Numerical analysis of heat transfer and material flow in reverse dual-rotation friction stir welding

Reverse dual-rotation friction stir welding (RDR-FSW) is a novel variant of FSW process in which the tool pin and the surrounding assisted shoulder rotate reversely, thus it has great potential to improve the weld quality and lower the welding loads through adjusting the rotating speeds of the tool pin and the assisted shoulder independently. In this study, a three-dimensional model of RDR-FSW process is developed to conduct the numerical simulation of the temperature profile and plastic material flow during the process. The predicted temperature difference between the advancing side and the retreating side is less than that in conventional FSW. The calculated streamlines demonstrate that at the upper portion of the workpiece the stronger material flow mainly develops on the advancing side of the tool, however at the lower part of the workpiece the stronger flow develops on the retreating side. The material flow near the shoulder gets stronger than that in conventional FSW under the same welding conditions, while near the pin at lower part of the workpiece it gets weaker. Due to the effects of the assisted shoulder the predicted shape and size of TMAZ (thermal mechanically affected zone) is decreased greatly compared to the conventional FSW. It lays solid foundation for optimizing the process parameters in RDR-FSW.