49
A
3
C
G2
is the system based on tensile strength and 27 J impact energy
gas metal arc welding
minimum weld metal tensile strength (in increments of MPa)
tested in the as-welded condition
impact test temperature _30
shielding gas CO 2
chemical composition of the wire electrode
0
For example, the wire designation above would have a minimum tensile strength of 490 MPa
and that the wire contains nominal amounts of zirconium, titanium and aluminum for
deoxidation purposes in addition to silicon and manganese. Such wires are capable of
producing sound welds in semi-killed and rimmed steels, especially using the short-circuiting
mode of metal transfer. Moreover, these wires can be used to produce acceptable welds even
when there is some rust present at the steel surface. Further guidance on the optimum use of
various carbon steel wires is given in the appendix to CSA Standard W48 . It should be noted
that CSA Standard W48 certifies GMAW wires based on tests with 100% CO, shielding gas
only. Certified wires are permitted to be used with argon rich gas mixtures but with certain
restrictions on the CO, and 0 , contents. Argon rich gas mixtures cause an increase in weld
metal manganese and silicon content while decreasing oxygen content. This occurs becasue
of a lack of "active gas" in the arc atmosphere. Electrodes certified with CO 2 are purposely
over alloyed with manganese and silicon to compensate for losses in a CO 2 arc environment.
USing shielding gas mixtures containing only small amounts of oxygen and/or carbon dioxide
will result in higher levels of these alloys in the weld. The additional amount of these alloys in
the weld will increase its yield and tensile strength properties. This mayor may not be
desirable depending on the type of load the weldment may be subjected to .
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The electrode diameters are commercially available in the range 0.9 to 1.6mm diameter. The
largest diameter electrode that can be used depends partly on the steel thickness to be
welded . It is usually 0.9mm diameter for workpiece thickness up to 10mm and 1.2mm up to
20mm thickness.
Gas metal arc welding is treated as a controlled hydrogen welding process as long as due
care is taken to ensure that the electrode and the jOint surfaces are clean. The shielding gas
used must have a low moisture content. Moisture content is evaluated by the temperature at
which condensation occurs. Welding grade gases usually have a dew point of -40 °C.
4.7
Application of Gas Metal Arc Welding Process
Virtually all weldable materials can be joined with the GMAW process. Nonferrous alloys
(aluminum, magnesium, copper, nickel, titanium and their alloys) are welded using spray or
pulsed spray mode, and successful procedure development depends mainly on selection of
the shielding gas and welding parameters. The filler metal is often designed to somewhat
match the base material in composition . Due to the inert shielding gas, no significant changes
in chemistry of the deposited weld metal should occur. There are exceptions of course;
aluminum filler metals are formulated to prevent hot cracking and do not normally match the
base metal chemistry.
Gas meta l arc welding of structural steels on the other hand can be more complex.
Considerations include filler metal composition, shielding gas and metal transfer mode, as well
as the metal thickness, joint design and welding position.
Similar jOint designs can be employed for gas metal arc welding as for shielded metal arc
welding except that groove angles can be reduced due to the smaller diameter of GMAW
wires . GMAW can be less forgiving than SMAW or FCAW particularly when using smaller wire
diameters. Good penetration and fusion is easily obtained directly beneath the arc, however,
in many cases increased oscillation is required to properly fuse into the sides of the joint,
whereas for the same joint FCAW or SMAW can produce satisfactory results without
oscillation. High argon content shielding gases create a directional penetration shape (finger
penetration) which is prone to incomplete fusion. Typical penetration shapes are illustrated in
Figure 4.16.
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Figure 4.16: Directional Penetration from High Argon Content
Steels in thickness from about 1 to 3mm can generally be butt welded with square edges in
one pass provided the gap is less than 3mm . For steel thickness ranging from 3 to 6mm, a
complete penetration groove jOint can be obtained with square edge preparation by welding
from both sides provided that there is adequ ate root gap (1 to 4mm). Above 6mm , it is
custom ary to prepare the joint edges. Thicknesses greater than 6mm usually require multiple
passes. Deposition rates achieved in gas metal arc welding using wires of various diameters
are shown in Figure 4.17.
30r---,---.---,----.---.---,
12
25
10
20
Deposition 15
Rate
6
(Ib/hr)
5
-
Deposition
Rate
(kg/hr)
2
°0~~1~00~~2~070~3~0~0--740~0~750~0~~600
Weldi ng Current (Amps)
Figure 4.17: GMAW Deposition Rate for Different Diameter Wires
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Weld Soundness in Gas Metal Arc Welding
Once an appropriate electrode and shielding gas have been selected, the soundness of weld
beads deposited using the GMAW process depends on cleanliness of the joint, optimum
setting of welding parameters (voltage, wire feed speed, electrode extension) and welder
technique .
Joint cleanliness is important in gas metal arc welding to avoid porosity. This is specially true
for aluminum alloys where elaborate chemical or mechanical cleaning measures may be
needed and time between cleaning and welding minimized in order to eliminate porosity.
Inadequate shielding gas flow rate, shielding gas contamination and excessive electrode
extension are other potential causes of porosity.
Incomplete fusion is a common flaw, in root areas and along groove faces during short
circuiting mode of metal transfer. There may be oxide islands present or the bead shape may
be convex, and in both cases interpass cleaning and grinding may be required so that
subsequent passes fuse completely. Narrow grooves, high travel speed, excessive electrode
extension, improper work and travel angles, are other possible causes for incomplete fusion
(see Figure 4.18).
Typical GMAW
Lack of Fusion
Figure 4.18: Incomplete Fusion (Lack of Fusion)
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Similarly, high travel speed, high voltage and current , improper gun angle, insufficient dwell
time at edge, can all cause undercutting.
4.8
Effect of Arc Welding Variables in Arc Welding Processes
Welding parameters influence weld bead shape and the creation of flaws in the weld zone.
These variables can be divided into three categories:
Primary Variables:
current
voltage
travel speed
These variables are adjusted most commonly to influence the weld characteristics and can
be varied over wide ranges and influence bead shape and penetration, arc stability and
deposition rate.
Secondary Variables:
tip to work distance
travel angle
work angle
These variables influence the weld characteristics indirectly by changing the primary variables.
Process Specific Variables:
electrode type and size
shielding gas composition (or flux)
current type and polarity
thickness and type of metal to be welded
joint design
welding position
desired heat input and/or the deposition rate
The effect of these variables is more or less similar in all of the continuous wire processes
(including gas shielded FCAW and SAW presented in the following chapters). The following
paragraphs describe the effects of these variables applied to gas metal arc welding .
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The effect of the three primary variables: current, arc voltage and travel speed on weld bead
characteristics (penetration, bead width and depth) are shown schematically in Figures 4.19 to
4.21. Continuous wire processes using a constant potential power source and a constant
speed wire feeder control current by wire feed speed . Qualified welding procedures should
always state the wire feed speed, wire size, shielding gas, etc. When setting up equipment to
implement a qualified welding procedure matching wire feed speed is the surest way to
reproduce the approved procedure.
Penetration
-r=:-<K?1
T
_
I ~
I
!
Travel Speed
(mm/sec)
Arc Voltage
(Volts)
Welding
Current
(Amps)
I
Shallow
/
..
Deep
Increasing Penetration
Figure 4.19: Influence of Current, Voltage and Travel Speed on Penetration
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r=
~
1-"" , , , , -
Bead Width
Travel Speed
(mm/sec)
::I
~_
-
Arc Voltage
(Volls)
Welding
Current
(Amps)
Narrow
-
Wide
Increasing Bea d Width
Figure 4.20: Influence of Current, Voltage and Travel Speed on Bead Width
I
Bead Height or _~
Reinforcement
~
I {......, . . . ., '-"
I ' ~' ~
I
~
I
,L ~
f-
Travel Speed
(mm/sec)
I
f-
-
-
Arc Voltage
(Valls)
-
-
-
~
~
-
Weld ing
Current
(Amps)
-
-
I
Low
High
Increasing Bead Heigh t
Figure 4.21: Influence of Current, Voltage and Travel Speed on Bead Height (Reinforcement)
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Assuming that all other parameters stay the same, an increase in current increases the wire
melt off rate (higher wire feed speed needed), deposition rate, heat input and penetration.
Increasing current also increases the minimum voltage that can be used. If current (WFS) is
increased too much, stubbing will occur indicating voltage needs to be increased. Excessive
current produces convex beads. Very low current also leads to unsatisfactory bead shape and
an unstable arc, and may also lead to incomplete fusion type of flaws.
Arc voltage is measured between the tip of the unmelted electrode and the surface of the weld
puddle. To some extent, it does depend on the welder technique . Provided voltage is within
the specified range of an approved welding procedure, acceptable welds should be obtained.
For a given current, too Iowan arc voltage (short arc length) leads to a convex and narrow
bead. Conversely, excess arc voltage (longer arc) yields a wider, flatter bead (Figure 4.22)
with a possibility of undercut, incomplete fusion and excessive spatter. In practice, arc voltage
is the main tool used to control the shape of the weld bead.
....--......J
r
\
Arc
Voltage
19
!
\
r---~~~------~~~--J
Arc 34
Voltage
th
'-----1.. _ 11 -~ L_
Bead
Width
j
!
\
\
-\
I.Bead Width.1
Ar c
Len gth
I
Figure 4.22: Weld Bead Width Related to Change In Arc Voltage
In the preceding paragraph, the term arc voltage has been used through out. It may be
different from the reading on the metre on the power source because of voltage drop from the
power source to the arc due to factors such as small diameter or long cable, loose
connections , uncalibrated metres, etc. Welding machines should be checked for proper
operation/calibration on a regu lar basis.
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Travel speed is the rate at which the arc is moved along the weld joint. Manually, the welder
determines the correct travel speed by judging the puddle size and keeping the arc at the
leading edge of the weld pool. Travel speeds are naturally faster than those for the shielded
metal arc welding process.
Excessive travel speed (lower heat input) leads to narrower, convex beads with limited
penetration and possibly undercut at the toes. However, too Iowa travel speed can also
produce a (large) weld with limited penetration since the arc force is absorbed by the molten
weld pool rather than the base materials at the leading edge of the pool. Low travel speed
when used on thinner materials can also cause burn-through due to heat build up.
Amongst the secondary variables, the Tip To Work distance (TTW) is the most important. Tip
To work distance is the sum of the electrode extension and the arc length (Figure 4.23(a)).
For a given preset voltage (arc length), an increase in Tip To Work distance means an
increase in the electrode extension which is described in Chapter 1, increases the resistance
in the circuit thus reducing the current (Figure 4.23(b)). Arc voltage is reduced because there
is an increase in the voltage drop over the longer electrode extension and the total voltage is
fixed due to the constant voltage output from the power source. Both these factors make the
arc colder and reduce the penetration. Further, if the wire feed speed is increased so that the
same arc current is maintained as with the smaller Tip To Work distance, then there will be an
increase in the deposition rate. The effects of reducing the electrical extension will be
opposite to those described above. The Tip To Work distance is suggested to be of the order
of 6 to 13 mm for short circuiting transfer and 13 to 22mm for spray transfer.
Electron Flow
...,.--·\111
TIp to
Work
Distance
I
/
\
/ 1
t
~
Electrode
ExtenSion
Arc
Length
Figure 4.23(a): Relationship Between Tip To Work Distance (TTW).
Electrode Extension and Arc Length
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\ II
Constant
Wire Feed
Speed
Normal
Distance
Normal
o
Cold
Vm =Total voltage across wire and arc (constant)
Va =Arc vol tage (from lip or wire to work piece)
A
=Welding cu rrent
Figure 4.23(b): Effect ofTip To Work Distance (TTW)
on Arc Voltage and Welding Current
In semi-automatic processes, the welder can make beneficial uses of these effects. The
welder controls the tip to work distance by moving the gun away from or nearer to the work
piece. When welding thin materials or over wider gaps, increased tip to work distance may
lead to an irregular arc, undesirable bead shape and lack of fusion types of flaws . Conversely,
a short electrical tip to work distance increases the current and can be helpful in ensuring a
hot start. Welding with a short tip to work distance will give better penetration but may also
lead to a build up of spatter in the nozzle.
Travel angle is the angle that the electrode makes with a reference line perpendicular to the
axis of the weld in the travel plane (Figure 4.24). When the electrode paints towards the start
of the weld, the travel angle is said to be a drag (trailing, lag, pull) angle and the practice is
referred to as backhand welding. When the electrode points in the direction of travel , then the
travel angle is said to be a lead (push) angle and the practice is called forehand welding . The
effect of travel angle on bead shape and penetration is shown in Figure 4.25.
The effects of welding variables described above are to a large extent sim ilar for flux cored
and submerged arc welding. Figure 4.26 displays fillet weld cross sections made using
different travel angles.
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Drag Angle -
Push Angle
Push Angle --1 __ _
/l
v'
-t---
~ _____
Drag Angle
Axis of Weld
Axis of Weld
Figure 4.24: Travel Angle for Push and Drag Techniques
Pulling
Angle
-30 '
-20'
-10'
0'
Pushing
Angle
+10'
()
\dtu@y
PulllDrag
Q
~
+20'
+30'
~
I
Push/Lead
Increasing Penetration
•
Figure 4.25: Effect of Travel Angle on Weld Bead Penetration
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20'Push
20 0 pull
Travel Angle
Figure 4.26: Comparison of Push/Pull Travel Angles
Work angle is the angle that the electrode makes with a vertical reference line in the work
plane (Figure 4.27). The work angle is selected to help control the bead shape and place it at
the required location in the joint, and to direct penetration. Figure 4.28 displays fillet weld
cross sections made using different travel and work angles.
Work Plane
r·························
......._..................................-..
~
~/ _..-.--_ii
Work Plane
I ---I
L •...... _ .•. _ .••.................___ ......................•••..•.•............. _...
I
_.1
......................................." ..........""...."."".... "...1
Figure 4.27: Definition of Work Angle for Fillet and Groove Joints
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Work Angle
(measured from horiz.)
Figure 4.28: Comparison of Varied Work Angles
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Chapter 5
FLUX CORED ARC WELDING
5.1
5.2
5.3
5.4
5.5
5.6
Principles of Operation ....... . ....... ... .... . . . . .. ... ..... ..... ... .. 110
Equipment .... ..... .. ... .... ..... .. ...... .... ... ... . •• ...... .... . .112
Advantages and Applications of the Cored Wire Processes ... .. ...... .. .... 113
Classification of Cored Wires ........... . . .. ................... . . . .... 114
Shielding Gases for Gas Shielded Tubular Electrodes ...... . . . .. . . .. . ..... 121
Gas Shielded Flux Cored Arc Welding of Carbon
and Low Alloy Steels .. . .... . ............. . . ... . . . •...... . .... . ..... 122
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FLUX CORED ARC WELDING
Principles of Operation
The gas shielded flux cored arc welding process combines specific features from both the
shielded metal arc and gas metal arc welding processes. A continuous filler metal electrode is
used but it has a hol low core. The core is filled with flux and other ingredients that perform the
same functions as the covering on the shielded metal arc welding electrodes, to stabilize the
arc, generate gases and provide a slag cover to shield the arc and weld metal from
atmospheric contamination, purify the weld metal, add alloying elements, shape the weld
bead, etc. Further protection is provided by an externally supplied shielding gas.
In operation, an arc is struck between the continuously fed tubular wire containing the fluxing
and other ingredients (flux cored wire) and the workpiece (see Figure 5.1). As in the GMAW
process, the heat generated by the arc melts the end of the electrode (the metal sheath and
the ingredients inside) and part of the base metal at the weld seam . The arc transfers the
molten metal from the tip of the melting electrode to the workpiece where it becomes the
deposited metal. The arc travel along the weld seam can be mechanized (automatic welding)
or manual (semi-automatic welding).
Solidified
Slag
•
Gas Nozzle
Flux Cored Electrode
Powdered Flux
t:JOa
~ Direction of Welding
Weld Pool
Figure 5.1: Schematic Representation of the
Gas Shielded Flux Cored Arc Welding Process
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Today, three groups of tubular electrodes are available for common use:
The first group of wires are called gas shielded flux cored wires and these are meant to
be used with an external gas shield foll owing the original developments in the mid 50's.
The first major variation of the gas shielded flux cored wire was the self-shielded flux
cored wire. With these wires, as the name implies, no external gas shield is used (Figure
5.2) and instead all the required shielding of the arc and the weld pool is provided by the
gases formed by the break down of flux ingredients in the core and the slag cover on the
weld metal. A certain amount of nitrogen pick-up from the atmosphere is unavoidable and
therefore denitriders or nitrogen fixers such as aluminum are added to the core
·ngredients.
Wire Guide and Contact Tube
Wr
Solidified
Slag
Tubular Electrode
Powdered Metal, Vapor Forming
Materials, Deoxidizers and Scavengers
Arc Shield Composed of Vaporized
and Slag Forming Compounds
Arc and Metal Transfer
t:JO/J
C) Direction of Welding
Weld Pool
Figure 5.2: Schematic Representation of the
Self-Shielded Flux Cored Arc Welding Process
Another group of tubular wires are called metal cored electrodes. These wires combi ne
features of flu x cored and gas metal arc welding wires. The continuously fed wire is
cored but does not contain fluxing ingredients. Instead the core only contains arc
stabilizing compounds, deoxidizers and metal powders. The shielding is therefore
provided only by the externally supplied shielding gas as in GMAW. Metal cored arc
welding electrodes are grouped with flu x cored arc welding wires in Canadian Standard
CSA W 48 but in the U.S., these wires are considered as a variation of gas metal arc
welding process.
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Equipment
The equipment arrangement for gas shielded FCAW and MCAW is essentially the same as for
GMAW and described in the previous chapter. It comprises a constant voltage power source,
a constant speed wire feeder and control system, the welding gun and a supply of shielding
gas. The power source and the gun of course must be rated for the current levels that are
likely to be used with the selected electrode. Since the flux cored arc welding process may
involve higher welding currents, guns for semi-automatic welding can be provided with an
attached protective hand shield . Most electrodes are designed for welding with direct current
electrode positive polarity. As in GMAW, the constant voltage power source and constant
speed wire feeder enable a constant arc voltage/arc length to be maintained .
In the case of self-shielded flux cored arc welding , the guns used are slightly different since
there is no need for an external gas supply. Most self-shielded flux cored arc welding wires
are designed for welding with direct current, electrode negative polarity and with longer
electrode extensions. Due to the latter reason , an insulated extension guide is attached to
the contact tube to ensure that the wire and the arc are directed at the intended location.
Resistance heating
of electrode
Tip to Work
Distance
Figure 5.3
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Advantages and Applications of the Cored Wire Processes
The cored wire processes offer a high quality weld deposit with higher deposition rate and
productivity than the SMAW process. Higher productivity is a result of a high duty cycle, high
deposition efficiency, and high travel speeds. Compared to GMAW, the cored wire processes
are more tolerant to small deviations in welding current, voltage, tip to work distance, etc., and
therefore are more likely to provide weld deposits free from incomplete fusion flaws.
Among the three cored wire variations covered here, the self shielded flux cored wires are
better able to tolerate air currents than the others and therefore are a more suitable candidate
for field work. In automatic applications, very high travel speeds are possible with self
shielded wires leading to high productivity. However, these wires should be properly selected
since some formulations are not designed for multipass welds. Metal cored electrodes
produce little if any slag or oxide similar to the GMAW process. However, the metal cored
wires provide a higher deposition rate than does GMAW, and also a wider, more rounded bead
shape when argon rich gas shielding is used (Figure 5.4).
16
19mm TTW
85% Ar, 15% CO 2 Shielding Gas
14
Metal
Cored
12
10
8
Deposition Rate 6
,#,#
~
........
.'.'
.'
.'
.'•
....
~
Solid Wire
GMAW
•
(Ibs/hr)
4
2
o 160 200
240
280
320
360
400 440
Arc Current, A
Figure 5.4: Comparative Deposition Rates for GMA and Metal
.,,,.-"cored Wire Welding with 1.6 mm Diameter Wire
~r",~ '
,fJl,,7 <" .....-
&<'" OS '"
Compared to GMAW, the main disadvantage of the cored wire processes is the amount of
fume generation . Self shielded tubular electrodes produce the greatest amount of particulate
fumes which in some cases may be more than covered electrodes. Gas shielded flux cored
and metal cored electrodes normally produce less fume than covered electrodes but more
than GMAW though th e rates can vary significantly from wire to wire. Secondly, there is a
need for interpass slag removal with the flu x cored wires. Finally, the weld quality of gas
shielded FCAW and MCAW welds ca n be impaired by the presence of air drafts as for GMAW.
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Tubular electrodes are available for welding several of the commercially significant metals and
alloys such as ca rbon and low alloy steels, stainless steels, nickel alloys, as well as for
hardfacing and surfacing applications. Depending on the wire size and the type of ingredients
in the core, cored wire processes can be applied for welding in all positions.
The flu x cored arc welding process is a more productive substitute for shielded metal arc
welding in most applications. It is commonly used for medium thickness workpieces which
may be considered as relatively thin for optimum application of the submerged arc welding
process and relatively thick for optimum application of sma ll diameter wire, gas metal arc
welding with CO, gas shield. Such applications are quite common in the fabrication of
construction equipment. General structural steel and industrial equipment fabrication (e.g.,
mach ine tool bases, ladles for the steel industry, etc.) are also a major user of the process.
More recently, the flu x cored arc welding process is being used for pipe welds. Applications in
the pressure vessel industry are also increasing gradually as newer wires provide lower weld
metal hydrogen content, better toughness and better control over excessive strength (Figure
5.5). Still, the weld metal toughness can be adversely affected by thermal stress relief, and
therefore weld tests should be performed to confirm that weld metal toughness is still
adequate after stress relieving .
5.4
Classification of Cored Wires
The requirements for carbon steel flux cored arc welding wires (both self-shielded and gas
shielded) are described in CSA Standard W48 and with some differences, in AWS
OJ 24
120
a
-
110
a
30
'Vi
6100
..-
0,
c: 90
Min· 20 ~
....................... ,................
§
.c
~~
'Vi
~
80
15
r-""
70 M.jn~ .
....... ....
25_
_~
1994
g>
o
10 iIi
'--....L......................I_-' 5
1977
~
1977
1994
• Minimum value specified by CSA W48
for the E491T-9 Classification
:5
20
iiiOJ
16
-g,
12
.s
Max'
.................................
e
I
Q)
:0
"en
€
8
o 4 '--_-'--'-.......-'-......1
1977
Figure 5.5: Improved control of Weld Metal Mechanical Properties
and Hydrogen Content In Recently Developed E491T-9 Wires
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• Maximum value specified by CSA W48
for the E491T-9 Classification
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Specification A5.20. Those for low alloy steels, stainless steels, and for surfacing are included
in AWS Specifications A5.29, A5 .22, and A5.21 , respectively.
Gas shielded carbon steel and low alloy steel flux cored wires are usually classified as rutile or
basic, depending on the flux chemistry. Metal transfer using rutile wires is in the spray mode
over a large operating current range and for all practical purposes, there is no globular to
spray transition current (Figure 5.6) . The deposited bead is generally smooth with excellent
penetration, and out of position welding capability is achieved by controlling the slag fluidity by
suitably designing the core ingredient mix. Recent improvements in the design of rutile wires
include lower weld metal hydrogen content and better notch toughness by microalloying the
weld metal with titanium and boron .
Basic flux cored electrodes have core ingredients rich in limestone and fluorspar, similar to the
covering on basic (E4918) electrodes. These electrodes do not readily operate in the spray
mode. Metal transfer occurs in the short circuiting mode at low currents and in globular mode
at high currents (Figure 5.7). While the penetration characteristics are comparable to that of
rutile wires, the arc is less stable with considerable spatter. More important, the slag is very
fluid making it difficult to use basic wires for out of position welding. Due to the basic nature of
flux ingredients, the weld deposit has relatively low hydrogen content and superior notch
toughness compared to rutile wires (Figure 5.8).
85~o Ar, 15% CO 2 Shielding Gas
I
350
I~
250
0
0
150
100
190
!
-
I
C'?' I
!j
,
-
!
I
J,
i
150
-
I
0
0
0
~
I..
200
!
!
~
i!
300
Wire Feed
Speed
(in/min)
i
19imm TTW
400
230
270
310
Current, (amperes)
350
-
390
Figure 5.6: Wire Feed Speed and Spray Transfer Mode
with 1.6 mm Diameter Rutile FCAW Wire
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19 mmTTW i
85% Ar. 15% ¢02
.
Spray
~
Shielding Gas!
400
Short
Circuit
~
g
1:!GlnbUla~ ~
U
L-...-.J
~
gF
300
Wire Feed
Speed
(in/min)
200
100
Usable Operating Range.
180
220
260
300
340
380
420
Current. (amperes)
Figure 5.7: Wire Feed Speed and Metal Transfer Mode
in 1.6 mm Diameter Basic FCAW Wire
160
0;
o
o
5
g
'"
10
~ 120
o
-,
-100
8
e
is
11 5
r-
11 6
-
80 I-
~!!...
" 40
~
I
,g
-11 8
@)
~ 60
~ 4
"
118
r-
o
~
iii0> 6
:0
'in
~
140 I-
2
36
~
r--
tl
~ 20
o J-.1...--.L......Jt.......JU
Basic
Rutile
E
o
Basic (E492T-5)
Rutile (E491T-9)
rl
Figure 5.8: Comparative Weld Metal Notch Toughness and Diffusible Hydrogen Levels
In Weld Metals Deposited by Basic and Rutile Wires
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I
19mm TTW
400
i
85% A r, 15% CO 2
Shielding Gas
Globular
I
Spray
!~
:
10
Short
Circuit
~
I
0
0
~
300
Wi re Feed
Speed
(in/min)
I
~
!
200
- t --
Transition
Current
100
180
220
260
340
300
380
420
460
Current, (amperes)
Figure '5.9: Wire Feed Speed and Metal Transfer Mode
in 1.6 mm Diameter Metal Cored Wire
Metal transfer in metal cored wires ca n be in any of the three modes - short circuiting,
globular or spray, depending on the welding parameters and shielding gas (Figure 5,9). In
practice , spray mode is used most often. Fo r out of position ap plications pulsed spray transfer
can be used.
Th e cored wire designation schemes followed in CSA Standard W48 and in AWS Specification
A5.20 for classification purposes are shown in Figures 5.10(a) and 5.10(b), respectively.
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Electrode
Slag System,
Cu rrent, Potarity
Shielding Gas
The leiter M designates that the
electrode is classified using 75 80'% argon , balance CO 2 or that
the electrode is sel f-shielded.
Minimum Tensile
Strength
Type of Wire:
43 = 430 MPa
49 = 490 MPa
T
C
-
XMJ
x
x
E
= Flux Co red Electrode
= Metal Cored Electrod e
HZ
+
Optional
designator about
controlled
hydrogen
~ z"
indicates the
maximum
diffusable
hydrogen per
1009 of
deposited weld
metal. Z can be
2,4.8 or 16.
Welding Pos ition§::
The letter J designates that the
electrode meets the requirements for
improved toughness of 27 J at 40°C. Absence of the letter J
indi cates normal impa ct
requirements as given in Table 16
= All Positions
2 = Flat & Flat & Horizonlal Fillets
Figure 5.1O(a): Classification Scheme for Flux Cored Wires In CSA Standard 48
x
E
x
XM
T
!I
~
Electrod e
- HZ
Slag System,
Current, Polarity
Shielding Gas
Minimum Tensile
Strength
=60 ksi
7 =70 ksi
Tubu larW ire
6
Welding Positions:
a = Fl at & Horizontal
1 = A ll Positions
I
Optional
designato rs
about controlled
hydrogen.
The leiter M designates that the
electrode is classified using 75 •
80% argon . balance CO2
shielding gas. When the le iter M
designator does not appear. it
signifies that either the shielding
gas used for classification is
100% CO 2 or that the electrode
is self-shielded .
Figure 5.10(b): Classification Scheme for Flux Cored Wires in AWS Specification A 5.20
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The main differences between the two schemes are:
Two digits are used in the GSA scheme to denote the minimum weld metal tensile
strength (in increments of 10 MPa) as opposed to a single digit (equal to ksi/10) used in
the AWS scheme;
For the welding position indicator, GSA Standard uses the digit 2 to indicate suitability
for flat and horizontal positions where as the AWS system uses 0 for the same purpose;
Metal cored wires are included in GSA Standard tables of GSA W48 dealing with
flux cored wires whereas in AWS, metal cored wires are included in tables GSA W48
dealing with solid wires for gas metal arc welding;
The slag system, current polarity and shielding gas are shown in Table 5.1.
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Table 5.1 : Shielding gas, current/polarity and slag system for electrodes of
different classification.
CSAW48
Classification
T·l"
T-2"
T-3
T-4
T-5"
T·6
T-7
- - T·B
T-S"
T-l0
f-------r:11 -~
--T-~ -
- - T-13
T-14
T-G
T·GS
- -C-3" -~-
I-
C-6"
C-G
C·GS
.-
Applicalion
Slag Syslem
Multiple Pass
Rutile
Rutile
Fluoride, rutile
Fluoride
Lime, fluoride
Basic oxide
Fluoride
Fluoride
Rutile
Fluoride
Fluoride
Rutile
c)
Single Pass
Single Pass
Multiple
Multiple
Multiple
Multiple
Pass
Pass
Pass
Pass
Multi~le Pass
Multiple Pass
~ glePass
~lePass
Multiple Pass
~glePas5
Single Pass
Multiple Pass
~lePass
Multiple Pass
Multiple Pass
Multiple Pass
Single Pass
CO-
b)
b) _ _
Nol applicable
Not applicable
Nol a~~licable
Nol applicable
--
Shielding Gas
'Current and Polari~
dc, electrode positive
dc, electrode positive
de , electrode positive
dc, electrode positive
dc, electrode positive
dc, electrode positive
dc, electrode nel1ative
dc, electrode negative
dc, electrode positive
dc, electrode negative
_
f- dc, electrode negative
CO2dc , electrode positive
None_ _ _~ectrode negative_
dc, electrode negative
None
a)
a)
a)
--~
CO 2•
Dc, electrode positive
CO,"
Dc. electrode positive
a)
&...1a)
a)
CO,"
CO,"
None
None
CO,"
None
None
None
CO,"
None
_ _ None
"The classificalion T-IM. T-2M . T-5M. T-9M. T-12M, C-3M and C-6M are po ssibl e if Ihe qualificalion lesls
are made wilh gas mixlures of 75- BO % argon. balance CO,.
(a)
(b)
(c)
(d)
As agreed upon between supplier and user.
Slag syslem developed by Ihe manufaclurer for specific applicalions.
Designed for root pass in pipeline girth welds.
Designed for welding of galvanized and aluminized sheel sleels.
The classification scheme for low alloy flux cored arc welding wires in AWS 5.29 Specification
is similar to that for carbon steel wires, the main difference being the higher weld metal tensile
strength and additional letters and numbers at the end used to indicate alloying elements
present in the weld metal. Similarly, metal cored wires for low alloy steels have the same
classification scheme as low alloy steel solid wires in AWS A5.28 except that S (denoting solid
wire) is replaced by C (indicating composite or metal cored wire). In comparison, stainless
steel flux cored arc welding wires are classified based primarily on the weld metal composition
and the shielding medium used during welding .
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Shielding Gases for Gas Shielded Tubular Electrodes
Carbon dioxide can be the shielding gas used when required, for classification purposes.
However, Argon-Carbon Dioxide (Ar-CO, ) mixtures are increasingly becoming popular as their
use with rutile wires provides less spatter, smoother beads, and better wetting action and
puddle control for out of position welding. Similarly, with basic wires, less spatter and
smoother beads are obtained. Less fumes are generated when compared with 100% CO,
shielding gas. Weld penetration is however reduced to some extent. For the reasons just
mentioned, the last revision of the Standard CSA W48 allows for a M9 ("Mixed gas")
designator in the classification which allows to classify the wires wiih gas mixtures having 75 80% argon, balance CO 2 , Metal cored wires are used mostly with Ar-CO, mixture as the
shielding gas and welding with 100%CO, is rare .
Since Argon (Ar) is an inert gas, it does not react with elements in the arc. Use of Ar-CO,
mixtures as a shielding gas causes less oxidation of Manganese (Mn) and Silicon (Si) present
in the wire , leading to higher Mn and Si content in the weld metal. This increases the weld
metal tensile strength, and may also reduce the elongation values (Figure 5,11), Similarly, the
amount of hydrogen retai ned in the weld metal can be larger compared with the use of CO,
gas, The wires can be designed to avoid excessive increase in weld meta l streng th and
impairment in elongation, and therefore the manufacturer should be consulted and/or
procedure qualification performed before embarking on the use of Ar-CO, mixture with flux
cored wires in fabrication . The shielding gas selected does not affect the deposition rate to
any significant extent.
100
90
'in
2'S.
~
~
80
-
28
25
-
30
25 _
?fi.
.<:
70 l-
"
60 I-
20 Co
~
50
15 g>
rn
~
'in
~
o
"
~ 40 -
10 iIi
5
C25
100% CO,
C25
100% CO,
Figure 5.11: Effect of Shielding Gas on Weld Metal Strength and Elongation
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Gas Shielded Flux Cored Arc Welding of Carbon and Low Alloy Steels
Joint Design
For groove welds, thicknesses up to 13 mm may be welded without edge preparation.
Greater thicknesses require beveled preparation. Self shielded electrodes do not penetrate as
deeply as gas shielded wires . Therefore the thickness that may be welded without bevel edge
preparation is limited to about 6 mm. The groove angles are usually smaller than those for
SMAW due to the smaller diameter of the FCAW wires compared to covered electrodes.
Joint designs for welding with the gas shielded flux cored and metal cored arc welding process
are similar to those used for the SMAW process. Suitable prequalified joint designs for use
are included in CSA Standard W59 and AWS 01 .1.
Electrode Diameter and Welding Position
For prequalified joints, CSA Standard W59 "Welded Steel Construction" limits the maximum
electrode size to 4 mm which in any case is the largest diameter electrode commercially
available. For welding in the vertical position, the electrode diameter is typically 1.6 mm or
less, and electrode selection limited to rutile electrodes since basic and metal cored electrodes
are not suitable for out of position welding with standard constant voltage power sources .
More recently however, pulsed power sources (see Figure 5.12) have enabled these
electrodes to be used for out of position welding . Additional advantages with pulsed power
sources are reduced spatter, reduced welding fumes and improved weld profile.
In metal cored electrodes, transition current increases with the electrode diameter as in the
GMAW process. Therefore the electrode diameter must be matched to the thickness of the
material being welded. For example , a 1.6 mm diameter wire with Ar-25% CO 2 gas needs to
operate at 300 A to achieve spray transfer. Burn through would be a problem unless the
material or root face had sufficient thickness .
Welding parameters
The principle welding parameters are; wire feed speed (or current), voltage and tip to work
distance . Other parameters controlled by the welder in semi-automatic operation include
travel speed , work angles, and the electrode position with respect to the joint. The effect of
these variables on weld bead characteristics is similar to that described for the GMAW
process.
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19 mm TTW
75% Ar, 25% CO, Shielding Gas
Standard Power Source
Globular/Short Circuiting
Pulsed Power Source
Spray Type Transfer
100
150
200
250
300
350
400
450
500
Current, (amperes)
Figure 5,12: Spray Transfer Over a Wider Current Range with Pulsed Power Source (1,6 mm
diameter Wire, Metal Cored Wire)
Table 5,2 shows typical current and voltage ranges for rutile wires using direct current
electrode positive polarity, CO, shielding gas, and the current - wire feed speed relationship for
a 1.6 mm diameter wire is shown in Figure 5.13. As current or wire feed speed increase,
voltage is also increased to maintain a constant arc length. Current and voltage values also
depend on the tip to work distance as explained in the previous section. Typical tip to work
distances for various electrode diameters are shown in Table 5.3. Increasing tip to work
distance without changing other settings decreases cu rrent as well as penetration. Arc
instability and spatter may increase, and gas shielding effectiveness may decrease. However,
as a result of the accompanying resistance heating of the longer electrode extension, moisture
and lubricants on the wire are reduced leading to some reduction in the weld metal hydrogen
content.
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Table 5.2 : Typical current ranges for rutile wires.
Wre
Diameter
Mm (inches)
1.1 (0.045)
1.6(1/16)
2.0 (5/64)
Flat Position
Vertical Position
Current, A
120 - 300
175 - 375
250 - 400
Voltage, V
24 - 33
25 -29
26 - 33
2.4 (3/32)
350 - 525
26 -35
Current, A
125 - 200
150 - 200
Not
recommended
Not
recommended
Voltage, V
22 - 25
21 - 24
Not
recommended
Not
recommended
Table 5.3 : Typical tip to work distance (mm) for rutile wires
VIIire Diameter, mm
Inches
1.1 (0.045)
1.6 (1/16
2.0 (5/64
2.4 (3/32
350
300
Wire Feed
Speed
(In/min)
Flat/Horizontal
12 15 15 20 -
20
25
25
30
Vertical
12 - 20
12 - 20
Not applicable
Not applicable
75% AI, 25% CO, Shielding Gas
25mmTIW
250
200
150
100'---'--'-..J--L.--l_.l.-.-'--'---l.---,'--.l.-.-'---'---'
150
190
230
270
310
350
390
Current, (A)
Figure 5.13: Wire Feed Speed - CUrrent Relationship
for a 1.6 mm Diameter Rutile FCAW Wire
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The deposition rate achieved with rutile wires at various current levels is shown in Figure 5.14.
The deposition rate is not affected significantly by changes in voltage. However, in selecting
current and electrode diameter, it should be kept in mind that for prequalified joints, CSA W59
standard limits single pass fillet size to 12 mm in the flat position and 10 mm in the horizontal
position . In groove welds , the layer thickness is restricted to 6 mm , except for the root and
surface layers. Split layer technique must be used when the groove width at the root exceeds
12 mm or if the layer width exceeds 22 mm in mU ltipass welds. These restrictions do not
apply to the surface layer. Finally, the effect of heat input on weld metal and heat affected
zone properties which varies from wire to wire and steel to steel, is also a consideration in
selecting the parameters.
I
I
I
I
I
I
I
I
14 12 3.2mm
10 8 Deposition Rate
(kgfhr)
6 4 2 0
I
0
I
200
I
I
I
400
J
I
600
I
800
Welding Current. (A) DCEP
Figure 5.14: Deposition Rate for Steel Flux Cored Wires
Welding Technique
Travel angle when welding with rutile and basic gas shielded flux cored electrodes is normally
20 to 30 degrees using a drag or backhand technique . Metal cored wires use a 10-20 degree
forehand or push technique . The work angle for groove welds is typically 0 degrees for all
wires though it can be increased to 15 degrees when placing a weld pass against the groove
face. For fillet welds, the work angle is typically 40 to 50 degrees. (See Fig 4.24 and 4.27)
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Weld Soundness
As with other semi-automatic manually applied arc welding process, welder skill in
manipulating the weld pool by properly directing the arc to the required location, and adjusting
the travel and work angles are of key importance in obtaining sound welds. Still, flaws can be
present in the welds due to other factors . These flaws and means to avoid them are:
Porosity: Potential causes include too high or too Iowa shielding gas flow rate; presen ce
of wind drafts; too long a tip to work distance; clogged nozzle; leaks in the gas line;
inadequate workpiece and/or wire cleanliness.
.~ .
Worm tracks: Caused by trapped hydrogen between slag and the weld metal, possibly
due to poor wire storage conditions and/or high humidity at the time of welding. It may
be reduced by decreasing current and voltage, switching to CO 2 shielding gas instead of
Ar rich gas mixtu re, and by increasing the tip to work distance. Unused portions of coils
or spools should not remain exposed to ambient conditions for extended periods of time.
Lubricants on the surfaces of some products readily pick up moisture.
Incomplete fusion/penetration: Incidence of incomplete fusion/penetration can be
minimized by proper interpass cleaning and by grinding/gouging, if necessary; by
ensuring that current setting is not unnecessarily low; reducing tip to work distance and
switching to CO 2 instead of Ar rich gases, especially for small diameter electrodes and
thick workpieces.
Cold (Hydrogen) Cracking: Cold cracking is countered by maintaining weld joint
cleanliness and ensuring that specified welding procedure (heat input, preheat and
interpass temperature) are followed. Electrode storage and selection (rutile or basic)
strongly influences diffusable hydrogen content.
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Chapter 6
SUBMERGED ARC WELDING (SAW)
6.1
6.2
6.3
6.4
6.5
6.6
Principles of Operation . ... ....... ... ..... . . ......................... 128
Current Type and Equipment ... ....... ........ ... ........... . . ....... 129
Advantages and Applications of Submerged Arc Welding .......... . ........ 130
Multiple Wire Submerged Arc Welding . ................... . . . . . .. . . . .. . .133
Wires and Fluxes for Submerged Arc Welding of
Carbon and Low Alloy Steels . ...... . ................. . . ........ . . .... 134
Submerged Arc Welding of Carbon and Low Alloy Steels .... .. .... . •.. ..... 138
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SUBMERGED ARC WELDING (SAW)
6.1 Principles of Operation
The submerged arc welding process is shown schematically in Figure 6.1. Compared to the
shielded metal arc welding process, the flu x to provide shielding is laid in granular form on the
unwelded seam ahead of the bare metal electrode. The electrode is fed continuou sly from a
coil, thus avoiding the interruptions inherent in the SMAW process to change electrodes. The
flux is quite effective in preventing the atmosp here from contaminating the molten weld metal
and no external shielding gas is required.
/
FluxFeed
Electrode
Arc Cavity
DDO¢
Molten Flux
Direction of Welding
Slag
~ Granular Flux Blanket
Solidified
Weld Metal
-~\
Ba se Metal -
1L2'.f.L.CLL'LL.C1.LfiI.'.f.L.CLL'LL.C1.LCLL'LL.C1.LfLI.'.f.L.CLLLLJ
Molten Weld Metal
Figure 6.1: Schematic Representation of the Submerged Arc Welding Process
The arc is struck beneath the flux between the bare electrode and the workpiece, which melts
a small amount of the flux. Although a non-cond uctor when cold, the flux becomes highly
conductive when molten (about 1300· C) providing a current path to sustain the arc between
the conti nuously fed metal electrode and the workpiece. The heat generated by the arc melts
the end of the electrode, the flu x, and part of the base metal at the weld seam. The arc
transfers the molten metal from the tip of the melting electrode to the workpiece where it
becomes the deposited metal. As the molten flu x combines with the molten metal, certain
chemical reactions occu r which remove some impurities and/or adjusts the chemical
compositlon of the weld metal.
While still molten, the flux which is lighter than the weld metal, rises to the surface of the weld
pool and protects it from oxidation and contam ination. On further cooling, the weld metal
solidifies at the trailing edge of the moving weld pool, and the weld bead usually has a smooth
surface due to the presence of the molten glass-like slag (molten flux resulting from all the
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chemical reactions) above it. The slag freezes next and continues to protect the weld metal
as it cools. Frozen or solidified slag is readily removable, sometimes popping off the bead
spontaneously. Excess, unmelted flux can be recovered and reused after proper processing.
The complete welding operation takes place beneath the flux without sparks, flash or spatter,
and it is for this reason that the process is called "submerged" arc welding. As a result, the
welding operator does not normally need a protective shield or helmet.
Since there is a need to lay granular flu x along the weld seam and the molten weld pool can
be quite large and fluid , submerged arc welding is best performed in the fla t position, and if
needed , in the horizonta l position. Also, since the operator can not see the arc or the weld
seam, submerged arc welding is best suited for situations where long welds with little or no
geometric variation are to be made in the flat position. The process can be mechanized or
used in a semi-automatic mode.
6.2
Current Type and Equipment
The equipment set-up for single wire submerged arc welding is shown in Figure 6.2. In
addition to the power supply, a submerged arc welding system requires a wire feeder to
maintain a continuous feed of the electrode wire through the torch. For single wire
submerged arc welding. direct current electrode positive (DeEP) is used for most applications
as it provides better control of bead shape , ease of arc initiation, and deeper penetration
welds with greater resistance to porosity.
Wire
Reel
Flux
HOpper\
-Electrode
Wire
Control
System
Electrode Lead
~Torch
II
Power
Source
=11=~~~~~j)
~
Direction of Travel ¢:XJOO
Base Metal- /.
~==::::::::==::~==~w~or~kJL~e~ad~~
Figure 6.2: Equipment Set-up for Single Wire Submerged Arc Welding
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Direct current electrode negative (DCEN) polarity is also occasionally used to provide a
greater deposition rate, However, penetration is reduced and there is some increased risk of
lack of fusion-type flaws, From a practical point of view, a change from DCEP to DCEN may
necessitate an increase in voltage of about 2 to 3V if a similar bead shape is to be maintained,
Both constant voltage and constant current (drooping voltage characteristics) power sources
can be used, Constant potential power sources , used in conjunction with constant speed wire
feeders, the arc length self-adjusts to a nearly constant value depending on the voltage, as in
GMAW, Wire feed speed and electrode diameter control welding current and the power
source controls voltage, By comparison, a constant current power source tries to simulate a
manual welder. Essentially, a voltage sensitive relay in a variable speed wire feeder
constantly adjusts the wire feed speed to maintain the target arc voltage and, therefore, a
constant arc length, The power source controls current, and arc voltage depends on wire feed
speed and electrode diameter. Modern power sources are available which operate in either
constant voltage or constant current mode,
Power sources are available that can deliver up to 1500 A. However, direct current is usually
kept below 1000 A since there can be excessive arc blow, Alternating current can be used to
reduce arc blow in high current applications, and other situations prone to arc blow, e,g"
multiwire and narrow gap welding , Alternating current power sources are usually constant
current type with a nea rl y square wave output voltage to assist in arc ignition at each polarity
reversal. Sq uare wave constant potential power sources have also become available that
provide both voltage and current in square wave form and therefore have less difficulty in arc
re-ignition at polarity reversals , The weld bead penetration obta ined with alternating current is
in between that for DCEP and DCEN ,
A coil and a hopper attached to the welding head respectively, provide a continuous feed of
the metal electrode from the coil through wire straighteners and a contact tip to the workpiece,
and flux in front of the metal electrode feed, The welding head is usually mounted on a
carriage where it moves at a predetermined travel speed, thus enabling complete
mechanization of the welding process, Alternatively, the welding head can be fixed and the
workpiece moves beneath it at a predetermined speed ,
6,3
Advantages and Applications of Submerged Arc Welding
By far, the greatest advantage of the submerged arc welding process is its high productivity,
resulting from high deposition rate and a high duty cycle, The high deposition rate is a
consequence of the mechanized nature of the process as it enables use of higher travel
speeds, and larger diameter wires and therefore higher currents than possible with semiautomatic processes, Variations such as use of multiple wires, addition of controlled amount
of iron powder to weld seams along with the granular flux can further increase depOSition rate,
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The weld deposit is considered to be a 'controlled hydrogen' type provided due care has been
taken in storage and handling of flux and wire. Heated flux storage units, similar to electrode
storage ovens are often used. Little fume is generated in the process and arc radiation and
spatter are generally not a problem. When the weld joint design is appropriate and welding
parameters are chosen correctly, sound welds with a smooth, uniform finish are easily
obtained.
The main limitation of the submerged arc welding process is that it is limited to welding in the
flat and horizontal positions only. The mechanized nature of the process implies more
expensive equipment and greater set up time.
Most submerged arc welding applications are for carbon and low alloy steels. The process is
also used for joining stainless steel and nickel based alloys. However, the fluxes are
proprietary in nature and flux manufacturers must be consulted for optimum flux selection.
Because of the mechanized nature of the process, it is most effectively used when numerous
similar welds are to be made (splicing of plates and panels in shipyards, fabricated structural
shapes, welding longitudinal or spiral seams of large diameter oil and natural gas pipelines
(see Figure 6.3) and when the thickness to be welded is large (circumferential and longitudinal
seams in thick wall pressure vessels) . Other applications of submerged arc welding include
overlaying (stainless steel overlay .on chromium-molybdenum steels for high temperature, high
pressure hydrogen applications) and rebuilding and hard surfacing.
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Figure 6.3a - Double submerged arc welding of spiral seam in
large diameter line pipe - inside (Weiland Pipe Inc.)
Figure 6.3b - Double submerged arc welding of spiral seam in
large diameter in line pipe - outside (Weiland Pipe Inc.)
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Multiple Wire Submerged Arc Welding
One of the great advantages of the submerged arc welding process is its adaptation to the use
of multiple electrodes fed into the same weld pool thus considerably increasing the deposition
rate. Some of these configurations (Figure 6.4) for multiple wire submerged arc welding are:
Parallel Electrode Welding : Also called twin wire welding, two electrode wires are connected
in parallel to the same power source. Both electrodes are fed by means of a single wire
feeder and through the same welding head. Welding current is the sum of currents for each
electrode and a single deep penetrating weld pool is obtained.
Multiple Arc Welding: Also called tandem welding, two (or more) electrodes can be
connected to individual power supplies and fed by separate drive rolls through separate
contact tips . The lead electrode in such cases is connected to a DC power source and the
trailing electrode to an AC source in order to reduce interaction between the magnetic fields of
the two arcs. It is important to ensure that the spacing between the arcs is not too large. The
trailing arc is usually positioned close enough to the leading arc that the slag cover does not
solidify between deposits. The total current in multiple wire welding can be as high as 2000 A
though in most applications it does not exceed 1200 A.
Series Arc Welding: The two electrodes fed through separate guide tubes are connected in
series. Separate sets of drive rolls and contact tips, insulated from each other need to be
employed. The current path is from one electrode to another, through the weld pool. The
weld bead has relatively shallow penetration, making this arrangement useful for overlay
welding.
+
+
Sin gle Wire
Twin Wi re Parall el Electrod es
Ta ndem El ectrodes
Series Arc
Figure 6.4: Submerged Arc Weldiing Process
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Wires and Fluxes for Submerged Arc Welding of Carbon
and Low Alloy Steels
Traditionally, solid wires similar to those for GMAW have been used for submerged arc
welding. The electrode size tends to be larger and the composition may be different since one
must consider the influence of the flux and the greater dilution from the base metal on the
weld metal composition. For this reason consumables for submerged arc welding are
selected as a wire-flux system rather than on an individual basis. More recently, composite
wires (tubular wires with alloy powder and other ingredients in the core) are being used for
submerged arc welding. The advantages of a tubular electrode is the wide range of deposit
chemistry possible and the ability for increasing travel speeds.
50% Filler Metal
50% Base Metal
~~~~------~
L -_ _ _ _ _ _ _
20% Filler Metal
80% Base Metal
70% Filler Metal
30% Base Metal
Figure 6.5: Dilution Ratios of Some Common Weld Joints
Fluxes for submerged arc welding can be categorized on method of manufacture or effects on
weld metal composition. There are two types of fluxes : fused fluxes and bonded fluxes. The
manufacture of fused fluxes involves melting together of various ingredients to provide a
homogeneous mixture which is then allowed to sOlidify by pouring onto a large chilling block.
The glass like, solidified particles are crushed, screened for sizing and then packaged for use.
The main advantages of fused fluxes are their chemical uniformity irrespective of the flux
particle size, resistance to moisture absorption and easy recycling without changes in particle
size or composition. The disadvantage of fused fluxes is that it is difficult to add deoxidizers
and ferroalloys. These compounds tend to oxidize during the melting process.
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In comparison, bonded fluxes are made by finely grinding the individual components of the
flux, mixing them in appropriate proportions and then adding a binder, typically potassium
and/or sodium silicate. The wet mixture is then baked at a relatively low temperature and
ground to size for packaging . The main advantage of bonded fluxes is that it is easier to add
deoxidizers and ferroalloys. On the negative side, such fluxes are prone to moisture pick up,
and to local changes in composition due to segregation or removal of fine mesh particles.
Fluxes that significantly influence the composition of the weld metal through slag/metal
reactions are termed active fluxes. Typically, these fluxes add manganese, silicon and
chromium to the weld metal. The extent of this addition increases with arc voltage since
higher arc voltage leads to increased flux consumption (Figure 6.6). Very active fluxes may be
used to deposit single or two pass welds only since the increase in the Si and Mn content of
subsequent passes may be sufficiently large to impair the weld metal ductility and also make it
more prone to hydrogen cracking. Certain active fluxes termed as alloy fluxes add elements
such as Ni and chromium . Such fluxes enable the welding of weathering steels (containing
chromium, nickel or copper) using carbon steel wires, and compensate for the loss of
chromium from the wire by oxidation when welding stainless steels.
1.4
1.2
1.0
0.8
Silicon
Weld Metal
%
Flux B
0.6
0.4
0.2
0
20
24
28
32
36
40
Arc Voltage
Figure 6.6: Effect of Arc Weld Metal Silicon Content for Two Active Fluxes
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Neutral fluxes also participate in slag-meta l reactions but the changes in silicon and
ma nganese are smaller and not dependent on arc voltage (Figure 6.7). There is little build up
of elements and such fluxes are therefore well suited for multi pass welds.
1.2
1.0
0.8
Silicon
Weld Metal
%
FluxC
0.6
0.4
Flux D
0.2
0
20
24
28
32
36
40
Arc Voltage
Figure 6.7: Effect of Arc Voltage on Weld Metal Silicon Content For Two Neutral Fluxes
Fluxes are also referred to as chemically basic, neutral or acidic. Chemically basic fluxes
have Ca lcium Oxide (CaO) and Magnesium Oxide (MgO) as the major ingredients. Chemically
acidic flu xes have Silicon Oxide (SiO,) as the main ingredient. When the ratio of basic oxides
to acidic oxides present is greater than 1, the flu x is chemically basic and when it is less than
1, it is chemically acidic. Ratios near 1 imply a chemically neutral flux. Basic fluxes transfer
smaller amounts of Si, Mn and oxygen to the weld metal, and therefore are preferred for
critical applications.
Requirements and selection for carbon steel wires and fluxes providing weld metal with
minimum specified ultimate strength of 490 MPa are detailed in CSA Standard W48, and for
higher strength weld metals, one can consult AWS Specification A5.23 . Submerged arc
welding wires are classified based on their composition whereas fluxes can only be classified
in conjunction with a welding wire and their classification indicates the weld metal strength and
toughness. The classification scheme for flux-wire combinations is shown in Figure 6.S. Thus,
flux-wire combination conforming to the designation F49A5-EM12K indicates that: (i) the
electrode wire has a medium (M) manganese content, nominally 0.12%C (12) and is made
from a silicon killed steel (K); and, (ii) when used with the specified flux in a standardized test ,
will provide weld metal that in the as welded condition (without post-weld heat treatment) will
meet the requirements of minimum 490 MPa ultimate tensile strength, and minimum 27 J
Charpy Vee notch impact strength at -50·C.
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. - - - - - - - - Designates a flux .
The first two digits indicate the minimum
. - - - - - - tensile strength of the weld in increments of
10 MPa.
~---
The next letter indicates the heallrealment
condition; "A" for Has·welded~, ~P" for the
post-weld heat-treated condition.
The fourth digit indicates the temperature at
which the impact strength of the weld metal
meets the requirements. A "Z" indicates
that impact testing is not requ ired . An "8"
indicates that the flux/electrode is suitable
for single pass only.
r
T~
--r
F XX X X ·E
Designates an electrode.
L XX
X
For solid electrodes, "L" indicates a low
manganese (Mn) (0.60% maximum),
"M" indicates a medium Mn (1.25% _ _ _ _ _ _ _---l
maximum), and "H" indicates high Mn
(2.25% maximum) content of the
electrode. "e" indicates a composite
electrode.
For solid electrodes, these one or two
digits indicate the nominal carbon
content of the etectrode. For composite _ _ _ _ _ _ _ _-.J
electrodes, they indicate the chemical
analysis of the weld metal made with a
specific flux.
For solid electrodes, a "K" indicates that _ _ _ _ _ _ _ _ _ _...J
the electrode is made of silicon killed
steel.
Figure 6.8: Classification System for Submerged Arc Welding Wires and Fluxes
(as per CSA W48)
It is important to note that as a result of the above classification scheme, a particular flux can
assume a different designation when used in conjunction with another wire. Another
consequence of the joint effect of wire and flux on weld metal properties is that once a specific
flux-wire system has been approved to a particular classification, then no other flu x or wire of
the same designation but different trade name may be substituted for it without a complete
new series of tests to demonstrate that all the requirements are still met. For more details on
submerged arc welding consumables, see Module 6.
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Flux Usage
Following are some of the precautions that should be taken in the storage and use of fluxes:
Fluxes can absorb moisture and thus compromise the controlled hydrogen characteristics
of the process. It is therefore important that once a flux bag is opened, it is stored in a
dry environment. If there is any doubt of its condition, the flux should be baked before
use, following the manufacturers recommendations.
Fluxes look alike and therefore if a flux transferred to a different container for proper
storage, it should be properly identified.
In recovering and reusing flux, it should be ensured that particle size distribution is
maintained. Too many fines in the flux make it difficult to feed, and loss of fines may
change the flux composition which may change the chemistry of the deposit.
When active or alloy fluxes are used, the specified welding parameters must be followed
diligently otherwise the weld deposit properties will be different from those expected.
Do not use an active or alloy flux where a neutral flux is required , and vice versa .
6.6
Submerged Arc Welding of Carbon and Low Alloy Steels
Joint Design
Because of the high currents and deep penetration possible with submerged arc welding,
steels up to 12 mm may be welded in one pass without any edge preparation. With edge
preparation, stee ls with thickness up to 25 mm are weldable in one pass. However, it
assumes that the joint is suitably designed to prevent burn-through and that the weld zone
mechanical properties achieved are acceptable. Figure 6.9 shows some joint designs that are
prequalified for use as per CSA Standard W59 subject to certain procedural constraints.
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82-2
/
s7S
60'
Effective Throat
=T
RF =6mm (1/4")
G=O
B-U2b-8
Figure 6.9 - Prequalified joint from eSA Standard W59 for submerged arc welding of carbon
and low alloy steels
We/ding Parameters and Deposition Rate
Recommended current ranges for electrodes of various diameters are shown in Table 6.1.
Approximate deposition rates obtained at various welding currents on mild steel with different
submerged arc welding arrangements are shown in Figure 6.10. Direct current electrode
negative polarity leads to higher deposition rates compared to dc electrode positive polarity.
Although reduced penetration increases susceptibility to incomplete penetration/fusion type of
flaws .
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96
Tandem Are, AC·AC
1/8" to 3/16"
90
84
78
72
Twin Arc DC
3/32" to 1/8"
66
60
54
Deposition Rate
(Ib/hr)
48
42
36
30
24
18
Single Electrode DC
5/54" to 7/32"
12
6
o L-~
o
__
L-~~
__
~~
__
~~
__
~~~
200 400 600 8001000120014001600180020002200
Amperes
Figure 6.10: Approximate Deposition Rates of Various Submerged-arc Welding Process
Variation (Upper line in each band is for DCEN polarity; lower line for DCEP polarity and larger diameter wire)
Table 6.1 : Current range for wires of different diameter
Electrode
Diameter, mm
2.0
2.4
3.2
4.0
4.B
5.6
6.4
Current Range, A
200 - 500
300 - 600
300 - BOO
400 - 900
500 -1200
600 - 1300
600 - 1600
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The deposition rate can be further increased by employing a longer electrode extension,
commonly referred to as 'longer electrical stick-out' . In effect, it implies a longer tip to work
distance (Figure 6.11). The approach relies on resistenace heating in the electrode extension
to increase the wire melt off rate. Typical tip to work distance employed with dcep is roughly 8
times the wire diamter, i.e., for a wire diameter of 4 mm, it is usually about 32 mm. Inlonger
stick out applications, it may be as high as 75 mm. Very long stick-out lengths create
problems with the electrode wandering unpredictably on and off the joint.
27.3 kg (60 Ib)
DC -7"
Slickout
22.7 kg (50 Ib)
DC- 5"
TTW
DC - Normal
TTW
1B.2 kg (40 Ib)
Deposilion Rale
(Ib/hr)
13.5 kg (30 Ib)
DC + Normal
TTW
9 kg (20 Ib)
10lb
0L-~
o
__-L__~__L-~__-L__~
500 600 700 BOO 900
400
Currenl
Figure 6.11: Effect of Polarity and Tip to Work Distance (TIW) on Deposition Rate
The resistance heating effect is more pronounced in smaller diameter electrodes than in larger
diameters. Wire diameter has only a small effect on deposition rate when using electrodes =
> 2mm in diameter. Two small diameter electrodes (2 mm diameter) used in twin wire (parallel
wire) configuration will provide higher deposition rates than a single 4 mm diameter wire
carrying the same welding current (see Figure 6.12).
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50
40
Deposition Rate
Twin 2 mm Diameter Wires
(Ib/hr)
30
~ngle
10'--- - - - ' - - - - ' - - - - - - L - - - '
600
800
1000
1200
Single 4 mm Diameter Wire
Amperes
Figure 6.12: Deposition Rates for Single and Twin Wire Submerged Arc Welding
The influence of other welding parameters like current, voltage, etc. on bead shape is similar
to that discussed earlier for GMAW process. The effect of changes in a single parameter can
be summarized as follows:
High welding current increases penetration, and also requires higher wire feed speed
(deposition rate); Excessive current can lead to a deep narrow bead prone to
solidification cracking;
DCEN polarity increases deposition rate but DCEP increases penetration;
Increasing arc voltage (increased arc length) produces a flatter, wider bead, increases
flu x consumption and effects pick up of alloying elements from the flux;
Increasing travel speed reduces heat input and penetration . High travel speeds at
constant heat input increase susceptibility to solidification cracking. Slow travel speed
also reduces penetration as the arc is cushioned by the molten weld pool.
The width and depth of the flux layer at the weld seam is another secondary variable that may
influence weld quality. Too deep a flu x layer gives a ropey bead surface. The weld bead
surface may be distorted by entrapped gases unable to escape the flux burden. Conversely,
too shallow a flux layer allows the arc to flash through the flux. The flux layer, being too thin is
unable to protect the weld pool. Spatter and porosity are potential consequences in such a
case.
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In case of multiple arc (tandem) welding, the spacing between the electrodes is another
important variable. If the spacing is less than about 50 mm, alternating current is used for the
trail electrode in order to avoid arc blow due to the interaction between the magnetic fields. If
spacing is more than 50 mm, direct current may be used for both electrodes. However, too
long a spacing can lead to slag entrapment as the second arc is not completely able to melt
through the slag layer from the first arc.
Welding Procedures
For welded construction in accordance with CSA Standard W59, the following limitations are
specified for pre-qualified joints:
fillet welds up to 12 mm may be deposited in a single pass in the flat position; in the
horizontal position, the maximum single pass fillet size is 8 mm; in any case, the
current must not exceed 1000 A for the single electrode and 1200 A for the parallel
electrode variation of the process;
in order to prevent burn-through, either appropriate backing bars should be used or the
root face must be at least 6 mm; for root face less than 6 mm, a shielded metal arc
weld pass may be manually deposited on the back side ;
the largest wire that may be used for submerged arc welding is 6 mm;
in groove welds, current for the root pass should be less than 10 times the groove
angle; this is to control the bead shape and dilution so as to reduce the likelihood of
weld metal solidification (centerline) cracking; for subsequent passes, welding
parameters should be chosen so that in cross section, the depth of the weld bead or its
width at any point along its depth does not exceed the surface width of the weld bead;
W<O
(b)
(a)
Figure 6.13
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with a single electrode wire, the layer thickness in groove welds is limited to 6 mm except
for the root and capping passes; this limitation does not apply to welds made with parallel
electrodes; also, split passes are required when the root opening is more than 13 mm, or
when the layer width exceeds 16 mm in multipass welds.
The limitations for multiple arc welds are slightly different; a fabricator can design welding
procedures outside the W59 limitations as long as a procedure qualification test is carried out
to demonstrate the adequacy of the welded joint.
As mentioned earlier, the submerged arc welding process is treated as a controlled hydrogen
process subject to proper storage and conditioning of the electrode and flux. Due to its
mechanized nature, it is capable of providing a sound weld deposit of uniform and consistent
properties. Taking advantage of these two process characteristics in some cases, allows for
the elimination of preheating. Minimum size fillet welds for steels of different thickness and
carbon equivalent, that may be deposited without preheat and without hydrogen induced heat
affected cold cracking are shown in Table 6.2. The minimum fillet size represents a certain a
minimum heat input that, depending on the steel thickness and carbon equivalent, is expected
to keep the heat affected zone hardness below a critical level for cold cracking.
Plate Thickness (t),
mm
T< 12 welded to > 40
T> 12weldedtot>40
'Carbon Equivalent
0.35
OAO
8
8
8
8
Carbon Equivalent"
0.45
0.50
8
10
10
10
0.55
10
12
0.60
12
16
= C + (Mn + Si)/6 + (Cr + Mo + V)/5 + (Ni+Cu)/15
Table 6.2 : Minimum single pass submerged arc filiet weld sizes to eliminate preheat (from
CSAW59)
When joining quenched and tempered steels, caution must be exercised in using high heat
inputs (high deposition rates). The accompanying slower cooling rate can adversely affect the
weld joint mechanical properties. Table 6.2 is not applicable to quenched and tempered
steels.
Welding parameters for certain jOints are shown in Figure 6.14.
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Weld Soundness
Possible flaws that can be present in submerged arc welds include:
Undercut: Caused by excessive voltage, incorrect electrode position or by too slow a
travel speed;
Porosity: Caused by trapped gases in the weld. Can be avoided by removing all rust
and contamination from the joint area and ensuring that flux is clean and dry. Changing
flux can also be helpful as some fluxes are better at controlling this problem;
Slag inclusions : Caused by insufficient interpass cleaning or a highly convex bead
shape. Bead shape may be improved by increasing voltage, by increasing speed or by
reducing current;
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I
25mmr
r"<-
~
25mm
(5 mm diameter y,irej
Side 1 Bead 1 Am ps Vollage Travel Speed
800
34V
Side
Amps Voltage Travel Speed
Bead
_,_ ,
40 cmfmin
.
I
T
50 emlmin
50 emlmin
~ 27
450
27
I
2
\.
t
( 1.6 mOl diameter wire)
~
(
10mm
(ons pass)
57mm
330101
7mm
'\
17mm
~
Conrlguration Parallel Electrodes Amps Venage Travel Speed
l ead
Two 2.5 mm Oia.
Trail
Two 2.5 rom Dia.
900
750
32
34
150 cm/mi~
150 cmfmin
Wire Dia: 4mm, Lead: DC + VE. Trail: AC
,,
550
3-12 700 Lead
2
15-18
Side Bead
Amps
Vollage Travel Speed
'-2
800 Trail
Side 1
-[
r
2
550
19-22 700 lead
800 trail
28
29
34
30 em/min
75 emlmin
28
40 Em/min
2.
34
75 em/min
- - -
60mm
IOG mm
l
7mm
Side 1
3
Tm
L--.....I'---'-=--'------.J_
Bead
Amps
Voltage
Travel Speed
1
1· 12
800-920
30
40 - 60 em/min
~ac k gouging
2
, ,
13·42
43-60
920
920
30
33
40 . 50 emlmin
-2 --
40 emfmin
,
4mm
4 mm
800
800
--~
8 mm
t~mm
-r
S;,.2 ~
60·
Side Bead Wire Dia . Amps
to sound weld metal)
4mm
-.!
X
20m{t
Side 2
Two Parallel Electrodes, 3.2 mm Diameter
Side
\!7
Vol tage Trave! Speed
35
35
80~~
50cm/min
(NO backgouging to sound metal on second side)
Figure 6.14: Typical Welding Parameters for Selected Joints
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Incomplete fusion/penetration: Normally submerged arc welding has sufficient penetration to
melt out small irregularities; possible causes include improper alignment of electrode with the
joint, not grinding or gouging to sound metal from Ihe second side, use of DC EN polarity
and/or long electrical stick out;
Bu rn-throug h: Burn-through is a consequence of excessive penetration. Can be countered
by redu cing gap or increasing root face, by decreasing current or increasing travel speed,
using SMAW in the root or providing a backing bar or flux back-up , etc.
Pock marks: These are oblong surface irregularities near the vertical toe of the fillet welds
caused by back pressure of the gases trapped between the molten weld pool and the
freezing slag; the solutions to counter porosity also apply to pock marks. Higher current
and/or lower travel speed may also be helpful by increasing the heat input and allowing the
slag to stay molten for a longer time;
Solidification cracking: Located at weld centerline, it is more prevalent in deep narrow beads
as well as weld passes deposited over large gaps; possible solutions lie in improved fit-up,
reducing current, reducing travel speed, and increasing groove angle. DC EN polarity can
also be helpful as dilution from the 'relatively higher carbon containing' base material is
reduced;
Hydrogen cracking : Caused by hydrogen absorbed by weld metal. Important precautions
are clean and dry flux and wire, and application of the stipulated preheatlpostheat.
Special Variations of Submerged Arc We/ding
Some of the special variations of the submerged arc welding process include:
Strip Welding : Here the electrode is in the form of a continuous thin strip of metal; it is used
for surfacing applications where the deposited weld bead has essentially the same width as
the strip electrode (typically 25 to 100 mm). The main advantage of the strip electrode is the
deposition rate when large surface areas to be overlayed, for example, the inside surface of
certain carbon steel and Cr-Mo steel vessels with stainless steel;.
Iron Powder Addition : Also referred to as bulk welding, iron powder or iron base
ingredients are added to the joint under the flux. The heat of the arc melts these additions
and adds to the deposited weld metal thus increasing deposition rate. Alloying elements can
also be added to the weld metal in this manner. The cooling rate is increased to some extent
and this can improve mechanical properties. The additions must be carefully selected to
avoid the introduction of unwanted elements (high oxygen) in the weld pool.
Hot wire addition : In this variation, used more frequently for gas tungsten arc welding, an
electric current is used to heat a separate wire which is fed into the normal submerged arc
arc/molten metal. The hot wire is not a part of the submerged arc welding equipment
electrical circuit. DepOSition rate can be increased by 50% or more without impairing the
weld metal properties.
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Chapter 7
RESISTANCE W ELDING (RW)
7.1
7.2
7.3
Principles of Operation ............... ..... . ......... . . .......... '..... 150
Process Parameters ......... . ... . ... .. ... . •......... . ......... . •. . .. 151
Types of Resistance Welding Processes ..................•.............. 152
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RESISTANCE WELDING (RW)
Principles of Operation
Resistance welding is a process in which faying surfaces held together under force are joined
in one or more spots by the heat generated due to the resistance to the flow of electric current
through the workpieces (see Figure 7.1). The degree of resistance depends primarily on the
nature of the materials joined, and the air gap at Ihe weld interface. It differs from arc welding
in that an external pressure is applied , and fluxes or filler metals are generally not used. (No
arc should be created.)
Force
;
Welding Temp.
End of
W,"
...
Water
"m'j____ _
II
,empera ture
/'
T
Water
...::;
/
t••
ACJ'
Contactor
Welding
Transformer
Force
Figure 7.1: Resistance Welding Principle
The maximum amount of heat will be generated at the point of maximum resistance. Under
ideal conditions, maximum resistance will be created at the surface between the parts joined.
Heat is also generated at the contact between the non-consumable copper welding electrodes
and the work. The amount should be small since the resistance between the high conductivity
electrode material and the work is low. In most applications, the electrodes are water cooled
to minimize the effect of heat generated at the contact points.
Materials that are eas ily welded with this process include carbon steels, stainless steel.
Aluminum, brass and copper can be resistance welded but with greater difficulty. The
difficulties encountered are due to low resistiVity and effects of surface compou nds.
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Process Parameters
There are four main controllable process variables involved in making resistance welds :
current that flows through the-workpieces .
pressure that the electrodes deliver to the work
the length of time the current flows through the work
the cross-sectional area of the electrode tip in contact with the work
A high current (up to 100,000 amps at low voltage) generates enough heat at the faying
surfaces (highest resistance point) such that the metal reaches a molten state. The force
applied before, during and after the current flow forges the heated parts so that coalescence
occurs. Pressure is maintained throughout the entire welding cycle to ensure a continuous
electrical circuit and help re fine the grain structure of the weld . The amount of current and
time to generate coalescence is related to the heat input required to increase the temperature
of the metal to the welding temperature and the amount of heat losses from the weld zone.
The cross-sectional area of the electrode tip is proportional to the level of current used and
size and type of weld required.
Figure 7.2: Cross-section of a Typical Resistance Spot Weld
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7.3 Types of Resistance Welding Processes
There are six main variations of the resistance welding process:
Resistance Spot Welding (RSW)
Resistance Projection Welding (RPW)
Resista nce Seam Welding (RSEW)
Flash Welding (FW)
Upset Welding (UW)
Percussion Welding (PEW)
Resistance Spot Welding (RSW)
Resistance spot welding is the most widely used resistance welding process. The contacting
surfaces in the region of current concentration are heated by a short time pulse of low voltage.
high amperage current to form a fused nugget of weld metal under applied force. When the
flow of current ceases, the electrode force is maintained while the weld metal rapidly cools.
The electrodes are then retracted after each weld cycle. A typical RSW set-up and joint type
are shown in Figure 7.3.
Figure 7.3: Resistance Spot Welding Process
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The equipment required for RSW can be simple and inexpensive or complex and costly
depending on its size and the degree of automation. It consists of a transformer power
source, two or more electrodes for conducting welding current, a controller/contactor, and in
most instances, means of applying necessary welding force.
The equipment can be considered in two categories: single-spot and multiple-spot machines.
Single spot machines, range from simple manually operated ones to stationary horn or rocker
arm types. Simple hand held single impulse manual machines rated at 2 kVA with a short
circuit current of 6000 A are capable of welding thin carbon steel up to 20 gauge. More exoti c
multiple impulse machines have extremely complex controllers that include steady or pulsed
current, preheat current , post heat current, varying levels of pressure for different time periods,
and other features. Examples of a single impulse and a multiple impulse resistance welding
program are shown in Figures 7.4 and 7.5, respectively.
Electrode Force
,,[:.--r Welding-Cu~~t\
//
".----
///
AAAAAAAA
AAA
rvv
VVVVVVVV
Squeeze
Time
Weld
Time
!Hold Off i
!Time!Time!
~~~------~·~~----------~
· ~ !~l
Welding Cycle
! Cycle Repeats
~.~----------------~-------------. i~~~--------------.~
Figure 7.4: A Single Impulse Resistance Welding Program
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Forge Force
/"--------'
o
I'
1/ ~-------------L..
Forg e Delay Time
.. i
.
§/
I
I
I
I
t'
~/
OJ/
I
,\
Tempering Current
\
-L--~~~~~HT~~~~~L---~~----- I
~!
:~
:
!
i squeeze! Preheat . Upslope! Weld !~! Weld! ~i Weld! ~ 1Quench ~Temper i Hold !
: Time
: Time
: Time
i Time !I=! Time !I=! Time ! ~ :Time iTime : Time !
~-----"
j "'"
,i " .. ,~:'u'
g ~g!..---..i
~ ~ ~i-------"i~
,
U
:01='
.
,
:
! Preweld Interval !
Weld Interval
.. !,.....
Pos tweld Interval
J"
~
.. !....
II
!
:..
Welding Cycl e
..!'
Figure 7.5: A Multiple Pulse Resistance Welding Program
The joint type most commonly used for RSW is the lap joint. There is a minimum requirement
for joint overlap which is related to the nugget size. The distance from the centerline of the
nugget to the edge of the sheet should be at least 1.5 times the nugget diameter. The
separation between the sheets to be welded should not exceed 10% of the thinner sheet
before pressure is applied.
The weld integrity is evaluated by pulling the joined parts apart. If the nugget pulls out of the
base metal it is considered a good-quality spot weld. The weld has shown a greater strength
than the surrounding base material. Many specifications require pull tests to be made after a
prescribed number of welds to test the consistency of weld quality, High quality welds though
require continuous maintenance of the electrode tip contour and must be redressed often to
maintain the proper shape.
RSW is extensively used in the automotive industry because of its high operating speeds and
its suitability for robotics and automation. It is also used widely for furniture , domestic
appliances, building products , and enclosures.
The main disadvantages of this process is the material thickness that can be welded. The
general thickness limit for carbon steel is about 3mm. Also , regular maintenance is required
on the electrodes to maintain weld quality.
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Resistance Projection Welding (RPW)
In this variation of resistance welding, current flow is induced at one or more points of contact
at predetermined protrusions, embossments, and interfaces on either one or both parts being
welded. These extensions, or projections are used to concentrate heat generation at the point
of contact. The process typically uses lower currents, lower forces, and shorter cycle times
than does the similar spot welding application . Typical joint configurations for this process are
shown in Figure 7.6.
(a) Spherical Projeclions
(b) Elongaled Projeclions
(c) Annular Projections
(d) Pyramidal Projeclions
Figure 7.6: Typical Projection Welding Configurations
Equipment used for RPW is sim ilar to that for spot welding with the exception of the electrode
contour. Large flat electrodes are most commonly used with the contact surface of the
electrode having the same diameter as the contact surface on the adjoining metals.
The major advantages of this process over spot welding is the increased electrode life
because of the larger contact surfaces used and the precise location of each weld.
Applications of this process include sheet-to-sheet joints, cross wire welds , annular
attachments, and nut or stud welds.
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Resistance Seam Welding (RSEW)
Resistance seam welding is employed to produce a continuous or non-continuous seam joint
between faying surfaces. In RSEW, a series of spot welds are generated using two electrode
wheels, a combination of one wheel and one flat bar type electrode , or a wire feed system in
which a copper wire is fed into a groove in the electrode wheel to provide a new surface for
contact with the work metal. The spot welds may be continuous to form a pressure type seal
or intermittent. The equipment is similar to other resistance welding machines other than the
shape and configuration of the electrodes.
Three types of seam welds are made with the RSEW process: Lap Seam Weld, Mash Seam
Weld, and Butt Seam Weld . Examples of these three welds are shown in Figure 7.7.
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0/
Radiused
"00",'.. ~ --.J:
/
o
~
I
Front View
Overlapping
Weld Nuggets
..,.
Travel
Side View
(a) Lap Seam Weld
I"'l/
U
o
Wide. Flat
Electrodes
.....
....
...... ......
....
C===~;::=;S;li~9~htlY LapCpe=d=~SS;;;==w=e: ld Nuggets
Sheets
After Welding
Before Welding
(b) Mash Seam Weld
n
u /
Two Abutting
Sheets I
Beveled
Electrodes
a
Before Welding
o
o
After Welding
(c) Butt Seam Weld (Front View)
Figure 7.7: Types of Resistance Seam Welds
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As with other resistance welding processes, weld speed, level of current, current waveform,
cooling , and electrode configuration and force must be chosen appropriately to obtain a quality
weld .
Advantages of this process are that leak proof seals can be accomplished, seam widths are
smaller than for spot welds, high speed welds are possible, and tooling costs are lower.
However, seam length and configuration are limited by the welding machine dimensions and
obstructions encountered along the electrodes path of travel.
Applications for this process include flange welds, and watertight joints on tanks.
Flash Welding (FW)
Flash welding is a process in which a weld is produced in a butt joint by flashing action and
the application of pressure . This process gets its name from the dramatic expulsion of molten
material from the joint when the workpieces are moved slowly together and make contact.
The action is the result of high current densities at small contact pOints between the adjoining
interfaces. The FW process is shown in Figure 7.8. The equipment most commonly used in
this process is a single phase AC welding power source.
Finished Flash Weld
Stationary
Clamp
Moveable
Clamp
I ------
I
;t-, Upsetting
Y Pressure
Welding
Transformer-==:,~~~~..J
A.C. Power
-~~~-II 'tIO~OO
i
Contactor
Figure 7.8: Flash Welding Process
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High electrical resistance is introduced when two square surfaces being flash welded are
slowly forced together and current flow is through points of isolated local contacts only. The
heat is generated by the flashing action and is localized in the area between the two parts.
The surfaces are brought to the melting temperature and expelled through the abutting area.
After this material is flashed away, another small arc and continued until the entire adjoining
surfaces reach the welding temperature . A higher degree of pressure is then applied and the
arcs are extinguished when upsetting occurs.
Some common applications of this process include chain links, wire joining operations, roll
form line, aircraft landing gear, band saw blades, aluminum-to-copper electrical transition for
power transmission , and mitre joints for automotive frames .
Upset Welding (UW)
Upset welding looks similar to flash welding but there are several differences between the two.
In upset welding (a) force is applied before the welding current is switched on, (b) typically no
melting occurs, (c) only parts with interfaces equal in cross-sectional area can be joined, and
(d) higher quality clean joints are required for adequate resistance heating.
Deformation occurs in the softened material when heated to the welding temperature and
when the upsetting force is applied. Generally, upset welds are solid state welds and if
melting does occur at the interface due to excessive heat, it is extruded out of the joint area
during upset pressure. The material expelled from the joint usually contains a large amount of
oxides and therefore the upsetting process is a cleaning action.
The equipment used for UW is similar to other resistance welding processes, utilizing a
stepdown transformer with high current and low voltage (typically less than 10 volts). The
output can be either alternating or direct current. A typical UW set-up is shown in Figure 7.9.
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Finished Upset Weld
{~-~CJ-"-! -~f
o
Upsetting
Pressure
Zone
j-~~t=~
Welding TranS!Ormer·-==::.
Contactor ~
A.C.
II
~
Power =~~~~~~~~dJ
Figure 7.9: Upset Welding Process
Advantages of this process include its ease of control and simple equipment, short welding
times, few weld defects, and enhanced weld properties.
Percussion Welding (PEW)
In percussion welding, an arc is produced by a rapid electrical discharge across a decreasing
air gap and pressure is applied "percussively" during or immediately after the discharge of
electrical energy. This process is similar to flash and upset welding in that interfaces with
similar cross sectional areas are used .
The advantage of this process is that the depth of heating is shallow and the time cycle is
proportionally short, but it is applicable to parts with modest cross sectional areas .
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Chapter 8
PLASMA ARC WELDING (PAW)
8.1
8.2
8.3
8.4
8.5
Principles of Operation .... . . ... ... . . . . .. ... . . ..... . . . .
Process Variables ............. .. .. . ...... ... . . . . .. . . .
Equipment for PAW ............. .. . . . . . . .. . . . ... . . . . .
Advantages and Disadvantages .......... . . . . . . ... . . . ..
Applications ....................... . .... .. ....... ...
. .... .. .. . ... .162
... ... . . . ..... 163
.. . . .. .•...... 164
.. . . ...••.. . .. 166
.... .. . . .. .... 166
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PLASMA ARC WELDING (PAW)
8. 1
Principles of Operation
Plasma arc welding, like GTAW, is a nonconsumable electrode process where the heat
required for welding is generated by an arc established between a non-consumable tungsten
electrode and the workpiece (transferred arc) or between the electrode and the constricting
nozzle (non-transferred arc). The weld pool is protected from atmospheric contamination
through a shielding gas exhausted from an auxiliary source or supplemented by the ionized
gas issuing from the torch.
DC Plasma Welding
The most common transferred arc or melt-in mode, utilizes a high frequency pilot arc struck
between a tungsten electrode and a copper alloy orifice within the torch. When the torch is
brought close to the work piece an electrical circuit is completed between the negative
electrode and positive workpiece thus initiating the selected welding current. The plasma gas,
most commonly argon, is fed through the torch at high pressure which becomes ionized by the
welding current forming a column of hot plasma. The orifice in the copper nozzle restricts the
high pressure column, forcing the hot ionized plasma towards the base material at high
velocity in the form of a jet. A weld pool is respectively formed and manipulated in a similar
manner as in the gas tungsten arc welding process, where filler metal may be added to the
pool to make the weld. The plasma arc welding process is shown schematically in Figure 8.1.
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Direction of Travel
DODQ
1- - - -
Nozzle
Constricting Nozzle
G~
as,-=:::::::___
_'.'P",laiSlsmrJla,,-,:
~-
Oriface to
Constrict Arc
-1-1'-'--/-/- - - -
r
Coolant
Shielding Gas
Tungsten Electrode
Base Metal
~~~l
Molten Weld Melal
J
Figure 8.1: Schematic Sketch of the Plasma Arc Welding Process
8.2
Process Variables
There are two distinct modes of operation in the PAW process: transferred and non-transferred
arc.
Transferred arc (melt-in mode) is used in a manner similar to the GTAW process but the PAW
process is more efficient in comparison . The reason for this is the higher temperature of the
plasma or constricted arc, and the higher velocity of the plasma jet that increases heat transfer
rate when compared to the GTAW process for the same current, resulting in faster welding
speeds and deeper penetration.
In the keyhole mode of welding, the plasma jet completely penetrates the workpiece and forms
a concentric hole through the thickness known as a "keyhole". Due to surface tension forces,
the 'molten metal flows around and into the keyhole as the torch traverses the workpieces, and
upon solidification forms the weld . This method can only be used where the plasma can
penetrate through the joint and is affected by base metal composition and the welding gases.
Typical thickness ranges that may be welded using keyhole plasma are 1.6 mm to 12 mm for
carbon and alloy steel and up to 19 mm for aluminum.
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One variation to the PAW process is Plasma MIG welding which is a combination of gas metal
arc welding and plasma arc welding. This process utilizes a modified plasma torch which
consists of a contact tip inserted in the centre of the torch with the tungten electrode placed
slightly off to the side. A continuous wire is fed through the contact tip and into the nozzle with
the plasma gas. The wire electrode and its arc are contained within the ionized plasma using
two separate power supplies, one for the plasma and the other for the electrode wire. In some
instances the wire is fed into the plasma column without a gas metal arc established. This
variation is used to join thinner materials when lower heat inputs are required .
8.3
Equipment for PAW
Basically, the PAW system consists of a power source, a plasma control console, a water
cooler, a welding torch, and a gas supply for the plasma and shielding gas. A typical PAW setup is shown in Figure 8.2.
Cold Water Supply
Drain
(
l'
-
Shielding Gas
Plasma Gas
1
Plasma
Console
[m
•
.
0
0
1=
Torch
"-'
~
~
LJ
Filler Rod
Base Metal
~,
/
0
000
00 0
~
00
00
00
~
"tle
dl
'C::!
Fool Pedal
(optional)
Power
Source
Gas Source
Work lead
Figure 8.2: Circuit Diagram for Plasma Arc Welding
The power source, which supplies the main power for the system, is usually the constant
current type with an open circuit voltage of at least 80 V to ensure reliable arc ignition and
maintain arc current transfer. It is in most instances supplemented with a sequence controller
and a control console. PAW power sources commonly operate on steady state or continuous
DCEN current. With power source developments/advances in recent years pulsed DCEN
current as well as Variable Polarity Plasma Arc (VPPA) with DCEN and DCEP cycling are also
employed. The VPPA is typically used when higher currents are employed for keyhole modes
and also when welding aluminum. The electrode positive component of the VPPA promotes
cathodic cleaning when welding aluminum alloys , allowing good weld pool flow characteristics.
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The sequence controller controls the timing of gas flow, arc initiation, main welding current
control, and any slope parameters. Gas flow for shielding and the plasma, as well as the high
frequency pilot arc are incorporated into the plasma control console.
Like most welding torches , PAW torches come in several types (manual or mechanized) and
sizes (different power ratings). In each case the design principles are the same. A cross
section of the basic torch is shown in Figure 8.3. The tungsten electrode, contain ing 2%
thorium oxide or additions of rare earths, is held in place by a collet within the torch body. The
electrode assembly is located in the plenum chamber where the plasma gas flows . The
copper nozzle is situated at the front of this chamber which contains the constricting orifice.
Shielding gas flows in a chamber surrounding the constri cting orifice which is contained by an
insulated ceramic gas nozzle. All hoses that supply the plasma and shielding gases, and the
torch coolant, are connected to the body and handle of the torch . The torch is electrically
connected to the power source which forms the negative pole of the electrical circuit for DC
welding.
Electrode
Collet
Electrode Cap
Plasma Gas
Shielding Gas
.. .................
...........................
Water In
1====#...::::::.---- Current (+)
~waterout
~~~~~~~~~~~
~
Current (-)
Nozzle
~Handle·
Constricting Nozzle /
Figure 8.3: Schematic Cross Section of a Plasma Torch Head
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Advantages and Disadvantages
The main advantages of this process are:
In the keyhole mode, greater thicknesses can be penetrated in a single pass, compared
with other processes, such as GTAW. Square butt jOints thus may be used for steels up to
12 mm thick leading to savings in jOint preparation costs.
The process can produce welds with the same quality as GTAW but in shorter times and
at lower costs.
Heat affected zones are smaller than GTAW welds, and the welds tend to have more
parallel sides which in turn reduce angular distortion.
The torch to work distance is not as critical as in GTAW, giving the welder more freedom
to observe and control the weld in manual operation.
The main disadvantages of the PAW process are:
There is a greater capital cost for equipment when compared to the GTAW process.
Due to the constricted arc and more narrow welds, joint fit-up and alignment is critica l.
The PAW torch requires more maintenance due to the greater number of parts and
complexity of the torch design.
The set back of the electrode tip with respect to the nozzle orifice must maintain accuracy
to produce consistent results .
The torch itself is slightly larger than a GTAW torch and can limit access in tight areas.
8.5
Applications
The PAW process is used to weld carbon and alloy steels, stain less steels, aluminum alloys,
titanium , copper and nickel alloys, zirconium, and tantalum. Some of the more common
applications includ e but are not limited to: welding thin sheet, manufacturing of pipe and tubing
using longitudinal and transverse welds, and welding aeronautic fuel containment cells.
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Chapter 9
ELECTRON BEAM WELDING (EBW)
9.1
9.2
9.3
9.4
Principles of Operation . . ...................... . ...... . . . ............ 168
Equipment ...... .. ... . .. . . .. .................•. . .... . . .... .. ... .. .169
Advantages and Disadvantages ... . ... . • .. . ............••..... . ... .... 170
Applications ....................... • ... ... . . ... . ...••............. 171
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ELECTRON BEAM WELDING (EBW)
9.1 Principles of Operation
Electron Beam Welding is a high-energy density fusion process that produces coalescence
between materials by the heat generated from a focused beam of high velocity electrons. The
process was developed in the 1950's initially for welding refractory and reactive metals for
nuclear applications. Since then, commercial units have been in use and improved over the
years. A typical high voltage electron beam welder is shown in Figure 9.1.
Insulating Gas
Electrical Feedthrough
High-Vollage Cable
High Voltage Insulator
High-Vacuum Chamber
Cathode Assembly
Ie
:]C?
_- --- r: 18I181'i~18I
_ ----
------it:~-~',,;.
Beam Deftection Coils
Beam Column Cutoff Valve
Effluent Gas
Standoff Distance -
~~~__-+. To Vacuum Pumps
L
"
Anode (at ground potential)
To Vacuum Pumps
Magnetic Lens
rI~~~~==:~:
:}
cc §g'\I)
,
To Vacuum Pumps
i-
Electron Beam
jI///#///.~\\\\\~\1}
Figure 9.1: Schematic Sketch of the Main Components
of an Electron Beam Welding Gun
The electron beam is formed by accelerating under high voltage electrons emitted from a hot
cathode(-) filament through an anode(+) in a vacuum chamber at velocities ranging from 0.3 to
0.7 times the speed of light. The cathode is usually tungsten or a tungsten alloy which is
heated to above its thermionic emission range. The beam is focussed to diameters ranging
from 0.25 to 1.3 mm as it passes through the magnetic lens (also known as an
electromagnetic focusing coil). The heat (thermal energy) is generated when the high velocity
electrons locally impact and penetrate into the workpiece resulting in almost instantaneous
melting of the weld seam. The beam vapourizes a small column of metal at the jOint. The
area immediately adjacent to the beam melts. As the beam is moved along the joint, capillary
action draws the liquified faces of the joint together forming the fused area.
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Electron Beam
~
Beam focal point
•
Small vapourized column
of base metal along joint
Figure 9.2: Electron Beam
Welding is most commonly done in a high vacuum chamber with vacuums better than 10-torr. Medium and non-vacuum systems have also been developed over the years due to the
demand for faster part production. In these systems, the electron gun is maintained under
high vacuum, and the electron beam is allowed to exit into medium vacuum or into the
ambient air where the workpiece is placed. In non-vacuum systems, the standoff distance is
very small to minimize electron scattering by the air, and the thickness that can be welded is
limited.
9.2
Equipment
All electron beam welding systems employ a gun, power supply, control system, one or more
vacuum pumping systems, and work handling mechanisms. Electron beam welders can be
designed as low-voltage systems (15-60 kV), or as high-voltage systems (100-200 kV) . The
low-voltage systems operate at higher currents with a typical beam current of 500 mA,
whereas the high-voltage systems operate at lower currents with a typical beam current of 40
mA.
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Low-voltage equipment is most commonly used in vacuum chambers and can weld material
up to 150 mm thick at a standoff distance (the distarice from the exit of the electron beam to
the work) as far as 660 mm. High-voltage systems are most suitable for non-vacuum welding
and are generally used to weld material less than 25 mm thick at a standoff distance of 37
mm. Welding speeds are generally lower for high-voltage systems but the size of the work is
is not limited by any vaccum chamber, work motion is easier, and vacuum does not have to be
redrawn as each weld is completed. However, if the components are welded in air, protective
shielding must be maintained to protect operators and personnel from potential radiation.
9.3
Advantages and Disadvantages
The major advantage of EBW is its ability to make full penetration welds in workpieces of large
thickness. It produces narrow parallel sided welds with heat affected zones much smaller than
those of any arc welding process, thus greatly minimizing distortion compared to most other
processes. Almost all metals can be
welded including super-alloys, refractory
metals, reactive metals, stainless steels,
and in some instances dissimilar metal
combinations.
The main disadvantage of EBW is the high
capital cost for equipment and operating
expenses, due to the high maintenance
required on the vacuum pumps. Fit up of
thick joints must be precise and beam
alignment must be perfect due to the
characteristic narrow nail head shaped
weld as shown in Figure 9.4, and the
potential for the electron beam to miss the
joint seam over a part of the thickness. A
potential problem when welding carbon
steels with the vacuum process is weld
porosity caused by the release of gases.
Gas levels can be reduced if a deoxidizing
filler material can be added. Other
potential flaws include: shrinkage cavities,
cracking, undercut, and cold shuts.
Photo courtesy of Nu- Tech Precision Metals tnc.
Figure 9.3: EBW Weld
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Figure 9.4: Typical Nail Head Profile of EB Welds,
and Potential for Missed Joint Seam
9.4
Applications
EBW is used to produce joints with
extremely high quality welds and
enormous penetration. Because of the
high cost of equipment. this process is
usually used in situations where
standards are high such as the nuclear
and aerospace industries. Typical
applications for EBW are: nuclear fuel
elements. pressure vessels for rocket
propulsion systems, special alloy jet
engine components, and hermetically
sealed vacuum devices.
Figure 9.5: Turbine Blade Repair
Photo courtesy of Nu· Tech Precision Metals Inc.
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Chapter 10
LASER BEAM WELDING (LBW)
10. 1 Principles of Operation ............. . . . ... . .. . .•.. . . ..... . .. . .... .. .. 174
10.2 Laser Types ......... . ............... . ... . . . .... . . . . . ..... . .... . .. 174
10.3 Advantages and Disadvantages .... .. ..... .. . .... •. ... .. . . ....... .. ... 177
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LASER BEAM WELDING (LBW)
10.1
Principles of Operation
Laser beam welding is a process that utilizes a beam of coherent optical light focused to
create a small spot of intense heat suitable for welding metallic and non-metallic materials.
"Laser"" is an acronym for "light amplification by stimulated emission of radiation." The laser
was first conceived in 1951 and the first laser device was demonstrated in 1960.
Basically, a laser beam is generated when a solid or a gas is excited through some means of
energy discharge which in turn excites its atomic particles. When allowed to return to their
original state, the atomic particles (either electrons or CO, gas molecules depending on the
laser type) emit active photons. Mirrors, one totally reflective and one partially transmissive,
are placed on both ends of the photon containment. The reflective mirror reflects the photons
thus transmitting them to the partially transmitting mirror through the unsilvered section
resulting in a focused beam. The beam impinges on the surface of the workpiece with such
concentration of energy that the surface is melted.
In laser welding of metals, the beam is focused at the surface or slightly below into the joint
interface, causing the metal to become molten. The molten metal takes on a radial
configuration known as "melt-in welding" which at high power densities forms a keyhole, the
same as in plasma or electron beam welding. This provides for deep penetration, and gives
the laser weld its characteristic high depth-to-width ratio .
10.2
Laser Types
There are two main types of lasers used in welding: the solid-state lasers which use a solid
med ium and gas lasers which use a CO, gas medium in a tube.
10.2.1 Solid-State Lasers
There are three types of solid-state lasers: (1) synthetic ruby laser, (2) glass laser, and (3) the
ND:YAG laser.
The ruby laser utilizes a synthetic ruby with chrom ium in aluminum oxide as the lasing
medium. The ruby is made into a rod approximately 300 mm in length and 25 mm in
diameter. Both ends are ground flat and parallel, and polished to extreme smoothness. Both
flat ends are then covered with mirrors to reflect light; however one end has a small opening
to allow the beam to exit the rod. The rod is surrounded by a high intensity light source,
usually a fl ash tube which when flashed excites the rubys' electrons. The flash lasts
approximately 2 milliseconds emitting an intense pulse of light which excites a high intensity
beam of coherent light from the opening at one end of the ruby rod. The burst of laser light is
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proportional to the flash, approximately 2 ms, and occurs each time the flash tube is flashed .
A continuous beam is not possible with this type of laser equipment because flashing the flash
bulb too fast will cause the ruby crystal and flash bulb to overheat.
The glass laser, which uses neodymium in glass, and the Nd:YAG laser, which uses a crystal
of yttrium aluminum garnet doped with neodymium, operate in the same manner as the ruby
laser system above. A typical solid-state ruby laser system is shown in Figure 10.1 .
Power
Supply
Totally
Reflecting \
Mirror
r
\
Partially
Reflecting
Mirror
Laser Beam
Figure 10.1: Schematic Sketch of a Ruby Laser
10.2.2 Gas Lasers
The gas laser utilizes a gas for its medium, mostly CO, . The basic CO, laser system is shown
in Figure 10.2.
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CO 2 Laser
Gas Supply
Power Supply
and Control
Gas Exhaus t
;'.
~
~
~
~
;f.
~,
~
~
~
~
1'.
U
TotallY\
Reflecting
Mirror
U
CO 2 Lasing
Medium
>-
;1
i
~
>-
Laser Beam
Partially
Reflecting
Mirror
Figure 10.2: Schematic Sketch of a CO2 Laser
Excitation of the CO, molecules is by means of high-voltage low current electric power. The
gas mixture is contained in a tube, which has mirrors on each end, one of which is totally
reflective while the other is partially reflective with a small transmissive area to allow the beam
to exit. Upon electrical discharge, the CO, molecules become excited and increase activity.
When allowed to return to their original state they emit photons which build up in the tube
cavity. The freed photons travel between the mirrors exciting the CO, molecules, starting a
chain reaction of photon emission. The photons form a continuous stream which exits through
the unsilvered opening in the one mirror in the form of a beam.
The out-put of the gas medium lasers range from 2 to 20 kW of power and can be run in
continuous wave or pulsed mode.
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Advantages and Disadvantages
There are several advantages to the LBW process, the most significant ones are:
By using mirrors, the user beam can be delivered to locations considered inaccessible for
other welding processes .
The characteristic of the laser welds are similar to those of electron beam welds viz.,
narrow and deep shape, narrow HAZ.
Welds can be made in a magnetic field.
All metals can be fused.
Distortion is minimal due to the narrow welds and heat affective zones produced.
Any flaw that occurs during laser welding can be repaired by the same system by
rewelding over the defective area, thus eliminating the need for other welding equipment.
The laser weld can be monitored by remote television which enables the use of adaptive
control tracking systems .
Welding is done in air ie., no vacuum is required .
The main disadvantages of the laser beam welding process are:
Cost of the equipment is high .
Welding is limited to an upper limit in material thickness of 20 mm for steel.
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Chapter 11
ELECTROSLAG WELDING (ESW)
11.1
11.2
11 .3
Principles of Operation ... . • •. ...... .. . . .... .. . .... ... .. . . .... . . . ... .180
Equipment . .... . .... . .. . ..•.... .. . ... ••. .................. . . .. .. :181
Applications .. . .... .. . . ... .. .
. .. . • • .. . . •...... .. . . . .... . . . ... 182
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ELECTROSLAG WELDING (ESW)
11. 1 Principles of Operation
In electroslag welding, the heat required to produce coalescence between two materials is
generated by the electrical resistance of a molten slag pool. The heat generated is sufficient
to melt the surfaces of the base materials as well a consumable filler.
An electrode is fed through a guide tube to the bottom of a vertical joint where the side walls
of the jOint are parallel to the axis of the electrode. An arc is initiated under a bed of flux as
the electrode makes contact with a starter tab, causing the conductive flux to become molten
which generates a slag bath . The arc extinguishes once the molten slag bath is formed.
Current continues to flow from the electrode through the molten slag and to the base material
creating extreme resistive heating. This causes the edges of the base metals, the electrode,
and in some instances a consumable guide tube to melt, fusing the two side walls as the filler
solidifies between copper shoes on each side. The copper shoes are usually water cooled,
and their shape is determined mainly by the joint type and final shape and size of the weld .
The process proceeds as the electrode is continually fed into the slag pool progressing the
weld up the joint as the gap fills. Due to its buoyancy, the slag floats on top of the molten filler
metal and progresses to the top as the joint is completed. The weld is considered finished
when the joint is filled.
Used mainly for carbon and low alloy steels and some stainless steels , the main disadvantage
with this process is that mechanical properties of completed joints are in most instances
mediocre. This is because the extreme heat involved with ESW allows the base metal and
weld to cool very slowly, thus resulting in large grained weld metal and heat affected zone .
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Equipment
The equipment used for ESW consists of a direct current type constant voltage power source.
a constant speed wire feeder, a guide tube (non-consumable or consumable), and copper
shoes.
The non-consumable guide method utilizes a welding head which moves up the joint as the
weld is made. Backing shoes move along the joint and rise with the welding head, see Figure
11.1.
Drive Unit Moved Up
By Rack and Pinion
Drive Unit _ _
Guide Tube ~
BaseMetal~
Electrode
Moving Water~cooled
Backing Shoes
Molten Flux
Molten Weld Metal
Solidified Weld Metal
Water Out-~ ~
Water In
Figure 11.1 Non-consumable Guide
(upward moving head) Electroslag Welding
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The consumable guide tube method uses backing shoes that are usually fixed and the welding
head is mounted on the work on top of the joint, see Figure 11 .2.
Electrode
BaseMetal~
Moving Water-cooled
Backing Shoes
~_
Consumable Guide Tube
Molten Flux
Molten Weld Metal
Solidified Weld Metal
Water Out------"")....:oWater In
Figure 11.2: Electroslag Welding Consumable Guide
In both methods, multiple electrode systems which can oscillate back and forth across the
width of the joint, may be employed for large thicknesses. The oscillating electrode method
requires additional equipment which moves the electrode(s) across the joint.
11 .3
Applications
Because of ESW's extremely high deposition rates, its major use has been in heavy plate
fabrication . Essentially, low volume joints between thick parts can be made in a single pass.
Single electrode processes can weld materials 75 mm in thickness, and up to 300 mm
thickness may be welded with two oscillating electrodes. Typical applications of ESW are:
cladding, subassemblies for steel buildings, joining of large electric motor housings, and
manufacturing of frames, bases, and metal working machinery.
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Chapter 12
DIFFUSION WELDING (DFW)
12.1
12.2
12.3
12.4
Principles of Operation ...... .. ..... . ......... ... ....... . . . .... . . . ... 184
Diffusion Welding Mechanisms ...... . .. .. ... . . •. ............. . ..•.... 184
Bonding Variables ...... .. .. . ...... . •...... . ........... • •......•.... 185
Applications ....... ... ..•.. . ...... ..... .. ...•... .. .. . . ... ... . ..... 186
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12. DIFFUSION WELDING
12.1
Principles of Operation
Diffusion welding is a solid state joining process (no melting of the parts occurs) that produces
an atomic bond between two surfaces through the application of pressure and heat. The bond
is considered complete when cavities and voids at the interface have fully closed.
The following conditions must be met to produce a diffusion weld:
joining must occur at temperatures below the melting point of the materials (usually .5
to .75 Tm where Tm is the melting point in degrees Kelvin)
coalescence of contacting surfaces is produced at loads insufficient to cause macrodeformation and
brittle surface oxides are removed to allow significant atom migration across the bond
interface
12.2
Diffusion Welding Mechanisms
In diffusion welding. coalescence occurs
through a 3- stage metallurgical sequence,
see Figure 12.1. Each stage is associated
with a particular metallurgical mechanism
that makes the dominant contribution to the
bonding process. These stages begin and
end gradually because the mechanisms
overlap in time.
(a) Initial Contact
During the first stage, the contact area
grows to a large fraction of the joint area by
localized deformation of the contacting
surface or asperities largely due to the
contact pressure . At completion of this
stage the interface boundary is no longer
planar, but consists of voids separated by
intimate areas of contact. In these areas of
contact. the joint becomes equivalent to a
grain boundary between the grains on each
surface.
(b)
Fig. 12.1: Diffusion Welding
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During the second stage, two changes
occur: all the voids at the interface shrink or
disappear and the interfacial grain
boundaries migrate out of the plane of the
joint. Creep and diffusion mechanisms are
important during the second stage as they
aid in the elimination of interfacial voids.
(c) Second Stage: Diffusion of Atoms
From one Grain to Another
During the third stage, grain boundaries
move and engulf any small voids still
present between the materials at the
interface. After the final stage is complete,
the only possible route for diffusion is
through the volume of grains themselves.
(d) Third Stage: Volume Diffusion of
Atoms to Voids
Figure 12.1: Diffusion Welding
12.3
Bonding Variables
There are three main process variables associated with diffusion welding: time, temperature,
and pressure. Other variables that affect the quality of the bond are surface roughness and
contamination.
Time
As time increases, the distance to which atoms diffuse increases. Time allows the three
mechanisms of diffusion to occur. Joint efficiencies may be poor if sufficient time is not
allowed.
Temperature
Temperature is the most important variable in diffusion. As temperature rises, the grains at the
interface are able to deform easily thus increasing the contact area. Also, the atoms are able
to diffuse much faster, thus reducing the time required to achieve an adequate bond.
Pressure
The degree of pressure determines how the first stage of deformation will function. If
deformation is insufficient due to inadequate pressure, large voids will remain present at the
interface and atoms will be unable to migrate across these.
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Surface Roughness
Surface roughness is a variable that determines the time required to collapse voids during the
second stage. The rougher the surface, the longer it will take to acquire full contact at the
interface.
Surface Contamination
Films of water, oil, or grease are injurious to the strength of diffusion welds . Non-oil-based
cleaners such as acetone should be used to pre-clean surfaces.
Oxide films, as found naturally on stainless steel , aluminum, and titanium , are hard and brittle
and prevent complete bond formation. During the deformation stages, the hard brittle oxide
continually fractures and reforms preventing the atoms from migrating across the interface.
Any oxide can be removed through ion bombardment immediately before being placed in the
bonding atmosphere. The bonding environment should consist of a vacuum or inert gas
chamber to eliminate the chances of re-oxidation at the elevated temperature needed for
diffusion welding.
12.4 Applications
Diffusion welding has been substituted for processes such as electron beam welding due to
its low equipment costs. Because a vacuum is generally required during the bonding process,
it has been considered for use in outer-space to make repairs on space stations and to
personnel carriers such as moon buggies. The most common applications for diffusion
welding involve titanium, nickel, and aluminum alloys as well as several dissimilar metal
combinations. Some items that have been welded thus far are:
The engine mount on the space shuttle had 28 diffusion welded titanium components
ranging from large frames to interconnecting boxes. This structure was designed to
withstand three million pounds of thrust.
Light weight cylindrical cases of titanium alloys for jet engines.
Dissimilar joints with metal combinations such as copper to aluminum, copper to titanium,
mild steel to stainless steel, and stainless steel to zirconium alloys.
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Chapter 13
EXPLOSION WELDING (EXW)
13.1
13.2
13.3
13.4
Principles of Operation .. . .. . ............. .. .. . . . . .. •... . . . . ...... .. .188
Explosion Welding Parameters and Variables . . . . ... . ... .. ... .... .. .. . . . .188
Mechanisms of EXW . . .. .. . . . ..... . ... .. .... .. ......... . ...•. . . . . .. 189
Applications . . . ..... . . . .. •..... ... . • . ... .. . ... .. . .. . . . . . ... . . . .... 189
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EXPLOSION WELDING (EXW)
13.1
Principles of Operation
Explosion welding is a solid state process in which coalescence is produced through high
speed impacting of two or more materials by controlled detonation. The explosion forces the
materials together at extremely high speed and pressure (velocity of detonation is about 5000
m/s [17000 IUs]). Bonding is achieved from the dynamic plastic interaction (flow of material)
and jetting at the interface. Unlike fusion-welding processes, there is no extreme heat transfer
involved in this bonding process and therefore no metallurgical changes take place at the
interface thus minimizing changes in mechanical properties. However, due to the plastic
deformation at the bond interface, this region can have a higher hardness than the base
material.
13.2
Explosion Welding Parameters and Variables
The set-up for EXW consists of a base component, a flyer plate (prime component), explosive
material, and a source of detonation as shown in Figure 13.1. The basic parameters involved
are flyer plate velocity, collision point velocity, and collision angle. Process variables that
affect these parameters are detonation rate, explosive quality and mass, flyer plate mass, and
standoff distance which controls the angle of impact. Each of these variables are obtimized
from trial and error bonding sessions, and through the interpretation of suitable inspection and
mechanical test methods.
Detonation
•
Explosive Material
Flyer Plale Standoff
I
,u---L.--f-- - - -...., Gap
Base
'--~I=~====~I
Component ' - -_
_
_
_
-'
Flyer
Plate
Figure 13.1: Schematic Diagram Showing Explosion Cladding Setup
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Mechanisms of EXIN
The explosive, commonly ammonium nitrate based, is the energy source in this process.
Energy is released through detonation in a very specific manner starting at the detonator
progressing at sonic velocities into the undetonated explosive (also known as velocity of
detonation). The characteristics of the detonation are controlled by the explosives
composition, particle size, packing density, layer thickness, moisture content, and in some
cases age.
The explosive material is distributed over the top of the prime component and is detonated
such that the detonation front progresses over the length of the prime component in a
tangential manner. Detonation accelerates the prime component across the standoff gap
colliding it into the base component. This acceleration is the result of a high pressure shock
front and trailing shock waves produced in the explosion.
Upon collision, the surfaces of the materials being joined locally deform and flow in the
immediate region of impact. Because of the angular geometry of the collision, a jetting action
accompanies the surface layer deformation which removes the surface material containing
oxides and contaminants that inhibit this solid state joining process. Most of this heat is
carried out of the joint interface, and the weld has a wavy cross-section .
13.4
Applications
Typical applications of EXW include the cladding of corrosion and wear resistant materials to
steel and stainless steel, and of transition joints for joining dissimilar metals (steel to aluminum
transition bars for ships). EXW is limited to flat and cylindrical surfaces and to materials with
suitable ductility and fracture toughness.
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Chapter 14
FRICTION WELDING (FRW)
14.1
14.2
14.3
14.4
Principles of Operation . ... . . .. .... . . .. . .. .... .. . ...... . ... . . . .. .. . .. 192
Process Variables . . ... . .. .• .. . .. ... . .. .. . . ... .. . . . . .. .... .. . .. . . ... 193
Metallurgical Parameters .. .. ... ..... .. ... .. . . . . . .•. . . . .. .... . . .. . ... 193
Applications . ... . . . . ... . .. . . .... .. . . . . . . . .. . ..... . .. . . .. .. . . . . .. . .194
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14.
FRICTION WELDING (FRW)
14.1
Principles of Operation
Friction welding is a process that uses frictional heat generated by the relative motion of two
materials in contact under compressive forces to form a solid-state metallurgical bond . It is
used to join a wide range of similar and dissimilar materials, ie. metals, ceramics, composites,
or polymers. There is no heat applied from any external means and the equipment used is
relatively simple.
The process involves two materials, whose adjoining ends are machined flat and parallel to
each other. The pieces are inserted into a machine resembling a lathe. Usually, with one
material rotating and the other stationary, the two are brought together under constant or
gradually increasing compressive forces to transform mechanical energy into thermal energy
by means of friction. Once a suitable welding temperature has been reached, the rotational
motion ceases and an upsetting force is applied and held allowing atomic diffusion to occur
forming a metallurgical bond between the parts. Figure 14.1 illustrates the friction welding
process.
Rotating
"""-I"""-#"• :' :'11
~
.. . Non Rotating
Non Rotating
. '.
"·';1
, ':.l
~
Pressure
Healing at
Interiace on
Contact
<oj : ,
~ Pressure
Non Rotating
~ Pressure
\1 .
t,/
..... "
~ Pressure
Figure 14.1: Friction Welding Process
Because of the localised heating and rapid cooling at the interface of adjoining metals,
mechanical properties may be sufficiently altered requiring post weld heat treatment to restore
ductility and strength.
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14.2
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Process Variables
There are two variations of the friction welding process: the conventional method, as
described above, and the inertia method. The inertia method stores the required energy for
joining in a flywheel system. The flywheel, connected to a motor, rotates one of the pieces to
be joined, until a predetermined speed is reached. The motor is disengaged and the pieces
brought in contact at a predetermined pressure and time and the kinetic energy of the flywheel
is converted into frictional heat at the jOint interface. The flywheel rotation ceases when the
required joining temperature is reached and an upset pressure is applied to complete the
weld.
Rotational speed, axial pressure, and welding time are the principle variables in both
variations of the process and each varies depending on the material type, composition and
size. These parameters must be precisely adjusted to ensure sufficient heating of the
materials into their plastic range where welding takes place . The heat generation is
proportional to the square root of the applied pressure and power dissipation is independent of
speed . High speeds give more rapid welding and narrower heat affected zones .
14.3
Metallurgical Parameters
There are two main requirements for materials to form a quality friction weld. First, the material
should be capable of being forged, and second, the material should be able to generate
sufficient heat at the interface required for welding.
The first factor eliminates brittle materials such as ceramics , cast irons, etc., because these
materials cannot support the load from the upsetting force. The second factor eliminates
materials that contain elements which provide dry lubrication such as free machining steels,
alloys containing substantial amounts of graphite, and lead alloys .
The heat generated by friction results in a weld without a weld metal region with characteristic
solidification structure. The process therefore avoids many of the problems associated with
certain weld metal solidification structures such as hot cracking .
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Applications
Similar metals are friction welded mainly because of economical considerations (short welding
times, high production rates for volume production), and the process has been used to
produce high quality joints in material such as copper, brass, bronze, low alloy steel, tool steel ,
stainless steel, nickel, tantalum, titanium, etc.
FRW has been used to join dissimilar metals as an alternate route to other welding methods
but mainly because no other methods can be used. Typical dissimilar metal joint combinations
include copper to 1018 steel , tool steel to 1045 steel , type 302 stainless steel to 1020 steel,
and aluminum 6061 to type 302 stainless steel. The parts that are joined by friction welding
usually have an axis which can be rotated perpendicular to the surface to be joined.
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Chapter 15
THERMIT WELDING (TW)
15.1
15.2
Principles of Operation . . ...... ..... . .. ... . .... .. . . .....•...... ..... .196
Applications .. ...... . ... ......... . •. .... .....•..... . ........ ...... 197
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15. THERMIT WELDING (TW)
15. 1 Principles of Operation
The thermit welding process uses the heat generated in a chemical reaction to join with or
without the application of pressure, see Figure 15.1.
Slag
Super
Hot Steel
Crucible
Slag
Tapping ---~'\P"
Device
Crucible
Parts to be
Welded
Preheated
to Red Heat
Figure 15.1: Thermit Welding Process
Heat as well as molten filler metal for welding are generated as a result of an exothermic
reaction between aluminum and iron oxide (thermit powder):
8 AI
+
3Fe 30 4
9Fe
+
4 AI,03 + Heat
(Filler metal)
The reaction occurs in a crucible mounted on top of a mold built around the parts to be
welded. In the example above, the parts to be welded are prepared with a square groove joint
and a predetermined root opening. A pre-calculated amount of thermit powder sufficient to fill
the joint and which may also contain appropriate alloying elements is added to the crucible. It
is then ignited by a special ignition powder producing a temperature of 1300C that is required
for the exothermic reaction to start. Once started, the temperature quickly increases to
2500C, almost twice the melting temperature of steel. Due to gravity, this molten metal flows
to the bottom of the crucible where it is tapped off into the mold and melts the edges of the
joint. An aluminum oxide slag produced in the reaction floats to the top, protecting the molten
metal from atmospheric contamination. Heat losses cool the molten metal causing it to
solidify thus forming a weld with a composition that depends on the alloying elements present
in the thermite powder.
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Applications
Rail welding is the most widely used application of thermit welding . It is used to join rails ,
replace rail defects, weld insulated joint assemblies into the track, and to make electrical
connections between rails for railway signal systems. Other applications have included
welding of ship frames , making large crankshafts, welding thick I-beams, welding reinforcing
bars, and joining copper and aluminum conducters for the electrical industry.
Fig 15.2: Thermit Welding of Copper
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GUIDES AND EXERCISES
MODULE 4
WELDING PROCESSES AND EQU IPMENT
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Guides and Exercises
To obtain maximum benefit from this module, we suggest that you follow this guide and complete
the exercises as indicated. It is important that you work through the text methodically, studying
each section thoroughly before moving on . The exercises are designed to give you an indication
whether you have learned the material and can move on or whether you need to go back and
study the section again.
Do the exercises honestly. They will not help you unless you take them seriously. If you get a
question wrong , go back through the text until you understand where you have gone wrong and
know the correct answer.
The length of time required to complete the module will vary from student to student. Find your
own pace. Do not rush. Remember you are trying to teach yourself something, not win a race.
Some people like to underline sections when they read a text. We suggest that you use caution
when you do this. What you think is important first time you read it may be different after reading
it three times. We suggest you read a section three times thoroughly before highlighting
anything.
The last exercise is designed to give you an indication of whether you are ready to take the CWB
Learning Centre closed book examination. The exercise questions are similar to the official
examination. Do not take the examination until you feel you are ready and you may wish to
study several Modules before taking the examination for each.
If you have difficulty with this module, do not hesitate to ask for help by contacting us at 1-800844-6790/905-542-2176.
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Guide 1
Carefully read Chapter 1 and answer the following questions:
1.
Welding is a joining process that by definition requires:
(a)the application of both heat and pressure
(b)the application of either heat or pressure
(c)the application of both heat and filler metal
(d)the application of both pressure and filler metal
(e)all of the above
2.
Name the two main methods of arc shielding.
3.
In which direction do the electrons flow in an electrical arc? From anode to cathode, or
cathode to anode?
4.
In arc welding, the electric arc is intensely hot with temperatures exceeding:
(a)
(b)
(c)
(d)
(e)
20000· C
30000· C
33000·C
40000· C
50000· C
5.
Which arc welding process has the highest arc efficiency? And what is its
approximate value?
6.
How do the arc voltage and arc stiffness change when the arc length is increased?
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Guide 2
Carefully read Chapter 2 and answer the following questions:
1.
What is the generic name for the electrode used in GTAW and does it melt to provide
filler metal for the weld?
2.
In the GTAW process what type of penetration and weld width can be expected when
using D.C. electrode positive?
3.
Which polarity is chosen most often for direct current GTA welding of steels and why?
4.
Why must the shielding gas for GTAW be completely inert?
5.
State two advantages of the GTAW process.
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Guide 3
Carefully read Chapter 3 and answer the following questions:
1.
What does SMAW stand for?
2.
Why is SMAW a very common ly used welding process?
3.
True or false? Constant voltage power sources are preferred for SMAW.
4.
Why is care necessary in proper storage and handling of electrodes for welding steels?
5.
True or false? Larger diameter electrodes are preferred for we lding the root passes in
V grooves and for out of position welds .
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Guide 4
Carefully read Chapter 4 and answer the following questions:
1.
True or false? Constant voltage power sources is generally used for GMAW.
2.
Which metal transfer mode in GMAW would be applied for welding (a) cast iron, (b)
aluminum alloy?
3.
Between 100% CO, and Ar-5%CO,. which shielding gas will you chose to ensure spray
transfer in semi-automatic welding of carbon steel?
4.
State three advantages of the GMAW process over the SMAW process.
5.
Which welding parameter is used most reliably to ensure that a welding procedure
developed with one machine is accurately duplicated on another machine?
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Guide 5
Carefully read Chapter 5 and answer the following questions:
1.
True or false? Welding equ ipment for gas shielded flux cored arc welding is essentially
the same as for gas metal arc welding.
2.
The most common gas used with gas shielded flux cored arc welding is:
(a)
(b)
(c)
(d)
Pure Argon
Pure Helium
75% Argon/25% CO 2
Argon with 10% hydrogen
3.
Which type of cored wire is most suitable for open air field work? Gas shielded flux
cored wire? Self shielded flux cored arc wire? Metal cored wire?
4.
True or false? Gas shielded flux cored wires with basic ingredients in the core are
ideally suited for out of position welding.
5.
Which technique - drag (backhand) or push (forehand) - is used for semi-automatic
welding with gas shielded flux cored arc welding?
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Guide 6
Carefully read Chapter 6 and answer the following questions:
1.
True or false? In the submerged arc welding process, granular flux is placed on the
unwelded seam behind the consumable electrode.
2.
What is the effect of change in polarity from DCEP to DC EN on a submerged arc weld
deposit?
3.
State the greatest advantage and disadvantage of the submerged arc welding process?
4.
True or false? Fused fluxes are prone to moisture pick-up.
5.
What are the effects of increasing the arc voltage when using an active flux for
submerged arc welding of carbon steels?
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Guide 7
Carefully read the remaining chapters and answer the following questions:
1.
What is the source of heat in the resistance welding process?
(a)
(b)
(c)
(d)
2.
Which of the following arc welding process(es) is (are) non-consumable electrode type?
(a)
(b)
(c)
(d)
(e)
(f)
3.
Wide, deep penetrating welds and heat affected zones
Narrow, deep penetrating weld and heat affected zones
Narrow, shallow penetrating welds and heat affected zones
Wide, shallow penetrating welds and heat affected zones
In thermite welding, what is the name of the reaction from which the required heat and
molten metal is obtained?
(a)
(b)
(c)
(d)
5.
GTAW
SMAW
GMAW
FCAW
SAW
PAW
The electron beam welding process results in:
(a)
(b)
(c)
(d)
4.
An electric arc
External pressure
Resistance of the copper electrode to electron flow
Electrical resistance of the workpieces
Exothermic
Duo Thermic
Interthermic
Endothermic
In electroslag welding , what is the usual cooling medium used on the copper shoes?
(a)
(b)
(c)
(d)
Dry ice
Air
Oil
Water
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Answers
Guide 1
1.
2.
3.
4.
5.
6.
(b)
Flux shielding and gas shielding.
Cathode to anode.
(b)
Submerged arc welding. 90 top 95%.
Arc voltage increases and arc stiffness decreases.
Guide 2
1.
2.
3.
4.
5.
Tungsten electrode. It does not melt during the welding process and that is why GTAW
is a "non-consumable electrode" arc welding process.
Shallow penetration and a wide weld .
Direct current electrode negative . More heat is produced at the workpiece leading to
deeper weld penetration.
Presence of active gases like oxygen and carbon dioxide will oxidize the tungsten
electrode and limit its life.
It gives high quality deposits, and the process can be used to weld virtually all metals
and alloys.
Guide 3
1.
2.
3.
4.
5.
Shielded Metal Arc Welding
It provides great flexibility in terms of the broad range of materials and their thicknesses
that can be joined in all welding positions.
False.
Improperly stored electrodes can pick up moisture leading to increased weld metal
hydrogen content and therefore increased risk of cold cracking in the weld zone .
False.
Guide 4
1.
2.
3.
4.
5.
True.
(a) Short circuiting mode, (b) Spray.
Ar-5%CO,.
High deposition rate, greater percentage of arc time due to infrequent electrode
changing, more economical use of filler metal in the absence of stub losses, potential
for cleaner, higher quality weld metal with low hydrogen.
Wire feed speed.
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Answers
Guide 5
1.
2.
3.
4.
5.
True.
(c).
Self shielded flux cored wire .
False.
Drag (backhand).
Guide 6
1.
2.
3.
4.
5.
False.
The deposition rate will increase, however, the penetration is reduced and the risk of
lack of fusion type flaws in groove welds is increased.
Greatest advantage is very high productivity possible due to high duty cycle
(mechanized process) and high deposition rate (multiple wires and high currents) . The
main disadvantage is that the process is not suitable for out of position welding.
False.
Increase in arc voltage increases the flux consumption and in the case of active fluxes,
the Mn and Si content of the weld metal is increased. As a result, such fluxes should
be used with caution for mUltipass welds since there is a potential for build up of Mn
and Si in the we ld metal, thereby decreasing the weld metal ductility and increasing the
risk of hydrogen cracking.
Guide 7
1.
2.
3.
4.
5.
(d)
(a) and (f)
(b)
(a)
(d)
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Module 4
Test
This test is designed to determine whether you are ready to attempt the formal examination.
Complete the ANSWER SHEET and compare the results with the test key. If you have a mark
less than 70%, you are advised to re-study the material.
1
.with a tungsten are, most of the current across the arc is carried by:
(a)
(b)
(c)
(d)
(e)
2.
The ch ief feature of an electron beam weld is:
(a)
(b)
(c)
(d)
(e)
3.
a smooth fillet weld
high sh rinkage
very shallow penetration and wide heat affected zone
a very narrow weld and heat affected zone
a high deposition rate
Which gases are used with the GMAW process?
(a)
(b)
(c)
(d)
(e)
4.
electrons
ions
nuclei
molecules of inert gas
there is no current
no gases are used
any gas including CO" hydrogen , fluorine, etc.
only CO" hydrogen and fluorine
argon and CO, and their mixtures in various proportions
only helium
Which of the following processes is a non-consumable electrode process?
(a)
(b)
(c)
(d)
(e)
SMAW
SAW
GMAW
GTAW
FCAW
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Module 4
Test
5.
Projection welding is a resistance spot welding process in which:
(a)
(b)
(c)
(d)
(e)
6.
Which of the following processes is not used in the overhead position?
(a)
(b)
(c)
(d)
(e)
7.
SAW
GMAW
SMAW
FCAW
GTAW
Which of the following shielding gas (mixtures) is suitable for spray transfer when
welding with 1.1 mm diameter GMAW wire?
(a)
(b)
(c)
(d)
(e)
8.
the parts are projected towards each other under high pressure
the current is localized by using a projection or embossment on the part to be
joined
the weld is made in a projection of the piece which is removed after
the current is localized using a high magnetic field
extra long electrodes are used
95% CO, - 5% Ar
75% CO, - 25% Ar
50% CO, - 50% Ar
25% CO, - 74% Ar
5% CO, - 95% Ar
The deposition rate of some SMAW electrodes is increased by including which of the
following in the flux coating:
(a)
(b)
(c)
(d)
(e)
basic fluxes
aluminum
deoxidants
iron powder
fluorides
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Module 4
Test
9.
The electrode in flux cored arc welding is:
(a)
(b)
(c)
(d)
(e)
10.
a
a
a
a
a
tubular wire containing gases
solid wire with flux coating
hand held rod with flux in the core
flux cored tungsten electrod e
tubular wire containing flux or other ingredients
What would be a typical current range for a 4mm (5/32") E4918 (E7018) electrode:
(a)
(b)
(c)
(d)
(e)
25 - 70 amperes
150 - 220 amperes
2.5 - 7 amperes
15 - 22 amperes
650 - 875 amperes
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Answer Sheet - Module 4
Complete the "Answer Sheet" and compare the results with the "Test Key". If you have a pass mark
less than 70%, you are advised to re-stud y the materi al.
Please circle only ONE letter corresponding to the answer you think is most correct.
QUESTION
1
2
3
4
5
6
7
8
9
10
a
a
a
a
a
a
a
a
a
a
b
b
b
b
b
b
b
b
b
b
ANSVVERS
c
c
c
c
c
c
c
c
c
c
d
d
d
d
d
d
d
d
d
d
e
e
e
e
e
e
e
e
e
e
The answer key below is provided for your use in the event that you wish to retest yourself.
QUESTION
1
2
3
4
5
6
7
8
9
10
a
a
a
a
a
a
a
a
a
a
ANSVVER
c
b
b
b
b
b
b
b
b
b
b
c
c
c
c
c
c
c
c
c
d
d
d
d
d
d
d
d
d
d
e
e
e
e
e
e
e
e
e
e
213
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eWB Learning Centre
Test Key - Module 4
Compare your answer sheet to this key.
QUESTION
1
2
3
4
5
6
7
8
ANSIJI.ERS
, a
a
a
a
Ibl
a
a
c
c
c
c
c
c
c
c
c
c
b
b
b
b
9
a
a
a
b
b
b
b
10
a
Uj
1
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d
d -,3
d ~
0 -4,
d
d
d
e
e
e
e
e
e
~e
0 1
e
d
d
~
e