Effects of Tool Plunging Path on the Welded Joint Properties of Pinless Friction Stir Spot Welding

Four tool plunging paths including a one-time plunging path and three step-by-step plunging paths were designed to study the effects of the tool plunging path on the welded joint properties of pinless friction stir spot welding (PFSSW). The appearance, cross-sectional microstructure, welding temperature, microhardness, and tensile shear failure load of the PFSSW of thin copper sheets under different tool plunging paths were explored. Furthermore, the fracture modes of welded joints under different tool plunging paths were analyzed. Studies showed that path 1 (plunge total depth at one time) produced the largest range of stirring zone, but the grains in the stirring zone were larger and the width of the thermal-mechanical affected zone was smaller. Path 1 obtained the highest peak temperature during the welding process, and path 3 (plunge 1/3 total depth + plunge 2/3 total depth) gained the lowest peak temperature. The greater the initial plunging amount of the tool, the faster the temperature rise rate in the welding stage. The tensile shear failure loads for path 1, path 2 (plunge 1/2 total depth + plunge 1/2 total depth), path 3, and path 4 (plunge 2/3 total depth + plunge 1/3 total depth) were 8.65 kN, 8.15 kN, 8.25 kN, and 8.85 kN, respectively. The tensile shear failure load of path 4 was 2.3% higher than that of path 1. The fracture modes of welded joints under different tool plunging paths were all nugget pullout fractures. The fracture morphology indicated that the fracture type was ductile fracture. The step-by-step plunging path proposed in this work extends the traditional PFSSW process. The findings of this study can provide a reference for the selection and design of tool plunging paths for PFSSW.