Die-casting alloy material for motor shell of hybrid vehicle and preparation method of die-casting alloy material

Abstract

The invention discloses a die-casting alloy material for a hybrid automobile motor shell and a preparation method of the die-casting alloy material, and relates to the technical field of alloys, and the preparation method specifically comprises the steps that pure aluminum is added into molten magnesium liquid to form matrix molten liquid; then Mg-Zn and Mg-Ca mother alloys are added, and interfacial active elements are introduced; multiple rare earth mother alloys are added in a graded mode according to the melting points from low to high, graded stirring and argon refining are carried out, and mixed melt is formed; then carrying out multi-stage injection molding; and finally performing low-temperature heat treatment on the casting. The prepared alloy material forms a unique strengthening phase-edge phase-continuous phase three-phase synergistic microstructure, and the three-phase structure enables the material to have excellent high-temperature strength and creep resistance, good thermal conductivity and excellent die-casting process performance at the same time; and the comprehensive use requirement of the hybrid vehicle motor shell under the long-term high-temperature working condition can be met.

Description

Technical Field

[0001] This invention relates to the technical field of alloys, and more particularly to a die-casting alloy material for the housing of a hybrid vehicle motor and its preparation method. Background Technology

[0002] Currently, with the global automotive industry undergoing a profound transformation towards green and intelligent manufacturing, hybrid vehicles have become a significant development trend in the current and future automotive market. Against this backdrop, lightweight vehicle design plays a crucial role in improving fuel economy, reducing emissions, and optimizing overall vehicle dynamics. As a core component of the powertrain, the lightweighting of the motor's housing is particularly critical. Die-cast magnesium alloys, with their significantly low density, demonstrate great potential in weight-sensitive components such as hybrid vehicle motor housings. However, hybrid motors often face continuous high-temperature conditions during actual operation, placing extremely stringent requirements on the comprehensive performance of the motor housing material, especially its high-temperature mechanical properties and thermal management capabilities.

[0003] To address the challenges of high-temperature applications of magnesium alloys, rare earth (RE) modification is widely considered one of the most effective ways to improve the overall performance of magnesium alloys, especially their high-temperature performance. Among these, the Mg-Al-RE series, by controlling the aluminum content at a low level, utilizes rare earth elements to suppress the softening of the magnesium-aluminum matrix while ensuring basic die-casting fluidity and room-temperature strength. This significantly improves the alloy's creep resistance and high-temperature strength, meeting the high-temperature performance requirements to a certain extent. However, as hybrid vehicle technology continues to evolve towards higher performance and more compact integration, existing magnesium alloy systems based on a single strengthening objective are gradually revealing their inherent limitations in achieving multi-dimensional performance synergy optimization. They struggle to simultaneously achieve high-temperature mechanical properties, room-temperature mechanical properties, thermal conductivity, and excellent die-casting process performance while maintaining cost control.

[0004] Therefore, it is necessary to improve the existing die-casting alloy materials and their preparation methods for hybrid vehicle motor housings in order to solve the above problems. Summary of the Invention

[0005] This invention overcomes the shortcomings of the prior art and provides a die-casting alloy material for the housing of a hybrid vehicle motor and its preparation method, aiming to solve the problem that magnesium alloy materials in the prior art are difficult to simultaneously achieve excellent high-temperature mechanical properties, good thermal conductivity and excellent die-casting processability.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: a method for preparing die-casting alloy material for the housing of a hybrid vehicle motor, comprising: S1. Add pure aluminum to the molten magnesium liquid and stir to dissolve it, forming a matrix melt with α-Mg matrix as the continuous phase; S2. Add Mg-Zn master alloy and Mg-Ca master alloy to the matrix melt and stir to dissolve, providing active elements at the Zn and Ca interface to form a transition melt; S3. Add Mg-Ce, Mg-La, Mg-Y, and Mg-Nd master alloys to the transition melt from low to high melting point and stir in stages. Introduce Ce, La, Y, and Nd as strengthening phase forming elements and edge phase auxiliary elements to form a mixed melt with uniformly distributed three-phase components. S4. The mixed solution is injected into the preheated mold through multi-stage pressure injection and then die-cast to form a casting with a preliminary three-phase structure. S5. Heat treatment is performed on the casting to strengthen the edge phase.

[0007] In a preferred embodiment of the present invention, in S1, the molten magnesium liquid is formed by melting industrial pure magnesium ingots at 680-700°C, and industrial pure aluminum ingots are added thereto, with aluminum accounting for 3.5-4.5% of the total weight of the alloy. The mixture is stirred continuously for 10-15 minutes at a stirring speed of 100-150 r / min, while a mixed protective gas is introduced at a flow rate of 5-10 L / min.

[0008] In a preferred embodiment of the present invention, in step S2, the temperature of the matrix melt is raised to 720-740°C, and Mg-Zn master alloy and Mg-Ca master alloy are added sequentially. The amount of Zn added accounts for 0.3-0.6% of the total weight of the alloy, and the amount of Ca added accounts for 0.05-0.15% of the total weight of the alloy. The mixture is stirred continuously for 5-10 minutes at a stirring speed of 120-180 r / min, and the master alloy is added in batches.

[0009] In a preferred embodiment of the present invention, in step S3, Mg-Ce master alloy and Mg-La master alloy are added to the transition melt, wherein the amount of Ce added accounts for 0.8-1.5% of the total weight of the alloy, and the amount of La added accounts for 0.4-0.8% of the total weight of the alloy, and the mixture is stirred continuously for 5-8 minutes at a stirring speed of 180-250 r / min.

[0010] In a preferred embodiment of the present invention, in step S3, Mg-Y master alloy and Mg-Nd master alloy are added, wherein the amount of Y added accounts for 0.2-0.5% of the total weight of the alloy, and the amount of Nd added accounts for 0.1-0.3% of the total weight of the alloy. The mixture is stirred continuously at a stirring speed of 180-250 r / min for 10-20 min. Then the temperature of the melt is lowered to 720-730℃ and allowed to stand for 5-10 min, and argon refining is performed. The refining gas flow rate is 0.5-1.0 L / min and the process lasts for 8-12 min.

[0011] In a preferred embodiment of the present invention, in S4, the temperature of the preheated mold is 200-250°C, and the injection process includes: a first stage of low-speed injection at a speed of 0.1-0.3 m / s, a second stage of high-speed injection at a speed of 3.0-5.0 m / s, and a third stage of pressurization at a pressure of 60-100 MPa for a duration of 100-300 ms. The casting is then removed after cooling in the mold to below 200°C.

[0012] In a preferred embodiment of the present invention, in step S5, the heat treatment is carried out under argon protection, wherein the casting is heated to 200-220°C at a rate of 5-8°C / min, held at that temperature for 4-6 hours, and then cooled to room temperature in the furnace at a rate of 2-3°C / min.

[0013] In a preferred embodiment of the present invention, in S3, the Mg-Ce, Mg-La, Mg-Y, and Mg-Nd master alloys are dried at 150-180°C for 3-4 hours before being added.

[0014] This invention provides a die-cast alloy material for the housing of a hybrid vehicle motor, comprising the following components by weight percentage: Al: 3.5-4.5% Zn: 0.3-0.6%, Ca: 0.05-0.15% Ce: 0.8-1.5% La: 0.4-0.8% Y: 0.2-0.5% Nd: 0.1-0.3% The balance is Mg and unavoidable impurities; Furthermore, the die-cast alloy material has a three-phase synergistic microstructure consisting of a reinforcing phase, an edge phase, and a continuous phase.

[0015] In a preferred embodiment of the present invention, the reinforcing phase is Al. 11 RE3 type and Mg 12 RE-type intermetallic compounds, wherein RE is a mixture of rare earth elements Ce, La, Y and Nd, the volume fraction of the reinforcing phase is 4-8%, the average size is 1-5 micrometers, and it is dispersed in spherical or short rod shapes; the edge phase is a coherent or semi-coherent transition layer with a thickness of 10-50 nm, containing Y-Zn, Nd-Zn, Y-Ca and Nd-Ca precipitates; the continuous phase is an α-Mg matrix with a grain size of 20-50 micrometers.

[0016] This invention addresses the shortcomings of the prior art and has the following beneficial effects: (1) This invention introduces Al, Zn, Ca, and various rare earth elements Ce, La, Y, and Nd, and employs a graded addition and stirring process to construct a three-phase synergistic microstructure in the alloy, consisting of a reinforcing phase, an edge phase, and a continuous phase. The reinforcing phase is composed of high-melting-point Al. 11 RE3 type and Mg 12 Predominantly composed of RE-type intermetallic compounds, dispersed throughout the matrix, these compounds effectively pin grain boundaries and hinder dislocation movement, thereby enhancing the alloy's high-temperature strength and creep resistance. Compared to existing Mg-Al-RE light rare earth alloys, their Al... 11 The RE3 phase is prone to microcracks at the interface with the matrix above 150℃, leading to performance degradation. However, this invention significantly suppresses the interface failure problem at high temperatures by optimizing the thermal stability and distribution uniformity of the phase through multi-element rare earth optimization. Furthermore, the edge phase, as a transition layer, further strengthens the interface bonding, enabling the alloy to maintain structural integrity and performance stability under long-term high-temperature conditions in the hybrid vehicle motor housing.

[0017] (2) This invention adds interfacial active elements such as Zn and Ca, which synergistically agglomerate with heavy rare earth elements Y and Nd at the interface between the reinforcing phase and the matrix to form a nanoscale coherent or semi-coherent edge phase. The edge phase can effectively buffer the interfacial stress caused by the difference in thermal expansion coefficients between the reinforcing phase and the α-Mg matrix, and reduce phonon scattering through lattice matching, thereby improving the thermal conductivity of the alloy and ensuring efficient heat dissipation of the motor casing. Compared with existing heavy rare earth alloys, due to the lack of an interfacial transition layer, the disordered agglomeration of the reinforcing phase not only scatters phonons and reduces thermal conductivity, but also causes interfacial stress concentration, resulting in insufficient room temperature strength. This invention achieves synergistic optimization of thermal management and mechanical properties through edge phase design; furthermore, the edge phase also enhances the interfacial load transfer efficiency, enabling the alloy to maintain high thermal conductivity at high temperatures while possessing excellent creep resistance.

[0018] (3) This invention precisely controls the solidification process and microstructure evolution of the alloy through multi-stage injection process and pressure control, combined with mold preheating and heat treatment. Low-speed injection avoids air entrapment, high-speed injection ensures filling of complex cavities, and the pressure stage compensates for solidification shrinkage and promotes element interface segregation. The alloy has good fluidity and high casting density, making it suitable for thin-walled motor housing molding. Compared with traditional die-casting alloys, the addition of rare earth elements often leads to an expansion of the solidification range or agglomeration of strengthening phases, resulting in insufficient filling or hot cracking defects. This invention avoids these problems by optimizing process parameters while refining the matrix grains. Furthermore, the heat treatment process eliminates internal stress and improves the edge phase structure.

[0019] (4) During heat treatment, a small amount of supersaturated rare earth elements, zinc and calcium atoms dissolved in the α-Mg matrix can be further precipitated in the form of nanoscale precipitates to form a small amount of nanoscale Al-RE type secondary strengthening phases, which form a primary-secondary synergistic strengthening system with the core strengthening phase, or promote the further growth and improvement of the interface synergistic strengthening edge phase, so that the edge phase distribution is more continuous, its structure is more stable and the interface bonding is more compact. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Figure 1 This is a flowchart of a preferred embodiment of the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0023] Application Overview: This application relates to die-cast alloy materials for high-performance hybrid vehicle motor housings; however, while existing Mg-Al-RE light rare earth alloys are cost-controllable, the resulting Al... 11 The RE3 phase has insufficient thermal stability, and microcracks are easily generated at the interface with the α-Mg matrix when it is in service above 150℃, which leads to the degradation of creep resistance and cannot meet the requirements of long-term high-temperature operation of motors. Although Mg-heavy rare earth alloys form a highly stable strengthening phase, the lack of an interface transition layer causes the interface stress caused by the difference in thermal expansion coefficients between the strengthening phase and the matrix to reduce the room temperature tensile strength compared with ordinary alloys. In addition, the agglomerated intermetallic compounds reduce the thermal conductivity, and the expansion of the solidification zone leads to insufficient filling during die casting.

[0024] When using conventional rare earth strengthening schemes, rare earth elements mainly dissolve in the matrix or form discrete, bulk intermetallic compounds. This disordered phase distribution leads to mutual constraints on performance. Traditional techniques attempt to reconcile these contradictions by simply adjusting the type and content of rare earth elements. In light rare earth systems, Al... 11 The RE3 phase is in direct contact with the α-Mg matrix. At high temperatures, the interfacial tensile stress far exceeds the phase boundary bonding strength, causing cracking and loss of strengthening effect. In the heavy rare earth system, the amount of addition is increased to improve high-temperature performance. The strengthening phase agglomerates disorderly, which not only scatters phonons and reduces thermal conductivity, but also forms low-melting-point eutectic between dendrites during solidification, which hinders melt flow. Moreover, at room temperature, the agglomerated phase becomes a crack propagation source, resulting in insufficient strength.

[0025] This application breaks through the conventional thinking of traditional alloy design by introducing a three-phase synergistic system of reinforcing phase, edge phase, and continuous phase. The reinforcing phase is a highly stable rare earth intermetallic compound that provides high-temperature strength. The edge phase is a coherent precipitate layer formed by the segregation of rare earth elements at the interface between the core phase and the magnesium matrix. It serves as a transition layer to alleviate the difference in thermal expansion coefficients and reduce interfacial stress. The continuous phase is an α-Mg matrix that ensures thermal conductivity, toughness, and formability. This results in a material with high strength, high thermal conductivity, and excellent formability.

[0026] Exemplary method: like Figure 1 As shown, a method for preparing a die-casting alloy material for a hybrid vehicle motor housing includes the following steps: S1. Add pure aluminum to the molten magnesium liquid and stir to dissolve it, forming a matrix melt with α-Mg matrix as the continuous phase; S2. Add Mg-Zn master alloy and Mg-Ca master alloy to the matrix melt and stir to dissolve, providing active elements at the Zn and Ca interface to form a transition melt; S3. Add Mg-Ce, Mg-La, Mg-Y, and Mg-Nd master alloys to the transition melt from low to high melting point and stir in stages. Introduce Ce, La, Y, and Nd as strengthening phase forming elements and edge phase auxiliary elements to form a mixed melt with uniformly distributed three-phase components. S4. The mixed solution is injected into the preheated mold through multi-stage pressure injection and then die-cast to form a casting with a preliminary three-phase structure. S5. Heat treatment is performed on the casting to strengthen the edge phase.

[0027] Step S1 establishes a continuous matrix framework for the alloy. By uniformly fusing aluminum with magnesium liquid, the matrix is ​​given basic mechanical properties through the solid solution strengthening effect of aluminum in the α-Mg matrix, ensuring its continuity as the main heat conductor. On the other hand, it reserves the necessary aluminum for subsequent reaction with rare earth elements to form an Al-RE type core strengthening phase.

[0028] Before step S1, industrial pure magnesium ingots with a purity of not less than 99.95% and industrial pure aluminum ingots with a purity of not less than 99.9% are required. Specific master alloys as carriers of alloying elements also need to be prepared, using pre-made master alloys, specifically including Mg-Zn master alloy, Mg-Ca master alloy, Mg-Ce master alloy, Mg-La master alloy, Mg-Y master alloy, and Mg-Nd master alloy, with all master alloys having a purity of not less than 99.9%.

[0029] In step S1, magnesium is the core raw material for constructing the α-Mg matrix; the amount of aluminum added accounts for 3.5-4.5% of the total weight of the alloy. Too low an aluminum content will result in insufficient room temperature strength and poor die-casting fluidity, while too high an aluminum content will lead to the formation of a large amount of thermally unstable Mg. 17 Al 12 This caused a sharp drop in the intensity of the high temperature.

[0030] An appropriate amount of aluminum can significantly lower the melting point of magnesium alloys and broaden the solidification range, thereby improving the fluidity of the alloy melt and enhancing its filling ability in complex thin-walled die-casting molds. This is crucial for manufacturing motor housings with complex geometries and uniform wall thickness. Aluminum enhances the room temperature strength of the α-Mg matrix through solid solution strengthening, compensating for the insufficient room temperature strength of existing alloys. Aluminum can form stable and high-melting-point Al-RE type intermetallic compounds with rare earth elements, serving as an important component of the strengthening phase and replacing the thermally unstable Mg. 17 Al 12 The phase provides the alloy with excellent high-temperature strength and creep resistance.

[0031] The melting tool is a graphite crucible with a protective coating. The crucible is made of graphite with a protective coating to prevent the graphite from reacting chemically with the molten magnesium and introducing impurities, thus avoiding impurities from damaging the continuity of the matrix.

[0032] The specific steps of S1 are as follows: Heat the crucible to 200-300℃, and after the crucible temperature stabilizes, put in industrial pure magnesium ingots; A mixed protective gas is continuously introduced above the crucible at a flow rate of 5-10 L / min to cover the surface of the liquid magnesium alloy and prevent oxidation and combustion. Heat the crucible to 680-700℃ and keep it at that temperature until the magnesium ingot is completely melted, forming a uniform molten magnesium liquid. Slowly add industrial pure aluminum ingots to the molten magnesium liquid and stir to dissolve them completely to form a matrix melt; the stirring speed is 100-150 r / min and continues for 10-15 min until the aluminum ingots are completely dissolved and uniformly mixed with the magnesium liquid.

[0033] The mixed protective gas consists of 99.5% argon and 0.5% sulfur hexafluoride. Because magnesium is highly reactive and easily oxidized and burned at high temperatures, the mixed protective gas can form a dense protective film on the surface of the molten metal, reducing magnesium loss, ensuring the accuracy of the composition, and solving the performance fluctuation problem caused by oxidation in traditional smelting.

[0034] Step S2 introduces two interfacial active elements, Zn and Ca, into the matrix melt. On the one hand, their atomic properties promote the nucleation and growth of the edge phase in the subsequent interface synergistic strengthening. On the other hand, it improves the performance of the die casting process by optimizing the solidification characteristics of the melt.

[0035] In step S2, the amount of Zn added accounts for 0.3-0.6% of the total weight of the alloy, and the amount of Ca added accounts for 0.05-0.15% of the total weight of the alloy; Zn and Ca are important interfacial active elements and microalloying elements in the alloy of the present invention.

[0036] During alloy smelting and solidification, Zn and Ca atoms exhibit high interfacial activity, preferentially adsorbing at the interface between the core strengthening phase and the magnesium matrix, thereby effectively reducing interfacial energy. Zn and Ca form specific low free energy compounds or solid solutions with heavy rare earth elements Y and Nd, including compounds such as Y-Zn, Nd-Zn, Y-Ca, and Nd-Ca. Due to their interfacial adsorption characteristics, Zn and Ca preferentially aggregate at the interface between the core strengthening phase and the matrix, reducing interfacial energy, promoting the nucleation and growth of the interfacial synergistic strengthening edge phase, enhancing the bonding strength of the phase interface, improving load transfer efficiency, and exhibiting excellent stability at high temperatures, thereby further enhancing the overall mechanical properties and creep resistance of the alloy.

[0037] Zn itself can provide effective solid solution strengthening by forming a solid solution in the α-Mg matrix, thereby improving the strength of the matrix. The intermetallic compounds such as Y-Zn and Nd-Zn formed with Y and Nd can act as effective dislocation barriers under the high power density vibration and stress that may be generated during the operation of the hybrid motor, preventing dislocation slip and proliferation, thereby further enhancing the comprehensive mechanical properties and fatigue life of the alloy. Ca has a significant grain-refining effect, promoting heterogeneous nucleation in the α-Mg matrix and thus refining the grains. At the same time, Ca can form stable Mg-Ca-Zn or Mg-Ca-RE phases with Zn and rare earth elements, which helps optimize the solidification path of the alloy and reduce the temperature difference between the liquidus and solidus. This significantly improves the die-casting fluidity, filling performance, and thermal crack resistance of the alloy. For manufacturing complex thin-walled motor housings, the excellent die-casting process performance can significantly reduce the scrap rate and improve the molding quality and surface finish.

[0038] The specific steps of S2 are as follows: Increase the temperature of the base melt to 720-740℃; The Mg-Zn master alloy and Mg-Ca master alloy are added sequentially. During the addition process, the melt is continuously stirred at a speed of 120-180 r / min for 5-10 min.

[0039] Preferably, the Mg-Zn master alloy and the Mg-Ca master alloy are added to the melt in batches and slowly, with each batch not exceeding 1 / 3 of the total weight. The next batch is added only after the previous batch of master alloy has been completely submerged and dissolved, so as to avoid sudden changes in local composition caused by a single addition. During stirring, ensure that the stirring paddle is immersed in the melt to a depth of 1 / 2 to 2 / 3 of the liquid level and rotates slowly along the inner wall of the crucible to avoid generating violent eddies.

[0040] By ensuring the uniform distribution of Zn and Ca elements in the α-Mg matrix, local enrichment and the formation of non-target phases are avoided, thus providing a uniform compositional environment for the subsequent synergistic formation of edge phases by rare earth elements and the two elements.

[0041] Step S2 ultimately forms a transitional melt with uniformly distributed Zn and Ca elements. The α-Mg matrix is ​​strengthened by the dual solid solution of Al and Zn, and its room temperature strength is initially improved. At the same time, the grain refinement effect of Ca element pre-refines the matrix grain size to 60-80 micrometers, laying the foundation for further refinement by rare earth elements.

[0042] In step S3, predetermined amounts of Mg-Ce master alloy, Mg-La master alloy, Mg-Y master alloy and Mg-Nd master alloy are added sequentially; the addition of rare earth master alloys is the key to forming the core strengthening phase and the interface synergistic strengthening edge phase.

[0043] In step S3, Ce accounts for 0.8-1.5% of the total alloy weight, La accounts for 0.4-0.8% of the total alloy weight, Y accounts for 0.2-0.5% of the total alloy weight, and Nd accounts for 0.1-0.3% of the total alloy weight. Therefore, the total die-casting alloy material consists of the following components by weight percentage: Al: 3.5-4.5%, Zn: 0.3-0.6%, Ca: 0.05-0.15%, Ce: 0.8-1.5%, La: 0.4-0.8%, Y: 0.2-0.5%, Nd: 0.1-0.3%, with the balance being Mg and other unavoidable impurity elements. This compositional design is based on the functional positioning and synergistic effects of each element in the alloy, ensuring an optimal balance of final material properties.

[0044] Specifically, Ce and La are light rare earth elements; Ce and La work synergistically with aluminum to preferentially form Al. 11(Ce,La)3 type high-melting-point intermetallic compounds possess high melting points and excellent thermal stability, with melting points exceeding 550℃. They maintain structural stability even at temperatures above 150℃ and exist in a dispersed form within the α-Mg matrix, constituting the main body of the core reinforcing phase in this invention. Under high-temperature conditions, rare-earth aluminum compounds can effectively pin grain boundaries, inhibit grain boundary slip and dislocation movement, and significantly improve the high-temperature strength and creep resistance of the alloy. This is crucial for motor housings that withstand the stresses generated by long-term high-temperature operation of motors.

[0045] Ce and La also have a significant grain-refining effect. During alloy solidification, these rare earth elements can act as heterogeneous nucleation sites, promoting the refinement of α-Mg matrix grains, reducing the 60-80 micrometer grains pre-refined by S2 to 20-50 micrometers. The fine grain structure can increase the number of grain boundaries, thereby effectively hindering dislocation movement and improving the room temperature strength and toughness of the alloy according to the Hall-Petch relationship.

[0046] Furthermore, Y and Nd are heavy rare earth elements; Y and Nd have relatively large atomic radii and low solid solubility in the magnesium matrix, which makes them prone to segregation at the phase interface during alloy solidification. Synergistically, with the added Zn and Ca elements, they form coherent or semi-coherent precipitates with specific crystal structures and lattice matching at the interface between the α-Mg matrix and the core strengthening phase through heterogeneous nucleation or solid-state precipitation. These include Y-Zn, Nd-Zn, Y-Ca, and Nd-Ca; among which, the core strengthening phase is mainly Al. 11 RE3 type and Mg 12 RE-type compounds.

[0047] The precipitated phase constitutes the interface synergistic strengthening edge phase of the present invention. The presence of the edge phase not only further enhances the overall strength and creep resistance of the alloy through the interface strengthening mechanism, but more importantly, its lattice matching characteristics significantly reduce the interfacial stress between the core strengthening phase and the α-Mg matrix due to the difference in thermal expansion coefficients, and improve the interfacial bonding force.

[0048] From a thermal conductivity perspective, this highly matched transition layer can effectively reduce phonon scattering at the phase interface, thereby reducing interfacial thermal resistance, significantly improving the overall thermal conductivity of the alloy, ensuring efficient heat dissipation of the motor housing, and maintaining long-term stable operation of the motor.

[0049] If the Y and Nd contents are below the range, it will be difficult to form a sufficient volume fraction or thickness of interfacial synergistic strengthening edge phase, and its synergistic strengthening and thermal conductivity optimization effects will be greatly reduced; conversely, if the contents are too high, it will not only significantly increase the material cost, but may also lead to the formation of coarse and brittle phases, which will deteriorate the plasticity and toughness of the alloy.

[0050] The specific steps for S3 are as follows: Maintain the temperature at 720-740℃, first mix the Mg-Ce master alloy and Mg-La master alloy, and slowly add the transition melt in batches; each batch should be added 5 minutes apart, and the next batch should be added only after the previous batch has completely dissolved, to avoid local enrichment due to similar melting points; Stir at a high speed of 180-250 r / min for 5-8 min to ensure that the light rare earth elements are initially fused with the melt, creating a uniform environment for the nucleation of the core strengthening phase; Mix the Mg-Y master alloy and the Mg-Nd master alloy, add them slowly in batches, and stir at a high speed of 180-250 r / min for 10-20 min to ensure that rare earth elements, Zn, Ca and matrix melt are uniformly mixed in all directions to promote the dispersed nucleation of the strengthening phase. After stirring, lower the temperature of the melt to 720-730℃, let it stand for 5-10 minutes, and remove the surface scum. Argon gas is used as the refining gas. It is introduced from the bottom of the melt through an immersion graphite tube at a flow rate of 0.5-1.0 L / min and refined for 8-12 minutes to remove hydrogen and non-metallic inclusions from the melt. After refining, the surface slag is removed again.

[0051] Step S3 follows the order of adding the master alloy with low melting point first and then high melting point. If the order is reversed, the low melting point master alloy may be locally overheated and burned when the high melting point master alloy is not completely dissolved, resulting in composition deviation. Before adding the rare earth master alloy, it needs to be dried in an oven at 150-180℃ for 3-4 hours to completely remove moisture and prevent the introduction of hydrogen gas to form pores, which would affect the density of the casting.

[0052] Step S3 ultimately yields a mixed melt with a uniform distribution of three phases, including: an α-Mg matrix, a reinforcing phase, and precursor elements for the edge phase. The reinforcing phase precipitates diffusely in spherical or short rod-shaped forms, with an average size of 1-5 micrometers and a volume fraction of 4%-8%, without significant agglomeration. The α-Mg matrix grains are refined to 20-50 micrometers. Through the synergistic effect of solid solution strengthening by Al and Zn and grain refinement, the room temperature tensile strength is further improved compared to the transition melt in S2. More importantly, Y and Nd form a uniform element-enriched region with Zn and Ca in the melt, providing precise compositional and structural conditions for the in-situ nucleation of the interface-synergistic strengthening edge phase during the S4 die-casting process. At the same time, the size and distribution of the core reinforcing phase are precisely controlled, avoiding the obstruction of the thermal conductivity path by excessive reinforcing phase, thus laying the foundation for subsequent optimization of thermal conductivity.

[0053] Step S4 is a key process step in realizing the transformation of the alloy's three-phase structure from the molten state to the solid state. Through the precise coordination of multi-stage injection parameters and mold preheating, it is necessary to ensure that the mixed melt is completely filled in the complex thin-walled mold cavity, and to promote the orderly precipitation and distribution of the core strengthening phase and the edge phase through solidification process control.

[0054] In step S4, the die-casting process parameters need to match the characteristics of the alloy melt and the structural requirements of the motor housing. The mold preheating temperature is controlled at 200-250℃. The multi-stage injection is divided into three stages: low-speed injection, high-speed injection, and pressurization. The low-speed injection speed is 0.1-0.3m / s, and the high-speed injection speed is increased to 3.0-5.0m / s. Pressurization is started immediately after the mold is completely filled. The pressurization pressure is maintained at 60-100MPa for 100-300ms. The casting needs to be cooled in the mold to below 200℃ before the mold is opened and the casting is removed.

[0055] Too low a mold preheating temperature will cause the molten metal to solidify rapidly upon contact with the cavity wall, resulting in insufficient filling or cold shut defects; too high a temperature will prolong the solidification time, easily leading to coarse grains and surface oxidation. Too high a slow injection speed can easily entrap gas and form pores, while too low a speed will result in low efficiency; a fast injection speed needs to match the filling requirements of the thin-walled area of ​​the outer shell, insufficient speed will result in incomplete filling at the end, while too fast speed will generate turbulent slag entrainment.

[0056] The die casting process has a decisive influence on the formation of the three-phase structure. The temperature gradient formed by the preheating of the mold causes the melt to solidify gradually from the cavity wall to the center, providing directional nucleation conditions for the core strengthening phase and promoting its uniform distribution along the grain boundaries. The pressure of the multi-stage injection causes the melt to solidify under high pressure, reducing porosity defects such as shrinkage cavities, while promoting the enrichment of Y, Nd, Zn, and Ca elements at the phase interface, and promoting the initial precipitation of edge phase precursors at the interface between the core strengthening phase and the α-Mg matrix. The continuous pressure during the holding stage ensures that the melt is replenished during solidification shrinkage, improving the density of the casting and providing a dense matrix for the uniform growth of the edge phase during subsequent heat treatment.

[0057] The specific steps of S4 are as follows: The temperature of the refined melt is lowered to 660-680℃, which is used as the pouring temperature before die casting. The molten metal is poured into the injection chamber of the die-casting machine; The die-casting mold is preheated to 200-250℃, and a layer of water-based release agent with a solid content of 5-8% is uniformly coated on the mold surface for 3-5 seconds to form a thin and uniform lubricating film, avoiding porosity in the casting due to excessive release agent residue. The mold structure design includes sufficient water cooling channels, and the temperature of each area of ​​the cavity is precisely controlled through the water cooling system to ensure that the casting achieves a controllable solidification rate during the die-casting process. This provides the necessary conditions for forming fine and uniform α-Mg matrix grains, controlling the size and dispersion distribution of the core strengthening phase, and promoting the orderly precipitation of the interfacial synergistic strengthening edge phase in specific areas. Start the die-casting machine's injection system and use a three-stage injection process for molding: The first stage is low-speed injection, with an injection speed of 0.1-0.3 m / s, which pushes the molten liquid to fill the injection chamber and gating smoothly. Sudden speed changes are avoided throughout the process to prevent the molten liquid from entraining air and forming porosity defects. Once the molten metal reaches the gate position, immediately switch to the second stage of high-speed injection, increasing the injection speed to 3.0-5.0 m / s, so that the molten metal can quickly fill the entire mold cavity, ensuring that the complex thin-walled structure is completely filled. At the same time, the cooling effect brought about by high-speed filling promotes the refinement of α-Mg matrix grains and the uniform distribution of fine core strengthening phases. After the mold is fully filled, the third stage of pressurization begins, maintaining the pressure at 60-100 MPa for 100-300 ms. This high pressure compensates for the solidification shrinkage of the melt, eliminating internal defects such as shrinkage porosity, further refining the grains and improving the density of the casting. Precise control at this stage is key to the efficient formation of the interface-synergistic strengthening edge phase. The combined effect of instantaneous high pressure and rapid solidification conditions promotes the rapid segregation of heavy rare earth elements Y, Nd, zinc, and calcium at the interface between the core strengthening phase and the α-Mg matrix, thereby forming the target coherent or semi-coherent precipitates. After pressurization is completed, the mold is kept closed, and the temperature is continuously regulated through the water cooling channel. After the casting cools to below 200°C in the mold, the mold opening mechanism is activated, and the casting is smoothly removed through the ejection device to avoid deformation or cracking caused by removing the casting at high temperature.

[0058] Step S5 involves strengthening and modifying the casting through heat treatment to promote the improvement and stability of the interfacial synergistic strengthening edge phase, while eliminating the internal stress generated during die casting. Ultimately, this achieves synergistic optimization of the alloy's mechanical and thermal properties, meeting the core requirement of long-term high-temperature service for hybrid vehicle motor housings.

[0059] In step S5, the casting is heated to 200-220°C under an argon protective atmosphere and held for 4-6 hours, then slowly cooled to room temperature in the furnace.

[0060] Too low a temperature will result in insufficient atomic diffusion motive force, failing to form enough strengthening phase; too high a temperature may cause edge phase coarsening or matrix grain growth, thus degrading performance. The holding time must ensure that the precipitation process is fully completed; too short a time will result in insufficient strengthening phase, while too long a time may cause the precipitated phase to aggregate and grow, weakening the strengthening effect.

[0061] During heat treatment, a small amount of supersaturated rare earth elements, zinc and calcium atoms dissolved in the α-Mg matrix can be further precipitated in the form of nanoscale precipitates, forming a small amount of nanoscale Al-RE type secondary strengthening phases. These phases form a primary-secondary synergistic strengthening system with the core strengthening phase, or promote the further growth and improvement of the interface synergistic strengthening edge phases, making the edge phase distribution more continuous, its structure more stable, and the interface bonding tighter.

[0062] The secondary precipitation strengthening mechanism can effectively pin dislocations, thereby significantly improving the room temperature and high temperature strength of the alloy and enhancing its creep resistance. At the same time, the heat treatment process eliminates lattice distortion and internal stress caused by rapid solidification during die casting through atomic diffusion, which helps to release residual stress generated during die casting, improve the dimensional stability of the casting, and reduce its crack susceptibility.

[0063] The specific steps of S5 include: Remove surface oil and oxide scale from the S4 casting, ultrasonically clean with anhydrous ethanol for 10-15 minutes, dry and then put into heat treatment fixtures to ensure uniform heating of all parts of the casting and avoid local temperature deviation caused by stacking. Place the tooling into the box-type heat treatment furnace, close the furnace door, and introduce argon gas at a flow rate of 2-3 L / min for 30 minutes to replace the air in the furnace and prevent oxidation of the casting surface during the heating process. Raise the furnace temperature to 200-220℃ at a rate of 5-8℃ / min, and maintain the temperature for 4-6 hours after reaching the target temperature. After the heat preservation is completed, argon gas is continuously introduced to allow the casting to cool slowly with the furnace at a rate of 2-3℃ / min until the furnace temperature drops below 50℃. Then, the argon gas supply is stopped, the furnace door is opened, and the casting is removed.

[0064] The final product is a die-cast alloy material suitable for use in the housing of hybrid vehicle motors; Exemplary materials: A die-cast alloy material for the housing of a hybrid vehicle motor, comprising: Al: 3.5-4.5% Zn: 0.3-0.6%, Ca: 0.05-0.15% Ce: 0.8-1.5% La: 0.4-0.8% Y: 0.2-0.5% Nd: 0.1-0.3% The balance is Mg and unavoidable impurities; Furthermore, the die-cast alloy material has a three-phase synergistic microstructure consisting of a reinforcing phase, an edge phase, and a continuous phase; The reinforcing phase is a high-melting-point intermetallic compound dispersed in the α-Mg matrix, mainly composed of Al. 11 RE3 type intermetallic compounds and Mg 12 Composition of RE-type intermetallic compounds; Among them, Al 11 RE3 type intermetallic compounds contain REs, which are a mixture of rare earth elements including Ce, La, Y, and Nd, and Mg. 12RE in RE-type intermetallic compounds is a mixture of rare earth elements, namely Ce and La; The volume fraction of the strengthening phase is 4-8%, the average size is 1-5 micrometers, and it is spherical or short rod-shaped. It is uniformly distributed in the α-Mg matrix. The strengthening phase has excellent high-temperature stability. Under working conditions with temperatures above 150℃, it can effectively pin grain boundaries, hinder dislocation movement and grain boundary slip, thereby providing the alloy with the main high-temperature strength and creep resistance. The marginal phase is a coherent or semi-coherent transition layer containing Y-Zn, Nd-Zn, Y-Ca and / or Nd-Ca precipitates, formed at the interface between the core phase and the continuous phase α-Mg matrix. The grain size of the α-Mg matrix is ​​20-50 micrometers.

[0065] The thickness of the edge phase is 10-50 nm, mainly composed of intermetallic compounds or solid solutions such as Y-Zn, Nd-Zn, Y-Ca and Nd-Ca enriched with heavy rare earth elements Y, Nd, zinc and calcium. The edge phase is formed by segregation at the interface between the reinforcing phase and the α-Mg matrix, serving as a transition layer. Its lattice structure has a high degree of matching with the α-Mg matrix and the reinforcing phase, thereby effectively alleviating the interfacial stress caused by the difference in thermal expansion coefficients between the reinforcing phase and the α-Mg matrix, and significantly enhancing the interfacial bonding force. At the same time, by establishing a lattice-matched transition region, it significantly reduces the phonon scattering efficiency at the interface, thereby improving the overall thermal conductivity of the alloy.

[0066] Example 1:

[0067] A method for preparing a die-casting alloy material for a hybrid vehicle motor housing includes the following steps: Prepare industrial pure magnesium ingots with a purity of not less than 99.95%, and industrial pure aluminum ingots with a purity of not less than 99.9%. Prepare Mg-Zn master alloy, Mg-Ca master alloy, Mg-Ce master alloy, Mg-La master alloy, Mg-Y master alloy, and Mg-Nd master alloy, with the purity of all master alloys not less than 99.9%.

[0068] Heat the crucible to 250°C. After the temperature stabilizes, place industrial pure magnesium ingots in it. Continuously introduce a mixed protective gas consisting of 99.5% argon and 0.5% sulfur hexafluoride at a flow rate of 7.5 L / min above the crucible. Heat the crucible to 690°C and maintain the temperature until the magnesium ingots are completely melted to form molten magnesium. Slowly add 4% (by weight of the total alloy) of industrial pure aluminum ingots to the molten magnesium. Stir at 125 r / min for 12.5 min until the aluminum ingots are completely dissolved and uniformly mixed with the magnesium to form the matrix melt.

[0069] The temperature of the molten base was raised to 730℃, and Mg-Zn and Mg-Ca master alloys were added sequentially. The amount of Zn added accounted for 0.45% of the total weight of the alloy, and the amount of Ca added accounted for 0.1% of the total weight of the alloy. The master alloys were added slowly in batches, with each batch not exceeding 1 / 3 of the total weight. The next batch was added only after the previous batch of master alloys had been completely submerged and dissolved. During the addition process, the melt was continuously stirred at a speed of 150 r / min for 7.5 min. During the stirring process, the stirring paddle was ensured to be immersed in the melt to a depth of 1 / 2 to 2 / 3 of the liquid level and to rotate slowly along the inner wall of the crucible to form a transition melt.

[0070] Maintain the temperature at 730℃, and dry the rare earth master alloy in an oven at 165℃ for 3.5 hours before adding it. First, mix the Mg-Ce master alloy and the Mg-La master alloy, with Ce accounting for 1.15% of the total alloy weight and La accounting for 0.6% of the total alloy weight. Add the transition melt slowly in batches, with an interval of 5 minutes between each batch, and add the next batch only after the previous batch has completely dissolved. Stir at high speed at 215 r / min for 6.5 min. Then mix the Mg-Y master alloy and the Mg-Nd master alloy, with Y accounting for 1.15% of the total alloy weight. Gold accounts for 0.35% of the total weight, and Nd accounts for 0.2% of the total weight of the alloy. The mixture is added slowly in batches and stirred at a high speed of 215 r / min for 15 min. After stirring, the temperature of the melt is reduced to 725℃ and allowed to stand for 7.5 min to remove surface slag. Argon is used as the refining gas and introduced from the bottom of the melt through an immersion graphite tube at a flow rate of 0.75 L / min. The refining process lasts for 10 min to remove hydrogen and non-metallic inclusions from the melt. After refining, surface slag is removed again to form a mixed melt.

[0071] The temperature of the refined molten metal was lowered to 670℃ as the pouring temperature before die casting. The molten metal was poured into the injection chamber of the die casting machine. The die casting mold was preheated to 225℃, and a layer of water-based release agent with a solid content of 6.5% was uniformly coated on the mold surface for 4 seconds. The die casting machine injection system was started, and a three-stage injection process was used for molding: the first stage was low-speed injection at a speed of 0.2 m / s; after the molten metal reached the gate position, the process was switched to the second stage of high-speed injection, with the injection speed increased to 4.0 m / s; after the mold was completely filled, the third stage of pressurization was initiated, with the pressurization pressure maintained at 80 MPa for 200 ms; after pressurization was completed, the mold was kept closed, and the temperature was continuously controlled through the water cooling channel. After the casting cooled to below 200℃ in the mold, the mold opening mechanism was activated, and the casting was smoothly removed through the ejection device.

[0072] Remove surface oil and oxide scale from the obtained castings, ultrasonically clean them with anhydrous ethanol for 12.5 minutes, dry them, and then place them into a special heat treatment fixture. Place the fixture into a box-type heat treatment furnace, close the furnace door, and introduce argon gas at a flow rate of 2.5 L / min for 30 minutes to replace the air in the furnace. Raise the furnace temperature to 210°C at a rate of 6.5°C / min, and maintain the temperature for 5 hours after reaching the target temperature. After the holding period, continue to introduce argon gas to allow the castings to cool slowly with the furnace at a cooling rate of 2.5°C / min until the furnace temperature drops below 50°C. Stop the argon supply, open the furnace door, and remove the castings.

[0073] The final die-casting alloy material has the following parameters: Al 4.0%, Zn 0.45%, Ca 0.1%, Ce 1.15%, La 0.6%, Y 0.35%, Nd 0.2%, with the balance being Mg and unavoidable impurities; it has a three-phase synergistic microstructure consisting of a reinforcing phase, an edge phase, and a continuous phase; the reinforcing phase has a volume fraction of 6%, an average size of 3 micrometers, and exhibits a spherical or short rod-like morphology, uniformly distributed in the α-Mg matrix; the grain size of the α-Mg matrix is ​​35 micrometers; the edge phase has a thickness of 30 nm and is a coherent or semi-coherent transition layer containing Y-Zn, Nd-Zn, Y-Ca, and Nd-Ca precipitates.

[0074] Example 2: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Zn added accounts for 0.3% of the total weight of the alloy, and the amount of Ca added accounts for 0.05% of the total weight of the alloy; the specifications of the remaining raw materials, preparation steps, and process parameters are the same as in Example 1. Example 3: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Zn added accounts for 0.37% of the total weight of the alloy, and the amount of Ca added accounts for 0.07% of the total weight of the alloy; the specifications of the remaining raw materials, preparation steps, and process parameters are the same as in Example 1. Example 4: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Zn added accounts for 0.43% of the total weight of the alloy, and the amount of Ca added accounts for 0.09% of the total weight of the alloy; the specifications of the remaining raw materials, preparation steps, and process parameters are the same as in Example 1. Example 5: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Zn added accounts for 0.47% of the total weight of the alloy, and the amount of Ca added accounts for 0.11% of the total weight of the alloy; the specifications of the remaining raw materials, preparation steps, and process parameters are the same as in Example 1. Example 6: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Zn added accounts for 0.53% of the total weight of the alloy, and the amount of Ca added accounts for 0.13% of the total weight of the alloy; the specifications of the remaining raw materials, preparation steps, and process parameters are the same as in Example 1. Example 7: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Zn added accounts for 0.6% of the total weight of the alloy, and the amount of Ca added accounts for 0.15% of the total weight of the alloy; the specifications of the remaining raw materials, preparation steps, and process parameters are the same as in Example 1. Example 8: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the Ce content accounts for 0.8% of the total alloy weight, and the La content accounts for 0.4% of the total alloy weight; the remaining raw material specifications, preparation steps, and process parameters are the same as in Example 1. Example 9: A method for preparing a die-casting alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the Ce content accounts for 1.0% of the total alloy weight, and the La content accounts for 0.5% of the total alloy weight; the specifications of other raw materials, preparation steps, and process parameters are the same as in Example 1. Example 10: A method for preparing a die-casting alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the Ce content accounts for 1.2% of the total alloy weight, and the La content accounts for 0.6% of the total alloy weight; the remaining raw material specifications, preparation steps, and process parameters are the same as in Example 1. Example 11: A method for preparing a die-casting alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the Ce content accounts for 1.3% of the total alloy weight, and the La content accounts for 0.7% of the total alloy weight; the remaining raw material specifications, preparation steps, and process parameters are the same as in Example 1. Example 12: A method for preparing a die-casting alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the Ce content accounts for 1.4% of the total alloy weight, and the La content accounts for 0.75% of the total alloy weight; the remaining raw material specifications, preparation steps, and process parameters are the same as in Example 1. Example 13: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the Ce content accounts for 1.5% of the total alloy weight, and the La content accounts for 0.8% of the total alloy weight; the remaining raw material specifications, preparation steps, and process parameters are the same as in Example 1. Example 14: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Y added accounts for 0.2% of the total alloy weight, and the amount of Nd added accounts for 0.1% of the total alloy weight; the specifications of other raw materials, preparation steps, and process parameters are the same as in Example 1. Example 15: A method for preparing a die-casting alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Y added accounts for 0.27% of the total weight of the alloy, and the amount of Nd added accounts for 0.15% of the total weight of the alloy; the specifications of the remaining raw materials, preparation steps, and process parameters are the same as in Example 1. Example 16: A method for preparing a die-casting alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Y added accounts for 0.43% of the total alloy weight, and the amount of Nd added accounts for 0.25% of the total alloy weight; the specifications of other raw materials, preparation steps, and process parameters are the same as in Example 1. Example 17: A method for preparing a die-casting alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Y added accounts for 0.5% of the total weight of the alloy, and the amount of Nd added accounts for 0.3% of the total weight of the alloy; the specifications of the remaining raw materials, preparation steps, and process parameters are the same as in Example 1. Example 18: A method for preparing a die-casting alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Y added accounts for 0.3% of the total alloy weight, and the amount of Nd added accounts for 0.22% of the total alloy weight; the specifications of other raw materials, preparation steps, and process parameters are the same as in Example 1. Example 19: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the amount of Y added accounts for 0.4% of the total alloy weight, and the amount of Nd added accounts for 0.18% of the total alloy weight; the specifications of other raw materials, preparation steps, and process parameters are the same as in Example 1. Example 20: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that the pressure boosting in the third stage of die casting is maintained at 60 MPa; the remaining raw material specifications, preparation steps, and process parameters are consistent with Example 1. Example 21: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that the pressure boosting in the third stage of die casting is maintained at 70 MPa; the remaining raw material specifications, preparation steps, and process parameters are consistent with Example 1. Example 22: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that the pressure boosting in the third stage of die casting is maintained at 75 MPa; the remaining raw material specifications, preparation steps, and process parameters are consistent with Example 1. Example 23: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that the pressure boosting in the third stage of die casting is maintained at 85 MPa; the remaining raw material specifications, preparation steps, and process parameters are consistent with Example 1. Example 24: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that the pressure boosting in the third stage of die casting is maintained at 90 MPa; the remaining raw material specifications, preparation steps, and process parameters are consistent with Example 1. Example 25: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that the pressure boosting in the third stage of die casting is maintained at 100 MPa; the remaining raw material specifications, preparation steps, and process parameters are consistent with Example 1.

[0075] Comparative Example 1: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: no Mg-Zn master alloy and Mg-Ca master alloy are added, i.e., the amount of Zn added accounts for 0% of the total weight of the alloy, and the amount of Ca added accounts for 0% of the total weight of the alloy; the specifications of other raw materials, preparation steps and process parameters are the same as in Example 1. Comparative Example 2: A method for preparing a die-casting alloy material for a hybrid vehicle motor housing differs from Example 1 in that: no Mg-Ce master alloy and Mg-La master alloy are added, i.e., the amount of Ce added accounts for 0% of the total weight of the alloy, and the amount of La added accounts for 0% of the total weight of the alloy; the specifications of other raw materials, preparation steps and process parameters are the same as in Example 1. Comparative Example 3: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: no Mg-Y master alloy and Mg-Nd master alloy are added, i.e., the amount of Y added accounts for 0% of the total weight of the alloy, and the amount of Nd added accounts for 0% of the total weight of the alloy; the specifications of other raw materials, preparation steps and process parameters are the same as in Example 1. Comparative Example 4: A method for preparing a die-cast alloy material for a hybrid vehicle motor housing differs from Example 1 in that: the third stage of die casting does not initiate effective pressurization, and the pressurization pressure is maintained at 10 MPa (far below the lower limit of the specified range, equivalent to no pressurization effect); the remaining raw material specifications, preparation steps, and process parameters are consistent with Example 1.

[0076] Experimental Example 1: Examples 1-13 and Comparative Examples 1-2 were selected for testing to obtain the room temperature strength, high temperature strength, thermal conductivity, and creep strain rate of the alloy materials. Room temperature strength refers to the tensile strength of the tested alloy material at 25°C. High-temperature strength refers to the tensile strength of the alloy material at 175℃. The creep strain rate is the creep strain rate of the test alloy material at 175℃ and under a load of 50MPa. Table 1. Performance test results of die-casting alloy materials in Examples 1-13 and Comparative Examples 1-2

[0077] The interfacial activity of Zn and Ca determines the variation of alloy properties with their content. In Example 1, the ratio of 0.45% Zn to 0.1% Ca maximizes the reduction of interfacial energy, promotes the formation of complete edge phases of precipitates such as Y-Zn and Nd-Zn, effectively alleviates the interfacial stress between the strengthening phase and the matrix, and at the same time, the grain refinement effect of Ca makes the matrix grains uniform. Therefore, the room temperature strength, high temperature strength, and thermal conductivity are all optimal, and the creep strain rate is the lowest. When the content deviates from this ratio, such as the low content in Example 2 or the high content in Example 7, the nucleation of edge phases is insufficient or locally enriched, the interfacial bonding force decreases, phonon scattering is enhanced, and the performance gradient deteriorates. In Comparative Example 1, when both are completely absent, the edge phase cannot be formed, and the performance is close to the lower limit of conventional alloys.

[0078] The content of Ce and La directly affects the formation and distribution of the strengthening phase. In Example 1, the combination of 1.15% Ce and 0.6% La generates a sufficient amount of Al11(Ce,La)3 type strengthening phase, which is spherically dispersed. This strengthens the phase by pinning grain boundaries and does not block the heat conduction path. If the content is too low, as in Example 8, the amount of strengthening phase is insufficient, and the high-temperature strengthening and grain refinement effects are weakened. If the content is too high, as in Example 13, the strengthening phase agglomerates into blocks, becoming a stress concentration source, and at the same time increases phonon scattering, resulting in a simultaneous decrease in strength and thermal conductivity. In Comparative Example 2, without both Ce and La, there is no stable strengthening phase to support high-temperature performance, and the creep strain rate increases significantly, failing to meet the requirements for high-temperature service.

[0079] Experimental Example 2: Examples 1, 14-25, and Comparative Examples 3-4 were selected for testing to obtain the room temperature strength, high temperature strength, thermal conductivity, and creep strain rate of the alloy materials. Room temperature strength refers to the tensile strength of the tested alloy material at 25°C. High-temperature strength refers to the tensile strength of the alloy material at 175℃. The creep strain rate is the creep strain rate of the test alloy material at 175℃ and under a load of 50MPa. Table 2 shows the test results of the die-casting alloy materials in Examples 1, 14-25, and Comparative Examples 3-4.

[0080] The contents of Y and Nd are core elements in the formation of the edge phase. In Example 1, the ratio of 0.35% Y to 0.2% Nd forms a coherent transition layer of about 30 nm at the interface between the reinforcing phase and the matrix, perfectly matching the crystal structures of both phases. This reduces interfacial stress, improves strength, and reduces phonon scattering, thus optimizing thermal conductivity. When the contents deviate from the optimal values, such as the low content in Example 14 leading to insufficient edge phase thickness, and the high content in Example 17 inducing a coarse and brittle phase, both disrupt the three-phase synergistic balance and significantly reduce performance. In Comparative Example 3, lacking both Y and Nd, there is no transition layer at the interface, and the reinforcing phase is in direct contact with the matrix. At high temperatures, the interface cracks, and the creep strain rate increases significantly.

[0081] The pressure applied during die casting influences performance by regulating density and phase distribution. In Example 1, the 80 MPa pressure fully compensates for solidification shrinkage, increasing casting density and simultaneously promoting the directional aggregation of elements such as Y and Nd towards the interface, thus facilitating the orderly precipitation of edge phases. When the pressure is below the optimal value (e.g., in Examples 20 and 4), the casting exhibits shrinkage defects, insufficient nucleation of edge phases, and a sharp drop in strength and thermal conductivity. Conversely, when the pressure is above the optimal value (e.g., in Example 25), excessive pressure leads to coarse grains, uneven distribution of the strengthening phase, and a reverse decline in performance. Examples 22 and 23, approaching the optimal pressure, maintain high performance levels, confirming the crucial regulatory role of pressure in the formation of three-phase structures.

[0082] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing a die-casting alloy material for a hybrid vehicle motor housing, characterized in that, include: S1. Add pure aluminum to the molten magnesium liquid and stir to dissolve it, forming a matrix melt with α-Mg matrix as the continuous phase; S2. Add Mg-Zn master alloy and Mg-Ca master alloy to the matrix melt and stir to dissolve, providing active elements at the Zn and Ca interface to form a transition melt; S3. Add Mg-Ce, Mg-La, Mg-Y, and Mg-Nd master alloys to the transition melt from low to high melting point and stir in stages. Introduce Ce, La, Y, and Nd as strengthening phase forming elements and edge phase auxiliary elements to form a mixed melt with uniformly distributed three-phase components. S4. The mixed solution is injected into the preheated mold through multi-stage pressure injection and then die-cast to form a casting with a preliminary three-phase structure. S5. Heat treatment is performed on the casting to strengthen the edge phase.

2. The method for preparing a die-casting alloy material for a hybrid vehicle motor housing according to claim 1, characterized in that: In S1, the molten magnesium liquid is formed by melting industrial pure magnesium ingots at 680-700℃, and industrial pure aluminum ingots are added to it, with aluminum accounting for 3.5-4.5% of the total weight of the alloy. The mixture is stirred continuously for 10-15 minutes at a stirring speed of 100-150 r / min, while a mixed protective gas is introduced at a flow rate of 5-10 L / min.

3. The method for preparing a die-casting alloy material for a hybrid vehicle motor housing according to claim 1, characterized in that: In step S2, the temperature of the matrix melt is raised to 720-740℃, and Mg-Zn master alloy and Mg-Ca master alloy are added sequentially. The amount of Zn added accounts for 0.3-0.6% of the total weight of the alloy, and the amount of Ca added accounts for 0.05-0.15% of the total weight of the alloy. The mixture is stirred continuously for 5-10 minutes at a stirring speed of 120-180 r / min, and the master alloy is added in batches.

4. The method for preparing a die-casting alloy material for a hybrid vehicle motor housing according to claim 1, characterized in that: In S3, Mg-Ce master alloy and Mg-La master alloy are added to the transition melt, wherein the amount of Ce added accounts for 0.8-1.5% of the total weight of the alloy and the amount of La added accounts for 0.4-0.8% of the total weight of the alloy. The mixture is stirred continuously for 5-8 minutes at a stirring speed of 180-250 r / min.

5. The method for preparing a die-casting alloy material for a hybrid vehicle motor housing according to claim 4, characterized in that: In step S3, Mg-Y master alloy and Mg-Nd master alloy are added, wherein the amount of Y added accounts for 0.2-0.5% of the total weight of the alloy and the amount of Nd added accounts for 0.1-0.3% of the total weight of the alloy. The mixture is stirred continuously at a stirring speed of 180-250 r / min for 10-20 min. Then the temperature of the melt is lowered to 720-730℃ and allowed to stand for 5-10 min. Argon refining is then carried out at a refining gas flow rate of 0.5-1.0 L / min for 8-12 min.

6. The method for preparing a die-casting alloy material for a hybrid vehicle motor housing according to claim 1, characterized in that: In S4, the preheating temperature of the mold is 200-250℃. The injection process includes: the first stage of low-speed injection, with a speed of 0.1-0.3m / s; the second stage of high-speed injection, with a speed of 3.0-5.0m / s; and the third stage of pressurization, with a pressure of 60-100MPa and a duration of 100-300ms. The casting is then removed after cooling in the mold to below 200℃.

7. The method for preparing a die-casting alloy material for a hybrid vehicle motor housing according to claim 1, characterized in that: In S5, the heat treatment is carried out under argon protection, where the casting is heated to 200-220°C at a rate of 5-8°C / min, held at that temperature for 4-6 hours, and then cooled to room temperature in the furnace at a rate of 2-3°C / min.

8. The method for preparing a die-casting alloy material for a hybrid vehicle motor housing according to claim 1, characterized in that: In S3, the Mg-Ce, Mg-La, Mg-Y, and Mg-Nd master alloys are dried at 150-180°C for 3-4 hours before being added.

9. A die-casting alloy material for a hybrid vehicle motor housing, based on a method for preparing a die-casting alloy material for a hybrid vehicle motor housing according to any one of claims 1-8, characterized in that, It consists of the following components by weight percentage: Al：3.5-4.5%、 Zn: 0.3-0.6%, Ca: 0.05-0.15% Ce: 0.8-1.5% La: 0.4-0.8% Y：0.2-0.5%、 Nd: 0.1-0.3% The balance is Mg and unavoidable impurities; Furthermore, the die-cast alloy material has a three-phase synergistic microstructure consisting of a reinforcing phase, an edge phase, and a continuous phase.

10. A die-casting alloy material for a hybrid vehicle motor housing according to claim 9, characterized in that: The strengthening phase is Al. 11 RE3 type and Mg 12 RE-type intermetallic compounds, wherein RE is a mixture of rare earth elements Ce, La, Y and Nd, the volume fraction of the reinforcing phase is 4-8%, the average size is 1-5 micrometers, and it is dispersed in spherical or short rod shapes; the edge phase is a coherent or semi-coherent transition layer with a thickness of 10-50 nm, containing Y-Zn, Nd-Zn, Y-Ca and Nd-Ca precipitates; the continuous phase is an α-Mg matrix with a grain size of 20-50 micrometers.

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