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2018
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Research Progress of Titanium Alloys for Aviation
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Introduction: Titanium alloy is widely used in the aerospace field due to its high specific strength and good corrosion resistance. This article introduces the research progress of major titanium alloys in recent years, including high-strength and high-toughness titanium alloys (Ti-1023, Ti-15-3, β21S and BT-22), high-temperature titanium alloys (IMI834, Ti-1100, BT36 and Ti -60), damage tolerance titanium alloys (TC21 and TC4-DT) and flame-retardant titanium alloys (Alloy C, BTT-1 and BTT-3), and look forward to the development direction of titanium alloys for aerospace use.
Introduction: Titanium alloy is widely used in the aerospace field due to its high specific strength and good corrosion resistance. This article introduces the research progress of major titanium alloys in recent years, including high-strength and high-toughness titanium alloys (Ti-1023, Ti-15-3, β21S and BT-22), high-temperature titanium alloys (IMI834, Ti-1100, BT36 and Ti -60), damage tolerance titanium alloys (TC21 and TC4-DT) and flame-retardant titanium alloys (Alloy C, BTT-1 and BTT-3), and look forward to the development direction of titanium alloys for aerospace use.
Titanium is an important structural metal widely used in the aerospace field. It has the obvious advantages of high specific strength, good corrosion resistance and wide operating temperature range (-269-600℃). The specific strength of titanium is -253 It is the first of the commonly used metal materials within ～600℃, so it was quickly used in the aviation industry. Aviation has always been the largest user in the world titanium market for 50 years. Figure 1 shows the main parts and components of A350 titanium. At present, the weight of titanium on advanced aircraft has reached 30% to 35% of the total weight of the aircraft structure, and titanium has become an indispensable structural material for modern aircraft. In recent years, the aviation industry has become increasingly urgent for new structural materials with high strength, high elastic modulus, high temperature resistance, and low density, which has greatly promoted the rapid development of titanium manufacturing. Therefore, this article first introduces the typical structure types of titanium alloys and the corresponding properties, and then briefly describes the research progress of the new titanium alloys developed in recent years, which are mainly divided into the following four categories: (1) High-strength and high-toughness titanium alloys; 2) High temperature titanium alloy; (3) Damage tolerance titanium alloy; (4) Flame retardant titanium alloy
Figure 1 Main parts and components of titanium for A350
One, the typical structure type of titanium alloy and the corresponding properties
The microstructure of titanium and titanium alloy castings is usually the transformed β structure formed by cooling down from the high temperature β zone. The structure maintains the original β grain boundary. The grain boundary consists of needle-like or flake-like α phases and net basket-like α phases. Composition, through deformation processing and heat treatment, different organizational structures can be obtained. Figure 2 shows the morphology of a typical titanium alloy. Generally, titanium alloy structure can be divided into Widmanstatten structure, bimorph structure, basket structure and isometric structure according to morphological characteristics. The formation of the above-mentioned structure mainly depends on the thermal deformation and heat treatment process.
(1) Widmanstatten structure: the characteristic is that the original β grains are large, clear and complete, and there are very obvious continuous α phases on the grain boundaries. The inside of the crystals is the α-phase bundle domains arranged in parallel and large and regular, and the inter-plates are β phases. (Figure 2a). This kind of structure will be formed when the start temperature and end temperature of alloy deformation are both in the β phase zone and the deformation is not large, or when the alloy is heated to the β phase zone and cooled slowly.
(2) Two-state structure: It is characterized by a certain number of discontinuous primary equiaxed alpha phase grains distributed on the matrix of the transformed beta structure, but the total content is less than 50%, so there are equiaxed primary alpha and beta transitions in the structure There are two forms of lamellar α in the tissue (Figure 2b). This structure can be formed when the alloy is heated and deformed at the upper temperature of the α+β two-phase zone. This type of organization has better comprehensive mechanical properties.
(3) Basket structure: It is characterized by the destruction of the original β-grain grain boundaries to varying degrees, the α-sheets in the crystal become shorter, the α-phase orientations are mixed, and they are staggered to form a woven basket-like structure (Figure 2c). This kind of structure is formed when the alloy undergoes large deformation near the β transformation point, and the deformation in the α+β two-phase region is not large enough. Its characteristics are: good plasticity, impact toughness, and good high temperature durability and creep resistance.
(4) Isometric structure: It is characterized by a large number (over 50%) of primary α grains, and a certain amount of β transformation structure, distributed at the boundary of the α phase, all of which are equiaxed or polygonal (Figure 2d) . After the flake α is deformed, it can be heated to form an equiaxed α. The degree of equiaxation is affected by the degree of deformation, heating temperature and holding time. The general trend is that as the degree of deformation increases, the heating temperature increases, the holding time increases, and the degree of equiaxation increases. This kind of organization is characterized by good comprehensive performance, especially high plasticity and impact toughness.
Figure 2 The basic structure of titanium alloy: (a) Widmanstatten structure, (b) basket structure, (c) isometric structure, (d) bimodal structure
Second, the new titanium alloy
1. High-strength and high-toughness β-type titanium alloy
Β-type titanium alloy has good processing properties, is easy to forge, roll, and weld, and can obtain higher strength and fracture toughness through solution aging treatment. At present, there are mainly the following types of high-strength and high-toughness β-type titanium alloys that have been practically used on aircraft:
Ti-1023 (Ti-10V-2Fe-3Al) titanium alloy was developed in 1971 by Timet Company of the United States. It is a forged titanium alloy with high structural benefit, high reliability and low manufacturing cost to adapt to the design principles of damage tolerance. The alloy has an Al equivalent of 4.0 and a Mo equivalent of 11.1, (α+β)/ The β phase transition temperature is 790～805℃, it has great hardenability and remarkable heat treatment strengthening effect, and has excellent forging performance. It can be forged at 760℃ isothermally, providing various near-net-type processed forgings. After heat treatment, the σb of the alloy is 965-1310MPa, and the KIC is 99-33 MPa·m1/2. It has been used in the landing gear main beam of Boeing 777 passenger aircraft and the main landing gear pillar of Airbus A380.
Ti-l5-3 (Ti-15V-3Cr 3Al-3Sn) high-strength beta titanium alloy is a near beta-type high-strength anti-corrosion alloy developed in the 1970s with funding from the US Air Force. The alloy’s Al equivalent is 5.0, Mo equivalent is 15.7, and the (α+β)/β phase transition temperature is 750～770℃. It has excellent cold deformability, age hardening performance and weldability, and its cold workability is better than Industrial pure titanium is good. It can be cold-formed for various complex parts in a solid solution state. It has less crack sensitivity and σb≥1310MPa after aging. This alloy is especially suitable for making rocket engine propellant tanks and ducts. Components, and have been applied to the application control system piping and fire extinguishing tanks on the Boeing 777.
β-21S (Ti-15Mo-3Al-2.7Nb-0.2Si) alloy is an anti-oxidation, ultra-high-strength titanium alloy developed by Timet Corporation of the United States in 1989. The alloy's Al equivalent is 4.0, Mo equivalent is 15.8, and the (α+β)/β phase transition temperature is 793～810℃. It has good oxidation resistance and can work at 540℃ for a long time; excellent cold and hot workability . After heat treatment, σb=1150～1350MPa, δ5=6%～8%. The alloy is suitable for engine liners and nozzles, etc., and has been used by the National Aeronautics and Space Administration (NASA) as the matrix material of silicon carbide/titanium composite materials.
BT22 (Ti-5Al-5Mo-5V-1Fe-1Cr) alloy is a high-strength near-beta titanium alloy successfully developed by Russia in the 1970s. The alloy’s Al equivalent is 6.0, Mo equivalent is 11.8, and the (α+β)/β phase transition temperature is 860～990℃. It has good processing performance and welding performance. It is mainly used for the production of die forgings, and its hardening depth is up to 200mm, its σb≥1105MPa. The alloy can be used to manufacture fasteners for fuselages, wing stress parts and operating systems that work at 350-400°C for a long time, as well as fan discs and blades for engines with operating temperatures below 350°C. And has been used in 1L-86 and 1L-96-300 fuselage, wings, landing gear and other high load-bearing parts.
2, high temperature titanium alloy
High-temperature titanium alloy is one of the key materials of modern aero-engines. The development demand of high-performance aero-engines drives the development of high-temperature titanium alloys. High-temperature titanium alloys are mainly used as compressor discs, blades, and casings of aircraft engines to reduce engine mass and increase thrust-to-weight ratio. Engines have very demanding requirements for high-temperature titanium alloys, which require materials to have a good match of room temperature performance, high temperature strength, creep performance, thermal stability, fatigue performance and fracture toughness.
The service temperature of Ti-6Al-4V alloy, which was first used in aero engines in the 1950s, generally did not exceed 350℃; subsequently, alloys such as IMI550 and BT3-l with a service temperature of about 400℃ were developed successively; in the 1960s, various countries successfully developed The use of IMI679, IMI685, Ti6246, Ti6242 and other alloys with a use temperature of 450 ℃ ~ 500 ℃; at present, high temperature titanium alloys with a maximum use temperature of 600 ℃ developed worldwide include IM1834 in the United Kingdom, Ti-1100 in the United States, and Russia BT36 and China’s Ti-60. The following mainly introduces several representative high-temperature titanium alloys:
IMI834 (Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si) alloy is a near-α-type titanium alloy developed by the United Kingdom in 1984. The alloy has an Al equivalent of 8.7, a Mo equivalent of 0.7, and a (α+β)/β phase transition temperature of 1045±10°C, which has good deformability. After heat treatment, the alloy's σb≥930MPa. It has been used in the large engine Trent700 of the Boeing 777 aircraft.
Ti-1100 (Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si) alloy is a high-heat-strength near-α-type titanium alloy developed by the United States in 1988. The Al equivalent of this alloy is 8.6, Mo equivalent is 0.4, and the (α+β)/β phase transition temperature is 1015°C. The alloy has good high-temperature creep properties, lower toughness and larger fatigue crack growth rate. It has been used in the high-pressure compressor disc and low-pressure turbine blades of the Lycoming T55-712 modified engine.
BT36 (Ti-6.2A1-2Sn-3.6Zr-0.7Mo-0.1Y-5.0W-0.15Si) alloy is a titanium alloy developed by Russia in 1992 with a service temperature of 600-650°C. The alloy has an Al equivalent of 8.5, a Mo equivalent of 2.7, and a (α+β)/β phase transition temperature of 1000-1025°C, and has good high-temperature creep properties.
Ti60 (Ti-5.8 Al-4.8 Sn-2.0Zr-1.0 Mo-0.35Si-0.85Nd) alloy is a modified design based on Ti55 alloy by the Institute of Metal Research of the Chinese Academy of Sciences in the late 1990s, with the participation of Baoji Nonferrous Metal Processing Plant A 600℃ high temperature titanium alloy developed. The alloy has an Al equivalent of 8.5, a Mo equivalent of 1.0, and a (α+β)/β phase transition temperature of 1025°C. It has good thermal stability and high-temperature oxidation resistance. Semi-industrial pilot tests (including compressor plate die forging) and comprehensive performance testing have been carried out.
3, damage tolerance titanium alloy
There are currently two typical damage tolerance titanium alloys, one is the high-strength and high-toughness damage-tolerant titanium alloy TC21 developed abroad, and the other is the medium-strength and high-toughness damage-tolerant titanium alloy TC4-DT developed in China.
TC21 (Ti-Al-Mo-Sn-Zr-Cr-Si-X) alloy is a high-strength, high-toughness, and damage-tolerant two-phase titanium alloy developed by the Northwest Nonferrous Metals Research Institute in 2001. It is developed by my country The first high-strength damage-tolerant titanium alloy with independent intellectual property rights, and the Beijing Institute of Aeronautical Materials is the application research unit of this alloy. The structure of the alloy in general use state is basket-like or three-state structure, with excellent strength, plasticity, toughness and low crack growth rate matching. Its typical mechanical properties are σb: 1100MPa, σ0.2: 1000MPa, δ: 8 %, ψ: 15%, E: 115 GPa, KIC: 70 MPa·m1/2, da/dN: 8×10-5 ~9×10-5 mm/cycle. Compared with Ti-6-22-22S (U.S.) and BT20 (Russian) titanium alloy widely used in U.S. F-22 aircraft, its comprehensive mechanical properties are more excellent, especially its very Excellent electron beam welding performance, suitable for manufacturing important load-bearing components such as large integral frames, pylons near the engine, beams, joints, and landing gear components.
TC4-DT alloy is a medium-strength, high-toughness, damage-tolerant, weldable titanium alloy jointly developed by Northwest Nonferrous Metals Research Institute and Beijing Institute of Aeronautical Materials on the basis of Ti-6Al-4VELI alloy in the United States. Its performance is comparable to Ti- 6Al-4VELI is equivalent. However, the alloy composition range of TC4-DT is more stringent than Ti-6AI-4VELI, specifically Al: 6.0-6. 35, V: 3.6 -4.4, Fe≤0.25, C≤0.05, O≤0.13, N≤0. 03, H≤0.0125, Ti allowance. The alloy has the characteristics of medium strength, high fracture toughness and high damage tolerance. Its typical mechanical properties are: σb: 895MPa, σ0.2: 795MPa, δ: 8%, ψ: 15%, KIC: 75MPa·m1/2, da/dN: 9×10-6 mm/cycle. The alloy is particularly suitable for manufacturing key load-bearing components such as large integral frames, beams and joints of aircraft. It has been applied to the stabilizer connection joints of Boeing 777 passenger aircraft and the body of F/A-22 aircraft.
4, flame-retardant titanium alloy
When used as aeroengine materials, ordinary titanium alloys may produce "titanium fire". In order to solve this problem, various countries have carried out the development of flame-retardant titanium alloys. The following mainly introduces several representative flame-retardant titanium alloys:
Alloy C (Ti-35V-15Cr) alloy is a β-type titanium alloy developed by American P&W Company in the 1980s. The alloy has a molybdenum equivalent of 47.5, which is currently the highest industrial beta titanium alloy with the highest molybdenum equivalent. It has good room temperature and high temperature plasticity, creep and fatigue properties, and can be made into plates, strips, bars and forgings. The alloy has been used in the high-pressure compressor casing, guide vanes and vector tail nozzle of the F119 engine (the power unit of the F/A-22 fighter), and successfully conducted flight experiments in January 1993.
BTT-1 and BTT-3 are flame-retardant titanium alloys developed by Russia from the friction mechanism, both of which are Ti-Cu-Al alloys (with addition of Mo, V, Zr).
BTT-l has good thermal processing properties, and can be made into engine parts with complex shapes, such as compressor casings and blades, with a working temperature of 450°C.
The process plasticity of BTT-3 alloy is better than that of BTT-1 alloy, and it is especially suitable for manufacturing various sheet and foil parts. The flame retardancy of BTT-3 is also higher than that of BTT-l. Under the same test conditions, the friction ignition temperature of Ti-6Al-4V is 100°C, that of BTT-l is 650°C, and that of BTT-3 is greater than 800°C.
3. Conclusions and prospects
my country’s titanium production ranks fourth in the world, but the production of high-end titanium for aerospace only accounts for about 10% of the total, which is still far from the world’s 50% level. Therefore, it is necessary to strengthen the comprehensive high-performance titanium alloy materials and Application of innovative research to improve the application level and dosage of aerospace titanium alloys. The development of high comprehensive performance titanium alloy materials and low-cost manufacturing technology are the two driving forces for expanding the application of titanium alloys. Using advanced manufacturing technology (such as laser near-net forming), replacing precious metal elements with lower-priced alloy elements to reduce the cost of titanium alloys, while improving the performance of titanium alloy components, and promoting the application level of titanium alloys in the aerospace field Simplifying the grades of titanium alloys, optimizing the properties of titanium alloys, and forming a backbone titanium alloy system with multiple uses of one material are the main development directions of titanium alloys in the aviation field in the future.
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