A Flow Stress Formulation of Magnesium Alloy at Elevated Temperature

Dynamic recrystallization (DRX) is the main softening mechanism of magnesium alloy AZ31B in hot deformation. Theoretically, in the flow rule the atomic diffusibility and the driving force of dislocation migration are dependent on the temperature, and the dislocation density and the cumulation of grain boundary energy are dependent on the strain rate. The peak stress will appear when the flow driving force and resistance force reach a balance, after which the stress descending will take place due to recrystallization fraction. Since the DRX is a thermally activated process, the recrystallized volume fraction can be regarded as the function of strain through Avrami equation. Based on this idea, the paper proposes a new constitutive model characterizing dynamic recrystallization for magnesium alloy AZ31B. The model is described by a peak stress and a strain softening rate, in which the peak stress depends only on Zener-Hollomon parameter and is determined through creep equation, and the strain softening rate is mainly dominated by the recrystallized volume fraction. As a result, the formulation of flow stress takes into account of temperature, strain and strain rate, and is characterized by DRX. The thermomechanical simulation tests of magnesium alloy AZ31B by using Gleeble-1500 were conducted, through which the parameters in flow stress model can be easily determined. Comparison shows that the maximum difference of flow stress between the model predictions and test values is approximately 2.32%. This indicates that the proposed constitutive model can be employed to represent the DRX behaviour of AZ31B.