A method for constructing a metal-composite interface connection structure suitable for a cryogenic environment

Abstract

The application discloses a kind of cryogenic environment metal-composite material interface connection structure's construction method, belongs to composite material environment interface connection and manufacturing technical field.The method includes the following steps: plasma treatment is carried out to metal substrate layer surface;Interface functional layer is constructed on metal surface using coupling agent;Preparation contains low thermal expansion or negative thermal expansion filler's bonding glue forms low thermal expansion connecting layer;Lay-up fiber reinforced composite material layer and solidification.The close combination of metal substrate layer to fiber reinforced composite material layer is realized by constructing five-layer coordination system.The experimental results show that the connection structure prepared by the method, the cryogenic shear strength reaches 29.2 MPa, increases by 123% compared with traditional process;In the thermal cycle test from-196 DEG C to room temperature, the cycle life is more than 100 times, effectively solve the interface cracking problem caused by thermal expansion coefficient mismatch under cryogenic environment.

Description

Technical Field

[0001] This invention relates to the field of composite material interface bonding and manufacturing technology in cryogenic environments, specifically to a method for constructing a metal-composite material interface bonding structure in cryogenic environments. The method includes steps such as metal surface pretreatment, surface activation, coupling agent interface construction, low thermal expansion bonding layer preparation, adhesive bonding and curing, and composite molding. Background Technology

[0002] With the rapid development of liquid hydrogen energy, commercial aerospace, deep space exploration, and cryogenic equipment, cryogenic pressure-bearing structures are continuously evolving towards lightweight, high load-bearing capacity, high reliability, and long service life. While traditional all-metal pressure-bearing structures possess excellent airtightness, processing performance, and engineering reliability, their high material density limits their further development in applications highly sensitive to structural quality, such as launch vehicles, aircraft, and liquid hydrogen storage and transportation systems. Therefore, hybrid pressure-bearing structures composed of metal materials and fiber-reinforced polymer matrix composites, fully leveraging the superior sealing performance of metal materials and the high specific strength, high specific stiffness, and low density of composite materials, have become an important development direction for cryogenic pressure-bearing equipment.

[0003] For the bonding between metals and composite materials, existing technologies have formed a relatively mature process system. Typically, processes such as metal surface pretreatment, interface activation, surface chemical modification, and adhesive molding are used to improve the bonding performance between the metal and the adhesive layer.

[0004] However, these mature processes are mainly designed for room temperature or general low-temperature environments and are difficult to adapt to extreme cryogenic environments. When applied to cryogenic environments, due to the huge difference in the thermal expansion coefficients of metals and composite materials, existing mature processes cannot effectively buffer the severe thermal shrinkage stress, leading to easy cracking or delamination at the interface. Therefore, existing mature process systems still have significant shortcomings in cryogenic pressure-bearing structures. How to construct an interface connection structure that combines low thermal expansion characteristics with high strength while meeting engineering manufacturing requirements, in order to solve the problem of thermal stress concentration at the metal-composite material interface in cryogenic environments, has become a key technical problem that urgently needs to be solved. Based on this need, this invention proposes a method for constructing a low thermal expansion metal-composite material connection structure suitable for cryogenic pressure-bearing structures. Summary of the Invention

[0005] Overall concept

[0006] This invention addresses the service requirements of connection areas in cryogenic pressure-bearing structures by proposing a method for constructing metal-composite material interface connection structures suitable for cryogenic environments. This invention is not limited to single interface modification or single adhesive layer optimization, but rather focuses on the overall construction of the connection area, building a multi-layered functional system consisting of an interface activation layer, an interface functional layer, and a low thermal expansion connection layer.

[0007] This system enables multi-scale and multi-functional collaborative operation in the connection area, effectively alleviating thermal stress concentration in cryogenic environments and improving the overall performance and long-term service reliability of the structure.

[0008] Technical solution

[0009] This invention provides a method for constructing a metal-composite material interface bonding structure suitable for cryogenic environments.

[0010] The overall process flow of the preparation of this invention is shown in Figure 2. Specifically, it includes the following steps: Step S1 (Metal Surface Pretreatment) The metal substrate surface undergoes graded cleaning and roughening treatment, employing steps such as ultrasonic overall cleaning and organic solvent degreasing to thoroughly remove oil, cutting residues, oxidation impurities, and contaminants from the metal surface. For surfaces with stubborn oxide layers, cross-directional mechanical grinding is used to further enhance the surface's mechanical bonding ability, ultimately obtaining a clean, highly active, and uniformly rough metal substrate layer.

[0011] Step S2 (Plasma Activation Treatment)

[0012] The purpose of interface activation treatment is to increase the surface energy of the metal and introduce active functional groups, creating conditions for the subsequent chemical bonding of coupling agents. A plasma surface treatment machine is used to activate the pretreated metal substrate, forming an interface activation layer on the metal surface.

[0013] Under the high-energy action of oxygen plasma, the following reactions occur on the metal surface: oxygen free radicals collide with the metal surface, introducing oxygen-containing functional groups such as hydroxyl and carboxyl groups; simultaneously, the plasma micro-etches the surface, increasing surface roughness and forming numerous micropores. These microstructural changes improve the wettability and adhesion of the metal surface. After treatment, the next step should be initiated immediately, and the post-activation resting time should be strictly controlled to avoid surface energy decay and ensure the metal surface remains in a highly active state.

[0014] Step S3 (Preparation of coupling agent solution and coating curing)

[0015] A coupling agent solution suitable for heterogeneous metal-polymer bonding was prepared. Acidic hydrolysis was used to regulate the full hydrolysis and activation of the coupling agent groups. A wetting agent was added to enhance the solution's wettability on the metal surface. The uniformly prepared coupling agent solution was then evenly coated onto the activated metal surface and pre-cured. During this process, the coupling agent groups underwent a dehydration condensation reaction with the hydroxyl groups on the metal surface, forming stable chemical bonds. The organic functional groups at the other end were exposed, providing reaction sites for subsequent chemical bonding with the epoxy resin adhesive layer.

[0016] Step S4 (Preparation of low thermal expansion coefficient adhesive)

[0017] By filling a polymer matrix with low or negative thermal expansion powders or particles as functional modified fillers, and supplementing them with surfactants to improve the uniformity of filler dispersion, modified polymeric adhesives with excellent rheological properties and controllable coefficients of thermal expansion are prepared. By adjusting the filler doping ratio, the thermal expansion characteristics of different metal substrates are adapted to achieve precise matching of the thermal expansion properties of the adhesive layer and the metal substrate, thereby fundamentally reducing the interfacial thermal stress caused by extreme cryogenic temperature differences and solving the problem of thermal deformation mismatch in heterogeneous materials.

[0018] Step S5 (Preparation of low thermal expansion adhesive layer)

[0019] In a constant temperature, humidity, and clean environment, a modified low-thermal-expansion adhesive is uniformly coated onto the surface of the coupling agent functional layer to form a low-thermal-expansion bonding layer with uniform thickness and a smooth transition. After coating, a vacuum negative pressure degassing process is used to completely eliminate micropores and pinhole defects inside the adhesive layer, ensuring that the adhesive layer is dense and uniform overall.

[0020] Step S6 (Composite Laying and Curing)

[0021] Lay out the carbon fiber prepreg to complete the overall assembly. The prepreg should be laid evenly to avoid warping of the structure after curing due to uneven layering. Place the stacked blanks into an autoclave or vacuum bag for curing.

[0022] After the above six processing steps, the final product is as follows: Figure 1 The cryogenic pressure-bearing connection structure is shown. From bottom to top, the structure includes a metal substrate layer 5, an interface activation layer 4, an interface functional layer 3, a low thermal expansion connection layer 2, and a fiber-reinforced composite material layer 1.

[0023] Beneficial effects

[0024] Compared with the prior art, the present invention has the following outstanding advantages: This invention employs a dual mechanism of "oxygen plasma activation + coupling agent chemical bonding" to construct a strong interfacial bond. Experiments show that the overlap shear strength reaches 29.2 MPa under cryogenic conditions, which is 123% higher than that of traditional processes, effectively overcoming the risk of interfacial delamination.

[0025] By precisely matching the low thermal expansion filler with the substrate, thermal mismatch stress is significantly reduced. The structure has a lifespan of over 100 cycles under cycling conditions from -196°C to room temperature, solving the problem of poor reliability during long-term service.

[0026] A controlled slow cooling process (≤2 ℃ / min) was used instead of traditional rapid cooling to avoid instantaneous high stress caused by thermal contraction. Comparative analysis showed that this method controlled structural warpage to <0.1 mm, ensuring dimensional accuracy and structural integrity. Attached Figure Description

[0027] Figure 1 is a cross-sectional schematic diagram of the cryogenic pressure-bearing connection structure of the present invention; Figure 2 is a flowchart of the manufacturing process of the cryogenic pressure-bearing structure of the present invention; Figure 3 is a bar chart comparing the cryogenic shear strength and thermal cycle life (number of cycles) of the examples and comparative examples; The meanings of the labels in the diagram are as follows: Figure 1. Labels: 1. Fiber-reinforced composite layer; 2. Low thermal expansion bonding layer; 3. Interface functional layer; 4. Interface activation layer; 5. Metal substrate layer.

[0028] Figure 2. Numbers: S1: Metal surface pretreatment, S2: Plasma activation treatment, S3: Coupling agent solution preparation and coating curing, S4: Preparation of low thermal expansion coefficient adhesive, S5: Preparation of low thermal expansion adhesive layer, S6: Composite laying and curing.

[0029] Figure 3, Chinese label: Bar chart X-axis: Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4 Y-axis: Shear strength (MPa), thermal cycle life (number of cycles) Detailed Implementation

[0030] To further illustrate the beneficial effects of the present invention, a detailed description is provided below in conjunction with the accompanying drawings, specific embodiments, and comparative examples.

[0031] Example 1: Fabrication of Cryogenic Metal / Composite Material Connection Structure

[0032] Substrate: 0.1 mm austenitic stainless steel (304L) sheet and carbon fiber reinforced epoxy resin prepreg T800 are selected. Austenitic stainless steel 304L sheet has excellent low temperature toughness, high yield strength and tensile strength.

[0033] Prepare the connection specimens according to steps S1~S6 above: S1 Metal Surface Pretreatment Use an ultrasonic cleaner to clean metal surfaces (both sheet metal and complex curved surfaces are suitable). Set the ultrasonic cleaning frequency to 40 kHz and the cleaning time to 12 minutes. Use deionized water as the cleaning solution. After cleaning, rinse thoroughly with deionized water and dry. For the subsequent organic solvent wiping step, use acetone as the first wiping solvent. Acetone effectively dissolves grease, cutting fluid, and other organic contaminants on the metal surface. After wiping, allow the acetone to evaporate naturally. Then, use anhydrous ethanol for a second wiping to further remove residual acetone and trace organic matter. After the ethanol evaporates, a clean base layer forms on the metal surface. For metal surfaces with oxide layers or stubborn contaminants, mechanical grinding can be used for supplementary treatment. Use 600-grit sandpaper and grind at a 45-degree angle to the direction of subsequent force to achieve a uniform surface roughness. After grinding, wipe clean with anhydrous ethanol and store properly to prevent recontamination.

[0034] S2 plasma activation treatment

[0035] A rotary plasma surface treatment machine was used to activate the pretreated metal surface. Oxygen with a purity of 99.999% was used at a flow rate of 75 sccm, a power of 900 W, and a treatment time of 240 seconds (area 120 cm²). 2 After treatment, the water contact angle is 8°.

[0036] S3 Preparation of silane coupling agent solution and coating curing

[0037] Anhydrous ethanol was used as a solvent, and an appropriate amount of γ-aminopropyltriethoxysilane (KH-550) was added as a silane coupling agent with a concentration of 0.8 wt%. Analytical grade glacial acetic acid solution was added dropwise to the ethanol solution, and the pH was monitored in real time with a precision pH meter until the pH value was adjusted to 4.8. To improve the wetting performance of the solution on the metal surface, polyoxyethylene sorbitan monolaurate was added as a wetting agent with a concentration of 0.1 wt%. The prepared solution was stirred on a magnetic stirrer for 1 hour to ensure that the components were fully mixed and uniform, forming a homogeneous silane solution. The silane solution was uniformly applied to the activated metal surface by spraying. The spraying pressure was controlled between 0.2 MPa and 0.4 MPa, and the distance between the nozzle and the metal surface was kept within the range of 15 cm to 25 cm to ensure the formation of a uniform thin layer. After spraying, the metal part was transferred to a vacuum drying oven, heated to 60 ℃ and kept at that temperature for 8 min to complete the construction of the interface functional layer (3).

[0038] Preparation of S4 low thermal expansion coefficient epoxy resin adhesive

[0039] First, weigh the epoxy resin adhesive (YUTAO E-4674) and the matching curing agent in a 3:1 weight ratio and mix them in a planetary mixer. Set the planetary mixer speed to 650 r / min and stir for 6 min to ensure that the epoxy resin and curing agent are fully and evenly mixed. Then, add 30 wt% of 2000 mesh SiO2 powder and 0.1 wt% of polyoxyethylene octylphenyl ether (Triton X-100) surfactant to assist dispersion. After stirring, determine the dispersion uniformity using the following methods: Place a small amount of slurry on a glass slide and observe it under an optical microscope (100x magnification). The standard deviation of the SiO2 powder distribution density should not exceed 15% of the average value; or use the sedimentation method: Dilute 10 g of slurry with 20 ml of acetone, let it stand for 24 hours, and the mass of the precipitate at the bottom should not exceed 0.5% of the total mass. Meeting either of the above conditions indicates that the dispersion is uniform.

[0040] The prepared low-thermal-expansion epoxy resin solution should remain in a flowable state, with its viscosity before curing controlled within the range of 3000 mPa·s to 4000 mPa·s (which can be measured using a rotational viscometer). This viscosity range ensures that the solution has good wetting and coating properties.

[0041] Preparation of S5 low thermal expansion adhesive layer

[0042] Under clean conditions with an ambient temperature of 20~25 ℃ and a relative humidity of <60%, the low thermal expansion coefficient epoxy resin liquid prepared in step S4 is uniformly applied to the surface of the metal substrate treated in step S3 by brushing or coating. The adhesive liquid is scraped to a thickness of 0.3 mm. After coating, vacuum degassing treatment is performed. The coated structural parts are placed in a vacuum drying oven and kept at a vacuum degree of 10 Pa for 8 min to completely eliminate micro bubbles inside the adhesive layer and ensure that the adhesive layer is dense and defect-free. After degassing, the vacuum is slowly released to avoid the regeneration of bubbles due to sudden pressure changes, forming a low thermal expansion bonding layer (2).

[0043] S6 Composite Laying and Curing

[0044] Carbon fiber prepreg was laid to complete the overall assembly. A symmetrical layup of [0° / 45° / 90° / -45°]s was adopted, with a total thickness of 2.0 mm and a layup ratio of 25% for each angle. The prepreg was compacted using a negative pressure vacuum bag with a vacuum degree of not less than 95 kPa. The temperature was increased to 60°C at a rate of 2°C / min and held for 4 hours to complete the initial cross-linking and curing of the interface. The temperature was then increased to 90°C at the same rate and held for 4 hours to achieve complete cross-linking of the resin system. After curing, the resin was slowly cooled to room temperature in the furnace at a rate of ≤2°C / min.

[0045] Comparative Example 1: Traditional epoxy adhesive bonding (without low thermal expansion layer, without silane functionalization)

[0046] Process differences: Only routine surface polishing and cleaning are performed, without plasma treatment or silane coupling agents; ordinary two-component epoxy adhesive (YUTAO E-4674) is used for bonding directly, without adding SiO2 powder filler.

[0047] Comparative Example 2: Traditional modified adhesive layer (with silane coupling agent but no low thermal expansion filler)

[0048] Process differences: Steps S1 to S3 (including plasma and silane) were performed, but pure epoxy resin (without SiO2 powder filler) was used directly in step S4. The rest was the same as in Example 1.

[0049] Comparative Example 3: Rapid natural cooling

[0050] Process differences: The process of Example 1 is followed, but after curing in step S6, the cooling rate is not controlled, and the vacuum bag is directly opened for natural air cooling to room temperature (cooling rate > 10~20 ℃ / min).

[0051] Comparative Example 4: Using a non-uniform layup scheme

[0052] Except for changing the layup sequence to [0° / 90°]s (i.e., only 0° and 90° layers, a total of 4 layers, with the total thickness remaining unchanged at 2.0 mm), all other process parameters (adhesive formulation, curing regime, etc.) are completely consistent with Example 1.

[0053] Test items and methods

[0054] Lap shear strength test: Referring to GB / T 7124-2008 "Determination of tensile shear strength of adhesives", the prepared lap shear specimens were immersed in a liquid nitrogen environment chamber at -196 ℃ for 24 h. Under the condition of maintaining a low temperature environment of -196 ℃, a universal testing machine equipped with a low temperature fixture was used to perform tensile shear test at a loading rate of 2 mm / min. The maximum shear force was recorded and the shear strength was calculated.

[0055] Cryogenic thermal cycle fatigue test: The temperature shock test method was adopted, with the temperature cycle range from -196 ℃ to room temperature. The heating rate and cooling rate were both controlled at 5 ℃ / min, and each cycle included a holding time of 10 min.

[0056] Warpage deformation measurement (for Comparative Example 4): The specimen was placed on a precision plate and a high-precision contact displacement sensor (resolution ≤ 0.001 mm) was used to measure the height deviation of its surface relative to the reference plane at multiple points along the diagonal direction of the specimen. The maximum value was taken as the maximum deflection.

[0057] Test Results and Data Analysis

[0058] Table 1 Performance Comparison Test Results of Different Process Schemes

[0059] (Note: N / A indicates that Comparative Examples 1-3 have cracked and warped after thermal cycling tests, making it impossible to complete the warping deformation test, hence marked N / A. Comparative Example 4 suffered severe warping due to layup issues.)

[0060] Data Analysis and Conclusions

[0061] To verify the technical effect of the present invention, the applicant conducted a comparative experiment. The test results are shown in Figure 3 and Table 1: The cryogenic shear strength of Example 1 (29.2 MPa) is 2.2 times that of Comparative Example 1 and 1.7 times that of Comparative Example 2. This fully demonstrates that the present invention effectively overcomes the risk of interfacial delamination caused by curing shrinkage stress and significantly enhances the interfacial bonding force under cryogenic conditions. Although Comparative Example 2 (no filler), Comparative Example 3 (rapid natural cooling), and Comparative Example 4 show improved strength, they are still far lower than those of the present invention.

[0062] In cryogenic thermal cycling tests, Example 1 showed almost no strength degradation after >100 cycles, while Comparative Example 1 suffered catastrophic failure after only 14 cycles. This result confirms that the low thermal expansion design of the present invention can effectively resist thermal fatigue damage in cryogenic environments, solving the problem of long-term service reliability.

[0063] Comparative Example 3 shows that controlled temperature cooling after curing is crucial for reducing curing shrinkage stress and further optimizes the integrity of the connection structure. Comparative Example 4 shows that the [0° / 45° / 90° / -45°]s symmetrical layup design adopted in this application ensures that thermal stress is uniformly released in all directions, thereby eliminating the risk of warping.

[0064] In summary, this invention successfully solves the problem of thermal stress concentration at the interface of stainless steel / composite materials in cryogenic environments through a multi-layered functional synergistic construction (interface activation + low thermal expansion filler + controlled temperature slow cooling). Experimental data show that compared with Comparative Example 1, this invention significantly improves the static shear strength (+123%) and increases the cryogenic cycle life from 14 cycles to over 100 cycles, fully verifying the necessity and advancement of the multi-layered synergistic technology solution of this invention.

[0065] Alternative solutions and process parameter adjustments: Explanation of stirring process parameters: In this embodiment, the planetary stirrer is preferably set to a speed of 650 r / min, and the stirring time is extended to 6 minutes to obtain optimal dispersion uniformity. However, through extensive experiments, the applicant has found that as long as the shear force is sufficient to break up SiO2 agglomerates without generating excessive bubbles or local overheating, the speed can be adjusted within a certain range. Therefore, a wider process window is 500 r / min to 1200 r / min.

[0066] If the rotation speed is lower than 500 r / min, it may cause uneven dispersion of the filler powder and agglomeration. If the rotation speed exceeds 1200 r / min, the filler powder may be excessively sheared and broken, reducing the reinforcing effect.

[0067] Correspondingly, the stirring time can also be adjusted according to the rotation speed, usually controlled between 5 min and 10 min, until the following dispersion uniformity standard is achieved.

[0068] Alternatives to dispersing agents: In this embodiment, Triton X-100 (a nonionic surfactant) was used at an addition amount of 0.1 wt% to assist in the wetting and dispersion of SiO2 powder in the resin matrix.

[0069] It should be understood that Triton X-100 is only one preferred surfactant. Those skilled in the art can choose other surfactants with similar surface activity and compatibility to achieve the same dispersion effect.

[0070] Applicable alternatives include, but are not limited to: Other nonionic surfactants include: Span series, Tween series, OP-10 (alkylphenol polyoxyethylene ether), etc. Coupling agents: such as γ-aminopropyltriethoxysilane (KH-550), titanate coupling agents (such as TT-20, TT-69, etc.), aluminate coupling agents, etc., can both play a coupling role and assist in dispersion; Adhesives: Various low-temperature resistant epoxy resins (such as bisphenol F type epoxy resin, alicyclic epoxy resin or toughened modified epoxy resin), polyurethane, silicone or acrylate.

[0071] Alternatives to functional fillers: This embodiment primarily uses silica (SiO2) powder as a low thermal expansion filler. To achieve precise matching of the thermal expansion coefficients of different metal substrates, other fillers with low thermal expansion coefficients or negative thermal expansion characteristics can be used as alternatives. Carbonaceous fillers: such as graphite powder and carbon fiber powder. These materials have excellent thermal conductivity and specific thermal expansion characteristics, making them suitable for scenarios requiring rapid heat dissipation or specific CTE matching; Negative thermal expansion ceramic fillers: such as zirconium tungstate (ZrW2O8) and aluminum tungstate (Al2(WO4)3). These materials exhibit a significant negative thermal expansion effect over a wide temperature range, which can more precisely offset the shrinkage of the resin matrix and achieve an optimal match with the metal substrate; Other inorganic oxides, such as zinc oxide and aluminum oxide, are powders with low expansion properties.

[0072] Explanation regarding filler addition amount

[0073] In this embodiment, the silica addition amount is set to 30 wt%, which is a relatively optimal balance point based on the matching coefficient of thermal expansion (CTE). Experiments have shown that when the addition amount is further reduced, it is difficult to achieve a coefficient of thermal expansion and contraction that matches the substrate metal, while further increasing the addition amount will lead to a decrease in the adhesive bonding strength and affect the interlayer bonding force.

[0074] Regarding the impact of cooling rate on cryogenic performance and the optimal range: Through in-depth research and process verification, the applicant has confirmed that the cooling rate is a key process parameter affecting the release of residual stress in the adhesive layer and the lifespan of cryogenic cycles.

[0075] Risks of excessively rapid cooling: When the cooling rate > 2 ℃ / min, a significant temperature gradient will be generated between the metal substrate and the resin matrix, resulting in insufficient release of curing shrinkage stress. Experimental observations show that the joint structure prepared under these conditions has excessive residual stress at the interface, and the cryogenic cycle life in the liquid hydrogen environment is significantly shortened (usually less than 50 cycles) before interface cracking or delamination failure occurs.

[0076] Suitable cooling window: When the cooling rate is controlled within the range of 1 ℃ / min to 2 ℃ / min, thermal stress is effectively released, and the internal structure of the adhesive layer is uniform and stable. Comparative experiments show that the specimens prepared under these conditions can achieve a cryogenic cycle life of over 100 cycles, meeting the requirements for cryogenic pressure equipment.

[0077] Therefore, this application limits the cooling rate to ≤2 ℃ / min (implying a lower limit as the minimum feasible rate in engineering), which is a scientific definition based on the aforementioned experimental phenomena and thermodynamic principles. Those skilled in the art can fully reproduce a highly reliable connection structure based on this parameter range.

[0078] Explanation of the basis for determining the thickness range of the adhesive coating: In a preferred embodiment of the present invention, the adhesive coating thickness is set to 0.3 mm. However, through in-depth analysis and process verification, the applicant discovered that this thickness is not the only limitation, but rather a process window with a clearly defined physical boundary. The 0.1~0.5 mm range described in the claims is determined based on a comprehensive consideration of the adhesive layer performance in cryogenic environments. When the coating thickness is less than 0.1 mm, microscopic defects such as localized missing adhesive, pinholes, or incomplete coverage are easily observed due to limitations in the uniformity of the coating process and the physical limits of vacuum degassing. In cryogenic environments, these defects can become stress concentration sources, leading to premature interface cracking or airtightness failure. Therefore, 0.1 mm is the critical lower limit for ensuring the continuity and density of the adhesive layer.

[0079] When the coating thickness exceeds 0.5 mm, two main risks are encountered: Uncontrolled exothermic curing: Epoxy resin curing is a strongly exothermic reaction. An excessively thick adhesive layer can cause internal heat to accumulate and become difficult to dissipate, generating enormous internal thermal stress and even causing microcracks within the adhesive layer.

[0080] Residual air bubbles: Air bubbles in the thick adhesive layer are difficult to completely escape within a limited vacuum time (5~10 min), and residual air bubbles will significantly weaken the mechanical strength of the bonding layer.

[0081] Therefore, 0.5 mm is the upper limit that balances curing quality, degassing efficiency, and avoidance of thermal stress damage.

[0082] Within the range of 0.1 mm to 0.5 mm, the flexibility of the adhesive layer slightly improves with increasing thickness, effectively absorbing some thermal stress while maintaining sufficient load-bearing area. Although 0.3 mm is the optimal balance point in this embodiment (balancing construction tolerance and mechanical performance), those skilled in the art will know from the above physical mechanism that as long as it is controlled within the above boundaries, an effective connection can be achieved without causing the technical solution to fail.

Claims

1. A method for constructing a metal-composite material interface bonding structure suitable for cryogenic environments, characterized in that, Includes the following steps: S1. Pre-treat the surface of the metal substrate to obtain a clean metal substrate layer; S2. The metal substrate layer is subjected to interface activation treatment using plasma to form an interface activation layer on the metal surface; S3. Coating a coupling agent onto the activated metal surface and curing it, so that the interface functional layer forms chemical bonds with the metal surface through the coupling agent; S4. Prepare a low thermal expansion adhesive, which is made by filling a base adhesive with a low thermal expansion or negative thermal expansion filler, wherein the amount of filler added is adjusted according to the thermal expansion coefficient of the metal substrate so that the thermal expansion coefficient of the composite adhesive layer matches the thermal expansion coefficient of the metal substrate. S5. Apply the low thermal expansion adhesive to the surface of the interface functional layer to form a low thermal expansion bonding layer; S6. Fiber-reinforced prepreg is laid on the surface of the low thermal expansion bonding layer, and after curing, a fiber-reinforced composite material layer is formed.

2. A cryogenic environment composite material-metal connection structure prepared by the construction method of claim 1, characterized in that, include: Metal substrate; An interface activation layer is disposed on the surface of the metal substrate layer; An interface function layer is disposed on the surface of the interface activation layer; A low thermal expansion bonding layer is disposed on the surface of the interface functional layer; A fiber-reinforced composite material layer is disposed on the surface of the low thermal expansion bonding layer.

3. The construction method according to claim 1, characterized in that, The pretreatment described in step S1 includes: performing graded cleaning treatment on the metal surface by ultrasonic cleaning and two-step organic solvent wiping; For the surface oxide layer, cross-directional mechanical grinding is used to form a uniform surface roughness.

4. The construction method according to claim 1, characterized in that, In step S2, oxygen plasma can be used, and the process parameters for the oxygen plasma activation treatment are as follows: Oxygen purity > 99%, processing time not less than 200 seconds per 100 cm².

5. The construction method according to claim 1, characterized in that, The construction of the interface functional layer in step S3 includes: Prepare a coupling agent solution, adjust the pH, add a wetting agent, stir, spray it onto the metal surface, and heat-insulate for curing.

6. The construction method according to claim 5, characterized in that, The coupling agent solution is prepared by the following steps: using anhydrous ethanol as solvent, a γ-aminopropyltriethoxysilane solution with a concentration of 0.5~1 wt% is prepared, the pH is adjusted to 4.0~5.5 with an acidic solution, 0.05-0.2 wt% wetting agent is added, and after thorough stirring, it is applied to the activated metal surface by spraying. After spraying, it is kept at 40~90℃ for curing.

7. The construction method according to claim 1, characterized in that, The low thermal expansion or negative thermal expansion filler mentioned in step S4 is selected from at least one of silica, graphite powder, carbon fiber powder, zirconium tungstate and aluminum tungstate.

8. The construction method according to claim 1, characterized in that, The adhesive matrix mentioned in step S4 is a low-temperature resistant adhesive; The adhesive is selected from at least one of epoxy resin, polyurethane, silicone or acrylate.

9. The construction method according to claim 1, characterized in that, The coating thickness described in step S5 is 0.1~0.5 mm, and the relative humidity during the coating process is below 60%.

10. The construction method according to claim 1, characterized in that, After curing is completed, a cooling step is also included: under vacuum bag negative pressure, the structural component is cooled to room temperature at a rate not exceeding 2 °C / min.

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