Method for reducing quenching residual stress of large aluminum alloy frame die forgings

Abstract

The invention relates to a method for reducing quenching residual stress of large aluminum alloy frame die forgings, and belongs to the technical field of aluminum alloy material machining. The method comprises the steps that solid solution quenching treatment is conducted, specifically, the die forgings are subjected to solid solution quenching; the die forgings obtained after solid solution quenching are transferred to a subzero treatment box to be subjected to subzero treatment, then the die forgings taken out of the box are put into a heating furnace to be subjected to heat preservation, circulation is conducted N times, and N is an integer larger than or equal to 1; cold pressing treatment is conducted, specifically, the die forging subjected to cold and hot circulation treatment is placed in a cavity of a cold pressing die or a cold pressing flat anvil to be subjected to cold pressing treatment; and aging treatment is conducted, specifically, the cold-pressed die forgings are placed in a heating furnace to be subjected to aging treatment. By means of the method, the solid solution quenching residual stress of the die forging can be effectively reduced, the conventional mechanical property of the die forging is improved, the purposes of keeping high mechanical property and remarkably reducing the residual stress are effectively achieved, operation is easy, the effect is remarkable, and the method is suitable for large-scale industrial application and popularization.

Description

Technical Field

[0001] This invention belongs to the field of aluminum alloy material processing technology, and specifically relates to a method for reducing the residual stress after quenching of large aluminum alloy frame forgings. Background Technology

[0002] Previously, aircraft fuselage frames were mostly made of titanium alloys. However, with the optimization of aluminum alloy properties, improvements in metal forming methods, and the need for cost control, high-strength 7-series aluminum alloys have become the preferred material to replace titanium alloys in the manufacture of large fuselage frames. However, compared to other materials, aluminum alloys have poor hardenability, and the frames are not only large in size but also have complex structures with significant thickness differences in different locations. This results in large aluminum alloy frame forgings having substantial residual stress after quenching. The presence of residual stress in forgings significantly affects their mechanical properties, fatigue resistance, and corrosion resistance. During forging processing, the redistribution of stress due to material cutting leads to dimensional instability in the workpiece, potentially causing deformation, warping, out-of-tolerance issues, or even scrapping. Many engineering accidents, besides the material's inherent structure and strength, are more or less related to residual stress. Therefore, effectively eliminating residual stress is a prerequisite for the safe production and service of large aluminum alloy frame workpieces.

[0003] In engineering practice, several methods have been summarized for eliminating residual stress, such as natural aging, hot aging, tensile aging, cold pressing, vibration aging, and cryogenic treatment. However, for large aluminum alloy frame forgings, any single method has irreconcilable drawbacks. The aluminum alloys used for frame forgings are mostly 7-series high-strength aluminum alloys, whose natural aging time is long and the stress relief effect after aging is not significant; hot aging often leaves the forging in an over-aged state, resulting in low stress reduction and decreased material properties; tensile aging is effective but only suitable for simple sheet and strip materials, and is difficult to effectively stretch for large and complex forgings; cold pressing also has a good effect on reducing residual stress, but it is difficult to accurately control the amount of deformation in actual operation; vibration aging is widely used in production, with strong applicability and simple process, especially suitable for large and complex structural parts, but the internal mechanism is unclear, and it is difficult to effectively determine the amplitude and frequency, making it difficult to stably eliminate residual stress; although cryogenic treatment can stably reduce residual stress in forgings, its effect is also weak.

[0004] Large aluminum alloy forgings are characterized by large effective cross-sectional thickness, complex distribution of ribs and webs, and localized high-profile protrusions. A single method cannot solve the residual stress problem in large aluminum alloy forgings. Patent application CN202410928749.6 discloses a method for reducing quenching residual stress in high-strength aluminum alloy frame forgings. This method uses a die-pressing method before aging treatment to subject the aluminum alloy frame forging to compressive deformation. After aging treatment, a vibration aging method is used to generate one or more vibration states in the metal structure, causing the residual stress in the forging to be superimposed with the vibration load, exceeding the material's yield limit and causing plastic deformation, thereby reducing stress. However, due to the aforementioned structural characteristics of large aluminum alloy forgings, the method disclosed in the patent cannot effectively reduce the quenching residual stress in large aluminum alloy forgings. Summary of the Invention

[0005] This invention provides a method for reducing the residual stress after quenching of large aluminum alloy frame forgings, which solves the technical problem that existing methods cannot effectively reduce the residual stress after quenching of large aluminum alloy forgings.

[0006] This invention is achieved through the following technical solution: a method for reducing residual stress during quenching of large aluminum alloy frame forgings, comprising:

[0007] Solution hardening: Solution hardening is performed on die forgings;

[0008] Hot and cold cycle treatment: The solution-quenched die forgings are transferred to a cryogenic treatment box for cryogenic treatment, and then the die forgings after exiting the box are placed in a heating furnace for heat preservation, and the cycle is repeated N times, where N is an integer greater than or equal to 1.

[0009] Cold pressing: The forgings that have undergone hot and cold cycling are placed in the cavity of a cold pressing die or a cold pressing anvil for cold pressing.

[0010] Aging treatment: The cold-pressed die forgings are placed in a heating furnace for aging treatment.

[0011] Furthermore, to better realize the present invention, the process of transferring the solution-quenched forging to a cryogenic treatment chamber for cryogenic treatment in the hot and cold cycling process includes:

[0012] The solution-quenched forgings are placed in a cryogenic treatment chamber at -30℃ to -100℃ and kept at that temperature for 2 to 8 hours. The cooling medium used in the cryogenic treatment chamber is liquid nitrogen.

[0013] Furthermore, to better realize the present invention, the process of placing the die-forged part after exiting the mold into a heating furnace for heat preservation during the hot and cold cycling treatment includes:

[0014] After defrosting, the forgings taken out of the cryogenic treatment box are placed in a heating furnace at 70℃~110℃ and kept at that temperature for 2h~8h. After that, they are transferred to the cryogenic treatment box for cryogenic treatment again or placed in the air for air cooling.

[0015] Furthermore, in order to better realize the present invention, the value of N in the hot and cold cycle treatment is in the range of 1 to 3.

[0016] Furthermore, in order to better realize the present invention, the time interval between the solution quenching treatment and the hot and cold cycling treatment shall not exceed 30 minutes.

[0017] Furthermore, in order to better realize the present invention, the heating temperature of the solution quenching treatment is 460℃~480℃, the holding time is 0.5h~10h, the quenching medium is warm water with a water temperature of 50℃~80℃, and the quenching medium is stirred during the quenching process.

[0018] Furthermore, in order to better realize the present invention, the fillet of the cold pressing die cavity is larger than the fillet of the corresponding die forging, the draft angle is larger than the draft angle of the corresponding die forging, and the lubricant used for cold pressing is lubricating oil, which is evenly applied to the die forging.

[0019] Furthermore, in order to better realize the present invention, the cold pressing deformation during the cold pressing process is 1% to 5%.

[0020] Furthermore, to better implement the present invention, the aging process is a two-stage aging process, including the following steps:

[0021] First-stage aging: Place the cold-pressed forgings in a heating furnace at 110℃~130℃ and hold for 5~7 hours;

[0022] Second stage aging: The die forgings that have undergone the first stage aging are heated to 150℃~180℃ and held for 4h~14h, and then air-cooled.

[0023] Furthermore, in order to better realize the present invention, the forging is a 7-series aluminum alloy frame forging.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] The method for reducing residual stress in large aluminum alloy frame forgings provided by this invention involves subjecting the forgings to a hot-cold cycle treatment after solution quenching. This subjects the forgings to multi-stage forces opposite to those experienced during solution quenching, thus offsetting the residual stress and specifically eliminating the non-uniform stress field caused by the complex structure. Furthermore, cold pressing is performed on the forgings after the hot-cold cycle treatment and before aging treatment, subjecting them to compressive deformation. This deformation, within the range of 1% to 5%, exhibits good results and effectively balances residual stress in different parts of the forging. After solution quenching, hot-cold cycle treatment, cold pressing, and aging treatment, large aluminum alloy frame forgings not only effectively reduce residual stress from solution quenching but also improve their conventional mechanical properties. This method effectively balances maintaining high mechanical properties with significantly reduced residual stress, is simple to operate, and yields significant results, making it suitable for large-scale industrial promotion and application. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0027] Example 1:

[0028] A large aluminum alloy frame-shaped forging with dimensions of 4000mm×2000mm×250mm was placed in a heating furnace for solution treatment. The alloy material was 7050 aluminum alloy, the solution temperature was 470℃, and the solution time was 7 hours. After the solution treatment, it was quickly placed in warm water at 70℃ for quenching, with constant stirring during the quenching process. Then, the quenched forging was placed in a cryogenic treatment chamber at -60℃ for 3 hours. After the cryogenic treatment, the forging was removed and defrosted. Then, the forging was transferred to a heating furnace at 80℃ for 4 hours. This cycle was repeated three times. After being removed from the furnace, the forging was air-cooled to room temperature. The cooled forgings were then placed in the cavity of a cold pressing mold for cold pressing, with a cold pressing amount of 5%. After cold pressing, the forgings were placed in a heating furnace for a two-stage aging treatment, with an aging regime of 120℃×6h+160℃×8h. The heating rate from 120℃ to 160℃ was 3℃ / min. The interval between the cold pressing and aging treatments was no more than 2h. The forgings were then air-cooled to room temperature to complete the process. The residual stress test values ​​and conventional mechanical properties of the large aluminum alloy frame forgings after the above steps are shown in Table 1 below. The residual stress of the sample decreased from 144.7MPa after solution aging to 79.1MPa. The tensile strength and yield strength increased by 19.5MPa and 26.9MPa respectively compared to aging, while the elongation remained the same.

[0029] It should be noted that the time interval between placing the die forgings into the cryogenic treatment chamber after quenching is 10 minutes.

[0030] Comparative Example 1:

[0031] A large aluminum alloy frame forging with dimensions of 4000mm×2000mm×250mm was placed in a heating furnace for solution treatment. The alloy material was 7050 aluminum alloy. The solution temperature was 470℃, and the solution time was 7 hours. After holding at this temperature, the forging was quickly quenched in warm water at 70℃ with constant stirring. After quenching, the forging was placed back into the heating furnace for a two-stage aging treatment at 120℃×6 hours + 160℃×8 hours. The heating rate from 120℃ to 160℃ was 3℃ / min. The forging was then air-cooled to room temperature to complete the process. The residual stress test values ​​and conventional mechanical properties are shown in Table 1 below. The residual stress of the sample was 144.7 MPa, the tensile strength and yield strength were 526.7 MPa and 460.5 MPa, respectively, and the elongation was 14.9%.

[0032] Comparative Example 2:

[0033] A high-strength aluminum alloy frame forging with dimensions of 4000mm×2000mm×250mm was solution-treated in a heating furnace using 7050 aluminum alloy at a solution temperature of 470℃ for 7 hours. After holding at this temperature, the forging was immediately quenched in 70℃ water with constant stirring. The quenched forging was then placed in a cryogenic treatment chamber at -60℃ for 3 hours. Next, it was transferred to a heating furnace at 80℃ for 4 hours, and this cycle was repeated three times. After removal from the furnace, the forging was air-cooled to room temperature. Following this thermal cycle, the forging was again placed in the heating furnace for a two-stage aging treatment at 120℃×6 hours + 160℃×8 hours. The heating rate from 120℃ to 160℃ was 3℃ / min. After removal from the furnace, the forging was air-cooled to room temperature, completing the process. The residual stress test values ​​and conventional mechanical properties are shown in Table 1 below. The residual stress of the specimen decreased from 144.7 MPa after solution aging to 133.7 MPa, with a residual stress reduction of approximately 8%. Meanwhile, the tensile strength and yield strength increased by 9.1 MPa and 15.3 MPa respectively compared to aging, while the elongation decreased slightly by 1.4%.

[0034] Comparative Example 3:

[0035] A high-strength aluminum alloy frame forging with dimensions of 4000mm×2000mm×250mm was solution-treated in a heating furnace. The alloy material was 7050 aluminum alloy. The solution temperature was 470℃, and the solution time was 7 hours. After holding at this temperature, the forging was quickly quenched in warm water at 70℃ with constant stirring. The quenched forging was then placed in a cold-pressing mold with a cold-pressing amount of 5%. After cold pressing, the forging was placed back in the heating furnace for a two-stage aging treatment: 120℃×6 hours + 160℃×8 hours. The heating rate from 120℃ to 160℃ was 3℃ / min, and the interval between cold pressing and aging was no more than 2 hours. The residual stress test values ​​and conventional mechanical properties are shown in Table 1 below. The residual stress of the sample decreased from 144.7 MPa after aging to 122.6 MPa, with approximately 15% of the residual stress eliminated. Compared with aging, the tensile strength and yield strength increased by 15.3 MPa and 20.1 MPa respectively, while the elongation decreased slightly by 2.5%.

[0036] Comparative Example 4:

[0037] A high-strength aluminum alloy frame forging with dimensions of 4000mm×2000mm×250mm was placed in a heating furnace for solution treatment. The alloy material was 7050 aluminum alloy, the solution temperature was 470℃, and the solution time was 7 hours. After the solution treatment, the forging was quickly placed in warm water at 70℃ for quenching, with constant stirring during the quenching process. The quenched forging was then placed in a cold pressing mold with a cold pressing amount of 5%. The cold-pressed forging was then placed in a cryogenic treatment chamber at a cryogenic temperature of -60℃ for 3 hours. Next, the forging was transferred to a heating furnace at a furnace temperature of 80℃ for 4 hours. This cycle was repeated three times. After being removed from the furnace, the forging was air-cooled to room temperature. The cooled forgings were placed in a heating furnace for a two-stage aging treatment, with an aging regime of 120℃×6h+160℃×8h. The heating rate from 120℃ to 160℃ was 3℃ / min. The interval between cold pressing and aging was no more than 2h. The forgings were then air-cooled to room temperature to complete the process. The residual stress test values ​​and conventional mechanical properties are shown in Table 1 below. The residual stress of the specimen decreased from 144.7MPa after aging to 112.7MPa, with approximately 22% of the residual stress eliminated. The tensile strength and yield strength increased by 23.7MPa and 34.1MPa respectively compared to aging, while the elongation decreased slightly by 2.8%.

[0038] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope described in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

[0039] Table 1. Effects of heat treatment process on mechanical properties and residual stress

[0040]

[0041] Example 2:

[0042] A large aluminum alloy frame-shaped forging with dimensions of 4000mm×2000mm×250mm was placed in a heating furnace for solution treatment. The alloy material was 7050 aluminum alloy, the solution temperature was 460℃, and the solution time was 0.5h. After the solution treatment, it was quickly placed in warm water at 50℃ for quenching, with constant stirring during the quenching process. Then, the quenched forging was placed in a cryogenic treatment chamber at -30℃ for 2h. After the cryogenic treatment, it was removed and defrosted. Then, the forging was transferred to a heating furnace at 70℃ for 2h. This cycle was repeated once. After being removed from the furnace, it was air-cooled to room temperature. The cooled forgings were then placed in the cavity of a cold pressing mold for cold pressing, with a cold pressing amount of 1%. After cold pressing, the forgings were placed in a heating furnace for a two-stage aging treatment, with an aging regime of 110℃×5h+150℃×4h. The heating rate from 110℃ to 150℃ was 3℃ / min. The interval between the cold pressing and aging treatments was no more than 2h. The forgings were then air-cooled to room temperature to complete the process. The residual stress of the sample decreased from 144.7MPa after solution aging to 93.2MPa. The tensile strength and yield strength decreased by 3.7MPa and 22.5MPa, respectively, compared to aging, and the elongation decreased by 3.9%.

[0043] It should be noted that the time interval between placing the die-forged parts into the cryogenic treatment chamber after quenching is 5 minutes.

[0044] Example 3:

[0045] A large aluminum alloy frame-shaped forging with dimensions of 4000mm×2000mm×250mm was placed in a heating furnace for solution treatment. The alloy material was 7050 aluminum alloy, the solution temperature was 480℃, and the solution time was 10 hours. After the solution treatment, the forging was quickly placed in warm water at 80℃ for quenching, with constant stirring during the quenching process. The quenched forging was then placed in a cryogenic treatment chamber at -100℃ for 8 hours. After the cryogenic treatment, the forging was removed and defrosted. Then, the forging was transferred to a heating furnace at 110℃ for 8 hours. This cycle was repeated three times. After being removed from the furnace, the forging was air-cooled to room temperature. The cooled forgings were then placed in the cavity of a cold pressing mold for cold pressing, with a cold pressing amount of 5%. After cold pressing, the forgings were placed in a heating furnace for a two-stage aging treatment, with an aging regime of 130℃×7h + 180℃×14h. The heating rate from 130℃ to 180℃ was 3℃ / min. The interval between the cold pressing and aging treatments was no more than 2h. The forgings were then air-cooled to room temperature to complete the process. The residual stress of the sample decreased from 144.7MPa after solution aging to 72.0MPa. The tensile strength and yield strength decreased by 32.7MPa and 41MPa, respectively, compared to aging, while the elongation decreased by 0.9%.

[0046] It should be noted that the time interval between placing the die-forged parts into the cryogenic treatment chamber after quenching is 25 minutes.

Claims

1. A method for reducing residual stress after quenching in large aluminum alloy frame-type die forgings, characterized in that, include: Solution hardening: Solution hardening is performed on die forgings; Hot and cold cycle treatment: The solution-quenched die forgings are transferred to a cryogenic treatment box for cryogenic treatment, and then the die forgings after exiting the box are placed in a heating furnace for heat preservation, and the cycle is repeated N times, where N is an integer greater than or equal to 1. Cold pressing: The forgings that have undergone hot and cold cycling are placed in the cavity of a cold pressing die or a cold pressing anvil for cold pressing. Aging treatment: The cold-pressed die forgings are placed in a heating furnace for aging treatment; The process of transferring the solution-quenched forgings to a cryogenic treatment chamber for cryogenic treatment in the hot and cold cycling process includes: The solution-quenched forgings are placed in a cryogenic treatment chamber at -30℃ to -100℃ and kept at that temperature for 2 to 8 hours. The cooling medium used in the cryogenic treatment chamber is liquid nitrogen. After defrosting, the forgings taken out of the cryogenic treatment box are placed in a heating furnace at 70℃~110℃ and kept at that temperature for 2h~8h. After that, they are transferred to the cryogenic treatment box for cryogenic treatment again or placed in the air for air cooling.

2. The method for reducing residual stress after quenching of large aluminum alloy frame forgings according to claim 1, characterized in that: The value of N in the hot and cold cycle process ranges from 1 to 3.

3. The method for reducing residual stress after quenching of large aluminum alloy frame forgings according to claim 1, characterized in that: The time interval between the solution quenching treatment and the hot and cold cycling treatment shall not exceed 30 minutes.

4. The method for reducing residual stress after quenching of large aluminum alloy frame forgings according to any one of claims 1-3, characterized in that: The solution quenching treatment is carried out at a heating temperature of 460℃~480℃, a holding time of 0.5h~10h, and the quenching medium is warm water at a temperature of 50℃~80℃. The quenching medium is stirred during the quenching process.

5. The method for reducing residual stress after quenching of large aluminum alloy frame forgings according to any one of claims 1-3, characterized in that: The fillet radius of the cold pressing die cavity is larger than the corresponding fillet radius of the forging part, and the draft angle is larger than the corresponding draft angle of the forging part. The lubricant used for cold pressing is lubricating oil, which is evenly applied to the forging part.

6. The method for reducing residual stress after quenching of large aluminum alloy frame forgings according to claim 5, characterized in that: The cold pressing deformation during the cold pressing process is 1% to 5%.

7. The method for reducing residual stress after quenching of large aluminum alloy frame forgings according to any one of claims 1-3, characterized in that, The timeliness processing is a two-level timeliness process, including the following steps: First-stage aging: Place the cold-pressed forgings in a heating furnace at 110℃~130℃ and hold for 5~7 hours; Second stage aging: The die forgings that have undergone the first stage aging are heated to 150℃~180℃ and held for 4h~14h, and then air-cooled.

8. The method for reducing residual stress after quenching of large aluminum alloy frame forgings according to any one of claims 1-3, characterized in that: The forging is a 7-series aluminum alloy frame forging.

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