Steel-aluminum bimetallic composite based on a powder interlayer and method for producing same

Abstract

The application relates to the technical field of bimetal composite materials, and discloses a steel-aluminum bimetal composite material based on a powder intermediate layer and a preparation method thereof, which comprises a steel base, a powder intermediate layer and an aluminum alloy. The powder intermediate layer is obtained by condensing CoCrFeNiAl high-entropy alloy powder on the surface of the steel base and sintering, and the thickness is controlled to be 10-20 mu m. The preparation process is completed through the following steps: hanging powder by pulling operation, glue removal sintering, hot dipping and casting of the aluminum alloy, and T6 heat treatment is additionally provided. The high-entropy alloy powder is used to hinder the rapid diffusion of aluminum and iron elements, the intermetallic compound at the interface is changed from a single-layer continuous form into a discontinuous double-layer distribution structure, stress concentration is reduced, and crack initiation is inhibited; meanwhile, the porous powder skeleton with a suitable thickness is convenient for the aluminum liquid to infiltrate and fill to avoid cavities, the performance of the aluminum alloy body is improved, and the good interface shear strength of the material is maintained.

Description

Technical Field

[0001] This invention relates to the field of bimetallic composite materials technology, specifically to a steel-aluminum bimetallic composite material based on a powder interlayer and its preparation method. Background Technology

[0002] In the process of developing lightweight weapons and equipment, the demand for lightweight materials is increasing. Aluminum-steel bimetallic composite materials combine the lightweight properties of aluminum alloys with the high strength characteristics of steel, making up for the performance defects of single aluminum alloys. For the production and manufacturing of complex parts such as composite road wheels made of tempered high-strength steel and end couplings, liquid-solid composite casting technology has been widely used due to its advantages of simple process and convenient operation.

[0003] Liquid-solid composite casting relies on contact heat transfer during aluminum alloy casting to complete the metallurgical bonding of the aluminum and steel interface. When a high-entropy alloy powder is pre-set as an intermediate layer at the interface, the inherent hysteresis diffusion effect and lattice distortion effect of the high-entropy alloy can hinder the thermal diffusion of aluminum elements during the composite process and reduce the formation rate of intermetallic compounds at the interface. After the high-entropy alloy participates in the interface reaction as a porous framework medium, it significantly reduces the thickness of the iron-aluminum intermetallic compound layer at the interface and promotes the formation of phase structure at the interface.

[0004] Current liquid-solid composite casting technologies, without the addition of a suitable intermediate layer, result in a single-layer continuous brittle intermetallic compound at the composite interface. Under stress, this leads to severe stress concentration at the interface, easily causing crack initiation and a decrease in interfacial shear strength. When introducing an intermediate layer, the thickness of the intermediate medium is difficult to precisely control under conventional operations. Excessive thickness causes molten aluminum alloy to seep into the powder gaps, resulting in insufficient filling and numerous voids and defects at the interface. Insufficient thickness, on the other hand, faces the problem of interfacial element blocking failure. Furthermore, to improve the mechanical properties of the aluminum alloy, the composite material needs to undergo prolonged solution treatment and other heat treatment processes. Under these prolonged high-temperature environments, aluminum elements migrate extensively to the interface, forming a large-area continuous brittle phase layer near the aluminum side. This not only disrupts the original metallurgical bonding state of the interface but also causes a significant decrease in the shear strength of the composite interface after heat treatment.

[0005] Therefore, the purpose of this invention is to provide a steel-aluminum bimetallic composite material based on a powder interlayer and its preparation method, so as to overcome the shortcomings of the prior art. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a steel-aluminum bimetallic composite material based on a powder interlayer and its preparation method. This invention solves the problems in existing liquid-solid composite casting technologies, such as the lack of a suitable intermediary medium leading to the formation of a single-layer continuous brittle intermetallic compound at the composite interface, resulting in a decrease in shear strength; the difficulty in accurately controlling the thickness of the intermediary medium during conventional operations causing insufficient aluminum filling, resulting in voids, gaps, or element blockage failure; and the degradation of the mechanical properties of the composite interface caused by long-term solution heat treatment promoting the migration of aluminum elements and the formation of a large-area continuous brittle phase layer near the aluminum side.

[0007] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a steel-aluminum bimetallic composite material based on a powder interlayer, employing the following technical solution: A steel-aluminum bimetallic composite material based on a powder interlayer includes a steel matrix, a powder interlayer, and an aluminum alloy; The powder intermediate layer is obtained by sintering high-entropy alloy powder condensed on the surface of a steel substrate to form a powder layer. The high-entropy alloy powder is CoCrFeNiAl alloy powder, and the thickness of the powder layer on the steel substrate surface is 10-20 μm. The steel-aluminum bimetallic composite material based on the powder interlayer is obtained by hot-dip galvanizing a steel substrate with a powder layer after sintering, assembling it with a steel mold, and then pouring an aluminum alloy solution into the mold and cooling and solidifying it.

[0008] By employing the above technical solution, this invention uses CoCrFeNiAl high-entropy alloy powder with a thickness of 10–20 μm as an intermediate layer, which helps to improve the phase structure of the composite interface and thus enhance its shear strength. Specifically, during hot-dip galvanizing and subsequent casting, the high-entropy alloy itself exhibits hysteresis diffusion and lattice distortion effects, which to some extent hinder the rapid thermal diffusion of liquid aluminum to the steel substrate side. Utilizing this multi-principal element characteristic, the reaction rate between aluminum and iron atoms is reduced, thereby controlling the amount and thickness of brittle intermetallic compounds formed at the interface.

[0009] Meanwhile, the use of a powder-pre-formed layer effectively alters the direct contact mode between the aluminum and steel sides. When the high-temperature molten aluminum penetrates the gaps in the powder layer, it only reacts locally with the high-entropy alloy particles, forming a physical barrier. This not only restricts the large-scale migration of elements but also prevents the formation of a large-area continuous brittle phase near the aluminum side during subsequent long-term solution treatment. Under the influence of the aforementioned interfacial reaction, phase reconstruction occurs at the interface, gradually forming a metallic phase structure with face-centered cubic and body-centered cubic phases as the base and a dispersed B2-reinforcing phase framework inside.

[0010] Under these conditions, the originally single and continuous intermetallic compound layer transforms into a discontinuous bilayer. This discontinuous gradient interface breaks the continuity of the structure, reducing stress concentration caused by the layered structure under stress, thus helping to suppress crack initiation. This allows the composite material to maintain good interfacial bonding performance even after T6 heat treatment.

[0011] Preferably, the high-entropy alloy powder has a particle size of 1–5 μm, and the high-entropy alloy powder is prepared by high-energy ball milling of CoCrFeNiAl alloy powder with an initial particle size of 20 μm. The steel substrate is made of 42CrMo steel or quenched and tempered high-strength steel; the aluminum alloy is made of commercially available ZL205A.

[0012] By adopting the above technical solution, the alloy powder grains after high-energy ball milling are refined, and its specific surface area and reactivity are correspondingly increased. This provides favorable conditions for the sintering and subsequent interface wetting of the powder layer. By controlling the powder particle size to 1–5 μm and matching it with a layer thickness of 10–20 μm, a moderately porous intermediate framework can be constructed on the steel substrate surface. During hot-dip galvanizing, the molten aluminum can easily penetrate and fill these capillary pores, reducing the risk of void defects caused by excessively large pores or uneven filling.

[0013] Secondly, the present invention provides a method for preparing a steel-aluminum bimetallic composite material based on a powder interlayer, employing the following technical solution: A method for preparing a steel-aluminum bimetallic composite material based on a powder interlayer includes the following steps: Pre-treatment of the steel substrate surface; The pretreated steel substrate is immersed in the prepared powder solution for lifting, and then naturally dried to allow the polyvinyl butyral and high-entropy alloy powders to condense and form a powder layer on the surface of the steel substrate. The steel substrate with the powder layer is placed in a tube furnace and subjected to debinding and sintering treatments in sequence; the sintered steel substrate is then hot-dip galvanized, wherein the steel substrate is immersed in a hot-dip galvanizing aluminum alloy solution. The preheated steel mold is assembled with the hot-dip galvanized steel substrate, and an aluminum alloy solution for casting is poured into the mold and then cooled and solidified. The composite material after cooling and curing is subjected to T6 heat treatment to obtain a steel-aluminum bimetallic composite material based on a powder interlayer.

[0014] By adopting the above technical solution, combined with surface powder coating and liquid-solid composite casting process, the pulling operation and subsequent natural drying enable the binder and high-entropy alloy powder to form a relatively stable solid pre-placed layer on the steel substrate. The main purpose of debinding and sintering is to remove organic matter and promote the initial bonding between powder particles, thereby constructing a porous intermediate layer structure with internal gaps.

[0015] After entering the hot-dip galvanizing stage, the residual gas in the powder layer is gradually expelled by the high temperature and fluidity of the liquid aluminum alloy, and an initial bond is formed at the interface. Subsequent mold preheating and casting, combined with T6 heat treatment, not only improve the mechanical properties of the aluminum alloy body, but also rely on the powder barrier formed in the early stage to limit the excessive diffusion of interface elements, which helps to maintain the overall stability of the composite interface.

[0016] Preferably, the powder solution is prepared from raw materials comprising the following parts by weight: 77.5 parts by weight of anhydrous ethanol, 2.5 parts by weight of PVB, and 20 parts by weight of CoCrFeNiAl alloy powder; the specific method for obtaining a powder layer on the surface of the steel substrate is as follows: The pretreated steel substrate is immersed in the prepared powder solution and pulled at a speed of 5 mm / s. The pulling operation is repeated 2, 3 or more times until a powder layer with a thickness of 10 to 20 μm is obtained on the surface of the steel substrate.

[0017] By adopting the above technical solution, anhydrous ethanol and PVB mixed in this ratio can form a suspension system with suitable viscosity, which is beneficial for the dispersion of alloy powder. When pulled at a speed of 5 mm / s, the liquid film on the steel substrate surface easily reaches dynamic equilibrium under the action of tension and gravity; combined with multiple pulling operations, the final thickness of the powder layer can be well controlled.

[0018] When the layer thickness is in the range of 10 to 20 μm, the aluminum liquid can more easily penetrate into the gaps through capillary action during the hot-dip plating stage. This reduces the probability of insufficient aluminum liquid filling and micropores caused by excessively thick powder layers, and also prevents the problem of weakened interface shielding effect caused by excessively thin powder layers.

[0019] Preferably, the process parameters for the debinding treatment include: under vacuum and high-purity argon gas, setting the heating rate to 1℃ / min, holding at 250℃ for 1h, then heating to 450℃ and holding for 1h, with degassing every 10min during the process; The sintering process parameters include: after the binder is removed and the air is vented, the heating rate is set to 6℃ / min, the temperature is held at 900℃ for 30min, and then cooled with the furnace.

[0020] By adopting the above technical solution, under vacuum and argon-filled conditions, the oxidation risk of the steel substrate and powder during heating can be reduced. Slow heating at 1℃ / min, combined with staged heat preservation and timed venting, helps the PVB binder to gradually decompose and be expelled, thereby reducing powder layer peeling or excess carbon residue caused by excessive gas expansion. Sintering at 900℃ facilitates neck growth between metal particles, improving the structural strength of the powder layer and making it less prone to large-area detachment when subjected to subsequent high-temperature molten aluminum slurry.

[0021] Preferably, the hot-dip galvanizing process is implemented as follows: the temperature of the hot-dip galvanizing process is controlled at 710°C, the steel substrate is immersed in the hot-dip galvanizing aluminum alloy solution and kept for 5 minutes before being taken out; the steel mold is preheated to 250°C, and the casting aluminum alloy solution is poured into the mold at 725°C.

[0022] By adopting the above technical solution, the hot-dip galvanizing temperature is set at 710℃. This utilizes the enthalpy of the liquid metal to promote its penetration into the pores of the powder skeleton, thereby establishing interfacial wetting between the steel, high-entropy alloy, and molten aluminum. Preheating the steel mold to 250℃ is mainly to reduce the interfacial cooling gradient during casting; while the casting temperature of 725℃ provides a certain thermodynamic driving force, which helps the newly poured molten aluminum to better fuse with the surface hot-dip galvanized layer, promoting a tighter bond between the liquid and solid phases.

[0023] Preferably, the heat treatment is T6 heat treatment, which includes solution treatment, quenching treatment, and annealing treatment in sequence. The conditions for solution treatment include: a controlled heating rate of 100℃ / h, a solution temperature of 540℃, and a solution time of 10–12h. The quenching treatment involves water quenching at 80℃. The conditions for annealing treatment include: holding at 175℃ for 5h and cooling to room temperature. The pretreatment includes, in sequence, degreasing and derusting the steel substrate surface, surface polishing to remove the surface stress layer, acid immersion, ultrasonic cleaning with alcohol, and drying.

[0024] By employing the above technical solution and performing solution treatment according to the set parameters, the strengthening phase atoms can be induced to dissolve into the solid solution matrix. Subsequent water quenching at 80°C helps retain the supersaturated solid solution state. Following a long-term annealing at 175°C, fine second phases are dispersed and precipitated, thereby improving the overall strength of the aluminum alloy portion. Furthermore, the initial surface pretreatment removes oxides, oil stains, and some processing stress layers from the substrate surface, exposing the metal substrate and helping to improve the initial adhesion of the powder solution to the steel surface.

[0025] This invention provides a steel-aluminum bimetallic composite material based on a powder interlayer and its preparation method. It has the following beneficial effects: 1. This invention establishes a powder interlayer obtained by sintering CoCrFeNiAl alloy powder, with the thickness of the powder interlayer on the steel substrate surface controlled at 10-20 μm. During hot-dip galvanizing and casting, the high-entropy alloy can hinder the diffusion of aluminum from the aluminum alloy solution towards the steel substrate, thereby reducing the reaction rate between aluminum and iron. The presence of the powder interlayer changes the contact mode between the aluminum alloy and the steel substrate, transforming the intermetallic compound generated at the interface from a single-layer continuous form to a discontinuous double-layer distribution. The discontinuous interface structure reduces stress concentration under stress, helps suppress crack initiation, and enables the bimetallic composite material to maintain good interfacial shear strength while achieving metallurgical bonding.

[0026] 2. This invention uses a pulling operation in the preparation method to obtain a powder layer on the surface of a steel substrate. By matching the pulling speed with the corresponding number of pulling operations, the powder solution can be stably adhered to the surface of the steel substrate, and the thickness of the final powder intermediate layer can be precisely controlled within the target range. This porous powder skeleton with appropriate thickness facilitates the penetration and filling of liquid aluminum alloy solution along the gaps in the powder during the subsequent hot-dip galvanizing process. This reduces the defects of insufficient aluminum liquid filling and voids caused by excessively thick powder layers, and also prevents the problem of interface element blocking failure caused by excessively thin powder layers.

[0027] 3. In this invention, the composite material is subjected to T6 heat treatment after casting, cooling and solidification. The barrier effect of the powder interlayer is combined with the heat treatment process. When conventional bimetallic composite materials undergo long-term solution treatment, a continuous brittle phase layer is generated at the interface, which leads to a serious decrease in shear strength. The high-entropy alloy powder particles restrict the migration of interface elements in physical space. While the composite material undergoes solution treatment, quenching and annealing steps to improve the mechanical properties of the aluminum alloy body, it avoids the formation of a large area of ​​continuous brittle phase near the aluminum side, thus improving the overall mechanical performance of the composite interface after heat treatment. Attached Figure Description

[0028] Figure 1 The image shows the microstructure of the composite interface of the steel-aluminum bimetallic composite material prepared in Example 1 of this invention. Figure 2 The image shows the microstructure of the composite interface of the steel-aluminum bimetallic composite material prepared in Example 2 of this invention. Figure 3 The image shows the microstructure of the composite interface of the steel-aluminum bimetallic composite material prepared in Example 3 of this invention. Figure 4 The image shows the microstructure of the composite interface of the steel-aluminum bimetallic composite material prepared in Comparative Example 1 of this invention. Figure 5The image shows the microstructure of the composite interface of the steel-aluminum bimetallic composite material prepared in Comparative Example 2 of this invention. Figure 6 This is a comparative bar chart showing the thickness of the intermetallic compound at the high-strength aluminum-steel bimetallic composite interface of the present invention. Figure 7 This is a line graph comparing the interfacial shear strength of the high-strength aluminum-steel bimetallic composite material of the present invention. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Preparation Examples 1-4: Preparation Example 1: This preparation example provides a method for preparing a pretreated steel substrate, including the following steps: Φ10mm×90mm 42CrMo steel or quenched and tempered high-strength steel bars were cut using wire cutting. The surface of the steel bars was then subjected to the following steps in sequence: degreasing, rust removal, surface polishing, acid immersion, ultrasonic cleaning with alcohol, and drying. The degreasing process involved burning at 200℃ for 2 hours; rust removal involved using an electric grinder or shot blasting until the surface was smooth and rust-free; surface polishing involved polishing with 240-grit and 400-grit sandpaper, and even further with 600-grit and 800-grit sandpaper, to remove the surface stress layer; acid immersion involved soaking in a 10wt.% sulfuric acid solution at room temperature for 10 minutes; ultrasonic cleaning with alcohol was performed at 50℃ and 40kHz for 10 minutes; and drying was done with warm air at 70℃ for 5 minutes.

[0031] Preparation Example 2: This preparation example provides a method for preparing CoCrFeNiAl alloy powder with a particle size of 1-5 μm, including the following steps: 100 parts of CoCrFeNiAl alloy powder with an initial particle size of 20 μm were weighed and placed into a planetary ball mill for high-energy ball milling. Two parts of anhydrous ethanol were added as a ball milling control agent, and high-purity argon gas was introduced as a protective atmosphere. The ball milling time was set to 20 h, the ball-to-powder mass ratio was 20:1, the rotation speed was 300 r / min, and the start-stop ratio was 120 min of operation and 30 min of shutdown. After ball milling, high-energy ball-milled CoCrFeNiAl alloy powder with a particle size of 1–5 μm was obtained.

[0032] Preparation Example 3: This preparation example provides a method for preparing a high-entropy alloy powder solution, including the following steps: Weigh out 77.5 parts of anhydrous ethanol, 2.5 parts of PVB, and 20 parts of the CoCrFeNiAl alloy powder prepared in Preparation Example 2. Add the PVB and CoCrFeNiAl alloy powder to the anhydrous ethanol, place the prepared solution in an ultrasonic instrument to mix it evenly, shake well and remove bubbles before use to obtain a powder solution.

[0033] Preparation Example 4: This preparation example provides a method for preparing an aluminum alloy solution, including the following steps: For hot-dip galvanizing aluminum alloy solution, commercially available ZL205A is melted at 720℃, argon is blown for 10 minutes, and the solution is refined, degassed, and slag is removed, and then held at the temperature for 15 minutes to obtain the hot-dip galvanizing aluminum alloy solution; for casting aluminum alloy solution, commercially available ZL205A is melted at 740℃, argon is blown for 10 minutes, and the solution is refined, degassed, and slag is removed, then cooled to 730℃ and held at the temperature for 15 minutes to obtain the casting aluminum alloy solution.

[0034] Examples 1-3: Example 1: This embodiment provides a method for preparing a steel-aluminum bimetallic composite material based on a powder interlayer, including the following steps: (1) Preparation of powder intermediate layer: The steel substrate pretreated in Preparation Example 1 was immersed in the powder solution prepared in Preparation Example 3 and pulled at a pulling speed of 5 mm / s for a total of 3 times. After being taken out, it was naturally dried to allow the polyvinyl butyral and high entropy alloy powder to condense and obtain a powder layer with a thickness of 20 μm on the surface of the steel substrate. (2) Debinding and sintering: The steel substrate with the powder layer is placed in a tube furnace, vacuumed and high-purity argon is introduced. Titanium powder is used to consume the residual oxygen in the furnace to perform debinding. The heating rate is set to 1℃ / min, and the temperature is held at 250℃ for 1h. Then the temperature is raised to 450℃ and held for 1h. During this period, the venting operation is performed every 10min. After the debinding and venting are completed, the sintering process is carried out. The heating rate is set to 6℃ / min, and the temperature is held at 900℃ for 30min. The furnace is then cooled. (3) Hot-dip galvanizing: The steel substrate after sintering is hot-dip galvanized. The temperature of the hot-dip galvanizing process is controlled at 710℃. The entire hot-dip galvanizing process takes 9 minutes. The steel substrate is immersed in the hot-dip galvanizing aluminum alloy solution prepared in Example 4 and kept for 5 minutes before being taken out. (4) Casting: The steel mold preheated to 250°C is assembled with the hot-dip galvanized steel substrate, and then the aluminum alloy solution for casting prepared in Example 4 is poured into the mold at 725°C and cooled and solidified. (5) Heat treatment: The composite material after cooling and solidification is subjected to T6 heat treatment. First, solution treatment is performed, with the heating rate controlled at 100℃ / h, the solution temperature at 540℃, and the solution time at 10h. Then, quenching treatment is performed, with water quenching at 80℃. Finally, annealing treatment is performed, with holding at 175℃ for 5h and cooling to room temperature to obtain the steel-aluminum bimetallic composite material based on the powder intermediate layer.

[0035] Example 2: This embodiment provides a method for preparing a steel-aluminum bimetallic composite material based on a powder interlayer, including the following steps: (1) Preparation of powder intermediate layer: The steel substrate pretreated in Preparation Example 1 was immersed in the powder solution prepared in Preparation Example 3 and pulled at a pulling speed of 5 mm / s for a total of 2 times. After being taken out, it was naturally dried to allow the polyvinyl butyral and high entropy alloy powder to condense and obtain a powder layer with a thickness of 10 μm on the surface of the steel substrate. (2) Debinding and sintering: The steel substrate with the powder layer is placed in a tube furnace, vacuumed and high-purity argon is introduced. Titanium powder is used to consume the residual oxygen in the furnace to perform debinding. The heating rate is set to 1℃ / min, and the temperature is held at 250℃ for 1h. Then the temperature is raised to 450℃ and held for 1h. During this period, the venting operation is performed every 10min. After the debinding and venting are completed, the sintering process is carried out. The heating rate is set to 6℃ / min, and the temperature is held at 900℃ for 30min. The furnace is then cooled. (3) Hot-dip galvanizing: The steel substrate after sintering is hot-dip galvanized. The temperature of the hot-dip galvanizing process is controlled at 710℃. The entire hot-dip galvanizing process takes 9 minutes. The steel substrate is immersed in the hot-dip galvanizing aluminum alloy solution prepared in Example 4 and kept for 5 minutes before being taken out. (4) Casting: The steel mold preheated to 250°C is assembled with the hot-dip galvanized steel substrate, and then the aluminum alloy solution for casting prepared in Example 4 is poured into the mold at 725°C and cooled and solidified. (5) Heat treatment: The composite material after cooling and solidification is subjected to T6 heat treatment. First, solution treatment is performed, with the heating rate controlled at 100℃ / h, the solution temperature at 540℃, and the solution time at 10h. Then, quenching treatment is performed, with water quenching at 80℃. Finally, annealing treatment is performed, with holding at 175℃ for 5h and cooling to room temperature to obtain the steel-aluminum bimetallic composite material based on the powder intermediate layer.

[0036] Example 3: This embodiment provides a method for preparing a steel-aluminum bimetallic composite material based on a powder interlayer, including the following steps: (1) Preparation of powder intermediate layer: The steel substrate pretreated in Preparation Example 1 was immersed in the powder solution prepared in Preparation Example 3 and pulled at a pulling speed of 5 mm / s. After being taken out, it was naturally dried. Through multiple pulling operations, the polyvinyl butyral and high entropy alloy powders were condensed to obtain a powder layer with a thickness of 40 μm on the surface of the steel substrate. (2) Debinding and sintering: The steel substrate with the powder layer is placed in a tube furnace, vacuumed and high-purity argon is introduced. Titanium powder is used to consume the residual oxygen in the furnace to perform debinding. The heating rate is set to 1℃ / min, and the temperature is held at 250℃ for 1h. Then the temperature is raised to 450℃ and held for 1h. During this period, the venting operation is performed every 10min. After the debinding and venting are completed, the sintering process is carried out. The heating rate is set to 6℃ / min, and the temperature is held at 900℃ for 30min. The furnace is then cooled. (3) Hot-dip galvanizing: The steel substrate after sintering is hot-dip galvanized. The temperature of the hot-dip galvanizing process is controlled at 710℃. The entire hot-dip galvanizing process takes 9 minutes. The steel substrate is immersed in the hot-dip galvanizing aluminum alloy solution prepared in Example 4 and kept for 5 minutes before being taken out. (4) Casting: The steel mold preheated to 250°C is assembled with the hot-dip galvanized steel substrate, and then the aluminum alloy solution for casting prepared in Example 4 is poured into the mold at 725°C and cooled and solidified. (5) Heat treatment: The composite material after cooling and solidification is subjected to T6 heat treatment. First, solution treatment is performed, with the heating rate controlled at 100℃ / h, the solution temperature at 540℃, and the solution time at 12h. Then, quenching treatment is performed, with water quenching at 80℃. Finally, annealing treatment is performed, with holding at 175℃ for 5h and cooling to room temperature to obtain the steel-aluminum bimetallic composite material based on the powder intermediate layer.

[0037] Comparative Examples 1-2: Comparative Example 1: Compared with Example 1, the difference is that there is no powder intermediate layer (i.e., no powder coating, debinding and sintering treatment is performed, and the pretreated steel substrate is directly hot-dip galvanized), and all other aspects are the same.

[0038] Comparative Example 2: Compared with Comparative Example 1, the difference is that T6 heat treatment was not performed after casting and cooling, but all other aspects are the same.

[0039] Test Examples 1-2: Test Example 1: Testing the thickness of the intermetallic compound (IMC) at the high-strength aluminum-steel bimetallic composite interface The specific experimental steps are as follows: (1) Metallographic samples were cut at the interface of the steel-aluminum bimetallic composite materials prepared in Examples 1-3 and Comparative Examples 1 and 2 using a wire cutting device.

[0040] (2) The cut metallographic sample was subjected to coarse grinding, fine grinding and polishing in sequence. Then, the polished surface was chemically etched with an etchant to prepare an interface metallographic structure sample that meets the observation requirements.

[0041] (3) The microstructure of the composite interface of each group of samples was observed using a metallographic microscope and a scanning electron microscope, with a focus on the growth morphology and distribution characteristics of the intermetallic compound layer at the interface; the microstructure of the composite interface of Examples 1, 2, 3, Comparative Example 1 and Comparative Example 2 are respectively as follows: Figure 1 , Figure 2 , Figure 3 , Figure 4 and Figure 5 As shown.

[0042] (4) The thickness of the intermetallic compound layer at the interface of each group of samples was measured at multiple points under a microscope using image processing software, and the average value of the measurements was taken as the final intermetallic compound thickness data. The test results are shown in Table 1.

[0043] Table 1. Thickness test results of high-strength aluminum / steel bimetallic composite intermetallic compounds prepared in Examples 1-3 and Comparative Examples 1 and 2 Conclusions and Analysis: Based on Table 1 and Figure 6 The data shows that in Comparative Example 2, without T6 heat treatment, the thickness of the intermetallic compound at the interface was 38.60 μm, while in Comparative Example 1, after T6 heat treatment, the thickness of the intermetallic compound at the interface increased to 85.67 μm. The data indicates that, without a powder interlayer, the conventional liquid-solid composite casting sample, after undergoing prolonged high-temperature solution treatment, exhibits more intense interdiffusion behavior of aluminum and iron elements at the interface, leading to rapid growth of the brittle intermetallic compound layer and a significant increase in thickness. This makes it easier for Al9Co2 and Al to form near the aluminum side. 13 Co4, Al 45 The continuous brittle phases such as Cr7 result in an excessively thick intermetallic compound layer at the interface, which is prone to becoming a crack initiation under stress.

[0044] Combination Figure 1 , Figure 2 , Figure 4 and Figure 5The microstructure shows that the microstructure changed after the introduction of the CoCrFeNiAl high-entropy alloy powder interlayer. In Example 2, when the powder layer thickness was 10 μm, the thickness of the intermetallic compound at the interface after heat treatment was controlled at 52.36 μm; in Example 1, when the powder layer thickness was 20 μm, the thickness of the intermetallic compound at the interface after heat treatment was further reduced to 32.60 μm, both significantly lower than the 85.67 μm in Comparative Example 1. This is because the pre-placed CoCrFeNiAl high-entropy alloy powder interlayer can block the elemental reaction between Al and Fe, hindering the thermal diffusion of aluminum during high-temperature treatment, thus preventing the single-layer continuous intermetallic compound (such as...) at the interface. Figure 4 The morphology shown transforms into a discontinuous bilayer intermetallic compound (such as...). Figure 1 , Figure 2 Comparative observation shows that a discontinuous gradient composite interface with FCC+BCC phase as the base and B2 phase as the skeleton is formed, which reduces the stress concentration phenomenon caused by the continuous layered structure.

[0045] According to Table 1 and Figure 6 The data shows that the thickness of the powder layer has a regulatory effect on interfacial bonding. When the powder layer thickness increases from 10 μm to 20 μm, the effect of inhibiting element diffusion becomes more significant, reducing the thickness of the intermetallic compound from 52.36 μm to 32.60 μm. However, when the powder layer thickness is further increased to 40 μm (as shown in Example 3), the test results show failure. Figure 3 The microstructure revealed numerous voids and gaps at the composite interface. This is because when the powder layer is too thick, the increased flow resistance of the molten aluminum during the subsequent hot-dip galvanizing process prevents it from completely filling the porous gaps between the high-entropy alloy powder particles, leaving numerous voids and gaps at the interface and hindering the formation of an effective metallurgical bonding interface. Therefore, controlling the thickness of the high-entropy alloy powder layer within the range of 10–20 μm can suppress the excessive growth of the intermetallic compound layer during heat treatment and improve the interface morphology.

[0046] Test Example 2: Testing the interfacial shear strength of high-strength aluminum-steel bimetallic composite materials The specific experimental steps are as follows: (1) Using a precision wire cutting device, standard shear specimens for mechanical property testing were cut at the interface of the steel-aluminum bimetallic composite materials prepared in Examples 1-3 and Comparative Examples 1 and 2.

[0047] (2) Referring to the test method for shear strength of bimetallic composites specified in "Interface formation and strengthening mechanisms of Al / Mgbimetallic composite via compound casting with rare earth Ce introduction" (Materials Science & Engineering A, Volume 854, 27 September 2022, 143830), the prepared shear specimens were assembled onto the special shear test fixture of the universal testing machine.

[0048] (3) Start the universal testing machine, set a fixed displacement loading rate, apply shear load to the interface of the shear specimen until the interface is completely broken, and the system automatically records the maximum shear load value at the moment of fracture.

[0049] (4) Based on the recorded maximum shear load value and the shear area of ​​the interface, the interfacial shear strength of each group of specimens was calculated, and the test results are shown in Table 2.

[0050] Table 2. Performance test results of the high-strength aluminum / steel bimetallic composite materials prepared in Examples 1-3 and Comparative Examples 1 and 2 Conclusions and Analysis: Based on Table 2 and Figure 7 The data shows that Comparative Example 2, without T6 heat treatment, has an interfacial shear strength of 95.09 MPa, while Comparative Example 1, after T6 heat treatment, has an interfacial shear strength that decreases to 32.97 MPa. Comparing the two sets of data indicates that, without a high-entropy alloy powder interlayer, conventional liquid-solid composite casting materials undergo thermal diffusion at the interface during prolonged high-temperature solution treatment and annealing. This element diffusion leads to the formation of a thicker intermetallic compound layer at the interface, with continuously distributed Al9Co2 and Al near the aluminum side. 13 Co4, Al 45 Brittle phases such as Cr7, when continuously distributed, easily induce stress concentration. When subjected to external loads, these phases evolve into crack initiation sites, ultimately leading to a decrease in the mechanical bonding properties of the composite material after heat treatment.

[0051] After introducing a high-entropy alloy powder interlayer (CoCrFeNiAl), the composite material maintained high shear strength after T6 heat treatment. Test data showed that in Example 2, with a powder layer thickness of 10 μm, the interfacial shear strength after heat treatment was 52.71 MPa; in Example 1, with a powder layer thickness of 20 μm, the interfacial shear strength increased to 82.10 MPa. This is because the high-entropy alloy powder interlayer distributed between the steel and aluminum interfaces hindered the thermal diffusion between aluminum and iron elements during high-temperature solution treatment, reduced the thickness of the intermetallic compound layer at the interface, and avoided the formation of continuous brittle phases (such as Al9Co2, Al...) near the aluminum side under long-term solution treatment. 13 Co4, Al 45 The formation of Cr7, etc. The interface changes from a single-layer continuous compound morphology to a discontinuous bilayer compound morphology. At the interface, a discontinuous gradient composite interface structure is formed with FCC and BCC phases as the base and dispersed B2 reinforcing phase as the framework. The discontinuous gradient network structure can reduce the stress concentration phenomenon caused by the layered continuous compound, hinder crack propagation, and enable the bimetallic interface to maintain high shear strength after high-temperature solid solution treatment.

[0052] Meanwhile, data shows that the thickness of the intermediate layer of the high-entropy alloy powder needs to be controlled within a reasonable range. When the powder layer thickness increases to 40 μm, as shown in Example 3, the interfacial shear strength decreases to 22.08 MPa. Figure 3 Microscopic morphological observation reveals that due to the excessive thickness of the powder layer, the flow resistance encountered by the molten aluminum alloy as it penetrates the porous powder skeleton increases during subsequent hot-dip galvanizing processes. The molten aluminum cannot completely fill the microscopic porous voids between the high-entropy alloy powder particles, leaving voids and gaps within the bonding interface. The presence of these internal defects prevents the hot-dip galvanizing process from forming a complete metallurgical bond, reducing the effective load-bearing area and causing a decrease in interfacial shear strength.

[0053] According to Table 2 and Figure 7 Data shows that controlling the thickness of the CoCrFeNiAl high-entropy alloy powder layer on the steel substrate surface within the range of 10 to 20 μm can effectively suppress element diffusion and the growth of brittle intermetallic compounds while ensuring the penetration and wetting of the aluminum alloy melt. The above parameter range allows the composite material to obtain improved interface microstructure and higher mechanical shear strength after T6 heat treatment. When the powder layer thickness is 20 μm, a balance is achieved between blocking element diffusion and ensuring molding quality, exhibiting the best mechanical properties.

Claims

1. A steel-aluminum bimetallic composite material based on a powder interlayer, characterized in that, Includes a steel substrate, a powder interlayer, and an aluminum alloy; The powder intermediate layer is obtained by sintering high-entropy alloy powder condensed into a powder layer on the surface of the steel substrate. The high-entropy alloy powder is CoCrFeNiAl alloy powder, and the thickness of the powder layer on the surface of the steel substrate is 10-20 μm. The steel-aluminum bimetallic composite material based on the powder intermediate layer is obtained by hot-dip galvanizing a steel substrate with a powder layer after sintering, assembling it with a steel mold, and then pouring an aluminum alloy solution into the mold and cooling and solidifying it.

2. The steel-aluminum bimetallic composite material according to claim 1, characterized in that, The high-entropy alloy powder has a particle size of 1–5 μm and is prepared by high-energy ball milling of CoCrFeNiAl alloy powder with an initial particle size of 20 μm.

3. The steel-aluminum bimetallic composite material according to claim 1, characterized in that, The steel substrate is made of 42CrMo steel or quenched and tempered high-strength steel; the aluminum alloy is made of commercially available finished product ZL205A.

4. A method for preparing a steel-aluminum bimetallic composite material based on a powder interlayer as described in any one of claims 1-3, characterized in that, Includes the following steps: Pre-treatment of the steel substrate surface; The pretreated steel substrate is immersed in the prepared powder solution for lifting, and then naturally dried to allow the polyvinyl butyral and high-entropy alloy powders to condense and form a powder layer on the surface of the steel substrate. The steel substrate with the powder layer is placed in a tube furnace and subjected to debinding and sintering processes in sequence. The steel substrate after sintering is hot-dip coated, wherein the steel substrate is immersed in a hot-dip coating aluminum alloy solution; The preheated steel mold is assembled with the hot-dip galvanized steel substrate, and an aluminum alloy solution for casting is poured into the mold and then cooled and solidified. The composite material after cooling and curing is subjected to T6 heat treatment to obtain the steel-aluminum bimetallic composite material based on the powder interlayer.

5. The preparation method according to claim 4, characterized in that, The powder solution is prepared from the following raw materials in parts by weight: 77.5 parts by weight of anhydrous ethanol, 2.5 parts by weight of PVB, and 20 parts by weight of CoCrFeNiAl alloy powder.

6. The preparation method according to claim 4, characterized in that, The specific method for obtaining a powder layer on the surface of a steel substrate is as follows: the pretreated steel substrate is immersed in the prepared powder solution, and the substrate is lifted at a lifting speed of 5 mm / s. The lifting operation is repeated 2 or 3 or more times until a powder layer with a thickness of 10 to 20 μm is obtained on the surface of the steel substrate.

7. The preparation method according to claim 4, characterized in that, The process parameters for the glue removal process include: under vacuum and high-purity argon gas, the heating rate is set to 1℃ / min, the temperature is held at 250℃ for 1 hour, and then the temperature is raised to 450℃ and held for 1 hour. During this period, the venting operation is performed once every 10 minutes. The process parameters for the sintering process include: after the glue is removed and the air is vented, the heating rate is set to 6℃ / min, the temperature is held at 900℃ for 30min, and then cooled with the furnace.

8. The preparation method according to claim 4, characterized in that, The specific implementation method of the hot-dip plating treatment is as follows: the temperature of the hot-dip plating process is controlled at 710℃, the steel substrate is immersed in the hot-dip plating aluminum alloy solution and kept for 5 minutes before being taken out. The steel mold is preheated to 250°C, and the aluminum alloy solution for casting is poured into the mold at 725°C.

9. The preparation method according to claim 4, characterized in that, The heat treatment is a T6 heat treatment, which includes solution treatment, quenching treatment, and annealing treatment in sequence. The conditions for solution treatment include: controlling the heating rate to be 100℃ / h, the solution temperature to be 540℃, and the solution time to be 10-12h. The quenching treatment is water quenching in water at 80℃. The conditions for annealing treatment include: holding at 175℃ for 5h and cooling to room temperature.

10. The preparation method according to claim 4, characterized in that, The pretreatment process includes, in sequence, degreasing, derusting, surface polishing to remove the surface stress layer, acid immersion, ultrasonic cleaning with alcohol, and drying of the steel substrate surface.

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