A method for pre-deforming high-strength aluminum alloy under low load and limited thermal expansion

Abstract

The application discloses a high-strength aluminum alloy low-load limited thermal expansion pre-deformation method, which comprises the following steps: material preparation and state freezing, die preparation and initial compression, low-temperature pre-compression, limited thermal expansion auxiliary pre-deformation, unloading and subsequent aging treatment, and performance and residual stress detection. The high-strength aluminum alloy material after solid solution quenching is first kept at low temperature, then is subjected to small compression amount initial pre-deformation, and is heated under the condition that the displacement of the die under compression or the gap between the dies is unchanged, so that the material is subjected to thermal expansion in a limited state, thereby obtaining additional compression deformation and stress relaxation effect, and finally realizing low-load pre-deformation, residual stress reduction and improvement of strength and plasticity after aging. The application combines low-load initial compression and limited thermal expansion auxiliary pre-deformation, improves the effective pre-deformation effect under the condition that the active compression amount is not significantly increased, and reduces the dependence of large-size, thick-section 7xxx series aluminum alloy and aluminum-lithium alloy forgings, thick plates or rolled plates on high-tonnage pressure equipment.

Description

Technical Field

[0001] This invention relates to a method for pre-deformation of high-strength aluminum alloy under low-load limited thermal expansion, belonging to the field of aluminum alloy casting technology. Background Technology

[0002] High-strength aluminum alloys possess advantages such as low density, high specific strength, high specific stiffness, and excellent comprehensive performance, making them promising candidates for applications in large aerospace components such as aircraft fuselage frames, wings, skins, and load-bearing joints. Among these, 7xxx series aluminum alloys and aluminum-lithium alloys are typical high-strength aluminum alloys widely used in aerospace structural components. 7xxx series aluminum alloys primarily rely on precipitated phases such as η′ and η phases for strengthening, while aluminum-lithium alloys primarily rely on precipitated phases such as T1 and θ′ phases for strengthening. Typical manufacturing processes for these alloys typically include casting, homogenization, hot deformation, solution treatment, quenching, pre-deformation, and aging treatment. Pre-deformation after solution quenching not only reduces residual quenching stress but also introduces dislocations and promotes heterogeneous nucleation and precipitation of strengthening phases during subsequent aging, thus significantly impacting the dimensional stability, strength, plasticity, and performance uniformity of high-strength aluminum alloy components. For large-sized, thick-section 7xxx series aluminum alloy and aluminum-lithium alloy forgings, plates, or rolled plates, conventional integral cold pressing, stretching, or compression pre-deformation typically requires high-tonnage equipment. When the component is large, thick, or has high deformation resistance, existing equipment may be unable to press or stretch it, or the pre-deformation amount may be insufficient. Insufficient pre-deformation leads to inadequate reduction of residual stress during quenching, further causing warping, decreased dimensional stability, and fluctuations in mechanical properties during subsequent machining processes.

[0003] While existing segmented cold-pressing pre-deformation can reduce the single-press load to some extent, it easily leads to strain transition zones and local stress concentrations between deformation areas, resulting in uneven distribution of pre-deformation within thick-section components. For 7xxx series aluminum alloys and aluminum-lithium alloys, uneven pre-deformation can further affect the precipitation behavior of strengthening phases during subsequent aging, thus causing inconsistencies in strength, plasticity, and residual stress state in different regions of the component.

[0004] Furthermore, existing cold-pressing pre-deformation methods typically rely on mechanical reduction to directly obtain the target pre-strain. When the target pre-deformation is large, the required forming load increases significantly; when limited by equipment tonnage and only a small reduction can be applied, it is difficult to simultaneously achieve sufficient stress relief and performance improvement. Therefore, there is an urgent need to develop a high-strength aluminum alloy pre-deformation method that can increase the effective pre-deformation, improve the age-strengthening effect, and enhance the residual stress reduction capability without significantly increasing the equipment load. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, the present invention aims to provide a method for pre-deformation of high-strength aluminum alloys under low-load limited thermal expansion, thereby solving problems such as high equipment load requirements, insufficient effective pre-deformation amount, inadequate residual stress reduction, and insufficient strength-plasticity matching after aging in the pre-deformation process of large-size, thick-section forgings, thick plates, or rolled plates of 7xxx series aluminum alloys and aluminum-lithium alloys.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a method for pre-deformation of high-strength aluminum alloy under low-load constrained thermal expansion, comprising the following steps:

[0008] Step S1, Material Preparation and Status Freezing:

[0009] S101 is used to solidify high-strength aluminum alloy forgings, thick plates or rolled plates to fully dissolve the alloying elements in the aluminum matrix and obtain a supersaturated solid solution structure.

[0010] S102. After solution treatment, the high-strength aluminum alloy material is quenched. The quenching transfer time is controlled within the range of 0–30 s. The quenching medium is water, low-temperature water or quenching medium with a cooling rate higher than that of room temperature water, in order to suppress the premature precipitation of strengthening phase during quenching.

[0011] S103. After quenching, the high-strength aluminum alloy material is transferred to a low-temperature environment for storage, including ordinary cold storage environment and deep cryogenic environment.

[0012] Step S2, Mold preparation and initial compression:

[0013] S201, Prepare a compression mold or pressing fixture with online heating function;

[0014] S202, the mold or pressing fixture is equipped with a temperature detection and control device;

[0015] S203, the mold or pressing fixture is equipped with a displacement holding device or a limiting device;

[0016] Step S3, Low-temperature pre-compression:

[0017] S301, the high-strength aluminum alloy material that has been kept at low temperature is taken out from the low temperature environment of a normal cold storage or a deep cryogenic environment, and transferred to a compression mold or pressing tooling before natural aging occurs significantly.

[0018] S302 involves applying initial compression pre-deformation to high-strength aluminum alloy materials, with the initial compression deformation amount being 0.5%-4.0% of the material's initial height.

[0019] S303, after reaching the predetermined initial compression deformation, the active pressing stops, and the mold is kept in the predetermined pressing position by means of press displacement control, mechanical limit block, servo locking device or mold gap holding device.

[0020] S302, applying initial compression pre-deformation to high-strength aluminum alloy material, wherein the initial compression deformation is 0.5%-4.0% of the initial height of the material, preferably 1.0%-3.0%;

[0021] S303, after reaching the predetermined initial compression deformation, the active pressing stops, and the mold is kept in the predetermined pressing position by means of press displacement control, mechanical limit block, servo locking device or mold gap holding device.

[0022] Step S4, restricted thermal expansion assisted pre-deformation:

[0023] Raise the mold temperature to the range of 60-180℃; keep it warm and in place for 10-60 minutes.

[0024] Step S5, Uninstallation and Subsequent Time-Limited Processing:

[0025] S501, after the pre-deformation assisted by limited thermal expansion is completed, the mold constraint is released and the high-strength aluminum alloy material is taken out;

[0026] S502 involves artificially aging the pre-deformed high-strength aluminum alloy material; for 7xxx series aluminum alloys, the artificial aging process is to hold at 121℃ for 6 hours and then at 177℃ for 6 hours; for aluminum-lithium alloys, the artificial aging process is to hold at 150℃ for 30 hours; after holding, the material is cooled in the furnace or air-cooled to room temperature.

[0027] S503, through the aforementioned artificial aging treatment, the dislocation, substructure, and residual stress adjustment state introduced during the restricted thermal expansion-assisted pre-deformation process are coupled with the subsequent precipitation process of strengthening phases; specifically, for 7xxx series aluminum alloys, the precipitation behavior of precipitated phases such as η′ phase and η phase is mainly controlled; for aluminum-lithium alloys, the precipitation behavior of precipitated phases such as T1 phase and θ′ phase is mainly controlled, thereby improving the precipitation distribution of strengthening phases and enhancing the material's strength and plasticity matching;

[0028] Step S6, performance and residual stress testing.

[0029] Furthermore, in S103, the temperature of the ordinary cold storage low-temperature environment is -20℃ to -40℃, the temperature of the cryogenic environment is -190℃ to -150℃, and the holding time is 0.5–3 h.

[0030] Furthermore, in S201, the mold includes an upper mold and a lower mold, and at least one mold is provided with a resistance heating device, an induction heating device, an oil temperature heating channel or other controllable heating structure inside the mold;

[0031] In step S202, the temperature detection device includes, but is not limited to, thermocouples, infrared temperature measuring devices, or embedded temperature sensors, to achieve real-time monitoring and closed-loop control of the mold temperature.

[0032] S203, the displacement holding device or limiting device is used to keep the mold gap unchanged or keep the pressing displacement unchanged after the high-strength aluminum alloy material has completed the initial compression, so that the material is in a state of restricted thermal expansion during the subsequent heating process.

[0033] Furthermore, in step S4, during the pre-deformation assisted by limited thermal expansion, the mold temperature is raised to the range of 130-160℃; the heat preservation and position holding time is 10-60 min.

[0034] Furthermore, the limited thermal expansion-assisted pre-deformation includes two methods;

[0035] Method 1: Cold pressing followed by online heating to maintain position;

[0036] S401, Place the high-strength aluminum alloy material that has been kept at low temperature in an unheated or low-temperature mold, and first perform low-load initial compression as described in S3;

[0037] S402, after the initial compression is completed, keep the mold pressing displacement unchanged or keep the mold gap unchanged, and start the mold online heating;

[0038] S403, the mold temperature is raised to the range of 60-180℃, preferably the range of 130-160℃; during the heating process, the high-strength aluminum alloy material gradually heats up from a low temperature state, and the material itself has a tendency to thermally expand; since the material is in the mold constraint state, its free thermal expansion is restricted, thereby forming a controlled additional compression effect in the thickness direction, and promoting the release of quenching residual stress and enhancing the pre-deformation effect.

[0039] S404: After the mold temperature reaches the set temperature, the pressing displacement or mold gap is kept unchanged, and the heat preservation time is 2-120 min, preferably 10-60 min.

[0040] Alternatively, method two can be adopted: compress and hold the mold in a preheated state;

[0041] S405, preheat the mold to a temperature range of 60-180°C, preferably 130-160°C;

[0042] S406, the high-strength aluminum alloy material after being held at low temperature is transferred to a preheated mold, and initial compression pre-deformation is immediately applied. The initial compression deformation is 0.5%-2.0% of the initial height of the material, preferably 1.0%-1.5%.

[0043] S407, after reaching the predetermined initial compression deformation, stop actively pressing down and keep the die pressing displacement unchanged or keep the die gap unchanged, so that the high-strength aluminum alloy material is in a state of restricted thermal expansion during the heating process;

[0044] S408, the heat preservation time is 2-120 minutes, preferably 10-60 minutes.

[0045] Furthermore, in step S6, the performance and residual stress detection includes:

[0046] S601, mechanical property sampling is performed on high-strength aluminum alloy materials after aging. Sampling locations include the 1 / 2 thickness position and other typical areas. Mechanical property test samples are prepared along the longitudinal, transverse and thickness directions, respectively.

[0047] S602 uses a universal testing machine to test its tensile properties, obtaining yield strength, tensile strength and elongation after fracture.

[0048] S603 uses the contour method, blind hole method, X-ray diffraction method, neutron diffraction method or other residual stress testing methods to detect residual stress in high-strength aluminum alloy materials after pre-deformation and aging.

[0049] S604. The test results are compared with those of conventional cold-pressed pre-deformed materials to evaluate the effect of the method of the present invention on residual stress reduction, strength improvement, plasticity improvement and performance uniformity.

[0050] This invention is based on a method for synergistically improving the strength and plasticity of high-strength aluminum alloys under low load stress relief. It employs a method where high-strength aluminum alloy material, after initial pre-deformation with a small amount of compression, undergoes restricted thermal expansion during heating via a mold holding or limiting mechanism. This achieves increased effective pre-deformation, reduced residual stress, and improved post-aging performance under relatively low active compression conditions. Specifically, the high-strength aluminum alloy material after solution quenching is first held at a low temperature, then subjected to initial pre-deformation with a small amount of compression. Heating is then performed while maintaining the mold's compression displacement or gap, causing thermal expansion of the material under restricted conditions. This results in additional compression deformation and stress relaxation, ultimately achieving low-load pre-deformation, reduced residual stress, and improved strength and plasticity after aging. Its main innovations include:

[0051] 1) A combined process of low-temperature holding—small compression pre-deformation—constrained thermal expansion assisted deformation:

[0052] This invention is applicable to high-strength aluminum alloy materials that require pre-deformation and artificial aging treatment after solution quenching, primarily including 7xxx series aluminum alloys and aluminum-lithium alloys. The solution-quenched high-strength aluminum alloy material is first held in a low-temperature environment, including a normal freezer environment of -20℃ to -40℃ and a cryogenic environment of -190℃ to -150℃, to suppress natural aging and stabilize the material's deformation resistance. Subsequently, a small amount of initial pre-deformation is applied to the material after low-temperature holding, the initial compression deformation being 0.5%-4.0% of the material's initial height, preferably 1.0%-3%. After reaching the predetermined initial compression amount, by maintaining the die's pressing displacement constant, maintaining the die gap constant, or using a limiting device, the material is kept in a state of restricted thermal expansion during subsequent heating, and the thermal expansion is converted into additional compression.

[0053] 2) Online heating mold and temperature holding control:

[0054] This invention employs a mold or tooling with online heating function, and uses a temperature detection and control device, as well as a displacement holding or limiting device, to achieve heating and positioning of materials under confined boundary conditions. The heating temperature of the mold or material is 60-180℃, preferably 130-160℃; the holding time is 2-120 min, preferably 10-60 min. This is used to ensure that the low-temperature material undergoes thermal expansion-assisted pre-deformation under confined conditions and to promote the release of residual stress.

[0055] 3) Two implementation methods for pre-deformation assisted by restricted thermal expansion:

[0056] This invention includes two implementation methods: First, the material, after being held at low temperature, is placed in an unheated or low-temperature mold for initial compression, and then the mold is heated online while maintaining its position. Second, the mold is preheated to a set temperature, and then the material, after being held at low temperature, is placed in a preheated mold for initial compression and then heated while maintaining its position. Both methods utilize the limited thermal expansion of the low-temperature material during the heating process to achieve additional pre-deformation and stress relaxation.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] 1. This invention does not rely on applying a large amount of mechanical reduction at once. Instead, it combines low-load initial compression with restricted thermal expansion-assisted pre-deformation to improve the effective pre-deformation effect without significantly increasing the amount of active reduction. This reduces the dependence of large-size, thick-section 7xxx series aluminum alloy and aluminum-lithium alloy forgings, thick plates or rolled plates on high-tonnage pressure equipment.

[0059] 2. This invention utilizes the thermal expansion trend generated during the process of heating high-strength aluminum alloy materials from a low-temperature holding state to 60–180 ℃, and transforms free thermal expansion into controlled compression by mold positioning or mold gap restriction, which is beneficial to further release quenching residual stress and improve stress relief effect.

[0060] 3. This invention suppresses natural aging by maintaining the material at low temperatures in a conventional freezer or by cryogenic preservation, thus keeping 7xxx series aluminum alloys and aluminum-lithium alloys in a low or stable deformation resistance state before pre-deformation, which is beneficial for completing the initial pre-compression under a lower load.

[0061] 4. This invention introduces a low-to-medium temperature retention stage during the pre-deformation process, enabling pre-deformation, stress relaxation, and microstructure regulation to occur simultaneously. This is beneficial for improving the precipitation behavior of strengthening phases during subsequent artificial aging. For 7xxx series aluminum alloys, the precipitation behavior of η′ phase and η phase can be controlled; for aluminum-lithium alloys, the precipitation behavior of T1 phase and θ′ phase can be controlled, thereby improving the strength and plasticity matching of the material after aging.

[0062] 5. Compared with conventional cold pressing pre-deformation, the present invention can not only reduce quenching residual stress, but also improve the effective pre-deformation amount through the auxiliary effect of limited thermal expansion, thereby reducing the performance risk and processing deformation risk caused by insufficient pre-deformation.

[0063] 6. Compared with the traditional segmented cold pressing method, the present invention does not rely on complex segmented pressing paths and segmented mold edge structure designs. The process route is more concentrated and it is suitable for low-load pre-deformation treatment of large-size, thick-section 7xxx series aluminum alloy and aluminum-lithium alloy forgings, thick plates and rolled plates. Attached Figure Description

[0064] Figure 1 This is a technical roadmap of the high-strength aluminum alloy low-load restricted thermal expansion pre-deformation method of the present invention;

[0065] Figure 2 In the image, (a) the dimensional characteristics of the forging and the actual object; (b) the forging compression process;

[0066] Figure 3 Compressive load-time curves of 2050 aluminum-lithium alloy grid forgings under different pre-deformation processes;

[0067] Figure 4 The residual stress test surface of a 2050 aluminum-lithium alloy grid forging and the residual stress distribution of the cross-section after different pre-deformation processes;

[0068] Figure 5 In the middle, (a) is a three-dimensional structural schematic diagram and physical image of a 7050 aluminum alloy block forging; (b) is a physical image of the forging undergoing fixed-stroke position-holding compression heating on a hydraulic press with a heating plate.

[0069] Figure 6 Compression load-time curves of 7050 aluminum alloy block forgings under different pre-deformation processes;

[0070] Figure 7 This is a schematic diagram of the detection location. Detailed Implementation

[0071] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0072] See Figure 1 The present invention discloses a method for pre-deformation of high-strength aluminum alloy under low-load restricted thermal expansion, comprising the following steps:

[0073] S1, Material preparation and status freeze;

[0074] S101 is used to solidify high-strength aluminum alloy forgings, thick plates or rolled plates to fully dissolve the alloying elements in the aluminum matrix and obtain a supersaturated solid solution structure.

[0075] S102. After solution treatment, the high-strength aluminum alloy material is quenched. The quenching transfer time is controlled within the range of 0–30 s. The quenching medium is water, low-temperature water or quenching medium with a cooling rate higher than that of room temperature water, in order to suppress the premature precipitation of strengthening phase during quenching.

[0076] S103. After quenching, the high-strength aluminum alloy material is transferred to a low-temperature environment for holding. This low-temperature environment includes a standard freezer environment and a cryogenic environment. The temperature of the standard freezer environment is -20℃ to -40℃, and the temperature of the cryogenic environment is -190℃ to -150℃, with a holding time of 0.5–3 hours. This low-temperature holding process suppresses natural aging of the high-strength aluminum alloy material before pre-deformation, avoids or reduces the increase in deformation resistance caused by room temperature storage, and ensures that the material maintains a low or stable deformation resistance state before subsequent low-load pre-deformation.

[0077] S2, mold preparation and initial compression (online heating pre-deformation);

[0078] S201, Prepare a compression mold or pressing fixture with online heating function. The mold includes an upper mold and a lower mold, and at least one mold is equipped with a resistance heating device, an induction heating device, an oil temperature heating channel, or other controllable heating structure inside the mold;

[0079] S202, the mold is equipped with a temperature detection and control device, the temperature detection device including but not limited to thermocouples, infrared temperature measuring devices or embedded temperature sensors, to realize real-time monitoring and closed-loop control of mold temperature;

[0080] S203, the mold is equipped with a displacement holding device or a limiting device. The displacement holding device or limiting device is used to keep the mold gap unchanged or keep the compression displacement unchanged after the high-strength aluminum alloy material has completed the initial compression, so that the material is in a state of restricted thermal expansion during the subsequent heating process.

[0081] S3, low temperature precompression (low load initial precompression).

[0082] S301 involves taking high-strength aluminum alloy material that has been kept at low temperatures out of a regular cold storage or cryogenic environment and transferring it to a pre-deformation mold before natural aging occurs significantly.

[0083] S302, applying initial compression pre-deformation to high-strength aluminum alloy material, wherein the initial compression deformation is 0.5%-4.0% of the initial height of the material, preferably 1.0%-3.0%;

[0084] S303: After reaching the predetermined initial compression deformation, the active pressing stops, and the mold is kept in the predetermined pressing position by means of press displacement control, mechanical limit block, servo locking device or mold gap holding device.

[0085] S4, restricted thermal expansion assisted pre-deformation;

[0086] This step includes two implementation methods.

[0087] Implementation method 1: Cold pressing followed by online heating to maintain position;

[0088] S401, Place the high-strength aluminum alloy material that has been kept at low temperature in an unheated or low-temperature mold, and first perform low-load initial compression as described in S3;

[0089] S402, after the initial compression is completed, keep the mold pressing displacement unchanged or keep the mold gap unchanged, and start the mold online heating;

[0090] Step S4, restricted thermal expansion assisted pre-deformation:

[0091] Raise the mold temperature to a range of 60-180℃; maintain the temperature and position for 10-60 minutes.

[0092] S403, the mold temperature is raised to the range of 60-180℃, preferably the range of 130-160℃; during the heating process, the high-strength aluminum alloy material gradually heats up from a low temperature state, and the material itself has a tendency to thermally expand; since the material is in the mold constraint state, its free thermal expansion is restricted, thereby forming a controlled additional compression effect in the thickness direction, and promoting the release of quenching residual stress and enhancing the pre-deformation effect.

[0093] S404: After the mold temperature reaches the set temperature, the pressing displacement or mold gap is kept unchanged, and the heat preservation time is 2-120 min, preferably 10-60 min.

[0094] Implementation Method 2: Compress and hold the mold in a preheated state;

[0095] S405, preheat the mold to a temperature range of 60-180°C, preferably 130-160°C;

[0096] S406, the high-strength aluminum alloy material after being held at low temperature is transferred to a preheated mold, and initial compression pre-deformation is immediately applied. The initial compression deformation is 0.5%-2.0% of the initial height of the material, preferably 1.0%-1.5%.

[0097] S407, after reaching the predetermined initial compression deformation, stop actively pressing down and keep the die pressing displacement unchanged or keep the die gap unchanged, so that the high-strength aluminum alloy material is in a state of restricted thermal expansion during the heating process;

[0098] S408, the heat preservation time is 2-120 minutes, preferably 10-60 minutes.

[0099] S5, uninstallation and subsequent time-sensitive processing;

[0100] S501, after the pre-deformation assisted by limited thermal expansion is completed, the mold constraint is released and the high-strength aluminum alloy material is taken out;

[0101] S502 involves artificially aging the pre-deformed high-strength aluminum alloy material; for 7xxx series aluminum alloys, the artificial aging temperature is 121℃ for 6 hours followed by 177℃ for 6 hours; for aluminum-lithium alloys, the artificial aging process is 150℃ for 30 hours; after the aging process, the material is cooled in the furnace or air-cooled to room temperature.

[0102] S503, through the aforementioned artificial aging treatment, the dislocation, substructure, and residual stress adjustment state introduced during the restricted thermal expansion-assisted pre-deformation process are coupled with the subsequent precipitation process of strengthening phases; specifically, for 7xxx series aluminum alloys, the precipitation behavior of precipitated phases such as η′ phase and η phase is mainly controlled; for aluminum-lithium alloys, the precipitation behavior of precipitated phases such as T1 phase and θ′ phase is mainly controlled, thereby improving the precipitation distribution of strengthening phases and enhancing the material strength and plasticity matching.

[0103] S6, Performance and Residual Stress Testing;

[0104] S601, mechanical property sampling is performed on high-strength aluminum alloy materials after aging. Sampling locations include the 1 / 2 thickness position and other typical areas. Mechanical property test samples are prepared along the longitudinal, transverse and thickness directions, respectively.

[0105] S602 uses a universal testing machine to test its tensile properties, obtaining yield strength, tensile strength and elongation after fracture.

[0106] S603 uses the contour method, blind hole method, X-ray diffraction method, neutron diffraction method or other residual stress testing methods to detect residual stress in high-strength aluminum alloy materials after pre-deformation and aging.

[0107] S604. The test results are compared with those of conventional cold-pressed pre-deformed materials to evaluate the effect of the method of the present invention on residual stress reduction, strength improvement, plasticity improvement and performance uniformity.

[0108] Example 1 (Example for a 2050 aluminum-lithium alloy grid forging):

[0109] This embodiment uses a 2050 aluminum-lithium alloy grid forging as the object, and employs the high-strength aluminum alloy low-load restricted thermal expansion pre-deformation method described in this invention to perform solution treatment, quenching, low-temperature holding, cold pressing with position holding, and artificial aging. The structural features and pre-deformation device of the grid forging are as follows... Figure 2 As shown. Figure 2 (a) is a three-dimensional structural diagram and physical image of the grid forging. The forging has a grid-like rib structure, with external dimensions of 150 mm × 120 mm × 50 mm and a web thickness of 20 mm. The cold-pressed surface is located at the top of the ribs of the forging. Figure 2 (b) is a physical image of a forging undergoing position-holding compression heating on a hydraulic press with a heating plate. The forging is placed between the upper and lower dies, which are connected to the heating plate and supported by a pad to achieve die heating and position-holding control during the compression pre-deformation process.

[0110] First, the 2050 aluminum-lithium alloy grid forging was solution treated at 525 ℃. After solution treatment, the forging was immediately transferred to room temperature stirred water for quenching. The quenching transfer time was 5 seconds, and the quenching immersion time was 5 minutes to ensure that the forging was fully cooled and obtained a supersaturated solid solution structure.

[0111] After quenching, the forgings are quickly transferred to a -40℃ freezer for cryogenic holding for 3 hours. This cryogenic holding process inhibits the natural aging of the 2050 aluminum-lithium alloy before pre-deformation, reduces premature precipitation of strengthening phases and increase in deformation resistance during room temperature storage, and ensures that the forgings maintain a low or stable deformation resistance state before subsequent cold pressing pre-deformation.

[0112] After the cryogenic holding period is complete, the forgings are removed from the freezer and transferred to a 300-ton hydraulic press equipped with a heating plate for pre-deformation. For example... Figure 2 As shown in (b), the forging is placed between the upper and lower dies, and the dies are heated using upper and lower heating plates. During the pre-deformation process, for Figure 2 (a) shows the cold-pressed surface of the top ribs of the grid forging, where a 3% compression is applied along the height direction. After reaching the predetermined compression, the active pressing is stopped, and a fixed-stroke positioning method is used to keep the pressing displacement constant, thus keeping the forging in a constrained state.

[0113] In the fixed-position holding state, the oil heating device of the hydraulic press heating plate is turned on to indirectly heat the mold and forging. The heating temperature is set to 155℃. During the heating process, contact thermocouples are used to monitor the temperature at five non-adjacent locations on the forging to characterize the temperature rise status of different areas of the forging. After each temperature measurement location reaches the set temperature range, it is maintained for 10 minutes. Because the forging is restricted by the mold and the compression displacement during the heating process, its free thermal expansion is constrained, thereby generating a restricted thermal expansion assisted compression effect in the height direction, which is beneficial to further improve the effective pre-deformation amount and promote the release of quenching residual stress. After the heat holding and positioning are completed, the mold constraint is released and the forging is unloaded and removed. To further illustrate the effect of the method of the present invention on reducing the pre-deformation forming load, the load changes of room temperature forgings and frozen forgings during the compression pre-deformation process are recorded, and the results are as follows. Figure 3 As shown. The room-temperature forgings are control samples that were cold-pressed directly at room temperature without undergoing -40℃ low-temperature holding and warm-up holding treatment; the frozen forgings are samples that were compressed and subjected to warm-up holding treatment after being held at -40℃ according to the method of this invention. Figure 3It can be seen that, under similar actual compression conditions, the compressive load of the frozen forgings is lower than that of the room-temperature forgings. This indicates that holding the temperature can suppress natural aging and reduce or stabilize the deformation resistance of the material before pre-deformation, thus facilitating pre-compression under lower forming force conditions. The final actual compression and maximum load of each sample are as follows: Frozen forging 1: 1.64 mm / 182.6 tons; Frozen forging 2: 1.59 mm / 184.7 tons; Room-temperature forging 1: 1.55 mm / 258.2 tons; Room-temperature forging 2: 1.51 mm / 251.1 tons. The above results show that the final compression of each sample is relatively small. Therefore, the load difference mainly comes from the material state before pre-deformation and the difference in the holding and heating process, rather than the difference in compression. In addition, the load of the frozen forgings changes slowly with time during the holding and heating process, indicating that the thermal expansion generated by the material heating is limited by the die compression displacement or die gap, further forming a restricted thermal expansion-assisted compression effect. The results verify the feasibility of the present invention in achieving low-load pre-deformation and additional deformation effects through low-temperature holding, fixed-range positioning and heating process without significantly increasing the amount of active compression.

[0114] Subsequently, the pre-deformed forgings underwent artificial aging treatment. The aging regime was 150℃ for 30 hours, followed by cooling to room temperature. After aging, mechanical property samples were taken from the forgings. The sampling location was at 1 / 2 thickness along the height direction of the forging. The preparation of mechanical property samples and tensile testing were performed according to GB / T 228.1-2021 to obtain tensile strength, yield strength, and elongation after fracture. Simultaneously, residual stress test surfaces were machined on samples from the same batch using a slow wire EDM method. After cutting, a coordinate measuring machine was used to collect the deformation point cloud of the cut surface, and the residual stress distribution of the forging cross-section was obtained by back-calculation using Abaqus finite element software.

[0115] To verify the technical effectiveness of the method of the present invention, control samples were set up. The control samples were 2050 aluminum-lithium alloy lattice forgings from the same batch, treated with conventional heat treatment and cold pressing processes, i.e., without -40℃ low-temperature holding or heating and position holding treatment, only the same amount of cold pressing in the height direction was applied. Samples treated with the method of the present invention were designated as "frozen samples," and samples treated with conventional processes were designated as "room temperature samples." Performance test samples 1-5 were parallel mechanical property test samples taken from the same forging sample.

[0116] The mechanical property test results are shown in Table 1. As can be seen from the results in Table 1, the frozen samples treated using the method of this invention exhibit higher strength and plasticity. The average tensile strength of the frozen samples was 549.98 MPa, the average yield strength was 497.78 MPa, and the average elongation after fracture was 7.02%; the average tensile strength of the room-temperature control samples was 526.52 MPa, the average yield strength was 480.58 MPa, and the average elongation after fracture was 5.27%. Compared with the room-temperature control samples, the average tensile strength of the frozen samples increased by approximately 23.45 MPa, the average yield strength increased by approximately 17.20 MPa, and the average elongation after fracture increased by approximately 1.75 percentage points.

[0117] Table 1. Room temperature tensile properties of 2050 aluminum-lithium alloy grid forgings after different pre-deformation processes.

[0118]

[0119] Residual stress test results as follows Figure 4 As shown. Figure 4 The left side shows a schematic diagram of the selection of the residual stress test surface for the grid-patterned forging. The test surface is located in the middle section area of ​​the forging. Figure 4 The right side shows the residual stress distribution contour plots of conventional forgings and frozen forgings on the same test surface. Figure 4 It can be seen that conventional forgings exhibit significant high residual stress concentration regions within their cross-sections, with local residual stress peaks reaching approximately 110-130 MPa. These high-stress areas are primarily located near the ribs. In contrast, the residual stress level of the frozen forgings treated using the method of this invention is significantly reduced, with local residual stress peaks reaching approximately 65-85 MPa. The range and intensity of the high-stress concentration regions are also weakened. This result demonstrates that, under the same cold pressing conditions, this invention, through low-temperature holding, stationary positioning, and the assisted effect of restricted thermal expansion during the heating process, can promote the redistribution and release of residual stress within the forgings, thereby significantly improving the quenching residual stress reduction effect. Combined with the aforementioned mechanical property results, it can be concluded that the method of this invention can improve post-aging strength and plasticity while reducing residual stress, exhibiting a good synergistic improvement effect in stress relief and plasticity.

[0120] Example 2 (Example for 7050 aluminum alloy block forging):

[0121] Example 2 uses a 7050 aluminum alloy block forging as the object, and applies the high-strength aluminum alloy low-load restricted thermal expansion pre-deformation method described in this invention to perform solution treatment, quenching, low-temperature holding, cold pressing and holding heating, and subsequent processing. The structural features and pre-deformation device of the 7050 aluminum alloy block forging are as follows: Figure 5 As shown. Figure 5(a) is a three-dimensional structural schematic diagram and physical image of a 7050 aluminum alloy block forging. The forging has external dimensions of 110mm×40mm×70mm, and the cold-pressed surface is located on the upper surface of the forging. Figure 5 (b) is a photograph of the forging undergoing fixed-stroke, position-holding compression heating on a hydraulic press with a heating plate. The forging is placed between the upper and lower dies, which are connected to the heating plate and supported by a pad to achieve die heating and fixed-stroke, position-holding control during the compression pre-deformation process. First, the 7050 aluminum alloy block forging is solution-treated at 477 ℃. After solution treatment, the forging is quickly transferred to 65 ℃ stirred water for quenching. The quenching transfer time is 5 seconds, and the quenching immersion time is 5 minutes to ensure sufficient cooling and obtain a supersaturated solid solution structure. After quenching, the forging is quickly transferred to a -40 ℃ freezer for cryogenic holding for 3 hours. This cryogenic holding process suppresses the natural aging of the 7050 aluminum alloy before pre-deformation, reduces premature precipitation of strengthening phases and increase in deformation resistance during room temperature storage, and maintains a low or stable deformation resistance state for the forging before subsequent cold pressing pre-deformation. After the cryogenic holding period is complete, the forgings are removed from the freezer and transferred to a 300-ton hydraulic press equipped with a heating plate for pre-deformation. For example... Figure 5 As shown in (b), the forging is placed between the upper and lower dies, and the dies are heated using upper and lower heating plates. During the pre-deformation process, for Figure 5 (a) The upper surface of the 7050 aluminum alloy block forging is cold-pressed, and a compression of 1.5% is applied along the height direction. After the predetermined compression is reached, the active pressing is stopped, and the pressing displacement is kept constant by a fixed-stroke position holding method, so that the forging is in a constrained state.

[0122] In the fixed-position holding state, the oil heating device of the hydraulic press heating plate is activated to indirectly heat the mold and forging. The heating temperature is set to 155 ℃. During the heating process, contact thermocouples are used to monitor the temperature at five non-adjacent locations on the forging to characterize the temperature rise of different areas of the forging. Once each temperature measurement location reaches the set temperature range, it is maintained for 10 minutes. Because the forging is constrained by the mold and the pressing displacement during the heating process, its free thermal expansion is restricted, resulting in a restricted thermal expansion-assisted compression effect in the height direction, which is beneficial to further improve the effective pre-deformation amount and promote the release of residual quenching stress. After the heat holding and positioning are completed, the mold constraints are released and the forging is unloaded and removed. Figure 6It can be seen that, under similar actual compression conditions, the compressive load of the frozen forging is lower than that of the room-temperature forging. This indicates that low-temperature holding can suppress natural aging and reduce or stabilize the deformation resistance of the material before pre-deformation, thus facilitating pre-compression under lower forming force conditions. The final actual compression and maximum load of each sample are as follows: room-temperature forging 1 approximately 1.05 mm / 82 tons, room-temperature forging 2 approximately 1.02 mm / 79 tons, frozen forging 1 approximately 1.09 mm / 65 tons, and frozen forging 2 approximately 1.11 mm / 64 tons. The above results indicate that the load difference mainly stems from the material state before pre-deformation and the differences in the holding and heating process, rather than the difference in compression. Furthermore, the load of the frozen forging increases slowly over time and gradually stabilizes during the holding and heating process, indicating that the thermal expansion generated by the material heating is limited by the die pressing displacement or die gap, further forming a restricted thermal expansion-assisted compression effect. This result verifies the feasibility of the present invention in achieving low-load pre-deformation and additional deformation effects through low-temperature holding, fixed-range holding, and heating processes without significantly increasing the active pressing amount.

[0123] Subsequently, the pre-deformed 7050 aluminum alloy block forgings underwent artificial aging treatment. The aging regime was 121℃ for 6 hours, followed by a further increase to 177℃ for 6 hours, and then cooling to room temperature. This two-stage artificial aging treatment coupled the dislocation, substructure, and residual stress adjustment state introduced during the restricted thermal expansion-assisted pre-deformation process with the subsequent precipitation of strengthening phases such as η′ and η phases, thereby improving the strength, plasticity, and residual stress state of the 7050 aluminum alloy forgings after aging. After aging, mechanical property samples were taken from the forgings. The sampling location was at 1 / 2 thickness along the height direction of the forging. The preparation and tensile testing of the mechanical property samples were performed according to GB / T 228.1-2021 to obtain tensile strength, yield strength, and elongation after fracture. Simultaneously, residual stress was measured on samples from the same batch using an X-ray residual stress analyzer to obtain residual stress values. The control samples were 7050 aluminum alloy square forgings from the same batch, treated with conventional heat treatment and cold pressing processes. This meant no -40℃ low-temperature holding or temperature-raising and position-holding treatment was performed; only the same amount of cold pressing was applied along the height direction. Samples treated using the method of this invention were designated "frozen samples," and samples treated with conventional processes were designated "room temperature samples." Performance test samples 1-3 were parallel mechanical property test samples taken from the same forging sample.

[0124] The mechanical property test results are shown in Table 2. As can be seen from Table 2, the frozen samples treated using the method of this invention exhibit higher tensile strength and yield strength. The average tensile strength of the frozen samples was 538.85 MPa, the average yield strength was 485.40 MPa, and the average elongation after fracture was 14.00%; the average tensile strength of the room-temperature control samples was 534.66 MPa, the average yield strength was 479.07 MPa, and the average elongation after fracture was 14.57%. Compared with the room-temperature control samples, the average tensile strength of the frozen samples increased by approximately 4.20 MPa, the average yield strength increased by approximately 6.33 MPa, while the elongation after fracture remained at approximately 14%. These results indicate that, under the same initial cold-pressing conditions, this invention, through low-temperature holding at -40 ℃, stationary positioning, and the assisted effect of limited thermal expansion during the heating process, improves the tensile strength and yield strength after aging while maintaining a high level of plasticity. This demonstrates its benefit in improving the strength-plasticity matching of 7050 aluminum alloy forgings after aging.

[0125] Table 2. Room temperature tensile properties of 7050 aluminum alloy block forgings after different pre-deformation processes:

[0126]

[0127] To further evaluate the effect of the method of this invention on the residual stress state of 7050 aluminum alloy block forgings, X-ray residual stress testing was used to inspect the compression contact surface of the forgings. The inspection locations are as follows: Figure 7 As shown in Table 3, six measuring points, P1-P6, were selected on the compression contact surface for residual stress testing. The test results are shown in Table 3.

[0128] As shown in Table 3, the room-temperature sample exhibits a high level of residual compressive stress at the compression contact surface. The residual stresses at measuring points P1, P3, and P4 reach -98.45 MPa, -118.35 MPa, and -104.28 MPa, respectively, with a maximum residual compressive stress of approximately -118.35 MPa. In contrast, the frozen sample treated with the method of this invention shows a lower overall residual stress level, with a maximum residual compressive stress of approximately -87.81 MPa, significantly lower than that of the room-temperature sample. Looking at the average absolute value of residual stress at each measuring point, the average absolute value of residual stress at measuring points P1–P6 of the frozen sample is approximately 57.04 MPa, while that of the room-temperature sample is approximately 73.11 MPa, indicating that the method of this invention can reduce the overall level of residual stress at the compression contact surface. Furthermore, the stress distribution range of the frozen sample is -87.81 MPa to 27.51 MPa, while that of the room-temperature sample is -118.35 MPa to 11.85 MPa, indicating that the concentration of high residual compressive stress in the frozen sample is reduced.

[0129] Table 3. X-ray residual stress test results of the compression contact surface of 7050 aluminum alloy block forgings:

[0130]

[0131] Therefore, for 7050 aluminum alloy block forgings, under similar compression conditions, the present invention can promote the redistribution and release of residual stress and reduce the level of local high residual stress by maintaining a low temperature of -40 ℃, keeping the position constant, and the auxiliary effect of limited thermal expansion during the heating process. This is beneficial to improving the dimensional stability and service reliability of the forgings in subsequent processing.

[0132] In summary, as can be seen from the embodiments, the present invention achieves its intended purpose:

[0133] 1) Under low initial compressive load conditions, high-strength aluminum alloy materials can obtain sufficient effective pre-deformation.

[0134] 2) By utilizing the temperature difference between the low-temperature holding state and the medium-low temperature heating state, the material undergoes controlled thermal expansion under confined boundary conditions, and part of the thermal expansion is converted into additional compressive deformation and internal stress relaxation.

[0135] 3) Improve the residual stress reduction effect of solution hardening of 7xxx series aluminum alloys and aluminum-lithium alloys without significantly increasing the active reduction amount and equipment peak load.

[0136] 4) By coupling low temperature holding, restricted thermal expansion assisted pre-deformation and subsequent aging treatment, the precipitation behavior of strengthening phase is improved, thereby simultaneously improving the strength and plasticity of high-strength aluminum alloys after aging.

[0137] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for pre-deformation of high-strength aluminum alloy under low-load restricted thermal expansion, characterized in that, Includes the following steps: Step S1, Material Preparation and Status Freezing: S101 is used to solidify high-strength aluminum alloy forgings, thick plates or rolled plates to fully dissolve the alloying elements in the aluminum matrix and obtain a supersaturated solid solution structure. S102. After solution treatment, the high-strength aluminum alloy material is quenched. The quenching transfer time is controlled within the range of 0–30 s. The quenching medium is water, low-temperature water or quenching medium with a cooling rate higher than that of room temperature water, in order to suppress the premature precipitation of strengthening phase during quenching. S103. After quenching, the high-strength aluminum alloy material is transferred to a low-temperature environment for storage, including ordinary cold storage environment and deep cryogenic environment. Step S2, Mold preparation and initial compression: S201, Prepare a compression mold or pressing fixture with online heating function; S202, the mold or pressing fixture is equipped with a temperature detection and control device; S203, the mold or pressing fixture is equipped with a displacement holding device or a limiting device; Step S3, Low-temperature pre-compression: S301, the high-strength aluminum alloy material that has been kept at low temperature is taken out from the low temperature environment of a normal cold storage or a deep cryogenic environment, and transferred to a compression mold or pressing tooling before natural aging occurs significantly. S302 involves applying initial compression pre-deformation to high-strength aluminum alloy materials, with the initial compression deformation amount being 0.5%-4.0% of the material's initial height. S303, after reaching the predetermined initial compression deformation, the active pressing stops, and the mold is kept in the predetermined pressing position by means of press displacement control, mechanical limit block, servo locking device or mold gap holding device. Step S4, constrained thermal expansion assisted pre-deformation: Raise the mold temperature to a range of 60-180℃; maintain the temperature and position for 10-60 minutes. Step S5, Uninstallation and Subsequent Time-Limited Processing: S501, after the pre-deformation assisted by limited thermal expansion is completed, the mold constraint is released and the high-strength aluminum alloy material is taken out; S502 involves artificially aging the pre-deformed high-strength aluminum alloy material; the artificial aging regime for 7xxx series aluminum alloys is 121℃ for 6 hours followed by 177℃ for 6 hours; the artificial aging regime for aluminum-lithium alloys is 150℃ for 30 hours; after the holding period, the material is cooled in the furnace or air-cooled to room temperature. S503, through the aforementioned artificial aging treatment, the dislocation, substructure, and residual stress adjustment state introduced during the restricted thermal expansion-assisted pre-deformation process are coupled with the subsequent precipitation process of strengthening phases; specifically, for 7xxx series aluminum alloys, the precipitation behavior of precipitated phases such as η′ phase and η phase is mainly controlled; for aluminum-lithium alloys, the precipitation behavior of precipitated phases such as T1 phase and θ′ phase is mainly controlled, thereby improving the precipitation distribution of strengthening phases and enhancing the material's strength and plasticity matching; Step S6, performance and residual stress testing.

2. The method for pre-deformation of high-strength aluminum alloy under low load-limited thermal expansion according to claim 1, characterized in that: In S103, the temperature of the ordinary cold storage low-temperature environment is -20℃ to -40℃, the temperature of the cryogenic environment is -190℃ to -150℃, and the holding time is 0.5–3 h.

3. The method for pre-deformation of high-strength aluminum alloy under low load-limited thermal expansion according to claim 1, characterized in that: In S201, the mold includes an upper mold and a lower mold, and at least one mold is provided with a resistance heating device, an induction heating device, an oil temperature heating channel or other controllable heating structure inside the mold; In step S202, the temperature detection device includes, but is not limited to, thermocouples, infrared temperature measuring devices, or embedded temperature sensors, to achieve real-time monitoring and closed-loop control of the mold temperature. S203, the displacement holding device or limiting device is used to keep the mold gap unchanged or keep the pressing displacement unchanged after the high-strength aluminum alloy material has completed the initial compression, so that the material is in a state of restricted thermal expansion during the subsequent heating process.

4. The method for pre-deformation of high-strength aluminum alloy under low load-limited thermal expansion according to claim 1, characterized in that: In step S4, during the pre-deformation assisted by limited thermal expansion, the mold temperature is raised to the range of 130-160℃; the heat preservation and positioning time is 10-60 min.

5. The method for pre-deformation of high-strength aluminum alloy under low load-limited thermal expansion according to claim 1, characterized in that: Step S4, the restricted thermal expansion-assisted pre-deformation includes two methods; Method 1: Cold pressing followed by online heating to maintain position; S401, Place the high-strength aluminum alloy material that has been kept at low temperature in an unheated or low-temperature mold, and first perform low-load initial compression as described in S3; S402, after the initial compression is completed, keep the mold pressing displacement unchanged or keep the mold gap unchanged, and start the mold online heating; S403 raises the mold temperature to the range of 60-180℃; during the heating process, the high-strength aluminum alloy material gradually heats up from a low temperature state, and the material itself tends to thermally expand; since the material is under the constraint of the mold, its free thermal expansion is restricted, thereby forming a controlled additional compression effect in the thickness direction, and promoting the release of quenching residual stress and enhancing the pre-deformation effect. S404: After the mold temperature reaches the set temperature, continue to maintain the pressing displacement or mold gap unchanged, and the heat preservation time is 2-120 min. Alternatively, method two can be adopted: compress and hold the mold in a preheated state; S405, preheat the mold to the range of 60-180℃; S406 involves transferring the high-strength aluminum alloy material, after being held at low temperature, into a preheated mold and immediately applying initial compression pre-deformation, with the initial compression deformation amount being 0.5%-2.0% of the material's initial height. S407, after reaching the predetermined initial compression deformation, stop actively pressing down and keep the die pressing displacement unchanged or keep the die gap unchanged, so that the high-strength aluminum alloy material is in a state of restricted thermal expansion during the heating process; S408, the heat preservation time is 2-120 minutes.

6. The method for pre-deformation of high-strength aluminum alloy under low load-limited thermal expansion according to claim 1, characterized in that: In step S6, the performance and residual stress testing includes: S601, mechanical property sampling is performed on high-strength aluminum alloy materials after aging. Sampling locations include the 1 / 2 thickness position and other typical areas. Mechanical property test samples are prepared along the longitudinal, transverse and thickness directions, respectively. S602 uses a universal testing machine to test its tensile properties, obtaining yield strength, tensile strength and elongation after fracture. S603 uses the contour method, blind hole method, X-ray diffraction method, neutron diffraction method or other residual stress testing methods to detect residual stress in high-strength aluminum alloy materials after pre-deformation and aging. S604. The test results are compared with those of conventional cold-pressed pre-deformed materials to evaluate the effect of the method of the present invention on residual stress reduction, strength improvement, plasticity improvement and performance uniformity.

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