钛合金材料在中国航空紧固件中的应用

Titanium alloy as an emerging structural material appeared and developed in the middle of the 20th century because of its excellent corrosion resistance, high specific strength, and non-magnetic. A series of advantages in the aerospace and other high-end industrial sectors have been widely used. In the 1950s, the United States began to use titanium alloy materials to manufacture aviation fasteners, China in the 1980s began to use titanium alloy materials to manufacture a small number of engineering aerospace fasteners; after entering the 21st century, with the overall level of China’s aerospace industry manufacturing technology, aviation fasteners made of titanium alloy materials and their processing and manufacturing technology to obtain a systematic and specialized development.

Titanium and titanium alloys have outstanding advantages, such as high specific strength, good corrosion resistance, and high temperature resistance, making them promising metal structure materials in the modern aerospace industry. Since the 1950s, in the United States, for the first time, the Ti-6Al-4V titanium alloy bolts used in B-52 bombers achieved a very good weight reduction effect; the aviation industry developed countries have launched titanium alloy fasteners research and engineering applications. Titanium alloy fasteners, instead of most of the lower strength than the steel fasteners, the aircraft weight reduction achieved very significant results. Such as the Boeing 747 aircraft fasteners to titanium instead of steel, its structural quality was reduced by 1814kg; Russia’s IL-96 aircraft, with 142,000 pieces of titanium alloy fasteners, steel weight reduction of up to 600kg; figure of 204 planes with 940kg BT16 titanium alloy fasteners, steel reduction of 688kg. Titanium alloys are positive potential performance that is the same as carbon fiber composites, matching the strength of titanium alloy fasteners. Titanium alloy’s positive potential performance matches carbon fiber composite materials, effectively preventing the fasteners from galvanic corrosion, making titanium alloy the best connecting material for composite materials. Therefore, with the increasing amount of titanium alloy and composite materials used in advanced military and civil aircraft, the demand for titanium alloy fasteners is increasing. The use of aluminum alloy temperature is higher than 150-200 ℃; for the structure of the aircraft, due to the high operating temperature and cannot use aluminum alloy fastener parts, titanium alloy will be a better choice. In addition, titanium alloy has inherent good elasticity and is non-magnetic to prevent the loosening of fastening bolts, and anti-magnetic interference also has a very important role.
In the United States military and civilian aircraft, titanium alloy fasteners have replaced alloy steel fasteners. The application of foreign titanium alloy fasteners has been very common; the amount of titanium alloy fasteners for large civil aircraft has reached hundreds of thousands of pieces, while various types of new titanium alloy fasteners are also constantly developed. China’s titanium alloy fastener development history can be traced back to 1965; Chengdu Aircraft Design Institute, according to the needs of the new aircraft, proposed the development of titanium alloy rivets, the 1970s relevant units of the titanium alloy rivets and application of the research work in the 1980s in some of China’s second-generation military aircraft began to a small number of titanium alloy rivets, bolts, and other fasteners. The late 1990s, with the third generation of foreign heavy combat aircraft production lines and titanium alloy rivets, bolts, and other fasteners. In the late 1990s, with the introduction of foreign third-generation heavy fighter production lines and the development of domestic third-generation fighters, as well as many aviation subcontracting production businesses, China’s aviation industry began to use some titanium alloy fasteners. In recent years with the development of China’s aerospace industry, the domestic relevant units targeted to carry out a large number of basic materials and fastener manufacturing technology research and development work, at present China’s independent research and development and production of titanium alloy fasteners have been in China’s modified aircraft and the new design of the aircraft to obtain a large number of engineering applications.

1. Titanium alloy materials for riveted fasteners

Fasteners commonly used in the aerospace industry mainly include rivets, bolts, and special fasteners in 3 categories. For rivets, the most important thing is the cold plasticity of the material; only the cold plasticity of the material manufactured rivets can be cold riveting installation. Usually, the strength requirements are not too high, and high corrosion resistance requirements of the parts of the titanium alloy rivets, β-type titanium alloy due to the solid solution state for a single β grain, and because it has a body-centered cubic arrangement of the atomic structure so that this type of alloy has very excellent cold working properties, very suitable for the manufacture of titanium alloy rivets.

1.1 TB2 titanium alloy

China’s titanium alloy fastener development is to TB2 titanium alloy rivets for the development of the starting point. 1965, Chengdu Aircraft Design and Research Institute, in the development of the new aircraft, the plan is to use titanium alloy rivets in the titanium structural components of the fuselage and put forward the demonstration and design. In 1970, with the cooperation of Tianjin Metallurgical Bureau Materials Research Institute and Nonferrous Metals Research Institute, Chengdu Aircraft Design Institute and Chengdu Aircraft Corporation jointly started the research on the development and application of TB2 titanium alloy rivets and successively completed the development of TB2 titanium alloy materials, research on the study of the cold upsetting of wire materials and rivets, riveting test studies, etc., and completed the technical appraisal of the relevant work in 1979, and formulated the provisional technical conditions. TB2 titanium alloy is a sub-stable β titanium alloy, alloy nominal composition Ti-3Al-8Cr-5Mo-5V. The alloy in the solid solution treatment state has excellent cold forming properties and welding performance; China is mainly used for the manufacture of satellite corrugated shells, star, and arrow connecting bands, and all kinds of cold upsetting rivets, and sometimes also used for the manufacture of small-size bolts. When manufacturing aviation fasteners, their use temperature is generally below 300 ℃; aerospace fasteners can be used in a short period to 500 ℃.
In 1986, China promulgated the first titanium alloy fasteners’ special standards, GJB120-1986 “titanium alloy rivets,” in 1990 China promulgated the second and third titanium alloy fasteners’ special technical standards GJB856-90 “tensile titanium alloy ring groove rivets specification” and GJB857.1-90 “100 ° countersunk head tensile titanium alloy into the ring groove rivets,” which three Standards are TB2 titanium alloy rivets special technical standards, on all types of specifications of TB2 rivets for a clear specification for its engineered batch production and application of the standard basis. TB2 manufactured titanium alloy rivets have been in China’s aerospace industry in several models to obtain a large number of applications; at the same time, in the aviation model, products have also obtained a certain number of applications and have achieved good results.

1.2 TB5 titanium alloy

TB5 titanium alloy is a sub-stable β-type titanium alloy; its nominal composition is Ti-15V-3Cr-3Sn-3Al. The alloy was initially developed under the U.S. Air Force’s financial support, Lockheed Martin determined the composition, and TIMET carried out the scale production. The alloy has excellent cold forming properties, its cold forming capacity, and pure titanium, can be in a solid solution state for a variety of complex parts of the cold forming (such as rivets riveted), aging room temperature tensile strength of up to 1000MPa or more, the alloy due to the high content of its V-element, antioxidant performance is poor, generally in the 200 ℃ below the working environment, but the alloy has excellent corrosion resistance.
Pratt & Whitney in the United States in its production of aero-engine on a large number of Ti-15V-3Cr-3Sn-3Al titanium alloy as a bracket, the U.S. B-1B bomber Ti-15V-3Cr-3Sn-3Al titanium alloy parts of the amount of more than 1,000, Ti-15V-3Cr-3Sn-3Al titanium alloy fasteners in the Boeing Airplane has also been used for many years. Our country uses TB5 titanium alloy to replace 30CrMnSiA to manufacture a certain fighter plane umbrella beam and manufacture satellite engine corrugated plate and other parts, at the same time using TB5 titanium alloy to manufacture fighter plane umbrella beam and satellite corrugated plate supporting the use of cold heading rivets.

1.3 Ti-45Nb alloy

Ti-45Nb alloy is a kind of special material for rivets; its outstanding advantages are high plasticity (elongation can be up to more than 20%, section shrinkage is as high as 60%-80%), excellent cold working performance, its shear strength (τ ≥ 350MPa) and tensile strength (σ_b ≥ 450MPa) are higher than that of pure titanium. The resistance to cold deformation is lower than that of pure titanium, which is suitable for making rivets for composite material connections. Material. The United States for the Ti-45Nb alloy has carried out a lot of basic research work; technology development is more mature, and in 1974 included in the AMS4982 specification, revised to AMS 4982C in 2002, has been widely used. In the United States, in aerospace rivet products, Ti-45Nb alloy has been fully replaced by pure titanium. The alloy and Ti-6Al-4V alloy, made of bimetallic rivets, has been in the Airbus and Boeing aircraft to obtain many applications.
For the requirements of high shear strength, the installation process does not allow the deformation of the rivet bar rivets, generally used bimetallic titanium rivets, bimetallic titanium rivets are composed of Ti-6Al-4V nail bar and Ti-45Nb head, after inertial friction welding, tightly fused to form a whole solid rivet. Only a small impact can make the Ti-45Nb rivet head plastic deformation in this bimetallic rivet riveting, while the Ti-6Al-4V rivet stem is not deformed. Bimetallic titanium alloy rivets in the B-1 bomber, Boeing, and other airplanes are widely used in riveted titanium alloy components and composite components. Such as the U.S. F-14 fighter wing’s leading edge uses 4000 bimetallic rivets; its fatigue performance and high lock bolts comparable to the cost can be reduced by 50%, 30% -40% lighter, the cost of such a bimetallic rivet to be lower than other β-type titanium alloy rivets. In recent years, China has also developed bimetallic and Ti-45Nb rivets, which have been in a new generation of aircraft composite skin riveting in engineering applications.

2. Bolt fasteners with titanium alloy materials

Aerospace fasteners in the largest amount of titanium alloy bolts, titanium alloy bolts can be divided into ordinary bolts, high lock bolts, and interference bolts according to their use. Since the manufacture of titanium alloy bolts, the general requirements of its heat treatment to obtain high tensile strength and shear strength usually require its strength level and 30CrMnSiA high-strength alloy steel comparable.

2.1 TC4 titanium alloy (σ_b ≥ 1100MPa level)

TC4 (U.S. grade Ti-6Al-4V/UNS R56400/ASTM Grade 5/Ti64) titanium alloy was first developed by the United States in 1954, has developed into an international titanium alloy, is currently the most comprehensive and in-depth study of its titanium alloy. Aviation, aerospace, civil and other industries have been widely used. It has been widely used in manufacturing beams, frames, landing gear, fasteners, aero-engine fans, compressor disks, magazines, blades, etc. It is also used in many other industries, accounting for more than half of the production of titanium alloys. The alloy has good process plasticity and superplasticity, alloy α + β / β transition temperature of 980-1010 ° C, and long-term working temperature of up to 400 ° C. Beginning in 1973, to cooperate with the turbofan-8 aero-engine TC4 titanium alloy blade development, China began the alloy’s research and engineering applications.
In 1956, the United States used TC4 (Ti-6Al-4V) titanium alloy, manufacturing the world’s first batch of titanium alloy bolts, first used in the B-52 bomber (instead of the original 30CrMnSiA bolts), as a result of the use of the effect is very good, and soon to be popularized. Almost all Western aircraft manufacture bolts in many TC4 (Ti-6Al-4V) titanium alloys. But because TC4(Ti-6Al-4V) is α+β duplex alloy, it can’t be formed by cold upsetting; its nail head must be heated upsetting, the heat treatment needs vacuum water quenching and aging, the processing equipment and technology requirements are high. In the late 1980s, China’s relevant units researched the hot heading technology of TC4 titanium alloy fasteners and developed hot heading machine tools successively. They realized the industrialization of TC4 titanium alloy fasteners in the 1990s. The industrialized production of TC4 titanium alloy fasteners was realized in the 1990s. At present, many aerospace standard parts factories in China have the hot heading equipment and technical ability for mass production of TC4 bolts, and the bolts made of TC4 titanium alloy have been used in a large number of engineering applications in China’s new-generation military planes, aerospace vehicles, and satellites.

2.2 TC6 titanium alloy (σ_b≥980MPa)

To meet the high-temperature requirements of aviation engines, Beijing Aviation Manufacturing Engineering Research Institute has developed a high temperature resistant to 500 ℃ below the TC6 (Russian material BT3-1) titanium alloy fasteners, compared with the TC4 titanium alloy; the material has a higher sensitivity to temperature, the fasteners are more difficult to manufacture. The alloy is the Soviet Union-developed BT3-1 titanium alloy, the nominal composition of Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si, currently widely used in Russia. China in 1979 with the WP13 aero-engine TC6 titanium alloy tail rod and other components and supporting the development of fasteners, the alloy’s imitation work, and application of research work.
TC6 alloy is a comprehensive performance of good martensitic α + β type duplex titanium alloy, generally used in the annealed state, and can also be strengthened with appropriate heat treatment. The alloy has good performance, oxidation resistance, and corrosion resistance is very excellent, and its manufactured parts can work for a long time at 400 ℃ for more than 6000h and 450 ℃ for a long time at 450 ℃ for more than 2000h. Isothermal annealing treatment room temperature tensile strength greater than 980MPa, yield strength greater than 840MPa, elongation greater than 10%, section shrinkage greater than 25%. 400 ℃ high temperature tensile strength greater than 720MPa, elongation greater than 14%, section shrinkage greater than 40%. It can also be a “solid solution + aging” treatment to improve its strength further.

2.3 TC16 (σ_b≥1030MPa level)

Currently, most of the steel fasteners are processed by cold heading, and only a few of the larger sizes are processed by hot heading. The cold heading process enables the fasteners to produce large quantities continuously. However, most industrial titanium alloys cannot be processed by cold heading due to their poor cold forming properties. Therefore, in Western countries, TC4 titanium alloy fasteners are mainly produced by the hot heading process; the disadvantages of the hot heading process are: local burns and overheating, and surface oxidization are easy to occur when the blanks are heated, at the same time, it is not easy to realize the automation of continuous upsetting, low production efficiency. To improve the production efficiency of titanium alloy fasteners and the stability of their quality, the former Soviet Union developed BT16 titanium alloy for the cold heading of fasteners, which realized the development of titanium alloy fasteners cold heading technology and leapfrog. It gained many engineering applications on IL76, IL86, IL96, AN124, Su27 series, and other Soviet (Russian) airplanes.
BT16 (our imitation grade TC16) titanium alloy nominal composition Ti-3Al-5Mo-4.5V, the alloy is a martensitic α + β-type duplex titanium alloy, β stabilization coefficient of 0.83, close to the critical composition. The titanium alloy is mainly used in the manufacture of aerospace fasteners with an operating temperature of 350°C or less, and the alloy has an α+β/β transition temperature of (860±20) °C. The smaller β grains and the higher α+β transition temperature of (860±20)°C in the titanium alloy are also important. Smaller β grain and in the annealed state of up to 25% of the volume fraction of the β phase determines the BT16 alloy has excellent room temperature process plasticity, so the alloy can be completed at room temperature conditions of the fastener head of cold upsetting fasteners, thus significantly improving the efficiency of its bolt production, reducing the cost of production, and subsequently in the subsequent solid solution aging heat treatment of its strength of up to 1030-1180 MPa. Russia (former Soviet Union) (Former Soviet Union) titanium alloy bolt fasteners are mainly manufactured using BT16 titanium alloy for decades without any quality incidents. China, the 1990s, Russia introduced the Su-27 aircraft production line; to meet the localization needs of Su-27 aircraft, the relevant domestic units immediately carried out the localization of BT16 titanium alloy and its fasteners; our country imitated the name of the TC16 titanium alloy. China’s independent development and production of TC16 titanium alloy bolts have been in the domestic third-generation fighters on many engineering applications.

2.4 TB3 (σ_b ≥ 1100MPa grade)

In the 1970s and 1980s, 1100MPa grade titanium alloy bolts for aerospace applications in Western industrialized countries such as the United States were mainly made of TC4 (Ti-6Al-4V) titanium alloy produced by hot heading forming process. At the end of the 1970s and early 1980s, our country was in urgent need of 1100MPa titanium alloy bolts and fasteners for composite structure connection, but due to the limitation of hot heading equipment (at that time, there was no hot heading forming equipment in China), we were unable to develop and produce 1100MPa grade TC4 (Ti-6Al-4V) bolts, and we mainly focused on the development of β-type titanium alloy which can be directly cold headed. TB3 titanium alloy was developed with this background; the composition of TB3 alloy was designed concerning Ti-8Mo-8V-2Fe-3Al titanium alloy, which was developed by the American Titanium Metal Company in the 20th century.
TB3 titanium alloy is a heat-treatable strengthened sub-stable β-type titanium alloy; the nominal composition is Ti-10Mo-8V-1Fe-3.5Al. The main advantage of this alloy is that the solid solution treatment state has excellent cold forming properties, its cold upsetting ratio (Dt/D0) is up to 2.8, “solid solution + aging” system can obtain high strength after treatment. After treatment, high strength can be obtained, mainly used to manufacture 1100MPa grade high-strength aerospace fasteners with a service temperature below 300°C. In October 1982, China began the development of TB3 titanium alloy bolts, and in 1985, the development work made progress and formed the relevant technical specifications. In the mid to late 1980s, the domestic TB3 titanium alloy was made into high lock bolts and interference bolts, installed in the aircraft composite structure and metal structure for China’s titanium alloy bolts and fasteners to find out the application of a certain experience base. The alloy is now widely used in 1100MPa titanium alloy bolt manufacturing and has been successfully applied to Y-7, J-8, and J-10 aircraft and some space vehicles. TB3 titanium alloy has become the main material of titanium alloy bolt fasteners for aerospace vehicles that can be industrially produced in China. At the same time, titanium alloy is also used in the manufacture of rivets; in China in 2006 promulgated the GJB120-2006 “titanium and titanium alloy rivets” in the TB3 titanium material as rivets formally included in the standard.

2.5 TB8 titanium alloy (σ_b ≥ 1280MPa grade)

With the rapid development of aerospace technology in the military and civil aircraft used in the mechanical connection technology requirements are higher and higher, the technical content of the standard parts used is also higher and higher, and its role in the aircraft is not only “fastening,” “connection” role. Still, it has become an important part of the realization of the performance of the aircraft. Rather, it has become an important structural component to realize the whole machine’s performance. The future development trend of aerospace technology requires a new type of fasteners with high specific strength, i.e., lightweight and high strength. Therefore, the United States, Russia, France, and other world aviation powers are actively developing tensile strength of 1200MPa above the high-strength titanium alloy materials and their fasteners. In recent years, Alcoa developed Timetal555 titanium alloy high-strength bolts, its solution aging tensile strength of 1300MPa or more, double shear strength greater than 745MPa, elongation greater than 10%, the performance indicators fully meet the typical 1250MPa cadmium-plated alloy steel fasteners specification requirements. PS Aviation Fastener Group uses SPSTITANTM761 titanium alloy processing and manufacturing of bolts Aerlite180, its tensile strength of up to 1240MPa, shear strength of up to 745MPa, reached many alloy steel, and corrosion-resistant alloys fasteners of the level of strength, and at the same time reduce weight by 40%.
To follow the international advanced aerospace titanium alloy fastener development trend, in recent years, XITU super crystal company and Xinyang Aerospace standard parts factory jointly developed a fastener with special TB8 titanium alloy rods and wires and its 1280MPa high-strength bolts and fasteners, its specifications from φ4-φ25. TB8 titanium alloy is China’s imitation of the United States of America’s β21S titanium alloy, the nominal composition of the Ti – 3Al – 2.7 Nb-15Mo, β21S alloy is the United States titanium metal company (Timent) in 1989 for the NASP program to develop a sub-stable β-type titanium alloy, β21S titanium alloy has excellent hot and cold machining properties, deep hardenability, high creep resistance, high oxidation resistance, and good corrosion resistance, so the alloy was recognized by the aircraft designers and manufacturers as an excellent aerospace structural materials in 1994. Excellent aerospace structural materials 1994 were first included in the U.S. ASTM standards; the United States mainly uses an alloy to manufacture space shuttles with titanium composite materials and Boeing 777 and other aircraft engine pod components. China, from the 1990s, began to carry out the alloy imitation work, completed a certain type of aircraft structural components with TB8 titanium alloy forgings and sheet metal parts of the development and application of research work, due to the eventual lack of access to engineering applications, only in the GB/T3620.1-2007 “titanium and titanium alloys grades and chemical composition,” the specification of its grades and compositions, its materials and products did not form the national standard, National military standard, and aviation standard specification.
Xinyang Aerospace Standard Parts Factory and XITU super-crystal company jointly developed 1280MPa grade TB8 titanium alloy bolts physical photo in Figure 1; the main performance indicators are shown in Table 1. Due to the alloy’s use of β-stabilizing elements for the high melting point oxidation-resistant molybdenum and niobium, rather than TB2, and TB3 titanium alloys used in the oxidation of poor vanadium, the alloy made of long-time use of the fastener Temperature up to 550 ℃, a complete solution to the traditional high-strength β titanium alloy fasteners using low temperature (not higher than 300 ℃) problem. At present, the development of the 1280MPa grade TB8 titanium alloy high-strength bolts has been used in China’s new generation of aircraft engineering applications, achieving good weight reduction and compatibility with composite materials.

Figure.1 1280MPa grade TB8 titanium alloy bolt photo
Table.1 Performance index of 1280MPa grade TB8 titanium alloy bolts.

Performance Items	Index	Remarks
Tensile strength/MPa	≥1280	According to GJB715.23A-2008 standard
Elongation rate/% (same furnace material)	≥8	The elongation of samples from the same batch of materials in the furnace is not required for bolts
Double shear strength/MPa	≥755	According to GJB715.26A-2008 standard
Fatigue life/time	≥130000	According to GJB715.30A-2002 standard, load: 12MPa

3. Summary

It can be seen that the research and application of aviation titanium alloy fasteners since the emergence of industrial titanium alloys has begun; the United States, Russia (the former Soviet Union), and other developed countries in the aviation industry in the very early stage have been formed in line with their technology fasteners with titanium alloy material system, titanium alloy fasteners in its aviation manufacturing field have gained a large number of applications. China’s aviation with titanium alloy fasteners research and application started late, are in the aircraft engine or aircraft copy, technology borrowing or introduction of the process of passive improvement, manufacturing fasteners titanium alloy materials are borrowed or imitation of Russia (the former Soviet Union), the United States, at the same time, our country’s aviation fasteners in the titanium fasteners in the possession of a relatively low amount of fasteners. With the rapid development of China’s aviation industry and the continuous improvement of the performance requirements of the aircraft, the future of high-performance aviation fasteners for the manufacture of titanium alloy materials put forward higher strength, higher fracture toughness, and higher fatigue performance requirements.
Author: Zhang Lijun, Wang Luck