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• Most accurate prediction to date of material transfer during dissimilar FSW of Al/Mg alloys is reported. • For the first time the role of maelstrom and straight-though flow pattern in redistribution of material is revealed. • Maelstrom flow causes downward transfer of Mg alloy from the top of advancing side. • Straight-through flow causes upward transfer of the balancing material. 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 model is employed to predict the material behavior 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. [Display omitted] [ABSTRACT FROM AUTHOR]

Pin thread is one of the most common geometrical features for the friction stir welding (FSW) tools. The main purpose of employing the pin thread is to improve the in-process material flow behaviors during FSW. However, it has not been fully understood how exactly the pin thread influences the material flow because of the lack of in-process observation. In this study, we aim to analyze the effect of pin thread on the in-process material flow during FSW of an Al-Mg-Zn alloy by using numerical simulation based on computational fluid dynamics (CFD). In our numerical simulation, the transient rotation of the threaded pin is implemented explicitly via fully transient control of the zone motion, and the mechanical interaction at the tool-workpiece interface is considered via the recent developed shear-stress-based frictional boundary condition. The numerical simulation has been validated by the experimental measured temperatures at 8 different locations, the distribution of marker materials and the geometry of deformation zone in the weld. Based on the numerical simulation results, three effects of the pin thread on the material flow have been elucidated. First, accelerated flow velocity and enhanced strain rate is induced owing to the use of the pin thread, which is attributed to the fact that the interfacial sticking is preferable inside the thread groove opening. Second, the pin thread has an effect to trap material in the high-velocity zone inside the thread groove opening, which causes a many-circle flow pattern around the threaded pin. Third, the pin thread contributes to a vertical pressure gradient, which is important for the in-process material transfer from the top to the bottom. The approaches and concepts in this study can be applied for further fundamental investigation of FSW and the computer aided design of the welding tools. [ABSTRACT FROM AUTHOR]

For better application of numerical simulation in optimization and design of friction stir welding (FSW), this paper presents a new frictional boundary condition at the tool/workpiece interface for computational fluid dynamics (CFD) modeling of FSW. The proposed boundary condition is based on an implementation of the Coulomb friction model. Using the new boundary condition, the CFD simulation yields non-uniform distribution of contact state over the tool/workpiece interface, as validated by the experimental weld macrostructure. It is found that interfacial sticking state is present over large area at the tool-workpiece interface, while significant interfacial sliding occurs at the shoulder periphery, the lower part of pin side, and the periphery of pin bottom. Due to the interfacial sticking, a rotating flow zone is found under the shoulder, in which fast circular motion occurs. The diameter of the rotating flow zone is smaller than the shoulder diameter, which is attributed to the presence of the interfacial sliding at the shoulder periphery. For the simulated welding condition, the heat generation due to friction and plastic deformation makes up 54.4 and 45.6% of the total heat generation rate, respectively. The simulated temperature field is validated by the good agreement to the experimental measurements. [ABSTRACT FROM AUTHOR]