High-performance titanium alloy surface tension tank and low-temperature superplastic forming method

Abstract

The application discloses a high-performance titanium alloy surface tension tank and a low-temperature superplastic forming method, and is based on the new demand of a carrying aircraft for a high-performance titanium alloy surface tension tank shell, and solves the defects of the prior art. Through alloying element regulation and special cogging / rolling combined deformation process optimization, high-performance, anisotropy-free superplastic forming titanium alloy plate material preparation is realized; on the basis, through low-temperature (less than or equal to 890 DEG C) superplastic forming process evaluation and positive and negative expansion surface control, the overall hot forming time is shortened; through high-temperature charging and high-temperature discharging combination, continuous high-temperature forming is realized, and meter-level diameter large titanium alloy thin-walled component low-temperature and high-efficiency superplastic near-net forming is realized; finally, the TC4 ball shell part body tensile strength is greater than or equal to 1010 MPa, which is higher than that of the currently commonly used titanium alloy superplastic forming and forging forming ball shell parts. Thus, the material-manufacturing process integration production of the high-performance TC4 titanium alloy surface tension tank shell is realized.

Description

Technical Field

[0001] This invention relates to a low-temperature superplastic forming method for high-performance titanium alloy surface tension tanks, belonging to the field of advanced manufacturing technology, and is mainly used for the manufacturing of structural components for the power system of launch vehicles. Background Technology

[0002] Titanium alloy surface tension tanks are key structural components of launch vehicle propulsion systems. With the increasing demands on the operating environment and performance of these tanks, new and higher requirements are being placed on titanium alloy materials and manufacturing technologies. Traditional TC4 forging is no longer sufficient to meet the requirements for lightweight products, necessitating the research and development of titanium alloy materials with higher mechanical properties and corresponding forming technologies. Furthermore, to increase the operating pressure of the tanks and meet structural strength requirements, the only way to improve structural strength is through wall thickness compensation, which undoubtedly increases the structural weight and fails to meet the lightweighting requirements. Superplastic forming technology is a precision plastic forming process that utilizes the superplasticity of materials to obtain the desired parts. This technology can significantly improve the formability of materials and has advantages such as high forming accuracy, near-zero springback, and no residual stress. It is also largely unrestricted by part size, making it the preferred near-net-shape forming process for titanium alloy tanks. However, because superplastic forming is a significant high-temperature hot deformation process, the prolonged thermal process inevitably leads to grain growth, thereby reducing the material's mechanical properties, and subsequent heat treatment cannot strengthen it. Currently, the tensile strength of large-size TC4 titanium alloy tank shells after superplastic forming is basically maintained at 900~950MPa, which can no longer meet the usage requirements. Summary of the Invention

[0003] The technical problem solved by this application is to overcome the shortcomings of the prior art and provide a high-performance titanium alloy surface tension tank and a low-temperature superplastic forming method to achieve the preparation of high-performance, anisotropic superplastic forming titanium alloy plates. Based on the titanium alloy plates, the tensile strength of the TC4 spherical shell part obtained by low-temperature (≤920℃) superplastic forming and positive and negative expansion is ≥1010MPa.

[0004] By controlling alloying elements and optimizing a special billet / rolling combined deformation process, high-performance, anisotropic superplastic forming of titanium alloy sheets is achieved. Based on this, through evaluation of low-temperature (≤920℃) superplastic forming processes and control of positive and negative bulging surfaces, the overall hot forming time is shortened, enabling low-temperature, high-rate near-net-shape superplastic forming of meter-sized large titanium alloy thin-walled components. Ultimately, a stable tensile strength of ≥1010MPa is achieved for TC4 spherical shell parts, exceeding that of commonly used titanium alloy superplastic forming spherical shell parts. This enables integrated material and manufacturing processes for high-performance TC4 titanium alloy surface tension tank shells. The project content involved in this invention patent can meet the service requirements of high-performance titanium alloy tanks for launch vehicles.

[0005] Due to the new performance requirements of next-generation surface tension tanks, traditionally manufactured spherical shell parts can no longer meet the demands. The titanium alloy sheet developed using this patent possesses excellent mechanical and formability properties, enabling near-net-shape superplastic forming under low-temperature, high-rate conditions, thus avoiding coarse grains and performance degradation after forming. Simultaneously, this technology can stably achieve a tensile strength of ≥1010 MPa for TC4 spherical shell parts, higher than commonly used superplastic formed titanium alloy spherical shell parts. Specifically, the concept of high-performance superplastic fine-grained titanium alloy sheet refers to a thickness of 8-16 mm, and large-size superplastic formed titanium alloy surface tension tank shells generally refer to tank shells with a diameter of 600-1200 mm.

[0006] The technical solution provided in this application is as follows:

[0007] A low-temperature superplastic forming method for a high-performance titanium alloy surface tension tank, such as Figure 3 As shown, it includes:

[0008] S1: The TC4 titanium alloy ingot is subjected to forging and slab manufacturing to obtain a forged slab.

[0009] S2: Perform multi-pass rolling on the forged slab to obtain the rolled slab;

[0010] S3: Heat-treat the rolled slab to obtain the original slab;

[0011] S4: Perform superplasticity testing on the original slab to obtain its average m value at 860~890℃; determine the original slab's 0.0001s value within this temperature range. -1 ~0.01s -1 If the average m value at the strain rate is greater than 0.3, then proceed to the next step of superplastic forming.

[0012] S5: Determine the structure of the superplastic forming mold. The superplastic forming mold includes an upper mold and a lower mold. The inner surface of the lower mold is consistent with the final forming surface of the tank shell blank. The upper mold has an annular groove that is coaxial with the axis of the final forming surface of the tank shell blank.

[0013] S6: Place the original slab between the upper and lower molds; perform reverse expansion forming to make the original slab fit against the surface of the upper mold to obtain the pre-formed surface;

[0014] S7: Perform positive expansion forming to make the pre-formed surface fit with the surface of the lower mold to obtain the TC4 titanium alloy tank shell blank.

[0015] In step S1, the TC4 titanium alloy ingot is subjected to forging and slab manufacturing to obtain a forged slab, including: the TC4 titanium alloy ingot is forged three times to obtain a forged slab; the temperature of the first forging is higher than the temperature of the second forging, and the temperature of the third forging is higher than the temperature of the second forging.

[0016] The three forging processes include: First forging: casting the TC4 titanium alloy ingot at the β phase transformation point T β The TC4 titanium alloy ingot is heated in the furnace at 50-150℃ for 4-6 hours. After being removed from the furnace, it is first upset to a height-to-diameter ratio ≤2, then held in the furnace for 1.5±0.5 hours, and then drawn to a height-to-diameter ratio of 2-2.5. After holding in the furnace for 1.5±0.5 hours, it is upset to 50-70% of the initial height of the TC4 titanium alloy ingot, thus obtaining a primary forging billet. During the forging process, the temperature is below T β At +30℃, place the TC4 titanium alloy ingot in an annealing furnace at 600~700℃ and hold for 6~8 hours, then air cool to room temperature.

[0017] Second forging: The primary forging billet, cooled to room temperature, is reheated to T. β Hold at 0~50℃ for 5~7 hours, then repeatedly upset and draw at this temperature 3~5 times, upsetting to 50~80% of the height of the first forging billet; if the billet surface temperature is below 900℃ during the upset and drawing process, reheat in the furnace and continue upset and drawing; if the billet surface temperature is ≥900℃ during the upset and drawing process, place it in an annealing furnace at 600~700℃ and hold for 6~8 hours; finally, air cool to room temperature to obtain the second forging billet;

[0018] Third forging: First, the secondary forging billet is placed in T... β Hold at 30~80℃ for 4~6 hours, then undergo one upsetting and drawing process after removal from the furnace. The deformation should be controlled at 45~60%. If the temperature does not fall below 900℃, continue forging to elongate the billet and reduce its thickness. If the temperature falls below 900℃, hold in the furnace for 1~1.5 hours at a holding temperature T. β ±30°C, continue forging at this temperature for 2-4 more times, and then the temperature should not be lower than 900°C. Cool to room temperature to obtain a forged slab.

[0019] In step S2, the forged slab is subjected to multiple rolling processes to obtain a rolled slab, including:

[0020] The first firing test, on the forged slab at T β The reversing rolling is carried out at 0~50℃, with the rolling directions of the previous and subsequent passes perpendicular. The deformation per pass is 10~15%, the rolling speed is 2~5m / s, and 3~5 passes are rolled. The total rolling deformation is 40~65%. If the temperature is lower than 900℃ during the rolling process, the temperature is returned to the furnace for reheating.

[0021] The second rolling process, with a rolling temperature of T. β + (0~50℃), the rolling direction is perpendicular to the final rolling direction of the first heat, the deformation per pass is 6~12%, the rolling speed is 2~5m / s, 3~5 passes are rolled, the total rolling deformation is 40~65%, and the rolling is finished by air cooling to room temperature;

[0022] The third rolling process, with a rolling temperature of T. β - (0~50℃), the deformation rate per pass is 10~20%, the rolling speed is 3~4m / s, and the final rolling temperature is 850±20℃ to obtain the rolled slab.

[0023] In step S3, the heat treatment of the rolled slab includes: holding the rolled slab at 600~700℃ for 240±10min and then air cooling.

[0024] The upper mold is located at the edge of the forming surface of the upper mold, and the edge of the annular groove and the surface of the upper mold facing the lower mold are rounded.

[0025] In step S6, the original slab is placed between the upper and lower molds; reverse expansion forming is performed, including: the heating rate of the superplastic forming mold is 55~75℃ / h; when the temperature of the superplastic forming mold reaches 650~750℃, the original slab is placed between the upper and lower molds again, and the temperature continues to rise; when the temperature of the superplastic forming mold reaches 860~890℃, the upper and lower superplastic forming molds are closed, maintaining a tonnage of 10~20t; reverse expansion air intake is performed to make the original slab fit with the upper mold, the air intake rate is 0.05~0.10MPa / min, the air pressure is raised to 2.0~3.0MPa, the pressure is held for 15~20min, and then the pressure is released.

[0026] The positive expansion forming includes: after the reverse expansion forming is completed and the air pressure is balanced, and the temperature of the superplastic forming mold is maintained at 860~890℃, positive expansion air intake is performed to make the original slab fit with the lower mold, the air intake speed is ≤0.05MPa / min, and the air pressure is increased to 1.5~2.0MPa; after the hemispherical blank formed by the original slab is completely fitted with the lower mold, the pressure is maintained for 15~20min, and then the pressure is released to balance the air pressure, while the upper mold is raised.

[0027] Based on a mass content of 100% for each component, the TC4 titanium alloy ingot comprises: Al: 6.55~6.75%, V: 4.20~4.50%, Fe: 0.10~0.30%, O: 0.10~0.20%, C≤0.08%; N≤0.05%, H≤0.015%, unavoidable impurities≤0.4%; the balance is Ti.

[0028] A high-performance titanium alloy surface tension tank is prepared according to any of the above-described methods for low-temperature superplastic forming of a high-performance titanium alloy surface tension tank.

[0029] The core of this invention is to achieve precise control over the microstructure and texture type of the sheet metal through optimization of the raw material preparation process, thereby obtaining mechanical properties superior to those of commonly used traditional TC4 titanium alloy sheets. Through superplastic forming mold design and optimization of the low-temperature superplastic forming process, high-performance sheets can be rapidly formed at low temperatures. The resulting large-size titanium alloy tank shell blank has a room temperature mechanical property Rm≥1010MPa, which is superior to the performance of current superplastic forming and forging products (900~950MPa), and has a large performance margin. It can meet the current and even future research and development needs of high-performance titanium alloy surface tension tanks in my country, and has high commercial application value.

[0030] In summary, this application includes at least the following beneficial technical effects:

[0031] (1) Based on the original TC4 alloy composition, the alloying elements are controlled to increase the content of alloying elements such as Al, V, and Fe. A titanium alloy ingot with high alloying element content and narrow composition range is produced by three vacuum melting processes. Special billet forging technology is further adopted to improve the ingot structure. Large deformation in the β single-phase region (Tβ+50~150℃) can refine the grains and effectively control the lamellar α phase; large deformation in the α / β phase region can achieve the breakage of Widmanstätten strip α phase and avoid strong texture. On this basis, through multi-fire reversing rolling in the α / β phase region, the microstructure and texture type are precisely controlled, thereby achieving high strength and anisotropy of TC4 titanium alloy plate. Taking the currently formed δ14×1200×1200mm as an example, the longitudinal and transverse tensile strength of the plate is ≥1150MPa, which is significantly improved compared with the commonly used δ14mm plate (GJB2505A-2018) plate.

[0032] (2) Full-scale process performance tests were conducted on the high-performance sheet metal. The microstructure and properties of each region of the sheet metal were stable, and the longitudinal and transverse properties were basically consistent, indicating good formability. The TC4 titanium alloy thick plate of this invention was tested at 830~890℃ / 0.001s. -1 It exhibits significant high-temperature superplasticity. Using the strain rate mutation method, the average m value of TC4 material at 860~890℃ was measured to be 0.48, which is better than the m value of traditional TC4 alloy under this condition.

[0033] (3) Based on the variable wall thickness and structural characteristics of the titanium alloy tank shell, it belongs to the category of parts with variable wall thickness and large depth ratio. Positive and negative bulging forming is required to make the pre-formed slab thinner at the edges and thicker in the middle, thereby compensating for the uneven wall thickness during the final bulging. This is fundamentally different from traditional unidirectional bulging, where the wall thickness varies greatly, being thinner in the middle and thicker at the edges, making part manufacturing impossible. Furthermore, compared to the currently commonly used superplastic bulging process, the forming temperature below 890℃ in this invention, compared to the commonly used 890~910℃ forming, enables low-temperature superplastic forming. High-temperature loading and unloading allow for continuous forming, shortening the billet's heating time. Overall, this reduces the impact of the thermal process on the material's mechanical properties, thereby achieving low-temperature, high-efficiency superplastic forming of meter-sized large titanium alloy shells.

[0034] (4) The core of this invention is to achieve fine control of the microstructure and texture type of the plate through the optimization of the raw material preparation process, thereby obtaining mechanical properties superior to those of commonly used traditional TC4 titanium alloy plates. Through the design of superplastic forming molds and the optimization of low-temperature superplastic forming process, high-performance plates can be rapidly formed at low temperatures. The final obtained large-size titanium alloy tank shell blank has room temperature mechanical properties Rm≥1010MPa, which is superior to the performance of current superplastic forming and forging products (900~950MPa), and has a large performance margin. It can be applied to the current and even future research and development needs of high-performance titanium alloy surface tension tanks in my country, and has high commercial application value. Attached Figure Description

[0035] Figure 1 The microstructure of the TC4 titanium alloy superplastic fine-grained plate prepared in the embodiments of the present invention;

[0036] Table 1 shows a comparison of the longitudinal and transverse room temperature mechanical properties of the TC4 titanium alloy superplastic fine-grained plates prepared in the embodiments of the present invention.

[0037] Figure 2 The superplastic forward and reverse forming mold surface designed for embodiments of the present invention;

[0038] Table 2 shows the mechanical properties of the TC4 titanium alloy surface tension tank shell prepared in the embodiments of the present invention;

[0039] Figure 3 This is a flowchart illustrating a specific implementation of an embodiment of the present invention. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments disclosed in the present invention will be described in further detail below with reference to the accompanying drawings.

[0041] A method for low-temperature superplastic forming of a high-performance titanium alloy surface tension tank, the method comprising the following steps:

[0042] The first step is alloy composition design and smelting. Based on TC4 titanium alloy, the composition is selected, which features high alloying element content, a narrow composition range, and low element range, including: Al: 6.55~6.75%, V: 4.20~4.50%, Fe: 0.10~0.30%, O: 0.10~0.20%, with the balance being Ti. The C / N / H content should meet the requirements of GJB2505A-2018 standard. High-alloying-element TC4 titanium alloy ingots are prepared using a three-stage vacuum arc melting process, and the smelting process parameters are optimized.

[0043] The second step involves specialized forging and slab manufacturing techniques. To ensure uniform and fine-grained titanium alloy forgings and effectively control the thickness of the α-phase lamellars, a "high-low-high" forging process is employed, consisting of three forging stages. The TC4 titanium alloy ingot is forged at the β-phase transformation point T... β The billet is held at 50-150℃ for 4-6 hours, heated in the furnace, and immediately transferred to a high-speed forging mill after exiting the furnace. It is first upset to a height-to-diameter ratio ≤2, then held in the furnace for 1.5±0.5 hours, then drawn to a height-to-diameter ratio of 2-2.5, held in the furnace for 1.5±0.5 hours, and then upset to 50-70% of its height. The final forging temperature is T. β The billet is heated to approximately +30℃, then placed in an annealing furnace at 600~700℃ for 6~8 hours, and finally air-cooled in a sand pit to room temperature, with the surface polished. The cooled billet is then reheated to T... β + (0~50℃) heat for 5~7h to allow for sufficient recrystallization, then repeatedly upset and draw at this temperature 3~5 times. Upsetting and drawing adopts the ordinary upset-drawing process combined with diagonal drawing process. Upset to 50~80% of the height, draw to a height-to-diameter ratio of 1.5~2.3. If the billet surface temperature is lower than 900℃ during the upsetting and drawing process, reheat in the furnace for 2±0.5h and continue upsetting and drawing. The final forging temperature is ≥900℃, then placed in an annealing furnace at 600~700℃ for 6~8h, and finally placed in a sand pit for air cooling to room temperature, and the surface is polished; the third forging is to first place the billet in T β Hold at 30~80℃ for 4~6 hours, then undergo one upsetting and drawing process after removal from the furnace. The deformation should be controlled at 45~60%. If the temperature does not drop below 900℃, continue forging to elongate the billet and reduce its thickness. If the temperature drops below 900℃, hold in the furnace for 1~1.5 hours at a holding temperature T. β ±30, continue forging at this temperature for 2 to 4 more times, with a final forging temperature of 900℃. After shaping, allow the slab thickness to be 150 to 200 mm, cool to room temperature, and polish the surface.

[0044] Specialized forging and slab manufacturing employ a combination of multiple deformation methods in the β single-phase region, involving large-scale cumulative deformation and controlled deformation rate. This approach efficiently and uniformly refines grains and effectively controls the thickness of the lamellar α-phase. In the α / β phase region, a combination of multiple deformation methods—large-scale cumulative deformation plus medium-temperature deformation—is used, with controlled deformation rate, to efficiently break up the strip-shaped α-phase, preventing overheating in the core and the formation of strong textures. This yields uniform and fine-grained titanium alloy forgings.

[0045] The third step involves multi-pass rolling and heat treatment to control the microstructure of the plate. A multi-pass cooling rolling process is employed, combined with numerical simulation technology, to analyze the strain field during rolling and determine the optimal deformation per pass and per heat treatment. The slab obtained in the second step is cut into 800-1000mm square billets. The first heat treatment is carried out at T... β The rolling process is carried out at + (0~50℃) with reversing rolling (the rolling directions of the previous and subsequent passes are perpendicular). If the temperature drops below 900℃ during rolling, the rolling process is repeated in the furnace for reheating. The deformation per pass is 10~15%, the rolling speed is 2~5m / s, and 3~5 passes are rolled. The total rolling deformation is 40~65%. The rolling direction of the second pass is perpendicular to the final rolling direction of the first pass, and the temperature is T. β + (0~50℃), the deformation per pass is 6~12%, the rolling speed is 2~5m / s, 3~5 passes are rolled, the total rolling deformation is 40~65%, and the slab is air-cooled to room temperature after rolling. The slab is cut into square billets of 800~1000mm; the third rolling is carried out at a rolling temperature T. β - (0~50℃), with a pass deformation rate of 10~20%, a rolling speed of 3~4m / s, and a final rolling temperature of 850±20℃, a high-performance ultrafine-grained titanium alloy sheet of (14~15)×1200×1200mm is finally rolled. The sheet is then held at 600~700℃ for 240±10min and air-cooled. Finally, it is straightened and precision-corrected to obtain the original slab. This process involves multi-pass cooling rolling, resulting in a sheet with fine grains and uniform properties. The tensile strength values ​​of the sheet in the transverse and longitudinal directions are almost identical, both exceeding 1150MPa.

[0046] The fourth step is to explore the low-temperature superplastic forming process. Full-size, zoned high-temperature tensile testing was conducted on the finished sheet metal. For TC4 titanium alloy thick plates, the testing was performed at 830~890℃ for 0.001s. -1 It exhibits significant high-temperature superplasticity, with fine grains after superplastic deformation; however, the grain size deteriorates significantly with increasing temperature, so high-temperature forming should be avoided. Using the strain rate mutation method, the average m value of TC4 material at 860~890℃ is ≥0.48 (greater than the superplasticity m value criterion), which is superior to the m value of traditional TC4 alloys under these conditions. This also reflects the uniformity of the forming performance of high-performance finished sheet metal.

[0047] Step 5: Superplastic forming mold design. The superplastic forming mold includes an upper mold and a lower mold. The inner surface of the lower mold is consistent with the final forming surface of the tank. The upper mold has an annular groove coaxial with the axis of the final forming surface of the tank. The edge of the annular groove is rounded with the surface of the upper mold facing the lower mold. The annular groove is located at the edge of the forming surface of the upper mold. Based on the variable wall thickness and structural characteristics of the titanium alloy tank shell, it belongs to a part with variable wall thickness and a large aspect ratio. Positive and negative bulging forming is required, so that the edge is thin and the middle is thick after the blank is bulged, thereby compensating for the unevenness of the wall thickness during the final bulging. Ni7N is selected as the mold material.

[0048] Step 6: Superplastic forming process settings. Heat the superplastic forming equipment to a target temperature of 860~890℃, with a heating rate of 55~75℃ / h. When the mold temperature reaches 650~750℃, open the furnace door, remove the shuttle car, and place the original slab 1 onto the superplastic mold using a hoisting method. Move the shuttle car in and close the furnace door. Do not apply pressure during heating, ensuring the distance between the upper and lower molds is 100~150mm. Continue heating.

[0049] Step 7, Superplastic Reverse Expansion Molding. When the average mold temperature is 870~890℃, and the upper mold temperature is ≥870℃, close the upper and lower superplastic molding molds, maintaining a tonnage of 10~20t. Execute the air intake program at an air intake rate of 0.05~0.10MPa / min, raising the air pressure to 2.0~3.0MPa, holding the pressure for 15~20min, then releasing the pressure to obtain the pre-formed profile 2. (Example...) Figure 2 As shown.

[0050] Step 8, Superplastic positive expansion forming. After the reverse expansion forming is completed and the air pressure is balanced, the positive expansion air intake program is started, with an air intake rate ≤0.05MPa / min, raising the air pressure to 1.5~2.0MPa. After the hemispherical blank is completely attached to the lower mold, the pressure is maintained for 15~20min, and then the pressure is released to balance the air pressure. At the same time, the upper mold is raised to obtain the TC4 titanium alloy tank shell blank 3.

[0051] Step 9: The TC4 titanium alloy tank shell blank is removed from the furnace. When the average temperature of the mold drops to 750°C, the furnace door is opened, the shuttle car is removed, the TC4 titanium alloy tank shell blank is removed from the mold, and placed in a sand pit to cool.

[0052] Step 10: Continue production according to step 6 to achieve high-efficiency manufacturing of TC4 titanium alloy tank shell blanks.

[0053] Example:

[0054] The first step was alloy composition design and smelting. TC4 titanium alloy was selected for composition design. Under the premise of meeting GJB2505A-2018, the composition was: Al% 6.6%, V% 4.3%, Fe% 0.25%, O% 0.18%, C 0.02%, N 0.004%, H 0.001%, with unavoidable impurities ≤0.25%; the balance was Ti. High-alloy ingots were prepared using a three-stage vacuum arc remelting process.

[0055] The second step involves special forging and slab manufacturing. To ensure uniform and fine titanium alloy forgings, the ingot is held at 1100℃ for 4.5 hours, then rapidly forged to a length-to-diameter ratio of 1.9. It is then held in the furnace for 1.5 hours, drawn to a length of 2.2, and held again for 1.5 hours to upset the ingot to 60% of its original height. It is then held at 700℃ for 6 hours, air-cooled in a sand pit, and surface cracks are removed. The temperature is then raised to 1000℃, held for 5.5 hours, and upset and drawn four times to reach 70% of its height. It is then held at 700℃ for 6 hours, air-cooled in a sand pit, and surface cracks are removed. Finally, the billet is held in a 1050℃ high-temperature furnace for 4 hours, with a deformation of 52%, and forged four times within the range of 960℃ to 1050℃, resulting in a final slab thickness of 180mm. Preheat the tooling before forging to ensure the tooling temperature is 200℃~240℃.

[0056] The third step involves multi-pass rolling and heat treatment to control the microstructure of the plate. The slab obtained in the second step is cut into 800-1000mm square billets. The first pass is a reversing roll at 1000℃ (the rolling directions of the previous and subsequent passes are perpendicular). If the temperature drops below 900℃ during rolling, the slab is returned to the furnace for reheating. The deformation per pass is 10-15%, the rolling speed is 3.1-3.5 m / s, and four passes are performed, resulting in a total deformation of 55%. The second pass is perpendicular to the final rolling direction of the first pass, with a deformation per pass of 6-12%, a rolling speed of 3.1-3.5 m / s, and five passes. The total rolling deformation was 60%. After rolling, the plates were air-cooled to room temperature, and the slabs were cut into square billets of 800-1000 mm. The third rolling pass was the same as the second. The final rolling pass was at 960℃, with a pass deformation rate of 15-18%, a rolling speed of 2.5-3.1 m / s, and a final rolling temperature of 850℃, resulting in a titanium plate with a thickness of 14±0.2 mm. This plate was then cut and shaped into a 1200×1200 mm high-performance ultrafine-grained titanium alloy plate. The plate was held at 600-700℃ for 240±10 min and then air-cooled. Finally, it was straightened and precision-fitted. The tensile strength of the plate was almost identical in both the transverse and longitudinal directions, both exceeding 1150 MPa.

[0057] The fourth step involves exploring the superplastic forming process of high-strength, fine-grained titanium alloy plates. Thick TC4 titanium alloy plates were formed at 830~920℃ for 0.01 seconds.-1 ~0.0001s -1 The longitudinal and transverse directions of 9 regions were subjected to high-temperature stretching at 830~890℃ / 0.01s. -1 Under these conditions, it exhibits significant superplasticity. Using the strain rate mutation method, the average m value of TC4 material at 860~890℃ was measured to be ≥0.48, which is better than the m value of traditional TC4 alloy under these conditions.

[0058] Step 5: Superplastic forming mold design. The hemispherical part has a thin-walled, variable-thickness characteristic, with a depth / diameter ratio ≥ 0.5 and a diameter of 1000 mm. A forward and reverse bulging forming process is used for mold design, and Ni7N is selected as the mold material.

[0059] Step 6: Superplastic forming process settings. Heat the superplastic forming equipment to a target temperature of 880℃, with a heating rate of 775℃ / h. Once the mold temperature reaches 700℃, open the furnace door, remove the shuttle car, and place the slab onto the superplastic mold using a hoisting method. Move the shuttle car in and close the furnace door. Do not apply pressure during heating, ensuring a 120mm distance between the upper and lower molds. Continue heating.

[0060] Step 7, Superplastic Reverse Expansion Molding. When the average mold temperature is 870~880℃, and the upper mold temperature is ≥870℃, close the upper and lower superplastic molding molds, maintaining a tonnage of 16t. Execute the air intake program at an air intake rate of 0.08MPa / min, raising the air pressure to 2.5MPa, holding the pressure for 20 minutes, and then releasing the pressure. Figure 2 As shown.

[0061] Step 8, Superplastic positive expansion forming. After the reverse expansion forming is completed and the air pressure is balanced, the positive expansion air intake program is started, with an air intake rate of 0.05MPa / min, raising the air pressure to 2.0MPa. After the hemispherical blank is completely attached to the lower mold, the pressure is maintained for 20 minutes, and then the pressure is released to balance the air pressure, while the upper platform is raised.

[0062] Step 9: Remove the hemispherical blank from the furnace. When the average temperature of the mold drops to 750℃, open the furnace door, remove the shuttle car, remove the hemispherical blank from the mold, and place it in a sand pit to cool.

[0063] Step 10: Continue production according to step 6 to achieve high-efficiency manufacturing of titanium alloy hemispherical blanks.

[0064] The microstructure of the TC4 titanium alloy superplastic fine-grained plate prepared in this embodiment is as follows: Figure 1As shown, the tensile strength and elongation of the sheet metal are almost identical in the RD and TD directions, with tensile strength values ​​ranging from 1160 to 1170 MPa, exhibiting good isotropic mechanical properties. Currently, GJB2505A-2018 specifies tensile strength of >10~100 mm thick sheets at 895~1100 MPa and yield strength ≥825 MPa; δ14 mm thick sheets meeting GJB2505A-2018 standards generally have tensile strengths ranging from 1030 to 1070 MPa.

[0065] Table 1 Performance of Finished Boards

[0066]

[0067] Table 2 Comparison of properties of TC4 titanium alloy tank shell blanks

[0068]

[0069] Comparative example:

[0070] The only difference from the example is that the mold temperature is 920°C during both reverse and forward bulging, while all other parameters remain the same. The resulting hemispherical properties are shown in the table below:

[0071] Table 2 Comparison of properties of TC4 titanium alloy tank shell blanks

[0072]

[0073] In summary, by optimizing the raw material preparation process, the performance of the sheet metal was improved, achieving mechanical properties superior to those of commonly used TC4 titanium alloy sheets. Optimization of the low-temperature superplastic forming process enabled the rapid low-temperature forming of high-performance sheet metal. The resulting large-size titanium alloy tank shell blank exhibits room-temperature mechanical properties Rm≥1010MPa, exceeding the performance of current superplastic forming and forging products (900~950MPa), and possesses a large performance margin. This makes it suitable for the current and future development needs of high-performance titanium alloy surface tension tanks in my country, demonstrating high commercial application value.

[0074] The contents not described in detail in this application specification are common knowledge to those skilled in the art.

[0075] The present application has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present application. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and implementation methods of the present application without departing from the spirit and scope of the present application, and all such modifications and improvements fall within the scope of the present application. The scope of protection of the present application is determined by the appended claims.

Claims

1. A low-temperature superplastic forming method for a high-performance titanium alloy surface tension tank, characterized in that, include: S1: The TC4 titanium alloy ingot is subjected to forging and slab manufacturing to obtain a forged slab. S2: Perform multi-pass rolling on the forged slab to obtain the rolled slab; S3: Heat treat the rolled slab to obtain the original slab; S4: Perform superplasticity testing on the original slab to obtain its average m value at 860~890℃; determine the original slab's 0.0001s value within this temperature range. -1 ~0.01s -1 If the average m value at the strain rate is greater than 0.3, then proceed to the next step of superplastic forming. S5: Determine the structure of the superplastic forming mold. The superplastic forming mold includes an upper mold and a lower mold. The inner surface of the lower mold is consistent with the final forming surface of the tank shell blank. The upper mold has an annular groove that is coaxial with the axis of the final forming surface of the tank shell blank. S6: Place the original slab between the upper and lower molds and perform reverse expansion forming to make the original slab fit with the surface of the upper mold and obtain the pre-formed surface; S7: Perform positive expansion forming to make the pre-formed surface fit with the surface of the lower mold to obtain the TC4 titanium alloy tank shell blank; In step S1, the TC4 titanium alloy ingot undergoes three forging processes, which include: First forging: The TC4 titanium alloy ingot is cast at the β phase transformation point T β The TC4 titanium alloy ingot is heated in the furnace at 50-150℃ for 4-6 hours. After being removed from the furnace, it is first upset to a height-to-diameter ratio ≤2, then held in the furnace for 1.5±0.5 hours, and then drawn to a height-to-diameter ratio of 2-2.

5. After holding in the furnace for 1.5±0.5 hours, it is upset to 50-70% of the initial height of the TC4 titanium alloy ingot, thus obtaining a primary forging billet. During the forging process, the temperature is below T β At +30℃, place the TC4 titanium alloy ingot in an annealing furnace at 600~700℃ and hold for 6~8 hours, then air cool to room temperature. Second forging: The primary forging billet, cooled to room temperature, is reheated to T. β Hold at 0~50℃ for 5~7 hours, then repeatedly upset and draw at this temperature 3~5 times, upsetting to 50~80% of the height of the first forging billet; if the billet surface temperature is below 900℃ during the upset and drawing process, reheat in the furnace and continue upset and drawing; if the billet surface temperature is ≥900℃ during the upset and drawing process, place it in an annealing furnace at 600~700℃ and hold for 6~8 hours; finally, air cool to room temperature to obtain the second forging billet; Third forging: First, the secondary forging billet is placed in T... β Hold at 30~80℃ for 4~6 hours, then undergo one upsetting and drawing process after removal from the furnace. The deformation should be controlled at 45~60%. If the temperature does not fall below 900℃, continue forging to elongate the billet and reduce its thickness. If the temperature falls below 900℃, hold in the furnace for 1~1.5 hours at a holding temperature T. β At ±30℃, continue forging for 2 to 4 more times at this temperature, and the temperature afterward shall not be lower than 900℃. Cool to room temperature to obtain a forged slab. In step S2, the forged slab is subjected to multiple rolling processes to obtain a rolled slab, including: The first firing test, on the forged slab at T β The reversing rolling is carried out at 0~50℃, with the rolling directions of the previous and subsequent passes perpendicular. The deformation per pass is 10~15%, the rolling speed is 2~5m / s, and 3~5 passes are rolled. The total rolling deformation is 40~65%. If the temperature is lower than 900℃ during the rolling process, the temperature is returned to the furnace for reheating. The second rolling process, with a rolling temperature of T. β + (0~50℃), the rolling direction is perpendicular to the final rolling direction of the first heat, the deformation per pass is 6~12%, the rolling speed is 2~5m / s, 3~5 passes are rolled, the total rolling deformation is 40~65%, and the rolling is finished by air cooling to room temperature; The third rolling process, with a rolling temperature of T. β - (0~50℃), the deformation rate per pass is 10~20%, the rolling speed is 3~4m / s, and the final rolling temperature is 850±20℃ to obtain the rolled slab; Based on a mass content of 100% for each component, the TC4 titanium alloy ingot comprises: Al: 6.55~6.75%, V: 4.20~4.50%, Fe: 0.10~0.30%, O: 0.10~0.20%, C≤0.08%; N≤0.05%, H≤0.015%, unavoidable impurities≤0.4%; the balance is Ti.

2. The low-temperature superplastic forming method for a high-performance titanium alloy surface tension tank according to claim 1, characterized in that, In step S1, the TC4 titanium alloy ingot is subjected to forging and slab manufacturing to obtain a forged slab, including: the TC4 titanium alloy ingot is forged three times to obtain a forged slab; the temperature of the first forging is higher than the temperature of the second forging, and the temperature of the third forging is higher than the temperature of the second forging.

3. The low-temperature superplastic forming method for a high-performance titanium alloy surface tension tank according to claim 1, characterized in that, In step S3, the rolled slab undergoes heat treatment, including: After rolling, the slab is held at 600~700℃ for 240±10min and then air-cooled.

4. The low-temperature superplastic forming method for a high-performance titanium alloy surface tension tank according to claim 1, characterized in that: The upper mold is located at the edge of the forming surface of the upper mold, and the edge of the annular groove and the surface of the upper mold facing the lower mold are rounded.

5. The low-temperature superplastic forming method for a high-performance titanium alloy surface tension tank according to claim 1, characterized in that, In step S6, the original slab is placed between the upper and lower molds for reverse expansion forming, including: the heating rate of the superplastic forming mold is 55~75℃ / h; when the temperature of the superplastic forming mold reaches 650~750℃, the original slab is placed between the upper and lower molds again, and the temperature continues to rise; when the temperature of the superplastic forming mold reaches 860~890℃, the upper and lower superplastic forming molds are closed, maintaining a tonnage of 10~20t; reverse expansion air intake is performed to make the original slab fit with the upper mold, the air intake rate is 0.05~0.10MPa / min, the air pressure is raised to 2.0~3.0MPa, the pressure is held for 15~20min, and then the pressure is released.

6. The low-temperature superplastic forming method for a high-performance titanium alloy surface tension tank according to claim 5, characterized in that, The positive expansion forming includes: after the reverse expansion forming is completed and the air pressure is balanced, and the temperature of the superplastic forming mold is maintained at 860~890℃, positive expansion air intake is performed to make the original slab fit with the lower mold, the air intake speed is ≤0.05MPa / min, and the air pressure is increased to 1.5~2.0MPa; after the hemispherical blank formed by the original slab is completely fitted with the lower mold, the pressure is maintained for 15~20min, and then the pressure is released to balance the air pressure, while the upper mold is raised.

7. A high-performance titanium alloy surface tension storage tank, characterized in that, The high-performance titanium alloy surface tension tank is prepared by a low-temperature superplastic forming method according to any one of claims 1-6.

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