A comparative investigation on conventional and stationary shoulder friction stir welding of Al-7075 butt-lap structure

Abstract

Both conventional friction stir welding (C-FSW) and stationary shoulder friction stir welding (S-FSW) were employed to join the Al-7075 butt-lap structure, then the microstructural evolution and mechanical characterization of all FSW joints were systematically studied. The C-FSW joint exhibited a rough surface with flashes and arc corrugations, while the surface of the S-FSW joint became smooth. Moreover, for the S-FSW joint, the shoulder-affected zone got eliminated and the material flow mode during FSW was changed owning to the application of stationary shoulder. Furthermore, in comparison to C-FSW, the lower welding heat input of S-FSW decreased the average grain size in the nugget zone and inhibited the coarsening of strengthening precipitates in the heat-affected zone, elevating the overall hardness for the S-FSW joint. In addition, the tensile strength of the S-FSW joint became higher compared to the C-FSW joint, and all the FSW joints failed inside the nugget zone attributing to the existence of hook defect. The sharp-angled hook defect deteriorated the plasticity of the C-FSW joint further, which was only 70% that of the S-FSW joint.