Corrosion-resistant aluminum alloy flange plate manufacturing process

Abstract

The invention discloses a manufacturing process of a corrosion-resistant aluminum alloy flange plate. The manufacturing process comprises the following steps that S1, alloy smelting and component control are conducted; s2, homogenizing heat treatment; s3, hot die forging forming; s4, solution treatment; s5, multi-stage artificial aging; and S6, surface treatment. According to the flange plate and the manufacturing method thereof, through the unique alloy component design and the combination of the whole-process optimization technology of precise smelting, homogenization, die forging, solid solution, multi-stage aging and micro-arc oxidation, cooperative regulation and control of intracrystalline strengthening and grain boundary purification are achieved on the microstructure, and finally the flange plate product with excellent comprehensive performance is obtained.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of mechanical component manufacturing, in particular to a corrosion-resistant aluminum alloy flange plate manufacturing process. BACKGROUND

[0002] As a key basic element for detachable connection in pipelines, pressure vessels and various equipment, the performance of a flange plate directly determines the sealing reliability, safe operation period and maintenance cost of the entire fluid conveying system. In typical fields such as offshore platforms, shipbuilding, coastal power stations and chemical and pharmaceutical industries, the flange plate is long-term served in harsh corrosive environments with high humidity, high salt mist, rich chloride ions or specific chemical media, and faces severe challenges such as pitting corrosion, crevice corrosion, intergranular corrosion and stress corrosion cracking. Therefore, the flange plate material not only requires sufficient mechanical strength to withstand internal pressure and bolt load, but also requires excellent and durable corrosion resistance.

[0003] At present, the industry mainly uses the following two types of materials for the flange plate in the above harsh working conditions:

[0004] The first type is austenitic stainless steel flange plate (such as 316L, 317L, etc.). This type of flange plate relies on the passivation film to provide corrosion resistance, and although it can resist uniform corrosion to some extent, in the presence of chloride ions, the passivation film is easily damaged locally, which can cause pitting and even stress corrosion cracking (SCC), posing a significant safety hazard. In addition, stainless steel materials have high density and high cost, and their corrosion resistance is limited in some media (such as concentrated sulfuric acid and halide solution), which restricts their application economy and scope of application.

[0005] The second type is a carbon steel or low alloy steel flange plate with a corrosion-resistant coating (such as electroplated zinc, hot-dip zinc, epoxy coating, etc.) applied to the surface. This scheme attempts to obtain protection performance at a lower material cost. However, this protection is "passive" protection, and its reliability completely depends on the integrity and density of the coating. Under the action of mechanical wear and tear, thermal cycling during transportation, installation and bolt tightening of the flange plate, the coating is easily damaged locally. Once the coating is damaged, the exposed steel substrate will form a corrosion couple with the coating (usually acting as a cathode), causing severe local corrosion of the substrate, and the perforation failure rate is often much higher than that of uncoated carbon steel, making it even less safe.

[0006] Aluminum alloy is considered an ideal alternative material for realizing the lightweight and long-life of flange plates due to its low density, high specific strength, no low-temperature brittleness and natural atmospheric corrosion resistance. However, the existing conventional aluminum alloy system has obvious technical bottlenecks in such high-performance applications:

[0007] The contradiction between strength and corrosion resistance: the commonly used 5xxx series (Al-Mg) alloy (such as 5083) has good resistance to marine atmospheric corrosion and weldability, but its strength (especially the tensile strength is usually lower than 300 MPa) is difficult to meet the pressure requirement of medium and high pressure pipeline system. The 6xxx series (Al-Mg-Si) alloy (such as 6061) can obtain medium strength (about 310 MPa) through T6 aging treatment, but its resistance to seawater corrosion, especially the pitting resistance, is poor, and it is difficult to work stably in harsh marine environment for a long time. The strength of high-strength 7xxx series (Al-Zn-Mg-Cu) alloy (such as 7075) is high, but its stress corrosion cracking resistance is poor, and the intergranular corrosion tendency is serious, so the safety cannot be guaranteed.

[0008] Limitations of traditional heat treatment process: the existing aluminum alloy flange plate is usually processed by single-stage peak aging after casting (T5 / T6 state) or forging (T6 state). In order to pursue the highest strength, this traditional T6 treatment often leads to continuous or semi-continuous precipitation of strengthening phase (such as Mg2Si, MgZn2, etc.) at grain boundary, and forms a solute-poor "precipitate-free zone" (PFZ) near the grain boundary. This microstructure makes the material form an activation-passivation cell between grain boundary and intragranular in the corrosion medium, and the corrosion rapidly expands along the grain boundary, resulting in a sharp deterioration of the intergranular corrosion and stress corrosion resistance. In other words, the traditional process inevitably sacrifices the corrosion resistance of the material while obtaining high strength, and this inherent contradiction has not been effectively solved for a long time.

[0009] Defects of surface treatment technology: in order to improve the corrosion resistance, the aluminum alloy flange plate is often subjected to anodic oxidation treatment. However, the film layer generated by conventional anodic oxidation (such as sulfuric acid anodic oxidation) has the defects of low hardness, insufficient toughness, limited adhesion to the substrate, etc. Under the working conditions of tightening or disassembling the flange plate bolts, or being subjected to vibration and friction, the film layer is easy to produce micro-cracks or peel off, thereby exposing the local substrate and causing pitting, and finally leading to the failure of protection.

[0010] Therefore, there is an urgent need in the art for a new technical solution, which can make fundamental innovation from the aspects of material system design and manufacturing process, break the shackles of the difficulty in balancing the strength and corrosion resistance of aluminum alloy, and develop a new type of flange plate product with high strength, high toughness and excellent corrosion resistance (especially excellent pitting resistance and stress corrosion resistance), so as to meet the increasing demand of modern industry for high-performance connecting components. SUMMARY

[0011] To overcome the shortcomings of existing technologies, this invention provides a manufacturing process for corrosion-resistant aluminum alloy flanges. Through unique alloy composition design and combined with a fully optimized process of "precision melting - homogenization - die forging - solution treatment - multi-stage aging - micro-arc oxidation", this invention achieves synergistic control of intragranular strengthening and grain boundary purification at the microstructure level, ultimately obtaining flange products with excellent comprehensive performance.

[0012] To solve the above-mentioned technical problems, the present invention provides the following technical solution: Firstly, a process for manufacturing a corrosion-resistant aluminum alloy flange, comprising the following steps:

[0013] S1: Alloy smelting and composition control;

[0014] S2: Homogenization heat treatment;

[0015] S3: Hot forging;

[0016] S4: Solution treatment;

[0017] S5: Multi-level manual efficiency;

[0018] S6: Surface treatment.

[0019] Furthermore, the alloy smelting and composition control includes raw material melting, refining, and degassing. The refining process employs a compound refining process using high-purity argon and hexachloroethane, with a refining time of 15-25 minutes.

[0020] Furthermore, the cooling method for the homogenization heat treatment is forced air cooling.

[0021] Furthermore, the preheating temperature of the forging die in the hot forging process is 280-350℃.

[0022] Furthermore, after the solution treatment, a quenching treatment is performed, wherein the quenching is water quenching.

[0023] Furthermore, the multi-level artificial efficiency process involves three levels of efficiency processing, and the three stages are carried out continuously.

[0024] Furthermore, the surface treatment involves machining the aged flange and then performing micro-arc oxidation on its surface to form a ceramic layer. The electrolyte used in the micro-arc oxidation process is a silicate system, and the current density is 5-15 A / dm³. 2 The processing time is 20-50 minutes, and the resulting ceramic layer has a thickness of 15-30 μm.

[0025] Secondly, a corrosion-resistant aluminum alloy flange, wherein the room temperature mechanical properties of the corrosion-resistant aluminum alloy flange meet the following requirements: tensile strength Rm≥380MPa, specified non-proportional elongation strength Rp0.2≥320MPa, and elongation after fracture A≥10%.

[0026] Furthermore, the corrosion-resistant aluminum alloy flange was subjected to a neutral salt spray test according to GB / T10125, and no pitting corrosion was observed on the surface after 1000 hours.

[0027] Compared with the prior art, the beneficial effects that this invention can achieve are:

[0028] 1. Scientifically designed matrix with superior corrosion resistance: This invention uses Mn and Mg as the main alloying elements and strictly controls the content of harmful impurity elements such as Fe, Cu, and Zn, thereby reducing the formation of cathodic intermetallic compounds at the source and lowering the driving force for galvanic corrosion. The addition of Cr and Ti for microalloying refines the grains and improves the uniformity of the microstructure. This composition system ensures high strength after aging treatment while endowing the alloy matrix with excellent inherent corrosion resistance.

[0029] 2. Multi-stage aging process as the core innovation, synergistically enhancing strength, toughness, and corrosion resistance: The invention's unique "low-temperature-medium-low-temperature" three-stage aging process is key to resolving the contradiction between strength and corrosion resistance. The first stage, low-temperature aging, forms a high-density GP region as a nucleation core; the second stage, medium-temperature aging, obtains a diffusely distributed metastable strengthening phase (β”), contributing the main strength; the third stage, low-temperature stabilization aging, promotes coarsening and increased spacing of grain boundary precipitates, effectively "purifying" the grain boundaries and significantly reducing intergranular corrosion and stress corrosion sensitivity. This process, while maintaining high strength (Rm≥380MPa), endows the material with extremely high corrosion resistance.

[0030] 3. Seamless process coordination ensures controllable microstructure and properties: From homogenization treatment to improve the hot workability of the ingot, to hot forging to obtain a dense fibrous structure, to rapid solution quenching to maintain supersaturation concentration, and finally to multi-stage aging, each step serves the final performance, forming an organic whole. This process ensures that the flange product has uniform, stable, and excellent comprehensive performance from core to surface.

[0031] 4. Strong and durable surface protection: Micro-arc oxidation technology replaces traditional anodizing, growing a high-hardness, high-wear-resistance ceramic oxide layer in situ on the flange surface, which is metallurgically bonded to the substrate. This ceramic layer has good insulation and stable chemical properties, providing the flange with physical and chemical barriers that surpass conventional methods, making it particularly suitable for harsh assembly and service environments involving friction and impact.

[0032] 5. Excellent overall product performance: The flange prepared by this invention successfully achieves a balance between high strength, high toughness, and extremely high corrosion resistance. Its mechanical properties meet the requirements of high-pressure pipelines, it shows no pitting corrosion after more than 1000 hours of neutral salt spray testing, has a long service life, and high safety and reliability. It is particularly suitable for applications with stringent requirements for weight and corrosion resistance, and has enormous market application value. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the manufacturing process of the corrosion-resistant aluminum alloy flange of the present invention. Detailed Implementation

[0034] Combination Figure 1 As shown, this invention provides a manufacturing process for corrosion-resistant aluminum alloy flanges. The fundamental goal of this method is to achieve a perfect combination of high strength, high toughness, and high corrosion resistance (especially resistance to intergranular corrosion and pitting corrosion) in the aluminum alloy matrix through the synergistic control of composition design, thermomechanical treatment, and surface modification. The method includes the following steps:

[0035] Step 1, Precision Alloy Melting and Composition Control: The core function of this step is to construct an alloy matrix with high inherent corrosion resistance potential. Through precise composition ratios and pure melting, the formation of harmful phases is avoided from the outset, providing a high-quality raw material foundation for all subsequent heat treatments and surface treatments.

[0036] Step 2, Homogenization Heat Treatment: The core function of this step is to eliminate intragranular segregation in the ingot and homogenize the distribution of solute atoms. The principle is to dissolve the low-melting-point eutectic phase generated by non-equilibrium solidification through high-temperature, long-term diffusion, preparing a uniform single-phase solid solution structure for subsequent hot working. This prevents cracking due to localized melting or stress concentration during hot working and ensures the maximization of the subsequent solution treatment effect.

[0037] Step 3, Hot Forging: The core function of this step is to obtain a fine, dense, fibrous streamlined forging structure by plastic deformation and dynamic recrystallization to break down the as-cast structure. The principle is to utilize triaxial compressive stress to weld together defects such as porosity and gas bubbles inside the ingot, significantly improving the material's density, mechanical properties (especially fatigue performance and toughness), and resistance to stress corrosion cracking.

[0038] Step 4, Solution Treatment: This step is a preparatory step for precipitation strengthening. The principle is to heat the forged workpiece to a temperature range where the second phase can dissolve in large quantities and hold it at that temperature, so that the strengthening phase elements (mainly Mg, Si, Mn, etc.) dissolve into the aluminum matrix to the maximum extent to form a supersaturated solid solution. Then, through rapid cooling (quenching), this metastable state is fixed to room temperature, providing sufficient driving force and solute atoms for subsequent aging precipitation.

[0039] Step 5, Multi-stage Artificial Aging: This step is the most critical core step in achieving synergistic optimization of strength and corrosion resistance. Its principle is to precisely control the nucleation, growth, and distribution of strengthening precipitates through a carefully designed three-stage aging path of "low temperature-medium temperature-low temperature." The goal is to obtain a large number of dispersed, fine strengthening phases within the grains to ensure strength, while simultaneously forming coarse, sparse precipitates at the grain boundaries to "purify the grain boundaries," significantly reducing intergranular corrosion and stress corrosion sensitivity.

[0040] Step Six, Surface Micro-arc Oxidation Treatment: This step provides the ultimate and most robust surface barrier protection. The principle is to generate micro-area plasma discharge on the aluminum substrate surface under a high electric field, growing an in-situ ceramicized alumina layer that is metallurgically bonded to the substrate. This layer possesses extremely high hardness, wear resistance, insulation, and chemical inertness, effectively isolating the flange from direct contact with corrosive media and providing durability protection surpassing conventional anodizing.

[0041] Steps one through six must be performed in sequence. There is a strict process inheritance and performance accumulation effect between them. Any omission of a step or reversal of the order will result in the failure to achieve the comprehensive performance matching expected by this solution.

[0042] The detailed process, parameter principles, and functions of alloy smelting and composition control in step one are as follows:

[0043] Precise design of alloy composition and its working principle:

[0044] Si (0.1-0.5%): Forms the Mg2Si strengthening phase with Mg, which is the main source of aging strengthening. If the content is below 0.1%, the strengthening effect is insufficient; if it is above 0.5%, coarse primary silicon phase will be formed, which will severely rupture the matrix and deteriorate toughness and corrosion resistance.

[0045] Fe (≤0.20%): A strictly limited harmful impurity. Fe readily combines with Al to form hard and brittle needle-like FeAl3 or AlCuFeMn phases, which are the source of fatigue cracks and significantly reduce corrosion resistance. This solution uses high-purity aluminum ingots and strictly controls their content to below 0.20%, which is key to ensuring high corrosion resistance.

[0046] Cu (≤0.10%): A strictly limited element. While Cu can improve strength, it severely deteriorates corrosion resistance because it forms high-potential Al₂CuMg or Al-Cu phases with Al, which act as cathodes in corrosive media, accelerating the dissolution of the surrounding aluminum matrix. This solution controls it to extremely low levels, fundamentally avoiding the driving force of galvanic corrosion.

[0047] Mn (0.8-1.5%): One of the core alloying elements. Mn has a high solid solubility in Al, resulting in significant solid solution strengthening. More importantly, Mn preferentially reacts with Fe to form the Al6(Mn,Fe) phase, transforming the harmful acicular Fe phase into a more equiaxed, Chinese character-shaped phase, thus mitigating its harmful effects. Mn can also increase the recrystallization temperature of the alloy and refine the grains after forging.

[0048] Mg (2.0-3.5%): A major alloying element. Mg has high solid solubility in Al, resulting in significant solid solution strengthening. It combines with Si to form the Mg2Si strengthening phase, which is the basis for age-hardening. However, when the Mg content is too high (>3.5%), the β phase (Al3Mg2) is easily precipitated continuously at the grain boundaries, leading to a sharp increase in stress corrosion cracking susceptibility.

[0049] Cr (0.10-0.25%): Microalloying element. Cr can form dispersed CrAl7 or (Cr,Fe)Al7 intermetallic compounds, effectively pinning grain boundaries and subgrain boundaries, inhibiting recrystallized grain growth, and refining the microstructure. At the same time, these dispersed phases can also refine precipitates and improve corrosion resistance.

[0050] Zn (≤0.05%): A strictly controlled impurity. Zn lowers the electrode potential of aluminum and accelerates corrosion. This protocol considers it a harmful impurity and strictly limits its use.

[0051] Ti (0.05-0.15%): Grain refiner. It is usually added in the form of Al-Ti-B wire. TiB2 and TiAl3 particles can act as non-spontaneous nucleation nuclei of α-Al, significantly refining the as-cast grains, thereby improving thermoplasticity, reducing segregation, and enhancing overall mechanical properties.

[0052] Detailed steps and principles of the smelting process:

[0053] a. Loading and melting: Use an electric resistance furnace or a gas furnace, place high-purity aluminum ingots (≥99.85%) at the bottom, and heat to 750-780℃ to completely melt them.

[0054] b. Addition of intermediate alloys: Add intermediate alloys in sequence, such as Al-20%Mn, Al-10%Mg, Al-5%Cr, and Al-3%Ti-B. The order of addition should follow the principle of "adding high melting points first, then low melting points". After each addition, stir thoroughly and keep warm for 10-15 minutes to ensure complete dissolution.

[0055] c. Refining: High-purity argon (99.999% purity) and hexachloroethane (C2Cl6) are used for combined refining. The principle is that when the argon gas bubbles rise in the melt, they can carry the tiny hydrogen bubbles and oxide inclusions suspended in the melt to the surface through partial pressure difference and surface adsorption. At the same time, solid hexachloroethane decomposes at high temperature (C2Cl6→2C+3Cl2), and the chlorine gas produced also has a hydrogen removal effect. The carbon produced by its decomposition can also partially reduce the Al2O3 oxide film. This combined refining lasts for 20 minutes to ensure the purity of the melt.

[0056] d. Settling and Slag Removal: After refining, adjust the melt temperature to 730-740℃ and let it stand for 20-30 minutes to allow residual impurities to float or sink fully. Then, use a scraper to thoroughly remove the slag from the surface of the melt.

[0057] e. Casting: Pure molten aluminum is poured into a cylindrical steel mold preheated to 300-400℃ within a temperature range of 720-750℃. After cooling, a round ingot with uniform composition and dense structure is obtained. If the casting temperature is too low, cold shuts are likely to occur; if it is too high, the grains will be coarse.

[0058] The detailed process, parameters, principles, and functions of the homogenization heat treatment in step two are as follows:

[0059] Under non-equilibrium solidification conditions, severe dendritic segregation occurs within the grains of an ingot. For example, elements such as Mn and Mg have low concentrations at the dendrite trunk center and high concentrations between dendrites and at grain boundaries. This compositional inhomogeneity leads to: (1) precipitation of non-equilibrium low-melting-point eutectic phases (such as Al8Mg5) at grain boundaries, which are prone to melting and cracking during hot working; (2) long diffusion paths of solute atoms during subsequent solution treatment, resulting in insufficient dissolution of the strengthening phase; and (3) affecting the uniformity of the final microstructure and deteriorating performance. Homogenization treatment involves holding the ingot at a temperature below the solidus temperature of the alloy but above the dissolution temperature of the non-equilibrium eutectic phase for a long time. This allows the composition within the grains to become more uniform through atomic diffusion and promotes the dissolution or transformation of the non-equilibrium phase into a more stable phase.

[0060] Temperature setting (540-570℃): This temperature range represents the optimal window verified by thermodynamic calculations and experiments. Below 540℃, the atomic diffusion rate is slow, the dissolution motive force of the non-equilibrium phase is insufficient, requiring extremely long holding times, which is uneconomical; above 570℃, there is a risk of overheating, and localized melting at grain boundaries will occur, resulting in irreversible damage. The preferred temperature is 555±5℃.

[0061] Holding time (8-16 hours): The diffusion distance is proportional to the square root of the time. Sufficient time is required to eliminate dendrite segregation on the scale of tens to hundreds of micrometers. 8 hours is the minimum requirement, while 16 hours ensures adequate homogenization. Longer times result in diminishing returns and increased energy consumption. A preferred time is 12 hours.

[0062] Cooling method (forced air cooling): Forced air cooling is used after homogenization instead of furnace cooling. The purpose is to quickly pass through the medium temperature range (approximately 300-500℃) and prevent the precipitation of coarse equilibrium phases (such as Mg2Si, Al6Mn, etc.) from the supersaturated solid solution during slow cooling. These coarse phases are difficult to dissolve again in subsequent solution treatment, wasting strengthening elements and reducing the final properties. Rapid cooling "freezes" solute atoms in the matrix, preparing it for hot forging and solution treatment.

[0063] The detailed process, parameters, principles, and functions of hot forging in step three are as follows:

[0064] The process transforms the homogenized as-cast microstructure into a fine forged microstructure. Through thermally activated plastic deformation, coarse as-cast grains and dendrite networks are broken down, and dynamic recrystallization forms new equiaxed fine grains. Simultaneously, the triaxial compressive stress effectively welds together any porosity or micropores that may exist in the ingot center, significantly improving the material's density. The streamlines formed during forging (grain extension along the deformation direction) give the flange anisotropic mechanical properties, resulting in higher strength in the direction of radial stress.

[0065] Billet heating temperature (450-500℃): This temperature ensures the alloy is in a single-phase region with high plasticity and low deformation resistance. If the temperature is too low, the deformation resistance is high, requiring larger tonnage equipment, and cracking is likely; if the temperature is too high, the grains coarsen, or even overheating occurs. Holding at this temperature for 1-3 hours is to ensure uniform temperature throughout the billet.

[0066] Mold preheating temperature (280-350℃): This is crucial to prevent surface cooling and cracking. If the mold is cold, the surface in contact with the hot billet will cool rapidly, causing a sharp decrease in plasticity and making it prone to surface cracks during subsequent deformation. Preheating the mold reduces temperature differences and ensures uniform deformation. Temperatures below 280℃ are insufficient, while temperatures above 350℃ reduce mold strength and lifespan.

[0067] Initial and final forging temperature control (≥420℃, ≥350℃): The initial forging temperature should not be lower than 420℃ to ensure a sufficient plastic deformation window. The final forging temperature should not be lower than 350℃ to prevent the "work hardening" effect. If forging is completed below the recrystallization temperature, the dislocations generated by deformation cannot be eliminated in time through dynamic recrystallization, leading to work hardening, increased internal stress, decreased toughness, and even cracking. Strict control of the final forging temperature is the guarantee for obtaining excellent post-forging microstructure.

[0068] The detailed process, parameters, principles, and functions of the solution treatment in step four are as follows:

[0069] The soluble second phase (mainly Mg2Si phase and Al6Mn, etc.) present in the microstructure after hot forging is dissolved into the aluminum matrix to the maximum extent to form a supersaturated solid solution. This is a prerequisite for subsequent age hardening.

[0070] Solution temperature (520-540℃): This temperature is lower than the eutectic temperature of the alloy, but higher than the Solvus Line of strengthening phases such as Mg2Si. Higher temperatures result in faster and more complete dissolution, but also bring the temperature closer to the overheating temperature, increasing the risk. This solution selects a safe and efficient range, preferably 530±5℃.

[0071] Solution treatment time (1-3 hours): The time is determined by the effective thickness of the part (flange thickness) to ensure that the core strengthening phase also dissolves fully. It is usually estimated as 1 hour of heat treatment for every 25 mm of thickness.

[0072] Quenching medium and transfer time (water quenching <15 seconds): Quenching is the decisive step in solution treatment. The goal is to quickly pass through the precipitation nose temperature range (approximately 250-400℃) to suppress premature precipitation of strengthening phases during cooling, thereby maximizing the preservation of supersaturated solid solubility. Water quenching has the fastest cooling rate and the best effect. A transfer time of less than 15 seconds is a strict requirement because the workpiece temperature drops rapidly after exiting the furnace. If the transfer is slow, partial precipitation will occur in the air, forming coarse phases, which severely weakens the age-strengthening potential. This loss cannot be compensated for in subsequent aging processes.

[0073] The detailed process, parameters, principles, and functions of the multi-stage artificial aging process in step five are as follows:

[0074] While conventional T6 state (single-stage peak aging) achieves the highest strength, the continuous distribution of grain boundary precipitates and the presence of precipitation-free zones (PFZs) near the grain boundaries result in extremely poor resistance to intergranular corrosion and stress corrosion cracking. This three-stage aging scheme aims to achieve a microstructure design of "high intragranular strength and high grain boundary corrosion resistance" through kinetic control.

[0075] The first stage of aging (110-130℃ / 6-10h): Its function is to form high-density GP regions. At lower temperatures, solute atoms in the supersaturated solid solution agglomerate, forming a large number of nanoscale Guinier-Preston (GP) regions. These GP regions serve as the heterogeneous nucleation cores for subsequent precipitates, ensuring the diffuse distribution of the precipitates. This stage lays the quantitative foundation for high strength.

[0076] The second stage of aging (150-170℃ / 4-8h): Its function is to complete the transformation of the main strengthening phase. Increasing the temperature provides higher energy for atomic diffusion, causing the GP zone to transform into the more stable metastable phase β" (or β'). This phase is coherent with the matrix, generating a strong elastic strain field, which is the main source of strength contribution. Most of the peak strength is obtained in this stage.

[0077] The third stage of aging (80-100℃ / 10-20h): its function is "stabilization" or "regression treatment." This is crucial for improving corrosion resistance. After prolonged holding at lower temperatures, the reinforcing phase within the grains becomes relatively stable with little change. However, at grain boundaries, due to higher energy and more defects, the precipitated phases continue to grow slowly, spheroidize, and increase in spacing (Ostwald ripening). Simultaneously, the PFZ width will moderately recover. This "grain boundary coarsening" disrupts the continuity of grain boundary precipitates, significantly prolonging the path of the corrosive medium along the grain boundaries and increasing the plasticity of the grain boundaries. This results in a qualitative leap in resistance to intergranular corrosion and stress corrosion cracking, while the strength loss is minimal (typically <5%).

[0078] Continuity: The three stages proceed continuously without cooling to room temperature. This ensures the continuity of tissue evolution and avoids additional internal stress or uncontrollable phase transitions that may be caused by cooling and reheating.

[0079] The detailed process, parameters, principles, and functions of the surface micro-arc oxidation treatment in step six are as follows:

[0080] Building upon the excellent corrosion resistance of the substrate, it provides an extremely robust physical barrier. Micro-arc oxidation (MAO) is a process that generates micro-area plasma discharge by instantaneously breaking down an insulating oxide film on the aluminum surface in an environmentally friendly electrolyte under high voltage. The temperature and pressure within the discharge channel are extremely high (~10℃). 4 K,10 2 The aluminum substrate is instantly melted by an electrolytic flux (MPa) and reacts with the electrolyte components to directly grow a ceramic layer mainly composed of α-Al₂O₃ and γ-Al₂O₃. This layer has a three-layer structure: an outer porous layer, a middle dense layer, and an inner transition layer, which is metallurgically bonded to the substrate, with a bonding strength far exceeding that of electroplating or spray coatings.

[0081] Electrolyte: A silicate system (such as sodium silicate or potassium silicate) is used. Its advantage is that the silicon dioxide generated by the reaction with aluminum can fill the pores of the film, making the film denser, harder, and more corrosion resistant.

[0082] Current density (5-15A / dm) 2 Current density directly affects discharge energy and film formation rate. Too low a density results in slow film formation and a thin film; too high a density leads to intense discharge, a rough film layer, and high porosity. This range represents the optimal values.

[0083] Processing time (20-50 minutes): directly determines the film thickness. The longer the processing time, the thicker the film. The target thickness for this solution is 15-30 μm. This thickness provides excellent protection without causing brittle cracking due to excessive internal stress.

[0084] Post-treatment: After treatment, remove the workpiece, rinse it thoroughly with deionized water, and then dry it by baking or air drying. No additional sealing treatment is required for the micro-arc oxidation film.

[0085] A corrosion-resistant aluminum alloy flange. This flange is characterized by its optimized microstructure, which features uniform composition, fine grains, dispersed nanoscale reinforcing phases within the grains, and coarse, discontinuous grain boundary precipitates. Furthermore, its surface is covered with a high-hardness, high-bonding ceramic oxide layer. Its room-temperature mechanical properties must meet the following requirements: tensile strength Rm ≥ 380 MPa, specified non-proportional elongation strength Rp0.2 ≥ 320 MPa, and elongation after fracture A ≥ 10%. This combination of properties indicates that the flange possesses both high strength and good toughness, sufficient to withstand the high pressure and mechanical impact in piping systems. Its corrosion resistance is demonstrated by a neutral salt spray test according to national standard GB / T10125. After 1000 hours of continuous spraying, no pitting corrosion was observed on the flange surface (especially at machined edges and sealing surfaces). This proves its comprehensive and superior corrosion resistance from the substrate to the surface.

[0086] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A manufacturing process for a corrosion-resistant aluminum alloy flange, characterized in that, Includes the following steps: S1: Alloy smelting and composition control; S2: Homogenization heat treatment; S3: Hot forging; S4: Solution treatment; S5: Multi-level manual efficiency; S6: Surface treatment.

2. The manufacturing process of the corrosion-resistant aluminum alloy flange according to claim 1, characterized in that, The alloy smelting and composition control includes raw material melting, refining, and degassing. The refining process uses high-purity argon and hexachloroethane for a total refining time of 15-25 minutes.

3. The manufacturing process of the corrosion-resistant aluminum alloy flange according to claim 1, characterized in that, The cooling method for the homogenization heat treatment is forced air cooling.

4. The manufacturing process of the corrosion-resistant aluminum alloy flange according to claim 1, characterized in that, The preheating temperature of the forging die in the hot forging process is 280-350℃.

5. The manufacturing process of the corrosion-resistant aluminum alloy flange according to claim 1, characterized in that, The solution treatment is followed by quenching, which is water quenching.

6. The manufacturing process of the corrosion-resistant aluminum alloy flange according to claim 1, characterized in that, The multi-level artificial efficiency refers to a three-level efficiency process, and the three stages are carried out continuously.

7. The manufacturing process of the corrosion-resistant aluminum alloy flange according to claim 1, characterized in that, The surface treatment involves machining the aged flange and then performing micro-arc oxidation to form a ceramic layer. The electrolyte used in the micro-arc oxidation is a silicate system, and the current density is 5-15 A / dm³. 2 The processing time is 20-50 minutes, and the resulting ceramic layer has a thickness of 15-30 μm.

8. A corrosion-resistant aluminum alloy flange manufactured by the corrosion-resistant aluminum alloy flange manufacturing process described in any one of claims 1 to 7.

9. The corrosion-resistant aluminum alloy flange according to claim 8, characterized in that, The room temperature mechanical properties of the corrosion-resistant aluminum alloy flange meet the following requirements: tensile strength Rm≥380MPa, specified non-proportional elongation strength Rp0.2≥320MPa, and elongation after fracture A≥10%.

10. The corrosion-resistant aluminum alloy flange according to claim 8, characterized in that, The corrosion-resistant aluminum alloy flange was subjected to a neutral salt spray test according to GB / T10125, and no pitting corrosion was observed on the surface after 1000 hours.

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