Application of Titanium Alloy in Chinese Aircraft Fastener

Due to its outstanding advantages of high specific strength, good corrosion resistance and high temperature resistance, titanium and titanium alloy have developed promising metal structural materials in the modern aerospace industry .Since the United States first applied ti-6al-4v titanium alloy bolts to b-52 bombers in the 1950s, and achieved a very good weight reduction effect, all countries with developed aviation industry have launched the research and engineering application of titanium alloy fasteners.After titanium alloy fasteners replace most steel fasteners with lower specific strength, the weight reduction of aircraft is very significant.For example, the fasteners of Boeing 747 aircraft were made of titanium instead of steel, and the structural mass was reduced by 1814 kg.Russia’s il-96 aircraft, one with 142,000 titanium alloy fasteners, weighs 600kg less than the steel parts;The 940kg BT16 titanium alloy fasteners are used in an aircraft as shown in FIG. 204, which are 688kg lighter than steel parts.The positive potential of titanium alloy is matched with that of carbon fiber composite, which effectively prevents galvanic corrosion of fasteners and makes titanium alloy the best connecting material for composite materials.Therefore, with the increasing consumption of titanium alloy and composite materials for advanced military aircraft, the demand for titanium alloy fasteners is increasing day by day.The operating temperature of titanium alloy is 150~200℃ higher than that of aluminum alloy, so it will be a better choice for parts where aluminum alloy fasteners cannot be used due to high operating temperature in aircraft structure.In addition, titanium alloy inherent good elasticity and non-magnetic, to prevent the loosening of fastening bolts and prevent the magnetic field interference also has a very important role.

Titanium alloy fasteners have largely replaced alloy steel fasteners in U.S. military and civilian aircraft.Abroad the application of titanium alloy fasteners has been very common, large civil aircraft use of titanium alloy fasteners reached hundreds of thousands of pieces, and various new types of titanium alloy fasteners have been constantly developed. Chinese titanium alloy fastener development history can be traced back to 1965, the chengdu aircraft design institute is put forward according to the need of new titanium alloys developed rivet, titanium rivet were applied research by relevant department in the 1970’s, in the 1980‘s in China started a small part of using titanium rivet, bolts and other fasteners on the second generation military aircraft.In the late 1990s, with the introduction of foreign third-generation heavy fighter production lines and the development of domestic third-generation fighter aircraft, as well as the development of a large number of aviation subcontract production business, China’s aviation industry began to use some titanium alloy fasteners.In recent years, with the development of China’s aerospace industry, domestic relevant departments have carried out a large number of targeted research and development of basic materials and fastener manufacturing technology. At present, China’s independent development and production of titanium alloy fasteners have been in China’s modified aircraft and newly designed aircraft in a large number of engineering applications.

1 Titanium alloy for rivet fasteners

The fasteners commonly used in aerospace industry mainly include rivets, bolts and special fasteners.For rivets, the most important is the cold plastic state of the material, only the cold plastic material made of rivets, can be cold riveting installation.Is usually on the strength requirement is not too high and the position with high requirements for corrosion resistance using titanium rivet, β type titanium alloy in the solid solution state into a single β grain, and because of its atoms with body centered cubic structure arrangement, so this kind of alloy has very excellent performance of cold working, very suitable for manufacturing titanium alloy rivets.

  • TB2 titanium alloy

The development of titanium alloy fasteners in China is based on the development of TB2 titanium alloy rivets.In 1965, chengdu aircraft design institute put forward the plan to use titanium alloy rivets on the titanium structural parts of the rear fuselage, and put forward the demonstration and design.Since 1970, in tianjin metallurgical bureau materials research institute and with the cooperation of non-ferrous metal research institute and other institues, the chengdu aircraft design institute with chengdu joint research on the development and application of TB2 titanium alloy rivets, successively completed TB2 titanium alloy material research development, the wire cold heading forming research, rivet experimental research work, completed the related technical appraisal work in 1979, formulate the interim technical conditions.TB2 titanium alloy is a kind of metastable β titanium alloy, its nominal component is ti-3al-8cr-5mo-5v. The alloy has excellent cold forming and welding properties in the state of solid solution treatment. At present, it is mainly used in the manufacture of satellite corrugated shell, satellite and arrow connecting belt and all kinds of cold heading rivets in China. Sometimes, it is also used in the manufacture of small bolts.When manufacturing aviation fasteners, the operating temperature is generally below 300℃. Aerospace fasteners can be used to 500℃ in a short time.

In 1986, China promulgated the first special standard for titanium alloy fasteners GJB120-1986 《titanium alloy rivet》, In 1990, China promulgated the second and third parts of the special technical standards for titanium alloy fasteners GJB856-90 《rivet specifications for tensile titanium alloy ring groove rivet》 and GJB857.1-90《100° countersunk head tensile titanium alloy ring groove rivet》,These three standards are the special technical standards of TB2 titanium alloy rivets. They have carried out clear specifications on various specifications of TB2 rivets, laying a standard foundation for their batch production and application in engineering.At present, the titanium alloy rivets made by TB2 have been widely used in the aerospace industry in China, and have also been used in a certain number of aviation products, all of which have achieved good results.

  • TB5 Titanium alloy

TB5 titanium alloy is a kind of metastable β titanium alloy, its nominal component is ti-15v-3cr-3sn-3al.The alloy was originally developed with funding from the U.S. air force, lockheed Martin identified the components, and TIMET began mass production.The alloy has excellent cold formability, its cold forming ability is the same as that of the pure titanium, can undertake all kinds of complicated parts in solid solution state of cold forming (rivet), room temperature tensile strength after aging can reach more than 1000 mpa, the alloy due to its high V element content, oxidation resistance is poorer, generally used in the work environment under 200 ℃, but the alloy has excellent corrosion resistance.HP company has used ti-15v-3cr-3sn-3al titanium alloy as a carrier in a large number of its aero-engines, and the ti-15v-3cr-3sn-3al titanium alloy parts on American b-1b bombers have reached more than 1,000. Ti-15v-3cr-3sn-3al titanium alloy fasteners have also been used in Boeing aircraft for many years.In China, TB5 titanium alloy is used to replace 30CrMnSiA to make the umbrella beam of a fighter plane and the corrugated plate of satellite engine. Meanwhile, TB5 titanium alloy is used to make the cold heading rivets matching the umbrella beam of fighter plane and the corrugated plate of satellite.

  • Ti-45Nb Titanium alloy

Ti – 45 nb alloy as a kind of riveting special purpose material, its optimal point is high plasticity(stretching rate can reach 20%, broken surface charge high shrinkage rate of 60% ~ 80%), good cold processing, its strong shear cut (tau acuity 350 mpa) and tensile strength (sigma b p 450 mpa) were higher than that of pure titanium, and cold deformation resistance is lower than pure titanium, very suitable for composite materials connections with rivet material.The United States has carried out a lot of basic research work on ti-45nb alloy, and its development technology is relatively mature. It was included in the AMS4982 specification in 1974 and revised to AMS4982C in 2002, and has been widely used up to now.In the us aerospace rivet products, ti-45nb alloy has completely replaced pure titanium.Its be made with ti-6al-4v become bimetallic rivets, have been used extensively in airbus and Boeing aircraft.

For rivets that require high shear strength and do not allow rivet rod deformation during installation, bimetallic titanium alloy rivets are generally used. Bimetallic titanium alloy rivets are composed of ti-6al-4v nail rod and ti-45nb head. After friction welding, they are closely fused together to form an integral solid rivet.This kind of bimetal rivet can produce plastic deformation of the ti-45nb rivet head with less impact force, while the ti-6al-4v rivet rod does not deform.Bimetallic titanium alloy rivets are widely used for riveting titanium alloy components and composite components in b-1 bombers, Boeing and other aircraft.For example, 4000 bimetallic rivets are used on the leading edge of the wings of American f-14 fighter. Their fatigue performance is similar to that of high-lock bolts, and the cost can be reduced by 50%, and the weight is 30% ~ 40% lighter. The cost of such bimetallic rivets is lower than that of other β titanium alloy rivets.In recent years, China has also developed bimetal rivets and ti-45nb rivets, which have been applied in the composite skin riveting of new generation aircraft.

2 Titanium alloy for bolt fasteners

Aerospace fasteners in the largest use of titanium alloy bolts, titanium alloy bolts according to its use can be divided into ordinary bolts, high-lock bolts and interference bolts.The titanium alloy material used to make bolts is generally required to obtain high tensile strength and shear strength after heat treatment, and its strength level is usually required to be equivalent to that of 30CrMnSiA high-strength alloy steel.

2.1  TC4 Titanium alloy

It has been widely used in aviation, aerospace and civil industries.It has been widely used in making beams, frames, landing gear, fasteners, aero-engine fans, compressor discs, casings, blades, etc., and in other industries. Currently, it accounts for more than half of the production of titanium alloy.The alloy has good process plasticity and superplasticity, and the transformation temperature of α+β/βthe alloy is 980~1010℃, and the long-term working temperature can reach 400℃.In 1973, in order to cooperate with the development of TC4 titanium alloy blade of turbofan-8 aero-engine, China started the research and engineering application of this alloy.

In 1956, the United States used TC4 (ti-6al-4v) titanium alloy to manufacture the first batch of titanium alloy bolts in the world, which were first used in b-52 bomber (instead of the original 30CrMnSiA bolts). Due to the very good use effect, they were soon promoted and applied. Now almost all western aircraft are using TC4 (ti-6al-4v) titanium alloy to manufacture their bolts.However, as TC4 (ti-6al-4v) isα+β two-phase alloy, it cannot be cold heading forming, and its nail head must be heated up, and the heat treatment requires vacuum water quenching and aging, which requires high requirements on processing equipment and technology.In the late 1980s, China’s relevant companies carried out research on hot upsetting technology for TC4 titanium alloy fasteners, and successively developed hot upsetting machine. In the 1990s, the industrial production of TC4 titanium alloy fasteners was realized.At present, many aerospace standard parts factories in China have the capability of hot upsetting equipment and technology for mass production of TC4 bolts. The bolts made of TC4 titanium alloy have been widely used in China’s new generation of military aircraft, aerospace vehicles and satellites.

2.2  TC6 titanium alloy (b ≥ 980MPa class)

In order to meet the requirements of high temperature use of aero-engines, Beijing aviation manufacturing engineering research institute has developed TC6 (Russian material bt3-1) titanium alloy fasteners that can withstand temperatures below 500 ℃. Compared with TC4 titanium alloy, this material has higher sensitivity to temperature, and its fasteners are more difficult to manufacture.The alloy is a bt3-1 titanium alloy developed by the former Soviet union, with the nominal composition of ti6al-2.5 mo-1.5 cr-0.5 fe-0.3 Si, which is now widely used in Russia.In order to cooperate with the development of WP13 aeroengine TC6 titanium alloy tail rod and other components and supporting fasteners in China in 1979, the imitation work and application research of this alloy were carried out.

TC6 alloy is a kind of α+β two-phase titanium alloy of mahalanite type, with good comprehensive performance. It is generally used in annealing condition, and can also be strengthened after heat treatment.The alloy has good performance, excellent oxidation resistance and corrosion resistance. Its components can work for more than 6000h at 400℃ for a long time and more than 2000h at 450℃ for a long time.After isothermal annealing treatment, the tensile strength at room temperature is greater than 980MPa, the yield strength is greater than 840MPa, the elongation is greater than 10%, and the reduction of area is greater than 25%.Tensile strength at high temperature of 400℃ is greater than 720MPa, elongation is greater than 14%, and reduction of area is greater than 40%.In order to further improve its intensity of use, it can also be treated with “solid solution + aging”.

2.3  TC16 (b ≥ 1030MPa class)

At present, for steel fasteners, most of them are processed by cold heading, and only a few of them are processed by hot heading,The cold heading technology enables the continuous mass production of fasteners.However, most industrial titanium alloys cannot be cold headed because of their poor cold forming properties.Therefore, in western countries, TC4 titanium alloy fasteners are mainly produced by hot upsetting forming process. The disadvantages of the hot upsetting process are as follows:During the heating of billet, local burn, overheat and surface oxidation are easy to occur, and it is not easy to realize automatic continuous upsetting and low production efficiency.In order to improve the stability of the titanium alloy fasteners production efficiency and quality, research and development of the former Soviet union fasteners in special cold heading with BT16 titanium alloy, titanium fasteners, cold heading technology development, and in 76, il 86, il, 96, 124, Su27 series soviet-made (Russian) on the plane for a large number of engineering application.

Nominal component of titanium alloy ti3al-5mo-4.5v BT16 (named TC16 in china) , this alloy is mahalanite type α+β two-phase titanium alloy, and the β stability coefficient is 0.83, which is close to the critical component.The titanium alloy is mainly used for manufacturing aviation fasteners with working temperature below 350 ° c, and the α+β/βalloy conversion temperature (860±20 ° c).The score determined that the BT16 alloy has excellent room temperature process plasticity, the alloy can complete the cold heading forming of the fastener head at room temperature, thus significantly improving the bolt production efficiency and reducing the production cost, and then its strength can reach 1030~1180MPa after the solid solution aging heat treatment.Russia (former Soviet union) titanium alloy bolt fasteners are mainly made of BT16 titanium alloy, which has been used for decades without any quality accidents.In the 1990s, China introduced the production line of su-27 aircraft from Russia. In order to meet the domestic demand of su-27 aircraft, relevant domestic department immediately carried out the localization of BT16 titanium alloy and its fasteners, which was named as TC16 titanium alloy .At present, TC16 titanium alloy bolts independently developed and produced by China have been widely used in the third generation fighter aircraft.

2.4 TB3 (b ≥ 1100MPa)

In the 1970s and 1980s, the 1100MPa titanium alloy bolts used in the aerospace industry in the United States and other western industrial developed countries were mainly made of TC4 (ti6al-4v) titanium alloy material, all of which were produced by hot upsetting forming process.In the late 1970 s and early ’80 s in our country should composite structure connection requirements, be badly in need of 1100 mpa titanium bolts kind of fasteners, due to the limitation of hot upsetting equipment (domestic didn’t hot upsetting forming equipment that time), to the development and production of 1100 mpa grade TC4 (Ti – 6, 4 v) bolt, mainly focus on developing can cold heading directly for this β type titanium alloy, TB3 titanium alloy is developed under the condition of this kind of background.The design of the TB3 alloy is based on ti-8mo-8v-2fe-3al titanium alloy developed by the titanium company in the 20th century.

TB3 titanium alloy is a kind of metastable titanium alloy which can be heat treated and strengthened. The nominal component of the alloy is ti-10mo-8v-1fe-3.5al.The main advantage of the alloy is that the solid solution treatment state has excellent cold forming performance, its cold heading ratio (Dt/D0) can reach 2.8, after the “solution + aging” system treatment can obtain high strength, mainly used for the manufacture of 1100MPa class high strength aerospace fasteners with the service temperature below 300℃.In October 1982, China started the development of TB3 titanium alloy bolts. In 1985, the development progress was made and the relevant technical specifications were formed.In the middle and late 1980s, the domestic TB3 titanium alloy was successively made into high-lock bolts and interference bolts, which were installed in the composite material structure and the metal structure of aircraft, providing a certain experience basis for the application of titanium alloy bolt fasteners in China.The alloy has been widely used in the manufacture of 1100MPa titanium alloy bolts, and has been successfully used in y-7, j-8, j-10 aircraft and some space vehicles.At present, TB3 titanium alloy has become the main material of titanium alloy bolt fasteners for aerospace aero-crafts which can be industrialized in China.At the same time, the titanium alloy is also used in the manufacture of rivets. In the GJB120-2006 《titanium and titanium alloy rivets》 issued by China in 2006, TB3 titanium material was officially included in the standard as the rivet material.

2.5  TB8 titanium alloy (b ≥ 1280MPa class)

With the rapid development of aviation and aerospace technology, the mechanical connection technology used in military and civilian aircraft has become more and more important, and its technical content become more and more high. Its role in the whole machine has not only been “fastening” and “connecting”, but also become an important structural part to realize the performance of the whole machine.The development trend of aerospace technology in the future requires high specific strength of new fasteners, that is, light weight and high strength.Therefore, the United States, Russia, France and other world aviation powers are actively developing high-strength titanium alloy materials with tensile strength of more than 1200MPa.In recent years, Alcoa has developed Timetal555 high-strength bolts in titanium alloy with tensile strength of more than 1300MPa, double shear strength of more than 745MPa and elongation of more than 10% after solid solution aging. All the performance indexes fully meet the requirements of the standard 1250MPa cadmium-plated alloy steel fasteners.The Aerlite180 bolts manufactured from SPS TITANTM761 titanium alloy have a tensile strength of 1240MPa and a shear strength of 745MPa, reaching the strength level of many alloy steel and corrosion-resistant alloy fasteners, and a weight reduction of 40%.

In order to keep up with the development trend of international advanced aerospace titanium alloy fasteners, in recent years, some of company in china have developed special TB8 titanium alloy bar wire material and 1280MPa high-strength bolt fasteners for fasteners, its specification from 4 ~ 25.TB8 titanium alloy is the generic beta 21 s titanium alloy in the United States, its name meaning into points for Ti – 3-2.7 Nb15Mo, beta is 21 s alloy titanium companies in the United States (1989 Timent) for NASP program developed by metastable beta type titanium alloy, beta 21 s titanium alloys have excellent hot and cold processing performance, deep hardenability, high creep resistance, high oxidation resistance and good corrosion resistance, so that the alloy is aircraft designer and manufacturer of recognition,As an excellent aerospace structural material, it was first included in the ASTM standard of the United States in 1994. The United States mainly USES this alloy to manufacture titanium matrix composites for the space shuttle and airplane engine pod parts such as Boeing 777.In China since the 1990 s to the imitation of the alloy, completed a certain type of aircraft structures with TB8 titanium alloy forging and sheet metal pieces of the development and application of the research work, because ultimately no for engineering application, only in the GB/T3620.1-2007 in titanium and titanium alloy type and chemical composition of their brand and component specification, the material and the product without the formation of a national standard, military specifications and on the AIDS to navigation.

The 1280MPa class TB8 titanium alloy bolts are shown in figure 1, and the main performance indexes are shown in table 1

Fig.1 Photos of 1280MPa TB8 Titanium bolt

Table 1 performance indexes of 1280MPa class TB8 titanium alloy bolts

Performance of the project indicators note
Tensile strength /MPa ≥ 1280

According to the GJB715.23 A – 2008

Standards

 

Elongation / %

(same furnace material)

 

≥ 8 Is the elongation of the same batch of material with the furnace sample, the bolt has no requirement on the elongation
Double shear strength /MPa ≥ 755 According to GJB715.26A-2008 standard
Fatigue life/times ≥ 130000 According to GJB715.30 a-2002 standard, load: 512MPa

 

3  summary

As you can see, aviation research from the application of titanium alloy fastener industry titanium alloy has been, the United States, Russia (Soviet) such as aviation industry in developed countries have formed in early technology system of fasteners with titanium alloy material, the titanium alloy fastener has won a large number of applications in the field of aviation manufacturing.And airlines in China started relatively late in research and application of titanium alloy fasteners, are in the aviation engine or an airplane, technology, or the introduction of passive in the improvement process, manufacturing fasteners titanium alloy material are basic reference or generic Russia (Soviet), the United States, at the same time, the uneven distribution of titanium alloy fasteners in our country aerospace fasteners is relatively low.With the rapid development of China’s aviation industry and the continuous improvement of aircraft performance requirements, the future high-performance aviation fasteners for the manufacture of titanium alloy materials put forward higher strength, higher fracture toughness, higher fatigue performance requirements.

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