Resistance welding involves the generation of heat by passing current
through the resistance caused by the contact between two or more metal surfaces.
Small pools of molten metal are formed at the weld area as high amounts of
current (1000-100000A) is passed through the metal. In general, resistance
welding methods are efficient and cause little pollution, but their applications
are somewhat limited and the equipment cost can be high.
Spot welding is a popular welding method used to join overlapping metal
sheets of up to 3mm thick. Two electrodes are simultaneously used to clamp the
metal sheets together and to pass current through the sheets. The advantages of
the method include efficient energy use, limited workpiece deformation, high
production rates, easy automation, and no required filler materials. Weld
strength is significantly lower than with other welding methods, making the
process suitable for only certain applications. It is used extensively in the
automotive industry – ordinary cars can have several thousand spot welds. A
specialized process, called shot welding, is used to spot weld stainless steel.
Like spot welding, seam welding relies on two electrodes to apply pressure
and current to join metal sheets. However, instead of pointed electrodes,
wheel-shaped electrodes roll along and often feed the workpiece, making it
possible to make long continuous welds. In the past, this process was used in
the manufacture of beverage cans, but now its uses are more limited.