Titanium is closely connected to the aviation industry. Since the first application of titanium materials in the engine nacelles and fire walls of DC-T aircraft by Douglas Corporation in 1953, the application of titanium in the aviation industry has gone through more than half a century of glorious history.

Since the industrial production of titanium in the 1940s, its high specific strength, excellent corrosion resistance, non-magnetism, and excellent welding performance have made titanium alloys widely used in many fields such as aerospace, military industry, marine development, petrochemical, power generation, and superconductivity. These advantages have earned titanium alloys the reputation of "versatile metal", "ocean metal", "third metal", "modern metal", and so on.

With the further exploration of the properties of titanium alloys, their application scope is still expanding, and titanium alloys are expected to become the third largest structural metal after steel and aluminum. This article comprehensively reviews the research and application progress of titanium alloys in the aviation and aerospace fields in countries such as the United States, Russia, the United Kingdom, Japan, and China, aiming to provide valuable reference and inspiration for the application and development of China's titanium industry in the aviation and aerospace fields.

1. Titanium alloy raw materials

Given the important role of titanium in defense, aviation, high-tech and other fields, it has been highly valued by military powers such as the United States, Russia, the United Kingdom, France, and Japan, and has been listed as a strategically significant structural metal for the 21st century. The development of titanium science and technology, including new alloys, new melting technologies, new T addition technologies, and application technologies, is undergoing rapid changes. China's titanium industry has gone through nearly 40 years of ups and downs, and with the support of the state, it has made great progress, establishing its own independent titanium industry system. Based on the production of 1751 tons of sponge titanium and 2206 tons of titanium processing materials in China in 2000, China produced 49632 tons of sponge titanium in 2008, an increase of 27.3 times in 8 years; In 2008, China produced 27737t of titanium butadiene materials, an increase of 11.6 times.

Due to the high cost of titanium alloy raw materials, 70-80% of titanium materials abroad are used in the aerospace industry. The demand for titanium alloys in China's aviation and aerospace industries is also particularly high. At present, the proportion of titanium alloy used in advanced aircraft under research in China is around 10% to 12%, and the proportion of titanium used in military aircraft is even higher, around 20% to 30%, while the proportion of titanium used in military aircraft engines is over 30%. The amount of titanium used in new rockets and missiles is also increasing.

2. Development and application of structural titanium alloys

With the gradual shift of aircraft design concepts from simple static strength to safety life, damage safety, and even modern damage tolerance design concepts, advanced titanium alloy materials are also gradually developing towards damage tolerance titanium alloys with high fracture toughness and low crack propagation rate. At present, developed countries abroad have taken the lead in the development of new damage tolerant titanium alloy materials and their application in advanced aircraft, especially in areas such as medium strength Ti-6Al-4V ELI and high-strength Ti-6-2222S, which have been successfully applied in the US F-22 In new generation aircraft such as F-35 and C-17. Greatly improving the service life and combat effectiveness of the aircraft. With the development of aircraft design concepts, the damage tolerance design concept of titanium alloy structures has also begun to receive attention in China. Since the 15th Five Year Plan, China has independently innovated and developed TC4-DT medium strength and high toughness damage tolerance titanium alloy and TC21 high strength and high toughness damage tolerance titanium alloy, and established β processing technology for damage tolerance titanium alloy, laying a material application technology foundation for the development of new aircraft in China. In order to meet the development needs of titanium alloys for aerospace structures, China has independently developed low strength and high toughness wire titanium alloys (NbTi) and pipe alloys (TAl8), as well as a series of ultra-high strength titanium alloys (TB8) ranging from 1300 MPa to 2000 MPa TBl9, TB20, etc. have initially formed a new type of titanium alloy material system with Chinese characteristics for aircraft structures, laying the foundation for the application framework of the new generation of titanium alloys for aviation and aerospace structures.

Ti-6Al-4V (TC4) is a medium strength alpha beta titanium alloy developed in the early 1960s, which has excellent comprehensive properties and is known as a universal alloy, TC4 titanium alloy is the earliest and most widely used universal titanium alloy for aerospace structures, including plates, bars, and forged castings. This alloy has good welding and machining properties, and fine-grained alloys have superplasticity. The use of superplastic forming/diffusion bonding (SPF/DB) combination process can manufacture complex components.

High strength structural titanium alloys generally refer to alloys with tensile strength above 1000MPa. Currently, high-strength titanium alloys that represent international advanced levels and have been practically applied in aircraft mainly include the metastable β - type alloy Ti-15-3 β321s， Near beta alloy Ti-1023 and alpha beta two-phase titanium alloy BT22. Replacing the commonly used 30CrMnSiA high-strength structural steel in aircraft structures with high-strength structural titanium alloys can reduce weight by more than 20%.

Ti-6Al-2Sn-2Zr-2Cr-2Mo (TC21) is a high-strength, high toughness, and damage tolerant two-phase titanium alloy developed in the 1970s. This alloy has advantages such as high strength, good damage tolerance, and excellent resistance to fatigue crack propagation after thermal mechanical treatment, making it suitable for manufacturing high-strength and high toughness load-bearing components. By adding Si element, the alloy maintains high strength at medium temperature, which is superior to Ti-6AI-4V. This alloy sheet can undergo superplastic forming at room temperature.

Ti-10V-2Fe-3Al (TB6) is a high-strength and high toughness near beta titanium alloy developed in the late 1970s. This alloy has advantages such as high specific strength, good fracture toughness, large quenching area, small anisotropy, good forging performance, and strong corrosion resistance. It also has many advantages of metastable beta titanium alloy without losing (the solid solution characteristics of IT-B titanium alloy can meet the requirements of damage tolerance design, high structural efficiency, high reliability, and low cost, with a maximum working temperature of 320 ℃. The main products of this alloy include bars, forgings, thick plates, and profiles. Through solid solution and timely heat treatment, a good match of strength, plasticity, and fracture toughness can be achieved, suitable for manufacturing structural parts with high requirements for strength and fracture toughness. Through thermal mechanical treatment, excellent toughness and low crack propagation rate can be obtained, suitable for fracture toughness. High demand structure.

3. Development and application of high-temperature titanium alloys

High temperature titanium alloys have been widely used in aviation engines due to their excellent thermal strength and high specific strength. High temperature titanium alloys are mainly used in fans and compressors of aircraft engines, such as compressor discs, blades, navigators, connecting rings, etc. Replacing the original nickel based high-temperature alloy with titanium alloy can reduce the weight of the compressor by 30-35%. The proportion of titanium used in advanced aircraft engines abroad has reached 25-39%, for example, the titanium alloy used in F100 engines accounts for 25% of the total engine weight, The V2500 engine is 31%, The F119 engine is 39%. The development demand of high-performance aircraft engines is driving the development of high-temperature titanium alloys, with the usage temperature gradually increasing from 400 ℃ represented by Ti-6Al-4V alloy in the 1950s to 600 ℃ represented by IMl834 alloy. The sharp decrease in creep resistance and high-temperature oxidation resistance above 600 ℃ is the two main obstacles limiting the development of titanium alloys towards higher temperatures. Therefore, 600 ℃ is considered as the "thermal barrier" temperature for the development of titanium alloys.

For many years, in order to meet the demand for high-performance aviation engines, developed countries in the aviation industry such as Europe, America, and Russia have attached great importance to the research and development of high-temperature titanium alloys, and have successively developed high-temperature titanium alloys for use at 350-600 ℃. The former Soviet Union developed BT6 in the late 1950s, BT3-l, BT8, BT9 and other titanium alloys were developed, and BTl8 was also developed in the 1960s and 1970s BT25 alloy. Afterwards, in order to improve the performance and service life of high-temperature titanium alloys, BTl8y was improved and developed on the basis of the original alloy, High temperature titanium alloys of grades BT25y, BT8M, BT8-1, and BT8M-1.

In recent years, BT36 titanium alloy has been developed for HK8, IIC90A and other engines. Similarly, the United States will also promote Ti64, Titanium alloys such as Ti811 and Ti6242 are used for JT90, respectively, Among advanced engines such as F-110.

The development of high-temperature titanium alloys in Russia is very complete and mature, forming a complete titanium alloy system. There are two or three optional high-temperature titanium alloy grades at a certain temperature level, such as BT8, which can be used at 500 ℃ The alloys used for BT9 and BT8-1550 ℃ include BT25 and BT25y, while the alloys used for 600 ℃ include BTl8y and BT36. Russia recommends BT25y for discs and rotor blades used in high-pressure compressors of aircraft engines at 450-550 ℃, and BTl8y for discs used at 550-600't3. Although BT36 has been developed, it seems to have not received the corresponding attention. China once imported BT36 alloy discs and bars produced in Russia. After analysis, it was found that there is a large amount of component segregation on the alloy discs and bars, and the problem of component uniformity has not been well solved. Moreover, its high-temperature performance has not reached the level of IMl834 alloy.

The development of high-temperature titanium alloys in the UK is the most mature, with its own independent system, forming a series of titanium alloy grades used at different temperatures. So far, IMl685 alloy is the most widely used and abundant high-temperature titanium alloy in aviation engines in the UK, such as the RB211 series engines used by Rolls Royes RBl99 engine Adour engine and M53 engine, etc. IMl829 alloy is used in the high-pressure compressor of RB211-535C engine. The rear third stage disc, drum, and rear shaft produced are integrated by electron beam welding, replacing the nickel based alloy material on RB211-535C, reducing the weight of the rotor by 30%. The successful development of IMl834 alloy has provided solid technical support for some high-performance engines. Although it has not been developed for a long time, it has been tested and applied in various engines. For example, the Trent700, a large civilian engine used in the Boeing 777 aircraft, uses IMl834 alloy for all wheels, drums, and rear axles of its high-pressure compressor, which are welded together using electron beam welding technology. Making Trent700 the first new civilian engine to use all titanium high-pressure compressor rotors significantly reduces the weight of the engine, The high-pressure compressor rotor of the EJ200 engine also uses IMl834 alloy. IMl834 is also being used on Pratt&Whitney's PW350 engine.

The development of high-temperature titanium alloys in the United States is also relatively mature, and currently the most widely used alloys in engines are Ti-6Al-4V and Ti-6242S.

Ti-1100 alloy is based on the composition of Ti-6242 S alloy by adjusting Al The content of Sn, Mo, and Si elements increases the maximum service temperature of the alloy to 600 ℃. It is understood that, Ti-1100 alloy has been used to manufacture high-pressure compressor discs and low-pressure turbine blades for the T55-712 modified engine of Lai Kang Ming Company.

The development of titanium alloys in China mainly follows the path of imitation, such as TC11 alloy corresponding to BT9 alloy, TA11、TA19、TC17， The corresponding US grades are Ti-811 Ti-6242S and Ti-17. In the past 20 years, China has started to follow the path of imitating and developing independently, such as high-temperature titanium alloy TA12 (Ti-55), with the addition of rare earth element Nd; Ti-60 alloy adds Al appropriately on the basis of TAl2 alloy The content of Sn and Si further improves the high-temperature creep performance and strength of the alloy, resulting in a service temperature of 600 ℃. On the basis of IMl829 alloy in the UK, rare earth element Gd was added to develop Ti-633G high-temperature titanium alloy at 550 ℃ domestically. Recently, on the basis of Ti-1100 alloy, about 0.1Y has been added and named Ti-600.

4. Development and application of low-temperature titanium alloys

Structural components used at low temperatures require good plasticity, low thermal conductivity, and excellent processing performance while maintaining a certain level of strength. The main low-temperature structural materials used domestically and internationally are stainless steel, aluminum alloy, titanium alloy, and nickel based alloy. Titanium alloys have excellent comprehensive properties at low temperatures and have been widely valued by countries around the world for many years. At low temperatures, the yield strength of titanium alloys significantly increases, about 3-6 times that of austenitic stainless steel; But the fracture toughness decreases with decreasing temperature, about 0.25~0.5 of that of austenitic stainless steel. Due to its much lower density than stainless steel, low thermal conductivity, low coefficient of expansion, and non-magnetism at low temperatures, titanium alloy is used as an important low-temperature engineering material in aerospace, superconductivity, and other fields.

β titanium alloys with Bee structure at low temperatures, like other body centered cubic metals, have a higher plastic to brittle transition temperature (TPR). As the temperature decreases, the ductility decreases, and they are generally not suitable for use at low temperatures. The TPR of alpha and near alpha titanium alloys is generally low, and they also have good plasticity at low temperatures. Therefore, some internationally recognized low-temperature titanium alloys are basically alpha and near alpha titanium alloys. In alpha beta titanium alloys, titanium alloys with less beta phase, such as Ti-6Al-4V ELI, can also be used well at liquid hydrogen temperature (22 K). Pure titanium and alpha titanium alloys such as Ti-5Al-2.5Sn ELl are ideal low-temperature structural materials at liquid helium temperature (4.2 K), but impurities outside the alloy composition, especially oxygen and iron content, must be controlled. The increase in iron and oxygen components increases the low-temperature brittleness of titanium materials. In addition, the increase in stable beta phase elements such as iron and manganese can easily lead to notch embrittlement of the material.

The former Soviet Union once led the world in the development and application of low-temperature titanium alloys, and its early development of alpha titanium alloy OT4, OT4-l, BT5-1KT, TT-3BKT and other alloys have been widely used in aerospace rocket equipment. These alloys increased their strength to 1400 MPa at 2 K, while their elongation remained above 10%. The low-temperature titanium alloys developed and applied in the United States mainly include Ti-5Al-2.5Sn Low temperature alpha titanium alloys such as Ti-8Al-1Mo-1V and Ti-6Al-3Nb-2Zr.

China started later than the United States and Russia in the research and application of low-temperature titanium alloys, and has continued to carry out research on existing TA7 technologies After conducting low-temperature performance testing and application research on titanium alloys such as TC1 and TC4, a titanium alloy suitable for low-temperature pipeline systems was developed during the Ninth Five Year Plan period. The alloy system is Ti Al Zr, Ti A1 Zr Mo, Ti AIL Sn Mo, Ti Al Zr Sn Mo, etc.

5. Development and application of titanium alloy for fasteners

The application of titanium alloy fasteners abroad has become very common, and various new types of fasteners are constantly emerging. The usage of titanium alloy fasteners for large-scale civilian use has reached hundreds of thousands. Under the same strength index, titanium fasteners are 70% lighter in weight than steel, and the fatigue strength and sensitivity to stress concentration of titanium alloys are superior to similar purpose steels. They have high corrosion resistance stability in various climate conditions. Therefore, the application of titanium fasteners is very important for aviation equipment.

1. Development of Titanium Alloy Fasteners

Titanium alloy fasteners mainly use three types of materials: the first type is a low Mo equivalent alpha beta type two-phase alloy, such as Ti-6Al-4V; The second type is metastable beta alloys, including beta III from the United States, Ti-44.5Nb, Ti-15-3 and TB2 in China, TB3 and TB8; The third type is α - β type two-phase alloys with subcritical composition, such as BT16l from Russia. The following table shows the characteristics of titanium alloy fastener materials.

Ti-6Al-4V is a low Mo equivalent α - β type two-phase alloy, with the lowest β stability coefficient (only 0.27) among the three types of alloys, and the highest aluminum equivalent (up to 6). So the beta phase content in the annealed state is only 7% (volume fraction). Its advantages are the lowest density, the best strength and fatigue performance, the simplest composition, and the lowest semi-finished product cost. However, due to the insufficient room temperature plasticity, induction heating is required for hot heading forming, as well as vacuum solution treatment and aging treatment when processing fasteners, resulting in higher processing costs.

The second type is beta alloys (such as TB2, TB3, TB5, TB8, etc., are completely different from α - β type alloys, The stability coefficient of B β is very high, ranging from 1.15 to 1.97, while the aluminum equivalent decreases to around 3. So during solid solution treatment, a single β phase can be obtained, allowing for cold forging of bolts and rivets at room temperature with low processing costs. The disadvantage is that the density is high, although the strength is comparable to Ti-6Al-4V, the fatigue performance is not as good as Ti-6Al-4V, and the composition is complex, resulting in high semi-finished product costs. Due to the need for vacuum aging treatment, the cost of finished fasteners is still higher than Ti-6Al-4V, and the operating temperature is also lower than Ti-6Al-4V.

The density of BT16 alloy is slightly higher than that of Ti-6Al-4V, but significantly lower than that of β alloy. The β stability coefficient of BT16 alloy is 0.83, which is between the above two categories and close to the critical composition (β stability coefficient is 1). In binary alloys composed of β - stable elements and Ti, as the content of β - stable elements increases, the grain size gradually decreases, and around the critical concentration of l, The number of alpha and beta phases is equal, and the grain size reaches the minimum. As the stable elements further increase, the grain size increases. The smaller p-grains and a high β phase content of up to 25% (volume fraction) in the annealed state determine the excellent room temperature T-process plasticity of BT16 alloy. So BT16 alloy has the condition to complete rapid upsetting of fastener heads at room temperature, that is, cold upsetting.

2. Application of Titanium Alloy for Fasteners

Ti-6A1-4V is a medium strength α - β type two-phase titanium alloy with excellent comprehensive properties. The semi-finished product specifications are complete, including bars, forgings, thick plates, thin plates, profiles, and wires. This alloy has a long-term working temperature of up to 400 ℃ and has been widely used in the aviation and aerospace industry. It is the main fastener material used in the aviation and aerospace sectors in the United States and Western European countries. Russian titanium alloy fasteners mainly use BT16 titanium alloy. BT16 alloy belongs to the Ti-Al-Mo-4V series α - β high-strength titanium alloy. The main semi-finished products are hot-rolled rods and polished rods and wires for cold heading, mainly used for manufacturing fasteners such as bolts, screws, nuts, and rivets. The maximum working temperature is 350 ℃. The strength of this alloy is slightly lower than that of Ti-6Al-4V alloy in the solid solution aging state. The main advantage is that it can be cold upset formed in the annealed state, which significantly improves production efficiency. Fasteners manufactured by cold deformation are widely used in Russia's mechanical manufacturing industry and are also the main standard component material used in Russia's aerospace and aviation sectors. They are also used in certain models of aircraft in the country. This alloy has two usage states: cold deformation strengthening without heat treatment and hot forging strengthening with solution aging treatment.

β III alloy was included in the AMS4977 specification as a fastener material in 1969 and has some applications in aircraft. However, in 1987, AMS4977B announced that the aerospace materials department recommended that β 11I alloy no longer be used as a standard component material for future new designs. According to recent reports, the alloy has ceased production. Ti-44.5Nb, as a special material for rivets, was included in the AMS4982 specification in 1974 and revised to AMS4982C in 2002. It is still in use today, but only a small section is welded to the head of Ti-6Al-4V rivets for cold riveting. Ti-15-3 (TB5) was first listed as a thin plate in the AMS4914 specification in 1984. TB5 and TB8 are used in China as matching rivets and screws for resistance parachute beams and wind deflectors (for high-temperature use) for a certain model of aircraft. TB2 and TB3 are domestically developed beta alloys in China. TB2 was initially used for sheet metal parts and later applied as rivets on certain models.

TB3 has been used as a material for bolt development since its inception and has also been applied in certain models.

6. Conclusion

Titanium is an important structural material for the development of national defense, aviation, high-tech and other fields in China, and has significant strategic significance. At present, China's research and development level, production capacity, and output of sponge titanium and titanium processing materials have reached the forefront of the world. The future development direction should focus on researching and developing higher performance alloys according to application needs and international development trends, improving the technical level of the titanium production industry, and moving from a titanium industry powerhouse to a titanium industry powerhouse.