A method for preparing A1-grade non-destructive flaw detection high impact toughness TC4 titanium alloy large-size rod

Abstract

The application discloses a preparation method of a high-impact toughness TC4 titanium alloy large-size rod with A1-level nondestructive flaw detection, and the method comprises the following steps: adopting a multi-pass high-temperature single-phase beta zone large deformation forging process in the breakdown stage of a secondary finished ingot, fully crushing and refining the as-cast grains through axial upsetting, lateral upsetting and diagonal upsetting and the like; then repeatedly forging through "high-low-high-low" at the temperature above and below the phase transition point, realizing grain homogenization and eliminating the original grain boundary through high-temperature dynamic recrystallization, effectively reducing the deformation dead zone of the corner area in combination with multi-pass diagonal upsetting and elongating forging; finally, carrying out multi-pass forging in the two-phase zone, further optimizing the microstructure morphology in combination with a heat treatment process, and controlling the lamellar alpha structure to guarantee the low-temperature impact performance. The application can significantly improve the uniformity of the TC4 alloy rod, meets the A1-level nondestructive detection requirement, has excellent axial and chordwise mechanical properties, and is suitable for the marine equipment and other high-end application fields with strict requirements on the comprehensive performance of the material.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy hot working technology in metal metallurgy, specifically to a method for preparing large-diameter TC4 titanium alloy bars with high impact toughness for A1-grade non-destructive testing in marine engineering. Background Technology

[0002] TC4 (Ti-6Al-4V) is an α+β type dual-phase titanium alloy developed in the United States in 1954. It has become one of the most widely used titanium alloys globally. TC4 composition is based on titanium (Ti), containing 6% α-stabilizing element aluminum (Al) and 4% β-stabilizing element vanadium (V). TC4 titanium alloy possesses excellent comprehensive properties and good processing performance, making it widely applicable in aerospace, petrochemical, and biomedical fields. In these different application scenarios, TC4 titanium alloy materials (such as plates and bars) need to achieve different mechanical properties; therefore, there are significant differences in alloy element content, microstructure characteristics, and comprehensive mechanical properties.

[0003] Taking the field of marine engineering as an example, it is a comprehensive technical science that applies basic marine science and related technologies to the development and utilization of the ocean, covering areas such as marine resource development, space utilization, energy development, and coastal protection. The demand for high-end equipment components such as large ship propulsion shafts, rudder stock, subsea pipeline flanges, high-strength bolts and pins for platform module connections, unmanned underwater vehicle structural components, seawater desalination high-pressure pump shafts, and ship engine connecting rods is increasing, creating an urgent need for large-size TC4 titanium alloy bars with A1-level non-destructive testing quality and high impact toughness. However, the preparation of such large-size TC4 titanium alloy bars still faces several technical challenges, the main problems of which are as follows:

[0004] 1) The challenge of matching strength and low-temperature impact toughness in large-diameter bars:

[0005] According to the GJB 944A-2018 standard, TC4 steel plates for naval vessels require a room temperature yield strength Rp0.2 ≥ 830 MPa, a tensile strength Rm ≥ 895 MPa, and an impact energy KV2 ≥ 20 J. Although the standard does not specify the KV2 performance at -5℃, it is well known that the impact energy KV2 of close-packed hexagonal metals typically decreases significantly with decreasing temperature. To ensure that TC4 titanium alloy materials achieve KV2 ≥ 22 J at -5℃, the Al and O contents must be strictly controlled to improve impact toughness; however, reducing these alloying elements can easily lead to insufficient tensile strength. Furthermore, lamellar structures have higher impact toughness than equiaxed structures, but lower tensile fracture strain; therefore, a suitable microstructure is crucial for ensuring mechanical properties. Thus, achieving both room temperature Rp0.2 ≥ 830 MPa and KV2 ≥ 22 J at -5℃ simultaneously presents a significant challenge in the design and process control of titanium alloy materials. Currently, no publicly available information on large-size bars in this area is available, representing a major challenge for the industry.

[0006] 2) Difficulties in improving the microstructure uniformity and non-destructive testing quality of large-diameter bars:

[0007] Titanium alloys have poor thermal conductivity, high deformation resistance, and their microstructure is extremely sensitive to hot working parameters. During forging, defects such as uneven deformation, localized overheating, grain coarsening, and bright bands easily occur, affecting the uniformity of the microstructure and the final properties. For example, during the forging of large-diameter bars, the surface undergoes severe deformation at a lower temperature, easily forming fine equiaxed grains; while the core deforms less, reaches a higher temperature, and cools slowly, easily forming coarse Widmanstätten or basketweave structures. This inconsistent microstructure severely affects the uniformity of the overall mechanical properties. Research shows that clutter signals in ultrasonic testing are closely related to the material's microstructure. Existing large-diameter TC4 bars often suffer from problems such as large grain size, insufficient α-phase spheroidization, and uneven microstructure distribution, leading to increased ultrasonic clutter and making it difficult to meet the requirements of high-standard non-destructive testing.

[0008] 3) Challenges in macroscopic texture control and anisotropy control of large-size bars:

[0009] Alpha titanium, a close-packed hexagonal metal, exhibits significant anisotropy. During processes such as drawing, the grains preferentially orient along the deformation direction, forming a texture. This texture is even more difficult to eliminate in large-diameter bars, leading to significant differences in radial and axial properties such as yield strength, elongation, reduction of area, and impact energy, affecting their reliability in critical components. Currently, complex forging and deformation processes are typically employed to ensure the isotropy of large-diameter bars. A repeated upsetting and drawing process with "multiple forgings and large deformations" is commonly used to break down the as-cast microstructure, refine the grains, and homogenize the composition. However, in actual production, excessive forging can lead to temperature drops, thickened absorbent layers, and low efficiency. Therefore, careful design of the deformation amount, rate, and temperature path for each forging (e.g., within the α+β two-phase region or the β phase region) is necessary to reconcile the conflict between microstructure refinement and property homogenization.

[0010] In addition to the main problems mentioned above, there is also the issue of deformation penetration. Ensuring a sufficiently large single reduction so that the deformation can be effectively transferred to the core is also a core technological challenge in large-scale forging.

[0011] To address the above problems, existing technologies have made some attempts, as follows:

[0012] 1) Invention patent CN119702920A describes a free forging method for preparing large-diameter TC4 alloy bars of AAA non-destructive testing grade. TC4 alloy bars with diameters ranging from Φ160 to Φ260 mm were prepared through multiple upsetting and drawing processes. Ultrasonic testing achieved AAA grade, and the axial and tangential room temperature tensile strengths of the bars were relatively close. Furthermore, as the bar diameter increased from Φ180 to Φ250 mm, the grain size of the bar increased, and both the mechanical strength and elongation after fracture decreased. However, this patent failed to provide impact mechanical properties for the bars, especially failing to meet the KV2 (Charpy V-notch impact energy, a measure of a material's toughness at low temperatures, measured in Joules (J)) performance at -5℃.

[0013] 2) A method for preparing large-diameter Ti6Al4V titanium alloy bars (patent CN114888219A) combines a large forging ratio of 3.0–4.0 and a small forging ratio of 1.5–3.0 to produce bars with diameters ranging from Φ250 to Φ450 mm. As the bar diameter increases from Φ180 to Φ250 mm, the grain size of the bar increases, and both mechanical strength and elongation after fracture decrease. The bar microstructure shows a high content of equiaxed α phase and a low content of lamellar α phase. However, the patent's description of the bar's non-destructive testing quality and -5℃ testing quality fails to meet the KV2 performance requirements under -5℃ conditions.

[0014] There is an urgent need for a new method for preparing large-size bars of high-impact toughness TC4 titanium alloy that can be applied to A1-level non-destructive testing. Summary of the Invention

[0015] In summary, addressing the numerous problems existing in the preparation of large-sized TC4 titanium alloy bars with high impact toughness for A1-level non-destructive testing under current technologies, this invention provides a method for preparing large-sized TC4 titanium alloy bars with high impact toughness for A1-level non-destructive testing. This invention, through a special forging process (multi-directional repeated upsetting and drawing, "high-low" cyclic forging, etc.), produces large-sized TC4 titanium alloy bars with uniform, fine equiaxed grain structure and some α-lamellae structure, stably meeting the A1-level (or higher) non-destructive testing standard, ensuring zero internal defects in the material. Furthermore, this invention requires strict control of the content of alloying elements such as oxygen and aluminum during smelting, as well as control of the microstructure morphology, while simultaneously achieving Rp0.2≥830MPa at room temperature and KV2≥22J at -5℃.

[0016] The present invention discloses a method for preparing large-size TC4 titanium alloy bars with high impact toughness and A1-grade non-destructive testing, comprising the following steps:

[0017] S1, Preparation of TC4 titanium alloy VAR ingot, as detailed below:

[0018] First, a Φ560 non-finished ingot is obtained through a VAR melting process, and then a Φ660 finished ingot is obtained through a second VAR melting process. This second finished ingot is used as the TC4 titanium alloy VAR ingot.

[0019] The primary purpose of the first VAR melting is to initially melt and solidify the raw materials into a dense ingot. However, this ingot may contain component segregation, inclusions, or microscopic defects. The Φ560mm ingot obtained from this first melting is usually not used directly for the final component manufacturing but is used as electrode material for the second VAR melting. Therefore, it is called a "first-stage non-finished ingot." After processing (such as trimming and cleaning), the Φ560mm ingot obtained from the first VAR melting is placed into a larger-sized vacuum arc remelting furnace for the second melting. During the second melting process, by more precisely controlling the melting rate, current, and cooling conditions, segregation and defects in the first ingot can be further eliminated, and the ingot structure can be made more uniform and dense. The final Φ660mm ingot has better quality (such as purity, component uniformity, and low defect rate) and is therefore called a "second-stage finished ingot."

[0020] In this invention, based on the design composition of TC4 titanium alloy ingots, Grade 1 standard granular sponge titanium, aluminum wire, aluminum-vanadium master alloy (produced via a two-step process), titanium dioxide, and iron are used as raw materials. The sponge titanium and aluminum-vanadium master alloy are mixed and pressed into electrode blocks. Subsequently, the prepared electrode blocks are welded into electrodes using a vacuum plasma welding box. The electrodes are then placed in a vacuum consumable arc furnace for melting. The metallurgical quality of the ingot is controlled by controlling key parameters such as the vacuum degree and leakage rate in the furnace, the melting voltage (furnace voltage), the melting current (arc current), and the temperature and flow rate of the crucible outlet. To control the cost of the ingot bar, a Φ560mm primary non-finished ingot is first obtained through a single VAR melting process, and then a secondary VAR melting process is performed to obtain a Φ660mm secondary finished ingot.

[0021] S2, the TC4 titanium alloy VAR ingot obtained in step S1 is subjected to high-temperature β-zone forging, as follows:

[0022] S21) First, brush two coats of glass insulation coating onto the TC4 titanium alloy VAR ingot. Then, place it into an electric heating furnace at a furnace temperature of 300-400℃ for heating for 6-8 hours. Confirm that the temperature reaches 1100-1150℃ and hold it for 7-9 hours. Next, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70%, forming a square billet with a side length of 500-540mm and a length of 1-1.2m. Then, shape the billet to obtain the first bar billet.

[0023] S22) The first forged billet is returned to the furnace and held at 1070-1100℃ for 2-3 hours. Then, it is subjected to 2-3 lateral upsetting and drawing, with an upsetting deformation of 50-70%. After that, the billet is shaped to obtain the second billet.

[0024] S23) The forged second billet is returned to the furnace and held at 1040-1070℃ for 2-3 hours. Then, it is upsetting and drawing diagonally 2-3 times with a deformation of 50-70%. Then, it is shaped into a square billet to obtain the third billet. Then, it is air-cooled to room temperature and ground.

[0025] S3 involves upsetting and forging the TC4 alloy billet across the phase region, as detailed below:

[0026] S31) First, brush the third billet obtained in step S23) with two coats of glass insulation coating, then load the furnace at a temperature of 300-400℃ and heat it for 6-8 hours. Confirm that the temperature reaches 930-980℃ and keep it at that temperature for 7-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70%. After that, perform billet shaping to obtain the fourth billet.

[0027] S32) The forged fourth billet is returned to the furnace and heated for 6 to 8 hours. The temperature is confirmed to reach 1000 to 1030℃ and held for 4 to 5 hours. Then, it is upsetting and drawing diagonally 2 to 3 times with an upsetting deformation of 50 to 70%. Then, it is square billet shaping to obtain the fifth billet.

[0028] S33) The forged fifth billet is returned to the furnace and heated for 6 to 8 hours until the temperature reaches 930 to 980°C. It is then held for 2 to 3 hours and subjected to 2 to 3 axial upsetting and drawing processes with a deformation of 50 to 70%. The billet is then shaped and the edges are beveled using a saw to take samples. The α-phase → β-phase transformation temperature of the billet is then tested. The billet is then air-cooled to room temperature and ground to obtain the sixth billet.

[0029] S4 involves upsetting and forging the TC4 alloy billet in the two-phase region, as detailed below:

[0030] S41) First, brush the sixth billet obtained in step S33) with two coats of glass insulation coating, then load the furnace at a temperature of 300-400℃ and heat for 6-8 hours to confirm that the temperature reaches 15-25℃ below the β phase transformation point, i.e., Tβ-15℃ to Tβ-25℃, where Tβ is the α→β phase transformation temperature, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with a deformation of 50-70% to form a square billet with a side length of 480-520mm and a length of 1-1.3m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the seventh billet.

[0031] S42) First, brush the seventh billet with two coats of glass insulation coating, then load the furnace at a temperature of 300-400℃ and heat it for 6-8 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold it for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with a deformation of 50-70%, forming a square billet with a side length of 310-330mm and a length of 2.6-3.1m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the eighth billet.

[0032] S5. The TC4 alloy billet is forged into a round shape by shoveling, as follows:

[0033] First, brush the eighth billet obtained in step S42) with two coats of glass insulation coating, then load the furnace at a temperature of 300-400℃ and heat it for 6-8 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃-Tβ-30℃, and hold it for 6-9 hours. Then, the eighth billet is formed into a rod. Specifically, the edges and faces of the eighth billet in the length direction are deformed into circumferential surfaces to form a cylindrical rod.

[0034] S6. The TC4 alloy bar is subjected to bar machining and heat treatment to obtain the final TC4 alloy bar.

[0035] According to a method for preparing large-size A1 grade non-destructive testing high impact toughness TC4 titanium alloy bars, the billet shaping in steps S2 to S4 specifically involves using a 60MN or 80MN fast forging machine to press the four edges of the rectangular billet along its length into a plane, forming an octagonal billet at the end face, and then flattening the end face.

[0036] According to the present invention, in a method for preparing large-size TC4 titanium alloy bars with high impact toughness and A1 grade non-destructive testing, the Tβ phase transformation temperature in step S4 is 1000~1005℃.

[0037] According to a method for preparing large-size TC4 titanium alloy bars with high impact toughness and A1 grade non-destructive testing according to the present invention, in steps S2 to S3, chamfering is performed after upsetting to prevent cracking and folding during subsequent forging deformation.

[0038] According to a method for preparing large-size TC4 titanium alloy bars with high impact toughness and A1 grade non-destructive testing according to the present invention, step S6 involves machining and heat treating the TC4 alloy bars. Specifically, the TC4 alloy bars obtained in step S5 are heat treated by holding them at 750-820℃ for 2 hours, then air-cooled to room temperature, and the black skin on the surface of the bars is removed and the surface cracks are polished by machine tool processing.

[0039] The heat treatment in this step has a significant impact on the microstructure of large-diameter forged bars, which in turn affects their mechanical properties. This is especially true for forging large-diameter bars with diameters of 200–400 mm. Appropriate post-forging heat treatment is crucial for controlling the microstructure and mechanical properties, mainly as follows:

[0040] 1) Eliminate the inhomogeneity of internal and external structures and properties:

[0041] During forging, the significant difference in deformation between the surface and core of large-diameter bars can easily lead to coarse grains and insufficient microstructure fragmentation in the core, forming so-called "fuzzy crystals" or "clear crystals," resulting in uneven microstructure across the entire cross-section. Annealing heat treatment can promote the recovery and recrystallization of uneven microstructures (such as fragmented α-phase), thereby forming a uniform and refined equiaxed microstructure and ensuring the consistency of mechanical properties between the core and surface of the bar.

[0042] 2) Eliminate residual stress:

[0043] Large-size bars tend to accumulate high residual stress after forging, which reduces the toughness of the bar and may cause cracking or deformation during subsequent processing. Releasing and eliminating residual stress helps to ensure dimensional stability.

[0044] 3) Adjusting overall mechanical properties:

[0045] Annealing heat treatment can adjust the matching of strength, plasticity and impact toughness within a certain range, providing the most favorable "bulk" state for subsequent final heat treatment or direct use.

[0046] According to a method for preparing large-size TC4 titanium alloy bars with high impact toughness and A1-level non-destructive testing according to the present invention, the size of the TC4 alloy bars obtained in step S6 is Φ280~Φ330mm.

[0047] According to a method for preparing large-size TC4 titanium alloy bars with high impact toughness and A1-level non-destructive testing according to the present invention, the composition of the TC4 titanium alloy bars obtained in step S6, by mass percentage, is as follows:

[0048] Al: 6.35-6.45%, V: 4.25-4.35%, Fe: 0.12-0.18%, O: 0.12-0.15%, C≤0.08%, N≤0.03%, H≤0.012%, balance is Ti and unavoidable impurities, individual impurities ≤0.10%, total impurities ≤0.30%.

[0049] According to the present invention, a method for preparing large-diameter TC4 titanium alloy bars with high impact toughness and A1-level non-destructive testing is provided. The axial room temperature tensile mechanical properties of the TC4 alloy bars obtained in step S6 meet the following requirements: tensile strength ≥ 939 MPa, yield strength ≥ 860 MPa, elongation ≥ 16%, reduction of area ≥ 35%. Simultaneously, the axial room temperature impact energy KV2 of the TC4 alloy bars is ≥ 30 J, and the impact energy KV2 at -5℃ is ≥ 22 J. The water immersion testing of the bars is A1 level.

[0050] According to the present invention, a method for preparing large-diameter TC4 titanium alloy bars with high impact toughness and A1-level non-destructive testing is provided. The tangential room temperature tensile mechanical properties of the TC4 alloy bars obtained in step S6 meet the following requirements: tensile strength ≥ 961 MPa, yield strength ≥ 844 MPa, elongation ≥ 12%, reduction of area ≥ 40%. At the same time, the tangential room temperature impact energy KV2 of the TC4 alloy bars is ≥ 24 J, and the impact energy KV2 at -5℃ is ≥ 22 J. The water immersion testing of the bars is A1 level.

[0051] The following beneficial effects were obtained by using the method for preparing large-size TC4 titanium alloy bars with high impact toughness for A1-level non-destructive testing according to the present invention:

[0052] 1. The present invention provides a method for preparing large-size TC4 titanium alloy bars with high impact toughness and A1 grade non-destructive testing, which can significantly improve the microstructure uniformity of TC4 alloy bars. The resulting bar microstructure consists of primary equiaxed α phase and secondary lamellar α phase, and the average grain size reaches grade 1 to 3 of GB / T 5168 standard.

[0053] 2. The method for preparing large-size TC4 titanium alloy bars with high impact toughness for A1-level non-destructive testing according to the present invention produces TC4 bars with diameters of Φ280~Φ350mm. These bars can meet the requirements of A1-level ultrasonic testing, and at -5℃, both the axial and chordal impact energy is not less than 22J. The bars have a uniform microstructure and can obtain excellent mechanical properties after heat treatment, meeting the existing requirements for the use of TC4 bars.

[0054] 3. The TC4 titanium alloy large-size bar material prepared by the method of A1 grade non-destructive testing of the present invention can meet the stringent requirements of polar ships, cold-region marine engineering equipment and low-temperature service environment for the material's resistance to brittle fracture, and can promote the technological progress and equipment upgrade in the fields of deep-sea exploration, marine resource development and high-end ship manufacturing. Attached Figure Description

[0055] Figure 1 Axial low-magnification metallographic image of TC4 bar prepared by a method for preparing large-size TC4 titanium alloy bars with high impact toughness for A1-level non-destructive testing according to the present invention.

[0056] Figure 2 A high-magnification metallographic image of the TC4 bar prepared by the method for preparing large-size TC4 titanium alloy bars with high impact toughness according to the present invention (A1 grade non-destructive testing).

[0057] Figure 3 The transverse low-magnification metallographic image of the TC4 bar prepared by the method for preparing large-size TC4 titanium alloy bars with high impact toughness according to the present invention (A1 grade non-destructive testing).

[0058] Figure 4 The image shows a high-magnification metallographic image of the transverse microstructure of a TC4 titanium alloy large-size bar prepared by a method for preparing A1-level non-destructive testing high-impact toughness TC4 titanium alloy bars according to the present invention. Detailed Implementation

[0059] The following description, in conjunction with the accompanying drawings and embodiments, further describes the technical means, creative features, achieved objectives, and effects of a method for preparing large-size A1-grade non-destructive testing high-impact toughness TC4 titanium alloy bars according to the present invention. The described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0060] Example 1:

[0061] S1, Preparation of TC4 titanium alloy VAR ingot, as detailed below:

[0062] First, a Φ560 non-finished ingot is obtained through a VAR melting process, and then a Φ660 finished ingot is obtained through a second VAR melting process. This second finished ingot is used as the TC4 titanium alloy VAR ingot.

[0063] S2, the TC4 titanium alloy VAR ingot obtained in step S1 is subjected to high-temperature β-zone forging, as follows:

[0064] S21) First, brush two coats of glass insulation coating on the TC4 titanium alloy VAR ingot, then put it into an electric heating furnace at a furnace temperature of 350℃ for 6.5 hours to confirm that the temperature reaches 1100~1150℃, and hold it for 7~9 hours. Then, perform 2~3 axial upsetting and drawing, with upsetting deformation of 50~70%, to form a square billet with a cross-sectional side length of 500~540mm and a cuboid shape with a length of 1~1.2m. Then, shape the billet to obtain the first bar billet.

[0065] S22) The first forged billet is returned to the furnace and held at 1070-1100℃ for 2-3 hours. Then, it is subjected to 2-3 lateral upsetting and drawing, with an upsetting deformation of 50-70%. After that, the billet is shaped to obtain the second billet.

[0066] S23) The forged second billet is returned to the furnace and held at 1040-1070℃ for 2-3 hours. Then, it is upsetting and drawing diagonally 2-3 times with a deformation of 50-70%. Then, it is shaped into a square billet to obtain the third billet. Then, it is air-cooled to room temperature and ground.

[0067] S3 involves upsetting and forging the TC4 alloy billet across the phase region, as detailed below:

[0068] S31) First, brush the third billet obtained in step S23) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 930-980℃ and hold for 7-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70%. After that, perform billet shaping to obtain the fourth billet.

[0069] S32) The forged fourth billet is returned to the furnace and heated for 6 to 8 hours. The temperature is confirmed to reach 1000 to 1030℃ and held for 4 to 5 hours. Then, it is upsetting and drawing diagonally 2 to 3 times with an upsetting deformation of 50 to 70%. Then, it is square billet shaping to obtain the fifth billet.

[0070] S33) The forged fifth billet is returned to the furnace and heated for 6 to 8 hours until the temperature reaches 930 to 980°C. It is then held for 2 to 3 hours and subjected to 2 to 3 axial upsetting and drawing processes with a deformation of 50 to 70%. The billet is then shaped and the edges are beveled using a saw to take samples. The α-phase → β-phase transformation temperature of the billet is then tested. The billet is then air-cooled to room temperature and ground to obtain the sixth billet.

[0071] S4 involves upsetting and forging the TC4 alloy billet in the two-phase region, as detailed below:

[0072] S41) First, brush the sixth billet obtained in step S33) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours to confirm that the temperature reaches 15-25℃ below the β phase transformation point, i.e., Tβ-15℃ to Tβ-25℃, where Tβ is the α→β phase transformation temperature, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70% to form a square billet with a cross-sectional side length of 480-520mm and a cuboid shape with a length of 1-1.3m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the seventh billet.

[0073] S42) First, brush the seventh billet with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with a deformation of 50-70%, forming a square billet with a side length of 310-330mm and a length of 2.6-3.1m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the eighth billet.

[0074] S5. The TC4 alloy billet is forged into a round shape by shoveling, as follows:

[0075] First, brush the eighth billet obtained in step S42) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold for 6-9 hours. Then, the eighth billet is formed into a rod. Specifically, the edges and faces of the eighth billet in the length direction are deformed into circumferential surfaces to form a cylindrical rod.

[0076] S6. Perform bar machining and heat treatment on the TC4 alloy bars, as follows:

[0077] In this step, the TC4 alloy bar obtained in step S5 is heat-treated by holding it at 820℃ for 2 hours, then air-cooled to room temperature, and the black skin on the surface of the bar is removed and the surface cracks are polished by machine tool processing.

[0078] The TC4 alloy bars prepared in Example 1 were subjected to mechanical property testing and non-destructive testing. The results are as follows:

[0079] According to GB / T 228.1-2021, the axial and tangential tensile mechanical properties of large-diameter TC4 titanium alloy bars were tested. According to GB / T 229-2020, the -5°C impact performance of large-diameter TC4 titanium alloy bars was tested. The water immersion test results for the bars were A1 grade. Specific results are shown in Table 1 below:

[0080]

[0081] Table 1 - Room temperature tensile properties and -5°C impact energy of large-diameter TC4 titanium alloy bars (Example 1)

[0082] Example 2:

[0083] S1, Preparation of TC4 titanium alloy VAR ingot, as detailed below:

[0084] First, a Φ560 non-finished ingot is obtained through a VAR melting process, and then a Φ660 finished ingot is obtained through a second VAR melting process. This second finished ingot is used as the TC4 titanium alloy VAR ingot.

[0085] S2, the TC4 titanium alloy VAR ingot obtained in step S1 is subjected to high-temperature β-zone forging, as follows:

[0086] S21) First, brush two coats of glass insulation coating on the TC4 titanium alloy VAR ingot, then put it into an electric heating furnace at a furnace temperature of 350℃ for 6.5 hours to confirm that the temperature reaches 1100~1150℃, and hold it for 7~9 hours. Then, perform 2~3 axial upsetting and drawing, with upsetting deformation of 50~70%, to form a square billet with a cross-sectional side length of 500~540mm and a cuboid shape with a length of 1~1.2m. Then, shape the billet to obtain the first bar billet.

[0087] S22) The first forged billet is returned to the furnace and held at 1070-1100℃ for 2-3 hours. Then, it is subjected to 2-3 lateral upsetting and drawing, with an upsetting deformation of 50-70%. After that, the billet is shaped to obtain the second billet.

[0088] S23) The forged second billet is returned to the furnace and held at 1040-1070℃ for 2-3 hours. Then, it is upsetting and drawing diagonally 2-3 times with a deformation of 50-70%. Then, it is shaped into a square billet to obtain the third billet. Then, it is air-cooled to room temperature and ground.

[0089] S3 involves upsetting and forging the TC4 alloy billet across the phase region, as detailed below:

[0090] S31) First, brush the third billet obtained in step S23) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 930-980℃ and hold for 7-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70%. After that, perform billet shaping to obtain the fourth billet.

[0091] S32) The forged fourth billet is returned to the furnace and heated for 6 to 8 hours. The temperature is confirmed to reach 1000 to 1030℃ and held for 4 to 5 hours. Then, it is upsetting and drawing diagonally 2 to 3 times with an upsetting deformation of 50 to 70%. Then, it is square billet shaping to obtain the fifth billet.

[0092] S33) The forged fifth billet is returned to the furnace and heated for 6 to 8 hours until the temperature reaches 930 to 980°C. It is then held for 2 to 3 hours and subjected to 2 to 3 axial upsetting and drawing processes with a deformation of 50 to 70%. The billet is then shaped and the edges are beveled using a saw to take samples. The α-phase → β-phase transformation temperature of the billet is then tested. The billet is then air-cooled to room temperature and ground to obtain the sixth billet.

[0093] S4 involves upsetting and forging the TC4 alloy billet in the two-phase region, as detailed below:

[0094] S41) First, brush the sixth billet obtained in step S33) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours to confirm that the temperature reaches 15-25℃ below the β phase transformation point, i.e., Tβ-15℃ to Tβ-25℃, where Tβ is the α→β phase transformation temperature, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70% to form a square billet with a cross-sectional side length of 480-520mm and a cuboid shape with a length of 1-1.3m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the seventh billet.

[0095] S42) First, brush the seventh billet with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with a deformation of 50-70%, forming a square billet with a side length of 310-330mm and a length of 2.6-3.1m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the eighth billet.

[0096] S5. The TC4 alloy billet is forged into a round shape by shoveling, as follows:

[0097] First, brush the eighth billet obtained in step S42) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold for 6-9 hours. Then, the eighth billet is formed into a rod. Specifically, the edges and faces of the eighth billet in the length direction are deformed into circumferential surfaces to form a cylindrical rod.

[0098] S6. Perform bar machining and heat treatment on the TC4 alloy bars, as follows:

[0099] In this step, the TC4 alloy bar obtained in step S5 is heat-treated by holding it at 780℃ for 2 hours, then air-cooled to room temperature, and the black skin on the surface of the bar is removed and the surface cracks are polished by machine tool processing.

[0100] The TC4 alloy bars prepared in Example 2 were subjected to mechanical property testing and non-destructive testing. The results are as follows:

[0101] According to GB / T 228.1-2021, the axial and tangential tensile mechanical properties of large-diameter TC4 titanium alloy bars were tested. According to GB / T 229-2020, the -5°C impact performance of large-diameter TC4 titanium alloy bars was tested. The water immersion test results for the bars were A1 grade. Specific results are shown in Table 2 below:

[0102]

[0103] Table 2 - Room temperature tensile properties and -5°C impact energy of large-diameter TC4 titanium alloy bars (Example 2)

[0104] As can be seen from Examples 1 and 2 above, the preparation method of A1-grade non-destructive testing high-impact toughness TC4 titanium alloy large-size bars of the present invention can significantly improve the microstructure uniformity of TC4 alloy bars, with the average grain size reaching level 1 to 3 of GB / T 5168 standard. Moreover, the Φ280-Φ350mm TC4 bars prepared by the present invention can meet the A1-grade ultrasonic testing requirements, with axial and tangential impact energy not less than 22J at -5℃. Furthermore, its microstructure is uniform, and excellent mechanical properties can be obtained after heat treatment, meeting the existing requirements for the use of TC4 bars. The TC4 bars prepared by the present invention can meet the stringent requirements for the material's resistance to brittle fracture in polar ships, cold-region marine engineering equipment, and low-temperature service environments, and can promote technological progress and equipment upgrades in fields such as deep-sea exploration, marine resource development, and high-end ship manufacturing.

[0105] Example 3:

[0106] S1, Preparation of TC4 titanium alloy VAR ingot, as detailed below:

[0107] First, a Φ560 non-finished ingot is obtained through a VAR melting process, and then a Φ660 finished ingot is obtained through a second VAR melting process. This second finished ingot is used as the TC4 titanium alloy VAR ingot.

[0108] S2, the TC4 titanium alloy VAR ingot obtained in step S1 is subjected to high-temperature β-zone forging, as follows:

[0109] S21) First, brush two coats of glass insulation coating on the TC4 titanium alloy VAR ingot, then put it into an electric heating furnace at a furnace temperature of 350℃ for 6.5 hours to confirm that the temperature reaches 1100~1150℃, and hold it for 7~9 hours. Then, perform 2~3 axial upsetting and drawing, with upsetting deformation of 50~70%, to form a square billet with a cross-sectional side length of 500~540mm and a cuboid shape with a length of 1~1.2m. Then, shape the billet to obtain the first bar billet.

[0110] S22) The first forged billet is returned to the furnace and held at 1070-1100℃ for 2-3 hours. Then, it is subjected to 2-3 lateral upsetting and drawing, with an upsetting deformation of 50-70%. After that, the billet is shaped to obtain the second billet.

[0111] S23) The forged second billet is returned to the furnace and held at 1040-1070℃ for 2-3 hours. Then, it is upsetting and drawing diagonally 2-3 times with a deformation of 50-70%. Then, it is shaped into a square billet to obtain the third billet. Then, it is air-cooled to room temperature and ground.

[0112] S3 involves upsetting and forging the TC4 alloy billet across the phase region, as detailed below:

[0113] S31) First, brush the third billet obtained in step S23) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 930-980℃ and hold for 7-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70%. After that, perform billet shaping to obtain the fourth billet.

[0114] S32) The forged fourth billet is returned to the furnace and heated for 6 to 8 hours. The temperature is confirmed to reach 1000 to 1030℃ and held for 4 to 5 hours. Then, it is upsetting and drawing diagonally 2 to 3 times with an upsetting deformation of 50 to 70%. Then, it is square billet shaping to obtain the fifth billet.

[0115] S33) The forged fifth billet is returned to the furnace and heated for 6 to 8 hours until the temperature reaches 930 to 980°C. It is then held for 2 to 3 hours and subjected to 2 to 3 axial upsetting and drawing processes with a deformation of 50 to 70%. The billet is then shaped and the edges are beveled using a saw to take samples. The α-phase → β-phase transformation temperature of the billet is then tested. The billet is then air-cooled to room temperature and ground to obtain the sixth billet.

[0116] S4 involves upsetting and forging the TC4 alloy billet in the two-phase region, as detailed below:

[0117] S41) First, brush the sixth billet obtained in step S33) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours to confirm that the temperature reaches 15-25℃ below the β phase transformation point, i.e., Tβ-15℃ to Tβ-25℃, where Tβ is the α→β phase transformation temperature, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70% to form a square billet with a cross-sectional side length of 480-520mm and a cuboid shape with a length of 1-1.3m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the seventh billet.

[0118] S42) First, brush the seventh billet with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with a deformation of 50-70%, forming a square billet with a side length of 310-330mm and a length of 2.6-3.1m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the eighth billet.

[0119] S5. The TC4 alloy billet is forged into a round shape by shoveling, as follows:

[0120] First, brush the eighth billet obtained in step S42) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold for 6-9 hours. Then, the eighth billet is formed into a rod. Specifically, the edges and faces of the eighth billet in the length direction are deformed into circumferential surfaces to form a cylindrical rod.

[0121] S6. Perform bar machining and heat treatment on the TC4 alloy bars, as follows:

[0122] In this step, the TC4 alloy bar obtained in step S5 is heat-treated by holding it at 700℃ for 2 hours, then air-cooled to room temperature, and the black skin on the surface of the bar is removed and the surface cracks are polished by machine tool processing.

[0123] The difference between Example 3 and Examples 1 and 2 is that the heat treatment temperature after bar forging (780℃ and 820℃) is different. Example 3 underwent heat treatment at 700℃. The mechanical properties and non-destructive testing of the TC4 alloy bar produced in Example 3 were performed, and the results are as follows:

[0124] According to GB / T 228.1-2021, the axial and tangential tensile mechanical properties of large-diameter TC4 titanium alloy bars were tested. According to GB / T 229-2020, the -5°C impact performance of large-diameter TC4 titanium alloy bars was tested. The water immersion test results for the bars were A1 grade. Specific results are shown in Table 3 below:

[0125]

[0126] Table 3 - Room temperature tensile properties and -5°C impact energy of large-diameter TC4 titanium alloy bars (Example 3)

[0127] Example 4:

[0128] S1, Preparation of TC4 titanium alloy VAR ingot, as detailed below:

[0129] First, a Φ560 non-finished ingot is obtained through a VAR melting process, and then a Φ660 finished ingot is obtained through a second VAR melting process. This second finished ingot is used as the TC4 titanium alloy VAR ingot.

[0130] S2, the TC4 titanium alloy VAR ingot obtained in step S1 is subjected to high-temperature β-zone forging, as follows:

[0131] S21) First, brush two coats of glass insulation coating on the TC4 titanium alloy VAR ingot, then put it into an electric heating furnace at a furnace temperature of 350℃ for 6.5 hours to confirm that the temperature reaches 1100~1150℃, and hold it for 7~9 hours. Then, perform 2~3 axial upsetting and drawing, with upsetting deformation of 50~70%, to form a square billet with a cross-sectional side length of 500~540mm and a cuboid shape with a length of 1~1.2m. Then, shape the billet to obtain the first bar billet.

[0132] S22) The first forged billet is returned to the furnace and held at 1070-1100℃ for 2-3 hours. Then, it is subjected to 2-3 lateral upsetting and drawing, with an upsetting deformation of 50-70%. After that, the billet is shaped to obtain the second billet.

[0133] S23) The forged second billet is returned to the furnace and held at 1040-1070℃ for 2-3 hours. Then, it is upsetting and drawing diagonally 2-3 times with a deformation of 50-70%. Then, it is shaped into a square billet to obtain the third billet. Then, it is air-cooled to room temperature and ground.

[0134] S3 involves upsetting and forging the TC4 alloy billet across the phase region, as detailed below:

[0135] S31) First, brush the third billet obtained in step S23) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 930-980℃ and hold for 7-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70%. After that, perform billet shaping to obtain the fourth billet.

[0136] S32) The forged fourth billet is returned to the furnace and heated for 6 to 8 hours. The temperature is confirmed to reach 1000 to 1030℃ and held for 4 to 5 hours. Then, it is upsetting and drawing diagonally 2 to 3 times with an upsetting deformation of 50 to 70%. Then, it is square billet shaping to obtain the fifth billet.

[0137] S33) The forged fifth billet is returned to the furnace and heated for 6 to 8 hours until the temperature reaches 930 to 980°C. It is then held for 2 to 3 hours and subjected to 2 to 3 axial upsetting and drawing processes with a deformation of 50 to 70%. The billet is then shaped and the edges are beveled using a saw to take samples. The α-phase → β-phase transformation temperature of the billet is then tested. The billet is then air-cooled to room temperature and ground to obtain the sixth billet.

[0138] S4 involves upsetting and forging the TC4 alloy billet in the two-phase region, as detailed below:

[0139] S41) First, brush the sixth billet obtained in step S33) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours to confirm that the temperature reaches 15-25℃ below the β phase transformation point, i.e., Tβ-15℃ to Tβ-25℃, where Tβ is the α→β phase transformation temperature, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70% to form a square billet with a cross-sectional side length of 480-520mm and a cuboid shape with a length of 1-1.3m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the seventh billet.

[0140] S42) First, brush the seventh billet with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with a deformation of 50-70%, forming a square billet with a side length of 310-330mm and a length of 2.6-3.1m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the eighth billet.

[0141] S5. The TC4 alloy billet is forged into a round shape by shoveling, as follows:

[0142] First, brush the eighth billet obtained in step S42) with two coats of glass insulation coating, then load the furnace at 350℃ and heat for 6.5 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold for 6-9 hours. Then, the eighth billet is formed into a rod. Specifically, the edges and faces of the eighth billet in the length direction are deformed into circumferential surfaces to form a cylindrical rod.

[0143] S6. Perform bar machining on the TC4 alloy bar, as follows:

[0144] In this step, after the TC4 alloy bar obtained in step 5) is cooled to room temperature, the black skin on the surface of the bar is removed and the surface cracks are polished by machine tool processing.

[0145] The difference between Example 4 and Examples 1, 2, and 3 is that the heat treatment temperatures (780℃, 820℃, and 700℃) after bar forging are different. In Example 4, the bar is directly cooled to room temperature. Mechanical properties and non-destructive testing were performed on the TC4 alloy bar produced in Example 4, and the results are as follows:

[0146] According to GB / T 228.1-2021, the axial and tangential tensile mechanical properties of large-diameter TC4 titanium alloy bars were tested. According to GB / T 229-2020, the -5°C impact performance of large-diameter TC4 titanium alloy bars was tested. The water immersion test results for the bars were A1 grade. Specific results are shown in Table 4 below:

[0147]

[0148] Table 4 - Room temperature tensile properties and -5°C impact energy of large-diameter TC4 titanium alloy bars (Example 4)

[0149] As can be seen from the room temperature tensile properties and -5°C impact energy of Examples 3 and 4 above (Tables 3 and 4), the method for preparing large-size TC4 titanium alloy bars with high impact toughness and A1 grade non-destructive testing according to the present invention differs in the final heat treatment process of the bars. Although it meets the standard of A1 grade for water immersion testing of bars, its -5°C impact energy KV2 has changed and is lower than 22J.

[0150] As can be seen from Examples 1, 2, 3, and 4 above, the annealing heat treatment temperature has a significant impact on the microstructure and properties of large-size titanium alloy bars. The specific differences are as follows:

[0151] 1) When annealing heat treatment was performed at 780℃ and 820℃ in Examples 1 and 2 respectively, the recrystallization of the forging deformation structure was more complete. At the same time, the proportion of β phase increased significantly at high temperature (α→β phase change was more complete), thus the impact toughness was significantly improved.

[0152] 2) In Examples 3 and 4, when annealing heat treatment was performed at 700℃ (or even lower temperatures) and cooled to room temperature, the microstructure evolution was mainly recovery. The deformed grains began to recover, but recrystallization was insufficient. A large number of deformation characteristics were still retained in the microstructure. At the same time, the primary α phase content was high, the secondary α phase precipitation was less, the microstructure was uneven, and stress concentration points were easily formed at the α / β interface. When subjected to impact load, the crack propagation path was relatively straight, resulting in low impact toughness.

[0153] In summary, for large-diameter TC4 bars, annealing heat treatment can homogenize the microstructure and promote the recovery and recrystallization of the forging deformation microstructure, eliminating residual stress. As the heat treatment temperature increases, recrystallization and α→β phase transformation become more complete, and the final microstructure evolves from "fine and non-uniform" to "larger, more uniform, and lamellar." Macroscopically, this manifests as a slight decrease in strength, but a significant improvement in impact toughness and microstructure uniformity.

[0154] This invention discloses a method for preparing large-size TC4 titanium alloy bars with high impact toughness for A1-level non-destructive testing. This method significantly improves the microstructure uniformity of the TC4 alloy bars, resulting in a microstructure composed of primary equiaxed α phase and secondary lamellar α phase, with an average grain size reaching grades 1-3 of GB / T 5168. Furthermore, the TC4 bars with diameters of Φ280-Φ350mm prepared by this invention can meet A1-level ultrasonic testing requirements, and at -5℃, both axial and tangential impact energy are not less than 22J. Their microstructure is uniform, and after heat treatment, they can achieve excellent mechanical properties, meeting the existing requirements for TC4 bars. In addition, the TC4 bars prepared by this invention can meet the stringent requirements for brittle fracture resistance in polar vessels, cold-region marine engineering equipment, and low-temperature service environments, promoting technological progress and equipment upgrades in fields such as deep-sea exploration, marine resource development, and high-end shipbuilding.

[0155] In the description of this invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0156] Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0157] Meanwhile, those skilled in the art should recognize that the above embodiments are only used to illustrate this application and are not intended to limit this application. Any changes or modifications to the above embodiments within the spirit and essence of this application will fall within the scope of the claims of this application.

Claims

1. A method for preparing large-diameter TC4 titanium alloy bars with high impact toughness and A1-grade non-destructive testing, characterized in that, The method includes the following steps: S1, Preparation of TC4 titanium alloy VAR ingot; S2, the TC4 titanium alloy VAR ingot obtained in step S1 is subjected to high-temperature β-zone forging, as follows: S21) First, brush two coats of glass insulation coating onto the TC4 titanium alloy VAR ingot. Then, place it into an electric heating furnace at a furnace temperature of 300-400℃ for heating for 6-8 hours. Confirm that the temperature reaches 1100-1150℃ and hold it for 7-9 hours. Next, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70%, forming a square billet with a side length of 500-540mm and a length of 1-1.2m. Then, shape the billet to obtain the first bar billet. S22) The first forged billet is returned to the furnace and held at 1070-1100℃ for 2-3 hours. Then, it is subjected to 2-3 lateral upsetting and drawing, with an upsetting deformation of 50-70%. After that, the billet is shaped to obtain the second billet. S23) The forged second billet is returned to the furnace and held at 1040-1070℃ for 2-3 hours. Then, it is upsetting and drawing diagonally 2-3 times with a deformation of 50-70%. Then, it is shaped into a square billet to obtain the third billet. Then, it is air-cooled to room temperature and ground. S3 involves upsetting and forging the TC4 alloy billet across the phase region, as detailed below: S31) First, brush the third billet obtained in step S23) with two coats of glass insulation coating, then load the furnace at a temperature of 300-400℃ and heat it for 6-8 hours. Confirm that the temperature reaches 930-980℃ and keep it at that temperature for 7-9 hours. Then, perform 2-3 axial upsetting and drawing operations with an upsetting deformation of 50-70%. After that, perform billet shaping to obtain the fourth billet. S32) The forged fourth billet is returned to the furnace and heated for 6 to 8 hours. The temperature is confirmed to reach 1000 to 1030℃ and held for 4 to 5 hours. Then, it is upsetting and drawing diagonally 2 to 3 times with an upsetting deformation of 50 to 70%. Then, it is square billet shaping to obtain the fifth billet. S33) The forged fifth billet is returned to the furnace and heated for 6 to 8 hours until the temperature reaches 930 to 980°C. It is then held for 2 to 3 hours and subjected to 2 to 3 axial upsetting and drawing processes with a deformation of 50 to 70%. The billet is then shaped and the edges are beveled using a saw to take samples. The α-phase → β-phase transformation temperature of the billet is then tested. The billet is then air-cooled to room temperature and ground to obtain the sixth billet. S4 involves upsetting and forging the TC4 alloy billet in the two-phase region, as detailed below: S41) First, brush the sixth billet obtained in step S33) with two coats of glass insulation coating, then load the furnace at a temperature of 300-400℃ and heat for 6-8 hours to confirm that the temperature reaches 15-25℃ below the β phase transformation point, i.e., Tβ-15℃ to Tβ-25℃, where Tβ is the α→β phase transformation temperature, and hold for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with a deformation of 50-70% to form a square billet with a side length of 480-520mm and a length of 1-1.3m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the seventh billet. S42) First, brush the seventh billet with two coats of glass insulation coating, then load the furnace at a temperature of 300-400℃ and heat it for 6-8 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃ to Tβ-30℃, and hold it for 6-9 hours. Then, perform 2-3 axial upsetting and drawing operations with a deformation of 50-70%, forming a square billet with a side length of 310-330mm and a length of 2.6-3.1m. The billet is then shaped, air-cooled to room temperature, and ground to obtain the eighth billet. S5. The TC4 alloy billet is forged into a round shape by shoveling, as follows: First, brush the eighth billet obtained in step S42) with two coats of glass insulation coating, then load the furnace at a temperature of 300-400℃ and heat it for 6-8 hours. Confirm that the temperature reaches 20-30℃ below the β phase transformation point, i.e., Tβ-20℃-Tβ-30℃, and hold it for 6-9 hours. Then, the eighth billet is formed into a rod. Specifically, the edges and faces of the eighth billet in the length direction are deformed into circumferential surfaces to form a cylindrical rod. S6. The TC4 alloy bar is subjected to bar machining and heat treatment to obtain the final TC4 alloy bar.

2. The method for preparing large-size A1-grade non-destructive testing high-impact toughness TC4 titanium alloy bars as described in claim 1, characterized in that, The preparation of TC4 titanium alloy VAR ingots is specifically as follows: first, a Φ560 non-finished ingot is obtained through a first VAR melting process, and then a Φ660 second finished ingot is obtained through a second VAR melting process. This second finished ingot is used as the TC4 titanium alloy VAR ingot.

3. The method for preparing large-size A1-grade non-destructive testing high-impact toughness TC4 titanium alloy bars as described in claim 1, characterized in that, The billet shaping in steps S2 to S4 specifically involves using a 60MN or 80MN high-speed forging machine to press the four edges of the rectangular billet along its length into a plane, forming an octagonal billet at the end, and then flattening the end face.

4. The method for preparing large-size A1-grade non-destructive testing high-impact toughness TC4 titanium alloy bars as described in claim 1, characterized in that, In step S4, the Tβ phase transition temperature is 1000–1005°C.

5. The method for preparing large-size A1-grade non-destructive testing high-impact toughness TC4 titanium alloy bars as described in claim 1, characterized in that, In steps S2 to S3, chamfering is performed after the corner is upset to prevent cracking and folding during subsequent forging deformation.

6. The method for preparing large-size A1 grade non-destructive testing high-impact toughness TC4 titanium alloy bars as described in claim 1, characterized in that, Step S6 involves machining and heat treating the TC4 alloy bar. Specifically, the TC4 alloy bar obtained in step S5 is heat treated at 750-820℃ for 2 hours, then air-cooled to room temperature, and the black skin on the surface of the bar is removed and the surface cracks are polished by machine tool processing.

7. The method for preparing large-size A1-grade non-destructive testing high-impact toughness TC4 titanium alloy bars as described in claim 1, characterized in that, The TC4 alloy rod obtained in step S6 has a size of Φ280~Φ330mm.

8. The method for preparing large-size A1 grade non-destructive testing high-impact toughness TC4 titanium alloy bars as described in claim 1, characterized in that, The composition of the TC4 titanium alloy bar obtained in step S6 is as follows by mass percentage: Al: 6.35-6.45%, V: 4.25-4.35%, Fe: 0.12-0.18%, O: 0.12-0.15%, C≤0.08%, N≤0.03%, H≤0.012%, with the balance being Ti and unavoidable impurities, with individual impurities ≤0.10% and total impurities ≤0.30%.

9. The method for preparing large-size A1 grade non-destructive testing high-impact toughness TC4 titanium alloy bars as described in claim 1, characterized in that, The axial room temperature tensile mechanical properties of the TC4 alloy bar obtained in step S6 meet the following requirements: tensile strength ≥939MPa, yield strength ≥860MPa, elongation at section ≥16%, reduction of area ≥35%. At the same time, the axial room temperature impact energy KV2 of the TC4 alloy bar is ≥30J, and the impact energy KV2 at -5℃ is ≥22J. The bar is classified as A1 grade in water immersion testing.

10. The method for preparing large-size A1-grade non-destructive testing high-impact toughness TC4 titanium alloy bars as described in claim 1, characterized in that, The tangential room temperature tensile mechanical properties of the TC4 alloy bar obtained in step S6 meet the following requirements: tensile strength ≥ 961 MPa, yield strength ≥ 844 MPa, elongation at section ≥ 12%, reduction of area ≥ 40%. At the same time, the tangential room temperature impact energy KV2 of the TC4 alloy bar is ≥ 24 J, and the impact energy KV2 at -5℃ is ≥ 22 J. The bar is classified as A1 grade in water immersion testing.

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