Microstructure and Mechanical Properties of Cu-Cu Joints Fabricated by RFSSW: Effect of Plunge Depth

Abstract

Due to the unique physicochemical properties of copper (Cu), conventional fusion welding faces certain challenges when welding this material. In this study, Cu-Cu joints were fabricated using refill friction stir spot welding (RFSSW) at five different plunge depths. The macroscopic morphologies of the welded joints (WJs) under various welding conditions were examined, and the microstructural differences in representative zones of the WJ were analyzed. The microhardness and tensile-shear failure load (T-SFL) of the WJs were evaluated to assess the mechanical performance, and the failure mechanisms were explored. The results indicated that with increasing plunge depth, the microstructure tended to coarsen, accompanied by a gradual decrease in microhardness. The T-SFL initially increased and then decreased with increasing plunge depth, with the optimal depth found to be 1.4 times the plate thickness. For Cu-Cu joints, the softening effect in the heat-affected zone (HAZ) outside the stirring zone is identified as a key factor limiting the load-bearing capacity of the WJs.