A local model for the thermomechanical conditions in friction stir welding

Abstract

The conditions under which the deposition process in friction stir welding is successful are not fully understood. However, it is known that only under specific thermomechanical conditions does a weld formation occur. If these conditions are not present, void formation will occur leading to a faulty weld. The objective of the present work is to analyse the primary conditions under which the cavity behind the tool is filled. For this, a fully coupled thermomechanical three-dimensional FE model has been developed in ABAQUS/Explicit using the arbitrary Lagrangian–Eulerian formulation and the Johnson–Cook material law. The model accounts for the compressibility by including the elastic response of the aluminium matrix. The contact forces are modelled by Coulomb's Law of friction, making the contact condition highly solution dependent. Furthermore, separation between the workpiece and the tool is allowed. This is often neglected in other models. Once non-recoverable separation is estimated by the model, a void develops. This is suggested as a preliminary criterion for evaluating the success of the deposition process. Of special interest is the contact condition along the tool/matrix interface, which controls the efficiency of the deposition process. In most models presented previously in the literature, the material flow at the tool interface is prescribed as boundary conditions. In all other contact models, the material is forced to keep contact with the tool. Therefore, the models are unable to predict when the suitable thermomechanical conditions and welding parameters are present. In the present work, the quasi-stationary thermomechanical state in the workpiece is established by modelling the dwell and weld periods. The different thermomechanical states in the colder, stiffer far-field matrix and the hotter, softer near-field matrix (under the tool) result in contact at the tool/matrix interface, thus, no void formation is observed. The steady-state model results are compared to the plunge force and heat generation observed in experimental welds in AA2024-T3.