Forming mechanism and processing of stationary upper shoulder BT-FSW

Taking the stationary upper shoulder bobbin tool friction stir welding (SSUBT-FSW) welded joint of AA6056 aluminum alloy as the research object, the distributions of temperature and strain fields have been clarified based on combining with experimental and numerical simulation methods. Results show that the upper shoulder of SSUBT-FSW is in a static state, which effectively increases the welding stability and decreases the maximum torque to 55 N·m (the maximum torque of conventional bobbin tool friction stir welding is 65 N·m). The pressure of the stationary upper shoulder on the weld surface reduces the height difference between the peak and trough of the arc corrugations from 60 μm to 10 μm, which significantly improves the surface finish of the joint. The diameter of plastic strain zone driven by the lower rotating shoulder is larger than that developed by the probe, promoting the plastic strain field shows a trapezoidal shape on the cross section of the joint, which is good agreement with the shape of the temperature field. In the SSUBT-FSW process, the material flow of the lower shoulder is significantly better than that of the stationary upper shoulder, and the material asymmetry flow characteristics are formed along the thickness direction of the joint.