Prediction of Dynamically Recrystallized Microstructure of AZ31 Magnesium Alloys in Hot Rolling Using an Expanded Dislocation Density Model

Microstructure simulation has been greatly expanded in recent years, particularly for the purpose of microstructural prediction and understanding mechanisms of microstructural evolution in thermal-mechanical processes. The application of microstructure simulation to hot rolling of magnesium alloy sheets will bring a great economic benefit. In this study, a multi-scale coupled dislocation density model of magnesium alloys was developed for predicting the microstructures of rolled AZ31 sheets. The simulations were performed by inserting the expanded dislocation density model into a VUSDFLD subroutine of ABAQUS software. The effects of rolling temperature and speed on dislocation densities of low angle grain boundary and high angle grain boundary and dynamic recrystallization (DRX) volume fraction were investigated by finite element simulations. The simulation results show that the dislocation densities of the high angle grain boundary and low angle grain boundary increase rapidly at the first rolling stage, but decrease slightly with rolling time. The changes of dislocation densities with the rolling time are complicated, which are influenced by an integrated mechanism of work-hardening, dynamic recovery and DRX. The dislocation densities can achieve the highest value when the rolling temperature is in range of 400-450 degrees C. Moreover, the DRX volume fraction is the largest in the surface layer and the smallest in the center layer of the rolled sheets. This is resulted from the distributions of rolling temperature, rolling strain and strain rate from the surface to the center of the rolled sheets. Rolling force decreases with the rolling temperature but increases slightly with the rolling speed. The predicted rolling forces are good in agreement with those of the experiments.