Deformation Behavior And Constitutive Model Of AZ31 And GW83 Magnesium Alloys During Double-pass Isothermal Compression

Wrought magnesium(Mg) alloys have important light-weight application potential in industries such as aerospace,high-speed transportation,and military equipment.However,for hot-rolled sheets and forgings of Mg alloys,multi-pass deformation is required and the evolutions of microscopic deformation mechanism and microstructure are complex,which makes it difficult to precisely match the external load and optimize the microstructure.In this research,Mg–3Al–1Zn(AZ31)and Mg–8Gd–3Y(GW83)Mg alloys are selected as the experimental materials.Firstly,double-pass isothermal compression will be carried out to study the flow behavior of Mg alloys under discontinuous thermal deformation with a temperature range of 350?–450?and a strain rate range of 0.001 s^-1–0.1 s^-1.Based on the dynamic material model,three-dimensional processing maps of AZ31 and GW83 Mg alloys will be established to comparatively study the evolutions of formability during double-pass deformation.Secondly,grain growth behavior and textural evolution during inter-pass holding are going to be studied using the electron backscatter diffraction(EBSD)technique.Finally,a new physical-based constitutive model will be established to describe the dynamic hardening and dynamic softening behavior of Mg alloy during hot deformation,laying a theoretical foundation for the subsequent multi-pass forming of complex components.Mg alloys exhibit a unique hardening behavior during double-pass deformation,that is,the second-pass peak stress is higher than the first-pass unloading stress.AZ31 Mg alloy exhibits the hardening behavior under various deformation conditions,while GW83Mg alloy presents both softening and hardening behavior.Different deformation behavior of the two alloys is related to the microstructural evolution during inter-pass holding.Rapid grain growth behavior is captured in AZ31 Mg alloy during holding,and the average grain size increased with the prolongation of time.Larger grain size causes higher peak stress in the second pass deformation,resulting in the hardening behavior.GW83Mg alloy undergoes precipitation and grain growth behaviors during inter-pass holding,causing the precipitation-induced hardening behavior in a temperature range of 350?–400?and a strain rate of 0.1 s^-1,and the grain-growth-induced hardening behavior at450?and a strain rate range of 0.01 s^-1–0.1 s^-1.Three-dimensional processing maps of the two Mg alloys illustrate a wider forming window under double-pass deformation.The inter-pass holding could improve the thermal formability of Mg alloys.For AZ31 Mg alloy,as the total strain is 80%,there exists no instability zone under single-pass and double-pass deformation when the temperature is lower than 375?.The alloy under double-pass deformation presents a narrower instability zone and a higher energy dissipation coefficient.For GW83 Mg alloy,double-pass deformation makes the material have better formability than the single-pass,which can also reduce deformation bands in the macrostructure.Comparing the processing maps of AZ31 and GW83 Mg alloy,it can be found that the instability zone of AZ31 Mg alloy mainly exists in a range with high temperature and large strain rate,while the instability zone of GW83 Mg alloy is concentrated in the low temperature and large strain rate.Therefore,large-size components of AZ31 Mg alloy are recommended to be produced by a low-temperature and multi-pass forming.The high-temperature and multi-pass deformation are advised for the hot processing of GW83 Mg alloy.Microstructural evolutions of Mg alloys during inter-pass holding show preferential grain growth behavior in a shorter time.For the holding treatment of AZ31 Mg alloy at400?,as the pre-strain exceeds 6%,the alloy could undergo rapid grain growth without an incubation period.There is no obvious change in the grain size of GW83 Mg alloy with a pre-strain of 10%under the 450?holding,and grain coarsening could be discovered at the pre-strain of 30%and 50%.Microstructural evolution during holding is related to the preferential growth of strain-free grains formed by pre-deformation,and the driving force comes from the difference in stored strain energy between deformed grains and strain-free grains.For AZ31 Mg alloy,two kinds of strain-free grains in the pre-strained microstructure are mainly responsible for the rapid increase in grain size.One is the undeformed grains at the pre-strain of 6%.The other is the dynamic recrystallized grains when the pre-strain is higher than 10%.For GW83 Mg alloy,the newly recrystallized grains produced by pre-deformation account for the microstructural evolution,showing the typical metadynamic recrystallization behavior.Textural evolutions of the two Mg alloys show inter-pass holding could strengthen the basal texture of AZ31 Mg alloy and maintain the weak texture of GW83 Mg alloy.For AZ31 Mg alloy,as c-axes of undeformed grains and dynamic recrystallized grains in pre-strained microstructure are parallel to the compression directions,preferential growth of strain-free grains results in the enhanced basal texture after holding.For GW83 Mg alloy,no preferential growth of grains with special orientation occurs,which allows the weak texture to be preserved.Therefore,a larger deformation amount(e.g.,over 30%)combined with faster cooling is favorable for obtaining fine grains and weak texture.A new physical-based constitutive model for isothermal deformation is established by integrating with dislocation density evolution,dynamic recrystallization,grain growth,and grain boundary sliding.The introduction of grain boundary sliding is proved to be reasonable by analyzing the evolution of flow stress,average grain size,and strain-free grain size during compression.The constitutive model successfully predicts the flow stress of AZ31 Mg alloy in the temperature range of 350?–450?and a strain rate range of 0.001 s^-1–0.1 s^-1.The correlation coefficient between predicted stress and experimental stress is 0.988,and the average relative deviation is 5.74%.Meanwhile,the model can effectively predict the average grain size during hot deformation.At 400?and a strain rate of 0.01 s^-1,the average relative deviation between predicted grain size and measured value is 4.7%.The new physical constitutive model presents good accuracy in the prediction of flow stress and average grain size.The simulation on deformation behavior and microstructure of Mg alloy during isothermal compression shows grain boundary sliding promotes the growth of fine recrystallized grains,resulting in a rapid increase of recrystallization fraction.Therefore,grain refinement can be achieved under a larger strain(e.g.more than 15%).The model is used to quantitatively analyze the effects of grain size and dislocation density on peak stress.The peak strain increases with the rise of grain size,which is caused by the combined effect of dynamic recrystallization and grain boundary sliding.It is found that the second-pass peak stress is mainly related to the evolution of grain size during inter-pass holding.A two-site mean field model of grain growth is introduced to describe the microstructural evolution during inter-pass holding,and the constitutive model successfully predicts the hardening behavior of AZ31 Mg alloy during double-pass deformation.The average relative deviation between the predicted stress and experimental stress is 10.0%.However,as the proposed constitutive model only covers the deformation behavior of one material point,it needs to be combined with the finite element software to study the thermal deformation behavior of Mg alloys.Also,the model does not involve the effect of second phase and dynamic precipitation on the thermal deformation behavior,and it cannot accurately predict the flow behavior of GW83 Mg alloy during isothermal compression.