Due to rotation and pressure of the tool frictional heat is produced and the parts soften plastically in the area of the rotating tool without reaching the melting point.
The pin of the tool immerses into the malleable material of the parts and produces by its rotation and shape a homogenous blend along the joining line. The shoulder of the tool forms and smoothes the welding line with only a marginal ridge.
After cooling there is a solid phase joint of the two parts. The process does not need filler wire, welding flux or gas shielding and produces a sealed weld without porosity and without inclusions.