Special type of welding

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Transcript Special type of welding

Special type of welding
Hareesha N Gowda
Lecturer, Dept of Aero Engg
DSCE, Blore-78
Special type of welding:
• Resistance welding – principles
• Seam welding,
• Butt welding
• Spot welding
• projection welding.
• Friction welding
• Explosive welding
• Thermit welding
• Laser welding
• Electron beam welding
RESISTANCE WELDING
Definition
• Resistance welding is a group of welding processes wherein coalescence is
produced by the heat obtained from resistance of the work to the flow of electric
current in a circuit of which the work is a part and by the application of pressure.
No filler metal is needed.
Fundamentals of Electric Resistance Welding
• The two factors or variables mainly responsible for resistance welding are
– The generation of Heat at the place where two pieces are to be joined.
– The application of pressure at the place where a weld joint is to be formed.
Principle
• When electric current flows through a material, it offers resistance to the
flow of current resulting in heating of the material.
• The heat generated is used to make a weld between two or more
workpieces.
• Resistance welding is based on the above principle.
• The heat generated in the material is given by Joules law:
HαI2RT
or H = k I2 RT
where H = heat generated in the material in Joules,
I = Flow of current through the material in Amperes
R = Electrical resistance of the material in Ohms
T = time for which the electric current flows through the material in seconds,
k = a constant, usually < 1 to account for heat loss through conduction and
radiation.
• High current is the primary requirement to produce a resistance weld.
• A step down transformer that converts the high voltage, low current
power line to a high current (upto 100,000 A) and low voltage (0.5 - 10 V)
power is used for the purpose.
Spot Welding
• Spot welding is a resistance welding process in which the two overlapping
workpieces held under pressure are joined together in one soot (location)
by the heat generated due to the resistance of the workpieces to the flow
of electric current through them.
• Figure shows the details of a resistance spot welding process.
Spot Welding
Description
• The two workpieces to be joined are cleaned to remove dirt, grease and other
oxides either chemically or mechanically to obtain a sound weld.
• The workpieces are then overlapped and placed firmly between two water
cooled cylindrically shaped copper alloy electrodes, which in turn are connected
to a secondary circuit of a step-down transformer.
• The electrodes carry high currents and also transmit the force/pressure to the
workpieces to complete the weld
Operation
• The welding current is switched ON. As the current passes through the
electrodes, to the workpiece, heat is generated in the air gap at the point of
contact of the two workpieces.
• The heat at this contact point is maximum with temperature varying from 815 930°C, and as a result melts the workpieces locally at the contact point to form a
spot weld.
• In order to obtain a strong bond, external pressure is applied to the workpiece,
through the electrode, by means of a piston-cylinder arrangement. The current is
switched OFF.
• In some cases, external pressure is not required and the holding pressure of the
two electrodes is just sufficient to create a good joint.
• Heat dissipates throughout the workpiece which cools the spot weld causing the
metal to solidify.
Spot Welding ( Continued….)
• The pressure is released and the workpiece is moved to the next location to make
another spot weld.
• In some spot welding machines, the workpiece remains stationary while the
electrode moves to the next location to make a weld.
• The duration of current flow varies from a fraction of second to a few seconds.
• Both the current and the duration of current flow form the important parameters
in spot welding and depend on the thickness and type of the workpieces being
welded.
• The shape and the surface condition of the electrode is another parameter in
obtaining a good weld.
Advantages
• Efficient energy use.
• Limited workpiece deformation. Also, workpiece is not melted to a larger extent.
Heat is concentrated only at the spot to be welded.
• High production rates.
• Suitable for automation.
• Filler metals are not required. Hence, no associated fumes or gas. This results in
clean weld.
Disadvantages
• Weld strength is significantly lower when compared to other processes.
This makes the process suitable for only certain applications.
• Silver and copper are difficult to weld because of their high thermal
conductivity.
Applications
• Extensively used for welding steels and especially in the automotive
industry for cars that requires several hundred spot welds made by
industrial robots.
Seam Welding
• Seam welding is a resistance welding process in which the overlapping
workpieces held under pressure are joined together by a series of spot welds
made progressively along the joint utilizing the heat generated by the electrical
resistance of the workpieces.
• Seam welding is similar to spot welding process, but, instead of pointed
electrodes, mechanically driven wheel shaped electrodes are used to produce a
continuous weld.
• Figure shows a seam welding process.
Description and Operation
• The two workpieces to be joined are cleaned to remove dirt, grease and other
oxides either chemically or mechanically to obtain a sound weld.
• The workpieces are overlapped and placed firmly between two wheel shaped
copper alloy electrodes, which in turn are connected to a secondary circuit of a
step-down transformer.
• The electrode wheels are driven mechanically in opposite directions with the
workpieces passing between them, while at the same time the pressure on the
joint is maintained.
• Welding current is passed in series of pulses at proper intervals through the
bearing of the roller electrode wheels
• As the current passes through the electrodes, to the workpiece, heat is generated
in the air gap at the point of contact (spot) of the two workpieces.
• This heat melts the workpieces locally at the contact point to form a spot weld.
• Pressure is applied by air, spring or hydraulically.
• Under the pressure of continuously rotating electrodes and the current flowing
through them, a series of overlapping spot welds are made progressively along the
joint as shown in the figure.
• The weld area is flooded with water to keep the electrode wheels cool during
welding.
Advantages
• A continuous overlapping weld produced by the process makes it suitable
for joining liquid or gas tight containers and vessels.
• Efficient energy use.
• Filler metals are not required. Hence, no associated fumes or gases. This
results in clean welds.
Disadvantages
• Requires complex control system to regulate the travel speed of
electrodes as well as the sequence of current to provide satisfactory
overlapping welds. The welding speed, spots per inch and the timing
schedule are all dependant on each other.
• Difficult to weld metals having thickness greater than 3 mm.
Applications
• Used to fabricate liquid or gas tight sheet metal vessels such as gasoline
tanks, heat exchangers.
Resistance Butt Welding
• Resistance butt welding or upset welding is a resistance welding process in
which the two parts to be joined are heated to elevated temperatures and
forged (by applying the desired pressure) together at that temperature
• Figure shows the equipment for resistance butt welding process.
Description and Operation
• The machine used for butt welding consists of two clamps mounted on a
horizontal slide.
• The clamps are made from a conducting material, usually copper alloy which serve
to carry high currents from a step-down transformer.
• The two workpieces to be joined are cleaned to remove dirt, grease and other
oxides either chemically or mechanically to obtain a sound weld.
• The workpieces are clamped rigidly on the welding machine.
• By applying external force, the work piece in the movable clamp is brought in tight
contact with the surface of the workpiece in the fixed clamp.
• High amperage current is then passed through the joint which heats the
neighboring surfaces.
• When the workpieces reaches a temperature of about 1600-1700°F, pressure is
increased to give a forging squeeze.
• Upsetting takes place while the current is flowing, and continues until the current
is switched OFF.
• After the metal has cooled, the pressure is released and the weld is completed.
• The weld joint obtained will be bulged and round due to the squeezing action of
the softened metal.
• This unwanted material can be removed later by machining process.
* Upset - To decrease the length and increase the cross-section
Advantages
• Joint obtained is clean, as filler metal is not used in this process.
• Produces defect free joint. Oxides, scales and other impurities are thrown
out of the weld joint due to the high pressure applied at elevated
temperatures.
Disadvantages
• The process is suitable for parts with similar cross-sectional area.
• Joint preparation is a must for proper heating of workpieces to take place.
Applications
• Used for producing joints in long tubes and pipes
Projection Welding
• Projection welding is a resistance welding process in which the workpieces are
joined by the heat generated due to the resistance of the workpieces to the flow
of electric current through them.
• The resulting welds are localized at predetermined points by projections,
embossments, or intersections.
• Figure shows the resistance projection welding process.
Description and Operation
• The process uses two flat, large cylindrically shaped water cooled copper
electrodes in which one electrode is fixed, while the other to which the pressure
is applied is movable.
• The electrodes are connected to a step-down transformer that provides the
required electric current for heating.
• One of the workpieces contains small projections or embossment (similar to a
pimple on a human face) made at a particular location where the joint is to be
made.
• The workpieces are cleaned to remove dust, scale and other oxides either
chemically or mechanically to obtain a sound weld.
• The workpieces are then placed between the two electrodes and held firmly
under external pressure.
• When the welding current is made to pass through the electrodes, to the
workpieces, maximum heat is generated at the point of contact of the two
workpieces, i.e., at the projections.
• This heat softens and melts the projections causing it to collapse under the
external pressure of the electrode thereby forming a spot weld. Refer figure.
• The current is switched OFF and the pressure on the workpiece is removed.
Advantages
• More than one spot weld can be made in a single operation.
• Welding current and pressure required is less.
• Suitable for automation.
• Filler metals are not used. Hence, clean weld joints are obtained.
Disadvantages
• Projections cannot be made in thin workpieces.
• Thin workpieces cannot withstand the electrode pressure.
• Equipment is costlier.
Applications
• A very common use of projection welding is the use of special nuts that have
projections on the portion of the part to be welded to the assembly.
• Also, used for welding parts of refrigerator, condensers, refrigerator racks, grills
etc.
FRICTION WELDING
• Friction Welding is a solid state welding process wherein coalescence is produced
by the heat obtained from mechanically induced sliding motion between rubbing
surfaces.
• The work parts are held together under pressure.
• Figure shows the arrangement for friction welding process.
Description and Operation
• The machine for friction welding is similar to a lathe which consists of a chuck held
in the spindle of the headstock.
• The chuck holds one of the workpieces and rotates it at high speeds (around 3000
rpm).
• The other workpiece is held stationary and in a movable clamp so that it can be
brought in contact with the rotating workpiece.
• The workpieces to be joined are prepared to have a smooth square cut surfaces.
• In operation, the stationary work piece is slowly brought in contact with the
rotating workpiece under an axial force. Refer figure.
• As the workpieces come in contact with each other, friction is generated at the
contact surface resulting in heating of the workpieces.
• The axial pressure to the stationary workpiece is increased until the friction
between the surfaces raises the heat to the welding temperature.
• At this moment, the rotation of the workpiece is stopped, but the pressure is
maintained or, in some cases increased to complete the weld.
• The weld joint obtained will be bulged due to the squeezing action of the softened
metal. The excess metal can be removed by machining.
Advantages
• Process is simple
• Low power requirements.
• Edge preparation is not required. The impurities are thrown away by the friction
generated between the two workpieces.
• No filler metal. Hence, the joint obtained is clean.
• Dissimilar metals can be easily welded.
Disadvantages
• The process is restricted to tubular parts and butt welds.
EXPLOSIVE WELDING
• Explosive welding is a solid state welding process in which detonation of explosives
is used to accelerate one of the workpieces to move towards another workpiece,
so that the impact creates a joint and completes the weld.
• Figure shows the arrangement for explosive welding process.
Description and Operation
• One of the workpiece called 'baseplate' rests on a rigid base or anvil,
while the other workpiece called 'flyer plate' is inclined at a pre-selected
angle (usually around 5°) to the base plate as shown in figure .
• A buffer usually made of a rubber or cardboard is placed above the flyer
plate to prevent the surface damage of the flyer plate due to the
detonation of explosives.
• An explosive material (TNT, RDX) in the form of a sheet is placed above
the buffer and is ignited from its lower edge.
• As the detonation force progresses across the flyer plate, a very high
compressive stress wave in the order of thousands of MPa sweeps across
the surface of the plate.
• This causes the flyer plate to move rapidly towards the base plate so that
the impact creates a joint and completes the weld.
• The entire operation is carried out in a chamber to prevent any accident
caused due to the detonation of explosives.
Advantages
• Bond strength of weld metals is very high.
• Edge preparation is not required.
• No melting of base metal.
• No filler metal is required.
• Dissimilar metals can be joined easily
Disadvantages
• Storage and use of explosives are dangerous.
• Detonation of explosives can damage the workpieces. Hence, work pieces with
high impact resistance only are suitable for this process.
• Not suitable for thick plates, as they require higher detonation velocities.
Applications
• Used for cladding (shell) of metals for the purpose of corrosion prevention.
• Also, dissimilar metals such as titanium to steel, aluminum to steel etc., can be
successfully welded with this process.
THERMIT WELDING
• Thermit welding or alumino-thermit welding is a fusion welding process in which
the workpieces are joined by the heat obtained from a chemical reaction of the
thermit mixture.
• Pressure may or may not be applied during the process.
• The thermit mixture is a mixture of iron oxide and aluminum powder, and when
this mixture is brought to its ignition temperature of about 1200°C, reaction
starts, producing molten iron and slag (A1203) releasing enormous amount of
heat.
3 Fe304 + 8 Al-> 9 Fe + 4 A1203 + heat
• The molten iron (Fe) obtained is poured into the cavity (gap between the two
workpieces) and upon solidification, complete fusion takes place.
• Figure shows the welding of rail joint (I-section) using thermit welding process.
Description and Operation
• The edges of the workpiece are cut flat and cleaned to remove dirt, grease and
other impurities to obtain a sound weld.
• A gap of about 1.5-6 mm is left between the edges of the two workpieces.
• A wax heated to its' plastic state is poured in the gap between the workpieces to
be joined and allowed to solidify.
• Excess wax solidified around
the joint is removed.
Description and Operation ( Continued…..)
• A mould box is placed around the joint and packed with sand providing necessary
gates and risers.
• A hole or heating gate is made in the mould connecting to the joint as shown in
figure.
• The wax material is melted out by means of a flame directed into the heating
gate, so that it leaves a cavity at the joint which will later be occupied by the
molten metal.
• The heating gate is then closed with a sand core or iron plug.
• A crucible containing the thermit mixture is suspended above the pouring cup.
• The mixture is ignited by lighting a magnesium ribbon or sparkler.
• Exothermic reaction occurs to form molten iron and slag which floats at the top.
• The temperature resulting from this reaction is approximately 2500°C.
• The reaction is allowed a specific time (around 60 seconds) and the slag is
removed from the surface of the molten metal.
• The plug at the bottom of the crucible is opened and the molten metal is poured
into the cavity.
• The molten metal acts as a filler metal, melts the edges of the joint and fuses to
form a weld.
• After the weld joint cools and solidifies, the mould is broken, risers are cut and
the joint is finished by machining and grinding.
Advantages
• Heat required for welding is obtained from the chemical reaction of the
thermit mixture. Hence, no costly power supply is required.
• The process is best suitable, particularly in remote locations where
sophisticated welding equipments and power supply cannot be arranged.
Disadvantages
• Process is applicable only to ferrous metal parts.
• Process consumes more time.
Applications
• Used in repair and welding of large forgings and castings, pipes, mill
housings and heavy rail sections.
LASER WELDING
• Laser welding or laser beam welding is a radiant energy welding process in which
the workpieces are joined by the heat obtained from the application of a
concentrated coherent (Consistent) light beam impinging upon the surfaces to be
joined.
• Figure shows the principle
and working of a laser beam
welding.
The operation is similar to
that, when the sun rays are
made to concentrate on a
single spot on a paper which
is placed below a magnifying
lens, the paper burns. The
laser energy source is a
refinement of this process
Theory
• The term 'LASER' stands for 'Light Amplification by Stimulated Emission of
Radiation'.
• The laser is a device that produces a light beam with the following properties:
– The light is nearly monochromatic (single wave length).
– The light is coherent with waves exactly in phase with one other.
– The laser beam is extremely intense (powerful).
– The laser light is highly collimated. It could travel a distance of about 3/4th of
million kilometers without any deviation.
– By virtue of the above properties, lasers find application in welding a variety
of materials.
Description and Operation
• Laser beam equipment consists of a cylindrical ruby crystal with both the ends
made absolutely parallel to each other.
• Ruby is aluminum oxide (Al02) with chromium dispersed throughout it.
• One of the end faces of the ruby crystal is highly silvered so that it reflects nearly
96% of the incident light.
• In order to tap the laser output, the other end face of the crystal is partially
silvered and contains a small hole through which the laser beam emerges.
• The ruby crystal is surrounded by a helical flash tube containing inert gas 'xenon'
which itself in turn is surrounded by a 'reflector' to maximize the intensity of the
incident light on the ruby crystal.
• The flash tube converts electrical energy into light energy.
• Cooling system, either gas or liquid is provided to protect the ruby crystal from
the enormous amount of heat generated.
• When the flash tube is connected to a pulsed high voltage source, xenon
transforms the electrical energy into white light flashes (light energy).
• As the ruby is exposed to the intense light flashes, the chromium atoms of the
crystal are excited and pumped to a high energy level.
• These chromium atoms immediately drop to an intermediate energy level with
the evolution of heat and eventually drop back to their original state with the
evolution of a discrete quantity of radiation in the form of red fluorescent light.
• As the red light emitted by one excited atom hits another excited atom, the
second atom gives off red light which is in phase with the colliding red light wave.
• The effect is enhanced as the silvered ends of the ruby crystal cause the red light
to reflect back and forth along the length of the crystal.
• The chain reaction collisions between the red light wave and the chromium
atoms becomes so numerous that, finally the total energy bursts and escapes
through the tiny hole as a 'LASER BEAM'.
• The laser beam is focused by an optical focusing lens on to the spot to be
welded.
• Optical energy as it impacts the workpiece is converted into heat energy.
• Due to the heat generated, the material melts over a tiny area and upon cooling,
the material within becomes homogeneous solid structure to make a stronger
joint.
Advantages
• Similar and dissimilar metals can be welded easily.
• Laser beam can be controlled to a great precision and hence, the welding spots
could also be located precisely. Certain locations in the material that are difficultto-reach can be welded easily by this process.
• Heating and cooling rates are much higher in this process. Also, heat affected
zone is very small. Hence, the process is ideal for locations which are surrounded
by heat sensitive components.
• Clean weld joints can be obtained by this process.
Disadvantages
• Slow welding speeds (25 - 250 mm/min).
• Rapid cooling rate cause problems such as cracking in high carbon steels.
• High equipment costs.
Applications
• Used in electronics industry for applications such as connecting wire leads to
small electronic components, to weld medical equipments, transmission
components in automobiles.
ELECTRON BEAM WELDING
• Electron beam welding is a radiant energy welding process in which the
workpieces are joined by the heat obtained from a concentrated beam composed
primarily of high-velocity electrons impinging on the surfaces to be joined.
• Figure shows the schematic of an electron beam welding.
Description and Operation
• The system consists of an electronic gun and a vacuum chamber inside which the
workpieces to be joined are placed.
• The electronic gun emits and accelerates the beam of electrons and focuses it on
the workpieces.
• When a tungsten filament is electrically heated in vacuum to approximately
2000°C, it emits electrons.
• The electrons are then accelerated towards the hollow anode by establishing a
high difference of voltage potential between the tungsten filament and a metal
anode.
• The electrons pass through the anode at high speeds (approximately half the
speed of light), then collected into a concentrated beam and further directed
towards the workpiece with the help of magnetic forces resulting from focusing
and deflection coils.
• The highly accelerated electrons hit the base metal and penetrate slightly below
the base surface.
• The kinetic energy of the electrons is converted into heat energy.
• The succession (Series) of electrons striking at the same place causes the
workpiece metal to melt and fuse together.
• It should be noted that, the greater the kinetic energy of the electrons, the
greater is the amount of heat released.
• Since electrons cannot travel well through air, they are made to travel in vacuum
which is the reason for enclosing the electron gun and the workpiece in a vacuum
chamber.
Advantages
• Any metals, including zirconium, beryllium or tungsten can be easily welded.
• High quality welds, as the operation is carried in vacuum.
• Concentrated beam minimizes distortion.
• Cooling rate is much higher.
• Heat affected zone is less.
• Shielding gas, flux or filler metal is not required.
Disadvantages
• High capital cost.
• Extensive joint preparation is required.
• Vacuum requirements tend to limit the production rate.
• Size of the vacuum chamber restricts the size of the workpiece being welded.
• Not suitable for high carbon steels. Cracks occur due to high cooling rates.
Applications
• Used in electronic industries, automotive and aircraft industries where the
quality of weld required forms the decisive factor.