A method for controlling low residual stress of an aluminum-lithium alloy die forging

Abstract

The present invention is a method for controlling low residual stress in aluminum-lithium alloy die forgings, which includes the following steps: (1) solution treatment and quenching; (2) cold pressing; (3) cryogenic treatment and thermal cycling; (4) aging; (5) cryogenic treatment and thermal cycling. This method controls the residual stress field in aluminum-lithium alloy die forgings by combining cold pressing and cryogenic thermal cycling. First, the die forging is cold-compressed in a precision mold to overall reduce the residual stress level, and then through cryogenic thermal cycling treatment, the crystal lattice expands and contracts during the cycle to obtain a stable residual stress state. The two methods coordinate and complement each other, controlling the residual stress in aluminum-lithium alloy die forgings at a relatively low level, thereby obtaining aluminum-lithium alloy die forgings with excellent performance and low residual stress. This process technology is applicable to the production and application of aluminum-lithium alloy die forgings for aerospace, ships, and automobiles.

Description

Technical Field

[0001] The present invention is a method for controlling low residual stress of aluminum-lithium alloy die forgings, belonging to the field of metal materials engineering. Background Art

[0002] Modern mechanical design pays more attention to low energy consumption and high service life. To meet the requirements of long flight time, high service life and economy of transportation machinery such as aircraft and ships, the demand for high specific strength and high specific modulus materials is increasing. Aluminum-lithium alloy materials are important materials with obvious structural weight reduction effects in metal materials. The present invention is a key process technology for reducing the residual stress of Al-Li-Cu-X series aluminum-lithium alloy die forgings, improving the machining qualification rate and material utilization rate of aluminum-lithium alloy parts.

[0003] In recent years, some high-performance aluminum-lithium alloys such as 2098 / 2198, 2097 series, 2196, 2195, 2099, etc. have been developed in this field. However, the control of residual stress in aluminum-lithium alloys mainly focuses on materials such as thick plates and open die forgings. At present, the pre-stretching method and flat anvil cold pressing method are mainly used to reduce the residual stress of thick plates and open die forgings. Although thick plates and open die forgings have the advantages of strong adaptability, low residual stress and stable process in the production of aircraft structural parts, they also have disadvantages such as the macroscopic streamline being cut off during the subsequent machining process of structural parts, mechanical property anisotropy, serious material waste and high cost. Summary of the Invention

[0004] The present invention is precisely designed and provided with a method for controlling low residual stress of aluminum-lithium alloy die forgings in view of the above-mentioned existing technical situation. Its purpose is to control the residual stress field of aluminum-lithium alloy die forgings by combining cold pressing and cryogenic thermal cycling. The two methods are coordinated and complementary to control the residual stress of aluminum-lithium alloy die forgings at a relatively low level, so as to obtain aluminum-lithium alloy die forgings with excellent performance and low residual stress.

[0005] The purpose of the present invention is achieved by the following technical solutions:

[0006] The method for controlling low residual stress of aluminum-lithium alloy die forgings is for Al-Cu-Li-X series alloys, and the steps of its process are as follows:

[0007] Step 1: Solution treatment and quenching

[0008] Heat the die forging to 500°C - 540°C for solution treatment, and then quench after solution treatment;

[0009] Step 2: Cold pressing

[0010] Put the die forging into the mold and perform cold pressing at room temperature, and require the compression deformation amount of each part of the die forging along the thickness direction to reach 2 - 4%;

[0011] Step 3: First cryogenic treatment and thermal cycling

[0012] Cool the cold-pressed die forgings in a cryogenic environment chamber to -90°C, hold for 0.5 h, take them out and leave them at room temperature for 1 h, and repeat this cycle 1 - 3 times;

[0013] Step 4: Artificial aging

[0014] The artificial aging is carried out by one of the following three methods:

[0015] The first method: Heat the die forgings to 160°C and hold for 18 - 24 h;

[0016] The second method: Heat the die forgings to 90°C - 135°C, hold for 10 h - 24 h, then continue to heat to 150°C - 195°C and hold for 4 h - 20 h;

[0017] The third method: Heat the die forgings to 90°C - 145°C, hold for 10 h - 24 h, then continue to heat to 150°C - 195°C and hold for 4 h - 20 h;

[0018] The first one is single-stage isothermal aging, such as holding at 160°C for 18 - 24 h;

[0019] The second one is multi-stage isothermal aging, such as first selecting a temperature point in the range of 90°C - 135°C, holding for 10 h - 24 h, and then continuing to select a temperature point in the range of 150°C - 195°C for heating and holding for 4 h - 20 h;

[0020] The third one is multi-stage non-isothermal aging, such as first heating from 90°C to 145°C at a certain heating rate in the range of 90°C - 145°C, taking 10 h - 24 h, and then continuing to heat from 150°C to 195°C at a certain heating rate in the range of 150°C - 195°C, taking 4 h - 20 h;

[0021] Step 5: Second cryogenic treatment and thermal cycling

[0022] Cool the die forgings after artificial aging in a cryogenic environment chamber to -90°C, hold for 0.5 h, take them out and leave them at room temperature for 1 h, and repeat this cycle 1 - 3 times.

[0023] In implementation, the chemical composition and weight percentage of the Al-Cu-Li-X series alloy are as follows: Cu 2.0 - 4.6%, Li 0.6 - 2.3%, Mn 0.10 - 0.80%, Zn 0.10 - 1.0%, Zr 0.04 - 0.20%, Mg 0.20% - 0.80%, Ag 0.1 - 0.7%, Si ≤ 0.10%, Fe ≤ 0.10%, Ti ≤ 0.12%, other impurities ≤ 0.05% individually, total ≤ 0.15%, and the balance is Al.

[0024] During implementation, the forging temperature of the die forging described in Step 1 is 510 °C.

[0025] During implementation, the transfer time for quenching after solution treatment in Step 1 does not exceed 30 seconds, and the temperature of the quenching medium is not higher than 60 °C.

[0026] During implementation, the cold pressing in Step 2 is carried out within 4 hours after the solution treatment and quenching in Step 1 are completed.

[0027] During implementation, the step-by-step cold pressing method is adopted in Step 2. First, the thicker part of the die forging is cold pressed, and then the die forging is cold pressed as a whole.

[0028] During implementation, the die forging targeted by the technical solution of the present invention is a multi-"H"-section combined structure for aircraft beams and frames. The dimensions of each part of the forging are inconsistent. Among them, the thickness of the upper and lower flange strips is 12 mm, the thickness of the rib strips is 10 mm, the height of the rib strips is 21 mm, and the thickness of the web is 4 mm and 8 mm. The dimensional tolerance requirement is ±0.2 mm.

[0029] During implementation, the die used for cold pressing in Step 2 is composed of two symmetrical upper and lower parts. The upper part includes an upper die holder (1), an upper die mating plate (2), and an upper die (3). The lower part includes a lower die holder (7), a lower die mating plate (6), and a lower die (5). A backing plate (4) is arranged between the upper die holder (1) and the lower die holder (7), and in combination with the upper die mating plate (2) and the lower die mating plate (6), the opening and closing gap between the upper die holder (1) and the lower die holder (7) is controlled.

[0030] The characteristics and beneficial effects of the technical solution of the present invention are as follows:

[0031] First, through cryogenic treatment and thermal cycling treatment after cold forming in the technical solution of the present invention, the crystal lattice expands and contracts during the cycle process, obtaining a stable residual stress state.

[0032] The technical solution of the present invention is directed to an Al-Li-Cu-X series aluminum-lithium alloy with a high Cu content (Cu content above 2%). The precipitation of the second phase is relatively complex. Generally, its precipitation process is considered as: GP zone → θ″ → θ′ → θ; at the same time, supersaturated solid solution → GP zone + δ′ → T1 + δ′ → T1. The main strengthening phases of the material are GP zone, δ′ and T1. The alloy contains other elements such as Mn, Zn, Mg, Zr, Ag, etc., and other second phases will also precipitate. With the precipitation of the second phase in stages such as aging, phase transformation and lattice distortion occur in the aluminum matrix, the material strength increases, and its residual stress also gradually increases. Generally, the aluminum-lithium alloy material reduces the residual stress by dislocation through cold deformation after quenching. After cold deformation, although the overall residual stress level of the material will decrease, there will be an unstable state.

[0033] In the technical solution of the present invention, cryogenic treatment is used as one of the important means to eliminate residual stress. The existing cryogenic technologies mainly include cryogenic rapid heating and thermal cycling processes. The workpiece is usually kept at a lower temperature first and then at a higher temperature. By using a large temperature difference between hot and cold, a thermal stress opposite to the residual stress after solution quenching is formed, so as to reduce the quenching residual stress of the workpiece. When keeping at a lower temperature, direct cryogenic treatment at -196°C with liquid nitrogen is often used. The direct contact between the workpiece and liquid nitrogen will form an obvious thermal shock effect, resulting in damage to the workpiece structure, and even causing irreversible damage such as low-temperature brittle fracture of some workpiece materials.

[0034] When implementing the technical solution of the present invention, the forging is a multi-"H"-shaped cross-section combined structure, and the thicknesses of the flange, rib, and web parts of the forging are different. An improper cryogenic treatment process not only cannot reduce the residual stress, but even has a negative effect of increasing the residual stress. Therefore, in the technical solution of the present invention, after cold pressing, the forging is placed in an environmental chamber, and the liquid nitrogen refrigerant does not directly contact the workpiece. The temperature of the workpiece is slowly and controllably reduced to a low temperature of -90°C, kept warm for 0.5 h, and then taken out and left to stand in the room-temperature air for 1 h. This first cryogenic and thermal cycling process measure will not form a large thermal shock effect, and the thermal expansion and contraction effect causes micro-plastic deformation, promoting the consumption of mobile dislocations and the entanglement and proliferation of fixed dislocations, thereby effectively reducing the residual stress. In addition, in the technical solution of the present invention, a second cryogenic and thermal cycling treatment is arranged after aging, so that the crystal lattice expands and contracts during the cycle, further releasing the internal residual stress of the workpiece, obtaining a stable residual stress state, and improving the dimensional stability of the workpiece.

[0035] Second, the technical solution of the present invention realizes low-residual stress processing of a forging with a multi-"H"-shaped cross-section combined structure.

[0036] The die forging of the Al-Li-Cu-X series aluminum-lithium alloy of the present invention has a multi-"H" cross-section combination structure. This die forging has a complex structure, which is based on the urgent need to reduce the weight of key load-bearing structures of military and civil aircraft. The overall design and preparation of components are adopted. The structure of the die forging is composed of multiple "H" shapes combined, and the thicknesses of each part are inconsistent. Among them, the thickness of the upper and lower flange bars of the die forging is 12 mm, the thickness of the rib bars is 10 mm, the height of the rib bars is 21 mm, and the thicknesses of the webs are 4 mm and 8 mm. The single pre-stretching method or flat anvil cold pressing method for eliminating residual stress is no longer applicable to the multi-"H" type complex structure die forging of the present invention. This die forging has typical characteristics of high ribs and thin webs, and non-negligible residual stress is generated during the forming and quenching processes. The high-rib and web parts with smaller thickness of the die forging are prone to deformation during machining and other processes, which adds additional sizing processes to the production of the die forging and even scrapps the workpiece. In addition to the aforementioned cryogenic and cold-hot cycle processes, the technical solution of the present invention also adopts a method combining cold pressing with cryogenic cold-hot cycles to control the residual stress of this multi-"H" type complex structure die forging at a relatively low level.

[0037] Due to the complex structure of the multi-"H" type die forging of the Al-Li-Cu-X series aluminum-lithium alloy with a high Cu content, it is difficult to control the uniformity and precision of the cold pressing deformation amount for eliminating residual stress. The technical solution of the present invention adopts a supporting cold pressing die and a step-by-step cold pressing method to solve this technical problem. There is a gap between the upper and lower dies of the conventional die forging tooling structure, and the upper and lower dies are not completely closed, belonging to open die forging. This structure makes it impossible to accurately and stably control the forging-down deformation amount during compression, resulting in excessive or insufficient deformation amounts in different parts of the multi-"H" type complex die forging with inconsistent dimensions of the flange bars, rib bars, and webs. This can cause problems such as low dimensional accuracy, unstable performance, and uneven elimination of residual stress of the die forging, and make the rib bar and web parts of the die forging prone to deformation during the machining process. For this reason, the die designed by the technical solution of the present invention is closed by adding a backing plate and jointly using adjustable upper die fitting plates and lower die fitting plates, belonging to closed die forging. This structure enables the forging-down deformation amount to be accurately and stably controlled during compression. Therefore, through the cold pressing of this die, it can be carried out step by step, and the precision cold pressing is completed in two steps or more than two steps according to the thicknesses of different parts of the die forging. Corresponding gaskets are designed and matched according to the different thicknesses of each part to ensure that the cold deformation amount after compression is 2-4%. During step-by-step compression, first place gaskets of a certain thickness at the flange bar and rib bar parts of the die forging to cause a certain degree of cold deformation in the flange bar and rib bar parts first, then take out the gaskets, and then complete the overall compression to cause cold deformation of the web. By adopting the step-by-step compression method, on the one hand, it ensures that the difficult-to-deform flange bar and rib bar parts have sufficient deformation amounts; on the other hand, it can ensure the uniformity of the cold compression deformation amounts of the flange bar, rib bar, and web parts, thereby effectively reducing the residual stress of each part of the forging. Description of the Drawings

[0038] Figure 1 Schematic diagram of the die structure used in the process of the present invention;

[0039] Figure 2 Schematic diagram of the forging product with a multi-"H" cross-sectional combined structure targeted by the process of the present invention

[0040] Figure 3 Three-dimensional schematic diagram of the forging product with a multi-"H" cross-sectional combined structure targeted by the process of the present invention

[0041] Figure 4 Photo of the forging product with a multi-"H" cross-sectional combined structure targeted by the process of the present invention

[0042] Figure 5 Comparison diagram of the residual stresses of die forgings under 5 processes of aging, aging after cold pressing, aging after deep cryogenic thermal cycling, aging after cold pressing - deep cryogenic thermal cycling, and cold pressing - deep cryogenic thermal cycling - aging - deep cryogenic thermal cycling. The die forging prepared by the cold pressing - deep cryogenic thermal cycling - aging - deep cryogenic thermal cycling process has the lowest residual stress. Detailed implementation manners

[0043] The technical solution of the present invention will be further described in detail below in conjunction with the drawings and embodiments:

[0044] This embodiment aims at preparing a qualified die forging of an Al-Li-Cu-X series aluminum-lithium alloy, and its alloy composition meets Cu 2.0 - 4.6%, Li 0.6 - 2.3%, Mn 0.10 - 0.80%, Zn 0.10 - 1.0%, Zr 0.04 - 0.20%, Mg 0.20% - 0.80%, Ag 0.1 - 0.7%, Si ≤ 0.10%, Fe ≤ 0.10%, Ti ≤ 0.12%, other impurities ≤ 0.05% individually, the total amount ≤ 0.15%, and the balance is Al. Its product is a multi-"H" cross-sectional combined structure, and its structure, shape, and dimensions are as Figure 2-4 shown.

[0045] The die used for cold pressing in this embodiment is as Figure 1 shown. The die consists of two symmetric upper and lower parts. The upper part includes an upper die base (1), an upper die mating plate (2), and an upper die (3). The lower part includes a lower die base (7), a lower die mating plate (6), and a lower die (5). A backing plate (4) is arranged between the upper die base (1) and the lower die base (7), and in combination with the upper die mating plate (2) and the lower die mating plate (6) to control the opening and closing gap between the upper die base (1) and the lower die base (7).

[0046] The process steps of Embodiment 1 are:

[0047] Step 1: Solution treatment and quenching

[0048] Heat the qualified Al-Li-Cu-X series aluminum-lithium alloy die forgings to 510°C for solution and quenching treatment. The transfer time for quenching after solution is no more than 30 seconds, and the temperature of the quenching medium is no higher than 60°C.

[0049] Step 2: Artificial aging

[0050] The artificial aging system is the third multi-stage non-isothermal aging. First, heat from 90°C to 145°C at a certain heating rate in the range of 90°C - 145°C, which takes 10h - 24h, and then continue to heat from 150°C to 195°C at a certain heating rate in the range of 150°C - 195°C, which takes 4h - 20h.

[0051] Use the X-ray method to measure the residual stress along the thickness direction of the cross-section of the forging. The test results are shown in Table 1. The results show that the maximum absolute value of the residual stress of the forging can reach about 150 MPa.

[0052] Table 1 Residual stress data of aluminum-lithium alloy die forgings after aging treatment

[0053]

[0054] The process steps of Example 2 are as follows:

[0055] Step 1: Solution and quenching

[0056] Heat the qualified Al-Li-Cu-X series aluminum-lithium alloy die forgings to 510°C for solution and quenching treatment. The transfer time for quenching after solution is no more than 30 seconds, and the temperature of the quenching medium is no higher than 60°C.

[0057] Step 2: Cold pressing

[0058] Place the die forging in the mold and perform cold pressing at room temperature. Adopt the step-by-step cold pressing method. First, cold press the thicker part of the die forging, and then cold press the whole die forging. The cold pressing deformation amount is 3%.

[0059] Step 3: Artificial aging

[0060] The artificial aging system is the third multi-stage non-isothermal aging. First, heat from 90°C to 145°C at a certain heating rate in the range of 90°C - 145°C, which takes 10h - 24h, and then continue to heat from 150°C to 195°C at a certain heating rate in the range of 150°C - 195°C, which takes 4h - 20h.

[0061] Use the X-ray method to measure the residual stress along the thickness direction of the cross-section of the forging. The test results are shown in Table 2. The results show that after adding 3% cold pressing treatment, the residual stress of the forging is significantly reduced, the maximum absolute value is not higher than 65 MPa, and the residual stress reduction rate reaches 72.47%.

[0062] Table 2 Residual stress data of aluminum-lithium alloy die forgings after cold pressing-aging treatment

[0063]

[0064] The process steps of Example 3 are as follows:

[0065] Step 1: Solution treatment and quenching

[0066] Heat the qualified Al-Li-Cu-X series aluminum-lithium alloy die forgings to 510°C for solution treatment and quenching. The transfer time for quenching after solution treatment does not exceed 30 seconds, and the temperature of the quenching medium is not higher than 60°C.

[0067] Step 2: First cryogenic treatment and thermal cycling

[0068] The die forgings are subjected to the first cryogenic treatment and thermal cycling in a cryogenic environment chamber, where the cryogenic temperatures are -90°C, -120°C, and -196°C respectively, the cryogenic times are 0.5 h and 1 h respectively, and the cryogenic cycle numbers are 1 time and 2 times respectively.

[0069] Step 3: Artificial aging

[0070] The artificial aging system is the third multi-stage non-isothermal aging. First, heat from 90°C to 145°C at a certain heating rate in the range of 90°C to 145°C, which takes 10 h to 24 h, and then continue to heat from 150°C to 195°C at a certain heating rate in the range of 150°C to 195°C, which takes 4 h to 20 h.

[0071] The residual stress in the thickness direction of the cross-section of the forgings is measured by the X-ray method, and the test results are shown in Table 3.

[0072] Table 3 Residual stress data of aluminum-lithium alloy die forgings after the first cryogenic thermal cycling-aging treatment

[0073]

[0074] The process steps of Example 4 are as follows:

[0075] Step 1: Solution treatment and quenching

[0076] Heat the qualified Al-Li-Cu-X series aluminum-lithium alloy die forgings to 510°C for solution treatment and quenching. The transfer time for quenching after solution treatment does not exceed 30 seconds, and the temperature of the quenching medium is not higher than 60°C.

[0077] Step 2: Cold pressing

[0078] The die forging is placed in a mold and cold pressed at room temperature. The step-by-step cold pressing method is adopted. First, the thicker part of the die forging is cold pressed, and then the whole die forging is cold pressed. The cold pressing deformation amount is 3%.

[0079] Step 3. First cryogenic treatment and thermal cycling

[0080] The die forging is subjected to the first cryogenic treatment and thermal cycling in a cryogenic environment chamber. The cryogenic temperatures are -90°C, -120°C, and -196°C respectively, the cryogenic times are 0.5 h and 1 h respectively, and the cryogenic cycle numbers are 1 time, 2 times, and 3 times respectively.

[0081] Step 4. Artificial aging

[0082] The artificial aging system is the third multi-stage non-isothermal aging. First, it is heated from 90°C to 145°C at a certain heating rate in the range of 90°C - 145°C, which takes 10 h - 24 h. Then it continues to be heated from 150°C to 195°C at a certain heating rate in the range of 150°C - 195°C, which takes 4 h - 20 h.

[0083] The X-ray method is used to measure the residual stress in the thickness direction of the cross-section of the forging. The test results are shown in Table 4. The results show that after the cold pressing deformation amount is 3%, and after the first different cryogenic thermal cycling process parameters and aging composite treatment, the overall residual stress of the forging shows an obvious decreasing trend.

[0084] Table 4 Residual stress data of Al-Li alloy die forgings after cold pressing - first cryogenic thermal cycling - aging treatment

[0085]

[0086]

[0087] The process steps of Example 5 are as follows:

[0088] Step 1. Solution treatment and quenching

[0089] The qualified Al-Li-Cu-X series Al-Li alloy die forging is heated to 510°C for solution treatment and quenching. The transfer time for quenching after solution treatment does not exceed 30 seconds, and the temperature of the quenching medium is not higher than 60°C.

[0090] Step 2. Cold pressing

[0091] The die forging is placed in a mold and cold pressed at room temperature. The step-by-step cold pressing method is adopted. First, the thicker part of the die forging is cold pressed, and then the whole die forging is cold pressed. The cold pressing deformation amount is 3%.

[0092] Step 3. First cryogenic treatment and thermal cycling

[0093] The die forgings are subjected to the first cryogenic and thermal cycling treatment in a cryogenic environmental chamber. The cryogenic temperatures are -90°C, -120°C, and -196°C respectively, the cryogenic times are 0.5 h and 1 h respectively, and the cryogenic cycle numbers are 1, 2, and 3 times respectively.

[0094] Step Four: Artificial aging

[0095] The artificial aging system is the third multi-stage non-isothermal aging. First, it is heated from 90°C to 145°C at a certain heating rate in the range of 90°C to 145°C, which takes 10 h to 24 h. Then, it continues to be heated from 150°C to 195°C at a certain heating rate in the range of 150°C to 195°C, which takes 4 h to 20 h.

[0096] Step Five: The second cryogenic and thermal cycling

[0097] The die forgings are subjected to the second cryogenic and thermal cycling treatment in a cryogenic environmental chamber. The cryogenic temperature is -90°C, the cryogenic time is 0.5 h, and the cryogenic cycle number is 1 time.

[0098] The X-ray method is used to measure the residual stress along the thickness direction of the cross-section of the forgings. The test results are shown in Table 5. The results show that after the combined treatment of 3% cold pressing deformation, the first cryogenic and thermal cycling, aging, and the second cryogenic and thermal cycling, the die forgings have the best residual stress reduction effect, the Cv value of the residual stress is relatively low, and the residual stress reduction rate can reach 90%.

[0099] Table 5 Residual stress data of Al-Li alloy die forgings after cold pressing - the first cryogenic and thermal cycling - aging - the second cryogenic and thermal cycling treatment

[0100]

[0101] Figure 5Figure for comparing residual stresses of Al-Li alloy die forgings under typical conditions of each embodiment. Embodiment 1 is the residual stress state of the die forgings of the present invention after artificial aging treatment, which is the residual stress state of the die forgings after conventional heat treatment. In the preparation process, the forgings are not subjected to cold pressing or cryogenic treatment to reduce residual stresses, and the residual stresses do not meet the standard of ±100 MPa. Embodiment 2 shows the influence of cold pressing on the residual stresses of the die forgings of the present invention, indicating that cold pressing significantly reduces the residual stresses of the die forgings, and the reduction rate of residual stresses reaches 72.47%. Embodiment 3 shows the influence of the first cryogenic treatment and thermal cycling on the residual stresses of the die forgings of the present invention, indicating that only applying the first cryogenic treatment and thermal cycling has a tendency to increase the overall residual stresses of the die forgings. Embodiment 4 shows the influence of the combined action of cold pressing and the first cryogenic treatment and thermal cycling on the residual stresses of the die forgings of the present invention, indicating that the combined action of cold pressing and the first cryogenic treatment and thermal cycling significantly reduces the overall residual stresses of the die forgings, and the maximum reduction rate of residual stresses can reach 81.86%. The first four embodiments summarize the residual stress levels of the die forgings of the present invention and the effects of existing residual stress reduction techniques. Embodiment 5 is a technical solution for reducing the residual stresses of die forgings proposed by the present invention, adopting a composite process of cold pressing, the first cryogenic thermal cycling, and the second cryogenic thermal cycling. The die forgings have the best residual stress reduction effect, the residual stresses meet the standard of ±100 MPa, the Cv value of the residual stresses is low, and the reduction rate of residual stresses is as high as 90%.

[0102] After the die forgings are treated by cold pressing - the first cryogenic thermal cycling - aging - the second cryogenic thermal cycling, the test results of their room temperature tensile properties and fracture toughness are shown in Table 6. The room temperature tensile test method for this forging is in accordance with GB / T 228.1 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature", and the fracture toughness test method is in accordance with HB 5487 "Test method for plane strain fracture toughness KIC of metallic materials".

[0103] Table 6 Properties of Al-Li alloy die forgings after cold pressing - the first cryogenic thermal cycling - aging - the second cryogenic thermal cycling

[0104]

[0105]

Claims

1. A method for controlling low residual stress of an aluminum-lithium alloy die forging, characterized in that: This method is applicable to Al-Cu-Li-X series alloys, and the steps of this method are as follows: Step 1: Solution treatment and quenching Heat the die forgings to 500°C - 540°C for solution treatment, and then perform quenching after solution treatment; Step 2: Cold pressing Place the die forgings in a mold and perform cold pressing at room temperature. It is required that the compression deformation amount of each part of the die forgings in the thickness direction reaches 2% - 4%; Step 3: First cryogenic treatment and thermal cycling Cool the cold-pressed die forgings in a cryogenic environment chamber to -90°C, hold for 0.5 h, take them out and let them stand at room temperature for 1 h, and repeat this cycle 1 - 3 times; Step 4: Artificial aging One of the following three methods is adopted for this artificial aging: The first method: Heat the die forgings to 160°C and hold for 18 - 24 h; The second method: Heat the die forgings to 90°C - 135°C, hold for 10 h - 24 h, then continue to heat to 150°C - 195°C and hold for 4 h - 20 h; The third method: Heat the die forgings to 90°C - 145°C, hold for 10 h - 24 h, then continue to heat to 150°C - 195°C and hold for 4 h - 20 h; Step 5: Second cryogenic treatment and thermal cycling Cool the artificially aged die forgings in a cryogenic environment chamber to -90°C, hold for 0.5 h, take them out and let them stand at room temperature for 1 h, and repeat this cycle 1 - 3 times.

2. The method for controlling low residual stress of an aluminum-lithium alloy die forging according to claim 1, wherein: The chemical composition and weight percentage of the Al-Cu-Li-X series alloy are as follows: Cu 2.0 - 4.6%, Li 0.6 - 2.3%, Mn 0.10 - 0.80%, Zn 0.10 - 1.0%, Zr 0.04 - 0.20%, Mg 0.20% - 0.80%, Ag 0.1 - 0.7%, Si ≤ 0.10%, Fe ≤ 0.10%, Ti ≤ 0.12%, the content of other single impurities ≤ 0.05%, the total content of impurities ≤ 0.15%, and the balance is Al.

3. The method for controlling low residual stress of an aluminum-lithium alloy die forging according to claim 1, wherein: The heating temperature of the die forgings in Step 1 is 510°C.

4. The method for controlling low residual stress of an Al-Li alloy die forging according to claim 1, characterized in that: The transfer time for quenching after solution treatment in Step 1 does not exceed 30 seconds, and the temperature of the quenching medium is not higher than 60°C.

5. The method for controlling low residual stress of an Al-Li alloy die forging according to claim 1, characterized in that: The cold pressing in Step 2 is carried out within 4 hours after the solution treatment and quenching in Step 1 are completed.

6. The method for controlling low residual stress of an aluminum-lithium alloy die forging according to claim 1, wherein: In Step 2, a step-by-step cold pressing method is adopted. First, cold press the thicker parts of the die forgings, and then perform overall cold pressing on the die forgings.

7. The method for controlling low residual stress of an Al-Li alloy die forging according to claim 1, wherein: The die forgings are a multi-"H" type cross-section combined structure for aircraft beams and frames. The dimensions of each part of the forgings are inconsistent. Among them, the thickness of the upper and lower flange strips is 12 mm, the thickness of the rib strips is 10 mm, the height of the rib strips is 21 mm, and the thickness of the web is 4 mm and 8 mm. The dimensional tolerance requirement is ±0.2 mm.

8. The method for controlling low residual stress of an aluminum-lithium alloy die forging according to claim 1, characterized in that: The mold used for cold pressing in Step 2 consists of two symmetric upper and lower parts. The upper part includes an upper mold base (1), an upper mold fitting plate (2) and an upper mold (3). The lower part includes a lower mold base (7), a lower mold fitting plate (6) and a lower mold (5). A backing plate (4) is arranged between the upper mold base (1) and the lower mold base (7), and in combination with the upper mold fitting plate (2) and the lower mold fitting plate (6), the opening and closing gap between the upper mold base (1) and the lower mold base (7) is controlled.

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