Composition Optimization,Microstructure And Property Control Of Mg-Al-Zn Series Deformed Magnesium Alloys

Lightweight metal structural material magnesium alloys are widely used in the area of automotive,aerospace,electronic products,biomedical and military.Despite of the high specific strength of magnesium alloys,the low absolute strength and ductility are still hindering their further application.So,huge number of investigators have paid attention to developing high strength and ductility magnesium alloys.Mg-Al-Zn series alloys（AZ series）are traditional commercial magnesium alloys.It cost less than traditional high strength and ductility magnesium alloys such as Mg-RE series alloys.In this paper,Mg-6Al-1Zn-0.15Mn wt.%(AZM610 commercial alloys are master alloys.In order to obtain high strength and ductility commercial alloys,unique hot extrusion process and adjusting alloying elements are necessary.On the one hand,the influence of extrusion condition on alloys’strength and ductility has been investigated.AZM610 alloys were melted to as-cast billets.Microstructure in as-cast samples and heat treatment condition were studied.After solution treatment,the samples were prepared by traditional isothermal extrusion（ITE）and differential thermal extrusion（DTE）.Various extrusion condition samples were obtained through changing the temperature of billets and mold.Microstructure characterization in these extrusion samples were explored and also its mechanical properties were tested.Compared to sample prepared by ITE samples,the sample prepared by DTE-450-90（billet is preheated at 450°C and mold temperature is90°C）exhibited highest tensile strength.The tensile yield strength（TYS）and ultimate tensile strength（UTS）are 386 MPa and 423 MPa,respectively,with the elongation to failure of12.0%.The high strength mainly contributes to fine grain strengthening（grain boundary strengthening）effects by sub-micron dynamic recrystallized（DRXed）grain,higher volume fraction of DRXed regions,dynamic precipitates(Mg₁₇Al₁₂)strengthening,dispersion strengthening and higher density od dislocations.On the other hand,the influence of alloying elements on alloys’strength and ductility has been investigated.Removing element Mn from the AZM610 alloy,the Mg-6Al-1Zn wt.%（AZ61）alloy was obtained;adding trace amounts of heavy rare earth element Y in 61 alloy,Mg-6Al-1Zn-0.2Y wt.%（AZW610）alloy was obtained;adding trace amounts of element Ca,light and heavy rare earth elements La and Y in AZM610 alloys,Mg-6Al-1Zn-0.15Mn-0.5Ca wt.%（AZMX6100）alloy,Mg-6Al-1Zn-0.15Mn-0.2La wt.%（AZMLa6100）alloy and Mg-6Al-1Zn-0.15Mn-0.2Y wt.%（AZMW6100）alloy were obtained.Microstructure in as-cast samples and heat treatment condition were discovered.The microstructure of as-cast and extruded samples was characterized,heat treatment condition was optimized and the mechanical properties of extruded samples were tested.Microstructure characterizations revealed that Al-Mn phases disappear in 61 alloy;new phase Al₂Ca generated in AZMX6100alloy;new second phases Al₂Y generated in AZW610 and AZMW6100 alloy with the adding of Y;new phases Al-La generated in AZMLa6100 alloy with the adding of La.After solution treatment,all samples prepared by DTE process.All samples had relatively same diameter（～1μm）of DRXed grain size but different volume fraction of DRXed regions.Comparing the mechanical properties of the alloys,AZMLa6100 alloys with DTE-420-130 process and AZMW6100 alloys with DTE-420-150 process exhibited better tensile strength.The increasing of strength resulted from grain boundary strengthening by grain refinement,second phase strengthening and dislocation strengthening.