Design And Research Of High Strength And Low Cost Mg-Zn-X Alloy

The addition of rare earths to magnesium alloys can significantly increase the high temperature strength of the alloy and improve the casting properties,but expensive rare earth elements increase the cost of the magnesium alloy.In order to improve the application of magnesium alloy,this paper designs magnesium alloy by adding a small amount of rare earth or no rare earth in the Mg-Zn alloy.In the end,the purpose of reducing the cost can be achieved when the strength is ensured.In this paper,the Mg-Zn system is used.In some experiments,Y and Zr elements are added to form a long-period stacking order（LPSO）phase,and a small amount of Sn element is added to regulate the LPSO phase.Another part of the experiment is carried out in the Mg-Zn-Cu alloy.Adding Al and Sn,using the double aging process,improves and promotes the aging effect of Zn.Theα-Mg,Mg₁₀YZn（18R-LPSO）and very small amount of Mg₃Y₂Zn₃（W）phase can be found in Mg-4.6Y-2.5Zn-0.6Zr as-cast alloy.The mechanical properties of the alloy are best during 8h in the heat treatment.After adding a small amount of the Sn alloy,the second phase of the as-cast alloy becomes fine,the secondary dendrite arm spacing（the SDAS）decreases.Adding 0.2 wt%of Sn to the alloy does not introduce a new phase.When the amount of Sn added is as high as 0.5 wt%,the Sn₃Y₅ phase can be detected in the alloy.The Sn addition may hinder the conversion of 18R-LPSO to 14H-LPSO in the alloy during the heat treatment.The addition of Sn has a refinement regulating effect on the second phase of the modified alloy.Theα-Mg,MgZn₂ and MgZnCu phases are mainly found in Mg-8Zn-0.5Cu as-cast alloys.With the addition of Al,the secondary dendrite arm spacing（SDAS）of the as-cast alloy first increases and then decreases,and Mg₃₂（Al,Zn）₄₉ phase and MgAlCu phase appear,and the as-cast mechanical properties decrease.The addition of Sn is only a mechanical addition to the Mg-Zn-Cu system and does not cause phase changes of other elements.The aging precipitation phase of Mg-8Zn-1Al-0.5Cu alloy under single aging are mainly the long rod-like phase and a small amount of block phase,and the precipitate phases under the double aging are the short thin rod phase and the small block phase.The latter is denser.After double aging,the alloy with the highest tensile strength is Mg-8Zn-2Al-0.5Cu,and the yield strength,tensile strength and elongation are 185 MPa,330 MPa and 6.7%,respectively.The alloy with the best comprehensive performance is Mg-8Zn-1Al-0.5Cu,and the yield strength,ultimate tensile strength and elongation are 191 MPa,321 MPa and 8.8%,respectively.Finally,the fracture characteristics are analyzed.It is found that there are many transgranular fractures and the fine cleavage surfaces in the fracture of Mg-8Zn-1Al-0.5Cu alloy,and it accords with the excellent mechanical properties.