Superplastic titanium alloy and forming process thereof

Abstract

The invention belongs to the technical field of titanium alloys, and particularly relates to a superplastic titanium alloy and a forming process thereof. The titanium alloy comprises the following chemical components in percentage by mass: 4.0%-6.0% of Al, 3.0%-4.5% of V, 0.05%-0.30% of Sc, 0.5%-1.2% of Fe, 0.08%-0.16% of O and the balance of titanium and inevitable impurities, and the total amount is 100%. The Sc element is introduced into the alloy, and a multi-step composite process of aging treatment, low-temperature high-speed pre-forming and high-temperature low-speed accurate forming is matched, so that a superplastic deformation window moves to low temperature, and excellent superplastic performance is obtained.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy technology, specifically relating to a superplastic titanium alloy and its forming process. Background Technology

[0002] Titanium alloys are widely used in aerospace, biomedical, and high-end equipment manufacturing fields due to their high specific strength, excellent corrosion resistance, and biocompatibility. However, the insufficient plasticity of titanium alloys results in poor formability. Superplastic forming technology provides an effective way to solve this problem.

[0003] Currently, the superplastic forming of titanium alloys generally involves first melting and thermomechanical treatment (such as multi-pass rolling, extrusion, or forging) to obtain a fine equiaxed grain structure. This pretreated titanium alloy material is then subjected to high temperature and low strain rate conditions for loading and forming. During this process, the material mainly relies on grain boundary slip mechanisms to achieve uniform large deformation. Superplasticity refers to the material exhibiting exceptionally high elongation without necking under the aforementioned high temperature and low strain rate conditions. However, existing superplastic titanium alloys face the following problems in achieving this large deformation: high temperature and insufficient superplastic deformation elongation. For example, the superplastic deformation temperature of Ti-6Al-4V is above 900℃, with a maximum elongation of approximately 900%. With the continuous expansion of titanium alloy applications, the requirements for the superplastic properties of titanium alloys are constantly increasing. How to enable titanium alloys to achieve greater superplastic deformation elongation at lower temperatures is a problem that needs to be solved. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a superplastic titanium alloy and its forming process. By improving the alloy composition and forming process, the titanium alloy exhibits excellent superplastic deformation elongation at relatively low temperatures. This invention introduces Sc element into the titanium alloy composition, forming thermally stable Sc-rich nanoclusters, which lowers the transformation temperature of the β phase and increases the recrystallization temperature of the α phase, thus shifting the superplastic deformation window to a lower temperature. Furthermore, during plastic deformation, the fine-grained billet undergoes aging treatment, followed by low-temperature high-speed preforming followed by high-temperature low-speed precision forming, resulting in a large superplastic deformation elongation.

[0005] The present invention is specifically implemented through the following technical solutions.

[0006] The first object of this invention is to provide a superplastic titanium alloy, which, by weight percentage, comprises the following chemical composition: Al: 4.0%~6.0%, V: 3.0%~4.5%, Sc: 0.05%~0.30%, Fe: 0.5%~1.2%, O: 0.08%~0.16%, with the balance being titanium and unavoidable impurities, totaling 100%.

[0007] When titanium alloys undergo superplastic forming, the various elements are melted and then subjected to thermomechanical treatment to obtain a fine-grained billet. The fine-grained billet is then aged at 550℃~650℃ to cause Sc atoms to segregate at grain boundaries and phase boundaries. The aged billet is then further aged at 610℃~630℃ and 5×10⁻⁶ ℃. -3 s -1 ~1×10 -2 s -1 Preforming was performed at a strain rate of 700℃~750℃ and then subjected to a strain rate of 1×10⁻⁶. -4 s -1 ~ 5×10 -4 s -1 Precise forming can be achieved under strain rate, followed by annealing.

[0008] In a preferred embodiment of the present invention, Sc: 0.10%~0.25%.

[0009] This invention introduces the element Sc into the titanium alloy composition. During intermediate heat treatment, Sc atoms preferentially agglomerate at grain boundaries and phase boundaries, forming Sc-rich nanoclusters with high thermal stability, thereby shifting the window for superplastic deformation to a lower temperature.

[0010] In a preferred embodiment of the present invention, the microstructure of the titanium alloy is an ultrafine-grained biphase (α+β) structure with an average grain size d≤2.0μm, and there are Sc-rich nanoclusters dispersed at the interface of the α and β phases.

[0011] A second objective of this invention is to provide a forming process for the aforementioned superplastic titanium alloy, comprising the following steps: After melting the various elements, a fine-grained billet is obtained through thermomechanical processing.

[0012] The fine-grained billet is aged at a temperature of 550℃~650℃ to cause Sc atoms to segregate at grain boundaries and phase boundaries.

[0013] The aged billet was subjected to a two-step superplastic forming process: first, at 610℃~630℃ and 5×10 -3 s -1 ~1×10 -2 s -1 Preforming was performed at a strain rate of 700℃~750℃ and then subjected to a strain rate of 1×10⁻⁶. -4 s -1 ~ 5×10 -4 s -1 Precise forming is performed at strain rates.

[0014] Afterwards, annealing is performed to relieve stress. The resulting material exhibits excellent superplastic elongation.

[0015] The specific steps described above are as follows: (1) Billet preparation and pretreatment: Fine-grained billets are obtained through thermomechanical treatment, specifically including the following steps: S1. The titanium alloy is batched according to the following composition: Al: 4.0%~6.0%, V: 3.0%~4.5%, Sc: 0.05%~0.30%, Fe: 0.5%~1.2%, O: 0.08%~0.16%, with the balance being titanium and unavoidable impurities, totaling 100%. Multiple melting processes are performed in a vacuum arc furnace to ensure uniform composition, resulting in an ingot.

[0016] S2. Hold the ingot at 1000℃~1020℃ for 8~24 hours, then furnace cool. This eliminates dendritic segregation and compositional inhomogeneity in the ingot, providing a uniform initial microstructure for subsequent hot deformation.

[0017] S3. Heat the homogenized ingot to 980℃~1000℃ and hold for 1~2 hours. Then perform multiple forging passes, controlling the total deformation to 60%~80%, with each pass involving approximately 20%~30% deformation. Return the ingot to the furnace between passes to restore temperature. This process thoroughly breaks down the coarse as-cast structure, introducing high-density dislocations and deformation bands, providing numerous nucleation sites for subsequent recrystallization. At this stage, Sc has begun to dissolve in the β matrix.

[0018] It should be noted that any existing forging technology can be used for the forging process, and the specific operation process is the same as that of existing technology. During the forging process, attention only needs to be paid to the deformation amount and heating temperature mentioned above in this invention. As a preferred embodiment of this invention, the following steps can be followed: Total deformation amount = [(H0-H1) / H0]×100%, where H0 is the initial height of the billet before forging, and H1 is the final height of the billet after multiple forging passes. The deformation amount per pass refers to the percentage of the height deformed in each pass relative to the initial height of the billet in that pass. In actual operation, the deformation per pass is achieved by controlling the reduction of the forging press. After each forging pass is completed, the billet is returned to the furnace for heat preservation until its temperature returns to 980℃~1000℃ before proceeding to the next pass.

[0019] S4. After forging, the billet is cooled to 750℃~800℃ and held for 0.5~1 hour, followed by multiple hot rolling passes, with the total deformation controlled at 70%~90%. The final rolling temperature is controlled at no less than 750℃. Large plastic deformation occurs in the (α+β) two-phase region, strongly refining the α and β phases. During deformation, dynamic recrystallization and phase transformation occur simultaneously, effectively obtaining an equiaxed or flattened ultrafine-grained structure with a size of 2~3μm. At this time, under the influence of intense plastic deformation and dislocations, Sc elements begin to migrate towards grain boundaries and phase boundaries. After this step, a fine-grained billet is obtained.

[0020] It should be noted that, in order to achieve the total deformation after multiple hot rolling passes and ensure that the final rolling temperature is not lower than 750℃, the total reduction is distributed across multiple passes. The reduction for each pass is controlled by the roll gap setting of the rolling mill. During the rolling process, an infrared thermometer is used to monitor the billet temperature in real time. If the temperature drops too quickly, the interval between passes can be appropriately shortened or the billet can be returned to the furnace for short-term reheating. The reduction is accumulated pass by pass, ensuring that the billet temperature remains above 750℃ when the last pass is completed.

[0021] (2) Intermediate heat treatment (aging treatment): The fine-grained billet is aged at a temperature of 550℃~650℃ for 2~8 hours. The purpose of this step is to promote the segregation of Sc atoms at grain boundaries and phase boundaries to form incoherent Sc-rich nanoclusters. These clusters will become strong grain boundary pinning points in subsequent superplastic deformation.

[0022] (3) Two-step superplastic forming: Step 1: Low-temperature high-speed preforming: at a relatively low temperature of 610℃~630℃ and a high initial strain rate of 5×10⁻⁶. -3 s -1 ~1×10 -2 s -1 The material undergoes initial shaping, resulting in a deformation of approximately 20% to 50%. During this stage, the Sc element is used to lower the grain boundary diffusion activation energy, thereby activating grain boundary slip.

[0023] Step 2: High-temperature, low-speed precision forming: Without unloading, the temperature is raised to 700℃~750℃, and the strain rate is reduced to 1×10⁻⁶. -4 s -1 ~ 5×10 -4 s -1 The final forming pressure is applied to complete the final precise forming of the component. This stage utilizes dynamic recrystallization and phase boundary slip to further refine the microstructure and eliminate defects.

[0024] (4) Annealing treatment.

[0025] In a preferred embodiment of the present invention, the two-step superplastic forming is performed on a pneumatic forming machine, wherein the forming pressure of the first step is 1.0 MPa to 2.0 MPa, and the forming pressure of the second step is 2.5 MPa to 5.0 MPa.

[0026] In a preferred embodiment of the present invention, the annealing temperature is 550°C to 600°C.

[0027] The above method enables titanium alloys to achieve high plasticity at relatively low temperatures (700℃~750℃), with a total elongation of more than 1500% during superplastic forming.

[0028] Compared with the prior art, the present invention has the following beneficial effects: This invention proposes a novel titanium alloy composition, consisting of the following chemical components: Al: 4.0%~6.0%, V: 3.0%~4.5%, Sc: 0.05%~0.30%, Fe: 0.5%~1.2%, O: 0.08%~0.16%, with the balance being titanium and unavoidable impurities, totaling 100%. The titanium alloy provided by this invention exhibits excellent superplastic properties. During plastic forming, due to the introduction of Sc into the alloy composition, Sc atoms preferentially agglomerate at grain and phase boundaries during aging treatment, forming thermally stable Sc-rich nanoclusters. These clusters effectively suppress grain growth even at recrystallization temperatures far exceeding those of conventional titanium alloys. This is the foundation for realizing the two-step superplastic forming process of this invention and is key to obtaining ultrafine grains (≤2.0 μm). Sc can lower the transformation temperature of the β phase and raise the recrystallization temperature of the α phase, shifting the superplastic deformation window to lower temperatures. There is an interaction between Sc and Fe and O. The addition of Sc alters the distribution of Fe and O within the crystal, causing them to tend to accumulate at grain boundaries and phase boundaries, forming a composite pinning structure together with Sc.

[0029] Existing superplastic forming technologies are all constant temperature, constant speed or simple pressure variation processes. The multi-step composite process proposed in this invention, which involves aging treatment of fine-grained billets, low-temperature high-speed preforming, and high-temperature low-speed precision forming, is specifically designed for titanium alloys containing Sc. Without stable nanoclusters formed by Sc, the grains will rapidly coarsen after the first step of low-temperature high-speed deformation, leading to the failure of the second step of forming.

[0030] This invention enables titanium alloys to exhibit excellent superplasticity at lower temperatures through the synergistic effect of the aforementioned components and processes. Attached Figure Description

[0031] Figure 1 This is a photograph of the billet bar material from Example 1.

[0032] Figure 2 This is a photograph of the product after superplastic molding in Example 1.

[0033] Figure 3 The images show the XRD patterns of alloys from Example 1 and Comparative Example 1.

[0034] Figure 4 This is a microstructure diagram of the alloy in Example 1.

[0035] Figure 5 This is a microstructure diagram of alloy 1 (Comparative Example). Detailed Implementation

[0036] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below with reference to specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention. Unless otherwise specified, the experimental methods and detection methods described in the following embodiments are conventional methods; unless otherwise specified, the reagents and materials described are commercially available.

[0037] The purpose of this invention is to achieve superplasticity in titanium alloys and reduce the temperature of plastic forming. To solve this problem, this invention achieves superplasticity at a lower temperature through improvements in alloy composition and corresponding forming processes.

[0038] The present invention will be specifically described below through the following embodiments and comparative examples.

[0039] Example 1 This embodiment provides a superplastic titanium alloy, which, by mass percentage, is composed of the following chemical composition: Al: 5.0%, V: 4%, Sc: 0.15%, Fe: 1.0%, O: 0.1%, balance being titanium and unavoidable impurities, totaling 100%.

[0040] The forming process of the above-mentioned superplastic titanium alloy includes the following steps: (1) Billet preparation and pretreatment: S1. Prepare the titanium alloy according to the above composition ratio, and perform multiple meltings in a vacuum arc furnace with a melting current of 1800A and a melting voltage of 20V to ensure uniform composition and obtain an ingot.

[0041] S2. Hold the ingot at 1020℃ for 12 hours, then furnace cool it.

[0042] S3. Heat the homogenized ingot to 1000℃ and hold for 1 hour. Then perform multiple forging passes, with the total deformation controlled at 80% and the deformation per pass at about 20%. The deformation per pass is achieved by controlling the reduction of the forging press. After each forging pass is completed, the billet is returned to the furnace for heat preservation to restore its temperature to 1000℃.

[0043] S4. After deformation, the billet is cooled to 800℃ and held at that temperature for 0.5 hours. Then, it undergoes multiple hot rolling passes, with the total deformation controlled at 80% and the final rolling temperature controlled at no less than 750℃. The reduction per pass is controlled at 20% by setting the roll gap of the rolling mill. During the rolling process, an infrared thermometer is used to monitor the billet temperature in real time. If the temperature drops too quickly, the billet is returned to the furnace for short-term reheating. The resulting billet / bar is shown in the image below. Figure 1 As shown.

[0044] (2) Intermediate heat treatment (aging treatment): The billet is aged at 600℃ for 4 hours to promote the segregation of Sc atoms at grain boundaries and phase boundaries to form non-coherent Sc-rich nanoclusters.

[0045] (3) Two-step superplastic forming on a pneumatic forming machine: Step 1: Low-temperature high-speed preforming: The forming pressure is 1.0 MPa, at 620℃ and an initial strain rate of 5 × 10⁻⁶. -3 s -1 The material is initially shaped to undergo a deformation of about 30%, and grain boundary slip is activated by utilizing the effect of Sc element to reduce the grain boundary diffusion activation energy.

[0046] Step 2: High-temperature, low-speed precision forming: Without unloading, raise the temperature to 720℃ and reduce the strain rate to 1×10⁻⁶. -4 s -1 A final forming pressure of 3 MPa is applied to complete the final precise forming of the component. Dynamic recrystallization and phase boundary slip are used to further refine the microstructure and eliminate defects.

[0047] (4) Afterwards, anneal at 550℃ for 1 hour to obtain the product as shown. Figure 2 As shown, the total elongation of the two-step superplastic forming is 1580%.

[0048] XRD pattern of superplastic titanium alloy as shown in the figure Figure 3 As shown, and after testing, as Figure 4 As shown, its microstructure is an equiaxed (α+β) microstructure with an average grain size of 1.7 μm and Sc-rich nanoclusters at the phase boundaries.

[0049] Comparative Example 1 Compared to Example 1, this example does not contain the Sc element.

[0050] This comparative example provides a titanium alloy, which, by mass percentage, consists of the following chemical composition: Al: 5.0%, V: 4%, Fe: 1.0%, O: 0.1%, balance being titanium and unavoidable impurities, totaling 100%.

[0051] The forming process of the above-mentioned superplastic titanium alloy includes the following steps: (1) Billet preparation and pretreatment: S1. Prepare the titanium alloy according to the above composition ratio, and perform multiple meltings in a vacuum arc furnace with a melting current of 1800A and a melting voltage of 20V to ensure uniform composition and obtain an ingot.

[0052] S2. Hold the ingot at 1020℃ for 12 hours, then furnace cool it.

[0053] S3. Heat the homogenized ingot to 1000℃ and hold for 1 hour. Then perform multiple forging passes, with the total deformation controlled at 70% and the deformation per pass at about 20%. The deformation per pass is achieved by controlling the reduction of the forging press. After each forging pass is completed, the billet is returned to the furnace for heat preservation to restore its temperature to 1000℃.

[0054] S4. After deformation, the billet is cooled to 800℃ and held at that temperature for 0.5 hours. Then, it undergoes multiple hot rolling passes, with the total deformation controlled at 80% and the final rolling temperature controlled at no less than 750℃. The reduction per pass is controlled at 20% by setting the roll gap of the rolling mill. During the rolling process, an infrared thermometer is used to monitor the billet temperature in real time. If the temperature drops too quickly, the billet is returned to the furnace for short-term reheating.

[0055] (2) Intermediate heat treatment (aging treatment): The billet is aged at 600℃ for 4 hours.

[0056] (3) Superplastic forming on a pneumatic forming machine: Step 1: Low-temperature high-speed preforming: The forming pressure is 1.0 MPa, at 620℃ and an initial strain rate of 5 × 10⁻⁶. -3 s -1 The material undergoes initial forming, resulting in approximately 30% deformation. An alloy billet with similar composition but no Sc is prepared using the same thermomechanical treatment. Its initial grain size is approximately 2.5 μm. After the same intermediate heat treatment, the microstructure remains largely unchanged due to the absence of Sc precipitation. During the first step of superplastic forming, the grains rapidly grow to 5.0 μm. Figure 5 As shown, this leads to a decrease in the material's plasticity, resulting in fracture after the first deformation step. The XRD pattern of the alloy after the first superplastic forming is shown below. Figure 3 As shown, compared with Example 1, due to the lack of Sc pinning, the grains grow more easily, the β(110) peak becomes slightly sharper, and the full width at half maximum (FWHM) decreases. Sc is a weak α stabilizer, and the proportion of the β phase increases relatively after the absence of Sc, making the role of Fe as a strong β stabilizer more prominent, resulting in an increase in the relative intensity of the β phase peak.

[0057] Comparative Example 2 Compared with Example 1, there is no aging process step, and the two-step forming is performed directly.

[0058] This comparative example provides a titanium alloy, which, by mass percentage, consists of the following chemical composition: Al: 5.0%, V: 4%, Sc: 0.15%, Fe: 1.0%, O: 0.1%, balance being titanium and unavoidable impurities, totaling 100%.

[0059] The forming process of the above-mentioned superplastic titanium alloy includes the following steps: (1) Billet preparation and pretreatment: S1. Prepare the titanium alloy according to the above composition ratio, and perform multiple meltings in a vacuum arc furnace with a melting current of 1800A and a melting voltage of 20V to ensure uniform composition and obtain an ingot.

[0060] S2. Hold the ingot at 1020℃ for 12 hours, then furnace cool it.

[0061] S3. Heat the homogenized ingot to 1000℃ and hold for 1 hour. Then perform multiple forging passes, with the total deformation controlled at 70% and the deformation per pass at about 20%. The deformation per pass is achieved by controlling the reduction of the forging press. After each forging pass is completed, the billet is returned to the furnace for heat preservation to restore its temperature to 1000℃.

[0062] S4. After deformation, the billet is cooled to 800℃ and held at that temperature for 0.5 hours. Then, it undergoes multiple hot rolling passes, with the total deformation controlled at 80% and the final rolling temperature controlled at no less than 750℃. The reduction per pass is controlled at 20% by setting the roll gap of the rolling mill. During the rolling process, an infrared thermometer is used to monitor the billet temperature in real time. If the temperature drops too quickly, the billet is returned to the furnace for short-term reheating.

[0063] (2) Two-step superplastic forming on a pneumatic forming machine: Step 1: Low-temperature high-speed preforming: The forming pressure is 1.0 MPa, at 620℃ and an initial strain rate of 5 × 10⁻⁶. -3 s -1 The material undergoes initial shaping, causing approximately 30% deformation.

[0064] Step 2: High-temperature, low-speed precision forming: Without unloading, raise the temperature to 720℃ and reduce the strain rate to 1×10⁻⁶. -4 s -1 Apply a final forming pressure of 3MPa to complete the final precise forming of the component.

[0065] (3) Then anneal at 550℃ for 1 hour.

[0066] Because Sc atoms failed to age and precipitate in time to form effective pinning points, the grain boundary migration resistance was insufficient during the first forming step, resulting in significant grain coarsening (from 2.0 μm to 4.5 μm). The final formed component exhibited an uneven microstructure and an elongation of only 650%, far lower than that of Example 1.

[0067] Example 2 This embodiment provides a superplastic titanium alloy, which, by mass percentage, is composed of the following chemical composition: Al: 4.0%, V: 4.0%, Sc: 0.2%, Fe: 0.5%, O: 0.08%, balance being titanium and unavoidable impurities, totaling 100%.

[0068] The forming process of the above-mentioned superplastic titanium alloy includes the following steps: (1) Billet preparation and pretreatment: S1. Prepare the titanium alloy according to the above composition ratio, and perform multiple meltings in a vacuum arc furnace with a melting current of 1800A and a melting voltage of 20V to ensure uniform composition and obtain an ingot.

[0069] S2. Hold the ingot at 1000℃ for 10 hours, then furnace cool it.

[0070] S3. Heat the homogenized ingot to 980℃ and hold for 1 hour. Then perform multiple forging passes, with the total deformation controlled at 70% and the deformation per pass at approximately 25%. The deformation per pass is achieved by controlling the reduction of the forging press. After each forging pass is completed, return the billet to the furnace for heat preservation to restore its temperature to 980℃.

[0071] S4. After deformation, the billet is cooled to 800℃ and held at that temperature for 1 hour. Then, it undergoes multiple hot rolling passes, with the total deformation controlled at 80% and the final rolling temperature controlled at no less than 750℃. The reduction per pass is controlled at 15% by setting the roll gap of the rolling mill. During the rolling process, an infrared thermometer is used to monitor the billet temperature in real time. If the temperature drops too quickly, the billet is returned to the furnace for short-term reheating.

[0072] (2) Intermediate heat treatment: The billet is aged at 550°C for 3 hours to promote the segregation of Sc atoms at grain boundaries and phase boundaries to form incoherent Sc-rich nanoclusters.

[0073] (3) Two-step superplastic forming on a pneumatic forming machine: Step 1: Low-temperature high-speed preforming: The forming pressure is 2.0 MPa, at 610℃ and an initial strain rate of 1×10⁻⁶. -2 s -1 The material is initially shaped to undergo approximately 40% deformation. The effect of Sc element in reducing the grain boundary diffusion activation energy is utilized to activate grain boundary slip.

[0074] Step 2: High-temperature, low-speed precision forming: Without unloading, raise the temperature to 750℃ and reduce the strain rate to 5×10. -4 s -1 A final forming pressure of 2.5 MPa is applied to complete the final precise forming of the component. Dynamic recrystallization and phase boundary slip are used to further refine the microstructure and eliminate defects. (4) Then anneal at 550℃ for 1 hour. The total elongation of the two-step superplastic forming was 1510%.

[0075] Upon testing, its microstructure is found to be an equiaxed (α+β) microstructure with an average grain size of 2.0 μm, and there are Sc-rich nanoclusters at the phase boundaries.

[0076] Example 3 This embodiment provides a superplastic titanium alloy, which, by mass percentage, is composed of the following chemical composition: Al: 6.0%, V: 4.5%, Sc: 0.3%, Fe: 1.2%, O: 0.16%, balance being titanium and unavoidable impurities, totaling 100%.

[0077] The forming process of the above-mentioned superplastic titanium alloy includes the following steps: (1) Billet preparation and pretreatment: S1. Prepare the titanium alloy according to the above composition ratio, and perform multiple meltings in a vacuum arc furnace with a melting current of 1800A and a melting voltage of 20V to ensure uniform composition and obtain an ingot.

[0078] S2. Hold the ingot at 1020℃ for 12 hours, then furnace cool it.

[0079] S3. Heat the homogenized ingot to 1000℃ and hold for 1 hour. Then perform multiple forging passes, with the total deformation controlled at 70% and the deformation per pass at about 20%. The deformation per pass is achieved by controlling the reduction of the forging press. After each forging pass is completed, the billet is returned to the furnace for heat preservation to restore its temperature to 1000℃.

[0080] S4. After deformation, the billet is cooled to 800℃ and held at that temperature for 0.5 hours. Then, it undergoes multiple hot rolling passes, with the total deformation controlled at 90% and the final rolling temperature controlled at no less than 750℃. The reduction per pass is controlled at 20% by setting the roll gap of the rolling mill. During the rolling process, an infrared thermometer is used to monitor the billet temperature in real time. If the temperature drops too quickly, the billet is returned to the furnace for short-term reheating.

[0081] (2) Intermediate heat treatment: The billet is aged at 650°C for 2 hours to promote the segregation of Sc atoms at grain boundaries and phase boundaries, forming non-coherent Sc-rich nanoclusters.

[0082] (3) Two-step superplastic forming on a pneumatic forming machine: Step 1: Low-temperature high-speed preforming: forming pressure of 2.0 MPa, at 630℃ and initial strain rate of 5 × 10⁻⁶. -3 s -1 The material is initially shaped to undergo approximately 50% deformation. The effect of Sc element in reducing the grain boundary diffusion activation energy is utilized to activate grain boundary slip.

[0083] Step 2: High-temperature, low-speed precision forming: Without unloading, raise the temperature to 700℃ and reduce the strain rate to 1×10⁻⁶. -4 s -1 A final forming pressure of 5 MPa is applied to complete the final precise forming of the component. Dynamic recrystallization and phase boundary slip are used to further refine the microstructure and eliminate defects.

[0084] (4) Then anneal at 550℃ for 1 hour. The total elongation of the two-step superplastic forming is 1530%.

[0085] Upon testing, its microstructure is found to be an equiaxed (α+β) microstructure with an average grain size of 1.9 μm, and there are Sc-rich nanoclusters at the phase boundaries.

[0086] Example 4 This embodiment provides a superplastic titanium alloy, which, by mass percentage, is composed of the following chemical composition: Al: 4.0%, V: 3.0%, Sc: 0.05%, Fe: 0.5%, O: 0.1%, balance being titanium and unavoidable impurities, totaling 100%.

[0087] The forming process of the above-mentioned superplastic titanium alloy includes the following steps: (1) Billet preparation and pretreatment: S1. Prepare the titanium alloy according to the above composition ratio, and perform multiple meltings in a vacuum arc furnace with a melting current of 1800A and a melting voltage of 20V to ensure uniform composition and obtain an ingot.

[0088] S2. Hold the ingot at 1020℃ for 12 hours, then furnace cool it.

[0089] S3. Heat the homogenized ingot to 1000℃ and hold for 1 hour. Then perform multiple forging passes, with the total deformation controlled at 70% and the deformation per pass at about 20%. The deformation per pass is achieved by controlling the reduction of the forging press. After each forging pass is completed, the billet is returned to the furnace for heat preservation to restore its temperature to 1000℃.

[0090] S4. After deformation, the billet is cooled to 780℃ and held at that temperature for 0.5 hours. Then, it undergoes multiple hot rolling passes, with the total deformation controlled at 80% and the final rolling temperature controlled at no less than 750℃. The reduction per pass is controlled at 20% by setting the roll gap of the rolling mill. During the rolling process, an infrared thermometer is used to monitor the billet temperature in real time. If the temperature drops too quickly, the billet is returned to the furnace for short-term reheating.

[0091] (2) Intermediate heat treatment: The billet is aged at 550°C for 4 hours to promote the segregation of Sc atoms at grain boundaries and phase boundaries, forming non-coherent Sc-rich nanoclusters.

[0092] (3) Two-step superplastic forming on a pneumatic forming machine: Step 1: Low-temperature high-speed preforming: The forming pressure is 1.0 MPa, at 620℃ and an initial strain rate of 5 × 10⁻⁶. -3 s -1 The material is initially shaped to undergo a deformation of about 20%, and grain boundary slip is activated by utilizing the effect of Sc element to reduce the grain boundary diffusion activation energy.

[0093] Step 2: High-temperature, low-speed precision forming: Without unloading, raise the temperature to 720℃ and reduce the strain rate to 1×10⁻⁶. -4 s -1 A final forming pressure of 3 MPa is applied to complete the final precise forming of the component. Dynamic recrystallization and phase boundary slip are used to further refine the microstructure and eliminate defects.

[0094] (4) Then anneal at 600℃ for 1 hour. The total elongation of the two-step superplastic forming is 1550%.

[0095] Upon testing, its microstructure is found to be an equiaxed (α+β) microstructure with an average grain size of 1.9 μm, and there are Sc-rich nanoclusters at the phase boundaries.

[0096] As can be seen from the above embodiments and comparative examples, the improved alloy composition of this invention is crucial. By introducing Sc into the alloy and employing a multi-step composite process of aging treatment, low-temperature high-speed preforming, and high-temperature low-speed precision forming, the superplastic deformation window shifts to a lower temperature, resulting in excellent superplastic properties. High plasticity can be achieved at relatively low temperatures (700℃~750℃), with a superplastic deformation elongation greater than 1500%.

[0097] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, it is intended to include any modifications and variations that fall within the scope of the claims and their equivalents.

Claims

1. A superplastic titanium alloy, characterized in that, It consists of the following chemical components by mass percentage: Al: 4.0%~6.0%, V: 3.0%~4.5%, Sc: 0.05%~0.30%, Fe: 0.5%~1.2%, O: 0.08%~0.16%, with the balance being titanium and unavoidable impurities, totaling 100%. When titanium alloys undergo superplastic forming, the various elements are melted and then subjected to thermomechanical treatment to obtain a fine-grained billet. The fine-grained billet is then aged at 550℃~650℃ to cause Sc atoms to segregate at grain boundaries and phase boundaries. The aged billet is then further aged at 610℃~630℃ and 5×10⁻⁶ ℃. -3 s -1 ~1×10 -2 s -1 Preforming was performed at a strain rate of 700℃~750℃ and then subjected to a strain rate of 1×10⁻⁶. -4 s -1 ~ 5×10 -4 s -1 Precise forming can be achieved under strain rate, followed by annealing.

2. The superplastic titanium alloy according to claim 1, characterized in that, The titanium alloy has an α+β microstructure with an average grain size d≤2.0μm and Sc-rich nanoclusters dispersed at the α and β phase interfaces.

3. A forming process for the superplastic titanium alloy according to claim 1, characterized in that, Includes the following steps: After melting the various elements, a fine-grained billet is obtained through thermomechanical treatment; The fine-grained billet is aged at a temperature of 550℃~650℃ to cause Sc atoms to segregate at grain boundaries and phase boundaries. The aged billet is then subjected to two-step superplastic forming: Step 1: At 610℃~630℃ and 5×10 -3 s -1 ~1×10 -2 s -1 Preliminary forming is carried out at a strain rate, causing the material to deform by 20% to 50%; Step 2: Without unloading, raise the temperature to 700℃~750℃ and reduce the strain rate to 1×10⁻⁶. -4 s -1 ~ 5×10 -4 s -1 Apply final forming pressure to complete the final precise forming of the component; Then annealing is performed.

4. The forming process according to claim 3, characterized in that, The processing time is 2 to 8 hours.

5. The forming process according to claim 3, characterized in that, The two-step superplastic forming is achieved on a pneumatic forming machine. The forming pressure of the first step is 1.0MPa~2.0MPa, and the forming pressure of the second step is 2.5MPa~5.0MPa.

6. The forming process according to claim 3, characterized in that, The annealing temperature is 550℃~600℃.

7. The forming process according to claim 3, characterized in that, After melting the various elements, a fine-grained billet is obtained through thermomechanical treatment, specifically including the following steps: The titanium alloy was batched according to the following composition: Al: 4.0%~6.0%, V: 3.0%~4.5%, Sc: 0.05%~0.30%, Fe: 0.5%~1.2%, O: 0.08%~0.16%, with the balance being titanium and unavoidable impurities, totaling 100%. Multiple melting processes were performed in a vacuum arc furnace to ensure uniform composition and obtain the ingot. The ingots are held at 1000℃~1020℃ for homogenization treatment, and then furnace cooled. The homogenized ingot is heated to 980℃~1000℃ and held for a period of time. Then, it is forged in multiple passes, with the total deformation controlled at 60%~80%. The ingot is then reheated between passes to restore the temperature. The forged billet is cooled to 750℃~800℃ and held at that temperature before being hot rolled in multiple passes. The total deformation is controlled at 70%~90%, and the final rolling temperature is controlled at no less than 750℃.

8. The forming process according to claim 7, characterized in that, The heat preservation time for homogenization treatment is 8 to 24 hours.

9. The forming process according to claim 7, characterized in that, Keep warm at 980℃~1000℃ for 1~2 hours.

10. The forming process according to claim 7, characterized in that, Keep warm at 750℃~800℃ for 0.5~1 hour.

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