Thermal expansion coating and method of spraying and assembling same

Abstract

The present application relates to the technical field of interference fit assembly in machining, in particular to a thermal expansion coating for shaft-hole interference fit assembly and a spraying method thereof. In the interference fit of the existing shaft parts and hole parts in the prior art, the press-in method is prone to cause scratches and micro-cracks on the fitting surface, the thermal assembly method is high in energy consumption, slow in heating and prone to cause thermal deformation and surface oxidation and decarburization, the cold assembly method is high in cost and has hidden dangers such as frost and cold brittleness, and it is difficult to realize fast and non-destructive interference assembly without changing the overall temperature of the parts and causing thermal deformation. A thermal expansion coating for mechanical part interference fit and a spraying and assembly method are provided to solve the above problems.

Description

Technical Field

[0001] This invention relates to the field of interference fit assembly technology in machining, and specifically to a thermal expansion coating and its spraying method for interference fit assembly of shaft and hole. Background Technology

[0002] In the machining and assembly processes of the machinery manufacturing industry, interference fits between shaft parts and hole parts are an extremely common assembly process. Traditional interference fit methods mainly include press-fitting and heat fitting (heating the hole part or cooling the shaft part). However, existing technologies have the following drawbacks: (1) Pressing can easily cause scratches and damage to the mating surfaces, and even cause micro-cracks in the parts, reducing assembly accuracy and fatigue life.

[0003] (2) Traditional hot fitting methods usually use oil bath furnaces, induction heating or flame heating to heat the hole-type parts to a high temperature to generate sufficient thermal expansion. Not only is the energy consumption high and the heating time long, but it can also cause thermal deformation and surface oxidation and decarburization of the parts, affecting dimensional accuracy and surface hardness; for complex parts that have been precision machined, the risk of thermal deformation is particularly fatal.

[0004] (3) Cold mounting methods (such as liquid nitrogen cooling shafts) are expensive and pose safety hazards such as frosting and cold brittleness.

[0005] Currently, the industry lacks a technical solution that can achieve rapid, non-destructive interference fit assembly through only local surface treatment without changing the overall temperature of the parts or causing thermal deformation. Summary of the Invention

[0006] To address the problems in existing interference fit technologies for shaft and hole parts, such as the press-fit method easily causing scratches, micro-cracks, and thermal deformation on the mating surfaces, the high energy consumption and slow heating of the hot fitting method, which is prone to thermal deformation and surface oxidation and decarburization, and the high cost and safety hazards of frosting and brittleness, and the difficulty in achieving fast and non-destructive interference fit without changing the overall temperature of the parts or causing thermal deformation, a thermal expansion coating is provided for interference fit of mechanical parts. The coating is applied to the mating surfaces of the parts to be assembled. The thermal expansion coating is a composite coating, which includes a bonding underlayer and a working surface layer from the inside to the outside. The bonding underlayer is formed by thermal spraying of nickel-aluminum alloy powder or nickel-chromium alloy powder, and the working surface layer is formed by thermal spraying of high thermal expansion composite powder. The high thermal expansion composite powder is composed of the following components in parts by weight: 50-65 parts aluminum-copper-zinc alloy powder, 20-30 parts zinc-tin alloy powder, 5-10 parts solid lubricant, and 0-15 parts unavoidable impurities.

[0007] In one embodiment, the coefficient of linear expansion of the working surface layer is (25-30) × 10⁻⁶. ⁻6 / ℃.

[0008] In one embodiment, the thickness of the bonding substrate is 0.05 mm to 0.1 mm.

[0009] In one embodiment, the working surface layer has a thickness of 0.1 mm to 0.3 mm.

[0010] In one embodiment, the nickel-aluminum alloy has a ratio of 80Ni:20Al, and the nickel-chromium alloy powder has a ratio of 80Ni:20Cr.

[0011] In one embodiment, the solid lubricant is graphite or molybdenum disulfide.

[0012] In one embodiment, the thermal spraying method is plasma spraying.

[0013] In one embodiment, the coefficient of linear expansion of the working surface layer is higher than that of the base material, and the coefficient of linear expansion of the bonding layer is between that of the working surface layer and the base material.

[0014] A method for preparing the thermally expandable coating by spraying includes the following steps: S1 Surface Pretreatment: The mating surfaces of the parts to be coated are degreased and cleaned, and then subjected to brown corundum sandblasting to achieve a surface roughness of Ra3.0μm~Ra5.0μm; S2 Substrate Preheating: Medium-frequency induction heating is used to preheat the spraying area. The preheating temperature is controlled at 120℃~150℃ to eliminate surface moisture and reduce spraying thermal shock stress. S3 Spray Coating Underlayer: Supersonic plasma spraying is used with a spraying power of 35kW~40kW, a spraying distance of 100mm~120mm, and a powder feeding rate of 30g / min~40g / min to form a bonding underlayer of a preset thickness on the substrate surface. S4 Spraying working surface layer: Change the powder under the same equipment, spray the high thermal expansion composite powder, spraying power 30kW~35kW, spraying distance 110mm~130mm, powder feeding rate 35g / min~45g / min, to form a working surface layer of preset thickness. S5 Sealing and Finishing: After natural cooling to room temperature, the micropores of the coating are sealed with a high-temperature resistant silicone sealant. Then, the coating is machined to the target assembly size and an interference fit is reserved through precision grinding or honing.

[0015] An interference fit assembly method for the thermal expansion coating includes the following steps: S1: Place the hole-type parts coated with the thermal expansion coating in a constant temperature heating device and heat them to 80℃~100℃, and keep them at that temperature for 20min~30min; S2: After the shaft-like parts are inserted and naturally cooled, the working surface layer shrinks to form an interference clamping force on the shaft-like parts.

[0016] Compared with the prior art, the present invention has the following significant advantages: (1) Achieve low-temperature assembly and eliminate thermal deformation: By utilizing the high expansion characteristics of the coating material, the assembly heating temperature is significantly reduced from the traditional 200℃ or above to 80℃-100℃, thus avoiding overall thermal deformation of machined parts and ensuring dimensional accuracy and geometric tolerance.

[0017] (2) Energy saving and environmental protection, and improved efficiency: The lower heating temperature directly shortens the heating and heat preservation time, saves electricity consumption, and avoids the surface oxidation and decarburization caused by high temperature, eliminating the need for a secondary descaling process after assembly.

[0018] (3) Protect the mating surfaces and improve the connection quality: The self-lubricating phase (graphite / molybdenum disulfide) contained in the coating plays a role in reducing friction during assembly and insertion, preventing surface scratches caused by the traditional press-fit method; the high strength of the coating after cooling can also ensure the connection strength of the interference fit.

[0019] (4) Wide applicability of the process: The supersonic plasma spraying / flame spraying process has strong bonding force and low porosity, and is suitable for shaft hole mating surfaces of various shapes. The coating thickness is controllable and the subsequent machining allowance is stable. Attached Figure Description

[0020] Appendix Figure 1 Schematic diagram of the cross-sectional structure of the thermal expansion coating; Appendix Figure 2 Schematic diagram of spray coating for perforated parts; Appendix Figure 3 Schematic diagram of the thermal expansion of hole-type parts Appendix Figure 4 Assembly diagram of shaft parts; Appendix Figure 5 Schematic diagram of cooling and shrinkage structure; 1-Hole-type parts, 2-Merging bottom layer, 3-Working surface layer, 4-Shaft-type parts Detailed Implementation

[0021] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0023] The thermal expansion coating for interference fit of mechanical parts described in this invention is applied to the mating surfaces of the parts to be assembled. The thermal expansion coating is a composite coating, comprising, from the inside out, a bonding underlayer and a working surface layer. The bonding underlayer is formed by thermal spraying of nickel-aluminum alloy powder or nickel-chromium alloy powder, and its thickness is 0.05mm to 0.1mm. Its main function is to form a metallurgical bond or strong mechanical interlock with the substrate and to alleviate residual stress caused by the difference in thermal expansion coefficients between the coating and the substrate. The working surface layer has a thickness of 0.1mm to 0.3mm. The working surface layer is formed by thermal spraying of high thermal expansion composite powder, which is composed of the following components by weight: 50-65 parts aluminum-copper-zinc alloy powder, 20-30 parts zinc-tin alloy powder, 5-10 parts solid lubricant, and 0-15 parts unavoidable impurities. The linear expansion coefficient of the working surface layer is (25-30) × 10⁻⁶. ⁻6 / ℃, much higher than that of ordinary steel / cast iron matrix (approximately 11-13) × 10 ⁻6 / ℃. Example

[0024] For the base layer 2, nickel-aluminum alloy powder is selected as the spraying material, with a nickel-aluminum alloy ratio of 80Ni:20Al. This ratio has a significant impact on the thermal spraying bonding base layer, directly determining the intensity of the exothermic reaction, porosity, high-temperature oxidation resistance, thermal shock stability, and surface layer compatibility. The ratio in Example 1 results in the highest bonding strength of the base layer, with plasma spraying bonding strength reaching 30–45 MPa; sufficient exothermic reaction, lower coating porosity, better density, and stronger ability to prevent oxygen and corrosive media from penetrating the substrate; optimal high-temperature oxidation resistance: a continuous, dense aluminum oxide passivation film is formed at high temperatures, with a long-term service temperature of 650–800℃, exhibiting excellent resistance to sulfidation and high-temperature flue gas corrosion; good toughness and shock resistance: perfectly buffering the difference in thermal expansion coefficients between the metal substrate and the surface layer, making it less prone to cracking under alternating hot and cold temperatures.

[0025] Meanwhile, in this embodiment, a plasma spraying process is used to prepare a bonding underlayer on the substrate surface, controlling the coating thickness to 0.08 mm. Before spraying, the substrate of the hole-type part 1 is roughened by sandblasting to improve surface roughness. During the spraying process, the high-temperature molten powder undergoes a local metallurgical reaction with the substrate. The substrate of the hole-type part is 45# steel. The coating and the 45# steel substrate simultaneously have a metallurgical bonding interface and a micro-mechanical interlocking structure. The coating bonding strength can reach 62 MPa, with no peeling or flaking defects. The difference in thermal expansion coefficient between the nickel-aluminum intermediate layer transition substrate and the upper working surface layer was tested. After cooling, the residual stress at the interface of the double-layer composite structure was reduced by 47% compared with the sample without the transition underlayer, completely avoiding the interface cracking problem caused by the mismatch of thermal expansion coefficients. The thickness of 0.08 mm ensures stress buffering capacity without causing insufficient overall coating rigidity and excessive cost due to an excessively thick underlayer.

[0026] Example 2 differs from Example 1 in that the nickel-aluminum alloy ratio is 95Ni:5Al. This ratio results in a mild reaction, a very wide spraying process window, and flame spraying is less prone to burn-out and cracking, making it highly stable for beginners and those with simple equipment. The high nickel content leads to better coating toughness and easier subsequent machining and polishing. It also results in lower stress, with thin-walled parts and easily deformable workpieces exhibiting less deformation after spraying.

[0027] Example 3 differs from Example 1 in that it uses a nickel-chromium alloy powder with a ratio of 80Ni20Cr. This example has top-tier high-temperature oxidation and sulfidation resistance, with a long-term service temperature of 900-950℃. Its coefficient of thermal expansion is between that of a carbon steel substrate and an alumina / chromium oxide ceramic surface layer, serving as a transitional underlayer that significantly reduces the risk of thermal cycling cracking and surface layer peeling. The coating has good toughness, low internal stress, and is not easily deformed in thin-walled parts. Its porosity is controllable, and plasma spraying can achieve low porosity, providing strong barrier properties against media penetration. Example 4 differs from Example 1 in that it uses a nickel-chromium alloy powder with a ratio of 95Ni5Cr. This example has an extremely low chromium content, a very thin oxide film, excellent coating plasticity, and excellent thermal shock resistance. It is not easy to flake off after repeated rapid cooling and heating. The thermal stress of spraying is minimal, and the deformation of ultra-thin substrates and precision small parts is minimal. The cost is lower, there is less spraying splatter, and the powder utilization rate is high.

[0028] Example 5 differs from either Example 3 or Example 4 in that a 0.06mm underlayer is prepared using nickel-chromium alloy powder, achieving the same stress relief effect as Example 1, and is more suitable for parts substrates under high-temperature conditions.

[0029] Example 6: The high thermal expansion composite powder for the working layer 3, by weight, consists of: 60 parts aluminum-copper-zinc alloy powder, 25 parts zinc-tin alloy powder, 8 parts molybdenum disulfide solid lubricant, and 7 parts unavoidable impurities. The working layer is then plasma-sprayed onto the surface of the bonding underlayer 2, with a total thickness of 0.2 mm per spray. After the powder is fully melted, it is uniformly deposited on the underlayer surface, forming a dense composite working coating. The measured coefficient of linear expansion of this working layer is 27.2 × 10⁻⁶. ⁻6 / ℃, falling within 25~30×10 ⁻6 Within the design temperature range; significantly higher than the cast iron matrix by 11~13×10℃. ⁻6 The linear expansion coefficient of / ℃, combined with the gradient of high and low expansion coefficients, allows for the gradual release of thermal stress in the bottom layer; In the example, 8 parts of molybdenum disulfide were used as solid lubricant, which reduced the dry friction coefficient of the coating to 0.12 and increased the wear resistance life by 2.3 times compared with ordinary alloy coatings. The total coating thickness of the 0.2mm working surface layer plus the 0.08mm bonding underlayer was 0.28mm, with tight interlayer bonding. No delamination or cracking was observed after 500 cycles of hot and cold cycling.

[0030] Example 7 differs from Example 6 in that the high thermal expansion composite powder of the working surface layer, by weight, comprises: 65 parts aluminum-copper-zinc alloy powder, 20 parts zinc-tin alloy powder, 5 parts graphite lubricant, and 10 parts unavoidable impurities. The measured coefficient of linear expansion is stable at 28.5 × 10⁻⁶. ⁻6 / ℃, graphite lubrication is suitable for low-speed, heavy-load friction conditions.

[0031] A method for preparing the above-mentioned thermal expansion coating by spraying includes the following steps: S1 Surface Pretreatment: The mating surfaces of the parts to be coated are degreased and cleaned, and then sandblasted with brown fused alumina to achieve a surface roughness of Ra3.0~Ra5.0um; S2 Substrate Preheating: Medium-frequency induction heating is used to preheat the spraying area. The preheating temperature is controlled at 120℃~150℃ to eliminate moisture on the substrate surface and reduce thermal shock stress during spraying. S3 spray coating combined with base layer: adopts supersonic plasma spraying technology, with spraying power controlled at 35-40kW, spraying distance 100-120mm, and powder feeding rate 30-40g / min; S4 Spraying Working Layer: Change powder under the same equipment, control the spraying power at 30-35kW, the spraying distance at 110-130mm, the powder feeding rate at 35-45g / min, and control the final coating thickness. S5 Sealing and Finishing: After spraying, allow it to cool naturally to room temperature. Use a high-temperature silicone sealant to seal the micropores in the coating. Then, use precision grinding or honing to process the coating size to the target assembly size, leaving an interference fit.

[0032] An assembly method based on the above-mentioned thermal expansion coating: During assembly, the hole-type part 1 with a thermal expansion coating is placed in a constant temperature heating chamber and heated to 80℃-100℃, which is much lower than the traditional heat assembly temperature. After holding at this temperature for 20-30 minutes, the shaft-type part is inserted. At this time, the inner diameter of the hole-type part expands with the increase in temperature, and the coating also tends to thicken inward. However, due to the circumferential constraint of the hole-type part's substrate, the actual thickening amount is less than the free expansion value of the coating. The hole-type part generates an assembly gap outward to satisfy the insertion of the shaft-type part 4. At the same time, preferably, the shaft-type part is frozen with liquid nitrogen. After insertion, it is naturally cooled, and the coating shrinks to form a strong interference fit.

[0033] Compared with the prior art, the present invention has the following significant advantages: Achieving low-temperature assembly and eliminating thermal deformation: By utilizing the high expansion characteristics of the coating material, the assembly heating temperature is significantly reduced from the traditional 200℃ or above to 80℃-100℃, avoiding overall thermal deformation of machined parts and ensuring dimensional accuracy and geometric tolerances.

[0034] Energy-saving and environmentally friendly, improving efficiency: Lower heating temperature directly shortens heating and heat preservation time, saving electricity consumption, while avoiding surface oxidation and decarburization caused by high temperature, eliminating the need for a secondary descaling process after assembly.

[0035] Protect mating surfaces and improve connection quality: The self-lubricating phase (graphite / molybdenum disulfide) contained in the coating plays a role in reducing friction during assembly and insertion, preventing surface scratches caused by traditional press-fit methods; the high strength of the coating after cooling can also ensure the connection strength of the interference fit.

[0036] Wide applicability of the process: The supersonic plasma spraying / flame spraying process has strong adhesion and low porosity, and is suitable for shaft and hole mating surfaces of various shapes. The coating thickness is controllable and the subsequent machining allowance is stable.

[0037] In the description of this invention, it should be noted that the terms "upper," "lower," "front," "rear," "left," and "right," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use. They are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. In addition, the terms "first," "second," and "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0038] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0039] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A thermally expanding coating, applied to the mating surfaces of parts to be assembled, characterized in that, The thermal expansion coating is a composite coating, which includes a bonding underlayer and a working surface layer from the inside out. The bonding underlayer is formed by thermal spraying of nickel-aluminum alloy powder or nickel-chromium alloy powder, and the working surface layer is formed by thermal spraying of high thermal expansion composite powder. The high thermal expansion composite powder is composed of the following components in parts by weight: 50-65 parts of aluminum-copper-zinc alloy powder, 20-30 parts of zinc-tin alloy powder, 5-10 parts of solid lubricant, and 0-15 parts of unavoidable impurities.

2. The thermal expansion coating according to claim 1, characterized in that, The coefficient of linear expansion of the working surface layer is higher than that of the part substrate, while the coefficient of linear expansion of the bonding layer is between that of the working surface layer and the part substrate.

3. The thermal expansion coating according to claim 1, characterized in that, The thickness of the bonding substrate is 0.05mm to 0.1mm.

4. The thermal expansion coating according to claim 1, characterized in that, The working surface layer has a thickness of 0.1mm to 0.3mm.

5. The thermal expansion coating according to claim 1, characterized in that, The nickel-aluminum alloy has a ratio of 80Ni:20Al, and the nickel-chromium alloy powder has a ratio of 80Ni:20Cr.

6. The thermal expansion coating according to claim 1, characterized in that, The solid lubricant is graphite or molybdenum disulfide.

7. The thermal expansion coating according to claim 1, characterized in that, The linear expansion coefficient of the working surface layer is (25~30)×10 ⁻6 / ℃.

8. The thermal expansion coating according to claim 1, characterized in that, The thermal spraying method is plasma spraying.

9. A method for spraying a thermally expanding coating as described in any one of claims 1 to 8, characterized in that, Includes the following steps: S1 Surface Pretreatment: The mating surfaces of the parts to be coated are degreased and cleaned, and then sandblasted with brown fused alumina. S2 substrate preheating: Medium frequency induction heating is used to preheat the spraying area, and the preheating temperature is controlled at 120℃~150℃; S3 spray coating bonding underlayer: spraying power 35kW~40kW, spraying distance 100mm~120mm, powder feeding rate 30g / min~40g / min, forming a bonding underlayer of a preset thickness on the substrate surface; S4 spraying working surface layer: After changing the powder, the spraying power is 30kW~35kW, the spraying distance is 110mm~130mm, the powder feeding rate is 35g / min~45g / min, forming a working surface layer of preset thickness. S5 Sealing and Finishing: After natural cooling to room temperature, the micropores of the coating are sealed with a high-temperature resistant silicone sealant. Then, the coating is machined to the target assembly size and an interference fit is reserved through precision grinding or honing.

10. An interference fit assembly method based on the thermal expansion coating according to any one of claims 1 to 9, characterized in that, Includes the following steps: S1: Place the hole-type parts coated with the thermal expansion coating in a constant temperature heating device and heat them to 80℃~100℃, and keep them at that temperature for 20min~30min; S2: After the shaft-like parts are inserted and naturally cooled, the working surface layer shrinks to form an interference clamping force on the shaft-like parts.

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