Material flow during dissimilar friction stir welding of Al/Mg alloys

Understanding of the intense in-process material flow during dissimilar friction stir welding (FSW) of Al/Mg alloys has long been limited by the accuracy of the numerical simulation model. In this paper, a computational fluid dynamics (CFD) simulation with state-of-the-art thermo-mechanical-flow fully coupled shear boundary condition is employed to predict the material during the dissimilar FSW of AA6061 Al alloy and AZ31B Mg alloy. The prediction on the three-dimensional temperature field and the flow field is obtained with the best accuracy to date. It is shown that the primary motion is rotation around the tool pin in the high-velocity zone (HVZ) near the welding tool, and there is a vertical component of velocity with a magnitude lower than 10% of the total velocity. It is shown that the material is significantly redistributed during the welding process, with Mg alloy accumulating at the bottom and Al alloy accumulating at the upper part of the joint. The material distribution behavior is attributed to the material transfer in the through-thickness direction. In the HVZ, most material flows in the straight-through pattern and is slightly elevated when it flows through the tool vicinity. A small amount of Mg alloy flows in the maelstrom pattern, encircles the tool pin multiple times, and is transferred to a lower position. In this way, The upward transfer of Al alloy and the downward transfer of Mg alloy lead to the through-thickness redistribution of each alloy.