Thermomechanical Modelling of Friction Stir Welding

Friction Stir Welding (FSW) is a fully coupled thermomechanical process and should in general be modelled as such. Basically, there are two major application areas of thermomechanical models in the investigation of the FSW process: i) Analysis of the thermomechanical conditions such as e.g. heat generation and local material deformation (often referred to as flow) during the welding process itself. ii) Prediction of the residual stresses that will be present in the joint structure post to welding. While the former in general will call for a fully-coupled thermomechanical procedure, however, typically on a local scale, the latter will very often be based on a semi-coupled, global procedure where the transient temperatures drive the stresses but not vice-versa. However, in the latter, prior knowledge about the heat generation must be obtained somehow, and if experimental data are not available for the FSW process at hand, the heat generation must either be prescribed analytically or based on a fully coupled analysis of the welding process itself. Along this line, a recently proposed thermal-pseudo-mechanical model is presented in which the temperature dependent yield stress of the weld material controls the heat generation. Thereby the heat generation is still numerically predicted but the cumbersome fully coupled analysis avoided. In the present work the formulation of all three mentioned modelling approaches as well as the very fundamental pure thermal models are briefly presented and discussed together with selected modelling results including prediction of material flow during welding, prediction of heat generation with the thermal-pseudo mechanical model as well as residual stress and deformation analysis combined with in-service loads.