The effects of tool geometry on static strength and the failure mode in micro-friction stir spot welding

Tool geometries and plunge depth play significant roles in the stirring process involved in micro-friction stir spot welding (mu FSSW). These two variables affect the hook and joint formation, which influence the static strength and failure mode characterization of a single spot weld. The present work investigated the influence of tool geometry and plunge depth on the static strength and the failure mode characterization in an AA1100 lap-shear joint with a 0.42-mm thickness by examining the macrostructure and mechanical properties. Seven tool designs and three plunge depth variations were applied to investigate the evolution of the hook and joint formation in each tool design and to determine the influence of the size and geometry on joint characteristics, respectively. Seven tool designs and three plunge depth variations were executed to obtain the highest tensile shear load. The regression and prediction interval analysis uncovered a correlation between tensile shear load (i.e., the maximum load) and real plunge depth. The results indicated that the formation of a hook, such as a straight or bending shape, depended on the features of the stir zone, which was affected by the tool's geometry. The hook's shape and location determined the possibility of the crack propagation path and ultimately affected the maximum load. The highest average max load was achieved for Tool 2 at total welding time (TWT) of 10 s at 330-400 mu m, and Tool 7 had the lowest among all the tools. Partial interfacial fracture (PIF) is a failure mode that has high joint strength, while pullout fracture (PF) is a failure mode that is considered to have medium joint strength. A conversion equation has been proposed to reveal the influence of tool geometry on the max load by considering the shoulder size, thickness, and material properties.