7050 aluminum alloy extruded rod for low altitude aircraft and method of making

Abstract

The application discloses a 7050 aluminum alloy extruded rod for low-altitude aircraft and a preparation method thereof, and belongs to the technical field of 7xxx high-strength aluminum alloy processing. The aluminum alloy is added with Zr and RE. The preparation steps include batching, smelting and refining, casting round ingot, four-stage gradient homogenization, gradient temperature preheating, extrusion, two-stage solid solution quenching in a vertical furnace, stretching and two-stage aging. Through the synergistic effect of the above-mentioned component formula, four-stage gradient homogenization, gradient temperature preheating isothermal extrusion and two-stage solid solution process, the T74511 state rod prepared has a coarse grain ring depth less than 1mm, a tensile strength greater than or equal to 520MPa, a yield strength greater than or equal to 443MPa, an elongation A50 after fracture greater than or equal to 12.5%, an exfoliation corrosion resistance reaching EA or EB level, and a processing material yield greater than or equal to 90%, and can meet the long service life and high safety requirements of load-bearing components of low-altitude aircraft.

Description

Technical Field

[0001] This invention belongs to the field of 7xxx series high-strength aluminum alloy processing technology, specifically relating to a 7050 aluminum alloy extruded bar for low-altitude aircraft and its preparation method. Background Technology

[0002] With the rapid development of the low-altitude economy, low-altitude aircraft are increasingly widely used in civilian logistics, emergency rescue, aerial sightseeing, and military reconnaissance. 7050 aluminum alloy belongs to the ultra-high strength 7xxx series Al-Zn-Mg-Cu alloys. With its relatively low density, high strength, excellent corrosion resistance, good toughness, and fatigue resistance, it has become a core material for load-bearing components of low-altitude aircraft, such as adapters, main beams, equipment supports, landing gear, and suspension connectors.

[0003] Current low-altitude aircraft components face stringent requirements for material uniformity, high strength, high toughness, high corrosion resistance, structural stability, and long service life. However, existing 7xxx aluminum alloy extruded bars in China generally suffer from technical challenges such as low strength, insufficient corrosion resistance, and significant coarse grain ring defects. The core bottleneck in current industrialized production of 7050 extruded bars lies in the tendency for coarse grain ring defects to form on the surface of the extruded bars. Coarse grain rings are large grain layers formed on the surface of the bar due to severe shear deformation and abnormal growth of recrystallized grains under the combined effects of extrusion deformation and solution heat treatment. Their formation stems from a combination of factors, including uneven temperature distribution inside and outside the ingot, excessive energy storage during surface deformation, uneven distribution of dispersed phases inhibiting recrystallization, and high-temperature solution treatment promoting grain growth.

[0004] For example, patent document CN102268621A discloses a method for producing aluminum alloy bars for aerospace applications. It points out that the extruded aluminum alloy bar semi-finished product is prone to grain growth during subsequent heat treatment, forming deep coarse grain rings or even coarse grains across the entire surface. This results in excessive coarse grain rings, decreased mechanical properties, and conventional production processes can hardly produce qualified aluminum alloy bars. This patent improves the degree of coarse grain rings by performing limited cold drawing on the bar semi-finished product after extrusion. However, this method mainly focuses on optimizing a single step in the cold drawing process and does not systematically address the coarse grain ring problem of 7050 aluminum alloy extruded bars from the perspective of alloy composition design and overall process coordination. According to this patent document, the acceptable coarse grain ring standard is ≤3mm, and the yield is only about 50%, indicating significant room for improvement.

[0005] Coarse grain rings bring three major fatal defects: First, mechanical properties delamination failure. The strength, toughness, and corrosion resistance of the surface coarse grain zone are far lower than those of the core of the bar. When the component is under stress, fatigue cracks are easily initiated along the coarse grain ring, which greatly shortens the service life of low-altitude aircraft components and makes it difficult to meet the long-life safety indicators of aircraft. Second, the yield is greatly reduced. Machining requires milling to remove the entire coarse grain ring layer, which greatly reduces the effective utilization rate of the bar (the yield of conventional processes in the industry is only 70%~80%), resulting in large raw material losses and significantly increased processing costs. Third, product stability is poor. The coarse grain ring is relatively deep (the depth of the coarse grain ring in industrial production often reaches the range of 3~9mm). The cross-sectional properties of the same batch and the same bar have large dispersion, which cannot meet the stringent requirements of low-altitude aircraft for uniformity of structure and consistency of performance.

[0006] Existing conventional improvement methods often only focus on optimizing homogenization or extrusion processes, without delving into the multi-factor coupling formation mechanism of coarse-grained rings. They lack a complete set of technologies for coordinated control of the entire process, including composition, homogenization heat treatment, isothermal extrusion, and solution aging treatment. They can only slightly reduce the depth of coarse-grained rings and cannot achieve stable mass production with no obvious coarse-grained rings and fine-grained uniform structure across the entire cross section.

[0007] In summary, the industry urgently needs to overcome the problem of coarse grain ring defects in 7050 extruded bars in order to meet the stringent requirements of long service life, high safety, and high consistency of materials for load-bearing components of low-altitude aircraft. Summary of the Invention

[0008] In view of the above, it is necessary to provide a 7050 aluminum alloy extruded bar for low-altitude aircraft and its preparation method. Through the design of RE and Zr composite microalloying composition, combined with the whole process of four-level gradient homogenization, gradient temperature preheating isothermal extrusion and two-stage solution quenching, the coarse grain ring defects on the surface of the extruded bar can be suppressed, thereby improving the mechanical properties, corrosion resistance and processing yield of the material.

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

[0010] A 7050 aluminum alloy extruded bar for low-altitude aircraft, wherein the aluminum alloy composition and mass percentage are as follows: Si≤0.08%, Fe≤0.10%, Cu: 2.10~2.35%, Mn: 0.05~0.08%, Mg: 1.95~2.20%, Cr: 0.02~0.04%, Zn: 6.00~6.40%, Ti: 0.02~0.05%, Zr: 0.11~0.14%, RE: 0.08~0.12%; wherein RE is a lanthanum-cerium mixed rare earth element; and Zn+Mg+Cu=10.2~10.7%, Zr+RE=0.19~0.24%; the remainder is Al and unavoidable impurities, and each unavoidable element is ≤0.05%, and the total amount of impurities is ≤0.10%; the surface coarse grain ring depth of the extruded bar is less than 1mm.

[0011] In this invention, the extruded bar is in T74511 condition, and the tensile strength of the extruded bar is ≥520MPa, the yield strength is ≥443MPa, the elongation after fracture (A50) is ≥12.5%, the anti-stripping corrosion grade reaches EA or EB, and the processing yield is ≥90%.

[0012] This invention also proposes a method for preparing 7050 aluminum alloy extruded bars for low-altitude aircraft as described above, characterized by comprising the following steps:

[0013] (1) Ingredients: Prepare raw materials according to the composition and mass percentage of the aluminum alloy.

[0014] (2) Smelting and refining: The raw materials are smelted and refined to obtain molten aluminum.

[0015] (3) Casting: The aluminum liquid is cast into a round ingot with a diameter of φ360~520mm.

[0016] (4) Homogenization heat treatment: The ingot is subjected to a four-stage gradient homogenization heat treatment with a maximum holding temperature of 472~476℃.

[0017] (5) Preheating: The ingot, mold and extrusion cylinder are preheated, and the preheating is ingot temperature gradient preheating.

[0018] (6) Extrusion: The ingot is extruded to obtain an extruded bar.

[0019] (7) Solution quenching: The extruded bar is subjected to two-stage solution treatment in a vertical quenching furnace, followed by rapid water quenching after heat preservation.

[0020] (8) Stretching: The extruded bar is stretched and straightened to relieve stress.

[0021] (9) Aging: The extruded bar is subjected to two-stage artificial aging.

[0022] In this invention, further, in step (1), the raw materials selected for batching are Al99.85 remelted aluminum ingots, Mg99.92 primary magnesium ingots, Zn99.995 metallic zinc ingots, Al-Cu master alloys, Al-Mn master alloys, aluminum-titanium-carbon master alloys, and Al-RE rare earth alloys; in step (2), the melting temperature is 720~760℃, the refining temperature is 710~730℃, and the argon flow rate is 4~8 Nm. 3 / h, chlorine flow rate is 0.1~0.35Nm 3 / h, refining time is 30~60min, electromagnetic stirring device is used to stir and degas and remove slag, after degassing the hydrogen content of the melt is ≤0.1mL / 100gAl, and after refining it is left to stand for 30~45min; in the step (3), the online grain refiner aluminum titanium carbon wire addition rate is 1.5~2.5kg / min, the casting material temperature is 690~710℃, the cooling water temperature is 26~32℃, and the casting speed is 18~28mm / min.

[0023] In this invention, further, in step (4), the four-stage gradient homogenization heat treatment is as follows: the first stage is heated from room temperature to 260℃±5℃ at a heating rate of 35℃ / h and held for 1.5~2.5h; the second stage is heated from 260℃ to 400℃±5℃ at a heating rate of 25℃ / h and held for 2~3h; the third stage is heated from 400℃ to 460℃±5℃ at a heating rate of 20℃ / h and held for 12~14h; the fourth stage is heated from 460℃ to 472~476℃ at a heating rate of 15℃ / h and held for 6~8h; after the holding period, the furnace is slowly cooled to below 220℃ before being removed from the furnace.

[0024] In this invention, further, in step (5), the ingot temperature gradient preheating is as follows: the ingot after removing surface impurities and segregation nodules from the car body is placed in an electromagnetic induction heating furnace for gradient preheating. The preheating temperature of the front end of the ingot is 400~430℃, and the temperature decreases every 300mm along the length of the ingot, with a gradient of 10~20℃; the mold preheating temperature is 400~430℃, and the extrusion cylinder preheating temperature is 400~430℃.

[0025] In this invention, further, in step (6), the extrusion ratio is 12~22, the extrusion speed is 0.4~2.0m / min, the die working zone is provided with a flow-blocking buffer and guiding structure, and the bar is water-cooled to below 80°C after extrusion.

[0026] In this invention, further, in step (7), the two-stage solution treatment is as follows: first, the bar is heated to 462°C and held for 90 minutes, then the temperature is raised to 472°C and held for 40 minutes, and after the holding is completed, it is immediately placed in circulating water for rapid water quenching. The water temperature of the bar is ≤40°C and the cooling rate is ≥180°C / min.

[0027] In this invention, further, in step (8), the bar is stretched within 24 hours after solution quenching, with a stretching rate of 1.5% to 3.0%.

[0028] In this invention, further, in step (9), the two-stage artificial aging is: the first stage aging temperature is 105℃±5℃, and the holding time is 7~8h; the second stage aging temperature is 175℃±5℃, and the holding time is 10~13h, to obtain T74511 state bar stock.

[0029] Compared with the prior art, the present invention has at least the following beneficial effects:

[0030] 1. This invention achieves a synergistic effect through the composite microalloying of RE and Zr. Experimental verification shows that Zr precipitates a Zr-containing dispersed phase during homogenization, exerting a Zener pinning effect on grain boundaries. Meanwhile, RE, with an atomic radius close to that of Al, can reduce the mismatch between the dispersed phase and the matrix, promoting uniform dispersion and precipitation of the dispersed phase, and delaying its coarsening behavior. This results in a Zr-RE composite dispersed phase with higher number density, smaller size, and more uniform distribution, achieving a stronger pinning effect on grain boundaries. Comparative example 5 shows that the depth of the coarse grain ring in the rod with the addition of equal amounts of Zr or RE alone reaches 2.8~3.5 mm, far inferior to the effect of the Zr+RE composite addition of this invention, fully demonstrating the significant synergistic effect between the two.

[0031] 2. This invention discovers a strict critical range for RE content. Experimental verification shows that when the RE content is below 0.08%, the precipitation driving force of the Zr-RE composite dispersed phase is insufficient, the number density of the dispersed phase is low, and it is difficult to form effective Zener pinning resistance. When the RE content is above 0.12%, excessive RE agglomerates at grain boundaries and forms coarse compounds, reducing the grain boundary bonding strength and providing favorable conditions for the migration of recrystallized grain boundaries, while deteriorating mechanical properties. Through the RE content gradient experiment of Comparative Example 4, it is verified that the setting of this critical range effectively balances the dispersed phase precipitation effect and grain boundary bonding strength, which is one of the core prerequisites for achieving coarse grain rings <1mm in this invention. The selection of RE content of 0.08~0.12% is based on the precise matching of the critical effect and synergistic mechanism verified by experiments, making the composition design of this invention different from the commonly used method of simply adding rare earth elements.

[0032] 3. This invention achieves multi-stage, step-by-step suppression of coarse-grained rings through the synergistic process of four-stage gradient homogenization, gradient-temperature preheating isothermal extrusion, and two-stage solution quenching. Experimental verification shows that four-stage gradient homogenization, by gradually increasing and decreasing the heating rate, ensures uniform precipitation of the Zr- and RE-containing composite dispersed phases across the entire ingot cross-section, eliminating the difference in internal and external gradients. Gradient-temperature preheating actively compensates for the surface friction temperature rise during extrusion, significantly reducing the deformation energy difference between the surface and the core, and eliminating the driving force for preferential recrystallization of the surface. Two-stage solution quenching balances the sufficient dissolution of the strengthening phase with the retention of the dispersed phase, avoiding the coarsening failure of the dispersed phase caused by a single high-temperature, long-term solution treatment. Comparison of Examples 1-3 shows that without any of the four-stage gradient homogenization, gradient-temperature preheating, or two-stage solution treatment processes, or without the use of RE and Zr composite microalloying composition design, the depth of the coarse-grained rings reaches 5-9 mm. This demonstrates that the synergistic effect of the above-mentioned composition and processes is a necessary condition for achieving the technical effect.

[0033] 4. Under the combined effect of the above mechanisms, compared with the existing industrialized 7050 aluminum alloy extruded bars with a coarse grain ring depth of 3~9mm and a processing yield of 70%~80%, the 7050 aluminum alloy extruded bars obtained by this invention achieve a significant performance improvement: the surface coarse grain ring depth is less than 1mm, the cross-sectional grain size is uniform, the tensile strength is ≥520MPa, the yield strength is ≥443MPa, the elongation after fracture A50 is ≥12.5%, the anti-stripping corrosion grade reaches EA or EB level, the processing yield is ≥90%, and the obtained bars are in T74511 condition, fully meeting the stringent requirements of long service life, high safety, and high consistency of materials for load-bearing components of low-altitude aircraft. Attached Figure Description

[0034] Figure 1 This is a low-magnification microstructure image of the 7050 aluminum alloy extruded bar obtained in Example 1 of the present invention.

[0035] Figure 2 This is a low-magnification microstructure diagram of the 7050 aluminum alloy extruded bar obtained in Example 2 of the present invention.

[0036] Figure 3 This is a low-magnification microstructure diagram of the 7050 aluminum alloy extruded bar obtained in Example 3 of the present invention.

[0037] Figure 4 This is a low-magnification microstructure diagram of the 7050 aluminum alloy extruded bar obtained in Comparative Example 1 of this invention.

[0038] Figure 5 This is a low-magnification microstructure diagram of the 7050 aluminum alloy extruded bar obtained in Comparative Example 2 of this invention.

[0039] Figure 6 This is a low-magnification microstructure diagram of the 7050 aluminum alloy extruded bar obtained in Comparative Example 3 of this invention.

[0040] Figure 7 This is a microstructure diagram of the surface layer of the 7050 aluminum alloy extruded bar obtained in Example 1 of the present invention.

[0041] Figure 8 This is a microstructure diagram of the surface layer of the 7050 aluminum alloy extruded bar prepared in Comparative Example 1 of this invention. Detailed Implementation

[0042] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0043] The main raw materials used in the embodiments and comparative examples of this invention are as follows.

[0044] Al99.85 aluminum ingots for remelting (Al content ≥ 99.85%), Mg99.92 primary magnesium ingots (Mg content ≥ 99.92%), Zn99.995 metallic zinc ingots (Zn content ≥ 99.995%), Al-50%Cu master alloy, Al-10%Mn master alloy, Al-5%Ti-0.2%C aluminum-titanium-carbon master alloy (hereinafter referred to as Al-Ti-C grain refiner), Al-10%RE rare earth alloy (RE is a lanthanum-cerium mixed rare earth, La:Ce mass ratio is about 1:1~1:3).

[0045] I. Implementation Examples

[0046] Example 1

[0047] The composition and mass percentage of the aluminum alloy in this embodiment are shown in Table 1, wherein Zn+Mg+Cu=10.55% and Zr+RE=0.22%. The preparation method includes the following steps.

[0048] (1) Batching: The raw materials are prepared according to the composition and mass percentage of the aluminum alloy shown in Table 1. The raw materials selected for batching are Al99.85 aluminum ingots for remelting, Mg99.92 primary magnesium ingots, Zn99.995 metallic zinc ingots, Al-50%Cu master alloy, Al-10%Mn master alloy, Al-Ti-C grain refiner and Al-10%RE rare earth alloy as raw materials for smelting the alloy.

[0049] (2) Smelting and refining: The raw materials are added to a smelting furnace for smelting at a temperature of 740°C. After the raw materials are completely melted, the molten aluminum is transferred to a refining furnace for refining at a temperature of 720°C. During the refining process, a mixture of argon and chlorine gas is introduced into the molten aluminum for degassing and refining, with an argon flow rate of 6 Nm³. 3 / h, chlorine flow rate 0.2Nm 3 The refining process lasts for 40 minutes, with an hourly refining time of 40 minutes. An electromagnetic stirrer is used during refining to promote degassing and slag removal. After refining, a sample is taken to test the hydrogen content of the melt; the hydrogen content is ≤0.1 mL / 100gAl. The melt is then allowed to stand for 40 minutes after refining.

[0050] (3) Casting: The degassed and slag-removed melt is cast into round ingots. During the casting process, Al-Ti-C grain refiner is added online at a rate of 2.0 kg / min. The casting temperature is 700℃, the cooling water temperature is 29℃, and the casting speed is 22 mm / min to prepare round ingots with a diameter of φ448 mm.

[0051] (4) Homogenization heat treatment: The round ingots are subjected to a four-stage gradient homogenization heat treatment. First stage: heating from room temperature to 260℃±5℃ at a heating rate of 35℃ / h and holding for 2h; Second stage: heating from 260℃ to 400℃±5℃ at a heating rate of 25℃ / h and holding for 2.5h; Third stage: heating from 400℃ to 460℃±5℃ at a heating rate of 20℃ / h and holding for 13h; Fourth stage: heating from 460℃ to 474℃ at a heating rate of 15℃ / h and holding for 7h. After the holding period, the ingots are slowly cooled to below 220℃ in the furnace before being removed from the furnace.

[0052] (5) Preheating: After removing surface impurities and segregation nodules from the ingot, place it into an electromagnetic induction heating furnace for gradient preheating. The preheating temperature at the front end of the ingot is 410℃, and the temperature decreases every 300mm along the length of the ingot, with a gradient of 15℃. The mold preheating temperature is 410℃, and the extrusion cylinder preheating temperature is 410℃.

[0053] (6) Extrusion: The ingot is extruded using a 75MN forward extrusion press. The extrusion ratio is 20, and the extrusion speed is 1.0 m / min. The die working zone is equipped with a flow-blocking and buffering structure to homogenize the metal flow and reduce surface shear deformation. The bar is immediately water-cooled to below 80°C after extrusion.

[0054] (7) Two-stage solution quenching: The extruded bars are subjected to two-stage solution treatment in a vertical quenching furnace. First stage: heating to 462℃ and holding for 90 min; Second stage: heating to 472℃ and holding for 40 min. After holding, the bars are immediately placed in circulating water for rapid water quenching. The water temperature of the bars is ≤40℃ and the cooling rate is ≥180℃ / min.

[0055] (8) Tensioning: The bar is stretched and straightened within 24 hours after solution quenching to relieve stress, with a stretching rate of 2.0%.

[0056] (9) Aging: The stretched bars were subjected to two-stage artificial aging. The first stage aging temperature was 105℃±5℃ and the holding time was 7.5h; the second stage aging temperature was 175℃±5℃ and the holding time was 11h, resulting in bars in the T74511 state.

[0057] Example 2

[0058] The composition and mass percentage of the aluminum alloy in this embodiment are shown in Table 1, wherein Zn+Mg+Cu=10.20% and Zr+RE=0.19%. The preparation method includes the following steps:

[0059] (1) Ingredients: Same as in Example 1.

[0060] (2) Melting and refining: Melting temperature 730℃, refining temperature 710℃, argon flow rate 4Nm 3 / h, chlorine flow rate 0.15Nm 3 / h, refining time 30min. After refining, let stand for 30min. The rest is the same as in Example 1.

[0061] (3) Casting: The online Al-Ti-C grain refiner was added at a rate of 1.6 kg / min, the casting temperature was 695°C, the cooling water temperature was 26°C, and the casting speed was 18 mm / min to prepare a round ingot with a diameter of φ448 mm. The rest was the same as in Example 1.

[0062] (4) Homogenization heat treatment: Same as in Example 1.

[0063] (5) Preheating: The preheating temperature of the front end of the ingot is 400℃, and the temperature decreases by 10℃ every 300mm along the length of the ingot. The preheating temperature of the mold is 400℃, and the preheating temperature of the extrusion cylinder is 400℃. The rest is the same as in Example 1.

[0064] (6) Extrusion: Extrusion ratio 20, extrusion speed 1.5m / min. The rest is the same as in Example 1.

[0065] (7) Two-stage solution quenching: Same as in Example 1.

[0066] (8) Tension: Tension rate 1.5%.

[0067] (9) Aging: The first aging temperature is 105℃±5℃ and the holding time is 7h; the second aging temperature is 175℃±5℃ and the holding time is 10h, to obtain T74511 state bar stock.

[0068] Example 3

[0069] The composition and mass percentage of the aluminum alloy in this embodiment are shown in Table 1, wherein Zn+Mg+Cu=10.69% and Zr+RE=0.24%. The preparation method includes the following steps:

[0070] (1) Ingredients: Same as in Example 1.

[0071] (2) Melting and refining: Melting temperature 750℃, refining temperature 730℃, argon flow rate 8Nm 3 / h, chlorine flow rate 0.3Nm 3 / h, refining time 60min. After refining, let stand for 45min. The rest is the same as in Example 1.

[0072] (3) Casting: The online Al-Ti-C grain refiner was added at a rate of 2.5 kg / min, the casting temperature was 710℃, the cooling water temperature was 32℃, and the casting speed was 28 mm / min to prepare a round ingot with a diameter of φ448 mm. The rest was the same as in Example 1.

[0073] (4) Homogenization heat treatment: Same as in Example 1.

[0074] (5) Preheating: The preheating temperature of the front end of the ingot is 430℃, and the temperature decreases by 20℃ every 300mm along the length of the ingot. The preheating temperature of the mold is 430℃, and the preheating temperature of the extrusion cylinder is 430℃. The rest is the same as in Example 1.

[0075] (6) Extrusion: Extrusion ratio 20, extrusion speed 0.6m / min. The rest is the same as in Example 1.

[0076] (7) Two-stage solution quenching: Same as in Example 1.

[0077] (8) Tension: Tension rate 2.5%.

[0078] (9) Aging: The first aging temperature is 105℃±5℃ and the holding time is 8h; the second aging temperature is 175℃±5℃ and the holding time is 12h, to obtain T74511 state bar stock.

[0079] II. Comparative Example

[0080] Comparative Example 1 (without rare earth RE elements, two-stage homogenization)

[0081] The composition and mass percentage of the aluminum alloy in this comparative example are shown in Table 1, which does not contain RE, and Zn+Mg+Cu=10.55%. The preparation method includes the following steps.

[0082] (1) Batching: Prepare raw materials according to the composition and mass percentage of aluminum alloy shown in Table 1. The raw materials selected for batching are Al99.85 aluminum ingot for remelting, Mg99.92 primary magnesium ingot, Zn99.995 metallic zinc ingot, Al-50%Cu master alloy, Al-10%Mn master alloy, and Al-Ti-C grain refiner as raw materials for smelting alloy (without adding Al-RE rare earth alloy).

[0083] (2) Smelting and refining: Same as in Example 1.

[0084] (3) Casting: Same as in Example 1.

[0085] (4) Homogenization heat treatment: The round ingot is subjected to a two-stage homogenization heat treatment. First stage: heated from room temperature to 400℃±5℃ at a heating rate of 25℃ / h and held for 2.5h; Second stage: heated from 400℃ to 470℃±5℃ at a heating rate of 20℃ / h and held for 13h. After the holding period, the ingot is slowly cooled to below 220℃ and then removed from the furnace.

[0086] (5) Preheating: Same as in Example 1.

[0087] (6) Extrusion: Extrusion ratio 20, extrusion speed 0.8m / min. The rest is the same as in Example 1.

[0088] (7) Two-stage solution quenching: Same as in Example 1.

[0089] (8) Stretching: Same as in Example 1.

[0090] (9) Aging: Same as in Example 1, to obtain bar stock in T74511 state.

[0091] Comparative Example 2 (Composition of this invention, two-stage homogenization, gradient-free preheating of ingot)

[0092] The composition and mass percentage of the aluminum alloy in this comparative example are shown in Table 1, where Zn+Mg+Cu=10.55% and Zr+RE=0.22%. The preparation method includes the following steps:

[0093] (1) Ingredients: Same as in Example 1.

[0094] (2) Smelting and refining: Same as in Example 1.

[0095] (3) Casting: Same as in Example 1.

[0096] (4) Homogenization heat treatment: Same as Comparative Example 1 (two-stage homogenization).

[0097] (5) Preheating: After removing surface impurities and segregation nodules from the ingot, place it into an electromagnetic induction heating furnace for gradient-free preheating. The overall preheating temperature of the ingot is 410℃. The preheating temperature of the mold is 410℃, and the preheating temperature of the extrusion cylinder is 410℃.

[0098] (6) Extrusion: Extrusion ratio 20, extrusion speed 1.4m / min. The rest is the same as in Example 1.

[0099] (7) Two-stage solution quenching: Same as in Example 1.

[0100] (8) Stretching: Same as in Example 1.

[0101] (9) Aging: Same as in Example 1, to obtain bar stock in T74511 state.

[0102] Comparative Example 3 (Components of this invention, single-stage solid solution)

[0103] The composition and mass percentage of the aluminum alloy in this comparative example are shown in Table 1, where Zn+Mg+Cu=10.55% and Zr+RE=0.22%. The preparation method includes the following steps:

[0104] (1) Ingredients: Same as in Example 1.

[0105] (2) Smelting and refining: Same as in Example 1.

[0106] (3) Casting: Same as in Example 1.

[0107] (4) Homogenization heat treatment: Same as Example 1 (four-level gradient homogenization).

[0108] (5) Preheating: Same as in Example 1.

[0109] (6) Extrusion: Extrusion ratio 20, extrusion speed 1.2m / min. The rest is the same as in Example 1.

[0110] (7) Single-stage solution quenching: The extruded bars are subjected to single-stage solution treatment in a vertical quenching furnace. The temperature is heated to 472℃ and held for 40 min. After the holding period, the bars are immediately placed in circulating water for rapid water quenching. The water temperature of the bars is ≤40℃ and the cooling rate is ≥180℃ / min.

[0111] (8) Stretching: Same as in Example 1.

[0112] (9) Aging: Same as in Example 1, to obtain bar stock in T74511 state.

[0113] Comparative Example 4 (Verification Test of Critical Range of RE Content)

[0114] To verify the critical range of RE content in this invention, three gradients of RE addition were set, namely 0%, 0.05% and 0.15%, while other components and all process parameters were kept the same as in Example 1.

[0115] Comparative Example 4-1 (RE=0%)

[0116] The composition and mass percentage of the aluminum alloy are the same as in Example 1, except that RE is not added (Zr remains at 0.12%). The preparation method is the same as in Example 1.

[0117] Comparative Example 4-2 (RE=0.05%)

[0118] The composition and mass percentage of the aluminum alloy are the same as in Example 1, except that the RE content is 0.05% and the Zr content is 0.12%. The preparation method is the same as in Example 1.

[0119] Comparative Example 4-3 (RE=0.15%)

[0120] The composition and mass percentage of the aluminum alloy are the same as in Example 1, except that the RE content is 0.15% and the Zr content is 0.12%. The preparation method is the same as in Example 1.

[0121] Comparative Example 5 (Verification Experiment of the Synergistic Effect of Zr and RE)

[0122] To verify the synergistic effect of Zr and RE in this invention, two comparative examples were set up: one with only Zr added (no RE) and the other with only RE added (no Zr). The total amount of Zr+RE added was fixed at approximately 0.22%, and all other components and process parameters were the same as in Example 1.

[0123] Comparative Example 5-1 (Zr only 0.22%, no RE)

[0124] The composition and mass percentage of the aluminum alloy are the same as in Example 1, except that the Zr content is 0.22% and RE is not added. The preparation method is the same as in Example 1.

[0125] Comparative Example 5-2 (RE only added 0.22%, no Zr)

[0126] The composition and mass percentage of the aluminum alloy are the same as in Example 1, except that the RE content is 0.22% and Zr is not added. The preparation method is the same as in Example 1.

[0127] 3. Alloy composition

[0128] Table 1 shows the composition and mass percentage (wt%) of the aluminum alloys of Examples 1-3 and Comparative Examples 1-5.

[0129]

[0130] Note: In Table 1, Zn+Mg+Cu = 10.2~10.7% (10.55% in Example 1, 10.20% in Example 2, and 10.69% in Example 3), and Zr+RE = 0.19~0.24% (0.22% in Example 1, 0.19% in Example 2, and 0.24% in Example 3).

[0131] IV. Performance Testing and Results

[0132] The aluminum alloy extruded bars obtained in Examples 1-3 and Comparative Examples 1-5 were subjected to performance testing according to the following methods.

[0133] Coarse grain ring depth: Tested according to GB / T 3246.2 "Methods for testing the microstructure of wrought aluminum and aluminum alloy products - Part 2: Low magnification microstructure test method". Take 3 points evenly on the edge of the cross-section of the bar to measure the coarse grain ring depth and take the maximum value.

[0134] Mechanical properties: Tested according to GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Test method at room temperature". Where Rm represents tensile strength (MPa), Rp0.2 represents yield strength (MPa) at a specified non-proportional elongation of 0.2%, and A50 represents elongation after fracture (%) at a gauge length of 50 mm.

[0135] Resistance to exfoliation corrosion: Tested according to GB / T 22639-2022 "Test Method for Exfoliation Corrosion of Aluminum Alloy Products".

[0136] Processing yield: calculated as (weight of qualified bars that have undergone machining to remove surface defects and meet the requirements of GB / T 3191 "Aluminum and Aluminum Alloy Extruded Bars" / weight of ingots cast) × 100%.

[0137] The test results are shown in Table 2.

[0138]

[0139] The results in Table 2 are analyzed as follows.

[0140] (a) Performance analysis of Examples 1-3.

[0141] As shown in Table 2, the coarse grain ring depth on the surface of the 7050 aluminum alloy extruded bars obtained in Examples 1-3 of this invention is only 0.1~0.9mm, all less than 1mm. Combined with... Figures 1-3 (Low-magnification microstructure images of Examples 1-3) It can be seen that the cross-sectional microstructure of the bars in Examples 1-3 is uniform, and no obvious coarse grain rings are seen on the surface. Figure 7The image shows the surface microstructure of the 7050 aluminum alloy extruded bar prepared in Example 1. It shows that the surface grains are small and uniform in size, and no abnormally large grains are observed.

[0142] The tensile strength Rm of Examples 1-3 is 520~535MPa, the yield strength Rp0.2 is 443~462MPa, the elongation after fracture A50 is 12.5%~13.9%, the anti-stripping corrosion grade reaches EA or EB, and the processing yield is 90%~93%.

[0143] As can be seen, this invention achieves a fine-grained, uniform microstructure across the entire cross-section of 7050 aluminum alloy extruded bars through the synergistic effects of RE+Zr composite microalloying, four-level gradient homogenization, gradient-temperature preheating isothermal extrusion, and two-stage solution quenching. The tensile strength difference between the edge and a quarter-diameter position is 0, indicating that the mechanical properties of all parts of the bar cross-section are highly consistent, eliminating the risk of mechanical property delamination failure caused by coarse grain rings.

[0144] (II) Comparative analysis of Comparative Examples 1-3.

[0145] Comparative Example 1 (without RE, two-stage homogenization): coarse grain ring depth reaches 6mm, tensile strength is only 490MPa, yield is only 77%, and the exfoliation corrosion resistance level is EC. Figure 4 (Low-magnification tissue diagram of Comparative Example 1) and Figure 8 (Comparative Example 1 surface microstructure diagram) It can be seen that obvious coarse grain rings are formed on the surface of the bar, and the grain size is large. This indicates that without the addition of RE element and using conventional two-stage homogenization, it is impossible to form a high-density Al3(Zr,RE) dispersed phase during the ingot homogenization process. The grain boundary pinning effect is insufficient during extrusion, the surface recrystallization driving force is large, and the coarse grain rings are severe.

[0146] Comparative Example 2 (with RE, but with two-stage homogenization + no gradient preheating): The coarse grain ring depth reaches 9mm, which is the most severe among all comparative examples. Figure 5 (The low-magnification microstructure of Comparative Example 2) shows that a coarse-grained ring with large grain size has formed on the surface of the bar. This indicates that although RE element was added, the bi-level homogenization could not fully dissolve the non-equilibrium phase at the grain boundaries and precipitate uniformly dispersed Al3(Zr,RE) particles, resulting in a gradient in the distribution of dispersed particles inside and outside the ingot. At the same time, the lack of gradient preheating caused the surface temperature to rise excessively due to friction during extrusion, resulting in excessive deformation energy storage. The driving force for preferential recrystallization on the surface is strong, which makes the coarse-grained ring even more severe.

[0147] Comparative Example 3 (with RE + four-stage homogenization + gradient preheating, but single-stage solid solution): coarse grain ring depth 5mm. Figure 6(The low-magnification microstructure of Comparative Example 3) shows that obvious coarse grain rings still exist on the surface of the bar. This indicates that although single-stage solid solution (472℃ / 40min) can partially dissolve the reinforcing phase, a single high-temperature short-time holding time cannot simultaneously ensure the full solid solution of the reinforcing phase and prevent the dispersion phase from coarsening. The dispersion phase coarsens at high temperature, loses its pinning effect on the grain boundaries, and the surface grains grow abnormally during the solid solution process.

[0148] The results of Comparative Examples 1-3 above fully demonstrate that the addition of RE, four-level gradient homogenization, gradient temperature preheating, and two-stage solid solution are an organic whole. Without any one of these steps, it is impossible to achieve the technical effect of coarse crystal rings <1mm.

[0149] (III) Analysis of Comparative Example 4 (Verification Test of Critical Range of RE Content).

[0150] Comparative Example 4-1 (RE=0%): coarse grain ring 5.5 mm, tensile strength 488 MPa, yield 76%. Compared with Example 1 (RE=0.10%, coarse grain ring 0.1 mm), the lack of RE resulted in a severely insufficient effect in inhibiting recrystallization.

[0151] Comparative Example 4-2 (RE=0.05%): coarse grain ring 2.5mm, tensile strength 505MPa, yield 82%. Although there is some improvement, the depth of the coarse grain ring is still greater than 1mm, which does not achieve the objective of this invention.

[0152] Comparative Example 4-3 (RE=0.15%): coarse grain ring 1.8 mm, tensile strength 510 MPa, yield 85%. Although the coarse grain ring is an improvement over Comparative Example 4-2, it still does not reach the target of <1 mm, and both tensile strength and yield are lower than in Example 1. Excessive RE (>0.12%) leads to the formation of coarse RE-containing compounds at the grain boundaries, which weakens the grain boundary bonding force and reduces mechanical properties and yield.

[0153] The above results indicate that there is a critical range for RE content: when it is too low (<0.08%), the precipitation density of the Al3(Zr,RE) dispersed phase is insufficient, resulting in poor suppression of recrystallization; when it is too high (>0.12%), excessive RE easily forms coarse compounds with Al, which deteriorates the mechanical properties and processing yield of the alloy. Only when the RE content is between 0.08% and 0.12% can it form the best composite refining effect with Zr, achieving the goal of coarse grain rings <1 mm.

[0154] (iv) Analysis of Comparative Example 5 (Verification Experiment of Synergistic Effect of Zr and RE)

[0155] Comparative Example 5-1 (Zr only 0.22%, no RE): coarse grain ring 2.8mm, tensile strength 508MPa, yield 80%. Although the addition of Zr alone can form some Al3Zr dispersed phase, the density of the dispersed phase is insufficient, the distribution is uneven, and the surface pinning effect is inadequate.

[0156] Comparative Example 5-2 (RE only 0.22%, no Zr): coarse grain ring 3.5mm, tensile strength 496MPa, yield 78%, and exfoliation corrosion resistance grade only EC. When RE is added alone, the solid solubility of RE in the aluminum matrix is ​​limited, which easily forms coarse RE-containing compounds, thus weakening the grain boundaries.

[0157] Example 1 (Zr 0.12% + RE 0.10%, total 0.22%): coarse grain ring 0.1mm, tensile strength 535MPa, yield 93%, anti-stripping corrosion grade EA.

[0158] The comparison shows that the combined addition of Zr and RE is far superior to the sum of the effects of adding equal amounts of Zr or RE alone. During the homogenization process, Zr and RE synergistically precipitate to form an Al3(Zr,RE) composite dispersed phase, which has the dual advantages of high-density precipitation and fine and uniform size. Zr provides the nucleation core, while RE reduces the mismatch between the dispersed phase and the matrix and promotes the uniform dispersion distribution of the dispersed phase. The synergistic effect of the two results in a higher number density, finer size, and more uniform distribution of the dispersed phase, thereby achieving the strongest pinning effect on grain boundaries.

[0159] In summary, this invention, by controlling the proportions of various alloying elements (especially the precise addition of RE at 0.08~0.12% and the synergistic ratio of Zr+RE=0.19~0.24%), and by rationally setting the process parameters for the entire process, including melting and casting, four-stage gradient homogenization, gradient preheating isothermal extrusion, two-stage solution quenching, stretching, and two-stage aging, produces a 7050 aluminum alloy extruded bar for low-altitude aircraft.

[0160] The 7050 aluminum alloy extruded bar produced by this invention has a coarse grain ring depth of less than 1 mm on the surface, uniform grain size in the cross section, tensile strength ≥520MPa, yield strength ≥443MPa, elongation after fracture A50 ≥12.5%, and anti-stripping corrosion grade ≥EB. The processing yield is increased to over 90%, significantly reducing milling processing losses, lowering production costs, and improving production efficiency. It fully meets the stringent requirements for long service life, high safety, and high consistency of materials for load-bearing components of low-altitude aircraft.

[0161] The above embodiments only illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention.

Claims

1. A 7050 aluminum alloy extruded bar for low-altitude aircraft, characterized in that, The composition and mass percentage of the aluminum alloy are as follows: Si ≤ 0.08%, Fe ≤ 0.10%, Cu: 2.10~2.35%, Mn: 0.05~0.08%, Mg: 1.95~2.20%, Cr: 0.02~0.04%, Zn: 6.00~6.40%, Ti: 0.02~0.05%, Zr: 0.11~0.14%, RE: 0.08~0.12%; wherein the RE is a lanthanum-cerium mixed rare earth element. And Zn+Mg+Cu=10.2~10.7%, Zr+RE=0.19~0.24%; The remainder consists of Al and unavoidable impurities, with each unavoidable element comprising ≤0.05% and the total impurity content ≤0.10%; the surface coarse grain ring depth of the extruded bar is less than 1 mm.

2. The 7050 aluminum alloy extruded bar for low-altitude aircraft according to claim 1, characterized in that, The extruded bar is in T74511 condition, with a tensile strength ≥520MPa, yield strength ≥443MPa, elongation after fracture A50 ≥12.5%, anti-stripping corrosion grade reaching EA or EB, and a processing yield ≥90%.

3. A method for preparing 7050 aluminum alloy extruded bars for low-altitude aircraft as described in claim 1, characterized in that, Includes the following steps: (1) Batching: Prepare raw materials according to the composition and mass percentage of the aluminum alloy; (2) Smelting and refining: The raw materials are smelted and refined to obtain molten aluminum; (3) Casting: The aluminum liquid is cast into a round ingot with a diameter of φ360~520mm; (4) Homogenization heat treatment: The ingot is subjected to a four-stage gradient homogenization heat treatment, with the highest holding temperature being 472~476℃; (5) Preheating: The ingot, mold and extrusion cylinder are preheated, and the preheating is ingot temperature gradient preheating; (6) Extrusion: The ingot is extruded to obtain an extruded bar; (7) Solution quenching: The extruded bar is subjected to two-stage solution treatment in a vertical quenching furnace, followed by rapid water quenching after heat preservation. (8) Stretching: The extruded bar is stretched and straightened to relieve stress; (9) Aging: The extruded bar is subjected to two-stage artificial aging.

4. The method for preparing 7050 aluminum alloy extruded bars for low-altitude aircraft according to claim 3, characterized in that: In step (1), the raw materials selected for batching are Al99.85 aluminum ingots for remelting, Mg99.92 primary magnesium ingots, Zn99.995 metallic zinc ingots, Al-Cu master alloy, Al-Mn master alloy, aluminum-titanium-carbon master alloy and Al-RE rare earth alloy. In step (2), the melting temperature is 720~760℃, the refining temperature is 710~730℃, and the argon flow rate is 4~8 Nm. 3 / h, chlorine flow rate is 0.1~0.35Nm 3 / h, refining time is 30~60min, using electromagnetic stirring device to stir and degas and remove slag, after degassing the hydrogen content of the melt is ≤0.1mL / 100gAl, and after refining, let stand for 30~45min; In step (3), the online grain refiner aluminum-titanium-carbon wire addition rate is 1.5~2.5 kg / min, the casting material temperature is 690~710℃, the cooling water temperature is 26~32℃, and the casting speed is 18~28 mm / min.

5. The method for preparing 7050 aluminum alloy extruded bars for low-altitude aircraft according to claim 3, characterized in that, In step (4), the four-stage gradient homogenization heat treatment is as follows: the first stage is heating from room temperature to 260℃±5℃ at a heating rate of 35℃ / h and holding for 1.5~2.5h; the second stage is heating from 260℃ to 400℃±5℃ at a heating rate of 25℃ / h and holding for 2~3h; the third stage is heating from 400℃ to 460℃±5℃ at a heating rate of 20℃ / h and holding for 12~14h; the fourth stage is heating from 460℃ to 472~476℃ at a heating rate of 15℃ / h and holding for 6~8h; after the holding is completed, the furnace is slowly cooled to below 220℃ before being removed from the furnace.

6. The method for preparing 7050 aluminum alloy extruded bars for low-altitude aircraft according to claim 3, characterized in that, In step (5), the ingot temperature gradient preheating is as follows: after removing surface impurities and segregation nodules from the ingot after the car body is removed, the ingot is placed in an electromagnetic induction heating furnace for gradient preheating. The preheating temperature of the front end of the ingot is 400~430℃, and the temperature decreases every 300mm along the length of the ingot, with a gradient of 10~20℃; the mold preheating temperature is 400~430℃, and the extrusion cylinder preheating temperature is 400~430℃.

7. The method for preparing 7050 aluminum alloy extruded bars for low-altitude aircraft according to claim 3, characterized in that, In step (6), the extrusion ratio is 12~22, the extrusion speed is 0.4~2.0m / min, the die working zone is equipped with a flow-blocking buffer and guiding structure, and the bar is water-cooled to below 80℃ after extrusion.

8. The method for preparing 7050 aluminum alloy extruded bars for low-altitude aircraft according to claim 3, characterized in that, In step (7), the two-stage solution treatment is as follows: first, the bar is heated to 462°C and held for 90 minutes, then the temperature is raised to 472°C and held for 40 minutes. After the holding period, the bar is immediately placed in circulating water for rapid water quenching. The water temperature of the bar is ≤40°C and the cooling rate is ≥180°C / min.

9. The method for preparing 7050 aluminum alloy extruded bars for low-altitude aircraft according to claim 3, characterized in that, In step (8), the bar is stretched within 24 hours after solution quenching, with a stretching rate of 1.5% to 3.0%.

10. The method for preparing 7050 aluminum alloy extruded bars for low-altitude aircraft according to claim 3, characterized in that, In step (9), the two-stage artificial aging is as follows: the first stage aging temperature is 105℃±5℃, and the holding time is 7~8h; the second stage aging temperature is 175℃±5℃, and the holding time is 10~13h, to obtain T74511 state bar stock.

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