Characteristics of the reverse dual-rotation friction stir welding conducted on 2219-T6 aluminum alloy

Reverse dual-rotation friction stir welding (RDR-FSW) has great potential to obtain appropriate welding conditions through adjusting the independently rotating tool pin and surrounding shoulder. The welding torque exerted on the workpiece by the reversely rotating shoulder also cancels off a part of the welding torque exerted by the rotating tool pin, thus the clamping requirement for the workpiece is also reduced. In the present paper, a tool system for the RDR-FSW was designed and successfully applied to weld high strength aluminum alloy 2219-T6, and then microstructures and mechanical properties of the optimized joint were investigated to demonstrate the RDR-FSW characteristics. The weld nugget zone was characterized by the homogeneity of refined grain structures, but there was a three-phase confluction on the advancing side formed by different grain structures from three different zones. The tensile strength of the optimized joint was 328 MPa (73.7% of the base material), showing an obvious improvement when compared with the optimized joint welded by the FSW without the reversely rotating assisted shoulder. The tensile fracture occurred in the ductile fracture mode and the fracture path propagated in the weakest region where the Vickers hardness is the minimum.