High-strength high-elongation high-corrosion-resistance die-casting magnesium alloy and preparation method thereof

Abstract

This invention discloses a high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy and its preparation method. The alloy's composition by weight percentage is: Al 8.0~12.0%, Zn 0.2~1.5%, Mn 0.1~1.0%, Be 0.002~0.03%, Ce 1.0~4.0%, La 0.5~3.0%, with the balance including Mg and unavoidable impurities. The total amount of unavoidable impurities is ≤0.2%, and the Ce / La ratio must simultaneously be 1~3:1. This invention achieves high strength (tensile strength ≥270MPa) and high elongation (≥9%) while also ensuring high corrosion resistance (salt spray corrosion rate ≤1 mg / cm³). 2 () / day), and effectively controlled costs, which has great application prospects in realizing the lightweighting of large-size structural components in new energy vehicles.

Description

Technical Field

[0001] This invention belongs to the field of magnesium alloy technology, specifically relating to a high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy and its preparation method. Background Technology

[0002] Magnesium alloys have a number of advantages; they are the lightest of commonly used metallic structural materials, with a density of approximately 1.7 g / cm³. 3 Magnesium is one-quarter the weight of steel and two-thirds the weight of aluminum, possessing a higher specific strength than both steel and aluminum alloys. It is abundant in resources and is currently the only lightweight metallic structural material on Earth that has not been extensively exploited. Magnesium alloys have excellent electromagnetic shielding properties, making them ideal casing materials for electronic devices such as computers and mobile phones, significantly reducing the harmful effects of electromagnetic radiation on the human body. Magnesium alloys exhibit better damping and vibration reduction performance than steel and aluminum alloys, with an average vibration reduction performance 30 times higher than aluminum. Magnesium alloys also possess excellent heat absorption and dissipation properties; their thermal conductivity is 200 times higher than that of plastics, while their thermal expansion is only half that of plastics, making them excellent heat exchange materials. Furthermore, magnesium alloys are recyclable and are the only biodegradable metallic material used in medicine. These numerous advantages make magnesium alloys a promising candidate for applications across various industries and sectors of the national economy.

[0003] Currently, AZ91D magnesium alloy boasts the best overall mechanical properties and excellent casting performance (good fluidity, low tendency to hot cracking, and relatively simple smelting and casting process), making it the earliest and most widely used magnesium alloy in industry. Although AZ91D magnesium alloy offers significant lightweight advantages, its low plasticity (elongation) severely limits its application in key automotive components, especially structural parts. This inherent brittleness makes traditional AZ91D die-cast parts more prone to brittle fracture under impact or complex alternating stress, rather than absorbing energy through plastic deformation. Therefore, they are typically only used to manufacture non-load-bearing shells or covers where toughness requirements are not high, failing to meet the stringent requirements of the automotive industry for high impact toughness and fatigue resistance in safety and structural components. This has become a major technical bottleneck hindering its expanded application. Therefore, developing low-cost, high-elongation die-cast magnesium alloy materials has become one of the key focuses in the magnesium alloy field. Adding inexpensive rare earth elements such as La and Ce to the AZ91D alloy composition, and further adjusting the Al content, can significantly refine the Mg content. 17 Al 12 By adjusting the grain size, the overall mechanical properties and corrosion resistance of the alloy can be improved simultaneously, showing high application potential.

[0004] Chinese patent CN101220432A discloses the composition and preparation method of a cerium-lanthanum-containing high-strength die-cast magnesium alloy. The composition by mass percentage is: Al 8.5%~9.5%, Zn 0.4%~0.9%, Mn 0.2%~0.6%, rare earth elements Ce 0.01%~1.5%, La 0.01%~1.5%, impurity elements Fe≤0.02%, Cu≤0.002%, Si≤0.01%, Ni≤0.001%, with the balance being magnesium. The alloying material used is cerium-lanthanum rare earth. The room temperature tensile yield strength, tensile strength, and elongation of this alloy are 150MPa, 250MPa, and 6%, respectively, indicating low mechanical properties and no performance advantage in the lightweight "magnesium-for-aluminum" process.

[0005] Chinese patent CN106756363A discloses a corrosion-resistant die-cast magnesium alloy and its preparation method. The composition and mass percentages of each component are as follows: Al: 7.0~11.5 wt.%, Zn: 0.5~2.0 wt.%, Mn: 0.3~0.4 wt.%, RE: 0.2~2.5 wt.%, Yb: 0.1~3.3 wt.%, Sm: 0.1~5.8 wt.%, Sr: 0.1~2.0 wt.%, with the balance being Mg and unavoidable impurity elements, and RE being mixed rare earth elements. However, the addition of microalloying elements such as Yb, Sm, and Sr to this alloy increases production costs, hindering the large-scale application of magnesium alloy die castings. Furthermore, the patent does not evaluate the tensile properties of the alloy at room temperature, especially its elongation, limiting its industrial application.

[0006] Chinese patent CN116607055A discloses a high-strength mixed rare earth die-cast magnesium alloy and its preparation method. The composition and mass percentages of each component are as follows: 3.95-4.05% RE, 2.95-3.06% Al, 0.45-0.52% Mn, and the balance Mg; wherein the RE is a mixture of La and Ce. However, due to the low Al content, this alloy has insufficient yield strength, and the low Al content also leads to poor alloy fluidity. During component manufacturing, it is prone to incomplete casting and hot cracking, affecting the product's pass rate and overall performance, thus greatly limiting its application in industrial production.

[0007] Chinese patent CN202511083929.X discloses a heat-treasure-free, high-strength, tough, flame-retardant die-cast magnesium alloy and its preparation method. The magnesium alloy, by weight percentage, comprises Mg and the following components: Al 5.2–8.8%, Mn 0.1–0.5%, Zn 0.01–1.0%, Sr 0.02–0.1%, Sn 0.001–0.5%, La and Ce mixed rare earth elements 0.3–3.9%, Nd 0.001–0.5%, Y 0.001–0.35%, Ho 0.001–0.05%, Ca 0.01–0.6%, and unavoidable impurities. Its tensile strength is 280–300 MPa, yield strength is 150–180 MPa, and elongation is 10–16%. Although this patent describes a high-strength and high-ductility alloy, it does not mention its resistance to salt spray corrosion.

[0008] In summary, existing die-cast magnesium alloys have shortcomings in terms of low strength, low elongation, and poor resistance to salt spray corrosion, which restricts their application in large-size structural components of new energy vehicles. Summary of the Invention

[0009] The purpose of this invention is to provide a high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy and its preparation method. This method achieves a synergistic improvement in both high strength (tensile strength ≥ 270 MPa) and high elongation (≥ 9%), while also ensuring high corrosion resistance (salt spray corrosion rate ≤ 1 mg / cm³). 2 Furthermore, this invention does not require the addition of precious rare earth elements, resulting in low alloy costs and significant application prospects for achieving lightweighting of large-size structural components in new energy vehicles.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows: A high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy has the following composition by weight percentage: Al: 8.0~12.0%, Zn: 0.2~1.5%, Mn: 0.1~1.0%, Be: 0.002~0.03%, Ce: 1.0~4.0%, La: 0.5~3.0%, with the balance including Mg and unavoidable impurities, the total amount of unavoidable impurities being ≤0.2%, and the above elements also need to simultaneously satisfy: Ce / La is 1~3:1.

[0011] Furthermore, the balance is Mg and unavoidable impurities.

[0012] The microstructure of the die-cast magnesium alloy of the present invention includes an α-magnesium matrix, a grain boundary second phase precipitated at the grain boundaries of the α-magnesium matrix, and an intragranular precipitate phase precipitated within the grains of the α-magnesium matrix; The intracrystalline precipitate is nanoscale Mg. 17 Al 12The phase and its complex precipitates with Zn, Ce and / or La are randomly distributed without a fixed orientation; The second grain boundary phase forms a network of grain boundary precipitation bands along the grain boundaries, with a width of 0.1~3 μm; the second grain boundary phase includes a network Mg-Al phase and a lath-like Mg-Al-Ce-La-Mn multi-component phase (Mn dissolved in Al). 11 (Ce,La)3); the width of the lath-shaped Mg-Al-Ce-La-Mn multi-component phase is ≤2μm, and the length is 0.5~15μm.

[0013] The die-cast magnesium alloy of this invention has a yield strength ≥165MPa, tensile strength ≥270MPa, and elongation ≥9% in the as-cast state; after a 100-hour neutral salt spray test at 35℃ with 5% NaCl, the corrosion rate is ≤1 mg / cm³. 2 ) / day.

[0014] In the composition design of the high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy described in this invention: Since the mechanical properties of an alloy are closely related to the quantity and type of solid solution atoms and precipitated phases in the alloy, in order to simultaneously improve the performance of die-cast magnesium alloys and achieve high strength, high elongation and high corrosion resistance, the present invention requires control of the alloy element content in the magnesium alloy.

[0015] (1) Strong plastic equilibrium of Al and Zn Al is the element that forms the strengthening phase Mg. 17 Al 12 It is an essential element for multi-element rare earth phases. If its content is below 8%, the total amount of reinforcing phase is insufficient, and the strength will not meet the requirements; if it is above 12%, coarse and continuous Mg will form. 17 Al 12 The network phase severely disrupts the matrix, leading to a sharp drop in plasticity. This invention controls the Al content to 8.0~12.0 wt.% to ensure the formation of sufficient reinforcing phases, while altering their morphology and distribution through the addition of Ce / La.

[0016] The microalloying of Zn aims to produce grain refinement and solid solution strengthening. Too low a Zn content (<0.2%) results in insufficient grain refinement; too high a content (>1.5%) significantly increases the alloy's susceptibility to hot cracking, impairing casting integrity and plasticity. This invention controls the Zn content at 0.2~1.5 wt.%, allowing it to form a small amount of fine intragranular precipitates with the remaining Al and Mg, thus aiding in strength enhancement.

[0017] (2) Mn has dual functions of structural bridging and purification. Structural function of bridges: Mn has a smaller atomic radius than Mg and is easily dissolved in Al. 11In the (Ce,La)3 phase, the addition of Mn further stabilizes the structure of the Mg-Al-Ce-La-Mn multi-component phase and promotes its precipitation at the grain boundaries in lath-like forms with a width ≤2μm. This lath-like Mg-Al-Ce-La-Mn multi-component phase distribution can effectively pin the grain boundaries, hinder dislocation movement (improving strength), and at the same time make the stress distribution more uniform (improving plasticity).

[0018] Purification effect: Mn has a very strong affinity for Fe impurities in the melt, forming high-melting-point Fe-Mn compounds that settle into the slag, thereby significantly reducing the content of harmful impurities such as Fe in the matrix. This is one of the reasons why the corrosion resistance of the alloy of this invention surpasses that of the traditional AZ91D alloy.

[0019] (3) Synergy between La and Ce Although La and Ce both belong to the light rare earth elements, their solid solubility, diffusion rate, and affinity for Al in a magnesium matrix differ. This invention reveals that strictly controlling the Ce / La mass ratio to 1-3:1 is crucial for forming the lath "Mg-Al-Ce-La-Mn multi-component phase." At this ratio, Ce preferentially acts: Ce has a larger negative enthalpy of mixing with Al (stronger thermodynamic driving force), preferentially forming fine Al particles in the early stages of solidification. 11 The Ce3 phase, acting as a heterogeneous nucleation core, effectively refines the α-Mg grains; subsequently, the precipitated La portion dissolves in Al. 11 In the Ce3 phase, more stable Al is formed. 11 The (Ce,La)3 phase. The addition of La can adjust the lattice constant and interfacial energy of this phase, maintaining its semi-coherent relationship with the magnesium matrix. This strengthens the matrix while preventing the interface from becoming a crack initiation point, which is key to improving plasticity. When La is in excess (Ce / La < 1), coarse Al will mainly form. 11 The La3 phase has a weak interface with the matrix, which leads to a decrease in both strength and plasticity.

[0020] By adding Ce / La in a specific ratio, a discontinuous lath-shaped Mg-Al-Ce-La-Mn grain boundary second phase was formed. This discontinuous Mg-Al-Ce-La-Mn grain boundary second phase greatly reduced the total area and continuity of the cathodic phase that would form a strong galvanic corrosion with the α-Mg matrix.

[0021] Meanwhile, the presence of elements such as Mn, Ce, and La in this multi-component phase optimizes the potential difference between it and the magnesium matrix, reducing the potential difference between the Mg-Al-Ce-La-Mn multi-component phase and the matrix, further weakening the driving force of micro-galvanic corrosion and improving the corrosion resistance of the alloy.

[0022] (4) Overall correlation of the quantity range of each element The content range of each element in this invention is an optimized range that complements each other: Ce: 1.0~4.0 wt.%, La: 0.5~3.0 wt.%, The total amount of Ce+La (1.5~7.0 wt.%) is directly related to the Al content (8.0~12.0 wt.%), ensuring sufficient Al to form the target multiphase with rare earth elements, rather than all of it forming Mg. 17 Al 12 Phase or free rare earth phase; The lower limit of Mn (0.1 wt.%) is determined by the minimum amount required to effectively purify impurities; the upper limit of Mn content (1.0 wt.%) is limited by the fact that excessive Mn will form coarse primary phases and impair plasticity. The addition of trace amounts of Be (0.002~0.03wt.%) aims to form a dense oxide film on the surface during alloy smelting, preventing excessive burning of active elements such as Mg, La, and Ce, and ensuring that the actual yield of the main alloying elements remains stable within the design range. This is the process guarantee for achieving precise composition control and reproducibility in large-scale production.

[0023] The multi-phase second phase at the grain boundaries of the aforementioned magnesium alloy can better pin the grain boundaries, controlling the grain size of the α-magnesium matrix in the alloy to be 1–30 μm, with more than 90% of the α-magnesium matrix grains having a size of 1–15 μm. This significant grain refinement effect further improves the mechanical properties of the alloy.

[0024] The preparation method of the high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy of the present invention specifically includes the following steps: S1. Material preparation: Prepare materials according to the above magnesium alloy composition, weigh the raw materials according to the weight percentage of each alloy element, and preheat; the raw materials include pure Mg, pure Al, pure Zn, magnesium- or aluminum-containing Mn master alloy, magnesium- or aluminum-containing Be master alloy, and La and Ce are prepared using pure La, pure Ce or Mg-La, Mg-Ce or Al-La, Al-Ce master alloys as raw materials. S2. Melting: First, heat the crucible to 400~500℃, put the pure Mg ingot into the crucible, and melt it under a protective gas or in a vacuum environment. After the pure Mg ingot melts, cover the surface of the melt with a covering agent. Then, heat the crucible to 750~780℃ and add Al-Be or Mg-Be master alloy. After the master alloy melts, add pure La, pure Ce or Mg-La, Mg-Ce or Al-La, Al-Ce master alloy. After the alloy is completely melted, cool the crucible to 720~750℃, and then add Mg-Mn or Al-Mn master alloy, pure Al, and pure Zn to melt and obtain the melt. S3. Refining: The obtained melt is heated to 740~760℃. Gas containing refining agent powder is introduced into the melt for powder spraying refining, slag removal and degassing treatment. The melt temperature is kept at 740~760℃ and left to stand. The slag on the surface of the melt is skimmed off. After the treatment, magnesium alloy melt is obtained. S4. Forming: The refined and slag-removed magnesium alloy melt is die-cast into a die casting.

[0025] Preferably, in step S1, the preheating temperature of the raw material is 180~240℃.

[0026] Preferably, in step S2, the protective gas is a mixture of N2 and SF6, a mixture of CO2 and SF6, or pure SF6 gas.

[0027] Preferably, in step S2, the covering agent has a density ≤1.58 g / cm³. 3 And it is a salt flux that does not contain Na ions.

[0028] Preferably, in step S3, the refining agent is a salt flux that does not contain Na ions.

[0029] Preferably, in step S3, the gas is argon, chlorine, or hexachloroethane.

[0030] Preferably, in step S4, the die-casting temperature is 660~720℃.

[0031] Preferably, in step S4, during the die casting process, the injection speed is 2~6m / s and the casting pressure is 20~80MPa.

[0032] The process steps and parameter ranges in the preparation method described in this invention are a systematic solution specifically designed for the alloy composition and aimed at obtaining specific microstructures and properties. The coordination and function of each step are as follows: Temperature control in the S2 smelting step: First, the temperature is controlled at 750~780℃ when Al-Be or Mg-Be master alloys and pure La, pure Ce, or Mg-La, Mg-Ce, or Al-La, Al-Ce master alloys are added. Then, at 720~750℃, Mg-Mn or Al-Mn master alloys, pure Al, and pure Zn are added until they are completely melted. The purpose is to ensure that the stated weight percentages of Al, Zn, and especially Ce and La alloying elements can be completely dissolved and fully diffused to obtain a melt with uniform composition, avoid initial segregation, and lay the foundation for the subsequent formation of a uniform and dispersed strengthening phase. Too high a smelting temperature will lead to excessive oxidation of the magnesium melt and dissolution of iron impurities; too low a smelting temperature will lead to incomplete melting of the high-melting-point rare earth master alloys.

[0033] The S3 refining and settling steps involve parameter coordination: refining is performed at 740~760℃, followed by settling the melt at 740~760℃. This operation not only allows the refining products to float and separate fully, but also further homogenizes the melt temperature and composition. Its purpose is to completely remove gases and inclusions from the melt, improve the melt cleanliness, and obtain a stable and pure melt state. This is a prerequisite for achieving subsequent stable solidification and obtaining a dense, defect-free as-cast structure.

[0034] S4 Molding: The melt is die-cast into a die casting. The die-casting temperature is controlled at 660~720℃, which is a key step in controlling the final microstructure. The inherently high cooling rate of the die-casting process, combined with the design of this lower die-casting temperature range, reduces the superheat of the melt. The melt with low superheat undergoes rapid cooling in the die-casting mold. The combination of low superheat and extremely high cooling rate produces two core effects: (1) significantly improves the nucleation rate, making the α-Mg matrix grain size significantly refined to 1~30μm; (2) greatly inhibits the long-range diffusion of Al, Ce, La and other atoms, forcing the Mg-Al-Ce-La-Mn multi-component strengthening phase to precipitate in a dispersed, lath-like form at the grain boundaries, rather than forming a coarse continuous network. At the same time, rapid solidification is also conducive to the formation of a nanoscale dispersed second phase within the grain.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention relates to a magnesium alloy composition based on conventional Mg-Al-RE die-cast magnesium alloys. By adjusting the contents of Al, La, and Ce elements and controlling the Al content (8.0~12.0 wt.%) and Ce / La ratio (1~3:1), under rapid solidification conditions during die casting, the grains are refined, promoting the formation of lath-like Mg-Al-Ce-La-Mn multi-component phases distributed at the grain boundaries. These multi-component phases effectively pin the grain boundaries, improving strength. Their discontinuous distribution and good interfacial bonding with the matrix provide space for dislocation movement, improving plasticity. Simultaneously, the Mg-Al-Ce-La-Mn multi-component phases significantly reduce the total area and continuity of the cathode phase that forms strong galvanic corrosion with the α-Mg matrix, and the reduced potential difference between the cathode and the magnesium matrix weakens the driving force of micro-galvanic corrosion. Combined with the effect of Mn in purifying impurities such as Fe in the melt, this significantly improves the salt spray corrosion resistance of the magnesium alloy while synergistically enhancing its strength and plasticity.

[0036] The magnesium alloy of this invention exhibits synergistic effects among its elements within the aforementioned ranges, achieving superior strength (tensile strength ≥ 270 MPa), plasticity (elongation ≥ 9%), and corrosion resistance (corrosion rate ≤ 1 mg / cm³ after 100 hours of neutral salt spray testing at 35°C with 5% NaCl). 2The invention optimizes the synchronous operation of the alloy by adding only low-cost rare earth elements such as La and Ce, without the need for expensive rare earth elements, thus effectively controlling the alloy cost. Attached Figure Description

[0037] Figure 1 This is a microstructure photograph of the die-cast magnesium alloy of Example 1 of the present invention; Figure 2 This is a microstructure photograph of the die-cast magnesium alloy of Comparative Example 4 of the present invention; Figure 3 This is a microstructure photograph of the die-cast magnesium alloy of Comparative Example 5 of the present invention. Detailed Implementation

[0038] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0039] The composition design of the die-cast magnesium alloy in the embodiments and comparative examples of the present invention is shown in Table 1. The balance is Mg and unavoidable impurities, and the total amount of unavoidable impurities is ≤0.2wt.%.

[0040] The process control of die-cast magnesium alloys in the embodiments and comparative examples of the present invention is shown in Table 2.

[0041] The properties of the resulting die-cast magnesium alloy parts are shown in Table 3.

[0042]

[0043]

[0044]

[0045] Figure 1 The image shows the microstructure of the die-cast magnesium alloy obtained in Example 1 of this invention. Figure 1 It can be seen that the microstructure of the magnesium alloy of the present invention includes an α-magnesium matrix, a grain boundary second phase precipitated at the grain boundaries of the α-magnesium matrix, and an intragranular precipitate phase precipitated within the grains of the α-magnesium matrix.

[0046] The intracrystalline precipitates are randomly distributed without a fixed orientation within the α-magnesium matrix. These intracrystalline precipitates can reduce the concentration of solid solution atoms, thereby improving the corrosion resistance of the alloy.

[0047] The grain boundary second phase forms a network of grain boundary precipitation bands with a width of 0.1~3μm along the grain boundaries, and the grain boundary second phase contains network Mg. 17 Al 12The lath-shaped Mg-Al-Ce-La-Mn multi-component phase has a width ≤2μm and a length of 0.5~15μm. Among them, the lath-shaped Mg-Al-Ce-La-Mn multi-component phase can better pin grain boundaries, simultaneously improve the strength and plasticity of the alloy, and reduce the total area and continuity of the cathode phase that forms strong galvanic corrosion with the α-Mg matrix. At the same time, it weakens the driving force of micro-galvanic corrosion and improves the alloy's salt spray corrosion resistance.

[0048] In addition, the grain size of the α-magnesium matrix in this alloy is 1~30μm, of which more than 90% of the α-magnesium matrix has a grain size of 1~15μm. The significant grain refinement effect further improves the mechanical properties of the alloy.

[0049] The die-cast magnesium alloy obtained by this invention achieves a synergistic improvement in alloy strength, plasticity, and salt spray corrosion resistance. Its yield strength is ≥165MPa, tensile strength is ≥270MPa, and elongation is ≥9%. After a 100-hour neutral salt spray test at 35℃ with 5% NaCl, the corrosion rate is ≤1 mg / cm³. 2 ) / day.

[0050] In Comparative Example 1, the alloy composition was 7.0% Al (below the lower limit), 2.0% Zn (above the upper limit), 1.2% Mn (above the upper limit), 0.5% Ce (below the lower limit), and 0.3% La (below the lower limit). The resulting alloy exhibited a yield strength of 133 MPa, a tensile strength of 223 MPa, an elongation of 8.5%, and a corrosion rate of 1.22. The microstructure of the alloy consisted of an α-Mg matrix, intragranular precipitates, and a second phase distributed along the grain boundaries. The α-Mg matrix exhibited uneven grain size distribution, with some regions showing coarse grains exceeding 30 μm. Due to insufficient Al content and low Ce and La contents, a sufficient amount of effective grain boundary strengthening phase was not formed. The grain boundary second phase consisted of discontinuous, coarse Mg... 17 Al 12 The microstructure is dominated by a single phase, with no diffusely distributed Mg-Al-Ce-La-Mn multi-phase phases observed. A small amount of non-oriented precipitates are present within the crystal structure. Due to insufficient total amount and poor distribution of the reinforcing phase, as well as coarsening of the matrix grains, the strength, plasticity, and corrosion resistance of this microstructure are significantly reduced.

[0051] In Comparative Example 2, the alloy composition was: Al 13.0% (above the upper limit), Zn 0.1% (below the lower limit), Mn 1.5% (above the upper limit), Be 0.05% (above the upper limit), Ce 0.2% (below the lower limit), and La 0.4% (below the lower limit). The resulting alloy exhibited a yield strength of 150 MPa, a tensile strength of 235 MPa, an elongation of 2.1%, and a corrosion rate of 1.69. The microstructure of the alloy showed an α-Mg matrix surrounded by a coarse, continuous, and complete network of Mg... 17 Al 12The phase is severely fragmented, with the width of the network phase exceeding 3 μm. Due to the extremely low rare earth element content, no dispersed Mg-Al-Ce-La-Mn multi-component phases were observed. Intragranular precipitates are few. The coarse, continuous network phase severely hinders dislocation movement, becoming a rapid pathway for stress concentration and crack propagation. This results in limited improvement in the alloy's yield strength and tensile strength, while the elongation deteriorates sharply to 2.1%. Corrosion resistance also decreases due to the increased area of ​​the grain boundary second phase.

[0052] In Comparative Example 3, the alloy composition was 6.0% Al (below the lower limit), 2.2% Zn (above the upper limit), 1.2% Mn (above the upper limit), and 4.0% La (above the upper limit). The resulting alloy exhibited a yield strength of 129 MPa, a tensile strength of 226 MPa, an elongation of 5.1%, and a corrosion rate of 1.87. The microstructure of the obtained alloy showed inhomogeneous α-Mg matrix grain size. Due to the excessively low Al content and excessively high La content, coarse, blocky Al particles primarily formed at the grain boundaries. 11 The La3 phase is not the discontinuous lamellar Mg-Al-Ce-La-Mn multi-phase phase as described in this invention. A certain number of precipitated phases exist within the crystal. Coarse, blocky Al... 11 The weak bonding interface between the La3 phase and the matrix is ​​not conducive to strength improvement and is prone to becoming a crack initiation point, resulting in unsatisfactory alloy strength, plasticity and corrosion resistance.

[0053] In Comparative Example 4, Al, Zn, Mn, Be, Ce, and La were all within the required range, but Ce / La was 8, which exceeded the limitation of this invention. The resulting alloy had a yield strength of 131 MPa, a tensile strength of 234 MPa, an elongation of 6.8%, and a corrosion rate of 1.51. Figure 2 The image shows the microstructure of the magnesium alloy obtained in Comparative Example 4. As can be seen from the image, La and Ce were not added in the required proportions in Comparative Example 4. Because the Ce content is much higher than that of La, only a small amount of coarse, blocky Al was formed. 11 The Ce3 phase was not formed, instead failing to create the dispersed lath-shaped Mg-Al-Ce-La-Mn multi-phase structure required in this invention. This coarse bulk structure provides limited strengthening of the alloy and contributes insufficiently to its plasticity and corrosion resistance.

[0054] Comparative Example 5 is a conventional AZ91D alloy without Ce and La, and the resulting alloy has a yield strength of 150 MPa, a tensile strength of 230 MPa, an elongation of 3.5%, and a corrosion rate of 1.10. Figure 3 The image shows the microstructure of the magnesium alloy obtained in Comparative Example 5. As can be seen from the image, the α-Mg matrix in the microstructure of the commercial AZ91D magnesium alloy is composed of coarse, continuous, network-like Mg atoms. 17 Al 12The network phase, consisting of a phase surrounding the Mg-Al-Ce-La-Mn matrix, has a width between 3 and 5 μm. A small amount of dispersed precipitates are present within the crystal structure. The discontinuous lamellar Mg-Al-Ce-La-Mn multi-component phase with pinning grain boundaries, formed by Ce / La addition in this invention, is completely absent. This microstructure results in alloys with generally low strength and continuous brittle Mg... 17 Al 12 The network phase severely impairs plasticity, resulting in low elongation and reduced corrosion resistance due to the continuous coarse network of Mg. 17 Al 12 The phase exists but is relatively poor.

[0055] As shown in Tables 2 and 3, Examples 1-8 followed the optimized composition and process window of this invention, and their mechanical properties and corrosion resistance were significantly better than those of the comparative examples. This confirms the key control role of the alloy composition and process parameters of this invention in obtaining the unique microstructure and comprehensive properties.

Claims

1. A high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy, characterized in that, Its composition by weight percentage is as follows: Al: 8.0~12.0%, Zn: 0.2~1.5%, Mn: 0.1~1.0%, Be: 0.002~0.03%, Ce: 1.0~4.0%, La: 0.5~3.0%, with the balance including Mg and unavoidable impurities. The total amount of unavoidable impurities is ≤0.2%, and the above elements must also satisfy the following: Ce / La ratio is 1~3∶1.

2. The high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy as described in claim 1, characterized in that, The balance is Mg and unavoidable impurities.

3. The high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy as described in claim 1 or 2, characterized in that, The microstructure of the die-cast magnesium alloy includes an α-magnesium matrix, a grain boundary second phase precipitated at the grain boundaries of the α-magnesium matrix, and an intragranular precipitate phase precipitated within the grains of the α-magnesium matrix. The intracrystalline precipitate is nanoscale Mg. 17 Al 12 The phase and its complex precipitates with Zn, Ce and / or La are randomly distributed without a fixed orientation; The second phase of the grain boundary forms a network of grain boundary precipitation bands along the grain boundary, with a width of 0.1~3μm; the second phase of the grain boundary includes a network Mg-Al phase and a lath Mg-Al-Ce-La-Mn multi-component phase; the lath Mg-Al-Ce-La-Mn multi-component phase has a width ≤2μm and a length of 0.5~15μm.

4. The high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy as described in claim 3, characterized in that, The α-magnesium matrix grain size in the microstructure of the die-cast magnesium alloy is 1~30μm, of which more than 90% of the α-magnesium matrix grain size is 1~15μm.

5. The high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy as described in claim 1, 2, 3, or 4, characterized in that, The die-cast magnesium alloy has a yield strength ≥165MPa, tensile strength ≥270MPa, and elongation ≥9%; after a 100-hour neutral salt spray test at 35℃ and 5% NaCl, the corrosion rate is ≤1 mg / cm³. 2 ) / day.

6. The method for preparing the high-strength, high-elongation, and high-corrosion-resistant die-cast magnesium alloy according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Material preparation: Prepare the magnesium alloy according to the composition of claim 1 or 2, weigh the raw materials according to the weight percentage of each alloy element, and preheat them; the raw materials include pure Mg, pure Al, pure Zn, magnesium- or aluminum-containing Mn master alloy, magnesium- or aluminum-containing Be master alloy, and La and Ce are prepared using pure La, pure Ce or Mg-La, Mg-Ce or Al-La, Al-Ce master alloys as raw materials. S2. Melting: First, heat the crucible to 400~500℃, put the pure Mg ingot into the crucible, and melt it under a protective gas or in a vacuum environment. After the pure Mg ingot melts, cover the surface of the melt with a covering agent. Then, heat the crucible to 750~780℃ and add Al-Be or Mg-Be master alloy. After the master alloy melts, add pure La, pure Ce or Mg-La, Mg-Ce or Al-La, Al-Ce master alloy. After the alloy is completely melted, cool the crucible to 720~750℃, and then add Mg-Mn or Al-Mn master alloy, pure Al, and pure Zn to melt and obtain the melt. S3. Refining: The obtained melt is heated to 740~760℃. Gas containing refining agent powder is introduced into the melt for powder spraying refining, slag removal and degassing treatment. The melt temperature is kept at 740~760℃ and left to stand. The slag on the surface of the melt is skimmed off. After the treatment, magnesium alloy melt is obtained. S4. Forming: The refined and slag-removed magnesium alloy melt is die-cast into a die casting.

7. The preparation method according to claim 6, characterized in that, In step S1, the preheating temperature of the raw materials is 180~240℃.

8. The preparation method according to claim 6, characterized in that, In step S2, the protective gas is a mixture of N2 and SF6, a mixture of CO2 and SF6, or pure SF6 gas.

9. The preparation method according to claim 6 or 8, characterized in that, In step S2, the covering agent has a density ≤1.58 g / cm³. 3 And it is a salt flux that does not contain Na ions.

10. The preparation method according to claim 6, characterized in that, In step S3, the refining agent is a salt flux that does not contain Na ions.

11. The preparation method according to claim 6 or 10, characterized in that, In step S3, the gas is argon, chlorine, or hexachloroethane.

12. The preparation method according to claim 6, characterized in that, In step S4, the die-casting temperature is 660~720℃.

13. The preparation method according to claim 6 or 12, characterized in that, In step S4, during the die casting process, the injection speed is 2~6m / s and the casting pressure is 20~80MPa.

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