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Friction Stir Powder Processing

Powder metallurgy is used to make alloys and composites of non-equilibrium compositions. The processing steps often involve powder compaction and further thermomechanical processing. For aluminum alloys, three microstructural features are very important; prior-particle boundaries, microstructural inhomogeneity, and size of primary intermetallic particles. Breakage of the aluminum oxide film on prior-particle boundaries by extrusion or forging is critical for ductility, fatigue and fracture toughness. Because of the material flow pattern, some microstructural inhomogeneity can not be eliminated in forging and extrusion. Friction stir processing provides opportunity to homogenize microstructure for subsequent forming operations or produce selectively reinforced regions. Because of the severe plastic deformation associated with friction stirring, the prior-particle boundaries and any powder scale microstructural or chemical inhomogeneity are eliminated. Using this approach P/M aluminum alloy with ~700 MPa strength and >10% ductility has been obtained.