In spot welding, the parts to be joined are normally over lapped. The work piece are clamped between two water cooled copper electrodes. On the passage of a high transient current, the interface melts over a spot and form the weld. The cooling at the electrode limits the size of the spot. A very high current is needed for a very short duration to complete the welding. The interfaces to be joined are initially cleaned by various methods, including scratch brushing and vapour decreasing. A spot weld normally contains some porosity at the weld centre, which unless excessive is harmless.
The spot welding process is difficult to use for highly conductive materials such as aluminum and magnesium.