Effects of travel speed on mechanical properties of AA7075-T6 ultra-thin sheet joints fabricated by high rotational speed micro pinless friction stir welding

Abstract

0.5 mm thick AA7075-T6 ultra-thin sheets were joined successfully by high rotational speed friction stir welding using a pinless tool. Friction stir welding was performed under a high rotational speed of 6000 rpm, and the travel speed ranged from 300 mm/min to 1200 mm/min. Zero thickness reduction and delamination phenomenon were observed in the nugget zone when the travel speed was 1200 mm/min. In this condition, the dynamic recrystallization occurred in the lower part of the nugget zone is incomplete. The mechanical properties gradually improved with the increasing in the travel speed. All the tensile specimens fractured along the boundary between the thermo-mechanically affected zone and the nugget zone, and the morphology showed a shear fracture mode. The maximum tensile strength of 482 MPa (equivalent to ∼90% of base metal) was obtained at the travel speed of 1200 mm/min, which presented a ductile fracture mode and a large amount of η phase (MgZn2) precipitates and thick tear ridges were detected on the fracture surfaces.