Review of tools for friction stir welding and processing

Friction stir welding (FSW) is a novel green manufacturing technique due to its energy efficiency and environmental friendliness. This solid state joining process involves a rotating tool consisting of a shoulder and/or a probe. The shoulder applies a downward pressure to the workpiece surface, constrains the plasticised material around the probe, generates heat through the friction and causes plastic deformation in a relatively thin layer under the bottom surface of the shoulder. The rotating probe mainly drags along, plasticises, and mixes the adjacent material in the stir zone, creating a joint without fusion. Friction stir processing (FSP), a variant of FSW, has been developed to manufacture composites, locally eliminate casting defects, refine microstructure and/or improve the associated mechanical and physical properties including strength, ductility, fatigue, creep, formability and corrosion resistance. However, major challenges such as tool design and wear currently limit the use of FSW/P for manufacturing applications, particularly for high melting temperature or high strength alloys. In this review, the FSW/P tools are briefly summarised in terms of the tool types, shapes, dimensions, materials and wear behaviours.