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Cast aluminum alloys A319 and A413 were friction stir welded using a rotating steel screw tool. The cast microstructures were characterized by numerous inclusions or second phase particles which were homogeneously stirred into the weld zone. TEM analysis illustrated the dynamic recrystallization which facilitates the solid-state flow, and digital microhardness measurements through the base metals and the weld zones illustrate the absence of any property degradation. In fact the comminution of particles in the weld zone decreased the interparticle spacing and increased the corresponding hardness by 15%.