Numerical simulation of weld formation in friction stir welding based on non-uniform tool-workpiece interaction: An effect of tool pin size

The weld formation in friction stir welding (FSW) determines the mechanical properties of the FSW joint. It is of great significant to predict the weld formation in FSW for optimizing the welding parameters. Thus, an integrated CFD model based on shear stress boundary with non-uniform distribution of the contact pressure was proposed to quantitatively analyze the plastic material flow and weld formation in friction stir welding. The formation mechanism of the void defect was systematically analyzed. The effects of tool pin size on plastic material flow and weld formation were quantitatively investigated. It is found that a significant decrease of contact pressure between the tool and workpiece at the rear of the tool pin side seriously reduces the frictional shear stress behind the tool leading to a severe decrease in plastic material flow at relatively high welding speed, resulting in the formation of void defects at the advancing side of the joint. The morphology of the void defects in the FSW joint is dependent on the tool pin size. The width of the void defect is increased while its height is decreased as the pin tip diameter increases. The larger pin tip diameter increases the temperature near the tool pin tip and enhances the plastic material flow, which increases both the horizontal and vertical migration distance of the plastic material in the middle of the workpiece, resulting in a void defect with lower height and larger width. The height of the void defect is decreased with an increase in pin root diameter because the vertical migration distances of plastic materials at the upper part of the workpiece are increased, while the width of the void defect does not show a clear trend with the change of the pin root diameter. The proposed CFD model was experimentally validated by using the measured weld formation and thermal cycles.