TC4 titanium alloy and method for improving low-temperature impact toughness thereof and application

Abstract

This invention discloses a method and application for improving the low-temperature impact toughness of TC4 titanium alloy, belonging to the field of titanium alloy material preparation technology. The method of this invention sequentially employs electron beam cold hearth melting and vacuum consumable arc melting to prepare TC4 ingots with an oxygen content of 0.11-0.13%. The TC4 ingots are then annealed at the α+β two-phase region temperature, followed by holding and cooling to obtain a TC4 titanium alloy with an equiaxed microstructure. This invention overcomes the defects of existing TC4 titanium alloys, such as decreased impact toughness and brittle fracture at low temperatures, by precisely controlling the oxygen content within a narrow range of 0.11%-0.13% and combining α+β two-phase region annealing to form an equiaxed microstructure. This improves the low-temperature impact toughness of TC4 titanium alloys and makes them suitable for manufacturing high-reliability components for aerospace, polar equipment, and other low-temperature service environments.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy material preparation technology, specifically to a TC4 titanium alloy and its method and application for improving low-temperature impact toughness. Background Technology

[0002] TC4 titanium alloy, a typical α+β type titanium alloy, is widely used in aerospace, medical devices, and marine engineering due to its high strength, excellent corrosion resistance, and machinability. However, conventional TC4 titanium alloy exhibits a significant decrease in impact toughness at low temperatures, making it prone to brittle fracture and severely impacting the safety and reliability of components under cryogenic conditions. Current technologies typically aim to improve the toughness of TC4 by optimizing heat treatment processes or adjusting alloy composition, but the effects are limited.

[0003] Therefore, how to solve the problem of low-temperature brittleness of TC4 titanium alloy has become a technical challenge that urgently needs to be overcome by those skilled in the art. Summary of the Invention

[0004] The purpose of this invention is to provide a TC4 titanium alloy and its method and application for improving low-temperature impact toughness, so as to overcome the problem of insufficient low-temperature impact toughness of conventional TC4 titanium alloy in the prior art.

[0005] The present invention solves the above-mentioned technical problems through the following technical solution: This invention provides a method for improving the low-temperature impact toughness of TC4 titanium alloy, comprising the following steps: TC4 ingots with an oxygen content of 0.11~0.13% were prepared by sequentially using electron beam cold bed melting and vacuum consumable arc melting. The TC4 ingot is annealed at a temperature in the α+β two-phase region, and then held and cooled sequentially to obtain a TC4 titanium alloy with an equiaxed microstructure.

[0006] A further improvement of this invention lies in the sequential use of electron beam cold bed melting and vacuum consumable arc melting to prepare TC4 ingots with an oxygen content of 0.11~0.13%, specifically including the following steps: The furnace charge is added to an electron beam cooling bed for melting to obtain the first smelting product. The furnace charge includes sponge titanium, aluminum-vanadium alloy, metallic aluminum briquettes, and TC4 recycled material. The mass of TC4 recycled material accounts for 20% to 40% of the total mass of the furnace charge. The first smelting product was subjected to vacuum arc melting to obtain TC4 ingots with an oxygen content of 0.11~0.13%.

[0007] A further improvement of this invention is that the process parameters for electron beam cold hearth melting are set to a melting power of 250~280kW and a melting speed of 70~140 kg·h. - ¹, Vacuum degree 3.0×10 - ²~2.0×10 - 2 Pa.

[0008] A further improvement of this invention is that the process parameters for vacuum consumable arc melting are set to a vacuum degree ≤ 1.0 × 10⁻⁶. - ¹ Pa, smelting current 10~20 kA, smelting voltage 25~35 V.

[0009] A further improvement of this invention is that the temperature range of the α+β two-phase region is 780℃±2℃.

[0010] A further improvement of this invention is that air cooling is used as the cooling method.

[0011] A further improvement of this invention is that the heat preservation time is 2 hours.

[0012] The present invention also provides a TC4 titanium alloy, which is prepared by the method described above for improving the low-temperature impact toughness of TC4 titanium alloy.

[0013] A further improvement of the present invention is that the average impact energy of the prepared TC4 titanium alloy at room temperature is not less than 58.2 J, and the average impact energy at -196℃ is not less than 22.8 J.

[0014] The present invention also provides an application of the TC4 titanium alloy as described above in key load-bearing structural components of aerospace, shipbuilding, or chemical equipment.

[0015] Compared with the prior art, the positive and progressive effects of the present invention are as follows: The method for improving the low-temperature impact toughness of TC4 titanium alloy provided by this invention reduces the weakening effect of interstitial atoms on grain boundaries by precisely controlling the oxygen content of TC4 ingots within the optimal range of 0.11%~0.13wt%, thereby suppressing the generation of low-temperature brittleness from the intrinsic material properties level. Simultaneously, it avoids a significant decrease in alloy strength caused by excessive oxygen removal, achieving a good match between strength and low-temperature impact toughness. The method employs a combined process of electron beam cold hearth melting (EB) and vacuum arc remelting (VAR), where electron beam cold hearth melting effectively removes harmful interstitial atoms such as oxygen and nitrogen. The absence of interstitial elements and the use of vacuum arc melting ensure the uniformity and density of the ingot composition. For low-oxygen composition systems of 0.11%~0.13wt%, a low-temperature annealing process in the α+β two-phase region is employed to obtain a uniform and fine equiaxed microstructure in the alloy. This effectively disperses the stress at the crack tip and hinders rapid crack propagation. It overcomes the defects of TC4 titanium alloy in the prior art, such as decreased impact toughness and brittle fracture at low temperatures, and improves the low-temperature impact toughness of TC4 titanium alloy. It is suitable for manufacturing high-reliability components in low-temperature service environments such as aerospace and polar equipment. Attached Figure Description

[0016] The accompanying drawings are provided to further understand the invention and constitute a part of this invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0017] Figure 1 This is a graph showing the heat treatment process of the present invention. Figure 2 Micrograph of the isometric structure of conventional TC4 in Comparative Example 1; Figure 3 Micrograph of TC4-DT isometric structure from Example 1; Figure 4 This is a diagram illustrating the comparison of impact energy. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0019] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0020] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. This is an explanation of the present invention and not a limitation thereof.

[0022] This invention provides a method for improving the low-temperature impact toughness of TC4 titanium alloy, comprising the following steps: TC4 ingots with an oxygen content of 0.11~0.13% were prepared by sequentially using electron beam cold bed melting and vacuum consumable arc melting. The TC4 ingot is annealed at a temperature in the α+β two-phase region, and then held and cooled sequentially to obtain a TC4 titanium alloy with an equiaxed microstructure.

[0023] The method for improving the low-temperature impact toughness of TC4 titanium alloy provided by this invention is based on the core logic of controlling the introduction of high-oxygen materials at the source and achieving precise deoxidation during the melting process through the synergistic combination of raw material ratio and vacuum melting process, thereby fundamentally eliminating the root cause of low-temperature brittleness. By precisely controlling the oxygen content of TC4 ingot within the optimal range of 0.11%~0.13wt%, the weakening effect of interstitial atoms on grain boundaries is reduced, suppressing the generation of low-temperature brittleness from the intrinsic material properties level, while avoiding a significant decrease in alloy strength due to excessive deoxidation, thus achieving a good match between strength and low-temperature impact toughness. A combined process of electron beam cold hearth melting (EB) and vacuum arc remelting (VAR) is employed, where electron beam cold hearth melting effectively removes harmful interstitial elements such as oxygen and nitrogen, and vacuum arc remelting... It ensures the uniformity and density of the ingot composition; for the low oxygen composition system of 0.11%~0.13wt%, the annealing process in the low temperature range of the α+β two-phase region is matched to obtain a uniform and fine equiaxed microstructure of the alloy, which can effectively disperse the stress at the crack tip and hinder the rapid crack propagation. It overcomes the defects of TC4 titanium alloy in the prior art, such as the decrease in impact toughness and easy brittle fracture at low temperature, and improves the low temperature impact toughness of TC4 titanium alloy. It is suitable for manufacturing high reliability components in low temperature service environments such as aerospace and polar equipment.

[0024] In the actual production of titanium alloys, a certain proportion of TC4 recycled material is usually added to the furnace charge to reduce costs. However, the surface of the recycled material is often covered with oxides and the internal oxygen content is relatively high, making it a source of high-oxygen materials. If the proportion of recycled material added is limited only at the furnace charge level without targeted process matching in the smelting stage, the conventional smelting kinetics cannot provide sufficient driving force and time window for the escape of excess oxygen, and the oxygen content of the ingot will still exceed the standard. Therefore, this invention emphasizes the necessity of synergy: under the premise of controlling an appropriate proportion of recycled material, a dual process of electron beam cold hearth smelting (EB) and vacuum arc remelting (VAR) must be used in synergy. EB smelting, with its high vacuum environment and adjustable molten pool superheat, can provide excellent thermodynamic and kinetic conditions for the volatilization and removal of oxygen; while VAR smelting performs deep refining under extremely high vacuum, further removing residual trace oxygen and other impurity gases, and ensuring the compactness of the ingot's crystalline structure. Only through the synergistic effect of this raw material ratio and duplex melting process can the interstitial oxygen content of TC4 ingots be precisely locked within an extremely narrow window of 0.11~0.13%. After obtaining the low-oxygen ingot, a suitable heat treatment process is needed to transform the as-cast microstructure into a high-performance equiaxed microstructure, thereby fully leveraging the toughness advantage brought by the low oxygen content. Figure 1 The heat treatment process curve shown in this embodiment indicates that the ingot is heated to the α+β two-phase region temperature for annealing. It should be understood that, although... Figure 1 The example shows specific temperature values, but in practice, as long as the annealing temperature is within the temperature range of the α+β two-phase region, the phase ratio can be reasonably controlled; it is not limited to a fixed value. After holding at the α+β two-phase region for a period of time, cooling is then performed. The choice of cooling method must take into account both microstructure transformation and stress control. For example, air cooling can achieve a moderate cooling rate, avoiding excessive residual stress due to excessively rapid cooling or coarsening of the microstructure due to excessively slow cooling.

[0025] Through the aforementioned synergistic oxygen control and heat treatment scheme, the TC4-DT alloy prepared in this embodiment exhibits significant advantages in both microstructure and macroscopic properties. Combined with... Figure 2 and Figure 3 The comparison clearly shows that... Figure 2 The equiaxed microstructure of the TC4 alloy with conventional oxygen content is shown, while Figure 3The equiaxed microstructure of the low-oxygen TC4-DT alloy in this embodiment is shown. The reduced oxygen content decreases the segregation of oxygen atoms at grain boundaries and the pinning effect on dislocation slip, fundamentally eliminating the hidden dangers of dislocation movement obstruction and grain boundary stress concentration at low temperatures. This low-oxygen content reduces grain boundary segregation and dislocation pinning, eliminating the causal chain of low-temperature brittleness. As a result, the TC4 titanium alloy prepared in this embodiment maintains high strength and low-cost recyclability while significantly improving its plasticity and crack propagation resistance at low temperatures, completely overcoming the technical bottleneck of conventional TC4 alloys being prone to brittle fracture under low-temperature conditions.

[0026] Specifically, TC4 ingots with an oxygen content of 0.11~0.13% are prepared by sequentially using electron beam cold hearth melting and vacuum consumable arc melting, including the following steps: The furnace charge is added to an electron beam cooling bed for melting to obtain the first smelting product. The furnace charge includes sponge titanium, aluminum-vanadium alloy, metallic aluminum briquettes, and TC4 recycled material. The mass of TC4 recycled material accounts for 20% to 40% of the total mass of the furnace charge. The first smelting product was subjected to vacuum arc melting to obtain TC4 ingots with an oxygen content of 0.11~0.13%.

[0027] The composition of the furnace charge and the proportion of recycled materials were more specifically defined, providing suitable kinetic conditions for subsequent smelting and degassing. In the raw materials for titanium alloy preparation, sponge titanium, aluminum-vanadium alloys, and metallic aluminum briquettes typically serve as supplementary sources of basic and alloying elements, with relatively controllable and low oxygen content. TC4 recycled materials, often derived from processing scraps or discarded parts, typically have a thick oxide scale on their surface and inevitably absorb oxygen during multiple thermal cycles, making them a major source of oxygen introduction into the furnace charge system. The reason this embodiment strictly limits the mass proportion of TC4 recycled materials to the range of 20% to 40% is based on a thorough consideration of matching the initial oxygen load with the kinetics of smelting and deoxidation.

[0028] On the one hand, if the proportion of recycled material exceeds 40%, for example, reaching 50% or 60%, the initial oxygen load of the furnace charge will rise sharply. Under excessively high initial oxygen concentrations, even if the subsequent EB and VAR dual-melting process is adopted, the volatilization driving force and escape time window of oxygen elements in the molten pool cannot meet the removal requirements of such a large amount of excess oxygen. The degassing kinetic matching relationship will be disrupted, ultimately leading to an inevitable over-limit of oxygen content in the ingot, which cannot be reduced back to the target range of 0.11~0.13%, thus failing to fundamentally eliminate the hidden danger of low-temperature brittleness.

[0029] On the other hand, in the industrial production of titanium alloys, the addition of recycled materials is a core means of reducing raw material costs. If the proportion of recycled materials is less than 20%, although the initial oxygen load is extremely low and the deoxidation pressure during smelting is small, the insufficient proportion of inexpensive materials in the furnace charge will lead to a significant increase in preparation costs, making the technical objective of this invention—balancing low cost and high toughness—uneconomical. Therefore, the proportion window of 20% to 40% ensures both the cost advantage brought by recycled materials and reserves sufficient degassing kinetic space for the duplex smelting process, representing the optimal balance between economy and technology.

[0030] Specifically, the process parameters for electron beam cold hearth melting are set as follows: melting power 250~280 kW, melting speed 70~140 kg·h. - ¹, Vacuum degree 3.0×10 - ²~2.0×10 - 2 Pa.

[0031] Specifically, the process parameters for vacuum consumable arc melting are set to a vacuum degree ≤ 1.0 × 10⁻⁶. - ¹ Pa, smelting current 10~20 kA, smelting voltage 25~35 V.

[0032] The above parameters are set based on a deep synergistic consideration of melting degassing kinetics and initial oxygen load. During the electron beam cold hearth melting stage, 3.0 × 10⁻⁶ - ²~2.0×10 - The high vacuum environment of 2 Pa provides extremely low gas phase partial pressure resistance for the volatilization of oxygen and its compounds, while the smelting power of 250~280 kW and the efficiency of 70~140 kg·h - The melting speed¹ is matched to ensure that the molten pool has sufficient superheat to break the bonds between oxygen atoms and the titanium matrix, while providing a sufficient residence time window for bubble nucleation, growth, and escape. If the power is too low or the speed is too high, insufficient superheat in the molten pool or too short a residence time will result in incomplete degassing; if the power is too high or the speed is too slow, it may cause boiling and splashing of the molten pool or extremely low production efficiency. In the vacuum consumable arc melting stage, ≤1.0×10 - The extremely high vacuum of ¹ Pa further solidifies the degassing environment, while the smelting current of 10–20 kA and the smelting voltage of 25–35 V together determine the stability of the electric arc and the depth of the molten pool. Appropriate current and voltage matching can ensure that the molten pool has a suitable depth and high temperature duration, achieving deep refining and removal of residual trace oxygen, and ensuring the density of the ingot's bottom-up sequential crystallization, avoiding defects such as porosity and shrinkage cavities.

[0033] Specifically, the temperature range of the α+β two-phase region is 780℃±2℃.

[0034] See Figure 1 The phase transformation point (Tβ) of TC4 titanium alloy is typically around 945℃, and 780℃ falls precisely in the low-temperature range of the α+β two-phase region. Annealing within this temperature range allows for precise control of the ratio of the primary α phase to the β-transformed phase. If the temperature is too low, the proportion of the primary α phase is too high, resulting in a lack of sufficient β-phase plastic buffer in the microstructure, thus limiting the improvement in toughness. If the temperature is too high, or even close to the phase transformation point, a large amount of the primary α phase dissolves, weakening the equiaxed microstructure characteristics and tending towards a basketweave or even Widmanstätten microstructure, which is also detrimental to the stable development of low-temperature impact toughness. Therefore, 780℃±2℃ is the optimal thermodynamic window for obtaining an ideal biphase equiaxed microstructure.

[0035] Specifically, air cooling is used.

[0036] Combination Figure 1 The heat treatment process curves shown clearly indicate that after the 2-hour holding period, the curve descends linearly until it intersects the horizontal axis, with the endpoint marked AC. Air cooling provides a moderate cooling rate, avoiding both the significant thermal stress caused by the rapid cooling of water cooling (thermal stress easily induces microcrack initiation at low temperatures) and the severe coarsening of the β-phase decomposition products caused by the extremely slow cooling of furnace cooling (coarsened microstructure reduces crack propagation resistance). Air cooling promotes the decomposition of the β-phase into fine and uniform secondary α-phase at a suitable cooling rate, perfectly complementing the primary equiaxed α-phase to form a... Figure 3 The isometric microstructure of TC4-DT is shown. Figure 3 In the middle, a large number of dark gray polygonal equiaxed grains are tightly packed, and bright white phases with network or discontinuous linear connections are distributed at the grain boundaries. This fine combination of two-phase structure is a microscopic manifestation of the combined effect of low oxygen content and suitable cooling rate.

[0037] Specifically, the heat preservation time is 2 hours.

[0038] The 2-hour holding time is set to ensure uniform temperature penetration within the large cross-section component, the ingot, and sufficient balance in the phase transformation process. If the holding time is too short, insufficient transformation of the core structure will lead to uneven performance; if the holding time is too long, it will not only waste energy but may also cause slight coarsening of the primary α phase, thus weakening the effect of fine grain strengthening and toughening.

[0039] Heat treatment is essential for leveraging the advantages of low oxygen levels. Only by combining the inherent potential of low oxygen with the external morphology of equiaxed microstructure can a qualitative leap in the low-temperature impact toughness of TC4 titanium alloy be achieved. In other embodiments, as long as the annealing temperature is located in the α+β two-phase region and the phase ratio can be precisely controlled, the holding time is sufficient to ensure temperature penetration, and the cooling rate is moderate to avoid stress concentration and microstructure coarsening, all should be considered within the scope of protection of this invention.

[0040] Based on the same inventive concept, the present invention also provides a TC4 titanium alloy, which is prepared by the method described above for improving the low-temperature impact toughness of TC4 titanium alloy.

[0041] Specifically, the average impact energy of the prepared TC4 titanium alloy at room temperature is not less than 58.2 J, and the average impact energy at -196℃ is not less than 22.8 J.

[0042] Based on the same inventive concept, the present invention also provides an application of the TC4 titanium alloy as described above in key load-bearing structural components of aerospace, shipbuilding or chemical equipment.

[0043] In the fields of aerospace, shipbuilding, and chemical equipment, there are numerous structural components that need to operate for extended periods in cryogenic or ultracryogenic environments and withstand complex vibrations and impact loads. While conventional TC4 alloys possess high strength at room temperature, under cryogenic conditions, the high interstitial oxygen content (0.18%~0.21%) leads to a sharp decline in the alloy's plasticity due to the segregation of oxygen atoms at grain boundaries and their strong pinning effect on dislocation slip. This makes the alloy highly susceptible to inducing microcracks at stress concentration points, which then propagate rapidly, resulting in catastrophic brittle fracture. The TC4-DT alloy prepared by this invention using a synergistic oxygen control method precisely locks the oxygen content at an extremely low level of 0.11~0.13%, fundamentally eliminating the risk of cryogenic brittleness. This allows the alloy to maintain an average impact energy of no less than 22.8 J even in extreme cryogenic environments at -196℃. This high toughness provides a solid material guarantee for safety and reliability under extreme conditions.

[0044] To more clearly illustrate the breadth and diversity of applications of this invention, specific application sub-categories are listed below for each of the three main application categories. It should be understood that these examples are illustrative only and not restrictive, and the scope of protection of this invention should not be limited to any single specific part model listed below: Example 1: Application in the aerospace field. Specifically, the TC4 titanium alloy of this invention can be applied to the support structure of fuel tanks in cryogenic environments. In cryogenic propellant tanks such as liquid oxygen or liquid hydrogen, the support structure is subjected to extremely cold environments of -196°C or even lower for extended periods, and must withstand the severe vibrations and impact loads during rocket launch and flight. Conventional TC4 has an impact energy of only about 19.2 J at this temperature, making it highly susceptible to brittle fracture and tank disintegration; while the TC4-DT alloy of this invention increases the impact energy to over 22.8 J at -196°C, an increase of 18.8%, effectively absorbing impact energy and inhibiting crack propagation, ensuring the safety of aerospace missions. Furthermore, this alloy can also be applied to critical load-bearing components such as cryogenic environment piping joints and cryogenic valve housings in spacecraft.

[0045] Example 2: Application in shipbuilding. Specifically, the TC4 titanium alloy of this invention can be applied to deep-sea pressure hull connectors. When deep-sea exploration equipment dives into the deep sea, it not only faces extremely high external water pressure, but also faces the risk of low-temperature brittle fracture of the hull connectors when operating in polar waters or deep-sea cold water environments. As a key stress transmission node in the pressure hull structure, brittle fracture of the connectors will cause the entire pressure chamber to instantly lose pressure and flood. The low-oxygen and high-toughness characteristics of the alloy of this invention enable the connectors to still have sufficient plastic deformation buffering capacity and crack initiation resistance threshold under the coupled low-temperature and high-pressure conditions of deep sea, greatly improving the life support capability of deep-sea equipment. Similarly, this alloy can also be used in pressure-resistant sealing flanges of deep-sea submersibles, low-temperature stress anchor chain components of polar icebreakers, etc.

[0046] Example 3: Application in the field of chemical equipment. Specifically, the TC4 titanium alloy of this invention can be applied to chemical pipeline valves in extremely cold regions. In chemical pipeline networks in polar or high-latitude regions, pipeline valves not only need to resist the erosion of internal corrosive media, but also need to withstand the impact of medium pressure pulsation and thermal expansion and contraction in sub-zero temperatures. Conventional TC4 valves are prone to valve body brittle fracture under extremely cold conditions, leading to hazardous chemical leakage accidents. The alloy of this invention, with its excellent low-temperature impact toughness, ensures the structural integrity of the valve under extremely cold opening and closing actions and pressure impacts, eliminating the risk of leakage caused by low-temperature brittle fracture. Similarly, this alloy can also be applied to key load-bearing components such as support legs of cryogenic liquefied gas storage tanks and stirring shafts of reactors in extremely cold environments.

[0047] It should be understood that although the above examples list specific components such as fuel tank support structures, deep-sea pressure hull connectors, and valves for chemical pipelines in extremely cold regions, this is by no means an exhaustive or limiting list of applications for this invention. In fact, any load-bearing structural component involving low-temperature, cryogenic, or impact conditions, and with stringent requirements for resistance to brittle fracture, can be manufactured using the TC4 titanium alloy prepared according to this invention. The core objective of this embodiment is to clearly point out that the technical value of this invention not only lies in the innovation of material preparation methods, but also extends directly to the safe service of end products. By leveraging the low-oxygen, high-toughness material properties, it empowers downstream key equipment, achieving a closed-loop technical protection system across the entire industry chain.

[0048] Example 1 The furnace charge is added to an electron beam cooling bed for melting to obtain the first smelting product. The furnace charge includes sponge titanium, aluminum-vanadium alloy, metallic aluminum briquettes, and TC4 recycled material. The mass of TC4 recycled material accounts for 20% to 40% of the total mass of the furnace charge. The first smelting product was subjected to vacuum consumable arc melting to obtain TC4 ingots with an oxygen content of 0.11~0.13%; The TC4 ingot was annealed at 780 (±2)℃ and air-cooled after holding at that temperature for two hours to obtain a TC4 titanium alloy (TC4-DT) with an equiaxed microstructure.

[0049] After three measurements by the instrument, the impact toughness of TC4-DT alloy at room temperature (20℃) was obtained as 57.1J, 60.7J and 56.7J respectively, and the average impact energy was calculated to be 58.2J. After three measurements by the instrument, the impact toughness of TC4-DT alloy at -40℃ was obtained as 48.6J, 47.9J and 43.6J respectively, and the average impact energy was calculated to be 46.7J. After three measurements by the instrument, the impact toughness of TC4-DT alloy at -90℃ was obtained as 40.7J, 42.4J and 39.6J respectively, and the average impact energy was calculated to be 40.9J. After three measurements by the instrument, the impact toughness of TC4-DT alloy at -196℃ was obtained as 22.8J, 21J and 24.7J respectively, and the average impact energy was calculated to be 22.8J.

[0050] Comparative Example 1 This comparative example aims to demonstrate, through counter-example, that when the proportion of recycled material exceeds the critical boundary of 40%, even with the specific duplex melting process and heat treatment methods described in this invention, it is impossible to achieve precise oxygen control and improved low-temperature toughness. The mass percentage of TC4 recycled material was adjusted to exceed the 20%~40% limit window, while the remaining furnace charge composition (sponge titanium, aluminum-vanadium alloy, metallic aluminum briquettes) and electron beam cold hearth melting process parameters (power 250~280 kW, speed 70~140 kg·h) were adjusted. - ¹, Vacuum degree 3.0×10 - ²~2.0×10 - ² Pa), vacuum self-consuming arc melting process parameters (vacuum degree ≤ 1.0 × 10⁻⁶ Pa), - The temperature and current (Pa, current 10~20 kA, voltage 25~35 V) and subsequent annealing heat treatment process (holding at 780℃±2℃ for 2 hours and air cooling) are completely consistent with those in Example 1, resulting in a TC4 titanium alloy (TC4) with an oxygen content of 0.18%~0.21%.

[0051] After three measurements by the instrument, the impact toughness of TC4-DT alloy at room temperature (20℃) was obtained as 57.1J, 49.6J and 51.1J respectively, and the average impact energy was calculated to be 52.6J. After three measurements by the instrument, the impact toughness of TC4-DT alloy at -40℃ was obtained as 32.8J, 37.9J and 38.5J respectively, and the average impact energy was calculated to be 36.4J. After three measurements by the instrument, the impact toughness of TC4-DT alloy at -90℃ was obtained as 32.7J, 28.9J and 32J respectively, and the average impact energy was calculated to be 31.2J. After three measurements by the instrument, the impact toughness of TC4-DT alloy at -196℃ was obtained as 18.4J, 19.2J and 20.1J respectively, and the average impact energy was calculated to be 19.2J.

[0052] The core mechanism by which an excessive proportion of recycled material leads to the complete failure of synergistic oxygen control lies in the disruption of the matching relationship between the initial oxygen load and degassing kinetics. When the proportion of recycled material exceeds 40%, the total amount of oxygen introduced into the furnace charge by the recycled material is too high, making it impossible for the oxygen concentration gradient and gas phase partial pressure difference within the molten pool to maintain sufficient volatilization driving force. Even if EB melting provides high vacuum and suitable superheat, and VAR melting performs deep refining, the degassing kinetics cannot meet the removal requirements of such a large oxygen load within the limited melt residence time and reaction window. Excess oxygen is forced to remain in the titanium matrix, inevitably causing the final ingot oxygen content to exceed the conventional high oxygen level of 0.18%~0.21%. This excessive oxygen content directly triggers a catastrophic dislocation pinning effect at the microscopic level. Under low-temperature conditions, excess oxygen atoms form high-density segregated hard points at the grain boundaries, like countless tiny roadblocks, exerting an extremely strong pinning and hindering effect on dislocation slip. Stress cannot be effectively dissipated through plastic deformation and can only accumulate at grain boundaries, eventually rapidly inducing microcrack initiation and unstable propagation, which macroscopically manifests as a sudden drop in impact energy and brittle fracture.

[0053] See Figure 4 By comparing the average impact energy of Example 1 and Comparative Example 1, it can be seen that the impact toughness is significantly improved from room temperature to -196℃. Specifically, at room temperature, the total impact energy of the low-oxygen TC4-DT titanium alloy is 10.6% higher than that of conventional oxygen content TC4, and at -196℃, the total impact energy of the low-oxygen TC4-DT titanium alloy is 18.8% higher than that of conventional oxygen content TC4, showing excellent low-temperature toughness and crack propagation resistance.

[0054] This data comparison fully demonstrates that when the proportion of recycled material exceeds 40%, the synergistic deoxidation capability of the smelting process is completely compromised, and the low-temperature toughness decreases significantly, making it impossible to achieve the technical goal of balancing low cost and high and low-temperature toughness pursued by this invention. Therefore, the proportion of recycled material of 20% to 40% is not a conventional parameter that can be arbitrarily exceeded, but rather a critical boundary that is deeply kinetically dependent on a specific duplex smelting process.

[0055] Finally, it should be noted that the embodiments listed above are merely one or more specific manifestations of the technical solution of this invention. Their purpose is to clearly illustrate the concept, principle, and application of this invention through specific examples, and is by no means intended to limit the scope of protection of this invention to these specific embodiments. In fact, the true value of this invention lies in its proposed technical ideas and innovations, rather than its manifestations or implementation methods.

[0056] For those skilled in the art, after thoroughly reading and understanding the technical solution of this invention, they are fully capable of making various changes, modifications, or equivalent substitutions to the specific implementation of the invention based on their own professional knowledge and skills. These changes may include, but are not limited to: adjusting the range of technical parameters, optimizing the algorithm flow to improve efficiency, and replacing some technical components to achieve better compatibility or reduce costs. As long as these modified technical solutions substantially retain the technical features claimed by the original invention, that is, they can still achieve the core functions and effects of this invention, then these changes should be considered to fall within the scope of protection of the pending claims of this invention.

[0057] Furthermore, with the continuous progress and development of technology, new technical means and methods are constantly emerging, which provides ample space for further improvement and perfection of this invention. Therefore, the scope of protection of this invention should also include reasonable and foresightful improvements and extensions based on existing technology. As long as these improvements and extensions do not depart from the basic principles and core concepts of this invention, they should be considered equivalents of this invention and are equally protected by patent rights.

Claims

1. A method for improving the low-temperature impact toughness of TC4 titanium alloy, characterized in that, Includes the following steps: TC4 ingots with an oxygen content of 0.11~0.13% were prepared by sequentially using electron beam cold bed melting and vacuum consumable arc melting. The TC4 ingot is annealed at a temperature in the α+β two-phase region, and then held and cooled sequentially to obtain a TC4 titanium alloy with an equiaxed microstructure.

2. The method for improving the low-temperature impact toughness of TC4 titanium alloy according to claim 1, characterized in that, TC4 ingots with an oxygen content of 0.11-0.13% were prepared by sequentially employing electron beam cold bed melting and vacuum consumable arc melting. The specific steps include: The furnace charge is added to an electron beam cooling bed for melting to obtain the first smelting product. The furnace charge includes sponge titanium, aluminum-vanadium alloy, metallic aluminum briquettes, and TC4 recycled material. The mass of TC4 recycled material accounts for 20% to 40% of the total mass of the furnace charge. The first smelting product was subjected to vacuum arc melting to obtain TC4 ingots with an oxygen content of 0.11~0.13%.

3. The method for improving the low-temperature impact toughness of TC4 titanium alloy according to claim 1, characterized in that, The process parameters for electron beam cold hearth melting were set as follows: melting power 250~280 kW, melting speed 70~140 kg·h. - ¹, Vacuum degree 3.0×10 - ²~2.0×10 - 2 Pa.

4. The method for improving the low-temperature impact toughness of TC4 titanium alloy according to claim 1, characterized in that, The process parameters for vacuum self-consuming arc melting are set to a vacuum degree ≤ 1.0 × 10⁻⁶. - ¹ Pa, smelting current 10~20 kA, smelting voltage 25~35 V.

5. The method for improving the low-temperature impact toughness of TC4 titanium alloy according to claim 1, characterized in that, The temperature range of the α+β two-phase region is 780℃±2℃.

6. The method for improving the low-temperature impact toughness of TC4 titanium alloy according to claim 1, characterized in that, Air cooling is used for cooling.

7. The method for improving the low-temperature impact toughness of TC4 titanium alloy according to claim 1, characterized in that, The heat preservation time is 2 hours.

8. A TC4 titanium alloy, characterized in that, It was prepared using the method for improving the low-temperature impact toughness of TC4 titanium alloy as described in any one of claims 1 to 7.

9. A TC4 titanium alloy according to claim 8, characterized in that, The average impact energy of the prepared TC4 titanium alloy at room temperature is not less than 58.2 J, and the average impact energy at -196℃ is not less than 22.8 J.

10. The application of the TC4 titanium alloy as described in claim 8 or 9 in key load-bearing structural components of aerospace, shipbuilding, or chemical equipment.

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