Die-casting alloy material for hybrid vehicle motor housing and preparation method thereof

Abstract

The application discloses a die-casting alloy material for a hybrid automobile motor shell and a preparation method thereof, relates to the technical field of alloys, and specifically comprises the following steps: adding pure aluminum into molten magnesium liquid to form a base melt; subsequently, Mg-Zn and Mg-Ca master alloys are added to introduce interface active elements; then, a plurality of rare earth master alloys are added in a low-to-high melting point order, and grading stirring and argon refining are carried out to form a mixed melt; subsequently, multi-stage injection molding is carried out; finally, low-temperature heat treatment is carried out on the casting. The alloy material prepared by the application forms a unique strengthening phase-edge phase-continuous phase three-phase synergistic microstructure, the three-phase structure enables the material to simultaneously have excellent high-temperature strength and creep resistance, good thermal conductivity and excellent die-casting process performance, and the material can meet the comprehensive use requirements of the hybrid automobile motor shell under long-term high-temperature working conditions.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of alloys, and particularly relates to a die casting alloy material for a hybrid vehicle motor shell and a preparation method thereof. BACKGROUND

[0002] At present, with the deep transformation of the global automobile industry towards green and intelligent direction, hybrid electric vehicles have become an important development trend in the current and future automobile market. Under this background, the lightweight design of vehicles is of great significance to improve fuel economy, reduce emissions and optimize vehicle dynamics performance. As one of the core components of the power system, the lightweight of the motor shell is crucial. Die casting magnesium alloy has great potential in the application of weight-sensitive components such as hybrid vehicle motor shells due to its significant low-density characteristics. However, hybrid motors often face continuous high-temperature working conditions during actual operation, which puts extremely strict requirements on the comprehensive performance of the motor shell material, especially the high-temperature mechanical properties and thermal management capability.

[0003] To cope with the challenges of magnesium alloy in high-temperature applications, rare earth (RE) element modification is widely considered as one of the most effective ways to improve the comprehensive performance of magnesium alloy, especially the high-temperature performance. Among them, the Mg-Al-RE series controls the aluminum content at a low level, while ensuring the basic die casting fluidity and room temperature strength, and uses rare earth elements to suppress the softening of the magnesium-aluminum matrix, thereby significantly improving the creep resistance and high-temperature strength of the alloy, and to some extent, meeting the demand for high-temperature performance. However, with the continuous evolution of hybrid vehicle technology towards higher performance and more compact integration, the existing magnesium alloy system based on a single strengthening target gradually exposes its inherent limitations in achieving multi-dimensional performance optimization, making it difficult to simultaneously consider high-temperature mechanical properties, room temperature mechanical properties, thermal conductivity and excellent die casting process performance under the premise of controllable cost.

[0004] Therefore, it is necessary to improve the die casting alloy material for hybrid vehicle motor shell and its preparation method in the prior art to solve the above problems. SUMMARY

[0005] The present application overcomes the shortcomings of the prior art and provides a die casting alloy material for a hybrid vehicle motor shell and a preparation method thereof, aiming to solve the problem that magnesium alloy materials in the prior art are difficult to simultaneously consider excellent high-temperature mechanical properties, good thermal conductivity and excellent die casting process performance.

[0006] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: a die casting alloy material preparation method for a hybrid vehicle motor shell, comprising:

[0007] S1, adding pure aluminum to the molten magnesium liquid and stirring to dissolve, forming a matrix melt with ɑ-Mg matrix as the continuous phase;

[0008] S2, adding Mg-Zn master alloy and Mg-Ca master alloy to the matrix melt and stirring to dissolve, providing Zn and Ca interfacial active elements, forming a transition melt;

[0009] S3, adding Mg-Ce, Mg-La, Mg-Y, Mg-Nd master alloys to the transition melt in order of increasing melting point and stirring in stages, introducing Ce, La, Y and Nd as strengthening phase forming elements and marginal phase auxiliary elements, forming a mixed melt with uniform distribution of three-phase components;

[0010] S4, multi-stage pressure injection of the mixed solution into a preheated mold and pressure casting to form a cast with a three-phase structure preliminarily constructed;

[0011] S5, heat treatment of the cast to strengthen the marginal phase.

[0012] In a preferred embodiment of the present application, in S1, the molten magnesium liquid is formed by melting industrial pure magnesium ingots at 680-700℃, and industrial pure aluminum ingots are added, with aluminum accounting for 3.5-4.5% of the total weight of the alloy, and stirring at a speed of 100-150 r / min for 10-15 min, while mixed protective gas is introduced at a flow rate of 5-10 L / min.

[0013] In a preferred embodiment of the present application, in S2, the temperature of the matrix melt is raised to 720-740℃, and Mg-Zn master alloy and Mg-Ca master alloy are added in sequence, with Zn added in an amount of 0.3-0.6% of the total weight of the alloy, and Ca added in an amount of 0.05-0.15% of the total weight of the alloy, and stirring at a speed of 120-180 r / min for 5-10 min, and the master alloys are added in batches.

[0014] In a preferred embodiment of the present application, in S3, Mg-Ce master alloy and Mg-La master alloy are added to the transition melt, with Ce added in an amount of 0.8-1.5% of the total weight of the alloy, and La added in an amount of 0.4-0.8% of the total weight of the alloy, and stirring at a speed of 180-250 r / min for 5-8 min.

[0015] In a preferred embodiment of the present application, in step S3, the Mg-Y master alloy and the Mg-Nd master alloy are continuously added, wherein the Y addition amount is 0.2-0.5% of the total weight of the alloy, the Nd addition amount is 0.1-0.3% of the total weight of the alloy, and the stirring speed is 180-250 r / min, and the stirring is continuously performed for 10-20 min; then the temperature of the molten liquid is reduced to 720-730 DEG C and the molten liquid is allowed to stand for 5-10 min, and argon refining is performed, the argon gas flow rate is 0.5-1.0 L / min, and the refining is continuously performed for 8-12 min.

[0016] In a preferred embodiment of the present application, in S4, the temperature of the preheated mold is 200-250 DEG C, and the injection process comprises: 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 pressure increase at a pressure of 60-100 MPa for a duration of 100-300 ms, and the castings are removed after being cooled to below 200 DEG C in the mold.

[0017] In a preferred embodiment of the present application, in S5, the heat treatment is performed under argon protection, the castings are heated to 200-220 DEG C at a rate of 5-8 DEG C / min, and then cooled to room temperature at a rate of 2-3 DEG C / min.

[0018] In a preferred embodiment of the present application, in S3, the Mg-Ce, Mg-La, Mg-Y and Mg-Nd master alloys are dried at 150-180 DEG C for 3-4 hours before being added.

[0019] The present application provides a die casting alloy material for a hybrid vehicle motor shell, which is composed of the following components in percentage by weight:

[0020] Al: 3.5-4.5%,

[0021] Zn: 0.3-0.6%,

[0022] Ca: 0.05-0.15%,

[0023] Ce: 0.8-1.5%,

[0024] La: 0.4-0.8%,

[0025] Y: 0.2-0.5%,

[0026] Nd: 0.1-0.3%,

[0027] the balance being Mg and inevitable impurities;

[0028] Furthermore, the die casting alloy material has a three-phase synergistic microstructure composed of a strengthening phase, an edge phase and a continuous phase.

[0029] In a preferred embodiment of the present application, the strengthening phase is Al 11 RE3 type and Mg 12 RE type intermetallic compound, wherein RE is a mixed rare earth element of Ce, La, Y and Nd, the volume fraction of the strengthening phase is 4-8%, the average size is 1-5 microns, and the strengthening phase is dispersedly distributed in a spherical or short rod shape; the edge phase is a coherent or semi-coherent transition layer with a thickness of 10-50 nm, and contains Y-Zn, Nd-Zn, Y-Ca and Nd-Ca precipitates; and the continuous phase is an α-Mg matrix with a grain size of 20-50 microns.

[0030] The present application solves the defects in the background art, and has the following beneficial effects:

[0031] (1) The present application introduces Al, Zn, Ca and multiple rare earth elements Ce, La, Y and Nd, and adopts a grading addition and stirring process to construct a three-phase synergistic microstructure composed of a strengthening phase, an edge phase and a continuous phase in the alloy. The strengthening phase is Al 11 RE3 type and Mg 12 The RE type intermetallic compound is mainly dispersedly distributed in the matrix, can effectively pin the grain boundary and hinder the dislocation movement, and improves the high temperature strength and creep resistance of the alloy. Compared with the existing Mg-Al-RE light rare earth alloy, the Al 11 RE3 phase is prone to produce microcracks at the interface with the matrix above 150℃, resulting in performance degradation, while the present application optimizes the thermal stability and distribution uniformity of the strengthening phase by multiple rare earth elements, significantly inhibits the interface failure problem at high temperature; further, the edge phase as a transition layer further strengthens the interface bonding, so that the alloy maintains structural integrity and performance stability under long-term high temperature working conditions of the hybrid vehicle motor shell.

[0032] (2) The present application adds interface active elements such as Zn and Ca, and cooperates with heavy rare earth Y and Nd at the interface between the strengthening phase and the matrix to form a nanoscale coherent or semi-coherent edge phase, which can effectively buffer the interface stress generated by the difference in thermal expansion coefficient between the strengthening 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 shell. Compared with the existing heavy rare earth alloy, the disordered agglomeration of the strengthening phase not only scatters phonons to reduce thermal conductivity, but also causes interface stress concentration to result in insufficient room temperature strength, and the present application realizes the synergistic optimization of thermal management and mechanical properties through the design of the edge phase; further, the edge phase also enhances the interface load transfer efficiency, so that the alloy not only has high thermal conductivity at high temperature, but also has excellent creep resistance.

[0033] (3) The present application 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 system. Low-speed injection avoids air entrapment, high-speed injection ensures complex cavity filling, and the pressure stage compensates for solidification shrinkage and promotes element interface segregation. The alloy has good flowability and high compactness of the casting, and is suitable for thin-walled motor shell forming. Compared with traditional die casting alloy, the solidification interval is expanded or the strengthening phase is aggregated due to the addition of rare earth elements, which causes filling shortage or thermal cracking defects. The present application avoids these problems by optimizing the process parameters and refines the matrix grains; further, the heat treatment process eliminates internal stress and improves the edge phase structure.

[0034] (4) During heat treatment, a small amount of supersaturated rare earth elements, zinc and calcium atoms dissolved in the a-Mg matrix can further precipitate in the form of nano-scale precipitates, forming a small amount of nano-scale Al-RE type secondary strengthening phase, forming a primary and secondary collaborative strengthening system with the core strengthening phase, or promoting the further growth and perfection of the interface collaborative strengthening edge phase, making the edge phase distribution more continuous, making its structure more stable, and the interface combination more closely. BRIEF DESCRIPTION OF DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings described below are only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings;

[0036] Figure 1 is a flowchart of the preferred embodiment of the present application. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0038] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein, therefore, the scope of protection of the present application is not limited by the specific embodiments disclosed below.

[0039] SUMMARY

[0040] The application is directed to a die casting alloy material for high-performance hybrid vehicle motor housings; however, the existing Mg-Al-RE light rare earth alloy, although its cost is controllable, has insufficient thermal stability of the RE3 phase, and microcracks are easily generated at the interface between the α-Mg matrix and the RE3 phase when serving above 150°C, which leads to attenuation of the creep resistance and cannot meet the long-term high-temperature working condition requirements of the motor; the Mg-heavy rare earth alloy forms a high-stability strengthening phase, but because there is no interface transition layer, the interface stress caused by the difference in the thermal expansion coefficient between the strengthening phase and the matrix reduces the room temperature tensile strength compared with ordinary alloys, and the agglomeration of intermetallic compounds reduces the thermal conductivity, and the expansion of the solidification interval leads to insufficient filling during die casting. 11 The RE3 phase directly contacts the α-Mg matrix, and the interface tensile stress at high temperatures far exceeds the interfacial bonding strength, causing cracking and losing the strengthening effect; the heavy rare earth system increases the addition amount to improve high-temperature performance, and the strengthening phase is disordered and agglomerated, which not only scatters phonons to reduce thermal conductivity, but also forms low-melting-point eutectics between dendrites during solidification, hindering the flow of the melt, and the agglomerated phase at room temperature becomes a crack propagation source, resulting in insufficient strength.

[0041] When a conventional rare earth strengthening scheme is used, the rare earth elements are mainly solid-solved in the matrix or form block-shaped intermetallic compounds that are dispersedly distributed, and the disordered phase distribution mode leads to mutual restraint of performance. The traditional technology adjusts the types and contents of rare earth elements to reconcile the contradictions. In the light rare earth system, Al 11 The RE3 phase directly contacts the α-Mg matrix, and the interface tensile stress at high temperatures far exceeds the interfacial bonding strength, causing cracking and losing the strengthening effect; the heavy rare earth system increases the addition amount to improve high-temperature performance, and the strengthening phase is disordered and agglomerated, which not only scatters phonons to reduce thermal conductivity, but also forms low-melting-point eutectics between dendrites during solidification, hindering the flow of the melt, and the agglomerated phase at room temperature becomes a crack propagation source, resulting in insufficient strength.

[0042] The application breaks through the mindset of traditional alloy design and introduces a three-phase synergistic system of strengthening phase-edge phase-continuous phase, wherein the strengthening phase is a high-stability rare earth intermetallic compound, providing high-temperature strength; the edge phase is a coherent precipitation layer formed by the segregation of rare earth elements at the interface between the core phase and the magnesium matrix, acting as a transition layer to alleviate the difference in the thermal expansion coefficient and reduce the interface stress; the continuous phase is the α-Mg matrix, ensuring thermal conductivity, toughness, and formability; so that the material has high strength, high thermal conductivity, and excellent formability.

[0043] An exemplary method:

[0044] As shown in Figure 1 A die casting alloy material preparation method for hybrid vehicle motor housings, comprising the steps of:

[0045] S1, adding pure aluminum to the molten magnesium liquid and stirring to dissolve, forming a matrix melt with the α-Mg matrix as the continuous phase;

[0046] S2, adding Mg-Zn master alloy and Mg-Ca master alloy to the matrix melt and stirring to dissolve, providing Zn and Ca interface active elements, forming a transition melt;

[0047] S3, add Mg-Ce, Mg-La, Mg-Y, Mg-Nd master alloys to the molten metal in transition from low to high melting point and grade stirring, introduce Ce, La, Y and Nd as strengthening phase forming elements and marginal phase auxiliary elements, and form a mixed melt with uniform distribution of three-phase components;

[0048] S4, multi-stage injection of the mixed solution into a preheated mold and pressure casting to form a cast with a three-phase structure for preliminary construction;

[0049] S5, heat treatment of the cast to strengthen the marginal phase.

[0050] Step S1 builds a continuous matrix framework of the alloy. Through the uniform fusion of aluminum and magnesium liquid, on the one hand, the solid solution strengthening effect of aluminum in the ɑ-Mg matrix is used to give the matrix basic mechanical properties and ensure its continuity as the main body of heat conduction; on the other hand, the necessary aluminum elements are reserved for the subsequent reaction with rare earth elements to form Al-RE type core strengthening phase.

[0051] Before step S1, an industrial pure magnesium ingot with a purity of not less than 99.95% is prepared; an industrial pure aluminum ingot with a purity of not less than 99.9% is prepared; and a specific component master alloy as an alloy element carrier is also prepared. The pre-prepared master alloys include 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, and the purity of all master alloys is not less than 99.9%.

[0052] In step S1, magnesium is the core raw material for building the ɑ-Mg matrix; the aluminum addition amount is 3.5-4.5% of the total weight of the alloy; too low aluminum content will result in insufficient room temperature strength and poor pressure casting fluidity, and too high aluminum content will form a large amount of thermally unstable Mg 17 Al 12 , causing a sharp drop in high-temperature strength.

[0053] The appropriate amount of aluminum can significantly reduce the melting point of the magnesium alloy, widen the solidification interval, and thus improve the fluidity of the alloy melt and enhance its filling capacity in complex thin-walled pressure casting molds, which is crucial for manufacturing motor housings with complex geometry and uniform wall thickness; aluminum improves the room temperature strength of the ɑ-Mg matrix through solid solution strengthening mechanism, making up for the defects of insufficient room temperature strength of existing alloys; aluminum can form stable and high-melting-point Al-RE intermetallic compounds with rare earth elements, which are an important part of the strengthening phase, replacing the thermally unstable Mg 17 Al 12 phase, and providing excellent high-temperature strength and creep resistance for the alloy.

[0054] The melting tool is a graphite crucible covered with a protective coating. The graphite crucible is selected from a graphite material covered with a protective coating, so as to prevent chemical reaction between the graphite and the molten magnesium liquid and introduction of impurities, and avoid damage to the continuity of the matrix.

[0055] The specific steps of S1 are as follows:

[0056] The crucible is heated to 200-300 DEG C, and after the temperature of the crucible is stabilized, the industrial pure magnesium ingot is put in;

[0057] The mixed protective gas is continuously introduced above the crucible, the flow rate is 5-10 L / min, the surface of the liquid magnesium alloy is covered, and oxidation and combustion are prevented;

[0058] The crucible is heated, the temperature is increased to 680-700 DEG C, and the magnesium ingot is completely melted to form a uniform molten magnesium liquid;

[0059] The industrial pure aluminum ingot is slowly added to the molten magnesium liquid, and stirring is performed to make it fully dissolved to form a matrix melt; the stirring speed is 100-150 r / min, and the stirring is continuously performed for 10-15 min, until the aluminum ingot is completely dissolved and uniformly mixed with the magnesium liquid.

[0060] The mixed protective gas is composed of 99.5% argon and 0.5% sulfur hexafluoride. Because magnesium is highly active and is easy to oxidize and burn at high temperature, the mixed protective gas can form a dense protective film on the surface of the melt, reduce the burning loss of the magnesium liquid, ensure the precision of the composition, and solve the performance fluctuation problem caused by oxidation in the traditional smelting.

[0061] In step S2, two interfacial active elements Zn and Ca are introduced into the matrix melt. On the one hand, the nucleation and growth of the subsequent interfacial synergistic strengthening edge phase are promoted by means of the atomic characteristics of the two elements; on the other hand, the die casting process performance is improved by optimizing the melt solidification characteristics.

[0062] In step S2, the addition amount of Zn accounts for 0.3-0.6% of the total weight of the alloy, and the addition amount of Ca accounts for 0.05-0.15% of the total weight of the alloy; Zn and Ca are important interfacial active elements and micro-alloying elements in the alloy.

[0063] During the alloy melting and solidification process, Zn and Ca atoms have high interface activity, which can be preferentially adsorbed at the interface of the core strengthening phase and the magnesium matrix, thereby effectively reducing the interface energy; Zn and Ca form specific low free energy compounds or solid solutions with heavy rare earth elements Y and Nd, including Y-Zn, Nd-Zn, Y-Ca, Nd-Ca and the like; Zn and Ca, by virtue of the interface adsorption characteristics, preferentially gather at the interface of the core strengthening phase and the matrix, reduce the interface energy, promote the nucleation and growth of the interface synergistic strengthening marginal phase, enhance the bonding strength of the phase interface, improve the load transfer efficiency, and exhibit excellent stability at high temperature, thereby further enhancing the comprehensive mechanical properties and creep resistance of the alloy.

[0064] Zn element itself can provide effective solid solution strengthening by forming a solid solution in the α-Mg matrix, and Y-Zn and Nd-Zn intermetallic compounds formed with Y and Nd can act as effective dislocation barriers to prevent dislocation slip and multiplication under the action of high power density vibration and stress generated during the operation of the hybrid motor, thereby further enhancing the comprehensive mechanical properties and fatigue life of the alloy.

[0065] Ca has a significant grain refinement effect, which can promote heterogeneous nucleation of the α-Mg matrix, thereby 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 to optimize the solidification path of the alloy, reduce the temperature difference between the liquidus and the solidus, thereby significantly improving the die casting fluidity, filling performance and hot cracking resistance of the alloy. For manufacturing complex thin-walled motor housings, excellent die casting process performance can significantly reduce the scrap rate and improve the forming quality and surface finish.

[0066] The specific steps of S2 are:

[0067] The temperature of the substrate melt is raised to 720-740℃;

[0068] The Mg-Zn master alloy and the Mg-Ca master alloy are added in turn, and during the addition process, the melt continues to be stirred, the stirring speed is adjusted to 120-180r / min, and the stirring is continued for 5-10min.

[0069] Preferably, the Mg-Zn master alloy and the Mg-Ca master alloy are added into the melt in batches, and the amount of each batch is not more than 1 / 3 of the total weight, and the next batch is added after the previous batch of master alloy is completely dissolved to avoid sudden local composition change caused by single addition;

[0070] During stirring, the stirring paddle is immersed in the melt to a depth of 1 / 2 to 2 / 3 of the liquid level, and slowly rotates along the inner wall of the crucible to avoid generating strong vortex.

[0071] By the uniform distribution of Zn and Ca elements in the α-Mg matrix, local enrichment to form non-target phases is avoided, and a uniform composition environment is provided for the subsequent rare earth elements and the two to form the edge phase.

[0072] Step S2 finally forms a transition melt with uniform distribution of Zn and Ca elements, and the α-Mg matrix is preliminarily strengthened in strength at room temperature through double solid solution strengthening of Al and Zn, and meanwhile, the grain size of the matrix is pre-refined to 60-80 microns by the grain refinement effect of Ca elements, laying a foundation for further refinement of rare earth elements.

[0073] In step S3, a predetermined amount of Mg-Ce master alloy, Mg-La master alloy, Mg-Y master alloy and Mg-Nd master alloy are sequentially added; the addition of rare earth master alloys is the key to forming the core strengthening phase and the interface synergistic strengthening edge phase.

[0074] In step S3, the addition amount of Ce accounts for 0.8-1.5% of the total weight of the alloy, the addition amount of La accounts for 0.4-0.8% of the total weight of the alloy, the addition amount of Y accounts for 0.2-0.5% of the total weight of the alloy, and the addition amount of Nd accounts for 0.1-0.3% of the total weight of the alloy.

[0075] Therefore, the total die casting alloy material is composed of the following components by weight percentage:

[0076] 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%, and the balance is Mg and other unavoidable impurity elements. This component ratio design is based on the functional positioning and synergistic effect of each element in the alloy, to ensure that the final material performance reaches the optimal balance.

[0077] Specifically, Ce and La are light rare earth elements; Ce and La synergistically act with aluminum elements to preferentially form Al 11 (Ce, La)3 type high-melting-point intermetallic compounds have high melting points and excellent thermal stability, with a melting point higher than 550℃, and can still maintain structural stability at high temperatures above 150℃, and exist in the form of dispersed distribution in the α-Mg matrix, constituting the main part of the core strengthening phase of the application. In high-temperature working conditions, rare earth aluminum compounds can effectively pin the grain boundaries, inhibit grain boundary sliding and dislocation movement, and significantly improve the high-temperature strength and creep resistance of the alloy, which is crucial for the motor shell to withstand the stress generated by the long-term high-temperature operation of the motor.

[0078] Ce and La also have significant grain refinement effect. During the alloy solidification process, these rare earth elements can act as heterogeneous nucleation core to promote the refinement of the α-Mg matrix grain, reducing the S2 pre-refined 60-80 micron grains to 20-50 microns. The small grain structure can increase the number of grain boundaries, effectively hinder the dislocation movement, and improve the room temperature strength and toughness of the alloy according to the Hall-Petch relationship.

[0079] Further, Y and Nd are heavy rare earth elements; Y and Nd have relatively large atomic radius and low solid solubility in the magnesium matrix, which makes them tend to segregate at the phase interface during the alloy solidification process, and cooperates with the added Zn and Ca elements to form coherent or semi-coherent precipitation layers with specific crystal structure and lattice matching degree at the interface of the α-Mg matrix and the core strengthening phase through heterogeneous nucleation or solid state precipitation, including: Y-Zn, Nd-Zn, Y-Ca and Nd-Ca; wherein, the core strengthening phase is mainly Al 11 RE3 type and Mg 12 RE type compound.

[0080] The precipitated phase constitutes the interface synergistic strengthening marginal phase of the present application. The existence of the marginal phase not only further improves the overall strength and creep resistance of the alloy through the interface strengthening mechanism, but more importantly, its lattice matching characteristics significantly reduce the interface stress between the core strengthening phase and the α-Mg matrix due to the difference in thermal expansion coefficient, and improve the interface bonding force.

[0081] From the perspective of heat conduction, this high-matching transition layer can effectively reduce the scattering of phonons at the phase interface, thereby reducing the interface thermal resistance and significantly improving the overall heat conduction capacity of the alloy, ensuring that the motor shell can efficiently dissipate heat and maintain long-term stable operation of the motor.

[0082] If the content of Y and Nd is lower than the range, it is difficult to form a sufficient volume fraction or thickness of the interface synergistic strengthening marginal phase, and the synergistic strengthening and heat conduction optimization effect will be greatly reduced; on the contrary, if the content is too high, not only will it significantly increase the material cost, but also may lead to the formation of coarse and brittle phases, thereby degrading the plasticity and toughness of the alloy.

[0083] The specific steps of S3 are:

[0084] The temperature is maintained at 720-740℃, the Mg-Ce master alloy and the Mg-La master alloy are mixed first, and the transition melt is slowly added in batches; each batch is separated by 5 minutes, and the next batch is added after the previous batch is completely dissolved to avoid local enrichment due to similar melting points;

[0085] High-speed stirring at 180-250r / min for 5-8min ensures the preliminary fusion of light rare earth elements with the melt, creating a uniform environment for the nucleation of the core strengthening phase;

[0086] Mixing Mg-Y master alloy and Mg-Nd master alloy, batch slow adding, high speed stirring for 10-20 min at 180-250 r / min, to ensure the full range of homogeneous mixing of rare earth elements, Zn, Ca and the base melt, to promote the nucleation of strengthening phase dispersion;

[0087] After stirring, the temperature of the melt is reduced to 720-730℃, and the surface scum is removed after 5-10 min of standing;

[0088] Argon is used as the refining gas, which is introduced from the bottom of the melt through an immersion type graphite tube, with a flow rate of 0.5-1.0 L / min, and the melt is refined for 8-12 min to remove hydrogen and non-metallic inclusions, and the surface scum is removed again after refining.

[0089] Step S3 follows the order of adding master alloys 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 out before the high melting point master alloy is completely dissolved, resulting in composition deviation. The rare earth master alloy needs to be dried in an oven at 150-180℃ for 3-4 hours before adding to completely remove moisture and prevent the introduction of hydrogen to form pores, affecting the density of the casting.

[0090] Step S3 finally obtains a mixed melt with uniform distribution of three-phase components, including: α-Mg matrix, strengthening phase, and edge phase precursor elements; the strengthening phase is dispersedly precipitated in spherical or short rod shape, with an average size of 1-5 microns and a volume fraction of 4%-8%, without obvious agglomeration; the α-Mg matrix grain is refined to 20-50 microns, and through the synergistic effect of solid solution strengthening and grain refinement of Al and Zn, the room temperature tensile strength is further improved compared to the S2 transition melt. More importantly, Y and Nd form a uniform element enrichment zone with Zn and Ca in the melt, which provides precise composition and structure conditions for the in-situ nucleation of the interface synergistic strengthening edge phase during the S4 die casting process, while the size and distribution of the core strengthening phase are precisely controlled, avoiding the blockage of the heat conduction path by excessive strengthening phase, laying a foundation for subsequent optimization of thermal conductivity.

[0091] Step S4 is a key process link for the transformation of alloy three-phase structure from melt to solid state, through precise coordination of multi-stage injection parameters and mold preheating, both to ensure complete filling of the mixed melt in the complex thin-walled mold cavity and to promote the ordered precipitation and distribution of the core strengthening phase and the edge phase through the solidification process.

[0092] In step S4, the die casting process parameters need to match the alloy melt characteristics and the motor housing structure requirements. The mold preheating temperature is controlled at 200-250°C. The multi-stage injection is divided into low-speed injection, high-speed injection and pressure boosting. The low-speed injection speed is 0.1-0.3 m / s, the high-speed injection speed is increased to 3.0-5.0 m / s, the pressure boosting is started immediately after the mold is completely filled, the pressure boosting pressure is maintained at 60-100 MPa, and the duration is 100-300 ms. The casting needs to be cooled to below 200°C in the mold before the mold is opened and taken out.

[0093] If the mold preheating temperature is too low, the melt will quickly solidify when it contacts the cavity wall, causing underfilling or cold shut defects. If it is too high, it will prolong the solidification time, easily leading to coarse grains and surface oxidation. If the slow injection speed is too high, it is easy to roll in gas and form pores. If it is too low, the efficiency is low. The fast injection speed needs to match the filling requirements of the thin-walled area of the housing. If the speed is not enough, it will not be filled at the end. If it is too fast, it will produce turbulent flow and roll slag.

[0094] The die casting process has a decisive influence on the formation of the three-phase structure. The temperature gradient formed by the mold preheating makes the melt gradually solidify from the cavity wall to the center, providing directional nucleation conditions for the core strengthening phase and promoting its uniform distribution along the grain boundary. The pressure action of multi-stage injection makes the melt solidify under high pressure, reducing shrinkage and other porosity defects, while promoting the enrichment of Y, Nd, Zn and Ca elements to the phase interface, promoting the preliminary precipitation of the edge phase precursor at the interface between the core strengthening phase and the α-Mg matrix. The sustained pressure in the pressure 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.

[0095] The specific steps of S4 are:

[0096] The temperature of the refined melt is reduced to 660-680°C as the pouring temperature before die casting;

[0097] Pour the melt into the injection chamber of the die casting machine;

[0098] The die casting mold is preheated to 200-250°C. The mold surface is uniformly coated with a layer of water-based release agent with a solid content of 5-8%, and the spraying time is 3-5 s to form a thin and uniform lubricating film, avoiding pores caused by excessive residue of the release agent. The mold structure design includes sufficient water cooling channels, which accurately regulate the temperature of each area of the cavity through the water cooling system to ensure that the casting realizes a controllable solidification rate during die casting, providing 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 interface to synergistically strengthen the edge phase to precipitate in a specific area;

[0099] Start the injection system of the die casting machine and use a three-stage injection process for molding:

[0100] The first stage is low-speed injection, the injection speed is 0.1-0.3 m / s, the molten metal is pushed to fill the injection chamber and the runner stably, the speed mutation is avoided throughout the process, and the air entrainment of the molten metal is prevented to form pores;

[0101] After the molten metal reaches the gate position, the second stage of high-speed injection is switched immediately, the injection speed is increased to 3.0-5.0 m / s, the molten metal is rapidly filled in the entire mold cavity, the complex thin-walled structure part is completely filled, and the chilling effect brought by high-speed filling is used to promote the grain refinement of the α-Mg matrix and the uniform distribution of the fine core strengthening phase;

[0102] After the mold is completely filled, the third stage of pressurization is entered, the pressurization pressure is maintained at 60-100 MPa, and the duration is 100-300 ms, the high pressure is used to compensate the solidification shrinkage of the molten metal, the internal defects such as shrinkage are eliminated, the grains are further refined, and the compactness of the casting is improved; the precise control of this stage is the key to the efficient formation of the interface synergistic strengthening edge phase, the transient high pressure and the rapid solidification conditions jointly act to promote the rapid segregation of the heavy rare earth elements Y, Nd, zinc and calcium at the interface between the core strengthening phase and the α-Mg matrix, and then form the target coherent or semi-coherent precipitate phase;

[0103] After the pressurization is completed, the mold is kept closed, the temperature is continuously controlled through the water cooling channel, after the casting is cooled to below 200℃ in the mold, the mold opening mechanism is started, and the casting is smoothly taken out through the ejection device, so as to avoid deformation or cracking caused by taking out under high temperature.

[0104] Step S5 strengthens and modifies the casting through a heat treatment process, promotes the perfection and stability of the interface synergistic strengthening edge phase, eliminates the internal stress generated in the die casting process, and finally realizes the synergistic optimization of the mechanical properties and thermal conductivity of the alloy, so as to meet the core needs of the long-term high-temperature service of the motor housing of the hybrid vehicle.

[0105] In step S5, the casting is heated to 200-220℃ under an argon protective atmosphere, and is kept for 4-6 hours, and then is slowly cooled to room temperature in the furnace.

[0106] Too low temperature will cause insufficient atomic diffusion power, and cannot form enough strengthening phase; too high temperature may cause edge phase coarsening or matrix grain growth, and thus deteriorate the performance. The holding time needs to ensure that the precipitation process is completed, too short will result in insufficient strengthening phase, and too long will cause the aggregation and growth of the precipitate phase, and weaken the strengthening effect.

[0107] During heat treatment, a small amount of supersaturated rare earth elements, zinc and calcium atoms dissolved in the α-Mg matrix can further precipitate in the form of nano-scale precipitates, forming a small amount of nano-scale Al-RE type secondary strengthening phase, forming a primary and secondary collaborative strengthening system with the core strengthening phase, or promoting the further growth and perfection of the interface collaborative strengthening edge phase, making the edge phase distribution more continuous, making its structure more stable, and the interface combination more closely.

[0108] The secondary precipitation strengthening mechanism can effectively pin dislocations, thereby further significantly improving the room temperature and high temperature strength of the alloy and enhancing its creep resistance. At the same time, during the heat treatment process, the lattice distortion and internal stress caused by rapid solidification during the die casting process are eliminated by atomic diffusion, which helps to release the residual stress generated during the die casting process, improves the dimensional stability of the casting, and reduces its cracking sensitivity.

[0109] The specific steps of S5 include:

[0110] The casting obtained in S4 is removed from the surface dirt and scale, ultrasonically cleaned with anhydrous ethanol for 10-15 minutes, dried, and then loaded into a heat treatment special tool to ensure uniform heating of each part of the casting and avoid local temperature deviation caused by stacking;

[0111] Put the tool into the box-type heat treatment furnace, and after closing the door, introduce argon gas with a flow rate of 2-3 L / min for 30 minutes to replace the air in the furnace to prevent the surface of the casting from oxidizing during heating;

[0112] Increase the furnace temperature to 200-220°C at a rate of 5-8°C / min, and maintain the target temperature for 4-6 hours

[0113] After the holding period ends, continue to introduce argon gas to allow the casting to cool slowly with the furnace at a cooling rate of 2-3°C / min, and stop the argon gas when the furnace temperature drops to below 50°C, open the door and take out the casting.

[0114] A die casting alloy material suitable for use in a hybrid vehicle motor housing is finally obtained;

[0115] Exemplary materials:

[0116] A die casting alloy material for a hybrid vehicle motor housing, comprising:

[0117] Al: 3.5-4.5%,

[0118] Zn: 0.3-0.6%,

[0119] Ca: 0.05-0.15%,

[0120] Ce: 0.8-1.5%,

[0121] La: 0.4-0.8%,

[0122] Y: 0.2-0.5%,

[0123] Nd: 0.1-0.3%,

[0124] the balance being Mg and inevitable impurities;

[0125] and the die casting alloy material has a three-phase synergistic microstructure composed of a strengthening phase, an edge phase and a continuous phase;

[0126] The strengthening phase is a high-melting intermetallic compound dispersed in the α-Mg matrix, mainly composed of Al 11 RE3-type intermetallic compounds and Mg 12 RE-type intermetallic compounds;

[0127] wherein Al 11 RE in the RE3-type intermetallic compound is a mixed rare earth element of Ce, La, Y and Nd, Mg 12 RE in the RE-type intermetallic compound is a mixed rare earth element of Ce and La;

[0128] The volume fraction of the strengthening phase is 4-8%, the average size is 1-5 microns, and the morphology is spherical or short rod, uniformly distributed in the α-Mg matrix, and the strengthening phase has excellent high temperature stability, which can effectively pin the grain boundary, hinder dislocation movement and grain boundary sliding at a working temperature higher than 150℃, thereby providing the alloy with main high temperature strength and creep resistance;

[0129] The edge 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 of the core phase and the continuous phase α-Mg matrix;

[0130] The grain size of the α-Mg matrix is 20-50 microns.

[0131] The thickness of the edge phase is 10-50 nm, mainly composed of Y-Zn, Nd-Zn, Y-Ca and Nd-Ca intermetallic compounds or solid solutions enriched with heavy rare earth elements Y, Nd, zinc and calcium; The edge phase is formed by segregation at the interface of the strengthening phase and the α-Mg matrix, as a transition layer, its lattice structure has high matching degree with the α-Mg matrix and the strengthening phase, thereby effectively relieving the interface stress between the strengthening phase and the α-Mg matrix due to the difference in thermal expansion coefficient, and significantly enhancing the interface bonding force, at the same time, by establishing a lattice-matched transition region, the scattering efficiency of phonons at the interface is significantly reduced, thereby improving the overall heat conduction capacity of the alloy.

[0132] Example 1:

[0133] A preparation method of die casting alloy material for hybrid electric vehicle motor shell, comprising the steps of:

[0134] Prepare industrial pure magnesium ingot with purity not less than 99.95%, industrial pure aluminum ingot with purity 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, Mg-Nd master alloy, and the purity of all master alloys is not less than 99.9%.

[0135] Heat the crucible to 250 DEG C, and after the temperature is stable, put the industrial pure magnesium ingot into the crucible, continuously introduce the mixed protective gas composed of 99.5% argon and 0.5% sulfur hexafluoride above the crucible, the flow rate is 7.5 L / min, heat the crucible to 690 DEG C, and continuously keep warm until the magnesium ingot is completely melted to form molten magnesium liquid, slowly add 4% of the total weight of the alloy of the industrial pure aluminum ingot to the molten magnesium liquid, stir at a speed of 125 r / min for 12.5 min, until the aluminum ingot is completely dissolved and uniformly mixed with the magnesium liquid to form a base melt.

[0136] Increase the temperature of the base melt to 730 DEG C, and sequentially add Mg-Zn master alloy and Mg-Ca master alloy, the Zn addition amount is 0.45% of the total weight of the alloy, and the Ca addition amount is 0.1% of the total weight of the alloy, the master alloys are slowly added to the melt in batches, the addition amount of each batch is not more than 1 / 3 of the total weight, and the next batch is added after the previous batch of master alloy is completely immersed and dissolved; during the addition process, the melt continues to be stirred, the stirring speed is adjusted to 150 r / min, and the stirring process lasts for 7.5 min, and during the stirring process, the stirring paddle is immersed in the melt to a depth of 1 / 2 to 2 / 3 of the liquid level, and slowly rotates along the inner wall of the crucible, forming a transition melt.

[0137] Maintain the temperature at 730 DEG C, dry the rare earth master alloy in an oven at 165 DEG C for 3.5 hours before adding; first mix Mg-Ce master alloy and Mg-La master alloy, the Ce addition amount is 1.15% of the total weight of the alloy, and the La addition amount is 0.6% of the total weight of the alloy, slowly add them to the transition melt in batches, with an interval of 5 minutes between each batch, and the next batch is added after the previous batch is completely dissolved; high-speed stirring at a speed of 215 r / min for 6.5 min; mix Mg-Y master alloy and Mg-Nd master alloy, the Y addition amount is 0.35% of the total weight of the alloy, and the Nd addition amount is 0.2% of the total weight of the alloy, slowly add them in batches, and high-speed stirring at a speed of 215 r / min for 15 min; after the stirring is completed, the temperature of the melt is reduced to 725 DEG C, and the melt is placed for 7.5 min, and the surface dross is removed; use argon as the refining gas, introduce it from the bottom of the melt through the immersed graphite tube, and control the flow rate at 0.75 L / min, refine for 10 min to remove hydrogen and non-metallic inclusions in the melt, remove the surface dross again after refining, and form a mixed melt.

[0138] The temperature of the refined melt is reduced to 670°C as the pouring temperature before die casting; the melt is poured into the injection chamber of the die casting machine; the die casting mold is preheated to 225°C, and the surface of the mold is uniformly coated with a layer of water-based release agent with a solid content of 6.5%, and the spraying time is 4s; start the injection system of the die casting machine, and use a three-stage injection process for molding: the first stage is low-speed injection, and the injection speed is 0.2m / s; after the melt reaches the gate position, switch to the second stage of high-speed injection, and the injection speed is increased to 4.0m / s; after the mold is completely filled, enter the third stage of pressure increase, and the pressure increase pressure is maintained at 80MPa for a duration of 200ms; after the pressure increase is completed, the mold is kept closed, and the temperature is continuously controlled through the water cooling channel, and after the casting is cooled to below 200°C in the mold, the mold opening mechanism is started, and the casting is smoothly taken out by the ejection device.

[0139] The obtained casting is removed from the surface dirt and oxide skin, ultrasonically cleaned with anhydrous ethanol for 12.5 minutes, dried and then loaded into a special tool for heat treatment; the tool is placed in a box-type heat treatment furnace, argon is introduced after the door is closed, the flow rate is 2.5L / min, and the air in the furnace is replaced for 30 minutes; the furnace temperature is raised to 210°C at a rate of 6.5°C / min, and the temperature is kept constant for 5 hours after reaching the target temperature; after the heat preservation is completed, the argon is continuously introduced to slowly cool the casting with the furnace, and the cooling rate is 2.5°C / min, and the argon is stopped when the furnace temperature drops to below 50°C, and the casting is taken out after the door is opened.

[0140] The final prepared die casting alloy material parameters are as follows: Al 4.0%, Zn 0.45%, Ca 0.1%, Ce 1.15%, La 0.6%, Y 0.35%, Nd 0.2%, and the balance is Mg and unavoidable impurities; having a three-phase synergistic microstructure composed of a strengthening phase, an edge phase and a continuous phase; the volume fraction of the strengthening phase is 6%, the average size is 3 microns, and the morphology is spherical or short rod, which is uniformly distributed in the ɑ-Mg matrix; the grain size of the ɑ-Mg matrix is 35 microns; the edge phase has a thickness of 30nm, which is a coherent or semi-coherent transition layer containing Y-Zn, Nd-Zn, Y-Ca and Nd-Ca precipitates.

[0141] Example 2:

[0142] A preparation method of a die casting alloy material for a hybrid vehicle motor shell, which is different from example 1 in that the addition amount of Zn accounts for 0.3% of the total weight of the alloy, and the addition amount of Ca accounts for 0.05% of the total weight of the alloy; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0143] Example 3:

[0144] A preparation method of a die casting alloy material for a hybrid vehicle motor shell, different from example 1 in that the Zn addition amount accounts for 0.37% of the total weight of the alloy, and the Ca addition amount accounts for 0.07% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0145] Example 4:

[0146] A preparation method of a die casting alloy material for a hybrid vehicle motor shell, different from example 1 in that the Zn addition amount accounts for 0.43% of the total weight of the alloy, and the Ca addition amount accounts for 0.09% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0147] Example 5:

[0148] A preparation method of a die casting alloy material for a hybrid vehicle motor shell, different from example 1 in that the Zn addition amount accounts for 0.47% of the total weight of the alloy, and the Ca addition amount accounts for 0.11% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0149] Example 6:

[0150] A preparation method of a die casting alloy material for a hybrid vehicle motor shell, different from example 1 in that the Zn addition amount accounts for 0.53% of the total weight of the alloy, and the Ca addition amount accounts for 0.13% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0151] Example 7:

[0152] A preparation method of a die casting alloy material for a hybrid vehicle motor shell, different from example 1 in that the Zn addition amount accounts for 0.6% of the total weight of the alloy, and the Ca addition amount accounts for 0.15% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0153] Example 8:

[0154] A preparation method of a die casting alloy material for a hybrid vehicle motor shell, different from example 1 in that the Zn addition amount accounts for 0.6% of the total weight of the alloy, and the Ca addition amount accounts for 0.15% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0155] Example 9:

[0156] A preparation method of the die-casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the Ce addition accounts for 1.0% of the total weight of the alloy, the La addition accounts for 0.5% of the total weight of the alloy; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0157] Example 10:

[0158] A preparation method of the die-casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the Ce addition accounts for 1.2% of the total weight of the alloy, the La addition accounts for 0.6% of the total weight of the alloy; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0159] Example 11:

[0160] A preparation method of the die-casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the Ce addition accounts for 1.3% of the total weight of the alloy, the La addition accounts for 0.7% of the total weight of the alloy; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0161] Example 12:

[0162] A preparation method of the die-casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the Ce addition accounts for 1.4% of the total weight of the alloy, the La addition accounts for 0.75% of the total weight of the alloy; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0163] Example 13:

[0164] A preparation method of the die-casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the Ce addition accounts for 1.5% of the total weight of the alloy, the La addition accounts for 0.8% of the total weight of the alloy; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0165] Example 14:

[0166] A preparation method of the die-casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the Y addition accounts for 0.2% of the total weight of the alloy, the Nd addition accounts for 0.1% of the total weight of the alloy; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0167] Example 15:

[0168] A preparation method of die casting alloy material for hybrid vehicle motor shell, different from example 1 is that the Y addition accounts for 0.27% of the total weight of the alloy, and the Nd addition accounts for 0.15% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0169] Example 16:

[0170] A preparation method of die casting alloy material for hybrid vehicle motor shell, different from example 1 is that the Y addition accounts for 0.43% of the total weight of the alloy, and the Nd addition accounts for 0.25% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0171] Example 17:

[0172] A preparation method of die casting alloy material for hybrid vehicle motor shell, different from example 1 is that the Y addition accounts for 0.5% of the total weight of the alloy, and the Nd addition accounts for 0.3% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0173] Example 18:

[0174] A preparation method of die casting alloy material for hybrid vehicle motor shell, different from example 1 is that the Y addition accounts for 0.3% of the total weight of the alloy, and the Nd addition accounts for 0.22% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0175] Example 19:

[0176] A preparation method of die casting alloy material for hybrid vehicle motor shell, different from example 1 is that the Y addition accounts for 0.4% of the total weight of the alloy, and the Nd addition accounts for 0.18% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0177] Example 20:

[0178] A preparation method of die casting alloy material for hybrid vehicle motor shell, different from example 1 is that the Y addition accounts for 0.4% of the total weight of the alloy, and the Nd addition accounts for 0.18% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0179] Example 21:

[0180] A preparation method of die casting alloy material for hybrid vehicle motor shell, different from example 1 is that the Y addition accounts for 0.4% of the total weight of the alloy, and the Nd addition accounts for 0.18% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0181] Example 22

[0182] A preparation method of the die casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the third stage pressure boosting pressure is maintained at 75 MPa; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0183] Example 23

[0184] A preparation method of the die casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the third stage pressure boosting pressure is maintained at 85 MPa; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0185] Example 24

[0186] A preparation method of the die casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the third stage pressure boosting pressure is maintained at 90 MPa; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0187] Example 25

[0188] A preparation method of the die casting alloy material for the hybrid vehicle motor shell, different from example 1 is that the third stage pressure boosting pressure is maintained at 100 MPa; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0189] Comparative example 1

[0190] A preparation method of the die casting alloy material for the hybrid vehicle motor shell, different from example 1 is that no Mg-Zn master alloy and Mg-Ca master alloy are added, that is, the Zn addition amount accounts for 0% of the total weight of the alloy, and the Ca addition amount accounts for 0% of the total weight of the alloy; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0191] Comparative example 2

[0192] A preparation method of the die casting alloy material for the hybrid vehicle motor shell, different from example 1 is that no Mg-Ce master alloy and Mg-La master alloy are added, that is, the Ce addition amount accounts for 0% of the total weight of the alloy, and the La addition amount accounts for 0% of the total weight of the alloy; the rest of the raw material specifications, preparation steps and process parameters are consistent with example 1.

[0193] Comparative example 3

[0194] A preparation method of the die casting alloy material for the hybrid vehicle motor shell, which is different from example 1 in that: no Mg-Y master alloy and Mg-Nd master alloy are added, i.e. the Y addition amount accounts for 0% of the total weight of the alloy, and the Nd addition amount accounts for 0% of the total weight of the alloy; the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0195] Comparative example 4:

[0196] A preparation method of the die casting alloy material for the hybrid vehicle motor shell, which is different from example 1 in that: the effective supercharging of the die casting third stage is not started, and the supercharging pressure is maintained at 10 MPa (much lower than the lower limit of the specified range, equivalent to no supercharging effect); the specifications of the remaining raw materials, the preparation steps and the process parameters are consistent with those of example 1.

[0197] Experimental example 1:

[0198] Examples 1-13 and comparative examples 1-2 are selected for testing to obtain the room temperature strength, high temperature strength, thermal conductivity and creep strain rate of the alloy material;

[0199] The room temperature strength is the tensile strength of the tested alloy material at 25°C;

[0200] The high temperature strength is the tensile strength of the tested alloy material at 175°C;

[0201] The creep strain rate is the creep strain rate of the tested alloy material at 175°C under a load of 50 MPa;

[0202] Table 1: Performance test results of the die casting alloy materials of examples 1-13 and comparative examples 1-2

[0203]

[0204] The interfacial activity of Zn and Ca determines the change rule of alloy performance with their content. In example 1, the ratio of 0.45% Zn and 0.1% Ca can maximize the reduction of interfacial energy, promote the formation of complete edge phase of Y-Zn, Nd-Zn and other precipitates, effectively relieve the interfacial stress between the strengthening phase and the matrix, and the grain refinement effect of Ca makes the matrix grain uniform, so 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 low content in example 2 or high content in example 7, the edge phase nucleation is insufficient or locally enriched, the interfacial bonding force decreases, the phonon scattering increases, and the performance gradient deteriorates. In comparative example 1, the edge phase cannot be formed when both of them are completely missing, and the performance is close to the lower limit of the conventional alloy.

[0205] 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.

[0206] Experimental Example 2:

[0207] 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.

[0208] Room temperature strength refers to the tensile strength of the tested alloy material at 25°C.

[0209] High-temperature strength refers to the tensile strength of the alloy material at 175℃.

[0210] The creep strain rate is the creep strain rate of the test alloy material at 175℃ and under a load of 50MPa.

[0211] Table 2 shows the test results of the die-casting alloy materials in Examples 1, 14-25, and Comparative Examples 3-4.

[0212]

[0213] 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.

[0214] The die-casting supercharging pressure affects the performance by regulating the density and phase distribution. The 80 MPa supercharging pressure of Example 1 can completely compensate for solidification shrinkage, improve the density of the casting, and promote the directional segregation of elements such as Y and Nd to the interface, and promote the ordered precipitation of the edge phase. When the pressure is lower than the optimal value, such as Example 20 and Comparative Example 4, the casting has shrinkage defects, the edge phase nucleation is insufficient, and the strength and thermal conductivity decrease sharply; when the pressure is higher than the optimal value, such as Example 25, excessive pressure leads to coarse grains, uneven distribution of strengthening phase, and reverse performance decline. Examples 22 and 23 are close to the optimal pressure, and the performance remains at a high level, confirming the key regulating effect of pressure on the formation of the three-phase structure.

[0215] The above is based on the ideal embodiment of the present application, and through the above description, relevant personnel can make various changes and modifications without deviating from the technical idea of the present application. The technical scope of the present application is not limited to the contents of the specification, and must be determined by the scope of the claims.

Claims

1. A method for preparing a die casting alloy material for a hybrid vehicle motor housing, characterized by, Comprise: S1, adding pure aluminum to the molten magnesium liquid and stirring to dissolve, forming a matrix melt with ɑ-Mg matrix as the continuous phase; S2, adding Mg-Zn master alloy and Mg-Ca master alloy to the matrix melt and stirring to dissolve, providing Zn and Ca interfacial active elements, forming a transition melt; S3, adding Mg-Ce, Mg-La, Mg-Y, Mg-Nd master alloys to the transition melt in order of increasing melting point and stirring in stages, introducing Ce, La, Y and Nd as strengthening phase forming elements and marginal phase auxiliary elements, forming a mixed melt with uniform distribution of three-phase components; S4, multi-stage injection of the mixed solution into a preheated mold and pressure casting to form a cast with a three-phase structure; S5, heat treatment of the cast to strengthen the marginal phase; In S5, the heat treatment is carried out under argon protection, the cast is heated to 200-220℃ at a rate of 5-8℃ / min, and then cooled to room temperature at a rate of 2-3℃ / min in the furnace; A die casting alloy material for hybrid vehicle motor housings, based on the preparation method of the die casting alloy material for hybrid vehicle motor housings, 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 being Mg and unavoidable impurities; And the die casting alloy material has a three-phase synergistic microstructure composed of a strengthening phase, a marginal phase and a continuous phase; the strengthening phase is Al 11 RE3 type and Mg 12 RE type intermetallic compound, wherein RE is a mixed rare earth element of Ce, La, Y and Nd, the volume fraction of the strengthening phase is 4-8%, the average size is 1-5 microns, and the strengthening phase is dispersedly distributed in the form of spheres or short rods; the edge phase is a coherent or semi-coherent transition layer with a thickness of 10-50 nm, and contains Y-Zn, Nd-Zn, Y-Ca and Nd-Ca precipitates; and the continuous phase is an α-Mg matrix with a grain size of 20-50 microns.

2. The method for preparing the die casting alloy material for the motor housing of the hybrid vehicle 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, with aluminum accounting for 3.5-4.5% of the total weight of the alloy, and stirring at a speed of 100-150r / min for 10-15min, while mixed protective gas is introduced at a flow rate of 5-10L / min.

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

4. The method for preparing die casting alloy material for hybrid electric 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, with Ce addition accounting for 0.8-1.5% of the total weight of the alloy and La addition accounting for 0.4-0.8% of the total weight of the alloy, and stirring at a speed of 180-250r / min for 5-8min.

5. The method according to claim 4, wherein the method is characterized in that: In step S3, the Mg-Y master alloy and the Mg-Nd master alloy are continuously added, wherein the Y addition amount is 0.2-0.5% of the total weight of the alloy, and the Nd addition amount is 0.1-0.3% of the total weight of the alloy, and the stirring is continuously carried out 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 the melt is statically treated for 5-10 min, and argon refining is carried out, the argon flow rate is 0.5-1.0 L / min, and the refining is continuously carried out for 8-12 min.

6. The method for preparing the die casting alloy material for the motor housing of the hybrid vehicle according to claim 1, characterized in that: In S4, the temperature of the preheated mold is 200-250 ℃, and the injection process comprises: 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 pressure increase at a pressure of 60-100 MPa for a duration of 100-300 ms, and the castings are taken out after being cooled to below 200 ℃ in the mold.

7. The method according to claim 1, wherein the method is characterized in that: In S3, the Mg-Ce, Mg-La, Mg-Y, and Mg-Nd master alloys are dried at 150-180 ℃ for 3-4 hours before being added.

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