7475 aluminum alloy heat treatment process method

Abstract

The invention discloses a 7475 aluminum alloy heat treatment process method which comprises the following steps: preserving heat for 30 minutes when the solid solution heat preservation time is 471 DEG C, heating to 494 DEG C, and preserving heat for 30 minutes; the first-stage aging temperature is 121 DEG C, the heat preservation time is 4 hours, the second-stage aging temperature is 163 DEG C, and the heat preservation time of the second-stage aging is 18-28 hours. The optimal secondary aging time is 20-26 hours, the secondary aging prolonging time cannot exceed 28 hours, and the method can be used as a rescue method for disqualification of the conductivity of the 7475 aluminum alloy after heat treatment. Under the conditions that the first-stage aging temperature and time are not changed and the second-stage aging temperature is fixed, along with the prolonging of the second-stage aging time, the strength level of the material can be obviously reduced, and the conductivity and the corrosion resistance are improved. By means of the heat treatment process, the aluminum alloy 7475 plate is high in strength, the conductivity and the ductility are obviously improved, and the comprehensive performance is good.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of 7475 aluminum alloy, and in particular relates to a heat treatment process method of 7475 aluminum alloy. Background Art

[0002] 7475 aluminum alloy belongs to the AI-Zn-Mg-Cu alloy system, and is mainly used for wing frames, stringers, and other parts that require both high strength and high fracture toughness. The domestic aluminum alloy 7475 is supplied in the annealed state, and the material is required to be in the solution treated over-aging state, so an appropriate heat treatment process must be carried out. Therefore, the heat treatment process technology used to meet the heat treatment state and ultrasonic testing requirements specified in the design becomes a key technology. According to the requirements of GJB1694A-2019 "Heat Treatment of Deformed Aluminum Alloys", the heat treatment of 7475 aluminum alloy faces the following problems:

[0003] The solution treatment temperature is constant and the solution time is determined;

[0004] The primary aging temperature and time are determined, the secondary aging temperature is determined, and the secondary aging time is uncertain;

[0005] 7475 is an Al-Zn-Mg-Cu aluminum alloy. Domestic aluminum alloy tests generally use a single-stage aging process. For example, the graded aging used in LC4 plates does not have a heating rate requirement. However, in the study of the heat treatment process parameters of 1933 forgings, the heating rate from the first stage to the second stage has a great influence on the performance of 1933 forgings. The selection of the heating time from the first stage to the second stage plays a vital role in the heat treatment process results. The heating rate from the first stage to the second stage of 1933 forgings is 10-15℃ / h. 7475 is a two-stage aging process, and the heating rate from the first stage to the second stage of aging has an uncertain effect on the performance.

[0006] When the conductivity of 7475 aluminum alloy semi-finished parts was checked after heat treatment in the workshop, it was found that the conductivity was low. Supplementary heat treatment was required and the secondary aging time of heat treatment was increased. The conductivity change was still uncertain. Summary of the invention

[0007] The technical problem solved by the present invention is to achieve a solution treatment over-aging state through an appropriate heat treatment process and obtain better performance of such material.

[0008] The technical solution of the present invention:

[0009] A 7475 aluminum alloy heat treatment process method, the method is used to make the 7475 aluminum alloy plate from the annealing state to the over-aged T651 state, comprising:

[0010] S1, the primary solution treatment temperature of 7475 aluminum alloy is 471℃, and the temperature is kept for 30 minutes;

[0011] S2, secondary solution treatment temperature of 7475 aluminum alloy is 494℃, and the temperature is kept for 30 minutes;

[0012] S3, the primary aging temperature of 7475 aluminum alloy is 121℃, and the primary aging holding time is 4 hours;

[0013] The secondary aging temperature of S4, 7475 aluminum alloy is 163°C, and the secondary aging holding time is 18 to 28 hours.

[0014] Furthermore, the solid solution test process of the processes described in S1 and S2 is as follows:

[0015] The primary solution treatment and secondary solution treatment of 7475 aluminum alloy are both carried out in an air furnace. The primary solution temperature is 471°C, and the primary solution holding time is 30 minutes, 1 hour, 2 hours, and 3 hours, respectively. The secondary solution treatment is heated to 494°C and kept for 30 minutes. Then, a double-stage aging treatment of 121°C×4h+163°C×18h is carried out in a programmable precision oven. After the double-stage aging treatment, air cooling is carried out, and the time from the primary aging temperature to the secondary aging temperature is 2h.

[0016] Furthermore, the following conclusions were obtained through the solid solution test process:

[0017] When the solution temperature is 471℃ and the holding time is more than 1 hour, the conductivity is lower than 39%IACS. When the solution temperature is 471℃ and the holding time is longer, the mechanical properties of the alloy will be affected, but the conductivity will be affected.

[0018] Furthermore, the heat preservation time test process of the processes described in S3 and S4 is as follows:

[0019] The primary solution and secondary solution treatment of 7475 aluminum alloy are both carried out in an air furnace. The primary solution temperature is 471°C, and the primary solution holding time is 30 minutes. The secondary solution treatment is heated to 494°C and kept for 30 minutes. Then, a double-stage aging treatment is carried out in a programmable precision oven. The primary aging temperature is 121°C and kept for 4h. The secondary aging temperature is 163°C. The holding times are 2h, 4h, 6h, 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h, 26h, 28h, 30h, and 34h respectively. After the double-stage aging treatment, air cooling is carried out, and the time from the primary aging temperature to the secondary aging temperature is 2h.

[0020] Furthermore, during the test, the time from the primary aging temperature to the secondary aging temperature was set to 0.5h, 2h, 4h, 6h, 8h, and 10h, respectively, to determine that when the two-stage aging temperature and time remain unchanged, changing the heating time from the primary aging temperature to the secondary aging temperature has no effect on the strength, elongation, and conductivity of the alloy.

[0021] Furthermore, the following conclusions were drawn from the heat preservation time test process:

[0022] When the primary aging temperature and time remain unchanged, as the secondary aging holding time increases, the tensile strength and yield strength show a downward trend, the tensile strength and yield strength are lost in this stage, the elongation fluctuates up and down, and the conductivity increases with the increase of the secondary aging time, showing an upward trend.

[0023] Furthermore, after S4, the method further includes: obtaining a 7475 aluminum alloy test piece after the processes from S1 to S4, preparing a metallographic sample along the plate direction from the aluminum alloy test piece, and checking under a quantitative metallographic microscope to see if the structure is normal.

[0024] Furthermore, the method further comprises:

[0025] When the electrical conductivity of the final 7475 aluminum alloy does not meet the requirements, the secondary aging insulation time is increased, but the total secondary aging insulation time is not more than 28 hours.

[0026] The technical solution of the present invention uses the heat treatment process parameters to realize the transformation of 7475 aluminum alloy from the annealed state to the state required for material use, that is, the solid solution treatment over-aging state. It is used for heat treatment part detection after part deformation, meets the detection requirements after heat treatment, meets the part performance requirements, avoids repeated heat treatment, and improves production efficiency. At the same time, a method for rescuing the unqualified conductivity of 7475 aluminum alloy after heat treatment is proposed to meet the part manufacturing requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A schematic diagram of a heat treatment process provided by an embodiment of the present invention;

[0028] Figure 2 A schematic diagram of the relationship between the secondary aging and heat preservation time and performance provided by an embodiment of the present invention;

[0029] Figure 3 A schematic diagram showing the relationship between the heating time from primary aging to secondary aging and the performance provided in an embodiment of the present invention;

[0030] Figure 4 A schematic diagram of the right end coating structure provided by an embodiment of the present invention;

[0031] Figure 5 A schematic diagram of the left end coating structure provided by an embodiment of the present invention;

[0032] Figure 6 A schematic diagram of a heat treatment process provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is described in detail below in conjunction with the accompanying drawings.

[0034] The embodiment of the present invention provides a heat treatment process for converting a 7475 aluminum alloy from an annealed state to a solution treated over-aging state, comprising:

[0035] (1) Heat treatment material requirements

[0036] The 7475 aluminum alloy used in this test has a plate thickness of δ1.8㎜ and a chemical composition of 6.04% Zn, 2.07% Mg, 1.67% Cu, 0.0048% Mn, 0.031% Fe, and 0.19% Cr. The material is in the annealed state.

[0037] (2) Process equipment requirements

[0038] RJJ-24-6 pit type tempering furnace for solution treatment

[0039] MCK-290CpRc-0-Mea programmable precision oven for aging treatment

[0040] OLYMPUS GX-71 quantitative metallographic microscope and image analysis system to observe microstructure

[0041] AG-xplus 250KN high-precision electronic tensile testing machine measures tensile properties (tensile strength, yield strength, elongation)

[0042] Sigmatest 2.067 eddy current conductivity meter was used to measure the conductivity.

[0043] (3) Heat treatment process steps The heat treatment process of 7475 aluminum alloy plate to achieve T651 state is as follows:

[0044] The primary solution treatment temperature is 471°C and kept at this temperature for 30 minutes;

[0045] The secondary solution treatment temperature is 494°C and kept at this temperature for 30 minutes;

[0046] The primary aging temperature is 121°C, and the temperature is kept for 4 hours;

[0047] The secondary aging temperature is 163℃, and the secondary aging holding time is 18 to 28 hours. The optimal secondary aging time is 20 to 26 hours;

[0048] The secondary aging extension time cannot exceed 28 hours.

[0049] The heating time from primary aging to secondary aging has little effect on the performance and conductivity of 7475 aluminum alloy after reaching T761 state.

[0050] Test process

[0051] The following are the heat treatment processes for testing various mechanical properties and electrical conductivity, such as Figure 1 As shown:

[0052] (1) Effect of solution time on performance

[0053] The 7475 aluminum alloy process test specimens were solution treated in an air furnace, and aging was carried out in a programmable precision oven. The solution temperature was 471°C, and the solution holding time was 30 minutes, 1 hour, 2 hours, and 3 hours, respectively. The temperature was raised to 494°C and kept for 30 minutes. The double-stage aging was 121°C × 4h + 163°C × 18h, and air-cooled after aging. The time from the primary aging temperature to the secondary aging temperature was 2h.

[0054] (2) Effect of secondary aging and heat preservation time on performance

[0055] The 7475 aluminum alloy process test specimens were solution treated in an air furnace, and aging was carried out in a programmable precision oven. The solution temperature was 471°C, the solution holding time was 30 minutes, the temperature was raised to 494°C, the holding time was 30 minutes, the primary aging temperature was 121°C, the holding time was 4 hours, the secondary aging temperature was 163°C, the holding time was 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, 24 hours, 26 hours, 28 hours, 30 hours, 34 hours, and air cooling after aging.

[0056] (3) Effect of heating time from primary aging to secondary aging on performance

[0057] The quenching of the 7475 aluminum alloy process test specimens was carried out in an air furnace, with a solution temperature of 471°C, a solution holding time of 30 minutes, a temperature rise to 494°C, a holding time of 30 minutes, a primary aging temperature of 121°C, a holding time of 4 hours, a secondary aging temperature of 163°C, a holding time of 18 hours, and air cooling after aging. The time from the primary aging temperature to the secondary aging temperature is 0.5h, 2h, 4h, 6h, 8h, and 10h, respectively.

[0058] (4) Microstructure inspection

[0059] The 7475 aluminum alloy process test specimens were solution treated in an air furnace, and aging was performed in a programmable precision oven. The solution treatment temperature was 471°C, the solution holding time was 30 minutes, the temperature was raised to 494°C, the holding time was 30 minutes, the primary aging temperature was 121°C, the holding time was 4 hours, the secondary aging temperature was 163°C, the holding time was 18 hours, and the aging was air cooled. After heat treatment, a high-magnification sample was taken from the test piece, and the aluminum alloy microstructure was observed by metallographic method.

[0060] Test results

[0061] (1) Effect of solution treatment holding time on performance

[0062] The relationship between solution time and tensile strength, yield strength, elongation and conductivity is shown in Table 1.

[0063] As shown in Table 1, when the solution temperature is 471°C and the holding time is more than 1 hour, the conductivity is lower than 39% IACS. When the solution temperature is 471°C and the length of the solution time has little effect on the mechanical properties of the alloy, it has some effect on the conductivity.

[0064] Table 1 Relationship between solution time and properties

[0065]

[0066]

[0067] (2) Effect of secondary aging and heat preservation time on performance

[0068] The relationship between the secondary aging holding time and the tensile strength, yield strength, elongation, and conductivity is shown in Figure 2 .

[0069] right Figure 2 Explanation: When the primary aging temperature and time remain unchanged, as the secondary aging insulation time increases, the tensile strength (hollow circle) and yield strength (solid point) show a significant downward trend, and the strength loss is large at this stage. The elongation (hollow triangle) fluctuates up and down, and the elongation is generally very high. The conductivity (solid triangle) increases significantly with the increase of the secondary aging time, showing an upward trend.

[0070] (3) Effect of heating time from primary aging to secondary aging on performance

[0071] The relationship between the heating time from primary aging to secondary aging and the tensile strength, yield strength, elongation, and conductivity is shown in Figure 3 .

[0072] The performance of 7475 aluminum alloy plate tested after using this heat treatment process method has the characteristics of high strength and good corrosion resistance, meeting the requirements of material acceptance standards. The main advantages are as follows:

[0073] (1) The heat treatment process parameters are accurate and can be used to detect heat-treated parts after deformation, which meets the requirements for post-heat treatment inspection, meets the performance requirements of parts, avoids repeated heat treatment, and improves production efficiency.

[0074] (2) A method to save the unqualified electrical conductivity of 7475 aluminum alloy after heat treatment is proposed. This method complies with the requirements of relevant technical standards and meets the requirements for parts manufacturing.

[0075] (3) The heat treatment process of 7475 aluminum alloy sheet can be applied to the manufacture of wing frames, trusses and other parts that require both high strength and high fracture toughness, solving the quality problem of heat treatment of key materials. 7475 aluminum alloy sheet has higher strength and better corrosion resistance, can be used in more severe environments, and enhances the safety performance of service.

[0076] right Figure 3 Explanation: When the temperature and time of the two-stage aging remain unchanged and the heating time from the primary aging to the secondary aging is changed, the strength, elongation and conductivity of the alloy do not change significantly with the extension of the heating time. The strength, elongation and conductivity are relatively high, and the changes are irregular and fluctuate within a certain range.

[0077] (4) Microstructure inspection

[0078] Metallographic samples were prepared along the plate direction on the tensile test piece, and the metallographic structure was displayed after grinding and polishing. The morphology of the coating layer on the right side (500 times) is shown in Figure 4 , the left end coating layer structure morphology (200 times) see Figure 5 , the organization is normal.

[0079] Effect of solution time on properties of 7475 aluminum alloy:

[0080] In the two-stage solid solution, the first-stage solid solution is the main factor affecting the mechanical properties of the plate. After the first-stage solid solution, the solid solution temperature is 471°C, the insulation time is 1 hour or more, the strength is qualified, and the conductivity is slightly lower than the requirement.

[0081] Effect of secondary aging time on the performance of 7475 aluminum alloy: The secondary aging time significantly affects the strength, plasticity and electrical conductivity of the alloy. When the primary aging temperature and time remain unchanged and the secondary aging temperature is constant, the tensile strength and yield strength show a significant downward trend, and the elongation fluctuates up and down. In general, the elongation is very high, and the electrical conductivity increases significantly with the increase of the secondary aging time, showing a significant upward trend.

[0082] When the secondary aging holding time of 7475 aluminum alloy is 12 hours and 20 hours, the elongation reaches a small peak value, while the strength is in the 34-hour holding range, and the tensile strength and yield strength are very high, far higher than the standard strength requirements (the standard requires tensile strength greater than 469MPa and yield strength greater than 393MPa). When the secondary aging time is greater than 10 hours, the conductivity is greater than the standard requirement of 39% IACS. It can be inferred that when the secondary aging time is extended indefinitely, the strength will be unqualified.

[0083] The standard requires that the conductivity is equal to or greater than 39% IACS, and the yield strength is higher than the specified minimum value and does not exceed 62MPa, then the sheet is qualified. Considering this technical indicator requirement, when the secondary aging temperature is 163℃, the insulation time is preferably 18 hours to 28 hours. The optimal time for the secondary aging is 20 to 26 hours, and the total extension time of the secondary aging cannot exceed 28 hours.

[0084] Effect of secondary aging time on electrical conductivity: When the primary aging time is constant, the electrical conductivity increases with the increase of the secondary aging time, and the electrical conductivity increases significantly. This shows that the secondary aging time has a greater influence on the electrical conductivity. When aged at 121°C, the α phase supersaturation is large, the lattice distortion continues to increase, and the electrical conductivity is low. As the temperature rises (to 163°C), the decomposition of the α phase increases, and the size of the GP zone grows, which significantly increases the electrical conductivity. The two interact with each other, and the lattice distortion has a great advantage. Therefore, during the secondary aging insulation, the electrical conductivity is very low. After over-aging, the lattice distortion decreases, and the second phase and strengthening particles at the grain boundary gather and grow. The two effects significantly increase the electrical conductivity. Due to Figure 3 It is known that with the extension of secondary aging time, the conductivity generally increases.

[0085] Based on the above aging process, it can be seen that the length of the secondary aging time has a great influence on the tensile strength, yield strength and electrical conductivity. Prolonging the secondary aging time will greatly reduce the tensile strength and yield strength indicators, and significantly increase the size of the electrical conductivity.

[0086] Influence of heating time from primary aging to secondary aging on performance and conductivity: When the temperature and time of the two-stage aging remain unchanged and the heating time from primary aging to secondary aging is changed, there is no obvious change in the strength, elongation and conductivity of the alloy with the extension of the heating time.

[0087] The best heat treatment process parameters for 7475 aluminum alloy are: solution heat preservation time at 471℃ for 30 minutes, then rise to 494℃ and keep warm for 30 minutes. The primary aging temperature is 121℃, and the temperature is kept warm for 4 hours. The secondary aging temperature is 163℃, and the secondary aging holding time is 18 to 28 hours. The best time for the secondary aging is 20 to 26 hours, and the extension time of the secondary aging cannot exceed 28 hours.

[0088] The heating time from primary aging to secondary aging has little effect on the T761 state performance and electrical conductivity of 7475 aluminum alloy.

[0089] When the primary aging temperature and time remain unchanged and the secondary aging temperature is constant, the material strength level can be significantly reduced and the electrical conductivity and corrosion resistance can be improved as the secondary aging time increases.

[0090] The present invention provides a heat treatment process method for converting a 7475 aluminum alloy from an annealing state to a solution treatment over-aging state: the solution treatment heat preservation time is 471°C for 30 minutes, then the temperature is raised to 494°C and the heat preservation time is 30 minutes. The primary aging temperature is 121°C and the heat preservation time is 4 hours. The secondary aging temperature is 163°C and the secondary aging heat preservation time is 18 to 28 hours. The optimal secondary aging time is 20 to 26 hours, and the secondary aging extension time cannot exceed 28 hours. This can be used as a rescue method for the unqualified electrical conductivity of the 7475 aluminum alloy after heat treatment. Under the condition that the primary aging temperature and time remain unchanged and the secondary aging temperature is constant, as the secondary aging time is extended, the material strength level can be significantly reduced, and the electrical conductivity and corrosion resistance can be improved. Figure 6 As shown in the figure, the heat treatment process makes the aluminum alloy 7475 sheet have high strength, significantly improved conductivity and elongation, and good comprehensive performance.

Claims

1. A 7475 aluminum alloy heat treatment process, characterized in that: The method is used to make the 7475 aluminum alloy plate reach the over-aged T651 state from the annealing state, comprising: S1, the primary solution treatment temperature of 7475 aluminum alloy is 471℃, and the temperature is kept for 30 minutes; S2, secondary solution treatment temperature of 7475 aluminum alloy is 494℃, and the temperature is kept for 30 minutes; S3, the primary aging temperature of 7475 aluminum alloy is 121℃, and the primary aging holding time is 4 hours; The secondary aging temperature of S4, 7475 aluminum alloy is 163°C, and the secondary aging holding time is 18 to 28 hours.

2. A 7475 aluminum alloy heat treatment process according to claim 1, characterized in that: The solid solution test process of the processes described in S1 and S2 is as follows: The primary solution treatment and secondary solution treatment of 7475 aluminum alloy are both carried out in an air furnace. The primary solution temperature is 471°C, and the primary solution holding time is 30 minutes, 1 hour, 2 hours, and 3 hours, respectively. The secondary solution treatment is heated to 494°C and kept for 30 minutes. Then, a double-stage aging treatment of 121°C×4h+163°C×18h is carried out in a programmable precision oven. After the double-stage aging treatment, air cooling is carried out, and the time from the primary aging temperature to the secondary aging temperature is 2h.

3. A 7475 aluminum alloy heat treatment process according to claim 2, characterized in that: The following conclusions were obtained through the solid solution test process: When the solution temperature is 471℃ and the holding time is more than 1 hour, the conductivity is lower than 39%IACS. When the solution temperature is 471℃ and the holding time is longer, the mechanical properties of the alloy will be affected, but the conductivity will be affected.

4. A 7475 aluminum alloy heat treatment process according to claim 1, characterized in that: The test process of holding time of the process described in S3 and S4 is as follows: The primary solution and secondary solution treatment of 7475 aluminum alloy are both carried out in an air furnace. The primary solution temperature is 471°C, and the primary solution holding time is 30 minutes. The secondary solution treatment is heated to 494°C and kept for 30 minutes. Then, a double-stage aging treatment is carried out in a programmable precision oven. The primary aging temperature is 121°C and kept for 4h. The secondary aging temperature is 163°C. The holding times are 2h, 4h, 6h, 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h, 26h, 28h, 30h, and 34h respectively. After the double-stage aging treatment, air cooling is carried out, and the time from the primary aging temperature to the secondary aging temperature is 2h.

5. A 7475 aluminum alloy heat treatment process according to claim 2 or 4, characterized in that: During the test, the time from the primary aging temperature to the secondary aging temperature was set to 0.5h, 2h, 4h, 6h, 8h, and 10h, respectively, to determine that when the two-stage aging temperature and time remain unchanged, changing the heating time from the primary aging temperature to the secondary aging temperature has no effect on the strength, elongation, and conductivity of the alloy.

6. A 7475 aluminum alloy heat treatment process according to claim 4, characterized in that: The following conclusions were drawn from the heat preservation time test process: When the primary aging temperature and time remain unchanged, as the secondary aging holding time increases, the tensile strength and yield strength show a downward trend, the tensile strength and yield strength are lost in this stage, the elongation fluctuates up and down, and the conductivity increases with the increase of the secondary aging time, showing an upward trend.

7. A 7475 aluminum alloy heat treatment process according to claim 5, characterized in that: After S4, the method further includes: obtaining a 7475 aluminum alloy test piece after the processes from S1 to S4, preparing a metallographic sample along the plate direction of the aluminum alloy test piece, and checking under a quantitative metallographic microscope to see if the structure is normal.

8. A 7475 aluminum alloy heat treatment process according to claim 1, characterized in that: The method further comprises: When the electrical conductivity of the final 7475 aluminum alloy does not meet the requirements, the secondary aging insulation time is increased, but the total secondary aging insulation time is not more than 28 hours.

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