Rare earth magnesium alloy with high tensile-compressive symmetry and soil corrosion resistance

Abstract

This invention provides a rare-earth magnesium alloy with high tensile-compressive symmetry and resistance to soil corrosion. The alloy composition by mass percentage is: Gd: 0.3~0.5 wt.%, Y: 0.3~0.5 wt.%, Dy: 0.3~0.5 wt.%, Zr: 0.3~0.5 wt.%, with the balance being Mg and unavoidable impurities. The total content of alloying elements does not exceed 2 wt.%, significantly reducing the manufacturing cost of the magnesium alloy. The alloy is prepared using a combination of semi-continuous casting, homogenization treatment, and hot extrusion. The alloy exhibits a fine equiaxed grain structure, a weak basal texture, and a small amount of nanoscale second-phase particles. The corrosion rate of this alloy is 0.063 / mm·y. ‑1 This is significantly lower than the lowest value reported so far for magnesium alloys in soil environments (Mg-2Nd: 0.128 / mm·y). ‑1 Compared to ordinary commercial AZ31 magnesium alloy, the compressive-tensile yield strength ratio is increased by 63%. Under low rare earth content conditions, it achieves a synergistic improvement in high tensile-compressive symmetry and high resistance to soil corrosion, making it suitable for components in agricultural machinery or geological exploration equipment that are frequently used in soil and humid environments.

Description

Technical Field

[0001] This invention relates to a magnesium alloy, and more particularly to a rare earth magnesium alloy with high tensile and compressive symmetry and resistance to soil corrosion. Background Technology

[0002] Magnesium and magnesium alloys are among the lightest engineering structural materials available today, possessing numerous advantages such as low density, high specific strength and stiffness, good casting properties, good damping, ease of machining, and recyclability. They have extremely important applications in electrical appliances, automobiles, agricultural machinery, and defense industries. However, compared to common materials like aluminum alloys, the large-scale application of magnesium alloys is still significantly limited. This is mainly because the close-packed hexagonal crystal structure of magnesium alloys means that the critical shear stress for basal slip is much lower than that of other slip systems. During room temperature deformation, only basal slip is generally initiated, and basal slip cannot provide the five independent slip systems required to satisfy the Von-Mises criterion, making it difficult to deform magnesium alloys at room temperature. After extrusion, rolling, and other deformation processes, magnesium alloys typically exhibit strong basal texture, resulting in significant anisotropy and tensile-compressive asymmetry, causing serious defects in subsequent bending, stamping, and other forming processes. Furthermore, magnesium has a low standard electrode potential of approximately -2.37 V vs. SHE, resulting in high corrosion susceptibility. Additionally, the naturally formed oxide film (MgO / Mg(OH)2) on its surface is loose and porous, making it difficult to form a dense passivation film similar to that of aluminum alloys. This makes it susceptible to corrosion by substances containing soil microorganisms and Cl. - and SO4 2- It is extremely sensitive to humid service environments.

[0003] Currently, the main methods to improve the tensile-compressive symmetry and corrosion resistance of magnesium alloys include adding large amounts of rare earth elements and using multi-directional repeated deformation to optimize the grain size, texture, and properties of surface corrosion product films, thereby improving the tensile-compressive symmetry and corrosion resistance of magnesium alloys. However, multi-directional repeated deformation and the addition of large amounts of rare earth elements result in extremely high manufacturing costs, and the multi-directional repeated deformation process is complex and has low production efficiency. This greatly limits the application range of magnesium alloys. Therefore, there is an urgent need to improve the tensile-compressive asymmetry and corrosion resistance of magnesium alloys to expand their application in agricultural machinery and equipment, geological exploration equipment, and other fields that frequently operate in soil and humid environments.

[0004] Rare earth elements have been proven to have excellent modifying effects in weakening texture and improving corrosion resistance. By rationally controlling the content of rare earth elements and effectively controlling the hot extrusion process, the trace addition of rare earth elements Gd, Y, and Dy can effectively reduce the precipitation of a large number of second-phase particles within the alloy, thereby reducing micro-galvanic corrosion nucleation sites and lowering manufacturing costs. The addition of rare earth elements can improve the structure of the product film on the alloy surface, significantly increasing the density of the corrosion product film and hindering the invasion of corrosive media. Simultaneously, the addition of rare earth elements can weaken the basal texture, reduce the difference in slip system activation during tension and compression, and improve tensile-compression symmetry. Furthermore, the addition of trace amounts of Zr can serve as heterogeneous nucleation sites during alloy solidification, refining the grain size. Grain refinement not only improves tensile-compression symmetry and enhances the mechanical properties of the alloy, but also increases the number of grain boundaries, providing sufficient rapid nucleation sites for corrosion products, promoting a uniform and dense coverage of corrosion products on the alloy matrix surface, and further improving the alloy's corrosion resistance. Therefore, by adding trace amounts of rare earth elements such as Gd, Y, Dy and Zr to regulate the grain size, texture and precipitated phases of magnesium alloys, it is expected to obtain magnesium alloys that combine high tensile and compressive symmetry and resistance to soil corrosion, thus expanding the application range of magnesium alloys. Summary of the Invention

[0005] To achieve the above objectives, the present invention provides a method for designing and preparing a rare earth magnesium alloy with high tensile-compressive symmetry and resistance to soil corrosion.

[0006] The present invention discloses a method for preparing a rare earth magnesium alloy with high tensile and compressive symmetry and resistance to soil corrosion, which is composed of Mg, Gd, Y, Zr and Dy, and the mass percentage of each component is as follows: Gd: 0.3~0.5 wt.%; Y: 0.3~0.5 wt.%; Dy: 0.3~0.5 wt.%; Zr: 0.3~0.5 wt.%; the remainder being Mg and unavoidable impurities.

[0007] In a specific implementation, the magnesium alloy comprises a dynamically recrystallized structure and fine second-phase particles. The recrystallized structure includes both fine equiaxed grains and coarse deformed grains, with the fine second-phase particles distributed within the grains.

[0008] In a specific implementation, the average grain size of the magnesium alloy is 2.5~3.2 μm. The size of the second phase particles is 50nm~200nm.

[0009] As part of the same inventive concept, this invention also provides a method for preparing a rare earth magnesium alloy with high tensile-compressive symmetry and resistance to soil corrosion, comprising the following steps: a) Alloy smelting and casting: Semi-continuous casting method is used. The raw materials used are industrial pure magnesium ingots, Mg-30%Gd, Mg-30%Y, Mg-30%Dy, and Mg-28.54%Zr master alloys. Under the protection of SF6 and CO2 mixed gas, the magnesium ingots are placed in a crucible and heated to 730~750℃ after melting to remove slag. The master alloy is added according to the alloy composition ratio. Before adding the master alloy, it is preheated at 150~200℃ for 20~40 minutes. When adding the master alloy, it is quickly pressed below the liquid surface. After melting, it is stirred for 3~6 minutes. After stirring evenly, it is allowed to stand at 730~750℃ for 20~40 minutes. After standing, the slag on the surface of the melt is skimmed off. When the temperature drops to 700~720℃, it is poured into an iron mold at 200~300℃. b) Homogenization: Homogenization is carried out at a temperature of 370~410℃ for 8~13h. c) Hot extrusion: Before extrusion, the homogenized ingot and extrusion die are preheated at 350℃~390℃ for 1~2 hours, and then hot extrusion is carried out at 350℃~400℃. The extrusion ratio is 25:1 and the extrusion rate is 0.60~1.20m / min.

[0010] Compared with the prior art, the present invention has the following advantages: 1) This invention incorporates trace amounts of rare earth elements Gd, Y, and Dy to randomize grain orientation and weaken texture intensity, thereby effectively suppressing the {10-12} tensile twinning behavior that leads to tensile-compressive asymmetry. Simultaneously, the addition of trace amounts of the grain refiner Zr refines the grains, further promoting the synergistic activation of various deformation mechanisms such as basal and non-basal slip, and grain boundary slip, further improving tensile-compressive symmetry. Furthermore, grain refinement significantly increases grain boundary density; grain boundaries have higher surface energy, preferentially undergoing dissolution reactions, providing numerous uniform nucleation sites for corrosion product formation, promoting the formation of continuously distributed corrosion products during corrosion, and hindering the invasion of corrosive media. Additionally, the oxides of rare earth elements Gd, Y, and Dy can effectively fill the pores in the Mg(OH)2 / MgO corrosion product film, enhancing the protective properties of the corrosion product film. The alloy prepared by this invention has a tensile yield strength of 204 MPa and a compressive yield strength of 196 MPa, with a tensile-compressive-yield ratio of 0.96. After exposure to humid soil for 180 days, its corrosion rate is 0.063 / mm·y. -1 This achieves a synergistic improvement in tensile-compressive symmetry and soil corrosion resistance; 2) In the magnesium alloy provided by this invention, the total amount of Gd, Y, and Dy does not exceed 1.5 wt.%, and the total amount of alloying elements does not exceed 2 wt.%, significantly reducing the preparation cost of the magnesium alloy. Furthermore, the process of this invention is simple, easy to operate and control, and the equipment used, such as melting furnaces and hot extrusion presses, are all conventional and general-purpose equipment, exhibiting strong portability. 3) The magnesium alloy prepared by this invention has the advantages of low cost, high tensile-compressive symmetry and high resistance to soil corrosion. It is particularly suitable for application scenarios with high requirements for weight, structural reliability and corrosion resistance, such as geological exploration equipment and agricultural machinery equipment. It can provide a high-performance magnesium alloy material with low cost, high tensile-compressive symmetry and high corrosion resistance for related fields, and expand the application prospects of magnesium alloy in diversified fields. Attached Figure Description

[0011] Figure 1 Metallographic micrographs of the magnesium alloys prepared in Examples 1-2;

[0012] Figure 2 These are transmission electron microscope images of the microstructure of the magnesium alloys prepared in Examples 1 and 2;

[0013] Figure 3 These are scanning images of the corrosion products on the surface of the magnesium alloys prepared in Examples 1 and 2;

[0014] Figure 4 These are macroscopic texture images of the magnesium alloys prepared in Examples 1 and 2;

[0015] Figure 5 The tensile and compressive stress-strain curves of the magnesium alloys prepared in Examples 1 and 2 at room temperature are shown. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that these embodiments are for illustrating the present invention and not for limiting the present invention. The scope of protection of the present invention is not limited to the following embodiments.

[0017] Example 1: The Mg-Gd-Y-Zr magnesium alloy of this example comprises the following components by weight percentage: Gd: 0.3 wt.%, Y: 0.3 wt.%, Zr: 0.4 wt.%, with the remainder being Mg and unavoidable impurities.

[0018] The magnesium alloy of this embodiment can be obtained by preparing it according to the above proportions and in the following manner: a) Alloy smelting and casting: Semi-continuous casting method is used. The raw materials used are industrial pure magnesium ingots, Mg-30%Gd, Mg-30%Y, and Mg-28.54%Zr master alloys. Under the protection of SF6 and CO2 mixed gas, the magnesium ingots are placed in a crucible and melted. The temperature is then raised to 730~750℃ to remove slag. The master alloy is added according to the alloy composition ratio. Before adding the master alloy, it is preheated at 150~200℃ for 20~40 minutes. When adding the master alloy, it is quickly pressed below the liquid surface. After melting, it is stirred for 3~6 minutes. After stirring evenly, it is allowed to stand at 730~750℃ for 20~40 minutes. After standing, the slag on the surface of the melt is skimmed off. When the temperature drops to 700~720℃, it is poured into an iron mold at 200~300℃. b) Homogenization: Homogenization is carried out at a temperature of 370~410℃ for 8~13h. c) Hot extrusion: Before extrusion, the homogenized ingot and extrusion die are preheated at 350℃~390℃ for 1~2 hours, and then hot extrusion is carried out at 350℃~400℃. The extrusion ratio is 25:1 and the extrusion rate is 0.60~1.20m / min.

[0019] Example 2: The Mg-Gd-Y-Dy-Zr magnesium alloy of this example comprises the following components by weight percentage: Gd: 0.3 wt.%, Y: 0.3 wt.%, Dy: 0.3 wt.%, Zr: 0.4 wt.%, with the remainder being Mg and unavoidable impurities. The alloy smelting, casting-homogenization-hot extrusion method for obtaining the magnesium alloy of this example is the same as in Example 1, except for the composition of the smelted alloy.

[0020] The magnesium alloy extruded bars from Examples 1 and 2 above were subjected to room temperature tensile and compression tests and corrosion tests. The measured properties are shown in Table 1. For ease of comparison, Table 1 also provides the mechanical property data of the extruded AZ31 magnesium alloy, the mechanical properties of the Mg-2Nd alloy, and the corrosion rate.

[0021] Table 1 Mechanical properties and corrosion rate of the magnesium alloy material described in this invention AZ31 141.5 / 82.2 / 0.58 / Mg-2Nd 86 190 85 380 1.01 0.128 Example 1 149 199 135 334 0.91 0.049 Example 2 204 229 196 364 0.96 0.063

[0022] As shown in Table 1, the alloys of this invention (Examples 1-2) exhibit higher tensile-compressive yield symmetry than commercial AZ31 magnesium alloys, with a tensile-compressive yield strength ratio of 0.96, representing an improvement of approximately 63% in tensile-compressive symmetry compared to AZ31 magnesium alloys. Although the tensile-compressive asymmetry of the Mg-2Nd alloy is almost eliminated, its tensile and compressive yield strengths are both below 100 MPa, making it difficult to meet the mechanical performance requirements of most components, and it also exhibits a high corrosion rate. In contrast, this invention maintains excellent tensile-compressive symmetry and high corrosion resistance while maintaining low rare earth element content and low cost.

[0023] Microstructural analysis of the extruded alloy shows that the alloy of this invention is mainly composed of fine equiaxed crystals, such as... Figure 1 As shown, fine equiaxed grains not only improve tensile-compressive symmetry and enhance the mechanical properties of the alloy, but also provide ample rapid nucleation sites for corrosion products due to the abundant and uniformly distributed grain boundaries. This promotes the uniform and dense coverage of corrosion products on the alloy matrix surface, hindering the intrusion of corrosive media.

[0024] Furthermore, in this invention, the rare earth elements Gd, Y, and Dy, added in trace amounts to the magnesium matrix, are mainly dissolved in the magnesium matrix, and only a small number of second-phase particles with a size of 50 nm to 200 nm were detected in the alloy, such as... Figure 2 As shown. The reduced number of second-phase particles lowers the micro-galvanic corrosion effect at its source; simultaneously, the addition of trace amounts of rare earth elements effectively improves the density of the MgO / Mg(OH)2 corrosion product film on the alloy surface (e.g., Figure 3 As shown in the figure, the protective ability of the corrosion product film is enhanced, thus improving corrosion resistance. Furthermore, the addition of rare earth elements Gd, Y, and Dy can significantly weaken the texture and promote the improvement of tensile-compressive yield asymmetry, such as... Figure 4 As shown. Under the combined effect, the alloy of the present invention maintains high tensile and compressive symmetry while also possessing high resistance to soil corrosion, thereby meeting the requirements of agricultural machinery equipment, geological exploration equipment and other fields for lightweight, high-performance magnesium alloy materials with good tensile and compressive symmetry and resistance to soil corrosion.

[0025] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A rare earth magnesium alloy with high tensile and compressive symmetry and resistance to soil corrosion, characterized in that, The composition of this magnesium alloy by weight percentage is: Gd: 0.3~0.5 wt.%; Y: 0.3~0.5 wt.%; Dy: 0.3~0.5 wt.%; Zr: 0.3~0.5 wt.%; the remainder is Mg and unavoidable impurities, with a total impurity content of less than 0.1 wt.%.

2. The rare earth magnesium alloy with high tensile and compressive symmetry and resistance to soil corrosion according to claim 1, characterized in that, The magnesium alloy comprises a dynamic recrystallization structure and fine second-phase particles. The recrystallization structure includes both fine equiaxed grains and coarse deformed grains, with the fine second-phase particles distributed within the grains.

3. The rare earth magnesium alloy with high tensile and compressive symmetry and resistance to soil corrosion according to claim 2, characterized in that, The average size of the grains is 2.5~3.2 μm, and the size of the second phase particles is 50nm~200nm.

4. The method for preparing a rare earth magnesium alloy with high tensile and compressive symmetry and resistance to soil corrosion according to claim 1, characterized in that, Includes the following steps: a) Alloy smelting and casting: Semi-continuous casting is used. The raw materials are industrial pure magnesium ingots, Mg-30%Gd, Mg-30%Y, Mg-30%Dy, and Mg-28.54%Zr master alloys. Under the protection of SF6 and CO2 mixed gas, the magnesium ingots are placed in a crucible and heated to 730~750℃ after melting to remove slag. The master alloy is added according to the alloy composition ratio. Before adding the master alloy, it is preheated at 150~200℃ for 20~40 minutes. When adding the master alloy, it is quickly pressed below the liquid surface. After melting, it is stirred for 3~6 minutes. After stirring evenly, it is allowed to stand at 730~750℃ for 20~40 minutes. After standing, the slag on the surface of the melt is skimmed off. When the temperature drops to 700~720℃, it is poured into an iron mold at 200~300℃. b) Homogenization: Homogenization is carried out at a temperature of 370~410℃ for 8~13h. c) Hot extrusion: Before extrusion, the homogenized ingot and extrusion die are preheated at 350℃~390℃ for 1~2 hours, and then hot extrusion is carried out at 350℃~400℃. The extrusion ratio is 25:1 and the extrusion rate is 0.60~1.20m / min.

5. The application of a rare earth magnesium alloy with high tensile and compressive symmetry and resistance to soil corrosion, characterized in that, Magnesium alloys prepared using the methods of claims 1-4 are used in components for geological exploration, agricultural machinery, and other applications that are frequently exposed to humid or soil-rich environments.

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