A numerical modelling approach to predict material flow and defect formation in refill friction stir spot welded joints

In this study, a fully coupled thermomechanical model of the refill friction stir spot welding process was developed using the coupled Eulerian-Lagrangian technique. The model was used to simulate joining of AA2024-T3 sheets with different welding times of 3 s and 1.5 s, and a fixed rotational speed of 2000 rpm. Model outputs of welding temperature, equivalent plastic strain, void content, and material flow were validated against experimental welds produced using the same welding parameters. Welding temperatures were accurately predicted for both welding times throughout the plunging stage yet overestimated during the refill stage compared with thermocouple measurements. Simulated regions of high plastic strain were found to correlate well with regions of higher grain refinement in experimental welds. Internal tunnel-like defects were predicted by the model in the shoulder plunge region; these predictions were validated by equivalent defects found in the microstructures of experimental welds. Stop-action analysis of experimental welds was used to validate the model’s ability to accurately capture material flow during the refill stage. This provides a valuable insight into refill flow behaviour and the formation mechanism of the internal tunnel defects, indicating that volume ratio is an important parameter for future study.