High-strength cast magnesium alloy and preparation method thereof

Abstract

The invention belongs to the technical field of magnesium alloy materials and casting, and particularly relates to a high-strength casting magnesium alloy and a preparation method thereof. The high-strength cast magnesium alloy is prepared from the following components in percentage by weight: 4.5 to 6.5 percent of Gd, 2.2 to 3.4 percent of Nd, 0.1 to 0.6 percent of Zn, 0.3 to 1.0 percent of Zr, 0 to 0.05 percent of Sr and the balance of Mg and inevitable impurity elements. According to the high-strength rare earth magnesium alloy and the preparation method thereof, the addition of the Y element of a conventional high-strength rare earth magnesium alloy is avoided, the total rare earth content of the conventional high-strength rare earth magnesium alloy is reduced, through the compound addition of Gd, Nd and Zn elements and scientific element content design optimization, the high strength of the alloy is guaranteed, meanwhile, excellent alloy plasticity is kept, the casting manufacturability of the alloy is improved, and the production cost is reduced. The alloy is excellent in fluidity, low in hot crack tendency and small in oxide inclusion tendency.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnesium alloy materials and casting, and particularly relates to a high-strength cast magnesium alloy and a preparation method thereof. Background Art

[0002] Magnesium alloys are widely used in aerospace, shipbuilding, rail transportation, weapons, electronics, and other fields due to their outstanding advantages, including high specific strength, high specific stiffness, high damping and shock absorption, and excellent electromagnetic shielding. In the aerospace and defense industries, the rapid development of high-end weaponry and equipment has led to an increasingly urgent demand for high-load, lightweight, and high-power-density structural components, which in turn places increasing demands on the mechanical properties of magnesium alloys.

[0003] Aircraft engine casings and aerospace missile cabins are both high-end equipment structural parts with highly integrated and complex structures. Therefore, casting is the main forming process for these magnesium alloy structural parts, which also places high demands on the casting process performance of magnesium alloy materials. At present, ZM6 alloy (Mg-Nd series) is a typical high-strength rare earth magnesium alloy application material for aerospace structural parts. It has good and mature casting processability, but the material mechanical properties (tensile strength of 230-260MPa, yield strength of 130-150MPa) cannot meet the strength requirements of the new generation of aerospace high-performance magnesium alloy castings. The British company Magnesium Elektron has developed a series of Mg-Y high-strength magnesium alloys, WE43 and WE54 alloys, with high mechanical properties (tensile strength of 265-295 MPa, yield strength of 175-190 MPa). They have been experimentally applied to a certain type of domestic aircraft casing. However, the casting processability of this material is extremely poor, especially the yttrium oxide inclusion defects that are easily generated during the casting process, which are difficult to overcome. This leads to an extremely low casting qualification rate, which seriously limits the application of Mg-Y alloys. In recent years, a series of new Mg-Gd and Mg-Gd-Y high-strength magnesium rare earth alloys have been developed both domestically and internationally, further improving the mechanical properties of the materials (tensile strength can reach 350 MPa, yield strength can reach 200 MPa). However, the rare earth element content of these alloys is too high (up to 12-18%), resulting in poor fluidity, high sensitivity to hot cracking, and a high tendency to oxidize. During the casting process, metallurgical defects such as porosity, cracks, segregation, and oxidation inclusions are easily formed, making the casting process extremely difficult and increasing production costs. Furthermore, the alloys have low elongation and a mismatch between strength and plasticity, making these materials unsuitable for mass production of high-performance, complex, thin-walled structural parts. Therefore, facing the technical requirements of high-end equipment structural parts in future aerospace and other fields, there is currently a lack of applicable high-strength cast magnesium alloy materials. Summary of the Invention

[0004] The purpose of the present invention is to provide a high-strength cast magnesium alloy and a preparation method thereof in response to the deficiencies in the existing technology, to solve the industry bottleneck problem that the mechanical properties and casting processability of the existing high-strength magnesium alloys cannot be taken into account at the same time, and to meet the application requirements of high-performance complex structural parts in the new generation of aerospace and other fields.

[0005] The technical solution of the present invention is:

[0006] According to a first aspect of the present invention, a high-strength cast magnesium alloy is provided, comprising the following components in percentage by weight: Gd: 4.5-6.5%, Nd: 2.2-3.4%, Zn: 0.1-0.6%, Zr: 0.3-1.0%, Sr: 0-0.05%, and the balance being Mg and unavoidable impurity elements.

[0007] Preferably, the above-mentioned high-strength cast magnesium alloy comprises the following components in weight percentage: Gd: 5.8-6.4%, Nd: 2.4-2.9%, Zn: 0.2-0.5%, Zr: 0.3-1.0%, Sr: 0.02-0.05%, and the remainder is Mg and inevitable impurity elements; the inevitable impurity elements include Fe, Cu, Si, Ni, etc., and the total amount of impurity elements is ≤0.1% by weight.

[0008] The present invention adopts Gd element and Nd element as the main alloying element for strengthening. Gd element and Nd element all have high solid solubility in Mg matrix, and solid solubility decreases rapidly with the decline of temperature, therefore all have significant aging strengthening ability. The composite addition of Gd and Nd elements can further reduce each other's solid solubility at room temperature, so that the alloy generates more fine and dispersed nano-strengthening phases during aging, realizes further enhancement of aging strengthening ability, and thus alloy strength is greatly improved. In addition, composite addition can also reduce the total rare earth content when added alone under the same mechanical property level on the one hand, reduce the formation of rare earth oxide inclusion defects, and can also increase the content of the high melting point second phase precipitated during solidification on the other hand, which means that the number of intercrystalline bridges is more, so that a more powerful pinning effect is formed between the crystals on both sides of the solidification process, and then realizes an effective reduction in hot cracking sensitivity.

[0009] The present invention adopts Zn element as a microalloying element to further enhance the strengthening effect and optimize the casting processability. The addition of Zn further enhances the precipitation of the prismatic rare earth-strengthening β' phase and promotes the formation of the basal γ" phase. This creates a cage-like, enclosed space structure composed of both prismatic and basal precipitates. This microstructure further hinders basal slip in the Mg crystal structure, enhancing the strengthening effect of the precipitates and, consequently, further improving the alloy's strength. Furthermore, it increases the fluidity of the magnesium alloy melt and improves the alloy's casting performance. However, excessive Zn addition increases the alloy's hot cracking susceptibility and stimulates the formation of LPSO phases in the matrix, which robs the rare earth atoms originally forming the rare earth-strengthening phases, reducing the number of strengthening precipitates in the matrix and significantly reducing the alloy's strength. Furthermore, excessive Zn addition can cause the formation of secondary phases in the Mg-Gd-Nd alloy matrix during solidification that are difficult to fully dissolve in the matrix during solution treatment. The coarse secondary phases remaining at the grain boundaries significantly degrade the alloy's plasticity.

[0010] Zr is a commonly used grain refiner in magnesium alloys. The present invention uses the combined addition of Zr and Sr as a microalloying method to further enhance grain refinement. The addition of Sr reacts with Mg to form high-melting-point compounds such as Mg2Sr. These compounds act as heterogeneous nucleation sites during solidification, resulting in grain refinement. Furthermore, Sr reduces the surface energy of the grains and, during solidification, is repelled to the solid-liquid interface, preventing grain growth.

[0011] Another aspect of the present invention provides a method for preparing a high-strength cast magnesium alloy, the method comprising the following steps:

[0012] (1) Ingredients: Pure Mg ingots, pure Zn ingots, Mg-30 wt.% Gd master alloys, Mg-30 wt.% Nd master alloys, Mg-30 wt.% Zr master alloys, and Mg-20 wt.% Sr master alloys were weighed and prepared according to the weight percentages of the chemical components;

[0013] (2) Melting: The melting furnace is powered on, and protective gas is introduced into the crucible. A pure Mg ingot is placed in the crucible of the melting furnace. After the pure Mg ingot is completely melted, a pure Zn ingot, a Mg-30wt.% Nd master alloy, a Mg-30wt.% Gd master alloy, a Mg-30wt.% Zr master alloy, and a Mg-20wt.% Sr master alloy are added in sequence. After all the metals in the crucible are completely melted, the mixture is stirred for 3 to 6 minutes to obtain a uniform alloy melt.

[0014] (3) Degassing: Lower the melt temperature to the degassing temperature range, introduce inert gas into the alloy melt for degassing, and after degassing is completed, perform slag removal on the melt surface;

[0015] (4) Refining: Raise the melt temperature to the refining temperature range, use a refining agent to refine the melt, clean the slag on the melt surface and crucible wall after refining, and then heat the melt to 800-820°C and let it stand for 20-30 minutes;

[0016] (5) Pouring: A protective gas is introduced into the mold cavity, and the melt temperature is reduced to 720-760°C. The magnesium alloy melt is then poured into the mold under a protective gas environment, and a casting or ingot is obtained after complete solidification.

[0017] (6) Heat treatment: After cleaning the alloy ingot or casting obtained in step (5), place it in a medium-high temperature heat treatment furnace for solution treatment, then transfer it to water for quenching, and finally place it in a low-temperature heat treatment furnace for aging treatment after the ingot or casting cools to room temperature. After aging is completed, take it out and air cool it to room temperature to obtain a high-strength cast magnesium alloy.

[0018] Preferably, in step (1), the pure Mg ingots, pure Zn ingots, Mg-30wt.% Gd master alloys, Mg-30wt.% Nd master alloys, Mg-30wt.% Zr master alloys and Mg-20wt.% Sr master alloys symmetrically taken in advance are sandblasted or polished to clean them, and then dried at a drying temperature of 200-300° C. for 1-2 hours.

[0019] Preferably, the protective gas in step (2) is a mixture of SF6 and CO2 gas in a volume ratio of 1:100, the protective gas temperature is 16-20°C, and the protective gas ventilation flow rate is 40-60 L / min.

[0020] Preferably, the addition temperature of the pure Zn ingot and the Mg-30wt.% Nd master alloy in step (2) is 740-760°C, the addition temperature of the Mg-30wt.% Gd master alloy is 760-780°C, the addition temperature of the Mg-30wt.% Zr master alloy is 780-800°C, and the addition temperature of the Mg-20wt.% Sr master alloy is 750-770°C.

[0021] Preferably, the degassing temperature in step (3) is in the range of 740 to 760° C.; the inert gas is high-purity argon; the inert gas flow rate is 10 to 20 L / min; and the degassing treatment time is 10 to 15 min.

[0022] Preferably, the refining temperature in step (4) is in the range of 750-760° C., the refining treatment time is 10-15 min, and the refining agent is RJ-5 flux.

[0023] Preferably, the refining treatment in step (4) is specifically to sink the refining stirrer to a depth of 2 / 3 to 3 / 4 of the melt for stirring, and at the same time sprinkle RJ-5 flux on the crest of the melt; the total amount of RJ-5 flux added is 1.0 to 2.0% of the total weight of the charge.

[0024] Preferably, the protective gas in step (5) is a mixture of SF6 and CO2 gas in a volume ratio of 1:100, the protective gas temperature is 16-20°C, and the protective gas ventilation flow rate is 20-40 L / min.

[0025] Preferably, the medium- and high-temperature heat treatment furnace in step (6) is a vertical quenching and solution furnace equipped with a fan, and CO2 protective gas needs to be introduced into the furnace; the low-temperature heat treatment furnace is a pit-type resistance furnace equipped with a fan, and a mixture of pyrite and sulfur powder in a weight ratio of 12:1 needs to be placed in advance in the furnace.

[0026] Preferably, the solution treatment in step (6) is a two-stage solution treatment, firstly keeping the temperature at 380-420°C for 2-3 hours, then heating to 510-520°C and keeping the temperature for 16-20 hours.

[0027] Preferably, the transfer time of the ingot or casting after the solution treatment to the water for quenching in step (6) is ≤15s; and the water temperature for quenching in the water is 20-80°C.

[0028] Preferably, the aging treatment temperature in step (6) is 220-230° C., and the aging treatment holding time is 18-26 h.

[0029] The advantages of the present invention are:

[0030] 1. The high-strength cast magnesium alloy provided by the present invention avoids the addition of Y element in conventional high-strength rare earth magnesium alloys and reduces the total rare earth content of conventional high-strength rare earth magnesium alloys. Through the composite addition of Gd, Nd and Zn elements and scientific element content design optimization, the alloy maintains excellent alloy plasticity while ensuring high strength, and also improves the casting processability of the alloy. The alloy has excellent fluidity, low hot cracking tendency, and low tendency of oxidation inclusion.

[0031] 2. The present invention adopts a melt composite purification treatment method that combines argon degassing and refining agent stirring refining, which not only effectively controls the loss of rare earth elements in the magnesium alloy, but also reduces the content of oxide inclusions in the melt, while also reducing the concentration of hydrogen in the melt and the tendency of shrinkage defects in the casting, thereby strengthening the purification of the rare earth magnesium alloy melt and facilitating the acquisition of high-performance, low-defect high-strength magnesium alloy castings.

[0032] 3. The present invention introduces protective gas into the mold cavity to replace the air in the cavity, thereby reducing the oxygen content in the cavity, strengthening the protection of the magnesium alloy melt during the pouring and filling process, and reducing the tendency of linear defects to form on the surface of the rare earth magnesium alloy casting.

[0033] 4. Through the high-strength cast magnesium alloy and its preparation method provided by the present invention, high-strength magnesium alloy castings with high mechanical properties and excellent metallurgical quality can be obtained. The high-strength magnesium alloy castings can achieve a tensile strength of 306-339 MPa, a yield strength of 207-255 MPa, and an elongation of 5-11% at room temperature, meeting the mechanical performance requirements of future lightweight and high-load aerospace components.

[0034] 5. The preparation method of the present invention is simple and convenient, has low operation difficulty, does not require special equipment, has low production cost, and is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a metallographic photograph of the high-strength cast magnesium alloy prepared in Example 1 after heat treatment; Figure 2 This is a microstructure photograph of the precipitated phase of the high-strength cast magnesium alloy prepared in Example 1 after heat treatment; Figure 3 This is a metallographic photograph of the high-strength cast magnesium alloy prepared in Example 3 after heat treatment; Figure 4 This is a metallographic photograph of the high-strength cast magnesium alloy prepared in Example 6 after heat treatment; Figure 5 This is a metallographic photograph of the high-strength cast magnesium alloy prepared in Example 7 after heat treatment; Figure 6 This is a microstructure photograph of the precipitated phase of the high-strength cast magnesium alloy prepared in Example 7 after heat treatment; Figure 7 This is a microstructure photograph of the precipitation phase of the high-strength cast magnesium alloy prepared in Comparative Example 1 after heat treatment; Figure 8 This is a metallographic photograph of the high-strength cast magnesium alloy prepared in Comparative Example 2 after heat treatment; Figure 9 This is a metallographic photograph of the magnesium alloy prepared in Comparative Example 2. DETAILED DESCRIPTION

[0036] The present invention will be described in detail below in conjunction with specific embodiments and accompanying drawings. The following embodiments are implemented under the premise of the technical solution of the present invention, which will help those skilled in the art to further understand the present invention. The following embodiments are only some embodiments of the present invention, not all embodiments, and the present invention is not limited to these embodiments.

[0037] Example 1

[0038] This embodiment relates to a high-strength cast magnesium alloy, the chemical composition of which is as follows by weight: Gd: 5.88%, Nd: 2.57%, Zn: 0.38%, Zr: 0.51%, Sr: 0.03%, the total amount of impurity elements is ≤0.1%, and the balance is Mg.

[0039] This embodiment relates to a method for preparing the aforementioned high-strength magnesium alloy, which specifically includes the following steps:

[0040] (1) Batching: First, the surface of the raw material ingots required for batching is polished clean. Then, the corresponding weights of pure Mg ingots, pure Zn ingots, Mg-30wt.% Gd master alloy, Mg-30wt.% Nd master alloy, Mg-30wt.% Zr master alloy, and Mg-20wt.% Sr master alloy are weighed according to the weight percentages of the above-mentioned alloy chemical compositions. Then, the weighed raw material ingots are placed in a drying oven at 250°C for 2 hours.

[0041] (2) Melting: The melting furnace is powered on, and a mixed protective gas of SF6 and CO2 gas with a volume ratio of 1:100 is introduced into the crucible. The protective gas temperature is 20°C and the ventilation flow rate is 50 L / min. A pure Mg ingot is placed in the crucible of the melting furnace. After the pure Mg ingot is completely melted, a pure Zn ingot and a Mg-30wt.% Nd master alloy are sequentially added at 750°C, a Mg-30wt.% Gd master alloy is added at 770°C, a Mg-30wt.% Zr master alloy is added at 790°C, and a Mg-20wt.% Sr master alloy is added at 760°C. After all the metals in the crucible are completely melted, the mixture is stirred for 5 minutes to obtain a uniform alloy melt.

[0042] (3) Degassing: The melt temperature was lowered to 750°C, and high-purity argon gas at a flow rate of 15 L / min was introduced into the alloy melt for degassing. After degassing for 15 minutes, the melt surface was subjected to slag removal.

[0043] (4) Refining: The melt temperature was raised to 755°C, and the refining stirrer was sunk to 2 / 3 of the melt depth and stirred for 15 min. At the same time, RJ-5 flux accounting for 1% of the total weight of the charge was continuously sprinkled on the crest of the melt flow for refining. After completion, the slag on the melt surface and the crucible wall was cleaned, and the melt temperature was raised to 800°C and allowed to stand for 30 min.

[0044] (5) Pouring: A mixed protective gas of SF6 and CO2 with a volume ratio of 1:100 is introduced into the mold cavity. The protective gas temperature is 20°C and the ventilation flow rate is 30 L / min. At the same time, the melt temperature is lowered to 720°C. The magnesium alloy melt is then poured into the mold under protective gas environment conditions. After complete solidification, an ingot is obtained.

[0045] (6) Heat treatment: After the alloy ingot is cleaned, it is placed in a vertical quenching solution furnace equipped with a fan and filled with CO2 protective gas for two-stage solution treatment. The solution treatment process is: first keep it at 400℃ for 2h, then heat it to 515℃ and keep it for 16h; then transfer it to 25℃ water for quenching, and the quenching transfer time is ≤15s; finally, after the ingot is cooled to room temperature, it is placed in a pit-type resistance furnace equipped with a fan for aging treatment. A mixture of pyrite and sulfur powder with a weight ratio of 12:1 needs to be placed in the furnace in advance. The aging treatment process is: keep it at 225℃ for 22h, take it out and air cool it to room temperature after aging is completed, and obtain a high-strength cast magnesium alloy.

[0046] The high-strength cast magnesium alloy prepared in this embodiment was tested for its flowability using a metal spiral mold. The flow length was 34.3 cm at a pouring temperature of 720°C.

[0047] The hot cracking sensitivity of the alloy in this example under constrained conditions was evaluated using a T-shaped hot cracking die. At a pouring temperature of 720°C, no obvious cracks appeared at the hot spot, and no visible signs of crack initiation appeared on the cross-section of the T-shaped specimen, indicating that the alloy in this case has low hot cracking sensitivity.

[0048] The metallographic structure of the magnesium alloy obtained in this embodiment is shown in FIG. Figure 1 , the grain size is about 49μm; the microstructure of the precipitated phase of the magnesium alloy obtained in this embodiment is shown in Figure 2 .

[0049] The room temperature mechanical properties of the high-strength cast magnesium alloy prepared in this embodiment are as follows: tensile strength of 335 MPa, yield strength of 220 MPa, and elongation of 9%.

[0050] Example 2

[0051] The chemical composition of the alloy in this embodiment is exactly the same as that in Example 1. The only difference is the heat treatment step in the preparation method. That is, in this embodiment, the alloy is quenched in 60° C. water after solution treatment. The rest of the preparation process is exactly the same.

[0052] The room temperature mechanical properties of the high-strength cast magnesium alloy prepared in this embodiment are: tensile strength of 333 MPa, yield strength of 218 MPa, and elongation of 10%. The room temperature mechanical properties of the alloy in this embodiment are basically equivalent to those of the alloy prepared in Example 1.

[0053] Example 3

[0054] The preparation method of this embodiment is exactly the same as that of Example 1, with the only difference being the chemical composition of the alloy, mainly that the Gd element content is slightly increased. That is, the chemical composition of the magnesium alloy in this embodiment is as follows by weight: Gd: 6.12%, Nd: 2.65%, Zn: 0.45%, Zr: 0.52%, Sr: 0.03%, the total amount of impurity elements is ≤0.1%, and the balance is Mg.

[0055] The metallographic structure of the magnesium alloy obtained in this embodiment is shown in FIG. Figure 3 The grain size is about 47 μm, which is basically the same as the grain size of Example 1.

[0056] The room-temperature mechanical properties of the high-strength cast magnesium alloy prepared in this example are: tensile strength of 336 MPa, yield strength of 232 MPa, and elongation of 11%. Because the Gd content in this example is higher than that in Example 1, the alloy generates more strengthening precipitates (β' phase) (Mg5(Gd,Nd)) after heat treatment, resulting in the alloy's higher strength while maintaining excellent plasticity.

[0057] Example 4

[0058] The preparation methods of this embodiment are exactly the same as those of Example 3, with the only difference being the chemical composition of the alloy, mainly that the Gd element content is further increased. That is, the chemical composition of the magnesium alloy in this embodiment is as follows by weight: Gd: 6.29%, Nd: 2.77%, Zn: 0.48%, Zr: 0.54%, Sr: 0.03%, the total amount of impurity elements ≤ 0.1%, and the balance is Mg.

[0059] The room-temperature mechanical properties of the high-strength cast magnesium alloy prepared in this example are: tensile strength of 339 MPa, yield strength of 245 MPa, and elongation of 7.5%. Because the Gd content in this example is higher than that in Example 3, heat treatment further promotes the precipitation of the strengthening β' phase, further enhancing the alloy's strength.

[0060] Example 5

[0061] The preparation methods of this embodiment are exactly the same as those of Example 3, with the only difference being the chemical composition of the alloy, mainly a slight increase in the Nd element content. That is, the chemical composition of the magnesium alloy in this embodiment is as follows by weight: Gd: 6.15%, Nd: 2.84%, Zn: 0.38%, Zr: 0.48%, Sr: 0.03%, the total amount of impurity elements ≤ 0.1%, and the balance being Mg.

[0062] The room temperature mechanical properties of the high-strength cast magnesium alloy prepared in this embodiment are as follows: tensile strength of 337 MPa, yield strength of 234 MPa, and elongation of 8%.

[0063] Example 6

[0064] The preparation methods of this embodiment are exactly the same as those of Example 4, with the only difference being the chemical composition of the alloy, mainly that the Nd content is significantly increased. That is, the chemical composition of the magnesium alloy in this embodiment is as follows by weight: Gd: 6.25%, Nd: 3.4%, Zn: 0.5%, Zr: 0.47%, Sr: 0.03%, the total amount of impurity elements ≤0.1%, and the balance being Mg.

[0065] The metallographic structure of the magnesium alloy obtained in this embodiment is shown in FIG. Figure 4 The grain size is about 49 μm, and there is a coarse second phase in the matrix that has not been dissolved.

[0066] The room-temperature mechanical properties of the high-strength cast magnesium alloy prepared in this example are: tensile strength of 317 MPa, yield strength of 255 MPa, and elongation of 5%. Because the Nd content in this example is increased compared to Example 4, the alloy forms more precipitates after heat treatment, resulting in a higher yield strength. However, the solid solubility of Nd in the Mg-Gd-Nd-Zn-Zr alloy is relatively low. Further increases in the Nd content lead to the formation of a second phase in the matrix that is difficult to dissolve back during the solid solution phase, reducing the alloy's plasticity and tensile strength. Overall, the alloy in this example still exhibits an excellent balance between strength and plasticity.

[0067] Example 7

[0068] The preparation method of this embodiment is exactly the same as that of Example 1, with the only difference being the chemical composition of the alloy, mainly that the Gd element content is reduced in comparison. That is, the chemical composition of the magnesium alloy in this embodiment is as follows by weight: Gd: 4.6%, Nd: 2.6%, Zn: 0.5%, Zr: 0.5%, Sr: 0.03%, the total amount of impurity elements is ≤0.1%, and the balance is Mg.

[0069] The metallographic structure of the magnesium alloy obtained in this embodiment is shown in FIG. Figure 5 The grain size is about 52 μm, which is basically the same as the grain size of Example 1.

[0070] The room temperature mechanical properties of the high-strength cast magnesium alloy prepared in this example are: tensile strength of 306 MPa, yield strength of 207 MPa, and elongation of 9.5%. Although the Gd content in this example is lower than that in Example 1, the strengthening precipitation phase formed in the alloy is relatively small, which reduces the strength of the alloy, but overall the alloy still has excellent mechanical properties. The microstructure of the precipitation phase of the magnesium alloy obtained in this example is shown in Figure 2. Figure 6 .

[0071] Example 8

[0072] The preparation method of this embodiment is exactly the same as that of Example 1, and the only difference is the chemical composition of the alloy. The main difference is that no Sr element is added in this embodiment. That is, the chemical composition of the magnesium alloy in this embodiment is as follows by weight: Gd: 5.91%, Nd: 2.6%, Zn: 0.35%, Zr: 0.52%, the total amount of impurity elements is ≤0.1%, and the balance is Mg.

[0073] The metallographic structure of the magnesium alloy obtained in this embodiment is shown in FIG. Figure 7 The grain size is about 57 μm, which is larger than that in Example 1.

[0074] The room temperature mechanical properties of the high-strength cast magnesium alloy prepared in this embodiment are as follows: tensile strength of 320 MPa, yield strength of 214 MPa, and elongation of 8%.

[0075] Comparative Example 1

[0076] The preparation method of this comparative example is exactly the same as that of Example 1, and the only difference is the chemical composition of the alloy. The main difference is that the Gd element content is significantly reduced in comparison. That is, the chemical composition of the magnesium alloy in this example is as follows by weight: Gd: 2.9%, Nd: 2.5%, Zn: 0.48%, Zr: 0.56%, Sr: 0.03%, the total amount of impurity elements is ≤0.1%, and the balance is Mg.

[0077] The room temperature mechanical properties of the alloy obtained in this comparative example are: tensile strength of 294 MPa, yield strength of 175 MPa, and elongation of 12.1%. Since the Gd content in this example is significantly lower than that in Example 1, the strengthening precipitation phase formed in the alloy is significantly reduced, resulting in a lower strength of the alloy. The microstructure of the precipitation phase of the magnesium alloy obtained in this example is shown in FIG. Figure 8 .

[0078] Comparative Example 2

[0079] The preparation method of this comparative example is exactly the same as that of Example 1, and the only difference is that the chemical composition of the alloy is different, mainly that the Gd element content is significantly increased. That is, the chemical composition of the magnesium alloy in this example is as follows by weight: Gd: 8.6%, Nd: 2.4%, Zn: 0.51%, Zr: 0.44%, Sr: 0.03%, the total amount of impurity elements is ≤0.1%, and the balance is Mg.

[0080] The metallographic structure of the magnesium alloy obtained in this embodiment is shown in FIG. Figure 9 The grain size is about 48 μm, and there is a coarse second phase in the matrix that has not been dissolved.

[0081] The room-temperature mechanical properties of the alloy obtained in this comparative example were: tensile strength of 331 MPa, yield strength of 246 MPa, and elongation of 3%. Since the Gd content in this example was significantly higher than that in Example 1, while the alloy strength was improved, the excessive Gd addition resulted in the formation of a second phase in the alloy that was difficult to dissolve back into the matrix, thus severely deteriorating the alloy's plasticity.

[0082] Comparative Example 3

[0083] The preparation method of this comparative example is exactly the same as that of Example 1, and the only difference is the chemical composition of the alloy. The main difference is that the Nd element content is significantly reduced in comparison. That is, the chemical composition of the magnesium alloy in this example is as follows by weight: Gd: 5.91%, Nd: 1%, Zn: 0.44%, Zr: 0.5%, Sr: 0.03%, the total amount of impurity elements is ≤0.1%, and the balance is Mg.

[0084] The room temperature mechanical properties of the alloy obtained in this comparative example are: tensile strength of 296 MPa, yield strength of 186 MPa, and elongation of 10.5%. Because the Nd content in this example is significantly lower than that in Example 1, the strengthening precipitation phase formed in the alloy is significantly reduced, resulting in the alloy having lower strength.

[0085] Comparative Example 4

[0086] The preparation method of this comparative example is exactly the same as that of Example 3, with the only difference being the chemical composition of the alloy, mainly that the Zn content is significantly increased. That is, the chemical composition of the magnesium alloy in this example is as follows by weight: Gd: 6.08%, Nd: 2.6%, Zn: 1.2%, Zr: 0.51%, Sr: 0.03%, the total amount of impurity elements ≤0.1%, and the balance being Mg.

[0087] The room-temperature mechanical properties of the alloy obtained in this comparative example were: tensile strength of 297 MPa, yield strength of 198 MPa, and elongation of 5.6%. Because the Zn content in this example was significantly higher than that in Example 1, it promoted the formation of the LPSO phase in the matrix. This reduced the amount of strengthening precipitates formed after heat treatment, lowering the alloy's strength. Furthermore, excessive Zn addition can lead to the formation of a difficult-to-resolve secondary phase at grain boundaries, thereby reducing the alloy's plasticity.

[0088] Comparative Example 5

[0089] This comparative example is for preparing a conventional ZM6 magnesium alloy. The chemical composition of the ZM6 magnesium alloy is as follows by weight: Nd: 2.3%, Zn: 0.3%, Zr: 0.6%, impurity elements Cu ≤ 0.1%, Ni ≤ 0.01%, the total amount of other impurity elements ≤ 0.3%, and the balance being Mg.

[0090] The preparation method of the ZM6 magnesium alloy in this comparative example is basically the same as that in Example 1, with the only difference being the batching, melting and heat treatment steps. The specific differences are as follows: in the batching and melting steps of this comparative example, the addition of the Mg-30wt.% Gd master alloy is not involved; in the heat treatment step of this comparative example, the heat treatment process is as follows: first, the alloy is kept at 400°C for 2 hours, then heated to 530°C and kept at this temperature for 14 hours for solution treatment, and then transferred to air for blow cooling after the solution treatment, with the quenching transfer time being ≤15 seconds; then, the alloy is kept at 200°C for 16 hours for aging treatment, and then removed from the alloy and air-cooled to room temperature after the aging treatment is completed.

[0091] The ZM6 magnesium alloy obtained in this comparative example was tested for alloy fluidity using a metal spiral mold. The flow length was 27.1 cm at a pouring temperature of 720° C., and the fluidity was lower than that of the alloy in Example 1.

[0092] The hot cracking sensitivity of the comparative alloy under constrained conditions was evaluated using a T-type hot cracking die. When the pouring temperature was 720°C, obvious through-type hot cracks were present in the hot node, and obvious crack extension was observed in the cross section of the T-type specimen, indicating that the comparative alloy had a higher hot cracking sensitivity.

[0093] The room temperature mechanical properties of the ZM6 magnesium alloy obtained in this comparative example are as follows: tensile strength of 244 MPa, yield strength of 139 MPa, and elongation of 10.5%.

[0094] Comparative Example 6

[0095] This comparative example is for preparing a WE43 magnesium alloy. The chemical composition of the WE43 magnesium alloy is as follows by weight: Gd: 0.6%, Nd: 2.2%, Y: 3.98%, Zn: 0.001%, Zr: 0.54%, with the impurity elements Cu ≤ 0.03%, Si ≤ 0.01%, Fe ≤ 0.01%, Ni ≤ 0.005%, and the total amount of the remaining impurity elements ≤ 0.3%, with the balance being Mg.

[0096] The preparation method of the WE43 magnesium alloy in this comparative example is basically the same as that in Example 1, with the only difference being the batching, melting and heat treatment steps. The specific differences are as follows: in the batching and melting steps of this comparative example, a Mg-30wt.%Y master alloy is added at a temperature of 770°C; in the heat treatment step of this comparative example, the heat treatment process is as follows: first, the alloy is kept at 400°C for 2 hours, then heated to 520°C and kept at this temperature for 8 hours for solution treatment, and after the solution treatment is completed, the alloy is transferred to air for blow cooling, with a quenching transfer time of ≤15s; then, the alloy is kept at 250°C for 7 hours for aging treatment, and after the aging is completed, the alloy is taken out and air-cooled to room temperature.

[0097] The WE43 magnesium alloy obtained in this comparative example was tested for its fluidity using a metal spiral mold. The flow length was 31.6 cm at a pouring temperature of 720° C., and its fluidity was lower than that of the alloy in Example 1.

[0098] The hot cracking sensitivity of the comparative alloy under constrained conditions was evaluated using a T-shaped hot cracking die. When the pouring temperature was 720°C, no obvious cracks appeared at the hot spot, and no visible crack initiation signs appeared on the cross section of the T-shaped specimen, indicating that the alloy in this case has low hot cracking sensitivity.

[0099] The room temperature mechanical properties of the WE43 magnesium alloy obtained in this comparative example are as follows: tensile strength of 284 MPa, yield strength of 186 MPa, and elongation of 8.5%.

[0100] Comparative Example 7

[0101] This comparative example is for preparing a high-strength cast magnesium alloy with a high Gd content. The chemical composition of the magnesium alloy is as follows by weight: Gd: 11.2%, Y: 0.1%, Zn: 0.1%, Zr: 0.39%, the total amount of other impurity elements is ≤ 0.1%, and the balance is Mg.

[0102] The preparation method of the magnesium alloy in this comparative example is basically the same as that in Example 1, with the only difference being the batching, melting and heat treatment steps. The specific differences are as follows: in the batching and melting steps of this comparative example, no Mg-30wt.%Nd master alloy is added, but Mg-30wt.%Y master alloy is added at a temperature of 770°C; in the heat treatment step of this comparative example, the heat treatment process is as follows: first, the alloy is kept at 400°C for 2 hours, then heated to 520°C and kept at this temperature for 10 hours for solution treatment, and after the solution treatment is completed, the alloy is transferred to air for blow cooling, with a quenching transfer time of ≤15 seconds; then, the alloy is kept at 225°C for 20 hours for aging treatment, and after the aging is completed, the alloy is taken out and air-cooled to room temperature.

[0103] The magnesium alloy obtained in this comparative example was tested for alloy fluidity using a metal spiral mold. The flow length was 26.2 cm at a pouring temperature of 720° C., and the fluidity was lower than that of the alloy in Example 1.

[0104] The hot cracking sensitivity of the comparative alloy under constrained conditions was evaluated using a T-type hot cracking die. When the pouring temperature was 720°C, obvious through-type hot cracks were present in the hot node, and obvious crack extension was observed in the cross section of the T-type specimen, indicating that the comparative alloy had a higher hot cracking sensitivity.

[0105] The room temperature mechanical properties of the magnesium alloy obtained in this comparative example are as follows: tensile strength of 334 MPa, yield strength of 216 MPa, and elongation of 2.5%.

[0106] The main chemical compositions of the magnesium alloys of the embodiments and comparative examples are shown in Table 1, and the mechanical properties of the obtained alloys are shown in Table 2.

[0107] It should be noted that, in the absence of conflicts, those skilled in the art may flexibly adjust the order of the above steps according to actual needs, or flexibly combine the above steps.

[0108] The methods and devices disclosed in the above embodiments can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the above-mentioned units is merely a division of logical functions, and there are other ways of division. For example, multiple important units can be combined or integrated into a device or system, or they can exist physically separately. Other unimportant features, units, devices, etc. can be ignored or not executed. The setting method of each unit in a specific device can be personalized according to actual needs.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should all be covered by the scope of protection of the present invention.

[0110] Table 1 Chemical composition of magnesium alloys in various embodiments and comparative examples (wt.%)

[0111]

[0112] Table 2 Room temperature mechanical properties of magnesium alloys in various embodiments and comparative examples

[0113]

[0114]

Claims

1. A high-strength cast magnesium alloy, characterized in that: The high-strength cast magnesium alloy comprises the following components in percentage by weight: Gd: 4.5-6.5%, Nd: 2.2-3.4%, Zn: 0.1-0.6%, Zr: 0.3-1.0%, Sr: 0-0.05%, and the balance is Mg and unavoidable impurity elements.

2. The high-strength cast magnesium alloy according to claim 1, characterized in that: The magnesium alloy components and their weight percentages are as follows: Gd: 5.8-6.4%, Nd: 2.4-2.9%, Zn: 0.2-0.5%, Zr: 0.3-1.0%, Sr: 0.02-0.05%, and the balance is Mg and inevitable impurity elements; the inevitable impurity elements include Fe, Cu, Si, and Ni, and the total amount of impurity elements is ≤0.1% by weight.

3. A method for preparing a high-strength cast magnesium alloy, for preparing a high-strength cast magnesium alloy according to any one of claims 1 to 2, characterized in that: The preparation method comprises the following steps: (1) Ingredients: Pure Mg ingots, pure Zn ingots, Mg-30 wt.% Gd master alloys, Mg-30 wt.% Nd master alloys, Mg-30 wt.% Zr master alloys, and Mg-20 wt.% Sr master alloys were weighed and prepared according to the weight percentages of the chemical components; (2) Melting: The melting furnace is powered on, and protective gas is introduced into the crucible. A pure Mg ingot is placed in the crucible of the melting furnace. After the pure Mg ingot is completely melted, a pure Zn ingot, a Mg-30wt.% Nd master alloy, a Mg-30wt.% Gd master alloy, a Mg-30wt.% Zr master alloy, and a Mg-20wt.% Sr master alloy are added in sequence. After all the metals in the crucible are completely melted, the mixture is stirred for 3 to 6 minutes to obtain a uniform alloy melt. (3) Degassing: Lower the melt temperature to the degassing temperature range, introduce inert gas into the alloy melt for degassing, and after degassing is completed, perform slag removal on the melt surface; (4) Refining: Raise the melt temperature to the refining temperature range, use a refining agent to refine the melt, clean the slag on the melt surface and crucible wall after refining, and then heat the melt to 800-820°C and let it stand for 20-30 minutes; (5) Pouring: A protective gas is introduced into the mold cavity, and the melt temperature is reduced to 720-760°C. The magnesium alloy melt is then poured into the mold under a protective gas environment, and a casting or ingot is obtained after complete solidification. (6) Heat treatment: After cleaning the alloy ingot or casting obtained in step (5), place it in a medium-high temperature heat treatment furnace for solution treatment, then transfer it to water for quenching, and finally place it in a low-temperature heat treatment furnace for aging treatment after the ingot or casting cools to room temperature. After aging is completed, take it out and air cool it to room temperature to obtain a high-strength cast magnesium alloy.

4. The method for preparing a high-strength cast magnesium alloy according to claim 3, wherein: In step (1), the ingredients need to be symmetrically taken in advance, pure Mg ingots, pure Zn ingots, Mg-30wt.% Gd master alloy, Mg-30wt.% Nd master alloy, Mg-30wt.% Zr master alloy and Mg-20wt.% Sr master alloy, sandblasted or polished clean, and then dried at a drying temperature of 200-300° C. for 1-2 hours.

5. The method for preparing a high-strength cast magnesium alloy according to claim 3, wherein: In step (2), the protective gas is a mixture of SF6 and CO2 in a volume ratio of 1:100, the protective gas temperature is 16-20°C, and the protective gas ventilation flow rate is 40-60 L / min; the addition temperature of the pure Zn ingot and the Mg-30wt.%Nd master alloy is 740-760°C, the addition temperature of the Mg-30wt.%Gd master alloy is 760-780°C, the addition temperature of the Mg-30wt.%Zr master alloy is 780-800°C, and the addition temperature of the Mg-20wt.%Sr master alloy is 750-770°C.

6. The method for preparing a high-strength cast magnesium alloy according to claim 3, wherein: In step (3), the degassing temperature range is 740-760° C.; the inert gas is high-purity argon; the inert gas ventilation flow rate is 10-20 L / min; and the degassing treatment time is 10-15 min.

7. The method for preparing a high-strength cast magnesium alloy according to claim 3, wherein: In step (4), the refining temperature range is 750-760°C, and the refining treatment time is 10-15 minutes; the refining agent is RJ-5 flux; the refining treatment is specifically as follows: sinking the refining stirrer to a depth of 2 / 3-3 / 4 of the melt for stirring, and at the same time sprinkling RJ-5 flux on the crest of the melt rolling; the total amount of RJ-5 flux added is 1.0-2.0% of the total weight of the charge.

8. The method for preparing a high-strength cast magnesium alloy according to claim 3, wherein: In step (5), the protective gas is a mixture of SF6 and CO2 gas in a volume ratio of 1:100, the protective gas temperature is 16-20°C, and the protective gas ventilation flow rate is 20-40 L / min.

9. The method for preparing a high-strength cast magnesium alloy according to claim 3, wherein: In step (6), the medium-high temperature heat treatment furnace is a vertical quenching and solution furnace equipped with a fan, and CO2 protective gas needs to be introduced into the furnace; the low-temperature heat treatment furnace is a pit-type resistance furnace equipped with a fan, and a mixture of pyrite and sulfur powder in a weight ratio of 12:1 needs to be placed in advance in the furnace.

10. The method for preparing a high-strength cast magnesium alloy according to claim 3, wherein: In step (6), the solution treatment specifically adopts a two-stage solution treatment, first keeping the temperature at 380-420°C for 2-3 hours, then heating to 510-520°C and keeping the temperature for 16-20 hours; the transfer time of the ingot or casting to water for quenching after the solution treatment is ≤15s; the water temperature of the water quenching is 20-80°C; the aging treatment temperature is 220-230°C, and the aging treatment holding time is 18-26 hours.

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