Effect of Pin Geometry on Static Strength of Friction Stir Spot Welds

Friction Stir Spot Welding (FSSW) is an emerging joining technique that has seen some successful automotive applications in the past few years. One of the most significant factors that influence the joint strength of a friction stir spot weld is the tool geometry. The tool geometry used in FSSW has been traditionally derived from friction stir linear welding and there has not been much focus on developing tool geometries specifically for FSSW. The main objective of this paper is to evaluate different pin geometries that are specifically catered towards maximizing the strength of friction stir spot welds. In order to evaluate the effect of only the pin, all tools considered had flat shoulders. Four different pin shapes were evaluated - baseline, thick, tapered and inverse tapered pins. Three different pin lengths were considered for each pin shape - 1.0, 1.2 and 1.4mm. Welding was done on typical automotive alloy A6022-T4 (1mm thick) and coupons were tested in cross-tension configuration. Cross-section of the as-welded sample for each pin shape was taken to observe the weld geometry. Hook formation and metallurgically bonded area played a very important role in determining the static strength of the FSSW joints. The thick pin configuration yielded welds with the highest static strength.