Modelling thermomechanical conditions at the tool/matrix interface in Friction Stir Welding

In friction stir welding the material flow is among others controlled by the contact condition at the tool interface, the thermomechanical state of the matrix and the welding parameters. The conditions under which the deposition process is successful are not fully understood and in most models presented previously in literature, the modelling of the material flow at the tool interface has been prescribed as boundary conditions, i.e. the material is forced to keep contact with the tool. The objective of the present work is to analyse the thermomechanical conditions under which a consolidated weld is obtained. A fully coupled thermo-mechanical 3D FE model has been developed in ABAQUS/Explicit using the ALE formulation and the Johnson-Cook material law. The contact forces are modelled by Coulomb’s law of friction making the contact condition highly solution dependent. The heat is generated by both frictional and plastic dissipation. Of special interest is the contact condition along the shoulder/matrix and probe/matrix interfaces, as especially the latter affects the efficiency of the deposition process. The thermo-mechanical state in the workpiece is established by modelling both the dwell and weld periods, thus, minimizing the number of prescribed boundary conditions. The sliding contact condition present during the start of the dwell period changes to a contact condition close to sticking prevailing throughout the welding period. Two procedures for controlling the tool is adopted i) as in the experiment and ii) simulating an offset in plunge depth. Both simulations reach similar steady state solutions. The model results are compared to the plunge force and heat generation observed in experimental welds in AA2024-T3.