A method for preparing an aluminum-based composite material in which silicon carbide nanocrystals and micron-sized particles are layered.

Abstract

A method for preparing an aluminum-based composite material in which silicon carbide nanocrystals and micron-sized particles are layered is disclosed. This invention addresses the problems of uncontrollable reinforcement distribution, difficulty in oriented whisker alignment, and difficulty in achieving dense impregnation of preforms under high ceramic content conditions in existing aluminum-based composite materials. The steps of this invention include: Step 1, acid washing of the whiskers and particles; Step 2, preparation of a slurry; Step 3, freeze casting to form a preform; Step 4, vacuum freeze drying; Step 5, sintering of the preform; and Step 6, extrusion impregnation of the aluminum-based composite material. This invention belongs to the field of aluminum-based composite material preparation technology.

Description

Technical Field

[0001] This invention relates to a method for preparing aluminum-based composite materials, belonging to the field of aluminum-based composite material preparation technology. Background Technology

[0002] Aluminum-based composites, due to their high specific strength, excellent thermal properties, and corrosion resistance, hold significant application potential in aerospace, electronic packaging, and high-end equipment manufacturing. Introducing ceramic reinforcements (such as particles, whiskers, and fibers) can significantly improve their strength, modulus, wear resistance, and high-temperature service performance. However, achieving aluminum-based composites with high strength, high modulus, and high fracture toughness hinges on the precise design of the reinforcement configuration, especially the spatial distribution and scale matching of the reinforcement. In recent years, hybrid reinforcement structures—such as micron-sized particles providing high load-bearing capacity and nano-whiskers providing toughening and crack propagation inhibition—have been proven to significantly improve the overall performance of composite materials.

[0003] Among various reinforcement configurations, layered structures have gradually become an important design direction in aluminum matrix composites due to their significant advantages in directional mechanical properties, crack propagation path control, and improved interfacial bonding quality. Layered structures can form stress redistribution and crack deflection mechanisms at the microscale, achieving a better balance between strength and toughness. Simultaneously, the controllability of the interlayer interface helps improve the reliability of materials under complex loading environments. Traditional aluminum matrix composite preparation methods, such as powder metallurgy, melt casting, and mechanical stirring, while capable of introducing particles or short fibers into the matrix, often result in a homogeneous reinforcement distribution, making it difficult to actively construct layered structures with orientation and periodic characteristics. Furthermore, whisker-type high aspect ratio reinforcements are prone to agglomeration during traditional mixing processes, leading to uncontrollable distribution and random orientation, thus failing to leverage the toughening and reinforcing effects of whiskers.

[0004] Cryo-casting technology offers a unique and effective approach for constructing layered structures. Its core mechanism utilizes the directional growth behavior of solvent ice crystals to drive the selective arrangement of reinforcements during freezing, thereby forming ceramic preforms with periodic lamellars. This method is particularly suitable for constructing ceramic networks with orientation requirements, facilitating precise spatial control of the reinforcements. However, when cryo-casting is applied to hybrid systems of "micron-sized SiC particles + nano-sized SiC whiskers," existing research and technologies face the following challenges:

[0005] (1) It is difficult to achieve stable dispersion and directional arrangement of whiskers. For example, although patent CN107815622A uses mechanical mixing and sintering to build whisker reinforcement, it does not address the dispersion problem of whiskers in the solvent ice crystal growth process in the cryogenic casting system, which leads to the easy agglomeration of whiskers and their inability to align with the preferred growth direction of ice crystals.

[0006] (2) The difference in behavior between particles and whiskers makes it impossible to naturally construct a layered structure. For example, although patent CN110818395B uses freeze casting to prepare layered aluminum matrix composites, the reinforcement is mainly particles obtained by ball milling, which does not involve the orientation difference caused by the aspect ratio of whiskers, and does not solve the technical problem that micron particles and nano whiskers have different rejection effects at the freezing interface and are difficult to form a stable layered interface in a coordinated manner.

[0007] (3) The lack of hierarchical control over the pore structure affects the infiltration process. For example, patent CN109112439B constructs a structure through the stacking of fine filaments, but the internal pore morphology is uniform and does not provide a combination of interlayer channels and intralayer fine pores suitable for the infiltration of molten aluminum; patents CN1246113C and CN120757369B belong to the powder metallurgy system and construct a layered structure through the difference between coarse and fine powders, but the pores are uncontrollable and cannot meet the hierarchical structure requirements generated by freeze casting. Therefore, they are difficult to use in the extrusion infiltration process with high ceramic content.

[0008] (4) Existing cryogenic casting only focuses on the layered ceramic structure itself and does not form an integrated metal infiltration design. For example, CN110818395B only realizes the ceramic layered structure, but does not consider the influence of the layered structure on the flow path of molten aluminum, the transmission of infiltration pressure, and the integrity of interlayers. This leads to problems such as incomplete infiltration and lamellar breakage in aluminum alloy systems with high ceramic content or high viscosity.

[0009] In summary, while existing technologies can prepare certain layered or oriented structures through freeze casting or other methods, in multi-scale hybrid reinforcement systems, problems such as difficulty in whisker orientation, difficulty in separating particles and whiskers, uncontrollable pore structure, and disconnection from the infiltration process are still prevalent, making it difficult to meet the true requirements of high-performance aluminum matrix composites for layered structures. Therefore, although layered structures have been proven to significantly improve the mechanical properties of reinforced aluminum matrix composites, how to construct ceramic layered preforms with controllable orientation, controllable pore structure, and infiltration capability in multi-scale hybrid reinforcement systems remains an unsolved problem.

[0010] Compared with the prior art, the method of the present invention has the following unique features: By utilizing the directional ice crystal growth of cryogenic casting, the whiskers are preferentially aligned along the freezing direction, constructing an orientation-reinforced structure that is difficult to obtain using traditional methods. Micron-sized SiC particles are enriched at the lamellar interfaces, forming natural "interlayer reinforcement bands" and constituting a unique hybrid layered system. By controlling parameters such as freezing rate and solid content, a hierarchical pore structure of "large-sized interlayer pores + small-scale intralayer pores" is formed, providing continuous channels for molten aluminum infiltration. Unlike the layered structure of powder metallurgy, the layered configuration of this method comes from the natural selective distribution of the reinforcement, rather than relying on the difference in powder particle size or mechanical film breaking, resulting in a more refined and controllable structure. This layered structure not only improves the efficiency of infiltration molding but also achieves multiple strengthening mechanisms in the final composite material, including crack propagation resistance, stress deflection, and orientation enhancement. While traditional cryogenic casting can induce ice crystals to grow along the temperature gradient, it has the following limitations for nanocrystals: the ice crystal arrangement force is weak, failing to provide sufficient torque to orient the whiskers along the freezing direction; nanocrystals aggregate before freezing, making "single-crystal orientation" impossible; whiskers at interlayer interfaces are more likely to be randomly distributed, hindering the formation of a continuous strengthening network; and if the whiskers are not well-oriented, toughening mechanisms such as bridging and crack deflection are difficult to fully utilize. Literature generally points out that "the degree of whisker orientation directly affects the strength, modulus, and crack propagation path of the composite material," but conventional cryogenic casting methods cannot achieve the required level of orientation control.

[0011] In conventional extrusion impregnation: micron-sized particles increase interlayer resistance, leading to incomplete impregnation; the whisker accumulation zone experiences concentrated stress, making it prone to brittle fracture; and the layered structure is damaged, twisted, or crushed by high pressure. Therefore, special designs are required for the impregnation direction, pressurization mode, preheating, and wetting strategies; otherwise, the integrity and compactness of the layered structure cannot be guaranteed.

[0012] Traditional ball milling methods have the following problems when preparing whisker / particle frozen casting systems: they cannot simultaneously meet the requirements for particle dispersion and whisker integrity; uncontrollable temperature leads to viscosity changes, which in turn affect the freezing behavior of the slurry; the air environment causes oxidation and high gas content, and solvent evaporation causes a shift in solid content; secondary defoaming causes abrupt changes in slurry temperature and viscosity, destroying the dispersion state. Summary of the Invention

[0013] To address the problems of uncontrollable reinforcement distribution, difficulty in oriented arrangement of whiskers, and difficulty in dense impregnation of preforms under high ceramic content in existing aluminum-based composite materials, this invention proposes a method for preparing aluminum-based composite materials in which silicon carbide nanocrystals and micron-sized particles are distributed in a layered manner.

[0014] The technical solution adopted by the present invention to solve the above problems is as follows: The steps of the present invention include: Step 1: Acid washing of whiskers and particles; Step 2: Prepare the slurry; Step 3: Freeze casting to form precast blocks; Step 4: Vacuum freeze drying; Step 5: Sintering the precast blocks; Step 6: Extrusion impregnation of the aluminum-based composite material.

[0015] Furthermore, step 1 specifically includes: Step 101: Select reinforcement: Select SiC particles with an average particle size of 1.5 μm and SiC whiskers with a length of 20-30 μm and a diameter of 0.5 μm as reinforcement; Step 102: Place SiC micron particles and SiC nanocrystals separately in an acidic pretreatment system to remove surface oxides and prepare surface active sites; The acid solution is formulated as follows: HF 1.0–1.5 vol%, HNO3 0.5–1.0 vol%, deionized water to bring the total to 100 vol%, and total acid content ≤2.5 vol%. Step 103: Perform chemical etching under stirring conditions for 5–20 min, maintaining the solution temperature at 15–25°C throughout the process, and using a magnetic stirring speed of 300–800 rpm; Step 104: Simultaneously apply ultrasound to assist in the removal of surface impurities and inhibit whisker entanglement; Step 105: After the chemical treatment is completed, use dynamic centrifugation-intermittent rinsing: centrifuge at a speed of 2000–3500 rpm, inject deionized water in batches and rinse alternately 3 to 6 times, each rinse lasting 2 to 5 minutes, until pH≈7. Step 106: The cleaned reinforcement is subjected to vacuum semi-drying treatment: temperature control 40~50℃, vacuum <1000 Pa, time 30–90 min. The purpose is to remove surface free water and maintain micro-wet dispersion, without complete drying to facilitate the next step of wet coating / ball milling.

[0016] Furthermore, step 2 specifically includes: Step 201: Solid phase ratio and solid content: Mix whiskers and particles at a volume ratio of 1:1, with a target total solid phase volume content of 15-25 vol%. Step 202: Dispersant and solvent: Prepare a solution matrix with a silica sol:deionized water volume ratio of 3:7, add sodium carboxymethyl cellulose as the main dispersant / thickener, with a CMC mass fraction of 0.1~0.4 wt% of the total slurry; and add 0.05~0.2 wt% of interface modifier to improve whisker wettability; Step 203, First stage: SiC micron particles, binder and dispersant are subjected to high-energy ball milling under vacuum and constant temperature conditions at a speed of 250~350 r / min for 6~10 h, with a ball-to-material ratio of 5:1~10:1, and the process is carried out at a temperature ≤ 40°C under a vacuum atmosphere to ensure that the particle surface and binder are fully coated and that no large amount of solvent volatilization occurs. Step 204, Second stage: First, cool the first stage slurry to 20~30°C, then add SiC whiskers, and stir at low speed, with a speed of 100~180 r / min for 4~10 h. Physical shearing and mild shear field promote the whisker to spread and be evenly distributed in the slurry, avoiding whisker breakage. Step 205: The ball milling process is kept at a constant temperature of 20~30°C and under short-term vacuum throughout, while low-amplitude ultrasonic assistance is used to remove micro-agglomerates without damaging whiskers; Step 206, Full Process Control: The ball-milled slurry can be used directly for cryogenic casting without the need for long-term degassing; if bubbles are present, use low vacuum degassing for 5-15 minutes to avoid excessive degassing that could lead to whisker agglomeration.

[0017] Furthermore, step 3 specifically includes: Step 301, Injection mold and initial conditions: Inject the prepared wet slurry into the polytetrafluoroethylene mold. The filling depth and the heat conduction parameters of the mold wall are pre-calibrated. After injection, let it stand for 1 to 3 minutes to release large air bubbles. Step 302, First Stage: Slow Freezing, Temperature Range: 10°C to 20°C; Holding time: 30s~10min; Target freezing rate at the interface: <2 μm / s; Step 303, Second Stage: Rapid Freezing, immediately and rapidly reduce the cold end temperature to 50°C to At 70°C, a freezing interface rate of 5~20 μm / s was achieved; the sample was kept frozen until it was completely frozen.

[0018] Furthermore, step 4 specifically includes: The frozen precast blocks are transferred to a vacuum freeze-drying device and a staged gradient sublimation drying method is used to avoid the collapse of the layered structure caused by strong sublimation in one go. Step 401, Drying temperature and vacuum control: Initial sublimation region: temperature 35℃~ At 25℃ and a vacuum of ≤20 Pa, maintain for 12~24 h to allow the ice crystals in the macropores to sublimate first. Mid-stage sublimation zone: Temperature slowly rises to 15℃~ At 5℃, under vacuum ≤15 Pa, maintain for 24~48 h to gradually stabilize the fine porous structure within the layer; Final fixation zone: Temperature is raised to 0℃~25℃, vacuum ≤10 Pa, and maintained for 12~36 h to completely remove bound water and stabilize the ceramic skeleton; Step 402, Structure Preservation Mechanism: The phased sublimation method allows the ice crystal surface to gradually recede, avoiding damage to the lamellar structure by a one-time high sublimation rate, ensuring the integrity of the particle-whisker network, and achieving the preservation of a dual-scale structure of interlayer channels and intralayer pores.

[0019] Furthermore, step 5 specifically includes: Step 501: After vacuum freeze drying is completed, the preform is placed in a sintering furnace with micro-oxygen control function for sintering treatment. Step 502: Use a micro-oxygen environment as the sintering atmosphere. By controlling the oxygen volume fraction in the furnace cavity within the range of 0.5% to 6%, the silica sol can be slowly converted into a continuous SiO2 thin layer during the sintering process, while effectively avoiding tip oxidation or structural damage of silicon carbide whiskers under high temperature conditions. Step 503: The sintering process adopts a dual-temperature zone heating strategy to ensure that the preform can maintain a stable layered structure during the debinding, necking, and main sintering stages. Step 503: Slowly heat the preform to 680-720°C at a heating rate of 1-3°C / min, and maintain it in this temperature range for 0.5-1.5 hours to allow the binder to completely decompose and be discharged, while simultaneously forming an initial neck structure between the silicon carbide particles and whiskers. Step 504: Continue to raise the temperature to 830-900℃ at a low rate, and keep it in this temperature range for 1-2 hours.

[0020] Furthermore, step 6 specifically includes: Step 601: Select 2024 aluminum alloy ingot as the matrix material for impregnation; Step 602: Heat the aluminum ingot to a temperature range of 780-830℃ to make it highly fluid but not excessively hot, and keep it in this temperature range for 20-40 minutes to allow the inclusions in the molten metal to float up fully, while reducing the hydrogen content in the molten metal to obtain a clean molten metal with good wettability. During this stage, a small amount of argon gas and a small amount of [unclear] are briefly introduced into the furnace cavity. The mixed gas is used to further reduce the thickness of the oxide film on the surface of the melt, thereby improving the wetting effect between the melt and the ceramic skeleton; Step 603: Preheat the sintered ceramic preform to a temperature range of 480-560℃ and maintain it for 15-30 minutes; Step 604: After completing the preparation of the aluminum melt and the preheating of the preform, place the ceramic preform in the extrusion impregnation device and bring it into contact with the aluminum melt, and then start the three-stage pressure impregnation process. First, apply an initial pressure of 1.5–3 MPa and maintain it for 1–2 minutes to allow molten aluminum to preferentially enter the interlayer main channels of the preform, forming a preliminary seepage path; The pressure is then increased to 4–6 MPa and maintained for 1.5–3 minutes, allowing the molten aluminum to further fill the mesoscale pores and gradually advance inward. Finally, the pressure is increased to 8–11 MPa and maintained for 4–8 minutes to ensure that the molten aluminum fully penetrates the fine pores within the layer, thereby achieving complete densification of the overall structure of the preform.

[0021] The beneficial effects of this invention are: 1. This invention constructs a layered structure preform of a whisker-particle hybrid system, wherein the whiskers are directionally arranged and the particles are enriched in the interlayer, thereby realizing the synergistic control of multi-scale reinforcement. 2. This invention utilizes cryogenic casting to achieve active design of the spatial distribution of reinforcements, solving the problem that nanocrystals are prone to agglomeration and difficult to orient in hybrid systems; 3. This invention forms a hierarchical pore structure of "large pore channels + small pore supports", which significantly improves the infiltration efficiency of molten aluminum and the densification of composite materials under high ceramic content conditions. 4. This invention achieves enhanced interfacial bonding, homogenized microstructure, and improved comprehensive mechanical properties through extrusion impregnation and subsequent heat treatment, resulting in an aluminum-based composite material with high modulus, high strength, and high fracture toughness. 5. This invention is applicable to aluminum-based composite materials with a volume fraction of 10 vol% to 40 vol% for both nano-SiC whiskers and micron-sized SiC particles. It also possesses good material scalability and can be extended to various hybrid reinforcement systems such as Al2O3, Si3N4, TiC, SiO2, and ZrO2, which combine nano-whiskers (or fibers) with micron-sized particles. Leveraging the ceramic preform configuration design capabilities obtained through freeze casting, this invention is particularly suitable for the preparation of metal-based composite materials requiring high strength and toughness, high modulus, and controllable anisotropic structures.

[0022] 6. This invention obtains a stable whisker / particle hybrid slurry through dispersion treatment. Utilizing the directional growth of ice crystals during freeze casting, nano-whiskers align along the freezing direction, and micron-sized particles naturally accumulate at the lamellar interfaces, forming a ceramic preform with distinct layered characteristics. This structure is then fixed during vacuum freeze-drying. Subsequent sintering yields a hierarchical porous structure with interlayer channels and intralayer micropores. Multi-stage pressure extrusion infiltration further allows the aluminum melt to fully penetrate the layered network, achieving high densification of the composite material. The composite material prepared by this invention simultaneously possesses whisker-oriented reinforcement, particle synergistic reinforcement, and the crack deflection effect of the layered structure, exhibiting high modulus, high strength, and excellent fracture toughness. It is suitable for high-performance structural components in aerospace, electronic heat dissipation, and lightweight equipment. Experimental results show that the whisker-particle hybrid reinforced aluminum matrix composite material prepared by this invention exhibits significant improvements in mechanical properties and structural uniformity. Compactness and porosity: By optimizing the cryogenic casting and extrusion impregnation processes, the composite material achieves a density of over 97% and a porosity controlled below 3%. The layered structure design effectively reduces potential defects in large-size precast structures.

[0023] Mechanical properties: The as-cast compressive strength reaches over 800 MPa, a significant improvement over traditional processes. As-cast K IC Increased to 20.5 MPa·m 1 / 2 The toughening mechanism of whiskers significantly improves the material's resistance to crack propagation.

[0024] Microstructure: The regular layered structure formed by freeze casting ensures the orderly distribution of the reinforcement, resulting in a uniform microstructure without significant particle aggregation or sedimentation. During sintering and extrusion infiltration, the interfacial bonding is excellent, and no obvious interfacial cracks or delamination are observed. This invention, through the combination of freeze casting and extrusion infiltration processes, not only achieves the synergistic effect of the reinforcing phases but also endows the composite material with excellent mechanical properties and structural stability, providing technical support for the engineering application of high-performance aluminum-based composite materials. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the macroscopic morphology of a ceramic preform formed by freeze casting. Figure 2 This is a schematic diagram of the transverse microstructure of a ceramic preform formed by freeze casting; Figure 3 This is a schematic diagram of the longitudinal microstructure of a ceramic preform formed by freeze casting. Figure 4 This is a schematic diagram showing the distribution of whiskers and particles in the preform; Figure 5This is a schematic diagram of the phase composition of the layered silicon carbide hybrid reinforced preform; Figure 6 These are scanning electron micrographs of the microstructure of the as-cast composite material. Figure 7 This is a schematic diagram of the compression test curve of the composite material; Figure 8 This is a schematic diagram of the bending strength test curve of the composite material; Figure 9 This is a schematic diagram of the fracture toughness test curve of the composite material. Detailed Implementation

[0026] Specific Implementation Method 1: The steps of the method for preparing an aluminum-based composite material in which silicon carbide nanocrystals and micron-sized particles are distributed in a layered manner as described in this implementation method include: Step 1: Acid washing of whiskers and particles; Reinforcement selection: SiC particles with an average particle size of 1.5 μm and SiC whiskers with a length of 20-30 μm and a diameter of 0.5 μm were selected as reinforcements; Acid washing was performed using a low-corrosion selective composite acid washing-centrifugal protective separation method: SiC micron particles and SiC nanocrystals were placed in an acidic pretreatment system to remove surface oxides and prepare surface active sites. Acid solution formulation (volume fraction range): HF (40%) 1.0–1.5 vol%, HNO3 (65%) 0.5–1.0 vol%, deionized water to 100 vol%, total acid content ≤ 2.5 vol%; Chemical etching is performed under stirring conditions (magnetic stirring 300–800 rpm) for 5–20 min (8–12 min recommended), maintaining the solution temperature at 15–25°C throughout the process to avoid damage to the whiskers due to temperature rise; Simultaneous application of ultrasound (200–400 W, intermittent 5 s ON / 5 s OFF) to assist in the removal of surface impurities and inhibit whisker entanglement; After the chemical treatment, dynamic centrifugation-intermittent rinsing was used: centrifugation speed 2000–3500 rpm, deionized water was injected in batches and rinsed alternately 3–6 times, each rinse for 2–5 minutes, until pH ≈7; The cleaned reinforcement is subjected to vacuum semi-drying treatment: temperature control 40–50℃, vacuum <1000 Pa, time 30–90 min (40–60 min recommended). The purpose is to remove surface free water and maintain micro-wet dispersibility, without complete drying to facilitate the next step of wet coating / ball milling. This method can significantly improve the surface cleanliness and dispersibility of SiC reinforcements, avoid whisker breakage and agglomeration problems, and provide a process basis for obtaining high-quality composite slurries and stable interfacial bonding.

[0027] Step 2: Prepare the slurry; Solid phase ratio and solid content: Mix according to whisker:particle volume ratio of 1:1 (can be adjusted in the range of 1:2~2:1), target total solid phase volume content of 15~25 vol% (20 vol% recommended); Dispersant and solvent: Prepare a solution matrix with a silica sol:deionized water volume ratio of 3:7, and add sodium carboxymethyl cellulose (CMC) as the main dispersant / thickener, with a CMC mass fraction of 0.1~0.4 wt% of the total slurry; an interface modifier (such as PVP) of 0.05~0.2 wt% can be added to improve whisker wettability; The first stage (particle coating, high-energy ball milling): SiC micron particles, binder and dispersant are subjected to high-energy ball milling under vacuum and constant temperature conditions at a speed of 250~350 r / min for 6~10 h (8 h recommended). The ball-to-material ratio is 5:1~10:1 (steel balls:material). The process is carried out at a temperature ≤40℃ under vacuum or inert atmosphere to ensure that the particle surface is fully coated with the binder and that no large amount of solvent volatilization occurs. The second stage (whisker introduction, low-speed gentle ball milling): First, cool the slurry from the first stage (to 20~30℃), then add SiC whiskers. Use low-speed stirring or low-speed ball milling (100~180 r / min) for 4~10 h (6~8 h recommended). Physical shearing and a gentle shear field promote whisker expansion and uniform distribution in the slurry, avoiding whisker breakage. The ball milling process should be kept at a constant temperature (20~30℃) and under short-term vacuum (intermittent vacuuming and exhaust). Low-amplitude ultrasonic assistance (short time, low power) can be used to remove micro-agglomerates without damaging the whiskers. Full process control: The ball-milled slurry can be used directly for cryogenic casting without the need for long-term degassing; if bubbles are present, low vacuum degassing (<2000 Pa) can be used for 5~15 minutes to avoid excessive degassing leading to whisker agglomeration; This method can significantly improve the uniformity and stability of whiskers and particles in the slurry, prevent the agglomeration and delamination of the reinforcing phase, and ensure the controllability and repeatability of the layered structure in the subsequent cryogenic casting process. Step 3: Freeze casting to form precast blocks; Injection mold and initial conditions: Inject the prepared wet slurry into a polytetrafluoroethylene (PTFE) mold or a mold with good thermal conductivity control. The filling depth and the thermal conductivity parameters of the mold wall are pre-calibrated. After injection, let it stand for 1 to 3 minutes to release large air bubbles. Phase 1: Slow Freezing (Redispersal and Untangling Phase) Temperature range: 10℃ to 20℃; Holding time: 30 s~10 min (depending on sample thickness); Target freezing interface rate: <2 μm / s (achieved through heat flow control between mold and cold source); Physical objective: The low temperature gradient results in a high nucleation density and slow growth of ice crystals. The liquid phase has a low degree of supercooling, which allows whiskers sufficient time to untangle and reorient with local flow. Micron particles can enter the ice crystal gaps and complete local redistribution under slow pushing conditions, significantly reducing the risk of whisker agglomeration. Second stage: Rapid freezing (directional lamination stage): Immediately and rapidly reduce the cold end temperature to... 50℃ to Set the temperature to 70℃ to achieve a freezing interface rate of 5~20 μm / s (adjustable); hold until the sample is completely frozen (5~30 min depending on the sample thickness, generally 5~30 min). Physical objective: Under a large temperature gradient, ice crystal growth is highly oriented, and the ice crystal lamellae generate strong tensile / pushing forces, which force the whiskers and particles to preferentially align and layer and enrich in the direction perpendicular to freezing, forming thinner, more regular lamellae and more uniform interlayer particle bands. During the freezing process, violent vibrations and air introduction should be avoided as much as possible; if the mold is thick, double-end cooling or controlled cold source translation can be used to ensure the consistency of the temperature gradient in unidirectional freezing. Step 4: Vacuum freeze drying; The frozen preforms are transferred to a vacuum freeze-drying device and a staged gradient sublimation drying method is used to avoid the collapse of the layered structure caused by strong sublimation in one go.

[0028] (1) Drying temperature and vacuum control: Initial sublimation region: temperature 35℃ ~ At 25℃, with a vacuum degree ≤ 20 Pa (preferably 3~10 Pa), maintain for 12~24h to allow the ice crystals in the large pores to sublimate first; Mid-stage sublimation zone: Temperature slowly rises to 15℃~ At 5℃ and vacuum ≤ 15 Pa, maintain for 24~48 h to gradually stabilize the fine porous structure within the layer; Final fixation zone: Temperature is raised to 0℃~25℃, vacuum ≤ 10 Pa, and maintained for 12~36 h to completely remove bound water and stabilize the ceramic skeleton.

[0029] (2) Structure retention mechanism: The phased sublimation method allows the ice crystal surface to gradually recede, avoiding the destruction of the lamellar structure by a one-time high sublimation rate, ensuring that the particle-whisker network remains intact, and achieving the preservation of the dual-scale structure of interlayer channels and intralayer pores; Step 5: Sintering the precast blocks; After vacuum freeze-drying, the preforms are placed in a sintering furnace with micro-oxygen control for sintering. This invention uses a micro-oxygen environment as the sintering atmosphere. By controlling the oxygen volume fraction in the furnace cavity within the range of 0.5% to 6%, or by continuously introducing air at a very low flow rate (0.1 to 0.5 L / min), a slightly oxidizing atmosphere is maintained. This allows the silica sol to slowly transform into a continuous SiO2 thin layer during sintering, while effectively preventing the silicon carbide whiskers from undergoing tip oxidation or structural damage at high temperatures.

[0030] The sintering process employs a dual-temperature zone heating strategy to ensure that the preform maintains a stable layered structure throughout the debinding, necking, and main sintering stages. First, the preform is slowly heated to 680–720°C at a heating rate of 1–3°C / min and held within this temperature range for 0.5–1.5 hours. This allows the binder to completely decompose and be expelled, while simultaneously forming an initial necking structure between the silicon carbide particles and whiskers. This stage provides the basic skeletal strength for subsequent high-temperature sintering.

[0031] The temperature was then increased to 830–900℃ using a low-rate heating method and held at this temperature for 1–2 hours. Within this temperature range, the silica sol inside the preform gradually decomposed and transformed into a continuously distributed silica thin layer, while simultaneously reacting with some alumina to form a SiO2–Al2O3 composite interface phase. Under the stabilizing effect of a micro-oxygen atmosphere, this interface phase could be uniformly generated along the layered ceramic sheets, making the interlayer bonding more stable and effectively improving the overall mechanical strength and thermal shock resistance of the preform. Through this dual-temperature sintering route, the ceramic skeleton not only maintains a clear and regular layered structure, but also achieves higher structural stability based on significant whisker bridging, providing a robust and reliable three-dimensional support network for subsequent aluminum alloy melt infiltration. Step 6: Extrusion impregnation of the aluminum-based composite material; After sintering, the preform is used in the extrusion infiltration step of aluminum alloy melt; a quasi-solid infiltration and multi-stage pressure coupling method is adopted to meet the infiltration requirements of multi-scale pores in the layered structure, while enabling the molten aluminum to fully fill the pore structure of each level inside the preform. First, 2024 aluminum alloy ingots were selected as the matrix material for impregnation; The aluminum ingot is heated to a temperature range of 780-830℃ to make it highly fluid but not excessively heated, and held at this temperature range for 20-40 minutes to allow the inclusions in the molten metal to float to the surface, while reducing the hydrogen content in the molten metal to obtain a clean molten metal with good wettability. During this stage, a small amount of argon gas or a mixture of argon gas and a small amount of argon gas can be briefly introduced into the furnace cavity. The mixed gas is used to further reduce the thickness of the oxide film on the surface of the melt, thereby improving the wetting effect between the melt and the ceramic skeleton; Meanwhile, the sintered ceramic preform is preheated within a temperature range of 480–560°C and maintained for 15–30 minutes. This preheating step aims to reduce the temperature difference between the ceramic preform and the molten aluminum, preventing interlayer cracks caused by thermal shock, while also improving interfacial wettability and flow driving force. Since the preform contains a multi-scale pore structure with both large interlayer pores and fine intralayer pores, proper preheating can help to equalize the overall structural temperature, thereby improving the stability of the melt entering the fine pore channels.

[0032] After the aluminum melt preparation and preheating of the preform are completed, the ceramic preform is placed in the extrusion impregnation device and brought into contact with the aluminum melt, and then the three-stage pressure impregnation process is started. First, apply an initial pressure of 1.5–3 MPa and maintain it for 1–2 minutes to allow molten aluminum to preferentially enter the interlayer main channels of the preform, forming a preliminary seepage path; The pressure is then increased to 4–6 MPa and maintained for 1.5–3 minutes, allowing the molten aluminum to further fill the mesoscale pores and gradually advance inward. Finally, the pressure is increased to 8–11 MPa and maintained for 4–8 minutes to ensure that the molten aluminum fully penetrates the fine pores within the layer, thereby achieving complete densification of the overall structure of the preform.

[0033] By employing a quasi-solid metallic state and a multi-stage pressure coupling process, the aluminum melt can penetrate sequentially in the order of "large interlayer pores → medium-scale pores → fine intralayer pores," effectively reducing the possibility of unimpregnated areas and closed pores. This results in an aluminum-based composite material with a highly uniform metal-ceramic interface and good overall density. The final composite material possesses high strength, high toughness, and high modulus, and its layered structure exhibits excellent crack propagation resistance under mechanical loads.

[0034] Working principle This invention utilizes the directional growth of solvent crystals to drive spatial rearrangement of silicon carbide reinforcement during freezing, resulting in preferred orientation of nanoscale SiC whiskers along the freezing direction, while micron-sized SiC particles form enrichment regions at the lamellar interfaces, thus obtaining a ceramic preform with a distinct layered structure. Subsequently, its multi-scale pore network is maintained through vacuum freeze-drying, and the hierarchical pore structure is utilized during sintering and extrusion infiltration to achieve rapid and thorough infiltration of molten aluminum, ultimately yielding an aluminum-based composite material with high modulus, high strength, and high toughness.

[0035] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent substitutions, and improvements made to the above embodiments without departing from the scope of the present invention, based on the technical essence of the present invention and within the spirit and principles of the present invention, shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing an aluminum-based composite material in which silicon carbide nanocrystals and micron-sized particles are layered and distributed, characterized in that, The specific steps include: Step 1: Acid washing of whiskers and particles; Step 2: Prepare the slurry; Step 3: Freeze casting to form precast blocks; Step 4: Vacuum freeze drying; Step 5: Sintering the precast blocks; Step 6: Extrusion impregnation of the aluminum-based composite material.

2. The method for preparing an aluminum-based composite material with silicon carbide nanocrystals and micron-sized particles in a layered distribution according to claim 1, characterized in that, Step 1 specifically includes: Step 101: Select reinforcement: Select SiC particles with an average particle size of 1.5 μm and SiC whiskers with a length of 20-30 μm and a diameter of 0.5 μm as reinforcement; Step 102: Place SiC micron particles and SiC nanocrystals separately in an acidic pretreatment system to remove surface oxides and prepare surface active sites; The acid solution was formulated as follows: HF 1.0–1.5 vol%, HNO3 0.5–1.0 vol%, deionized water to bring the total to 100 vol%, and total acid content ≤2.5 vol%. Step 103: Perform chemical etching under stirring conditions for 5–20 min, maintaining the solution temperature at 15–25°C throughout the process, and using a magnetic stirring speed of 300–800 rpm; Step 104: Simultaneously apply ultrasound to assist in the removal of surface impurities and inhibit whisker entanglement; Step 105: After the chemical treatment is completed, use dynamic centrifugation-intermittent rinsing: centrifuge at a speed of 2000–3500 rpm, inject deionized water in batches and rinse alternately 3 to 6 times, each rinse lasting 2 to 5 minutes, until pH≈7. Step 106: The cleaned reinforcement is subjected to vacuum semi-drying treatment: temperature control 40~50℃, vacuum <1000 Pa, time 30–90 min. The purpose is to remove surface free water and maintain micro-wet dispersion, without complete drying to facilitate the next step of wet coating / ball milling.

3. The method for preparing an aluminum-based composite material with silicon carbide nanocrystals and micron-sized particles in a layered distribution according to claim 1, characterized in that, Step 2 specifically includes: Step 201: Solid phase ratio and solid content: Mix whiskers and particles at a volume ratio of 1:1, with a target total solid phase volume content of 15-25 vol%. Step 202: Dispersant and solvent: Prepare a solution matrix with a silica sol:deionized water volume ratio of 3:7, add sodium carboxymethyl cellulose as the main dispersant / thickener, with a CMC mass fraction of 0.1~0.4 wt% of the total slurry; and add 0.05~0.2 wt% of interface modifier to improve whisker wettability; Step 203, First stage: SiC micron particles, binder and dispersant are subjected to high-energy ball milling under vacuum and constant temperature conditions at a speed of 250~350 r / min for 6~10 h, with a ball-to-material ratio of 5:1~10:1, and the process is carried out at a temperature ≤ 40°C under a vacuum atmosphere to ensure that the particle surface and binder are fully coated and that no large amount of solvent volatilization occurs. Step 204, Second stage: First, cool the first stage slurry to 20~30°C, then add SiC whiskers, and stir at low speed, with a speed of 100~180 r / min for 4~10 h. Physical shearing and mild shear field promote the whisker to spread and be evenly distributed in the slurry, avoiding whisker breakage. Step 205: The ball milling process is kept at a constant temperature of 20~30°C and under short-term vacuum throughout, while low-amplitude ultrasonic assistance is used to remove micro-agglomerates without damaging whiskers; Step 206, Full Process Control: The ball-milled slurry can be used directly for cryogenic casting without the need for long-term degassing; if bubbles are present, use low vacuum degassing for 5-15 minutes to avoid excessive degassing that could lead to whisker agglomeration.

4. The method for preparing an aluminum-based composite material with silicon carbide nanocrystals and micron-sized particles in a layered distribution according to claim 1, characterized in that, Step 3 specifically includes: Step 301, Injection mold and initial conditions: Inject the prepared wet slurry into the polytetrafluoroethylene mold. The filling depth and the heat conduction parameters of the mold wall are pre-calibrated. After injection, let it stand for 1 to 3 minutes to release large air bubbles. Step 302, First Stage: Slow Freezing, Temperature Range: 10°C to 20°C; Holding time: 30s~10min; Target freezing rate at the interface: <2 μm / s; Step 303, Second Stage: Rapid Freezing, immediately and rapidly reduce the cold end temperature to 50°C to At 70°C, a freezing interface rate of 5~20 μm / s was achieved; the sample was kept frozen until it was completely frozen.

5. The method for preparing an aluminum-based composite material with silicon carbide nanocrystals and micron-sized particles in a layered distribution according to claim 1, characterized in that, Step 4 specifically includes: The frozen precast blocks are transferred to a vacuum freeze-drying device and a staged gradient sublimation drying method is used to avoid the collapse of the layered structure caused by strong sublimation in one go. Step 401, Drying temperature and vacuum control: Initial sublimation region: temperature 35℃~ At 25℃ and a vacuum of ≤20 Pa, maintain for 12~24 h to allow the ice crystals in the macropores to sublimate first. Mid-stage sublimation zone: Temperature slowly rises to 15℃~ At 5℃, under vacuum ≤15 Pa, maintain for 24~48 h to gradually stabilize the fine porous structure within the layer; Final fixation zone: Temperature is raised to 0℃~25℃, vacuum ≤10 Pa, and maintained for 12~36 h to completely remove bound water and stabilize the ceramic skeleton; Step 402, Structure Preservation Mechanism: The phased sublimation method allows the ice crystal surface to gradually recede, avoiding damage to the lamellar structure by a one-time high sublimation rate, ensuring the integrity of the particle-whisker network, and achieving the preservation of a dual-scale structure of interlayer channels and intralayer pores.

6. The method for preparing an aluminum-based composite material with silicon carbide nanocrystals and micron-sized particles in a layered distribution according to claim 1, characterized in that, Step 5 specifically includes: Step 501: After vacuum freeze drying is completed, the preform is placed in a sintering furnace with micro-oxygen control function for sintering treatment. Step 502: Use a micro-oxygen environment as the sintering atmosphere. By controlling the oxygen volume fraction in the furnace cavity within the range of 0.5% to 6%, the silica sol can be slowly converted into a continuous SiO2 thin layer during the sintering process, while effectively avoiding tip oxidation or structural damage of silicon carbide whiskers under high temperature conditions. Step 503: The sintering process adopts a dual-temperature zone heating strategy to ensure that the preform can maintain a stable layered structure during the debinding, necking, and main sintering stages. Step 503: Slowly heat the preform to 680-720°C at a heating rate of 1-3°C / min, and maintain it in this temperature range for 0.5-1.5 hours to allow the binder to completely decompose and be discharged, while simultaneously forming an initial neck structure between the silicon carbide particles and whiskers. Step 504: Continue to raise the temperature to 830-900℃ at a low rate, and keep it in this temperature range for 1-2 hours.

7. The method for preparing an aluminum-based composite material with silicon carbide nanocrystals and micron-sized particles in a layered distribution according to claim 1, characterized in that, Step 6 specifically includes: Step 601: Select 2024 aluminum alloy ingot as the matrix material for impregnation; Step 602: Heat the aluminum ingot to a temperature range of 780-830℃ to make it highly fluid but not excessively hot, and keep it in this temperature range for 20-40 minutes to allow the inclusions in the molten metal to float up fully, while reducing the hydrogen content in the molten metal to obtain a clean molten metal with good wettability. During this stage, a small amount of argon gas and a small amount of [unclear] are briefly introduced into the furnace cavity. The mixed gas is used to further reduce the thickness of the oxide film on the surface of the melt, thereby improving the wetting effect between the melt and the ceramic skeleton; Step 603: Preheat the sintered ceramic preform to a temperature range of 480-560℃ and maintain it for 15-30 minutes; Step 604: After completing the preparation of the aluminum melt and the preheating of the preform, place the ceramic preform in the extrusion impregnation device and bring it into contact with the aluminum melt, and then start the three-stage pressure impregnation process. First, apply an initial pressure of 1.5–3 MPa and maintain it for 1–2 minutes to allow molten aluminum to preferentially enter the interlayer main channels of the preform, forming a preliminary seepage path; The pressure is then increased to 4–6 MPa and maintained for 1.5–3 minutes, allowing the molten aluminum to further fill the mesoscale pores and gradually advance inward. Finally, the pressure is increased to 8–11 MPa and maintained for 4–8 minutes to ensure that the molten aluminum fully penetrates the fine pores within the layer, thereby achieving complete densification of the overall structure of the preform.

Images