Anti-quenching stress relief heat treatment method

Abstract

The invention relates to an anti-quenching stress relief heat treatment method, belongs to the technical field of non-ferrous metal processing, and solves the problems of non-uniform performance of a core part and a surface layer and large residual stress caused by low hardenability of a wrought aluminum alloy part and large machining deformation caused by large residual stress of an aluminum alloy casting. A quenched part is blow-dried and then immediately put into a deep cooling box, nitrogen serves as a cooling medium, the temperature is slowly reduced to-120 DEG C to-90 DEG C at the speed of 2-5 DEG C / min, heat preservation is conducted according to (2-2.5) delta min, the quenched part is transferred to an air circulation furnace within 2 min, the quenched part is heated to 120-140 DEG C at the speed of 10-15 DEG C / min, air cooling is conducted after heat preservation is conducted according to (1-1.5) delta min, circulation is conducted for 2-3 times, and by precisely regulating and controlling the parameters such as the cooling rate, the heating rate, the temperature, the heat preservation time and the number of times of circulation and the like, the quenching quality of the quenched part is improved. And multi-parameter synergistic effect is achieved, residual stress is remarkably reduced, the performance uniformity and comprehensive performance of parts are improved, and the method is particularly suitable for aluminum alloy parts of complex structures and large sizes.

Description

Technical Field

[0001] The present invention relates to the technical field of non-ferrous metal processing, and particularly to a reverse quenching stress-relieving heat treatment method. Background Art

[0002] Due to its excellent properties such as low density, high strength, and good corrosion resistance, aluminum alloy is widely used in fields such as aerospace, automotive manufacturing, and mechanical equipment, and is a key material for modern industry. Aluminum alloy parts are mainly divided into wrought aluminum alloy parts and aluminum alloy castings, and they each have their own characteristics in manufacturing processes and performance. However, with the development of technology, aluminum alloy parts face higher performance requirements and also expose many problems.

[0003] In the research and development of wrought aluminum alloy parts, as the content of alloying elements increases, although the strength of the aluminum alloy continuously increases, the residual stress also increases accordingly. Currently, the commonly used heat treatment states include Txx51 and Txx52, and these states reduce the residual quenching stress to a certain extent through plastic deformation (such as stretching or compression), thereby ensuring the material performance. However, the increase in alloying elements increases the quenching sensitivity of the aluminum alloy. Especially in thick-walled, large-sized, or complex-structured parts, the cooling rate difference between the core and the surface is significant, resulting in non-uniform tissue performance and affecting the overall performance consistency of the parts. In order to ensure the uniformity and consistency of the performance of the core and the surface of the parts, the method of rough machining first and then heat treatment is usually adopted, but this makes it impossible to perform plastic deformation treatment (such as stretching or compression) during the heat treatment process. In addition, although existing heat treatment technologies (such as local heat treatment) can improve the forming performance of aluminum alloy, they have problems such as complex processes, low efficiency, high energy consumption, and poor surface quality, which limit their application in complex-structured parts.

[0004] In the research and development of aluminum alloy castings, as the strength and size specifications of the castings increase, the residual stress after quenching increases significantly, resulting in more prominent machining deformation problems. Since aluminum alloy castings usually cannot reduce the residual stress through plastic deformation treatment (such as stretching or compression), although existing stress-relieving methods (such as spray quenching, thermal cycling treatment, cryogenic treatment, etc.) have certain effects, it is difficult to achieve efficient and uniform stress relief in high-strength, large-sized castings.

[0005] In summary, there are many deficiencies in the existing technology in solving the problem of residual stress in aluminum alloy parts, and there is an urgent need for a more efficient and more applicable stress-relieving method to meet the requirements of high performance and high precision of aluminum alloy parts in modern industry. Summary of the Invention

[0006] In view of the above analysis, the embodiments of the present invention aim to provide an anti-quenching stress-relieving heat treatment method, which is particularly applicable to deformed aluminum alloy parts and aluminum alloy castings, so as to at least solve one of the following problems in the prior art: (1) The existing heat treatment technology cannot effectively eliminate the residual stress of thick-walled, large-sized or complex-structured parts, resulting in uneven part performance. (2) Cold drawing and cold pressing treatments are not applicable to thick-walled, large-sized or complex-structured parts, restricting their application in aluminum alloy castings. (3) Existing stress-relieving methods (such as spray quenching, thermal cycling treatment, cryogenic treatment, etc.) are difficult to achieve efficient and uniform stress elimination in high-strength, large-sized castings. (4) The existing technology has problems such as complex process, low efficiency, high energy consumption, and poor surface quality, restricting its application in complex-structured parts. To achieve the above object, the present invention adopts the following technical solutions:

[0007] On the one hand, the embodiments of the present invention provide an anti-quenching stress-relieving heat treatment method, and the steps include:

[0008] S1. Part pre-treatment: After drying the quenched parts, put them into a cryogenic box;

[0009] S2. Cryogenic treatment: Pass nitrogen into the cryogenic box for cooling, and the cooling rate is 2°C / min to 5°C / min. After cooling to the cryogenic temperature, keep it warm;

[0010] S3. Heating treatment: Transfer the parts from the cryogenic box to an air circulation furnace for heating. After heating to a certain temperature, keep it warm, and finally take it out and air-cool;

[0011] S4. Circulation treatment: Put the parts into the cryogenic box again, and repeat the operations of S2 and S3.

[0012] Further, the transfer time of the quenched parts to the cryogenic box in S1 is ≤ 4h.

[0013] Further, in S2, the cryogenic temperature is -120°C to -90°C.

[0014] Further, in S2, the holding time is (2 - 2.5)δ min, where δ is the maximum wall thickness of the part in mm.

[0015] Further, in S3, the transfer time does not exceed 2 min.

[0016] Further, in S3, the heating rate is 10°C / min to 15°C / min.

[0017] Further, in S3, heat the temperature to 120°C to 140°C.

[0018] Further, in S3, the heat preservation time is (1 - 1.5)δ min, where δ is the maximum wall thickness of the part in mm.

[0019] Further, in S4, the number of times of repeated operation is 2 - 3 times.

[0020] In the second aspect, the present invention provides an aluminum alloy material obtained by the above-mentioned reverse quenching stress-relieving heat treatment method.

[0021] In the third aspect, the present invention provides a processing method for deformed aluminum alloy parts or aluminum alloy castings, and the processing method includes performing the above-mentioned reverse quenching stress-relieving heat treatment method after solution quenching.

[0022] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects for aluminum alloy parts:

[0024] 1) Broaden the application range to thick-walled, large-sized or complex-structured parts:

[0025] For the elimination of residual stress in thick-walled, large-sized or complex-structured parts, traditional methods still have certain difficulties. The present invention uses nitrogen as the cooling medium and realizes the synergistic effect of multiple parameters by finely regulating the key parameters of cryogenic treatment and heat treatment (such as cooling rate, heating rate, cryogenic temperature, heating temperature, heat preservation time and number of cycles, etc.), significantly eliminating the residual stress in thick-walled, large-sized or complex-structured parts and meeting the quality requirements of high-performance aluminum alloy parts in high-end manufacturing.

[0026] 2) Eliminate stress efficiently without cold drawing or cold pressing:

[0027] Traditional methods for stress relief mainly rely on cold drawing or cold pressing. However, for thick-walled structural parts, large-sized parts or parts with complex shapes, due to the significant difference in properties between the core and the surface layer, it is usually necessary to perform rough machining first and then heat treatment to improve the performance uniformity. But this process flow limits the implementation of subsequent cold drawing or cold pressing for stress relief. Although the existing cryogenic and rapid heating processes can eliminate residual stress to a certain extent, for thick-walled, large-sized or complex-structured parts, the effect is not thorough enough, and other stress relief methods such as cold drawing or cold pressing still need to be combined to ensure the effect. The present invention effectively eliminates residual stress without cold drawing or cold pressing by optimizing the synergistic effect of cryogenic and rapid heating. Further simplifies the process flow, reduces the deformation risk during machining, and thus reduces the production cost and scrap rate.

[0028] 3) Improve the dimensional stability of parts:

[0029] In existing methods that combine cryogenic treatment and heat treatment, most are simple one-time sequential treatments, making it difficult to completely eliminate residual stress and prone to causing deformation of workpieces during subsequent processing or use. The present invention adopts a cyclic treatment process of cryogenic treatment and rapid heating. Through the repeated thermal expansion and contraction effects inside the material, the residual stress is more thoroughly eliminated, solving the problem of uneven stress distribution in one-time treatment. It significantly reduces the deformation risk of workpieces, greatly improves the dimensional stability and machining accuracy of parts, and reduces the rejection rate.

[0030] 4) Improve the comprehensive performance of parts:

[0031] The present invention uses nitrogen as the cooling medium, enhancing safety, achieving precise control of temperature and time during the reverse quenching process. While effectively reducing residual stress, it significantly enhances the strength, toughness, and fatigue resistance of parts. In addition, the present invention combines the reverse quenching stress relief treatment with the aging process. By heating to the aging precipitation temperature, maximizing the temperature difference between hot and cold, and promoting the dispersed precipitation of the second phase, the comprehensive performance of the parts is further strengthened.

[0032] 5) Applicable to large-sized aluminum alloy parts and mass production:

[0033] Existing technologies are often limited by the size of the liquid nitrogen tank and difficult to meet the requirements of large-sized parts and mass production. The present invention uses nitrogen in combination with a cryogenic tank with a flexibly designed size for cryogenic treatment, successfully solving this problem. It is applicable to large-sized parts and mass production, improving production efficiency and flexibility.

[0034] 6) High process flexibility to meet diverse requirements:

[0035] The heat treatment process of the present invention has high adjustability and can flexibly control parameters such as temperature and time according to different aluminum alloy grades and part characteristics to meet the requirements of various complex scenarios. This high adaptability enables the technology to be widely applied in various complex scenarios, enhancing the practicality and application scope of the process.

[0036] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combination schemes. Other features and advantages of the present invention will be described in the subsequent specification. Moreover, some advantages can be made obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be realized and obtained from the content specifically pointed out in the specification and the drawings. Description of the Drawings

[0037] The drawings are only for the purpose of showing specific embodiments and are not considered to be a limitation of the present invention. Throughout the drawings, the same reference signs represent the same components.

[0038] Figure 1Schematic diagram of traditional stress-relieving heat treatment by cold drawing or cold pressing

[0039] Figure 2 Schematic diagram of stress-relieving heat treatment by reverse quenching of the present invention

[0040] Figure 3 Schematic diagram of the cooling device of the present invention

[0041] Reference numerals

[0042] 1 - Solution heating; 2 - Quenching; 3 - Cold drawing or cold pressing at room temperature; 4 - Aging treatment; 5 - Blowing dry; 6 - Stress relief by reverse quenching; 7 - Deep cold box body; 8 - Box door; 9 - Temperature sensor; 10 - Circulation fan; 11 - Intelligent temperature control system; 12 - Nitrogen injection port; 13 - Low-temperature solenoid valve; 14 - Main pipeline; 15 - Gasification tower; 16 - Liquid nitrogen tank Detailed implementation manners

[0043] The preferred embodiments of the present invention will be specifically described below with reference to the accompanying drawings. The accompanying drawings form a part of this application and are used together with the embodiments of the present invention to explain the principles of the present invention, rather than to limit the scope of the present invention

[0044] It should be noted that the present invention mainly targets deformed aluminum alloy parts and aluminum alloy castings, and is particularly suitable for parts with thick walls (wall thickness 30 - 150 mm), large sizes (size 1000 - 5000 mm) and / or complex shapes. These parts usually have large residual stresses or non-uniform stresses due to the manufacturing or processing process, which affect their performance and service life

[0045] On the one hand, the present invention discloses a stress-relieving heat treatment method by reverse quenching, as Figure 2 shown, including the following steps

[0046] S1. Part pre-treatment: After quenching, the parts are blown dry and then put into the deep cold box

[0047] S2. Deep cold treatment: Nitrogen is introduced into the deep cold box for cooling at a cooling rate of 2°C / min - 5°C / min, and after cooling to the deep cold temperature, heat preservation is carried out

[0048] S3. Heating treatment: The parts are transferred from the deep cold box to an air circulation furnace for heating, and after heating to a certain temperature, heat preservation is carried out, and finally they are taken out of the furnace and air-cooled

[0049] S4. Circulation treatment: The parts are put into the deep cold box again, and the operations of S2 and S3 are repeated

[0050] It should be noted that for deformed aluminum alloy parts, the traditional process usually uses cold drawing or cold pressing after quenching to reduce residual stresses. As Figure 1As shown, the parts are subjected to cold drawing or cold pressing at room temperature after quenching. Although this method can partially relieve stress, its operation process is cumbersome, the stress relief effect is limited, and new stress states may be introduced. Therefore, it is mainly applicable to ordinary aluminum alloy parts. For thick-walled, large-sized or complex-structured parts, the traditional method has poor effects and its application is significantly limited. The main reason is that during the heat treatment process, there is a significant temperature gradient between the core and the surface of the parts, resulting in large differences in tissue properties. Although this difference can be reconciled through rough machining and heat treatment, this often limits the application of subsequent cold drawing or cold pressing, making it difficult for the traditional method to meet the quality requirements of high-performance aluminum alloy parts.

[0051] For aluminum alloy castings, with the increase in the strength and size specifications of the castings, the residual stress after quenching increases significantly. Due to the complex shape and large size of aluminum alloy castings, cold drawing or cold pressing is usually not possible to reduce the residual stress. Therefore, in the existing technology, it is difficult to achieve efficient and uniform stress relief when dealing with high-strength, large-sized aluminum alloy castings.

[0052] To solve the above problems, the present invention proposes an innovative reverse quenching stress relief heat treatment method, as Figure 2 shown. The specific steps are as follows:

[0053] After quenching, instead of using the traditional cold drawing or cold pressing treatment, reverse quenching stress relief treatment is immediately carried out. First, the parts are slowly cooled to cryogenic temperature and held for a period of time, and then rapidly heated to the heating temperature and held. This reverse quenching stress relief treatment needs to be cycled multiple times until the residual stress is completely eliminated or a satisfactory effect is achieved. Finally, aging treatment is carried out to further optimize the stress state of the parts and improve their comprehensive mechanical properties.

[0054] By finely regulating the key parameters of cryogenic treatment and rapid heat treatment (such as cooling rate, heating rate, cryogenic temperature, heating temperature, holding time, and number of cycles, etc.), the synergistic effect of multiple parameters is achieved, significantly eliminating the residual stress of thick-walled, large-sized or complex-structured parts. This method does not rely on traditional cold drawing or cold pressing operations, simplifies the process flow, is especially suitable for aluminum alloy castings that cannot be plastically deformed, and at the same time solves the problem that the stress relief effect in cryogenic and rapid heat treatment of thick-walled, large-sized or complex-structured parts is not thorough enough, providing reliable technical support for the manufacture of high-performance aluminum alloy parts.

[0055] Furthermore, in S1, the transfer time of the quenched parts to the cryogenic box ≤ 4h, such as 10min, 20min, 30min, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h.

[0056] The quenched parts should be transferred to the cryogenic box as soon as possible. The shorter the transfer time, the better, generally not exceeding 4 hours. Utilizing the characteristic that the organizational structure of the alloy is not yet fully stable in the quenched soft state, stress relief treatment by reverse quenching is carried out immediately after quenching, which can obtain better stress reduction effect. Compared with under-aged, peak-aged or over-aged states, no aging precipitation strengthening occurs in the quenched state, the strength of the parts is low, and it is easier to reduce the residual stress. Drying the quenched parts can prevent the residue of moisture or impurities, thus ensuring the quality of cryogenic treatment. When drying, dry compressed air or argon is preferably used, and the drying time is generally several minutes to more than ten minutes.

[0057] Furthermore, in S2, before the nitrogen enters the cryogenic box, it needs to be filtered and dried to ensure the purity > 99.9%, so as to prevent moisture and impurities from affecting the treatment effect and the equipment life. Moisture will freeze and affect the cooling uniformity and damage the surface of the parts, and impurities may cause chemical reactions and corrode the equipment.

[0058] During operation, a high-precision filter (such as a filter element with a pore diameter of 0.1μm or smaller) can be installed on the conveying pipeline to filter solid impurity particles. Adsorption drying (filling the drying tower with adsorbents such as molecular sieves and silica gel to adsorb moisture and regenerating after saturation) or freeze drying (refrigerating to condense moisture into ice and then filtering and removing it) is used to remove moisture, and professional instruments such as gas chromatographs and mass spectrometers are used to detect the nitrogen purity in real time. When the purity is insufficient, the filtering and drying equipment is checked and adjusted in time.

[0059] Furthermore, in S2, the present invention uses nitrogen instead of traditional liquid nitrogen as the cooling medium for cryogenic treatment.

[0060] Although the cooling rate of liquid nitrogen is fast, its vaporization is intense and it is difficult to precisely control the temperature. Especially when processing thick-walled or large-sized high-strength aluminum alloy parts and aluminum alloy parts with complex shapes, this rapid cooling method is likely to cause a large thermal stress impact and increase the cracking risk. In addition, the size limitation and high cost of the liquid nitrogen box also limit its application in large-sized parts and mass production.

[0061] In contrast, the present invention cools by the volatilization of liquid nitrogen or the cryogenic box refrigeration system combined with liquid nitrogen. Low-temperature nitrogen is generated by the liquid nitrogen tank and the vaporization tower, and the temperature is controlled at -196°C to -140°C. The liquid nitrogen box is changed to a cryogenic box with a larger volume, which not only avoids the above problems, but also greatly reduces the safety hazards such as asphyxiation or frostbite that may be caused during the operation of liquid nitrogen, providing a more precise, economical cooling solution suitable for large-sized and mass production.

[0062] Further, in S2, the cooling rate is precisely controlled between 2°C / min and 5°C / min, such as 2°C / min, 2.5°C / min, 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, or 5°C / min.

[0063] The slow cryogenic cooling device of the present invention uses the method of introducing nitrogen into the cryogenic box for cryogenic treatment, and can maintain the cooling rate within the above range. Compared with direct cooling with liquid nitrogen, the process of using nitrogen for cooling is more gentle, which helps to keep the temperature gradient inside the part in a reasonable state and avoid thermal stress concentration caused by sudden temperature changes and possible part deformation.

[0064] Specifically, as Figure 3 shown, the cooling device of the present invention mainly consists of the following parts: a liquid nitrogen supply system, a main pipeline 14, a low-temperature solenoid valve 13, a cryogenic box body 7, a box door 8, a temperature sensor 9, a circulation fan 10, and an intelligent temperature control system 11.

[0065] The liquid nitrogen supply system includes a liquid nitrogen tank 16 for storing liquid nitrogen and a vaporization tower 15 for converting liquid nitrogen into low-temperature nitrogen, providing the necessary low-temperature medium for the entire cooling process. The main pipeline 14 transports the low-temperature nitrogen generated by the vaporization tower 15 to the cryogenic box body 7. The main pipeline 14 is connected to the cryogenic box body 7 through three branch pipelines, and the flow rate and flow direction of nitrogen are controlled by low-temperature solenoid valves 13 installed on the three branch pipelines.

[0066] The cryogenic box body 7 is the main working area of the cooling device. It is internally provided with a plurality of nitrogen injection ports 12 to achieve uniform distribution of low-temperature nitrogen in the box. The design of the box door 8 ensures the convenience of operation and maintenance. The cryogenic box body 7 is provided with nitrogen injection ports 12. The nitrogen injection ports 12 inject nitrogen into the cryogenic box body 7 under the control of the low-temperature solenoid valve 13. A circulation fan 10 is installed in the cryogenic box body 7 to promote the air flow in the box, ensuring uniform distribution of nitrogen throughout the box body, thereby improving the cooling efficiency. A temperature sensor 9 is also provided in the cryogenic box body 7 to monitor the temperature inside the cryogenic box body 7 in real time and feed the data back to the intelligent temperature control system 11, which automatically adjusts the opening and closing of the low-temperature solenoid valve 13 according to the preset temperature range, controls the nitrogen flow rate, and realizes precise control of the temperature inside the box.

[0067] In summary, the cooling device of the present invention realizes precise control of the temperature inside the cryogenic box through the cooperation of liquid nitrogen vaporization, pipeline transportation, solenoid valve control, nitrogen injection, and circulation fan, as well as the precise regulation of the intelligent temperature control system. The device has a reasonable structural design and is easy to operate, effectively meeting the requirements of various low-temperature environments. This gentle and precise cooling method can be optimized according to different aluminum alloy grades and part characteristics. By extending the cooling and heat preservation time, the residual stress is fully released, providing reliable support for the processing of complex-shaped and large-sized parts.

[0068] Further, in S2, a cryogenic box is used instead of the traditional liquid nitrogen box.

[0069] Although the traditional liquid nitrogen box can provide extremely low temperatures, its size is usually limited and it is difficult to meet the processing requirements of large-sized alloy parts. The present invention uses a cryogenic box instead of a liquid nitrogen box, which can achieve a larger processing space. For example, the size of existing cryogenic boxes on the market can reach 3000 mm in length, 2000 mm in width, and 2000 mm in height, or even larger. In addition, the modular combined cryogenic box can be flexibly adjusted according to customer needs. It can either operate independently with small boxes or be combined into large boxes to meet the processing requirements of large parts or large quantities of parts. This design not only improves the flexibility and adaptability of the equipment, but also reduces the liquid nitrogen consumption cost by optimizing the refrigeration system and thermal insulation materials.

[0070] Further, in S2, the cryogenic temperature is -120°C to -90°C, such as -120°C, -115°C, -110°C, -105°C, -100°C, -95°C, -90°C. Usually, when the wall thickness of the part is greater than 100 mm, the cryogenic temperature is -120°C.

[0071] The cryogenic temperature range of -120°C to -90°C avoids the safety hazard of liquid nitrogen at -196°C. Moreover, within this range, significant changes occur in the internal microstructure of the alloy. The cryogenic treatment promotes the dispersion precipitation of the second-phase particles, increasing their quantity and making their distribution more uniform. The size of the precipitated phase is smaller and finer, which can improve the strength and hardness of the material, fully release the internal stress of the part, and reduce the residual stress. This not only ensures the safety of the treatment but also meets the process requirements, providing guarantee for the performance optimization of alloy parts. When the wall thickness of the part exceeds 100 mm, a lower temperature can significantly shrink the part, balance the stress difference, release the stress, and promote the uniform transformation of the structure, addressing the problems of large and complex residual stress distribution and uneven structure.

[0072] Further, in S2, the heat preservation time of the cryogenic treatment is (2 - 2.5)δ min, where δ is the maximum wall thickness of the part, in mm.

[0073] This calculation method of the heat preservation time fully considers the heat transfer characteristics of parts with different wall thicknesses, especially thick-walled structural parts. Since the larger the wall thickness, the slower the heat dissipation and the more complex the residual stress distribution inside the part, it is difficult to meet the requirements of parts with different wall thicknesses by using a fixed heat preservation time, which may lead to non-uniform tissue properties or even an increase in residual stress. By determining the heat preservation duration based on the maximum wall thickness, it can accurately adapt to the heat conduction rhythm of the part, ensuring that both the core and the surface of the part can fully respond to cryogenic treatment at an appropriate duration, thus making the tissue properties tend to be uniform and minimizing the high residual stress caused by wall thickness differences. This not only helps to improve the static mechanical properties of the part but also significantly enhances its fatigue life.

[0074] Further, in S3, the transfer time does not exceed 2 min, such as 0.5 min, 0.8 min, 1 min, 1.3 min, 1.5 min, 1.8 min, 2 min.

[0075] After cryogenic treatment, the microstructure inside the part is in a relatively stable but fragile state. If exposed to the external environment for too long at this time, it may cause the surface temperature of the part to rise, resulting in a change in the temperature gradient and generating new thermal stress. Rapid transfer to an air circulation furnace for heating can ensure that the part quickly heats up in a short time, avoiding stress redistribution or accumulation caused by too slow temperature change.

[0076] Further, in S3, the heating rate is 10°C / min to 15°C / min, such as 10°C / min, 11°C / min, 12°C / min, 13°C / min, 14°C / min, 15°C / min.

[0077] Compared with the ordinary heating rate (3°C / min to 4°C / min), the present invention adopts a higher heating rate. The purpose is to quickly increase the temperature of the part in a short time, cause the part to expand rapidly in volume, thereby generating a large reverse thermal stress, effectively offsetting the residual stress inside the part. In addition, rapid heating can also reduce the residence time of the part at high temperature, avoiding the degradation of material properties due to long-term high-temperature exposure. At the same time, rapid heating helps to form a temperature gradient inside the part, promoting the release of residual stress and avoiding the situation where the residual stress cannot be fully offset due to too slow heating, thereby optimizing the mechanical properties of the part.

[0078] However, for large parts, due to their large volume and mass, the power and heat transfer efficiency of the heating equipment limit the heating rate and it is impossible to achieve a rapid heating rate exceeding 15°C / min. Therefore, the heating rate of 10°C / min to 15°C / min adopted in the present invention takes into account both the performance optimization brought by rapid heating and the limitations of equipment capabilities and part sizes in practical applications, and has high practicality and economy.

[0079] In one embodiment, an air circulation furnace is used for heating. By means of high-power strong convection air, the air circulation is accelerated to achieve rapid and uniform heating of large components. This air circulation furnace has a relatively large internal space and strong load capacity, and is suitable for heat treatment of large or batch parts.

[0080] The specific operation steps are as follows: First, set the heating temperature according to the alloy grade and heat treatment state; after the temperature in the furnace reaches the set value, quickly transfer the parts from the deep-freezing box to the air circulation furnace, heat them to the set temperature and hold for a certain time; finally, take the parts out of the furnace and air-cool them to room temperature.

[0081] To compensate for the possible temperature loss during the transfer process, before quickly transferring the parts, the air circulation furnace can be preheated to 1.0 - 1.2 times the target temperature, and the furnace temperature is set 20 - 30 °C higher than the final heating temperature to compensate for the temperature drop when opening the furnace door. The air circulation furnace is equipped with an intelligent temperature control system, which can monitor and accurately adjust the temperature in real time, so that the temperature fluctuation is controlled within ±3 °C, ensuring the stability and consistency of the heat treatment process.

[0082] Furthermore, in S3, the heating temperature is 120 °C - 140 °C, such as 120 °C, 122 °C, 125 °C, 128 °C, 130 °C, 132 °C, 135 °C, 138 °C, 140 °C. Within this temperature range, the residual stress of the aluminum alloy can be effectively released, and at the same time, the strength and performance of the material are maintained. If the temperature exceeds 140 °C, although the residual stress and deformation will be further reduced, the material strength will be significantly decreased.

[0083] Furthermore, in S3, the holding time is (1 - 1.5)δ min, where δ is the maximum wall thickness of the part, and the unit is mm.

[0084] This design of the holding time is based on the heat treatment empirical formula, ensuring that it is proportional to the wall thickness of the part and tending to be as short as possible, so as to quickly carry out stress relief treatment in the soft state of alloy quenching, thereby effectively releasing the residual stress. This design can not only promote the uniform distribution of temperature inside the part and the transformation of the microstructure, but also avoid the performance degradation caused by too long holding time, ensuring the reliability and stability of the part in subsequent processing and use, especially suitable for large parts with a wall thickness exceeding 150 mm.

[0085] In one embodiment: In the S3 stage, the specific steps of the furnace discharging and air cooling are as follows: First, by precisely controlling the cooling rate of the air circulation furnace, the in-furnace cooling is carried out at a set rate (for example, 1 - 2 °C / min). When the temperature in the furnace drops below 80 °C, the parts are taken out of the furnace for air cooling. This optimized cooling method can effectively avoid the deformation and stress concentration problems caused by excessive temperature gradient during the rapid cooling of the parts, while ensuring the stability of the microstructure of the parts during the cooling process, and further improving the performance uniformity and consistency of the parts.

[0086] Further, the reverse quenching stress relief treatment 6 can be combined with the aging treatment 4, such as Figure 2 shown, setting the holding temperature of the last heat treatment as the aging temperature, which can be selected according to the characteristics of different aluminum alloys, such as 120 °C, 160 °C or 180 °C, etc., and extending the holding time to the aging holding time. This combination method realizes the efficient integration of the reverse quenching stress relief treatment and the aging process. By heating to the aging precipitation temperature, maximizing the temperature difference between hot and cold, and promoting the dispersion precipitation of the second phase, the comprehensive performance of the parts is further enhanced.

[0087] Further, in S4, the number of repeated treatments is 2 - 3 times.

[0088] This design is mainly optimized for the characteristics of thick-walled or large-sized high-strength aluminum alloy structural parts. Due to the large wall thickness and complex internal structure of such parts, the residual stress is usually large. It is difficult to fully release the residual stress only through one cryogenic treatment and heat treatment, and it is also impossible to effectively solve the problem of part performance uniformity.

[0089] In the traditional method of combining cryogenic treatment and heat treatment, most are simple one-time sequential treatments, and most are rapid cooling with liquid nitrogen. Although the time is short and the cooling rate is fast, due to the large temperature gradient, it may cause internal thermal stress concentration in the parts, and even lead to problems such as material cracking, especially not suitable for thick-walled or large-sized parts that require multiple cycle treatments.

[0090] The present invention adopts slow nitrogen cryogenic treatment. By precisely controlling the cooling rate, it avoids the risk of material cracking caused by rapid cooling, and at the same time is more suitable for multiple cycle stress relief treatments, which can ensure that the residual stress of thick-walled or large-sized high-strength aluminum alloy structural parts is fully released, thereby significantly improving the performance uniformity and stability of the parts.

[0091] Under normal circumstances, if the maximum wall thickness of the part does not exceed 100 mm, after 2 cycles of operation, it can basically meet the requirements of subsequent processing or use. However, for parts with extremely complex shapes or significant wall thickness differences, the number of cycles can be increased as appropriate according to the actual situation, but generally not exceeding 4 times. For example, if the maximum wall thickness of the part is greater than 100 mm, to ensure that the residual stress is fully eliminated and the part performance reaches a uniform effect, the number of cycles is usually set to 3 times.

[0092] Furthermore, after each cycle of treatment, the part can be subjected to residual stress detection. If it is detected that the residual stress is 10% higher than the target value, then in the next cycle, the process parameters can be adjusted by increasing the holding time of cryogenic treatment by 10%-20% and increasing the heating rate of heat treatment by 1 °C / min - 2 °C / min to enhance the stress elimination effect.

[0093] Furthermore, after all the cycle treatments are completed, a comprehensive performance test can be carried out on the part, and the test contents cover multiple key indicators such as residual stress, hardness, strength, and toughness. If the test results show that the part performance does not meet the standard, the parameters such as cryogenic temperature, holding time, and heating rate can be adjusted specifically according to the specific performance deviation. For example, if the residual stress is still high, the cryogenic temperature can be appropriately reduced or the holding time can be extended; through continuous adjustment and treatment, until the part performance fully meets the standard, it can enter the next process or be put into use after leaving the factory.

[0094] In the second aspect, the present invention proposes an aluminum alloy material, which is obtained by treating with the above-mentioned reverse quenching stress relief heat treatment method.

[0095] In the third aspect, the present invention proposes a method for treating a wrought aluminum alloy part or an aluminum alloy casting, which includes performing the above-mentioned reverse quenching stress relief heat treatment after solution quenching.

[0096] In the prior art, residual stress often exists in wrought aluminum alloy parts or aluminum alloy castings after solution quenching, which will cause problems such as deformation or non-uniform performance of the parts during subsequent processing or use. Traditional methods such as cold drawing, cold pressing, or simple cryogenic treatment have limited effects on eliminating residual stress, especially for thick-walled, large-sized, or complex-structured parts, it is difficult to completely solve the problem.

[0097] The present invention provides a method for treating a wrought aluminum alloy part or an aluminum alloy casting, and the specific steps are as follows:

[0098] Step 1: Solution quenching

[0099] Perform solution quenching treatment on the wrought aluminum alloy part or the aluminum alloy casting, and the specific parameters are as follows:

[0100] Solution temperature: It is determined according to the specific composition of the aluminum alloy material. For wrought aluminum alloy parts, it is usually in the range of 468°C to 475°C, and for aluminum alloy castings, it is usually in the range of 520 to 540°C.

[0101] Insulation time: It is determined according to the part size and wall thickness, generally (3 - 4)δmin, where δ is the maximum wall thickness of the part, and the unit is mm.

[0102] Quenching medium: Circulating room temperature water or warm water at 30°C to 60°C is used as the quenching medium.

[0103] Step 2: Reverse quenching stress relief heat treatment

[0104] The reverse quenching stress relief heat treatment method described in the present invention is used for treatment, and the specific steps are as follows:

[0105] Part pre - treatment: After drying the solution - quenched parts, put them into a cryogenic box, and the transfer time should be controlled within 4 hours (≤4h).

[0106] Cryogenic treatment: Nitrogen is introduced into the cryogenic box for cooling, the cooling rate is 2°C / min to 5°C / min, the cryogenic temperature is - 120°C to - 90°C, and the insulation time is (2 - 2.5)δmin.

[0107] Heating treatment: Transfer the parts from the cryogenic box to an air - circulation furnace for heating, the heating rate is 10°C / min to 15°C / min, the heating temperature is up to 120°C to 140°C, the insulation time is (1 - 1.5)δmin, and finally take them out of the furnace and air - cool.

[0108] Circulation treatment: Put the parts into the cryogenic box again, repeat the above operations of cryogenic treatment and heating treatment, and the number of cycles is 2 to 3 times.

[0109] Compared with the prior art, a reverse quenching stress relief heat treatment method provided in this embodiment uses nitrogen as the cooling medium and strictly controls key parameters such as the cooling rate, heating rate, cryogenic temperature, heating temperature, insulation time, and number of cycles, realizing the synergistic effect of multiple parameters. By cryogenic treatment, the internal microstructure of the parts is stabilized, and then rapid heating is used to cause the parts to expand in volume and generate reverse stress, thereby offsetting the residual stress. After circulation treatment, the residual stress is further reduced. This method can significantly reduce the residual stress, improve the uniformity of the part performance, and enhance its comprehensive performance, especially suitable for thick - wall, large - size or complex - structure parts. For example, in Example 1, the average equivalent stress of the parts is reduced from 118.2 MPa to 38.6 MPa, and the effect is remarkable.

[0110] The present invention will be further described below with embodiments in conjunction with the specification, but the described embodiments are only for the present invention and do not limit the present invention.

[0111] Example 1

[0112] Deformed aluminum alloy part: a 7A99 ultra-high-strength aluminum alloy cabin section of metal material, with an outer contour of Φ350mm and multiple steps on the inner wall. It is a representative of complex structural parts. Due to its complex structure and uneven wall thickness (the maximum wall thickness is 45mm, and the thinnest wall thickness is 30mm), cold drawing or cold pressing cannot be carried out during the heat treatment process.

[0113] In view of the poor hardenability of the 7A99 alloy, direct solution quenching may lead to uneven quenching, which in turn affects the performance consistency between the core and the surface of the part. Therefore, before solution quenching, the cabin section is first rough-machined to reduce the wall thickness of the workpiece to an appropriate thickness to improve the hardenability. After the thinning treatment, the workpiece is then solution quenched, effectively solving the problem of uneven performance between the core and the surface caused by insufficient hardenability. Finally, the residual stress is removed by reverse quenching stress relief heat treatment to ensure the overall performance uniformity and stability of the part.

[0114] The reverse quenching stress relief heat treatment method, the specific steps are as follows:

[0115] S1. Part pre-treatment: The 7A99 ultra-high-strength aluminum alloy cabin section after solution heating and room temperature water quenching is dried with dry compressed air to ensure that there is no moisture residue on the surface, and then immediately and steadily placed into a cryogenic box.

[0116] S2. Cryogenic treatment: Inject dry and filtered nitrogen into the cryogenic box body 7. Control the nitrogen flow rate and injection time through the intelligent temperature control system 11 and the low-temperature solenoid valve 13 on the main pipeline 14. Use the temperature sensor 9 to monitor the temperature inside the box and regulate the cooling rate at 5°C / min. The cooling continues until the temperature inside the box reaches -90°C. During the process, the circulation fan 10 operates to ensure the uniform distribution of nitrogen. After reaching -90°C, start 100 minutes of heat preservation, and the intelligent temperature control system 11 adjusts the solenoid valve 13 according to the feedback of the sensor 9 to maintain the temperature stability.

[0117] S3. Heating treatment: After the heat preservation is completed, transfer the part to an air circulation furnace preheated to 100°C within 1 minute. The air circulation furnace uses strong convection air for heating, and the heating rate is set at 12°C / min. When the temperature inside the furnace reaches 120°C, maintain the heating and heat preservation state for 55 minutes. After the heat preservation is completed, take the part out of the furnace and perform natural air cooling.

[0118] S4 Cycle treatment: In order to further enhance the stress relief effect, put the part into the cryogenic box again and repeat the operations in steps S2 and S3. The entire cryogenic and reverse quenching heat treatment process needs to be repeated 3 times to ensure that the residual stress inside the cabin section is eliminated to the greatest extent.

[0119] After the anti-quenching stress relief heat treatment is completed, the T74 aging treatment is then carried out, and subsequent processing or use of the parts can be carried out.

[0120] After the treatment, it is detected that the average equivalent stress of the cabin section is significantly reduced, and the deformation amount of the finish machining of the outer contour is controlled within a very small range, meeting the usage requirements of high-performance structural parts.

[0121] Example 2

[0122] Deformed aluminum alloy part: a 7A99 ultra-high-strength aluminum alloy cabin section of metal material. The difference from Example 1 is only that during the S2 cryogenic treatment, the cryogenic box cools down at a slow cooling rate of 2°C / min.

[0123] Example 3

[0124] Deformed aluminum alloy part: a 7A99 ultra-high-strength aluminum alloy cabin section of metal material. The difference from Example 1 is only that during the S3 heat treatment, the heating rate is set to 10°C / min.

[0125] Example 4

[0126] Deformed aluminum alloy part: a 7A85 ultra-high-strength aluminum alloy box of metal material. The difference from Example 1 is that the material is a 7A85 ultra-high-strength aluminum alloy box, with a length of 1000 mm, a width of 800 mm, a height of 400 mm, and there are multiple steps on the inner wall. Cold drawing or cold pressing treatment cannot be carried out during the heat treatment process. First, rough machining of the box is carried out, and the maximum wall thickness is 150 mm, which is representative of thick-walled structural parts. First, heat the 7A85 alloy to the solution temperature and hold for a period of time to ensure that the solution elements in the alloy are fully dissolved. Subsequently, quenching is carried out.

[0127] After that, anti-quenching stress relief heat treatment is carried out, and the steps are as follows:

[0128] S1. Part pre-treatment: After drying the quenched 7A85 ultra-high-strength aluminum alloy part with argon, put it into the cryogenic box.

[0129] S2. Cryogenic treatment: Cool down to -120°C at a rate of 3°C / min, and the holding time is 300 min.

[0130] S3. Heat treatment: Transfer to a preheated air circulation furnace within 2 min and heat to 130°C at a rate of 10°C / min, and the holding time is 225 min.

[0131] S4. Cycle treatment: Cycle 3 times.

[0132] After the treatment, the residual stress of the part is significantly reduced, and the performance uniformity is improved, proving that the method of the present invention also has good treatment effects on large-size aluminum alloy parts.

[0133] Example 5

[0134] Aluminum alloy casting: The metal material is ZL205A aluminum alloy crossbeam casting. The maximum length of the crossbeam is 2500 mm, which is representative of large-sized parts. The maximum wall thickness is 22 mm, and the thinnest wall thickness is 15 mm. After solution heating and water quenching at 50 °C, reverse quenching stress relief heat treatment is carried out. The specific steps are as follows:

[0135] S1. Part pre-treatment: Blow dry the quenched ZL205A aluminum alloy crossbeam casting parts to ensure that there is no residual moisture on the surface, and then place them steadily into the cryogenic box for cryogenic treatment.

[0136] S2. Cryogenic treatment: Introduce nitrogen as the cooling medium into the cryogenic box. The cryogenic box cools down at a slow cooling rate of 4 °C / min until the set cryogenic temperature of -120 °C is reached. At this temperature, maintain the heat preservation state for 55 min to ensure that the residual stress inside the crossbeam casting is initially released.

[0137] S3. Heating treatment: After the heat preservation is completed, quickly take out the crossbeam casting from the cryogenic box and transfer it to the air circulation furnace within 2 min. The air circulation furnace uses strong convection air for heating up, and the heating rate is set at 15 °C / min. When the temperature in the furnace reaches 140 °C, maintain the heating and heat preservation state for 30 min to further eliminate the residual stress inside the crossbeam casting. After the heat preservation is completed, take out the crossbeam casting from the furnace and perform natural air cooling.

[0138] S4. In order to more comprehensively eliminate the residual stress inside the crossbeam casting and improve its strength and stability, put the crossbeam casting into the cryogenic box again, and repeat the operations of S2 and S3 steps. The entire cryogenic and reverse quenching heat treatment process needs to be repeated 2 times to ensure that the crossbeam casting reaches the best heat treatment effect.

[0139] After the reverse quenching stress relief heat treatment is completed, then perform T5 aging treatment, and then the parts can be processed or used subsequently.

[0140] After treatment, the residual stress of the crossbeam casting is significantly reduced, the strength and stability are improved, and the actual use requirements are met.

[0141] Comparative Example 1

[0142] The difference from Example 1 is only that the reverse quenching stress relief heat treatment part is missing, and the residual stress of the parts is not significantly eliminated, and the performance is poor.

[0143] Comparative Example 2

[0144] The difference from Example 1 is only that liquid nitrogen quenching is used.

[0145] Comparative Example 3

[0146] The difference from Example 1 is only that in S2, the cryogenic box is cooled at a rate of 10 °C / min.

[0147] Comparative Example 4

[0148] The difference from Example 1 is only that in S2, the set cryogenic temperature is -60 °C.

[0149] Comparative Example 5

[0150] The difference from Example 1 is only that in S3, the heating rate is set to 5 °C / min.

[0151] Comparative Example 6

[0152] The difference from Example 1 is only that in S3, when the temperature in the furnace reaches 100 °C, the heating and heat preservation state is maintained.

[0153] Comparative Example 7

[0154] The difference from Example 1 is only that in S4, the cryogenic and reverse quenching heat treatment processes are only repeated 1 time.

[0155] Comparative Example 8

[0156] The difference from Example 1 is only that in S2, the heat preservation state is maintained for 45 min.

[0157] Comparative Example 9

[0158] The difference from Example 1 is only that in S1, the time for transferring to the cryogenic box after air drying is 5.2 h.

[0159] Comparative Example 10

[0160] The difference from Example 5 is only that the reverse quenching stress relief heat treatment part is missing.

[0161] Comparative Example 11

[0162] The difference from Example 1 is only that in S2, the set cryogenic temperature is -150 °C.

[0163] Comparative Example 12

[0164] The difference from Example 1 is only that in S2, the heat preservation state is maintained for 150 min.

[0165] Performance Characterization

[0166] Table 1 shows that the average equivalent stress and the finishing deformation of the outer contour of the cabin section in Examples 1 to 5 are both at a relatively low level. For example, for the wrought aluminum alloy material 7A99, the average equivalent stress < 40 MPa, and the finishing deformation of the outer contour of the cabin section is -0.05 to 0.05 mm; for the wrought aluminum alloy material 7A85, the average equivalent stress < 47 MPa, and the finishing deformation of the outer contour of the cabin section is -0.05 to 0.08; for the aluminum alloy casting material ZL205A, the average equivalent stress < 23 MPa, and the finishing deformation of the outer contour of the cabin section is -0.05 to 0.15.

[0167] Compared with Comparative Examples 1 to 9, Example 1 has significantly reduced average equivalent stress and finishing deformation of the outer contour, more uniform stress distribution, and higher machining accuracy, indicating that the present invention has a significant stress elimination effect on wrought aluminum alloy parts and complex structural parts. The results of Example 5 and Comparative Example 10 show the same trend, indicating that the present invention has a significant stress elimination effect on aluminum alloy castings and large-sized parts.

[0168] The results of Comparative Examples 11 to 12 are similar to those of Example 1, indicating that when the cryogenic temperature drops to -120 °C, the stress elimination effect is close to saturation. Although further reducing the temperature has a slight improvement, it will prolong the cooling time and reduce the economy; similarly, prolonging the holding time can also reduce the residual stress, but the effect will saturate, prolong the production cycle, and reduce the economy. This shows that the parameter settings of the present invention are scientific and reasonable, and can effectively balance the relationship between the stress elimination effect, economy, and production efficiency.

[0169] In summary, the present invention shows significant advantages in eliminating the residual stress of wrought aluminum alloy parts and aluminum alloy castings, and is particularly suitable for thick-walled, large-sized, and complex structural parts. By optimizing the cryogenic treatment and holding time, while ensuring the stress elimination effect, the economy and production efficiency are also taken into account, providing an efficient and reliable solution for the manufacture of high-performance aluminum alloy parts.

[0170] Table 1

[0171] Category Average equivalent stress / MPa Finish machining deformation of the outer contour of the cabin section / mm Example 1 38.6 -0.04～0.05 Example 2 38.2 -0.03～0.05 Example 3 39.6 -0.05～0.05 Example 4 46.5 -0.05～0.08 Example 5 22.7 -0.05～0.15 Comparative Example 1 118.2 -0.12～0.20 Comparative Example 2 45.2 -0.06～0.07 Comparative Example 3 56.6 -0.07～0.10 Comparative Example 4 52.2 -0.08～0.09 Comparative Example 5 48.2 -0.07～0.07 Comparative Example 6 49.6 -0.07～0.08 Comparative Example 7 51.3 -0.08～0.08 Comparative Example 8 46.3 -0.07～0.07 Comparative Example 9 39.2 -0.05～0.05 Comparative Example 10 49.0 -0.20～0.25 Comparative Example 11 38.3 -0.04～0.05 Comparative Example 12 38.4 -0.04～0.05

[0172] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for heat treatment of reverse quenching and stress relief, characterized in that the steps include: S1. Parts pretreatment: Blow dry the quenched parts and put them into a deep freezer; S2. Cryogenic treatment: nitrogen is introduced into the cryogenic box to cool down the temperature at a rate of 2°C / min to 5°C / min, and the temperature is kept warm after cooling down to the cryogenic temperature; S3. Heating treatment: transfer the parts from the deep freezer to an air circulation furnace for heating, keep them warm after they reach a certain temperature, and finally take them out of the furnace for air cooling; S4. Cycle processing: put the parts into the deep freezer again and repeat the operations of S2 and S3.

2. The heat treatment method according to claim 1, characterized in that In S1, the transfer time of the quenched parts to the deep freezing box is ≤4h.

3. The heat treatment method according to claim 3, characterized in that: In S2, the deep cooling temperature is -120°C to -90°C.

4. The heat treatment method according to claim 4, characterized in that: In S2, the holding time is (2-2.5) δmin, where δ is the maximum wall thickness of the part, in mm.

5. The heat treatment method according to claim 1, characterized in that: In S3, the transfer time does not exceed 2 minutes.

6. The heat treatment method according to claim 6, characterized in that: In S3, the heating rate is 10°C / min to 15°C / min.

7. The heat treatment method according to claim 7, characterized in that: In S3, the heating temperature is raised to 120°C to 140°C.

8. The heat treatment method according to claim 8, characterized in that: In S3, the holding time is (1-1.5) δmin, where δ is the maximum wall thickness of the part, in mm.

9. An aluminum alloy material, characterized in that: The aluminum alloy material is obtained by using the reverse quenching and stress relief heat treatment method described in any one of claims 1 to 8.

10. A method for processing a deformed aluminum alloy part or an aluminum alloy casting, characterized in that: The treatment method comprises performing the reverse quenching stress relief heat treatment method according to any one of claims 1 to 8 after solution quenching.

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