Abstract: The reverse dual-rotation friction stir welding (RDR-FSW) has the capability to adjust the heat generation because of the separately designed tool shoulder and tool pin. The welding torque exerted on the workpiece by the reversely rotating shoulder is opposite to that exerted by the rotating tool pin, so the total welding torque is reduced, which is beneficial to reducing the clamping requirement of workpieces. In the present paper, a RDR-FSW joint was welded in a condition similar to the optimal welding condition of conventional FSW, and microstructures in various zones were investigated by comparison, aiming to highlight effects of the reversely rotating assisted shoulder. Due to the heat conduction of the middle cylinder and the bottom end cover on which the assisted shoulder was machined, the thermal effect of RDR-FSW was smaller than that of the conventional FSW. Moreover, the effect of assisted shoulder on the plastic flow or deformation of material or was constrained in a thin layer near the weld top surface, and thus the flow of material especially along the thickness direction was clearly decreased in the RDR-FSW. In the heat-affected zone (HAZ), the precipitate coarsening was the main evolution and was completed through the dissolution of small precipitates and the continuous growth of large precipitates. By contrast, the dissolution degree of precipitates increased significantly in the thermo-mechanically affected zone (TMAZ), and a small amount of original meta-stable precipitates transformed to block-shaped stable precipitates. Precipitate evolutions in the shoulder affected zone (SAZ) and the weld nugget zone were similar, i.e. the majority of original meta-stable precipitates dissolved into the matrix and the remainder transformed to stable precipitates, though the dissolution degree was greater in the SAZ. Compared with the conventional FSW joint, the coarsening degrees of precipitates in the HAZ and TMAZ of RDR-FSW joint were much smaller, as well as the dissolution degrees of precipitates in all four specified zones.

Key words: Friction stir welding, Reverse dual-rotation friction stir welding, High-strength aluminum alloy, Microstructures, Precipitates