A method of joining metals by melting the two parts together, pressing them together, or both (See also solder). 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. Molten filler material runs between the two heated edges and solidifies to form the joint. Another method, similar to gas welding, is electric-arc welding. The filler rod forms one electrode and the metal itself another. Current passes by arcing or sparking across the gap between them, melting the rod and the metal edges. Arc welding is generally used for thicker pieces of metal and higher temperatures than gas welding and for small delicate jobs lasers have been used. A great deal of skill is required to produce a strong reliable weld.