Effect Of Alloying On Microstructure And Mechanical Properties Of AZ91 Wrought Magnesium Alloy

Magnesium alloy has the advantages of low density and high specific strength.It is a promising structural metal material that has been widely used in aerospace,automotive parts,portable electronic equipment and other fields.In order to further improve the application fields and scope of magnesium alloys,researchers around the world are improving various properties of magnesium alloys by various methods,such as alloying,heat treatment and thermal deformation.Among the magnesium alloys,AZ series?Mg-Al-Zn?magnesium alloys have excellent corrosion resistance,good mechanical properties,and good cast ability.Among them,the AZ91 alloy has medium strength and toughness,which can be used in both the as-cast and deformed.By introducing rare earth elements,its shape can be changed and its mechanical properties can be further improved.In this study,the microstructure of AZ91magnesium alloy was modified by adding alloying elements,and the purpose of improving its mechanical properties was achieved.In this paper,AZ91 magnesium alloy is used as the base alloy.Based on the addition of a certain amount of Ca element,the content of mixed rare earth alloy elements?Y and Sm?with different total contents when the ratio of the rare earth elements Y and Sm is constant is investigated.AZ91,AZ91-1Ca-x Y-y Sm?x=0,0.33,0.67,1;y=0,0.17,0.33,0.5wt.%?alloys were prepared by traditional casting methods,and the microstructure and properties of alloys by heat treatment and extrusion were studied.The results show that,through XRD and EDS analysis,the AZ91 alloy with Ca,Y,and Sm alloying elements forms?-Mg phase,eutectic?-Mg₁₇Al₁₂ phase,Al₂Ca phase,Al₂Y phase and Al₂Sm phase during the casting process.After solution treatment,the?-Mg₁₇Al₁₂ phase in the alloy is basically dissolved into the matrix.After aging,a large amount of?-Mg₁₇Al₁₂ phase is uniformly distributed in the grain boundaries and intragrains.Some Ca,Y,and Sm elements can dissolve into the matrix after aging to the role of solid solution strengthening.Lamellar?-Mg₁₇Al₁₂ phase was precipitated in the alloy after solution aging,and granular?-Mg₁₇Al₁₂ phase was precipitated near the grain boundary.The yield strength and tensile strength of aging AZ91 and AZ91-1Ca-x Y-y Sm?x=0,0.33,0.67,1;y=0,0.17,0.33,0.5wt.%?series alloys show a tend of first increase and then decrease.Among them,AZ91-1Ca-0.67Y-0.33Sm alloy has the best mechanical properties,and its yield strength,tensile strength and elongation are 172 MPa,243MPa and 8.9%,respectively.Through the analysis of extrusion experiments at a certain temperature and extrusion ratio,it can be known that as the content of rare earth elements increases,AZ91 and AZ91-1Ca-x Y-y Sm?x=0,0.33,0.67,1;y=0,0.17,0.33,0.5wt.%?alloy grain size gradually decreased.AZ91-1Ca-0.67Y-0.33Sm alloy has the best mechanical properties.Its yield strength,tensile strength and elongation of the alloy are 225 MPa,321 MPa,and 16%,respectively.During the extrusion process,the intermetallic compounds Al₂Y and Al₂Sm phases promote dynamic recrystallization and nucleation to refine the grains.At the same time,the distribution of high-temperature stable phases also hinders the movement of dislocations,thereby improving the mechanical properties of the alloy.When there are too many Al₂Sm phases,it is easy to form hard and brittle phases at the grain boundaries and reduce the mechanical properties of the alloy.When the total content of Y and Sm is 1%,the peak ageing AZ91-1Ca-0.67Y-0.33Sm alloy has yield strength,tensile strength,and elongation of 223 MPa,350 MPa,and 13%,respectively.