16. Solid State Welding

The welding processes that produce sound joints at temperatures much below the parent metal melting temperature are referred to as solid state welding. Since there is no melting taking place, solid-state welded joints are usually free from all the defects that may occur in case of fusion welding, like, porosity, slag inclusion, hot cracking, undercut, etc. At the same time it does not require any shielding medium as well as any kind of filler metal. Using solid state welding, dissimilar materials can also be effectively joined which may not be possible through fusion welding. Friction stir welding is one such method of solid state welding. In the heart of this method lies a cylindrical shouldered tool with a profiled pin. The pin while rotating is inserted into the joint line between the two pieces, clamped rigidly to a base plate. The welds are created by the combined action of frictional heating and mechanical deformation due to the rotating tool. The majority of the heat is generated from the friction. It softens the material but does not attain the melting temperature. The pin traverses the entire joint resulting in the required bonding.