Despite the remarkable capabilities of friction stir welding (FSW) in joining dissimilar materials, the numerical simulation of FSW is predominantly limited to the joining of similar materials. The material mixing and defects’ prediction in FSW of dissimilar materials through numerical simulation have not been thoroughly studied. The role of progressive tool wear is another aspect of practical importance that has not received due consideration in numerical simulation. As such, we contribute to the body of knowledge with a numerical study of FSW of dissimilar materials in the context of defect prediction and tool wear. We numerically simulated material mixing and defects (surface and subsurface tunnel, exit hole, and flash formation) using a coupled Eulerian–Lagrangian approach. The model predictions are validated with the experimental results on FSW of the candidate pair AA6061 and AZ31B. The influence of tool wear on tool dimensions is experimentally investigated for several sets of tool rotations and traverse speeds and incorporated in the numerical simulation to predict the weld defects. The developed model successfully predicted subsurface tunnel defects, surface tunnels, excessive flash formations, and exit holes with a maximum deviation of 1.2 mm. The simulation revealed the substantial impact of the plate position, on either the advancing or retreating side, on the defect formation; for instance, when AZ31B was placed on the AS, the surface tunnel reached about 50% of the workpiece thickness. The numerical model successfully captured defect formation due to the wear-induced changes in tool dimensions, e.g., the pin length decreased up to 30% after welding at higher tool rotations and traverse speeds, leading to surface tunnel defects.

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The friction stir welding (FSW) process is often associated with various defects, which may develop due to the inappropriate choice of process parameters. The visualisation of the defects is often impossible by means of simple experiments. The present work develops a finite element model using a coupled Eulerian-Lagrangian (CEL) approach to investigate commonly occurred defects in the FSW process. The numerical model is framed, in general, to predict multiple defects such as the improper joining of substrates, formation of tunnel defect, excessive flash, voids and other surface irregularities. In principle, the flow of material within and out of the solution domain allows predicting both the surface and subsurface defects. A suitable comparison between simulation and experiments shows the robustness of the developed model.

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