Surface treatment method for improving sensitization resistance of high-magnesium 5000 series aluminum alloy plate

Abstract

A surface treatment method for improving sensitization resistance of a high-magnesium 5000-series aluminum alloy plate comprises the following steps: 1) in the 5000-series aluminum alloy plate, the content of Mg is 5.0-6.0 wt%; (2) the 5000-series aluminum alloy plate is placed in a low-temperature environment, the temperature of the 5000-series aluminum alloy plate is reduced to-100 DEG C to-50 DEG C from the room temperature, and the temperature reduction speed is 30-150 DEG C / min; (3) small-reduction-rate finishing is conducted on the surface of the 5000-series aluminum alloy plate, the reduction rate is 0.1-1.5%, and the plate finishing temperature ranges from-50 DEG C to-3 DEG C; (4) the plate coiling temperature ranges from-40 DEG C to 0 DEG C, and the temperature rise speed ranges from 30 DEG C / min to 150 DEG C / min; and (5) after coiling, low-temperature pre-precipitation treatment is conducted, the low-temperature pre-precipitation temperature ranges from-5 DEG C to 5 DEG C, and heat preservation is conducted for 240 min to 720 min. The surface resistance of the obtained 5000-series aluminum alloy plate is smaller than 8 microohms, and after simulation aging is conducted for 1000 hours at the temperature of 70-130 DEG C, the intergranular corrosion resistance rate of the 5000-series aluminum alloy plate is still equivalent to the intergranular corrosion resistance rate of an untreated plate and is smaller than or equal to 45 mg / cm < 2 >. After being subjected to surface treatment, the surface treatment method is suitable for being in long-term contact with new energy battery aluminum packages or hybrid power and fuel oil vehicle chassis components or structural components in service scenes higher than the room temperature, and the components are arranged in areas of other heat sources of an engine, an exhaust device or a motor vehicle.

Description

TECHNICAL FIELD

[0001] The present application relates to the surface treatment technology of aluminum alloy material, and in particular to a surface treatment method for improving the sensitization resistance of high-magnesium 5000 series aluminum alloy plate. BACKGROUND

[0002] 5000 series aluminum alloy has high strength, high elongation, weldability and high corrosion resistance, and is widely used in automotive coverings and battery package structural parts in harsh marine environments and atmospheric corrosion environments. The trend of automobile lightweighting requires that material design must consider higher strength and other mechanical properties on the basis of continuous weight reduction.

[0003] Aluminum alloy mainly realizes high strength through solid solution strengthening of alloying element Mg. According to the Al-Mg binary phase diagram, the solid solubility of Mg in Al decreases rapidly with decreasing temperature, and the solubility of Mg is 3% to 6% under the condition of rapid cooling of semi-continuous casting, so Mg is usually in a supersaturated state in 5000 series aluminum alloy. The atomic radius of magnesium (0.145 nm) is larger than that of aluminum (0.118 nm), so it can form strong solid solution strengthening. When the magnesium content is 1% to 6%, the solid solution strengthening effect increases with the increase of magnesium content. Therefore, the mechanical properties increase by 30 MPa for every 1% increase in magnesium content. Therefore, weight reduction and strength improvement are the inevitable trend of future aluminum alloy development, and the bottlenecks encountered are as follows:

[0004] 1. In the medium and low temperature service temperature range (50-200℃), this temperature usually represents the influence of environmental heat source, internal combustion engine, battery heat source; also represents the local processing condition of the heat affected zone of welding. In this temperature range, Mg atoms are often precipitated from the supersaturated solid solution in the form of β phase. However, due to the difficulty in nucleation of β precipitated phase, the core is small, the size of the precipitated phase is easy to increase with the service time, and it is distributed at the grain boundary position, which leads to a sharp increase in the sensitivity of the material to intergranular corrosion and stress corrosion. Relative to the anode of α-Al matrix, preferential electrochemical corrosion occurs in the corrosion environment, becoming one of the key local corrosion sources of 5000 series aluminum alloy, which has more harm than benefit on the mechanical properties of the material.

[0005] 2. Under the trend of green environmental protection, the use of recycled materials is inevitable. Various alloys continuously accumulate in the production process and cannot be eliminated, which breaks the existing potential system and becomes the source of "dirty" alloy, posing new challenges to the corrosion resistance mechanism and research and development of future 5000 series aluminum alloy. At present, the international leading aluminum company Hydro has produced "zero carbon" aluminum materials with 100% recycled materials, which are temporarily used in relatively simple service environments. High magnesium leads to more magnesium enrichment on the surface of the plate, bringing more severe corrosion challenges.

[0006] US2018 / 0112297A1 Hydronaluminiun discloses a hot forming process, strain hardening and reverse annealing, temperature range 190-250℃, at least 30 minutes. ICG mass loss <15mg / cm after pre-sensitization treatment 2 , to meet certain corrosion requirements. But this process requires higher atmosphere protection, otherwise it will damage the structure of the oxidation film, which is not conducive to coating compatibility. The case also involves the consideration of grain size. The process reverses the sensitization behavior through reverse annealing, but it does not really solve the problem of the material itself.

[0007] Chinese patent CN107787376A Hydronaluminiun Steel discloses an unage hardening aluminum alloy plate, 3.6%≤Mg≤6%, Si≤0.4%, Fe≤0.5%, Cu≤0.15%, 0.1%≤Mn≤0.4%, Cr<0.05%, Zn≤0.20%, Ti≤0.20%, the rest is aluminum and unavoidable impurities, the maximum single impurity is 0.05%, and the total amount is not more than 0.15%. Through high recovery annealing at 220℃ to 240℃, the stability of the component in the case of possible thermal load during operation is ensured.

[0008] Chinese patent CN202111655831.9 Southwestern Aluminum discloses a method for improving the intergranular corrosion of 5083H116 sheet, which uses a gas cushion type continuous heat treatment furnace instead of a box type furnace for stabilization treatment, solving the problems of unstable intergranular corrosion, poor surface quality, low production efficiency and high comprehensive cost. Currently, continuous annealing is a common production process for high-surface-performance aluminum plates, but it cannot solve the essential problem of β phase precipitation.

[0009] German patent DE10231437A1 proposes adding a small amount of Zr(<0.25%) to achieve control of intergranular corrosion, which significantly reduces the susceptibility to intergranular corrosion ASTM G67 test value even after thermal sensitization through specific alloy composition, but introduces a new alloy component Zr, which poses a new challenge to waste recycling.

[0010] Chinese patent CN 104797727A discloses a 5000 series aluminum alloy with intergranular corrosion resistance, only with a slight tendency of intergranular corrosion, while providing high strength and good deformability and containing standard alloying components, thus simplifying the recycling of the aluminum alloy. The patent is achieved by an alloy having the following composition: 2.91% < Mg < 4.5%; the alloying components Zn, Cr, Cu and Mn satisfy the relationship: (2.3 x %Zn + 1.25 x %Cr + 0.65 x %Cu + 0.05 x %Mn) + 2.4 > %Mg. The design principle lies in the introduction of a new τ phase, which competes for the survival space of β phase, avoiding the source of intergranular corrosion. The competing ability of different alloying components for Mg is modified by a compensation factor. However, the aluminum alloy still allows zirconium with a content of up to 0.25 wt.%, which is considered to be detrimental to the recycling of the aluminum alloy.

[0011] European patent EP2888382A1 discloses a high-strength intergranular corrosion-resistant 5000 series aluminum alloy with Mg content > 4 wt.%. The first teaching aluminum alloy strip according to the invention has a recrystallized structure, and the microstructure satisfies the relationship between the Mg content and the microstructure: Gs > 22a + 2 * c_mg.

[0012] Chinese patent 201811330504.4 discloses a pulse current treatment process to improve the intergranular corrosion resistance of 5xxx aluminum alloy. The process directly treats components that are about to fail due to corrosion or have already failed due to corrosion in engineering applications, without disassembly and in-situ treatment.

[0013] The above disclosed patents and research papers mainly focus on the micro-alloy compensation of the composition system, the reverse treatment of sensitization, or the development of new coatings for the 5000 series intergranular corrosion resistance. The introduction of new element types by micro-alloying has little research on the toxic negative effect in the recycling stage. The optimization of heat treatment process mainly focuses on the improvement of grain size, but the control range is limited by the requirement range of mechanical properties. The new coating sets up a barrier layer for the external environment, without improving the problem itself. SUMMARY

[0014] The purpose of the present invention is to provide a surface treatment method for improving the sensitization resistance of high-magnesium 5000 series aluminum alloy sheet. The obtained 5000 series aluminum alloy sheet still has a intergranular corrosion rate comparable to that of untreated sheet after 1000 hours of simulated aging at 70-130℃: ≤45mg / cm 2 ; The present invention does not add micro-alloying elements, and does not produce or increase the risk of poisoning in subsequent recycling procedures. After surface treatment according to the present invention, the material is suitable for new energy battery aluminum packaging, high-strength automotive interior panels and other materials that are in contact with higher than room temperature service scenarios for a long time.

[0015] To achieve the above purpose, the technical scheme of the present invention is:

[0016] A surface treatment method for improving the sensitization resistance of high-magnesium 5000 series aluminum alloy plate, comprising the following steps:

[0017] 1) The Mg content in the 5000 series aluminum alloy plate satisfies 5.0-6.0wt%;

[0018] 2) The 5000 series aluminum alloy plate is placed in a low-temperature environment, so that the 5000 series aluminum alloy plate is cooled from room temperature to -100℃ to -50℃, and the temperature drop speed is 30-150℃ / min;

[0019] 6) The surface of the 5000 series aluminum alloy plate is finished with a small reduction ratio, the reduction ratio is 0.1-1.5%, and the finishing temperature is -50℃ to -3℃;

[0020] 3) The plate coiling temperature is -40℃ to 0℃, and the temperature rise speed is 30-150℃ / min;

[0021] 4) After the plate is coiled, pre-precipitation low-temperature pre-precipitation treatment is carried out, the low-temperature pre-precipitation temperature is -5℃ to 5℃, and the holding time is 240-720min.

[0022] Further, the thickness of the 5000 series aluminum alloy plate is ≤4mm.

[0023] Preferably, the low-temperature environment is cooled by heat convection, semiconductor, thermoelectric or superconducting methods.

[0024] Preferably, the low-temperature environment is cooled by liquid nitrogen or liquid helium intermedium gasification.

[0025] The surface resistance of the aluminum alloy plate is <8μΩ; after 1000 hours of simulated aging at 70-130℃, the intergranular corrosion resistance of the aluminum alloy plate is ≤45mg / cm 2 .

[0026] In the surface treatment method described in the application:

[0027] The plate is subjected to low-temperature treatment, and the plate is cooled from room temperature to low temperature. According to the Mg content of the aluminum alloy material, the thickness of the plate, and the mechanical properties, appropriate cooling process parameters are selected, and the strip temperature is reduced to -100℃ to 0℃ through continuous low-temperature treatment by heat conduction, and the temperature drop speed is 30-150℃ / min. Most of the solid solution Mg atoms in the plate have low migration rate and are almost frozen in the supersaturated position.

[0028] Through the low-temperature surface small reduction finishing material, the movable dislocation and vacancy of the strip surface can proliferate, low-energy grain boundaries are formed, high-energy grain boundaries are de-structured, and the channel conditions for non-continuous beta phase precipitation in the subsequent service process are formed. The surface morphology and roughness meet the requirements of 1% to 10% stamping forming lubrication and high surface for subsequent part production process.

[0029] In the pre-precipitation treatment stage, the temperature of the plate is restored to -5 to 5 DEG C, and the temperature rising speed is 30 to 150 DEG C / min. Part of Mg atoms are captured by vacancies in this stage, effectively reducing the total amount of Mg to be precipitated, and reducing the risk of high Mg grain boundary precipitation.

[0030] According to the atomic diffusion and lattice thermodynamics and dynamics theory, the diffusion speed of magnesium atoms in aluminum alloy is a complex process affected by multiple factors, including temperature, crystal structure, alloy composition, grain size and grain boundary, defect density and stress state, etc. At room temperature, if the defect density is improved, such as vacancy, the mobility will be promoted.

[0031] The surface treatment described in the application is divided into three stages:

[0032] The first stage is to perform rapid low-temperature treatment on the high-magnesium plate. At low temperature, the lattice vibration, that is, the migration driving force of Mg atoms in the aluminum alloy, is reduced, so that the Mg atoms are almost 'frozen' in the supersaturated region and do not undergo relative displacement during the deformation process in the second stage, and the diffusion speed is extremely low.

[0033] The second stage is to use small reduction to regulate the dispersion of the shallow grain boundary type of the plate under the condition of 'frozen' Mg atoms at low temperature, to scatter the original grain boundary (greater than or equal to 15 degrees) of the Mg atom string diffusion path, to break the large space network of Mg element aggregation in the service stage, to reduce the possibility of forming large-size continuous beta phase, to proliferate the narrow dispersion diffusion path such as small-angle grain boundary, dislocation, etc., and to form a large number of dislocations and vacancies in the crystal, which become a dispersion trap field for capturing Mg atom precipitation.

[0034] The third stage is the coiling and pre-precipitation of the surface control process and a certain time of pre-precipitation low-temperature pre-precipitation stage. With the rapid increase of the pre-precipitation temperature to the target temperature, the rapid increase of the temperature increases the lattice vibration energy, greatly driving the Mg atoms frozen in the shallow solid solution position, and migrating to the shallow large and small angle grain boundaries, dislocations, vacancies and other defect traps, on the one hand, reducing the total Mg amount of the plate in the shallow solid solution, reducing the risk of aggregation precipitation in the subsequent service aging stage, on the other hand, the pre-precipitated Mg atoms are dispersedly distributed in the shallow surface, not aggregated, not in series, when corrosion begins, the probability of grain boundary corrosion initiation is replaced by dispersed pitting, the network corrosion diffusion speed is greatly reduced, thereby effectively reducing the corrosion rate.

[0035] The design effect of the present application is finally reflected in the temperature zone of the part service stage of 50-200 DEG C. With the passage of time, the total amount of Mg element precipitation does not change, but the spatial distribution position of the dispersion is changed, the amount of intergranular string precipitation is effectively reduced, and the dispersion precipitation is significantly improved. The low-temperature treatment process is carried out at low temperature, effectively resists the thickening phenomenon of the oxide film introduced by temperature and atmosphere when the traditional stabilization treatment is used to improve the corrosion resistance of the high-magnesium 5000 series, and greatly guarantees the compatibility of the subsequent coating, welding, forming and other user backends.

[0036] The above design idea makes the present application seamlessly grafted after the continuous annealing process on the basis of the traditional manufacturing process, and the surface structure of the material is modulated at the minimum cost, the Mg element aging precipitation network is homogenized, dispersed and randomized, the intergranular corrosion resistance of the high-Mg deformed 5000 series alloy plate is greatly improved, and a green solution is provided for the future research and development of 5000 series materials with high proportion of recycled materials. The low-temperature surface treatment unit can be reasonably embedded after the continuous annealing line to realize continuous production. In the continuous low-temperature environment, the intrinsic characteristics of high thermal conductivity of aluminum alloy are fully utilized, the cooling speed is fast, the automation degree is high, and the surface low-temperature treatment of batch sheet materials is stably realized.

[0037] Compared with the prior art, the present application has the following advantages:

[0038] The low-temperature treatment process of the present application mainly utilizes the excellent thermal conductivity of aluminum alloy, the diffusion characteristics of Mg atoms in the aluminum alloy matrix, the growth stagnation of the aluminum surface oxide film under low-temperature environment and other characteristics, effectively reduces the lattice vibration and migration rate of various elements of the aluminum alloy through heat conduction and other cooling means, and realizes the dispersion modulation of the network grain boundary of the beta phase precipitation network easily formed on the surface of the 5000 series aluminum alloy.

[0039] The present application adopts low-temperature surface control treatment, greatly reduces unnecessary medium-high temperature stabilization heat treatment, is beneficial to the control of the thickness of the oxide film, maintains a low-temperature environment during the modulation process, and the oxide film of the plate is not thickened compared with before the treatment, the surface resistance is less than 8 mu omega, which guarantees that the plate after low-temperature treatment does not affect the subsequent coating, welding and forming compatibility.

[0040] The present application can be combined with the air cushion furnace continuous annealing unit for production, is compatible with traditional continuous annealing heat treatment and other traditional rapid continuous process flow, and has high compatibility with traditional equipment and small equipment investment.

[0041] The low-temperature surface control technology of the present application has low carbon emission and no medium in the furnace except air. The continuous impact on the environment is low.

[0042] The low-temperature surface control technology of the present application is suitable for various deformed aluminum alloy materials and has good universality and adaptability.

[0043] The low-temperature surface control technology is suitable for aluminum alloy materials of various thicknesses and has high compatibility with total reduction rate.

[0044] The low-temperature pre-precipitation treatment is newly added, so that energy waste caused by high-temperature stabilization heat treatment and deterioration of surface performance of the plate caused by heat treatment under a high-temperature environment are greatly reduced. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 The figure is a surface structure photo of Example 1 after 500 hours of sensitization.

[0046] Figure 2 The figure is a surface structure photo of Comparative Example 1 after 500 hours of sensitization. DETAILED DESCRIPTION

[0047] The application will be further described below in combination with examples and drawings.

[0048] The examples and comparative examples of the application are shown in Table 1 and Table 2.

[0049] Example 1

[0050] Taking the Mg content of 5.54wt% as an example, the 0.9mm-thick 5000-series deformed aluminum alloy O-state plate after low-temperature surface treatment and pre-precipitation treatment is subjected to -76℃ low-temperature surface treatment, 0.4% small reduction finishing treatment at -31℃ to -21℃, and pre-precipitation treatment at -2℃ for 560 minutes, and the ASTM G67 mass loss of the plate after 130℃ sensitization treatment for 1000h is 24.9mg / cm 2 .

[0051] The corrosion weight loss is slower in the later period, and the control in the early period is particularly important. Therefore, for the aluminum alloy with high Mg content, the surface structure of the example and the comparative example after 500 hours of sensitization is compared.

[0052] Referring to Figure 1 , the figure shows the surface structure of the plate of Example 1 after 500 hours of sensitization at 130℃, from Figure 1 It can be seen that the β phase mainly exists in the form of extremely fine and dispersed distribution, and has not formed obvious grain boundary aggregation.

[0053] In comparison, Comparative Example 1, which has not been subjected to low-temperature surface treatment and pre-precipitation treatment but has the same Mg content and plate thickness as Example 1, has a grain boundary corrosion corrosion rate of 55.6mg / cm 2 , which is 123% higher than that of Example 1. As Figure 2As shown, in the metallographic structure of the sheet metal surface of Comparative Example 1 (500 hours), string-like β-phase segregation was observed to form a severe network structure on the rolled surface. If processed into battery pack parts, it is extremely easy to accelerate corrosion under the combined effects of stress and corrosive media, causing electrolyte leakage, battery system open circuits and other faults, posing a great threat to the safe service of the vehicle body.

[0054] As the comparison shows, the absolute amount of Mg is crucial in traditional processing technology; reducing the amount of Mg to meet corrosion resistance requirements would sacrifice the strength of the plate. This invention, however, argues that the segregation mode of Mg has a greater impact on the final corrosion resistance. By employing low-temperature surface treatment to achieve precise spatial control of Mg atoms, the segregation channels of the risk factor-β phase are specifically modulated and reprocessed, thus breaking through the bottleneck of strength improvement.

[0055] Table 1

[0056]

[0057] Table 2

[0058]

Claims

1. A surface treatment method for improving the susceptibility of high-magnesium 5000-series aluminum alloy sheet, characterized by: The method comprises the following steps: 1) the Mg content of the 5000 series aluminum alloy plate is 5.0-6.0 wt%; 2) the 5000 series aluminum alloy plate is placed in a low-temperature environment, so that the 5000 series aluminum alloy plate is cooled from room temperature to -100℃ to -50℃ at a temperature drop speed of 30-150℃ / min; 3) the surface of the 5000 series aluminum alloy plate is finished with a small reduction ratio, the reduction ratio is 0.1-1.5%, and the finishing temperature is -50℃ to -3℃; 4) the plate coiling temperature is -40℃ to 0℃, and the temperature rise speed is 30-150℃ / min; 5) the plate is subjected to pre-precipitation low-temperature pre-precipitation treatment after coiling, the low-temperature pre-precipitation temperature is -5℃ to 5℃, and the holding time is 240-720 min.

2. The surface treatment method for improving the susceptibility of a high-magnesium 5000-series aluminum alloy sheet according to Claim 1, characterized by, The thickness of the 5000 series aluminum alloy plate is ≤4 mm.

3. The surface treatment method for improving the susceptibility of a high-magnesium 5000-series aluminum alloy sheet according to Claim 1, characterized by, The low-temperature environment is realized by heat convection, semiconductor, thermoelectricity or superconductivity.

4. The surface treatment method for improving the susceptibility of a high-magnesium 5000-series aluminum alloy sheet according to Claim 1, characterized by, The low-temperature environment is realized by liquid nitrogen or liquid helium medium gasification.

5. The surface treatment method for improving the susceptibility of a high-magnesium 5000-series aluminum alloy sheet according to claim 1 or 2, characterized by, The surface resistance of the aluminum alloy plate after surface treatment is < 8 μΩ; and the intergranular corrosion resistance of the aluminum alloy plate is ≤ 45 mg / cm2 after 1000 hours of simulated aging at 70-130 °C 2 .

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