Resistance welding incorporates a group of processes- Spot, Seam, Projection and Butt Welding – in which the heat for welding is generated by the resistance to the flow of electrical current, through the parts being joined. Resistance welding differs from other forms of welding, in that no extraneous materials, such as fluxes, filler rods, etc are used. Resistance welding further differs from the fusion welding process by utilizing the application of mechanical pressure to forge the heated parts together. The effect of pressure is to refine the grain structure, thus producing a weld with physical properties in most cases equal to parent metal and sometimes even superior.

Resistance welding machines and their operation often appear mysterious to the layman, when he sees good welds made so quickly and easily.

In order to better understand the nature of resistance welding and how it is accomplished, it is best to consider a simple spot weld. The basic principle will apply to all other resistance welding and heating methods except Flash butt welding.