Improvement of local tensile properties in cast aluminum alloy by friction stir processing (FSP) was examined. Small tensile specimens were machined out from nine locations inside of the stir zone manufactured by the FSP condition of tool rotation speed 1000 rpm and tool travel speed 500 mm/min, and tensile strength and ductility were locally evaluated. Although the plate as casted condition showed very brittle manner with only 2 or 3% fracture ductility, some of specimens machined from the stir zone achieved more than 20% of fracture ductility with local necking. However, the retreating side was not improved, and its fracture behavior was brittle similarly with the plate as casted condition. Fractography revealed that the ductility in cast aluminum alloy was controlled by the coarse brittle second phase, and an improvement of ductility resulted from the crashed brittle second phase to be fine. The stir zone, which was manufactured by tool travel speed 1000 mm/min, showed higher ductility than that by tool travel speed 500 mm/min. The retreating side was also improved in fast travel speed of 1000 mm/min. The mechanism to achieve an improvement of fracture ductility by FSP was discussed from the viewpoint from heat input.
