Modelling of the Contact Condition at the Tool/Matrix Interface in Friction Stir Welding

The objective of the present paper is to investigate the heat generation and contact condition during Friction Stir Welding (FSW). For this purpose, an analytical model is developed for the heat generation and this is combined with a Eulerian FE-analysis of the temperature field. The heat generation is closely related to the friction condition at the contact interface between the FSW tool and the weld piece material as well as the material flow in the weld matrix, since the mechanisms for heat generation by frictional and plastic dissipation are different. The heat generation from the tool is governed by the contact condition, i.e. whether there is sliding, sticking or partial sliding/sticking. The contact condition in FSW is complex (dependent on alloy, welding parameters, tool design etc.), and previous models (both analytical and numerical) for simulation of the heat generation assume a known contact condition at the contact interface, e.g. either as pure sliding or sticking. The present model uses Coulomb’s law of friction for the sliding condition and the material yield shear stress for the sticking condition to model the contact forces. The model includes heat generation contributions from both the conical shoulder and tool probe, and enables a contact condition which could be partial sliding/sticking. Furthermore, the analytical results are compared to experiments by an assessment of specific global variables, i.e. plunge forces and torques. The analytical model combined with the experimental results is used to deduce which contact condition is most likely to be present during welding.