Microstructure Evolution And Properties Control Of AZ91D—(Gd,Y) Rare Earth Magnesium Alloy

Through the control of material composition and organization,obtaining excellent performance to meet the use of materials in various fields has been the goal pursued by many scholars.The methods of controlling the properties of alloys include alloying,heat treatment and cold deformation.The composition and performance of the material are controlled from the aspects of composition and process.Magnesium alloy has the advantages of light weight,high specific strength and specific rigidity,and has a large storage capacity in the earth's crust,which is widely used in fields such as chemical equipment,automotive,aerospace,military and other fields.Currently the lightest metal engineering material.However,magnesium alloys still have some defects that limit their applications.For example,it is unstable at high temperatures,has poor performance,and has the disadvantages of being active and easily oxidized,which limits the use of magnesium alloys in certain fields..But to make full use of the excellent properties of magnesium alloys,it is necessary to improve the defects of magnesium alloys.This requires controlling the structure and properties of the alloy from the perspective of the application of magnesium alloys,such as controlling the composition of alloys by adding alloy elements and performing a series of heat treatments on the alloy to control its structure and properties.The structure of the alloy can be controlled by large cold deformation,thereby controlling the properties of the alloy.Studies have shown that due to the unique chemical properties of rare earth elements and the arrangement of extra-nuclear electrons,the structure and properties of magnesium alloys can be improved.Therefore,by adding rare earth elements to the magnesium alloy to change the composition of the alloy,the structure and properties of the alloy can be controlled,the structure of the alloy can be optimized,and the effect of the material control method can be intuitively reflected by the properties of the alloy.Therefore,it is of great significance to study the distribution ratio of rare earth magnesium alloys and the control principle of rare earth elements on the micro-structure of the alloys.In this paper,different contents of rare earth elements Gd and Y are added to the widely used AZ91D die-casting magnesium alloy in the industry,the alloy composition is controlled,and the micro-structure of the alloy is controlled by different heat treatment processes to achieve the purpose of controlling alloy properties.Different testing techniques such as OM,XRD,transmission electron microscopy,scanning electron microscopy,energy spectrum analysis,and tensile and hardness measurements are used to observe the alloy structure and analyze its mechanical properties.The amount of rare earth elements added was determined by analyzing the structure and properties of the alloy to control the composition of the alloy.At the same time,the treatment process of the alloy was selected.The control variable method was used to study the principle of controlling the structure and properties of AZ91D magnesium alloy under different processing conditions.The experiment is divided into the addition of a single Gd element and the addition of Gd and Y elements to change the composition of the alloy.The research shows that:When the composition of the alloy is controlled by the single addition of Gd element,for the as-cast alloy,the structure of the alloy is mainly composed of?-Mg matrix,?-Mg₁₇Al₁₂2 phase and rare earth phase Al₂Gd.After the Gd and Y elements are added in combination,the alloy is detected To a small amount of bulk Al6Mn phase and rare earth phase Al₂Y.With the increase of the rare earth added in the alloy,the continuous network of?-Mg₁₇Al₁₂2 phase in the alloy is gradually decomposed and refined,and it transforms into rod,block and granular.Diffuse distribution and matrix.With the increase of rare earth,the grains of the alloy gradually refine.After the solution treatment,the?-Mg₁₇Al₁₂2 phase in the alloy gradually decomposes and dissolves into the matrix,and after the aging treatment,the?phase in the matrix will precipitate outward.And it is distributed in the matrix in a granular form.After the addition of rare earth elements,the Al₂Gd and Al₂Y phases formed will be distributed in the form of lumps or particles near the grain boundaries of the alloy or inside the crystal.The study of controlling the structure and properties of a single Gd element added alloy through different heat treatment processes found that:for the as-cast magnesium alloy structure,the addition of rare earth element Gd is 1.0%,which has the most obvious effect on the structure of the alloy,and the alloy has the best mechanical properties.At this time,the tensile strength is 268.40 MPa,the elongation is 6.2%,and the hardness value is 100.9 HV.After the alloy is subjected to solid solution treatment,the mechanical properties of the alloy are optimal when the amount of Gd added to the solid solution alloy is 1.0%.The tensile strength reached288.47 MPa,the elongation reached 11.33%,and the hardness value was 93.2 HV.The tensile strength and elongation have been significantly improved,and the hardness of the alloy has decreased to a certain extent compared to the as-cast alloy.After alloying with rare earth elements,after solid solution aging treatment,the alloy has the best performance when the Gd content is 0.8%,the peak tensile strength is294.38 MPa,the elongation is 10.27%,and the hardness is 109 HV.The hardness and tensile strength of the alloy are increased,while the elongation is decreased.The study of controlling the micro-structure and properties of the alloy with the addition of rare earth elements through different heat treatment processes found that when the content of Y is 0.8%and the addition of Gd and Y is 1.8%,the properties of the as-cast and solid-solution alloys reach the best.The as-cast alloy has a tensile strength of 283.73 MPa,an elongation of 9.2%,and a hardness of 97.1 HV.After solution treatment,the tensile strength of the alloy was 308.68 MPa,the elongation was 14.94%,and the hardness was 94.2 HV.