CN110195181A - A kind of diecast magnesium alloy and its manufacturing method with high-temperature heat-resistance performance - Google Patents

Abstract

The invention discloses a die-casting magnesium alloy with high temperature and heat resistance performance. The mass percentages of chemical elements are: Ca 1-4wt%, Al 5-9wt%, RE 0.5-3wt%, Sr 2-5wt%, Mn 0.1- 0.5wt%, the balance is Mg and other unavoidable impurities. In addition, the invention also discloses a method for manufacturing the above-mentioned die-casting magnesium alloy with high temperature and heat resistance. The die-casting magnesium alloy has high-temperature heat resistance, high strength and good thermal conductivity, which well solves the problems of insufficient thermal conductivity, low strength, low heat resistance and poor die-casting performance of existing magnesium alloys.

Description

A kind of die casting magnesium alloy with high temperature heat resistance and its manufacturing method

technical field

The invention relates to an alloy and a manufacturing method thereof, in particular to a magnesium alloy and a manufacturing method thereof.

Background technique

The Mg-Al-RE series magnesium alloy has excellent casting performance and mechanical properties, and the high melting point Al ₁₁ RE ₃ phase formed by the combination of RE and Al is beneficial to improve the room temperature and high temperature mechanical properties of the alloy, resulting in good room temperature strength, toughness and High temperature heat resistance. On the other hand, the generated Al ₁₁ RE ₃ can significantly reduce the aluminum content in the alloy and improve the thermal conductivity of the alloy. Therefore, the AE series magnesium alloy is a potential cast magnesium alloy with high thermal conductivity. However, the thermal conductivity of the currently developed Mg-Al-RE casting alloys is lower than 100W/m·K, and the strength at room temperature needs to be improved. Therefore, the development of new cast magnesium alloys with high thermal conductivity and high strength with good casting performance is of great significance to meet the needs of electronic products for light weight and high thermal conductivity radiators. At present, the rare earths added to Mg-Al-RE magnesium alloys are mainly cerium-rich light mixed rare earths.

The addition of alkaline earth elements to magnesium-aluminum-based alloys will precipitate Al-Ca or Al-Sr phases with high melting points in the matrix. These high-melting point phases are very stable at high temperatures, and have a good pinning effect on the grain boundaries, which can effectively improve the high-temperature mechanical properties of magnesium alloys, especially the creep resistance. Therefore, the development of low-cost heat-resistant magnesium alloys containing alkaline earth elements such as Sr and Ca is of great significance to promote the application of magnesium alloys in heat-resistant parts around automobile engines. Based on the limitations of the application of heat-resistant magnesium alloys such as AZ and AS series in automotive power systems, Canada Norenda has developed AJ (Mg-Al-Sr) series of heat-resistant magnesium alloys, such as AJ50x, AJ51x, AJ52x , AJ62x and AJ62Lx and other brands of alloys, among which AJ62x has been successfully used to produce thin-walled magnesium alloy parts such as oil pans and valve covers. The maximum working temperature of AJ62x heat-resistant magnesium alloy can reach 175°C, and its room temperature tensile strength and creep properties are significantly better than AZ91 die-casting magnesium alloy. In recent years, the peripheral parts of automobile engines have put forward higher requirements on the heat resistance of heat-resistant magnesium alloys, and it is necessary to develop high-strength heat-resistant die-casting magnesium alloys with a heat-resistant temperature of 200 °C.

While pure magnesium has a high thermal conductivity, about 157W/m·K, but its strength is too low, and the tensile yield strength in the as-cast state is about 21MPa. Adding alloying elements to magnesium can significantly improve its mechanical properties, but the thermal conductivity usually decreases significantly due to the addition of alloying elements. Therefore, in recent years, some heat-resistant magnesium alloys with high thermal conductivity and high strength have been gradually developed in China, but these alloys are basically deformed alloys and are not suitable for die-casting.

For example: the publication number is CN1804083, the publication date is July 19, 2006, and the Chinese patent document named "high-strength heat-resistant rare earth magnesium alloy" discloses a technical solution related to a high-strength heat-resistant rare earth magnesium alloy. But it is not suitable for die casting magnesium alloy. In addition, since the alloy contains: 2-10% by mass of gadolinium element, 3-12% by mass of yttrium element, and the combined mass of these two elements reaches 13-14% by mass, the rest is composed of magnesium and unavoidable impurities, that is, the The alloy composition is expensive and the specific gravity is large, resulting in low thermal conductivity of the alloy.

Another example: the publication number is CN101376938, the publication date is March 4, 2009, and the Chinese patent document titled "A Novel Flame-Retardant High-Strength Heat-Resisting Magnesium Alloy and Its Preparation Method", the technical solution disclosed in this patent document involves A novel flame-retardant, high-strength, heat-resistant magnesium alloy and its preparation method. In the technical scheme, the alloy composition is designed to add 0.3-1.0% by mass of Ca, 0.2-0.8% of RE and 0.05- 0.15% Sr; where RE is a common La-rich mixed rare earth. The mechanical properties of the alloy at room temperature, 150°C, and 200°C have been significantly improved; however, the strength level is not high enough, especially the high-temperature performance (the alloy strength at 150°C is less than 185MPa), and its thermal conductivity is low, which cannot meet the needs of gears. Practical application requirements in working environments such as boxes.

Based on this, it can be seen that there are few reports on the composition design of high thermal conductivity, high strength and heat resistant die-casting magnesium alloys at home and abroad. The development of high-strength heat-resistant magnesium alloys and related preparation technologies is lagging behind, which cannot meet the urgent needs of the user market for high-temperature and high-strength heat-resistant die-casting magnesium alloys. Therefore, it is expected to obtain a magnesium alloy with high temperature and heat resistance. The performance is good, and the problems of insufficient thermal conductivity, low strength, low heat resistance and poor die-casting performance of the existing magnesium alloy are well solved.

Contents of the invention

One of the purposes of the present invention is to provide a die-casting magnesium alloy with high temperature heat resistance, which has high temperature heat resistance, high strength and good thermal conductivity, which can well solve the problem of thermal conductivity of existing magnesium alloys , Low strength, low heat resistance and poor die casting performance.

In order to achieve the above object, the present invention proposes a die-casting magnesium alloy with high temperature and heat resistance, the mass percentage of its chemical elements is:

Ca 1～4wt%,

Al 5～9wt%,

RE 0.5～3wt%,

Sr 2～5wt%,

Mn 0.1～0.5wt%,

The balance is Mg and other unavoidable impurities.

For the technical solution described in the present invention, the inventor of this case obtained a die-casting magnesium alloy with high temperature and heat resistance through reasonable optimization of alloying elements, and according to the influence of the addition of alloying elements in the die-casting alloy on the final performance of the alloy, As well as the need for high thermal conductivity alloying of magnesium alloys, the inventors of this case studied the effects of various alloying elements in magnesium and the influence of various factors on the die-casting formability of alloys through a large number of experiments, and found that when alkaline earth is added to magnesium alloys Metals (such as Sr, Ca) and rare earth elements, die-casting magnesium alloys have excellent room temperature strength and high temperature creep resistance, good die-casting performance and high thermal conductivity. In addition, due to the addition of alkaline earth metals and rare earth elements to the die-casting magnesium alloy, it generates a variety of high-melting point and small second phases, thus improving the strength and heat resistance of the die-casting magnesium alloy and further improving the die-casting of the die-casting magnesium alloy. performance.

In addition, the characteristics of die-casting alloys are generally low melting point, good fluidity of the melt, small crystallization temperature range, and small tendency of rapid cooling and thermal cracking, so as to meet the requirements of extremely high mold filling speed and fast cooling speed during die-casting. And die-casting into complex shapes, thin-walled castings can fill complex mold cavities, therefore, in order to obtain suitable die-casting magnesium alloys, it is also necessary to consider adding alloying elements that are beneficial to obtain these characteristics when designing magnesium alloy compositions. Therefore, the composition of the magnesium alloy can be reasonably designed.

Based on this, the inventor of this case proposed a die-casting alloy with high temperature and heat resistance, the mass percentage of chemical elements is: Ca 1-4wt%, Al 5-9wt%, RE 0.5-3wt%, Sr 2-5wt%, Mn 0.1～0.5wt%, the balance is Mg and other unavoidable impurities, and the design principle of each chemical element in the die-casting magnesium alloy is as follows:

Ca: one of the alkaline earth elements, which works synergistically with Sr to produce grain refinement in magnesium, and can also inhibit the oxidation of molten magnesium. It also has a flame retardant effect, can increase the ignition temperature of the alloy melt, and can Improve the creep properties of the alloy. This element can form a second phase with other elements in magnesium, especially to obtain an ordered single-layer nanostructure with a high strengthening effect, which is very effective in improving the mechanical properties of magnesium alloys. In order to control the amount and type of the second phase to maintain good thermal conductivity, the mass percentage of Ca in the die-casting magnesium alloy with high temperature and heat resistance of the present invention is controlled at 1-4wt%.

Al: Al is the most commonly used alloying element in magnesium alloys. Its density is small, and Al can form a limited solid solution with magnesium, which can improve the casting performance while improving the alloy strength and hardness of die-casting magnesium alloys. In addition, Al can also produce aging strengthening through heat treatment, and at the same time, Al can widen the solidification zone, improve casting performance, especially improve the flow properties of die-casting magnesium alloys, and optimize the die-casting performance. According to the findings of the inventors of this case, the thermal conductivity of magnesium-aluminum alloys decreases with the increase of the number of solid-solution atoms. Therefore, it is necessary to control the content of aluminum elements to maintain good thermal conductivity, so as to avoid that Al elements will seriously reduce the thermal conductivity of magnesium alloys. thermal conductivity. In order to obtain a magnesium alloy with high thermal conductivity, too much Al cannot be added as an alloying element. When the mass percentage of Al increases to 9%, the thermal conductivity of the obtained magnesium alloy is reduced to one-third of that of pure magnesium . Therefore, in the die-casting magnesium alloy with high temperature and heat resistance of the present invention, the mass percentage of Al is controlled at 5-9 wt%.

RE: Rare earth element (RE) is an important alloying element, which has the functions of purifying the alloy solution, refining the alloy structure, improving the room temperature and high temperature mechanical properties of the alloy, etc. However, rare earth elements are expensive, and in order to control the cost of the alloy, their addition amount should not be too much. Rare earth elements have poor atomic diffusion ability, which can increase the recrystallization temperature of magnesium alloys and precipitate a stable second phase, thereby greatly improving the high temperature strength and heat resistance of magnesium alloys. In order to further optimize the performance of the alloy, the technical scheme of the present invention adopts the addition of rare earth elements with a mass percentage of 0.5-3wt% on the basis that the die-casting magnesium alloy of this case is a Mg-Al-Sr-Ca-Mn multi-element alloy, thereby improving Improve the fluidity of the alloy, reduce the hot cracking tendency of the alloy, and at the same time generate an appropriate amount of nano-scale strengthening phase in the alloy, so that the die-casting magnesium alloy with high temperature and heat resistance of the present invention has good die-casting performance and can take into account high thermal conductivity. Excellent mechanical properties of high strength and toughness, especially high temperature mechanical properties.

Sr: A kind of alkaline earth element, which works synergistically with Ca to produce grain refinement in magnesium, and can also inhibit the oxidation of molten magnesium. It also has a flame retardant effect, can increase the ignition temperature of the alloy melt, and can improve Alloy creep properties. This element can form a second phase with other elements in magnesium, especially to obtain an ordered single-layer nanostructure with a high strengthening effect, which is very effective in improving the mechanical properties of magnesium alloys. In order to control the amount and type of the second phase to maintain good thermal conductivity, the mass percentage of Sr in the die-casting magnesium alloy with high temperature and heat resistance of the present invention is controlled at Sr2-5wt%.

Mn: For the die-casting magnesium alloy described in the present invention, due to the presence of Fe element impurities in the process of adding alloy elements, and Fe element impurities are the main reason for the poor corrosion resistance of magnesium alloys, so the addition of Mn elements makes the precipitation of Fe-Mn Compounds can be used to control the content of iron impurities, thereby improving the corrosion resistance of magnesium alloys; at the same time, Mn elements in magnesium can increase heat resistance, refine the grain structure of magnesium alloys, and strengthen the alloys. In the die-casting magnesium alloy with high temperature and heat resistance performance of the present invention, after adding 0.1-0.5% of Mn element by mass percentage, the creep resistance of the die-casting magnesium alloy is significantly increased, and the heat resistance is improved.

Further, in the die-casting magnesium alloy with high temperature and heat resistance according to the present invention, its microstructure includes a magnesium matrix and a precipitated second phase, and the second phase includes an Al-Ca phase, an Al-Sr phase, an Al -RE phase and Al-Mn phase.

In the technical solution of the present invention, the thermal conductivity of the magnesium alloy is closely related to the quantity and type of the solid solution atoms and the second phase in the magnesium alloy. Therefore, in order to obtain a die-cast magnesium alloy with high temperature and heat resistance, To improve the thermal conductivity of die-casting magnesium alloys, this case properly controls the number of solid-solution atoms in the magnesium alloys, while ensuring that the size and quantity of the precipitated phases are not too large.

It should be noted that, in this case, the Al-Ca phase, Al-Sr phase, Al-RE phase and Al-Mn phase are represented by the elements included in the second phase, but this expression does not mean that the second phase The atomic ratio of the elements included is 1:1. Specifically, taking the Al-Ca phase as an example, the Al-Ca phase means that the second phase includes Al and Ca elements, but it does not mean that the atoms of Al and Ca are necessarily It is 1:1.

Further, in the die-casting magnesium alloy with high temperature and heat resistance according to the present invention, the Al-Ca phase includes at least Al ₂ Ca.

Further, in the die-casting magnesium alloy with high temperature and heat resistance according to the present invention, the RE element includes at least one of La, Gd and Y.

Further, in the die-casting magnesium alloy with high temperature and heat resistance according to the present invention, the content of Al element is 6-9 wt%.

Further, in the die-casting magnesium alloy with high temperature and heat resistance according to the present invention, Sr+Ca≥4wt%.

Further, in the die-casting magnesium alloy with high temperature and heat resistance according to the present invention, the RE element content is 0.9-3wt%.

Furthermore, in the die-casting magnesium alloy with high temperature heat resistance according to the present invention, its creep rate at 200°C and 70MPa is ≤9.1×10 ^-9 s ^-1 .

Further, in the die-casting magnesium alloy with high temperature and heat resistance according to the present invention, its thermal conductivity at room temperature is greater than 85 W/m·K.

Further, in the die-casting magnesium alloy with high temperature heat resistance according to the present invention, its yield strength at room temperature is ≥158 MPa, its tensile strength is ≥248 MPa, and its elongation is ≥4.5%.

Further, in the die-casting magnesium alloy with high temperature heat resistance according to the present invention, its yield strength at 150° C. is ≥138 MPa, its tensile strength is ≥200 MPa, and its elongation is ≥10%.

Further, in the die-casting magnesium alloy with high temperature heat resistance according to the present invention, its yield strength at 200° C. is ≥130 MPa, its tensile strength is ≥188 MPa, and its elongation is ≥13%.

Correspondingly, another object of the present invention is to provide a method for manufacturing the above-mentioned die-casting magnesium alloy with high temperature and heat resistance. The die-casting magnesium alloy obtained by the manufacturing method has high strength and good thermal conductivity.

In order to achieve the above object, the present invention proposes a method for manufacturing the above-mentioned die-casting magnesium alloy with high temperature and heat resistance, comprising steps:

(1) Put the crucible into a heating furnace to preheat, and then spray a release agent on the inner wall of the crucible;

(2) Put the pure magnesium ingot into the crucible and feed the mixed gas of _SF6 and _CO2 , raise the temperature to completely melt the pure magnesium, keep it for a period of time, then cool down to 750℃±5℃, add pure Al, Mg-Mn master alloy , Mg-RE master alloy, Mg-Ca master alloy and Mg-Sr master alloy;

(3) After it is completely melted, lower the temperature to 730°C±5°C, stir, remove slag and let stand;

(4) the crucible is cooled, and the magnesium alloy ingot is taken out;

(5) Melting the magnesium alloy ingot in a furnace of a die-casting machine and keeping it warm, and then injecting the molten magnesium alloy into a die-casting mold to obtain the high-strength, high-thermal-conductivity, heat-resistant die-casting magnesium alloy.

In the manufacturing method of the present invention, in order to obtain a die-casting magnesium alloy with high temperature and heat resistance, in addition to considering the reasonable design of alloy elements, the setting of process conditions, especially temperature conditions, is also very important for this case , the inventors of the present case found through experimental research that under the above temperature conditions, the required properties of the die-casting magnesium alloy can be obtained.

Further, in the manufacturing method of the present invention, in the step (1), the crucible is preheated to 300°C±5°C, and then the mold release agent is sprayed on the inner wall of the crucible.

Further, in the manufacturing method of the present invention, in the step (2), when the temperature of the crucible reaches 500°C±5°C, put the pure magnesium ingot into the crucible and inject the mixed gas of _SF6 and _CO2 , Then raise the temperature to 770°C±5°C, wait until the pure magnesium is completely melted, and keep it warm for 10-20min.

Further, in the manufacturing method of the present invention, in the step (3), stirring is carried out for 10-20 minutes, and standing for 25-35 minutes after removing slag.

Further, in the manufacturing method of the present invention, in the step (4), the crucible is cooled by an annular spray cooling system, and the crucible is completely immersed in water after the surface of the magnesium alloy is solidified, so that the magnesium alloy is cast The ingot is released from the crucible.

Further, in the manufacturing method of the present invention, in the step (5), the holding temperature is 710°C±5°C, and the molten magnesium alloy is injected into the die-casting mold at an injection speed of 55-65m/s Among them, the temperature of the die-casting mold is 250°C±5°C, and the casting pressure is 60±5MPa.

The die-casting magnesium alloy with high temperature and heat resistance of the present invention optimizes the alloy composition by adding alkaline earth metals and rare earth elements, and realizes that the die-casting magnesium alloy has excellent room temperature strength and high temperature creep resistance, and also has good Die casting performance and high thermal conductivity.

In addition, due to the addition of alkaline earth metals and rare earth elements to the die-casting magnesium alloy with high temperature and heat resistance of the present invention, a variety of high-melting point and small second phases are formed, which improves the strength and heat resistance of the alloy, and further improves the die-casting of the alloy. performance.

The die-casting magnesium alloy that can be obtained by the manufacturing method of the invention has high mechanical properties, good heat resistance, high thermal conductivity and good die-casting performance.

Detailed ways

The die-casting magnesium alloy with high temperature and heat resistance and its manufacturing method described in the present invention will be further explained and illustrated in conjunction with specific examples below. However, the explanation and illustration do not constitute an improper limitation to the technical solution of the present invention.

Embodiment 1-6 and comparative example 1

Table 1 lists the mass percentages of each chemical element in the die-casting magnesium alloys with high temperature heat resistance of Examples 1-6 and the comparative magnesium alloy of Comparative Example 1.

Table 1. (wt%, the balance is Mg and other unavoidable impurities)

The manufacturing method of the die-casting magnesium alloy with high temperature and heat resistance of Examples 1-6 and the comparison magnesium alloy of Comparative Example 1 is obtained by the following steps:

(1) Put the crucible into the heating furnace to preheat, preheat the crucible to 300°C±5°C, and then spray the release agent on the inner wall of the crucible;

(2) When the temperature of the crucible reaches 500°C±5°C, put the pure magnesium ingot into the crucible and inject the mixed gas of SF ₆ and CO ₂ , then raise the temperature to 770°C±5°C, wait until the pure magnesium is completely melted, and keep it warm for 10- 20min, then cool down to 750℃±5℃, add pure Al, Mg-Mn master alloy, Mg-RE master alloy, Mg-Ca master alloy and Mg-Sr master alloy;

(3) After it is completely melted, lower the temperature to 730°C±5°C, stir for 10-20min, and let stand for 25-35min after removing the slag;

(4) The crucible is cooled by an annular spray cooling system, and the crucible is completely submerged in water after the surface of the magnesium alloy is solidified, so that the magnesium alloy ingot is separated from the crucible for cooling, and finally the magnesium alloy ingot is taken out;

(5) Melt the magnesium alloy ingot in the furnace of the die-casting machine and keep it warm at a temperature of 710°C±5°C, then inject the melted magnesium alloy into the die-casting mold at an injection speed of 55-65m/s, and the die-casting mold The temperature is 250°C±5°C, and the casting pressure is 60±5MPa to obtain the high-strength, high-thermal conductivity and heat-resistant die-casting magnesium alloy.

Table 2 lists the specific process parameters of the manufacturing method of the die-casting magnesium alloys with high temperature heat resistance of Examples 1-6 and the comparison magnesium alloy of Comparative Example 1.

Table 2.

Performance tests were performed on the obtained die-casting magnesium alloys of each embodiment and the comparison magnesium alloy of Comparative Example 1, and the test results are listed in Table 3.

table 3.

Combining Tables 1 to 3, it can be seen that the die-casting magnesium alloys of each embodiment of this case are manufactured by adopting appropriate alloy composition design and suitable manufacturing method, therefore, the die-casting magnesium alloys of each embodiment have excellent mechanical properties and heat resistance. Good, high thermal conductivity, creep rate at 200°C and 70MPa ≤ 9.1×10 ^-9 s ^-1 , thermal conductivity at room temperature greater than 85W/m·K, and yield strength at room temperature ≥ 158MPa , Tensile strength ≥ 248MPa, elongation ≥ 4.5%.

At the same time, the performance of the die-casting magnesium alloys of each embodiment at high temperatures is far superior to that of Comparative Example 1. The yield strength of each embodiment at 150°C is ≥138MPa, the tensile strength is ≥200MPa, and the elongation is ≥10%. The yield strength at 200°C is ≥130MPa, the tensile strength is ≥188MPa, and the elongation is ≥13%.

It should be noted that the prior art part in the scope of protection of the present invention is not limited to the embodiments given in the application documents, and all prior art that does not contradict the solution of the present invention, including but not limited to the prior art Patent documents, prior publications, prior public use, etc., can all be included in the scope of protection of the present invention.

In addition, the combination of the technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific examples. All the technical features recorded in this case can be freely combined or combined in any way, unless conflict with each other.

It should also be noted that the above-mentioned embodiments are only specific embodiments of the present invention. Apparently, the present invention is not limited to the above embodiments, and the similar changes or deformations made thereupon can be directly obtained or easily thought of by those skilled in the art from the disclosed content of the present invention, and all should belong to the protection scope of the present invention .

Claims (18)

1. A die-casting magnesium alloy with high temperature heat resistance, characterized in that its chemical element mass percentage is: Ca 1～4wt%, Al 5～9wt%, RE 0.5～3wt%, Sr 2～5wt%, Mn 0.1～0.5wt%, The balance is Mg and other unavoidable impurities. 2. The die-casting magnesium alloy with high temperature and heat resistance as claimed in claim 1, characterized in that its microstructure includes a magnesium matrix and a precipitated second phase, and the second phase includes an Al-Ca phase, an Al-Sr phase, Al-RE phase and Al-Mn phase. 3. The die-casting magnesium alloy with high temperature and heat resistance according to claim 2, characterized in that, the Al-Ca phase includes at least Al ₂ Ca. 4. The die-casting magnesium alloy with high temperature and heat resistance according to claim 1, wherein the RE element comprises at least one of La, Gd and Y. 5. The die-casting magnesium alloy with high temperature and heat resistance according to claim 1, characterized in that the content of Al element is 6-9 wt%. 6. The die-casting magnesium alloy with high temperature and heat resistance as claimed in claim 1, characterized in that Sr+Ca≥4wt%. 7. The die-casting magnesium alloy with high temperature and heat resistance according to claim 1, characterized in that the RE element content is 0.9-3wt%. 8. The die-casting magnesium alloy with high temperature and heat resistance according to claim 1, characterized in that its creep rate at 200°C and 70 MPa is ≤9.1×10 ^-9 s ^-1 . 9. The die-casting magnesium alloy with high temperature heat resistance as claimed in claim 1, characterized in that its thermal conductivity at room temperature is greater than 85 W/m·K. 10. The die-casting magnesium alloy with high temperature and heat resistance as claimed in claim 1, characterized in that its yield strength at room temperature is ≥ 158 MPa, its tensile strength is ≥ 248 MPa, and its elongation is ≥ 4.5%. 11. The die-casting magnesium alloy with high temperature and heat resistance according to claim 1, characterized in that its yield strength at 150°C is ≥138 MPa, its tensile strength is ≥200 MPa, and its elongation is ≥10%. 12. The die-casting magnesium alloy with high temperature and heat resistance as claimed in claim 1, characterized in that its yield strength at 200°C is ≥130 MPa, its tensile strength is ≥188 MPa, and its elongation is ≥13%. 13. The method for manufacturing a die-casting magnesium alloy with high temperature heat resistance as claimed in any one of claims 1-12, characterized in that it comprises the steps of: (1) Put the crucible into a heating furnace to preheat, and then spray a release agent on the inner wall of the crucible; (2) Put the pure magnesium ingot into the crucible and feed the mixed gas of _SF6 and _CO2 , raise the temperature to completely melt the pure magnesium, keep it for a period of time, then cool down to 750℃±5℃, add pure Al, Mg-Mn master alloy , Mg-RE master alloy, Mg-Ca master alloy and Mg-Sr master alloy; (3) After complete melting, lower the temperature to 730°C±5°C, stir, remove slag and let stand; (4) the crucible is cooled, and the magnesium alloy ingot is taken out; (5) Melting the magnesium alloy ingot in a furnace of a die-casting machine and keeping it warm, and then injecting the molten magnesium alloy into a die-casting mold to obtain the high-strength, high-thermal-conductivity, heat-resistant die-casting magnesium alloy. 14. The manufacturing method according to claim 13, characterized in that, in the step (1), the crucible is preheated to 300°C±5°C, and then the mold release agent is sprayed on the inner wall of the crucible. 15. The manufacturing method according to claim 13, characterized in that, in the step (2), when the crucible temperature reaches 500°C ± 5°C, put the pure magnesium ingot into the crucible and feed SF ₆ and CO ₂ Mix the gas, then raise the temperature to 770°C±5°C, wait until the pure magnesium is completely melted, and keep it warm for 10-20min. 16. The manufacturing method according to claim 13, characterized in that, in the step (3), stirring for 10-20 min, and standing for 25-35 min after removing slag. 17. The manufacturing method according to claim 13, characterized in that, in the step (4), the crucible is cooled by an annular jet cooling system, and the crucible is completely submerged in water after the surface of the magnesium alloy is solidified, so that The magnesium alloy ingot is released from the crucible. 18. The manufacturing method as claimed in claim 13, characterized in that, in the step (5), the holding temperature is 710°C±5°C, and the molten magnesium alloy is injected at an injection speed of 55-65m/s into the die-casting mold, the temperature of the die-casting mold is 250°C±5°C, and the casting pressure is 60±5MPa.