A low-expansion die-casting aluminum alloy and a preparation method thereof

Abstract

This invention provides a low-expansion die-cast aluminum alloy with the following composition and component content by mass percentage: Zn: 40-45%; Mg: 15-20%; Y: 0-0.5%; balance Al. This invention also discloses the smelting and preparation process, vibration casting process, and high-pressure die-casting process of the above-mentioned aluminum alloy. The specific steps are: high-temperature melting of the aluminum alloy, melt modification treatment, refining and slag removal, mechanical vibration treatment, and high-pressure die casting. The preparation process of the eutectic Al-Zn-Mg die-cast aluminum alloy of this invention is simple and easy to implement, the amount of alloying elements added is easy to control, the comprehensive performance effect is significant, the industrial production cost is low, and it has wide applicability. This invention uses element modification and mechanical vibration to refine the α-Al phase, achieving multi-dimensional optimization of the alloy's thermal expansion coefficient, thermal conductivity, and flow filling properties, obtaining a low-expansion eutectic Al-Zn-Mg die-cast aluminum alloy for electronic packaging materials.

Description

Technical Field

[0001] This invention relates to the field of die-cast aluminum alloy technology, and in particular to a low-expansion die-cast aluminum alloy and its preparation method. Background Technology

[0002] The high integration of electronic devices places stringent requirements on the thermal expansion and thermal conductivity properties of thermal management materials in the electronic packaging field. Localized temperature rises in devices can easily lead to failure due to thermal mismatch between the chip and the substrate. Therefore, thermal management materials must possess both a low coefficient of thermal expansion and high thermal conductivity. Currently, thermal management materials are mainly aluminum-based composites, but their complex manufacturing processes limit their large-scale commercial application. In-situ generation of a high-volume-fraction second phase is key to preparing low-expansion aluminum alloys. Designing an aluminum alloy that combines low expansion and excellent flow properties is a practical engineering problem that urgently needs to be solved. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of the aforementioned background technology by providing a die-cast aluminum alloy with both low expansion and excellent fluidity, as well as a preparation method thereof, in order to achieve a balanced optimization of the alloy's thermal expansion and flow filling performance.

[0004] To achieve the above objectives, the present invention provides a low-expansion die-cast aluminum alloy, the composition of which and the content of each component by mass percentage are as follows:

[0005] Zn: 40-45%;

[0006] Mg: 15-20%;

[0007] Y: 0–0.5%;

[0008] The margin is Al.

[0009] This invention also provides a method for preparing a low-expansion die-cast aluminum alloy, specifically including the following steps:

[0010] S1, melting aluminum alloy, using industrial pure Al, high-purity Zn and high-purity Mg as raw materials, the aluminum alloy is prepared and melted according to the target composition. After the alloy is completely melted, it is manually stirred for a period of time to make its composition uniform, and the aluminum alloy melt is obtained and kept at a constant temperature for a period of time.

[0011] S2, the modifier elements are melt-treated, the slag on the surface of the aluminum alloy melt is removed, Al-10Y master alloy is added and stirred thoroughly until the composition is uniform, and the modified aluminum alloy melt is obtained and kept at a constant temperature for a period of time.

[0012] S3, Melt Refining and Slag Removal: The melt after S2 treatment is refined and slag removed by adding refining agent and slag remover, and then left to stand and keep warm for a period of time.

[0013] S4, high pressure casting: The melt after S3 treatment is subjected to high pressure casting. The die casting mold is installed on the cold chamber die casting machine. The machine is turned on, and the melt after S3 treatment is taken out using a pouring bag. Mechanical vibration treatment is performed, and after vibration, high pressure casting is performed to obtain Al-Zn-Mg series aluminum alloy die castings.

[0014] Furthermore, the manual stirring time in S1 is 5-10 minutes, and the standing and heat preservation time is 10-30 minutes.

[0015] Furthermore, the melting temperature in S1 is 720–750°C.

[0016] Furthermore, the stirring time and the standing and heat preservation time in S2 are both 5 to 20 minutes.

[0017] Furthermore, the degradation treatment temperature in S2 is 700–720℃.

[0018] Furthermore, in S3, a refining agent and a slag remover are added using a nitrogen injection method. The commercial brands of the refining agent and the slag remover are YT-J-1 and YT-D-4, respectively. They are mixed evenly at a ratio of 1:1, with the total addition amount being 1% of the melt weight. The treatment temperature is controlled at 680–700℃, the treatment time is 2–10 min, and the standing time is 5–15 min.

[0019] Furthermore, the vibration frequency of the mechanical vibration in S4 is 50–200 Hz, and the vibration time is 30–60 s.

[0020] The above-described solution of the present invention has the following beneficial effects:

[0021] This invention achieves high volumetric Al6Zn composition through alloy composition design. 11 Mg 11 The precipitated phase has a relatively low coefficient of expansion, which significantly improves the expansion performance of pure aluminum; and the Mg and Zn contents are located near the Al-Zn-Mg ternary eutectic point, ensuring the flow and filling performance of the alloy, thus achieving dual optimization of the expansion and flow properties of the Al-Zn-Mg alloy.

[0022] In this invention, by adding the modifying element Y and mechanically vibrating the melt, the synergistic effect of modification and external vibration field is brought into play, and the microstructure of the alloy is refined simultaneously, so as to achieve multiple balances of alloy expansion, thermal conductivity and flow properties.

[0023] The alloy system involved in this invention is easy to prepare and operate, the amount added is easy to control, it adopts composite modification treatment, there is no pollutant discharge, the operation process is simple, the alloy system elements used are inexpensive, and the overall performance is excellent.

[0024] Other beneficial effects of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0025] Figure 1 This is the low-magnification as-cast optical microstructure of the Al-43Zn-15Mg alloy in the comparative example of this invention;

[0026] Figure 2 This is the high-magnification as-cast optical microstructure of the Al-43Zn-15Mg alloy in the comparative example of this invention;

[0027] Figure 3 The coefficient of thermal expansion of the as-cast Al-43Zn-15Mg alloy in the comparative example of this invention;

[0028] Figure 4 The low-magnification as-cast optical microstructure of the Al-43Zn-15Mg alloy in Example 1 of this invention;

[0029] Figure 5 This is the high-magnification as-cast optical microstructure of the Al-43Zn-15Mg alloy in Example 1 of this invention;

[0030] Figure 6 The coefficient of thermal expansion of the as-cast Al-43Zn-15Mg alloy in Example 1 of this invention;

[0031] Figure 7 The low-magnification as-cast optical microstructure of the Al-43Zn-15Mg alloy in Example 2 of this invention;

[0032] Figure 8 This is the high-magnification as-cast optical microstructure of the Al-43Zn-15Mg alloy in Example 2 of the present invention;

[0033] Figure 9 The coefficient of thermal expansion is given by the as-cast Al-43Zn-15Mg alloy in Example 2 of this invention. Detailed Implementation

[0034] To make the technical problems, solutions, and advantages of this invention clearer, a detailed description will be provided below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Furthermore, the technical features involved in the different embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0035] Embodiments of the present invention provide a eutectic Al-Zn-Mg alloy die-casting aluminum alloy with low expansion and excellent fluidity, the composition of which and the content of each component by mass percentage are as follows:

[0036] Zn: 40-45%;

[0037] Mg: 15-20%;

[0038] Y: 0–0.5%;

[0039] The margin is Al.

[0040] The smelting process for this low-expansion Al-Zn-Mg series die-cast aluminum alloy specifically includes the following steps:

[0041] S1, molten aluminum alloy. The added elements are Zn and Mg. The specific steps are as follows:

[0042] The alloy composition is designed according to performance requirements. Industrially pure Al, high-purity Zn, and high-purity Mg are used as raw materials to prepare and melt the aluminum alloy according to the target composition. After the alloy is completely melted, it is manually stirred for 5-10 minutes to make its composition uniform, and the aluminum alloy melt is obtained and held at a constant temperature for 10-30 minutes.

[0043] S2, the modified element undergoes melt treatment. The added element is Y. The specific steps are as follows:

[0044] Remove the slag from the surface of the aluminum alloy melt, add Al-10Y master alloy to the aluminum alloy melt and stir thoroughly until the composition is uniform to obtain the modified aluminum alloy melt, and let it stand for 5 to 20 minutes.

[0045] S3, melt refining and slag removal.

[0046] The melt after S2 treatment is then refined and deslag-removed. Refining and deslag-removing agents are added, and the mixture is allowed to stand for 5–15 minutes.

[0047] S4, formed by high pressure casting.

[0048] The S3-treated melt is subjected to high-pressure casting. The die-casting mold is installed on the cold chamber die-casting machine, the machine is turned on, and the S3-treated melt is taken out using a casting bag. It is then subjected to mechanical vibration treatment at a frequency of 50–200 Hz for 30–60 s. After vibration, it is subjected to high-pressure casting to obtain Al-Zn-Mg series aluminum alloy die castings.

[0049] In a preferred embodiment, the melting temperature in S1 and S2 is 720–750°C; the stirring time in S2 is 5–20 min; the deterioration treatment temperature in S2 is 700–720°C, and the stirring time is 2–20 min.

[0050] In a preferred embodiment, the refining and slag removal in S4 is carried out by adding refining agent and slag remover by nitrogen injection. The commercial brands of refining agent and slag remover are YT-J-1 and YT-D-4, respectively. They are mixed evenly in a 1:1 ratio, with the total addition amount being 1% of the melt weight. The treatment temperature is controlled at 680-700°C, and the treatment time is 2-10 minutes.

[0051] Among them, Mg and Zn are often used as strengthening elements. When the Zn content is 40-45% and the Mg content is 15-20%, the alloy composition is close to the ternary eutectic composition, and the flow properties of the alloy are well guaranteed. Secondly, according to the simulation results of thermodynamic phase precipitation behavior, in the actual solidification process, a high volume fraction of Al6Zn is generated in situ. 11 Mg 11 Regarding the ternary eutectic point of this alloy, the second phase mass fraction of the Al-43.26wt.%Zn-15.49wt.%Mg alloy is as high as 65%, and Al6Zn 11 Mg 11 The phase exhibits low expansion characteristics, which can effectively reduce the thermal expansion properties of pure aluminum, making it meet the requirements of electronic packaging for thermal management materials.

[0052] Elements like γ can act as reactive elements. During actual solidification, γ tends to accumulate in areas with high Zn and Mg atom concentrations, reducing its migration rate in the melt and thus inhibiting Al6Zn. 11 Mg 11 Phase growth. This modification effect can weaken the influence of the second phase on thermally conductive charge carriers, increase the mean free path of electron and phonon motion, improve the thermal conductivity and mechanical properties of the alloy, and have a relatively small impact on the alloy's expansion and flow filling properties.

[0053] Compared with existing technologies, this embodiment improves the morphology and distribution of the alloy's microstructure by selecting Y as a modifying element through compositional design. This alloy exhibits better flow and filling properties, meeting the requirements of large-scale industrial die casting production. Furthermore, the high volume fraction of Al6Zn... 11 Mg 11 This phase can significantly reduce the expansion properties of industrial pure aluminum, meeting the requirements of high-performance electronic packaging for thermal management materials with good expansion properties.

[0054] To more intuitively illustrate the effects of the present invention, the preparation process, microstructure, and performance characteristics of the high-strength, high-thermal-conductivity aluminum alloy of the present invention are described in conjunction with the accompanying drawings, comparative examples, and embodiments.

[0055] Comparative Example 1:

[0056] The materials used in this comparative example are industrial pure aluminum, high-purity Zn, and high-purity Mg, with the following composition by mass percentage: Zn: 43%, Mg: 15%, and the remainder being Al. The alloy preparation process and parameters are as follows: The weighed industrial pure aluminum is melted at a melting temperature of 720℃. After complete melting, high-purity Zn and high-purity Mg are added, and the mixture is manually stirred for 5 minutes to ensure uniform composition. The mixture is then allowed to stand for 10 minutes. Commercially available YT-J-1 refining agent and YT-D-4 slag remover are mixed in a 1:1 ratio, allowed to stand for 10 minutes, cooled to 680℃, and the slag is removed. The mixture is then high-pressure die-cast using a cold chamber die-casting machine. The melt is poured into a die-casting mold preheated to 300℃, and after cooling, samples are taken from the die-cast parts for analysis.

[0057] To characterize the various properties of the alloy, the coefficient of thermal expansion was tested according to GB / T 4339-2008, the thermal conductivity was tested according to GB / T 22588-2008, and the flow and filling performance was tested using the spiral method. The length of the spiral cavity was 1200 mm, the width of the flow channel cross-section was 10 mm, the height was 4 mm, and there was a vent hole with a diameter of 10 mm at the end of the flow channel. The mass of the alloy in each test was 500 g, the melt pouring temperature was 750℃, and the preheating temperature of the flow test mold was 200℃. Under the same conditions, the same alloy composition was tested 5 times, and the arithmetic mean of the flow length of the 5 tests was taken as the index for evaluating the flow and filling performance of the alloy.

[0058] Figure 1 and Figure 2 The images show the low-magnification and high-magnification as-cast optical microstructures of the comparative Al-43Zn-15Mg alloy, respectively. Combining the microstructure with the Al-Zn-Mg ternary phase diagram, it can be seen that the as-cast microstructure of the Al-43Zn-15Mg alloy mainly consists of α-Al and Al6Zn. 11 Mg 11 Phase composition: the primary α-Al phase exhibits a typical dendritic structure, with secondary dendrite arm spacing of approximately 20 μm. High-magnification microstructure clearly shows the eutectic Al6Zn. 11 Mg 11 The phase is distributed in a fibrous form on the α-Al matrix and exhibits a clear orientation relationship, that is, fibrous Al6Zn inside different grains. 11 Mg 11 The orientation of the phases is different.

[0059] The coefficient of thermal expansion of the as-cast Al-43Zn-15Mg alloy is as follows: Figure 3 As shown, when the ambient temperature is 100℃, the coefficient of thermal expansion of this alloy is 16.2 × 10⁻⁶. -6 K -1 When the ambient temperature is 150℃, the coefficient of thermal expansion of this alloy is 18.4×10⁻⁶. - 6 K-1 When the ambient temperature is 200℃, the coefficient of thermal expansion of this alloy is 19.9 × 10⁻⁶. -6 K -1 The alloy exhibits a thermal conductivity of 75 W / (m·K). Furthermore, it demonstrates excellent flow and filling properties; flowability tests show that the Al-43Zn-15Mg alloy has a flow length of 650 mm, superior to the commercially available ADC12 alloy (600 mm). This comparative example, through reasonable compositional design, yields an aluminum alloy for electronic packaging that combines low expansion and excellent flow and filling properties, but its thermal conductivity has room for further improvement.

[0060] Example 1: High-pressure casting of Al-43Zn-15Mg alloy after vibration

[0061] The materials used in this embodiment are industrial pure aluminum, high-purity Zn, and high-purity Mg. The composition by mass percentage is: Zn: 43%, Mg: 15%, and the remainder is Al.

[0062] The alloy preparation process in this embodiment is basically the same as that in Comparative Example 1, except that the alloy melt is subjected to mechanical vibration, and the melting process parameters are also different, as detailed below:

[0063] Weighed industrial pure aluminum was melted at a melting temperature of 730℃. After complete melting, high-purity Zn and high-purity Mg were added, and the mixture was manually stirred for 10 minutes to ensure homogeneity. The mixture was then allowed to stand for 20 minutes. Commercially available YT-J-1 refining agent and YT-D-4 slag remover were mixed in a 1:1 ratio and allowed to stand for 20 minutes. After cooling to 690℃, the slag was skimmed off. The molten metal was collected using a casting bag and quickly placed on a mechanical vibration table for vibration treatment at a frequency of 200Hz for 30 seconds. Then, it was die-cast using a cold chamber die-casting machine under high pressure. The molten metal was poured into a die-casting mold preheated to 300℃. After cooling, samples were taken from the die-casting for analysis.

[0064] Figure 4 and Figure 5 The images show the low-magnification and high-magnification as-cast optical microstructures of the Al-43Zn-15Mg alloy after mechanical vibration in the examples. After mechanical vibration, the grains of the Al-43Zn-15Mg alloy were significantly refined, with a secondary dendrite arm spacing of approximately 10 μm. The refinement mechanism is mainly attributed to the cavitation effect of mechanical vibration, which breaks up the dendritic α-Al phase, increases the nucleation sites for α-Al, and thus refines the α-Al phase. Secondly, mechanical vibration reduces casting defects such as shrinkage cavities and porosity in die castings, improving the strength and stiffness of the alloy. Furthermore, after mechanical vibration, the fibrous Al6Zn... 11 Mg 11 The phases still show a clear orientational relationship.

[0065] The coefficient of thermal expansion of the as-cast Al-43Zn-15Mg alloy after mechanical vibration is as follows: Figure 6 As shown, when the ambient temperature is 100℃, the coefficient of thermal expansion of this alloy is 16.8 × 10⁻⁶. -6 K -1 When the ambient temperature is 150℃, the coefficient of thermal expansion of this alloy is 19.2×10⁻⁶. -6 K -1 When the ambient temperature is 200℃, the coefficient of thermal expansion of this alloy is 20.5×10⁻⁶. -6 K -1 After mechanical vibration, the coefficient of thermal expansion of the Al-43Zn-15Mg alloy in Example 1 was comparable to that of Comparative Example 1. The thermal conductivity of the alloy after mechanical vibration was 89 W / (m·K), an improvement of 19% compared to Comparative Example 1. This comparative example achieved improved thermal conductivity of the eutectic Al-Zn-Mg alloy by mechanically vibrating the melt, without significantly changing the alloy's coefficient of thermal expansion, thus obtaining an aluminum alloy for electronic packaging that combines thermal conductivity, low expansion, and excellent flow and filling properties.

[0066] Example 2: High-pressure casting of Al-43Zn-15Mg-0.3Y alloy after vibration

[0067] The materials used in this embodiment are industrial pure aluminum, high-purity Zn, high-purity Mg, and Al-10Y master alloy. The composition by mass percentage is: Zn: 43%, Mg: 15%, Y: 0.3%, and the remainder is Al.

[0068] The alloy preparation process in this embodiment is basically the same as that in Example 1, except that the alloy melt is subjected to Y element modification treatment, and the smelting process parameters are also different, as detailed below:

[0069] Weighed industrial pure aluminum was melted at 750℃. After complete melting, high-purity Zn and high-purity Mg were added, and the mixture was manually stirred for 20 minutes to ensure homogeneity. The mixture was then allowed to stand for 10 minutes. Floating slag was removed from the surface of the aluminum alloy melt. Al-10Y master alloy was added and stirred thoroughly until homogeneous, resulting in a modified aluminum alloy melt. This melt was then allowed to stand for 10 minutes. Commercially available YT-J-1 refining agent and YT-D-4 slag remover were mixed in a 1:1 ratio and allowed to stand for 10 minutes. After cooling to 700℃, the slag was removed. The melt was collected using a casting bag and quickly placed on a mechanical vibration table for vibration treatment at a frequency of 50Hz for 60 seconds. High-pressure die casting was then performed using a cold chamber die casting machine. The melt was poured into a die casting mold preheated to 300℃. After cooling, samples were taken from the die casting for analysis.

[0070] Figure 7 and Figure 8The images show the high-magnification and low-magnification as-cast optical microstructures of the Al-43Zn-15Mg-0.3Y alloy after mechanical vibration in the examples. After the synergistic treatment of mechanical vibration and Y modification, the grains of the Al-43Zn-15Mg-0.3Y alloy were further refined, with a secondary dendrite arm spacing of approximately 8 μm. The refinement mechanism is mainly attributed to the synergistic effect of the cavitation effect of mechanical vibration and the adsorption and poisoning of Y elements. Mechanical vibration breaks up the dendritic α-Al phase, increasing the nucleation sites for α-Al. The highly reactive Y element adsorbs on the surface of the α-Al phase, reducing its surface energy and inhibiting its growth, thereby synergistically refining the α-Al phase. Furthermore, after mechanical vibration, the fibrous Al6Zn... 11 Mg 11 The phases still show a clear orientational relationship.

[0071] The coefficient of thermal expansion of the as-cast Al-43Zn-15Mg-0.3Y alloy after mechanical vibration is as follows: Figure 9 As shown, when the ambient temperature is 100℃, the coefficient of thermal expansion of this alloy is 15.9 × 10⁻⁶. -6 K -1 When the ambient temperature is 150℃, the coefficient of thermal expansion of this alloy is 18.2×10⁻⁶. -6 K -1 When the ambient temperature is 200℃, the coefficient of thermal expansion of this alloy is 19.8×10⁻⁶. -6 K -1 After mechanical vibration and elemental modification, the coefficient of thermal expansion of the Al-43Zn-15Mg-0.3Y alloy in this embodiment is lower than that of Comparative Example 1. The thermal conductivity of the alloy after mechanical vibration is 101 W / (m·K), an increase of 35% compared to Comparative Example 1. Furthermore, this alloy exhibits excellent flow and filling properties; flowability tests show that the flow length of the Al-43Zn-15Mg alloy is 675 mm, an increase of 4% compared to Comparative Example 1. This comparative example achieves improved thermal conductivity of the eutectic Al-Zn-Mg alloy and a slight decrease in the coefficient of thermal expansion by combining mechanical vibration and elemental modification treatment of the melt, resulting in an aluminum alloy for electronic packaging that combines high thermal conductivity, low expansion, and excellent flow and filling properties.

[0072] This embodiment improves the thermal expansion, thermal conductivity, and flow filling properties of the alloy simultaneously through a melt composite treatment involving mechanical vibration and elemental modification, thereby achieving the preparation of a low-expansion and high-thermal-conductivity die-cast aluminum alloy that can meet the performance requirements of thermal management materials in the electronic packaging field.

[0073] In summary, all the aforementioned embodiments exhibit both low expansion and excellent flow and filling properties. In particular, Embodiment 2 demonstrates the best overall performance, achieving a simultaneous improvement in the coefficient of thermal expansion, thermal conductivity, and flow and filling properties of the eutectic Al-43Zn-15Mg die-cast aluminum alloy compared to the ordinary as-cast Al-43Zn-15Mg alloy in Comparative Example 1. This meets the performance requirements of thermal management materials in the electronic packaging field, as detailed in Table 1.

[0074] Table 1. Properties of each alloy in the comparative examples and embodiments.

[0075]

[0076] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A low-expansion die cast aluminum alloy characterized by comprising, in mass %, The composition and the content of each component by mass percentage are as follows: Zn: 40~45%; Mg: 15~20%; Y：0.3%； The balance is Al; The low-expansion die-cast aluminum alloy is formed by high-pressure casting after vibration.

2. A method of producing a low-expansion die-cast aluminum alloy according to claim 1, characterized by, Includes the following steps: S1, melting aluminum alloy, using industrial pure Al, high-purity Zn and high-purity Mg as raw materials, the aluminum alloy is prepared and melted according to the target composition. After the alloy is completely melted, it is manually stirred for a period of time to make its composition uniform, and the aluminum alloy melt is obtained and kept at a constant temperature for a period of time. S2, the modifier elements are melt-treated, the slag on the surface of the aluminum alloy melt is removed, Al-10Y master alloy is added and stirred thoroughly until the composition is uniform, and the modified aluminum alloy melt is obtained and kept at a constant temperature for a period of time. S3, Melt Refining and Slag Removal: The melt after S2 treatment is refined and slag removed by adding refining agent and slag remover, and then left to stand and keep warm for a period of time. S4, high pressure casting: The melt after S3 treatment is subjected to high pressure casting. The die casting mold is installed on the cold chamber die casting machine. The machine is turned on, and the melt after S3 treatment is taken out using a pouring bag. Mechanical vibration treatment is performed, and after vibration, high pressure casting is performed to obtain Al-Zn-Mg series aluminum alloy die castings.

3. The method for preparing a low-expansion die-cast aluminum alloy according to claim 2, characterized in that, The manual stirring time in S1 is 5-10 minutes, and the standing and heat preservation time is 10-30 minutes.

4. The method for preparing a low-expansion die-cast aluminum alloy according to claim 2, characterized in that, The melting temperature in S1 is 720~750℃.

5. The method for preparing a low-expansion die-cast aluminum alloy according to claim 2, characterized in that, The stirring time and the standing and heat preservation time in S2 are both 5~20 minutes.

6. The method for preparing a low-expansion die-cast aluminum alloy according to claim 2, characterized in that, The degradation treatment temperature in S2 is 700~720℃.

7. The method for preparing a low-expansion die-cast aluminum alloy according to claim 2, characterized in that, In S3, refining agent and slag remover are added by nitrogen injection. The commercial brands of refining agent and slag remover are YT-J-1 and YT-D-4, respectively. They are mixed evenly at a ratio of 1:

1. The total addition amount is 1% of the melt weight. The treatment temperature is controlled at 680~700℃, the treatment time is 2~10min, and the standing time is 5~15min.

8. The method for preparing a low-expansion die-cast aluminum alloy according to claim 2, characterized in that, The mechanical vibration frequency in S4 is 50~200Hz, and the vibration time is 30~60s.

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