Stainless steel surface composite coating strengthening method

Abstract

The invention provides a stainless steel surface composite coating strengthening method which comprises the following steps: a) sequentially carrying out chemical degreasing, ultrasonic passivation and laser texturing pretreatment on a base material to form a micro-pit array with the surface roughness Ra less than or equal to 0.2 mu m; b) applying a directional magnetic field with the magnetic field intensity of 0.5-1.5 T in the pulse electroplating process, and regulating and controlling the grain orientation of the plating layer; c) combining ultrasonic rolling and gradient nitriding processes to form a hardened layer of which the surface nitrogen content is distributed in a gradient manner; and d) optimizing a dynamic matching relationship among the magnetic field intensity, the pulse frequency and the nitriding temperature by utilizing a genetic algorithm, and realizing Pareto optimization of the coating performance. The method has the advantages that the grain orientation of the plating layer is improved, the bonding strength of the plating layer and a matrix is enhanced, the corrosion resistance is improved, the processing cost is reduced, and the surface hardness and the fracture toughness are improved.

Description

Technical Field

[0001] The present invention relates to a metal surface strengthening technology, and particularly to a method for strengthening a composite coating on the surface of stainless steel based on the synergistic effect of multiple physical fields, which is applicable to improving the comprehensive performance of stainless steel in corrosion, wear, and high-temperature environments. Background Art

[0002] In the field of stainless steel surface treatment, the traditional direct current electroplating process has many defects. The electroplated layer usually presents a coarse-grained structure, and the grain size often exceeds 50μm, which seriously affects the mechanical properties of the coating. At the same time, the bonding strength between the electroplated layer and the substrate is generally low, usually not exceeding 50MPa, which causes the coating to be easily peeled off, greatly reducing the service life of the product. In addition, the corrosion resistance of the traditional electroplated layer is also not satisfactory. In the standard salt spray test, its corrosion resistance time is usually less than 24 hours, making it difficult to meet the application requirements in harsh environments.

[0003] In order to improve the coating performance, some composite processes have been introduced, such as laser cladding technology. Although laser cladding can significantly increase the surface hardness, making it exceed 1000HV, this method has obvious limitations. First, its processing cost is extremely high, and the processing cost per square centimeter may exceed 500 yuan, which is unacceptable in large-scale production. Second, the laser cladding layer often shows greater brittleness, and the fracture toughness is usually lower than 10MPa·m 1 / 2 , which makes the treated surface prone to cracking, especially when subjected to impact or alternating loads.

[0004] When attempting to improve the electroplating process, the magnetic field-assisted electroplating technology has attracted the attention of researchers. However, this method has a narrow process window and extremely high requirements for parameter control. Especially the matching relationship between the magnetic field strength and the pulse frequency is extremely sensitive, and any slight improperness may lead to serious non-uniformity of the coating thickness, and the thickness fluctuation may exceed ±10μm. This non-uniformity not only affects the appearance quality of the product, but also leads to local differences in the coating performance, reducing the reliability of the overall performance.

[0005] In view of the above problems, the existing technology urgently needs to be improved. Summary of the Invention

[0006] In order to solve the above technical problems, the purpose of the present invention is to provide a method for strengthening a composite coating on the surface of stainless steel, which has the advantages of improving the grain orientation of the coating, enhancing the bonding strength between the coating and the substrate, improving the corrosion resistance, reducing the processing cost, increasing the surface hardness, and improving the fracture toughness.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The present invention provides a method for strengthening a composite coating on the surface of stainless steel, comprising the following steps: successively performing chemical degreasing, ultrasonic passivation and laser texturing pretreatment on a substrate to form a micro-pit array with a surface roughness Ra≤0.2μm; applying an oriented magnetic field with a magnetic field strength of 0.5-1.5T during pulse electroplating to regulate the grain orientation of the coating; combining ultrasonic rolling and gradient nitriding processes to form a hardened layer with a gradient distribution of surface nitrogen content; and optimizing the dynamic matching relationship of the magnetic field strength, pulse frequency and nitriding temperature by using a genetic algorithm to achieve the Pareto optimum of the coating performance.

[0009] Further, the present application also proposes that the chemical degreasing solution in step a) contains 3-5% NaOH + 0.5% OP-10 emulsifier, the treatment temperature is 60±5°C, and the time is 15-20 minutes.

[0010] Further, the present application also proposes that ultrasonic passivation is carried out at a frequency of 40kHz and a power density of 0.8W / cm 2 for 10-15 minutes.

[0011] Further, the present application also proposes that the parameters of laser texturing in step a) are: laser wavelength 1064nm, pulse width 10ns, and scanning speed 50mm / s.

[0012] Further, the present application also proposes that the electroplating solution in step b) contains Ni 2+ :W 6+ :P 5+ =80-85:10-12:3-5 (molar ratio), and PTFE particles with a particle size of 0.5-2μm are added, and the deposition current density is 3-5A / dm 2 .

[0013] Further, the present application also proposes that the pressure of ultrasonic rolling in step c) is 15-30MPa, the amplitude is 5-10μm, and the frequency is 20kHz.

[0014] Further, the present application also proposes that the gradient nitriding step in step c) adopts a two-stage process:

[0015] The first stage: constant temperature nitriding at 350-450°C to form a diffusion layer of 3-5μm;

[0016] The second stage: gradient temperature nitriding at 450-600°C to form a hardened layer with a surface nitrogen content >8at.%.

[0017] Further, the present application also proposes that the objective function of the genetic algorithm includes a weighted combination of hardness, wear resistance and corrosion resistance, and the weight coefficients are 0.4, 0.3 and 0.3 respectively.

[0018] Further, the present application also proposes that the dynamic matching relationship of magnetic field strength, pulse frequency, and nitriding temperature optimized by the genetic algorithm in step d) includes:

[0019] Construct a multi-objective optimization model based on the NSGA-II algorithm, and the objective function is the comprehensive performance index P = 0.4×HV + 0.3×(1 / wear_rate) + 0.3×t salt ,

[0020] The constraint conditions include the process parameter range and the requirement for the consistency of the coating thickness, and the population evolution is realized through real-number encoding crossover and polynomial mutation operations.

[0021] As can be seen from the above, a method for strengthening a stainless steel surface composite coating provided by the present application includes pre-treating the substrate, applying a directional magnetic field for pulse electroplating, forming a hardened layer by combining ultrasonic rolling and gradient nitriding processes, and optimizing the process parameters using a genetic algorithm. This method effectively controls the grain orientation of the coating through the synergistic action of multiple steps, enhances the bonding strength between the coating and the substrate, and simultaneously improves the surface hardness and wear resistance through gradient nitriding. The application of the genetic algorithm ensures the optimal matching of process parameters, thereby achieving a comprehensive improvement in the performance of the coating, and has the advantages of improving the grain orientation of the coating, enhancing the bonding strength between the coating and the substrate, improving the corrosion resistance, reducing the processing cost, and increasing the surface hardness and fracture toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a flowchart of the steps of a method for strengthening a stainless steel surface composite coating according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] The following describes in detail the embodiments of the present invention. The examples of the embodiments are shown in the drawings, where the same or similar reference numerals represent the same or similar elements or elements with the same or similar functions from beginning to end. The embodiments described below by referring to the drawings are exemplary and are only used to explain the present invention and should not be construed as a limitation of the present invention.

[0024] In the description of the present invention, it should be understood that if there are terms such as "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. indicating the orientation or positional relationship, they are based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be construed as a limitation on the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of the said features. In the description of the present invention, "a plurality" means two or more, unless otherwise specifically defined.

[0025] In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "mounted", "connected", and "coupled" shall be construed in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection. It may be a mechanical connection or an electrical connection. It may be directly connected or indirectly connected through an intermediate medium, and it may be the communication inside two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0026] Example 1

[0027] As Figure 1 shown, the stainless steel surface coating technology has broad application prospects in industrial applications, but there are some limitations in the existing technology. Traditional DC electroplated coatings have problems such as coarse grains, low bonding strength, and poor corrosion resistance. Although laser cladding can improve hardness, it has high cost and high brittleness. In magnetic field-assisted electroplating, improper matching of magnetic field strength and pulse frequency easily leads to uneven coating thickness. In order to overcome these problems, a new method for strengthening the stainless steel surface composite coating is proposed.

[0028] This method is achieved through the following steps: sequentially performing chemical degreasing, ultrasonic passivation, and laser texturing pretreatment on the substrate to form a micro-pit array with a surface roughness Ra≤0.2μm; applying an oriented magnetic field with a magnetic field strength of 0.5-1.5T during pulse electroplating to control the grain orientation of the coating; combining ultrasonic rolling and gradient nitriding processes to form a hardened layer with a gradient distribution of surface nitrogen content; using genetic algorithms to optimize the dynamic matching relationship of magnetic field strength, pulse frequency, and nitriding temperature to achieve Pareto optimality of the coating performance.

[0029] The insufficient strength and corrosion resistance of the coating on the stainless-steel surface are problems that need to be solved urgently. Through chemical degreasing, ultrasonic passivation, and laser texturing pretreatment to form a micro-pit array and increase the surface roughness, it helps to improve the adhesion and bonding strength of the coating. During pulse electroplating, applying a directional magnetic field to regulate the grain orientation of the coating helps to form a fine and uniform grain structure, thereby improving the strength and corrosion resistance of the coating. Combining ultrasonic rolling and gradient nitriding processes to form a hardened layer with a gradient distribution of surface nitrogen content further enhances the surface hardness and wear resistance. Using the genetic algorithm to optimize the dynamic matching relationship of magnetic field strength, pulse frequency, and nitriding temperature, and achieving the Pareto optimum of the coating performance through multi-objective optimization to ensure that the coating reaches the best state in terms of hardness, wear resistance, and corrosion resistance.

[0030] Chemical degreasing is to treat the substrate in a solution containing 3-5% NaOH and 0.5% OP-10 emulsifier at 60±5°C for 15-20 minutes to remove the oil stain on the surface. Ultrasonic passivation uses a frequency of 40 kHz and a power density of 0.8 W / cm 2 to treat for 10-15 minutes to remove the oxide layer on the surface. Laser texturing uses a laser with a wavelength of 1064 nm, a pulse width of 10 ns, and a scanning speed of 50 mm / s to form a micro-pit array on the surface. The pulse electroplating solution contains Ni 2+ :W 6+ :P 5+ =80-85:10-12:3-5 (molar ratio), and PTFE particles with a particle size of 0.5-2 μm are added, and the deposition current density is 3-5 A / dm 2 . The pressure of ultrasonic rolling is 15-30 MPa, the amplitude is 5-10 μm, and the frequency is 20 kHz. The gradient nitriding step adopts a two-stage process: the first stage is constant-temperature nitriding at 350-450°C to form a diffusion layer of 3-5 μm; the second stage is gradient heating nitriding at 450-600°C to form a hardened layer with a surface nitrogen content > 8 at.%.

[0031] Compared with the prior art, the present invention significantly improves the comprehensive performance of the coating through multi-step pretreatment and optimization. The traditional direct-current electroplated coating has problems such as coarse grains, low bonding strength, and poor corrosion resistance, while the present invention solves these problems by regulating the grain orientation of the coating and forming a hardened layer with a gradient distribution of surface nitrogen content. Although laser cladding can improve hardness, it has high cost and high brittleness, while the present invention combines ultrasonic rolling and gradient nitriding processes, which not only improves hardness but also avoids the problem of high brittleness. In magnetic-field-assisted electroplating, improper matching of magnetic field strength and pulse frequency easily leads to uneven coating thickness, while the present invention optimizes the dynamic matching relationship of magnetic field strength, pulse frequency, and nitriding temperature through the genetic algorithm to ensure the consistency of coating thickness.

[0032] Through the above steps, the technical solution of this application can effectively solve the problems of insufficient strength and corrosion resistance of the stainless-steel surface coating, and significantly improve the comprehensive performance of the stainless-steel surface. The chemical degreasing, ultrasonic passivation, and laser texturing pretreatment steps increase the adhesion and bonding strength of the coating by removing surface impurities and forming a micro-pit array. During the pulse electroplating process, a directional magnetic field is applied to regulate the grain orientation of the coating, forming a fine and uniform grain structure, which improves the strength and corrosion resistance of the coating. Combining ultrasonic rolling and gradient nitriding processes forms a hardened layer with a gradient distribution of surface nitrogen content, further enhancing the surface hardness and wear resistance. Using the genetic algorithm to optimize the dynamic matching relationship of the magnetic field strength, pulse frequency, and nitriding temperature, and achieving the Pareto optimality of the coating performance through multi-objective optimization, ensuring that the coating reaches the best state in terms of hardness, wear resistance, and corrosion resistance.

[0033] Further, this application also proposes that the chemical degreasing solution in step a) contains 3-5% NaOH + 0.5% OP-10 emulsifier, the treatment temperature is 60 ± 5°C, and the time is 15-20 minutes.

[0034] The composition ratio and treatment conditions of the chemical degreasing solution play a key role in solving the optimization problems of the effective composition ratio and treatment conditions during the chemical degreasing process. By setting the concentration of NaOH at 3-5%, the concentration of OP-10 emulsifier at 0.5%, the treatment temperature at 60 ± 5°C, and the time at 15-20 minutes, the effect of chemical degreasing can be ensured, removing the oil and impurities on the substrate surface and providing a clean and uniform surface for subsequent pretreatment and coating strengthening processes.

[0035] NaOH is a strong base that can effectively saponify grease, while the OP-10 emulsifier helps to emulsify and disperse the oil in the degreasing solution, thereby improving the degreasing effect. The setting of the treatment temperature and time is to avoid excessive corrosion of the substrate while ensuring the degreasing effect. Controlling the treatment temperature at 60 ± 5°C can accelerate the chemical reaction rate, and the treatment time of 15-20 minutes ensures the full progress of the degreasing process.

[0036] Through the optimization of the composition ratio and treatment conditions of this chemical degreasing solution, the degreasing efficiency and effect can be significantly improved, ensuring that there are no residual oil and impurities on the substrate surface, thereby providing a good foundation for subsequent ultrasonic passivation and laser texturing pretreatment, and further improving the overall effect of the entire stainless-steel surface composite coating strengthening method. Compared with traditional degreasing methods, this optimized chemical degreasing solution composition ratio and treatment conditions can achieve high-efficiency and thorough degreasing effects in a shorter time, while avoiding excessive corrosion of the substrate, showing obvious superiority.

[0037] Further, this application also proposes that in step a), the ultrasonic passivation uses a frequency of 40 kHz and 0.8 W / cm2 Power density, treatment time is 10 - 15 minutes.

[0038] This technical feature ensures the effective removal of the oxide layer and other impurities on the stainless - steel surface during the ultrasonic passivation process by selecting specific frequencies and power densities, thereby improving the effect of subsequent process steps. The selection of the frequency and power density of ultrasonic passivation directly affects the passivation effect and treatment time. By optimizing these parameters, efficient and stable surface pretreatment can be achieved, ensuring the quality and performance of the final coating.

[0039] Based on the understanding of ultrasonic passivation in step a), the ultrasonic passivation uses a frequency of 40 kHz and a power density of 0.8 W / cm 2 The power density, and the treatment time of 10 - 15 minutes is to find the best balance between the treatment time and the passivation effect. Specifically, a frequency of 40 kHz can provide sufficient energy to break the surface oxide layer, while a power density of 0.8 W / cm 2 ensures the uniform distribution of energy, avoiding over - treatment or surface damage. The treatment time of 10 - 15 minutes is the optimal time range determined through experiments to ensure the passivation effect while avoiding resource waste caused by over - long treatment.

[0040] Furthermore, as a preferred implementation, ultrasonic passivation can be carried out in a constant - temperature water bath to maintain a stable treatment temperature, thereby further improving the consistency and controllability of the passivation effect. Thus, a uniform microstructure can be formed on the stainless - steel surface, providing a good foundation for subsequent electroplating and other treatment steps.

[0041] Compared with the prior art, the ultrasonic passivation solution of this application has the following advantages: First, by optimizing the frequency and power density, surface impurities can be efficiently removed, improving the passivation effect; second, the treatment time is reasonable, ensuring the efficiency and stability of the passivation process; finally, further combined with auxiliary measures such as a constant - temperature water bath, the consistency and controllability of the treatment effect can be further improved. Thus, the technical solution of this application improves the surface pretreatment effect of stainless steel while ensuring the quality and performance of subsequent process steps.

[0042] Furthermore, this application also proposes that the parameters of laser texturing are: laser wavelength 1064 nm, pulse width 10 ns, and scanning speed 50 mm / s.

[0043] The parameter selection for laser texturing includes laser wavelength, pulse width, and scanning speed. A laser wavelength of 1064 nm can ensure good absorption of the laser on the stainless-steel surface, thus effectively performing surface treatment. A pulse width of 10 ns can control the energy density of the laser and avoid overheating and damaging the material. A scanning speed of 50 mm / s can ensure the uniform movement of the laser on the surface, forming a consistent micro-pit array. These parameters work together to ensure the formation of a micro-pit array with a surface roughness Ra ≤ 0.2 μm, thereby optimizing the strengthening effect of the composite coating on the stainless-steel surface.

[0044] Furthermore, a laser wavelength of 1064 nm is achieved by selecting a suitable laser source, such as using a Nd:YAG laser. A pulse width of 10 ns can be achieved by adjusting the pulse parameters of the laser. Specifically, the pulse width can be precisely adjusted through the control system of the laser. A scanning speed of 50 mm / s can be set through the control software of the laser scanning system to ensure the uniform movement of the laser beam on the stainless-steel surface. In addition, to ensure the stability and consistency of the laser texturing process, a high-precision motion control system and a real-time monitoring system can be adopted to adjust and optimize the process parameters at any time.

[0045] By adopting the above parameters of laser texturing, this application can form a micro-pit array with a surface roughness Ra ≤ 0.2 μm on the stainless-steel surface. Compared with the prior art, it significantly improves the bonding strength and wear resistance of the composite coating, and solves the problems such as coarse grains, low bonding strength, and poor corrosion resistance existing in traditional electroplated coatings. Thus, this application has made significant technological progress in optimizing the stainless-steel surface treatment process.

[0046] Furthermore, this application also proposes to apply a directional magnetic field with a magnetic field intensity of 0.5 - 1.5 T during the pulse electroplating process to regulate the grain orientation of the coating. The electroplating solution contains Ni 2+ :W 6+ :P 5+ = 80 - 85:10 - 12:3 - 5 (molar ratio), and PTFE particles with a particle size of 0.5 - 2 μm are added. The deposition current density is 3 - 5 A / dm 2 .

[0047] Applying a directional magnetic field with a magnetic field intensity of 0.5 - 1.5 T during the pulse electroplating process optimizes the composition and structure of the coating by regulating the composition of the electroplating solution and the deposition current density. Specifically, the electroplating solution contains specific proportions of Ni 2+ , W 6+ , P 5+ , and PTFE particles are added to improve the wear resistance and corrosion resistance of the coating. By controlling the deposition current density, the thickness and uniformity of the coating can be further optimized.

[0048] The regulation of the composition of the electroplating solution and the deposition current density is a key step in achieving optimized coatings. In the electroplating solution, the molar ratio of Ni 2+ , W 6+ , P 5+ is 80 - 85:10 - 12:3 - 5. Through this specific ratio, the composition of the coating can be effectively controlled, endowing it with excellent physical and chemical properties. The addition of PTFE particles with a particle size between 0.5 - 2 μm can significantly improve the wear resistance and corrosion resistance of the coating. In addition, when the deposition current density is controlled at 3 - 5 A / dm 2 , the thickness of the coating can be ensured to be uniform, avoiding coating defects caused by too high or too low current density.

[0049] Through the above technical means, this application provides an effective solution to the problem of optimizing the composition and structure of the coating during the electroplating process. Compared with traditional DC electroplated coatings, the technical solution of this application significantly improves the wear resistance and corrosion resistance of the coating by applying a directional magnetic field and optimizing the composition of the electroplating solution. At the same time, by controlling the deposition current density, the thickness and uniformity of the coating are further optimized, overcoming the problem of uneven coating thickness in traditional processes. Thus, the technical solution of this application has high operability and practical value while improving the coating performance.

[0050] Furthermore, this application also proposes that in step c), the pressure of ultrasonic rolling is 15 - 30 MPa, the amplitude is 5 - 10 μm, and the frequency is 20 kHz.

[0051] The technical features included in this application are the pressure, amplitude, and frequency parameters of ultrasonic rolling. The pressure of ultrasonic rolling is 15 - 30 MPa, the amplitude is 5 - 10 μm, and the frequency is 20 kHz. These parameters play a key role in the uniformity and performance of the coating. By adjusting the pressure, amplitude, and frequency of ultrasonic rolling, the uniformity of the coating can be effectively controlled, thereby enhancing the overall performance of the coating. Through the above technical means, the problems of the influence of pressure and amplitude on the uniformity and performance of the coating during ultrasonic rolling are solved. Specifically, appropriate pressure and amplitude can ensure the uniform distribution of the coating during ultrasonic rolling, and a suitable frequency can further improve the densification and hardness of the coating. The optimized combination of these parameters enables the coating to achieve the best results in terms of uniformity and performance.

[0052] The pressure of ultrasonic rolling being 15 - 30 MPa means that when operating within this range, sufficient mechanical force can be provided to promote the uniform distribution of the coating, while avoiding coating damage caused by too high pressure. The amplitude of 5 - 10 μm can ensure an appropriate vibration amplitude generated by the ultrasonic wave during rolling, thus contributing to the densification of the coating. The frequency of 20 kHz can enable the ultrasonic wave to generate high-frequency vibration during rolling, improving the densification and hardness of the coating.

[0053] By adopting the above technical means, the present application effectively solves the problem of the influence of pressure and amplitude on the coating uniformity and performance during the ultrasonic rolling process. Compared with the prior art, by optimizing the parameter combination of ultrasonic rolling, the present application enables the coating to achieve the best effect in terms of uniformity and performance, significantly improving the overall performance of the coating.

[0054] Furthermore, the present application also proposes that the gradient nitriding step adopts a two-stage process: the first stage is isothermal nitriding at 350 - 450 °C to form a diffusion layer with a thickness of 3 - 5 μm; the second stage is gradient heating nitriding at 450 - 600 °C to form a hardened layer with a surface nitrogen content > 8 at.%.

[0055] Through the above technical means, the isothermal nitriding in the first stage can form a stable diffusion layer on the stainless steel surface, providing a uniform basis for the subsequent gradient heating nitriding. The gradient heating nitriding in the second stage further increases the surface nitrogen content to form a hardened layer, thereby improving the hardness and wear resistance of the stainless steel surface.

[0056] Specifically, the isothermal nitriding process in the first stage can ensure the uniform diffusion of nitrogen atoms on the stainless steel surface to form a diffusion layer with a thickness of 3 - 5 μm. The existence of this diffusion layer helps nitrogen atoms to further penetrate and concentrate on the surface during the gradient heating nitriding process in the second stage, forming a hardened layer with a nitrogen content greater than 8 at.%. This hardened layer significantly improves the hardness and wear resistance of the stainless steel surface.

[0057] As a preferred implementation manner, the isothermal nitriding in the first stage can adopt gas nitriding or plasma nitriding processes, and specific process parameters such as nitrogen flow rate and pressure can be adjusted according to actual requirements. The gradient heating nitriding in the second stage can be carried out by gradually increasing the temperature, and the rate of temperature increase and time can be optimized according to the characteristics of the stainless steel material. In addition, other surface treatment technologies, such as ultrasonic-assisted nitriding, can be combined to further improve the nitriding effect.

[0058] It can be seen that the present application solves the problem of forming a hardened layer with a gradient distribution of surface nitrogen content during the process of composite coating on the stainless steel surface through a two-stage gradient nitriding process. Compared with the prior art, the method of the present application not only improves the hardness and wear resistance of the stainless steel surface, but also ensures the uniformity and stability of the nitrided layer, having significant technical advantages.

[0059] Furthermore, the present application also proposes that the objective function of the genetic algorithm includes a weighted combination of hardness, wear resistance, and corrosion resistance, and the weight coefficients are 0.4, 0.3, and 0.3 respectively.

[0060] The objective function of the genetic algorithm includes a weighted combination of hardness, wear resistance, and corrosion resistance, with weight coefficients of 0.4, 0.3, and 0.3 respectively. By optimizing these performance indicators through the genetic algorithm, the comprehensive performance of the coating is improved. The genetic algorithm gradually optimizes the objective function through operations such as selection, crossover, and mutation, thereby finding the best balance point among hardness, wear resistance, and corrosion resistance. This method effectively solves the problem of how to optimize the hardness, wear resistance, and corrosion resistance of the stainless steel surface composite coating.

[0061] Specifically, the objective function of the genetic algorithm can be implemented in various ways. For example, a multi-objective optimization model can be constructed through the NSGA-II algorithm, and the objective function is the comprehensive performance index P = 0.4×HV + 0.3×(1 / wear_rate) + 0.3×t salt . The constraint conditions of this model include the process parameter range and the requirement of coating thickness consistency. The population evolution is realized through real-number encoding crossover and polynomial mutation operations. Thus, the dynamic matching relationship between the magnetic field strength, pulse frequency, and nitriding temperature can be effectively optimized, and the Pareto optimality of the coating performance can be achieved. Under the efficiency of this algorithm, compared with the gradient descent method, NSGA-II converges to the Pareto front within 50 iterations, and the calculation time is reduced by 60%. At the same time, this algorithm has multi-objective trade-offs: the solution set shows the existence of a trade-off between hardness and wear resistance (such as solution 1: HV = 1120 but wear rate = 12; solution 2: HV = 1050 but wear rate = 8), providing optional solutions for practical applications. Engineering applicability: The parameter solution space is compact (the optimal solutions are concentrated in [0.8 - 1.4T, 20 - 50Hz, 550 - 700°C]), avoiding extreme process conditions.

[0062] Utilize the genetic algorithm to optimize the dynamic matching relationship between the magnetic field strength, pulse frequency, and nitriding temperature. Construct a multi-objective optimization model through the NSGA-II algorithm, and the objective function is the comprehensive performance index P = 0.4×HV + 0.3×(1 / wear_rate) + 0.3×t salt . The population evolution is realized through real-number encoding crossover and polynomial mutation operations to ensure the process parameter range and the consistency requirement of the coating thickness. The function of this technical feature is to optimize the electroplating process parameters through the genetic algorithm, so that the coating performance reaches Pareto optimality, thereby improving the hardness, wear resistance, and corrosion resistance of the coating.

[0063] This application optimizes the weighted combination of hardness, wear resistance, and corrosion resistance through a genetic algorithm, effectively solving the problems of traditional DC electroplated layers such as large grain size, low bonding strength, and poor corrosion resistance, and avoiding the limitations of high cost and high brittleness of laser cladding. In addition, through magnetic field-assisted electroplating, the problem of uneven coating thickness caused by improper matching of magnetic field strength and pulse frequency is solved. Compared with the prior art, this application significantly improves the comprehensive performance of the stainless steel surface composite coating.

[0064] This model can be trained with experimental data to determine the optimal weight coefficients and the form of the objective function. The constraint conditions include the process parameter range and the requirement for coating thickness consistency. The process parameter range can be set according to actual production conditions, and the requirement for coating thickness consistency can be calibrated based on experimental measurement results. Real-coded crossover and polynomial mutation operations are used to achieve population evolution. The real-coded crossover operation can generate new individuals by cross-recombining the genes of parent individuals; the polynomial mutation operation can increase the diversity of the population by randomly mutating the genes of individuals.

[0065] Thus, by using a genetic algorithm to optimize the dynamic matching relationship among magnetic field strength, pulse frequency, and nitriding temperature, the problem of dynamic matching among magnetic field strength, pulse frequency, and nitriding temperature can be effectively solved. This technical solution ensures the Pareto optimality of the coating performance by constructing a multi-objective optimization model, thereby improving the hardness, wear resistance, and corrosion resistance of the coating. Compared with the prior art, the technical solution of this application has significant advantages in optimizing electroplating process parameters and can achieve higher process stability and coating quality.

[0066] Specifically, the objective function of the genetic algorithm includes a weighted combination of hardness, wear resistance, and corrosion resistance, and the weight coefficients are 0.4, 0.3, and 0.3 respectively. The parameter adjustment during the optimization process can be varied according to actual needs. For example, in different electroplating environments, the range of magnetic field strength can be adjusted based on experimental data to ensure that the thickness and performance of the coating reach the best state.

[0067] Thus, by using a genetic algorithm to optimize the dynamic matching relationship among magnetic field strength, pulse frequency, and nitriding temperature, the comprehensive performance of the coating can be significantly improved, achieving an optimal balance in terms of hardness, wear resistance, and corrosion resistance. This method not only solves the problems of large grain size, low bonding strength, and poor corrosion resistance in traditional electroplating processes but also overcomes the limitations of high cost and high brittleness of composite processes, providing an efficient and economical coating strengthening method.

[0068] Example 2

[0069] Step a: Multi-stage pretreatment

[0070] Chemical degreasing: NaOH solution (50 g / L) + Na₂SiO₃ (5 g / L), treated at 85 °C for 20 min, the surface oil removal rate > 99.5%.

[0071] Ultrasonic passivation: HF acid (5%) + H₂O₂ (30%), ultrasonic power 200 W, treated for 15 min, forming a 1 - 5 nm passivation film.

[0072] Laser texturing: Using picosecond laser (1064 nm, pulse width 10 ns), scanning speed 50 mm / s, forming a micro - pit array with Ra ≤ 0.2 μm (micro - pit diameter 5 - 20 μm, depth 1 - 3 μm).

[0073] Step b: Magnetic - field - assisted pulse electroplating

[0074] Electrolyte: CrCl₃·6H₂O (200 g / L) + NiCl₂·6H₂O (100 g / L), pH 2.5, current density 5 A / dm² 2 。

[0075] Magnetic - field regulation: 0.5 - 1.5 T directional magnetic field, making the coating grains preferentially grow along the <110> crystal system, and the grain size is refined to 2 - 10 μm.

[0076] Step c: Gradient nitriding hardening

[0077] Ultrasonic rolling: Pressure 15 - 30 MPa, amplitude 5 - 10 μm, frequency 20 kHz, treated for 5 min, the density is increased to 98%.

[0078] Nitriding process: 550 °C × 8 h (N concentration gradient 0 - 0.3 wt%) + 850 °C × 2 h (N concentration gradient 0.3 - 0.8 wt%), the surface hardness reaches 1200 HV.

[0079] Step d: Establish a multi - objective optimization model based on genetic algorithm:

[0080] Optimization variables: Magnetic - field intensity (0.5 - 1.5 T), pulse frequency (10 - 100 Hz), nitriding temperature (500 - 800 °C).

[0081] Objective function: Maximize the comprehensive performance index P = 0.4×HV + 0.3×(1 / wear_rate) + 0.3×t salt 。

[0082] Technical effects

[0083]

[0084]

[0085] It should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. However, any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for strengthening a composite coating on a stainless steel surface, characterized in that: The following steps are involved: a) chemical degreasing, ultrasonic passivation and laser texturing pretreatment are sequentially performed on the substrate to form a micro-pit array with a surface roughness Ra≤0.2μm; b) applying a directional magnetic field with a magnetic field strength of 0.5-1.5T during the pulse electroplating process to regulate the grain orientation of the coating; c) Combining ultrasonic rolling and gradient nitriding process to form a hardened layer with gradient distribution of nitrogen content on the surface; d) Genetic algorithm is used to optimize the dynamic matching relationship among magnetic field intensity, pulse frequency and nitriding temperature to achieve Pareto optimal coating performance.

2. The method for strengthening the stainless steel surface composite coating according to claim 1, characterized in that: In step a) The chemical degreasing liquid contains 3-5% NaOH + 0.5% OP-10 emulsifier, the treatment temperature is 60±5℃, and the treatment time is 15-20 minutes.

3. The method for strengthening the stainless steel surface composite coating according to claim 1, characterized in that: In step a) Ultrasonic passivation uses a frequency of 40kHz and 0.8W / cm 2 Power density, processing time 10-15 minutes.

4. The method for strengthening the stainless steel surface composite coating according to claim 1, characterized in that: In step a) The parameters of laser texturing are: laser wavelength 1064 nm, pulse width 10 ns, and scanning speed 50 mm / s.

5. The method for strengthening the stainless steel surface composite coating according to claim 1, characterized in that: In step b), the electroplating solution contains Ni 2+ :W 6+ :P 5+ =80-85:10-12:3-5 (molar ratio), and add PTFE particles with a particle size of 0.5-2μm, and the deposition current density is 3-5A / dm 2 .

6. The method for strengthening the stainless steel surface composite coating according to claim 1, characterized in that: In step c) The pressure of ultrasonic rolling is 15-30MPa, the amplitude is 5-10μm, and the frequency is 20kHz.

7. The method for strengthening the stainless steel surface composite coating according to claim 1, characterized in that: In step c) The gradient nitriding step adopts a two-stage process: The first stage: nitriding at a constant temperature of 350-450℃ to form a 3-5μm diffusion layer; The second stage: 450-600℃ gradient temperature nitriding to form a hardened layer with a surface nitrogen content of >8at.%.

8. The method for strengthening the stainless steel surface composite coating according to claim 1, characterized in that: The objective function of the genetic algorithm contains a weighted combination of hardness, wear resistance, and corrosion resistance, with weight coefficients of 0.4, 0.3, and 0.3, respectively.

9. The method for strengthening the stainless steel surface composite coating according to claim 1, characterized in that: In step d) The use of genetic algorithms to optimize the dynamic matching relationship between magnetic field intensity, pulse frequency and nitriding temperature includes: A multi-objective optimization model is constructed based on the NSGA-II algorithm, and the objective function is the comprehensive performance index P = 0.4 × HV + 0.3 × (1 / wear_rate) + 0.3 × t salt , The constraints include the range of process parameters and the consistency requirements of coating thickness, and population evolution is achieved through real-coded crossover and polynomial mutation operations.

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