Effects of tool rotation direction on microstructure and mechanical properties of 6061 aluminum alloy joints by the synergistically double-sided friction stir welding

In order to improve welding efficiency and joint performance of friction stir welded aluminum alloy plates of different thickness, a new synergistically double-sided friction stir welding (SDS-FSW) technique was developed and investigated. The microstructure and mechanical properties of SDS-FSW, conventional single-sided FSW (SSFSW) and conventional sequentially double-sided FSW (CDS-FSW) 6 mm thick 6061 aluminum alloy joints were systematically investigated and compared. The temperature field and material flow of these joints during welding were studied with numerical modelling. Results show that the microstructure of SDS-FSW joints is uniform and sound, with grains and second-phase particles in the weld nugget zone refined. When compared with the other two FSW processes, there is higher heat input and improved material flow in the SDS-FSW process at 1400 rpm and 600 mm/min. The microhardness and microstructure of the SDS-FSW joints are uniform along the weld thickness. The tensile strength is comparable to BM, while considerably higher than that of SS-FSW joints. In addition, when the upper and lower tools rotate in opposite directions, joint microstructure improved further.