Study On The Microstructure And Mechanical Properties Of ZM6 Magnesium Alloy During Hot Deformation

ZM6 heat-resistant magnesium alloy has good heat resistance and corrosion resistance,and has higher strength and better creep resistance at high temperature.However,it is difficult to deform at room temperature due to casting structure defects,which limits the wide application of ZM6 alloy.The heat deformation treatment of alloy at high temperature can improve the casting defects of alloy and obtain better microstructure,improve the comprehensive mechanical properties of alloy,and expand the application scope of alloy.Therefore,it is of great significance to develop ZM6deformed magnesium alloy.In this paper,ZM6 heat-resistant magnesium alloy was used as the experimental material.Firstly,the as-cast ZM6 alloy was treated by solid solution treatment,and then the thermal simulation compression deformation experiment was carried out,by analyzing the hot deformation behavior of ZM6 alloy,the constitutive equation and hot processing map and the best hot working range of ZM6 alloy was obtained.Equal channel angular pressing（ECAP）with different extrusion temperatures and extrusion passes was conducted to refine the grain size of the alloy.Finally,the alloy was aged at low temperature to further explore the effect of ECAP deformation conditions on the microstructure and properties of the alloy.The microstructure evolution and room temperature mechanical properties of the alloy were analyzed by metallographic observation（OM）,X-ray diffraction（XRD）,electron backscatter diffraction（EBSD）,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.The thermal simulation compression deformation of solution ZM6 alloy was carried out at 350～440℃and 0.001～1 s^-1,with the increase of strain rate and the decrease of deformation temperature,the flow stress of ZM6 alloy increased.The flow stress curve and microstructure of ZM6 alloy showed obvious characteristics of dynamic recrystallization.The activation energy of ZM6 alloy was 217.5204 KJ/mol,the hot compression constitutive equation was （?） and （?）.The hot deformation instability region of the alloy was mainly in the region of low temperature and high strain rate,and with the increase of strain,the instability region gradually decreased and advanced to the direction of high speed and high temperature.The safe hot working parameters of ZM6 alloy were two processing areas as follows:409～440℃of deformation temperature（T）and 0.001～0.025 s^-1of deformation rate(_?),376～389℃of T and0.001～0.002 s^-1of_?.The cast and SA state ZM6 alloy were mainly composed ofα-Mg matrix and Mg₁₂（Nd,Zn）phase,SEA state ZM6 alloy precipitated Mg₄₁Nd₅ and Mg₁₂（Nd,Zn）phases.The grain size of the alloy was greatly refined after ECAP deformation.With the increase of extrusion temperature,the average grain size of the alloy incresed,the content of second phase precipitates decreased,the morphology of the second phase in the alloy mainly changes from long rod shape to a large number of flakes,rod shape and then to short rod shape,the hardness,tensile strength and yield strength of the alloy decreased gradually,while the elongation increased gradually.The average grain size of the SEA 2P alloy was 13.2μm.After 4 passes ECAP deformation and aging at different extrusion temperatures,with the increase of extrusion passes,the average grain size of SEA state ZM6 alloy was obviously refined and more uniformed,the total content of the second phase gradually increased,and the size of the second phase gradually increased.The morphology of the second phase in the alloy was mainly from granular,long rod and flake to a large number of flake,rod,and then to basically all long rod;the hardness of the alloy increased gradually,while the tensile strength,yield strength and elongation first increased and then decreased.When other conditions were constant,the comprehensive mechanical properties of the alloy are the best after 4 passes ECAP deformation at 425℃and aging.The ultimate tensile strength of SEA 4P alloy was 244.32 MPa and the elongation was 14.19%,which was 125.6%higher than SA state ZM6 alloy.After ECAP deformation under different conditions and aging,the fracture surface of SEA state ZM6 alloy was typical ductile fracture,and there were many dimples on the fracture surface,and there were obvious precipitation of second phase particles in the dimples.