Resistance Spot Welding
In this process, a spot of weld is made between overlapping sheets by means of two cylindrical copper alloy electrodes, one on top and the other at the bottom, which carry a high current. The electrodes also clamp the work and apply pressure when the metal at the joint gets sufficiently heated by electrical resistance. A tiny button of fused metal results at the sheet interface and it is called nugget. The electrodes are retracted after the weld is completed. The process is used in large scale in automobile production.