Numerical analysis of reverse dual-rotation friction stir welding process

A three-dimensional model of reverse dual-rotation friction stir welding （RDR-FSW） is developed to conduct the numerical simulation of heat generation and material flow during the process. The reverse rotation of the assisted shoulder and the tool pin is considered to model the heat generation rate. The predicted temperature difference between the advancing side and the retreating side in RDR-FSW is less than that in conventional FSW. There are two reverse flows during the RDR-FSW which is beneficial to the uniformity of the temperature profile. Due to the reverse rotation effects of the assisted shoulder, the predicted shape and size of thermal-mechanically affected zone （TMAZ） based on the iso-viscosity line are decreased greatly compared to the conventional FSW. It lays solid foundation for optimizing the process parameters in RDR-FSW.

Effects of the Reversely Rotating Assisted Shoulder on Microstructures During the Reverse Dual-rotation Friction Stir Welding

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 thermomechanically 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.

逆向差速搅拌摩擦焊接材料塑性流变和热场的数值模拟

逆向差速搅拌摩擦焊接是一种搅拌针与轴肩分离并逆向差速旋转的搅拌摩擦焊接新工艺。为了实现逆向差速搅拌摩擦焊接的工艺优化,掌握焊接工艺参数对接头组织和性能的影响规律,需要深入研究这一新工艺涉及的热物理机制。综合考虑焊接过程中的摩擦产热和塑性变形做功作用,建立逆向差速搅拌摩擦焊接过程中材料流动与热分布的三维耦合数理模型,研究焊接过程中温度分布特征、材料流动规律、搅拌头附近的应变速率分布以及黏度分布情况,获取焊接过程中搅拌头附近的材料流动规律、应变速率以及黏度分布的规律。对比分析逆向差速搅拌摩擦焊与常规搅拌摩擦焊在温度分布、材料流动以及搅拌头附近应变速率和黏度分布的差异。焊缝横断面的数值模拟结果与试验实测结果基本吻合。