Resistance welding is a group of welding processes in which
coalescence is produced by the heat obtained from resistance of the
work piece to electric current in a circuit of which the work piece is a
part and by the application of pressure .
In resistance welding a low voltage (typically 1 V and very high current
(typically 15000 A is passed through the joint for very short time typically
This high temperature heats the joint due to contact resistance at the
joint and melts it.
The pressure on joint is continuously maintained and metal fuses
together under this pressure.
The heat generated in resistance welding is expressed as
Where H= total heat generated in work J
t=time for which the electric current is passing through joint,s
R= resistance of the joint,ohms
k=constant on account of heat losses from the welded joint.
The resistance of the joint R is a complex factor to know as it is
(a)resistance of electrode
(b)the contact resistance between workpiece and electrode
(c )contact resistance between two workpiece plates
(d)the resistance of workplates
The amount of heat released is directly proportional to
The main requirement of process is the low voltage and high
current which is obtained by means of step down transformer.
The electric supply is to be closely controlled so that heat
released is just enough to melt the joint and the subsequent
fusion takes place due to force (forge welding) on the joint
The force required can be joined either mechanically or
hydraulically or pneumatically
Variables in Resistance welding
Contact resistance between two workpiece plates
and their resistance themselves. Contact resistance
is affected by
1. Surface finish on plates since rougher surface
have higher contact resistance
2.Cleanliness of the surface. Oxides and other
contaminants if present should be removed before
resistance welding is done.
Forging Pressure It decreases contact resistance
and there by increases the welding current for
The important characteristic of Resistance welding
is the Transfer of heat to two parts being joined
differently so that proper fusion is obtained for
dissimilar metal or of different thickness.
When two dissimilar metals with different electrical
resistively or thermal conductivity are joined the heat
liberated in lower resistively metal is less and
therefore it is necessary to use electrode of higher
resistively near metal or use electrode diameter of
larger contact area near metal which has higher
Electrodes For resistance
The function of electrodes are:
1.Carry high current required for fusion
2.Transmit the mechanical force to keep plates
under pressure and in alignment during fusion (high
3.Helps to remove heat from weld zone thus
preventing over heating and surface fusion of work.
Hence electrodes should have
High electrical conductivity
Pure Copper----- Low strength
conductivity but low strength hence used for
welding of non ferrous metals like Al, Mg
conductivity and high strength used for low
strength steels as mild steel ,low alloy steel
Cu+(Co+Be)--- Low conductivity but high
strength hence used for heat resisting alloys
as stainless steel, steel with tungsten and
other alloying elements.
Electrode tips are available in number of
different configuration for different welding
Since large amount of heat is liberated in
electrode it is necessary to cool electrodes to
maintain their strength. Hence cooling water
is circulated through electrodes.
Advantages of Resistance
Very little skill is required to operate the
resistance welding. Mostly all m/cs are
semiautomatic or automatic
Suitable for mass production and gives high
No consumable are used except electric
power and small electrode wear
Heating of w/p is confined to very small area
so less distortion
Welding m/c is complex with various elements as
transformers, electrodes and heavy conductors for
carrying high current. And m/c is expensive
Certain resistance welding machines are limited to
lap joints. lap joints have inherent crevices which are
responsible for stress concentration and loss of
fatigue life. It also increases chances of corrosion.
The electrode point should always be kept clean. If
points are dirty or scaly there will be excessive heat
in these points and cause burning or splitting.
Various types of Resistance
Seven important resistance-welding processes.
1. Resistance spot welding,
2. Resistance seam welding
3. Projection welding
4. Flash welding
5. Upset welding
6.High-frequency resistance welding,
Spot welding is a type of resistance welding used to
weld various sheet metals. Typically the sheets are in
the 0.5-3.0 mm thickness range.
The process uses two copper alloy electrodes to
concentrate welding current into a small "spot" and to
simultaneously clamp the sheets together.
Forcing a large current through the spot will melt the
metal and form the weld.
The attractive feature of spot welding is a lot of energy
can be delivered to the spot in a very short time (say a
few hundred milliseconds). That permits the welding to
occur without excessive heating to the rest of the sheet.
Diameter of the weld spot is in the range 1/8” - 1/2” (3 12 mm).
The amount of heat (energy) delivered to the spot is
determined by1. the resistance between the electrodes
2. amplitude of current
3. and duration of the current.
The amount of energy is chosen to match the sheet's
2.its thickness, and
3. type of electrodes.
Applying too little energy won't melt the metal or will
make a poor weld.
Applying too much energy will melt too much metal make
a hole rather than a weld.
Energy delivered to the spot can be controlled to
produce reliable welds.
Function of Electrode
The electrodes do not take part in
They keep the parts aligned and in place.
They apply the pressure required for developing the
correct surface resistance at the interface, for
containing the molten metal avoiding weld expulsion
and for forging the nugget near the end of the cycle.
They convey the electric welding current and, being
water cooled, they also dissipate excess heat once
the weld is completed, avoiding surface melting.
The diameter of electrode determines the diameter
of fusion zone The diameter of fusion zone is given
De = electrode diameter
T=thickness of plate.
The Squence of Resistance spot
welding consist of
1.Squeeze Time : It is time required for electrode to
align and clamp two workpiece together under them
and provide necessary electrical contact.
2.Weld Time: is time of current flow through
workpiece till they are heated to melting
3.Hold time: is time when pressure is maintained on
molten metal w/o electric current. During this time
pieces are expected to forge weld
4.Off time: is time during which pressure on
electrodes is taken off so that plates can be
positioned for next spot.
Forging Pressure is applied to ensure that
during the solidification nugget does not
develop any porosity or crack. Porosity may
also develop if forging pressure is released
Forging pressure depends on
(i)Thickness of metal
(ii)Goemetry of weld.
1.Too high current will cause
Weld explusion,cavitation and weld cracking
Reduced mechanical properties
Electrode embedment in the surface
2. Too Low Current
Unfused surface and poor weld.
More time of application of current may lead
to boiling, porosity and growth of nugget upto
It is non labor-intensive and can be easily
Adaptable to a wide variety of electrically
Applicable to a variety of thicknesses
Very short cycle times
A robust process
Tolerant to fit-up variations
In the automobile manufacturing industry,
where it is used almost universally to weld
the sheet metal to form a car. Spot welders
can also be completely automated, and many
of the industrial robots found on assembly
lines are spot welders (the other major use
for robots being painting).
Spot welding straps to nickel-cadmium or
nickel-metal-hydride cells in order to make
Fast Welding speed
Low cost per weld if large production is done
Less operational skills required.
1.Limited to metal that range in thickness from
2.Mats with high thermal and electrical conductivity are
difficult to weld by Spot welding.
3. Spot weld should not be made too close to the edge
otherwise metal will become overheated and
cracking will result around the weld.
Resistance Seam Welding
Seam Welding is a Resistance Welding (RW)
process of continuous joining of overlapping sheets
by passing them between two rotating electrode
wheels.It is specialised process of spot welding.
Here disc electrodes are used which are continoulsy
rotated so that w/p gets advanced underneath them
while at same time pressure on joint is maintained.
Heat generated by the electric current flowing
through the contact area and pressure provided by
the wheels are sufficient to produce a leak-tight
The current is applied through heavy copper
electrodes in series of pulses at proper
intervals. The timing is adjusted so that
pulses overlap each other and form a
continuous seam joint.
Seam welding is restircted to welding of thin
materials thickness range from
Can be carried out for steels, Al, Mg and Ni
alloys but not for Copper or its alloys
Adaptable to a variety of electrically
Ability to produce leak-tight welds
Fast processing times
Projection welding is a variation of resistance
welding in which current flow is concentrated at the
contact surfaces of interest by an embossed, cold
headed, or machined projection.
The projections are generally very small of theorder
of 0.8mm and are obtained by means of embossing.
The projection(s) effectively localize the current,
forcing the parts to heat predominately at the mating
This rapid interfacial heating allows for the
application of resistance projection welding across a
wide range of applications not feasible by
conventional resistance spot welding.
The advantages of projection welding include
It is versatile process
high speed and ability to automate,
the ability to make a number of welds
minimization of marking on one side of joints in
Welds can be placed closer than possible in spot
The electrodes can be made to take shape of
Proper heat balance can be easily obtained in projection welding
by making projection in thicker plate while welding sheet of
different thicknesses. Ability to join sheets of widely dissimilar
thicknesses For welding dissimilar metals the projections are
made on material having higher electrical conductivity to provide
proper heat balance.
Because of larger sized electrodes used the life of electrodes is
much longer than that of spot welding electrodes
Can be used to minimize surface marking when joining sheets
Increased energy efficiency
Minimal cycle time
Due to the efficiency of power transfer, thicker materials can be
successfully welded. Materials as thick as 3 mm (0.125 in) can
be successfully welded
Solid state welding
It is a group of welding processes which
produces coalescence at temperatures
essentially below the melting point of the
base materials being joined, without the
addition of brazing filler metal. Pressure may
or may not be used.
Cold Welding (CW)
Cold welding is a solid state welding process which
uses pressure at room temperature to produce
coalescence of metals with substantial deformation
at the weld.
Welding is accomplished by using extremely high
pressures on extremely clean interfacing materials.
Sufficiently high pressure can be obtained with
simple hand tools when extremely thin materials are
When cold welding heavier sections a press is
usually required to exert sufficient pressure to make
a successful weld
Indentations are usually made in the parts
being cold welded. The process is readily
adaptable to joining ductile metals.
Aluminum and copper are readily cold
welded. Aluminum and copper can be joined
together by cold welding.
Flash welding is a development of resistance
butt welding and is particularly suitable for
butt welding complex or larger sections.
It is used for a wide range of component
shapes and sizes from bicycle wheel rims to
In flash welding, the components are clamped between
dies and brought together slowly with the current
switched on, see Fig.1.
In this process ends of rods (tubes, sheets) are heated
and fused by an arc struck between them and then
forged (brought into a contact under a pressure)
producing a weld.
The welded parts are held in electrode clamps, one of
which is stationary and the second is movable
After a pre-set material loss has occurred, sufficient to
heat the material behind the interface to its plastic state,
the components are forged together to expel melted
material and contaminants.
Careful setting up and control of the welding sequence,
particularly flashing and upset, are required to ensure
freedom from weld discontinuities.
Automatic monitoring of process parameters, particularly
current, force and displacement, can be used to verify
conformance with the set sequence, and provide a guide
to weld quality.
Flash welds in steels have a low impact toughness
performance and require post-weld heat treatment for
applications where impact toughness is specified.
Certain weld interface flaws, such as flat spots, are not
readily detected by non-destructive testing techniques
Flash welding is ideally suited to producing butt
welds in large or complex sections.
Weld time is relatively short, from a few seconds for
the thinnest sections to a few minutes for the
Flash Welding can be used for joining many ferrous
and nonferrous alloys and combinations of dissimilar
metals. Practically Flash-welding can be applied to
any metal that can be forged
Economical in operation and in the use of
Suitable for mass production.
Unskilled workforce needed.
Strong welds obtained.
Good fatigue properties available.
No special symmetry requirements (different
from for Friction welding).
Removal of flash required (same as for Friction welding).
Process may affect or remove any strain hardening (cold
Heat treatment after Flash-welding may be needed to
develop full properties.
Costly maintenance of equipment due to flashing.
Fire hazards present.
Electric power and upsetting force in available equipment
limit the weldable size.
Heat unbalance from different materials may cause
Shunt effect for closed rings rolled and flash welded may
High accuracy alignment is required.
Upset welding (UW)
It is a resistance welding process which produces
coalescence simultaneously over the entire area of
abutting surfaces or progressively along a joint, by
the heat obtained from resistance to electric current
through the area where those surfaces are in
Pressure is applied before heating is started and is
maintained throughout the heating period
The equipment used for upset welding is very
similar to that used for flash welding
The difference from flash welding is that the parts
are clamped in the welding machine and force is
applied bringing them tightly together.
High-amperage current is then passed through the
joint, which heats the abutting surfaces.
When they have been heated to a suitable forging
temperature an upsetting force is applied and the
current is stopped.
The high temperature of the work at the abutting
surfaces plus the high pressure causes coalescence
to take place.
After cooling, the force is released and the weld is
Many different shapes like wire, bar, strip
and tubing of various materials can be
joined end to end by Upset-welding
Ease of parameters control (only current, time and
High quality, absence of typical fusion defects
Metallurgical properties comparable to those of hot
Simple, sturdy and reliable equipment operated by
Tolerance for minor alloy deviations
Large selection of materials, including difficult to
Equipment generally suitable to one type of
High Frequency current in range of 300-1000KHz is
applied to the welding area and squeeze force is added
to workpiece heated from the heat resistance.
In this process the high frequency welding current is
introduced into the metal at the surfaces to be welded
but prior to their contact with each other.
The High Frequency electric resistance welding method
is classified, depending on method applied in application
of High frequency current on workpiece:
1.High frequency Induction
2.High Frequency Resistance Welding.
In High Frequency Induction ERW method
Induction coil is used in inducting high
frequency current to generate heat.
It is used for joining small diameter steel
In High Frequency Resistance Welding,
contactor is applied to work piece to directly
provide current. Used for large diameter steel
This process is ideally suited for making pipe,
tubing, and structural shapes.
It is used for other manufactured items made
from continuous strips of material.
Percussion Welding uses electrical energy
stored in a condenser to produce an intense
momentary power discharge to provide the
localized heating at the interface.
It is suitable for joining dissimilar metals that
are not weldable by flash butt welding, or
when flash is not desirable at the weld joint
Pressure is applied progressively during or
immediately following the electrical discharge. This
process is quite similar to flash welding and upset
welding, but is limited to parts of the same geometry
and cross section
Arc is produced at the abutting surfaces by the very
rapid discharge of stored electrical energy across a
rapidly decreasing air gap.
This is immediately followed by application of
pressure to provide an impact bringing the two parts
together in a progressive percussive manner.
There is an extremely shallow depth of
heating and time cycle is very short.
It is used only for parts with fairly small
It is a solid state welding process which
produces coalescence of the faying surfaces
by the application of pressure and elevated
The process does not involve microscopic
deformation melting or relative motion of the
The process is used for joining refractory metals at
temperatures that do not affect their metallurgical
Successful welds have been made on refractory metals
at temperatures slightly over half the normal melting
temperature of the metal.
To accomplish this type of joining extremely close
tolerance joint preparation is required and a vacuum or
inert atmosphere is used.
The process is used quite extensively for joining
The process is considered diffusion brazing when a layer
of filler material is placed between the faying surfaces of
the parts being joined.
These processes are used primarily by the aircraft and
The characteristic process parameters are
the welding temperature,
the welding time.
To get a surface free of oxidation the process
is done in vacuum or inert gas atmosphere.
It is a solid state welding process in which
coalescence is effected by high-velocity
movement together of the parts to be joined
produced by a controlled detonation.
Even though heat is not applied in making an
explosion weld it appears that the metal at
the interface is molten during welding.
Explosion welding creates a strong weld
between almost all metals.
It has been used to weld dissimilar metals
that were not weldable by the arc processes.
The weld apparently does not disturb the
effects of cold work or other forms of
mechanical or thermal treatment .
The strength of the weld joint is equal to or
greater than the strength of the weaker of the
two metals joined.
One of the most widely used applications of
explosion welding has been in the cladding of
base metals with thinner alloys.
Joining of tube-to-tube sheets for the
manufacture of heat exchangers.
The process is also used as a repair tool for
repairing leaking tube-to-tube sheet joints.
Another and new application has been the
joining of pipes in a socket joint.
It is a solid state welding process which produces
coalescence of metals by heating them in a forge
and by applying pressure or blows sufficient to
cause permanent deformation at the interface
This is one of the older welding processes and at
one time was called hammer welding.
Forge welds made by blacksmiths were made by
heating the parts to be joined to a red heat
considerably below the molten temperature
It is a solid state welding process which produces
coalescence of materials by the heat obtained from
mechanically-induced sliding motion between
The work parts are held together under pressure.
This process usually involves the rotating of one
part against another to generate frictional heat at the
When a suitable high temperature has been
reached, rotational motion ceases and additional
pressure is applied and coalescence occurs.
Ability to produce high quality welds in a short
No filler metal is required and flux is not used.
The process is capable of welding most of
the common metals.
It can also be used to join many
combinations of dissimilar metals
Factors affecting Friction
There are three
Welding important factors involved in making a friction
The rotational speed which is related to the material to be
welded and the diameter of the weld at the interface.
The pressure between the two parts to be welded. Pressure
changes during the weld sequence. At the start it is very low, but
it is increased to create the frictional heat. When the rotation is
stopped pressure is rapidly increased so that forging takes place
immediately before or after rotation is stopped.
The welding time. Time is related to the shape and the type of
metal and the surface area. It is normally a matter of a few
seconds. The actual operation of the machine is automatic and is
controlled by a sequence controller which can be set according
to the weld schedule established for the parts to be joined
Application Of Friction
Lightweight tanks from aluminium alloys or
aluminium based metal matrix composites (MMCs)
Military bridges and amphibious personnel carriers
Steel tanks that could be friction stir welded from
extra high strength armour plate using two passes
from two sides after further tool and parameter
Titanium lightweight field howitzers
Hot pressure welding
It is a solid state welding process which produces
coalescence of materials with heat and the
application of pressure sufficient to produce
macro-deformation of the base metal.
In this process coalescence occurs at the interface
between the parts because of pressure and heat
which is accompanied by noticeable deformation.
The deformation of the surface cracks the surface
oxide film and increases the areas of clean metal.
Welding this metal to the clean metal of the abutting
part is accomplished by diffusion across the
interface so that coalescence of the faying surface
This type of operation is normally carried on
in closed chambers where vacuum or a
shielding medium may be used. It is used
primarily in the production of weldments for
the aerospace industry
It is a solid state welding process which
produces coalescence of metals by heating
and by applying pressure with rolls sufficient
to cause deformation at the faying surfaces.
This process is similar to forge welding
except that pressure is applied by means of
rolls rather than by means of hammer blows.
Coalescence occurs at the interface between
the two parts by means of diffusion at the
One of the major uses of this process is the
cladding of mild or low-alloy steel with a
high-alloy material such as stainless steel.
It is also used for making bimetallic materials
for the instrument industry.
It is a solid state welding process which
produces coalescence by the local
application of high-frequency vibratory energy
as the work parts are held together under
Welding occurs when the ultrasonic tip or
electrode, the energy coupling device, is
clamped against the work pieces and is made
to oscillate in a plane parallel to the weld
The combined clamping pressure and
oscillating forces introduce dynamic stresses
in the base metal.
This produces minute deformations which
create a moderate temperature rise in the
base metal at the weld zone.
This coupled with the clamping pressure
provides for coalescence across the interface
to produce the weld.
Ultrasonic energy will aid in cleaning the weld
area by breaking up oxide films and causing
them to be carried away.
This process is used extensively in the
electronics, aerospace, and instrument
industries. It is also used for producing
packages and containers and for sealing