Research On The Microstructure And Mechanical Properities Of Mg-Al-Ca Magnesium Alloys With High Alloying Content

As the lightest structural material,magnesium alloy has excellent specific strength,electrical and thermal conductivity and damping properties.These excellent characteristics make magnesium alloys gradually used in aerospace,automotive,electronics and medical fields.However,the lower elasticity modulus?45 GPa?of magnesium alloys severely limits the wide application of magnesium alloys in engineering.In this paper,Mg-Al-Ca alloy with high alloying content was prepared by traditional smelting casting process.A series of high-performance low-cost Mg-Al-Ca wrought magnesium alloys were prepared by homogenization treatment and extrusion process.The effects of Ca/Al ratio,Ca+Al content,Ca content and homogenization treatment on the microstructure and mechanical properties of the alloys were systematically studied.In the Mg-Al-Ca alloy system,the change in Ca/Al ratio significantly affects the composition and quantity of the second phase.When the total content of Ca+Al is 23wt.%and Ca/Al ratio is 0.15,the second phase in the alloy is C36-?Mg,Al?₂Ca phase and A12-Mg₁₇Al₁₂ phase.When Ca/Al ratio is increased to 0.75,the second phase in the alloy is only C36 phase.When Ca/Al ratio is 2.8,the second phase has only the C14-Mg₂Ca phase.The C36 phase in the as-cast microstructure of Mg-13Al-10Ca alloy exhibits a network structure.After 500°C×9h homogenization treatment and 350°C extrusion process,the structure of the particle band and the magnesium matrix phase is formed,and the yield strength is 260 MPa,the tensile strength is 324 MPa,the elongation is 6.6%,and the elastic modulus is 49 GPa.The elastic modulus of the A12 phase in the Mg-Al-Ca alloy is 70 GPa,the elastic modulus of the C14 phase is about 64 GPa,and the elastic modulus of the C36 phase is100 GPa.Therefore,in order to obtain an alloy of high modulus and high strength,it is desirable to have more C36 phases in the structure.The volume fraction of C36 phase in AX208 as-cast alloy is 42.9%,the yield strength after extrusion deformation is 274MPa,the tensile strength is 309MPa,the elongation is 1%,the elastic modulus is 51GPa;The volume fraction of C36 phase in AX2013 as-cast alloy is 50.1%.After extrusion process,the tensile strength is 233 MPa,the elongation is 0.5%,and the elastic modulus is as high as 54 GPa.When the Ca/Al ratio is maintained at 0.75,the kind of the second phase in the alloy does not change substantially with the increase of the total Ca+Al alloying content,but the amount of the second phase increases remarkably,and the elastic modulus of the alloy also increases.However,the presence of too much second phase reduces the elongation of the alloy.The homogenization heat treatment can modify the shape of the second phase in the alloy.By adjusting the homogenization treatment process,the secondary homogenization process of 400°C×24h+500°C×9h is designed to produce an alloy with high strength and elastic modulus.The yield strength of the alloy is 214 MPa and the tensile strength is 297MPa,the elongation is 2%and the elastic modulus is as high as 51 GPa.