| As the international community continues to place greater emphasis on energy and pollution issues,more and more countries are focusing on the development of lightweight metal materials.At the same time,the demand for high performance lightweight metals is also increasing.The research,development,and application of lightweight metal materials are highly compatible with China’s goals of achieving"carbon peaking"and"carbon neutrality".Magnesium and its alloys have great application prospects in the field of lightweight metal materials and have received high attention from scholars both at home and abroad.Due to its advantages such as high specific strength,high specific stiffness,good noise reduction and vibration damping performance,recyclability,and non-pollution to the environment,it has been widely applied in many fields.However,the low tensile strength and poor ductility of magnesium alloys have become urgent problems that need to be solved in order to further expand their application range.Therefore,improving the mechanical properties of magnesium alloys has become a hot and key topic in the research of lightweight metal materials.To achieve high performance of magnesium alloys,investigating their deformation behavior and corresponding deformation mechanisms is the most fundamental work.Mg-Al-Ca-Mn-Zn alloy exhibits good properties as a non-rare earth magnesium alloy.Therefore,this paper focuses on Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn alloy,and uses rolling as the main plastic deformation method to systematically analyze the effects of different deformation conditions on the formability,microstructure,mechanical properties,and corresponding deformation mechanisms.Suitable annealing processes were used to heat-treat the rolled plates,and then plates with fine-grained structures were selected for high-temperature superplasticity behavior.The main research contents and results are summarized as follows in this paper:（1）Elevated rolling temperature within a certain range is beneficial to improve the formability and mechanical properties of Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn alloy during multi-pass rolling experiments.Moreover,higher temperature provides conditions for dynamic recrystallization and improves the uniformity of grain distribution.（2）Changing the direction of rolling can weaken the consistency of grain orientation in the sheet and reduce the strength of the basal texture.When the Route D cross-rolling path is used to roll the sheet,the formability is optimal,the edge cracks are completely eliminated,the average grain size is reduced from 24.57μm in unidirectional rolling to 14.3μm,and the grain size distribution is uniform.At the same time,the K values of the magnesium alloy sheet on the（0002）、（1012—）and（101—4）crystal planes are close to 1,and the anisotropy is greatly improved.The tensile strength and elongation of the RD and TD directions are similar,and the average elongation is 19%.（3）Proper intermediate heat treatment between passes can promote recrystallization,refine grain size,reduce the hardness of the sheet,and improve the formability of the final rolled sheet.The optimal intermediate heat treatment process is determined to be 200℃×24h+480℃×2 h,with a tensile strength of 293.03 MPa and an elongation rate of 12.56%.（4）The properties of Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn alloy sheet are significantly affected by rolling temperature,rolling strain rate,reduction in thickness,and pre-rolling with large strains.It was ultimately determined that the best overall properties of the magnesium alloy sheet were achieved by subjecting it to a small deformation pre-rolling,changing the rolling direction by 90°,and then directly reducing the 10 mm sheet to 2 mm thickness with 80%thickness reduction at a temperature of 450℃and a rolling strain rate of 20 s-1.Under these conditions,the main K values of the magnesium alloy sheet are close to 1 and the anisotropy of the sheet was improved,the ultimate tensile strength was 270.2MPa,elongation was 22.67%,and the fracture mode was ductile fracture.（5）After annealing Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn alloy sheet that underwent multiple rolling passes at 400℃for 180 minutes,the microstructure was composed of equiaxed grains with an average grain size of 12.63μm.If the temperature is further increased or the annealing time is extended,abnormal grain growth will occur in the microstructure,which destroys the uniformity of the structure.This indicates that this annealing process is the optimal process.The ultimate tensile strength was 237.2 MPa,and the elongation was 19.86%,which was a 314%improvement over the elongation of the rolled state（4.8%）.（6）The optimal annealing process for Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn alloy sheet produced by single-pass rolling is 350℃/1 h.At this state,the average grain size mainly falls between 5-10μm,and the ultimate tensile strength is 251.57 MPa with an elongation of 26.16%.（7）Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn magnesium alloy reaches its maximum elongation at a deformation temperature of 623 K and a strain rate of 1.67×10-4 s-1,with a value of296.9%.During high-temperature tensile testing,a decrease in strain rate or an increase in temperature will cause varying degrees of decrease in ultimate tensile strength but an increase in elongation for Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn alloy.Additionally,magnesium alloys undergo three stages during high-temperature tensile testing,namely strain hardening stage,dynamic balance steady-state rheology stage,and softening stage.In this study,the maximum and minimum values of the strain rate sensitivity exponent（m）are 0.4 and 0.17,respectively.（8）The superplastic deformation mechanism of Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn alloy was studied,the Q was calculated to be 98.7～140.2 k J/mol,and the n was 3.A normalized curve was constructed,and a constitutive equation for superplastic deformation mechanism was established. |
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