A heat treatment method for reducing residual stress in 7075 aluminum alloy

Abstract

The present invention discloses a heat treatment method for reducing the residual stress of 7075 aluminum alloy. Step 1: Rough machining: Rough machine the aluminum alloy plate. Step 2: First stress relief annealing: Put the aluminum alloy plate obtained in Step 1 into a box-type tempering furnace with nitrogen protection. Step 3: Semi-finishing machining: Perform semi-finishing machining on the annealed aluminum alloy plate. Step 4: Second stress relief annealing. Step 5: Finish machining: Machine the aluminum alloy plate annealed for the second time in Step 4 to the finished product size. Step 6: Fixture manufacturing: Manufacture a middle frame pressing plate. Step 7: Part clamping: Clamp the aluminum alloy plate annealed for the second time with the middle frame pressing plate. Step 8: Use a high and low temperature treatment box to process the aluminum alloy plate annealed for the second time clamped in Step 7. Step 9: Remove the fixture: Remove the middle frame pressing plate. The present invention adopts a cold and hot cycle heat treatment for precision products with relatively strict dimensional requirements. This process can eliminate 80% of the processing stress, especially in the cryogenic section, which is more obvious.

Description

Technical Field

[0001] The present invention relates to the technical field of heat treatment, and specifically to a heat treatment method for reducing the residual stress of 7075 aluminum alloy. Background Technique

[0002] 7075 aluminum alloy is a kind of super-hard aluminum alloy of the Al-Zn-Mg-Cu system, belonging to the best products in aluminum alloys. Its excellent mechanical properties even exceed those of many low-carbon steels. 7075 aluminum alloy has good mechanical properties below 150 °C, and 7075 aluminum alloy is also an aluminum alloy that can be strengthened by heat treatment. After heat treatment, the tensile strength can reach 540 MPa, and the Brinell hardness can reach 150 - 160 HB. With an excellent weight-strength ratio, it is an ideal choice for high-stress parts. Therefore, this material is widely used in many fields such as aerospace, high-end electronic equipment, precision machinery, and optoelectronic instruments.

[0003] The main frame structure of this equipment is made of 7075 aluminum alloy. The operating environment temperature of the equipment varies within the range of -10 °C and 55 °C. During the optical debugging and operation after assembly, affected by the ambient temperature, the structural dimensions will change slightly, resulting in the misalignment of the optical instrument and the need for re-calibration, which causes many inconveniences during equipment debugging and actual use, seriously affecting work efficiency and service performance.

[0004] There are two main reasons for this deformation. One is the thermal stress generated during the quenching of aluminum alloy heat treatment and the tissue stress generated during artificial aging. The other is the processing stress generated during the machining of parts.

[0005] Due to temperature changes, the structure of the optical instrument deforms, increasing the number of calibrations and the difficulty of actual work, affecting the normal use of the equipment. The original processing process was: rough machining → semi-finishing → finishing, and the dimensional stability of the equipment structure was poor, with high temperature sensitivity. Summary of the Invention

[0006] The present invention provides a heat treatment method for reducing the residual stress of 7075 aluminum alloy to solve the problems raised in the background technique.

[0007] A heat treatment method for reducing the residual stress of 7075 aluminum alloy according to the present invention includes the following steps:

[0008] Step 1: Rough machining: The equipment uses a CNC850 machining center to rough machine the aluminum alloy plate, leaving a 4-mm allowance on one side of the plane.

[0009] Step 2: First stress relief annealing: Put the aluminum alloy sheet in Step 1 into a box-type tempering furnace with nitrogen protection. The heating rate in the furnace is 3 hours, the holding temperature is 220 °C, and the holding time is 8 hours. After the holding is completed, open the furnace door halfway. After 3 hours, take it out and air-cool it to obtain an annealed aluminum alloy sheet;

[0010] Step 3: Semi-finishing: Use a CNC850 machining center to semi-finish the annealed aluminum alloy sheet in Step 2, leaving a machining allowance of 1.5 mm on one side of the plane to obtain a semi-finished aluminum alloy sheet;

[0011] Step 4: Second stress relief annealing: Use a box-type tempering furnace with nitrogen protection. Put the semi-finished aluminum alloy sheet in Step 3 into the furnace. The heating rate in the furnace is 3 hours, the holding temperature is 220 °C, and the holding time is 8 hours. After the holding is completed, open the furnace door halfway. After 3 hours, take it out and air-cool it to obtain a second-annealed aluminum alloy sheet;

[0012] Step 5: Finishing: Use a CNC850 machining center to machine the second-annealed aluminum alloy sheet in Step 4 to the finished size. The surface roughness of the upper and lower surfaces is Ra1.6, and the flatness is 0.05;

[0013] Step 6: Tooling production: Use a CNC850 machining center and a surface grinder to produce a middle frame pressing plate. The flatness of the middle frame pressing plate meets the requirement of ≤0.03 mm, and the roughness is Ra0.8;

[0014] Step 7: Part clamping: Use high-strength bolts and nuts of M30×380, with the strength of the bolts and nuts being 10.9 grade, and use the middle frame pressing plate to clamp the second-annealed aluminum alloy sheet;

[0015] Step 8: Thermal cycling stabilization treatment: Use a high and low temperature treatment box to process the second-annealed aluminum alloy sheet clamped in Step 7. First, keep it at -65 °C for 4 h; then raise the temperature to 175 °C and keep it for 4 h, and cycle twice; after the holding at 175 °C is completed, cool it in the furnace to room temperature;

[0016] Step 9: Remove the tooling: After cooling to room temperature, remove the middle frame pressing plate, inspect the workpiece and make records.

[0017] Preferably, the middle frame pressing plate used includes a pressing plate. A circular hole is opened in the middle of the pressing plate, and strip-shaped holes are opened at the four corners and the middle edge positions of the pressing plate. A second-annealed aluminum alloy sheet is installed in the middle of multiple pressing plates, and then high-strength bolts of M30×380 are passed through the strip-shaped holes, and then the heads of the M30×380 high-strength bolts are threadedly connected to nuts.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] In view of this defect, the present invention starts from the manufacturing process of the material and adds two stress relief annealing processes and two thermal cycling heat treatment processes.

[0020] For 7075 aluminum alloy material, a heat treatment process for eliminating residual stress is adopted. In order to ensure that the mechanical properties of the material do not decrease too much, low-temperature stress relief annealing is selected to reduce the internal stress of the material.

[0021] The present invention adopts thermal cycling heat treatment, which is more suitable for components working at different temperatures and precision products with strict dimensional requirements. The process adopts a pressing method fixed by tooling, and a heat treatment system with two thermal cycles from -65°C to 175°C. This process can eliminate about 80% of the processing stress, especially in the cryogenic section, which is more obvious. See Table 2 in the specific implementation manner. It can be seen from Table 2 that the change in the flatness of the middle frame after thermal cycling heat treatment is controlled within 0.03 mm, and the average change in flatness is 0.021 mm, while the average change before stress relief by heat treatment is 0.108 mm. From the data, it can be seen that after thermal cycling stabilization treatment, the flatness is improved by 80.77% compared with before.

[0022] Reducing the processing stress of 7075 aluminum alloy by heat treatment has a positive effect and effect in reducing the temperature sensitivity of the material and ensuring its dimensional stability.

[0023] It can be seen from the test results of the specimens that the processing stress of the parts can be effectively reduced by heat treatment. The same process is adopted for similar main structural parts during production. Finally, after the entire equipment is assembled, high and low temperature tests are carried out. The tests are carried out in the range of -20°C to 60°C. After long-term (48-hour heat and cold alternation) test, the equipment fully meets the design and use requirements, and successfully solves the problem of frequent calibration due to temperature influence. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The technical solution of the present invention will be further described in detail below in conjunction with the drawings and embodiments.

[0025] Figure 1 is a schematic structural diagram of an aluminum alloy plate of a heat treatment method for reducing the residual stress of 7075 aluminum alloy according to the present invention;

[0026] Figure 2 is a change curve of the stress relief annealing process of a heat treatment method for reducing the residual stress of 7075 aluminum alloy according to the present invention;

[0027] Figure 3 is a change curve of the thermal cycling stabilization treatment process of a heat treatment method for reducing the residual stress of 7075 aluminum alloy according to the present invention;

[0028] Figure 4It is a schematic clamping diagram of the thermal cycle stabilization treatment of a heat treatment method for reducing the residual stress of 7075 aluminum alloy according to the present invention;

[0029] Figure 5 It is a structural diagram of the middle frame pressing plate of a heat treatment method for reducing the residual stress of 7075 aluminum alloy according to the present invention.

[0030] In the figure: pressing plate (1), circular hole (101), strip hole (102). Specific embodiments

[0031] The following will disclose multiple embodiments of the present invention in the form of diagrams. For the sake of clarity, many physical details will be described together in the following narrative. However, it should be understood that these physical details are not used to limit the present invention. That is to say, in some embodiments of the present invention, these physical details are unnecessary. In addition, for the sake of simplifying the diagrams, some conventional structures and components will be shown in a simple schematic manner in the diagrams.

[0032] In addition, the technical solutions between various embodiments can be combined with each other, but it must be based on the fact that those skilled in the art can implement them. When the combination of technical solutions conflicts with each other or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope required by the present invention.

[0033] Please refer to Figures 1-5 , a heat treatment method for reducing the residual stress of 7075 aluminum alloy according to the present invention, the specific steps are as follows:

[0034] Step 1: Rough machining: The equipment uses a CNC850 machining center to rough machine the aluminum alloy plate, leaving a 4-mm allowance on one side of the plane; mainly to quickly and efficiently remove the blank allowance and prepare for subsequent machining;

[0035] Step 2: First stress relief annealing: Put the aluminum alloy plate obtained in Step 1 into a box-type tempering furnace with nitrogen protection. The heating rate in the furnace is 3 hours, the holding temperature is 220 °C, and the holding time is 8 hours. After the holding is completed, open the furnace door halfway. After 3 hours, take it out of the furnace and air-cool it to obtain an annealed aluminum alloy plate; the main purpose of Step 2 is to remove the machining stress generated during rough machining;

[0036] Step 3: Semi-finishing machining: Use a CNC850 machining center to semi-finish machine the annealed aluminum alloy plate obtained in Step 2, leaving a 1.5-mm allowance on one side of the plane to obtain a semi-finished machined aluminum alloy plate; it can reduce the errors left in rough machining and make the machining surface reach a certain machining accuracy, providing the best machining conditions for high-speed cutting in finish machining;

[0037] Step 4: Secondary stress relief annealing: Use a box tempering furnace with nitrogen protection to heat the semi-finished aluminum alloy sheet in Step 3. The heating rate in the furnace is 3 hours, the holding temperature is 220 °C, and the holding time is 8 hours. After the holding is completed, open the furnace door halfway. After 3 hours, take it out of the furnace and air-cool it to obtain a secondary annealed aluminum alloy sheet. The process is the same as the first stress relief annealing. The main purpose is to eliminate the stress of semi-finishing. At the same time, it is also a supplement to the first stress relief annealing, which can eliminate the processing stress to the greatest extent;

[0038] Step 5: Finish machining: Use a CNC850 machining center to machine the secondary annealed aluminum alloy sheet in Step 4 to the finished size. The surface roughness of the upper and lower surfaces is Ra1.6, and the flatness is 0.05; this can ensure the final dimensional accuracy and surface quality of the workpiece;

[0039] Step 6: Tooling production: Use a CNC850 machining center and a surface grinder to produce a middle frame pressing plate. The flatness of the middle frame pressing plate is required to be ≤0.03mm, and the roughness is Ra0.8, both of which are higher than the requirements of the aluminum alloy sheet. Only in this way can it be ensured that the parts can meet the technical requirements;

[0040] Step 7: Part clamping: Use high-strength bolts and nuts of M30×380, and use the middle frame pressing plate to clamp the secondary annealed aluminum alloy sheet to ensure the flatness requirement of the part through external force constraint;

[0041] Step 8: Thermal cycling stabilization treatment: Use a high and low temperature treatment box to process the secondary annealed aluminum alloy sheet clamped in Step 7. First, keep it at -65 °C for 4 hours; then raise the temperature to 175 °C and keep it for 4 hours, and cycle twice; after the holding at 175 °C is completed, cool it in the furnace to room temperature; the key to ensuring the dimensional stability of the part is to apply external force constraints through the temperature changes of hot and cold alternation.

[0042] Step 9: Remove the tooling: After cooling to room temperature, remove the middle frame pressing plate, inspect the workpiece and make records.

[0043] The specific implementation process method is as follows:

[0044] 2.1 The middle frame pressing plate is made of 7075T651 material, and the processing technology of the middle frame pressing plate is roughly as follows:

[0045] 2.2 Rough machining → First stress relief annealing → Semi-finishing → Secondary stress relief annealing → Finish machining → (Tooling production) → Clamp the aluminum alloy sheet → Thermal cycling stabilization treatment (Tooling fixation, see the tooling schematic Figure 4 , Figure 5 ) → Remove the tooling;

[0046] 2.3 Rough machining, with a single-sided allowance of 4 mm; semi-finishing machining, with a single-sided allowance of 1.5 mm. In order to eliminate machining stress as much as possible, there are two stress relief annealings interspersed in the machining process, both using the same heat treatment process. Slowly heat to 220 °C, and the time from room temperature to the holding temperature does not exceed 3 h. After heating to 220 °C, hold for 8 h. After the holding is completed, open the furnace door halfway for 3 h and then take out and air cool. See Figure 2 。

[0047] 2.4 After the finish machining of the aluminum alloy plate, it is installed on a special fixture and the nuts are tightened. To ensure the flatness requirement of 0.05 for the frame pressing plate during machining, the flatness of the fixture - frame pressing plate is designed to be 0.03, and the requirement of the fixture is higher than that of the part. This device can keep the aluminum alloy plate with stable dimensions and fully release the stress under the constraint of external force.

[0048] 2.5 The thermal cycling stabilization treatment is carried out by holding at -65 °C for 4 h and 175 °C for 4 h, doing two cycles, and then cooling to room temperature in the furnace. See Figure 3 。The purpose is to make the dimensions of the aluminum alloy plate more stable through the changes of thermal cycling.

[0049] 3. Tracking records

[0050] 3.1 According to the above process method, conduct tests and make detailed records of the test results. Select several groups of frame pressing plates as test samples, and use a coordinate measuring machine for the finished product inspection of the aluminum alloy plate. Some of the results are shown in Table 1.

[0051] Table 1 Frame flatness record (unit: mm) Flatness

[0052]

[0053]

[0054] It can be seen from the test results that for the first stress relief annealing, the deformation of the part is relatively large, up to 0.09 mm; after the second stress relief annealing, the deformation gradually decreases to 0.06 mm; and the deformation drops to within 0.03 mm after the thermal cycling stabilization treatment.

[0055] Table 2 Machining data before heat treatment

[0056]

[0057] Comparing with the previous data (Table 2), it can be seen that the change of flatness is controlled within 0.03 mm, the average change of flatness is 0.021 mm, while the average change before stress relief by heat treatment is 0.108 mm. From the data, it can be seen that after the thermal cycling stabilization treatment, the flatness is improved by 80.77% compared with before. Therefore, this process can eliminate about 80% of the machining stress.

[0058] It can be seen that the process method of eliminating stress by heat treatment is effective.

[0059] The above is only the implementation mode of the present invention and is not used to limit the present invention. For those skilled in the art, various changes and modifications can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of the claims of the present invention.

Claims

1. A heat treatment method for reducing the residual stress of 7075 aluminum alloy, characterized in that, It includes the following steps: Step 1: Rough machining: Using a CNC850 machining center, rough machine the aluminum alloy sheet, leaving a 4-mm allowance on the unilateral plane; Step 2: First stress relief annealing: Put the aluminum alloy sheet from Step 1 into a box-type tempering furnace with nitrogen protection. It takes 3 hours to heat the furnace temperature to 220°C, then hold for 8 hours. After the holding is completed, open the furnace door halfway. After 3 hours, take it out and air cool to obtain an annealed aluminum alloy sheet; Step 3: Semi-finishing machining: Using a CNC850 machining center for the annealed aluminum alloy sheet from Step 2, semi-finish machine the annealed aluminum alloy sheet, leaving a 1.5-mm allowance on the unilateral plane to obtain a semi-finished machined aluminum alloy sheet; Step 4: Second stress relief annealing: Put the semi-finished machined aluminum alloy sheet from Step 3 into a box-type tempering furnace with nitrogen protection. It takes 3 hours to heat the furnace temperature to 220°C, then hold for 8 hours. After the holding is completed, open the furnace door halfway. After 3 hours, take it out and air cool to obtain a second annealed aluminum alloy sheet; Step 5: Finishing machining: Using a CNC850 machining center, machine the second annealed aluminum alloy sheet from Step 4 to the finished product size, with the surface roughness of the upper and lower surfaces being Ra1.6 and the flatness being 0.05 mm; Step 6: Tooling production: Using a CNC850 machining center and a surface grinder, produce the middle frame pressing plate, with the flatness of the middle frame pressing plate meeting the requirement of ≤0.03 mm and the roughness being Ra0.8; Step 7: Part clamping: Use high-strength bolts and nuts of M30×380, and use the middle frame pressing plate to clamp the aluminum alloy sheet machined to the finished product size; Step 8: Thermal cycling stabilization treatment: Use a high and low temperature treatment box to treat the clamped aluminum alloy sheet in Step 7. First, keep it at -65°C for 4 h; then raise the temperature to 175°C and hold for 4 h, and cycle twice; after the holding at 175°C is completed, cool it in the furnace to room temperature; Step 9: Remove the tooling: After cooling to room temperature, remove the middle frame pressing plate, inspect the workpiece and make records; The middle frame pressing plate adopted includes a pressing plate (1). A circular hole (101) is opened in the middle of the pressing plate (1), and strip-shaped holes (102) are opened at the four corners and the middle edge positions of the pressing plate (1). Multiple pressing plates (1) are used to install the aluminum alloy sheet machined to the finished product size in the middle, and then use high-strength bolts of M30×380 to pass through the strip-shaped holes (102), and then the threaded part of the head of the high-strength bolt of M30×380 is threadedly connected to the nut; The strength of the high-strength bolts and nuts of M30×380 is 10.9 grade.

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