Study on the Flow Field of Friction Stir Welding of AZ31 Magnesium Alloy Based on the Temperature Variation

In this study, AZ31 magnesium alloy plates were butt welded by friction stir welding to investigate the flow field characterization, and temperature variation was measured. “Marker insert technology” and “stitch welding of dissimilar materials technique” were used to make the flow field visualization. The results indicated that both the temperature variation and the flow way of material are three dimensional asymmetric. In terms of temperature distribution, along the welding direction, the temperature is gradually increased during the process; along the thickness direction, the peak temperature is high in the upper part and low in the lower; perpendicular to the welding direction, the peak temperature of the advancing side is slightly higher than the retreating side. In terms of the flow field, the flow way of the material in the advancing side is different from retreating side. Material from the advancing side crosses the pin and is divided into two directions: part is dragged forward by the shoulder while part is extruded backward. However, materials in retreating side move backward only. At different depths, the flow range of plastic material is different. The closer to the bottom of the weld, the flow range of plastic material is smaller. Along the welding direction, the flow range of the plastic material changes larger from the initial stage to the end stage. On the cross section of the weld, plastic material that adjacent to the pin is squeezed down while part of the material at the bottom is forced to move upwards. The rising material converged with the downward current and formed a cycle. Considering the deformation tendency of the lapped surface, the advancing side is more intense than the retreating side.