Titanium alloy materials for aerospace fasteners

Fastener, as an important universal basic component, plays an important role in industry, and is called “the rice of industry”. Mr. Wu Guanghui, the famous chief designer of ARJ21-700 aircraft in China, also praised fasteners as “tens of thousands of types, groups, connecting structures, connecting systems, small things and big things [1].

According to the types, fasteners can be divided into 13 categories: bolt, screw, stud, nut, wood screw, self tapping screw, washer, rivet, pin, retaining ring, link pair and fastener assembly; according to the application field, fasteners can be divided into general purpose fasteners and aerospace fasteners. In the field of aviation, the connection mode of aircraft is still mainly mechanical connection, and the connection and assembly of aircraft rely on a large number of various fasteners; in the field of aerospace, the connection between aircraft parts also depends on fasteners. With the development of lightweight equipment, more and more aerospace fasteners prefer titanium alloy materials. In foreign countries, the application history of titanium alloy fasteners can be traced back to the 1950s. The United States took the lead in applying Ti-6Al-4V alloy bolts to B-52 bombers, and achieved remarkable weight reduction effect. Therefore, the application of titanium alloy fasteners in the field of aerospace started. At present, the United States, France and other developed countries in Europe and the United States, more than 95% of titanium alloy fasteners are made of internationally recognized Ti-6Al-4V materials, and some advanced models of titanium alloy fasteners have completely replaced 30CrMnSiA steel. After the C-5A military transport aircraft adopted titanium alloy fasteners, the mass was reduced by about 4500kg; after the Boeing 747 fasteners of civil aircraft replaced steel with titanium, the mass was reduced by 1814kg [2]. Russian titanium alloy fasteners and alloy systems have been used in Il-76, il-86, il-96, Tu-204, an-72 and An-124 aircraft models, significantly reducing the weight of the aircraft. For example, in figure-204, 940 kg BT16 titanium alloy fasteners are used in the aircraft, with a weight reduction of 688 kg; in figure-76, 142000 titanium alloy fasteners are used in the aircraft, with a weight reduction of 600 kg [2-3]. The development history of titanium alloy fasteners in China can be traced back to 1965. In the 1970s, relevant units carried out research on titanium alloy rivets and their application; in the 1980s, a small amount of titanium alloy fasteners, such as rivets and bolts, began to be used in some second-generation military aircraft in China; in the late 1990s, with the introduction of the third generation heavy fighter production line abroad and the third generation war in China In recent years, with the development of China’s aerospace industry, various units have carried out the research and development of titanium alloy materials for fasteners and fastener manufacturing technology. Titanium alloy fasteners are the first to be widely used in the field of aerospace [4], and the consumption in civil aircraft is also very considerable. According to the data, each domestic C919 aircraft needs about 200000 titanium alloy fasteners, and if the annual production of 150 large aircraft is planned in 2018, 30 million titanium alloy fasteners are needed every year [2].

Advantages

Table 1 shows the performance comparison between titanium alloy and steel materials for fasteners. Titanium alloy materials have the following advantages in the application of fasteners.
Tab. 1 Comparison in properties of different materials used for fastener[5]

Material name Density/
(g·cm-3)
Melting point/℃ Modulus of elasticity/GPa Relative impact strength/MPa Permeability/
(H·m-1)
Coefficient of thermal expansion/℃-1 Specific strength/cm Yield ratio
Ti-5111 4.43 110 2.3 1.0 9.4 17.5×105 0.88
Ti-6Al-4V 4.43 1 649 114 2.6 1.0 9.2 19.8×105 0.83
Ti-6Al-4VEIL 4.43 1 649 110 2.5 1.0 9.6 19.0×105 0.86
416 7.80 1 500 200 1.4 700~1 000 11.0 10.9×105 0.75
SAE Grade 5 7.80 1 140 212 1.0 500~2 500 13.0 8.4×105 0.77
SAE Grade 8 7.80 1 140 212 1.5 1 500~2 500 13.0 11.7×105 0.86

(1) The density is small. The density of titanium alloy is significantly lower than that of steel, so the weight of titanium alloy fastener is lighter than that of steel fastener.
(2) High specific strength. Titanium alloy is one of the common metal materials with high specific strength. Using the advantages of high specific strength, titanium alloy can also be used to replace the light aluminum alloy material. When the external load is the same, the geometric size of titanium alloy parts is smaller, which can effectively save space. This material utilization concept is of great significance to the aerospace field.
(3) High melting point. The melting point of titanium alloy is significantly higher than that of steel, so the heat resistance of titanium alloy fastener is better than that of steel fastener.
(4) The coefficient of thermal expansion and modulus of elasticity are small. According to the calculation formula of thermal stress:
20200122141205 13381 - Titanium alloy materials for aerospace fasteners (1)
Where: E is the modulus of elasticity; α is the coefficient of thermal expansion; ΔT is the temperature change.
It can be seen from formula (1) that the coefficient of thermal expansion and modulus of elasticity of titanium alloy are smaller than those of nickel alloy and steel. In the same temperature range, the thermal stress produced by titanium alloy is very small, so titanium alloy has higher thermal fatigue performance.
(5) No magnetism. The permeability of titanium alloy is very small, almost negligible, so titanium alloy fasteners are non-magnetic, which can effectively prevent the interference of magnetic field. Austenitic stainless steel is also non-magnetic, but subsequent cold working will increase its magnetism, and the hot or cold working of titanium alloy will not change its magnetism, which makes titanium alloy can be used in avionics equipment.
(6) The ratio of yield to strength is higher. Yield strength is the critical strength standard of fastener design under tensile load, and then tensile strength, because once the fastener produces yield deformation, it will lose the fastening effect. Compared with steel, the yield strength of titanium alloy is close to the tensile strength, and the yield strength is higher, so the safety of titanium alloy fastener is higher.
(7) The electrode potential matches the carbon fiber composite. In fasteners, the important reason for the huge amount of titanium alloy is that the electrode potential of titanium alloy matches the electrode potential of carbon fiber composite, which effectively prevents the occurrence of galvanic corrosion.
(8) In addition, titanium alloy has the advantages of excellent corrosion resistance and high creep resistance.

General situation

Titanium alloy materials for fasteners and performance overview

Titanium alloy materials for fasteners are closely related to the manufacturing process and application of fasteners. On the one hand, the manufacturing process of titanium alloy fastener mainly includes three parts: first, plastic deformation, such as upsetting, reducing and rolling thread; second, surface strengthening, such as the strengthening of bolt bearing surface and straight bar transition area; finally, machining, such as turning, milling and grinding. On the other hand, due to the different purposes of fasteners, the performance requirements of the required materials are also different, which requires the use of different titanium alloy materials. Taking rivets and bolts as examples, rivets need to be upset at one or both ends in the installation process, so the riveting process requires high plasticity of materials. Bolts are generally required to have high strength, and their strength level is close to that of 30CrMnSiA high strength alloy steel, so high strength titanium alloy material is usually used. Based on the above two factors, titanium alloy materials for fasteners are mainly divided into three types: industrial pure titanium, (α + β) type and β type titanium alloy. See Table 2 for details. It can be seen from table 2 that industrial pure titanium is mainly TA1 and TA2. (α + β) titanium alloy mainly includes TC4, TC6 and ti-662. The main type of β – titanium alloy is metastable β – titanium alloy, because the molybdenum equivalent of metastable β – titanium alloy is about 10%. The heat treatment strengthening effect of near β titanium alloy with Mo equivalent less than 10% is insufficient; the stable β titanium alloy with Mo equivalent more than 10% has high β phase stability and is difficult to decompose during aging heat treatment, so the strengthening effect of sub stable β titanium alloy is the most obvious. In addition, metastable β – titanium alloy has excellent cold formability, can be cold upset, avoid using professional heating equipment and gas protection medium, high production efficiency and material utilization, high dimensional accuracy and surface quality of formed fasteners. The (α + β) titanium alloy fastener can only be formed by hot upsetting, which requires special heating equipment and gas medium. The production efficiency and material utilization rate are low, and the heating temperature is easy to be uneven.

Tab. 2 Titanium alloys used for fastener
Alloy grade Nominal chemical composition Alloy type
TA1 Commercially pure titanium α type
TA2 Commercially pure titanium α type
TC4 Ti-6Al-4V (α+β) type
TC6(BT3-1) Ti-6Al-1.5Cr-2.5Mo-0.5Fe-0.3Si (α+β) type
Ti-662 Ti-6Al-6V-2Sn (α+β) type
Ti-62222 Ti-6Al-2Cr-2Mo-2Fe-2Sn (α+β) type
Ti-5111 Ti-5Al-1Sn-1Zr-1V-0.8Mo (α+β) type
TC16(BT16) Ti-2.5Al-5.0Mo-5.0V (α+β) type
SP-700 Ti-4.5Al-3V-2Fe-2Mo Near β type
Ti-555 Ti-5Al-5V-5Mo-3Cr Near β type
VT16-1 Ti-3Al-5V-3Cr-5Mo Near β type
TB6(Ti-10-2-3) Ti-10Fe-2V-3Al Near β type
Ti-3253 Ti-3Al-2V-5Mo-3Fe Metastable β
β-Ⅲ Ti-11.5Mo-6Zr-4Sn Metastable β
TB2 Ti-5Mo-5V-8Cr-3Al Metastable β
TB3 Ti-10Mo-8V-1Fe-3.5Al Metastable β
B120VCA Ti-13V-11Cr-3Al Metastable β
TB4(Ti-47121) Ti-4Al-7Mo-10V-2Fe-1Zr Metastable β
TB5(Ti-15-3) Ti-15V-3Cr-3Al-3Sn Metastable β
TB8(β21S) Ti-15Mo-3Al-2.7Nb-0.2Si Metastable β
TB9(βC) Ti-3Al-8V-6Cr-4Mo-4Zr Metastable β
Ti45Nb Ti45Nb Stable β type

The mechanical properties of titanium alloy materials for rivets and bolts are listed in Table 3 and table 4 respectively. It can be seen from table 3 that the tensile strength of pure titanium used for rivets is more than 350 MPa, and the shear strength is 240-350 MPa; (α + β) type titanium alloy rivets are used in annealed state, and β type titanium alloy is used in solution state, and the tensile strength of the two alloys is basically the same, 800-950 MPa, and the shear strength is more than 600 MPa. Except for TC4 titanium alloy, the titanium alloy materials for bolts are all metastable β titanium alloy, and they are all used in solution + aging state. Except for TB8, tb9 and ti-555 alloy materials, the tensile strength can reach more than 1200 MPa. Most β titanium alloy materials generally have tensile strength of about 1100 MPa and shear strength of 650-700 MPa.

Alloy grade State Rm/MPa A/% Ψ/% τ/MPa
TA1(CP40) Annealing 345 25 50 240
TA1(CP55) Annealing 440 25 40 350
BT16 Annealing 830~950 16 60 640
TB2 Solid solution 880~980 20 62 640
TB3 Solid solution 840~940 20 65 650
βⅢ Solid solution 800~900 18 65 620
Ti-45Nb Annealing 450 25 60
Note: RM is the tensile strength; a is the elongation; ψ is the reduction of area; τ is the shear strength.

Tab. 4 Mechanical properties of titanium alloys used for bolts[2, 4, 6-7]

Alloy grade Rm/MPa RP0.2/MPa A/% Ψ/% τ/MPa
TC4 1 100 1 000 10 20 665
BT16 1 030~1 180 12 30 705
TB2 1 100 12 30 700
TB3 1 100 1 000 10 30 690
TB8 ≥1 280 ≥8 ≥755
TB9 1 325 1 158 11 28 ≥650
Ti-555 ≥1 309 >10 ≥779
Note: RM is the tensile strength; Rp0.2 is the yield strength; a is the elongation; ψ is the reduction of area; τ is the shear strength.

Several important titanium alloy materials for fasteners

TC4 titanium alloy

TC4 titanium alloy is a kind of two-phase titanium alloy with medium strength, which is also the most researched and applied titanium alloy material. Most of the titanium alloy materials for fasteners are TC4 titanium alloy. When TC4 titanium alloy is used to make fasteners, only hot upsetting can be used, and special hot upsetting equipment and heating equipment must be used, which not only affects the production efficiency, but also has low material utilization. For high-strength fasteners, the strength of TC4 titanium alloy fasteners can not meet the requirements. The maximum tensile strength of the alloy after solution aging is 1100 MPa, and the shear strength is about 650 MPa. Due to the poor hardenability of TC4 titanium alloy, the section size of TC4 titanium alloy fasteners during solution aging is generally below 19 mm. TC4 titanium alloy fasteners include bolts, high lock bolts, pop rivets, screws and ring groove rivets, most of which have been widely used in domestic aircraft, engines, airborne equipment, aerospace vehicles and satellites.

TC6 titanium alloy

TC6 titanium alloy is a kind of martensitic (α + β) biphasic titanium alloy with excellent comprehensive properties. Its nominal composition is ti-6al-2.5mo-1.5cr-0.5fe-0.3si. The alloy is generally used in annealing state, can also be strengthened by heat treatment, and has good oxidation resistance.

TC16 Titanium Alloy

TC16 Titanium alloy is a typical solution aged two-phase titanium alloy with the nominal composition of Ti-3Al-5Mo-4.5V. After solution treatment, the alloy has high room temperature plasticity, so it has good cold heading performance, and the ratio of upsetting to forging reaches 1:4. In fastener manufacturing, TC16 Titanium alloy can be either directly cold upset or hot upset. At present, TC16 Titanium alloy fasteners include bolts, screws and self-locking nuts.

TB2 titanium alloy

TB2 titanium alloy is a metastable β – type titanium alloy. The nominal composition of the alloy is ti-3al-8cr-5mo-5v. In the solution state, TB2 titanium alloy has excellent cold formability and weldability. At present, it is mainly used for manufacturing satellite corrugated shell, satellite arrow connecting belt, all kinds of cold heading rivets and bolts, especially TB2 titanium alloy rivets have been widely used in key products in aerospace field.

Tb3 titanium alloy

Tb3 titanium alloy [4] is a metastable β – titanium alloy which can be heat treated and strengthened. The nominal composition of the alloy is ti-10mo-8v-1fe-3.5al. The main advantage of the alloy is that it has excellent cold formability in solution treatment state, and its cold upset ratio can reach 2.8. After solution aging, the alloy can obtain higher strength, which is mainly used for manufacturing 1100 MPa High Strength aerospace fasteners.

TB5 titanium alloy

TB5 titanium alloy is a metastable β – type titanium alloy with the nominal composition of ti-15v-3cr-3sn-3al. TB5 titanium alloy has excellent cold formability, which can be compared with pure titanium. After solution, many kinds of fasteners can be cold formed, and the tensile strength at room temperature can reach 1 000 MPa after aging. Boeing company has applied TB5 titanium alloy fasteners to Boeing aircraft, and China also uses TB5 titanium alloy to manufacture cold heading rivets for matching with fighter umbrella beam and satellite corrugated plate [4].

TB8 titanium alloy

TB8 titanium alloy is a metastable β 21s titanium alloy. Its nominal composition is ti-3al-2.7nb-15mo. This titanium alloy has excellent cold and hot working properties, good hardenability, excellent creep resistance and corrosion resistance. Due to the adoption of homocrystalline β stable elements Mo and Nb with high melting point and small self diffusion coefficient, TB8 titanium alloy has high temperature oxidation resistance, which is 100 times higher than that of Ti-15-3 alloy. See Table 5 for specific data. At present, TB8 titanium alloy high-strength bolts have been widely used in key products in the aviation field in China.
Tab. 5 Comparison in oxidation data between TB8 and Ti-15-3 titanium alloys[8]

Alloy grade Temperature/℃ Increased quality/(mg·cm-2)
24 h 32 h
Ti-15-3 649 3.39 4.79
815 102.60 172.30
TB8 649 0.14 0.23
815 1.21 1.75

Ti-45Nb aloy

Ti-45nb alloy is a stable β – type titanium alloy, which is a special titanium alloy for rivets. At first, titanium alloy materials for rivets are mainly pure titanium, but the strength of pure titanium fasteners is too low. In some high bearing parts, pure titanium fasteners can not meet the requirements, so it is urgent to have a titanium alloy with plasticity close to pure titanium, and strength higher than pure titanium. The commonly used metastable β titanium alloy has large deformation resistance, and the room temperature plasticity is quite different from pure titanium. Later, ti-45nb alloy was developed, which has high plasticity at room temperature, elongation at room temperature up to 20%, reduction of area up to 60%, and excellent cold working ability. Compared with pure titanium, ti-45nb alloy has higher tensile strength and shear strength, reaching 450 MPa and 350 MPa respectively.

Future development trend

Ultra high strength titanium alloy fasteners

With the development of China’s aerospace industry, the connection technology level of new aircraft and space vehicles is constantly improving, which also puts forward new requirements for new fasteners. The future development of ultra-high strength titanium alloy fasteners with tensile strength of 1200-1500 MPa and shear strength ≥ 750 MPa is one of the trends in the future.

High temperature resistant titanium alloy fasteners

At present, the use temperature of titanium alloy materials for fasteners is not high, see Table 5 for details. In the field of Aeronautics and Astronautics, the service temperature of materials is required to increase with the continuous improvement of the flight speed of new aircraft and aircraft. Therefore, the high temperature resistant titanium alloy fastener is also the future development trend, especially in the aerospace field, the new high temperature titanium alloy material is required to be able to serve in 600 ~ 800 ℃ for a short time. In general, Ti2AlNb alloy is used to replace the heavier superalloy, and its deformation is relatively serious, while Ti2AlNb alloy is used to replace other titanium alloy materials, which can not meet the weight reduction requirements; Ti Al based intermetallic compound process plasticity is poor, and its maturity is poor. Therefore, in the future, the high temperature titanium alloy materials used for fasteners are still mainly two-phase titanium alloy of near α type and high aluminum equivalent. At high temperature, the improvement of strength and creep resistance of titanium alloy mainly depends on the solution strengthening effect of Al, Sn and Zr. However, due to the limitation of aluminum equivalent, the content of these elements can not be increased indefinitely. Therefore, under the proper control of the content of Al, Sn and Zr, titanium alloy is designed by multi-element composite alloying. The stable element Mo has solution strengthening effect on the high temperature strength and creep strength of high temperature titanium alloy, and Nb, Cr and V have similar effect. The addition of a small amount of β stable elements can also prevent the embrittlement of the alloy. In addition, the content of Si is very important to the properties of titanium alloy. After adding about 0.2% Si, the ellipsoidal silicide will precipitate on the boundary of α sheet in a non-uniform and discontinuous manner, which can effectively block the movement of dislocation, produce dispersion strengthening effect, and greatly improve the creep resistance of the alloy. However, the appearance of silicide also has a harmful effect on the thermal stability of the alloy structure, which not only reduces the plasticity of the alloy, but also enhances the ordering degree of the alloy and promotes the formation of Ti3Al phase. Therefore, the Si content should be controlled at a lower level, and the general mass fraction should not be more than 0.5%. Therefore, the multi-element composite strengthening is still the development direction of new high temperature titanium alloy design.

Tab. 6 Usage temperatures of titanium alloys for common fastener

Alloy grade TC4 TC16 β-Ⅲ Ti-45Nb TB2 TB3 TB5 TB8
Service temperature/℃ 400 350 370 425 300 300 290 450

Source: China Fasteners Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Reference

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Summary
titanium alloy materials for aerospace fasteners - Titanium alloy materials for aerospace fasteners
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Titanium alloy materials for aerospace fasteners
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More than 95% of titanium alloy fasteners are made of internationally recognized Ti-6Al-4V materials, and some advanced models of titanium alloy fasteners have completely replaced 30CrMnSiA steel.
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