Numerical simulation and experimental investigation of band patterns in bobbin tool friction stir welding of aluminum alloy

A coupled Eulerian-Lagrangian (CEL) model was developed to investigate the formation mechanism of band pattern (BP) during bobbin tool friction stir welding (BTFSW) process. The Johnson-Cook constitutive model which combines the strain rate and temperature effects in the material flow stress was used. The workpiece was modeled by the Eulerian formulation, while the tool was characterized by the Lagrangian formulation. The numerical analysis results agree well with experimental observations. The band structures with various sizes were discovered in the BP zone. Both the experimental and numerical analysis results indicate the BP zone forms on the advancing side of the joint. This is because the converged material flow is produced by both shoulders while being driven by the probe. The peak temperature, along the thickness of the BP zone, increased at the beginning followed by a decrease further away from the upper surface. The peak temperature obtained near the lower surface is slightly higher than that near the upper surface.