Method for refining grains of magnesium-rare earth alloy and magnesium-rare earth alloy

Abstract

The application discloses a magnesium rare earth alloy grain refining method and a magnesium rare earth alloy, and belongs to the technical field of metal material processing. In view of the problems of low yield strength and large performance dispersion of the existing EV31A magnesium rare earth alloy, the application provides a magnesium rare earth alloy containing 2.3%-3.1% of neodymium, 1.3%-1.7% of gadolinium, 0.2%-0.4% of zinc, 0.3%-0.5% of zirconium and 0.1%-0.5% of strontium in percentage by weight, and the magnesium rare earth alloy is prepared by the following method: after raw materials are melted under a protective atmosphere, magnesium-zirconium intermediate alloy is added at a first temperature, magnesium-strontium intermediate alloy is added at a second temperature which is lower than the first temperature, two-stage solid solution treatment is carried out after refining and pouring, and finally aging treatment is carried out at a fifth temperature which is lower than a third temperature. The application cooperatively improves the yield strength of the alloy and reduces the performance dispersion under the condition of not adding yttrium elements through the coupling regulation of strontium modification and smelting and heat treatment processes, the process is simple and efficient, and the application can be widely applied to aerospace and automobile load-bearing castings.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, specifically, it relates to a method for refining the grains of magnesium rare earth alloys and the magnesium rare earth alloys prepared by this method. Background Technology

[0002] Magnesium alloys, as one of the lightest metallic structural materials, offer significant weight reduction advantages in aerospace, automotive, and other equipment fields. In aerospace load-bearing castings, magnesium rare-earth alloys are widely used due to their excellent high-temperature and casting properties. Among them, EV31A magnesium alloy, due to its yttrium-free content and superior casting performance, has become an important material selection for aerospace load-bearing castings in recent years.

[0003] However, with the increasing demands for reliability in aerospace products, the shortcomings of EV31A magnesium alloy in terms of mechanical properties have become increasingly apparent. Compared to WE43A magnesium alloy, EV31A has a yield strength that is about 10% lower, and its mechanical properties exhibit greater dispersion, resulting in insufficient batch stability of castings and making it difficult to meet the requirements of high-reliability load-bearing structures. Therefore, how to further improve the yield strength and reduce the property dispersion of EV31A magnesium alloy while maintaining its original casting performance advantages has become an urgent technical problem to be solved in this field. Summary of the Invention

[0004] To address the issues of low yield strength and high performance dispersion in existing EV31A magnesium alloys, this invention provides a method for refining the grains of magnesium rare earth alloys and a magnesium rare earth alloy itself. The aim is to achieve grain refinement and microstructure homogenization through the coupled control of strontium modification treatment with smelting and heat treatment processes, thereby improving the yield strength of the alloy and reducing performance dispersion.

[0005] The present invention adopts the following technical solution: A method for grain refinement of a magnesium rare earth alloy, wherein the magnesium rare earth alloy contains, by weight percentage: neodymium 2.3%~3.1%, gadolinium 1.3%~1.7%, zinc 0.2%~0.4%, zirconium 0.3%~0.5%, strontium 0.1%~0.5%, with the balance being magnesium and unavoidable impurities, wherein the sum of all impurity elements by weight is less than 0.3%; the method includes the following steps: Under a protective atmosphere, raw materials containing magnesium, zinc, neodymium, and gadolinium are melted, heated to a first temperature, and magnesium-zirconium master alloy is added. After melting, the mixture is stirred, and then the temperature is adjusted to a second temperature lower than the first temperature. Magnesium-strontium master alloy is added, and after melting, the mixture is stirred to obtain a melt. The melt is refined and then poured to obtain a casting. The casting is subjected to solution treatment, first held at a third temperature lower than the second temperature, then heated to a fourth temperature higher than the third temperature but lower than the second temperature and held thereafter, and then cooled by water quenching. The solution-treated castings are then aged at a fifth temperature, below the third temperature.

[0006] Preferably, before melting the raw materials containing magnesium, zinc, neodymium, and gadolinium, the method further includes a raw material preparation step: preparing the raw materials according to the weight percentage content, and preheating the raw materials and melting tools at 240~300°C for 2~3 hours.

[0007] Preferably, the protective atmosphere is a mixture of sulfur hexafluoride and carbon dioxide; and / or, after adding the magnesium-zirconium master alloy, the mixture is stirred for 8-12 minutes, and after adding the magnesium-strontium master alloy, the mixture is stirred for 5-8 minutes.

[0008] Preferably, the refining is a compound refining, which includes: first refining with RJ5 refining agent, then refining with argon gas; after refining, standing for 25 to 35 minutes and removing deposited impurities; wherein the refining temperature is 750 to 760°C and the refining time is 8 to 12 minutes.

[0009] Preferably, the pouring temperature is 750~760℃.

[0010] Preferably, the first temperature is 780~820℃, the second temperature is 740~770℃, the third temperature is 330~350℃, the fourth temperature is 510~530℃, and the fifth temperature is 190~210℃.

[0011] Preferably, the solution treatment is carried out under a carbon dioxide protective atmosphere, first at 35~345℃ for 1.5~3.0 hours, and then at 515~525℃ for 8~12 hours.

[0012] Preferably, the water quenching cooling is quenching cooling in water at 40~60℃.

[0013] Preferably, after water quenching and cooling, a drying process is further included, wherein the casting is dried by blowing hot air or by drying in a furnace at a temperature of 110~120℃.

[0014] The present invention also provides a magnesium rare earth alloy, wherein the magnesium rare earth alloy is obtained by grain refinement by the above-mentioned grain refinement method, wherein the magnesium rare earth alloy contains, by weight percentage: neodymium 2.3%~3.1%, gadolinium 1.3%~1.7%, zinc 0.2%~0.4%, zirconium 0.3%~0.5%, strontium 0.1%~0.5%, and the balance being magnesium and unavoidable impurities.

[0015] Beneficial effects: This invention achieves grain refinement and microstructure homogenization by adding strontium to magnesium rare earth alloys and by optimizing the smelting and heat treatment processes.

[0016] Furthermore, this invention achieves the synergistic effect of precipitation strengthening and grain refinement without adding yttrium, maintaining the excellent casting properties of magnesium rare earth alloys. The preparation process is simple, efficient, and easy to operate, and can be widely used in load-bearing castings in aerospace, automotive and other fields. Attached Figure Description

[0017] The above and other aspects, features, and advantages of embodiments of the present invention will become clearer from the following description taken in conjunction with the accompanying drawings, in which: Figure 1 This is a flowchart of a method for refining the grain size of magnesium rare earth alloys according to the present invention; Figure 2 This is a metallographic image of the magnesium rare earth alloy material prepared in Example 1 of the present invention. Detailed Implementation

[0018] Hereinafter, specific embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention can be implemented in many different forms, and should not be construed as limited to the specific embodiments set forth herein. Rather, these embodiments are provided to explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the various embodiments of the invention and various modifications suitable for particular intended applications.

[0019] This invention addresses the technical problem of existing EV31A magnesium rare earth alloys used in aerospace load-bearing castings having a yield strength approximately 10% lower than WE43A and greater performance dispersion. It provides a grain refinement method through strontium modification treatment combined with optimized smelting and heat treatment processes. The core idea behind solving this technical problem is to refine the grains through the dual thermodynamic and kinetic modification effects of strontium, and to achieve microstructure homogenization and precipitation strengthening through the coupled control of smelting and heat treatment processes. This synergistically improves yield strength and reduces performance dispersion without the addition of yttrium.

[0020] Specifically, the addition of strontium lowers the liquidus and solidus temperatures of the alloy, reducing the undercooling of the molten metal under the same cooling conditions. This leads to necking, melting, and ionization of secondary dendrites in cellular dendrites, promoting the transformation of cellular dendrites into equiaxed dendrites and equiaxed crystals. Furthermore, strontium is a surface-active element with extremely low solubility in α-magnesium. During solidification, it is pushed to the solid-liquid interface front, where the enriched strontium inhibits the growth of cellular dendrites, providing more time for nucleation and promoting the formation of equiaxed crystals. Grain refinement effectively reduces the total area of ​​dislocation slip, hindering dislocation slip and resulting in a significant increase in the material's yield strength.

[0021] During the smelting stage, this invention employs a step-by-step feeding and gradient temperature control strategy—first adding a magnesium-zirconium master alloy at a first temperature to ensure that zirconium is fully dissolved and exerts its grain refinement and heat resistance effects, and then adding a magnesium-strontium master alloy at a second temperature lower than the first temperature, which reduces the high-temperature burn-off of strontium and ensures its uniform dispersion in the melt—thus providing a stable and effective source of modification for the solidification process; combined with refining treatment under a protective atmosphere, it fully removes oxide inclusions and impurities, preventing inclusions from becoming sources of crack initiation and performance fluctuations, thereby reducing the performance dispersion of the final casting from both melt purity and compositional uniformity.

[0022] In the heat treatment stage, a two-stage solution treatment is adopted: first, pre-holding at a third temperature lower than the second temperature to fully dissolve or pre-treat the low-melting-point phase; then, holding at a fourth temperature higher than the third temperature but lower than the second temperature to fully dissolve the neodymium, gadolinium, and zinc-containing reinforcing phases into the matrix; followed by rapid water quenching to form a supersaturated solid solution; finally, aging is performed at a fifth temperature lower than the third temperature to promote the precipitation of fine and dispersed reinforcing phases. This coupling mechanism of solution treatment and aging works synergistically with the fine-grained structure produced by strontium modification: the fine-grained structure improves the yield strength and homogenizes deformation behavior through grain boundary strengthening, while precipitation strengthening further improves the strength by hindering dislocation movement. The combined effect of these two factors allows the alloy to simultaneously improve yield strength and reduce the dispersion of mechanical properties without the addition of yttrium.

[0023] The grain refinement method of the present invention will be described in detail below with reference to the accompanying drawings. Figure 1 This is a flowchart of a method for refining the grain size of magnesium rare-earth alloys according to the present invention. (Refer to...) Figure 1 The method for refining the grain size of magnesium rare earth alloys according to the present invention includes steps S1 to S6. The specific details of each step are as follows:

[0024] S1. Raw material preparation

[0025] The raw materials are prepared according to their weight percentage content, including magnesium-neodymium master alloy, magnesium-zirconium master alloy, magnesium-gadolinium master alloy, magnesium-strontium master alloy, pure zinc, and pure magnesium. Preferably, the raw materials and melting tools are preheated at 240-300°C for 2-3 hours to remove moisture and prevent splashing during subsequent melting.

[0026] S2, Smelting Under a protective atmosphere, raw materials containing magnesium, zinc, neodymium, and gadolinium (i.e., raw materials other than magnesium-zirconium master alloys and magnesium-strontium master alloys) are added to a melting furnace for melting. Preferably, the protective atmosphere is a mixture of sulfur hexafluoride and carbon dioxide. After all the furnace charge has melted, the temperature is raised to a first temperature, and magnesium-zirconium master alloys are added. After all the alloys have melted, the mixture is stirred. Then, the temperature is adjusted to a second temperature lower than the first temperature, and magnesium-strontium master alloys are added. After all the alloys have melted, the mixture is stirred to obtain a melt.

[0027] The first temperature is preferably 780~820℃, and the second temperature is preferably 740~770℃. After adding the magnesium-zirconium master alloy, stirring is preferably performed for 8~12 minutes; after adding the magnesium-strontium master alloy, stirring is preferably performed for 5~8 minutes. When the magnesium liquid is burning, an appropriate amount of FLUX5 solvent can be added.

[0028] S3, Refined The melt obtained in step S2 is refined to obtain a refined liquid. Preferably, the refining is a composite refining process, which includes refining with RJ5 refining agent first, followed by refining with argon gas. More preferably, the refining temperature is 750~760℃, the refining time is 8~12 minutes, and after refining, the mixture is allowed to stand for 25~35 minutes and deposited impurities are removed to fully remove inclusions in the melt and reduce performance dispersion.

[0029] S4, Pouring The refined liquid obtained in step S3 is poured into a mold, and after solidification, it is removed to obtain a casting. Preferably, the pouring temperature is 750~760℃.

[0030] S5, Solution treatment The casting obtained in step S4 is subjected to solution treatment. First, it is kept at a third temperature lower than the second temperature, then heated to a fourth temperature higher than the third temperature but lower than the second temperature and kept at that temperature, and then cooled by water quenching.

[0031] The third temperature is preferably 330~350℃, and the fourth temperature is preferably 510~530℃. Preferably, the solution treatment is carried out under a carbon dioxide protective atmosphere, first holding at 335~345℃ for 1.5~3.0 hours, then raising the temperature to 515~525℃ and holding for 8~12 hours. After solution treatment, the casting is quenched in water at 40~60℃. More preferably, the water quenching process further includes a drying treatment, which involves blowing the casting dry with hot air or drying it in a furnace at a temperature of 110~120℃.

[0032] S6. Timeliness Processing The solution-treated casting obtained in step S5 is then subjected to aging treatment. The aging treatment is carried out at a fifth temperature, lower than the third temperature. Preferably, the fifth temperature is 190-210°C, and more preferably 195-205°C.

[0033] Through the above steps, the present invention achieves the synergistic effect of strontium modification, smelting and heat treatment processes, and obtains magnesium rare earth alloys with fine grains and uniform structure, effectively improving yield strength and reducing performance dispersion.

[0034] The present invention is further illustrated below with specific embodiments. These embodiments are for illustrative purposes only and do not limit the scope of the invention. In the embodiments below, the total amount of all impurity elements in the magnesium rare earth alloy is less than 0.3% by weight.

[0035] Example 1 This embodiment provides a method for refining the grain size of a magnesium rare earth alloy by adding strontium. The magnesium rare earth alloy contains, by weight percentage: neodymium 2.9%, gadolinium 1.70%, zinc 0.4%, zirconium 0.5%, strontium 0.2%, with the remainder being magnesium and unavoidable impurity elements.

[0036] The magnesium rare earth alloy material and grain refinement method of this embodiment are prepared by the following steps: S1. Raw material preparation: Prepare raw materials according to weight percentage content. The raw materials are magnesium-neodymium master alloy, magnesium-zirconium master alloy, magnesium-gadolinium master alloy, magnesium-strontium master alloy, pure zinc and pure magnesium. Place the raw materials and smelting tools in a drying oven for preheating treatment for later use.

[0037] S2. Melting: Melt the raw material described in step S1 under the protective conditions of a mixed gas of sulfur hexafluoride (0.2%) and carbon dioxide (99.8%). Before the melting step, heat the crucible and melting tools at 720°C for 1.2 hours until the crucible turns dark red. Except for the magnesium-zirconium master alloy and the magnesium-strontium master alloy, all the furnace charge prepared in step S1 is added to the melting furnace for melting. At the same time, a small amount of FLUX5 protective agent is added. After all the furnace charge has melted, the temperature is adjusted to 800℃, and the magnesium-zirconium master alloy is added in batches. After the magnesium-zirconium master alloy has completely melted, it is stirred for 10 minutes. Then the temperature is adjusted to 755℃, and the magnesium-strontium master alloy is added. After the master alloy has completely melted, it is stirred for 5 minutes. The magnesium-strontium master alloy is pressed into the melt to 1 / 2 of its depth using a bell jar and moved in a circular motion in a certain direction. After the master alloy has completely melted, it is stirred for 5-8 minutes. At this time, stirring ensures that the liquid surface fluctuates slightly and the oxide film on the surface does not break, thus obtaining the melt. When the magnesium liquid is burning, an appropriate amount of FLUX5 solvent can be added.

[0038] S3. Refining: The melt obtained in step S2 is subjected to compound refining. First, it is refined with RJ5 for 11 minutes. During refining, it is necessary to ensure that the melt is fully in contact with the refining agent (RJ5). Then, argon gas is introduced for refining for 8 minutes. After refining, it is allowed to stand for 25 minutes. Then, the impurities deposited at the bottom of the crucible are removed. After standing for another 20 minutes, the refined liquid is obtained.

[0039] S4. Pouring: Pour the refined liquid obtained in step S3 into the mold at a pouring temperature of 750℃. After solidification, remove the mold to obtain the casting.

[0040] S5. Solution Treatment: Solution treatment is performed in a vacuum atmosphere heat treatment furnace. Before solution treatment, the heat treatment furnace is dried at a temperature of 120~150℃ for 1~1.5 hours. After drying, the casting is placed in the heat treatment furnace, and a vacuum is drawn. When the vacuum reaches -35Kpa, carbon dioxide gas is introduced. When the pressure reaches 20KPa, the process is stopped, and the above steps are repeated once to achieve the carbon dioxide protection conditions. The casting obtained in step S4 is then subjected to solution treatment under the above protective conditions at a temperature of 340℃ for 2.5 hours, followed by a rise to 520℃ and a holding time of 9 hours. After solution treatment, the casting is quenched in water at 50℃, cooled, and then dried to obtain a solution-treated casting. The drying process involves blowing the casting with hot air or drying it in a furnace at a temperature of 110℃.

[0041] S6. Aging treatment: The casting obtained from step S5 after solution treatment is subjected to aging treatment at a temperature of 200℃ for 15 hours.

[0042] The mechanical properties of the material obtained in this embodiment are as follows: tensile strength 269 MPa, yield strength 172 MPa, elongation 5%. The room temperature tensile test method for metallic materials is GB / T228.1-2021.

[0043] Figure 2 This is a metallographic image of the magnesium rare earth alloy material prepared in Example 1 of the present invention. From... Figure 2 As can be seen, this embodiment yields uniform alloy grains. The material in this embodiment also effectively reduces the total area of ​​dislocation slip, hindering dislocation slip. The more difficult the dislocation slip, the higher the material's performance, demonstrating improved alloy strength. Therefore, it can be confirmed that the magnesium alloy of Embodiment 1 of this invention, through controlled melting and heat treatment processes, can achieve precipitation strengthening and grain refinement without the addition of yttrium.

[0044] Example 2 Compared with Example 1, this embodiment reduces the strontium content while keeping the other components unchanged.

[0045] This embodiment provides a method for refining the grain size of a magnesium rare earth alloy by adding strontium. The magnesium rare earth alloy contains, by weight percentage: 2.9% neodymium, 1.70% gadolinium, 0.4% zinc, 0.5% zirconium, 0.1% strontium, with the remainder being magnesium and unavoidable impurity elements.

[0046] The magnesium rare earth alloy material and grain refinement method of this embodiment are prepared by the following steps: S1. Raw material preparation: Prepare raw materials according to weight percentage content. The raw materials are magnesium-neodymium master alloy, magnesium-zirconium master alloy, magnesium-gadolinium master alloy, magnesium-strontium master alloy, pure zinc and pure magnesium. Place the raw materials and smelting tools in a drying oven for preheating treatment for later use.

[0047] S2. Melting: Melt the raw material described in step S1 under the protective conditions of a mixed gas of sulfur hexafluoride (0.2%) and carbon dioxide (99.8%). Before the melting step, heat the crucible and melting tools at 720°C for 1.2 hours until the crucible turns dark red. Except for the magnesium-zirconium master alloy and the magnesium-strontium master alloy, all the furnace charge prepared in step S1 is added to the melting furnace for melting. At the same time, a small amount of FLUX5 protective agent is added. After all the furnace charge has melted, the temperature is adjusted to 800℃, and the magnesium-zirconium master alloy is added in batches. After the magnesium-zirconium master alloy has completely melted, it is stirred for 10 minutes. Then the temperature is adjusted to 755℃, and the magnesium-strontium master alloy is added. After the master alloy has completely melted, it is stirred for 5 minutes. The magnesium-strontium master alloy is pressed into the melt to 1 / 2 of its depth using a bell jar and moved in a circular motion in a certain direction. After the master alloy has completely melted, it is stirred for 5-8 minutes. At this time, stirring ensures that the liquid surface fluctuates slightly and the oxide film on the surface does not break, thus obtaining the melt. When the magnesium liquid is burning, an appropriate amount of FLUX5 solvent can be added.

[0048] S3. Refining: The melt obtained in step S2 is subjected to compound refining. First, it is refined with RJ5 for 11 minutes. During refining, it is necessary to ensure that the melt is fully in contact with the refining agent (RJ5). Then, argon gas is introduced for refining for 8 minutes. After refining, it is allowed to stand for 25 minutes. Then, the impurities deposited at the bottom of the crucible are removed. After standing for another 20 minutes, the refined liquid is obtained.

[0049] S4. Pouring: Pour the refined liquid obtained in step S3 into the mold at a pouring temperature of 750℃. After solidification, remove the mold to obtain the casting.

[0050] S5. Solution Treatment: Solution treatment is performed in a vacuum atmosphere heat treatment furnace. Before solution treatment, the heat treatment furnace is dried at a temperature of 120~150℃ for 1~1.5 hours. After drying, the casting is placed in the heat treatment furnace, and a vacuum is drawn. When the vacuum reaches -35Kpa, carbon dioxide gas is introduced. When the pressure reaches 20KPa, the process is stopped, and the above steps are repeated once to achieve the carbon dioxide protection conditions. The casting obtained in step S4 is then subjected to solution treatment under the above protective conditions at a temperature of 340℃ for 2.5 hours, followed by a rise to 520℃ and a holding time of 9 hours. After solution treatment, the casting is quenched in water at 50℃, cooled, and then dried to obtain a solution-treated casting. The drying process involves blowing the casting with hot air or drying it in a furnace at a temperature of 110℃.

[0051] S6. Aging treatment: The casting obtained from step S5 after solution treatment is subjected to aging treatment at a temperature of 200℃ for 15 hours.

[0052] The mechanical properties of the material obtained in this embodiment are as follows: tensile strength 260 MPa, yield strength 168 MPa, elongation 3%. The room temperature tensile test method for metallic materials is GB / T228.1-2021.

[0053] Example 3 Compared with Example 1, this embodiment increases the strontium content while keeping the other components unchanged.

[0054] This embodiment provides a method for refining the grain size of a magnesium rare earth alloy by adding strontium. The magnesium rare earth alloy contains, by weight percentage: 2.9% neodymium, 1.70% gadolinium, 0.4% zinc, 0.5% zirconium, 0.5% strontium, with the remainder being magnesium and unavoidable impurity elements.

[0055] The magnesium rare earth alloy material and grain refinement method of this embodiment are prepared by the following steps: S1. Raw material preparation: Prepare raw materials according to weight percentage content. The raw materials are magnesium-neodymium master alloy, magnesium-zirconium master alloy, magnesium-gadolinium master alloy, magnesium-strontium master alloy, pure zinc and pure magnesium. Place the raw materials and smelting tools in a drying oven for preheating treatment for later use.

[0056] S2. Melting: Melt the raw material described in step S1 under the protective conditions of a mixed gas of sulfur hexafluoride (0.2%) and carbon dioxide (99.8%). Before the melting step, heat the crucible and melting tools at 720°C for 1.2 hours until the crucible turns dark red. Except for the magnesium-zirconium master alloy and the magnesium-strontium master alloy, all the furnace charge prepared in step S1 is added to the melting furnace for melting. At the same time, a small amount of FLUX5 protective agent is added. After all the furnace charge has melted, the temperature is adjusted to 800℃, and the magnesium-zirconium master alloy is added in batches. After the magnesium-zirconium master alloy has completely melted, it is stirred for 10 minutes. Then the temperature is adjusted to 755℃, and the magnesium-strontium master alloy is added. After the master alloy has completely melted, it is stirred for 5 minutes. The magnesium-strontium master alloy is pressed into the melt to 1 / 2 of its depth using a bell jar and moved in a circular motion in a certain direction. After the master alloy has completely melted, it is stirred for 5-8 minutes. At this time, stirring ensures that the liquid surface fluctuates slightly and the oxide film on the surface does not break, thus obtaining the melt. When the magnesium liquid is burning, an appropriate amount of FLUX5 solvent can be added.

[0057] S3. Refining: The melt obtained in step S2 is subjected to compound refining. First, it is refined with RJ5 for 11 minutes. During refining, it is necessary to ensure that the melt is fully in contact with the refining agent (RJ5). Then, argon gas is introduced for refining for 8 minutes. After refining, it is allowed to stand for 25 minutes. Then, the impurities deposited at the bottom of the crucible are removed. After standing for another 20 minutes, the refined liquid is obtained.

[0058] S4. Pouring: Pour the refined liquid obtained in step S3 into the mold at a pouring temperature of 750℃. After solidification, remove the mold to obtain the casting.

[0059] S5. Solution Treatment: Solution treatment is performed in a vacuum atmosphere heat treatment furnace. Before solution treatment, the heat treatment furnace is dried at a temperature of 120~150℃ for 1~1.5 hours. After drying, the casting is placed in the heat treatment furnace, and a vacuum is drawn. When the vacuum reaches -35Kpa, carbon dioxide gas is introduced. When the pressure reaches 20KPa, the process is stopped, and the above steps are repeated once to achieve the carbon dioxide protection conditions. The casting obtained in step S4 is then subjected to solution treatment under the above protective conditions at a temperature of 340℃ for 2.5 hours, followed by a rise to 520℃ and a holding time of 9 hours. After solution treatment, the casting is quenched in water at 50℃, cooled, and then dried to obtain a solution-treated casting. The drying process involves blowing the casting with hot air or drying it in a furnace at a temperature of 110℃.

[0060] S6. Aging treatment: The casting obtained from step S5 after solution treatment is subjected to aging treatment at a temperature of 200℃ for 15 hours.

[0061] The mechanical properties of the material obtained in this embodiment are as follows: tensile strength 270 MPa, yield strength 169 MPa, elongation 2.5%. The room temperature tensile test method for metallic materials is GB / T228.1-2021.

[0062] Example 4 Compared with Example 1, this embodiment increases the strontium content while keeping the other components unchanged.

[0063] This embodiment provides a method for refining the grain size of a magnesium rare earth alloy by adding strontium. The magnesium rare earth alloy contains, by weight percentage: 2.9% neodymium, 1.70% gadolinium, 0.4% zinc, 0.5% zirconium, 0.3% strontium, with the remainder being magnesium and unavoidable impurity elements.

[0064] The magnesium rare earth alloy material and grain refinement method of this embodiment are prepared by the following steps: S1. Raw material preparation: Prepare raw materials according to weight percentage content. The raw materials are magnesium-neodymium master alloy, magnesium-zirconium master alloy, magnesium-gadolinium master alloy, magnesium-strontium master alloy, pure zinc and pure magnesium. Place the raw materials and smelting tools in a drying oven for preheating treatment for later use.

[0065] S2. Melting: Melt the raw material described in step S1 under the protective conditions of a mixed gas of sulfur hexafluoride (0.2%) and carbon dioxide (99.8%). Before the melting step, heat the crucible and melting tools at 720°C for 1.2 hours until the crucible turns dark red. Except for the magnesium-zirconium master alloy and the magnesium-strontium master alloy, all the furnace charge prepared in step S1 is added to the melting furnace for melting. At the same time, a small amount of FLUX5 protective agent is added. After all the furnace charge has melted, the temperature is adjusted to 800℃, and the magnesium-zirconium master alloy is added in batches. After the magnesium-zirconium master alloy has completely melted, it is stirred for 10 minutes. Then the temperature is adjusted to 755℃, and the magnesium-strontium master alloy is added. After the master alloy has completely melted, it is stirred for 5 minutes. The magnesium-strontium master alloy is pressed into the melt to 1 / 2 of its depth using a bell jar and moved in a circular motion in a certain direction. After the master alloy has completely melted, it is stirred for 5-8 minutes. At this time, stirring ensures that the liquid surface fluctuates slightly and the oxide film on the surface does not break, thus obtaining the melt. When the magnesium liquid is burning, an appropriate amount of FLUX5 solvent can be added.

[0066] S3. Refining: The melt obtained in step S2 is subjected to compound refining. First, it is refined with RJ5 for 11 minutes. During refining, it is necessary to ensure that the melt is fully in contact with the refining agent (RJ5). Then, argon gas is introduced for refining for 8 minutes. After refining, it is allowed to stand for 25 minutes. Then, the impurities deposited at the bottom of the crucible are removed. After standing for another 20 minutes, the refined liquid is obtained.

[0067] S4. Pouring: Pour the refined liquid obtained in step S3 into the mold at a pouring temperature of 750℃. After solidification, remove the mold to obtain the casting.

[0068] S5. Solution Treatment: Solution treatment is performed in a vacuum atmosphere heat treatment furnace. Before solution treatment, the heat treatment furnace is dried at a temperature of 120~150℃ for 1~1.5 hours. After drying, the casting is placed in the heat treatment furnace, and a vacuum is drawn. When the vacuum reaches -35Kpa, carbon dioxide gas is introduced. When the pressure reaches 20KPa, the process is stopped, and the above steps are repeated once to achieve the carbon dioxide protection conditions. The casting obtained in step S4 is then subjected to solution treatment under the above protective conditions at a temperature of 340℃ for 2.5 hours, followed by a rise to 520℃ and a holding time of 9 hours. After solution treatment, the casting is quenched in water at 50℃, cooled, and then dried to obtain a solution-treated casting. The drying process involves blowing the casting with hot air or drying it in a furnace at a temperature of 110℃.

[0069] S6. Aging treatment: The casting obtained from step S5 after solution treatment is subjected to aging treatment at a temperature of 200℃ for 15 hours.

[0070] The mechanical properties of the material obtained in this embodiment are as follows: tensile strength 272 MPa, yield strength 170 MPa, elongation 4%. The room temperature tensile test method for metallic materials is GB / T228.1-2021.

[0071] Example 5 Compared with Example 1, this embodiment increases the solution treatment time, while the rest remains the same.

[0072] This embodiment provides a method for refining the grain size of a magnesium rare earth alloy by adding strontium. The magnesium rare earth alloy contains, by weight percentage: neodymium 2.9%, gadolinium 1.70%, zinc 0.4%, zirconium 0.5%, strontium 0.2%, with the remainder being magnesium and unavoidable impurity elements.

[0073] The magnesium rare earth alloy material and grain refinement method of this embodiment are prepared by the following steps: S1. Raw material preparation: Prepare raw materials according to weight percentage content. The raw materials are magnesium-neodymium master alloy, magnesium-zirconium master alloy, magnesium-gadolinium master alloy, magnesium-strontium master alloy, pure zinc and pure magnesium. Place the raw materials and smelting tools in a drying oven for preheating treatment for later use.

[0074] S2. Melting: Melt the raw material described in step S1 under the protective conditions of a mixed gas of sulfur hexafluoride (0.2%) and carbon dioxide (99.8%). Before the melting step, heat the crucible and melting tools at 720°C for 1.2 hours until the crucible turns dark red. Except for the magnesium-zirconium master alloy and the magnesium-strontium master alloy, all the furnace charge prepared in step S1 is added to the melting furnace for melting. At the same time, a small amount of FLUX5 protective agent is added. After all the furnace charge has melted, the temperature is adjusted to 800℃, and the magnesium-zirconium master alloy is added in batches. After the magnesium-zirconium master alloy has completely melted, it is stirred for 10 minutes. Then the temperature is adjusted to 755℃, and the magnesium-strontium master alloy is added. After the master alloy has completely melted, it is stirred for 5 minutes. The magnesium-strontium master alloy is pressed into the melt to 1 / 2 of its depth using a bell jar and moved in a circular motion in a certain direction. After the master alloy has completely melted, it is stirred for 5-8 minutes. At this time, stirring ensures that the liquid surface fluctuates slightly and the oxide film on the surface does not break, thus obtaining the melt. When the magnesium liquid is burning, an appropriate amount of FLUX5 solvent can be added.

[0075] S3. Refining: The melt obtained in step S2 is subjected to compound refining. First, it is refined with RJ5 for 11 minutes. During refining, it is necessary to ensure that the melt is fully in contact with the refining agent (RJ5). Then, argon gas is introduced for refining for 8 minutes. After refining, it is allowed to stand for 25 minutes. Then, the impurities deposited at the bottom of the crucible are removed. After standing for another 20 minutes, the refined liquid is obtained.

[0076] S4. Pouring: Pour the refined liquid obtained in step S3 into the mold at a pouring temperature of 750℃. After solidification, remove the mold to obtain the casting.

[0077] S5. Solution Treatment: Solution treatment is performed in a vacuum atmosphere heat treatment furnace. Before solution treatment, the heat treatment furnace is dried at a temperature of 120~150℃ for 1~1.5 hours. After drying, the casting is placed in the heat treatment furnace, and a vacuum is drawn. When the vacuum reaches -35Kpa, carbon dioxide gas is introduced. When the pressure reaches 20KPa, the process is stopped, and the above steps are repeated once to achieve the carbon dioxide protection conditions. The casting obtained in step S4 is then subjected to solution treatment under the above protective conditions at a solution temperature of 340℃ for 2.5 hours, followed by raising the temperature to 520℃ and holding for 12 hours. After solution treatment, the casting is quenched in water at 50℃, cooled, and then dried to obtain the solution-treated casting. The drying process involves blowing the casting with hot air or drying it in a furnace at a temperature of 110℃.

[0078] S6. Aging treatment: The casting obtained from step S5 after solution treatment is subjected to aging treatment at a temperature of 200℃ for 15 hours.

[0079] The mechanical properties of the material obtained in this embodiment are as follows: tensile strength 280 MPa, yield strength 175 MPa, and elongation 1.5%. The method for room temperature tensile testing of metallic materials is described.

[0080] In summary, this invention provides a method for refining the grain size of magnesium rare earth alloys and the magnesium rare earth alloys prepared by this method. By introducing strontium modification treatment into a specific composition system (neodymium 2.3%~3.1%, gadolinium 1.3%~1.7%, zinc 0.2%~0.4%, zirconium 0.3%~0.5%, strontium 0.1%~0.5%), and coupling gradient temperature control and refining in the melting stage with a two-stage solution treatment and aging process in the heat treatment stage, significant grain refinement and microstructure homogenization are achieved. This method effectively improves the yield strength of magnesium rare earth alloys and reduces the dispersion of mechanical properties without adding yttrium. The resulting material has a tensile strength of 260~280 MPa and a yield strength of 168~175 MPa. The preparation process of this invention is simple and efficient, easy to implement in industrial production, and can be widely used in the manufacture of load-bearing castings in aerospace, automotive, and other fields.

[0081] The optional embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the embodiments of the present invention are not limited to the specific details in the above embodiments. Within the scope of the technical concept of the embodiments of the present invention, various simple modifications can be made to the technical solutions of the embodiments of the present invention, and these simple modifications all fall within the protection scope of the embodiments of the present invention.

Claims

1. A method for refining the grain size of a magnesium rare earth alloy, characterized in that, The magnesium rare earth alloy contains, by weight percentage: neodymium: 2.3~3.1%, gadolinium: 1.3~1.70%, zinc: 0.2~0.4%, zirconium: 0.3~0.5%, strontium: 0.1~0.5%, with the remainder being magnesium and unavoidable impurities; The grain refinement method includes the following steps: Under a protective atmosphere, raw materials containing magnesium, zinc, neodymium, and gadolinium are melted, heated to a first temperature, and magnesium-zirconium master alloy is added. After melting, the mixture is stirred, and then the temperature is adjusted to a second temperature lower than the first temperature. Magnesium-strontium master alloy is added, and after melting, the mixture is stirred to obtain a melt. The melt is refined and then poured to obtain a casting. The casting is subjected to solution treatment, first held at a third temperature lower than the second temperature, then heated to a fourth temperature higher than the third temperature but lower than the second temperature and held thereafter, and then cooled by water quenching. The solution-treated castings are then aged at a fifth temperature, below the third temperature.

2. The grain refinement method according to claim 1, characterized in that, Before melting the raw materials containing magnesium, zinc, neodymium, and gadolinium, the grain refinement method further includes a raw material preparation step: preparing the raw materials according to the weight percentage content, and preheating the raw materials and melting tools at 240~300℃ for 2~3 hours.

3. The grain refinement method according to claim 1, characterized in that, The protective atmosphere is a mixture of sulfur hexafluoride and carbon dioxide; and / or, after adding magnesium-zirconium master alloy, stir for 8-12 minutes, or after adding magnesium-strontium master alloy, stir for 5-8 minutes.

4. The grain refinement method according to claim 1, characterized in that, The refining process is a compound refining process, which includes: first refining with RJ5 refining agent, then refining with argon gas; after refining, standing for 25 to 35 minutes and removing deposited impurities; wherein the refining temperature is 750 to 760°C and the refining time is 8 to 12 minutes.

5. The grain refinement method according to claim 1, characterized in that, The pouring temperature is 750~760℃.

6. The grain refinement method according to claim 1, characterized in that, The first temperature is 780~820℃, the second temperature is 740~770℃, the third temperature is 330~350℃, the fourth temperature is 510~530℃, and the fifth temperature is 190~210℃.

7. The grain refinement method according to claim 1 or 6, characterized in that, The solution treatment is carried out under a carbon dioxide protective atmosphere, first at 335~345℃ for 1.5~3.0 hours, and then at 515~525℃ for 8~12 hours.

8. The grain refinement method according to claim 1, characterized in that, The water quenching cooling is quenching and cooling in water at 40~60℃.

9. The grain refinement method according to claim 1, characterized in that, After water quenching and cooling, the grain refinement method further includes a drying process, which involves blowing the casting dry with hot air or drying it in a furnace at a temperature of 110~120℃.

10. A magnesium rare earth alloy, characterized in that, The magnesium rare earth alloy is prepared by the grain refinement method according to any one of claims 1-9, wherein the magnesium rare earth alloy contains, by weight percentage: neodymium: 2.3~3.1%, gadolinium: 1.3~1.70%, zinc: 0.2~0.4%, zirconium: 0.3~0.5%, strontium: 0.1~0.5%, with the remainder being magnesium and unavoidable impurities.

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