Investigations Of The Microstructure Evolution And Mechanical Properties Of Wrought Mg-Al-Zn/sn Alloys

Magnesium alloys have attracted much attention as the lightest engineering structural materials because of their low density,high specific strength combined with good properties of electromagnetic shielding and available recycling.However,wrought magnesium alloys are well known for poor formability at room temperature due to their limited slip systems and easily formed basal texture.Moreover,the formability of magnesium alloys is significantly infunenced by the morphology,size and volume fraction of second phases since coarse second phases with high volume fraction are deleterious to plastic deformation,which results in the severe cracks of rolled sheets.Therefore,the industrial appication of magnesium alloys is still confronted with challenges.Aiming at the challenges in the wrought magnesium alloys such as difficulty of controlling microstructure,serious cracking and low productivity,etc,we employ the intermediate-heat-treatment assisted rolling,high-ratio extrusion,and short-process rolling respectively on the Mg–Al–Zn/Sn alloys,and investigate the influence of alloy composition,second phases and grain size on the microstructure involution and mechniacal properties.Simultaneously,the industrial process for achieving high strength and high plasticity of magnesium alloys is also explored.The main conclusions are shown as follows:?1?A process of intermediate-heat-treatment assisted rolling is presented.Influence of pre-rolling pass,pre-rolling temperature and rolling direction on the microstructure and mechanical properties is investigated in the TRC ATZ421 alloy.It is revealed that pre-rolling with 2 passes assisted with intermediate-heat-treatment can effectively solve the difficulty of solid diffusion for the serious segregative phases in the TRC alloy;the yield strength shows apparent anisotropy in the tensile direction,which can be weakened by rolling along TD of the TRC alloy.The tensile properties of ATZ421 has been optimized,showing tensile strength of 313 MPa,yield strength of 255 MPa and elongation of 23.7% at room temperature,and excellent tensile strength of 214,154,93 MPa with corresponding elongation of 44.3%,68.5%,101.5%,respectively from 100 to 150 ?.?2?It is found that intermediate-heat-treatment assisted rolling can facilitate the solid diffusion of Mg₁₇Al₁₂ and Mg₂Sn eutectic phases to a-Mg,resulting in the dissolution of segregative phases in the TRC ATZ421 alloy.The anisotropy of tensile yield strength is mainly attributed to the distinction of schmid factors of prismatic <a> slip in the RD and TD;the greater the difference of their schmid factors,the greater the difference of yield strength.The good thermostability of Mg?17?Sn plays an important role in the high tensile strength and elongation of ATZ421 alloy.?3?A high-ratio extruded AZ61 sheet has been prepared in one step with an extrusion speed of 2.1 m·min-1 with excellent formability.Effect of solid solution and aging and direct aging on the microstructure and mechanical properties of extruded alloy is studied.It is revealed that the tensile strength and yield strength is enhanced from 314 to 336 MPa and 169 to 191 MPa repectively after direct aging with relatively high elongation?23.3%?,while solid solution and aging plays no role in improving strength,but decreasing ductility;the tensile deformability of extruded alloy at 300 ? is also remarkably improved by direct aging,showing superpalsticity of 306%.The yield strength of extrued AZ61 alloy is optimized with an increase of 68 MPa by low temperature rolling.?4?It is found that the high density of dislocations and vacancies produced by high-ratio extrusion can act as the effective nucleation sites of Mg₁₇Al₁₂ of continuous precipitation within grains,which improves the precipitation hardening and dispersion hardening,and also suppresses the growth of discontinuous Mg₁₇Al₁₂ precipitates from grain boundaries to grain interiors,resulting in the high strength and high ductility of the direct-aged AZ61 alloy.It is revealed that the enhanced yield strength of rolled AZ61 mainly results from the precipitation hardening and grain size strengthening.?5?A conventional process of short-process rolling is provided.Influence of grain size on the microstructure evolution and mechanical properties is investigated in the rolled AZ31 alloy.It is revealed that the dynamic recrystallization efficiency gradually decreases with the grain size increasing in the range from 200 to 370 mm,nevertheless,all the samples can finally obtain the uniform refined microstructure;the sample of 370 mm presents better tensile strength of 283 MPa,yield strength of 213 MPa and elongation to failure of 30.7% with relatively finer grains of 2.9 mm.The average grain size of AZ31 alloy is further optimized to 2.6 mm by decreasing the short-process rolling temperature,which significantly improves the yield strength to 238 MPa.?6?It is found that the sample of smaller grain size can provide more boundary nucleation sites for recrystallization and promotes dynamic recrystallization,while the sample of coarser grain size contains limited areas of high local strain for activating {10???1} compression twins or { 10???1 }-{ 10???2 } double twins due to the deformation nonuniformity,which delays recrystallization.The fraction of highly deformed grains and sub-structure grains of the final as-rolled samples increases with grain size increasing,resulting in finer annealed grains in the samples of coarse grain size than those of smaller grains and basal texture weakening,which further enhances the tensile strength and ductility.Overall,the investigation of microstructure evolution and mechanical properties of the Mg–Al–Zn/Sn alloys presents effective methods of intermediate-heat-treatment assisted rolling,high-ratio extrusion and short-process rolling to obtain magnesium alloy sheets with uniform microstructure and excellent tensile properties,which provides references for solving the practical difficulties of poor formability,easy cracking because of severe segregation,low productivity and cumbersome production process and so on.