Welding is the joining together of materials (typically metals or thermoplastics), usually by a fusion process. Most commonly, workpieces are welded by melting both of them and adding more molten metal or plastic to form a pool that cools to form a strong joint. The energy to form the joint between metal workpieces most often comes from a flame (e.g. oxyacetylene) or an arc, but welding by laser beam, electron beam and friction processes is well established. Energy for welding of thermoplastics typically comes from direct contact with a heated tool or a hot gas.