Effect of tool geometry on static strength of friction stir spot-welded aluminum alloy

Friction stir spot welding is performed on 5083 Al alloy using tools with a conventional cylindrical pin and the proposed triangular pin. Partial metallurgical bond (called as 'hook') is formed in the weld region between the overlapped metal sheets. The tool-pin geometry significantly affects the hook shape. Under the same process condition, welds made with the cylindrical pin have a continuous hook which bypasses the stir zone and points downward towards the weld bottom. By contrast, for welds made with the triangular pin, the hook is directed upwards and then arrested at the periphery of the stir zone. The difference in the hook shape could be attributed to the asymmetric rotation of the triangular pin that may cause the material in the vicinity of the pin to move back and forth in the radial direction resulting in the hook being broken-up (dispersed) in the stir zone. In addition, the triangular pin results in a finer grain structure in the stir zone compared to the cylindrical pin. Static strength of welds made with the triangular pin is twice that of welds made with the cylindrical pin. which is attributed to the finer grain size as well as tensile failure mode as a result of the arrested hook. (C) 2008 Elsevier Ltd. All rights reserved.