Effect Of Microelement Calcium On Thermal Deformation Behavior And Dynamic Recrystallization Behavior Of AZ31 Magnesium Alloy

Magnesium alloys have broad application prospects in the fields of aerospace,electronic communication and medical devices,due to their low density,high specific strength,good shock absorption and recyclability.However,magnesium alloys exhibit poor plasticity and formability at room temperature,which restricts their large-scale commercial application.Adding element Ca can improve the texture type,the plasticity and formability of magnesium alloys.In recent years,many scholars have focused on the effect of Ca on the microstructure and mechanical properties of magnesium alloys.However,there have been few reports on the effect of microelement Ca on the thermal deformation behavior and dynamic recrystallization behavior of magnesium alloys.In this paper,AZ31 magnesium alloy and Mg-3Al-1Zn-0.2Ca(AZ31-0.2Ca)magnesium alloy were taken as the research objects.The effect of microelement Ca on the rheological behavior,thermal processing property and dynamic recrystallization evolution of AZ31 magnesium alloy were studied through isothermal compression simulation experiment,metallographic microscopy,electron backscatter diffraction technology and scanning electron microscopy.The main research contents and conclusions were as follows:(1)The flow stress curves of AZ31 and AZ31-0.2Ca alloys were obtained under the deformation temperature of 300-500?and strain rates of 0.001-1 s^-1 through the isothermal compression tests.The effects of deformation temperature and strain rate on the flow stress of alloys during hot deformation were analyzed.The effects of microelement Ca on the flow stress of AZ31 under different hot deformation conditions were studied.The results showed that the addition of Ca did not change the flow mechanism of the flow stress curves,but affected the stress of AZ31 alloy.(2)Based on the flow stress curves of AZ31 and AZ31-0.2Ca alloys,a constitutive model with the hyperbolic sine equation was established and the activation energy Q values were obtained.The results showed that the addition of Ca had slight effect on the activation energy and did not increase the difficulty of thermal deformation of AZ31alloy.(3)The hot processing maps of AZ31 and AZ31-0.2Ca alloys were established based on dynamic material model.The effects of Ca element on the efficiency of power dissipation and the instability parameter of AZ31 alloy were studied.The optimal processing zone of AZ31-0.2Ca alloy at strain 0.4 was determined.The results showed that the optimal processing areas were in the temperature range of 400-490?,strain rate range of 0.001-0.01s^-1 and in the temperature range of 420-480?,strain rate range of 0.2-1 s^-1.(4)The critical strains of dynamic recrystallization of AZ31 and AZ31-0.2Ca alloys under different deformation conditions were determined using the method of working hardening rate.The effects of thermal deformation parameters on the critical strain of alloys were revealed.The effects of Ca element on the critical strain and microstructure evolution of AZ31 alloy were analyzed.The results showed that the Ca element reduced the critical strain of dynamic recrystallization and increased the volume fraction of dynamic recrystallization of AZ31 alloy.With the addition of the Ca element,the Al₂Ca phase promoted the formation of dynamic recrystallization grains of AZ31 alloy.(5)The microstructures of AZ31 and AZ31-0.2Ca alloys under different strains at temperature of 450?and strain rate of 0.001s^-1 were studied using electron backscattered diffraction.The dynamic recrystallization mechanism of alloys was analyzed.The results showed that the addition of Ca did not change the dynamic recrystallization mechanism of AZ31 alloy.The dynamic recrystallization mechanisms were continuous dynamic recrystallization and discontinuous dynamic recrystallization,and discontinuous dynamic recrystallization mechanism was dominant.