Fusion welding is a generic term for welding processes that rely on melting to join materials of similar compositions and melting points.[1] Due to the high-temperature phase transitions inherent to these processes, a heat-affected zone is created in the material[1]: 755 (although some techniques, like beam welding, often minimize this effect by introducing comparatively little heat into the workpiece[2]).
In contrast to fusion welding, solid-state welding does not involve the melting of materials.
In the great majority of applications, the interatomic bond is established by melting. When the workpiece materials (base or parent materials) and the filler (if used at all) have similar but not necessarily identical compositions and melting points, the process is referred to as fusion welding or simply welding.