Advanced welding

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Advanced Welding
Content
Soldering
Brazing
Welding
Gas welding
Metal arc welding
Advanced welding techniques: Magnetic arc welding,
Friction welding, Explosive welding, Ultrasonic welding,
electron beam welding, Laser welding
Soldering
Joining processes which produces coalescence of
materials by heating them to a suitable
temperature and by using a filler metal having a
liquidus not exceeding 450 oC and below the
solidus of the base materials.
The filler metal (usually of lead and tin) is
distributed between the closely fitted surfaces of
the joint by capillary attraction
Brazing
Brazing is the joining of metal without melting them, using a
filler metal which has a melting point above 4500C but below
that of the parent metal, and which fills the joint by capillarity
Advantages
- Brazing is a non-fusion techniques, as base materials does
not melt, low distortion
- Usually does not effect the properties of the parent metal.
So, post heat treatment are rarely required
- Semi-skilled/unskilled labour can be used because of ease of
automation
-Wide range of filler metal heating methods are available
Brazing procedure
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1.
2.
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Mechanical and chemical cleaning
Heat components
Flux and filler metal melting
Borax (used above 7500C)-less corrosive than 2
Fluoride (used below 7500C)- used in silver brazing
Post-braze heat treatment
Post-braze cleaning
Inspection
Disadvantages or Difficulties
• The nature of the braze component is complex. The
most important consideration as regards strength is
the continuity of the bond, which can vary from 0100%, as it is dependent on the ability of the brass
metal to wet the surfaces of the gap
• In general, liquid braze metals will not wet, clean
unfilmed surfaces unless
• (a) the liquid metal is intersoluble with the base
(parent) metal
• (b) the liquid and solid metal react to form an
intermetallic compounds
Fluxes
• Most common method of ensuring good wetting
• Generally achieved by dissolving oxides
• Same fluxes also deposit metals on to the surface of the
parent metal and reacts with the surfaces, thus preparing
it chemically (e.g.:ZnCl2 flux-Zn is deposited on Fe
surfaces giving tinning effect)
• Flux also has a blanketing effect on the surface keeping
O2 out
• Fluxes are applied over heated area or filler rod is
coated in flux
Brazing
Induction heating
Inductor is placed close to the parts to be brazed. In most cases the
coil surrounds the components. A high frequency current in the
inductor induces a heating current in the work piece.
The brazing cycle can be precisely controlled using timing
equipment built into the HF generator.
Advantage of induction heating
Rapid and uniform heat-rate
Can be used in inert atmosphere or in vacuum
Good heating techniques for high quality
Mostly used for steel components
Cleaning
Mechanical cleaning
–Usually abrasion will be necessary on large
components
–It is usually less efficient and more costly than
chemical cleaning when large numbers of small
components are involved in the production
process.
–Other mechanical methods generally employed
are chipping and scratch brushing, rinsing or
scrubbing with water, acid or other chemical
Cleaning
Chemical cleaning
1. Degreasing using (a) Solvent (Petroleum or chlorinated
hydrocarbons)
or
(b) Vapour degreasing using stabilised
trichloroethylene, carbon tetrachloric or acetone
2. Scale or oxide removal can than take place by acid cleaning or
pickling
(salt pickling can also be used)
e.g. : Iron and steel – 10% H2SO4
Brass – 10% H2SO4 acid for 10 min maxm
Stainless steel – 7% HNO3 + 21% H2SO4 in water
Welding
• The process of permanently joining two or more metal parts,
by melting both materials. The molten materials quickly cool,
and the two metals are permanently bonded.
Advantage:
• Higher mechanical properties
• Fixing stress cracks
• Reinforcing weak joints
• Cutting or shaping new parts
Equipments used in gas
and oxy-acetylene
welding processes
Oxygen
Acetylene
Steel cylinder
Steel cylinder
Contained in
compressed
form
High pressure acetylene
is not stable so it
dissolved in acetone,
which has the ability to
absorb a large volume of
gas and release it as the
pressure falls.
Supplied 3.4, 5
and 6.8 m3
capacities
Mild steel-13,
660 kN/m2
Alloy steel-17,
240kN/m2
R. H. thread in
valve
1 volume acetone-25
volume acetylene
Pressure 1, 552 kN/m2
Danger of explosionporous substance
Welding gas mixture
Fuel Gas
Maximum Flame temperature
with air (degree C) with oxygen (degree C)
Acetylene
1 755
3 200
Butane
1 750
2 730
Coal gas
1 600
2 000
Hydrogen
1 700
2 300
Propane
1 750
2 500
Oxy-acetylene flame
Neutral flame
Oxidising flame
Carburising flame
www.twi.co.uk
Gas welding
Effect of welding on the structure
Ref: Basic fabrication and welding enginering, F. J. M. Smith, LST
Temperature distribution during oxyacetylene welding 10 mm thick mild
steel
Arc welding
The most common and economical method is AC arc welding
Arc
•Highly luminous and intensely hot discharge of
electricity between two electrodes
•Discovered early 19th cent. by Sir Humphry Davy
•High current and low voltage
•When electrodes are parted, strong electric forces
draw electrons from one electrode to the other,
initiating the arc
Shielding gases in arc welding
Tungsten inert gas
welding (TIG)
•Tungsten electrode30000C
•Argon and Helium
•Filler material is added as
in gas welding
Schematic of TIG
Shielding gases in arc welding
• Metal inert gas welding
(MIG)
• Consumable electrode
• Argon, Helium and
Carbon Dioxide
• No filler materials
MIG weld area:
Positions and arc welding
Risk involved in arc welding
1. Exposure to radiation
2. Flying sparks
3. Electric shock
4. Fumes
5. Damage to eyes
6. Burns
Safety
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•
•
•
•
•
•
Make sure to work on a dry floor.
Wear thick rubber shoes and dry leather welding gloves.
Be sure to use insulated electrode holders.
Check to make sure that your equipment is all properly
grounded.
Keep your work area properly ventilated to avoid
inhaling any potentially toxic fumes.
Be on the look out for flying bits of melted metal.
Most importantly, be aware of any other people who are
around you.
Types of weld
Types of weld
Welding processes-groups-letter designation
Magnetic Arc Welding
• Arc is rotated around the weld line by the force
which results from the interaction between the
magnetic field and the current
• CO2 or inert gas shielding is used
Steps in MIAB
• Faces to be joined are brought together and internal
magnetic coil is put in place
• Welding current, magnetic coil system is put in place
and shielding gas are turned on
• Work pieces are retracted to a defined gap to produce
the arc
• Arc rotates about interface-melting faces to be joined
• Faces are pressed together
• Welding current, magnetic field and shielding gas are
switched off
Magnetic Arc Welding
MIAB
Faster than arc fusion welding
and conventional welding
Used industrially
Accurate-No further finishing
machining operation are
required
Allows quality control
MIAF
Non-consumable electrode
Suitable for welding of thin
wall pipes or tubes certain
pressed sheet fabrication
Friction welding
-Friction heat caused by the motion of one surface
against another enables plastic deformation and
atomic diffusion at the interface
-Used by the automotive industry for decades in the
manufacture of a range of components
-The weld is formed across the entire cross-sectional
area of the interface in a single shot process
Advantages of friction welding
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•
•
•
•
•
•
Narrow HAZ
Dissimilar metals can be joined
No fusion zone
Can be used under water
Very high reproducibility - an essential requirement for a mass production
industry
Excellent weld quality, with none of the porosity that can arise in fusion
welding
environmentally friendly, because no fumes or spatter are generated, and
there is no arc glare or reflected laser beams with which to contend
Variations of friction welding
• Rotary Friction Welding
• Linear Friction Welding
• Friction stir welding
Direct or continuous drive
Pre-determined time of
motion determined by the
size and type of material
Inertia friction welding
One of the work pieces is connected to a flywheel and the
other is restrained from rotating
Flywheel used to provide energy and is disengaged
before the work pieces are pushed together
Less drive power required than with direct drive
welding
Linear friction welding
±1-3mm
Frequency 25125Hz
Maximum
axial force
150kN
Friction surface / Friction stir welding
Steps in friction stir welding
• A non-consumable rotating tool is pushed into the
materials to be welded and then the central pin, or
probe, followed by the shoulder, is brought into
contact with the two parts to be joined.
• The rotation of the tool heats up and plasticises the
materials it is in contact with and, as the tool moves
along the joint line, material from the front of the
tool is swept around this plasticised annulus to the
rear, so eliminating the interface.
Explosive Welding
•Welding produced by explosively forcing one plate
(or component) against the one to which it is to be
joined at an approximate angle of incidence, known as
the impact angle
•Methods: 1. Inclined gap method
2. Parallel gap method
In parallel gap method, detonation velocity should be
equal to or less than the speed of sound in the metal
being welded
Explosives
Explosive
Detonation
velocity, m/s
RDX (Cyclotrimethylene trinitramine, C3H6N6O6
8100
PETN (Pentaerythritol tetranitrate, C5H8N12O4)
8190
TNT (Trinitrotoluene, C7H5N3O6)
6600
Tetryl (Trinitrophenylmethylinitramine,
C7H5O8N5)
7800
Lead azide (N6Pb)
5010
Detasheet
7020
Ammonium nitrate (NH4NO3)
2655
Explosive welding
Inclined gap method
• Various detonation speeds are possible with the inclined gap
method
• A jet is formed. The jet is a thin layer of metal stripped from
the surfaces of both plates, which in turn exposes the
uncontaminated metal surfaces which are then welded in the
high pressure zone, known as stagnation point
• Typically the weld surfaces are wavy
• Weld is mainly solid state with small pockets of melted jet
material (on the front and back slopes of the waves)
• Some welding may also be enhanced by friction due to the
difference in the velocity of the plates
Application of explosive welding
•
•
•
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Cladding plates
Joining of pipes and tubes
Major areas of the use of this method are heat exchanger
tube sheets and pressure vessels
Tube Plugging
Remote joining in hazardous environments
Joining of dissimilar metals - Aluminium to steel,
Titanium alloys to Cr – Ni steel, Cu to stainless steel,
Tungsten to Steel, etc.
Attaching cooling fins
Other applications are in chemical process vessels, ship
building industry, cryogenic industry, etc.
Advantages of explosive welding
1.
2.
3.
4.
5.
6.
7.
Can bond many dissimilar, normally unweldable metals.
Minimum fixturing/jigs.
Simplicity of the process.
Extremely large surfaces can be bonded.
Wide range of thicknesses can be explosively clad
together.
No effect on parent properties.
Small quantity of explosive used.
Disadvantages of explosive welding
1. The metals must have high enough impact
resistance, and ductility.
2. Noise and blast can require operator protection,
vacuum chambers, buried in sand/water.
3. The use of explosives in industrial areas will be
restricted by the noise and ground vibrations
caused by the explosion.
4. The geometries welded must be simple – flat,
cylindrical, conical.
5. Area should be cleaned and sound grounded for
explosion
6. Licences are necessary to hold and use explosives
Ultrasonic welding
•
•
•
•
A solid state process for metal and plastics
Energy required comes in the form of mechanical vibrations
Most operates at 20, 30, 40 kHz
Weld is produced when the work pieces are clamped together
between an anvil and a high frequency vibration probe
(sonotrode)
• Empirical relation for a ultrasonic welding:
E=k(HT)3/2
Where, E = Electrical energy
k = Constant for given welding system
H = Vickers hardness
T = Thickness of the work piece in contrast with the
sonotrode
Types of ultrasonic welding
Direct couple methods
Wedge-Reed method – where the
transducer is coupled through a
resonant bar
Ultrasonic welding
• Sonotrode induces lateral vibration and local
movement between the frying surfaces
• This tends to disrupt any surface oxide film
present and also raises the temperature, extending
an area of plastic flow, and a solid-phase type of
pressure is formed
• Morphology of the weld is similar to the friction
weld
Variants:
• Spot welding- elliptical “spots”
• Ring welding – hollow sonotrode tip
• Line welding – linear sonotrode tip
• Continuous welding – Rotating wheel shaped sonotrode
and a roller type of anvil
Application:
• Largest growth area for ultrasonic welding is micro
miniature welding and micro joining in micro electric
applications
• Capable of joining very fine wires to electrical
components
Ultrasonic welding control
Ultrasonic welding control
Ultrasonic welding control
Ultrasonic welding
Advantages:
•Energy efficiency
•High productivity with low costs and ease of automated assembly line
production
Disadvantages:
The maximum component length that can be welded by a single horn is
approximately 250 mm. This is due to limitations in the power output
capability of a single transducer, the inability of the horns to transmit very
high power, and amplitude control difficulties due to the fact that joints of
this length are comparable to the wavelength of the ultrasound.
Electron beam (EB) welding
• EB welding is a fusion joining process in which the work
piece is embedded with a dense stream of high velocity
electrons. Welding usually takes place in an evacuated
chamber.
• Advantage: Very deep penetration can be achieved. For
example, joining of 200 mm aluminium plates requires 600
passes when conventional gas metal arc process requires
over 100 passes even using specially developed narrowgrove process. By using the EB process, the same plate can
be welded in only 2 passes.
• Disadvantage: Dealing with the vacuum needed for the
process
Laser welding
• Possible application is the fabrication of stiffened
panel structures commonly used for ships, aircraft,
and other structures. Stiffeners can be laser welded
on to panels with no filler materials.
• No doubt that laser will be used in various ways in
metal fabrication industries.
• It is still difficult to predict how extensively they
will be used and how soon.
Ref: Metals hand book. Ninth edition. Vol 6: Welding brazing and soldering
Welding
defects
Oxides in welding
Difficulties:
Form tenacious film
Melting point oxides higher than the parent metal
Rapid formation
Unless the oxides are removed:
Fusion may be difficult
Inclusions may be present in the weld metal
Joining will be weakened
Factors that
contribute to the
weld distortion
and their relation
to each other and
to the total
distortion
Ref: International series on
materials Science and
Technology. V33: Analysis
of welded structures
Combating distortion
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