EP4733427A1

EP4733427A1 - High-toughness and high-thermal-conductivity die-cast magnesium alloy and preparation method therefor

Info

Publication number: EP4733427A1
Application number: EP24829722.8A
Authority: EP
Inventors: Zhuo Wang; Weineng TANG; Yong Hu
Original assignee: Baosteel Metal Co Ltd
Current assignee: Baosteel Metal Co Ltd
Priority date: 2023-06-28
Filing date: 2024-01-10
Publication date: 2026-04-29
Also published as: WO2025001059A1; CN118773498A

Abstract

A high-strength, high-toughness, and high-thermal-conductivity die-cast magnesium alloy and its preparation method, consisting of the following components in weight percentage: Al: 2.5-4.0%, RE: 3.5-5.5%, Mn: 0.1-0.3%, Be: 0.002-0.003%, Ca: 0.1-1.0%, Gd: 0.05-0.3%, where RE is one or both of La and Ce, and the balance is Mg and other unavoidable impurities, wherein the above-mentioned elements simultaneously meet the following requirement: 0.7≤Al/RE≤0.8. A new acicular Al₃RE second phase is introduced to enhance the mechanical properties of the material through a more excellent second-phase strengthening effect.

The formation of the second phase reduces the negative impact of Al on the thermal conductivity of the magnesium alloy. Meanwhile, it inhibits the formation of the brittle Mg₁₇Al₁₂ phase, avoiding a significant decrease in thermal conductivity. Thus, a magnesium alloy with both high thermal conductivity and excellent room-temperature mechanical properties is obtained. The resulting magnesium alloy has a room-temperature yield strength of 140-150MPa, an elongation of 8-12%, and a thermal conductivity of 100-110W/(m·K).

Description

Technical Field

The present invention relates to the technical field of magnesium alloy materials, in particular to a high-strength, high-toughness, and high-thermal-conductivity die-cast magnesium alloy and its preparation method.

Background Technology

Heat dissipation system structures such as mobile phone middle plates, laptop casings, 5G base station housings, automotive headlight bases and mobile energy storage devices not only require the material to have the characteristics of low density and high strength but also excellent thermal conductivity/heat dissipation performance. Magnesium alloy is currently the metal material with the lowest density among engineering structural materials. Meanwhile, the magnesium alloy has excellent thermal conductivity, damping performance and electromagnetic performance, so it is regarded as a structural-functional integrated material with great potential. Due to its lightweight characteristics and excellent physical properties, the magnesium alloy has broad application prospects in the aforementioned fields of 3C products and automotive parts.
The thermal conductivity of pure magnesium at room temperature is about 154.5W/(m·K), but its low yield strength makes it unsuitable to be used as an engineering material. Wrought magnesium alloy has excellent mechanical properties, but the conventional extrusion, rolling and forging of magnesium alloy have high costs and cannot produce magnesium alloy products with complex structural shapes. Therefore, more than 80% of magnesium alloy products adopt die casting as the forming process. Die casting is the most widely used magnesium alloy casting process, which has the advantages of low manufacturing cost and being able to form products with complex structures. Products such as 3C product casings, 5G base station housings, automotive headlight bases and mobile energy storage devices need to have both excellent thermal conductivity/heat dissipation performance and mechanical properties while being lightweight, so as to maintain high working stability and service life of the products. Further, such products usually have complex external structures, and die casting is an ideal choice considering the cost factor. AZ91D (Mg-9Al-1Zn), the most widely used die-cast magnesium alloy, is fabricated by adding specific contents of Al, Zn and other elements to pure magnesium to improve its mechanical properties. However, its thermal conductivity is significantly reduced to merely 51.2 W/(m·K), which can no longer meet the increasingly demanding requirements for magnesium alloy products. Therefore, it is urgent to conduct research on the composition design and forming technology of new magnesium alloys, aiming to develop die-cast magnesium alloy products with high strength, high toughness and high thermal conductivity.
Chinese Patent CN 105463280 A discloses "A Magnesium Alloy with High Thermal Conductivity and its Preparation Method", which produces an Mg-Al-RE-Mn die-cast magnesium alloy. Its chemical composition in mass percentage is: Al: 3.0-6.8%, Mn: 0.3-0.8%, mixed rare earths (La, Ce, Pr and Nd): 2.8-4.8%, and the balance is Mg. The thermal conductivity of this alloy is higher than 100 W/(m·K), but the patent does not mention the mechanical properties of the magnesium alloy.
Chinese Patent CN 107604228 A discloses "A Corrosion-resistant Diecast Magnesium Alloy of High Heat Conduction and its Preparation Method", which develops an Mg-Al-Ce-Mn-Be die-cast magnesium alloy. Its chemical composition in mass percentage is: Ce: 3.5-5.0%, Al: 2.5-3.5%, Mn: 0.5%, Be: 0.05%, and the balance is Mg. This alloy has a thermal conductivity of more than 100 W/(m·K) and a tensile strength of 250-260 MPa, but key mechanical property indicators such as yield strength and elongation at room temperature of the magnesium alloy are not mentioned.
Chinese Patent CN 114351022 A discloses "A High-heat-conductivity Magnesium Alloy Containing High-solid-solubility Rare Earth Elements and its Preparation Method", which prepares an Mg-RE-Al cast magnesium alloy. Its chemical composition is: RE: 4-16% (mass percentage), Al: the atomic ratio of Al to RE is 0.3-4%, and the balance is Mg. The thermal conductivity of this alloy is ≥ 100 W/(m·K). However, it contains a large amount of Sm, a precious rare earth element, leading to high cost, and its mechanical properties are not mentioned either.
The die-cast magnesium alloy materials reported in the prior art fail to simultaneously achieve high strength, high toughness, high thermal conductivity and low processing cost. Thus, it is imperative to carry out research on the composition design and new forming technology of new die-cast magnesium alloys so as to develop new die-cast magnesium alloys with high strength, high toughness and high thermal conductivity and expand the application scope of magnesium alloys.

Contents of the invention

The objective of the present invention is to provide a high-strength, high-toughness, and high-thermal-conductivity die-cast magnesium alloy and its preparation method. Without adding expensive rare earth elements or other alloying elements, a low-cost alloy formula is adopted. While ensuring the alloy's good castability, the invention solves the problem that existing magnesium alloys cannot simultaneously achieve high mechanical properties and high thermal conductivity. The magnesium alloy has a room-temperature thermal conductivity of 100-110W/(m·K), a room-temperature yield strength of 140-150MPa, and a room-temperature elongation of 8-12%. It is also capable of manufacturing magnesium alloy products with complex structures. This process boasts low cost and facilitates large-scale mass production, making it widely applicable to heat dissipation/thermal conduction components in the 3C product and automotive parts sectors.
To achieve the above purpose, the technical solution of the present invention is:
A high-strength, high-toughness, and high-thermal-conductivity die-cast magnesium alloy, consisting of the components in weight percentage: Al: 2.5-4.0%, RE: 3.5-5.5%, Mn: 0.1-0.3%, Be: 0.002-0.003%, Ca: 0.1-1.0%, Gd: 0.05-0.3%, where RE is one or both of La and Ce, and the balance is Mg and other unavoidable impurities, and the above-mentioned elements also simultaneously meet the following requirements: 0.7≤Al/RE≤0.8.
Preferably, the balance comprises Mg and other unavoidable impurities.
In the die-cast magnesium alloy matrix of the present invention, second phases such as Al₁₁RE₃, Al₂RE, Al₂Ca, (Mg,Al)₂Ca, Al₂Gd, and acicular Al₃RE are uniformly distributed.
The die-cast magnesium alloy in the present invention has a room-temperature yield strength of 140-150MPa, an elongation of 8-12%, and a thermal conductivity of 100-110W/(m·K).
Al element is the most commonly used alloying element in magnesium alloys, and Mg-Al series alloys possess excellent castability. However, the atomic volume of Al is smaller than that of Mg. As Al atoms form a solid solution in Mg, the lattice volume gradually decreases. The smaller the lattice volume, the higher the likelihood of free electron scattering, and the more notable the negative impact on thermal conductivity. Additionally, the difference in valence electrons between Al and Mg atoms also results in a significant reduction in the thermal conductivity of magnesium alloys when Al is dissolved in Mg as a solid solution. For instance, the thermal conductivity of the commercial die-cast magnesium alloy AZ91D is merely 51.2W/(m·K).
In the present invention, by adding elements (e.g., La, Ce, Ca, and Gd) with strong affinity for Al, second phases including Al₁₁RE₃, Al₂RE, Al₂Ca, (Mg,Al)₂Ca, and Al₂Gd are formed, and a new acicular second phase Al₃RE is introduced. Compared with the Al₁₁RE₃ second phase, this acicular second phase has a smaller size and offers a more superior second-phase strengthening effect, thereby significantly enhancing the mechanical properties of the material. These second phases not only improve the mechanical properties of the material, but also consume the Al element in the Mg matrix, increase the lattice volume, reduce the probability of free electron scattering, and prevent the decrease in thermal conductivity of the material caused by the solid solution of Al element in the magnesium alloy. Consequently, the magnesium alloy provides both high mechanical properties and high thermal conductivity.
In Mg-Al series alloys, when the Al content is less than 2.5wt-%, the die-casting performance is impaired, and the fluidity of the magnesium melt is poor. When the Al content exceeds 4wt-%, the thermal conductivity decreases remarkably. Therefore, the Al content in the present invention is controlled within the range of 2.5 -4.0wt-%.
La and Ce react with Al to form a large quantity of Al₁₁RE₃ second phase and a small amount of Al₂RE second phase. Among them, the added Al and RE elements are controlled to satisfy 0.7 ≤ Al/RE ≤ 0.8, so that Al and RE exist in the forms of fine lamellar Al₁₁RE₃, ultra-fine acicular Al₃RE second phase, and granular Al₂RE second phase. On the basis of the existing Al₁₁RE₃ and Al₂RE second phases, the new second phase Al₃RE is introduced, which produces a more excellent second-phase strengthening effect and significantly improves the mechanical properties of the material. Moreover, the formation of secondary phases Al₁₁RE₃, Al₃RE, and Al₂RE largely eliminates the presence of aluminum as solute atoms in the magnesium alloy, minimizing the adverse effect of Al on the alloy's thermal conductivity. As a result, the material's thermal conductivity remains unaffected, ensuring the magnesium alloy integrates both outstanding thermal conductivity and mechanical properties. Economical rare earth elements La and Ce also function to purify the magnesium alloy melt. Insufficient addition of La and Ce elements cannot generate an adequate volume of Al-RE secondary phases to enhance material performance. In contrast, when the content of La and Ce exceeds 5.5wt-%, the die-casting capability of the magnesium alloy declines markedly, accompanied by the reduced melt fluidity. Excessive RE additions also drive up material costs. Hence, the RE (La and Ce) content is controlled within the range of 3.5-5.5wt-% in the present invention.
In Mg-Al alloys, Ca element reacts with Al to form fine, dispersed secondary phases Al₂Ca and (Mg,Al)₂Ca, which remarkably improve the room-temperature mechanical properties of the magnesium alloy while preserving excellent ductility and toughness. Notably, the existence of Ca in the form of Al₂Ca secondary phases in the magnesium alloy does not significantly compromise the material's thermal conductivity. Additionally, Ca imparts oxidation resistance and flame-retardant properties to magnesium alloys, inhibiting surface oxidation of the melt during die-casting and drastically reducing inclusions in the die-cast magnesium alloy. However, excessive Ca content degrades the fluidity of the magnesium melt during die-casting, increases hot cracking susceptibility and brittleness, and undermines mechanical performance. Therefore, the Ca content is restricted to 0.1-1wt-% in the present invention.
Mn element exhibits extremely low solid solubility in magnesium alloys and can effectively reduce the concentration of harmful elements such as Fe, Cu, and Ni in the magnesium alloy, thereby enhancing the alloy's corrosion resistance. Furthermore, an appropriate amount of Mn refines the as-cast microstructure of the magnesium alloy, boosting the ductility and toughness with negligible negative impact on thermal conductivity. That said, adding more than 0.3wt-% Mn in the present invention weakens the grain refinement effect. Consequently, the Mn content is limited to 0.1-0.3wt-% in the present invention.
Be has a stronger affinity for oxygen than Mg. A trace amount of Be can form a dense oxide film on the surface of the magnesium alloy melt, preventing melt oxidation. When the Be content is below 0.002wt-%, the oxidation of the magnesium alloy melt intensifies significantly. When it exceeds 0.003wt-%, the material's microstructure becomes noticeably coarser, leading to reduced mechanical properties. Thus, the Be content is controlled between 0.002-0.003wt-% in the present invention, which reduces slang and inclusions in the magnesium alloy melt, improves mechanical properties, and avoids the degradation of thermal conductivity caused by inclusions and oxides.
When Gd element is added to Mg-Al alloy, a large number of reticular Mg₁₇Al₁₂ second phases precipitate at the grain boundaries of conventional Mg-Al alloy during the solidification process. These second phases distributed at the grain boundaries severely destroy the periodic arrangement of the magnesium matrix lattice and cause scattering of free electrons, thereby reducing the thermal conductivity of the material. In the present invention, the added Gd element can form Al₂Gd second phases with Al element, which inhibits the formation of the brittle Mg₁₇Al₁₂ phase and thus avoids a significant decrease in thermal conductivity. Meanwhile, the formed Al₂Gd phases have an obvious strengthening effect, which improves the strength of the magnesium alloy without significantly reducing the plasticity of the material. In addition, the Al₂Gd phases can also promote the heterogeneous nucleation process, increase the nucleation rate during the die-casting solidification process, and refine the grain size and microstructure. An insufficient amount of Gd element in the magnesium alloy cannot produce the effects of strengthening and inhibiting the formation of Mg₁₇Al₁₂, while an excessive addition of Gd element will reduce the die-casting performance of the magnesium alloy. Therefore, the content of Gd element is controlled within the range of 0.05-0.3wt-% in the present invention.
Thus, through composition control, the present invention obtains a die-cast magnesium alloy with both high strength and toughness while maintaining the excellent casting performance of the magnesium alloy.
The preparation method of the high-strength, high-toughness and high-thermal-conductivity die-cast magnesium alloy according to the present invention includes the following steps:

1) Ingredients
Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Mg-Ca, Mg-Gd, Al-Be master alloys are used as raw materials, and processed according to the magnesium alloy composition described above.
2) Smelting
Pure Mg ingots are put into a crucible, heated to 680-710°C, and completely melted under shielding gas. Then pure Al ingots and pure Mn powder are added and stirred under shielding atmosphere until the alloy is completely melted. After that, the temperature is raised to 740-750°C, and pure La ingots and pure Ce ingots are added in sequence, followed by stirring under a shielding atmosphere until the alloy is completely melted. Subsequently, the temperature is lowered to 690-710°C, and Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. After the master alloys are completely melted, a magnesium alloy melt is obtained by tiring for 5-10min.
3) Refining
Magnesium alloy flux is added to the obtained magnesium alloy melt for refining for 5-10min, and the surface slang is removed. Then the temperature is raised to 700-720°C and kept for 10-15min.
4) Molding
After refining, the magnesium alloy melt is cast into ingots. Then, the ingots are placed in the melting furnace of a die-casting machine and heated to 680-700°C for melting. The melt enters the mold cavity through the injection system for filling, where the mold temperature is 200-250°C; the casting pressure is 100-110MPa, and the injection speed is 3-5m/s.

Preferably, in Step 3), the magnesium alloy flux is RJ-2 flux and RJ-3 flux.
Preferably, in Step 2), the protective gas is a mixed gas of CO₂ and SF₆.
Preferably, in Step 4), the pouring temperature is 680-700°C.
In the die-casting forming process of the present invention, the furnace temperature is controlled at 680-700°C. This temperature range ensures good fluidity and mold-filling property of the magnesium alloy melt, while avoiding the formation of coarse dendrites in the material, which would affect the mechanical properties of the material. When the temperature is lower than 680°C, the fluidity of the magnesium alloy melt deteriorates, and surface defects appear on the mold-filled product. When the temperature is higher than 700°C, coarse dendrites appear in the microstructure of the mold-filled product, leading to a decrease in the mechanical properties of the material.
The mold temperature is controlled at 200-250°C. Within this temperature range, the alloy can achieve the purpose of complete mold filling; defects such as cracks and pores are significantly reduced, and the material has excellent mechanical properties. When the temperature is lower than 200°C, the large temperature difference between the high-temperature melt and the mold causes surface defects on the mold-filled product, and meanwhile, the plasticity of the product decreases. When the mold temperature is higher than 250°C, the range of the fine-grain zone on the product surface decreases, resulting in a reduction in mechanical properties.
A casting pressure of 100-110MPa is employed to ensure the product's proper mold filling, providing the material with a dense microstructure, superior mechanical properties and excellent thermal conductivity.
The injection speed is regulated within the range of 3-5m/s. An excessively low injection speed will result in incomplete mold filling by the material, while an excessively high injection speed will lead to higher porosity of the material, causing a reduction in both mechanical properties and thermal conductivity.
Compared with existing technology, the beneficial effect of the present invention is:
In the high-strength, high-toughness and high-thermal-conductivity magnesium alloy of the present invention, Al, La, Ce, Ca and Gd elements are added to the magnesium matrix, forming second phases such as Al₁₁RE₃, Al₂RE, Al₂Ca, (Mg, Al)₂Ca and Al₂Gd. Meanwhile, the contents of the added Al and RE elements are controlled to meet the requirement of 0.7≤Al/RE≤0.8, introducing a new acicular second phase Al₃RE. This produces a more outstanding second-phase strengthening effect, thereby enhancing the material's mechanical properties. Moreover, the formation of these second phases largely prevents Al from existing in the magnesium alloy in the form of solute atoms, mitigating the negative impact of Al element on the thermal conductivity of the magnesium alloy and thus avoiding a decrease in the material's thermal conductivity. In addition, the added Gd element can form Al₂Gd second phases with Al element, which inhibits the formation of the brittle Mg₁₇Al₁₂ phase and thus avoids a significant decrease in thermal conductivity. Accordingly, a magnesium alloy that simultaneously balances high thermal conductivity and excellent room-temperature mechanical properties is obtained.
The alloying elements added in the present invention not only guarantee the material's performance, but also take into account the die-casting manufacturability. On the basis of the compositional design, the present invention further matches appropriate melting furnace temperature, mold temperature, casting pressure and injection speed, enabling the magnesium alloy to exhibit favorable fluidity and mold filling capacity during die-casting. The resulting products feature a dense and defect-free microstructure and low process cost, and allow for large-scale production of complex structural components such as 3C casings and automotive parts with complex shapes.
In the present invention, the magnesium alloy has a room-temperature thermal conductivity of 100-110 W/(m·K), a room-temperature yield strength of 140-150 MPa, and a room-temperature elongation of 8-12%.

Specific implementation modes

The technical solution of the present invention is described in detail below through embodiments. These embodiments are implemented on the premise of the technical solution of the present invention, providing detailed implementation modes and specific operation processes. However, the scope of the present invention is not limited to the following embodiments.
The composition of the magnesium alloy in the embodiments of the present invention is shown in Table 1, with the remainder being Mg and other unavoidable impurities.

Embodiment 1

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 700°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 740°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 710°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 5 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 10 minutes to remove surface slag. Finally, the melt is held at 700°C for 15 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 680°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 216°C, the casting pressure set at 105MPa, and the injection speed at 3.5m/s.

The properties of the resulting die-cast magnesium alloy components are presented in Table 3.

Embodiment 2

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 690°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 741°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 700°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 6 minutes, and then the magnesium alloy flux RJ-3 is added for refining for 7 minutes to remove surface slag. Finally, the melt is held at 710°C for 12 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 692°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 220°C, the casting pressure set at 100MPa, and the injection speed at 3m/s.

Embodiment 3

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 710°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 750°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 705°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 10 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 10 minutes to remove surface slag. Finally, the melt is held at 705°C for 11 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 700°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 239°C, the casting pressure set at 101MPa, and the injection speed at 4m/s.

Embodiment 4

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 695°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 749°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 708°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 10 minutes, and then the magnesium alloy flux RJ-3 is added for refining for 9 minutes to remove surface slag. Finally, the melt is held at 710°C for 15 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 695°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 250°C, the casting pressure set at 110MPa, and the injection speed at 3.3m/s.

Embodiment 5

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 705°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 748°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 706°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 10 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 10 minutes to remove surface slag. Finally, the melt is held at 720°C for 10 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 688°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 208°C, the casting pressure set at 108MPa, and the injection speed at 4.3m/s.

Embodiment 6

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 680°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 745°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 690°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 5 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 5 minutes to remove surface slag. Finally, the melt is held at 705°C for 11 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 690°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 200°C, the casting pressure set at 109MPa, and the injection speed at 5m/s.

Embodiment 7

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 700°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 743°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 695°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 8 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 8 minutes to remove surface slag. Finally, the melt is held at 713°C for 11 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 695°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 226°C, the casting pressure set at 102MPa, and the injection speed at 4.6m/s.

Embodiment 8

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 690°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 744°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 705°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 7 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 7 minutes to remove surface slag. Finally, the melt is held at 720°C for 15 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 698°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 239°C, the casting pressure set at 103MPa, and the injection speed at 3.1m/s.

Embodiment 9

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 710°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 746°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 694°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 7 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 6 minutes to remove surface slag. Finally, the melt is held at 715°C for 11 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 685°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 203°C, the casting pressure set at 104MPa, and the injection speed at 3.9m/s.

Embodiment 10

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 705°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 747°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 698°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 5 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 5 minutes to remove surface slag. Finally, the melt is held at 718°C for 15 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 689°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 245°C, the casting pressure set at 106MPa, and the injection speed at 4.8m/s.

The properties of the resulting die-cast magnesium alloy components are presented in Table 3.
For Comparative Examples 1 and 2, commercial magnesium alloy grades AZ91D and AM60 were adopted, respectively, and the forming process of these two alloys followed the traditional die-casting method. In contrast, the present invention employs a design principle for high-strength, high-toughness, and high-thermal-conductivity magnesium alloys, with die-casting as the forming process.

Comparative Example 1

The composition of the magnesium alloy in percentage is specified as follows: 9wt-% Al, 1wt-% Zn, and the remainder being Mg.
The pure Mg ingots are placed into a crucible of a melting furnace and heated to 695°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Zn ingots are added, and the mixture is stirred for 8 minutes under the shielding gas. Once the alloy is completely melted, the magnesium alloy flux RJ-2 is then added for refining for 7 minutes to remove surface slag. Finally, the melt is held at 720°C for 15 minutes and cast into magnesium alloy ingots. Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 680°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 206°C, the casting pressure set at 105MPa, and the injection speed at 3.5m/s.

Comparative Example 2

The composition of the magnesium alloy in percentage is specified as follows: 6wt-% Al, 0.5wt-% Mn, and the remainder being Mg.
The pure Mg ingots are placed into a crucible of a melting furnace and heated to 710°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred for 10 minutes under the shielding gas. Once the alloy is completely melted, the magnesium alloy flux RJ-2 is then added for refining for 10 minutes to remove surface slag. Finally, the melt is held at 720°C for 10 minutes and cast into magnesium alloy ingots. Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 700°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 209°C, the casting pressure set at 105MPa, and the injection speed at 3.9m/s.

Comparative Example 3

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 703°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 747°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 695°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 5 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 5 minutes to remove surface slag. Finally, the melt is held at 718°C for 15 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 695°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 220°C, the casting pressure set at 100MPa, and the injection speed at 3.5m/s.

Comparative Example 4

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be, Mg-Ca and Mg-Gd master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 705°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 740°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 700°C, after which Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 5 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 5 minutes to remove surface slag. Finally, the melt is held at 718°C for 15 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 700°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 230°C, the casting pressure set at 103MPa, and the injection speed at 3m/s.

Comparative Example 5

1) Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Al-Be and Mg-Ca master alloys are used as raw materials, which are proportioned in accordance with the weight percentage of the designed magnesium alloy composition.
2) The pure Mg ingots are placed into a crucible of a melting furnace and heated to 709°C. Under the protection of a mixed shielding gas of CO₂ and SF₆, the pure Mg ingots are completely melted to form a melt. Subsequently, pure Al ingots and pure Mn powder are added, and the mixture is stirred under the shielding gas until the alloy is fully melted. The temperature is then increased to 739°C, followed by the sequential addition of pure La ingots and pure Ce ingots. The mixture is stirred under the shielding gas until the alloy is fully melted, and then allowed to stand and cool to 698°C, after which Al-Be and Mg-Ca master alloys are added to the melt. Once the alloy is completely melted, it is stirred for 5 minutes, and then the magnesium alloy flux RJ-2 is added for refining for 5 minutes to remove surface slag. Finally, the melt is held at 718°C for 15 minutes and cast into magnesium alloy ingots.
3) Magnesium alloy ingots are placed into the melting furnace of a 650T die-casting machine and melted at 710°C. The molten metal is injected into the mold cavity via the injection system for filling, with the mold temperature maintained at 215°C, the casting pressure set at 101MPa, and the injection speed at 3.6m/s.

The properties of the resulting die-cast magnesium alloy components are presented in Table 3.
According to the data in Table 3, the magnesium alloy in the present invention has a room-temperature thermal conductivity of 100-110 W/(m·K), a room-temperature yield strength of 140-150 MPa, and a room-temperature elongation of 8-12%. Compared with the conventional AZ91D and AM60B magnesium alloys, the magnesium alloy obtained by the present invention demonstrates significantly enhanced room-temperature mechanical properties and thermal conductivity.
Traditionally, the die-cast AZ91D and AM60B magnesium alloys utilize Al as the primary alloying element, which achieves the purpose of solid solution strengthening and second-phase (Mg₁₇Al₁₂) strengthening to improve the room-temperature yield strength of the material. Additionally, a small amount of Zn and Mn are added to further optimize the comprehensive mechanical properties. However, this strengthening effect is limited. Meanwhile, the solid solution of a relatively high content of Al element in the magnesium matrix remarkably reduces the material's thermal conductivity.
As indicated in the Comparative Examples in Table 3, both the die-cast AZ91D magnesium alloy (Comparative Example 1) and the die-cast AM60B magnesium alloy (Comparative Example 2) exhibit a thermal conductivity of less than 65W/(m·K), a room-temperature yield strength of less than 150MPa, and an elongation of less than 10%, indicating that they cannot simultaneously demonstrate excellent mechanical properties and thermal conductivity.
In Comparative Example 3, the low contents of Al and RE elements lead to a weak second-phase strengthening effect. Consequently, the material has a yield strength of only 131MPa, an elongation of 10%, and a thermal conductivity of 95W/(m·K).
In Comparative Example 4, at an Al/RE mass ratio of 1, a substantial amount of Al element dissolves in the Mg matrix or forms numerous Mg₁₇Al₁₂ phases. The alloying elements exert a significant negative effect on the thermal conductivity, leading to the material's yield strength of 138 MPa, elongation of 11%, and thermal conductivity of 90 W/(m·K).

In Comparative Example 5, without the addition of Gd element, Al element forms a large number of Mg₁₇Al₁₂ phases at the grain boundaries. As a result, the material achieves a yield strength of 139 MPa, elongation of 10.6%, and thermal conductivity of 91 W/(m·K).

Table 1

Unit: Weight percentage
Serial No.	Al	Zn	La	Ce	Mn	Be	Ca	Gd	Mg
Embodiment 1	2.5	-	3	0.5	0.25	0.002	0.7	0.25	Balance
Embodiment 2	2.6	-	0.7	3	0.12	0.0025	0.8	0.15	Balance
Embodiment 3	2.8	-	2.5	1.2	0.15	0.003	0.9	0.05	Balance
Embodiment 4	3	-	1.6	2.5	0.26	0.0021	0.6	0.1	Balance
Embodiment 5	3.1	-	0.6	3.8	0.1	0.0023	1.0	0.2	Balance
Embodiment 6	3.3	-	4.5	0.2	0.3	0.0024	0.5	0.3	Balance
Embodiment 7	3.5	-	3.6	0.95	0.18	0.0027	0.3	0.12	Balance
Embodiment 8	3.7	-	2.2	2.5	0.23	0.0022	0.2	0.23	Balance
Embodiment 9	3.9	-	5.5	-	0.29	0.0026	0.1	0.26	Balance
Embodiment 10	4.0	-	-	5.3	0.11	0.0029	0.4	0.18	Balance
Comparative Example 1 (AZ91D)	9	1	-	-	-	-	-	-	Balance
Comparative Example 2 (AM60)	6	-	-	-	0.5	-	-	-	Balance
Comparative Example 3	2	-	1	2.2	0.2	0.002	0.5	0.1	Balance
Comparative Example 4	4	-	1.6	2.4	0.15	0.003	0.3	0.2	Balance
Comparative Example 5	3.1	-	1.6	2.8	0.1	0.002	0.4	-	Balance

Table 2

No.	Forming process	Casting temperature °C	Mold temperature °C	Casting pressure MPa	Injection speed m/s
Embodiment 1	Die-casting	680	216	105	3.5
Embodiment 2	Die-casting	692	220	100	3
Embodiment 3	Die-casting	703	239	101	4
Embodiment 4	Die-casting	695	250	110	3.3
Embodiment 5	Die-casting	688	208	108	4.3
Embodiment 6	Die-casting	705	200	109	5
Embodiment 7	Die-casting	710	226	102	4.6
Embodiment 8	Die-casting	715	239	103	3.1
Embodiment 9	Die-casting	720	203	104	3.9
Embodiment 10	Die-casting	717	245	106	5
Comparative Example 1 (AZ91D)	Die-casting	680	206	105	3.5
Comparative Example 2 (AM60)	Die-casting	700	209	105	3.9
Comparative Example 3	Die-casting	695	220	100	3.5
Comparative Example 4	Die-casting	700	230	103	3
Comparative Example 5	Die-casting	710	215	101	3.6

Table 3

No.	Room temperature (2.5°C)
No.	Yield strength MPa	Elongation %	Thermal conductivity W/ (m·K)
Embodiment 1	141	10.1	108
Embodiment 2	146	9.8	109
Embodiment 3	150	8	110
Embodiment 4	147	10.3	107
Embodiment 5	143	11.2	106
Embodiment 6	148	12	102
Embodiment 7	140	8.6	103
Embodiment 8	145	9.3	105
Embodiment 9	144	11.5	101
Embodiment 10	149	10.9	100
Comparative Example 1 (AZ91D)	150	5	51.2
Comparative Example 2 (AM60)	130	9.8	63.1
Comparative Example 3	131	10	95
Comparative Example 4	138	11	90
Comparative Example 5	139	10.6	91

Claims

A high-strength, high-toughness, and high-thermal-conductivity die-cast magnesium alloy, consisting of the components in weight percentage: Al: 2.5-4.0%, RE: 3.5-5.5%, Mn: 0.1-0.3%, Be: 0.002-0.003%, Ca: 0.1-1.0%, Gd: 0.05-0.3%, where RE is one or both of La and Ce, and the balance is Mg and other unavoidable impurities, wherein the above-mentioned elements simultaneously meet the following requirement: 0.7≤Al/RE≤0.8.
The high-strength, high-toughness and high-thermal-conductivity die-cast magnesium alloy according to claim 1,
characterized in that
the balance is Mg and other unavoidable impurities.
The high-strength, high-toughness and high-thermal-conductivity die-cast magnesium alloy according to claim 1 or 2,
characterized in that
the matrix of the die-cast magnesium alloy is uniformly distributed with second phases including Al₁₁RE₃, Al₂RE, Al2Ca, (Mg,Al)₂Ca, Al₂Gd and acicular Al₃RE.
The high-strength, high-toughness and high-thermal-conductivity die-cast magnesium alloy according to claim 1, 2 or 3,
characterized in that
the die-cast magnesium alloy has a room-temperature yield strength of 140-150MPa, an elongation of 8-12%, and a thermal conductivity of 100-110W/(m·K).
The preparation method of the high-strength, high-toughness and high-thermal-conductivity die-cast magnesium alloy according to any one of claims 1 to 4,
characterized by comprising the following steps:
1) Ingredients
Pure Mg ingots, pure Al ingots, pure La ingots, pure Ce ingots, pure Mn powder, and Mg-Ca, Mg-Gd, Al-Be master alloys used as raw materials are processed as the magnesium alloy composition described in claim 1 or 2;

2) Smelting
Pure Mg ingots are put into a crucible, heated to 680-710°C, and completely melted under the protection of a shielding gas; then pure Al ingots and pure Mn powder are added and stirred under shielding atmosphere until the alloy is completely melted; thereafter, the temperature is raised to 740-750°C, and pure La ingots and pure Ce ingots are added sequentially, followed by stirring under shielding atmosphere until the alloy is completely melted; subsequently, the temperature is lowered to 690-710°C, and Al-Be, Mg-Ca and Mg-Gd master alloys are added to the melt; after the master alloys are completely melted, a magnesium alloy melt is obtained by stirring for 5-10min;

3) Refining
Magnesium alloy flux is added to the obtained magnesium alloy melt for refining for 5-10min, and the surface slang is removed; then the temperature is raised to 700-720°C and kept for 10-15min;

4) Die Casting Molding
After refining, the magnesium alloy melt is cast into ingots; then, the ingots are placed in the melting furnace of a die-casting machine and heated to 680-700°C for melting; the melt enters the mold cavity through the injection system for filling, where the mold temperature is 200-250°C; the casting pressure is 100-110MPa, and the injection speed is 3-5m/s.
The preparation method according to claim 5,
characterized in that:
in Step 3), the magnesium alloy flux is RJ-2 flux and RJ-3 flux.
The preparation method according to claim 5,
characterized in that:
in Step 2), the protective gas is a mixed gas of CO₂ and SF₆.
The preparation method according to claim 5,
characterized in that:
in Step 4), the pouring temperature is 680-700°C.