Preparation method of low-stress 7xxx series aluminum alloy large forge piece

Abstract

The invention relates to the technical field of non-ferrous metal processing, and particularly provides a preparation method of a low-stress 7xxx series aluminum alloy large forge piece. The preparation method comprises the following steps: S1, carrying out forging treatment on an aluminum alloy ingot blank by adopting an isothermal forging process to obtain a forge piece; s2, the forge piece is subjected to solution treatment; s3, fixing the forge piece subjected to solution treatment by using a profiling tool, and putting the forge piece into a quenching solution for quenching treatment; wherein the quenching solution is a polyalkylene glycol aqueous solution; and S4, the forged piece subjected to quenching treatment is subjected to cold pressing treatment and graded aging treatment, and a finished product is obtained. The graded solution treatment process is combined with the controllable quenching technology, and the subsequent cold pressing treatment is matched, so that the residual stress level of the forge piece is effectively controlled.

Description

Technical Field

[0001] The present invention relates to the technical field of nonferrous metal processing, and in particular to a method for preparing a low-stress 7xxx series aluminum alloy large forging. Background Art

[0002] With the rapid development of the aviation industry, aircraft structural components are moving towards integrated, lightweight, and high-performance design. Compared with traditional riveted or welded structures, the use of integral die forgings significantly reduces the number of joints, reduces structural weight, improves fatigue life and reliability, and enhances manufacturing efficiency. However, the manufacture of large, complex aluminum alloy forgings still faces significant technical challenges, among which residual stress control is a key factor affecting the quality of the finished product.

[0003] High-strength 7xxx series aluminum alloys have become the preferred material for primary load-bearing components in the aviation industry, such as aircraft joints, beams, partitions, wing frames, etc., due to their excellent specific strength and good corrosion resistance. In traditional manufacturing processes, high-strength 7xxx series aluminum alloy forgings need to undergo solution quenching treatment to obtain the required mechanical properties. However, due to the high degree of alloying of this series of alloys, rapid quenching must be used to ensure that the alloying elements are fully dissolved, but this will cause large residual stresses inside the forgings. For large, thin-walled, weakly rigid structures, such as integral frame forgings, this residual stress will not only cause deformation or even cracking during the quenching process, but will also cause dimensional deviations due to stress release during subsequent machining, and in severe cases, even cause the workpiece to be scrapped. In addition, the presence of residual stress will also reduce the fatigue properties and stress corrosion resistance of the material, affecting the service safety and life of the component.

[0004] Currently, commonly used methods for reducing residual stress include stress relief annealing, cold deformation, vibration aging, and cryogenic treatment. Although stress relief annealing can reduce residual stress, it requires long-term high-temperature treatment, which not only increases energy consumption and costs, but may also cause material overaging, resulting in a decrease in strength. Although cold deformation does not require heating, it is not suitable for forgings with complex shapes and has limited stress relief effects. Although vibration aging is simple to operate and is not subject to workpiece size requirements, its relief effect is unstable and may cause defects such as cracks due to improper vibration parameters. Although cryogenic treatment can further release residual stress, it requires large equipment investments and complex processes, making it difficult to apply on a large scale. Therefore, developing an efficient, stable, and suitable low-stress manufacturing technology for complex structures is of great significance to improving the quality and reliability of aviation aluminum alloy forgings. Summary of the Invention

[0005] In response to the above problems, the present invention provides a method for preparing large low-stress 7xxx series aluminum alloy forgings. By combining a graded solid solution treatment process with a controllable quenching technology and a subsequent cold pressing process, the residual stress level of the forgings can be effectively controlled.

[0006] To solve the above technical problems, in a first aspect, the present invention provides a method for preparing a low-stress 7xxx series aluminum alloy large forging, comprising the following steps: S1. An aluminum alloy ingot is forged using an isothermal forging process to obtain a forging; S2. Solution treat the forgings at 410-460°C for 1-6 hours, then at 430-490°C for 1-10 hours; S3. Using a profiling tool to fix the forging after solution treatment, place it in a quenching solution for quenching; wherein the quenching solution is a polyalkylene glycol aqueous solution having a concentration of 2wt%-12wt% and a temperature of 15-60°C; S4. The quenched forging is subjected to cold pressing and graded aging treatment to obtain a finished product; wherein the deformation amount of the cold pressing treatment is 1%-5%.

[0007] The present invention combines a graded solution treatment process with a controllable quenching technology and cooperates with subsequent cold pressing to achieve effective control of residual stress in forgings. In the solution treatment stage, by precisely controlling the solution temperature and time parameters, the uniform distribution of alloying elements is promoted, providing a good organizational basis for subsequent aging treatment. During the quenching process, the coating effect formed by the polyalkylene glycol quenching solution can accurately regulate the cooling rate, which not only ensures the necessary quenching speed, but also avoids the micro-stress concentration caused by drastic phase change, thereby enabling the forgings to obtain a uniform cooling effect, significantly reducing the generation of quenching residual stress, and effectively reducing the risk of forging deformation and cracking. At the same time, the present invention uses a contouring tool to accurately constrain the forgings, effectively suppressing the shape distortion caused by thermal stress during the quenching process. In the subsequent cold pressing process, by introducing 1-5% plastic deformation, a dislocation network is formed inside the material. These dislocations interact with the residual stress field to achieve stress redistribution and effective release. During the aging treatment stage, through two-stage temperature control, the GP zone is first formed at low temperature as the nucleation site, and then the high temperature is used to promote the uniform precipitation of the η' phase, thereby improving the strength while maintaining good toughness.

[0008] Preferably, in step S1, the forging process includes a preliminary forging process and a final forging process.

[0009] Preferably, in step S1, the pre-forging treatment is: heating the aluminum alloy ingot and the pre-forging die to 400-480°C, keeping the temperature for 8-24 hours, and then pre-forging at a pressing speed of 0.1-4 mm / s to obtain a pre-forged part.

[0010] Preferably, in step S1, the final forging treatment is: after removing the flash, heating the pre-forged piece and the final forging die to 400-480°C, keeping the temperature for 8-24 hours, and then performing final forging at a pressing speed of 0.1-4 mm / s to obtain a forging.

[0011] Preferably, in step S3, the support point of the profiling tool is set at the corner position of the forging, and the space between the profiling tool and the forging is filled with a heat insulating material. Preferably, the heat insulating material is asbestos or graphite felt.

[0012] In the quenching process of the present invention, the workpiece is fixed on a special profiling tool designed based on the structural characteristics of the forging. The support points of the tool on the forging mainly act on the corner positions of the forging, effectively supporting and fixing the entire frame structure. At the same time, asbestos or graphite felt and other materials are filled between the tool and the forging to ensure sufficient contact between the tool and the corner positions. At the same time, a supporting frame beam structure of at least 300 mm is retained outside the frame outline to avoid collision between the workpiece and the water tank wall, thereby ensuring smooth circulation of the quenching liquid.

[0013] Preferably, in step S3, during the quenching treatment, the temperature rise of the quenching solution is controlled within 5°C.

[0014] Preferably, in step S4, the deformation amount of the cold pressing treatment is 2%-3%.

[0015] Preferably, in step S4, the graded aging treatment is: first, keeping warm at 80-160°C for 4-36 hours, and then keeping warm at 120-180°C for 3-18 hours.

[0016] In a second aspect, the present invention provides a 7xxx series aluminum alloy large forging, which is prepared according to the above-mentioned method for preparing the low-stress 7xxx series aluminum alloy large forging.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The present invention adopts triple regulation of graded solid solution + controlled quenching + cold pressing deformation to achieve precise control of residual stress in large aluminum alloy forgings. The present invention first adopts a solid solution treatment process with stepped temperature increase to ensure that the alloy elements are fully homogenized; then, the cooling rate is precisely controlled by adding a controllable quenching technology of a specific quenching solution; finally, a cold pressing treatment with a specific deformation amount is applied to achieve effective release of residual stress. The process of the present invention is simple and feasible, and only appropriate changes need to be made to the traditional preparation process, making it suitable for industrial large-scale production. At the same time, the present invention has good applicability and can be widely used in the manufacture of various 7xxx series aluminum alloy forgings such as 7050, 7055, and 7065, and is particularly suitable for the precision forming of large-scale integral frame forgings with high ribs, thin walls, and weak rigidity. In addition, the recyclable use of the quenching solution greatly reduces production costs, achieving good economic benefits while ensuring the quality of the finished product. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1Graphs showing equivalent stress simulation results of a 7055 large forging before and after cold pressing in Example 1 of the present invention; (a) after cold pressing, and (b) after quenching. DETAILED DESCRIPTION

[0019] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the specific implementation methods will be briefly introduced below. Obviously, the embodiments described below are some implementation methods of the present invention. For ordinary technicians in this field, other implementation methods can be obtained based on these embodiments without paying creative work.

[0020] If specific experimental steps or conditions are not specified in the examples, the procedures or conditions of conventional experimental steps described in the literature in the field can be followed. All raw materials and instruments used are commercially available, including but not limited to those used in the examples of this application.

[0021] Example 1 This embodiment provides a method for preparing a low-stress 7055 aluminum alloy large forging, comprising the following steps: (1) 7055 aluminum alloy was formed into the required billet size for the pre-forging die by billet forming. The billet was heated to 425°C and held for 10 h. The pre-forging die was heated to 425°C and held for 24 h. Pre-forging was performed at a reduction rate of 2.5 mm / s. After cooling, the flash was removed. The pre-forged piece was heated to 425°C and held for 10 h. The final forging die was heated to 425°C and held for 24 h. Final forging was performed at a reduction rate of 1.5 mm / s to obtain a forging.

[0022] (2) Solution treatment of the forgings was performed, first at 455°C for 5 h, then at 470°C for 10 h.

[0023] (3) The forgings after solution treatment were fixed on the profiling fixture for quenching. The quenching solution was a 7 wt% polyalkylene glycol aqueous solution. The initial temperature of the solution was 57 °C. After quenching, the temperature rise of the solution was ≤5 °C (measured at 60 °C).

[0024] (4) After quenching treatment, the forgings are cold pressed with a cold pressing die 4 hours later to eliminate residual stress, and the cold pressing elongation is 3%.

[0025] (5) The forgings after cold pressing are subjected to aging treatment, first at 115 ° C for 8 hours, then at 155 ° C for 16 hours, and then air-cooled after aging to obtain the finished product.

[0026] According to the test, the maximum residual stress of the forging after quenching in this embodiment is about 300 MPa, and the residual stress after cold pressing is less than 200 MPa, as shown in FIG1 .

[0027] Example 2 This embodiment provides a method for preparing a low-stress 7050 series aluminum alloy large forging, comprising the following steps: (1) 7050 aluminum alloy was formed into the required billet size for the pre-forging die by billet forming. The billet was heated to 420°C and held for 8 h. The pre-forging die was heated to 420°C and held for 20 h. Pre-forging was performed at a reduction rate of 3 mm / s. After cooling, the flash was removed. The pre-forged piece was heated to 420°C and held for 8 h. The final forging die was heated to 420°C and held for 20 h. Final forging was performed at a reduction rate of 2 mm / s to obtain a forging.

[0028] (2) Solution treatment of the forgings was performed, first at 450 °C for 4 h, then at 470 °C for 8 h.

[0029] (3) The forgings after solution treatment were fixed on the profiling fixture for quenching. The quenching solution was a 6 wt% polyalkylene glycol aqueous solution. The initial temperature of the solution was 52 °C. After quenching, the temperature rise of the solution was ≤5 °C (measured 55 °C).

[0030] (4) After quenching treatment, the forgings are cold pressed with a cold pressing die 4 hours later to eliminate residual stress, and the cold pressing elongation is 2%.

[0031] (5) The forgings after cold pressing are subjected to aging treatment, first at 120 ° C for 6 hours, then at 165 ° C for 12 hours, and then air-cooled after aging to obtain the finished product.

[0032] Example 3 This embodiment provides a method for preparing a low-stress 7065 series aluminum alloy large forging, comprising the following steps: (1) 7065 aluminum alloy was formed into the required billet size for the pre-forging die by billet forming. The billet was heated to 425°C and held for 6 h. The pre-forging die was heated to 425°C and held for 20 h. Pre-forging was performed at a reduction rate of 4 mm / s. After cooling, the flash was removed. The pre-forged piece was heated to 425°C and held for 6 h. The final forging die was heated to 425°C and held for 20 h. Final forging was performed at a reduction rate of 3 mm / s to obtain a forging.

[0033] (2) Solution treatment of the forgings was performed, first at 455°C for 3 h, then at 485°C for 7 h.

[0034] (3) The forgings after solution treatment were fixed on the profiling fixture for quenching. The quenching solution was an 8 wt% polyalkylene glycol aqueous solution. The initial temperature of the solution was 57 °C. After quenching, the temperature rise of the solution was ≤5 °C (measured 61 °C).

[0035] (4) After quenching treatment, the forgings are cold pressed with a cold pressing die 4 hours later to eliminate residual stress, and the cold pressing elongation is 3%.

[0036] (5) The forgings after cold pressing are subjected to aging treatment, first at 125 ° C for 5 hours, then at 165 ° C for 10 hours, and then air-cooled after aging to obtain the finished product.

[0037] Comparative Example 1 The difference between this comparative example and Example 1 is that deionized water is used as the quenching solution instead of the polyalkylene glycol aqueous solution. Other conditions are the same as those in Example 1.

[0038] Comparative Example 2 The difference between this comparative example and Example 1 is that no cold pressing treatment is performed, and the forgings after quenching treatment are directly subjected to aging treatment. Other aspects are the same as Example 1.

[0039] Comparative Example 3 The difference between this comparative example and Example 1 is that the forging is not clamped by a profiling tool during the quenching treatment, nor is it subjected to cold pressing treatment. Other aspects are the same as Example 1. The preparation method comprises: (1) 7055 aluminum alloy was formed into the required billet size for the pre-forging die by billet forming. The billet was heated to 425°C and held for 10 h. The pre-forging die was heated to 425°C and held for 24 h. Pre-forging was performed at a reduction rate of 2.5 mm / s. After cooling, the flash was removed. The pre-forged piece was heated to 425°C and held for 10 h. The final forging die was heated to 425°C and held for 24 h. Final forging was performed at a reduction rate of 1.5 mm / s to obtain a forging.

[0040] (2) Solution treatment of the forgings was performed, first at 455°C for 5 h, then at 470°C for 10 h.

[0041] (3) Use a high-temperature resistant sling to hook the two ends of the forging after solution treatment and immerse it in the quenching solution for quenching. The quenching solution is a 7 wt% polyalkylene glycol aqueous solution. The initial temperature of the solution is 57 °C. After quenching, the temperature rise of the solution is ≤5 °C (measured 60 °C).

[0042] (4) The forgings after quenching treatment are subjected to aging treatment, first at 115 ° C for 8 hours, then at 155 ° C for 16 hours, and then air-cooled after aging to obtain the finished product.

[0043] Performance Testing The performance of the finished aluminum alloy forgings prepared in Examples 1-3 and Comparative Examples 1-3 was tested. The residual stress along the shape direction of the web of the forging was measured by X-ray diffraction (XRD). The test standard was based on GB / T 7704-2017. The tensile strength, yield strength, and elongation were tested based on GB / T 16865-2013. The results are shown in Table 1 below.

[0044] Table 1 As can be seen from the data in Table 1, the triple-control process of graded solid solution + controlled quenching + cold pressing deformation provided by the present invention can reduce deformation and residual stress levels, while ensuring high strength, and achieving excellent comprehensive performance of residual stress <50MPa and elongation >10%. Comparative Example 1 suffers from stress concentration due to the excessively rapid cooling rate, resulting in a residual stress as high as 163MPa; Comparative Example 2 suffers from extremely high residual stress in the forging due to the lack of cold pressing. Comparative Example 3 uses hanger quenching, resulting in uneven deformation of the forging, and the lack of cold pressing results in severe stress concentration.

[0045] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing a low-stress 7xxx series aluminum alloy large forging, characterized in that: The following steps are included: S1. An aluminum alloy ingot is forged using an isothermal forging process to obtain a forging; S2. Solution treat the forgings at 410-460°C for 1-6 hours, then at 430-490°C for 1-10 hours; S3. Using a profiling tool to fix the forging after solution treatment, place it in a quenching solution for quenching; wherein the quenching solution is a polyalkylene glycol aqueous solution having a concentration of 2wt%-12wt% and a temperature of 15-60°C; S4. The quenched forging is subjected to cold pressing and graded aging treatment to obtain a finished product; wherein the deformation amount of the cold pressing treatment is 1%-5%.

2. The method for preparing a low-stress 7xxx series aluminum alloy large forging according to claim 1, wherein: In step S1, the forging process includes a preliminary forging process and a final forging process.

3. The method for preparing a low-stress 7xxx series aluminum alloy large forging according to claim 2, wherein: In step S1, the pre-forging treatment is as follows: heating the aluminum alloy ingot and the pre-forging die to 400-480° C., keeping the temperature for 8-24 hours, and then pre-forging at a pressing speed of 0.1-4 mm / s to obtain a pre-forged part.

4. The method for preparing a low-stress 7xxx series aluminum alloy large forging according to claim 2, wherein: In step S1, the final forging process is as follows: after removing the flash, the pre-forged piece and the final forging die are heated to 400-480°C, kept at this temperature for 8-24 hours, and then final forging is performed at a pressing speed of 0.1-4 mm / s to obtain a forging.

5. The method for preparing a low-stress 7xxx series aluminum alloy large forging according to claim 1, wherein: In step S3, the support points of the profiling tooling are set at the corners of the forging, and the space between the profiling tooling and the forging is filled with heat-insulating material.

6. The method for preparing a low-stress 7xxx series aluminum alloy large forging according to claim 5, wherein: The heat insulation material is asbestos or graphite felt.

7. The method for preparing a low-stress 7xxx series aluminum alloy large forging according to claim 1, wherein: In step S3, during the quenching treatment, the temperature rise of the quenching solution is controlled within 5°C.

8. The method for preparing a low-stress 7xxx series aluminum alloy large forging according to claim 1, wherein: In step S4, the deformation amount of the cold pressing treatment is 2%-3%.

9. The method for preparing a low-stress 7xxx series aluminum alloy large forging according to claim 1, wherein: In step S4, the graded aging treatment is: first, keeping the temperature at 80-160° C. for 4-36 hours, and then keeping the temperature at 120-180° C. for 3-18 hours.

10. A 7xxx series aluminum alloy large forging, characterized in that: The low-stress 7xxx series aluminum alloy large forging is prepared according to the preparation method of any one of claims 1-9.

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