Heat flow in welding
Subjects of Interest
• Heat sources
• Heat source and melting efficiency
• Analysis of heat flow in welding
• Effects of welding parameter
• Weld thermal simulator
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Sep-Dec 2007
Objectives
• This chapter provides information of heat flow during
welding, which can strongly affect phase transformation,
microstructure, and properties of the welds.
• Students are required to indicate heat source and power
density used in different welding methods, which affect the
melting efficiency.
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Welding heat sources
Electrical sources
Mechanical sources
• Arc welding
• Resistance welding
• Electroslag
Heat intensity ~ 106-108 Wm-2
• Friction (stir) welding
Heat intensity ~ 104-106 Wm-2
• Ultrasonic welding (15-75 KHz)
• Explosion welding (EXW)
Chemical sources
Other sources
• Oxyfuel gas welding
• Thermit welding
Heat intensity ~
106-108
Wm-2
• Diffusion welding
High energy sources
• Laser beam welding
• Electron beam welding
Heat intensity ~ 1010-1012 Wm-2
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Welding Arc
Characteristics
• A welding arc consists of a sustained electrical discharge
through a high temperature, conducting plasma, producing
(ionic gas or plasma sufficient thermal energy as to be useful for the joining of metal by
with electric current
fusion.
passing through)
• Gaseous conductor changes electrical energy into heat.
• Arc produces sources of heat + radiation (careful required
proper protection)
http://en.wikipedia.org
bell shaped arc
Welding arc
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Gas metal arc welding
Sep-Dec 2007
Emission of electron at cathode
Emission of electrons at cathode occurs when an amount
of energy required to remove the electron from a material
(liquid or solid). This amount of energy per electron is
called ‘work function’. (analogous to ionization potential)
Material
Work function, eV
Al
3.8-4.3
Cu
1.1-1.7
Fe
3.5-4
Mg
3.1-3.7
W
4.3-5.3
BaO, SrO
0.95
Thoria
2.5
CsO
0.75
Al2O3
<2.5
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Emission occurs mainly by two processes;
1) Cold cathode
At low pressure, high voltage
conditions, positive ions are accelerated
toward the cathode and bombard the
cathode with relatively high energy.
2) Thermal emission
At high temperature some electrons
acquire enough thermal energy to
overcome the work function and
become free electrons.
Sep-Dec 2007
Plasma formation
www.fronius.com
States of matter
Solid
Melting
• Plasma consists of ionized state of a
gas composed of nearly equal
numbers of electrons and ions, which
can react to electric or magnetic fields.
Liquid
Vaporization
Gas
(neutral
atoms/molecules)
Ionization
Plasma
• Electrons, which support most of the
current conduction, flow from cathode
terminal (-) to anode terminal (+).
(negative charges • Neutral plasma can be established
and positive ions) by thermal means by collision
process, which requires the attainment
of equilibrium temperature according to
ionization potential of the materials.
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Ionization potential
Energy
Element/Compound
Ionization Potential (Volts or eV)
He
24.6
Ar
15.8
H2
15.4
N2
15.6
O2
12.1
CO2
13.8
CO
14.1
C
11.3
Si
8.2
Fe
7.9
Ni
7.6
Na
5.1
K
4.3
Cs
3.9
Ionization potential, Vi, required to strip an
electron from an outer shell of and atom or M+.
Plasma temperature = Ionization potential x 1000 K
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Power in arc
•The electrical power is dissipated in three
regions of the arc: anode, cathode and plasma
column.
Cathode -
•The area at cathode and anode has strong
effects on arc configuration, the flow of the
heat energy to the terminal affecting shape
and depth of the fusion zone.
Arc area is mainly divided into
three zones;
Pc
Power (Parc)
Heat
1) Anode
Pa = IE a
2) Cathode
Pa
Anode +
Energy dissipation in the arc
Pc = IE c
3) Plasma arc column
Parc = I (dE arc / dl )l
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Note: Most heat goes to the
anode/cathode and most is lost
radially from the arc
Sep-Dec 2007
Temperature in the arc and heat loss
www.geocities.com
• The arc temperature ~ 5000-30,000 K
depending on the nature of plasma and
current.
• The arc temperature is determined by
measuring the spectral radiation
emitted.
Heat losses in the arc
• Energy losses by heat conduction
and convection, radiation and
diffusion.
• In Ar gas, radiation loss ~ 20%
while in other welding gas, radiation
loss <10%.
Plasma temperature contour in the arc
Temp
Radiation loss
Heat loss
Note: The use of fluxing reduces radiation lost
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Polarity
There are three different types of current used in arc welding
1) Direct-Current Electrode Negative (DCEN)
2) Direct-Current Electrode Positive (DCEP)
3) Alternating current (AC)
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Direct-Current Electrode Negative (DCEN)
• Also called straight polarity.
• Electrons are emitted from the negative
tungsten electrode and accelerated while
travelling through the arc.
• Most commonly used in GTAW.
• Relatively narrow and deep weld pool is
produced due to high energy.
• DCEN in GMAW makes the arc unstable
and causes excessive spatter, large droplet
size of metal and the arcs forces the droplets
away from the workpiece. This is due to a
low rate of electron emission from the negative
electrode.
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Direct-Current Electrode Positive (DCEP)
• Also called reverse polarity.
• The electrode is connected to the positive
terminal of the power source, therefore the
heating affect is now at the tungsten electrode
rather than the workpiece. shallow weld for
welding thin sheets.
• At low current in Ar, the size of the droplet ~ the
size of the electrode Globular transfer.
• The droplet size is inversely proportional to the
current and the droplets are released at the rate
of a few per second.
• At above the critical current the droplets are
released at the rate of hundreds per second
(spray mode).
• Positive irons clean off the oxide surface.
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Surface cleaning action
DCEP can be employed to clean the surface of the workpiece by knocking
off oxide films by the positive ions of the shielding gas.
Ex: cleaning of Al2O3 oxide film
(Tm ~2054oC) on aluminium to
make melting of the metal
underneath the oxide film easier.
Surface cleaning action in GTAW with
DC electrode positive.
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Alternating Current (AC)
• Reasonably good penetration and
oxide cleaning action can be both
obtained.
• Often used for welding aluminium
alloys.
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Heat source efficiency
In the case of arc welding, having a constant voltage E and a
constant current I, the arc efficiency can be expressed as;
η=
Where
Qt weld
Qt weld
Q
=
=
Qno min al t weld EIt weld EI
Eq.2
Q
is the rate of heat transfer
Qnominal is the heat input
tweld
is the welding time
In cases of electron beam and laser beam welding, Qnominal is the power
heat source of the electron beam and laser beam respectively.
The term, heat input per unit length of weld often refers to
Qno min al
EI
, or
V
V
Where
Qnominal or EI
V
Qnominal / V
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Eq.3
is the heat input
is the welding speed
is heat input per unit length of weld
Sep-Dec 2007
Heat source efficiency measurement
• Heat source efficiency can be measured using
a calorimeter (by measuring the heat transfer
from the heat source to the workpiece and then to
the calorimeter).
• The temperature rise in the cooling water
(Tout-Tin) can be measured using thermocouples
or thermistors. Heat transfer from the workpiece
to the calorimeter is given by
Eq.4
α
α
0
0
Qt weld = ∫ WC (Tout − Tin )dt ≈ WC ∫ (Tout − Tin )dt
Where
W
C
Tout
Tin
t
is the mass flow rate of water
is the specific heat of water
is the outlet water temperature
is the inlet water temperature
is time
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Note: This integral corresponds
to the shaded area, and can be
used to calculated the arc
efficiency η.
Sep-Dec 2007
Heat source efficiency measurement
• The arc efficiency can also be measured
using Seebeck envelope calorimeter. This
technique utilises thermocouple junctions for
sensing temperature difference.
• The heat transfer from the workpiece to
the calorimeter can be determined by
measuring the temperature different ∆T and
hence gradient across a gradient layer of
material of known thermal conductivity k
and thickness L.
α
Qt weld = A∫ k
0
Where
A
∆T/L
∆T
dt
L
Eq.5
is the area for heat flow
is temperature gradient
Layer of temperature gradient for heat
source efficiency measurement.
Note: this type of calorimeter is used to determine the arc
efficiencies in PAW, GMAW, and SAW.
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Heat source efficiency measurement
• In GMAW the arc, metal droplets, and the
cathode heating contribute to the efficiency
of the heat source.
• Lu and Kou used a combination of three
calorimeters to estimate the amounts of
heat transfer from the arc, filler metal
droplets and the cathode heating to the
workpiece in GMAW of aluminium.
(a) Measured results, (b) breakdown of power inputs.
(a) Heat transfer from metal droplets
(b) Total heat inputs
(c) Heat inputs from arc and metal droplets.
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Heat source efficiency in various
welding processes
LBW
Heat source efficiency is low
because of the high
reflectivity.
PAW
Heat source efficiency is
much higher than LBW (no
reflectivity).
SAW
EBW
Heat source efficiencies in several
welding processes.
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Heat source efficiency is
higher than GTAW or SMAW
since the arc is covered with
thermally insulating blanket of
molten slag and granular flux.
Heat source efficiency is high
due to the keyhole acting like
a black body trapping the
energy from electron beam.
Sep-Dec 2007
Melting efficiency
Aweld = Afiller +Abase
Melting efficiency is the ability of the heat source to
melt the base metal (as well as the filler metal).
Cross section of weld
The melting efficiency of the arc ηm can be defined as follows
Where
V
Hbase
Hfiller
tweld
ηm =
( AbaseVt weld ) H base + ( A filler Vt weld ) H filler
ηEIt weld
Eq.7
V
is the welding speed
ηm
is the energy required to raise a unit volume of
tweld
base metal to the melting point and melt it.
is the energy required to raise a unit volume of
filler metal to the melting point and melt it.
is the welding time.
Note: the quantity inside the parentheses represents the volume of material
melted while the denominator represents the heat transfer from the heat
source to the workpiece.
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Melting efficiency
(a) shallow welds of
lower melting
efficiency,
(b) (b) deeper weld of
higher melting
efficiency.
Low heat input
Low welding speed
High heat input
High welding speed
Aweld = Afiller +Abase
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Power density distribution of heat source
Power density distribution is influenced by
1) Electrode tip angle
2) Electrode tip geometry
Sharp electrode
• Arc diameter
• Power density distribution
Blunter electrode
• Arc diameter
• Power density distribution
Effect of electrode tip angle on shape and power
density distribution of gas-tungsten arc.
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Effect of electrode tip angle on shape of
gas tungsten arc and power density
Conical angle of
electrode tip
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The arc becomes
more constricted
Sep-Dec 2007
Analysis of heat flow in welding
Heat or temperature distribution occurring during welding greatly affect
microstructure of the weld, hence, the weld properties
•The temperature-distance profile
shows that the heat source travels
along the weld in the direction A-A’ at
a constant speed.
• As the heat source moves on, the
cooling rates around the weld are very
high.
• A more intense heat source will give
a steeper profile and the HAZ, which
will be confined to a narrower region.
Temperature distribution round a typical weld
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Effect of temperature gradient on
weld microstructure
The temperature gradients in the liquid weld material are substantially higher
than in most casting processes. This leads to high solidification rates which
produce a finer dendritic structure than that observed in most castings.
Microstructures occurring in a weld and its HAZ.
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Effect of welding parameters
• Effect of heat input Q and welding
speed V on the weld pool.
• Effect of heat input on cooling rate.
• Effect of the power density
distribution of the heat source on the
weld shape.
• Heat sink effect of workpiece.
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Effect of heat input and welding
speed on the weld pool
• The shape and size of the weld pool is
significantly affected by heat input Q and
the welding speed V.
Heat input
Welding speed
The weld pool
becomes more
elongated.
Note: the cross indicates the
position of the electrode.
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Effect of heat input on cooling rate
The cooling rate in ESW (high Q/V)
is much smaller than that in arc
welding.
Heat input per
unit length EI/V
Cooling rate
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Effect of power density distribution
on weld shape
Power density
Weld penetration
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Heat sink effect of the workpiece
• The cooling rate increases with the
thickness of the workpiece due to
the heat sink effect.
• Thicker workpiece acts as a better
heat sink to cool the weld down.
Brass with a higher melting point than
that of aluminium is used as a heat sink
to increase the cooling rate in
aluminium welding.
Blass heat sink is clamped behind
aluminium to be welded.
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References
• Kou, S., Welding metallurgy, 2nd edition, 2003, John Willey and
Sons, Inc., USA, ISBN 0-471-43491-4.
• Gourd, L.M., Principles of welding technology, 3rd edition, 1995,
Edward Arnold, ISBN 0 340 61399 8.
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