The application of a rotating anvil in friction stir spot welding: A numerical and experimental study

A rotating anvil, similar to a pinless friction stir welding (FSW) tool, can be applied to friction stir spot welding (FSSW) of thin metal plates. FSSW is a solid state joining process that is currently being used by automotive manufacturers as an alternative to rivets and traditional resistance spot welding. The principal detractor of this process is the keyhole left by pin extraction, which can be detrimental to the weld strength. A pinless tool can be used to eliminate the keyhole. However, this approach is limited to joining thin sheet (< 1 mm). A rotating anvil with the pinless FSSW process permits the joining of thicker cross sections, decreases the cycle time and reduces the reaction forces and torques acting on the spot welding frame. The use of a rotating anvil for FSSW is currently not presented in the literature and it is the objective of this work to establish the ideal conditions for creating mechanically sound spot welds. Tensile shear tests, macrosection analysis and a numerical model of the process are used to evaluate the spot welds. Macrosection and numerical analysis reveals that the material flow between the pinless tool and rotating anvil is complex and unique to this process. It has been found that the use of a rotating anvil for FSSW is a viable means to repeatably create quality spot welds in thicker weldements.