The flow and slip rate of material in friction stir welding mainly determined the weld quality, flow rate depended not only on the shape of pin and welding parameters but also on slip, and an instantaneous centre of speed was generated by rotating speed and welding speed. The relative speed of contact interface is generally an important factor in material flow. The material experienced different viscous slips when rotating around the pin, which disengaged pin at a larger slip and adhered to pin at a smaller slip, resulting in large differences in two sides. The first derivative of the dividing line at the boundary of SZ was extracted to investigate material flow.
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