Material Flow, Heat Transfer Modelling and Theoretical Investigation of Microstructure Evolution in AA6082 during Friction Stir Spot Welding

Friction stir spot welding (FSSW) as a variant of the linear friction stir welding is implemented in automotive industry as a partial replacement of resistance spot welding for aluminium. FSSW as a solid state joining technology, primarily takes advantage of severe thermoplastic deformation, to achieve the joining between two parts, which can be from the same material or even dissimilar. In this project, AA6082 will be welded by FSSW and simultaneous, the FSSW process will be simulated by finite element method. From this model, some important state variables can be derived, such as temperature, strain, and strain rate etc. The model will be calibrated by comparing temperature history obtained from the simulation with experimental data. To validate the simulation results, effective strain and strain rate also will be taken from the simulation and then implemented into Gleeble for hot torsion investigation. By comparing the grain size of dynamic recrystallization between FSSW specimen and hot torsion specimen, we can confirm if we got the right strain and strain rate from the simulation. The shape of friction stir zone can also be predicted by using particle tracking method. Besides, a physically based internal state variable model will be implemented into the commercial Finite Element (FE) analysis package DEFORM. The model allows predicting dislocation density and grain size after FSSW. The grain structure evolution will be analysed by both optical microscope and electron backscatter diffraction (EBSD).