Simulation of material plastic flow driven by non-uniform friction force during friction stir welding and related defect prediction

Material flow during welding is an important factor that affects the quality of friction stir welding (FSW) joints. In this study, a new welding tool/work piece contact model is proposed to simulate material flow near the FSW tool. The material around the tool flows under the driving force of friction non-uniformly distributed on the interface. Friction force is calculated based on modified Coulomb friction law in this model. Wormhole defects under various welding parameters are graphically predicted by the distribution of tracing particles added in numerical models. Five groups of FSW experiments were conducted to obtain temperature and joint morphology results. Simulation results, including temperature and wormhole defects prediction, corresponded with the experimental results. Further analyses imply that under the process parameters selected in this study, an area with very low friction force occurred behind the tool pin. When the material in front of the tool pin moved to this area from the retreating side, the material speed significantly decreased due to the insufficient driving force, and it became difficult for the material to move to the advancing side along the circular path. The study reveals that the non-uniform friction force model provides a potential tool for predicting FSW defects. (C) 2016 Elsevier Ltd. All rights reserved.