Research On Microstructure And Mechanical Properties Of Mg-Gd-Y-Zn-Zr Alloy Via Centrifugal Casting,Extrusion And Rolling

The high-strength Mg-Gd-Y-Zn-Zr alloy has broad application prospects in key fields such as aerospace,national defense and military industry,and transportation manufacturing.In order to broaden the application fields of magnesium alloys,this thesis is the preparation of high-strength Mg-Gd-Y-Zn-Zr alloys by using centrifugal casting technology;we improve the uniformity of the structure and properties of the cast alloy by approptiate homogenization treatment;high-strength magnesium alloy plates are prepared by different extrusion processes;the extruded billet alloy via large-strain high-efficiency rolling process to achieve high-efficiency and large-strain preparation of high-strength and tough plates;finally,we successfully explored a new preparation process of magnesium alloys:the centrifugal casting large-strain high-efficiency rolling process,which has important reference significance for the forming technology of high strength and toughness magnesium alloy.The as-cast Mg-8Gd-4Y-1Zn-0.4Zr alloy is mainly composed ofα-Mg,Mg₂₄（Gd,Y）₅,Mg₃（Gd,Y）and LPSO phases.Compared with gravity casting,the grains of the alloy prepared by centrifugal casting are obviously refined,the volume fraction of the second phase increases and the distribution of the dispersion is refined;with the increase of the centrifugal radius,the alloy grain size gradually decreases,the second phase is refined and its distribution becomes more uniformly dispersed,and the strength gradually increases.The optimal homogenization treatment process of the centrifugal as-cast Mg-8Gd-4Y-1Zn-0.4Zr alloy was explored.After homogenization treatment at 510 ℃ for 12 hours,non-equilibrium Mg₃（Gd,Y）eutectic compounds were transformed to equilibrium Mg₅（Gd,Y）phase,the strength and elongation of the as-cast alloy were improved.Compared with the gravity casting alloy,the yield strength of the alloy at the largest centrifugal radius increased from 88MPa to 119MPa.The increment is about 35%.The ultimate tensile strength（UTS）increased from 161MPa to 226MPa.The increment is about about 40%.The elongation increased from 3.3%to 4.8%.The extruded Mg-8Gd-4Y-1-Zn-0.4Zr alloy is composed ofα-Mg matrix,Mg₅（Gd,Y）and LPSO phases.The structure is composed of deformed large grains and fine recrystallized grains when the extrusion temperature is 420 ℃.The alloy extruded at 450℃ is all fine and uniform recrystallized grains.However,the grains grow up when the extrusion temperature is 480℃.As the extrusion temperature increases,the size of the recrystallized grains gradually increases,and the volume fraction of the second phase gradually decreases.The 420 ℃ extruded sheet has the best tensile strength of 345MPa,and the 450 ℃ extruded sheet has the best comprehensive mechanical properties,with a UTS of 329 MPa and an elongation rate of 11%.Compared with the alloy with an extrusion ratio of 7,the grains of the alloy with an extrusion ratio of 22 grow abnormally,and the mechanical properties decrease.Based on the as-extruded Mg-Gd-Y-Zn-Zr alloy,the high-strength alloy sheets were prepared by large-strain high-efficiency rolling process,with 50%reduction per pass.The cumulative reduction of 80%and 90%can be obtained by merely 2 rolling passes.The rolled alloy is composed ofα-Mg matrix,Mg₅（Gd,Y）and LPSO phase.The microstructure and mechanical properties at three rolling temperatures of 390 ℃,420 ℃ and 450 ℃ are studied.As the rolling temperature increases,the occurrence of recrystallization and the growth of grains are promoted.The number and volume fraction of second phases gradually decrease.Compared with 80%reduction,the recrystallized grains of the sheet with 90%reduction are finer with more dispersed precipitates.The plate with low rolling temperature and large deformation has the highest strength.The alloy with 390 ℃ rolling reduction has a tensile strength of 418Mpa and an elongation of6.1%.By using centrifugal casting+large-strain high-efficiency rolling process,high-strength magnesium alloy plates are efficiently prepared.The alloys that are directly rolled in the centrifugal as-cast state and rolled after homogenization are all mixed crystal structures composed of coarse deformed grains and fine recrystallized grains.Compared with the direct rolling,the rolled alloy after homogenization has finer grains.The comprehensive mechanical properties are higher,the tensile strength reaches 367Mpa,and the elongation is 9.2%.