Hard oxidation process for aluminum alloy

Abstract

The invention discloses an aluminum alloy hard oxidation process, and relates to the technical field of aluminum alloy part surface treatment, the aluminum alloy hard oxidation process comprises the following steps: S1, pretreatment: oil removal, alkali corrosion treatment and acid pickling neutralization; s2, hard oxidation is conducted, specifically, the pretreated workpiece is put into an oxidation tank with electrolyte and a stirring system to be subjected to stirring and oxidation treatment; and S3, post-treatment: cleaning with hot water, sealing with nickel acetate and drying in a drying oven. The process mainly comprises the following steps: carrying out pretreatment of oil removal by an organic solvent, alkali etching and acid pickling, combining sulfuric acid-citric acid-monopotassium phosphate composite electrolyte, pulsed power supply and gradient boosting oxidation, carrying out ultrasonic stirring and precise temperature control hard oxidation, and finally carrying out nickel acetate sealing aftertreatment. The four industrial problems of poor batch consistency of oxidation films of aluminum alloy workpieces, non-uniformity of complex workpieces, high energy consumption and poor compatibility to high alloy materials are thoroughly solved, and the oxidation film which is high in hardness, uniformity and corrosion resistance and saves energy is obtained.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy surface treatment technology, specifically to an aluminum alloy hard anodizing process. Background Technology

[0002] Hard anodizing processes typically offer advantages such as improved wear resistance, corrosion resistance, and hardness, and can be applied in the following fields:

[0003] In the aerospace field, where there are stringent requirements for lightweight and high-performance materials, hard anodized materials can be used to manufacture aircraft landing gear, aircraft structural components, engine parts, and more. For example, after titanium alloys undergo hard anodizing treatment, the film hardness of the landing gear can reach approximately HV550, and fatigue life can be increased by 40%.

[0004] Automotive Industry: It can facilitate lightweight automotive design and enhance component performance. It can process engine blocks, pistons, braking system components, and body structural parts. After treatment, piston ring friction power consumption can be reduced, which helps improve fuel efficiency, and the corrosion resistance and wear resistance of braking system components can also be enhanced.

[0005] In the field of precision instruments and electronic equipment: hard anodized aluminum is commonly used in robotic arms for transporting semiconductor wafers. By processing it, its surface roughness is controlled to below Ra0.1μm, eliminating the effects of static electricity on the wafers. Additionally, hard anodized aluminum is used in 5G base station heat sinks to meet the dual requirements of heat dissipation and electromagnetic shielding.

[0006] In the military and marine engineering fields: components such as submarine hydraulic system valve bodies, seawater hydraulic transmission systems, offshore helicopter platforms, and oil exploration drill pipes that are susceptible to seawater corrosion or need to work in extreme environments can have their seawater corrosion resistance lifespan extended and ensure stable operation under low temperature and high pressure after treatment.

[0007] Mechanical Industry: Bearings, gears, guide rails and other frequently contacted and easily worn parts of mechanical equipment can have their surface hardness improved, wear reduced and equipment failure rate lowered after hard anodizing treatment.

[0008] In the field of metal surface treatment, hard anodizing is one of the core technologies for improving the surface hardness, wear resistance, corrosion resistance, and insulation of metals (especially aluminum alloys), and it has been widely used in aerospace, machinery manufacturing, and electronic equipment industries. Currently, the mainstream hard anodizing processes in the industry mainly revolve around "electrolyte preparation - process parameter control - pretreatment / posttreatment," and the specific technical solutions and their shortcomings are as follows:

[0009] Electrolyte system: Current processes generally use a single sulfuric acid electrolyte or a binary system composed of sulfuric acid and a small amount of inorganic additives (such as aluminum sulfate or oxalic acid), with a sulfuric acid concentration typically between 10% and 20%. This type of electrolyte has a simple formulation and low cost, but lacks effective pH buffering components. Its pH value is easily affected by factors such as workpiece corrosion and moisture evaporation, causing fluctuations (usually deviating from the optimal range of 1.5-3.0), which in turn affects the film formation rate and density of the oxide film. Some processes attempt to add corrosion inhibitors, but these are mostly single-component agents, making it difficult to simultaneously meet the dual requirements of "reducing workpiece corrosion" and "optimizing film structure." Defects such as pinholes and pitting are prone to appear on the oxide film surface.

[0010] Process parameter control: Temperature control primarily employs conventional refrigeration equipment to maintain the electrolyte temperature within the range of 20℃-26℃, but temperature fluctuations are significant (typically ±3℃ or more). High temperatures tend to accelerate the dissolution rate of the oxide film, resulting in a porous film layer; low temperatures may reduce ion migration efficiency, leading to slow film thickness growth and making it difficult to achieve stable control of film thickness and hardness. Power supply and current control: DC power is primarily used, with current densities typically fixed at 1-3 A / dm³. 2 DC power supply can easily cause uneven current distribution on the workpiece surface, and the lack of low current density guidance in the pre-oxidation stage can lead to problems such as localized excessively fast or slow growth of the initial oxide film. This results in film thickness deviations of more than ±5μm between different batches of products, and hardness fluctuations exceeding HV50.

[0011] Mixing and workpiece compatibility: Traditional processes often use single-circulation pumps or air mixing, resulting in fixed mixing rates and limited mass transfer. For workpieces with complex structures such as grooves, corners, and blind holes, the electrolyte cannot fully penetrate these areas, leading to insufficient current density and oxide film thickness that is 30%-50% thinner than in flat areas, or even areas without film, severely impacting the overall protective performance of the workpiece. For example, precision molds in machinery manufacturing and irregularly shaped parts in aerospace are prone to premature failure due to weak localized film layers when processed using existing processes.

[0012] Process flow and energy consumption: Pretreatment: Degreasing is mostly done with alkaline degreasing agents (such as a mixture of sodium carbonate and sodium silicate), which takes 15-20 minutes and easily leaves alkaline substances on the workpiece surface; after alkaline etching, cold water is mostly used for rinsing and neutralization, but incomplete neutralization can easily lead to a decrease in the adhesion of the subsequent oxide film.

[0013] Oxidation stage: Due to poor electrolyte stability and low stirring efficiency, the film thickness increase rate is slow, typically requiring 4-6 minutes to increase the film thickness by 1 μm. Processing a 20 μm film-thickness workpiece requires 80-120 minutes, resulting in high energy consumption (approximately 0.8-1.2 kWh / dm²). 2 ).

[0014] Post-treatment: Hot water sealing or potassium dichromate sealing is often used, with sealing time as long as 25-30 minutes. However, potassium dichromate sealing agent poses an environmental pollution risk and does not meet the current requirements for green production.

[0015] In summary, the current process has the following technical shortcomings:

[0016] 1. Unstable oxide film quality and batch consistency: Existing processes are sensitive to environmental and parameter fluctuations, easily affected by factors such as electrolyte composition, temperature, current, and voltage, leading to significant variations in key oxide film indicators such as thickness, hardness, and porosity. For example, a ±2% fluctuation in the sulfuric acid concentration of the electrolyte may cause an oxide film hardness deviation exceeding HV50; insufficient temperature control precision (e.g., ±5℃) may result in a film thickness uniformity error exceeding ±5μm for the same batch of workpieces. Consequences: In demanding fields such as aerospace and precision instruments, this instability can lead to decreased component assembly precision, shortened service life, and even safety hazards.

[0017] 2. Poor processing effect on complex-shaped workpieces and insufficient film uniformity: For workpieces with complex structures such as grooves, corners, and blind holes, the current process suffers from uneven electrolyte flow and current distribution, resulting in either excessively thin film layers with insufficient protection or excessively thick film layers that are prone to cracking. For example, in the inner corners of an automotive engine block, the oxide film thickness may be more than 30% thinner than on flat areas under traditional stirring methods, and defects such as pinholes and porosity are prone to occur. Consequences: The overall protective performance of the workpiece decreases, complex areas are more susceptible to preferential wear or corrosion, increasing scrap rate and maintenance costs.

[0018] 3. High energy consumption, long processing cycle, and poor economic efficiency: Traditional hard anodizing mostly uses DC power and requires maintaining high voltage (e.g., 30-40V) for extended periods. Simultaneously, the electrolyte cooling efficiency is low, resulting in significantly high energy consumption. Furthermore, the film growth rate is slow (e.g., 4-5 minutes per μm), leading to a long processing cycle. For example, processing 100 identical aluminum alloy workpieces using traditional processes consumes 20%-30% more energy than the optimized solution in this patent, and the processing time per batch is 2-3 hours longer. Consequences: Increased production costs for enterprises, inconsistent with energy conservation and environmental protection trends, and difficulty in adapting to large-scale mass production needs.

[0019] 4. Insufficient process compatibility and poor adaptability to high-alloy materials: Existing processes are prone to poor oxide film adhesion and insufficient hardness when processing aluminum alloys with high silicon and copper content (such as 2024 and 7075) due to interference from alloying elements. Additional adjustments to the electrolyte are required, but the results are unstable. For example, when processing 7075 aluminum alloy, traditional sulfuric acid electrolytes easily cause film "scorching," requiring a reduction in current density, which slows film thickness growth, and the final hardness only reaches around HV350 (lower than the application requirement of HV400). Consequences: This limits the application of hard anodizing processes on high-alloy workpieces and makes it difficult to meet the diverse material performance requirements of high-end fields.

[0020] Therefore, existing hard anodizing processes have significant limitations in terms of oxide film quality stability, adaptability to complex workpieces, energy consumption, and environmental friendliness, making it difficult to meet the demands of high-end manufacturing for high-precision, high-efficiency, and low-energy surface treatment technologies. Summary of the Invention

[0021] The purpose of this invention is to provide a hard anodizing process for aluminum alloys, aiming to solve the aforementioned technical problems. The process of this invention mainly involves a pretreatment of degreasing with organic solvents, alkaline etching, and pickling, combined with a sulfuric acid-citric acid-potassium dihydrogen phosphate composite electrolyte, pulse power supply and gradient voltage boosting oxidation, as well as a hard anodizing process with ultrasonic stirring and precise temperature control. Finally, a nickel acetate sealing posttreatment is performed. This completely solves the four major industry problems of poor batch consistency of oxide films on aluminum alloy workpieces, unevenness of complex workpieces, high energy consumption, and poor compatibility with high alloy materials, resulting in an oxide film with high hardness, high uniformity, high corrosion resistance, and energy saving.

[0022] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0023] A hard anodizing process for aluminum alloys includes the following steps:

[0024] S1. Pretreatment: Immerse the workpiece in an organic solvent to remove oil. After degreasing, immerse the workpiece in an alkaline solution for alkaline etching to remove the natural oxide film and minor scratches. Then immerse the workpiece in an acidic solution for pickling to neutralize the residual alkaline solution and clean the surface.

[0025] S2, Hard anodizing: The pretreated workpiece is placed into an anodizing tank with an electrolyte and a stirring system for stirring and anodizing.

[0026] S3. Post-treatment: After completing the above stirring and oxidation treatment, the workpiece is cleaned with hot water to remove residual electrolyte; then the workpiece is placed in nickel acetate sealant to fill the oxide film pores; after completion, the workpiece is placed in an oven to dry and remove surface moisture.

[0027] Preferably, in the pretreatment step S1, the organic solvent is acetone or ethanol, etc., and the soaking time in the organic solvent is 5-10 min; the alkaline solution is a 40-60 g / L sodium hydroxide solution, and the alkaline etching is carried out at 50-60℃ for 1-2 min; the acidic solution is a 20%-30% nitric acid solution, and the acid washing and neutralization is carried out for 2-3 min. Acetone and ethanol, as polar organic solvents, possess strong dissolving and penetrating abilities for non-polar and weakly polar oil stains such as mineral oil, grease, and cutting fluid. They effectively wet, penetrate, and break down the oil film on the workpiece surface, causing it to detach from the aluminum alloy matrix and dissolve or disperse in the solvent. Compared to traditional alkaline degreasing, organic solvent degreasing is faster and more efficient, thoroughly removing stubborn, high-viscosity oil stains in a short time, avoiding the problem of incomplete degreasing due to excessive oil contamination that can occur with alkaline degreasing. Furthermore, acetone and ethanol have excellent volatility, leaving almost no residue after subsequent water washing or evaporation, avoiding the introduction of new contaminants due to surfactant residue. This provides an absolutely clean starting point for subsequent alkaline etching. Effective batch-to-batch consistency is ensured; neither acetone nor ethanol reacts with the aluminum alloy substrate, and the process is carried out at room temperature without corroding or eroding the aluminum alloy substrate, thus avoiding damage to the dimensional accuracy of precision workpieces due to excessive pretreatment; soaking in organic solvent for 5-10 minutes effectively ensures that the solvent has sufficient time to penetrate and break down the firmly attached oil film. If less than 5 minutes, degreasing may be incomplete, especially for workpieces with complex structures and blind holes, where residual oil will directly hinder subsequent alkaline etching and pickling, becoming the root cause of defects such as oxide film blistering, peeling, and mottling. If more than 10 minutes, the degreasing effect will no longer be significantly improved, but it will lead to a decrease in production efficiency and an increase in solvent evaporation loss.During alkaline etching, a sodium hydroxide solution of 40-60 g / L is used as the alkaline solution. This concentration range provides a sufficiently fast etching rate to ensure production efficiency while maintaining a controllable and uniform reaction. If the concentration is below 40 g / L, the reaction rate will be too slow, resulting in low production efficiency. Furthermore, it may not be able to completely remove the copper-rich or silicon-rich layers formed by alloy element segregation, leading to uneven oxide film and poor adhesion. If the concentration is above 60 g / L, the reaction will be too vigorous, making it difficult to control the uniformity of etching. It may also easily lead to over-etching, causing workpiece dimensional deviations, surface roughness, and even irreversible phenomena such as pear-skin texture or grain boundary corrosion. This directly leads to undesirable phenomena such as roughness, looseness, and decreased hardness of the oxide film. Controlling the alkaline etching temperature at 50-60℃, combined with a 40-60g / L sodium hydroxide solution, ensures the reaction rate remains within a controllable range. This avoids the reduced solubility of the reaction product (sodium aluminate) below 50℃, which would cause aluminum hydroxide precipitate to re-adhere to the workpiece surface, forming a grayish-white residue that is difficult to remove even with subsequent acid washing. This severely damages the oxide film quality. If the temperature exceeds 60℃, the reaction rate increases dramatically, volatility intensifies, the working environment deteriorates, and over-corrosion and residue formation are also likely. The alkaline etching time is controlled at 1-2 minutes, combined with a 40-60g / L sodium hydroxide solution and a controlled temperature of 50-60℃, allowing the reaction to precisely remove 1-3μm of surface material. During the pickling and neutralization process, the nitric acid solution has a strong oxidizing effect on aluminum, dissolving the slightly soluble compounds or ash residues of alloying elements such as silicon, copper, and magnesium remaining on the aluminum surface after alkaline etching. The acidic solution uses a 20%-30% nitric acid solution. This concentration of nitric acid has the best "brightening" effect, effectively dissolving impurities without excessively corroding the aluminum substrate. If the concentration is below 20%, the brightening ability is insufficient, making it difficult to effectively remove the black ash residue remaining after alkaline etching. This residue will seriously affect the appearance, uniformity, and adhesion of the oxide film. If the concentration is above 30%, the volatilization of nitric acid will be accelerated, forming a large amount of nitrogen oxides. Yellow fumes from nitric acid pollution can pollute the environment and harm workers' health. Furthermore, excessively high concentrations can cause over-passivation of the aluminum surface, forming a dense passivation film that inhibits subsequent hard oxidation reactions, making oxide film formation difficult or extremely slow. A neutralization time of 2-3 minutes is sufficient to complete the neutralization reaction and remove the ash completely. If the neutralization time is less than 2 minutes, neutralization or brightening may be incomplete, potentially leaving residual alkalinity or impurities in certain areas. If the neutralization time exceeds 3 minutes, excessive soaking in high-nitric acid concentrations is not beneficial and can increase the chemical dissolution loss of aluminum and surface roughness.

[0028] Preferably, in step S2 hard oxidation, the electrolyte is a mixture of sulfuric acid, organic acid, and buffer, with the pH value controlled between 1.8 and 2.5. This is the optimal balance point between film formation and dissolution, which can stabilize the film quality of the oxide film. If the pH value is lower than 1.8, the H+ in the solution will increase. + Excessive concentration intensifies the chemical dissolution of the oxide film, resulting in a loose film with low hardness. If the pH value is higher than 2.5, the film formation rate will be faster, but the quality of the film will decrease, and a powdery film will be easily produced, resulting in poor adhesion of the film.

[0029] Preferably, in step S2 hard oxidation, the sulfuric acid concentration is 12%-18%, the organic acid is citric acid with a concentration of 3-8 g / L, and the buffer is potassium dihydrogen phosphate with a concentration of 5-10 g / L. The sulfuric acid serves as the main electrolyte, providing the H₂ required for the oxidation reaction. + With SO42- ions, the sulfuric acid concentration is controlled at 12%-18%. This ensures normal oxide film formation while reducing excessive corrosion of the workpiece. If the sulfuric acid concentration is below 12%, the solution conductivity will be insufficient, requiring a higher voltage to achieve the predetermined current density, increasing energy consumption, slowing film growth, and potentially resulting in a softer film. If the sulfuric acid concentration is above 18%, the solubility of the film is too strong, reducing film formation efficiency, making the film porous and porous with high surface roughness, and actually decreasing hardness and wear resistance. The citrate ions in the citric acid are excellent complexing agents, capable of reacting with Al in the electrolyte. 3+ Ions bind together to form stable complexes, effectively reducing free Al. 3+ The concentration of citric acid is important, as citric acid is a weak acid that can form a buffer system with sulfuric acid, helping to stabilize the pH value. The concentration of citric acid is 3-8 g / L, which can improve the microstructure of the oxide film, making it more dense and uniform, and also has a corrosion-inhibiting effect, reducing defects such as pinholes and pitting. If the concentration of citric acid is below 3 g / L, the complexing effect is insufficient, and it cannot effectively inhibit Al. 3+ The stability improvement effect is not significant due to interference from foreign metal ions. If the citric acid concentration is higher than 8 g / L, it will cause changes in the electrolyte conductivity, which may inhibit the film formation reaction, slow down film growth, increase costs, and the excessive organic matter content may accelerate electrolyte aging. The potassium dihydrogen phosphate can greatly enhance the electrolyte's buffering capacity against pH, resisting the effects of added acids, alkalis, or H2 produced by the reaction. +Both exhibit strong neutralizing capabilities, maintaining pH values ​​within a very narrow range. Furthermore, phosphate ions can be partially incorporated into the oxide film structure, altering the film's crystal structure and contributing to improved hardness and wear resistance. The concentration of potassium dihydrogen phosphate is controlled at 5-10 g / L to stabilize the electrolyte pH between 1.8 and 2.5, preventing pH fluctuations from affecting the oxide film quality. If the potassium dihydrogen phosphate concentration is below 5 g / L, the buffer capacity in the solution will be insufficient to resist the natural pH decrease during production; if the concentration is above 10 g / L, excessive potassium (K) will be introduced. + This may have an adverse effect on the performance of the film.

[0030] Preferably, in step S2, hard anodizing, a refrigeration cycle system is used to control the electrolyte temperature at 18±0.5℃. In this invention, the refrigeration cycle system precisely controls the electrolyte temperature at 18℃. The lower temperature environment is conducive to the formation of an oxide film with high hardness and good density, slows down the film dissolution rate, and makes the film thickness growth stable. For example, at 18℃, the oxide film hardness of 2024 aluminum alloy can reach HV450 or higher, and the film thickness uniformity error is controlled within ±2μm.

[0031] Preferably, a pulse power supply is used, with a pulse frequency of 500-1000Hz and a duty cycle of 30%-50%. In this invention, the pulse power supply is intermittent. During the pulse conduction period, a high current forms an oxide film; during the pulse turn-off period, the heat and bubbles accumulated in the reaction interface region are diffused and discharged. The pulse frequency is controlled between 500-1000Hz to avoid the phenomenon that when the pulse period is below 500Hz, the heat generation during conduction is still too concentrated, and the reaction interface is excessively cooled during the turn-off period, which is not conducive to stable film formation. It also avoids the phenomenon that when the pulse frequency is above 1000Hz, it approaches the DC effect and loses the diffusion and cooling advantages of the pulse power supply. The duty cycle is controlled between 30%-50% to avoid the phenomenon that the effective current time is too short and the film formation efficiency is too low when it is below 30%. It also avoids the phenomenon that when it is above 50%, it approaches the DC effect, the conduction period is too long, and the heat and bubble accumulation is severe.

[0032] Preferably, in step S2 hard oxidation, the oxidation process includes a pre-oxidation process and a gradient pressurization process, wherein the pre-oxidation process is to first apply pressure at 0.5-1 A / dm 2 Pre-oxidation is performed at a current density of 5-10 minutes to form a uniform initial film; the gradient voltage increase process involves gradually increasing the current density to 2-3 A / dm² after pre-oxidation. 2 The voltage matching material is adjusted to 18-25V. In this invention, the pre-oxidation process uses 0.5-1A / dm³. 2The current density can form an effective and continuous initial film layer, and can uniformly form a high-quality, high-resistivity initial oxide film on the surface of the aluminum substrate workpiece, providing a uniform basis for the subsequent "gradient boost" so that the current can be distributed more evenly under high current density.

[0033] Preferably, in step S2, hard anodizing, synchronous stirring is also required using a stirring system. This stirring system combines ultrasonic stirring and a circulating pump. The ultrasonic stirring frequency is 20-40 kHz, and the circulating pump flow rate is 5-10 L / min. In this invention, to address the problem of uneven electrolyte distribution on complex-shaped workpieces, a unique combination of ultrasonic stirring and circulating pump stirring is employed. The ultrasonic generator produces 20-40 kHz ultrasonic waves, which are transmitted through a transducer, causing cavitation and microfluidic effects in the electrolyte, improving its fluidity and mass transfer on the surface of complex-shaped workpieces. The circulating pump circulates the electrolyte at a flow rate of 5-10 L / min, further enhancing uniform distribution. Through this stirring method, even workpieces with complex internal structures can obtain a uniform oxide film, with film thickness deviation controlled within ±3 μm.

[0034] Preferably, in the post-treatment step S3, the hot water temperature is controlled at 60-70℃, and the washing time is 3-4 minutes; the concentration of nickel acetate is 6-10 g / L, and the oven is sealed at 85-95℃ for 25-30 minutes; the oven temperature is controlled at 60-70℃, and the oven is dried for 10-15 minutes. In this invention, hot water effectively dissolves and washes away trace amounts of residual sulfuric acid electrolyte on the surface and in the pores of the workpiece, preventing residual acidic substances from corroding the substrate or contaminating the subsequent sealing bath solution, thus ensuring stable sealing quality. Furthermore, the high-temperature water causes alumina to undergo a certain degree of hydration, forming hydrated alumina. This hydrate has a larger volume than alumina, allowing it to slightly expand and partially seal the micropores of the film. This hydration reaction provides initial sealing of the pores, laying a more uniform foundation for subsequent high-quality nickel salt sealing, reducing the consumption of the final sealing agent. Hot water rinsing also keeps the pores open and activated, which is more conducive to the adsorption and reaction of the subsequent sealing agent. The sealing agent used is nickel acetate. At high temperatures, nickel ions undergo hydrolysis, and the resulting nickel hydroxide is deposited in the micropores of the oxide film. The nickel hydroxide further reacts with the alumina to generate spinel-structured compounds such as NiAl2O4 or other complexes. These compounds are structurally stable and can work together with nickel hydroxide to effectively and permanently fill and seal the pores of the oxide film.

[0035] The aluminum alloy hard anodizing process of the present invention has the following beneficial effects:

[0036] 1. Stable oxide film quality and high batch consistency: The aluminum alloy hard anodizing process of this invention precisely controls the electrolyte composition, with sulfuric acid concentration fluctuations controlled within ±0.5%, and potassium dihydrogen phosphate stabilizing the pH value within a very small range, avoiding film quality fluctuations due to composition changes; an advanced refrigeration cycle system is used to control the electrolyte temperature with an accuracy of ±0.5℃, solving the influence of temperature on film thickness and hardness; a pulse power supply is used to precisely regulate the current and voltage, ensuring that the oxide film thickness deviation of the same batch of workpieces is controlled within ±1μm and the hardness deviation is controlled within HV20, meeting the needs of aerospace, precision instruments and other fields with extremely high requirements for oxide film performance consistency;

[0037] 2. Excellent processing effect on complex-shaped workpieces with good film uniformity: The aluminum alloy hard anodizing process of this invention uses ultrasonic waves and a circulating pump for coordinated stirring. The ultrasonic cavitation effect allows the electrolyte to penetrate into the fine parts of complex structures, and the circulating pump ensures uniform circulation of the electrolyte. When processing complex workpieces such as automobile engine blocks, the film thickness deviation between special parts such as grooves, corners, and blind holes and flat parts can be controlled within ±1μm. The film layer is free of defects such as pinholes and porosity, which significantly improves the overall protective performance and service life of the workpiece and reduces the scrap rate.

[0038] 3. Low energy consumption, short processing cycle, and good economic efficiency: The pulse power supply method saves 20%-30% more energy than the traditional DC power supply. Furthermore, by optimizing process parameters, the film growth rate can be increased by 30%-50%. For example, the growth time for each 1μm film thickness is shortened to less than 2 minutes. When processing the same number and specifications of aluminum alloy workpieces, the processing time is shortened by 1-2 hours, reducing production costs, which is in line with the trend of energy conservation and environmental protection, and improves the production efficiency and market competitiveness of enterprises.

[0039] 4. Strong process compatibility and excellent adaptability to high alloy materials: The unique electrolyte formula and process parameters effectively overcome the oxidation problems of high silicon and high copper aluminum alloys (such as 2024 and 7075). When treating 7075 aluminum alloy, the hardness of the oxide film can reach HV450 or above, and the adhesion reaches the 0-level standard. It solves the problems of easy "burning", poor adhesion and insufficient hardness of the film layer in traditional processes, and broadens the application range of hard anodizing process on high alloy workpieces, meeting the diverse needs of high-end fields for material performance.

[0040] 5. In the aluminum alloy hard anodizing process of this invention, compared with traditional alkaline degreasing, organic solvent degreasing is faster and more efficient in the pretreatment degreasing process. It can thoroughly remove stubborn, high-viscosity oil stains in a short time, avoiding the problem of incomplete degreasing due to excessive oil stains in alkaline degreasing. Furthermore, acetone and ethanol have excellent volatility, leaving almost no residue after subsequent water washing or evaporation, avoiding the introduction of new pollution sources due to surfactant residues. This provides an absolutely clean starting point for subsequent alkaline etching, achieving the effect of removing no residue from the workpiece surface. The acetone and ethanol do not corrode or erode the aluminum alloy substrate at room temperature, avoiding damage to the dimensional accuracy of precision workpieces due to excessive pretreatment, and thus protecting the substrate.

[0041] 6. The aluminum alloy hard anodizing process of this invention achieves controllable and uniform corrosion depth through precise control of concentration, temperature, and time during the pretreatment alkaline etching process. This eliminates surface inhomogeneity caused by differences in raw material conditions or previous processing, ensuring that all workpieces are at a completely consistent starting point before oxidation, greatly improving the consistency of film quality between batches. The newly exposed aluminum surface possesses extremely high chemical activity, ensuring that the subsequent hard anodizing film formation reaction can be initiated rapidly and uniformly. Especially for difficult-to-process areas such as grooves and blind holes in complex workpieces, the activated surface can effectively compensate for slight unevenness in current distribution, thereby significantly improving the uniformity of film thickness. Furthermore, it effectively eliminates potentially hidden trace impurities and microscopic defects on the surface, reducing the probability of pinholes, porosity, and other defects in the oxide film from the source.

[0042] 7. In the aluminum alloy hard anodizing process of the present invention, during the acid neutralization process of the pretreatment, aluminum is mainly oxidized to aluminum oxide by nitric acid solution. The aluminum oxide is then dissolved by excess nitric acid, thus forming a dynamic oxidation-to-dissolution process. On the one hand, it can thoroughly remove alkaline residues and avoid problems such as electrolyte contamination and local pH fluctuations caused by alkaline substances being introduced into the oxidation tank. On the other hand, through the dynamic oxidation-to-dissolution process, the microscopic surface is further smoothed to obtain a real surface with uniform reactivity. It can preferentially dissolve the black ash formed after alkaline etching, exposing the pure aluminum substrate, significantly improving the adhesion between the subsequent oxide film and the substrate, preventing the film layer from "burning" or becoming loose, and effectively solving the problem of "insufficient process compatibility" in the background technology. Furthermore, the highly clean surface after nitric acid pickling allows for the formation of a dense, defect-free anodic oxide film in the subsequent oxidation process, thereby improving the hardness and corrosion resistance of the oxide film on the aluminum substrate surface.

[0043] 8. The aluminum alloy hard anodizing process of this invention adopts a dual-effect system of "citric acid complexation + phosphate buffering" to keep the film-forming environment highly stable throughout the entire production cycle, fundamentally ensuring the consistency of oxide film performance (thickness, hardness) of different batches of workpieces; citric acid can effectively complex dissimilar metal ions, preventing them from being locally enriched at the oxide film interface, thereby completely avoiding the occurrence of "burning" phenomenon and improving the hardness of the film layer;

[0044] 9. The aluminum alloy hard anodizing process of this invention utilizes the "relaxation effect" of the pulse power supply and the uniform initial film formed by pre-oxidation to ensure uniform current distribution throughout complex structures, completely solving the problem of excessively thin film layers in grooves and corners, reducing thickness uniformity error from over 30% in the traditional method to within ±10%; the average current density of the pulse power supply is lower than that of the DC power supply, significantly reducing heat generation; and simultaneously, using 2-3 A / dm 2 The current density is low without burning the workpiece, so the film growth rate can reach 2-3 min / μm, shortening the processing cycle by 20%-30% and reducing the overall energy consumption; during the pulse interval, the film has time to cool and reorganize, resulting in fewer structural defects and lower porosity, thus achieving higher hardness and corrosion resistance.

[0045] 10. The aluminum alloy hard anodizing process of this invention adopts a combination of ultrasonic stirring and circulating pump stirring. The circulating pump provides macroscopic and forced electrolyte flow, which removes heat and bubbles from the workpiece surface and renews the interface solution. The ultrasonic waves utilize the cavitation effect to generate microscopic and extremely high-intensity disturbances, which can penetrate the complex structure of the workpiece (such as blind holes and deep holes) and completely eliminate the stagnant layer there, ensuring the microscopic uniformity of heat and concentration. This invention completely solves the mass transfer and heat transfer problems at both macroscopic and microscopic scales, and is a key technical guarantee for obtaining an ultra-uniform film layer. It is especially suitable for complex structural parts with deep holes and micropores in aerospace, precision hydraulics and other fields. Attached Figure Description

[0046] Figure 1 A schematic diagram of the surface of a workpiece that has not undergone hard anodizing;

[0047] Figure 2 A schematic diagram of the workpiece surface after applying the aluminum alloy hard anodizing process of the present invention. Figure 1 ;

[0048] Figure 3 A schematic diagram of the workpiece surface after applying the aluminum alloy hard anodizing process of the present invention. Figure 2 ;

[0049] Figure 4 This is a schematic diagram of the aluminum alloy hard anodizing process of the present invention. Detailed Implementation

[0050] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments and accompanying drawings.

[0051] It should be noted that 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 specification of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0052] Example 1: A 6061 aluminum alloy flat workpiece with dimensions of 100mm × 100mm × 5mm.

[0053] like Figure 4 As shown, an aluminum alloy hard anodizing process, using 6061 aluminum alloy flat workpiece, includes the following steps:

[0054] S1. Pretreatment: Immerse the workpiece in acetone solution for 10 minutes to remove oil. After degreasing, immerse the workpiece in 50g / L sodium hydroxide solution and etch it at 55℃ for 1.5 minutes to remove the natural oxide film and minor scratches. Then, immerse the workpiece in 25% nitric acid solution for pickling to neutralize the residual alkaline solution and clean the surface.

[0055] S2, Hard Anodizing: The workpiece is placed in an oxidation tank equipped with a specific electrolyte and stirring system. The electrolyte is prepared by taking an appropriate amount of 15% sulfuric acid solution, adding citric acid and potassium dihydrogen phosphate to the sulfuric acid solution at a ratio of 5g citric acid and 8g potassium dihydrogen phosphate per liter of sulfuric acid solution, and controlling the pH value of the solution to 2.2 to form the electrolyte. A high-efficiency refrigeration circulation system is used in the oxidation tank to precisely control the electrolyte temperature at 18℃. Then, a pulse power supply with a pulse frequency of 800Hz and a duty cycle of 40% is used to oxidize the workpiece in the oxidation tank. During the oxidation process, initially at 0.8A / dm 2 Pre-oxidation for 8 minutes to form a uniform initial film; then gradually increase to 2.3 A / dm. 2 The voltage matching material was adjusted to 20V to ensure that the film thickness increased at the expected rate and the adhesion was improved. At the same time, a combination of ultrasonic stirring and circulating pump stirring was used for stirring. The frequency of ultrasonic stirring was 30kHz and the flow rate of circulating pump stirring was 8L / min, so that the film thickness deviation of the workpiece was controlled within ±3μm.

[0056] S3. Post-treatment: After completing the above stirring and oxidation treatment, first wash the workpiece with hot water at 70℃ for 3 minutes to remove residual electrolyte, then put it into nickel acetate sealant with a concentration of 8g / L and seal it at 90℃ for 25 minutes to fill the oxide film pores and improve corrosion resistance and insulation. Finally, dry it in an oven at 65℃ for 12 minutes to remove surface moisture.

[0057] After completing the above oxidation process, the workpiece was inspected, and the results are as follows:

[0058]

[0059]

[0060] The surface data of the workpiece after the oxidation process in Example 1 are shown in Table 1 above. The results show that the hard oxidation process of the present invention enables the oxide film thickness formed on the workpiece surface to meet the standard, and the deviation can be controlled within ±2μm. The microhardness exceeds the standard by 20%, that is, the surface oxide film hardness of the workpiece is significantly improved, the workpiece has excellent corrosion resistance, the film thickness uniformity exceeds the standard by 60%, and the oxidation efficiency of the workpiece is improved by 33-50%, which can significantly improve the production efficiency.

[0061] Example 2: Complex internal cavity part made of 2024 aluminum alloy (internal cavity 30mm×20mm×10mm)

[0062] like Figure 4 As shown, a hard anodizing process for aluminum alloys, using 2024 aluminum alloy for complex internal cavity parts, includes the following steps:

[0063] S1. Pretreatment: Immerse the workpiece in acetone solution for 10 minutes to remove oil. After degreasing, immerse the workpiece in 50g / L sodium hydroxide solution and etch it at 55℃ for 1.5 minutes to remove the natural oxide film and minor scratches. Then, immerse the workpiece in 25% nitric acid solution for pickling to neutralize the residual alkaline solution and clean the surface.

[0064] S2. Hard Anodizing: The workpiece is placed in an oxidation tank equipped with a specific electrolyte and stirring system. The electrolyte is prepared by taking an appropriate amount of 13% sulfuric acid solution, adding citric acid and potassium dihydrogen phosphate at a ratio of 6g citric acid and 6g potassium dihydrogen phosphate per liter of sulfuric acid solution, and controlling the pH of the solution to 2.0. A high-efficiency refrigeration circulation system is used in the oxidation tank to precisely control the electrolyte temperature at 18℃. Then, a pulse power supply with a pulse frequency of 600Hz and a duty cycle of 45% is used to oxidize the workpiece in the oxidation tank. During the oxidation process, a pulse power supply of 0.6A / dm is initially applied. 2 Pre-oxidation for 6 minutes to form a uniform initial film; then gradually increase to 2.2 A / dm.2 The voltage matching material was adjusted to 21V to ensure that the film thickness increased at the expected rate and the adhesion was improved. At the same time, a combination of ultrasonic stirring and circulating pump stirring was used for stirring. The frequency of ultrasonic stirring was 30kHz and the flow rate of circulating pump stirring was 8L / min, so that the film thickness deviation of the workpiece was controlled within ±3μm.

[0065] S3. Post-treatment: After completing the above stirring and oxidation treatment, first wash the workpiece with hot water at 70℃ for 3 minutes to remove residual electrolyte, then put it into nickel acetate sealant with a concentration of 8g / L and seal it at 90℃ for 25 minutes to fill the oxide film pores and improve corrosion resistance and insulation. Finally, dry it in an oven at 65℃ for 12 minutes to remove surface moisture.

[0066]

[0067] The surface data of the workpiece after the oxidation process in Example 2 are shown in Table 2 above. The results show that the hard oxidation process of the present invention enables the oxide film thickness formed on the workpiece surface to meet the standard, and the deviation can be controlled within ±3μm. The microhardness exceeds the standard by 14%, that is, the surface oxide film hardness of the workpiece is significantly improved, the workpiece has excellent corrosion resistance, the film thickness uniformity exceeds the standard by 40%, and the oxidation efficiency of the workpiece is improved by 33-50%, which can significantly improve the production efficiency.

[0068] Reference Appendix Figure 1 As can be seen, before hard anodizing, aluminum alloy workpieces will have many scratches on their surface, and some dirt will adhere to the surface of the workpiece. (Refer to the attached document.) Figure 2 It is evident that after the workpiece undergoes the hard anodizing process of this invention, its surface is uniform and free of scratches, with minimal contaminant adhesion. Therefore, the aluminum alloy hard anodizing process of this invention results in a stable oxide film quality on the workpiece surface, with deviations controllable within ±2-3μm. It is suitable for both conventional and complex workpieces, reduces energy consumption by more than 30%, and utilizes environmentally friendly sealing agents. This effectively addresses the three major pain points of existing processes and can be widely used in high-end manufacturing scenarios such as aerospace aluminum alloy parts, precision mechanical molds, and metal casings for electronic devices.

Claims

1. A hard anodizing process for aluminum alloys, characterized in that: Includes the following steps: S1. Pretreatment: Immerse the workpiece in an organic solvent to remove oil. After degreasing, immerse the workpiece in an alkaline solution for alkaline etching to remove the natural oxide film and minor scratches. Then immerse the workpiece in an acidic solution for pickling to neutralize the residual alkaline solution and clean the surface. S2, Hard anodizing: The pretreated workpiece is placed into an anodizing tank with an electrolyte and a stirring system for stirring and anodizing. S3. Post-treatment: After completing the above stirring and oxidation treatment, the workpiece is cleaned with hot water to remove residual electrolyte; then the workpiece is placed in nickel acetate sealant to fill the oxide film pores; after completion, the workpiece is placed in an oven to dry and remove surface moisture.

2. The aluminum alloy hard anodizing process according to claim 1, characterized in that: In the pretreatment step S1, the organic solvent used is acetone or ethanol, and the soaking time in the organic solvent is 5-10 minutes. The alkaline solution used is a 40-60 g / L sodium hydroxide solution, which is subjected to alkaline etching at 50-60℃ for 1-2 minutes. The acidic solution used is a 20%-30% nitric acid solution, and the acid washing and neutralization takes 2-3 minutes.

3. The aluminum alloy hard anodizing process according to claim 1, characterized in that: In step S2, hard oxidation, the electrolyte is a mixture of sulfuric acid, organic acid and buffer, with the pH controlled between 1.8 and 2.

5.

4. The aluminum alloy hard anodizing process according to claim 3, characterized in that: In step S2, hard oxidation, the sulfuric acid concentration is 12%-18%, the organic acid is citric acid with a concentration of 3-8 g / L, and the buffer is potassium dihydrogen phosphate with a concentration of 5-10 g / L.

5. The aluminum alloy hard anodizing process according to claim 4, characterized in that: In step S2, hard oxidation, a refrigeration cycle system is used to control the electrolyte temperature at 18±0.5℃.

6. The aluminum alloy hard anodizing process according to claim 5, characterized in that: It is powered by a pulse power supply with a pulse frequency of 500-1000Hz and a duty cycle of 30%-50%.

7. The aluminum alloy hard anodizing process according to claim 6, characterized in that: In step S2, hard oxidation, the oxidation process includes a pre-oxidation process and a gradient pressurization process. The pre-oxidation process involves first applying pressure at 0.5-1 A / dm³. 2 Pre-oxidation is performed at a current density of 5-10 minutes to form a uniform initial film; the gradient voltage increase process involves gradually increasing the current density to 2-3 A / dm² after pre-oxidation. 2 The voltage matching material has been adjusted to 18-25V.

8. The aluminum alloy hard anodizing process according to claim 7, characterized in that: In step S2, hard anodizing, synchronous stirring is also required by a stirring system. The stirring system adopts a combination of ultrasonic stirring and circulating pump stirring. The frequency of ultrasonic stirring is 20-40kHz, and the flow rate of circulating pump stirring is 5-10L / min.

9. The aluminum alloy hard anodizing process according to claim 1, characterized in that: In the post-processing step S3, the hot water temperature is controlled at 60-70℃, and the washing time is 3-4 minutes. The concentration of the nickel acetate is 6-10 g / L, and it is sealed at 85-95℃ for 25-30 min; The oven temperature is controlled at 60-70℃, and the drying time is 10-15 minutes.

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