The effect of the welding parameters and tool size on the thermal process and tool torque in reverse dual-rotation friction stir welding

• A 3D thermo-mechanical model of RDR-FSW process is developed.
• Effect of the welding parameter on contact conditions has been considered.
• Effect of process parameters on the thermal processes and torque has been analyzed.
• A detailed understanding of thermal process and tool torque in RDR-FSW is provided.
• The calculated thermal cycles agrees well with the experimental results.

Reverse dual-rotation friction stir welding (RDR-FSW) is a novel variant of conventional friction stir welding (FSW) process. The key feature is that the tool pin and the assisted shoulder are separated and rotate reversely and independently during welding process, thus it has great potential to improve the weld quality and lower the welding loads through adjusting the rotation speeds of the tool pin and the assisted shoulder independently. A 3D model of RDR-FSW process is developed to analyze the effect of welding parameters and tool size on the thermal process and the tool torque quantitatively. The model considers the effect of the welding parameters on the dimensionless slip rate and the friction coefficient between the tool-workpiece contact interfaces. It is found that with an increase of the radial distance, the locations of peak and valley values of heat generation rate at the shoulder-workpiece contact interfaces vary from the retreating side (RS) to the advancing side (AS) and from the AS to the RS, respectively. Although the reverse rotation of the tool pin and the assisted shoulder has little effect on the total heat generation, the corresponding material flow pattern and the distribution of heat generation rate lead to a more homogeneous temperature distribution and a much lower torque exerted on the workpiece in RDR-FSW process. The model is experimentally validated by comparing the measured thermal cycles with the calculated data.