Finite-element analysis of the tool tilt angle effect on the formation of friction stir welds

The tool tilt angle is one of the key process parameters determining the weld quality during friction stir welding. However, the influence of tilt angle on the weld formation has not been fully understood. Hence, in this paper, a three-dimensional thermal-mechanical coupled finite element model is established, in which the tilt angle is considered in the geometric model. This model is shown to be able to predict the in-process thermal-mechanical state variables (e.g. temperature, flow path and stress) during the welding and the post-welding morphology of the weld. The predicted temperature distribution and weld morphology agree well with the corresponding experimental results. It is shown by the simulation results that wormhole defects appear in the weld with a zero tilt angle, while no such defects are observed in the case that the tilt angle of 2° is employed. In order to get a deeper understanding of the weld formation mechanism, the thermal-mechanical condition around the tool during welding with/without tilt angle are compared and analysed. It is found from our simulation that both the peak temperature and compressive normal stress on material in the rear side of the tool are elevated by including a tilt angle of 2°. This temperature increase induced by the tilt angle will significantly soften the workpiece, meanwhile higher frictional driving force in the rear side will be generated by the increased compressive force. Therefore, the material flow in the rear side of the tool with the title angle is significantly enhanced, which contributes to avoid the generation of wormhole defect on the advancing side.