1.Introduction

Since the industrial production of titanium in the 1940s, it has been widely used in aerospace because of its high specific strength, good corrosion resistance, non-magnetic, good welding performance and other advantages, as well as superconductivity, hydrogen storage, memory and other advantages. , military industry, marine development, petrochemical, power generation, superconductivity and other fields, has the reputation of "all-round metal", "marine metal", "third metal", "modern metal" and so on. With the continuous exploration of the excellent properties of titanium, its application scope is still expanding, and it will become the third structural metal after steel and aluminum. In view of the important role of titanium in national defense, aviation, high-tech and other fields, it has been highly valued by the United States, Russia, Britain, France and other military powers and Japan and other countries, and listed as a key structural metal with strategic significance in the 21st century. The development of titanium science and technology, including new alloys, new smelting technology, new T technology and application technology, is undergoing rapid changes. China's titanium industry has experienced ups and downs for nearly 40 years. With the support of the state, it has made great progress and established its own independent titanium industry system. In 2000, China produced 1,751 t of titanium sponge and 2,206 t of titanium processed materials. In 2008, China produced 49,632 t of titanium sponge, an increase of 27.3 times in 8 years. In 2008, China produced 27,737 t of titanium processed materials, an increase of 11.6 times.

Due to the high cost of titanium alloy raw materials, 70% to 80% of titanium materials abroad are used in the aviation and aerospace industries. The demand for titanium alloys in my country's aviation and aerospace fields is also particularly large. At present, the proportion of titanium alloys used in advanced aircraft under development in my country is about 10% to 12%. The proportion of titanium used in military aircraft is higher, at about 20% to 30%, and the proportion of titanium used in military aircraft engines is more than 30%. . The amount of titanium used in new rockets and missiles is also increasing.

This paper mainly summarizes the progress of the research and application of titanium in the aviation and aerospace fields in the United States, Russia, Britain, Japan and China, which can serve as a reference for the application and development of my country's titanium industry in the aviation and aerospace fields.

2. Development and application of structural titanium alloys

With the gradual change of the aircraft design concept from the simple static strength in the past to the safety-life, damage-safety, and even the modern damage tolerance design concept. Advanced titanium alloy materials are also gradually developing towards damage-tolerant titanium alloys with high fracture toughness and low crack growth rate. At present, foreign developed countries have taken the lead in the development of new damage-tolerant titanium alloy materials and their application in advanced aircraft, especially those such as medium-strength Ti-6Al-4V ELI and high-strength Ti-6-222S etc., has been successfully applied in the new generation of aircraft such as American F-22, F-35 and C-17. Greatly improve the service life and combat effectiveness of the aircraft. With the development of aircraft design concepts, the damage tolerance design ideas of titanium alloy structures have also begun to receive attention in my country. Since the "Tenth Five-Year Plan", my country has independently developed TC4-DT medium-strength and high-toughness damage-tolerant titanium alloy and TC21 high-strength and high-toughness damage-tolerant titanium alloy, and established the β machining of damage-tolerant titanium alloy. technology, which has laid the foundation of material application technology for the development of new aircraft in my country. In order to meet the development needs of titanium alloys for aviation and aerospace structures, my country has independently developed low-strength and high-toughness wire titanium alloys (NbTi) and tube alloys (TAl8), 1300 MPa~2 000 MPa series ultra-high-strength titanium alloys (TB8) , TB19, TB20), etc., a new titanium alloy material system with Chinese characteristics for aircraft structures has been initially formed, and the application frame structure of a new generation of titanium alloys for aviation and aerospace structures has been established. The specific properties are shown in Table 1.

Ti-6Al-4V (TC4) is a medium-strength α-β titanium alloy developed in the early 1960s. It has excellent comprehensive properties and is known as a universal alloy. It is the earliest and most widely used in aviation and aerospace structures. General-purpose titanium alloys, including plates, titanium rods, and forgings, etc. The alloy has good welding and machinability properties, and the fine-grained alloy has superplasticity, and complex components can be fabricated by the combined process of superplastic forming/diffusion bonding (SPF/DB).

High-strength structural titanium alloys generally refer to alloys with a tensile strength of more than 1000MPa. At present, high-strength titanium alloys that represent the international advanced level and have been practically used in aircraft mainly include metastable β-type alloys Ti-15-3, β21S, near β-type alloy Ti-1023 and α-β-type two-phase titanium alloy BT22. Replacing the 30CrMnSiA high-strength structural steel commonly used in aircraft structures with high-strength structural titanium alloys can reduce the weight by more than 20%.

Ti-6Al-2Sn-2Zr-2Cr-2Mo (TC21) is a high-strength, high-toughness, damage-tolerant two-phase titanium alloy developed in the 1970s. After thermomechanical treatment, the alloy has the advantages of high strength, good damage tolerance and excellent resistance to fatigue crack growth, and is suitable for the manufacture of high-strength and high-toughness load-bearing components. By adding Si element, the alloy maintains high strength at medium temperature, which is better than Ti-6Al-4V. The alloy sheet can be superplastically formed at room temperature.

Ti-10V-2Fe-3Al(TB6) is a high-strength, high-toughness near-beta titanium alloy developed in the late 1970s. The alloy has the advantages of high specific strength, good fracture toughness, large hardening area, small anisotropy, good forging performance and strong corrosion resistance, and has many advantages of metastable B titanium alloy without losing α-β titanium alloy. The solid-solution characteristics can meet the requirements of damage tolerance design and high structural efficiency, high reliability and low cost. The maximum working temperature is 320 °C. The main products of this alloy are bars, forgings, thick plates and profiles. Through solution and aging heat treatment, a good match of strength, plasticity and fracture toughness can be achieved, and it is suitable for manufacturing structural parts with high requirements on strength and fracture toughness. Excellent toughness and low crack growth rate can be obtained by thermomechanical treatment, which is suitable for structures with high fracture toughness requirements.

3. Development and application of high temperature titanium alloys

High-temperature titanium alloys have been widely used in aero-engines due to their excellent thermal strength and high specific strength. High-temperature titanium alloys are mainly used in fans and compressors of aero-engines, such as compressor discs, blades, navigators, connecting rings, etc. Replacing the original nickel-based superalloy with titanium alloys can reduce the weight of the compressor by 30% to 35%. The proportion of titanium used in foreign advanced aero-engines has reached 25~39%. For example, the titanium alloy content of the F100 engine accounts for 25% of the total engine weight, the V2500 engine is 31%, and the F119 engine is 39%. The development demand of high-performance aero-engines has led to the development of high-temperature titanium alloys, and the operating temperature has gradually increased, from 400 °C represented by Ti-6Al-4V alloy in the 1950s to 600 °C represented by IMl834 alloy. Above 600°C, the sharp decline of creep resistance and high temperature oxidation resistance are the two main obstacles restricting the development of titanium alloys to higher temperatures. Therefore, 600°C is considered to be the "thermal barrier" temperature for the development of titanium alloys.

Over the years, in order to meet the needs of high-performance aero-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 used at 350~600℃. The former Soviet Union developed titanium alloys of BT6, BT3-l, BT8, BT9 and other grades in the late 1950s, and developed BT18 and BT25 alloys in the 1960s and 1970s. Since then, in order to improve the performance and working life of high-temperature titanium alloys, high-temperature titanium alloys of BT18y, BT25y, BT8M, BT8-1 and BT8M-l are improved and developed on the basis of the original alloy. In recent years, BT36 titanium alloy has been developed, which are used in HK8, IIC90A and other engines respectively. Similarly, the United States also uses Ti64, Ti811, Ti6242 and other titanium alloys in advanced engines such as JT90 and F-110, respectively. The main technical indicators of typical high temperature titanium alloys are shown in Table 2.

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. For example, the alloys that can be used at 500 °C are BT8, BT9 and BT8-1, the alloys used at 550 °C are BT25 and BT25y, and the alloys used at 600 °C are There are BTl8y and BT36. Russia recommends BT25y for roulettes and rotor blades used in aero-engine high-pressure compressors at 450~550℃, and recommends BT18y for roulettes used at 550~600℃. Although BT36 has been developed, it does not seem to have received corresponding attention. my country has introduced BT36 alloy plates and bars produced in Russia. After analysis, there is a large amount of composition segregation on the alloy plates and bars. The problem of composition uniformity has not been well solved, and its high temperature performance is not good. Reach the level of IMl834 alloy.

High-temperature titanium alloys in the UK are the most mature and have their own independent systems, forming a series of titanium alloy grades used at different temperatures. So far, IMl685 alloy is a high-temperature titanium alloy with the widest and largest number of applications in aero-engines in the UK, such as RB211 series engines, RBl99 engines, Adour engines and M53 engines used in Rolls-Royes. IMl829 alloy is used in high pressure compressor of RB211-535C engine. The produced rear 3-stage disc, drum and rear axle are integrated by electron beam welding, which replaces the nickel-based alloy material on RB21l-535C and reduces the weight of the rotor by 30%. The successful development of IM1834 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, such as the large civil engine Trent700 selected by Boeing 777 aircraft. (Tunda), all the discs, drums and rear axles of its high-pressure compressors are made of IMl834 alloy and welded together by electron beam welding process. It makes Trent700 the first engine to use all-titanium high-pressure compressor rotor among the new civil engines, which 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 in Pratt & Whitney's PW350 engine.

The development of high-temperature titanium alloys in the United States is also relatively mature. At present, the most widely used alloys in engines are Ti-6Al-4V and Ti-6 242 S. Ti-1 100 alloy is based on the composition of Ti-6 242 S alloy, and by adjusting the content of Al, Sn, Mo and Si elements, the maximum service temperature of the alloy is increased to 600 ℃. It is understood that Ti-1 100 alloy has been used to manufacture parts such as high-pressure compressor discs and low-pressure turbine blades of Lycoming's T55-712 modified engine.

The development of titanium alloys in my country is mainly based on imitation. For example, TC11 alloy corresponds to BT9 alloy, TAll, TA19, TC17, and the corresponding American grades are Ti-811, Ti-6 242 S and Ti-17. In the past 20 years, my country has begun to take the route of self-development while imitating, such as high-temperature titanium alloy TA12 (Ti-55), adding rare earth element Nd; Ti-60 alloy on the basis of TAl2 alloy, adding AI, Sn and Si appropriately. The content of the alloy further improves the high temperature creep performance and strength of the alloy, so that the service temperature of the alloy reaches 600 ℃. Domestically, on the basis of the British IMl829 alloy, the rare earth element Gd was added, and the 550 ℃ high temperature titanium alloy Ti-633 was developed. Recently, on the basis of Ti-1 100 alloy, about 0.1Y was added, and it was named Ti-600.

4. Development and application of low temperature titanium alloys

Structural parts used at low temperatures require good plasticity, low thermal conductivity and excellent processability while maintaining a certain strength. The structural materials for low-temperature use at home and abroad are mainly 113J such as stainless steel, aluminum alloy, titanium alloy and nickel-based alloy. Titanium alloy has good comprehensive properties at low temperature, and has been widely valued by countries all over the world for many years. At low temperature, the yield strength of titanium alloy increases greatly, which is about 3-6 times that of austenitic stainless steel; but the fracture toughness decreases with the decrease of temperature, which is about 0.25~0.5 of austenitic stainless steel. Because the density of titanium alloy is much lower than that of stainless steel, and it has low thermal conductivity, small expansion coefficient and non-magnetic properties at low temperature, it is used as an important low-temperature engineering material in aerospace, superconductivity and other fields.

The β-titanium alloy with bee structure at low temperature, like other body-centered cubic metals, has a high plastic-brittle transition temperature (TPR). The TPR of α and near-α titanium alloys is generally very low, and they also have good plasticity at low temperatures. Therefore, some low-temperature titanium alloys recognized internationally are basically α and near-α titanium alloys. Among α-β titanium alloys, titanium alloys with less β phase, such as Ti-6Al-4V ELI, can also be used well at liquid hydrogen temperature (22K). Alpha titanium alloys such as pure titanium and Ti-5Al-2.5Sn ELl are an ideal low-temperature structural material at liquid helium temperature (4.2K), but impurities other than alloy composition must be controlled, especially the content of oxygen and iron. The increase of iron and oxygen components increases the low temperature brittleness of titanium materials, and the increase of β-phase stabilizing elements such as iron and manganese is easy to cause notch embrittlement of the material.

The former Soviet Union once took the lead in the development and application of low-temperature titanium alloys in the world. Its early developed alpha titanium alloys OT4, OT4-1, BT5-1KT, HT-3BKT and other alloys have been widely used in aerospace rocket equipment. The strength of these alloys increased to 1400 MPa at 2K, while the elongation remained above 10%. The low-temperature titanium alloys developed and applied in the United States mainly include low-temperature alpha titanium alloys such as Ti-5Al-2.5Sn, Ti-8Al-1Mo-1V, and Ti-6A1-3Nb-2Zr.

China started a little later than the United States and Russia in the development and application of low-temperature titanium alloys. After my country has carried out low-temperature performance testing and application research on existing titanium alloys such as TA7, TC1, and TC4, it was developed during the "Ninth Five-Year Plan" period. Ti-Al-Zr, Ti-Al-Zr-Mo, Ti-Al-Sn-Mo, Ti-Al-Zr-Sn-Mo, etc. The typical properties of the low-temperature titanium alloys developed in China are shown in Table 3.

5. Development and application of titanium alloys for fasteners

The application of titanium alloy fasteners abroad has been very common, and various new types of fasteners are emerging. The amount of titanium alloy fasteners used in large civil aircrafts reaches hundreds of thousands of pieces. Under the same strength index, the quality of titanium fasteners is 70% lighter than that of steel, and the fatigue strength and sensitivity to stress concentration of titanium alloys are better than those of similar steels. High corrosion resistance, so the application of titanium fasteners is very important for aviation equipment.

5.1 Development of Fastener Titanium Alloy

Titanium alloy fasteners mainly use three types of materials: the first type is α-β type two-phase alloy with low Mo equivalent, such as Ti-6Al-4V; the second type is metastable β alloy.

Gold, there are βIII, Ti-44.5Nb, Ti-15-3 in the United States and TB2, TB3 and TB8 in my country; the third type is the α-β type two-phase alloy with subcritical composition, such as BT16 in Russia. The following table shows the properties of titanium alloy fastener materials.

Ti-6Al-4V is a low Mo equivalent α-β type two-phase alloy. Among the three types of alloys, the β stability coefficient is the lowest (only 0.27), while the aluminum equivalent is the highest (up to 6). Therefore, the B phase content in the annealed state is only 7% (volume fraction). Its advantages are the lowest density, the best strength and fatigue properties, the simplest composition, and the lowest cost of semi-finished products. However, since the room temperature plasticity is not high enough, induction heating is required for hot upsetting, vacuum solution treatment and aging treatment when processing fasteners, and the cost of galvanizing is relatively high.

The second type is β alloys (such as TB2, TB3, TB5, TB8, etc.), which are completely different from α-β alloys, and the β stability coefficient is very high, ranging from 1.15 to 1.97, while the aluminum equivalent is reduced to about 3. Therefore, a single β phase can be obtained during solution treatment, so that bolts and rivets can be formed by cold heading at room temperature. The processing cost is low. 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 ingredients are complex, and the cost of semi-finished products is high. Since vacuum aging treatment is also required, the cost of finished fasteners is still higher than that of Ti-6Al-4V, and the operating temperature is also lower than that of Ti-6Al-4V.

The density of BTl6 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 is close to the critical composition (β stability coefficient is 1). In binary alloys composed of β-stabilizing elements and Ti, with the content of β-stabilizing elements

With the increase of , the grain size gradually decreases, and the number of α and β phases is equal near the l critical concentration, and the grain size reaches the minimum. When the stabilizing element is further increased, the grain size increases. The smaller β grains and the β phase content as high as 25% (volume fraction) in the annealed state determine the excellent room temperature ductility of the BT16 alloy. Therefore, the BTl6 alloy has the conditions to complete the rapid upsetting of the fastener head at room temperature, that is, cold upsetting.

5.2 Application of fastener titanium alloy

Ti-6Al-4V is a medium-strength α-β type two-phase titanium alloy with excellent comprehensive properties and complete specifications of semi-finished products, including bars, forgings, thick plates, thin plates, profiles and wires. The alloy has a long-term working temperature of up to 400 ° C, and has obtained the most extensive application in the aviation and aerospace industries. It is the main fastener material used in the aerospace and aerospace sectors in the United States and Western European countries. Russian titanium alloy fasteners mainly use BT16 titanium alloy. BT16 alloy belongs to Ti-Al-Mo-V series α-β type high-strength titanium alloy. The main semi-finished products are hot-rolled rods and polished rods and wires for cold heading. They are mainly used for the manufacture of fasteners, such as bolts, screws, Nuts and rivets, etc. The maximum working temperature is 350℃. The strength of the alloy in the solution aging state is slightly lower than that of the Ti-6Al-4V alloy. The main advantage is that it can be formed by cold heading in the annealed state, thus significantly improving the production efficiency. Fasteners manufactured by cold deformation are widely used in Russia. The machinery manufacturing industry is widely used, and it is also the main standard material used in the Russian aviation and aerospace sector, and it is also used in some models of aircraft in the country. The alloy has two use states: cold deformation strengthening without heat treatment and hot heading forming strengthening solution aging treatment.

βIII alloy was listed in the AMS4977 specification as a fastener material in 1969 and had some applications in aircraft, but it was announced in 1987 in AMS4977B that the aerospace materials department recommended that βIII alloy no longer be used as a standard material for future new designs. According to recent reports, the alloy has been discontinued. As a special material for rivets, Ti-44.5Nb was included in the AMS4982 specification in 1974, and was revised to AMS4982C in 2002. It is still used today, but only a small section is welded on the head of the Ti-6Al-4V rivet to make it cold riveted. Ti-15-3 (TB5) was first listed in the AMS4914 specification in 1984 as a thin plate. TB5 and TB8 are respectively used as matching rivets and screws for a certain type of aircraft in China as a resistance umbrella beam and a wind deflector (for high temperature use). TB2 and TB3 are β alloys developed by our country. TB2 was initially used for sheet metal parts and later used as rivets on some models. TB3 was originally developed as a material for bolts and has also been used in some models.

6.Conclusion

Titanium is an important structural material in my country's development of national defense, aviation, high-tech and other fields, and has important strategic significance. At present, the research and development level and production capacity of titanium sponge and titanium processing materials in my country have ranked among the top in the world. The future development direction should focus on research and development of higher-performance alloys according to application needs and combined with international development trends, so as to improve the technical level of the titanium production industry. , from a titanium industry power to a titanium industry power forward.