Microstructure Prediction Of Thermal Deformation Of AZ31 Magnesium Alloy Based On Dislocation Density Model

Magnesium alloys are widely used in automobile,aviation,electronic equipment and medical equipment industries due to their advantages such as low density,high specific strength and stiffness,good weldability and electromagnetic shielding.Since the magnesium alloys are Hexagonal Close-Packed(HCP)structure,there are few slip systems that can be launched at room temperature,which greatly limits the room temperature formability of the magnesium alloys.However,slip system of magnesium alloys can be increased at elevated temperatures,and the formability can be improved.Nevertheless,the mechanism of microstructure evolution of magnesium alloys is complex at the elevated temperature forming.Establishing the microstructure model for microstructure prediction not only helps to grasp the microstructure evolution mechanism,but also can better control the forming process.In this paper,the AZ31 magnesium alloy is used as the research object.Thermal-mechanical behavior of AZ31 magnesium alloy extruded bar at deformation temperatures of 300,400 and 500? and strain rates of 0.1,0.01 and 0.001 s-1 was studied by thermal compression experiments.Based on the Arrhenius equation,a constitutive model of rheological behavior was established,in which the activation energy Q was 132.45 KJ·mol-1 and the strain hardening coefficient n was 4.67.According to the mechanism of dynamic recrystallization(DRX)of AZ31 magnesium alloy at high temperature deformation,a macroscopic deformation-microstructure multi-scale coupled dislocation density model for magnesium alloy at high temperature deformation was established.This model considers the effects of time,strain rate,temperature and DRX volume fraction on the variation of dislocation density,and can reflect the interactions among work hardening,low angle grain boundaries(LAGB)transition to high angle grain boundaries(HAGB)and the dynamic recovery(DRV)during the hot working process.The finite element simulation of the compressive thermal process was carried out using the VUSDFLD subroutine in ABAQUS software.Numerical simulation results of DRX volume fraction,dislocation density of HAGB and LAGB and the compressive load were obtained.Metallographic observation,X-ray diffraction(XRD)test and Electron Back-Scattered Diffraction(EBSD)analysis of microstructure after thermal compression.As a result,at the same strain rate,the higher the temperature,the lower the dislocation density of HAGB and LAGB,and the lower the stress;At the same temperature,the higher the strain rate,the higher the dislocation density of HAGB and LAGB,and the higher the stress;The average misorientation of LAGB decreases with increasing temperature,and the average misorientation of HAGB increases with increasing temperature.The experimental compressive loads are the same as the simulation results.Dislocation densities measured by XRD are consistent with the simulation result trend,which the rationality of the dislocation density model.Finally,the finite element simulation of the hot rolling process of AZ31 magnesium alloy was performed by utilizing the established dislocation density model to study the influence of sheet heating temperature,rolling speed and reduction on the microstructure evolution.