Austempering heat treatment process for 7175 aluminum alloy for aerospace

Abstract

The application belongs to the field of metal heat treatment, and particularly relates to a 7175 aluminum alloy retrogression and re-aging heat treatment process for aviation, the retrogression and re-aging process is characterized by comprising the following steps: pre-aging treatment, heating 7175 aluminum alloy extruded profiles to 120 DEG C at a heating rate of 10 DEG C / min and keeping for 10 h; retrogression treatment, heating the 7175 aluminum alloy subjected to the pre-aging treatment to 185 DEG C at a heating rate of 40 DEG C / min and keeping for 40 min; re-aging treatment, cooling the 7175 aluminum alloy extruded profiles subjected to the retrogression treatment to room temperature, re-heating to 120 DEG C, keeping for 10 h and heating at a heating rate of 10 DEG C / min. The retrogression and re-aging process can greatly improve the corrosion resistance, effectively shorten the aging treatment time, and is beneficial to expanding the application range of 7175 in the aviation field, improving the product production efficiency and saving the cost.

Description

Technical Field

[0001] This invention belongs to the field of metal heat treatment, and particularly relates to a re-aging heat treatment process for 7175 aluminum alloy for aerospace applications. Background Technology

[0002] The 7000 series aluminum alloys belong to the Al-Zn-Mg-Cu alloy family. Due to their low density, high strength, and good processing and welding properties, they are widely used in aerospace and civilian industries. To meet the higher performance requirements of the rapidly developing modern aerospace and transportation industries, some scholars have conducted extensive research on the heat treatment process of these alloys and achieved many considerable results.

[0003] 7000 series aluminum alloys are typical age-hardening alloys. The purpose of aging treatment is to precipitate a second phase from the supersaturated solid solution to strengthen the alloy matrix. The size, quantity, and distribution of the precipitated phase determine the strength, toughness, and stress corrosion resistance of the alloy. Typical aging treatments include: peak aging (T6), two-stage aging, reversion aging (RAA), and bimodal aging.

[0004] Peak aging (T6) is an aging process aimed at achieving high strength. After peak aging, GP zones and fine, dispersed η' phases precipitate within the grains, while the eutectic, continuously distributed η phase is distributed along the grain boundaries. This grain boundary microstructure is highly sensitive to stress corrosion cracking and exfoliation corrosion cracking. After this treatment, the strength reaches its peak, but the resistance to stress corrosion cracking is poor, which significantly limits the optimal performance matching capability of 7000 series alloys.

[0005] Two-stage aging consists of two phases: 1. Nucleation stage (low-temperature aging); 2. Stabilization stage (high-temperature aging). Uniform disk-shaped phases mainly form within the alloy, while larger η' phases precipitate, and coarse, stable η phases form at high-angle grain boundaries. With prolonged aging, the intragranular η' phase coarsens, and the stable η phase at the grain boundaries grows, resulting in distinct non-precipitated zones at the grain boundaries. After two-stage aging, discontinuous bulk precipitates are distributed at the grain boundaries, improving stress corrosion resistance. However, the strengthening phases in the matrix grow and coarsen, causing a decrease in alloy strength of approximately 10%-15%, and varying degrees of reduction in plasticity and toughness. To improve the two-stage aging treatment of 7000 series aluminum alloys, researchers proposed a T736 heat treatment with strength close to T6 and stress corrosion resistance between T73 and T76.

[0006] To achieve a better balance between mechanical properties and corrosion resistance, Regression Reaging (RRA) was developed. RRA mainly consists of three stages: The first stage is T6 peak aging, after which semi-coherent η' is dispersed and precipitated within the grains, and chain-like incoherent η phases are formed at the grain boundaries; The second stage is high-temperature short-time treatment, after which η' within the grains is re-dissolved, and the continuous chain-like precipitates on the grain boundaries begin to fuse and are no longer continuously distributed. This grain boundary structure improves the resistance to stress corrosion and exfoliation corrosion, but the re-dissolved η' phase within the grains greatly reduces the strength of the alloy; The third stage is T6 aging again, at which the strength reaches its peak, and partially coherently dispersed η' precipitates within the grains, while discontinuous incoherent η phase particles still exist on the grain boundaries.

[0007] In recent years, research on the RRA process for 7000 series alloys has been very popular. J. Long et al. studied the evolution of the microstructure and mechanical properties of 7A55 aluminum alloys after continuous RRA treatment. However, due to the high secondary aging temperature and short holding time (redissolution) of the RRA process, as well as many complex processes and the inability to apply it in industry, it remains a problem that urgently needs to be solved. Summary of the Invention

[0008] This invention addresses the problems of high secondary aging temperature and short holding time in the existing RRA process, which makes it unsuitable for industrial application. It provides a regression re-aging process for 7175 aluminum alloy extruded profiles. This aging process can shorten the aging time and save costs. At the same time, regression re-aging can improve the corrosion resistance of 7175 aluminum alloy while maintaining its strength. Moreover, this process can be applied in actual production.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A heat treatment process for the re-aging and re-hardening of 7175 aluminum alloy for aerospace applications includes the following steps: solution treatment, pre-aging treatment, re-aging treatment, and re-aging treatment. The solution treatment further includes quenching, specifically water quenching. The pre-aging treatment has an aging temperature of 115-125℃, a heating rate of 10℃ / min, and a holding time of 8-12h; this pre-aging treatment is a peak aging process, a high-strength aging process. The re-aging treatment has an aging temperature of 165-185℃ and a holding time of 40-60min; this re-aging treatment is a high-temperature, short-time treatment, a method for dissolving precipitated phases. The re-aging treatment has an aging temperature of 100-120℃ and a holding time of 8-10h.

[0011] Preferably, in the water quenching process, the water temperature is 25°C, the water temperature change before and after cooling does not exceed 5°C, and the quenching transfer time does not exceed 25 seconds.

[0012] Preferably, after the pre-aging treatment, the temperature is rapidly increased to the aging temperature of the regression treatment, with a heating rate of 40℃ / min.

[0013] As a preferred method, furnace cooling is used after the regression treatment.

[0014] Preferably, the product is removed from the furnace after aging treatment and then air-cooled at room temperature.

[0015] As a preferred option, the optimal conditions for its re-aging process are: pre-aging treatment at 120℃ for 10 hours; re-aging treatment at 185℃ for 40 minutes; and re-aging treatment at 120℃ for 10 hours. Under these conditions, the alloy exhibits a tensile strength of 639.9 MPa, a yield strength of 566.8 MPa, an elongation of 12.5%, an electrical conductivity of 22 S / m, a hardness of 250 HV, and resistance to stress corrosion cracking: no signs of fracture were observed after 50 days of loading at 350 MPa. The exfoliation corrosion rating is EB.

[0016] This invention discloses a re-aging heat treatment process for 7175 aluminum alloy used in aerospace applications, the design principle of which is as follows:

[0017] 7000 series aluminum alloys are typical age-hardening alloys. The purpose of aging treatment is to precipitate a second phase from the supersaturated solid solution to strengthen the alloy matrix. The size, quantity, and distribution of the precipitated phase determine the strength, toughness, and stress corrosion resistance of the alloy. Typical aging treatments include peak aging (T6), two-stage aging, regressive aging (RAA), and bimodal aging. To achieve a better balance between mechanical properties and corrosion resistance, this invention employs the regressive aging process, whose greatest advantage is that it can improve corrosion resistance while maintaining strength.

[0018] Among them: the pre-aging treatment is mainly to obtain the highest strength (T6), and the precipitates in the microstructure are dispersed in the grain and grain boundary. The re-aging treatment is to dissolve the fine precipitates on the grain boundary, and some larger phases continue to grow at the grain boundary, showing an obvious discontinuous distribution state (coarse phase). The re-aging treatment will re-disperse the phases that were dissolved in the grain after the high temperature and short time treatment, and at the same time, the phases on the grain boundary will grow further, so that the 7175 aluminum alloy has the microstructure characteristics of both strength and corrosion resistance.

[0019] The difficulty of the re-aging process lies in the selection of re-aging temperature and holding time. Too long a time or too high a temperature will lead to grain growth, excessive growth of precipitates, and re-dissolution. Insufficient temperature and time will result in insufficient re-dissolution of chain precipitates on grain boundaries and insufficient re-dissolution of η' phase within the grains. During re-aging, the precipitates within the grains will grow excessively, thus affecting the strength.

[0020] Ultimately, after the re-aging process of this invention, the intragranular structure of the aluminum alloy is similar to that of T6 aging, and the grain boundary structure is similar to that of double-stage aging. Due to the combination of the advantages of peak aging and double-stage aging, this process can give the alloy good resistance to stress corrosion, strength and toughness.

[0021] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0022] 1. The regression re-aging process provided by this invention applies regression re-aging treatment to 7175 aluminum alloy extruded profiles. Compared with existing technologies, this not only shortens the treatment time but also improves the corrosion resistance of 7175 aluminum alloy while ensuring strength. This invention perfectly overcomes the shortcomings of previous processes, enabling 7175 profiles to be better applied in the aerospace field.

[0023] 2. Through extensive experimentation, this invention has determined the optimal re-aging process to be: 120℃ / 10h + 185℃ / 40min + 120℃ / 8.5h. Under these conditions, the alloy exhibits a tensile strength of 639.9MPa, a yield strength of 566.8MPa, an elongation of 12.5%, an electrical conductivity of 22S / m, a hardness of 250HV, and resistance to stress corrosion cracking: no signs of fracture were observed after 50 days of loading at 350MPa. The exfoliation corrosion rating is EB, and its mechanical properties and corrosion resistance completely exceed the performance requirements for 7175 aluminum alloy used in aerospace applications. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 Image (EB) after peeling and etching in Example 1;

[0026] Figure 2 Example 2: Photograph of the re-aging and peeling corrosion process (EB);

[0027] Figure 3 The top view (EA) after re-aging and exfoliation corrosion in Example 3;

[0028] Figure 4 This is a side view (EA) after regression and re-aging of the corrosion in Example 3;

[0029] Figure 5 Image of Comparative Example 1 after peeling and erosion (ED);

[0030] Figure 6Comparative Example 2: Image after peeling and corrosion (EC);

[0031] Figure 7 Comparative Example 3: Image after peeling and corrosion (EC);

[0032] Figure 8 The TEM bright-field phase of the 7175 aluminum alloy re-aging in Example 1;

[0033] Figure 9 The bright-field phase of the 7175 aluminum alloy unipolar aging TEM in Comparative Example 1;

[0034] Figure 10 The bright-field phase of the TEM in the 7175 aluminum alloy with dual-stage aging in Comparative Example 2;

[0035] Figure 11 This is a schematic diagram of the re-aging process and its structure. Detailed Implementation

[0036] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0037] Numerous specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways than those described herein, and therefore the invention is not limited to the specific embodiments disclosed in the following specification.

[0038] Example 1, as Figure 1 As shown,

[0039] A re-aging heat treatment process for 7175 aluminum alloy used in aerospace:

[0040] The 7175 aluminum alloy used consists of 5.8% zinc, 2.9% magnesium, 1.5% copper, 0.12% manganese, 0.17% chromium, 0.10% iron, 0.10% silicon, 0.1% titanium, and 0.05%-0.15% other components, with the balance being aluminum.

[0041] The re-aging heat treatment method for 7175 aluminum alloy extruded profiles used in aviation is as follows:

[0042] First, a solution treatment is performed at a temperature of 475℃ for 2.5 hours. The quenching method is water quenching at a temperature of 25℃, with a temperature difference of less than 5℃ between the water before and after cooling. The quenching transfer time is 13 seconds.

[0043] After solution quenching, a re-aging treatment is performed. During the re-aging treatment, a pre-aging treatment is first performed. The aging temperature of the pre-aging treatment is 120℃, the heating rate is 10℃ / min, and the holding time is 10h.

[0044] After the heat preservation was completed, the temperature was raised to 177℃ using a continuous heating method for regression treatment. The heating rate was 40℃ / min, and the heat preservation was carried out for 52 minutes. After the regression treatment, the cooling method was furnace cooling.

[0045] Then, a second aging treatment is performed at a temperature of 120℃, a heating rate of 10℃ / min, and a holding time of 8.5h. The second aging treatment is cooled at room temperature to obtain 7175 aluminum alloy profiles.

[0046] Example 2, as Figure 2 As shown, a re-aging heat treatment process for 7175 aluminum alloy used in aerospace applications is described:

[0047] The 7175 alloy used consists of 5.8% zinc, 2.9% magnesium, 1.5% copper, 0.12% manganese, 0.17% chromium, 0.10% iron, 0.10% silicon, 0.1% titanium, and 0.05%-0.15% other components, with the balance being aluminum.

[0048] The re-aging heat treatment method for 7175 aluminum alloy extruded profiles used in aviation is as follows:

[0049] First, a solution treatment is performed at a temperature of 477℃ for 2.5 hours. The quenching method is water quenching at a temperature of 25℃, with a temperature difference of less than 5℃ between the water before and after cooling. The quenching transfer time is 14 seconds.

[0050] After solution quenching, a re-aging treatment is performed. During the re-aging treatment, a pre-aging treatment is first performed. The aging temperature of the pre-aging treatment is 117℃, the heating rate is 10℃ / min, and the holding time is 11h.

[0051] After the heat preservation is completed, the temperature is raised to 183℃ using a continuous heating method for regression treatment. The heating rate is 40℃ / min, and the heat preservation is carried out for 40min. After the regression treatment, the cooling method is furnace cooling.

[0052] Then, a second aging treatment was performed at a temperature of 108℃, a heating rate of 10℃ / min, and a holding time of 8.5h. The second aging treatment was carried out by room temperature cooling to obtain 7175 aluminum alloy profiles.

[0053] Example 3, as Figure 3 , Figure 4 As shown, a re-aging heat treatment process for 7175 aluminum alloy used in aerospace applications is described:

[0054] The 7175 alloy used consists of 5.3% zinc, 2.7% magnesium, 1.6% copper, 0.11% manganese, 0.20% chromium, 0.13% iron, 0.15% silicon, 0.1% titanium, and 0.05%-0.15% other components, with the balance being aluminum.

[0055] The re-aging heat treatment method for 7175 aluminum alloy extruded profiles used in aviation is as follows:

[0056] First, a solution treatment is performed at a temperature of 476℃ for 2.5 hours. The quenching method is water quenching at a temperature of 25℃, with a temperature difference of less than 5℃ between the water before and after cooling. The quenching transfer time is 15 seconds.

[0057] After solution quenching, a re-aging treatment is performed. During the re-aging treatment, a pre-aging treatment is first performed. The aging temperature of the pre-aging treatment is 125℃, the heating rate is 10℃ / min, and the holding time is 9h.

[0058] After the heat preservation is completed, the temperature is raised to 188℃ using a continuous heating method for regression treatment. The heating rate is 40℃ / min, and the heat preservation is carried out for 45min. After the regression treatment, the cooling method is furnace cooling.

[0059] Then, a second aging treatment was performed at an aging temperature of 117℃, a heating rate of 10℃ / min, and a holding time of 8h. The second aging treatment was cooled at room temperature to obtain 7175 aluminum alloy profiles.

[0060] Comparative Example 1, such as Figure 5 As shown, a heat treatment process for 7175 aluminum alloy used in aviation:

[0061] The 7175 alloy used consists of 4.9% zinc, 2.0% magnesium, 1.3% copper, 0.09% manganese, 0.17% chromium, 0.20% iron, 0.16% silicon, 0.10% titanium, and 0.05%-0.15% other components, with the balance being aluminum.

[0062] The traditional heat treatment method for 7175 aluminum alloy extruded profiles used in aviation is as follows:

[0063] First, a solution treatment is performed at a temperature of 475℃ for 2.5 hours. The quenching method is water quenching at a temperature of 25℃, with a temperature difference of less than 5℃ between the water before and after cooling. The quenching transfer time is 15 seconds.

[0064] After solution quenching, a single-stage peak aging treatment (T6) was performed at an aging temperature of 120℃, a heating rate of 10℃ / min, and a holding time of 24h. The cooling method used was room temperature cooling, resulting in 7175 aluminum alloy profiles.

[0065] Comparative Example 2, such as Figure 6As shown, a heat treatment process for 7175 aluminum alloy used in aviation:

[0066] The 7175 alloy used consists of 4.7% zinc, 2.2% magnesium, 1.1% copper, 0.11% manganese, 0.15% chromium, 0.19% iron, 0.17% silicon, 0.11% titanium, and 0.05%-0.15% other components, with the balance being aluminum.

[0067] The traditional heat treatment method for 7175 aluminum alloy extruded profiles used in aviation is as follows:

[0068] First, a solution treatment is performed at a temperature of 475℃ for 2.5 hours. The quenching method is water quenching at a temperature of 25℃, with a temperature difference of less than 5℃ between the water before and after cooling. The quenching transfer time is 11 seconds.

[0069] After solution quenching, a two-stage aging treatment is performed. The first stage aging temperature is 115℃ and the holding time is 12h.

[0070] After the heat preservation is completed, the temperature is raised to 160℃ for the second stage of aging treatment, and the heat preservation time is 8 hours. The cooling method used is room temperature cooling, resulting in 7175 aluminum alloy profiles.

[0071] Comparative Example 3, such as Figure 7 As shown, a heat treatment process for 7175 aluminum alloy used in aviation:

[0072] The 7175 alloy used consists of 4.7% zinc, 2.4% magnesium, 1.4% copper, 0.16% manganese, 0.20% chromium, 0.19% iron, 0.13% silicon, 0.10% titanium, and 0.05%-0.15% other components, with the balance being aluminum.

[0073] The three-stage aging heat treatment method for 7175 aluminum alloy extruded profiles used in aviation is as follows:

[0074] First, a solution treatment is performed at a temperature of 475℃ for 2.5 hours. The quenching method is water quenching at a temperature of 25℃, with a temperature difference of less than 5℃ between the water before and after cooling. The quenching transfer time is 15 seconds.

[0075] After solution quenching, a re-aging treatment is performed. During the re-aging treatment, a pre-aging treatment is first performed. The aging temperature of the pre-aging treatment is 125℃, the heating rate is 10℃ / min, and the holding time is 9h.

[0076] After the heat preservation is completed, the temperature is raised to 140℃ using a continuous heating method, and then a regression treatment is performed. The heating rate is 40℃ / min, and the heat preservation is carried out for 2 hours. After the regression treatment and aging, the cooling method is furnace cooling.

[0077] Then, a second aging treatment is performed at a temperature of 110℃ for 8 hours. The second aging treatment is cooled at room temperature to obtain 7175 aluminum alloy profiles.

[0078] The aluminum alloy sheets obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to mechanical property testing, conductivity testing, stress corrosion testing, and exfoliation corrosion testing. The results are shown in Tables 1 and 2.

[0079] Table 1 shows the mechanical properties and conductivity test results of Examples 1 to 3 and Comparative Examples 1 to 3.

[0080] Table 1 shows the mechanical properties and electrical conductivity of Examples 1-3 and Comparative Examples 1-3.

[0081]

[0082] Table 2 shows the stress corrosion and exfoliation corrosion performance test results of Examples 1 to 3 and Comparative Examples 1 to 3.

[0083] Table 2. Stress corrosion and exfoliation corrosion performance of Examples 1-3 and Comparative Examples 1-3

[0084]

[0085] The 7175 aluminum alloys for aerospace applications prepared in Examples 1 to 3 of this invention completely exceed the performance requirements for 7175 aluminum alloys used in aerospace aircraft: tensile strength ≥ 600 MPa; yield strength ≥ 550 MPa; elongation ≥ 10%; hardness ≥ 200 HV; and electrical conductivity ≥ 20 S / m. When the applied stress was 350 MPa and the loading time was fifty days, the 7175 aluminum alloys prepared in Examples 1 to 3 did not fracture, and the exfoliation corrosion rating was EB or higher.

[0086] As can be seen from the comparison of Tables 1 and 2, the biggest advantage of adopting the regression re-aging process is that it improves the corrosion resistance while ensuring the strength (T6 strength), so that 7175 profiles can be better used in the aerospace field.

[0087] Depend on Figures 8-11 It can be seen that, Figure 8 The TEM bright-field phase of the 7175 aluminum alloy obtained in Example 1 after regression and reaging shows that the 7175 aluminum alloy after regression and reaging exhibits discontinuous distribution of incoherent η phases at the grain boundaries. After high-temperature short-time treatment during the regression stage, the η' phases within the grains dissolve back, and the continuous chain-like precipitates at the grain boundaries begin to fuse, no longer continuously distributed. This grain boundary structure improves the resistance to stress corrosion and exfoliation corrosion. Figure 9 The bright-field TEM phase of 7175 aluminum alloy with a single peak aging was obtained in Comparative Example 1. Figure 10 The TEM bright-field phase of the 7175 aluminum alloy obtained by comparative example 2 under two-stage aging shows that, due to the lack of a regression treatment process, the finished product does not possess the grain boundary structure after regression and re-aging treatment, i.e., the distribution pattern of precipitates on the grain boundaries. In comparative examples 1 and 2, the precipitates are continuously distributed, forming continuous corrosion channels, which leads to intensified corrosion. Moreover, the generation of potential difference is mainly caused by the precipitates at the grain boundaries, which is also the inducing factor for anodic dissolution. Therefore, the 7175 aluminum alloy without regression and re-aging treatment does not have the corresponding resistance to stress corrosion and exfoliation corrosion.

[0088] Comparative Example 3 illustrates a process parameter used in exploring the regression-re-aging process. The challenge of this process lies in selecting the regression temperature and holding time. Excessive time or temperature can lead to grain growth, excessive precipitate growth, and re-dissolution. Insufficient temperature and time result in inadequate re-dissolution of the chain-like precipitates at grain boundaries and the η' phase within the grains. Furthermore, during re-aging, this can cause excessive growth of the precipitates within the grains, thereby affecting strength.

[0089] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments that can be applied to other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A re-aging heat treatment process for 7175 aluminum alloy used in aviation, comprising the following steps: The invention comprises solution treatment, pre-aging treatment, regression treatment, and re-aging treatment, characterized in that the solution treatment further includes quenching, wherein the quenching method is water quenching; the pre-aging treatment has an aging temperature of 120℃, a heating rate of 10℃ / min, and a holding time of 10h; the regression treatment has an aging temperature of 185℃ and a holding time of 40min; and the re-aging treatment has an aging temperature of 120℃ and a holding time of 8.5h. In the water quenching process, the water temperature is 25℃, the water temperature change before and after cooling does not exceed 5℃, and the quenching transfer time does not exceed 25s. After pre-aging treatment, the temperature is rapidly increased to the aging temperature of regression treatment at a rate of 40℃ / min. After the regression treatment, furnace cooling was used for cooling. After further aging treatment, the product is removed from the furnace and cooled in air at room temperature. The specific performance requirements for the 7175 aluminum alloy obtained by the regression and re-aging heat treatment process for aerospace applications are as follows: tensile strength ≥ 600 MPa; yield strength ≥ 550 MPa; elongation ≥ 10%; hardness ≥ 200 HV; electrical conductivity ≥ 20 S / m; under the conditions of a loading stress of 350 MPa and a loading time of fifty days, the aerospace 7175 aluminum alloy did not fracture, and the exfoliation corrosion level was above EB.

Images