Thermomechanical modelling and force analysis of friction stir welding by the finite element method

Friction stir welding (FSW) is a solid-state jointing technology, in which the butted plates are heated, plasticized and jointed locally by the plunged probe and shoulder moving along the joint line. The residual stresses due to the thermomechanical performance of the material and the constraint of the welded plates by the fixture are one of main concerns for this process. A prediction of the clamping force applied on the plates during FSW is expected to be helpful in controlling the residual stresses and weld quality. Furthermore, the prediction of the force history in FSW will be beneficial to understand the mechanics of the process and to provide valid models for controlling the process, especially in the case of robotic FSW. In this paper, a three-dimensional model based on a finite element method is proposed to study the thermal history and stress distribution in the weld and, subsequently, to compute mechanical forces in the longitudinal, lateral and vertical directions. The proposed model includes a coupled thermomechanical modelling. The parametric investigation of the effects of the tool rotational and longitudinal speed on the longitudinal, lateral and vertical forces is also conducted in order to compute the appropriate clamping force applied on the plates. Measurements by the load cells in the longitudinal, lateral and vertical directions are presented and reveal a reasonable agreement between the experimental results and the numerical calculations.