Friction stir processing is an emerging processing technique based on the principles of friction stir welding. Friction stir welding is a relatively new joining process, developed initially for aluminum alloys, by The Welding Institute (TWI) of UK (Thomas et al., 1991). It is a solid-state joining technique that is energy efficient, environment-friendly, and versatile. It is being touted as the most significant development in metal joining in a decade. The basic concept of friction stir processing is remarkably simple. A rotating tool with pin and shoulder is inserted in the material to be joined and traversed along the line of interest (Figure 1).

Figure 1

The heating is localized and generated by friction between the tool and the workpiece, with additional adiabatic heating from metal deformation. A processed zone is produced by the movement of material from the front of the pin to the back of the pin. As mentioned later, the pin and shoulder of the tool can be modified in a number of ways to influence the material flow and microstructural evolution. The following unique features of friction stirring can be utilized to develop new processes:

• Low amount of heat generated,
• Extensive plastic flow of material,
• Very fine grain size in the stirred region,
• Random misorientation of grain boundaries in stirred region,
• Mechanical mixing of the surface layer,
• Large forging pressure, and
• A controlled flow of material.

In this brief overview of friction stir technologies, an outline of several new concepts developed in the last few years is presented, which make friction stir processing (FSP) a generic tool for localized microstructural modification and manufacturing.

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