CN116240473A - Low residual stress control method for aluminum lithium alloy die forging - Google Patents
Low residual stress control method for aluminum lithium alloy die forging Download PDFInfo
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Abstract
The invention relates to a low residual stress control method for an aluminum lithium alloy die forging, which comprises the steps of (1) solid solution and quenching; (2) cold pressing; (3) cryogenic and thermal cycle; (4) aging; (5) The method adopts a cold pressing and deep cooling and heating circulation combined method to control the residual stress field of the aluminum lithium alloy die forging, the die forging firstly completes cold compression in a precision die to reduce the residual stress level wholly, and then the crystal lattice stretches and changes in the circulation process through the deep cooling and heating circulation treatment to obtain a stable residual stress state. The two methods complement each other in a coordinated way, and the residual stress of the aluminum-lithium alloy die forging is controlled at a lower level, so that the aluminum-lithium alloy die forging with excellent performance and lower residual stress is obtained. The process technology is suitable for the production and application of the aluminum-lithium alloy die forging for aerospace, ships and automobiles.
Description
Technical Field
The invention relates to a low residual stress control method for an aluminum lithium alloy die forging, and belongs to the field of metal material engineering.
Background
The invention relates to a key process technology for reducing residual stress of Al-Li-Cu-X series aluminum lithium alloy die forgings and improving processing qualification rate and material utilization rate of aluminum lithium alloy parts, which is characterized in that the modern mechanical design is more and more focused on low energy consumption and long service life, and the requirements of long voyage, long service life and economy of transportation machinery such as aircrafts, ships and the like are met, and the requirements of high specific strength and high specific modulus materials are also more and more increased.
In recent years, some high performance aluminum lithium alloys such as 2098/2198, 2097 series, 2196, 2195, 2099, etc. have been developed in the art, but the control of residual stress of aluminum lithium alloys has been mainly focused on materials such as thick plates and free forgings. At present, residual stress of thick plates and free forgings is mainly reduced by a prestretching method and a flat anvil cold pressing method. Although the thick plate and the free forging have the advantages of strong adaptability, low residual stress, stable process and the like in the aspect of the production of the aircraft structural member, the defects of the structural member such as cut-off of macroscopic streamline, anisotropic mechanical property, serious material waste, high cost and the like in the subsequent machining process are also present.
Disclosure of Invention
The invention provides a low residual stress control method for an aluminum-lithium alloy die forging, which aims to control the residual stress field of the aluminum-lithium alloy die forging by adopting a cold pressing and deep cooling and heat and cold circulation combined method, and the two methods are mutually coordinated and supplemented to control the residual stress of the aluminum-lithium alloy die forging at a lower level, so that the aluminum-lithium alloy die forging with excellent performance and lower residual stress is obtained.
The aim of the invention is realized by the following technical scheme:
the low residual stress control method of the aluminum lithium alloy die forging aims at Al-Cu-Li-X alloy, and comprises the following process steps:
step one, solutionizing and quenching
Heating the die forging to 500-540 ℃ for solid solution, and quenching after solid solution;
step two, cold pressing
Putting the die forging into a die, and cold pressing at room temperature, wherein the compression deformation of each part of the die forging along the thickness direction is required to reach 2-4%;
third, the first deep cooling and cold and hot circulation
Cooling the cold-pressed die forging to-90 ℃ in a cryogenic environment box, preserving heat for 0.5h, taking out, and standing for 1h at room temperature, and circulating for 1-3 times;
step four, artificial aging
The artificial aging adopts one of the following three methods:
first kind: heating the die forging to 160 ℃, and preserving heat for 18-24 hours;
second kind: heating the die forging to 90-135 ℃, preserving heat for 10-24 h, continuously heating to 150-195 ℃ and preserving heat for 4-20 h;
third kind: heating the die forging to 90-145 ℃, preserving heat for 10-24 hours, continuously heating to 150-195 ℃, and preserving heat for 4-20 hours;
the first is single-stage isothermal aging, such as heat preservation at 160 ℃ for 18-24 h;
the second is multistage isothermal aging, such as selecting a temperature point in the range of 90-135 ℃ and preserving heat for 10-24 hours, then continuing to select a temperature point in the range of 150-195 ℃ and heating, and preserving heat for 4-20 hours;
the third is multi-stage non-isothermal aging, such as heating from 90 ℃ to 145 ℃ at a certain heating rate in the interval of 90-145 ℃ for 10-24 hours, then continuously heating from 150 ℃ to 195 ℃ at a certain heating rate in the interval of 150-195 ℃ for 4-20 hours;
fifth, second deep cooling and cold-hot circulation
Cooling the artificially aged die forging to-90 ℃ in a cryogenic environment box, preserving heat for 0.5h, taking out, and standing for 1h at room temperature, thus circulating for 1-3 times.
In implementation, the Al-Cu-Li-X alloy comprises the following chemical components in percentage by weight: 2.0 to 4.6 percent of Cu, 0.6 to 2.3 percent of Li, 0.10 to 0.80 percent of Mn, 0.10 to 1.0 percent of Zn, 0.04 to 0.20 percent of Zr, 0.20 to 0.80 percent of Mg, 0.1 to 0.7 percent of Ag, less than or equal to 0.10 percent of Si, less than or equal to 0.10 percent of Fe, less than or equal to 0.12 percent of Ti, less than or equal to 0.05 percent of other impurities, less than or equal to 0.15 percent of the total amount, and the balance of Al.
In practice, the die forging heating temperature in step one is 510 ℃.
In the implementation, the transfer time of quenching after solid solution in the first step is not more than 30 seconds, and the temperature of a quenching medium is not higher than 60 ℃.
In the implementation, the cold pressing in the second step is performed within 4 hours after the solid solution and quenching are completed in the first step.
In the implementation, a step-by-step cold pressing mode is adopted in the second step, the thicker die forging part is firstly subjected to cold pressing, and then the die forging part is subjected to integral cold pressing.
In implementation, the die forging provided by the technical scheme of the invention is of a multi-H-shaped cross section combined structure for an airplane beam and a frame, and all parts of the forging are inconsistent in size, wherein the thickness of an upper edge strip and a lower edge strip is 12mm, the thickness of a rib strip is 10mm, the height of the rib strip is 21mm, the thickness of a web plate is 4mm and 8mm, and the dimensional tolerance requirement is +/-0.2 mm.
When the cold pressing die is implemented, the die used in the second step consists of an upper symmetrical structure and a lower symmetrical structure, wherein the upper structure comprises an upper die holder (1), an upper die anastomosis plate (2) and an upper die (3), the lower structure comprises a lower die holder (7), a lower die anastomosis plate (6) and a lower die (5), and a base plate (4) is arranged between the upper die holder (1) and the lower die holder (7) to be combined with the upper die anastomosis plate (2) and the lower die anastomosis plate (6) so as to control the opening and closing gaps of the upper die holder (1) and the lower die holder (7).
The technical scheme of the invention has the characteristics and beneficial effects that:
1. according to the technical scheme, the crystal lattice is subjected to expansion and contraction change in the circulation process through deep cooling and cold and heat circulation treatment after cold forming, so that a stable residual stress state is obtained.
Aiming at the Al-Li-Cu-X aluminum lithium alloy with high Cu content (Cu content is more than 2 percent), the second phase precipitation is more complex, and the precipitation process is generally considered as follows: GP zone → θ "→ θ' → θ; simultaneously supersaturated solid solution→gp region+δ '→t1+δ' →t1. The main strengthening phases of the material are GP zone, delta' and T1, other elements such as Mn, zn, mg, zr, ag and the like are contained in the alloy, and other second phases are precipitated. With the precipitation of the second phase in the aging stage, the phase change and lattice distortion occur in the aluminum matrix, the strength of the material is increased, and the residual stress of the material is gradually increased. In general, aluminum lithium alloy materials are subjected to quenching and then cold deformation to generate dislocation so as to reduce residual stress. After cold deformation, the residual stress level of the material is reduced as a whole, but an unstable state exists.
In the technical scheme of the invention, the deep cooling treatment is used as one of important means for eliminating residual stress. The existing deep cooling technology mainly adopts a deep cooling and rapid heating and cold-hot circulation process, a workpiece is always kept at a lower temperature and then kept at a higher temperature, and a larger cold-hot temperature difference is utilized to form thermal stress opposite to residual stress after solution quenching, so that the quenching residual stress of the workpiece is reduced. At lower temperature, liquid nitrogen is often directly adopted for cryogenic treatment at-196 ℃, and the direct contact of a workpiece and the liquid nitrogen can form obvious thermal shock effect, so that the workpiece tissue is damaged, and even some material workpieces are irreversibly damaged such as low-temperature brittle fracture.
When the technical scheme of the invention is implemented, the die forging is of a multi-H-shaped cross section combined structure, the thicknesses of the forging edge strips, the ribs and the web plate parts are different, and the improper cryogenic treatment process cannot reduce residual stress and even has the negative effect of increasing the residual stress. Therefore, the technical scheme of the invention is that after cold pressing, the die forging is placed in an environment box, a liquid nitrogen refrigerant is not in direct contact with a workpiece, the temperature of the workpiece is slowly and controllably reduced to the low temperature of-90 ℃, the temperature is kept for 0.5h, and then the die forging is taken out and is stopped in room temperature air for 1h. The first deep cooling and cold-hot circulation process measures can not form a larger thermal shock effect, and the thermal expansion and cold contraction effect induces micro plastic deformation, so that the movable dislocation consumption, the fixed dislocation entanglement and proliferation are promoted, and the residual stress is effectively reduced. In addition, the technical scheme of the invention arranges the second deep cooling and cold and hot circulation treatment after aging, so that the crystal lattice stretches and changes in the circulation process, the residual stress in the workpiece is further released, a stable residual stress state is obtained, and the dimensional stability of the workpiece is improved.
2. The technical scheme of the invention realizes the low residual stress processing of the die forging with the multi-H-shaped section combined structure.
The die forging of the Al-Li-Cu-X aluminum lithium alloy is of a multi-H-shaped section combined structure, the die forging structure is complex, the structure is based on urgent requirements of weight reduction of a force bearing structure of a military and civil engineering, the integrated design and the preparation of parts are adopted, the die forging structure is formed by combining a plurality of H-shaped structures, the dimensions and the thicknesses of all parts are inconsistent, wherein the thicknesses of upper and lower edge strips of the die forging are 12mm, the thicknesses of ribs are 10mm, the heights of the ribs are 21mm, and the thicknesses of webs are 4mm and 8mm. The single prestretching method or the flat anvil cold pressing method for reducing residual stress is not applicable to the multi-H-shaped complex-structure die forging. The die forging has the characteristic of typical high ribs and thin webs, non-negligible residual stress is generated in the forming and quenching processes, and the high ribs and web parts with smaller thickness of the die forging are easy to deform in the machining processes and the like, so that extra shaping processes are added in the production of the die forging, and even workpieces are scrapped. Besides the deep cooling and cold-hot circulation process, the technical scheme of the invention also adopts a method of combining cold pressing and deep cooling and cold-hot circulation, so that the residual stress of the multi-H-shaped complex-structure die forging is controlled at a lower level.
Because the multi-H-shaped die forging piece of the Al-Li-Cu-X aluminum lithium alloy with high Cu content has a complex structure, the uniformity of cold pressing deformation and the difficulty in controlling precision for reducing residual stress are high, the technical scheme of the invention adopts a matched cold pressing die and a step-by-step cold pressing method to solve the technical problem. Gaps are reserved between the upper die and the lower die of the conventional die forging tool structure, the upper die and the lower die are not completely closed, the structure ensures that the pressing deformation during compression cannot be accurately and stably controlled, the phenomenon that deformation of all parts with inconsistent dimensions of a plurality of H-shaped complex die forging strips, ribs and webs is overlarge or undersize is caused, and the problems of low dimensional accuracy, unstable performance and uneven reduction of residual stress of the die forging can be caused, so that the ribs and the webs of the die forging are easy to deform in a machining process. Therefore, the die designed by the technical scheme of the invention is combined with the adjustable upper die anastomosis plate and the lower die anastomosis plate by adding the backing plate, so that the upper die and the lower die are closed, and the die belongs to closed die forging. Therefore, the cold pressing of the die can be performed step by step, the precision cold pressing is completed in two steps or more than two steps according to the thickness of different parts of the die forging, and the cold deformation amount after compression is ensured to be 2-4% according to the design of different thicknesses of all parts and matching with corresponding gaskets. When the gasket is compressed step by step, the gasket with certain thickness is firstly arranged at the edge strip and the rib strip of the die forging, so that the edge strip and the rib strip are firstly subjected to cold deformation to a certain extent, then the gasket is taken out, and then the whole compression is completed, so that the web is subjected to cold deformation. By adopting a step-by-step compression method, on one hand, the edge strip and the rib strip which are difficult to deform are ensured to have enough deformation; on the other hand, the uniformity of cold compression deformation of the edge strip, the rib and the web plate can be ensured, so that residual stress of each part of the forging is effectively reduced.
Drawings
FIG. 1 is a schematic diagram of a mold used in the process of the present invention;
FIG. 2 is a schematic illustration of a forging product of a multi-H-section composite structure for which the process of the present invention is directed
FIG. 3 is a three-dimensional schematic view of a forging product of a multi-H-section composite structure for which the process of the present invention is directed
FIG. 4 is a photograph of a forging product of a multi-H-section composite structure for which the process of the present invention is directed
FIG. 5 is a graph showing the residual stress of a die forging in 5 processes of aging, aging after cold pressing, aging after deep cooling and heating cycle, aging after cold pressing-deep cooling and heating cycle, and cold pressing-deep cooling and heating cycle-aging-deep cooling and heating cycle, wherein the die forging prepared by the cold pressing-deep cooling and heating cycle-aging-deep cooling and heating cycle process has the lowest residual stress.
Detailed Description
The technical scheme of the invention is further described in detail below with reference to the attached drawings and examples:
the embodiment aims at preparing qualified Al-Li-Cu-X aluminum lithium alloy die forgings, wherein alloy components of the die forgings meet the requirements of 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 less than or equal to 0.10%, fe less than or equal to 0.10%, ti less than or equal to 0.12%, other impurities are singly less than or equal to 0.05%, the total amount is less than or equal to 0.15%, and the balance is Al. The product is a multi-H-shaped section combined structure, and the structure, the shape and the size of the product are shown in figures 2-4.
The cold pressing mold adopted in this embodiment is shown in fig. 1, and is composed of an upper and a lower symmetrical two-part structure, wherein the upper structure comprises an upper mold base (1), an upper mold anastomosis plate (2) and an upper mold (3), the lower structure comprises a lower mold base (7), a lower mold anastomosis plate (6) and a lower mold (5), and a base plate (4) is arranged between the upper mold base (1) and the lower mold base (7) to be combined with the upper mold anastomosis plate (2) and the lower mold anastomosis plate (6) so as to control the opening and closing gaps of the upper mold base (1) and the lower mold base (7).
The process steps of example 1 are:
step one, solutionizing and quenching
And heating the qualified Al-Li-Cu-X aluminum lithium alloy die forging to 510 ℃ for solid solution and quenching treatment. The transfer time of quenching after solid solution is not more than 30 seconds, and the temperature of the quenching medium is not higher than 60 ℃.
Step two, artificial aging
The artificial aging system is a third multi-stage non-isothermal aging, wherein the temperature is firstly increased from 90 ℃ to 145 ℃ at a certain temperature increasing rate in a range of 90-145 ℃ for 10-24 hours, and then the temperature is continuously increased from 150 ℃ to 195 ℃ at a certain temperature increasing rate in a range of 150-195 ℃ for 4-20 hours.
The residual stress of the cross section of the forging piece along the thickness direction is measured by an X-ray method, and the test result is shown in Table 1. The result shows that the maximum value of the absolute value of the residual stress of the forging can reach about 150MPa.
Table 1 residual stress data after aging treatment of aluminum lithium alloy die forgings
The process steps of example 2 are:
step one, solutionizing and quenching
And heating the qualified Al-Li-Cu-X aluminum lithium alloy die forging to 510 ℃ for solid solution and quenching treatment. The transfer time of quenching after solid solution is not more than 30 seconds, and the temperature of the quenching medium is not higher than 60 ℃.
Step two, cold pressing
And putting the die forging into a die, cold pressing at room temperature, and adopting a step-by-step cold pressing mode to cold press the thicker die forging part, and then integrally cold pressing the die forging, wherein the cold pressing deformation is 3%.
Step three, artificial aging
The artificial aging system is a third multi-stage non-isothermal aging, wherein the temperature is firstly increased from 90 ℃ to 145 ℃ at a certain temperature increasing rate in a range of 90-145 ℃ for 10-24 hours, and then the temperature is continuously increased from 150 ℃ to 195 ℃ at a certain temperature increasing rate in a range of 150-195 ℃ for 4-20 hours.
The residual stress of the cross section of the forging piece along the thickness direction is measured by an X-ray method, and the test result is shown in Table 2. The result shows that after 3% cold pressing treatment is added, the residual stress of the forging piece is obviously reduced, the maximum absolute value is not higher than 65MPa, and the residual stress relief rate reaches 72.47%.
Table 2 residual stress data after cold press-aging treatment of aluminum lithium alloy die forgings
The process steps of example 3 are:
step one, solutionizing and quenching
And heating the qualified Al-Li-Cu-X aluminum lithium alloy die forging to 510 ℃ for solid solution and quenching treatment. The transfer time of quenching after solid solution is not more than 30 seconds, and the temperature of the quenching medium is not higher than 60 ℃.
Step two, first deep cooling, cold and hot circulation
The die forging is subjected to primary cryogenic cooling and cold-hot circulation treatment in a cryogenic environment box, wherein the cryogenic temperatures are respectively-90 ℃, -120 ℃, -196 ℃, the cryogenic times are respectively 0.5h and 1h, and the cryogenic circulation times are respectively 1 time, 2 times and 3 times.
Step three, artificial aging
The artificial aging system is a third multi-stage non-isothermal aging, wherein the temperature is firstly increased from 90 ℃ to 145 ℃ at a certain temperature increasing rate in a range of 90-145 ℃ for 10-24 hours, and then the temperature is continuously increased from 150 ℃ to 195 ℃ at a certain temperature increasing rate in a range of 150-195 ℃ for 4-20 hours.
The residual stress of the cross section of the forging piece along the thickness direction is measured by an X-ray method, and the test result is shown in Table 3.
Table 3 residual stress data after first deep cooling heat cycle-aging treatment of aluminum lithium alloy die forgings
The process steps of example 4 are:
step one, solutionizing and quenching
And heating the qualified Al-Li-Cu-X aluminum lithium alloy die forging to 510 ℃ for solid solution and quenching treatment. The transfer time of quenching after solid solution is not more than 30 seconds, and the temperature of the quenching medium is not higher than 60 ℃.
Step two, cold pressing
And putting the die forging into a die, cold pressing at room temperature, and adopting a step-by-step cold pressing mode to cold press the thicker die forging part, and then integrally cold pressing the die forging, wherein the cold pressing deformation is 3%.
Third, the first deep cooling and cold and hot circulation
The die forging is subjected to primary cryogenic cooling and cold-hot circulation treatment in a cryogenic environment box, wherein the cryogenic temperatures are respectively-90 ℃, -120 ℃, -196 ℃, the cryogenic times are respectively 0.5h and 1h, and the cryogenic circulation times are respectively 1 time, 2 times and 3 times.
Step four, artificial aging
The artificial aging system is a third multi-stage non-isothermal aging, wherein the temperature is firstly increased from 90 ℃ to 145 ℃ at a certain temperature increasing rate in a range of 90-145 ℃ for 10-24 hours, and then the temperature is continuously increased from 150 ℃ to 195 ℃ at a certain temperature increasing rate in a range of 150-195 ℃ for 4-20 hours.
The residual stress of the cross section of the forging piece along the thickness direction is measured by an X-ray method, and the test result is shown in Table 4. The result shows that after the cold pressing deformation amount is 3%, different deep cooling and heating cycle technological parameters and ageing compound treatment are adopted for the first time, the residual stress of the forging is obviously reduced overall.
Table 4 residual stress data after cold pressing-first sub-zero heat cycle-aging treatment of aluminum lithium alloy die forgings
The process steps of example 5 are:
step one, solutionizing and quenching
And heating the qualified Al-Li-Cu-X aluminum lithium alloy die forging to 510 ℃ for solid solution and quenching treatment. The transfer time of quenching after solid solution is not more than 30 seconds, and the temperature of the quenching medium is not higher than 60 ℃.
Step two, cold pressing
And putting the die forging into a die, cold pressing at room temperature, and adopting a step-by-step cold pressing mode to cold press the thicker die forging part, and then integrally cold pressing the die forging, wherein the cold pressing deformation is 3%.
Third, the first deep cooling and cold and hot circulation
The die forging is subjected to primary cryogenic cooling and cold-hot circulation treatment in a cryogenic environment box, wherein the cryogenic temperatures are respectively-90 ℃, -120 ℃, -196 ℃, the cryogenic times are respectively 0.5h and 1h, and the cryogenic circulation times are respectively 1 time, 2 times and 3 times.
Step four, artificial aging
The artificial aging system is a third multi-stage non-isothermal aging, wherein the temperature is firstly increased from 90 ℃ to 145 ℃ at a certain temperature increasing rate in a range of 90-145 ℃ for 10-24 hours, and then the temperature is continuously increased from 150 ℃ to 195 ℃ at a certain temperature increasing rate in a range of 150-195 ℃ for 4-20 hours.
Fifth, second deep cooling and cold-hot circulation
And performing secondary cryogenic cooling and cold-hot circulation treatment on the die forging in a cryogenic environment box, wherein the cryogenic temperature is-90 ℃, the cryogenic time is 0.5h, and the cryogenic circulation times are 1.
The residual stress of the cross section of the forging piece along the thickness direction is measured by an X-ray method, and the test result is shown in Table 5. The result shows that after the cold pressing deformation is 3%, the first deep cooling and heating cycle, aging and the second deep cooling and heating cycle are combined, the reduction effect of the residual stress of the forging piece is optimal, the Cv value of the residual stress is lower, and the reduction rate of the residual stress can reach 90%.
Table 5 residual stress data after cold pressing-first sub-zero heat cycle-aging-second sub-zero heat cycle treatment of aluminum lithium alloy die forgings
FIG. 5 is a graph comparing residual stresses of aluminum lithium alloy die forgings under typical conditions of the respective examples. Example 1 is the residual stress state of the die forging after artificial aging treatment, which is the residual stress state of the die forging after conventional heat treatment, and the forging is not subjected to cold pressing or deep cooling and other residual stress reduction treatment in the preparation process, and the residual stress does not reach the standard of +/-100 MPa. Example 2 is the effect of cold pressing on the residual stress of the die forging, and shows that the cold pressing obviously reduces the residual stress of the die forging, and the residual stress relief rate reaches 72.47%. Example 3 is an illustration of the effect of a first sub-cooling, cold and hot cycle on the die forging residual stress of the present invention, showing that the application of only the first sub-cooling, cold and hot cycle tends to increase the overall die forging residual stress. Example 4 shows the effect of cold pressing and first deep cooling and cold-hot circulation combined action on the residual stress of the die forging piece, and shows that the cold pressing and first deep cooling and cold-hot circulation combined action leads the total residual stress of the die forging piece to be obviously reduced, and the residual stress reduction rate can reach 81.86 percent. The four previous examples outline the residual stress levels of the die forgings of the present invention and the effects of existing residual stress relief techniques. Embodiment 5 is a technical scheme for reducing residual stress of a die forging, which is provided by the invention, by adopting a cold pressing, first cryogenic cooling and heating cycle and second cryogenic cooling and heating cycle composite process, the residual stress reducing effect of the die forging is optimal, the residual stress reaches +/-100 MPa standard, the residual stress Cv value is lower, and the residual stress reducing rate is as high as 90%.
The test results of room temperature stretching and fracture toughness of the die forging after cold pressing, first deep cooling and heating cycle, aging and second deep cooling and heating cycle are shown in table 6. The room temperature tensile test method of the forging comprises the following steps of: room temperature test methods, fracture toughness test methods according to HB 5487, "method of metal material plane strain fracture toughness KIC test".
Table 6 performance of aluminum lithium alloy die forgings after cold pressing-first cryogenic cooling and heating cycle-aging-second cryogenic cooling and heating cycle treatment
Claims (8)
1. A low residual stress control method for an aluminum lithium alloy die forging is characterized by comprising the following steps of: the method aims at Al-Cu-Li-X alloy, and comprises the following steps:
step one, solutionizing and quenching
Heating the die forging to 500-540 ℃ for solid solution, and quenching after solid solution;
step two, cold pressing
Putting the die forging into a die, and cold pressing at room temperature, wherein the compression deformation of each part of the die forging along the thickness direction is required to reach 2-4%;
third, the first deep cooling and cold and hot circulation
Cooling the cold-pressed die forging to-90 ℃ in a cryogenic environment box, preserving heat for 0.5h, taking out, and standing for 1h at room temperature, and circulating for 1-3 times;
step four, artificial aging
The artificial aging adopts one of the following three methods:
first kind: heating the die forging to 160 ℃, and preserving heat for 18-24 hours;
second kind: heating the die forging to 90-135 ℃, preserving heat for 10-24 h, continuously heating to 150-195 ℃ and preserving heat for 4-20 h;
third kind: heating the die forging to 90-145 ℃, preserving heat for 10-24 hours, continuously heating to 150-195 ℃, and preserving heat for 4-20 hours;
fifth, second deep cooling and cold-hot circulation
Cooling the artificially aged die forging to-90 ℃ in a cryogenic environment box, preserving heat for 0.5h, taking out, and standing for 1h at room temperature, thus circulating for 1-3 times.
2. The method for controlling the low residual stress of the aluminum lithium alloy die forging according to claim 1, wherein the method comprises the following steps of: the Al-Cu-Li-X alloy comprises the following chemical components in percentage by weight: 2.0 to 4.6 percent of Cu, 0.6 to 2.3 percent of Li0.10 to 0.80 percent of Mn, 0.10 to 1.0 percent of Zn, 0.04 to 0.20 percent of Zr, 0.20 to 0.80 percent of Mg, 0.1 to 0.7 percent of Ag, less than or equal to 0.10 percent of Si, less than or equal to 0.10 percent of Fe, less than or equal to 0.12 percent of Ti, less than or equal to 0.05 percent of other impurities, less than or equal to 0.15 percent of the total amount, and the balance of Al.
3. The method for controlling the low residual stress of the aluminum lithium alloy die forging according to claim 1, wherein the method comprises the following steps of: the heating temperature of the die forging in the first step is 510 ℃.
4. The method for controlling the low residual stress of the aluminum lithium alloy die forging according to claim 1, wherein the method comprises the following steps of: and (3) transferring time for quenching after solid solution in the step one is not more than 30 seconds, and the temperature of a quenching medium is not higher than 60 ℃.
5. The method for controlling the low residual stress of the aluminum lithium alloy die forging according to claim 1, wherein the method comprises the following steps of: the cold pressing in the second step is carried out within 4 hours after the solid solution and quenching are completed in the first step.
6. The method for controlling the low residual stress of the aluminum lithium alloy die forging according to claim 1, wherein the method comprises the following steps of: and step two, adopting a step-by-step cold pressing mode, firstly carrying out cold pressing on the thicker die forging part, and then carrying out integral cold pressing on the die forging.
7. The method for controlling the low residual stress of the aluminum lithium alloy die forging according to claim 1, wherein the method comprises the following steps of: the die forging is of a multi-H-shaped section combined structure for an airplane beam and a frame, the sizes of all parts of the forging are inconsistent, the thickness of an upper edge strip and a lower edge strip is 12mm, the thickness of a rib strip is 10mm, the height of the rib strip is 21mm, the thickness of a web plate is 4mm and 8mm, and the dimensional tolerance is required to be +/-0.2 mm.
8. The method for controlling the low residual stress of the aluminum lithium alloy die forging according to claim 1, wherein the method comprises the following steps of: the die used for cold pressing in the second step consists of an upper symmetrical structure and a lower symmetrical structure, wherein the upper structure comprises an upper die holder (1), an upper die anastomosis plate (2) and an upper die (3), the lower structure comprises a lower die holder (7), a lower die anastomosis plate (6) and a lower die (5), and a base plate (4) combined with the upper die anastomosis plate (2) and the lower die anastomosis plate (6) is arranged between the upper die holder (1) and the lower die holder (7) so as to control the opening and closing gap of the upper die holder (1) and the lower die holder (7).
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