Cryogenic treatment process of 7xxx series high-strength aluminum alloy pipe

Abstract

The invention belongs to the technical field of aluminum alloy material processing, and discloses a cryogenic treatment process of a 7xxx series high-strength aluminum alloy pipe, the process sequentially comprises three core steps of solution treatment, cryogenic treatment and aging treatment, and the 7xxx series high-strength aluminum alloy pipe is obtained by accurately controlling key parameters of each step. The technical problems that in a traditional T6 heat treatment process, a 7xxx series aluminum alloy pipe is uneven in precipitated phase distribution, grain refinement is insufficient, the strength and plasticity are difficult to consider, and internal stress is large are solved. After being treated by the process, the tensile strength of the pipe is improved to 705MPa, the yield strength is improved to 678MPa, the ductility is improved to 11.8%, the size of a precipitated phase in a microstructure is smaller, distribution is more uniform, the internal stress is remarkably reduced, the size stability is greatly improved, industrial batch production can be realized, and the requirements of the fields of aerospace, rail transit and the like on the high-performance aluminum alloy pipe are met.

Description

Technical Field

[0001] This invention belongs to the field of aluminum alloy material processing technology, and relates to a treatment of 7xxx series high-strength aluminum alloy pipes, specifically a cryogenic treatment process for 7xxx series high-strength aluminum alloy pipes. Background Technology

[0002] 7xxx series aluminum alloys belong to the Al-Zn-Mg-Cu series of ultra-high strength aluminum alloys. Due to their excellent strength characteristics, they are widely used in high-end equipment manufacturing fields such as aerospace and rail transportation, and are often processed into tubular components to bear critical loads.

[0003] Currently, the industry commonly uses the T6 heat treatment process (solution treatment + aging) to strengthen 7xxx series aluminum alloy tubes. Although this process can improve the material strength to a certain extent, it has significant technical defects: On the one hand, the distribution of precipitated phases inside the material is uneven after heat treatment, and the grain refinement effect is not good, which leads to the mutual restriction between the strength and plasticity of the tube, making it difficult to meet the requirements for the synergistic performance of strength and plasticity under complex load scenarios; on the other hand, the internal stress of the tube is large after traditional process treatment, which is prone to deformation during subsequent machining, resulting in poor dimensional stability and affecting the product's processing accuracy and reliability.

[0004] Existing research on cryogenic treatment technology in the field of 7xxx series aluminum alloys is mostly focused on plates, with less specific research on tubes. Furthermore, there are problems such as broad process parameters and no unified standard for the number of cycles, resulting in a lack of precise process solutions that can be directly applied to industrial applications.

[0005] Due to the fundamental differences between tubes and plates in terms of structural morphology and stress distribution, the cryogenic treatment parameters for plates cannot be directly applied to tube processing, thus limiting the performance improvement of 7xxx series aluminum alloy tubes. Therefore, developing a cryogenic treatment process with clearly defined parameters, high stability, and the ability to significantly improve the strength-ductility synergy and dimensional stability of 7xxx series aluminum alloy tubes has become a pressing technical challenge in this field. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a cryogenic treatment process for 7xxx series high-strength aluminum alloy tubing.

[0007] The technical problem solved by this invention is achieved through the following technical solution:

[0008] A cryogenic treatment process for 7xxx series high-strength aluminum alloy tubing, characterized by the following steps:

[0009] Step 1, Solution treatment: Heat the 7xxx series aluminum alloy tube to 470-490℃, hold for 2-4 hours, then cool to room temperature with water, and then isothermal in a constant temperature environment of 20-30℃ for 1-2 hours.

[0010] Step 2, cryogenic treatment: Place the isothermal pipe into a cryogenic device with a rotating device and rotate it at a constant speed of 5-10 rpm. First, cool it down to -80℃ at a rate of 5-8℃ / min and hold it for 0.5-1 hour. Then, cool it down to -115-125℃ at a rate of 3-5℃ / min and hold it for 1-3 hours. Finally, heat it up to room temperature at a rate of 5-15℃ / min. Repeat this cryogenic heating process 3 times.

[0011] Step 3, Aging treatment: The first stage is to keep the temperature at 120-125℃ for 4-6 hours, and the second stage is to keep the temperature at 135-140℃ for 4-6 hours, followed by air cooling to room temperature;

[0012] Step 4: Low-temperature stress relief treatment: Heat the aged pipe to 80-100℃, keep it at that temperature for 2-3 hours, and then cool it to room temperature in the furnace.

[0013] Furthermore, the solution treatment in step 1 is performed at a temperature of 480°C for 3-4 hours.

[0014] Furthermore, the rotation speed in step 2 is 8 rpm, the first stage cooling rate is 8℃ / min, the holding time is -80℃ for 1 hour, the second stage cooling rate is 4℃ / min, the cryogenic temperature is -120℃, the holding time is 2 hours, and the heating rate is 10℃ / min.

[0015] Furthermore, in step 3, the first stage aging temperature is 122℃ and the holding time is 5 hours, while the second stage aging temperature is 138℃ and the holding time is 5 hours.

[0016] Moreover, the water-cooled quenching transfer time in step 1 does not exceed 30 seconds, the water temperature before quenching is 10-30℃, and the water temperature after quenching does not exceed 40℃.

[0017] Furthermore, in step 4, the low-temperature stress relief temperature is 90℃, the holding time is 2.5 hours, and the furnace cooling rate is ≤5℃ / min.

[0018] The advantages and positive effects of this invention are:

[0019] 1. The cryogenic treatment process of this 7xxx series high-strength aluminum alloy pipe significantly improves the synergistic performance of strength and plasticity: After treatment, the tensile strength of the 7xxx series aluminum alloy pipe reaches 705MPa, which is 6MPa higher than that of the traditional T6 process; the yield strength reaches 678MPa, which is 19MPa higher than that of the traditional T6 process; and the elongation reaches 11.8%, which is 0.6% higher than that of the traditional T6 process. This achieves simultaneous optimization of strength and plasticity, meeting the usage requirements under complex load scenarios.

[0020] 2. The cryogenic treatment process for this 7xxx series high-strength aluminum alloy tube significantly optimizes the microstructure: By precisely controlling the cryogenic treatment parameters, the precipitated phase inside the tube is mainly η'-MgZn2, and the average equivalent radius is reduced from 1.2nm in the traditional T6 process to 1.0nm, while the number of precipitated phases per unit volume is increased from 4.53×10 24 pcs / m 3 Increased to 7.55×10 24 pcs / m 3 The precipitated phases are smaller in size and more uniformly distributed, with no obvious elemental segregation, which lays the microstructural foundation for improving the material's strength and plasticity.

[0021] 3. The cryogenic treatment process for this 7xxx series high-strength aluminum alloy tubing reduces internal stress and improves dimensional stability: During cryogenic treatment, the atomic arrangement inside the material tends to stabilize, and internal stress is effectively released. After subsequent machining, the roundness of tubing treated with this process can still be controlled within ±0.2mm, while the roundness of tubing treated with the traditional T6 process is mostly between ±0.5-1mm. The deformation is significantly reduced, significantly improving product machining accuracy and reliability.

[0022] 4. The cryogenic treatment process for 7xxx series high-strength aluminum alloy tubes has high process stability and can be industrialized: This invention clarifies the key parameter ranges and optimal values ​​for each treatment step, has good process repeatability, is applicable to the processing of 7xxx series aluminum alloy tubes of different specifications, and can realize industrialized mass production, solving the problem of unclear parameters and difficulty in promotion and application of existing cryogenic treatment processes. Attached Figure Description

[0023] Figure 1 This is a comparison chart of the strength and ductility properties of 7xxx series aluminum alloy T6 and T6-DCT (-180 ℃) of the present invention, where the horizontal axis represents the percentage reduction and the vertical axis represents the yield strength.

[0024] Figure 2 These are TEM images of the microstructure of 7xxx series aluminum alloy T6 and T6-DCT (-180 ℃) of the present invention, wherein Figures (a), (c), and (e) are schematic diagrams of the heat treatment of T6; and Figures (b), (d), and (f) are schematic diagrams of the heat treatment of T6-DCT (-180 ℃).

[0025] Figure 3 This is a schematic diagram of the grain size change of 7xxx series aluminum alloys after T6 and T6-DCT (-120℃) treatment according to the present invention. Figures (a) and (c) are schematic diagrams of T6 heat treatment; Figures (b) and (d) are schematic diagrams of T6-DCT (-120℃) heat treatment. In Figures (c) and (d), the horizontal axis represents the grain size and the vertical axis represents the area fraction.

[0026] Figure 4 This is a schematic diagram showing the changes in the size of precipitated phases in 7xxx series aluminum alloys after T6 and T6-DCT (-120℃) treatment according to the present invention. The horizontal axis represents the atomic size inside the alloy, and the vertical axis represents the average size of the precipitated phase within that atomic range. This data shows that the average volume of the precipitated phase decreases and the number per unit volume increases, leading to the following conclusions: (1) The nucleation rate of the precipitated phase increases. (2) The driving force for the nucleation and growth of the precipitated phase increases. Both of these can demonstrate the superiority of cryogenic processing at the microscopic level. Detailed Implementation

[0027] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings:

[0028] The cryogenic treatment process for this 7xxx series high-strength aluminum alloy tubing is innovative in that it includes three steps: solution treatment, cryogenic treatment, and aging treatment. The specific details of each step are as follows:

[0029] Step 1, Solution treatment: Heat the 7xxx series aluminum alloy tube to 470-490℃ (preferably 480℃), hold for 2-4 hours (preferably 3 hours, extended to 4 hours if the wall thickness is >10mm), and then water cool to room temperature;

[0030] Among them, the quenching transfer time shall not exceed 30s, the water temperature before quenching shall be controlled at 10-30℃, and the water temperature after quenching shall not exceed 40℃, so as to ensure the stability of the solution treatment effect.

[0031] Immediately after water cooling, place the pipe in a constant temperature environment of 20-30℃ and hold it at that temperature for 1-2 hours to eliminate the temperature gradient between the surface and the core generated during the water cooling process and reduce the initial residual stress.

[0032] Step 2, cryogenic treatment: The isothermalized pipe is placed in a liquid nitrogen cryogenic furnace with a rotating device. The pipe is rotated at a constant speed of 5-10 rpm to ensure uniform circumferential temperature.

[0033] The first stage of cooling: the temperature is reduced to -80℃ at a rate of 5-8℃ / min, and then held for 0.5-1 hour to alleviate the stress change caused by the low temperature shock;

[0034] The second stage of cooling: the temperature is reduced to -115-125℃ (preferably -120℃) at a rate of 3-5℃ / min, and held for 1-3 hours (preferably 2 hours) to promote uniform nucleation of the precipitated phase;

[0035] Temperature recovery: Heat to room temperature at a rate of 8-12℃ / min to complete one cryogenic cycle;

[0036] Repeat the above cryogenic heating cycle three times to promote the uniform nucleation and refinement of the precipitated phase through multiple cycles;

[0037] Step 3, Aging Treatment: First Stage Aging: Heat the cryogenically cooled pipe to 120-125℃ and hold for 4-6 hours to promote uniform nucleation of the η'-MgZn2 phase;

[0038] The second stage of aging involves heating to 135-140℃ and holding for 4-6 hours to allow the precipitated phase to grow moderately and distribute stably. This two-stage aging process can optimize the growth rhythm of the precipitated phase.

[0039] Then, it is naturally air-cooled to room temperature at a rate of ≤10℃ / min to avoid secondary stress and ensure the stable existence of the refined precipitates, thereby further improving the strength and plasticity of the material.

[0040] Step 4, Low-temperature stress relief treatment: Heat the aged pipe to 80-100℃ and keep it at that temperature for 2-3 hours to slowly release the trace stress remaining during the aging process;

[0041] The furnace is cooled to room temperature at a rate of ≤5℃ / min to ensure sufficient stress release without affecting the stability of the precipitated phase.

[0042] The isothermal pretreatment after solution treatment in this invention solves the surface-core temperature difference stress caused by water cooling, providing a prerequisite for cryogenic uniformity; the staged cooling avoids low-temperature shock, and the rotating device overcomes the technical bottleneck of uneven circumferential temperature in large-diameter pipes; the two-stage aging process optimizes the nucleation and growth rhythm of precipitates, solving the problem of uneven precipitate size in traditional single-stage aging; the low-temperature stress relief treatment is designed for the characteristics of aluminum alloys, with a gentle treatment of 80-100℃, which releases stress without damaging the precipitates.

[0043] Example

[0044] Extrusion: The hollow aluminum alloy ingot of the 7xxx series is placed in a high-temperature electric furnace for heating and heat preservation. The heating temperature of the ingot is controlled at 435±5℃. The heating coil of the extrusion cylinder is adjusted to 445±5℃, and the mold temperature is controlled at 445±5℃. After the ingot is heated for 1 hour, it is placed in the extrusion cylinder of a 3600T reverse extrusion press for hot extrusion. The extrusion speed is 0.5mm / s, the extrusion ratio is 15, and a pipe with a specification of Ф150mm×6mm is obtained.

[0045] Pre-straightening: Roller straightening equipment is used to straighten the extruded pipe to ensure that the roundness of the pipe is controlled within ±0.2mm.

[0046] Solution treatment: Place the pre-straightened pipe into a quenching furnace, raise the furnace temperature to 470℃ and stabilize it for 30 minutes, load the pipe and start timing, hold for 90 minutes and then perform water quenching; the water temperature before quenching is 10-30℃, the water temperature after quenching does not exceed 40℃, and the quenching transfer time does not exceed 30 seconds.

[0047] Cryogenic treatment: The quenched pipe is placed in a liquid nitrogen cryogenic furnace and rotated at 8 rpm. First, it is cooled to -80°C at a rate of 8°C / min and held for 1 hour. Then, it is cooled to -120°C at a rate of 4°C / min and held for 2 hours. Subsequently, it is heated to room temperature at a rate of 10°C / min. This cryogenic heating cycle is repeated 3 times.

[0048] Aging treatment: Place the cryogenically treated pipes into an aging furnace with a temperature control accuracy of ±3℃, raise the temperature to 122℃, hold for 5 hours, raise the temperature to 138℃, hold for 5 hours, and then remove and allow to air cool to room temperature.

[0049] Low-temperature stress relief: Hold at 90℃ for 2.5 hours, then cool with the furnace.

[0050] Performance testing and machining verification:

[0051] Performance testing: Tensile tests were conducted using a universal testing machine in accordance with GB / T 228.1-2010 standard. The results showed that the pipe had a tensile strength of 705 MPa, a yield strength of 678 MPa, and an elongation of 11.8%. Microstructure was observed using a transmission electron microscope with an accelerating voltage of 200 kV. The precipitated phase was mainly η'-MgZn2, which was small in size and uniformly distributed, with no obvious elemental agglomeration.

[0052] Machining verification: The finished pipe was machined using a double-blade cutting method. After machining, the pipe was removed and its roundness was still controlled within ±0.2mm, with no obvious deformation.

[0053] Comparative example (traditional T6 process)

[0054] The extrusion, pre-straightening, and solution treatment steps are the same as in the example.

[0055] Aging treatment: The solution-treated pipe is placed directly into an aging furnace, heated to 130°C, held for 10 hours, and then air-cooled to room temperature.

[0056] Performance testing and machining verification:

[0057] Performance testing: tensile strength 699 MPa, yield strength 659 MPa, elongation 11.2%; although the precipitated phase in the microstructure is mainly η'-MgZn2, the uniformity of distribution is not as good as that of the example.

[0058] Machining verification: After machining, the pipes rebounded significantly. The roundness of some pipes was controlled within ±0.5mm, and the roundness of most pipes reached ±1mm. The deformation was significantly greater than that in the example.

[0059] As can be seen from the comparison of the embodiments and comparative examples, the cryogenic treatment process of the present invention has significant advantages in improving the strength-plasticity synergy of 7xxx series aluminum alloy tubes, optimizing the microstructure and improving dimensional stability. The process parameters are clear and highly repeatable, making it suitable for industrial application.

[0060] This invention is based on the structural characteristics of aluminum alloy tubing (hollow structure, circumferential stress sensitivity) and the evolution of its microstructure (temperature dependence of precipitate nucleation and growth). Through multi-step synergistic design, it achieves a "1+1>2" effect: for example, isothermal pretreatment lays the foundation for cryogenic uniformity, staged cryogenic treatment combined with rotation addresses temperature inhomogeneity, two-stage aging optimizes precipitates, and low-temperature stress relief locks in performance. Each step is interconnected and indispensable. This invention not only solves many shortcomings of existing technologies but also achieves simultaneous breakthroughs in strength, plasticity, dimensional stability, and applicability. In particular, its stable processing capability for large-diameter tubing fills a gap in the industry, expanding the application boundaries of 7xxx series aluminum alloy tubing in high-end equipment, and possesses significant industrial value and technological leadership.

[0061] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A cryogenic treatment process for 7xxx series high-strength aluminum alloy tubing, characterized in that: Includes the following steps: Step 1, Solution treatment: Heat the 7xxx series aluminum alloy tube to 470-490℃, keep it at that temperature for 2-4 hours, then cool it to room temperature with water, and then isothermally heat it in a constant temperature environment of 20-30℃ for 1-2 hours. Step 2, cryogenic treatment: Place the isothermal pipe into a cryogenic device with a rotating device and rotate it at a constant speed of 5-10 rpm. First, cool it down to -80℃ at a rate of 5-8℃ / min and hold it for 0.5-1 hour. Then, cool it down to -115-125℃ at a rate of 3-5℃ / min and hold it for 1-3 hours. Finally, heat it up to room temperature at a rate of 5-15℃ / min. Repeat this cryogenic heating process 3 times. Step 3, Aging treatment: The first stage is to keep the temperature at 120-125℃ for 4-6 hours, and the second stage is to keep the temperature at 135-140℃ for 4-6 hours, followed by air cooling to room temperature; Step 4: Low-temperature stress relief treatment: Heat the aged pipe to 80-100℃, keep it at that temperature for 2-3 hours, and then cool it to room temperature in the furnace.

2. The cryogenic treatment process for 7xxx series high-strength aluminum alloy tubing according to claim 1, characterized in that: The solution treatment in step 1 is performed at a temperature of 480°C for 3-4 hours.

3. The cryogenic treatment process for 7xxx series high-strength aluminum alloy tubing according to claim 1, characterized in that: The rotation speed in step 2 is 8 rpm, the first stage cooling rate is 8℃ / min, the holding temperature is -80℃ for 1 hour, the second stage cooling rate is 4℃ / min, the cryogenic temperature is -120℃, the holding temperature is 2 hours, and the heating rate is 10℃ / min.

4. The cryogenic treatment process for 7xxx series high-strength aluminum alloy tubing according to claim 1, characterized in that: In step 3, the first stage aging temperature is 122℃ and the holding time is 5 hours. The second stage aging temperature is 138℃ and the holding time is 5 hours.

5. The cryogenic treatment process for 7xxx series high-strength aluminum alloy tubing according to claim 1, characterized in that: The water-cooled quenching transfer time in step 1 shall not exceed 30s, the water temperature before quenching shall be 10-30℃, and the water temperature after quenching shall not exceed 40℃.

6. The cryogenic treatment process for 7xxx series high-strength aluminum alloy tubing according to claim 1, characterized in that: In step 4, the low-temperature stress relief temperature is 90℃, the holding time is 2.5 hours, and the furnace cooling rate is ≤5℃ / min.

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