High-strength and high-toughness cast TiAl alloy and preparation method thereof

Abstract

The invention belongs to the field of TiAl intermetallic compounds, and particularly relates to a high-strength and high-toughness cast TiAl alloy and a preparation method thereof. The TiAl alloy comprises the following components in atomic percent: 43 to 47 percent of Al, 1.6 to 3.2 percent of Mn, 1.5 to 2.5 percent of Nb, 0.6 to 1.0 percent of B and the balance of Ti. The TiAl alloy is prepared from the following components in percentage by mass: 28.0 to 31.0 percent of Al, 2.3 to 4.5 percent of Mn, 4.0 to 5.3 percent of Nb, 0.20 to 0.28 percent of B and the balance of Ti. The alloy component has the remarkable characteristics that the neutral Mn element is adopted to replace beta-phase stabilizing elements such as Mo, Cr, W and Ta, a brittle beta / B2 phase in a matrix is eliminated, an as-cast microstructure is in a fine full-lamellar morphology, and the comprehensive performance of the material is obviously improved. The nickel-based high-temperature alloy can replace nickel-based high-temperature alloy to stably work for a long time in the range of 600-700 DEG C, has excellent tensile strength and fracture toughness, has good casting fluidity and solidification feeding performance, and is suitable for preparing hot-end components in the aerospace field through a lost-wax investment casting method or a metal hard mold casting method.

Description

Technical Field

[0001] The present invention belongs to the field of TiAl intermetallic compounds, specifically a high-strength, high-toughness cast TiAl alloy and a preparation method thereof. The alloy is suitable for preparing lightweight, high-temperature-resistant thin-walled components such as blades, impellers, and airfoils by lost wax investment casting or metal hard mold casting methods, and can be used for a long time at an operating temperature of 600-700°C. Background Art

[0002] Lightweight, high-temperature resistant TiAl alloys have broad application prospects in the aerospace field. The iconic second-generation TiAl alloy grade 4822 (Ti-48Al-2Cr-2Nb at%) was commercialized by GE in 2006 to replace the denser nickel-based alloy blades. Currently, the number of aircraft engines equipped with TiAl alloy low-pressure turbine blades is over 5,000, specifically the GENX, Leap-1A, and Leap-1B aircraft engines. However, the TiAl-4822 alloy has disadvantages such as large grain size, low strength, and strong anisotropy. Figure 1 ), with the improvement of aviation engine efficiency and the increase in blade speed, which in turn requires higher material properties, the demand for improved comprehensive performance of TiAl alloys has become urgent. The third generation of TiAl alloys uses refractory elements such as Mo / W / Ta to improve the comprehensive performance of the alloy. Therefore, the three generations of high-strength TiAl generally add appropriate amounts of the above elements. With the deepening of iteration, researchers found that the addition of Mo / W / Ta refractory elements not only leads to increased density, but also has serious segregation behavior in the cast structure, and the β phase content in the matrix is ​​significantly increased ( Figure 2 ), while increasing the strength of the material, it also reduces the room temperature plasticity and high temperature durability of the material. For example, the most mature Austrian brand TNM alloy: Ti-43.5Al-4Nb-1Mo-0.15B (at%), this alloy is used on the low-pressure turbine blades of the PW1100GTF aircraft engine. Because the matrix contains a certain amount of β phase, it gradually transforms into a brittle ω phase during long-term use, resulting in deterioration of blade performance. More than 1,000 aircraft engines produced were exposed to poor toughness during service. Subsequently, the low-pressure turbine blades of this type of aircraft engine had to be replaced with nickel-based alloy blades. Another element optimization method to improve matrix performance is to add neutral Mn elements instead of Cr elements for strengthening. Not only will the matrix strength be significantly improved, but the formed full lamellar structure will significantly improve the fracture toughness of the matrix ( Figure 3However, what is worrying is that Mn has a high vapor pressure and is very easy to volatilize during vacuum melting. This is especially true for TiAl alloys containing more than 3wt% manganese. The loss value fluctuates greatly and does not follow the subtraction law, making it difficult to accurately control the elemental composition. The melting process is also affected by the volatilization of low-melting-point aluminum in the matrix, making it particularly difficult to accurately control the overall alloy composition. Especially for sensitive components in the aerospace field, the stringent quality requirements for alloy composition accuracy and batch consistency have become a major obstacle to the actual industrial production and application of high-strength, high-toughness TiAl master alloys containing Mn, and have become a technical bottleneck that researchers in this field urgently need to solve. Summary of the Invention

[0003] The purpose of the present invention is to provide a high-strength and high-toughness cast TiAl alloy containing highly volatile Mn as the main element and a preparation method thereof, so as to solve the problems of high vapor pressure of Mn element, very easy volatility during vacuum melting, and difficulty in precise control of element composition. Compared with the second-generation typical alloy grade 4822, the alloy microstructure has better high-temperature comprehensive mechanical properties and is suitable for the preparation of lightweight, high-temperature resistant thin-walled components such as blades, impellers, and airfoils in the aerospace field.

[0004] The technical solution of the present invention is:

[0005] A high-strength and high-toughness casting TiAl alloy has the following components in atomic percentage: Al: 43-47%, Mn: 1.6-3.2%, Nb: 1.5-2.5%, B: 0.6-1.0%, and the balance is Ti.

[0006] A high-strength and high-toughness casting TiAl alloy has the following components by mass percentage: Al: 28.0-31.0%, Mn: 2.3-4.5%, Nb: 4.0-5.3%, B: 0.20-0.28%, and the balance is Ti.

[0007] The preparation method of the high-strength and high-toughness cast TiAl alloy adopts 2-3 times of vacuum arc consumable melting; or adopts a melting combination mode combining 2-3 times of vacuum arc consumable melting and vacuum arc shell melting.

[0008] In the preparation method of the high-strength and high-toughness cast TiAl alloy, during vacuum melting, high-purity argon is input to form an atmosphere higher than the saturated vapor pressure of the Mn element to suppress component volatilization and achieve precise control of the composition.

[0009] The preparation method of the high-strength and high-toughness cast TiAl alloy is as follows: the raw materials are added with sponge titanium, AlMn master alloy or elemental Mn, AlNb master alloy, TiB2 or AlB master alloy and Al beans, which are uniformly mixed and pressed into dense electrodes according to the required percentage of components, and then vacuum arc combined melting is carried out.

[0010] In the preparation method of the high-strength and high-toughness cast TiAl alloy, the TiAl alloy uses neutral Mn elements to replace β-phase stabilizing elements.

[0011] In the preparation method of the high-strength and high-toughness cast TiAl alloy, the β-phase stabilizing elements include Mo, W, Ta or Cr.

[0012] The design concept of the present invention is:

[0013] The present invention discloses a TiAl alloy composition containing highly volatile Mn as a primary element and a preparation method thereof. A TiAl alloy containing titanium, aluminum, niobium, manganese, and boron is obtained by two to three vacuum combined smelting processes. The present invention emphasizes that the Al content is within the range of 43 to 47 at%, and that matrix-neutral Mn is added to replace Mo / W / Ta or excess Nb to prevent the precipitation of β / B2 phase in the matrix. Within the range of medium to low Al content (43 to 47 at%), the alloy prevents the formation of β phase structure along the γ solidification path. Simultaneously, maintaining a reasonable B content nucleates and refines the as-cast structure, preventing the formation of dendritic structure in the matrix, thereby ensuring the material's isotropy and good solidification and shrinkage feeding capabilities. A moderate content of Nb / Mn elements in the matrix can ensure sufficient performance improvement without causing serious β-phase segregation in the structure, which leads to deterioration of high-temperature endurance and room-temperature brittleness. The key concept of this alloy design is to improve the defects of the highly alloyed TiAl matrix by replacing β-phase / B2 stabilizing elements with Mn, while optimizing and iterating the content of Al, Nb and B elements to achieve a balanced improvement in the microstructure and mechanical behavior of the cast TiAl alloy.

[0014] The advantages and beneficial effects of the present invention are:

[0015] 1. The Mn-containing TiAl alloy material of the present invention is characterized by a preparation process route using a smelting combination method of 2 to 3 vacuum arc consumable melting (VAR) or 2 to 3 arc consumable melting combined with vacuum arc shell melting (VSM). A fine, fully lamellar (α2+γ) structure with no β / B2 phase between grain boundaries is formed along the γ solidification route of the phase diagram. The advantage of this TiAl alloy microstructure containing Mn and a medium-low Al content is that there is no high-temperature soft β phase in the matrix and its brittle B2 phase that is orderly transformed at room temperature, which overcomes the disadvantages of the material's high-temperature durability and insufficient room-temperature plasticity. At the same time, the fine, fully lamellar structure formed by the TiAl alloy containing a medium-low Al content of B refining element is conducive to overcoming crack propagation and extension, improving grain coordinated deformation and high-temperature strength performance. A vacuum melting method similar to that of ordinary titanium alloys can be used 2 to 3 times, without the risk of brittle cracking and element segregation. The purpose of homogeneous melting can be achieved by curbing element volatilization under a certain vacuum degree, and industrial mass production is easy to achieve.

[0016] 2. Under the condition that the density of the TiAl alloy containing Mn element of the present invention is equivalent to that of the traditional 4822 alloy (Ti-48Al-2Cr-2Nb) (adding high-density Mo\W\Ta or increasing the Nb content will increase the matrix density, resulting in a decrease in the material specific strength), in the operating temperature range of 600-700°C, the 4522XD alloy of the present invention has an isotropic fine lamellar structure. Compared with the performance of the 4822 alloy, the room temperature and high temperature strength are increased by 30% (~150MPa), and the fracture toughness K ΙC Increase by 25% (~10MPa.m 1 / 2 ), which is very beneficial for the preparation of high-performance complex load-bearing components from low-plasticity TiAl intermetallic compound materials.

[0017] 3. Lightweight, high-temperature-resistant TiAl alloys have excellent high-temperature performance and lightweight characteristics, and are the most promising materials for replacing nickel-based high-temperature alloys in hot-end components such as hypersonic aircraft, advanced aircraft engines, and internal combustion engines. The TiAl-4522XD alloy of the present invention has the advantages of low density, high strength, good fracture toughness, and isotropy. Compared with traditional TiAl alloys, it has more balanced performance and is easy to melt, precision cast, and machine. It has great application potential in the aerospace field. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 : Coarse full-lamellar microstructure of TiAl-4822 alloy.

[0019] Figure 2 : Microstructure of fine three-phase (lamellar + β + γ) of TiAl-TNM alloy.

[0020] Figure 3: Microstructure of fine full lamellar TiAl-4522XD alloy.

[0021] Figure 4 : High Mach aircraft 4522XD alloy casting lightweight rudder diagram.

[0022] Figure 5 : Microstructure of various parts of traditional TiAl-4822 alloy aero-engine low-pressure turbine blade.

[0023] Figure 6 : Microstructure diagram of various parts of TiAl-4522XD alloy aero-engine low-pressure turbine blade of the present invention.

[0024] Figure 7 : TiAl-4522XD alloy casting square rod (a) and machined compressor blade (b) of the present invention. DETAILED DESCRIPTION

[0025] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0026] Example 1

[0027] In this embodiment, the TiAl-4522XD alloy is composed of Ti-43Al-1.6Nb-2.0Mn-0.6B (at%) or Ti-28.0Al-4.0Nb-2.8Mn-0.2B (wt%). The raw materials are uniformly mixed according to the composition ratio and pressed into 4 kg electrodes. Eighteen electrodes are then vacuum welded together and then subjected to a single vacuum arc remelting (VAR) process to produce a pre-alloyed ingot with a diameter of 180 mm and a length of 600 mm. The materials used to press the electrodes are titanium sponge, pure aluminum, AlMn, AlNb, and TiB2 master alloy particles prepared according to the above composition. The raw material particle sizes are: titanium sponge 6-25.4 mm, pure aluminum (99.8 wt%) 6-12 mm, AlMn master alloy 2-5 mm, AlNb master alloy 1-3 mm, and TiB2 master alloy less than 100 mesh.

[0028] The two primary pre-alloyed ingots are welded together in a vacuum or argon atmosphere and then melted again by secondary vacuum consumable arc melting (VAR) to form a 144 kg ingot with a diameter of 220 mm.

[0029] In this embodiment, after obtaining TiAl-4522XD alloy by a combined smelting method of primary vacuum arc consumable melting (VAR) and secondary vacuum arc consumable melting (VAR), the secondary smelted ingot was sawn into three portions. Each portion, approximately 45 kg, was placed in a vacuum induction water-cooled copper crucible (ISM) to melt and cast a TiAl alloy rudder skeleton. The skeleton casting has a fine and uniform full lamellar structure. The grain size of the thinnest part at the front and rear trailing edges of the rudder is close to that of the thickest part at the skeleton mounting edge (50-90 μm). At a temperature of 700°C, the material has a yield strength σ 0.2 ≥440MPa, breaking strength σ b ≥630MPa, elongation ≥3%, fatigue limit strength ≥320MPa, all aspects of performance are 20-30% higher than the second generation typical alloy grade 4822, and the performance distribution difference of each area of ​​the component is small ( Figure 4 ).

[0030] Example 2

[0031] In this embodiment, the TiAl-4522XD alloy is composed of Ti-44Al-2.2Nb-2.5Mn-0.8B (at%) or Ti-29.2Al-5.0Nb-3.5Mn-0.28B (wt%). The raw materials are uniformly mixed according to the required composition and then pressed into 22 kg electrodes. Six electrodes are then vacuum welded together and then subjected to a single vacuum arc remelting (VAR) process to produce a pre-alloyed ingot with a diameter of 220 mm and a length of 900 mm. The materials used to press the electrodes are titanium sponge, pure aluminum, AlMn, AlNb, and AlB master alloy particles prepared according to the above composition. The raw material particle sizes are: titanium sponge 3-12.7 mm, pure aluminum (99.9 wt%) 6-9 mm, AlMn master alloy 1-3 mm, AlNb master alloy 1-2 mm, and AlB master alloy less than 80 mesh.

[0032] The pre-alloyed ingot is welded with auxiliary guide rods in a vacuum or argon atmosphere and then melted and cast again through vacuum skull melting (VSM) to form 27 split mold ingots with a diameter of 70 mm and a weight of 3 to 5 kg. This eliminates the need for sawing and facilitates accurate weight feeding in the next step of precision casting.

[0033] In this embodiment, after obtaining TiAl-4522XD alloy by vacuum arc consumable melting VAR + vacuum arc shell melting VSM combined melting method, the ingot of secondary shell melting is divided into corresponding parts according to the required weight, such as 15 kg placed in a water-cooled copper crucible to melt and cast TiAl alloy low-pressure turbine blades. The blade body and the thinner intake and exhaust edges are all fine full-lamellar structure with grain size in the range of 30-70μm, showing good microstructural uniformity. At a temperature of 600℃, the impeller material has a yield strength σ 0.2 ≥500MPa, breaking strength σ b ≥650MPa, elongation ≥1.0%, fatigue limit strength ≥400MPa, and its performance is significantly better than that of traditional TiAl-4822 alloy.

[0034] like Figure 5 As shown in Figure 2, the low-pressure turbine blades (blade body and tenon) of traditional TiAl-4822 alloy aircraft engines are coarse and their microstructure is anisotropic. Figure 6 As shown, the low-pressure turbine blades (blade body, tenon) of the TiAl-4522XD alloy aero-engine of the present invention are small and have an isotropic microstructure.

[0035] Example 3

[0036] In this embodiment, the TiAl-4522XD alloy is composed of Ti-47Al-2.5Nb-2.0Mn-0.4B (at%) or Ti-32.0Al-5.8Nb-2.8Mn-0.14B (wt%). After uniformly mixing the raw materials according to the required composition, they are pressed into 22 kg electrodes. Six electrodes are then vacuum welded together and then subjected to a single vacuum arc remelting (VAR) process to produce a pre-alloyed ingot with a diameter of 220 mm and a length of 900 mm. The materials used to press the electrodes are titanium sponge, pure aluminum, AlMn, AlNb, and TiB2 master alloy particles prepared according to the above composition. The raw material particle sizes are: titanium sponge 3-6 mm, pure aluminum (99.7 wt%) 3-8 mm, AlMn master alloy 2-4 mm, AlNb master alloy 2-3 mm, and TiB2 master alloy less than 60 mesh.

[0037] The two primary pre-alloyed ingots are welded together in a vacuum or argon atmosphere, and auxiliary guide rods are welded to them. They are then melted again through a second vacuum consumable arc melting (VAR) process to form a 264 kg ingot with a diameter of 280 mm. The two secondary melted ingots are then welded together in a vacuum or argon atmosphere, and auxiliary guide rods are welded to them. They are then melted again through a third vacuum consumable arc melting (VAR) process to form a 528 kg ingot with a diameter of 380 mm.

[0038] In this embodiment, after the TiAl-4522XD alloy is obtained by three vacuum arc consumable melting VAR combined melting method, the three-melting ingot is sawed into 10 parts, each part is 40 kg after peeling machine processing, and a reasonable steel mold is designed to cast it into Round bars or 50-80mm square bars are directly machined into parts such as aircraft engine high-pressure compressor blades or aircraft wing surfaces after hot isostatic pressing ( Figure 7 ).

[0039] Table 1 Comparison of as-cast properties of three TiAl alloys

[0040]

[0041] As can be seen from the data in Table 1, the present invention addresses the performance deficiencies of TiAl-4822 and third-generation high-strength TiAl-TNM by proposing a high-strength, high-toughness TiAl alloy with optimized composition. This alloy eliminates the harmful β phase in the matrix and forms a fine, fully lamellar structure, exhibiting excellent comprehensive performance indicators. Under working conditions, the material exhibits high strength and durability, and significantly improves fracture toughness.

[0042] Implementation results indicate that the alloy's notable characteristic is the use of neutral Mn in place of β-phase stabilizing elements such as Mo, Cr, W, and Ta, eliminating the brittle β / B2 phase in the matrix. The as-cast microstructure exhibits a fine, fully lamellar morphology, significantly improving the material's overall performance. This alloy exhibits long-term stable operation in the 600-700°C range, replacing nickel-based superalloys. The alloy exhibits excellent tensile strength and fracture toughness, along with excellent casting fluidity and solidification feeding properties. This makes it suitable for the fabrication of lightweight, high-temperature-resistant, thin-walled components for aerospace applications, such as blades, impellers, and airfoils, using lost-wax or hard-metal casting methods.

Claims

1. A high-strength and high-toughness cast TiAl alloy, characterized in that: Calculated by atomic percentage, the composition of the TiAl alloy is as follows: Al: 43-47%, Mn: 1.6-3.2%, Nb: 1.5-2.5%, B: 0.6-1.0%, and the balance is Ti.

2. A high-strength and high-toughness cast TiAl alloy, characterized in that: The composition of the TiAl alloy is as follows, by mass percentage: Al: 28.0-31.0%, Mn: 2.3-4.5%, Nb: 4.0-5.3%, B: 0.20-0.28%, and the balance is Ti.

3. The method for preparing a high-strength and high-toughness cast TiAl alloy according to any one of claims 1 to 2, characterized in that: Adopt 2-3 times of vacuum arc consumable melting; or adopt a melting combination method combining 2-3 times of vacuum arc consumable melting and vacuum arc shell melting.

4. The method for preparing a high-strength and high-toughness cast TiAl alloy according to claim 3, characterized in that: During the vacuum melting process, high-purity argon gas is input to form an atmosphere higher than the saturated vapor pressure of the Mn element to curb the volatilization of components and achieve precise control of the composition.

5. The method for preparing a high-strength and high-toughness cast TiAl alloy according to claim 3, characterized in that: The raw materials are titanium sponge, AlMn master alloy or elemental Mn, AlNb master alloy, TiB2 or AlB master alloy and Al beans, which are uniformly mixed and pressed into dense electrodes according to the required composition percentages, and then vacuum arc combined melting is carried out.

6. The method for preparing a high-strength and high-toughness cast TiAl alloy according to claim 5, characterized in that: TiAl alloy uses neutral Mn element to replace β-phase stabilizing elements.

7. The method for preparing a high-strength and high-toughness cast TiAl alloy according to claim 6, characterized in that: The β-phase stabilizing element includes Mo, W, Ta or Cr.

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