Friction stir processing (FSP) is a method of changing the properties of a metal through intense, localized plastic deformation. This deformation is produced by forcibly inserting a non-consumable tool into the workpiece, and revolving the tool in a stirring motion as it is pushed laterally through the workpiece. The precursor of this technique, friction stir welding, is used to join multiple pieces of metal without creating the heat affected zone typical of fusion welding.
When ideally implemented, this process mixes the material without changing the phase (by melting or otherwise) and creates a microstructure with fine, equiaxed grains. This homogeneous grain structure, separated by high-angle boundaries, allows some aluminium alloys to take on superplastic properties. Friction stir processing also enhances the tensile strength and fatigue strength of the metal. In tests with actively cooled magnesium-alloy workpieces, the microhardness was almost tripled in the area of the friction stir processed seam (to 120–130 Vickers hardness).