A method for residual stress relief of aluminum alloy die forgings

Abstract

The present application belongs to the technical field of aluminum alloy die forging manufacturing, and provides a method for eliminating residual stress of aluminum alloy die forgings, which can solve the problems of large residual stress and uneven distribution of residual stress of complex die forgings with frame rib structure. The method is to use a composite cold pressing die composed of a metal die and a flexible die to perform cold deformation treatment on the aluminum alloy die forgings, so that the maximum value of the bulk residual stress of the core of the aluminum alloy die forgings is controlled to be below 150 MPa, and the unevenness is controlled to be below 50 MPa. Specifically, during the cold deformation treatment, the residual stress at positions such as webs in the aluminum alloy die forgings can be eliminated by the metal die, and the residual stress in areas such as frame ribs and round corners of the aluminum alloy die forgings which cannot directly contact the metal die can be eliminated by the flexible die, thereby effectively reducing the machining deformation caused by the release of residual stress during the machining process of the frame rib type parts.

Description

Technical Field

[0001] This invention belongs to the field of aluminum alloy forging manufacturing technology, and specifically relates to a method for eliminating residual stress in aluminum alloy forgings. Background Technology

[0002] Aluminum alloys, due to their advantages such as low density, high specific strength, low cost, and ease of machining, are widely used in the aerospace field as the main load-bearing components of aircraft. Driven by the development needs for weight reduction and efficiency improvement in aircraft, advanced aircraft commonly employ large-scale integral die forgings to reduce assembly weight and increase overall structural rigidity. However, die forgings typically have complex structures and generally suffer from high levels of residual stress. During the machining process of forgings into parts, deformation can occur due to the release of residual stress, potentially leading to quality problems such as out-of-tolerance geometric shapes.

[0003] Traditional cold pressing is performed using rigid molds made of materials such as cast steel or die steel. During cold pressing, the rigid mold displaces upon contact with the surface of the forging material, causing plastic deformation and redistributing residual stress. This cold deformation method can thus eliminate residual stress in the forging. However, complex forgings often have intricate structures such as ribs. Because the mold can only move vertically during cold pressing, it cannot effectively eliminate residual stress in the rib structure. Consequently, the forging deforms during machining due to the release of residual stress. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for eliminating residual stress in aluminum alloy forgings. This method solves the problems of high residual stress and uneven residual stress distribution in complex forgings with frame rib structures. The method utilizes a composite cold-pressing die composed of a metal die and a flexible die to perform cold deformation treatment on the aluminum alloy forging, controlling the maximum residual stress in the core of the forging to below 150 MPa and the non-uniformity to below 50 MPa. Specifically, during the cold deformation process, residual stress in areas such as the web of the aluminum alloy forging can be eliminated using the metal die, while residual stress in areas such as the frame ribs and fillets that cannot directly contact the metal die can be eliminated using the flexible die. This effectively reduces machining deformation caused by residual stress release during the machining of frame rib-type parts.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A method for eliminating residual stress in aluminum alloy forgings, the method comprising the following steps:

[0007] S1) Design a composite cold pressing die that matches the shape of the aluminum alloy forging based on the shape of the aluminum alloy forging;

[0008] The composite cold pressing die includes an upper die and a lower die. After the upper die and the lower die are closed, cold pressing deformation of the aluminum alloy forging can be achieved. The upper die and the lower die each include a metal die and a flexible die. The flexible die is nested on the metal die and ensures that the metal die contacts the top surface of the web and frame ribs of the aluminum alloy forging during the cold deformation process. The flexible die contacts areas of the aluminum alloy forging and the metal die that cannot directly contact each other during the cold deformation process.

[0009] S2) By using finite element simulation technology, the residual stress distribution of aluminum alloy forgings is analyzed, the main parameters of the composite cold pressing die are adjusted, and the profile of the composite cold pressing die is iteratively optimized so that the maximum residual stress of the aluminum alloy forgings is less than 150MPa.

[0010] The main parameters include the cold pressing deformation amount 'a' and the flexible die interference dimension 't'; wherein, the cold pressing deformation amount 'a' satisfies a=2c / T, where c is the cold pressing deformation reduction amount in mm; T is the cross-sectional thickness of the aluminum alloy forging (the thickness of the web of the aluminum alloy forging), in mm; and the flexible die interference dimension 't' is in mm.

[0011] S3) Fix the iteratively optimized composite cold pressing mold onto the forging equipment, place the solution-quenched aluminum alloy forging on the lower mold of the composite cold pressing mold, and control the upper mold to close the mold through the forging equipment to realize the cold pressing deformation of the aluminum alloy forging, and finally realize the elimination of residual stress in the aluminum alloy forging.

[0012] According to an embodiment of the present invention, in step S1), the aluminum alloy forging includes a web and frame ribs, the frame ribs being disposed on the web, and the web and frame ribs being integrally formed by forging; the number of frame ribs is not particularly limited and can be selected according to the application field of the aluminum alloy forging.

[0013] According to an embodiment of the present invention, in step S1), the area where the aluminum alloy forging and the metal mold cannot directly contact each other is, for example, the side wall and rounded corner of the frame rib of the aluminum alloy forging (such as the connection between the web and the frame rib).

[0014] According to the embodiment of the present invention, in step S2), the residual stress distribution of the aluminum alloy forging is analyzed by finite element simulation technology, the main parameters of the composite cold pressing die are adjusted, the cold pressing deformation a and the interference dimension t of the flexible die are determined, and the profile of the composite cold pressing die is iteratively optimized so that the maximum residual stress of the aluminum alloy forging is less than 150MPa.

[0015] According to an embodiment of the present invention, in step S2), the cold pressing deformation amount a is 1.2~2.5%, for example, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4% or 2.5%.

[0016] According to an embodiment of the present invention, in step S2), the interference dimension t of the flexible mold is 1.5~8mm, for example, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm, 5.5mm, 6mm, 6.5mm, 7mm, 7.5mm or 8mm.

[0017] For example, by using finite element simulation technology to iteratively optimize the profile of the composite cold pressing die, when the cold pressing deformation a of the aluminum alloy forging is 1.5~2.0% and the interference dimension t of the flexible die is 3~6mm, the maximum residual stress of the aluminum alloy forging can be lower than 150MPa.

[0018] For example, by using finite element simulation technology to iteratively optimize the profile of the composite cold pressing die, when the cold pressing deformation a of the aluminum alloy forging is 1.6~2.2% and the interference dimension t of the flexible die is 2~4 mm, the maximum residual stress of the aluminum alloy forging can be lower than 150MPa.

[0019] According to an embodiment of the present invention, in step S3), the aluminum alloy forging after solution quenching is cold-pressed and deformed within 4 hours.

[0020] According to an embodiment of the present invention, in step S3), during the cold pressing deformation process, the metal mold will squeeze the flexible mold to generate pressure, causing the aluminum alloy forging to undergo plastic deformation, especially causing plastic deformation at the side walls and rounded corners of the frame ribs of the aluminum alloy forging, thereby eliminating the residual stress of the complex structure aluminum alloy forging, controlling the maximum residual stress of the complex structure aluminum alloy forging to below 150 MPa, and controlling the non-uniformity to below 50 MPa.

[0021] According to an embodiment of the present invention, in step S1), the metal mold is made of mold steel, such as Cr12MoV, H13, etc. The flexible mold is made of polyamide, thermosetting resin, or polyoxymethylene plastic, etc.

[0022] According to an embodiment of the present invention, the maximum residual stress of the aluminum alloy forging after being treated by the method is controlled below 150 MPa, and the non-uniformity is controlled below 50 MPa.

[0023] According to an embodiment of the present invention, the method is implemented based on a composite cold pressing die composed of a metal mold and a flexible mold. First, the profiles of the metal mold and the flexible mold are designed according to the shape and size of the aluminum alloy forging. Then, using finite element simulation technology, the residual stress distribution of the aluminum alloy forging after cold pressing deformation is calculated. The main parameters of the composite cold pressing die are adjusted, and the first step is repeated to optimize the profile structure of the metal mold and the flexible mold in the composite cold pressing die. Through iterative optimization of the profile of the composite cold pressing die, the maximum residual stress of the aluminum alloy forging is reduced to below 150 MPa. Finally, the solution-quenched aluminum alloy forging is placed on the lower die of the composite cold pressing die, and the upper die is closed by controlling the forging equipment to achieve cold pressing deformation of the aluminum alloy forging, ultimately eliminating the residual stress of the aluminum alloy forging.

[0024] The beneficial effects of this invention are:

[0025] For aluminum alloy forgings with complex profiles, traditional metal mold cold pressing dies cannot effectively eliminate residual stress inside structures with large aspect ratios, such as frame ribs, due to pressure transmission issues. The metal-flexible composite cold pressing die proposed in this invention can effectively solve the problem of controlling residual stress inside frame rib structures. During the forging process, aluminum alloy forgings typically retain a portion of material allowance to ensure complete filling during cold deformation, but this results in inconsistencies in the profile of each forging. Therefore, the amount of deformation during cold pressing needs to be considered in the design of the cold pressing die. Using the same cold pressing die to control aluminum alloy forgings with different profile errors will lead to uneven deformation, resulting in uneven distribution of residual stress. By using the flexible mold in the metal-flexible composite cold pressing mold of the present invention, cold pressing deformation can be performed on areas such as frame rib structure and rounded corners that cannot be directly contacted by the metal mold. With the elastic deformation of the flexible mold (material such as resin and plastic), it can be effectively attached to the aluminum alloy forging and apply a uniform load, avoiding the problem of uneven deformation of the aluminum alloy forging. This allows the maximum residual stress of the obtained complex aluminum alloy forging to be controlled below 150 MPa and the non-uniformity to be controlled below 50 MPa, effectively reducing the processing deformation caused by the release of residual stress during the machining of frame rib type parts. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of a preferred embodiment of the composite cold pressing mold of the present invention;

[0027] Figure 2 This is a schematic diagram of the structure of a preferred embodiment of the composite cold pressing mold of the present invention;

[0028] The attached figures are labeled as follows: 1. Metal mold; 2. Aluminum alloy forging; 3. Flexible mold. Detailed Implementation

[0029] The method of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0030] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0031] Example 1:

[0032] The composite cold-pressing die selected for S100 includes an upper die and a lower die. When the upper and lower dies are closed, they enable cold-pressing deformation of the aluminum alloy forging. The upper and lower dies each include a metal die and a flexible die. The flexible die is nested within the metal die, ensuring that the metal die contacts the top surface of the web and frame ribs of the aluminum alloy forging during cold deformation. The flexible die contacts areas where the aluminum alloy forging and the metal die cannot directly contact each other during cold deformation. The metal die is made of Cr12MoV, and the flexible die is made of polyamide. The aluminum alloy forging is made of 7050 aluminum alloy, and the web thickness T is 50 mm.

[0033] Based on the designed die material and cold pressing parameters, S200 uses the finite element simulation software ABAQUS to predict the residual stress of aluminum alloy forgings. Through simulation optimization, the stress level is lowest when the bottom fillet radius R = 15 mm, the cold pressing deformation a is set to 1.5~2.0%, and the flexible die interference dimension t is set to 3~6 mm, resulting in a residual stress level below 100 MPa. When the cold pressing deformation a is set to 2.0%, the cold pressing deformation reduction c is 0.5 mm, and the flexible die interference dimension t is set to 5 mm, the stress level is lowest. At this point, the simulation results show that the residual stress in the web core of the forging is 84.6 MPa, and the residual stress in the frame rib core is 73.6 MPa.

[0034] S300 manufactures the mold according to design parameters. During production, before solution quenching of the forging, the mold is installed on the forging equipment. The forging is solution heat treated at 470℃, followed by quenching in a 50℃ water bath. Once the surface temperature of the forging drops to approximately 60℃, it is quickly transferred to the forging equipment and cold-pressed according to the predetermined cold-pressing deformation (2.0%). The time from quenching to cold pressing completion must be controlled within 4 hours. After cold pressing, the residual stress on the web surface of the forging is tested using the pinhole method. The maximum residual stress is 79.2 MPa, and the minimum is 36.4 MPa, both below 100 MPa; the non-uniformity is 42.8 MPa, less than 50 MPa. X-ray testing shows that the residual stress on the surface of the frame ribs of the forging is 93.0 MPa, also below 100 MPa.

[0035] Comparative Example 1:

[0036] The other operations are the same as in Example 1, except that in step S200, the interference dimension t of the flexible mold is set to 8 mm.

[0037] After cold pressing, X-ray testing revealed that the residual stress on the surface of the forged frame rib was 181.7 MPa, while the residual stress on the frame rib surface in Example 1 was 93.0 MPa. The reason for the increased stress is that excessive deformation of the flexible mold leads to increased deformation at the frame rib position. Due to the constraint of the metal mold, the internal stress of the frame rib increases, resulting in increased residual stress.

[0038] Comparative Example 2:

[0039] The other operations are the same as in Example 1, except that in step S300, the cold pressing deformation amount a is set to 3.0%.

[0040] After cold pressing, the maximum residual stress on the web surface of the forging was measured using the orifice method, and it was found to be 153.1 MPa, which is higher than the 79.2 MPa in Example 1. The reason for the increased stress is that due to the structural constraints of the forging, excessive cold pressing deformation leads to an increase in the internal stress of the part, which in turn leads to an increase in residual stress.

[0041] Example 2:

[0042] The other operations are the same as in Example 1, except that in step S100, the flexible mold is made of thermosetting resin.

[0043] In step S200, when the cold pressing deformation amount 'a' is set to 1.5~2.0% and the flexible die interference dimension 't' is set to 1.7~3.3 mm, the residual stress level is below 100 MPa. When the cold pressing deformation amount 'a' is set to 2.0%, the cold pressing deformation reduction is 0.5 mm, and the flexible die interference dimension 't' is set to 2.3 mm, the stress level is the lowest.

[0044] After cold pressing, the maximum residual stress on the web surface of the forging was measured using the orifice method. The maximum residual stress was 78.6 MPa, which is close to the 79.2 MPa result in Example 1. The non-uniformity was 41.8 MPa, which is close to the 42.8 MPa result in Example 1. Furthermore, because the elastic modulus and strength of thermosetting resin are higher than those of polyamide, there is a difference between the flexible mold interference dimension t in Example 1 and the flexible mold interference dimension t in Example 2.

[0045] Example 3:

[0046] The composite cold-pressing die selected for S100 includes an upper die and a lower die. When the upper and lower dies are closed, they enable cold-pressing deformation of the aluminum alloy forging. The upper and lower dies each include a metal die and a flexible die. The flexible die is nested within the metal die, ensuring that the metal die contacts the top surface of the web and frame ribs of the aluminum alloy forging during cold deformation. The flexible die contacts areas where the aluminum alloy forging and the metal die cannot directly contact each other during cold deformation. The metal die is made of H13 aluminum alloy, and the flexible die is made of polyoxymethylene (POM). The aluminum alloy forging is 7A85 aluminum alloy, and the web cross-sectional thickness T is 60 mm.

[0047] Based on the designed die material and cold pressing parameters, S200 uses the finite element simulation software ABAQUS to predict the residual stress of aluminum alloy forgings. Through simulation optimization, the stress level is lowest when the bottom fillet radius R = 17 mm, the cold pressing deformation a is set to 1.6~2.2%, and the flexible die interference dimension t is set to 2~4 mm, resulting in a residual stress level below 100 MPa. When the cold pressing deformation a is set to 1.8%, the cold pressing deformation reduction c is 0.54 mm, and the flexible die interference dimension t is set to 2.6 mm, the stress level is lowest. At this point, the simulation results show that the residual stress in the web core of the forging is 84.0 MPa, and the residual stress in the frame rib core is 73.0 MPa.

[0048] S300 manufactures the mold according to the design parameters. During production, before solution quenching of the forging, the mold is installed on the forging equipment. The forging is solution heat treated at 473℃, followed by quenching in a 40℃ water bath. Once the surface temperature of the forging drops to approximately 65℃, it is quickly transferred to the forging equipment and cold-pressed according to the predetermined cold-pressing deformation amount (1.8%). The time from quenching to cold pressing completion must be controlled within 4 hours. After cold pressing, the residual stress on the web surface of the forging is tested using the orifice method. The maximum residual stress is 81.7 MPa, below 100 MPa; the non-uniformity is 31.6 MPa, less than 50 MPa.

[0049] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for eliminating residual stress in aluminum alloy forgings, wherein, The method includes the following steps: S1) Design a composite cold pressing die that matches the shape of the aluminum alloy forging based on the shape of the aluminum alloy forging; The composite cold pressing die includes an upper die and a lower die. After the upper die and the lower die are closed, cold pressing deformation of the aluminum alloy forging can be achieved. The upper die and the lower die each include a metal die and a flexible die. The flexible die is nested on the metal die and ensures that the metal die contacts the top surface of the web and frame ribs of the aluminum alloy forging during the cold deformation process. The flexible die contacts areas of the aluminum alloy forging and the metal die that cannot directly contact each other during the cold deformation process. S2) By using finite element simulation technology, the residual stress distribution of aluminum alloy forgings is analyzed, the main parameters of the composite cold pressing die are adjusted, and the profile of the composite cold pressing die is iteratively optimized so that the maximum residual stress of the aluminum alloy forgings is less than 150MPa. The main parameters include the cold pressing deformation amount 'a' and the flexible die interference dimension 't'; wherein, the cold pressing deformation amount 'a' satisfies a=2c / T, where c is the cold pressing deformation reduction in mm; T is the thickness of the web of the aluminum alloy forging in mm; and the flexible die interference dimension 't' is in mm. S3) Fix the iteratively optimized composite cold pressing mold onto the forging equipment, place the solution-quenched aluminum alloy forging on the lower mold of the composite cold pressing mold, and control the upper mold to close the mold through the forging equipment to achieve cold pressing deformation of the aluminum alloy forging, and finally eliminate the residual stress of the aluminum alloy forging. In step S1), the areas where the aluminum alloy forging and the metal mold cannot directly contact each other are the side walls and rounded corners of the frame ribs of the aluminum alloy forging. In step S1), the metal mold is made of mold steel, and the flexible mold is made of polyamide, thermosetting resin or polyoxymethylene plastic. In step S2), the cold pressing deformation amount a is 1.2~2.5%; the interference dimension t of the flexible mold is 1.5~8mm.

2. The method according to claim 1, wherein, In step S1), the aluminum alloy forging includes a web and frame ribs, the frame ribs are disposed on the web, and the web and frame ribs are integrally formed by forging.

3. The method according to claim 1, wherein, In step S2), the residual stress distribution of the aluminum alloy forging is analyzed by finite element simulation technology, the main parameters of the composite cold pressing die are adjusted, the cold pressing deformation a and the interference dimension t of the flexible die are determined, and the profile of the composite cold pressing die is iteratively optimized so that the maximum residual stress of the aluminum alloy forging is less than 150MPa.

4. The method according to claim 1, wherein, By using finite element simulation technology, the profile of the composite cold pressing die is iteratively optimized. When the cold pressing deformation a of the aluminum alloy forging is 1.5~2.0% and the interference dimension t of the flexible die is 3~6mm, the maximum residual stress of the aluminum alloy forging can be lower than 150MPa.

5. The method according to claim 1, wherein, By using finite element simulation technology, the profile of the composite cold pressing die is iteratively optimized. When the cold pressing deformation a of the aluminum alloy forging is 1.6~2.2% and the interference dimension t of the flexible die is 2~4 mm, the maximum residual stress of the aluminum alloy forging can be lower than 150MPa.

6. The method according to claim 1, wherein, In step S3), the aluminum alloy forgings after solution quenching are cold-pressed and deformed within 4 hours.

7. The method according to any one of claims 1-6, wherein, The maximum residual stress of the aluminum alloy forgings treated by the method is controlled below 150 MPa, and the non-uniformity is controlled below 50 MPa.

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