Resistance welding refers to a group of welding processes that produce coalescence of faying surfaces where heat to form the weld is generated by the resistance of the flow of welding current through the workpieces. Some factors influencing heat or welding temperatures are the proporties of the workpieces, the electrode materials, electrode geometry, electrode force, weld current and weld time, etc. Small pools of molten metal are formed at the weld area as a high amount of current (1000–100 000 A) 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 resistance welding method used to join two to four overlapping metal sheets which are up to 3 mm thick each. In some applications with only two overlapping metal sheets, the sheet thickness can be up to 6 mm. 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. Spot welding is used in preference to more costly mechanical fastening, such as riveting and screwing, when disassembly for maintenance is not required. While weld strength at each weld spot is high, the fact that the weld spots do not form a continuous seam means that the overall strength is often 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, can be 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. Other resistance welding methods include flash welding, projection welding, and upset welding.