Resistance welding are types of welding processes in which welding is achieved by the heat obtained from the electrical resistance of the work piece to be joint.

Obviously, the work piece is a part of the circuit. There are seven important resistance-welding processes as described below.

Resistance spot welding (RSW) - This process produces a welding joint in one spot and the work parts are held together under pressure by welding electrodes. The size and shape of the welds are determined by the size and shape of the electrodes. The spot welding machines are of two general types: the horn or rocker arm, which is used for thinner gauges and the press type, which are used for larger machines.

Projection welding (RPW) - In this process welds are obtained by concentrating heat at points of projections, embossments, or intersections in the work pieces. Types of projections are: the button or dome type, elongated projections, ring projections, shoulder projections, cross wire, and radius projection.

Resistance seam welding (RSEW) - In this process the weld is a series of overlapping spot welds made progressively along a joint rotating the electrodes. Electrodes are in the form of wheels. It has two variations: (1) Roll resistance spot welding, where the spots are not overlapped enough to produce gaslight welds and (2) Mash seam welding where the pressure is 300 times normal pressure, the lap is comparatively narrow and the electrode wheel twice as wide as used for standard seam welding. 

Flash Welding (FW) - In this process welds are produced over the entire area of the welding surfaces by an intense flashing that generates heat and the surfaces are brought to the melting point. As soon as this material is flashed away another small arc is formed, and this continues until the joining surfaces are at the melting temperature. At this stage the surfaces are pressed and welding occurs.

Upset welding (UW) - In this process, welding surfaces are pressed before heating is started and pressure is maintained throughout the heating period. This is used only if the parts to be welded have equal cross-sectional area. A very high current is passed which heats the welding surfaces. When the surfaces reach forging temperature force is applied and the current is stopped. The weld is completed after cooling and releasing the pressure.

Percussion welding (PEW) - This is used only for parts with relatively small cross-sectional areas. This process uses heat from an arc produced by a rapid discharge of electrical energy across a rapidly decreasing air gap between the surfaces to be welded. An impact is created bringing the two parts together in a progressive percussive manner. In this process, heating is very shallow and very short.

High frequency resistance welding (HFRW) - This process uses a very high frequency alternating current, typically in the 10,000 to 500,000 hertz range. Because of such high frequency of current, it flows along the metal surface. As the current flows along the surface, the heating is concentrated on the surface of the welding edges. Welds are formed by application of pressure at the welding areas.