Fine-grain cast Al-Mg-Zn-Cu alloy prepared based on rare earth composite refiner Al-Ti-B-RE and preparation method of fine-grain cast Al-Mg-Zn-Cu alloy

Abstract

The invention belongs to the technical field of aluminum alloys, and particularly discloses a fine grain casting Al-Mg-Zn-Cu alloy prepared based on a rare earth composite refiner Al-Ti-B-RE and a preparation method of the fine grain casting Al-Mg-Zn-Cu alloy. The rare earth composite refiner Al-Ti-B-RE comprises the following components in percentage by mass: 4.0 to 6.5 percent of Ti, 0.7 to 2.5 percent of B, 2.1 to 11.5 percent of RE and the balance of Al and impurity elements; the Al-Mg-Zn-Cu alloy is prepared from the following components in percentage by mass: 3.00 percent to 5.50 percent of Mg, 2.50 percent to 5.00 percent of Zn, 0.4 percent to 1.60 percent of Cu, 0.01 percent to 0.15 percent of Si, 0.01 percent to 0.15 percent of Fe, 0.02 percent to 0.20 percent of Ti, 0.004 percent to 0.15 percent of B, 0.005 percent to 0.35 percent of RE and the balance of Al and impurity elements. According to the invention, the composite refiner Al-Ti-B-RE is used for replacing a traditional Al-Ti-B refiner, and alloy component optimization is combined, so that effective refinement of Al-Mg-Zn-Cu alloy grains is realized.

Description

Technical Field

[0001] The invention relates to the technical field of aluminum alloys, and in particular to a fine-grained cast Al-Mg-Zn-Cu alloy prepared based on a rare earth composite refiner Al-Ti-B-RE and a preparation method thereof. Background Art

[0002] With the rapid development of new energy vehicles, the endurance of new energy vehicles is mainly proportional to the weight of the battery. Using lighter aluminum alloys instead of steel can greatly reduce the weight of the vehicle body, thereby increasing the weight of the battery of new energy vehicles. Therefore, the research on lightweight and high-strength cast aluminum alloys is of great significance. Aluminum alloys have the characteristics of low density, high specific strength, good fatigue resistance and excellent resistance to stress corrosion cracking. Therefore, as a lightweight material, they have great application potential in the aerospace and automotive industries. Al-Mg-Zn-Cu alloys have high strength and low density, so they have been widely used in the aviation and automotive industries.

[0003] Grain refinement is an important means to improve the toughness of aluminum alloys. It can improve strength and plasticity at the same time. In addition, grain refinement can also reduce the solidification defects of the alloy and improve the casting performance of the alloy. Therefore, the research on grain refinement has important practical significance. At present, the commonly used refiner is Al-5Ti-1B refiner, but Al-Ti-B refiner still has some problems, such as TiB2 particles are easy to aggregate and precipitate, which affects the grain refinement effect. Therefore, the development of more efficient refiners has important research and practical significance.

[0004] The present invention takes high-strength and toughness Al-Mg-Zn-Cu alloy as the object, and adds rare earth composite refiner Al-Ti-B-RE during the smelting process. After the high-strength and toughness Al-Mg-Zn-Cu alloy is refined by the rare earth composite refiner Al-Ti-B-RE, the grain size is significantly reduced, thereby developing a fine-grained high-strength and toughness casting Al-Mg-Zn-Cu alloy. Summary of the invention

[0005] The present invention provides a fine-grained cast Al-Mg-Zn-Cu alloy prepared based on a rare earth composite refiner Al-Ti-B-RE and a preparation method thereof, wherein the element content of the rare earth composite refiner Al-Ti-B-RE is optimized, and the optimal element content and the optimal addition amount in the alloy are designed.

[0006] In order to achieve the above object, the specific technical solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a rare earth composite refiner Al-Ti-B-RE and a fine-grained cast Al-Mg-Zn-Cu alloy prepared based on the rare earth composite refiner Al-Ti-B-RE. The rare earth composite refiner Al-Ti-B-RE comprises the following components by mass fraction: Ti 4.0-6.5%, B 0.7-2.5%, RE 2.1-11.5%, and the remainder is Al and impurity elements, and the total mass percentage of the impurity elements is less than or equal to 0.50%. The fine-grained cast Al-Mg-Zn-Cu alloy prepared based on the rare earth composite refiner Al-Ti-B-RE comprises the following components in mass fraction: Mg 3.00-5.50%, Zn 2.50-5.00%, Cu 0.4-1.60%, Si 0.01-0.15%, Fe 0.01-0.15%, Ti 0.02-0.20%, B 0.004-0.15%, RE 0.005-0.35%, and the remainder is Al and impurity elements, and the total mass percentage content of the impurity elements is less than or equal to 0.50%; wherein Ti, B and RE are added in the form of Al-Ti-B-RE.

[0008] Furthermore, the fine-grained cast Al-Mg-Zn-Cu alloy prepared based on the rare earth composite refiner Al-Ti-B-RE includes the following components in mass fraction: Mg 3.50-5.50%, Zn 2.50-4.50%, Cu 0.5-1.50%, Si 0.01-0.15%, Fe 0.01-0.12%, Ti 0.02-0.20%, B 0.004-0.15%, RE 0.005-0.35%, and the remainder is Al and impurity elements, and the total mass percentage of the impurity elements is less than or equal to 0.50%.

[0009] Furthermore, the rare earth composite refiner Al-Ti-B-RE comprises the following components in mass fractions: Ti 4.2-5.9%, B 0.8-2.0%, RE 2.1-10.5%, and the remainder is Al and impurity elements, and the total mass percentage of the impurity elements is less than or equal to 0.50%.

[0010] Furthermore, the rare earth composite refiner Al-Ti-B-RE is added to the Al-Mg-Zn-Cu alloy in an amount of 0.10 to 3.50%, preferably 0.10 to 3.00%.

[0011] Furthermore, the rare earth includes at least one of La, Ce, Y, Pm, Er, Nd, and Sm.

[0012] In a second aspect, the present invention provides a method for preparing the fine-grained cast Al-Mg-Zn-Cu alloy prepared based on the rare earth composite refiner Al-Ti-B-RE, comprising the following steps:

[0013] S1. Provide prefabricated raw materials of pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner, where M includes Cu, Si and Fe; the component content of the Al-Ti-B-RE composite refiner is: Ti 4.0-6.5%, B 0.7-2.5%, RE 2.1-11.5%, and the remainder is Al and impurity elements, and the total mass percentage of the impurity elements is less than or equal to 0.50%; the mass proportion of each element in the prefabricated raw material based on 100 parts by mass of all the above prefabricated raw materials is: Mg 3.00-5.50%, Zn 2.50-5.00%, Cu 0.4-1.60%, Si 0.01-0.15%, Fe 0.01-0.15%, Ti 0.02-0.20%, B 0.004-0.15%, RE 0.005-0.35%, the balance being Al and impurity elements, the sum of the mass percentages of the impurity elements being less than or equal to 0.50%;

[0014] S2. Before smelting, pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner are placed in a drying oven at a temperature of 150-250 ° C and dried for 1.5-2.5 hours for use;

[0015] S3. Adding a pure aluminum ingot into a smelting furnace, setting the furnace temperature to 720°C, and completely melting the aluminum ingot to obtain an aluminum melt;

[0016] S4. Al-M master alloy, pure Zn and pure Mg are sequentially added to the aluminum melt to obtain an aluminum alloy melt;

[0017] S5. Raise the temperature of the aluminum alloy melt to 730°C-740°C, add a special refining agent for aluminum-magnesium alloy to the bottom of the melt, stir the aluminum alloy melt for 5-10 minutes, and then let it stand for 5-10 minutes;

[0018] S6. After slagging, the temperature of the aluminum alloy melt is raised to 740°C-760°C, and an Al-Ti-B-RE rare earth composite refiner is added to the refined aluminum alloy melt, and the mixture is stirred for 3-5 minutes and then allowed to stand for 5-20 minutes;

[0019] S7. Then, the furnace temperature is lowered to 710°C-720°C, and the aluminum alloy melt is cast into a metal mold to obtain an Al-Mg-Zn-Cu alloy containing rare earth.

[0020] Furthermore, the refining agent includes at least one of CaF2, K2SiF6, KCl, MgCl2, TiO2, C2Cl6, and K2CO3.

[0021] The present invention has the following beneficial effects:

[0022] (1) RE can inhibit the aggregation of TiB2 particles in Al-Ti-B. TiB2 has the lowest nucleation energy and is a very effective nucleating agent for aluminum. Therefore, the addition of RE is beneficial to the refining effect of the composite rare earth refiner Al-Ti-B-RE.

[0023] (2) Adding a small amount of RE to the Al-Ti-B refiner can form Ti2Al 20 RE phase, Ti2Al 20 There are 8 matching relationships between RE phase and Al phase, so Ti2Al 20 RE has a better nucleation effect than TiAl3, so a small amount of rare earth can improve the refining effect of the composite rare earth refiner Al-Ti-B-RE. As the amount of RE added to the Al-Ti-B refiner increases, RE can form Ti2Al on the surface of the TiAl3 phase. 20 RE phase, Ti2Al 20 The TiAl3 phase surrounded by RE phase also has a better nucleation effect.

[0024] (3) The content of RE in the composite rare earth refiner Al-Ti-B-RE cannot be too high. When the content of RE in the composite rare earth refiner Al-Ti-B-RE is too high, the excess RE will react with Ti2Al 20 RE phase reaction generates a large amount of network Al4RE phase, thereby reducing the Ti2Al 20 RE phase nucleates particles, while the network Al4RE phase does not have a nucleation effect, so it is necessary to control the content of RE in the composite rare earth refiner Al-Ti-B-RE.

[0025] (4) The amount of rare earth composite refiner added to the Al-Mg-Zn-Cu alloy cannot be too high. Excessive rare earth composite refiner particles tend to settle at the bottom of the crucible, resulting in the formation of sludge and reducing the overall refining efficiency. Therefore, it is necessary to control the content of RE in the rare earth composite refiner Al-Ti-B-RE and the amount of rare earth composite refiner Al-Ti-B-RE added.

[0026] (5) The addition of an appropriate amount of RE has a certain purification ability, which can reduce the hydrogen content in the aluminum melt and reduce harmful metal inclusions and oxides in the aluminum alloy, thereby playing a role in degassing and purifying the melt.

[0027] (6) The solidification range of Al-Mg-Zn-Cu alloy is relatively large, so loose shrinkage cavities will appear during solidification during the smelting process. The addition of rare earth composite refiner Al-Ti-B-RE greatly refines the alloy grain size and the size of the second phase, ensuring the uniformity of the structure. In addition, grain refinement leads to a decrease in the porosity and pore size of the aluminum alloy, thereby improving the mechanical properties of the alloy. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the invention of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 This is the microstructure of the alloy anode coating in Example 1 of the present invention.

[0030] Figure 2 This is the microstructure of the alloy anode coating of Comparative Example 1 of the present invention.

[0031] Figure 3 This is the microstructure of the alloy anode coating of Comparative Example 2 of the present invention.

[0032] Figure 4 This is the scanning microstructure of the rare earth composite refiner in Example 1 of the present invention.

[0033] Figure 5 This is the scanning microstructure of the refiner in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0035] The inventors found that the TiB2 particles of the Al-Ti-B grain refiner tend to aggregate, which results in a decrease in its aluminum alloy refinement effect. The addition of rare earth RE can make the TiB2 particles more evenly distributed in the Al-Ti-B structure, thereby improving the non-uniform nucleation effect of the TiB2 particles. i2 Al 20 RE phase, and Ti2Al 20 The lattice matching degree between RE phase and α-Al is good, so Ti2Al 20RE phase can also serve as an effective nucleation site for α-Al. Based on this, the present invention combines the optimization of alloy components and contents, so that the grain size of Al-Mg-Zn-Cu alloy is significantly reduced after being refined by the composite refiner Al-Ti-B-RE, the second phase in the structure is significantly refined, and the number of oxidation and metal inclusions in the structure is also reduced, which is beneficial to the improvement of mechanical properties and processing properties. In addition, the addition form of the composite refiner Al-Ti-B-RE of the present invention is also necessary. When Ti, B, and RE elements are added to the Al-Mg-Zn-Cu alloy separately at the same content, the refining effect when added as the composite refiner Al-Ti-B-RE and the improvement level of the alloy mechanical properties and processing properties cannot be achieved.

[0036] The following describes the preparation method of Al-Mg-Zn-Cu aluminum alloy in combination with specific embodiments and comparative examples, and the mass percentage of the impurity elements in each embodiment and comparative example is ≤0.50%. Those skilled in the art will understand that the preparation method described in the present invention is only an example, and any other suitable preparation method is within the scope of the present invention.

[0037] Example 1

[0038] A prefabricated raw material of pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner is provided, wherein M comprises Cu, Si and Fe, and the mass proportion of each element in the prefabricated raw material based on 100 parts by mass of all the above prefabricated raw materials is as follows: Mg: 5.10%, Zn: 3.20%, Cu: 1.10%, Si: 0.06%, Fe: 0.09%, Ti: 0.027%, B: 0.005%, Ce: 0.028%, and the balance is Al and impurity elements.

[0039] The rare earth composite refiner Al-Ti-B-RE includes the following components in mass fractions: Ti: 5.6%, B: 1.1%, Ce: 5.8%, and the balance is Al and impurity elements.

[0040] Before smelting, pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner are placed in a drying oven at a temperature of 200° C. and dried for 2.0 hours for use.

[0041] Add pure aluminum ingots into a smelting furnace, set the furnace temperature to 720°C, and completely melt the aluminum ingots to obtain aluminum melt.

[0042] An aluminum alloy melt is obtained by sequentially adding aluminum-M master alloy, pure Zn and pure Mg into an aluminum melt.

[0043] The temperature of the aluminum alloy melt was raised to 740°C, a special refining agent for aluminum-magnesium alloy was added to the bottom of the melt, and the aluminum alloy melt was stirred for 8 minutes and then allowed to stand for 8 minutes.

[0044] After slagging, the temperature of the aluminum alloy melt was raised to 750°C, and 0.5% by mass of Al-Ti-B-RE rare earth composite refiner was added to the refined aluminum alloy melt, and the mixture was stirred for 5 minutes and then allowed to stand for 10 minutes.

[0045] Subsequently, the furnace temperature is lowered to 715°C, and the aluminum alloy melt is cast into a metal mold to obtain a fine-grained cast Al-Mg-Zn-Cu alloy containing rare earth. The addition of the rare earth composite refiner reduces the grain size of the alloy, and its grain size is smaller than that of the cast Al-Mg-Zn-Cu alloy refined by the Al-Ti-B refiner in Comparative Example 1.

[0046] Example 2

[0047] A prefabricated raw material of pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner is provided, wherein M comprises Cu, Si and Fe, and the mass proportion of each element in the prefabricated raw material based on 100 parts by mass of all the above prefabricated raw materials is as follows: Mg: 5.20%, Zn: 3.10%, Cu: 1.20%, Si: 0.12%, Fe: 0.08%, Ti: 0.033%, B: 0.007%, La: 0.034%, and the balance is Al and impurity elements.

[0048] The rare earth composite refiner Al-Ti-B-RE includes the following components in mass fractions: Ti: 5.7%, B: 1.2%, La: 5.7%, and the balance is Al and impurity elements.

[0049] Before smelting, pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner are placed in a drying oven at a temperature of 200° C. and dried for 2.0 hours for use.

[0050] Add pure aluminum ingots into a smelting furnace, set the furnace temperature to 720°C, and completely melt the aluminum ingots to obtain aluminum melt.

[0051] An aluminum alloy melt is obtained by sequentially adding aluminum-M master alloy, pure Zn and pure Mg into an aluminum melt.

[0052] The temperature of the aluminum alloy melt was raised to 730°C, a special refining agent for aluminum-magnesium alloy was added to the bottom of the melt, and the aluminum alloy melt was stirred for 8 minutes and then allowed to stand for 8 minutes.

[0053] After slagging, the temperature of the aluminum alloy melt was raised to 750°C, and an Al-Ti-B-RE rare earth composite refiner with a mass fraction of 0.6% was added to the refined aluminum alloy melt, and the mixture was stirred for 5 minutes and then allowed to stand for 10 minutes.

[0054] Subsequently, the furnace temperature is lowered to 715°C, and the aluminum alloy melt is cast into a metal mold to obtain a fine-grained cast Al-Mg-Zn-Cu alloy containing rare earth. The addition of the rare earth composite refiner reduces the grain size of the alloy, and its grain size is smaller than that of the cast Al-Mg-Zn-Cu alloy refined by the Al-Ti-B refiner in Comparative Example 1.

[0055] Example 3

[0056] A prefabricated raw material of pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner is provided, wherein M comprises Cu, Si and Fe, and the mass proportion of each element in the prefabricated raw material based on 100 parts by mass of all the above prefabricated raw materials is as follows: Mg: 5.10%, Zn: 3.30%, Cu: 1.00%, Si: 0.11%, Fe: 0.07%, Ti: 0.038%, B: 0.009%, Y: 0.018%, and the balance is Al and impurity elements.

[0057] The rare earth composite refiner Al-Ti-B-RE includes the following components in mass fractions: Ti: 6.0%, B: 1.6%, Y: 2.5%, and the balance is Al and impurity elements.

[0058] Before smelting, pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner are placed in a drying oven at a temperature of 200° C. and dried for 2.0 hours for use.

[0059] Add pure aluminum ingots into a smelting furnace, set the furnace temperature to 720°C, and completely melt the aluminum ingots to obtain aluminum melt.

[0060] An aluminum alloy melt is obtained by sequentially adding aluminum-M master alloy, pure Zn and pure Mg into an aluminum melt.

[0061] The temperature of the aluminum alloy melt was raised to 730°C, a special refining agent for aluminum-magnesium alloy was added to the bottom of the melt, and the aluminum alloy melt was stirred for 8 minutes and then allowed to stand for 8 minutes.

[0062] After slagging, the temperature of the aluminum alloy melt was raised to 750°C, and an Al-Ti-B-RE rare earth composite refiner with a mass fraction of 0.7% was added to the refined aluminum alloy melt, and the mixture was stirred for 5 minutes and then allowed to stand for 10 minutes.

[0063] Subsequently, the furnace temperature is lowered to 715°C, and the aluminum alloy melt is cast into a metal mold to obtain a fine-grained cast Al-Mg-Zn-Cu alloy containing rare earth. The addition of the rare earth composite refiner reduces the grain size of the alloy, and its grain size is smaller than that of the cast Al-Mg-Zn-Cu alloy refined by the Al-Ti-B refiner in Comparative Example 1.

[0064] Comparative Example 1

[0065] A prefabricated raw material of pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B refiner is provided, wherein M comprises Cu, Si and Fe, and the mass proportion of each element in the prefabricated raw material based on 100 parts by mass of all the above prefabricated raw materials is as follows: Mg: 5.20%, Zn: 3.10%, Cu: 1.10%, Si: 0.12%, Fe: 0.07%, Ti: 0.024%, B: 0.006%, and the balance is Al and impurity elements.

[0066] The refiner Al-Ti-B includes the following components in mass fractions: Ti: 5.1%, B: 1.2%, and the balance is Al and impurity elements.

[0067] Before smelting, pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B refiner are placed in a drying oven at a temperature of 200° C. and dried for 2.0 hours for use.

[0068] Add pure aluminum ingots into a smelting furnace, set the furnace temperature to 720°C, and completely melt the aluminum ingots to obtain aluminum melt.

[0069] An aluminum alloy melt is obtained by sequentially adding aluminum-M master alloy, pure Zn and pure Mg into an aluminum melt.

[0070] The temperature of the aluminum alloy melt was raised to 730°C, a special refining agent for aluminum-magnesium alloy was added to the bottom of the melt, and the aluminum alloy melt was stirred for 8 minutes and then allowed to stand for 8 minutes.

[0071] After slagging, the temperature of the aluminum alloy melt was raised to 750°C, and an Al-Ti-B refiner with a mass fraction of 0.5% was added to the refined aluminum alloy melt. The mixture was stirred for 5 minutes and then allowed to stand for 10 minutes.

[0072] The furnace temperature was then lowered to 715°C, and the aluminum alloy melt was cast into a metal mold to obtain a cast Al-Mg-Zn-Cu alloy.

[0073] Comparative Example 2

[0074] Prefabricated raw materials of pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy and refining agent are provided, wherein M includes Cu, Si and Fe, and the mass proportion of each element in the prefabricated raw materials based on 100 parts by mass of all the above prefabricated raw materials is as follows: Mg: 5.10%, Zn: 3.20%, Cu: 1.10%, Si: 0.12%, Fe: 0.07%, and the remainder is Al and impurity elements.

[0075] Before smelting, pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B refiner are placed in a drying oven at a temperature of 200° C. and dried for 2.0 hours for use.

[0076] Add pure aluminum ingots into a smelting furnace, set the furnace temperature to 720°C, and completely melt the aluminum ingots to obtain aluminum melt.

[0077] An aluminum alloy melt is obtained by sequentially adding aluminum-M master alloy, pure Zn and pure Mg into an aluminum melt.

[0078] The temperature of the aluminum alloy melt was raised to 730°C, a special refining agent for aluminum-magnesium alloy was added to the bottom of the melt, and the aluminum alloy melt was stirred for 8 minutes and then allowed to stand for 8 minutes.

[0079] The furnace temperature was then lowered to 715°C, and the aluminum alloy melt was cast into a metal mold to obtain a cast Al-Mg-Zn-Cu alloy.

[0080] Examples 1-3 can all obtain uniform fine-grained structures, and Example 1 has the smallest grain size and the best refinement effect. Comparing the microstructures of the alloy anode coatings of Example 1, Comparative Example 1 and Comparative Example 2, as shown in FIG. Figure 1-3 As shown, it can be seen that the degree of grain refinement in Example 1 is significantly better than that in Comparative Examples 1 and 2; comparing the scanning microstructures of the refiner in Example 1 and Comparative Example 1, as shown in FIG. Figure 4-5 As shown in the figure, it can be seen that compared with comparative example 1, the aggregation phenomenon of TiB2 particles in Example 1 is improved, and the core-shell TiAl3 formed by RE i2 Al 20 RE phase and network Al4RE phase are less, which are helpful to improve the grain refinement effect.

Claims

1. A fine-grained cast Al-Mg-Zn-Cu alloy prepared based on a rare earth composite refiner Al-Ti-B-RE, characterized in that: The rare earth composite refiner Al-Ti-B-RE comprises the following components by mass fraction: Ti 4.0-6.5%, B 0.7-2.5%, RE 2.1-11.5%, and the remainder is Al and impurity elements, and the total mass percentage of the impurity elements is less than or equal to 0.50%; The fine-grained cast Al-Mg-Zn-Cu alloy prepared based on the rare earth composite refiner Al-Ti-B-RE includes the following components in mass fraction: Mg 3.00-5.50%, Zn 2.50-5.00%, Cu 0.4-1.60%, Si 0.01-0.15%, Fe 0.01-0.15%, Ti 0.02-0.20%, B 0.004-0.15%, RE 0.005-0.35%, and the remainder is Al and impurity elements, and the total mass percentage content of the impurity elements is less than or equal to 0.50%.

2. The fine-grained cast Al-Mg-Zn-Cu alloy according to claim 1, characterized in that: It includes the following components in mass fraction: Mg 3.50-5.50%, Zn 2.50-4.50%, Cu 0.5-1.50%, Si0.01-0.15%, Fe 0.01-0.12%, Ti 0.02-0.20%, B 0.004-0.15%, RE 0.005-0.30%, and the balance is Al and impurity elements.

3. The fine-grained cast Al-Mg-Zn-Cu alloy according to claim 1, characterized in that: The rare earth composite refiner Al-Ti-B-RE comprises the following components by mass fraction: Ti 4.2-5.9%, B 0.8-2.0%, RE 2.1~10.5%, the balance is Al and impurity elements.

4. The fine-grained cast Al-Mg-Zn-Cu alloy according to claim 1 or 3, characterized in that: The rare earth composite refiner Al-Ti-B-RE is added in an amount of 0.10-3.50% in the Al-Mg-Zn-Cu alloy.

5. The fine-grained cast Al-Mg-Zn-Cu alloy according to claim 4, characterized in that: The rare earth composite refiner Al-Ti-B-RE is added in an amount of 0.10-3.00% in the Al-Mg-Zn-Cu alloy.

6. The fine-grained cast Al-Mg-Zn-Cu alloy according to claim 1, characterized in that: The rare earth includes at least one of La, Ce, Y, Pm, Er, Nd, and Sm.

7. A method for preparing fine-grained cast Al-Mg-Zn-Cu alloy based on rare earth composite refiner Al-Ti-B-RE, characterized in that: The following steps are involved: S1. Provide a prefabricated raw material of pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner, M includes Cu, Si, Fe, the component content of the Al-Ti-B-RE composite refiner and the mass proportion of each element in the prefabricated raw material based on 100 parts by mass of all the above prefabricated raw materials as shown in claim 1; S2. Before smelting, pure aluminum, pure magnesium, pure zinc, aluminum-M master alloy, refining agent and Al-Ti-B-RE composite refiner are placed in a drying oven at a temperature of 150-250 ° C and dried for 1.5-2.5 hours for use; S3. Adding a pure aluminum ingot into a smelting furnace, setting the furnace temperature to 720°C, and completely melting the aluminum ingot to obtain an aluminum melt; S4. Al-M master alloy, pure Zn and pure Mg are sequentially added to the aluminum melt to obtain an aluminum alloy melt; S5. Raise the temperature of the aluminum alloy melt to 730°C-740°C, add a special refining agent for aluminum-magnesium alloy to the bottom of the melt, stir the aluminum alloy melt for 5-10 minutes, and then let it stand for 5-10 minutes; S6. After slagging, the temperature of the aluminum alloy melt is raised to 740°C-760°C, and an Al-Ti-B-RE rare earth composite refiner is added to the refined aluminum alloy melt, and the mixture is stirred for 3-5 minutes and then allowed to stand for 5-20 minutes; S7. Then, the furnace temperature is lowered to 710°C-720°C, and the aluminum alloy melt is cast into a metal mold to obtain an Al-Mg-Zn-Cu alloy containing rare earth.

8. The preparation method according to claim 7, characterized in that: The refining agent includes at least one of CaF2, K2SiF6, KCl, MgCl2, TiO2, C2Cl6, and K2CO3.

9. A rare earth composite refiner Al-Ti-B-RE suitable for preparing fine-grained cast Al-Mg-Zn-Cu alloy, characterized in that: The invention comprises the following components by mass fraction: Ti 4.0-6.5%, B 0.7-2.5%, RE 2.1-11.5%, and the remainder is Al and impurity elements, wherein the total mass percentage of the impurity elements is less than or equal to 0.50%.

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