Low-cost die-casting magnesium alloy with high toughness and excellent casting fluidity and preparation method of low-cost die-casting magnesium alloy

Abstract

The invention discloses a low-cost die-casting magnesium alloy with high toughness and excellent casting fluidity and a preparation method thereof, and belongs to the technical field of die-casting magnesium alloy part preparation and performance modification thereof. The magnesium alloy comprises the following components in percentage by weight: 6.2 to 10.7 wt.% of Al; % of Si: 1.2 to 2.7 wt.%; % of Sr: 0.3 to 1.5 wt.%; % of Mg, 0.1-1.2 wt.% of Er, an NbB2 ceramic refined phase introduced through an Al-Nb-B refiner, and the balance of Mg. The preparation method comprises the steps that element raw materials for producing the die-casting magnesium alloy are prepared, all intermediate alloys are smelted and melted completely, then the prepared NbB2 ceramic refined phase is added into melt, and uniform distribution of the ceramic refined phase is achieved along with mechanical stirring and ultrasonic direct / indirect vibration treatment; and then a simple die-casting process is used for production. The magnesium alloy has excellent die-casting mold-filling fluidity and room-temperature tensile mechanical properties, is high in surface quality and high in size precision, can obtain excellent mechanical properties without heat treatment, and is suitable for large-scale industrial production and application.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of die-cast magnesium alloy components and their performance modification, and particularly relates to a die-cast magnesium alloy with low cost, high strength and toughness, and excellent casting fluidity, and a preparation method thereof. Background Art

[0002] With the progress of industrial technology, especially in the fields of automobiles, aerospace, electronic equipment, etc., lightweight design has become an important means to improve energy efficiency, reduce carbon emissions and enhance performance. As a lightweight structural material, magnesium alloy, with its low density (about 2 / 3 of aluminum alloy and 1 / 4 of steel) and high specific strength, especially in the automotive industry, the application of magnesium alloy can effectively reduce the weight of the whole vehicle, thereby improving fuel economy and reducing carbon dioxide emissions, which conforms to the current global development trend of green environmental protection and energy conservation and emission reduction.

[0003] With the booming development of new energy vehicles, the lightweight trend of automobile bodies is also developing towards a diversified material design system, and the lightweight coefficient of the whole vehicle will gradually decrease. Therefore, a large number of automobile component manufacturers have gradually shifted the research and development focus to the development of magnesium alloy automotive die-castings. Only a more diversified material system design can maximize lightweight. Due to the excellent material properties of magnesium alloy, its application in vehicle bodies and closed components has increased rapidly. Especially in new energy vehicles, many components are used, such as seat frames and front frames, engine cylinder head covers and wheels, etc., all of which have the characteristics of bearing low loads. Especially with the rapid development of new energy vehicles, die-cast magnesium alloy components for automobiles are gradually developing towards thinner walls, more complex shapes and higher strength and toughness, which requires die-cast magnesium alloy to have higher strength and toughness and excellent casting performance. Excellent casting fluidity can not only effectively ensure the filling property and surface quality of castings, but also ensure the precise forming of complex shapes and thin-wall structures, thereby reducing the defect incidence rate and improving the mechanical properties and reliability of castings. Especially during the die-casting process, the fluidity of magnesium alloy directly affects the casting production efficiency and the yield rate. Therefore, developing a magnesium alloy with both high strength and toughness and excellent casting fluidity is the key to improving its performance and application range. It not only helps to improve the stability of the production process, but also can reduce the production cost, thereby enhancing its market competitiveness.

[0004] Patent 200710306663.6 discloses "yttrium-rich rare earth high-strength heat-resistant creep-resistant die-cast magnesium alloy". In addition to low-cost elements A1 and Zn, the composition of this magnesium alloy also contains heavy rare earth elements such as Y, Er, Ho, etc. Although this magnesium alloy has both excellent strength and plasticity, it requires high-cost rare earth elements such as Y, Er, Ho, etc., and its die-casting fluidity is not outstanding. When preparing large thin-wall die-castings, it cannot be perfectly filled, resulting in defects, which is not conducive to commercial applications.

[0005] Based on the literature data survey, the die-cast magnesium alloy materials reported in the prior art cannot simultaneously take into account high strength and toughness, excellent casting fluidity and processing costs. Therefore, it is urgent to achieve significant cost reduction without sacrificing performance through the reasonable design and optimization of alloy elements, thereby promoting the popularization and application of magnesium alloys in the mid- and low-end markets. In summary, the development of a low-cost die-cast magnesium alloy with high strength and toughness and excellent casting fluidity will not only help improve the application performance of magnesium alloys in the automotive, aerospace and electronics industries, but also promote the green development and technological progress of related industries. Summary of the invention

[0006] In view of the problem that the die-cast magnesium alloy materials reported in the prior art cannot simultaneously take into account high strength and toughness, excellent casting fluidity and processing costs, the purpose of the present invention is to develop a low-cost, high strength and toughness and excellent casting fluidity die-cast magnesium alloy through reasonable design and optimization of alloy elements and advanced melt control technology, so as to improve the application performance of magnesium alloys in the automotive, aerospace and electronics industries.

[0007] The purpose of the present invention is achieved by the following technical solution: a low-cost, high-toughness and excellent casting fluidity die-casting magnesium alloy, the magnesium alloy comprises, by weight percentage: Al: 6.2-10.7 wt.% aluminum; 1.2-2.7 wt.% Si; 0.3-1.5 wt.% Sr; 0.1-1.2 wt.% Er; 0.01-0.05 wt.% NbB 2 Ceramic refined phase; Mg is the balance.

[0008] Further, the magnesium alloy comprises the following components calculated in weight percentage: Al: 7.0-9.5 wt.%; Si: 1.5-2.2 wt.%; Sr: 0.5-1.0 wt.%; Er: 0.3-0.8 wt.% and the mass percentage of NbB introduced by the Al-Nb-B refiner is 0.02-0.04 wt.% 2 Ceramic refined phase, Mg is the balance.

[0009] Further, the magnesium alloy comprises the following components calculated in weight percentage: Al: 8.0-9.0 wt.%; Si: 1.7-2.0 wt.%; Sr: 0.8-1.0 wt.%; Er: 0.5-0.7 wt.% and 0.03 wt.% NbB introduced by Al-Nb-B refiner 2 Ceramic refined phase, Mg is the balance.

[0010] Preferably, the magnesium alloy comprises the following components calculated in weight percentage: Al: 8.0-9.0 wt.%; Si: 1.7-2.0 wt.%; Sr: 0.9 wt.%; Er: 0.6 wt.% and the mass percentage of NbB introduced by the Al-Nb-B refiner is 0.03 wt.% 2 Ceramic refined phase, Mg is the balance.

[0011] The preparation method of the above alloy comprises the following steps:

[0012] (1) Raw materials for preparing die-cast magnesium alloys: pure magnesium as magnesium source; pure aluminum as aluminum source; Al-10Si master alloy as Si source; Al-5Sr master alloy as Sr source; Al-5Er master alloy as Er source; 2 The Al-Nb-B refining agent of the ceramic refining phase acts as a carrier for adding the NbB2 ceramic refining phase;

[0013] (2) Grind away the oxide scale on the surface of pure magnesium, pure aluminum, Al-10Si master alloy, Al-5Sr master alloy, and Al-5Er master alloy, and perform drying treatment;

[0014] (3) Weigh the raw materials according to the composition ratio; place the pure magnesium in an induction melting furnace at 710℃~760℃ for melting, and introduce SF 6 and CO 2 The mixed gas is used as the protective gas. After all the pure magnesium is melted, the Al-10Si master alloy, Al-5Sr master alloy, and Al-5Er master alloy prepared in proportion are then added. After all are melted, argon is introduced for refining and exhaust. (4) After the melt is stable, the Al-Nb-B refiner prepared as required is added to introduce NbB 2 The ceramic refining phase plays a role in grain refinement; after the Al-Nb-B refining agent is completely melted, mechanical stirring and ultrasonic vibration treatment are applied;

[0015] (5) After the temperature of the magnesium alloy melt drops to 700-720°C, it is kept stable and warm; then, the magnesium alloy melt is poured into the pressure chamber of the die-casting machine for die-casting, the initial temperature of the mold is 200-250°C, the initial temperature of the magnesium alloy solution in the pressure chamber is 700-720°C, and the injection speed is 1-3m / s; after die-casting and demolding, the die-cast magnesium alloy is obtained.

[0016] The ultrasonic vibration treatment is carried out in two ways. One is to directly apply the ultrasonic probe to the melt to promote the NbB 2 The other method is to apply the ultrasonic probe to the crucible wall and use the high-frequency vibration of ultrasound to indirectly disturb the melt and promote the NbB 2Disperse distribution of ceramic refined phase.

[0017] The Al-Nb-B refiner is selected from Al-Nb-B prepared by solid-liquid reaction technology. 3 Nb and NbB 2 The raw materials are Nb metal powder and KBF. 4 Fluoride salt reacts at 850°C to generate an Al-Nb-B refiner containing a refined particle phase, the composition of which is Al-(1~6)wt.% Nb-(0.3~1)wt.%B in mass percentage.

[0018] The die-casting process uses a CSV900 vacuum vertical die-casting machine for die-casting experimental preparation. The basic information of the die-casting is as follows: the punch diameter is 50 mm, the pressure chamber diameter is 50 mm, the length is 200 mm, the pressure chamber filling degree is 50%; the first-stage injection stroke is 300 mm, and the second-stage injection stroke is 102 mm.

[0019] Specifically, the preparation method includes the following steps:

[0020] (1) Grinding off the surface oxide scale of the selected pure magnesium, pure aluminum, Al-10Si master alloy, Al-5Sr master alloy, and Al-5Er master alloy, and performing drying treatment;

[0021] (2) Weigh the raw materials according to the specific composition ratio; first, weigh a certain amount of pure magnesium and place it in an induction melting furnace at 710℃~760℃ for melting, while introducing SF 6 and CO 2 The mixed gas is used as the protective gas. After all the pure magnesium is melted, the temperature is measured to stabilize the temperature of the magnesium melt at 750°C. Then, Al-10Si master alloy, Al-5Sr master alloy, and Al-5Er master alloy prepared in proportion are added. After all are melted, argon gas is introduced for refining and exhaust. Then, the oxidized residue on the surface of the melt is removed with a slag scoop.

[0022] (4) After the melt is stabilized, add the Al-Nb-B refiner prepared according to the requirements to introduce NbB 2 The ceramic refining phase plays a role in grain refinement. After the Al-Nb-B refiner is completely melted, mechanical stirring and ultrasonic vibration treatment are applied. The speed of the stirring paddle during mechanical stirring is 100r~500r / min. There are two ways for the ultrasonic device to act on the melt. One is to directly apply the ultrasonic probe to the melt to promote NbB by ultrasonic cavitation effect and disturbance. 2 The other method is to apply the ultrasonic probe to the crucible wall and use the high-frequency vibration of ultrasound to indirectly disturb the melt and promote the NbB 2The dispersion distribution of ceramic refined phase; the power of ultrasonic treatment is 500W~2000W; after mechanical stirring and ultrasonic action, the residue on the surface of the melt is removed;

[0023] (5) After the temperature of the magnesium alloy melt drops to 710°C, keep it stable for 5 minutes; then, pour the magnesium alloy melt into the pressure chamber of the die-casting machine for die-casting. The die-casting process is as follows: In this study, a CSV900 vacuum vertical die-casting machine was selected for die-casting experimental preparation. The basic information of die-casting is as follows: the punch diameter is 50mm, the pressure chamber diameter is 50mm, the length is 200mm, and the pressure chamber filling degree is 50%. The first-stage injection stroke is 300mm, and the second-stage injection stroke is 102mm. The initial temperature of the mold is 250°C, the initial temperature of the magnesium alloy solution in the pressure chamber is 710°C, and the injection speed is 2.5m / s; after die-casting and demolding, the die-cast magnesium alloy is obtained.

[0024] Compared with the existing magnesium alloys, the present invention has the following obvious substantial characteristics and advantages:

[0025] The composition and preparation method of the Mg-Al-Si-Sr series die-casting magnesium alloy provided by the present invention adopts adjustable Al element, Si element and Sr, Er modifying elements, etc., and is assisted by ceramic particle refinement and ultrasonic vibration melt treatment combined with die-casting process. Aluminum element and silicon element are beneficial to the improvement of casting fluidity of magnesium alloy; the addition of strontium element and erbium element can modify the coarse second phase in magnesium alloy, and promote the refinement and homogenization of the second phase; NbB 2 The addition of ceramic refining phase can play a role in refining the grains of magnesium alloys; ultrasonic treatment can not only promote the uniform distribution of refined particle phases, but also further refine the grains of magnesium alloys. As a complete technical solution, the synergistic effect of each component of this alloy can not only achieve the improvement of casting fluidity, but also the enhancement of the as-cast room temperature tensile properties. The Mg-Al-Si-Sr die-cast magnesium alloy prepared by this process has good surface quality, high dimensional accuracy, good casting fluidity, and excellent room temperature mechanical properties. Its comprehensive performance is significantly better than commercial die-cast magnesium alloys such as AM60 and AE44. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a die-casting model used to test the casting fluidity of the die-cast magnesium alloy in Example 1 of the present invention.

[0027] Figure 2 This is a flow diagram of the die-casting of the Mg-Al-Si-Sr alloy in Examples 1 to 3 of the present invention.

[0028] Figure 3 Mg-6.5Al-1.5Si-0.5Sr-0.3Er-0.02NbB in Example 1 of the present invention 2Metallographic structure diagram.

[0029] Figure 4 Mg-7.5Al-2.0Si-1.0Sr-0.5Er-0.03NbB in Example 2 of the present invention 2 Metallographic structure diagram.

[0030] Figure 5 Mg-10.0Al-2.5Si-1.2Sr-1.0Er-0.05NbB in Example 3 of the present invention 2 Metallographic structure diagram.

[0031] Figure 6 Mg-10.0Al-2.5Si-1.2Sr-1.0Er-0.05NbB in Example 3 of the present invention 2 Scanning electron microscope microstructure.

[0032] Figure 7 1 is the room temperature tensile curve of the Mg-Al-Si-Sr alloy in Examples 1 to 3 of the present invention. DETAILED DESCRIPTION

[0033] The present invention is further described in detail below with reference to examples, but the embodiments of the present invention are not limited thereto:

[0034] Example 1

[0035] The purpose of the present invention is achieved by the following technical solution: a low-cost, high-toughness and excellent casting fluidity die-casting magnesium alloy, characterized in that the die-casting magnesium alloy composition with excellent comprehensive performance includes, by weight percentage: 6.2~10.7 wt.% Al; 1.2~2.7 wt.% Si; 0.3~1.5 wt.% Sr; 0.1~1.2 wt.% Er; 0.01~0.05 wt.% NbB 2 Ceramic refined phase; Mg is the balance.

[0036] The preparation method comprises the following steps:

[0037] (1) Grinding off the surface oxide scale of the selected pure magnesium, pure aluminum, Al-10Si master alloy, Al-5Sr master alloy, and Al-5Er master alloy, and performing drying treatment;

[0038] (2) Weigh the raw materials according to the specific composition ratio; first, weigh a certain amount of pure magnesium and place it in an induction melting furnace at 710℃~760℃ for melting, while introducing SF 6 and CO 2The mixed gas is used as the protective gas. After all the pure magnesium is melted, the temperature is measured to stabilize the temperature of the magnesium melt at 750°C. Then, Al-10Si master alloy, Al-5Sr master alloy, and Al-5Er master alloy prepared in proportion are added. After all are melted, argon gas is introduced for refining and exhaust. Then, the oxidized residue on the surface of the melt is removed with a slag scoop.

[0039] (4) After the melt is stabilized, add the Al-Nb-B refiner prepared according to the requirements to introduce NbB 2 The ceramic refining phase plays a role in grain refinement. After the Al-Nb-B refiner is completely melted, mechanical stirring and ultrasonic vibration treatment are applied for 30 minutes. The speed of the stirring paddle during mechanical stirring is 1000r / min. There are two ways for the ultrasonic device to act on the melt. One is to directly apply the ultrasonic probe to the melt to promote NbB by ultrasonic cavitation effect and disturbance. 2 The other method is to apply the ultrasonic probe to the crucible wall and use the high-frequency vibration of ultrasound to indirectly disturb the melt and promote the NbB 2 Dispersion distribution of ceramic refined phase; after mechanical stirring and ultrasonic action, the residue on the melt surface is removed;

[0040] (5) After the temperature of the magnesium alloy melt drops to 710°C, it is kept stable for 5 minutes. Subsequently, the magnesium alloy melt is poured into the pressure chamber of the die-casting machine for die-casting. The die used for die-casting is a standard concentric spiral die. The purpose is to examine the die-casting fluidity of the magnesium alloy. The structural model is shown in the figure. Figure 1 As shown. The die-casting process is as follows: In this study, a CSV900 vacuum vertical die-casting machine was selected for die-casting experimental preparation. The basic information of die-casting: punch diameter 50mm, pressure chamber diameter 50mm, length 200mm, pressure chamber fullness 50%. The first-stage injection stroke is 300mm, and the second-stage injection stroke is 102mm. The initial temperature of the mold is 250℃, the initial temperature of the magnesium alloy solution in the pressure chamber is 710℃, and the injection speed is 2.5m / s; after die-casting and demolding, the composition is Mg-6.5Al-1.5Si-0.5Sr-0.3Er-0.02NbB 2 The die-casting flow length of the magnesium alloy exceeds 1100mm. Figure 2 Mg-6.5Al-1.5Si-0.5Sr-0.3Er-0.02NbB 2 The metallographic microstructure of die-cast magnesium alloy is as follows: Figure 3 shown.

[0041] Example 2

[0042] (1) Pure magnesium, pure aluminum, Al-10Si master alloy, Al-5Sr master alloy, and Al-5Er master alloy are smelted according to the smelting method in Example 1;

[0043] (2) Pure aluminum, Al-10Si master alloy, Al-5Sr master alloy, Al-5Er master alloy and Al-Nb-B refiner are weighed according to the characteristic ratio, and after being prepared, they are placed in an induction melting furnace at 710° C. to 760° C. for melting. The process is referred to Example 1;

[0044] (3) After the melt is stabilized, mechanical stirring and ultrasonic vibration treatment are applied for 20 minutes. The speed of the stirring paddle during mechanical stirring is 500 r / min. There are two ways to act on the melt with the ultrasonic device. One is to directly act on the melt with the ultrasonic probe; the other is to act on the crucible wall with the ultrasonic probe to promote NbB 2 Dispersion distribution of ceramic refined phase; after mechanical stirring and ultrasonic action, the residue on the melt surface is removed;

[0045] (5) After the temperature of the magnesium alloy melt drops to 710°C, it is kept stable for 5 minutes. Subsequently, the magnesium alloy melt is poured into the pressure chamber of the die casting machine for die casting. The initial temperature of the mold is 250°C, the initial temperature of the magnesium alloy solution in the pressure chamber is 710°C, and the injection speed is 2.5 m / s. After die casting and demolding, the composition of the obtained material is Mg-7.5Al-2.0Si-1.0Sr-0.5Er-0.03NbB 2 The die-casting flow length of the magnesium alloy exceeds 1100mm. Figure 2 Mg-7.5Al-2.0Si-1.0Sr-0.5Er-0.03NbB 2 The metallographic microstructure of die-cast magnesium alloy is as follows: Figure 4 shown.

[0046] Example 3

[0047] (1) Pure magnesium, pure aluminum, Al-10Si master alloy, Al-5Sr master alloy, and Al-5Er master alloy are smelted according to the smelting method in Example 1;

[0048] (2) Pure aluminum, Al-10Si master alloy, Al-5Sr master alloy, Al-5Er master alloy and Al-Nb-B refiner are weighed according to the characteristic ratio, and after being prepared, they are placed in an induction melting furnace at 710° C. to 760° C. for melting. The process is referred to Example 1;

[0049] (3) After the melt is stabilized, mechanical stirring and ultrasonic vibration treatment are applied for 10 minutes. The speed of the stirring paddle during mechanical stirring is 300 r / min. There are two ways to act on the melt with the ultrasonic device. One is to directly act on the melt with the ultrasonic probe; the other is to act on the crucible wall with the ultrasonic probe to promote NbB 2 Dispersion distribution of ceramic refined phase; after mechanical stirring and ultrasonic action, the residue on the melt surface is removed;

[0050] (5) After the temperature of the magnesium alloy melt drops to 710°C, it is kept stable for 5 minutes. Subsequently, the magnesium alloy melt is poured into the pressure chamber of the die casting machine for die casting. The initial temperature of the mold is 250°C, the initial temperature of the magnesium alloy solution in the pressure chamber is 710°C, and the injection speed is 2.5 m / s. After die casting and demolding, the composition of the obtained material is Mg-10.0Al-2.5Si-1.2Sr-1.0Er-0.05NbB 2 The die-casting flow length of the magnesium alloy exceeds 1100 mm. Figure 2 Mg-10.0Al-2.5Si-1.2Sr-1.0Er-0.05NbB 2 The microstructure of die-cast magnesium alloys is Figure 5 and 6 shown.

[0051] The room temperature tensile curves of the die-cast magnesium alloys obtained in the three embodiments of this patent are as follows: Figure 5 The specific mechanical properties data are shown in Table 1. It can be seen that the cast tensile strength of the three alloys exceeds 280MPa, the yield strength also exceeds 190MPa, and the plasticity exceeds 10%.

[0052] Table 1 Summary of room temperature tensile mechanical properties of die-cast magnesium alloys in three embodiments of this patent

[0053]

[0054] From the results in all the above examples, it can be seen that the prepared Mg-Al-Si-Sr series die-casting magnesium alloy not only has excellent die-casting filling ability, but also this series of alloys also has excellent room temperature mechanical properties, and is suitable for large-scale applications in lightweight automotive parts.

[0055] The above describes the embodiments of the present invention in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments. Any other modifications, modifications, substitutions, combinations, and simplifications made without violating the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A low-cost die-cast magnesium alloy with high strength and excellent casting fluidity, characterized in that: The magnesium alloy includes the following components calculated in weight percentage: Al: 6.2~10.7 wt.%; Si: 1.2~2.7 wt.%; Sr: 0.3~1.5 wt.%; Er: 0.1~1.2 wt.% and NbB2 ceramic refining phase introduced by Al-Nb-B refining agent: 0.01~0.05 wt.%, and Mg is the balance.

2. The low-cost die-cast magnesium alloy with high toughness and excellent casting fluidity according to claim 1, characterized in that: The magnesium alloy includes the following components calculated in weight percentage: Al: 7.0-9.5 wt.%; Si: 1.5-2.2 wt.%; Sr: 0.5-1.0 wt.%; Er: 0.3-0.8 wt.% and 0.02-0.04 wt.% NbB2 ceramic refining phase introduced by Al-Nb-B refining agent, with Mg as the balance.

3. The low-cost die-cast magnesium alloy with high toughness and excellent casting fluidity according to claim 1, characterized in that: The magnesium alloy includes the following components calculated in weight percentage: Al: 8.0-9.0 wt.%; Si: 1.7-2.0 wt.%; Sr: 0.8-1.0 wt.%; Er: 0.5-0.7 wt.% and 0.03 wt.% NbB2 ceramic refining phase introduced by Al-Nb-B refining agent, with Mg as the balance.

4. The low-cost die-cast magnesium alloy with high toughness and excellent casting fluidity according to claim 1, characterized in that: The magnesium alloy includes the following components calculated in weight percentage: Al: 8.0-9.0 wt.%; Si: 1.7-2.0 wt.%; Sr: 0.9 wt.%; Er: 0.6 wt.% and 0.03 wt.% NbB2 ceramic refining phase introduced by Al-Nb-B refining agent, and Mg is the balance.

5. The method for preparing a die-cast magnesium alloy with low cost and high toughness and excellent casting fluidity according to any one of claims 1 to 4, characterized in that: The preparation method and steps are as follows: (1) Raw materials for preparing die-cast magnesium alloys are selected: pure magnesium as magnesium source; pure aluminum as aluminum source; Al-10Si master alloy as Si source; Al-5Sr master alloy as Sr source; Al-5Er master alloy as Er source; Al-Nb-B refiner containing NbB2 ceramic refinement phase as carrier for adding NbB2 ceramic refinement phase; (2) Grind away the oxide scale on the surface of pure magnesium, pure aluminum, Al-10Si master alloy, Al-5Sr master alloy, and Al-5Er master alloy, and perform drying treatment; (3) Weigh the raw materials according to the composition ratio; place the pure magnesium in an induction melting furnace at 710℃~760℃ for melting, and introduce SF6 and CO2 mixed gas as protective gas. After all the pure magnesium is melted, add Al-10Si master alloy, Al-5Sr master alloy and Al-5Er master alloy prepared in proportion. After all are melted, introduce argon gas for refining and exhaust; (4) After the melt is stable, add Al-Nb-B refiner prepared according to the requirements to introduce NbB2 ceramic refinement phase to play a role in grain refinement; after the Al-Nb-B refiner is completely melted, apply mechanical stirring and ultrasonic vibration treatment; (5) After the temperature of the magnesium alloy melt is reduced to 700-720℃, stabilize and keep the temperature; Subsequently, the magnesium alloy melt is poured into the pressure chamber of the die-casting machine for die-casting, the initial temperature of the mold is 200-250°C, the initial temperature of the magnesium alloy solution in the pressure chamber is 700-720°C, and the injection speed is 1-3m / s; after die-casting and demolding, the die-cast magnesium alloy is obtained.

6. The method for preparing a die-cast magnesium alloy with high strength and toughness and excellent casting fluidity at low cost according to claim 5, characterized in that: The ultrasonic vibration treatment adopts two methods: one is to apply the ultrasonic probe directly to the melt to promote the dispersion distribution of the NbB2 ceramic refined phase by ultrasonic cavitation effect and disturbance; the other is to apply the ultrasonic probe to the crucible wall to indirectly disturb the melt by the high-frequency vibration of ultrasound to promote the dispersion distribution of the NbB2 ceramic refined phase.

7. The method for preparing a die-cast magnesium alloy with high strength and toughness and excellent casting fluidity at low cost according to claim 5, characterized in that: The Al-Nb-B refiner is selected from a master alloy containing Al3Nb and NbB2 particle phases prepared by solid-liquid reaction technology. The reaction raw materials are Nb metal powder and KBF4 fluoride salt, which react at 850°C to generate an Al-Nb-B refiner containing a refined particle phase, and the composition is Al-(1-6) wt.% Nb-(0.3-1) wt.% B by mass percentage.

8. The method for preparing a die-cast magnesium alloy with high strength and toughness and excellent casting fluidity at low cost according to claim 5, characterized in that: The die-casting process uses a CSV900 vacuum vertical die-casting machine for die-casting experimental preparation. The basic information of the die-casting is as follows: the punch diameter is 50 mm, the pressure chamber diameter is 50 mm, the length is 200 mm, the pressure chamber filling degree is 50%; the first-stage injection stroke is 300 mm, and the second-stage injection stroke is 102 mm.

Images