In-depth understanding of material flow behavior and refinement mechanism during bobbin tool friction stir welding

Bobbin tool friction stir welding (BT-FSW) is a promising solid-state welding process for fabricating closed or hollow profiles due to its self-supporting nature, which is achieved using a combined tool consisting of two shoulders and a penetrating pin, releasing the backing plate. The joint reliability is mainly affected by its macro-/micro-features including geometric defects and non-uniform grains. However, the underlying thermo-physical process has not been fully understood to clarify how the defects form and grains evolve during welding and how to control them. In this paper, a 3D thermo-mechanically coupled Eulerian-Lagrangian model was developed to analyze the material flow behavior and help understand the defect forming mechanism and recrystallization behavior during BT-FSW of aluminum alloy, wherein tracing particles were specially embedded. The calculated results revealed a complex material migration in the domain driven by the rotating tool. The flow behavior in horizontal/vertical directions at local regions was asynchronous, and eventually converged on the advancing side (AS) to normally form a sound joint, where the combined effects of shearing and squeezing of the material flow throughout thickness affected the morphology of S-line defect. In addition, the non-uniform thermo-mechanical cycling caused an abrupt change in grain structure near the TMAZ/SZ-AS transition region due to the combined continuous and discontinuous dynamic recrystallization, and geometrical effect of strain. The final joint failure was the result of competition between the two softening regions, S-line defect and TMAZ/SZ-AS. These can be applied for further fundamental investigation of parameter optimization or welding structure design, and promote the exploration of post-processing methods to improve the joint performance.