Investigation on the welded joint properties of pinless friction stir spot welding of copper under different tool grooves

Abstract

Three types of pinless friction stir spot welding (PFSSW) tools were designed employing Fibonacci spiral curve, involute curve, and Archimedean spiral curve to create corresponding grooves. Subsequently, PFSSW experiments were conducted on copper sheets using these three distinct tools. The surface morphology, cross-sectional macrostructure and microstructure, microhardness, joint strength, fracture type, and fracture surface morphology of the joints were analyzed under the influence of different tools. The research findings indicate that the Fibonacci spiral curve groove (FSCG) tool exhibits the highest disparity in three-dimensional morphology within the central zone of the joint, followed by the involute curve groove (ICG) tool and the Archimedean spiral curve groove (ASCG) tool. The ICG tool generates the widest stirring zone (SZ) in the joint, while the FSCG tool produces the deepest SZ. The grains of SZ under all three tools is coarser compared to the base material. The microhardness values under the ASCG tool are relatively higher, whereas those under the FSCG tool are lower. Among the welded specimens, those created using the FSCG tool exhibit the highest average tensile-shear failure load of 8.21 kN, followed by the ICG tool of 7.80 kN, and the ASCG tool yielding the lowest average tensile-shear failure load of 7.77 kN. The fracture type of the specimens welded by FSCG tool is semi-nugget pull-out fracture, while the joint specimens of ICG tool and ASCG tool are both nugget pull-out fracture. The fracture mode of the joints under all three tools is characterized as ductile fracture. The conducted research can not only enrich the variety of PFSSW tool grooves, but also enhance the comprehension of joint properties under distinct tool groove configurations.