Mechanical properties, microstructure, and fracture behavior of friction stir welded AA7075 joints with conical pin and conical threaded pin type tools

This study investigates the mechanical properties, microstructure, and fracture behavior of friction stir welded (FSWed) AA7075 joints considering the influence of process parameters and tool geometry. FSWed joints are produced with a conical pin and conical threaded pin type tools using tool rotational speeds of 1400 and 2000 rpm and welding speeds of 20 and 40 mm/min. The FSWed joint showed higher values of the tensile strength (188 MPa), percentage elongation (5.7%), and microhardness (137 HV) with the conical threaded pin type tool. However, the conical pin type tool produced a minimum surface roughness of 9.59 μm. A comparatively lower tensile strength observed for the FSWed joint with conical pin type tool could be attributed to their coarser, elongated, and heterogeneous grain distribution and porosity defects in the welding zone. No significant difference has been observed in the microhardness with the conical pin and conical threaded pin type tools. The fracture of the FSWed joint predominantly occurred in the heat-affected zone during the tensile test. The FSWed joint produced with a conical threaded pin showed better mechanical properties, favorable microstructure, and ductile type fracture behavior at a welding speed of 40 mm/min and tool rotation of 2000 rpm.