A surface zinc-aluminum alloy material and a reverse gradient zinc infiltration process for an aluminum alloy material

Abstract

The application discloses a surface zinc-alloyed aluminum material and a reverse gradient zinc-alloying process for an aluminum material surface. The surface zinc-alloyed aluminum material comprises an aluminum alloy base and a zinc-alloyed layer on the surface of the aluminum alloy base. The zinc element content in the zinc-alloyed layer is distributed in a gradient from the surface layer of the zinc-alloyed layer to the aluminum alloy base, and the zinc element content gradually decreases. The main phase of the zinc-alloyed layer is an alpha-Al(Zn) substitution solid solution. The total depth of the zinc-alloyed layer is less than or equal to 400 microns, and the reverse gradient depth is less than or equal to 20 microns. The application solves the problem that the zinc-alloyed aluminum material is preferentially corroded and horizontally diffused in the middle high-zinc area of the zinc-alloyed layer due to the reverse gradient distribution of the zinc element, and the outer reverse gradient layer is peeled off without corrosion. Meanwhile, the application solves a series of problems, such as the loss of the protection of the zinc-alloyed layer, the significant reduction of the utilization rate of the surface zinc-alloyed layer, the reduction of the actual use thickness of the aluminum alloy component, the poor control effect of the corrosion resistance of the zinc-alloyed aluminum material and the shortening of the service life.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy surface protection technology, specifically to a zinc-diffused aluminum alloy material and a reverse gradient zinc-diffusion process for the surface of aluminum alloy materials. Background Technology

[0002] Localized corrosion is the main form of corrosion in aluminum alloys. The corrosion is small and deep, making it difficult to detect. Once corrosion perforation occurs, it will lead to the failure of heat exchange equipment.

[0003] Existing publicly available technologies mainly focus on aluminum alloy surface protection, with surface zinc spraying being a key aspect.

[0004] For example, the prior art discloses an aluminum alloy heat exchange tube with a 0.5µm-0.3µm Zn layer on its surface, and discloses a surface zinc content set to be less than or equal to 6g / m. 2 Parallel flow heat exchange tubes.

[0005] These technologies all focus on the design of zinc spraying amount and zinc spraying process, but do not control the zinc diffusion curve of the zinc spraying alloy, especially the reverse gradient distribution.

[0006] When zinc penetrates into aluminum alloys, it forms a gradient distribution (the zinc content on the surface is higher than that inside). Zinc lowers the corrosion potential of aluminum alloys, accelerates corrosion in areas with high zinc concentration on the surface, and gradually slows down the corrosion rate as the zinc content decreases, thus controlling the corrosion behavior of aluminum alloys.

[0007] The presence of a reverse gradient (where the zinc content on the surface is lower than that inside) causes the highest zinc content in the middle of the zinc-diffused aluminum alloy layer to corrode preferentially and spread laterally. This causes the outermost reverse gradient layer to be separated from the inner aluminum alloy substrate by the corrosion of the middle high-zinc zone, resulting in the reverse gradient layer peeling off without corrosion.

[0008] This peeling phenomenon not only causes the anti-gradient layer to lose its due corrosion protection function and significantly reduces the utilization rate of the surface zinc-diffused layer, which has an adverse effect on the corrosion resistance control of zinc-diffused aluminum alloy materials, but also directly reduces the actual thickness of aluminum alloy components, further shortening the service life of aluminum alloy components used for heat exchange.

[0009] Therefore, how to control the distribution of zinc element during the zinc diffusion process and reduce or even eliminate the depth of the anti-gradient layer has become a key technical problem that needs to be solved to improve the corrosion resistance of zinc-diffused aluminum alloy materials for heat exchange. Summary of the Invention

[0010] To overcome the shortcomings of the prior art, the purpose of this application is to provide a surface zinc-infiltrated aluminum alloy material, which solves the problem that the high zinc zone in the middle of the zinc-infiltrated layer is preferentially corroded and spread laterally due to the reverse gradient distribution of zinc, resulting in the outer reverse gradient layer peeling off without corrosion. At the same time, it solves a series of problems caused by this peeling phenomenon, such as loss of the protective function of the zinc-infiltrated layer, a significant reduction in the utilization rate of the surface zinc-infiltrated layer, a reduction in the actual thickness of the aluminum alloy component, poor corrosion resistance control effect of the zinc-infiltrated aluminum alloy material, and shortened service life.

[0011] To solve the above problems, the technical solution adopted in this application is as follows: This application provides a surface zinc-diffused aluminum alloy material, comprising an aluminum alloy substrate and a zinc-diffused layer covering the surface of the aluminum alloy substrate. The zinc content in the zinc-diffused layer is distributed in a gradient that continuously and gradually decreases from the surface of the zinc-diffused layer towards the aluminum alloy substrate. The main phase of the zinc-diffused layer is an α-Al(Zn) substitution solid solution. The total depth of the zinc-diffused layer is ≤400μm, and the reverse gradient depth is ≤20μm.

[0012] As a further preferred embodiment, the mass fraction of zinc in the zinc-infiltrated layer described in this application embodiment does not exceed 5 wt%.

[0013] As a further preferred embodiment, the reverse gradient depth of the zinc diffusion layer described in this application embodiment is 0μm-5μm.

[0014] As a further preferred embodiment, the surface zinc-diffused aluminum alloy material described in this application has a cross-sectional corrosion depth ≤30μm after 1000h SWAAT testing; a tensile strength of 95-98MPa and a yield strength of 35-38MPa at room temperature.

[0015] This application also provides a reverse gradient zinc diffusion process for aluminum alloy materials. By controlling and eliminating the reverse gradient distribution of zinc elements, the protection failure caused by the reverse gradient distribution of zinc elements in the zinc diffusion layer is solved, avoiding the process defects of traditional high-temperature zinc diffusion process, such as serious zinc volatilization and oxidation loss, and low zinc diffusion efficiency.

[0016] The process includes the following steps. Zinc plating: A zinc plating layer is formed on the surface of the aluminum alloy substrate, and the mass fraction of zinc in the composite zinc layer is not less than 50 wt%. Zinc diffusion heat treatment: Place the aluminum alloy substrate that has been zinc coated above into a heat treatment furnace, seal it and introduce protective gas, raise the temperature inside the furnace to 580℃-620℃, and hold it at this temperature for 15-50 minutes; after holding, cool it to room temperature to complete the zinc diffusion heat treatment. Diffusion heat treatment: The aluminum alloy substrate after zinc diffusion heat treatment is transferred into a heat treatment furnace and subjected to heat diffusion treatment in the temperature range of 420℃-580℃. After the heat diffusion treatment is completed, it is cooled to room temperature in the furnace or by air to obtain the surface zinc-dipped aluminum alloy material.

[0017] As a further preferred embodiment, the reverse gradient zinc diffusion process on the surface of the aluminum alloy material described in this application embodiment employs a two-stage heat treatment, the specific steps of which include... Main diffusion section: The temperature in the heat treatment furnace is raised to 500℃-580℃, and the holding time at this temperature is 30-100min; Homogenization section: The temperature of the heat treatment furnace is reduced to 420℃-500℃, and the temperature is maintained at this temperature for 30-100 minutes.

[0018] As a further preferred embodiment, the reverse gradient zinc diffusion process on the surface of the aluminum alloy material described in this application embodiment employs a three-stage heat treatment, specifically including the following steps: Pre-diffusion section: The temperature inside the heat treatment furnace is raised to 420℃-480℃ at a heating rate of 3-5℃ / min, and held at this temperature for 50-100min; Main control section: Increase the heat treatment temperature to 520℃-580℃ at a heating rate of 5-10℃ / min, and hold at this temperature for 60-150min; Homogenization and finishing stage: The temperature of the heat treatment furnace is reduced to 450℃-500℃ at a cooling rate of 3-5℃ / min, and held at this temperature for 60-150min.

[0019] As a further preferred embodiment, in the reverse gradient zinc diffusion process on the surface of aluminum alloy materials described in this application embodiment, the pre-diffusion section, the main control section, and the homogenization and finishing section are continuously completed in the same heat treatment furnace, and the volume fraction of oxygen in the heat treatment furnace is ≤0.1%.

[0020] As a further preferred embodiment, in the reverse gradient zinc diffusion process on the surface of aluminum alloy materials described in this application embodiment, in the zinc coating treatment step, the zinc raw material is pure zinc or a zinc alloy with a zinc content of not less than 50wt%; a zinc coating layer is formed on the aluminum alloy substrate by one of hot-dip galvanizing, vacuum sputtering, electroplating, or cold rolling composite; the thickness of the zinc coating layer is 2-10μm.

[0021] As a further preferred embodiment, in the reverse gradient zinc diffusion process on the surface of aluminum alloy materials described in this application, the protective gas is one or a mixture of two or more of argon, nitrogen, and helium.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The surface zinc-diffused aluminum alloy material described in this application has a gradient distribution that gradually decreases from the surface of the zinc-diffused layer to the substrate, and the reverse gradient depth is ≤20μm. This avoids the problem of preferential transverse corrosion in the high zinc zone of the subsurface of traditional reverse gradient zinc-diffused materials and the overall peeling off of the surface layer before it can play a protective role. This structure can preferentially cause uniform corrosion to provide sacrificial anode protection for the substrate, and the corrosion rate gradually slows down as the zinc content decreases towards the interior of the substrate, so as to achieve full control of corrosion behavior and structurally eliminate the risk of premature corrosion perforation failure of aluminum alloy components.

[0023] 2. In the zinc-diffused aluminum alloy material described in the embodiments of this application, the main phase of the zinc-diffused layer is an α-Al(Zn) substitution solid solution, with zinc atoms uniformly dissolved in the aluminum matrix lattice. There is no large amount of brittle zinc-aluminum intermetallic compound or independent zinc phase precipitation, nor is there a physical interface of mechanical coating. The zinc-diffused layer forms an atomic-level metallurgical bond with the aluminum alloy matrix, and the bonding strength is much higher than that of traditional zinc spraying and hot-dip galvanizing coatings.

[0024] Meanwhile, the thermal expansion coefficient and plastic deformation capacity of the solid solution structure are highly matched with those of the matrix, making it suitable for subsequent forming processes such as bending and expanding heat exchange components. It will not cause defects such as peeling, microcracks, or detachment of the zinc-infiltrated layer, and will also avoid intergranular corrosion and intergranular cracking caused by zinc-rich phases at grain boundaries, thus greatly improving the service stability of the material.

[0025] 3. In the zinc-diffused aluminum alloy material described in this application embodiment, the total depth of the zinc-diffused layer is ≤400μm and the reverse gradient depth is ≤20μm. The controllable thickness of the zinc-diffused layer ensures sufficient effective protective thickness, providing long-term stable corrosion protection for the material, without reducing the effective load-bearing wall thickness of the aluminum alloy substrate due to excessive zinc-diffused layer thickness. At the same time, the preparation process corresponding to this structure does not require long-term ultra-high temperature heat treatment, and will not cause abnormal grain growth in the aluminum alloy substrate. Finally, the core mechanical properties of the material, such as tensile strength, yield strength, and elongation, are not significantly different from those of the original aluminum alloy substrate, perfectly meeting the core usage requirements of heat exchange equipment for the material's pressure-bearing capacity and forming performance.

[0026] 4. The reverse gradient zinc diffusion process on the surface of aluminum alloy materials described in the application embodiment precisely controls the distribution of zinc elements through graded diffusion heat treatment, and stably controls the reverse gradient depth of the zinc diffusion layer to ≤20μm. This fundamentally eliminates the phenomenon of the zinc diffusion layer peeling off before it can play a protective role. The resulting material shows no peeling corrosion after 1000h cyclic acidic seawater test (SWAAT), with a maximum corrosion depth of ≤30μm, which greatly extends the service life of aluminum alloy components.

[0027] The graded diffusion heat treatment does not require an inert protective atmosphere and can be completed in an ordinary heat treatment furnace under normal air pressure without the need for special high-end equipment. At the same time, it is compatible with a variety of conventional zinc coating processes such as hot-dip galvanizing, electroplating, and vacuum sputtering. The graded process parameters can be flexibly adjusted to adapt to large-scale continuous production, which greatly reduces equipment investment and production energy consumption costs.

[0028] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 Metallographic images of the alloy cross-sections before and after rapid zinc infiltration.

[0031] Figure 2 The zinc diffusion curves are cross-sectional views of Examples 1-3 and Comparative Examples 1-3.

[0032] Figure 3 The surface corrosion morphology of Comparative Example 1 after 1000 hours of salt spray corrosion with SWAAT is shown.

[0033] Figure 4 For comparison of zinc penetration curves of cross sections before and after 1000h of salt spray corrosion (SWAAT) in Comparative Example 1.

[0034] Figure 5 The surface corrosion morphology after 1000 hours of salt spray corrosion (SWAAT) in Example 1 is shown.

[0035] Figure 6 This is a comparison of the zinc penetration curves of the cross section before and after 1000h of salt spray corrosion (SWAAT) in Example 1. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of this application, but not all embodiments.

[0037] Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0038] The term "comprising" and other equivalent descriptive terms used in the specification and claims of this application are intended to cover a non-exclusive inclusion, which includes both the contents explicitly described in the specification and claims and steps or units that are not described in the specification and claims but are inherent in the product, method or structure.

[0039] This application provides a zinc-diffused aluminum alloy material, comprising an aluminum alloy substrate and a zinc-diffused layer covering the surface of the aluminum alloy substrate. The aluminum substrate can be selected from 3-series or 6-series aluminum alloys, preferably 3-series aluminum alloys, such as 3003, 3004, 3102, 3005, etc., or 5052, 6061, etc.

[0040] The zinc content in the zinc-diffused layer described in this application embodiment exhibits a gradient distribution with a continuous and gradually decreasing trend from the surface of the zinc-diffused layer towards the aluminum alloy substrate. This continuously decreasing gradient distribution creates a gradient of potential that steadily increases from negative to positive from the surface to the substrate. The surface layer has the highest zinc content and the most negative potential, thus preferentially undergoing corrosion. It acts as a sacrificial anode to continuously protect the internal substrate, without any localized abrupt changes in potential, completely avoiding the micro-area galvanic corrosion caused by sudden changes in zinc content in traditional processes. At the same time, it ensures that corrosion starts from the surface and progresses uniformly and gradually inward, without forming transverse corrosion channels, thus eliminating the occurrence of exfoliation corrosion from the structural root. In addition, the continuous and gradually decreasing distribution of zinc content ensures a continuous and stable transition of the coefficient of thermal expansion of the zinc-diffused layer from the surface to the substrate and the aluminum alloy substrate. There is no difference in the coefficient of thermal expansion caused by sudden changes in composition. Under heat exchange conditions with alternating temperatures, there will be no thermal stress concentration, eliminating defects such as microcracks, peeling, and interface delamination in the zinc-diffused layer, ensuring the integrity of the bond during long-term service.

[0041] In the embodiments of this application, the main phase of the zinc-infiltrating layer is an α-Al(Zn) substitution solid solution. The single-phase structure formed by zinc atoms replacing aluminum atoms in the α-Al phase lattice of the aluminum alloy matrix is ​​completely matched with the crystal structure of the main phase of the matrix, with no obvious phase interface and physical delamination. This achieves atomic-level metallurgical bonding between the zinc-infiltrating layer and the matrix, avoiding the risks of zinc layer peeling, slag shedding, and overall detachment. Combined with the control of zinc content gradient, the highest mass fraction of zinc in the zinc-infiltrating layer does not exceed 5wt%, which is far below the room temperature solid solubility limit of zinc in aluminum. Almost all zinc atoms are dissolved in the aluminum lattice to form a single-phase solid solution, with no brittle phase precipitation, thus eliminating local corrosion susceptibility sources and forming defects from a structural perspective.

[0042] The α-Al(Zn) solid solution has a high degree of plasticity and formability matching with the matrix. There is no work hardening or decrease in plasticity caused by the hard and brittle phase. The zinc-infiltrated material can still smoothly complete subsequent forming processes such as pipe bending, pipe expansion, and stamping. At the same time, the mechanical properties of the matrix will not be affected by the modification of the zinc-infiltrated layer.

[0043] In this embodiment, the total depth of the zinc infiltration layer is ≤400μm, and the reverse gradient depth is ≤20μm. Controlling the total depth of the zinc infiltration layer to within 400μm can prevent excessive zinc infiltration into the matrix, which would lead to a reduction in the effective load-bearing wall thickness of the component, thus ensuring the pressure-bearing capacity, deformation resistance, and structural safety of the heat exchange tube. In this embodiment, the reverse gradient depth is controlled to ≤20μm, which means that the zinc content on the surface of the material is high enough to give priority to the sacrificial anode protection function. This can avoid the problem of internal corrosion first and surface peeling before it can play its role in the traditional process. After 1000h of SWAAT testing, no peeling corrosion phenomenon was observed, while the comparative examples with excessive reverse gradient depth all showed severe peeling.

[0044] More preferably, the reverse gradient depth of the zinc-infiltrated layer in the embodiments of this application is 0μm-5μm.

[0045] As a further preferred option, the surface zinc-diffused aluminum alloy material described in this application embodiment has a cross-sectional corrosion depth of ≤30μm after 1000h SWAAT testing; this means that the total corrosion depth does not exceed 300μm and is always within the protection range of the zinc-diffused layer, thus solving the problems of uncontrollable corrosion process and sudden perforation failure of traditional zinc-diffused materials.

[0046] The zinc-aluminum alloy material with surface diffusion described in this application has a tensile strength of 95-98 MPa and a yield strength of 35-38 MPa at room temperature.

[0047] This application also provides a reverse gradient zinc diffusion process for aluminum alloy materials. By controlling and eliminating the reverse gradient distribution of zinc elements, the protection failure caused by the reverse gradient distribution of zinc elements in the zinc diffusion layer is solved, avoiding the process defects of traditional high-temperature zinc diffusion process, such as serious zinc volatilization and oxidation loss, and low zinc diffusion efficiency.

[0048] The process includes the following steps. Zinc plating treatment: A zinc plating layer is formed on the surface of the aluminum alloy substrate, and the mass fraction of zinc in the composite zinc layer is not less than 50wt%. The zinc plating layer with a zinc mass fraction of ≥50wt% ensures a sufficient supply of zinc atoms, avoiding the problem of insufficient diffusion layer thickness and insufficient protective performance caused by insufficient zinc source during subsequent high-temperature diffusion. Compared with traditional powder zinc diffusion and paste zinc diffusion, it solves the problem of local insufficient zinc diffusion and large fluctuations in diffusion layer thickness caused by uneven zinc source contact. Zinc diffusion heat treatment: The zinc-coated aluminum alloy substrate is placed in a heat treatment furnace, sealed, and a protective gas is introduced. The furnace temperature is raised to 580℃-620℃ and held at this temperature for 15-50 minutes. After holding, it is cooled to room temperature to complete the zinc diffusion heat treatment. In this step, the high temperature of 580℃-620℃ can increase the diffusion coefficient of zinc atoms in the aluminum substrate. Combined with the holding time of 15-50 minutes, this allows the surface zinc coating to completely disappear and all zinc atoms to penetrate into the substrate, while avoiding prolonged zinc diffusion. High temperatures lead to excessive zinc volatilization and abnormal growth of substrate grains. At high temperatures, zinc and aluminum atoms diffuse into each other, eliminating the physical interface between the coating and the substrate and forming an atomic-level metallurgical bond. This avoids the risk of coating peeling and flaking during subsequent service from the structural root. The sealed protective gas provides a low-oxygen environment, which significantly reduces the oxidation and volatilization loss of zinc at high temperatures. It also keeps the initial reverse gradient depth formed after zinc diffusion stably controlled within a correctable range of 20-40μm, avoiding the irreversible problem of excessively deep reverse gradients in traditional processes that cannot be eliminated later. Diffusion heat treatment: The aluminum alloy substrate after zinc diffusion heat treatment is transferred to a heat treatment furnace and subjected to staged heat treatment. Thermal diffusion treatment is carried out within a temperature range of 420℃-580℃. After the thermal diffusion treatment, the substrate is cooled to room temperature either in the furnace or by air. This yields a zinc-dipped aluminum alloy material. In this step, staged thermal activation is achieved through graded heat treatment. For example, pre-diffusion at medium and low temperatures fills the zinc concentration gap in the surface layer, followed by primary diffusion at medium and high temperatures to reduce the peak zinc concentration in the subsurface layer. Finally, low-temperature homogenization and solidification of the gradient distribution allows the zinc atoms enriched in the subsurface layer to diffuse directionally towards the surface layer, correcting the anti-gradient structure. The temperature range of 420℃-580℃ ensures sufficient diffusion activation energy for effective diffusion of zinc atoms while avoiding excessive zinc volatilization and oxidation at high temperatures, thus preventing the formation of a new anti-gradient. Ultimately, a normal gradient of zinc elements is formed, continuously and gradually decreasing from the surface layer to the substrate, achieving a smooth transition of corrosion potential and avoiding the preferential corrosion and lateral expansion of the high-zinc zone in the middle of traditional materials, which leads to the peeling failure of the diffusion layer.

[0049] In some embodiments, the reverse gradient zinc diffusion process on the surface of the aluminum alloy material employs a two-stage heat treatment, specifically including the following steps: Main diffusion stage: The temperature in the heat treatment furnace is raised to 500℃-580℃ and held at this temperature for 30-100 minutes. In this step, high-temperature diffusion of zinc in the aluminum matrix is ​​achieved, quickly breaking the reverse gradient concentration balance of "low zinc in the surface layer and high zinc in the subsurface layer". This drives zinc atoms in the high zinc region of the subsurface layer to diffuse directionally to the low zinc region of the surface layer. The 30-100 minute holding time can precisely reduce the peak zinc concentration in the subsurface layer, rapidly compressing the initial reverse gradient depth to a controllable range. At the same time, the high-temperature holding time is strictly controlled to avoid the large-scale volatilization or oxidation of surface zinc caused by long holding time in a single-stage high-temperature process, thus preventing the vicious cycle of eliminating the old reverse gradient and forming a new one. Homogenization stage: The temperature of the heat treatment furnace is reduced to 420℃-500℃ and held at this temperature for 30-100 minutes. In this step, at the temperature of 420℃-500℃, the oxidation and volatilization loss of surface zinc in the air environment is almost negligible, while still having sufficient diffusion activation energy. On the one hand, it fills the remaining zinc concentration gap on the surface, completely eliminates the reverse gradient, and finally stabilizes the reverse gradient depth at ≤20μm, with the optimal 0μm complete elimination. On the other hand, the local concentration fluctuations caused by the rapid diffusion in the homogenization main stage allow the zinc element to form a normal gradient that decreases continuously and steadily from the surface to the matrix, without concentration abrupt changes or local zinc enrichment, which meets the gradient design requirements for corrosion resistance protection.

[0050] In some embodiments employing two-stage heat treatment, the resulting continuous zinc gradient achieves a smooth transition of corrosion potential from the surface to the substrate, eliminating the intermediate high-zinc-low-potential zone of traditional anti-gradient materials. This avoids preferential corrosion of the intermediate layer, lateral expansion to form peeling channels, and overall peeling off of the surface layer before corrosion occurs. After 1000 hours of SWAAT testing, the treated material showed no peeling, bulging, cracking, or flaking corrosion.

[0051] The homogenization section eliminates local concentration anomalies such as zinc-rich grain boundaries and zinc-rich dot clusters, avoiding local abnormal corrosion such as pitting corrosion and intergranular corrosion. The corrosion front is flat and continuous, showing a characteristic of uniformly advancing inward from the surface to the substrate. After 1000h SWAAT test, the corrosion depth is stable at ≤30μm, which is far superior to materials treated by single-stage process (corrosion depth 50-70μm), and the service life is increased by more than 1 times.

[0052] In other embodiments, the reverse gradient zinc diffusion process on the surface of the aluminum alloy material employs a three-stage heat treatment, the specific steps of which include... Pre-diffusion stage: The temperature in the heat treatment furnace is raised to 420℃-480℃ at a rate of 3-5℃ / min and held at this temperature for 50-100min. In this step, the low-speed heating ensures uniform temperature of the inner and outer walls of the aluminum alloy billet, preventing uneven diffusion caused by local temperature differences. At the same time, the diffusion activity of zinc atoms is slowly activated to avoid rapid heating causing zinc atoms to preferentially diffuse into the interior of the aluminum alloy matrix rather than migrate directionally to the low-zinc area on the surface. The low temperature range of 420℃-480℃ is the critical activation temperature for zinc atom diffusion. This ensures that zinc atoms have sufficient directional diffusion ability, and because the saturated vapor pressure of zinc is extremely low, the oxidation and / or volatilization loss of surface zinc in the air environment is almost negligible, and there will be no secondary decrease in surface zinc concentration at high temperatures. The holding time of 50-100min allows zinc atoms in the high-zinc area of ​​the subsurface to diffuse stably into the low-zinc area of ​​the surface, completing the pre-filling of the zinc concentration gap on the surface, while preventing excessive diffusion of zinc atoms into the matrix and exceeding the total diffusion depth. Main control section: The heat treatment temperature is increased to 520℃-580℃ at a heating rate of 5-10℃ / min, and held at this temperature for 60-150min. Since the billet temperature uniformity and surface zinc substrate have been completed in the pre-diffusion section, the heating rate of 5-10℃ / min can quickly reach the target temperature, improve production efficiency, and avoid local thermal stress caused by excessive heating. The medium-high temperature range of 520℃-580℃ improves the zinc atom diffusion coefficient, enabling efficient diffusion. At the same time, the temperature is strictly controlled within the upper limit of 580℃ to avoid excessive zinc volatilization and substrate overheating. The holding time of 60-150min is to match the diffusion requirements of anti-gradient elimination, which can both allow zinc atoms in the high zinc region of the subsurface to complete bidirectional diffusion, smooth out the peak zinc concentration in the subsurface, and avoid abnormal growth of substrate grains and loss of surface zinc caused by prolonged high temperature holding. Homogenization final stage: The temperature of the heat treatment furnace is reduced to 450℃-500℃ at a rate of 3-5℃ / min, and held at this temperature for 60-150min. In this step, slow cooling can avoid thermal stress concentration caused by rapid cooling, prevent microcracks in the diffusion layer and deformation of components, and slow temperature reduction can prevent zinc atom diffusion from being frozen instantly, thus preventing zinc segregation at grain boundaries and sudden changes in local concentration. The medium-low temperature range of 450℃-500℃ retains sufficient zinc atom diffusion activity to achieve short-range homogenization diffusion, and because the temperature is far below the high-temperature volatilization threshold of zinc, there is no oxidation or volatilization loss of surface zinc, and no new reverse gradient will appear. Controlling the holding time allows zinc atoms to complete homogenization diffusion from the grain to the grain boundary, eliminating local concentration fluctuations and forming a completely smooth concentration gradient, while further optimizing the interface bonding.

[0053] As a further preferred embodiment, in the reverse gradient zinc diffusion process on the surface of aluminum alloy materials described in this application embodiment, the pre-diffusion section, the main control section, and the homogenization and finishing section are continuously completed in the same heat treatment furnace, and the volume fraction of oxygen in the heat treatment furnace is ≤0.1%.

[0054] In the embodiments of this application, single-stage isothermal heat treatment can also be used. However, experimental studies have shown that two-stage heat treatment and three-stage heat treatment have significant advantages. In particular, the three-stage heat treatment method has lossless filling of the zinc gap in the surface layer in the pre-diffusion stage, efficient smoothing of the concentration peak in the main control stage, and smoothing of the gradient in the homogenization stage. Throughout the process, it ensures that the diffusion flux of zinc atoms to the surface layer is always greater than the zinc loss flux in the surface layer, avoids repeated reverse gradients, and effectively solves the contradiction between diffusion efficiency and surface zinc loss.

[0055] Through three-stage heat treatment, the final zinc element gradient is an ideal gradient that is completely continuous and steadily decreasing.

[0056] As a further preferred embodiment, in the reverse gradient zinc diffusion process for aluminum alloy materials described in this application embodiment, the zinc raw material in the zinc coating treatment step is pure zinc or a zinc alloy with a zinc content of not less than 50 wt%; the thickness of the zinc coating layer is 2-10 μm; pure zinc or a zinc alloy with a zinc content of not less than 50 wt% can form a large zinc concentration difference between the zinc coating layer and the aluminum alloy substrate, providing sufficient diffusion driving force for zinc atoms, ensuring that deep diffusion is completed in a short time, and avoiding the problems of insufficient zinc diffusion, surface zinc layer residue, and substandard diffusion layer thickness due to insufficient concentration difference in low zinc content zinc coating layers; by controlling the zinc coating layer thickness and zinc content to accurately calculate the total zinc supply, on the one hand, it can avoid the problem that excessive zinc source will cause the zinc content of the diffusion layer to exceed the room temperature solid solubility limit of aluminum, precipitating Zn-Al brittle intermetallic compounds, causing diffusion layer cracking, decreased interface bonding, and local preferential corrosion; on the other hand, it can also avoid the problem that insufficient zinc source has to increase the thickness of the zinc coating layer, resulting in uneven diffusion layer thickness, large fluctuations in zinc distribution, and inability to achieve precise quantity control.

[0057] In the embodiments of this application, a zinc coating layer is formed on an aluminum alloy substrate using one of the following methods: hot-dip galvanizing, vacuum sputtering, electroplating, or cold rolling composite.

[0058] As a further preferred embodiment, in the reverse gradient zinc diffusion process on the surface of aluminum alloy materials described in this application, the protective gas is one or a mixture of two or more of argon, nitrogen, and helium.

[0059] Example 1 This embodiment provides a zinc-plated aluminum alloy material, which is processed using the following process: Substrate pretreatment: The 3003 aluminum alloy sheet is degreased by alkaline washing, descaled by acid washing, and dried by water washing; Zinc plating treatment: Hot-dip galvanizing to prepare a pure zinc plating layer with a thickness of 4μm and a zinc element mass fraction of 99.99%; Zinc-dipped heat treatment: After zinc coating, the workpiece is placed in an atmosphere furnace, sealed and protected by high-purity argon gas, and heated to 600℃ at 8℃ / min and held for 30min; then cooled to room temperature with the furnace to obtain a zinc-dipped billet with an initial reverse gradient depth of 25μm. Diffusion heat treatment: A two-stage heat treatment process is used in an air environment without a protective atmosphere. Main diffusion section: Heat to 550℃ at 5℃ / min and hold for 45min; Homogenization section: The furnace is cooled to 480℃ at a rate of 3℃ / min and held for 45min. After the heat preservation is completed, the material is cooled to room temperature in the furnace to obtain a zinc-plated aluminum alloy material.

[0060] Example 2 This embodiment provides a zinc-plated aluminum alloy material, which is processed using the following process: Substrate pretreatment: The 3003 aluminum alloy sheet is degreased by alkaline washing, descaled by acid washing, and dried by water washing; Zinc plating treatment: Electroplating is used to prepare a pure zinc plating layer with a thickness of 5 μm and a zinc element mass fraction of 99.99%; Zinc diffusion heat treatment: After zinc coating, the workpiece is placed in an atmosphere furnace, sealed and protected by high-purity nitrogen gas, and heated to 610℃ at 8℃ / min and held for 40min; then cooled to room temperature with the furnace to obtain a deep reverse gradient zinc diffusion billet with an initial reverse gradient depth of 37μm. Diffusion heat treatment: A three-stage heat treatment process is employed in an air environment without a protective atmosphere. Pre-diffusion section: Heat to 450℃ at a rate of 4℃ / min and hold for 80min; Main control section: The furnace is heated to 560℃ at a rate of 8℃ / min and held for 120min; Homogenization and finishing stage: The furnace is cooled to 480℃ at a rate of 4℃ / min and held for 100min. After the heat preservation is completed, the material is cooled to room temperature in the furnace to obtain a zinc-plated aluminum alloy material.

[0061] Example 3 This embodiment provides a zinc-plated aluminum alloy material, which is processed using the following process: Substrate pretreatment: The 3003 aluminum alloy sheet is degreased by alkaline washing, descaled by acid washing, and dried by water washing; Zinc plating treatment: A zinc-aluminum alloy zinc-plated layer with a thickness of 5 μm was prepared by cold rolling and composite processing. The alloy composition was Zn-30Al, and the zinc element mass fraction was 70 wt%. Zinc diffusion heat treatment: After zinc coating, the workpiece is placed in an atmosphere furnace, sealed and purged with a mixture of argon and helium protective gas, and heated to 590℃ at 8℃ / min, and held for 25min; then cooled to room temperature with the furnace to obtain a zinc diffusion billet with an initial reverse gradient depth of 22μm. Diffusion heat treatment: A two-stage heat treatment process is used in an air environment without a protective atmosphere. Main diffusion section: Heat to 530℃ at 5℃ / min and hold for 60min; Homogenization section: The furnace is cooled to 450℃ at a rate of 3℃ / min and held for 60min. After the heat preservation is completed, the material is cooled to room temperature in the furnace to obtain a zinc-plated aluminum alloy material.

[0062] Example 4 A zinc-infiltrated aluminum alloy material is provided, and the process is as follows: the substrate pretreatment, zinc coating treatment, and zinc infiltration heat treatment are completely consistent with those in Example 1; a single-stage constant temperature process is adopted, the billet is transferred into a conventional heat treatment furnace, heated from room temperature to 550°C at a heating rate of 5°C / min, held at 550°C for 90min, and cooled to room temperature with the furnace to obtain the surface zinc-infiltrated aluminum alloy material.

[0063] Comparative Example 1 A zinc-infiltrated aluminum alloy material is provided. After a 2μm thick pure zinc layer is laminated on the surface of 3003 aluminum alloy in the same manner as in Example 1, the material is directly subjected to rapid zinc infiltration heat treatment at 600℃ for 30 minutes under argon protection to obtain a zinc-infiltrated aluminum alloy material with obvious reverse gradient distribution on the surface and a reverse gradient depth of 25μm.

[0064] Comparative Example 2 A zinc-infiltrated aluminum alloy material is provided. After a 4μm thick pure zinc layer is laminated onto the surface of 3003 aluminum alloy using the same method as in Example 1, the material is subjected to rapid zinc infiltration heat treatment at 600℃ for 3 minutes under argon protection. Subsequently, the temperature is increased to 590℃ at a rate of 5℃ / min in an unprotected atmosphere and held at this temperature for 60 minutes to obtain a zinc-infiltrated aluminum alloy material with a clear reverse gradient distribution on the surface and a reverse gradient depth of 30μm.

[0065] Comparative Example 3 A zinc-infiltrated aluminum alloy material is provided. After a 4μm thick pure zinc layer is laminated onto the surface of 3003 aluminum alloy in the same manner as in Example 1, the material is subjected to rapid zinc infiltration heat treatment at 620℃ for 30 min under argon protection. Subsequently, it is subjected to heat treatment at 400℃ for 30 min in an unprotected atmosphere. The resulting zinc-infiltrated aluminum alloy material has a clear reverse gradient distribution on the surface, with a reverse gradient depth of 37μm.

[0066] Comparative Example 4 A zinc-infiltrated aluminum alloy material is provided, and the process is as follows: the substrate pretreatment is the same as in Example 1; the zinc coating treatment is hot-dip galvanizing to prepare a zinc-aluminum alloy zinc coating layer with a thickness of 5μm, the alloy composition is Zn-70Al, and the zinc element mass fraction is 30wt%; the zinc infiltration heat treatment and diffusion heat treatment are completely consistent with Example 1, and a zinc-infiltrated aluminum alloy material with an anti-gradient depth of 18μm is obtained.

[0067] Performance test results The performance of the zinc-plated aluminum alloy material obtained in the examples was tested, wherein... 1. The specific testing of zinc element distribution, reverse gradient depth, and highest mass fraction of zinc element is as follows: After the sample is mounted, polished, and etched, the elemental plane is scanned from the surface to the matrix along the material cross section using a scanning electron microscope-energy dispersive spectroscopy (SEM-EDS). 2. The total depth of the zinc infiltration layer was determined by metallographic method and cross-sectional microscopic observation. The equipment used was a metallographic microscope (OM), and the standard was GB / T32420. 3. Microstructure, phase structure, grain boundary and interface features were observed using scanning electron microscopy (SEM); 4. Corrosion resistance includes corrosion morphology, corrosion depth, and whether exfoliation corrosion occurs. A cyclic acidic seawater spray corrosion test is conducted according to standard ASTM G85-A3, with a test duration of 1000 hours. 5. The room temperature mechanical properties were tested using a room temperature uniaxial tensile test. The testing equipment was an electronic universal testing machine, and the standard was GB / T228.1—2021 "Metallic materials - Tensile testing - Room temperature test method".

[0068] The test results are shown in Table 1 and Figures 1-6 .

[0069] Table 1: Figure 1 The images show metallographic photographs of the alloy cross-sections before and after rapid zinc diffusion heat treatment. The left side shows the image before rapid zinc diffusion heat treatment, where an independent and continuous pure zinc layer is visible on the surface of the aluminum alloy. The interface between the pure zinc layer and the 3003 aluminum alloy substrate is clearly defined, with no signs of element diffusion. The substrate is a pure 3003 aluminum alloy structure. The right side shows the image after rapid zinc diffusion heat treatment, where the surface pure zinc layer completely disappears. Zinc atoms diffuse into the aluminum alloy substrate lattice at high temperature, forming a zinc-diffused aluminum alloy layer of 3003 aluminum alloy. The zinc-diffused layer and the substrate are metallurgically bonded, with no obvious gaps at the interface, achieving a structural transformation from a surface coating to a substrate diffusion layer.

[0070] Figure 2 The process curves for rapid zinc diffusion heat treatment and diffusion heat treatment are shown in the cross-sectional zinc diffusion curves of Examples 1-4 and Comparative Examples 1-3. Examples 1 and 2 show no obvious reverse gradient, and the zinc element shows a normal gradient distribution with a continuous and stable decrease from the surface layer to the interior. Examples 3 and 3 show only a slight reverse gradient. Comparative Examples 1, 2, and 3 all show obvious reverse gradient distribution, with the zinc element first rapidly increasing to a peak value from the surface layer to the interior, and then rapidly decreasing.

[0071] Figure 3 To show the surface corrosion morphology of Comparative Example 1 after 1000 hours of salt spray corrosion with SWAAT, the photograph shows severe exfoliation corrosion on the sample surface, with large areas of peeling, bulging and dense pitting. The corrosion products are in blocky form, and the surface morphology is uneven, which directly reflects the characteristics of zinc penetration layer erosion failure caused by reverse gradient distribution.

[0072] Figure 4 The figure shows a comparison of the cross-sectional zinc penetration curves before and after 1000 hours of salt spray corrosion with SWAAT in Comparative Example 1. The figure shows that the reverse gradient distribution caused non-uniform exfoliation corrosion of the sample, which completely destroyed the gradient distribution of zinc and lost the protective effect of the sacrificial anode.

[0073] Figure 5The image shows the surface corrosion morphology after SWAAT salt spray corrosion for 1000 hours in Example 1. The sample surface in the image shows no obvious peeling, blistering, bulging, or pitting. It only shows a uniform corrosion morphology. The corrosion products are uniformly and densely attached to the material surface. There are no localized peeling or product detachment areas. This directly demonstrates the excellent corrosion resistance of the material after the process of this invention and effectively avoids peeling corrosion failure.

[0074] Figure 6 For the comparison of cross-sectional zinc penetration curves before and after 1000h of salt spray corrosion with SWAAT in Example 1, the uncorroded curve showed no reverse gradient distribution, and the zinc element showed a normal gradient that decreased continuously and steadily from the surface to the interior. The curve after corrosion still maintained the complete gradient distribution characteristics, with only the zinc content on the surface slightly decreasing due to uniform corrosion. The zinc element continued to decrease steadily with the increase of zinc penetration depth, without any sudden drop in content or disordered distribution, and the corrosion front was flat.

[0075] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A surface zinc-aluminide alloy material, characterized by, It includes an aluminum alloy substrate and a zinc-infiltrating layer covering the surface of the aluminum alloy substrate. The zinc content in the zinc-infiltrating layer is distributed in a gradient that shows a continuous and gradually decreasing trend from the surface of the zinc-infiltrating layer towards the aluminum alloy substrate. The main phase of the zinc-infiltrating layer is an α-Al(Zn) substitution solid solution. The total depth of the zinc-infiltrating layer is ≤400μm, and the reverse gradient depth is ≤20μm.

2. The surface zinc-aluminized alloy material according to claim 1, characterized by, The mass fraction of zinc in the zinc-impregnated layer does not exceed 5 wt%.

3. The zinc-plated aluminum alloy material according to claim 2, characterized in that, The reverse gradient depth of the zinc-infiltrated layer is 0μm-5μm.

4. The surface-zinc-diffused aluminum alloy material according to any one of claims 1-3, characterized in that, The cross-sectional corrosion depth after 1000h SWAAT test is ≤30μm; the tensile strength at room temperature is 95-98MPa, and the yield strength is 35-38MPa.

5. A reverse gradient zinc diffusion process for the surface of aluminum alloy materials, characterized in that, Includes the following steps, Zinc plating: A zinc plating layer is formed on the surface of the aluminum alloy substrate, and the mass fraction of zinc in the composite zinc layer is not less than 50 wt%. Zinc diffusion heat treatment: Place the aluminum alloy substrate that has been zinc coated above into a heat treatment furnace, seal it and introduce protective gas, raise the temperature inside the furnace to 580℃-620℃, and hold it at this temperature for 15-50 minutes; after holding, cool it to room temperature to complete the zinc diffusion heat treatment. Diffusion heat treatment: The aluminum alloy substrate after zinc diffusion heat treatment is transferred into a heat treatment furnace and subjected to heat diffusion treatment in the temperature range of 420℃-580℃. After the heat diffusion treatment is completed, it is cooled to room temperature in the furnace or by air. A zinc-plated aluminum alloy material was obtained.

6. The reverse gradient zinc diffusion process for aluminum alloy materials according to claim 5, characterized in that, A two-stage heat treatment is adopted, and the specific steps include: Main diffusion section: The temperature in the heat treatment furnace is raised to 500℃-580℃, and the holding time at this temperature is 30-100min; Homogenization section: The temperature of the heat treatment furnace is reduced to 420℃-500℃, and the temperature is maintained at this temperature for 30-100 minutes.

7. The reverse gradient zinc diffusion process for aluminum alloy materials according to claim 5, characterized in that, A three-stage heat treatment process is employed, and the specific steps include: Pre-diffusion section: The temperature inside the heat treatment furnace is raised to 420℃-480℃ at a heating rate of 3-5℃ / min, and held at this temperature for 50-100min; Main control section: Increase the heat treatment temperature to 520℃-580℃ at a heating rate of 5-10℃ / min, and hold at this temperature for 60-150min; Homogenization and finishing stage: The temperature of the heat treatment furnace is reduced to 450℃-500℃ at a cooling rate of 3-5℃ / min, and held at this temperature for 60-150min.

8. The reverse gradient zinc diffusion process for aluminum alloy materials according to any one of claims 7, characterized in that, The pre-diffusion section, main control section, and homogenization and finishing section are completed continuously in the same heat treatment furnace, and the volume fraction of oxygen in the heat treatment furnace is ≤0.1%.

9. The reverse gradient zinc diffusion process for aluminum alloy materials according to claims 5-8, characterized in that, In the zinc coating process, the zinc raw material is pure zinc or a zinc alloy with a zinc content of not less than 50 wt%; a zinc coating layer is formed on the aluminum alloy substrate by one of the following methods: hot-dip galvanizing, vacuum sputtering, electroplating, or cold rolling; the thickness of the zinc coating layer is 2-10 μm.

10. The reverse gradient zinc diffusion process for aluminum alloy materials according to any one of claims 5-8, characterized in that, The protective gas is one or a mixture of two or more of argon, nitrogen, and helium.

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