Numerical modelling of microstructure in friction stir welding of aluminium alloys

Mechanical properties like strength and hardness depend largely on microstructure. The conventional methods to evaluate microstructure such as optical and electron-based microscopy require a substantial amount of time and are expensive as well. To deal with this issue, the present work reports evaluation of the microstructure via numerical modelling in friction stir welding (FSW). This includes a 3-D thermo-mechanical model built on the Lagrangian implicit formulation. It has been experimentally validated for different processing conditions. A coupled approach combining Cellular Automaton (CA) and Laasraoui and Jonas (LJ) with the thermo-mechanical model is followed. Temperature, strain and strain rate form the inputs from the developed model to predict the microstructure. Nucleation and grain growth have also been considered in the model. The results have been validated by comparing the experimentally obtained grain size results at the weld zones namely stir zone, thermo-mechanically affected zone and heat-affected zone; and the percentage errors are 7.3%, 10.6%, and 8.5%, respectively. The effects of two key process parameters (tool rotation (omega) and welding speed (v)) on temperature and effective strain have been investigated and correlated with the obtained grain size.