This article discusses three distinct mechanisms of bonding for solid-state (forge) welding processes, namely, contaminant displacement/interatomic bonding, dissociation of retained oxides, and decomposition of the interfacial structure. It explains the processes that can be characterized as having two stages: heating and forging. The article also includes a table that illustrates weld strengths as a function of annealing temperature for a range of materials.

Friction welding (FRW) is a solid-state welding process in which the heat for welding is produced by the relative motion of the two interfaces being joined. This article provides an outline of the mechanisms of friction heating and discusses the two principal FRW methods: direct-drive welding and inertia-drive welding. It summarizes the similar and dissimilar metals that can be joined by FRW and discusses the metallurgical considerations that govern the properties of the resulting weld.