A method for the production of a layered composite material

Abstract

This invention relates to a method for preparing a layered composite material, comprising the following steps: S1, pretreating the surfaces of an aluminum plate and a dissimilar metal plate; S2, coating the surfaces of the aluminum plate and the dissimilar metal plate treated in S1 with a paste-like aluminum flux, wherein the melting point and minimum activity temperature of the aluminum flux are both lower than the diffusion reaction temperature of the aluminum plate and the dissimilar metal plate; S3, alternately stacking the aluminum plate and the dissimilar metal plate treated in S2 to form a stack, wherein aluminum flux is present between adjacent aluminum plates and dissimilar metal plates, and subjecting the stack to vacuum heating surface treatment; S4, subjecting the stack treated in S3 to further vacuum hot-press diffusion treatment, thereby obtaining an aluminum flux-assisted metal / aluminide layered composite material. This method significantly improves the diffusion reaction rate and reduces the probability of Kirkendal voids; it avoids the "centerline" defect that easily forms in the original technology, which leads to incomplete conversion of the aluminum layer into aluminides, allowing the layered composite material to be used under conditions higher than the melting point of aluminum.

Description

Technical Field

[0001] This invention belongs to the field of layered composite material technology, and specifically relates to a method for preparing layered composite materials. Background Technology

[0002] Metal / aluminide layered composite materials are biomimetic composite materials designed based on shell structures. They are mainly produced by cyclically stacking aluminum foil and dissimilar metal foils, and allowing element interdiffusion under thermodynamic conditions to cause a phase transformation in the aluminum layer, resulting in a high-strength and hard intermetallic compound layer. This achieves a balance between strength and toughness between the layers, and has advantages such as controllable density, adjustable strength and toughness, and good dynamic filtering performance. As a result, they are mainly used in fields such as armor and aerospace where material strength and filtering performance need to be matched.

[0003] Currently, the preparation of metal / aluminide layered composites mainly employs the vacuum hot-press diffusion method. High temperatures activate atoms, enabling them to diffuse over long distances, while high pressure enhances the diffusion behavior along the diffusion direction. The hot-press diffusion method offers advantages such as simple operation, high controllability, and low pollution, making it suitable for mass production. However, in current research on iron-based metal / aluminide layered composites, the "centerline" is a major problem limiting its application and performance improvement. The "centerline" arises primarily from the presence of oxide films at the dissimilar metal interface. During interdiffusion of interfacial atoms, the breached oxide film migrates with the main diffusing atoms, forming an oxide impurity defect at the center of the original aluminum layer, originating from the oxide films on both sides. The presence of the "centerline" leads to stress concentration points within the layered composite, which easily become crack initiations under stress, causing unstable propagation and a sharp decrease in the strength of the layered composite, resulting in the loss of its original strength characteristics.

[0004] To address the aforementioned problem, improving surface treatment methods or shortening the interval between oxide film removal and vacuuming is insufficient to effectively improve the "centerline" defect. This is mainly due to the high chemical reactivity of aluminum, which is extremely prone to oxidation. During the hot pressing process, a new oxide film forms on the aluminum surface after the oxide film has been removed. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing a layered composite material that solves the problem of residual oxide film at the interface and ensures strength properties, in light of the current state of the prior art.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: a method for preparing layered composite materials, characterized by comprising the following steps in sequence:

[0007] S1. Perform surface pretreatment on aluminum plates and dissimilar metal plates that are different from aluminum plates;

[0008] S2. Coat the surfaces of the aluminum plate and the dissimilar metal plate treated in S1 with a paste-like aluminum flux. The melting point and minimum activity temperature of the aluminum flux are both lower than the diffusion reaction temperature of the aluminum plate and the dissimilar metal plate.

[0009] S3. The aluminum plate and dissimilar metal plate treated in S2 are stacked alternately to form a stack body, wherein there is aluminum flux between adjacent aluminum plates and dissimilar metal plates, and the stack body is subjected to vacuum heating surface treatment.

[0010] S4. The stack body after S3 is further subjected to vacuum hot-press diffusion treatment to obtain aluminum flux-assisted metal / aluminide layered composite material.

[0011] The coating thickness in step S2 above is between 0.6 mm and 1 mm. The choice of thickness depends on the solubility of the alkali and acid solutions during step S1. The higher the solubility of the alkali and acid solutions, the thinner the coated aluminum flux paste will be.

[0012] Preferably, in step S1, the surface pretreatment is as follows: the surfaces of the aluminum plate and the dissimilar metal plate are sanded with sandpaper, then the aluminum plate and the dissimilar metal plate are subjected to alkaline washing and acid washing under ultrasonic vibration, and finally vacuum drying treatment is performed.

[0013] To thoroughly clean the surfaces of aluminum plates and dissimilar metal plates, alkaline washing with 6–10 mol / L NaOH solution and acid washing with 8–12% HCl solution are performed before vacuum drying. After acid and alkaline washing, ultrasonic cleaning with anhydrous ethanol is then carried out.

[0014] Preferably, the amplitude of the ultrasonic wave is 4-12 μm, the acid washing and alkaline washing time is 3 min, and the ultrasonic cleaning time is 45 s.

[0015] Preferably, in step S2, deionized water or distilled water is mixed with chloride-based or fluoride-based aluminum flux powder at a mass ratio of 1:1 to 1:3, and then stirred to obtain a paste-like aluminum flux. The selection of the aforementioned mass ratio depends on the number of layers in the stack and the thickness of the dissimilar metal plate. The more layers and the thicker the dissimilar metal plate, the denser the paste-like aluminum flux needs to be to resist pressure without deformation.

[0016] Preferably, in step S2, the first surface of the aluminum plate and the first surface of the dissimilar metal plate are coated with aluminum flux; in step S3, the first surface of the aluminum plate is in contact with the uncoated surface of the dissimilar metal plate, and the uncoated surface of the aluminum plate is in contact with the first surface of the dissimilar metal plate. Alternatively, the outer surface of the aluminum plate or the dissimilar metal plate may be coated with aluminum flux.

[0017] Preferably, in step S3, the first and last layers of the stack are both dissimilar metal plates, which are high-entropy alloy plates, pure iron plates, or 304 stainless steel plates. Using dissimilar metal plates for the first and last layers ensures that each aluminum plate is under the same thermo-pressure diffusion conditions, allowing the aluminum layer to transform into an aluminide layer within the same timeframe. Furthermore, since the aluminide layer is a hard and brittle layer, prone to cracking under service conditions, using dissimilar tough metals at the first and last layers avoids exposing the aluminide layer, reducing the likelihood of aluminide layer cracking and failure during use.

[0018] Preferably, in step S3, the stacked body is placed in a vacuum hot press furnace for vacuum heat treatment. The vacuum heat treatment process is as follows: the vacuum hot press furnace is evacuated to a vacuum degree ≤1×10⁻⁶. -1 The temperature is increased at a rate of 2–6 °C / min, with a pressure of 0 MPa during the heating phase. After reaching 450–600 °C, the temperature is held for 5–20 minutes, during which the pressure is gradually increased to 1–3 MPa. No pressure is applied during the heating phase to prevent the flux from being squeezed out due to interlayer pressure before it takes effect. When the temperature reaches the holding phase, i.e., when the flux melts and takes effect, the pressure is gradually increased to allow the decomposed oxide film fragments to be smoothly squeezed out of the interlayer, ensuring a reduction in interlayer oxide inclusions and avoiding the occurrence of "centerline" defects.

[0019] Preferably, in step S4, the vacuum hot-press diffusion process is as follows: continue evacuating until the vacuum degree is ≤1×10⁻⁶. - 2 The aluminum alloy is heated to 645–655°C at a rate of 6–10°C / min and held for 2–6 hours. During this holding period, the pressure is simultaneously increased to 6–10 MPa and held. The temperature is then cooled to 300–400°C at a rate of 10–20°C / min, followed by a further cooling to 30–50°C at a rate of 5–10°C / min. Simultaneously, the pressure is reduced to 0 MPa. By performing hot-pressing and holding near the melting point of aluminum, aluminum atoms and dissimilar metal atoms are kept in a state of maximum activation, improving the hot-pressing diffusion rate. A segmented cooling process is used to avoid interlayer deformation caused by the difference in thermal diffusivity between the dissimilar metal and the aluminide layer, which could lead to defects at the interface and reduced material mechanical properties. The pressure reduction along with the cooling process is also designed to prevent interlayer deformation during cooling, as pressure is applied during the cooling process.

[0020] To prevent diffusion reaction and bonding between the dissimilar metal plates and the graphite mold, in step S3, graphite paper covering at least the corresponding surfaces is provided on both the upper and lower surfaces of the stack.

[0021] Compared with existing technologies, the advantages of this invention are as follows: In this preparation method, aluminum flux is used to dissolve or destroy the residual or newly formed oxide film on the surface of the pretreated aluminum plate. The oxide is extruded into the interlayer between the aluminum plate and the dissimilar metal plate under high pressure along with the molten aluminum flux, thus solving the problem of difficult oxide film removal and obtaining a metal / aluminide layered composite material with low defects and high strength and toughness. By alternately stacking aluminum plates and dissimilar metal plates, a layered composite material with shell-like biomimetic properties can be prepared, exhibiting multi-interface effects and multi-layer filtering effects. Due to the unique layered structure and special failure mode of this material, in addition to its excellent properties of high strength, high modulus, and low density, it also has a strong ability to absorb impact energy.

[0022] Because the melting point and minimum active temperature of aluminum flux are both lower than the diffusion reaction temperature of aluminum plates and dissimilar metal plates, and it has good thermal stability and molten fluidity, and because aluminum and oxide film have different coefficients of thermal expansion, the oxide film will crack under heating, allowing the aluminum flux to penetrate. Aluminum flux using chloride-based or fluoride-based flux reacts with aluminum through chloride or fluoride salts, breaking the bond between the oxide film and aluminum. Simultaneously, the generated AlCl3 sublimates into gas at high temperatures, escaping from between the oxide film and aluminum, promoting oxide film breakage. Meanwhile, the fluorides in the flux effectively dissolve the oxide film.

[0023] By using aluminum flux to deeply treat the oxide film, atomic diffusion between the aluminum plate and the dissimilar metal plate can reduce the process of breaking through the interface oxide film. That is, in a low activation state, atoms on both sides of the interface can interdiffusion. At the same time, in the subsequent diffusion reaction, the diffusion interface front of the atoms also reduces the obstruction of oxide film fragments, greatly improves the diffusion reaction rate, and reduces the probability of Kirkendal pores.

[0024] The use of aluminum flux to assist in the preparation of metal / aluminide layered composite materials overcomes the challenge of "centerline" defects, ensuring that the aluminum layer is completely converted into aluminides while maintaining high strength and toughness, thus reducing the risk of aluminide layer fracture failure. Solving the "centerline" defect in the aluminide layer allows for wider application of metal / aluminide layered composite materials in high-temperature devices. This avoids the predicament of existing technologies that easily generate "centerline" defects, resulting in incomplete conversion of the aluminum layer into aluminides and residual aluminum at the center, limiting the operating temperature to the aluminum melting point. This allows the layered composite material to be used under conditions exceeding the aluminum melting point. In other words, by using an aluminum flux-assisted method to solve problems such as the difficulty in removing the aluminum oxide film during the preparation of metal / aluminide layered composite materials, leading to "centerline" defects and slow diffusion reaction rates, a metal / aluminide layered composite material with low defects, high strength-toughness matching, high diffusion reaction rate, and a wider operating temperature range can be obtained.

[0025] In addition, the preparation method of the present invention has low cost, simple process, low noise and air pollution, and good performance of finished product. Attached Figure Description

[0026] Figure 1 This is a front view of the dissimilar metal plates after being coated with a paste-like aluminum brazing flux according to this embodiment;

[0027] Figure 2 This is a schematic diagram of the stacked structure in this embodiment;

[0028] Figure 3 This is a schematic diagram of the thermo-pressure diffusion reaction in this embodiment;

[0029] Figure 4 This is a microstructure diagram of the layered composite material prepared in Example 1.

[0030] Figure 5 This is a diagram showing the flexural strength of the layered composite material prepared in Example 1.

[0031] Figure 6 This is a growth kinetics index diagram of the layered composite material prepared in Example 1.

[0032] Figure 7 This is a microstructure diagram of the layered composite material prepared in Example 2.

[0033] Figure 8 This is a diagram showing the flexural strength of the layered composite material prepared in Example 2.

[0034] Figure 9 This is a growth kinetics index diagram of the layered composite material prepared in Example 2;

[0035] Figure 10 This is a microstructure diagram of the layered composite material prepared in Example 3;

[0036] Figure 11 This is a diagram showing the flexural strength of the layered composite material prepared in Example 3.

[0037] Figure 12 This is a growth kinetics index diagram of the layered composite material prepared in Example 3. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0039] Example 1:

[0040] like Figures 1 to 6 The image shown is a first preferred embodiment of the present invention.

[0041] The preparation method of the layered composite material in this embodiment includes the following steps in sequence:

[0042] S1. Perform surface pretreatment on aluminum plate 3 and dissimilar metal plate 1 (different from aluminum plate). In this embodiment, dissimilar metal plate 1 is a high-entropy alloy plate. For surface heat treatment, first polish the surfaces of aluminum plate and high-entropy alloy plate with 600-grit sandpaper. Then, under ultrasonic vibration with an amplitude of 4μm, perform alkaline washing on aluminum plate and dissimilar metal plate with 6mol / L NaOH solution, and acid washing on aluminum plate and dissimilar metal plate with 8% HCl. The time for the aforementioned alkaline washing and acid washing is 3min. Subsequently, use anhydrous ethanol for ultrasonic cleaning for 45s. Finally, quickly place aluminum plate and high-entropy alloy plate into a vacuum drying oven for vacuum drying treatment to ensure that there is no anhydrous ethanol residue on the surface.

[0043] The above uses 600-grit sandpaper to prevent molten aluminum flux from remaining in the sanding scratches in S3.

[0044] S2. Coat the first surfaces of the aluminum plate 3 and the high-entropy alloy plate treated in S1 with a paste-like aluminum flux 2, with a coating thickness of 1 mm. The aforementioned paste-like aluminum flux is formed by mixing deionized water and chloride-based aluminum flux powder at a mass ratio of 1:1 and stirring with a glass rod until a paste is formed. The melting point and minimum activity temperature of the aforementioned aluminum flux are both lower than the diffusion reaction temperature of the aluminum plate and the dissimilar metal plate. The chloride-based aluminum flux is self-made, and its composition by mass percentage is: KCl 36.2%, NaCl 23.1%, ZnCl 21%, LiCl 11.2%, LiF 4.4%, CdCl 24.1%.

[0045] S3. The aluminum plate 3 and the high-entropy alloy plate treated in S2 are alternately stacked to form a stack 5, with the first and last layers of the stack 5 being high-entropy alloy plates. The first surface of the aluminum plate is in contact with the surface of the high-entropy alloy plate that is not coated with flux, and vice versa. Graphite paper 8, 0.1 mm thick, is applied to both the upper and lower surfaces of the stack, covering the corresponding surfaces. The stack 5 is then placed in a vacuum hot press furnace for vacuum heating surface treatment, wherein, as... Figure 3 As shown, the stacked body 5 is disposed between two pressure columns 6, and a graphite mold 7 is disposed between the pressure columns 6 and the stacked body 5. The aforementioned graphite paper 8 covers the surface of the graphite mold 7. The structure and working principle of the vacuum hot press furnace are the same as those of the prior art, and will not be described in detail in this embodiment. In this embodiment, the vacuum heating surface treatment process is as follows: the vacuum hot press furnace is evacuated to a vacuum degree ≤1×10⁻⁶. -1 The pressure is increased at a rate of 2℃ / min, with a pressure of 0MPa during the heating stage. After the temperature reaches 450℃, it is held for 20min, and the pressure is gradually increased to 1MPa during the holding stage.

[0046] S4. The stack 5 processed in S3 is further subjected to vacuum hot-press diffusion treatment. The vacuum hot-press diffusion treatment process is as follows: continue to evacuate the vacuum hot-press furnace until the vacuum degree is ≤1×10 -2 Pa, based on the heat preservation temperature in step S3, the temperature is increased to 645℃ at a rate of 6℃ / min and then kept at that temperature for 6 hours. During the heat preservation stage, the pressure is simultaneously increased to 10MPa and then kept at that pressure. Then, the temperature is first cooled to 400℃ at a rate of 10℃ / min, and then cooled to 50℃ at a rate of 10℃ / min. During the cooling process, the pressure is simultaneously reduced to 0MPa, thereby obtaining the aluminum flux-assisted metal / aluminide layered composite material.

[0047] like Figure 4 As shown, the high-entropy alloy-based metal / aluminate layer 11 generated by the complete diffusion reaction of high-entropy alloy 10 and aluminum has no "centerline" defects, but only a small amount of Kirkendall pores caused by the difference in diffusion rates of different elements.

[0048] like Figure 5 As shown, the flexural strength of high-entropy alloy-based metal / aluminide layered composite materials can reach over 1000 MPa, significantly improving the flexural strength of layered composite materials in which the aluminum layer is completely transformed into an aluminide layer, and avoiding the difficulty in demonstrating material strength due to "centerline" defects. Figure 6 As shown, the growth kinetic index of the high-entropy alloy-based metal / aluminide layered composite material is 0.72. Since the diffusion process eliminates the obstruction of oxide film and oxide impurities, the atomic diffusion process skips the "grain boundary diffusion" type and directly enters the "mixed diffusion of interface diffusion and bulk diffusion" type, which greatly improves the atomic interdiffusion rate and reduces the reaction time.

[0049] Example 2:

[0050] like Figures 7 to 9 The image shows the second preferred embodiment of the present invention.

[0051] The only differences between this embodiment and Embodiment 1 above are: 1. The dissimilar metal plate is a pure iron plate; 2. The process parameters are different, as follows:

[0052] In step S1, the amplitude is 8 μm, and the device is washed with 8 mol / L NaOH solution for alkaline washing and 10% HCl solution for acid washing.

[0053] In step S2, the paste-like aluminum flux is prepared by mixing deionized water and fluoride-based aluminum flux powder at a mass ratio of 1:2; the coating thickness is 0.8 mm. The fluoride-based aluminum flux is homemade, and its composition by mass percentage is: KF 42%, AlF 358%.

[0054] In the vacuum heating surface treatment process of step S3, the temperature is increased at a rate of 4℃ / min, the holding temperature is 520℃, and the holding time is 12min. During the holding stage, the pressure is increased synchronously to 2MPa.

[0055] In step S4, the vacuum hot-pressing diffusion treatment, the temperature is increased to 650°C at a rate of 8°C / min and then held for 4 hours. During the holding period, the pressure is simultaneously increased to 8 MPa and held for 4 hours. Afterwards, under vacuum, the temperature is cooled to 350°C at a rate of 15°C / min, and then further cooled to 40°C at a rate of 7°C / min, with the pressure decreasing directly to 0 MPa during the cooling phase.

[0056] like Figure 7 As shown, the pure iron-based metal / aluminide layer 13 formed by the complete diffusion reaction of pure iron 12 and aluminum has no "centerline" defect.

[0057] like Figure 8 As shown, the flexural strength of pure iron-based metal / aluminide layered composite materials can reach over 1000 MPa, significantly improving the flexural strength of layered composite materials in which the aluminum layer is completely transformed into an aluminide layer, and avoiding the difficulty in demonstrating material strength due to "centerline" defects. Figure 9 As shown, the growth kinetic index of pure iron-based metal / aluminide layered composite material is 0.85. Since the diffusion process eliminates the obstruction of oxide film and oxide impurities, the atomic diffusion process skips the "grain boundary diffusion" type and directly enters the "mixed diffusion of interface diffusion and bulk diffusion" type, which greatly improves the atomic interdiffusion rate and reduces the reaction time.

[0058] Example 3:

[0059] like Figures 10 to 12 The image shown represents the third preferred embodiment of the present invention.

[0060] The only differences between this embodiment and Embodiment 1 above are: 1. The dissimilar metal plate is 304 stainless steel plate; 2. The process parameters are different, as follows:

[0061] In step S1, the amplitude is 12 μm, and the device is washed with 10 mol / L NaOH solution for alkaline washing and 12% HCl solution for acid washing.

[0062] In step S2, the paste-like aluminum flux is prepared by mixing deionized water and fluoride-based aluminum flux powder at a mass ratio of 1:3, and the coating thickness is 0.6 mm. The fluoride-based aluminum flux is homemade, and its composition by mass percentage is: KF 42%, AlF 358%.

[0063] In the vacuum heating surface treatment process of step S3, the temperature is increased at a rate of 6℃ / min, the holding temperature is 600℃, and the holding time is 5min. During the holding stage, the pressure is increased synchronously to 3MPa.

[0064] In step S4, the vacuum hot-pressing diffusion treatment, the temperature is increased to 655°C at a rate of 10°C / min and then held for 2 hours. During the holding period, the pressure is simultaneously increased to 6 MPa and held for 2 hours. Afterwards, under vacuum, the temperature is cooled to 300°C at a rate of 20°C / min, and then further cooled to 30°C at a rate of 5°C / min, with the pressure decreasing directly to 0 MPa during the cooling phase.

[0065] like Figure 10 As shown, the 304 stainless steel base metal / aluminide layer 15 formed by the complete diffusion reaction of 304 stainless steel base metal 14 and aluminum has no "centerline" defect.

[0066] like Figure 11 As shown, the flexural strength of 304 stainless steel-based metal / aluminide layered composite material can reach over 1000 MPa, significantly improving the flexural strength of layered composite materials in the state where the aluminum layer has completely transformed into an aluminide layer, and avoiding the difficulty in demonstrating material strength due to "centerline" defects. Figure 12 As shown, the growth kinetic index of the 304 stainless steel-based metal / aluminide layered composite material is 0.79. Since the diffusion process eliminates the obstruction of oxide film and oxide impurities, the atomic diffusion process skips the "grain boundary diffusion" type and directly enters the "mixed diffusion of interface diffusion and bulk diffusion" type, which greatly improves the atomic interdiffusion rate and reduces the reaction time.

Claims

1. A method for preparing a layered composite material, characterized in that, The steps are as follows: S1. Perform surface pretreatment on aluminum plates and dissimilar metal plates that are different from aluminum plates; S2. Coat the surfaces of the aluminum plate and the dissimilar metal plate treated in S1 with a paste-like aluminum flux. The melting point and minimum activity temperature of the aluminum flux are both lower than the diffusion reaction temperature of the aluminum plate and the dissimilar metal plate. S3. The aluminum plate and dissimilar metal plate processed in S2 are stacked alternately to form a stack body, wherein aluminum flux is placed between adjacent aluminum plates and dissimilar metal plates, and the stack body is subjected to vacuum heating surface treatment; the first and last layers of the stack body are dissimilar metal plates, which are high-entropy alloy plates, pure iron plates or 304 stainless steel plates. In step S3, the stacked body is placed in a vacuum hot press furnace for vacuum heat treatment. The vacuum heat treatment process is as follows: the vacuum hot press furnace is evacuated to a vacuum degree ≤1×10⁻⁶. -1 Pa, heating at a rate of 2~6℃ / min, with a pressure of 0MPa during the heating stage, and holding at 450~600℃ for 5~20min, gradually increasing the pressure to 1~3MPa during the holding stage; S4. The stack body after S3 is further subjected to vacuum hot-press diffusion treatment to obtain aluminum flux-assisted layered composite material. In step S4, the vacuum hot-press diffusion process is as follows: continue evacuating until the vacuum degree is ≤1×10 -2 Pa, the temperature is increased to 645-655℃ at a rate of 6-10℃ / min and then held for 2-6 hours. During the holding period, the pressure is simultaneously increased to 6-10MPa and held. Then, the temperature is first cooled to 300-400℃ at a rate of 10-20℃ / min, and then cooled to 30-50℃ at a rate of 5-10℃ / min. During the cooling process, the pressure is simultaneously reduced to 0MPa.

2. The preparation method according to claim 1, characterized in that: In step S1, the surface pretreatment is as follows: the surface of the aluminum plate and the dissimilar metal plate is sanded with sandpaper, then the aluminum plate and the dissimilar metal plate are subjected to alkaline washing and acid washing under ultrasonic vibration, and finally vacuum drying treatment is performed.

3. The preparation method according to claim 2, characterized in that: Before vacuum drying, the product is washed with 6-10 mol / L NaOH solution for alkaline washing and 8-12% HCl solution for acid washing. After acid and alkaline washing, it is ultrasonically cleaned with anhydrous ethanol.

4. The preparation method according to claim 3, characterized in that: The amplitude of the ultrasonic wave is 4~12μm, the acid washing and alkaline washing time is 3min, and the ultrasonic cleaning time is 45s.

5. The preparation method according to claim 1, characterized in that: In step S2, deionized water or distilled water is mixed with chloride-based or fluoride-based aluminum flux powder at a mass ratio of 1:1 to 1:3, and then stirred to obtain a paste-like aluminum flux.

6. The preparation method according to claim 1, characterized in that: In step S2, the first surface of the aluminum plate and the first surface of the dissimilar metal plate are coated with aluminum flux. In step S3, the first surface of the aluminum plate is in contact with the side of the dissimilar metal plate that is not coated with aluminum flux, and the side of the aluminum plate that is not coated with aluminum flux is in contact with the first surface of the dissimilar metal plate.

7. The preparation method according to claim 1, characterized in that: In step S3, the upper and lower surfaces of the stack are each provided with graphite paper that at least covers the corresponding surfaces.

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