Friction stir welding/processing (FSW/P) is an innovative solid-state joining/processing technique (Thomas et al. 1991; Mishra and Ma 2005). The basic concept of FSW/P is very simple, as shown in Fig. 21.1 (Park et al. 2003a). A specially designed tool rotating at high speed is plunged into work pieces to be joined/processed and then is traversed along the weld seam, or in a direction of interest in the case of friction stir processing (FSP). The rotating tool produces frictional heat which softens the material so that it is readily extruded around the tool. The simultaneous rotational and translation motion of the tool forces the material to flow around the tool, filling a cavity at the rear of the tool and thus creating a solid-state joint. During the flow, the material undergoes extreme levels of plastic deformation and thermal exposure, which drastically changes the microstructure in the center of the processed zone.