Spot welding
A resistance spot welding process where sheet metals are bonded locally through electrical current flow and mechanical contact pressure. It forms the backbone of automotive body manufacturing.
In spot welding, you press the overlapping sheets between two copper electrodes and send a short, strong current pulse through the contact point. The electrical resistance there heats the material so much that it melts locally and forms a weld nugget. The electrodes must be clean and the sheets bare, otherwise the current will scatter or the joint won't hold. If the current is too low or the time too short, the weld won't hold; with too much current or pressure, you burn through or get deep indentations. A clean spot is evenly round with a slight bulge – you hear and see that right at the machine.