In forge welding, which is used to make steel chains, the two parts are heated and then hammered together.
Electrical methods use an electric current passed through two metal surfaces in close contact.
The temperature rises at the interface because of the high electrical resistance and welds the surfaces together. In spot welding, point contact electrodes press the metal surfaces together. In seam welding, the electrodes are in the form of rollers. Both these methods are used in mass production. Gas welding uses an oxy-acetylene flame to heat the metal and a rod of metallic filler material.