Advances In Titanium Alloys For Aerospace Applications

      Titanium element is widely distributed because its content is more than 0.4% of the mass of the earth’s crust, the world’s proven reserves of about 3.4 billion tons, in all elements in the content of the 10th (oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium, hydrogen, titanium).

      American scientists in 1910 using “sodium method” (sodium reduction TiCl4) to obtain the first metal titanium, but the titanium industry did not immediately develop with the discovery of titanium.

      It wasn’t until 1948, after World War II, that the titanium industry took off after a “magnesium process” (magnesium reduction of TiCl4) developed by scientists in Luxembourg was used in the United States.

      Titanium is 40 percent less dense than steel and titanium is as strong as steel, which improves structural efficiency. At the same time, titanium has good heat resistance, corrosion resistance, elasticity, elasticity and forming ability. Because titanium has the above characteristics, titanium alloy has been applied to the aviation industry since its appearance. In 1953, titanium material was used for the first time on the engine fireproof wall and short cabin of DC-T machine produced by Douglas Company of the United States, beginning the history of titanium alloy application in aviation.

      The space shuttle is the primary and most widely used aircraft. Titanium is the main structural material of aircraft, as well as the preferred material for important components such as aero-engine fans, compressor discs and blades, known as “space metal”. The more advanced the aircraft is, the more titanium is used. For example, the titanium content of F22 fourth-generation aircraft in The United States is 41% (mass fraction) and the titanium content of ITS F119 engine is 39%, which is the aircraft with the highest titanium content at present. Titanium alloy research originated from aviation and the development of aviation industry also promoted the development of titanium alloy. The research of titanium alloy for aviation has always been the most important and active branch in the field of titanium alloy, but its development is extremely difficult, for example, people spend more than ten years of energy to overcome the “thermal barrier” problem of titanium alloy for aeroengine.

      In this paper, titanium alloys are classified from the Angle of matrix phase composition. Taking aircraft as the representative of aircraft, the application and research of titanium alloy in aero-engine, aircraft fuselage and aviation fasteners are introduced emphatically. Finally, the problems existing in the development of titanium alloy for aviation are analyzed.

      1.Classification Of Titanium Alloys

      The classification of titanium alloys in the United States, Britain, Russia, France, Japan and other countries is mostly made by the manufacturer, with various names. Some companies directly use chemical symbols and numbers of elements to replace the names of alloy elements and their content, such as Ti-6Al-4V (equivalent to TC4 in China). The comparison and chemical composition of different countries are listed in Table 1. According to the phase composition of titanium alloy can be divided into: dense row hexagonal structure (HCP) α type titanium alloy (including near α alloy) – that is, the domestic brand TA, two phase mixture α+β type titanium alloy – that is, the domestic brand TC and body center cubic structure (BCC) β type titanium alloy (including near β alloy) – that is, the domestic brand TB.

      Figure 1 Comparison Of Titanium Alloy Grades In Various Countries

      1.1αTitanium Alloy

      The single-phase solid solution alloy with α ti as matrix in annealing state is α type titanium alloy, which mainly contains Al, Sn and other elements. Al is an important alloying element in titanium alloys, which can increase the tensile and creep strength of alloys, reduce the density of titanium alloys and improve the specific strength. In order to maximize the solution strengthening effect of aluminum and avoid alloy embrittlement caused by excessive Al, alloying of high-temperature titanium alloy should follow the equivalent empirical formula proposed by ROSENBERG. Only in this way can the alloy maintain good thermal stability while improving heat resistance strength. These elements in α titanium alloy stabilize by inhibiting or increasing the phase transition temperature. Compared with β titanium alloy, α alloy has good creep resistance, strength, weldability and toughness and is the preferred alloy for use at high temperature. At the same time, the α alloy does not have cold brittleness, it is also suitable for use in low temperature environment, extending its application range. The forging defects of α alloy are easy to occur due to its poor forging ability. The forging defects can be controlled by reducing the processing rate per pass and frequent heat treatment. The α matrix is a stable phase. For the alloy with a given composition, the change of its properties is mainly due to the change of grain size, because the yield strength and creep strength are related to grain size and the energy stored during deformation. The strength of α titanium alloy can not be improved by heat treatment and the strength of α titanium alloy has little or no change after annealing. Some alloys contain more Al, Sn, Zr and a small amount of β -stable elements (generally less than 2%). Although these alloys contain β phase, the matrix is mainly composed of α phase, which is very close to α alloy in heat treatment sensitivity and machining performance and is called near α type titanium alloy. The near-α alloys were developed on the basis of the high creep strength obtained by strengthening the α matrix with solid dissolved gold elements. Most of the near-α alloys have become important types of high-temperature titanium alloys due to their good thermal stability. Its strengthening mechanism is that the atoms in the β phase diffuse quickly and creep easily and the β stabilizing elements also inhibit the embrittlement of α phase (that is, delay the formation of ordered phase in α).

      Common α titanium alloys (including near α alloys) Ti811 (Ti-8Al-1Mo-1V), Ti-6Al-2Zr-1Mo-1V, Ti-679 (Ti-2.25Al-11Sn-5Zr-1Mo-0.25Si), BT18 (Ti-7.7Al-11Zr-0.6Mo-1NB-0.3Si), Ti811 (Ti-8Al-1Mo-1V), Ti-6Al-2Zr-1Mo-1V, Ti-679 (Ti-2.25Al-11Sn-5Zr-1Mo-0.25Si), BT18 (Ti-7.7Al-11Zr-0.6Mo-1NB-0.3Si) And Ti6242S (Ti-6Al-2Sn-4Zr-2Mo-0.1Si), its composition and properties are listed in Table 2.

      1.2α+β Titanium Alloy

      In order to improve the strength and toughness of titanium alloys, α+β titanium alloys have been developed. Compared with other titanium alloys, α+β alloys are strengthened by adding both α stable and β stable elements. α+β alloy has excellent comprehensive properties, such as its room temperature strength is higher than α alloy, good thermal processing performance, can be strengthened by heat treatment, so it is suitable for aviation structural parts. The annealed microstructure of α+β titanium alloys is α+β phase, the content of β phase is generally 5%~40%. But its structure is not stable, the highest temperature can only reach 500℃, welding performance and heat resistance is lower than α titanium alloy.

      α+β type titanium alloys are mainly TC4 (Ti-6A L-4V), TC 6 (T-I-6A L-1.5C R -2.5Mo0.5Fe-0.3Si), TC11 (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si), TC17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr), TC19 (Ti-6Al-2Sn-4Zr-6Mo) and TC21 (Ti-6.2Al) 2.8 Mo nb – 2-2 sn – 2.1 – Zr – 1.3 (Cr), etc. Among them, TC11 alloy is also called near β alloy.

      ZHOU proposed a TC11 alloy processing process, in which the alloy was first heat treated at a temperature below 15° β -transition temperature, followed by rapid water cooling and then toughened and strengthened at high and low temperatures to obtain a new microstructure. The matrix of the new structure consists of 15% equiaxed α grains, 50%~60% layered α grains and transformed β grains. The results show that the alloy exhibits high fatigue resistance, long creep fatigue life, high toughness and excellent high-temperature service performance without decreasing plasticity and thermal stability.

      Figure 2 Properties Of Severalα+β Titanium Alloy Alloys

      The experimental principle of the new process and strengthening and toughening mechanism is discussed. The key problem of the practical application of this process is the accurate control of temperature.

      This TC11 titanium processing process has been used to produce reliable aero-engine compressor discs, rotators and other components.

      1.3 β Titanium Alloy

      β -type titanium alloys are obtained when β -stable element content is high enough and β -phase is rapidly cooled after solution treatment and retained to room temperature. β titanium alloy can be classified into stable β titanium alloy and metastable β titanium alloy according to the stable state structure type, as shown in FIG. 1. In Figure 1, MS is the temperature line of martensitic transformation, βC is the minimum content of β stable elements in metal stable alloy and βS is the minimum content of β stable elements in stable alloy.

      Figure 1. The Relationship Between β Stabilizer Content And Titanium Alloy Phase Composition

      β alloy has good cold forming property, hardenability and heat treatment response in solution state.

      The commonly used heat treatment method is first solid solution treatment and then aged at 450~650℃, the fine α phase will precipitate in the original β matrix, forming a dispersed second phase, which is the strengthening mechanism of β alloy. Because β titanium alloy precipitates more α phase and contains more α-β phase interface to hinder dislocation movement, β titanium alloy has the highest strength at room temperature.

      The ability of metallic materials to absorb energy during deformation and fracture is called toughness and the more energy a material absorbs, the better its toughness. Fracture toughness is an indicator of the toughness of materials, reflecting the resistance of materials to crack and other sharp defects. Generally speaking, the fracture toughness of titanium alloys is inversely proportional to the strength, that is, the fracture toughness decreases as the strength increases. To study the application of β titanium alloy in aerospace industry, it is necessary to design the microstructure, processing technology and heat treatment system with good strength and fracture toughness. Alloy composition and microstructure are two main factors determining the fracture toughness of β titanium alloy. Alloy composition determines the amount of β phase in the alloy, as well as the type and fracture toughness of the alloy. The morphology, quantity and volume of microstructure also affect the fracture toughness of the alloy. Fu Yanyan believed that β stable element and medium element Zr of β titanium alloy could improve the strength and reduce the fracture toughness of the alloy. The fine β grains do not improve the strength of aged β titanium alloy effectively and decrease the fracture toughness of Ti-15-3 alloy, but have no obvious effect on the fracture toughness of β-C and Ti-1023 alloy.

      The strength of aged β titanium alloy is mainly determined by the content and size of the secondary α phase precipitated during aging. In the case of the same primary α phase, the fine secondary α phase can significantly improve the strength of the alloy.

The coarsening of the primary α phase and the transformation of the primary α phase from spherical to flake lead to the decrease of plasticity and increase of fracture toughness of β titanium alloy. The two-state structure of β titanium alloy has good matching of strength, plasticity and toughness.

      β titanium alloy is widely used because of its high strength and high plasticity advantages that other titanium alloys cannot match after aging. At the same time, β titanium alloy is gradually replacing α+β titanium alloy as the preferred structural material for aircraft fuselage and wing due to its heat treatable strengthening and deep quenching ability and it is playing an increasingly important role in the aerospace industry.

      2.Development And Application Of Titanium Alloys For Aviation

      In the 1950s, military aircraft entered the supersonic age and the original aluminum and steel structures could not meet the new demand. Titanium alloy entered the industrial development stage just at this time. Titanium alloy due to low density, high specific strength, corrosion resistance, high temperature resistance, no magnetic, welding, wide temperature range (269~600℃) and other excellent performance and can be a variety of parts forming, welding and mechanical processing in the aerospace field is soon widely used. In the early 1950s, military aircraft began to use industrial pure titanium to manufacture the heat shield, tail cover, reduction plate and other structural parts of the rear fuselage with less stress. In the 1960s, titanium alloy was further applied to the main structural parts of aircraft, such as flap rolling, load bearing frame, middle wing box beam, landing gear beam and so on. By the 1970s, the application of titanium alloy in aircraft structure expanded from fighter aircraft to military large bombers and transport planes and began to use a large number of titanium alloy structure in civil aircraft.

      Since 1980s, titanium used in civil aircraft has gradually increased and has exceeded that used in military aircraft. The more advanced the plane, the more titanium it uses. Fig 3-5 lists the mass fraction of titanium materials used by the 3rd and 4th generation fighter planes, advanced bombers and transport planes, titanium alloy types used by general aircraft and the amount of titanium alloy and composite materials used by Airbus aircraft respectively. It can be seen from Table 5 that titanium material used in Airbus A380 has reached 10% and titanium material has become an indispensable structural material for modern aircraft. According to different uses, titanium alloys for aviation can be divided into titanium alloys for aircraft engines, titanium alloys for aircraft fuselage and titanium alloys for aviation fasteners. In recent years, the applications of titanium alloys in the above three aspects have been studied deeply.

      Figure 3 The Quality Fraction Of Materials Used In The Third And Fourth Generation Fighter Planes And Advanced Bombers And Transport Planes Of USA

      Figure 4 Titanium Alloy Used In General Aircraft

      Figure 5 Titanium Alloy And Composite Materials Used In Airbus Aircraft

      Figure 6 Titanium Alloy For Aircraft Engines Of Various Countries

      2.1Titanium Alloy For Aeroengines

      The engine is the heart of an airplane. The rotating parts of the engine, such as fans, high-pressure compressor discs and blades, must not only bear great stress, but also have certain heat resistance. Such operating conditions are too hot for aluminum; It’s too dense for steel. Titanium is the best choice, titanium has good high temperature strength, creep resistance and oxidation resistance at 300~650℃. At the same time, an important performance index of the engine is the thrust-to-weight ratio, that is, the ratio of the thrust generated by the engine to its mass. The thrust-to-weight ratio of the earliest engines was 2-3, but now it can reach 10. The higher the thrust-weight ratio, the better the engine performance. Using titanium alloy instead of nickel base superalloy can reduce the engine mass and greatly improve the thrust-to-weight ratio of aircraft engine. Titanium is increasingly used in aircraft engines. In foreign advanced aeroengines, the amount of high-temperature titanium alloy has accounted for 25%~40% of the total mass of the engine, such as the amount of titanium alloy in the third generation of engine F100 is 25% and the amount of titanium alloy in the fourth generation of engine F119 is 40%.

Aeroengine parts require titanium alloy to have good instantaneous strength, heat resistance, durable strength, high temperature creep resistance and microstructure stability in the temperature range from room temperature to high temperature. Although β and near-β titanium alloys have high tensile strength at room temperature to about 300 ° C, their creep resistance and heat stability decline sharply at higher temperatures, so β titanium alloys are rarely used in aircraft engines. α and near α titanium alloys have good creep, durability and weldability, suitable for use in high temperature environments.

      α+β titanium alloy not only has good hot working performance, but also has good comprehensive performance in medium and high temperature environment. Therefore, α, near α and α+β titanium alloys are widely used in aeroengines. Table 6 lists titanium alloys for aircraft engines developed by various countries in the world.

      At present, the highest working temperature of high-temperature titanium alloy used in aero-engine has been increased from 350℃ to 600℃, which can meet the material demand of advanced engine. After half a century of efforts by titanium alloy researchers around the world, developed Ti811 (Ti-8Al-1Mo-1V), Ti-6Al-2Zr-1Mo-1V, Ti-679 (Ti-2.25Al-11Sn-5Zr-1Mo-0.25Si), TC6 (Ti-6Al-1.5Cr-2.5Mo-0.5Fe-0.3Si), TC17 (Ti) -5Al-4Mo-4Cr-2Sn-2Zr), TC19 (Ti-6Al-2Sn-4Zr-6Mo), TC21 (Ti-6.2Al-2.8Mo-2Nb-2Sn-2.1Zr-1.3Cr), Ti1100 (Ti-6Al-2.75Sn-4Zr-4Mo-0.4) 5Si), IMI834 (Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C) and other alloys.

      Ti811 (Ti-8Al-1Mo-1V) alloy has many advantages such as low density, high elastic modulus, excellent vibration damping performance, good thermal stability, good welding performance and good molding performance and its specific stiffness is the highest in all industrial titanium alloys. Zhao Yongqing conducted in-depth research on the thermal stability and high-temperature fatigue performance of Ti811 alloy and studied the influence of microstructure and sample surface state on the thermal stability of Ti811 alloy. The results show that Ti811 alloy with equiaxed structure and bimodal structure has good thermal stability. The existence of needle-like structure worsens the thermal stability of Ti811 alloy. In addition, it is found that the surface oxidation layer and exposure time of Ti811 alloy have no obvious influence on the thermal stability of the alloy at 425℃.

      Gao Guangrui studied the effects of temperature, displacement amplitude, contact pressure and other factors on the high-temperature fretting fatigue (FF) behavior of Ti811 titanium alloy by using a high frequency fatigue test machine and a self-made high temperature fretting fatigue device. The results show that:

      At 350℃ and 500℃, the fretting fatigue sensitivity of Ti811 alloy increases with the increase of temperature. Creep is an important factor affecting the FF failure of Ti811 alloy at high temperature and displacement amplitude changes affect the role and mechanism of fatigue stress factors and wear in FF process.

      Ti-6al-2zr-1mo-1v is a universal alloy successfully developed by the former Soviet Union in the 1960s. The alloy can work at 300~500 ℃ and is mainly used in the production of aircraft engine casings.

      OUYANG have done a lot of work in studying the recrystallization behavior of Ti-6Al-2Zr-1Mo-1V titanium alloy at different temperatures and strain rates. The results show that:

      When the deformation temperature is higher than 1050℃ and the strain rate is lower than 0.01s-1, the dynamic recrystallization mechanism is mainly discontinuous dynamic recrystallization. When the deformation temperature is lower than 1050℃ and the strain rate is higher than 0.01s-1, the dynamic recrystallization mechanism of the alloy is mainly continuous dynamic recrystallization, with a small amount of discontinuous dynamic recrystallization. In addition, the phase transformation orientation of Ti-6Al-2Zr-1Mo-1V alloy is different from other titanium alloys. HE studied the factors affecting phase transformation orientation of the alloy. The results show that the transformation of external factors, such as deformation stress, strain rate and cooling rate, obeies Burgers shift rule in β → α phase. However, strain rate and cooling rate can significantly affect the morphology of α precipitated phase.

      Ti-679 alloy is low aluminum and high tin, adding zirconium, molybdenum, silicon and other alloying elements to get, can be used as engine high-pressure compressor blade and disk. In its alloying elements, the role of aluminum is to improve the alloy strength, but easy to lead to poor shape, with low aluminum and high tin, can get better shape and strength; The function of molybdenum is to avoid forming too much β phase, so that the creep strength decreases. The role of zirconium is to complement and strengthen the alpha phase. Ti-679 alloy has good creep resistance and thermal stability and its operating temperature can reach 450℃.

      TC6 titanium alloy has good thermal strength and thermal stability and its mechanical behavior and microstructure changes at high temperature have attracted extensive attention from researchers all over the world. Bai Xinfang conducted thermal insulation heat treatment of TC6 titanium alloy at 990℃ to study the effect of oxygen atom and alloying element distribution changes on the surface microstructure and hardness. The results show that after heat treatment at 990℃, the microhardness of the oxygen-rich α layer on the inner surface of the sample shows a low-high-low variation from the edge to the interior of the matrix and reaches a maximum of 449HV1 at about 55μm away from the edge. The change of inner surface microhardness is caused by the distribution change of alloying elements and the enrichment of oxygen atoms. Sun Kun studied the dynamic mechanical behavior of four typical microstructure TC6 titanium alloy samples under high strain rate loading condition (1× 103S-1). The results show that the flow stress of TC6 titanium alloy with different microstructure increases rapidly with the increase of strain.

      TC17 titanium alloy is a transitional two-phase titanium alloy rich in β stable elements. The alloy has the advantages of high creep resistance, good hardenability and high fracture toughness at medium temperature (300~450℃) and is widely used in the manufacture of aero-engine fan disks and compressor disks. As a two-phase titanium alloy, TC17 can adjust its microstructure through heat treatment to improve its comprehensive mechanical properties. The standard heat treatment process is :(840℃, 1hAC) + (800 ℃, 4hWQ) + (630 ℃, 8hACTC4). Sun Xiaomin studied the microstructure of TC17 titanium alloy after laser melting deposition and solution aging. The results show that when the solution temperature increases from 800℃ to 835℃, the volume fraction of primary α phase decreases from 53% to 34% and the photo layer thickens significantly with the width of 0.7~0.8μm after aging. The content of secondary α phase increases gradually with the solution temperature increasing. TC19 titanium alloy is a β -rich α+β type titanium alloy developed in the United States in the 20th century. It is developed on the basis of Ti-6242 alloy (Ti-6Al-2Sn-4Zr-2Mo). It is a kind of titanium alloy with high strength and toughness. Compared with Ti-6242 alloy, TC19 alloy improves the tensile properties at room temperature and high temperature by increasing Mo content. With the addition of Sn and Zr, the phase transformation behavior of the alloy becomes very slow. Zhu Baohui studied TC19 titanium alloy bars prepared by different forging processes. The results show that both the conventional forging process and the high-low-high forging process can be used to forge TCl9 alloy bars, but the mechanical properties of the bars obtained by the high-low-high forging process are superior to the conventional forging process.

      TC21 alloy is a new type of high strength and toughness two-phase titanium alloy developed by China with independent intellectual property rights. It is used as an important structural material in aviation and aerospace field. Many studies have been carried out on the relationship between cooling rate, heat treatment and microstructure of the alloy. Wang Yihong proposed that when the cooling rate is greater than 122E /s, the β phase transforms into orthogonal martensite. When the cooling rate is between 122 and 3℃ /s, the block transformation takes place, the cooling rate continues to decrease and the phase transformation is controlled by diffusion, forming two kinds of weistensite lamellar with different morphology. The research results of Song Yinggang show that:

      An elastoplastic deformation layer was formed on the surface of TC21 titanium alloy after shot peening. During the strengthening process, the dislocation density increases due to the initiation of the slip systems of the base plane, cylinder plane and cone plane of densely packed hexagonal crystals and the dislocation morphology in phase A presents A network. The hardness of nanoindentation was 3.2GPa before and 6.7GPa after strengthening, which was more than double. A high macroscopic residual compressive stress is formed in the reinforcing layer and it is manifested by a gradual decreasing gradient from the surface inward. The depth of strengthening layer reaches 370μm. Gong Xuhui studied the dynamic tensile behavior of TC21 titanium alloy at high temperature. The results show that when the strain rate is 0.001 and 0.05s-1, the yield stress-temperature curve has a turning point and the turning point temperature increases with the increase of strain rate. When the temperature is lower than the turning point temperature, the yield stress of TC21 titanium alloy and polycrystalline pure titanium with the same oxygen content has a similar temperature correlation. Qu Henglei conducted compression tests on TC21 titanium alloy with strain rates ranging from 0.01 to 50s-1 and temperatures ranging from 973-1373K and concluded that there were uneven deformation structures in different parts of the sample and re-crystallization and dynamic re-crystallization occurred in the deformation of the alloy at different temperatures. Re-crystallization results in grain coarsening (about 100~200μm in size).

      Dynamic re-crystallization results in grain refinement (minimum size 1~2μm).

      The above alloys are titanium alloys used in conventional aero-engines and their operating temperatures are all below 650 ℃.

      At present, the practical performance of heat-resistant titanium alloys are Ti1100 and IMI834, which have been used in EJ2000 and 55-712 variants respectively. Due to the occurrence of “titanium fire” accident, flame retardant titanium alloy has attracted more and more attention. New titanium alloys with good flame retardancy have been developed in USA and Russia. The high strength and flame retardant titanium Alloy C developed by Pratt & Whitney has been used as the vector nozzle part of F119 engine. The nominal composition of the Alloy is Ti-35V-15Cr (mass fraction, %). The Alloy contains a large amount of expensive metal vanadium. Further increase in material prices. Lower cost Ti-Cu alloys have been studied in Russia and BT25 and BT36 alloys have been reported. Chinese researchers have systematically summarized and appraised the previous research work on titanium alloys for engines.

      2.2Titanium alloy for aircraft fuselage

      Aircraft engines require alloys with good thermal strength and specific strength, while the fuselage requires alloys with good strength, corrosion resistance and light weight at moderate temperatures. Titanium alloy can well meet these requirements, using titanium alloy as the fuselage material has the following five advantages: 1) replacing steel and nickel-based superalloy can greatly reduce the aircraft mass. The high thrust-to-weight ratio allows titanium to replace stronger steel in aircraft parts. 2) Can meet the aircraft strength requirements.

      Compared with aluminum alloys, titanium alloys with about 60% mass can achieve the same strength. Titanium alloy can replace aluminum alloy when the operating temperature exceeds 130℃, because this temperature is the limit applicable temperature of traditional aluminum alloy. 3) Good corrosion resistance. Most aircraft support structures are under kitchens and toilets, which are prone to corrosion and titanium alloys do not require anti-corrosion coating or coating. 4) Good electrochemical compatibility with polymer composites. 5) Space limitation, replacing steel and aluminum alloy. A typical example of titanium being used for space constraints is the titanium landing gear beam of the Boeing 747. This beam is the largest titanium alloy forgings, although other alloys (such as 7075) cost less, but when the load requires mass, aluminum landing gear volume beyond the wing is not acceptable. Steel is strong enough to hold mass, but it adds a lot of mass to the plane. Figure 2 is a schematic diagram of the fuselage materials used by Boeing 777 aircraft. Titanium alloys widely used in aircraft fuselage include β-21S (Ti-15Mo-3Al-2.7NB-0.2Si), Ti-10-2-3 (Ti-10V-2Fe-3Al), Ti-15-3 (Ti-15V-3Cr-3Al-3Sn), Ti-3Al-8V-6Cr-4Mo-4 Zr, etc. BOYER has summarized the application of titanium alloys in airframes and the authors of this article discuss only the first two alloys.

      β-21S (Ti-15Mo-3Al-2.7Nb-0.25Si) alloy is developed by American Timet company for the National Space Shuttle. It can be made into strips with oxidation resistance and can be used as composite materials.

      It has better high temperature properties and better creep resistance than Ti-6-4 (the creep resistance of β alloy is not good at high temperature).

      The β-21S has been used by Boeing and P&W at temperatures up to 650 ° C instantaneously and has a continuous operating temperature of 480 ° C to 565 ° C. The outstanding advantage of β-21S alloy is that it can better resist the corrosion of high temperature hydraulic press liquid. This hydraulic fluid is one of the few substances that can corrode titanium alloys in the space environment. At temperatures above 130℃, it breaks down and forms a phosphoric acid containing organometrics, which corrodes titanium alloys and, more importantly, causes serious cracks in engine pumps that contain a lot of hydrogen. β -21s is the only metal that is resistant to this corrosive agent because β -21s contains molybdenum and niobium, which are used in engine nacelles and jet engine parts (originally steel or nickel-based alloys). In addition, β -21s can reduce the mass of the nozzles, plugs, skins and various rail structures used in the manufacture of the three engines used in the 777 (P&W4084, GE90 and Trent800), which can reduce the mass of each aircraft by 74 kg.

      Ti-10-2-3 (Ti-10V-2Fe-3Al) is one of the most widely used titanium alloys with high strength and toughness. It was first developed by American Timet company in 1971. It is a forged titanium alloy with high structural benefit, high reliability and low cost resulting from the design principle of damage tolerance. V and Fe are the main β stable elements. In order to improve the forging performance and fracture toughness of the alloy, Fe content is less than 2% and O content is limited to less than 0.13%.

      The tensile strength of the forgings can reach 11901Mpa and the mass of each aircraft can be reduced by 270kg with Ti-10-2-3.

      Boeing produces the aircraft using high-strength alloys to minimize mass. This titanium alloy is the largest β titanium alloy used in Boeing 777. The landing gear of the aircraft is almost entirely made of this alloy, with only the inner and outer cylinders and axles made of 4340M (strength of 1895MPa). The main landing gear struts of the Airbus A380 are also ti-10-2-3. The alloy also has good fatigue resistance and can eliminate stress corrosion cracking caused by using steel. McDonnellDouglas uses ti-10-2-3 (1105 MPa) for cargo doors, engine nacelle, tail fins and other parts of the c-17 transport aircraft. The advantage of TI-10-2-3 in fatigue strength also makes it widely used in helicopters. Companies such as Bell, Westland, Sikorsky and Eurocopter use Ti-10-2-3 for their rotor systems.

      2.3Titanium Alloy For Aviation Fasteners

      In addition to metal components, there are many carbon fiber composite materials on both military and civilian aircraft and spacecraft.

      The electrode potential of titanium and carbon fiber composites is similar and titanium alloy is the only connecting material of the composites. Therefore, with the increasing use of titanium alloy and composite materials for advanced military and civil aircraft, the demand for titanium alloy fasteners is increasing day by day. Titanium alloy used as aerospace fasteners has at least the following four advantages: 1) good weight reduction effect. A Russian il-96 aircraft uses 142,000 fasteners, which can reduce the mass by nearly 600kg. The use of titanium alloy fasteners in China’s aerospace system also has obvious weight reduction effect. Reducing the mass of aircraft and spacecraft can improve thrust, increase range and save fuel, reduce launch costs and so on.

      2)The excellent corrosion resistance of titanium alloy especially can match with its positive carbon fiber composite material, which can also effectively prevent galvanic corrosion of fasteners. 3）

      Aluminum cannot be used for fastener parts due to high temperature, only titanium alloy can be used. 4）Titanium suspension has good elasticity and no magnetic field, which is very important to prevent loosening of fastening bolts and magnetic field interference.

      Modern aircraft use a variety of titanium alloy fasteners, including ordinary titanium bolts, interference bolts, special fasteners and so on. In developed countries such as the United States and France, more than 95% of titanium alloy fasteners are made of Ti-6Al-4V (TC4) material. In addition, there are TB2, β III, TI-44.5, TI-15-3 (TB5), TB8 and TB3, whose typical performance parameters are listed in Table 7.

      The β stability coefficient of Ti-6Al-4V (TC4) alloy is the lowest, which is 0.27. Its advantages are the lowest density, good strength and fatigue performance, simple alloy composition, the lowest cost of semi-finished products. But because the room temperature plasticity is not high enough, it is necessary to use induction heating for hot upsetting forming when processing fasteners and the processing cost of vacuum solution treatment and aging treatment is high.

      TB2, TB3, TB8 and TB16 are metastable β titanium alloys with higher β stability coefficient than alloy. Their disadvantages are higher density, similar to ti-6Al-4V in strength, but inferior to Ti-6Al-4V in fatigue performance, complex composition and high cost of semi-finished products. The cost of finished fasteners is higher than that of TI-6Al-4V due to the same vacuum aging process.

      Figure 7 Some Typical Performance Parameters Of Titanium Alloys For Fasteners

      3.Existing Problems And Prospects

      Titanium is a kind of excellent performance and abundant reserves of metal, known as “modern metal”, after half a century of development, titanium alloy preparation technology and application research have made great progress, especially in the aerospace field has been widely used. However, some existing problems are also gradually exposed and the further development of titanium alloy for aviation is facing no small challenges, mainly manifested in the following three aspects:

      (1) Dosage. Whether for military or civil aircraft, titanium alloy dosage directly reflects a country’s aviation level. At present, aero-engine titanium consumption is low. The difficulty of using quantity is still quite large in order to further increase to about 50%,.

      (2) Performance. Like other aerospace structural materials, high performance requires good matching of properties, that is, mechanical properties, physical properties, chemical properties, process properties and the controllability of defects must be considered comprehensively. The creep resistance and high temperature oxidation resistance of existing titanium alloys above 600℃ are two major obstacles to the expansion of titanium alloys.

      In this paper, the author believes that in the whole process of the development and application of aerospace titanium alloy technology, new manufacturing technologies will be the focus of development and research, such as superplastic forming and powder metallurgy forming method.

      (3) Manufacturing Cost. Although some achievements have been made in reducing the cost of titanium alloys, there are still many fields to be researched and developed. Taking flame retardant titanium Alloy as an example, although Alloy C invented in the United States had excellent flame retardant characteristics and high temperature mechanical properties, it was only formally applied in F119 engine because it needed to add a large number of expensive V and poor malleability, resulting in a high price.

      Due to backward management and technology, the price of domestic titanium alloy products is poor in the international competitiveness, which is not conducive to further expanding the application in China. Therefore, first of all, we must seriously discuss the way to reduce the cost of titanium products and determine the near, medium and long-term development plan. Secondly, China should establish its own titanium alloy system to ensure that there are a variety of alloy options for each use and gradually get rid of the long-term dependence of aviation key materials on foreign countries, forming backbone materials or general materials, fundamentally laying the foundation for the realization of low-cost manufacturing. Finally, to replace expensive alloy elements with cheaper elements and reduce the cost of titanium alloy parts through technological approaches is an important topic in the future titanium alloy research work.

      To sum up, titanium alloy has high thrust-to-weight ratio, high toughness, good strength and weldabi0.lity and is a kind of aviation material with excellent comprehensive properties. In the past few decades, the alloying theory, comprehensive strengthening and toughening technology and heat treatment process of titanium alloys for aviation have been developed greatly. At present, the research of titanium alloy mainly focuses on thermal stability, creep resistance and low cost titanium alloy design and manufacturing process at high temperature. With the deepening of the research, the high-end application of aviation will drive the technological progress of low-cost processing of titanium alloy, so as to fundamentally break the cost bottleneck restricting the improvement of the amount and application level of aviation titanium alloy. An all-titanium airplane may become a reality in the near future.