A method for preparing high-entropy alloy gradient coatings based on layer-by-layer solid-phase densification

Abstract

This invention belongs to the field of high-entropy alloy gradient coating preparation technology, specifically relating to a method for preparing high-entropy alloy gradient coatings based on layer-by-layer solid-phase densification, comprising the following steps: S1: Selecting a metallic material as the substrate, and cleaning and roughening the substrate surface; S2: Quantitatively calculating the gradient coating components and determining the number of layers, with a total number of coating layers n≥3; S3: Cyclic manufacturing of layer-by-layer spraying deposition-solid-phase densification; S4: Final surface treatment. This invention combines plasma spraying and friction stir processing in alternating cycles, effectively eliminating interlayer defects and constructing a compositional gradient through a cyclic construction mode of layer-by-layer spraying and solid-phase densification. This results in high densification, high bonding strength, and controllable thickness.

Description

Technical Field

[0001] This invention belongs to the field of high-entropy alloy gradient coating preparation technology, specifically relating to a method for preparing high-entropy alloy gradient coatings based on layer-by-layer solid-phase densification. Background Technology

[0002] Gradient coatings are a type of coating that integrates function and structure, characterized by a continuous or quasi-continuous variation in the composition, microstructure, and properties of the coating along a certain dimension. Compared to traditional coatings with uniform composition, gradient design can effectively alleviate interfacial stress concentration caused by abrupt changes in performance between the coating and the substrate, achieving a smooth transition from the substrate to the surface, thereby significantly improving the coating's bonding strength, toughness, and overall service reliability. Therefore, developing high-performance gradient coatings is of great significance for meeting the application requirements under extreme conditions in aerospace, energy equipment, and other fields.

[0003] In recent years, with the continuous emergence of new materials, high-entropy alloys have provided entirely new possibilities for the design and preparation of gradient coatings. High-entropy alloys are a novel type of alloy material composed of five or more metallic elements in equal or near-equal proportions. Due to their high mixing entropy, high-entropy alloys are more likely to form solid solution structures rather than brittle intermetallic compounds. Therefore, high-entropy alloys possess excellent comprehensive properties such as high strength, high hardness, superior high-temperature stability, wear resistance, and corrosion resistance, making them ideal materials for preparing high-performance gradient coatings.

[0004] Plasma spraying has become a common method for preparing high-entropy alloy coatings due to its advantages such as low substrate temperature rise, wide range of applicable materials, and high deposition efficiency. However, during the spraying process, the sprayed powder is in a molten or semi-molten state under the high temperature of the plasma jet and impacts the substrate at high speed. These molten or semi-molten sprayed powders spread, solidify, and stack layer by layer on the substrate surface to form a lamellar structure, ultimately forming the coating. These characteristics inevitably lead to defects such as porosity, oxide inclusions, and microcracks within the coating. These defects severely affect the density of the coating and weaken its mechanical properties.

[0005] To address the inherent defects of traditional plasma spraying, such as continuous oxide films, high porosity, and low interlayer bonding strength caused by the high temperature of the spraying flame, resulting in oxidation of the sprayed powder particles, researchers have explored the introduction of friction stir processing (FSM) as a post-processing technique. FSM is a solid-state material surface modification technology developed based on the principle of friction stir welding. It utilizes a rotating stirring head to generate frictional heat and intense plastic deformation on the material surface, breaking down the continuous oxide film and dispersing it into fine reinforcing particles. Simultaneously, it can achieve pore closure and refine and densify the material's microstructure. Because its processing temperature is below the material's melting point, FSM effectively avoids secondary defects caused by melting and solidification, demonstrating significant potential for improving the quality of sprayed coatings.

[0006] However, the aforementioned techniques still face certain technical bottlenecks in preparing high-performance, thick, and precisely gradient high-entropy alloy coatings. For example, Chinese invention patent application CN118932328A discloses a method for preparing a high-entropy alloy coating and its in-situ formation. This method involves first cold-spraying a first high-entropy alloy coating onto the surface of a metal substrate such as aluminum, magnesium, copper, or titanium, followed by a penetrating stirring friction treatment (stirring depth greater than the coating thickness). This causes the first high-entropy alloy to undergo in-situ alloying with the substrate metal, forming a second high-entropy alloy coating, thereby significantly improving the coating's bonding strength, plasticity, and density. The design intent and applicability of this method are limited to in-situ modification of single-layer or thin coatings. Furthermore, the one-time penetrating stirring has a significantly weaker effect on the plastic deformation and densification of the underlying region than on the surface layer, making it difficult to achieve uniform microstructure modification along the thickness direction.

[0007] Chinese invention patent application CN118563303A discloses a gradient material and its preparation method. This method involves sequentially depositing powders of different compositions layer by layer on an AZ31 magnesium alloy substrate using three cold spraying passes to form a gradient structure. A single shallow-indentation friction stir process is then employed to refine the microstructure and densify the coating. However, this method, which involves uniform stirring after all spraying is completed, struggles to promptly eliminate accumulated oxides and pores from the spraying process. This can lead to residual defects in the underlying layers, disrupting the continuity of the gradient structure and reducing the coating's bonding strength and overall performance. Furthermore, the through-processing can trigger intense vertical mass exchange between layers, disrupting the original gradient composition and microstructure distribution, and affecting the maintenance of the gradient effect. Especially in thicker coatings, the stirring head has limited shearing and plastic deformation effects on the lower layers, further exacerbating the difficulty in removing underlying defects, thus impacting the coating's density and service reliability.

[0008] Ye Dongming's Master's Thesis (Cold Spray Friction Composite Additive Manufacturing CoCrFeNi) pA method for cold-sprayed friction composite additive manufacturing is disclosed in "Research on Fatigue Behavior of Al (6061Al), Xi'an University of Architecture and Technology, 2023". This method first uses cold spraying to deposit a layer containing CoCrFeNi. p High-entropy alloy particles of 6061Al composite powder were used, and the coating was immediately subjected to friction stirring without a stirring needle. This process of cold spraying and friction stirring was repeated multiple times to prepare CoCrFeNi. p Particle-reinforced 6061Al composite material. However, in this method, each layer uses composite powder with the same composition ratio, resulting in a material with uniform overall chemical composition, but without involving functional gradient design in the thickness direction; at the same time, the cold spraying process has a low temperature, and basically no continuous lamellar oxide film is formed during the deposition process. Friction stir processing is mainly used to close the micropores between particles and promote particle bonding. The relevant process parameters of friction stir processing are difficult to be directly applied to the full breakup and dispersion of continuous oxides formed by plasma spraying, and it is still difficult to meet the application requirements of preparing functional gradient high entropy alloy coatings with precise composition gradient changes and full breakup and dispersion of interlayer oxides.

[0009] Chinese invention patent application CN118106601A discloses a method for preparing high-entropy alloy-reinforced aluminum-based gradient composite materials using a stir friction method. The method first forms an aluminum alloy layer by overlay welding (fusion welding), and then uses a stirring head with a stirring needle to soften and press filamentous fillers with different high-entropy alloy contents into the aluminum alloy surface under the action of axial force and frictional heat to form a single-layer reinforcement layer with a thickness of about 5 mm. Subsequently, the overlay welding and wire feeding stir friction steps are repeated to achieve a gradient structure by changing the high-entropy alloy content in the filament layer by layer. However, each aluminum alloy layer in this method is still formed by overlay welding. The filamentary reinforcement needs to be softened at high temperature or even partially melted before being filled. Inevitably, defects such as porosity, hot cracking, burn-off and segregation of high-entropy alloying elements, which are common in fusion welding, will exist during the process. At the same time, since each layer needs to undergo secondary heat input from the high temperature of overlay welding and stirring friction, the cumulative heat input is large, resulting in large residual thermal stress and deformation. It is still difficult to meet the application requirements of preparing functional gradient high-entropy alloy coatings with no fusion welding defects, low thermal stress, precise and controllable composition, and sufficient fragmentation and dispersion of continuous thick oxides between layers.

[0010] The aforementioned existing technologies still struggle to achieve the layer-by-layer complete fragmentation and dispersion of continuous oxides between layers in high-entropy alloy gradient coatings based on plasma spraying processes, the precise maintenance of compositional gradients, and the improvement of overall coating density. Summary of the Invention

[0011] The technical problem to be solved by this invention is to address the shortcomings of existing technologies by providing a method for preparing high-entropy alloy gradient coatings based on layer-by-layer solid-phase densification. This invention combines plasma spraying and friction stirring processes in an alternating cycle. Through a cyclical construction mode of layer-by-layer spraying and layer-by-layer solid-phase densification, interlayer defects are effectively eliminated layer by layer and a composition gradient is constructed. The result is a high degree of densification, high bonding strength, and controllable thickness.

[0012] This solution is achieved through the following technical measures: a method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification, comprising the following steps: S1: Select a metal material as the substrate and clean and roughen the surface of the substrate; S2: Quantitative calculation of gradient coating components and determination of the number of layers, total number of coating layers. ≥3; S3: Cyclic manufacturing process of layer-by-layer spray deposition-solid phase densification: (I) Perform the following two procedures sequentially on each layer: Process A: Coating deposition is performed by plasma spraying; Process B: Coating densification treatment is carried out by friction stirring. (II) Cyclic Operation and Interlayer Processing: After completing process B in the current layer, check the forming quality of the processing area. If the forming quality meets the requirements, repeat process A and process B until the current layer is completed. Preparation of the coating layer; S4: Final surface treatment.

[0013] Preferably, in step S2, a high-entropy alloy powder is mixed with a matrix homogeneous powder, and the mass percentage of the high-entropy alloy powder is adjusted layer by layer. High entropy alloy powder mass percentage of the layer Calculate using the following formula: ; In the formula: The current level number, 1 ≤ ≤ ; This refers to the total number of coating layers; For the current number The mass percentage of high-entropy alloy powder in the layer; This represents the mass percentage of the underlying high-entropy alloy powder, ranging from 10% to 30%. This represents the mass percentage of the high-entropy alloy powder on the surface layer, ranging from 80% to 100%. This represents a fixed increment in the high-entropy alloy content between coating layers.

[0014] Preferably, in step S3, process A, the current layer coating is deposited using an atmospheric plasma spraying device, and the thickness of a single layer is [not specified]. The value range is 0.1 mm ~ 1.0 mm.

[0015] Preferably, the spraying process parameters in step S3, process A, are as follows: Spraying current: 450 A ~ 550 A; Spraying voltage: 40 V ~ 55 V; The flow rate of the main gas Ar is 35 L / min ~ 45 L / min; Flow rate of auxiliary gas H2: 5 L / min ~ 10 L / min; Powder delivery rate: 20 g / min ~ 30 g / min; Spraying distance: 90 mm ~ 110 mm; Coating cooling: After spraying, the coating surface is cooled by compressed air or by natural cooling. Only after the coating temperature drops below 100℃ can step B be performed.

[0016] Preferably, in step B of step S3, a needleless stirring head made of hard alloy WC-Co or polycrystalline cubic boron nitride is used to perform friction stirring. The shoulder diameter of the needleless stirring head is 10 mm to 15 mm, and the end face of the shoulder of the needleless stirring head has a concave structure with a concave angle of 3° to 8°.

[0017] Preferably, the method for densifying the coating in step B of step S3 is as follows: The coating surface is cooled until the temperature drops below 100°C. Then, a needleless stirring head is used to perform friction stirring on the current layer. During the friction stirring process, a local inert gas is introduced for protection. The flow rate of the protective gas is 15 L / min ~ 25 L / min. The protected area of ​​the inert gas is the friction stirring area.

[0018] Preferably, the shoulder pressing depth of the needle-free stirring head is... It needs to be based on the thickness of the single-layer spray coating. The settings are dynamically configured, and the specific calculation formula is as follows: ; In the formula: ; This refers to the thickness of a single-layer spray coating. The densification coefficient ranges from 0.25 to 0.5.

[0019] Preferably, the rotational speed of the needleless stirring head is 1000 rpm ~ 1500 rpm, and the travel speed is 60 mm / min ~ 100 mm / min; the shoulder tilt angle of the needleless stirring head is 2° ~ 2.5°.

[0020] Preferably, the method for checking the forming quality of the processing area after completing process B of the current layer in step S3 is as follows: (1) Edge treatment of the stirring friction processing area: If there are flashes caused by stirring and extrusion at the edge of the coating, the flashes only need to be removed by mechanical or manual grinding. (2) The surface condition of the friction stir processing area is preserved, and the microscopic arc-shaped concave and convex texture of the surface is preserved; The final surface treatment in step S4 is as follows: Once the preset total number of layers is reached, the outermost surface is mechanically ground or polished according to the final dimensional accuracy and surface quality requirements of the product to obtain a high-entropy alloy gradient coating with a dense internal structure and a gradient distribution of components.

[0021] Preferably, in step S1, the substrate surface is cleaned and roughened in the following manner: (1) The substrate surface is roughened by sandblasting with corundum sand of 24-60 mesh, and the sandblasting air pressure is controlled at 0.5 MPa-0.7 MPa; (2) Use acetone or anhydrous ethanol to ultrasonically clean the substrate for 10 min to 15 min, and blow it dry after cleaning.

[0022] The beneficial effects of this invention are: 1. This invention significantly improves the densification of the coating and overcomes the inherent defects of sprayed coatings: Compared with the coatings prepared by existing technologies, which generally have pores and oxide inclusions, this invention introduces a layer-by-layer stirring friction densification process (step B), which uses intense plastic deformation to promote the closure of pores and break the oxide film in the original coating into fine dispersed particles, thereby improving the density of the final coating and enhancing the mechanical properties of the coating. 2. This invention achieves a transformation from physical / mechanical bonding to metallurgical bonding between the coating and the underlying layer / substrate, thereby improving the coating bonding strength: This invention accurately calculates the shoulder pressing depth of the needle-free stirring head through a densification model. This ensures that the thermoplastic deformation zone generated during the friction stir process can penetrate the current deposition layer and act on the adjacent lower layer or substrate surface, promoting atomic diffusion and recrystallization between interfaces, forming a high-quality metallurgical bonding interface, and improving the coating bonding strength. 3. This invention constructs a smooth gradient with precise and controllable composition, effectively alleviating and releasing the internal stress of the coating: This invention uses a linear gradient formula to precisely control the content of high-entropy alloy components in each layer, achieving a smooth transition from the substrate to the surface layer and avoiding stress concentration sources; at the same time, under the combined effect of layer-by-layer solid-state processing, the internal stress of the coating is controlled at a low level, thereby improving the coating's resistance to thermal fatigue and impact. 4. This invention breaks through the thickness limitations of traditional processes and expands application scenarios: By adopting a cyclic process route of layer-by-layer deposition and layer-by-layer densification, the problem of difficult elimination of bottom layer defects in traditional one-time post-processing processes is effectively avoided, and internal stress is effectively controlled. Therefore, this invention can break through the thickness limitations of traditional processes and stably prepare thick, crack-free gradient coatings. 5. The process of this invention is simplified and the material utilization rate is improved: there is no need to grind and level in the thickness direction between layers, only the flash needs to be removed before the next layer is sprayed, which greatly reduces the material loss and labor cost caused by interlayer machining. 6. This invention can achieve low oxide content control: In step B, this invention introduces inert gas protection, which, combined with the physical breaking and dispersion effect of stirring friction processing on the original oxide film, minimizes the oxide content and defects inside the coating.

[0023] Therefore, it can be seen that the present invention has outstanding substantive features and significant progress compared with the prior art. Attached Figure Description

[0024] Figure 1 This is a process flow diagram of the present invention.

[0025] Figure 2 This is a schematic diagram of the needleless stirring head in this invention.

[0026] Figure 3 This is a schematic diagram of the spraying process A in this invention.

[0027] Figure 4 This is a schematic diagram of process B in this invention.

[0028] Figure 5 This is a macroscopic fracture morphology diagram of the tensile specimen from Example 1.

[0029] Figure 6 The image shows a scanning electron microscope (SEM) image of the tensile fracture morphology of the gradient coating in Comparative Example 1.

[0030] In the figure: 1-Needleless stirring head, 11-Stirring head clamping part, 12-Shoulder, 13-Concave structure, 2-Plasma spray gun, 3-Plasma spraying flame, 4-Plasma spraying gradient coating, 41-First coating layer, 42-Second coating layer, 43-Third coating layer, 44-Fourth coating layer, 45-Fifth coating layer, 5-Substrate. Detailed Implementation

[0031] To clearly illustrate the technical features of this solution, the following detailed implementation method, in conjunction with its accompanying drawings, will be used to describe the solution.

[0032] A method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification includes the following steps: S1: Select a metallic material (e.g., aluminum alloy) as the substrate 5, and clean and roughen the surface of the substrate 5. The specific treatment method is as follows: (1) The surface of the substrate 5 is roughened by sandblasting with corundum sand of 24-60 mesh, and the sandblasting air pressure is controlled at 0.5 MPa-0.7 MPa to increase the roughness of the surface of the substrate 5, which is conducive to the physical and mechanical bonding between the first coating layer 41 and the substrate 5. (2) After sandblasting, use acetone or anhydrous ethanol to ultrasonically clean the substrate 5 for 10 min to 15 min to remove residual oil and sand on the surface of the substrate 5. After cleaning, blow the substrate 5 dry for later use.

[0033] S2: Quantitative calculation of gradient coating components and determination of the number of layers, setting the total number of coating layers as follows: , The value is determined based on the total thickness of the coating design and the thickness of each single layer. Considering the mathematical constraints of this gradient calculation formula and the design requirements for a smooth gradient, the total number of coating layers is set. ≥3; A method of mixing high-entropy alloy powder with homogeneous matrix powder is adopted, and the mass percentage of high-entropy alloy powder is adjusted layer by layer. To ensure uniform composition transition, the first... High entropy alloy powder mass percentage of the layer Calculate using the following formula: ; In the formula: The current level number, 1 ≤ ≤ ; This refers to the total number of coating layers; For the current number The mass percentage of high-entropy alloy powder in the layer; This represents the mass percentage of the high-entropy alloy powder in the bottom layer (first layer), ranging from 10% to 30%. For the surface layer (the first) The percentage of high-entropy alloy powder (in layers), ranging from 80% to 100%. It is a fixed increment of the high-entropy alloy content between coatings, that is, starting from the second layer, the proportion of high-entropy alloy powder added to each layer compared to the previous layer.

[0034] S3: Cyclic manufacturing process of layer-by-layer spray deposition-solid phase densification: (I) Perform the following two procedures sequentially on each layer: Process A: Coating deposition is performed by plasma spraying. Specifically, atmospheric plasma spraying equipment is used to deposit the current layer coating.

[0035] Regarding the thickness of single-layer spray coating deposition Definition: The thickness of the single-layer spray coating deposition This refers to the cumulative coating thickness achieved through one or more spraying processes before the initial friction stir processing. To balance spraying efficiency with the densification quality of subsequent friction stir processing, the thickness of a single spraying layer is... The value range is 0.1 mm to 1.0 mm, and the preferred single-layer spray deposition thickness is... The value range is 0.3 mm ~ 0.6 mm.

[0036] The spraying process parameters are as follows: Spraying current: 450 A ~ 550 A; Spraying voltage: 40 V ~ 55 V; The flow rate of the main gas Ar is 35 L / min ~ 45 L / min; Flow rate of auxiliary gas H2: 5 L / min ~ 10 L / min; Powder delivery rate: 20 g / min ~ 30 g / min; Spraying distance: 90 mm ~ 110 mm; Coating Cooling: After spraying, the coating surface is cooled by compressed air or naturally. Operation B can only proceed after the coating temperature drops below 100℃. Step S3 strictly controls the coating temperature to prevent heat accumulation that could lead to excessive softening of the substrate and abnormal grain growth, ensuring that subsequent friction stir densification is carried out under stable substrate support and a suitable temperature field.

[0037] Step B: Coating densification treatment by friction stir processing; specifically, friction stir processing is performed using a needleless stirring head 1 made of hard alloy WC-Co or polycrystalline cubic boron nitride (PCBN). The diameter of the shoulder 12 in the needleless stirring head 1 is 10 mm to 15 mm, and the end face of the shoulder 12 in the needleless stirring head 1 is a concave structure 13 with a concave angle of 3° to 8°. During the friction stir processing, the concave structure 13 can apply an inward radial extrusion force to the thermoplastic softened material below the shoulder 12, thereby effectively gathering and constraining the plastically deformed material in the area below the needleless stirring head 1, avoiding material splashing outward or forming flash defects at the processing edge during high-speed rotation and pressing.

[0038] The coating surface is cooled (e.g., by air cooling or natural cooling). Once the surface temperature drops below 100°C, the current layer is subjected to friction stirring using a needle-free stirring head 1. To prevent secondary oxidation of the coating material caused by the high temperature during the friction stirring process, a localized inert gas (e.g., argon) is introduced for protection. The flow rate of the protective gas is 15 L / min ~ 25 L / min, and the protected area is the friction stirring processing area.

[0039] In process B, the following two key control parameters must be strictly followed: (a) Shoulder depth of the needleless stirring head This is to ensure interlayer metallurgical bonding.

[0040] Shoulder depth of needle-free stirring head It needs to be based on the thickness of the single-layer spray coating. To ensure that the plastic deformation zone generated by the needleless stirring head 1 can completely penetrate the currently loose sprayed layer and exert sufficient forging and extrusion effect on the interlayer interface, this invention introduces a densification coefficient through dynamic settings. Shoulder depth of needle-free stirring head The specific calculation formula is as follows: ; In the formula: ; This refers to the thickness of a single-layer spray coating. The densification coefficient ranges from 0.25 to 0.5.

[0041] Densification coefficient The additional downward pressure ratio of the needle-free stirring head 1 relative to the single-layer spray deposition thickness was determined. Densification coefficient. The setting is not a simple empirical value, but a key control variable based on the law of volume conservation and the solid-state bonding mechanism of stirring friction. Densification coefficient The physical meaning includes the following two essential engineering components: (1) Volume shrinkage compensation component: Plasma-sprayed coatings, as a layered stacked structure, typically contain 5% to 15% porosity in their initial state. Under the intense thermoplastic deformation of friction stir processing, the pores inside the coating close, leading to macroscopic volume shrinkage (thickness reduction). Densification coefficient It can dynamically compensate for the thickness loss during the transition from loose to dense, ensuring that the needle-free stirring head 1 always maintains close contact with the shrunken coating surface; (2) Interface penetration component: To achieve interlayer metallurgical bonding, simply densifying the coating itself is insufficient. Densification coefficient It can also force the thermoplastic deformation zone generated by the needleless stirring head 1 to penetrate the bottom interface of the current deposited layer and cut into the underlying material at a predetermined depth. The mechanical forging and shearing action generated by this cutting action is key to breaking the continuous oxide film between layers and promoting the diffusion of elements across the interface.

[0042] The value range is set to 0.25 ~ 0.5, which is the optimal process window that can balance the relationship between full densification quality, interlayer metallurgical bond strength, and material utilization / equipment load. When When the pressure increment of the needleless stirring head 1 is less than 0.25, most of the pressure increase is consumed by the volume shrinkage compensation component of the coating, leaving insufficient interfacial penetration component. At this point, although the needleless stirring head 1 can compact the current layer, it cannot effectively break down the oxide barrier at the interlayer interface, easily leading to interlayer cold overlap or weak bonding defects. However, when... At a depth of >0.5, although sufficient interfacial mixing can be ensured, excessive pressure depth will cause the rheological material to be unable to be confined below the shoulder 12, resulting in outward overflow and severe flash, which wastes the effective deposited material. At the same time, excessive axial resistance will significantly increase the mechanical load of the mixing equipment and increase the wear risk of the needleless mixing head 1.

[0043] (b) Heat input ratio to ensure that the oxide film breaks down without melting.

[0044] To ensure sufficient plastic flow without melting, the ratio of the rotational speed to the travel speed of the needleless stirring head 1 needs to be controlled. The rotational speed of the needleless stirring head 1 is set to 1000 rpm ~ 1500 rpm, and the travel speed is set to 60 mm / min ~ 100 mm / min; the tilt angle of the shoulder 12 of the needleless stirring head 1 is 2° ~ 2.5°.

[0045] The specific combination of process parameters for friction stir processing (coating densification) is designed for the specific application of plasma-sprayed porous coatings. This invention achieves a dual purpose distinct from traditional friction stir welding by strictly limiting the heat input ratio (rotation speed / travel speed): firstly, it provides adequate thermoplastic flow to close the microscopic pores within the coating, rather than achieving macroscopic bonding; secondly, it utilizes a lower heat input level to prevent abnormal grain coarsening or recrystallization of amorphous phases in high-entropy alloy coatings due to overheating, thereby maximizing the preservation of the coating's excellent performance in its deposited state.

[0046] (II) Cyclic Operation and Interlayer Processing: After completing process B of the current layer, the forming quality of the processing area is checked. The inspection method is as follows: (1) Edge treatment of the stirring friction processing area: If there are flashes caused by stirring and extrusion at the edge of the coating, only mechanical or manual grinding is needed to remove the flashes to ensure that the surface flatness does not affect the mechanical avoidance of the next process. (2) The surface condition of the friction stir processing area is preserved, eliminating the need for grinding the surface arc-shaped texture of the friction stir processing area and retaining the microscopic arc-shaped concave-convex texture of the surface. The reason is that: (a) the upsetting action of the shoulder 12 in the needleless stirring head 1 has ensured the macroscopic flatness of the current coating surface. (b) The microscopic arc-shaped concave-convex texture of the surface is preserved to utilize the increased surface area effect and mechanical interlocking structure provided by the arc-shaped concave-convex texture to effectively increase the interlocking area and interfacial bonding strength when the next layer of sprayed coating is deposited. This method avoids material loss caused by interlayer mechanical grinding, simplifies the process flow, and also improves the overall anti-peeling performance of the multilayer coating by utilizing the specific morphology formed by solid-state processing.

[0047] After the molding quality inspection is completed, if the molding quality meets the requirements, repeat steps A and B until the final step is completed. The preparation of the coating layers involves sequentially performing steps A and B for each layer until all layers are prepared.

[0048] S4: Final surface treatment.

[0049] Once the preset total number of layers is reached, the outermost surface is mechanically ground or polished according to the final dimensional accuracy and surface quality requirements of the product. This removes the arc-shaped patterns and trace amounts of surface oxidation caused by stirring friction, resulting in a dense, high-entropy alloy gradient coating with a gradient distribution of composition.

[0050] Example 1 This embodiment provides a method for preparing an AlCoCrFeNi high-entropy alloy gradient coating on the surface of 6061 aluminum alloy, comprising the following steps: S1: Select a 6061-T6 aluminum alloy sheet with a length of 100mm, a width of 50mm, and a thickness of 10mm as the substrate 5, and clean and roughen the surface of the substrate 5: (1) The surface of the substrate 5 was roughened by sandblasting with brown corundum sand with a particle size of 46 mesh. The sandblasting air pressure was controlled at 0.6 MPa and the sandblasting angle was about 75° until the surface of the substrate 5 showed a uniform dark gray matte metallic luster and the surface roughness Ra was about 5~7 μm. (2) Place the sandblasted substrate 5 in anhydrous ethanol for ultrasonic cleaning for 15 min to remove residual oil and sand on the surface of the substrate 5. After taking out the substrate 5, blow it dry with compressed air for later use.

[0051] S2: Quantitative calculation of gradient coating components and layer design: (1) Set the total number of coating layers = 5, the total thickness of the coating is 2.0 mm (single layer thickness) (0.4 mm); (2) Spherical AlCoCrFeNi high-entropy alloy powder (particle size 15~45 μm) was selected as the gradient functional component, and 6061 aluminum alloy powder (particle size 15~45 μm) was selected as the matrix homogeneous component. (3) Set the mass percentage of the high-entropy alloy powder at the bottom layer. =20%, the mass percentage of high-entropy alloy powder on the surface layer. =100%; (4) According to the gradient calculation formula Calculate the mass percentage of each layer of high-entropy alloy powder: 20%; 20% + 1 / 4 × 80% = 40%; : 20% + 2 / 4 × 80% = 60%; 20% + 3 / 4 × 80% = 80%; : 20% + 4 / 4 × 80% = 100%.

[0052] Five sets of spray powder were prepared according to the above proportions and mechanically mixed for later use.

[0053] S3: Layer-by-layer spraying deposition-solid phase densification cycle manufacturing.

[0054] Preparation of the first coating layer: Process A: Coating deposition treatment via plasma spraying: The first batch of spray powder (i.e., high-entropy alloy powder with a mass percentage of 20%) was loaded into the powder feeder. Spraying was performed using an atmospheric plasma spraying system. The spraying process parameters were: spraying current 500 A, spraying voltage 48 V, main gas Ar flow rate 40 L / min, auxiliary gas H2 flow rate 7 L / min, powder feed rate 25 g / min, and spraying distance 100 mm. The single-layer spray coating thickness was achieved using the plasma spraying system. Controlled within 0.4 mm;

[0055] Coating cooling: After the spraying is completed, the surface of the first coating layer 41 is cooled by compressed air. The surface temperature is monitored in real time using an infrared thermometer. Once the temperature drops below 100℃, the operation of process B is immediately carried out.

[0056] Step B: Coating densification treatment via friction stirring.

[0057] A needleless stirring head 1 with a shoulder 12 having a diameter of 12 mm and made of WC-Co hard alloy is used. The end face of the shoulder 12 of the needleless stirring head 1 is a concave structure 13 with a concave angle of 3°.

[0058] The process parameters for friction stir machining are set as follows: the rotational speed of the needleless stirring head 1 is 1200 rpm, the travel speed is 80 mm / min, and the tilt angle of the shoulder 12 of the needleless stirring head 1 is 2.5°. To avoid high-temperature oxidation, local argon gas protection is used during the machining process. The protected area is the friction stir machining area, and the argon gas flow rate is set to 20 L / min.

[0059] The compaction depth is precisely set according to the densification model of the present invention: Current single-layer spray coating thickness =0.4 mm. Densification coefficient = 0.4. According to the formula Calculate the shoulder depth of the needleless stirring head. = 0.56 mm. Therefore, in the actual processing, the needleless stirring head 1 completely compacts the current 0.4 mm coating (compensating for pore shrinkage), allowing the thermoplastic deformation zone to completely penetrate the first coating layer 41 and penetrate 0.16 mm into the substrate 5, achieving a high-quality metallurgical bond between the coating and the substrate 5.

[0060] Preparation of the second coating layer 41 to the fifth coating layer 45: Repeat steps A and B above. Replace the powder with the second group of spray powder (i.e., high entropy alloy powder with a mass percentage of 40%), the third group of spray powder (i.e., high entropy alloy powder with a mass percentage of 60%), the fourth group of spray powder (i.e., high entropy alloy powder with a mass percentage of 80%), and the fifth group of spray powder (i.e., high entropy alloy powder with a mass percentage of 100%) in sequence.

[0061] In each subsequent process B, the shoulder depth of the needle-free stirring head is... The setting is always 0.56 mm relative to the surface of the current deposited layer to ensure that each processing penetrates the current layer and acts on the top of the previous coating, achieving a dense interlayer bond. A strict cooling procedure to below 100°C is performed before each processing step.

[0062] S4: Final surface treatment.

[0063] After completing the friction stirring densification process of the fifth coating layer 45, the outermost surface is mechanically ground according to the final dimensional accuracy and surface quality requirements of the product to remove the arc-shaped patterns and trace amounts of surface oxidation caused by friction stirring, thereby obtaining a high-entropy alloy gradient coating with a dense internal structure and a gradient distribution of composition.

[0064] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not introduce the layer-by-layer stirring friction densification process of Example 1.

[0065] The preparation process of Comparative Example 1 is as follows: Step 1: Matrix 5 and material preparation.

[0066] The same 6061-T6 aluminum alloy sheet as in Example 1 was used as substrate 5, and the same sandblasting roughening treatment was performed, resulting in a surface roughness Ra of approximately 5~7 μm. Five sets of spray powders with the same composition and gradient ratio as in Example 1 were used.

[0067] Step 2: Plasma spray coating preparation.

[0068] The preparation process involves only atmospheric plasma spraying deposition, without any stirring friction densification process (i.e., step B in Example 1 is omitted). The specific operation is as follows: (1) The first coating layer 41 (i.e., high entropy alloy powder with a mass percentage of 20%) was sprayed according to the plasma spraying process parameters of step A in Example 1 (spraying current 500 A, spraying voltage 48 V, main gas Ar flow rate 40 L / min, auxiliary gas H2 flow rate 7 L / min, powder feeding rate 25 g / min, spraying distance 100 mm).

[0069] (2) After the first coating layer 41 is sprayed, it is cooled to room temperature using compressed air (this is a conventional and necessary cooling measure taken in traditional multi-layer spraying processes to prevent heat accumulation from causing the substrate to overheat and the coating to crack), and then the next coating layer is prepared immediately;

[0070] (3) Repeat the above process until all five coating layers are completed.

[0071] Step 3: Final surface treatment.

[0072] After the gradient coating was prepared, the same post-processing standard as in Example 1 was used to obtain a high-entropy alloy gradient coating;

[0073] The samples of Example 1 and Comparative Example 1 were tested using industry-standard testing methods.

[0074] Coating porosity testing: The average porosity of the coating was determined using the Archimedes drainage method;

[0075] Bonding strength test: The coated sample was bonded to the paired tensile bar using high-strength epoxy resin adhesive for tensile testing;

[0076] Microhardness testing: The surface hardness of the coating was tested using a microhardness tester. The test load was set to 100 g, and the loading time was 10 s.

[0077] Thermal shock performance test: The high-temperature water quenching method was used. The sample was heated to 500 ℃, held at that temperature for 15 min, and then quickly immersed in flowing water at 25 ℃ to cool it. This completed one thermal shock cycle. The cycle was repeated until cracks appeared on the coating surface or the coating peeled off.

[0078] Table 1: Performance test results of Example 1 and Comparative Example 1 Coating thickness (mm) 2.02 ± 0.10 1.95 ± 0.05 Average porosity (%) 8.5 ± 1.5 < 0.8 Bond strength (MPa) 35 ± 5 >80 (Adhesive failure) Microhardness (HV) 420 ± 30 580 ± 20 Number of thermal shock cycles (times) 12 > 30 The test results in Table 1 show that: (1) Densification mechanism: The average porosity of Comparative Example 1 (conventional process) is as high as 8.5±1.5%. This is determined by the characteristics of the atmospheric plasma spraying process itself: when molten or semi-molten sprayed powder impacts the surface of the substrate 5 at high speed and flattens and stacks, pores and oxide inclusions will inevitably remain between layers, resulting in low density.

[0079] In contrast, the porosity of Example 1 is below 0.8%, which is due to the layer-by-layer stirring friction processing step introduced in this invention: the strong closed extrusion stress generated by the concave shoulder stirring head is used to perform intense thermoplastic processing and upsetting on the deposited layer, successfully closing the inherent pores in the plasma spray coating and fundamentally eliminating the layered stacking defects.

[0080] (2) Interface integration mechanism: The bonding strength test showed that the bonding strength of Comparative Example 1 was only 35±5 MPa. Figure 5 The image shown is a scanning electron microscope image of the tensile fracture morphology of Comparative Example 1. A small number of shallow dimples were observed on the fracture surface, but the overall fracture was still mainly characterized by brittle interface peeling. The fracture mode was a mixed ductile-brittle fracture, indicating that the bonding strength between the coating and the substrate still mainly depends on mechanical bonding.

[0081] The tensile bond strength of Example 1 exceeds 80 MPa, which is significantly higher than that of Comparative Example 1. Figure 5 This is a macroscopic fracture morphology image of the tensile specimen from Example 1. (From...) Figure 5 As can be seen, the fracture surface after the tensile test was covered by residual adhesive of uneven thickness and exhibiting severe tearing, while the coating body and the coating-substrate interface did not peel or fracture. This indicates that the fracture path was entirely within the adhesive, manifesting as adhesive failure. This suggests that the bonding strength between the high-entropy alloy gradient coating prepared in this invention and the substrate exceeds the strength limit of the adhesive used.

[0082] This is because, during the friction stir machining process, the shoulder pressing depth of the needleless stirring head is precisely controlled. This allows the thermoplastic deformation zone to penetrate the current deposition layer and act deeply on the lower layer or substrate 5. At the same time, the stirring and upsetting action of the needleless stirring head 1 effectively breaks up the continuous oxide inclusions in the coating, promotes the interdiffusion and recrystallization of elements across the interface, and thus transforms the original interface bonding mode, which is mainly mechanical interlocking, into a high-strength bonding interface, which is mainly metallurgical bonding.

[0083] (3) Hardness and thermal shock resistance: The surface microhardness of Example 1 was 580 HV, which was nearly 40% higher than that of Comparative Example 1. This was due to the extremely high density (which eliminated the pore softening effect) and the fine grain strengthening effect caused by dynamic recrystallization during the friction stir process.

[0084] In the thermal shock test, Example 1 withstood more than 30 thermal shock cycles without any surface damage. This indicates that the gradient coating method of the present invention effectively alleviates the internal stress of the coating, and at the same time, the original coating is changed from mechanical bonding to metallurgical bonding, which effectively improves the thermal shock resistance and service reliability of the coating.

[0085] Technical features not described in this invention can be implemented using existing technologies and will not be elaborated upon here. This invention is not limited to the specific embodiments described above; any changes, modifications, additions, or substitutions made by those skilled in the art within the scope of this invention should also fall within the protection scope of this invention.

Claims

1. A method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification, characterized in that, Includes the following steps: S1: Select a metal material as the substrate and clean and roughen the surface of the substrate; S2: Quantitative calculation of gradient coating components and determination of the number of layers, total number of coating layers. ≥3; The high-entropy alloy powder is mixed with a homogeneous matrix powder, and the mass percentage of the high-entropy alloy powder is adjusted layer by layer. High entropy alloy powder mass percentage of the layer Calculate using the following formula: ； In the formula: The current level number, 1 ≤ ≤ ; This refers to the total number of coating layers; For the current number The mass percentage of high-entropy alloy powder in the layer; This represents the mass percentage of the underlying high-entropy alloy powder, ranging from 10% to 30%. This represents the mass percentage of the high-entropy alloy powder on the surface layer, ranging from 80% to 100%. This represents a fixed increment in the content of high-entropy alloys between coating layers; S3: Cyclic manufacturing process of layer-by-layer spray deposition-solid phase densification: (I) Perform the following two procedures sequentially on each layer: Process A: Coating deposition is performed by plasma spraying; Process B: Coating densification treatment is carried out by friction stirring. (II) Cyclic Operation and Interlayer Processing: After completing process B in the current layer, check the forming quality of the processing area. If the forming quality meets the requirements, repeat process A and process B until the current layer is completed. Preparation of the coating layer; S4: Final surface treatment.

2. The method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification according to claim 1, characterized in that, In step S3, process A, the current layer coating is deposited using atmospheric plasma spraying equipment, and the thickness of a single layer is [not specified]. The value range is 0.1 mm ~ 1.0 mm.

3. The method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification according to claim 2, characterized in that, The spraying process parameters in step S3, process A are as follows: Spraying current: 450 A ~ 550 A; Spraying voltage: 40 V ~ 55 V; The flow rate of the main gas Ar is 35 L / min ~ 45 L / min; Flow rate of auxiliary gas H2: 5 L / min ~ 10 L / min; Powder delivery rate: 20 g / min ~ 30 g / min; Spraying distance: 90 mm ~ 110 mm; Coating cooling: After spraying, the coating surface is cooled by compressed air or by natural cooling. Only after the coating temperature drops below 100℃ can step B be performed.

4. The method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification according to claim 3, characterized in that, In step S3, process B uses a needleless stirring head made of hard alloy WC-Co or polycrystalline cubic boron nitride to perform friction stirring. The shoulder diameter of the needleless stirring head is 10 mm to 15 mm, and the end face of the shoulder of the needleless stirring head has a concave structure with a concave angle of 3° to 8°.

5. The method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification according to claim 4, characterized in that, The method for densifying the coating in step S3, process B, is as follows: The coating surface is cooled until the temperature drops below 100°C. Then, a needleless stirring head is used to perform friction stirring on the current layer. During the friction stirring process, a local inert gas is introduced for protection. The flow rate of the protective gas is 15 L / min ~ 25 L / min. The protected area of ​​the inert gas is the friction stirring area.

6. The method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification according to claim 5, characterized in that, The shoulder pressing depth of the needleless stirring head It needs to be based on the thickness of the single-layer spray coating. The settings are dynamically configured, and the specific calculation formula is as follows: ； In the formula: ; This refers to the thickness of a single-layer spray coating. The densification coefficient ranges from 0.25 to 0.

5.

7. The method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification according to claim 6, characterized in that, The needleless stirring head has a rotational speed of 1000 rpm to 1500 rpm and a travel speed of 60 mm / min to 100 mm / min; the shoulder tilt angle of the needleless stirring head is 2° to 2.5°.

8. The method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification according to claim 7, characterized in that, The method for checking the forming quality of the processing area after completing process B of the current layer in step S3 is as follows: (1) Edge treatment of the friction stir processing area: If there are flashes caused by stirring and extrusion at the edge of the coating, the flashes need to be removed by mechanical or manual grinding. (2) The surface condition of the friction stir processing area is preserved, and the microscopic arc-shaped concave and convex texture of the surface is preserved; The final surface treatment in step S4 is as follows: Once the preset total number of layers is reached, the outermost surface is mechanically ground or polished according to the final dimensional accuracy and surface quality requirements of the product to obtain a high-entropy alloy gradient coating with a dense internal structure and a gradient distribution of components.

9. The method for preparing a high-entropy alloy gradient coating based on layer-by-layer solid-phase densification according to claim 8, characterized in that, In step S1, the substrate surface is cleaned and roughened in the following manner: (1) The substrate surface is roughened by sandblasting with corundum sand of 24-60 mesh, and the sandblasting air pressure is controlled at 0.5 MPa-0.7 MPa; (2) Use acetone or anhydrous ethanol to ultrasonically clean the substrate for 10 min to 15 min, and blow it dry after cleaning.

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