The Influence Of Different Casting Methods On The Microstructure And Property Of Mg-Gd-Y-Ag-Zr Casting Alloy

As the lightweight structure metal,magnesium alloys are becoming increasingly attractive for structural in aerospace,automotive and electronic industries for their low density,high specific strength,good vibration attenuation and shock resistant performance,good thermal and electrical conductivity and well electromagnetic shielding performance.In addition,magnesium alloy has good recyclability,known as the "green engineering materials in the 21 st century".However,the mechanical properties of the pure magnesium is poor,it is difficult to meet the requirement of engineering structure material.The alloying is most efficient way to improve the mechanical properties of magnesium alloy.Except that,magnesium alloy chemical properties are lively,the standard electrode potential is only-2.36 V.Corrosion of magnesium alloy is easy to occur in the humid air and the generated oxide film is porous,which can't protect the magnesium matrix.At present,corrosion resistance and mechanical properties of magnesium alloys has become a main factor hinder the application of magnesium alloys.Rare earth elements have larger solid solubility in manesium,and the solid solubility of magnesium drops sharply with the decreas of temperature.The join of rare earth elements can effectively improve the organization of the alloy,increase strength and corrosion resistance,rare earth elment is the most effective alloying element in magnesium alloys.The representatives of the rare earth magnesium alloys are Mg-Nd,Mg-Y and Mg-Gd alloy.Among them,the Mg-Gd-Y alloys show good room temperature strength and high temperature creep resistance,which received wide attention of scholars both at home and abroad.This paper selects the Mg-Gd-Y-Ag-Zr alloy as the research object and mainly compared the mechanical properties,microstructure and corrosion resistance of Mg-Gd-Y-Ag-Zr alloy in low pressure casting or gravitiy casting..The results show that:(1)The defects in as-cast organization of low pressure casing Mg-Gd-Y-Ag-Zr alloy is less than gravity casting alloy,the low pressure casing alloy show better mechanical properties than that of gravity casting alloy in as-cast,cast-T4 and cast-T6 condition.(2)In solidification process,the solidification speed of low pressure casting Mg-GdY-Ag-Zr alloy is slow,the ingredient transformation of solid phase and liquid phase is similar to the equilibrium solidification process,the Gd,Y elements in the alloy tended to form Mg5(Gd,Y)and Mg24(Gd,Y)5 phase.During the solidification process of gravity casting alloy,some Mg3(Gd,Y)phase form for the composition fluctuations and the elment diffusion of Mg3(Gd,Y)phase is hardly due to the quick temperature decrease.The residues in as-cast organization contain more high melting Mg3(Gd,Y)phase,and it makes the solution effect of gravity casting alloy poor.The higher content of Gd and Y element in low pressure casting alloy makes the precipitation strength of low preesure casting alloy better than that of gravity casting alloy.(3)The corrosion types of Mg-Gd-Y-Ag-Zr alloys is galvanic corrosion,which composed of particle phase and ?-Mg matrix.The particle phase is made up of Mg3(Gd,Y)phase and Mg5(Gd,Y)phase.(4)In the as-cast and cast-T6 conditon,the rate of evolved hydrogen of low pressure casting Mg-Gd-Y-Ag-Zr alloy is faster than that of gravity casting alloy,but slower in castT4 condition.