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.
There are at least seven important resistance-welding processes. These are flash welding,
high-frequency resistance welding, percussion welding, projection welding, resistance
seam welding, resistance spot welding, and upset welding. They are alike in many
respects but are sufficiently different.
Resistance spot welding (RSW) is a resistance welding process which produces
coalescence at the faying surfaces in one spot by the heat obtained from resistance
to electric current through the work parts held together under pressure by electrodes.
The size and shape of the individually formed welds are limited primarily
by the size and contour of the electrodes. The equipment for resistance spot
welding can be relatively simple and inexpensive up through extremely
large multiple spot welding machines. The stationary single spot welding
machines are of two general types: the horn or rocker arm type and
the press type.
The horn type machines have a pivoted or rocking upper electrode arm, which
is actuated by pneumatic power or by the operator`s physical power.
They can be used for a wide range of work but are restricted to 50 kVA and
are used for thinner gauges. For larger machines normally over 50 kVA,
the press type machine is used. In these machines, the upper electrode
moves in a slide. The pressure and motion are provided on the upper
electrode by hydraulic or pneumatic pressure, or are motor operated.
For high-volume production work, such as in the automotive industry, multiple
spot welding machines are used. These are in the form of a press on which
individual guns carrying electrode tips are mounted. Welds are made in a
sequential order so that all electrodes are not carrying current at the same time.
Projection welding (RPW) is a resistance welding process which produces
coalescence of metals with the heat obtained from resistance to electrical
current through the work parts held together under pressure by electrodes.
The resulting welds are localized at predetermined points by projections,
embossments, or intersections. Localization of heating is obtained by a
projection or embossment on one or both of the parts being welded.
There are several types of projections: (1) the button or dome type,
usually round, (2) elongated projections, (3) ring projections, (4)
shoulder projections, (5) cross wire welding, and (6) radius projection.
The major advantage of projection welding is that electrode life is
increased because larger contact surfaces are used. 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.
Resistance seam welding (RSEW) is a resistance welding process
which produces coalescence at the faying surfaces the heat obtained from
resistance to electric current through the work parts held together
under pressure by electrodes.
The resulting weld is a series of overlapping resistance spot welds
made progressively along a joint rotating the electrodes.
When the spots are not overlapped enough to produce gaslight welds it is
a variation known as roll resistance spot welding. This process
differs from spot welding since the electrodes are wheels.
Both the upper and lower electrode wheels are powered.
Pressure is applied in the same manner as a press type welder.
The wheels can be either in line with the throat of the machine or
transverse. If they are in line it is normally called a longitudinal
seam welding machine. Welding current is transferred through the
bearing of the roller electrode wheels. Water cooling is not provided
internally and therefore the weld area is flooded with cooling water
to keep the electrode wheels cool.
In seam welding a rather complex control system is required. This involves
the travel speed as well as the sequence of current flow to provide for
overlapping welds. The welding speed, the spots per inch, and the timing
schedule are dependent on each other. Welding schedules provide the pressure,
the current, the speed, and the size of the electrode wheels.
This process is quite common for making flange welds, for making watertight
joints for tanks, etc. Another variation is the so-called mash seam welding
where the lap is fairly narrow and the electrode wheel is at least twice as wide as
used for standard seam welding. The pressure is increased to approximately 300
times normal pressure. The final weld mash seam thickness is only 25% greater
than the original single sheet.
Flash Welding (FW) is a resistance welding process which produces
coalescence simultaneously over the entire area of abutting surfaces,
by the heat obtained from resistance to electric current between the
two surfaces, and by the application of pressure after heating is
Flashing and upsetting are accompanied by expulsion of metal from the joint.
During the welding operation there is an intense flashing arc and heating
of the metal on the surface abutting each other. After a predetermined
time the two pieces are forced together and coalescence occurs at the interface,
current flow is possible because of the light contact between the two parts being
The heat is generated by the flashing and is localized in the area between
the two parts. The surfaces are brought to the melting point and expelled through
the abutting area. As soon as this material is flashed away another small arc is
formed which continues until the entire abutting surfaces are at the melting
temperature. Pressure is then applied and the arcs are extinguished and
Upset welding (UW) 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 contact.
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. It can be used only
if the parts to be welded are equal in cross-sectional area.
The abutting surfaces must be very carefully prepared to provide
for proper heating.
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 completed.
Percussion welding (PEW) is a resistance welding process which
produces coalescence of the abutting members using heat from an arc
produced by a rapid discharge of electrical energy.
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. It is more complex than the other
two processes in that heat is obtained from an arc 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. The advantage of the
process is that there is an extremely shallow depth of heating and
time cycle is very short. It is used only for parts with fairly small
High frequency resistance welding (HFRW) is a resistance
welding process which produces coalescence of metals with the
heat generated from the resistance of the work pieces to a
high-frequency alternating current in the 10,000 to 500,000
hertz range and the rapid application of an upsetting force
after heating is substantially completed. The path of the current
in the work piece is controlled by the proximity effect.
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. 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.
Current is introduced by means of sliding contacts at the edge of the joint.
The high-frequency welding current flows along one edge of the seam to the
welding point between the pressure rolls and back along the opposite edge
to the other sliding contact.
The current is of such high frequency that it flows along the metal surface
to a depth of several thousandths of an inch. Each edge of the joint is the
conductor of the current and the heating is concentrated on the surface of
these edges. At the area between the closing rolls the material is at the
plastic temperature, and with the pressure applied, coalescence occurs.