Electric resistance welding is a thermo-electric process in which heat is generated at the interface of the parts to be joined by passing an electrical current through the parts for a precisely controlled time under a controlled pressure (also called force). Resistance'' welding derives from the fact that the resistance of the work pieces and electrodes are used in combination or contrast to generate the heat at their interface.
Seam welding is a process that produces a serial of welds between two metals. The seam may be a butterfly joint or an overlap joint. Seam welding can be as roll spot welding whereby there is a certain distance between the welding spot or as a continuous weld whereby the single spots overlapping each other (produce a closed weld). The process differs from spot welding by using welding wheels instead of welding electrodes, produced out of bars.
Butt welding is a process that joins two pieces of material together along a single edge in a single plane. When two sheets of steel are laid side-by-side and joined together along a single joint, this is an example of butt welding.
Spot welding is the most commonly used form of resistance welding. Usually it is used to weld various sheet metals with reduced thickness. The weld is discontinued and limited to one or more spots, where the work pieces usually overlap. It is produced by the generation of heat and pressure, without filler metal.
With projection welding the current will be concentrated by using dents, or projections in one of the work pieces. The heat will be concentrated at the projections, which gives the possibility to weld more welds at a closer spacing or heavier sections. Sometimes projection welding will be used to avoid damage to one of the work piece surfaces due to the fact that flat electrodes are used in this process.