Development Status of titanium alloy materials for Aviation fasteners
Titanium alloy fasteners are widely used in aerospace field due to their low density, high ratio and corrosion resistance. This paper focuses on the development of titanium alloy materials for Aviation fasteners. Combined with the development of titanium alloy for fasteners, the application status of titanium alloy for fasteners at home and abroad is introduced, and the performance characteristics of titanium alloy materials for fasteners are compared and analyzed. At the same time, combined with the requirements of advanced aircraft for high-performance fasteners, several high-strength titanium alloy materials for fasteners and their fasteners processing technology are introduced.
Bolts, studs, screws, nuts, washers, pins, rivets and other fasteners are used in a large amount on the aircraft, and the fasteners and elastic elements used in an aircraft are at least hundreds of thousands of pieces, there are a few million pieces. For example, an Il-96 plane in Russia uses 142,000 pieces of fasteners, and a single Airbus A380 uses more than 1 million pieces of fasteners, boeing 787 has experienced delayed delivery due to shortage of fasteners. With the improvement of the advanced nature of the aircraft, the requirements for fastener materials are getting higher and higher. Titanium alloys with high weight loss, corrosion resistance, no magnetism and good compatibility with composite materials are gradually becoming the first choice for advanced aircraft fastener materials [1-3], titanium alloy fasteners are widely used in military fighters and transport aircrafts such as F-16, F-18, F-35, F-22, C- 17, etc. and have achieved good results. Titanium alloy fasteners used in F-15 fighters account for 73% of the fasteners used in the whole machine, c-17 large military transport aircraft used 423000 titanium alloy pins and 241000 titanium alloy bolts; Figure-204 The airliner used a 940kg BT16 Titanium Alloy Fastener, which reduced the weight by 688kg, after replacing alloy steel fasteners with titanium alloy fasteners, the total weight of the stand-alone plane is reduced by 1814kg.
In recent years, the alloy steel fasteners on the us military and civilian aircraft have been basically replaced by titanium alloy fasteners, while china’s space fasteners with titanium alloy material technology developed late and depended on imports for a long time. With the acceleration of the localization process of titanium alloy for fasteners, it is necessary to further sort out the development status of titanium alloy materials and technology for fasteners.
On the basis of reviewing the application status of titanium alloy fasteners at home and abroad, this paper compares and analyzes the performance characteristics of titanium alloy materials used for fasteners and combines the demand of advanced aircraft for high-performance fasteners, this paper introduces several high-strength titanium alloy materials for fasteners and the processing technology of fasteners.
Development and application of titanium alloy materials for fasteners
Table of Contents
- Development and application of titanium alloy materials for fasteners
- Research progress of titanium alloy for high strength fasteners
Development and application of titanium alloy for fasteners abroad
A large number of bolts are used in fasteners. Titanium alloy bolts require shear strength and tensile strength to reach the level of 30crmnsia of high strength steel. The first use of titanium alloy fasteners dates back to the 50’s of the 20th century. The United States first used Ti-6Al-4V(Ti-64) bolts on B- 52 bombers and achieved remarkable weight loss effect . The β stability coefficient of Ti-64 is 0.27, with low density, good strength and fatigue performance, simple alloy composition and low cost of semi-finished products, so it has been widely used and developed. In 1955, 1 million Ti-64 titanium alloy screw bolts were used, and in 1958, it reached 20 million, and gradually became the main fastener material used in the aviation and aerospace sectors of the united states and western europe. However, the cold plasticity of the Ti-64 is very poor, and the forming of its fasteners can only be hot forging, and the production cost of special equipment such as vacuum solid solution (water cooling) and aging needs to be improved. At the same time, due to its poor hardenability, the performance consistency under large cross-section cannot be guaranteed, which leads to the limit of Bolt size in production, generally no more than φ19mm. Subsequently, the United States began to use Ti-3Al-8V-6Cr-4Mo-4Zr(β-c) for the preparation of fasteners, whose strength level reached 1150MPa, and due to its good hardenability, large-size fasteners with a range of 38mm can be produced .
The fasteners in Russia mainly adopt BT16(Ti-3Al-5Mo-4.5V), the metal α + β type high-strength titanium alloy, whose strength level is 1030MPa. The main semi-finished products are hot rolled bar, polished Rod and wire for cold heading, it is mainly used to manufacture fasteners, such as bolts, screws, nuts and rivets. The highest working temperature is 350℃. The strength of BT16 titanium alloy under solid solution aging is slightly lower than that of Ti-64 alloy. Its main advantage is that it can be formed by cold heading under annealing condition, which obviously improves the production efficiency [6-7]. Therefore, BT16 fasteners manufactured by cold deformation are widely used in the machinery manufacturing industry in russia and have become the main standard component materials used in the russian aviation and aerospace sector.
With the promotion of advanced civil aircrafts such as A380 and so on, european and american countries have successively started to study the alternative alloys of high-strength fasteners that can be replaced by such superalloys as inonel718, A286 and MP35N, including β-lcb, Ti-153, β, however, there is no report of its actual application in fasteners.
Development and application of titanium alloy for fasteners in China
The development of titanium alloy fasteners in China started late. In the middle of the 1960 s, chengdu aircraft design and research institute began to study the use of TB2 titanium alloy rivets in titanium alloy aircraft fuselage, and completed the technical appraisal of related work in the late 1970 s . Until the late 1980 s, the research on hot heading technology of TC4 titanium alloy fasteners was gradually carried out . At the same time, in order to overcome the problem of TC4 alloy head forming, Ti-45Nb rivets with good plasticity are developed according to foreign bi-metal titanium alloy rivets and connected with TC4 rivet by friction welding Ti-45Nb rivets. In order to follow the development trend of international advanced aerospace fasteners, china has also successively imitated a series of fasteners using titanium alloy, such as TC6 (BT3-1), which is imitated according to Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si alloy of the former soviet union. Mals’ α/β two-phase titanium alloy, TC16(Ti-3Al-5Mo-4.5V) titanium alloy imitating BT16, TB3 alloy developed referring to Ti-8Mo-8V-2Fe-3Al alloy in the United States, etc. However, the quality of china’s titanium alloy fasteners is unstable and most of them still rely on imports. It is not only more expensive, but also often due to insufficient procurement and supply, the development or production is in a state of “shutdown. Therefore, the amount of titanium alloy fasteners independently developed in China is very small on advanced fighter planes.
In terms of high-strength fasteners, the fasteners for active-duty fighters in our country are mostly imported high-strength steel 30crmnsia. In recent years, imported Ti-64 titanium alloy wire bars have been gradually adopted to manufacture 1100MPa grade fasteners. TC16 and TB8 titanium alloy fasteners have also been adopted and have achieved good weight loss effect. Thereinto, TB8(β21S,Ti-15Mo-3Al-2.7Nb-0.2Si) titanium alloy is a kind of substable β-type titanium alloy [10-11] developed by american Timet company in 1989 according to the requirements of national aerospace program NASP on antioxidant metal and composite material matrix, it not only has the creep resistance ability similar to Ti-64 alloy, 100 times higher oxidation resistance and excellent corrosion resistance than Ti-153 alloy, moreover, like Ti-153 alloy, it has the same cold deformation performance as industrial pure titanium, and is very easy to process into plates, strips, foil, wire bars, etc, it is one of the ideal titanium alloy materials for manufacturing 1250MPa grade fasteners. However, it is worth noting that TB8 titanium alloy easily leads to composition segregation due to containing 15% (mass fraction) Mo element, and the production of large-size ingots (>1t) is difficult, limited the scale production and further promotion and application .
Performance Comparison of titanium alloy for fasteners
Table 1 lists the properties of several commonly used titanium alloys for fasteners at present. Titanium alloy fasteners made of TC4, BT16, TB2, TB3, TB5, etc. are designed to replace aluminum alloy and alloy steel fasteners, and the strength level is more than 1000MPa; the fasteners made of TB8 alloy have gradually replaced the high-strength steel and TC4 fasteners, and the strength level is above 1250MPa.
Beta-C, as one of the fasteners frequently used at present, has successfully entered commercial application and been applied in the large-size fasteners of φ38mm, the main reason is that the alloy has good hardenability and cold forming ability. In addition, the regulation of heat treatment process on the microstructure and properties of the alloy has also been fully studied, and the results are shown in Table 2. It can be seen that the heat treatment process has a great influence on the mechanical properties of titanium alloy. The tensile strength of Beta-C alloy can be adjusted in the range of 1200 ~ 1550MPa, correspondingly, the variation range of elongation is 9%~ 12%.
Research progress of titanium alloy for high strength fasteners
Requirements of Aviation fasteners for mechanical properties of materials
Aviation fasteners are not only affected by static load during the service period, but also subject to high-speed rotation of rotating parts due to the take-off and landing of the aircraft, the vibration of the engine, the alternating load generated by motorized flight and sudden wind and other factors requires high mechanical properties of materials, and the performance package that must be tested
Including tensile strength, double shear strength and fatigue properties. In terms of the detection technology, the detection of the tensile strength of the fastener is different from that of the material performance test. There is no need to prepare a standard sample, but to install the finished product of the titanium alloy fastener on the stretching machine with a special fixture for detection, according to the different head shape of the fastener, the fixture used in the test is different. Before testing it is required that there should be at least two incomplete threads at the end of non-spinning screw thread bolt below the bearing surface of the mounting nut, which should stretch out from the top of the nut.
The measured value of fracture force is its tensile strength. The theoretical tensile strength of general countersunk bolts is 90% of the tensile strength of convex bolts . The double shear strength test of fasteners also adopts the formed fasteners to be placed on the specific shear tooling, and the maximum pressure that makes the material fracture by pressurized the shear tooling is the double shear strength.
Because titanium alloy fasteners adopt many thermal mechanical processing methods which can improve the fatigue life, and the material microstructure and processing methods have influence on the fatigue life, therefore, the fatigue life of fasteners is not tested with raw materials or semi-finished products, but generally with finished fasteners for testing. The common high-strength fasteners require a certain test load (determined by relevant technical conditions or order documents) at R = 0.1, under the fatigue condition with test frequency not exceeding 210Hz, 130,000 cycles will not fail . With the improvement of the advanced nature of aircraft and the development of aviation material technology, higher weight loss requirements are put forward for fasteners and their materials. Ferrero proposes two-step development goals for fastener materials, as shown in table 3. In the first stage, it is required that the tensile strength and shear strength of fasteners should reach the levels of 1241MPa and 703MPa respectively. The corresponding tensile strength, shear strength and elongation of the material should reach the levels of 1379MPa, 745MPa and 10% respectively. In the second stage, the tensile strength and shear strength of the fastener are expected to reach 1517MPa and 862MPa respectively, which requires higher mechanical properties of the material.
Table 1 performance of titanium alloy for common Aviation fasteners
|Alloy||Ingredients||Diameter /mm||State||Tensile strength/MPa||Elongation/%||Shear strength/MPa||Pier bulk ratio|
Table 2 room temperature mechanical properties of Beta-C under different treatment processes
|Process Engineering||Tensile strength/MPa||Yield strength/MPa||Elongation/%||Section shrinkage/%|
|Direct aging, 496℃ /2h/AC, PAM||1555||1410||9||10|
|STA, 760℃ /30min/AC+496℃ /24h, PAM||1215||1105||12||16|
|STA, 815℃ /1h/AC+496℃ /20h/AC, VAR||1325||1185||11||38|
|STA, AMS 4958||>1240||—||>8||—|
Table 3 requirements of Aviation High-strength fasteners and material performance
|Class||Tensile strength/MPa of fasteners||Fastener shear strength/MPa||Tensile strength of the alloy/MPa||Alloy shear strength/MPa||Elongation/%|
Development of high toughness titanium alloy technology
Table 4 lists several materials with high strength titanium alloy developed in recent years and their room temperature properties. It is easy to find that near β titanium alloy is expected to be the best candidate material for high-strength fasteners due to its excellent hardenability and shear performance, better cold forming ability and potential to achieve higher strength. Among them, the strength level of alloy such as BT22 in russia and Ti-5553 in the united states has reached 1250MPa. It has been successfully applied to key structures such as landing gears and is undergoing extensive application research. In recent years, Alcoa company has used Ti-5553 to manufacture aviation fastener alloy, whose tensile limit can be adjusted between 1179 ~ 1496MPa. The mechanical properties of the pins of the Ti-5553 series of AERO-LITE alloy MIL-STD-1312 were tested under the standard. The results show that the minimum tensile limit of the alloy is 63.6kN, and the double shear strength is 745MPa, the fatigue life exceeds 130000 times under load ratio of 22kN/2.2kN and frequency of 10Hz, and the tensile limit load of thread exceeds 94.3kN. This indicates that the Ti-5553 has high strength and fatigue properties, but the plasticity is less than 8% when the strength exceeds 1200MPa. For this reason, in recent years, the application of Ti-5553 alloy modified based on Ti-3553 as fasteners can obtain 75% cold deformation capacity, and the tensile strength and shear strength are 25%[15-16] higher than that of Ti-64 alloy.
Table 4 room temperature strength of several kinds of high strength titanium alloy
|Alloy||Status + Craft||Tensile strength/MPa||Yield strength/MPa||Elongation/%||Shear strength/MPa|
|β-C||φ8.8mm, 510℃ /6h, AC||1489||1372||7||896|
|Ti-6222||φ7 mm, 899℃ /1h, WQ+510℃ /8H, AC||1475||1327||10||834|
|Ti-5553||φ11mm, 815℃ /1h, WQ+537℃ /8H, AC||1503||1465||9||786|
Ti-7333(Ti-7Mo-3Nb-3Cr-3Al) is a new type of near-β titanium alloy with independent intellectual property rights developed by northwest university of technology, after 50min/AC + 580℃ 8h/AC), the tensile strength is greater than 1350MPa, the elongation is greater than 8%, and the fracture toughness KIC is greater than 90MPa m-1/2, the comprehensive performance is better than that of Ti-5553 titanium alloy bars with the same specification. <15~85mm bar after solution aging (820 ℃,50min/AC +520~540 ℃,6h/AC treatment, its tensile strength is more than 1400MPa, the elongation is more than 8%, and the strength level is better than TB8 and other titanium alloy bars, as shown in the figure. At present, Ti-7333 alloy is being applied to high-strength fasteners. Du et al.  have reported a kind of high-strength near β titanium alloy Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe, whose properties are excellent after hot rolled in two-phase zone, solid solution in two-phase zone and aging at lower temperature, the tensile strength can reach 1503MPa, and the elongation can reach 15%. It is also expected to become an excellent titanium alloy material for fasteners.
In short, titanium alloy used in aviation fasteners has basically completed 970MPa(Ti-64, TB3), 1100MPa(β-c, Ti-153), 1250MPa(TB8) strength grade material research, at present, the application research of 1370MPa grade Ti-3553, Ti-7333 alloy fasteners is being carried out, as shown in the figure.
Fig. 1 strong plastic contrast of 6 kinds of high strength titanium alloy after heat treatment
Figure 2 titanium alloy development for Aviation fasteners
Progress in processing technology of titanium alloy wire and fasteners
While continuously developing higher-strength titanium alloy materials, both at home and abroad also attach great importance to in-depth research on thermo-mechanical treatment process and microstructure stability control technology, such as TI M ETAL-LCB, Ti-153, in the study on the strengthening mechanism of high-strength titanium alloys such as β-21s and BT22, it is found that the appropriate solution aging + thermo-mechanical treatment process can produce small (the size is about 10μm) while the best match between ultra-high strength (tensile strength of 1500~1600M Pa) and plasticity (elongation of 8%) is obtained by uniformly distributed α + β fine crystal structure [19-21].
Reasonable thermal deformation process has an important influence on obtaining wire rods with good microstructure and comprehensive mechanical properties. The hot deformation technology included in the production process of wire rod includes heating temperature, deformation and rolling speed. For example, in order to make BT16 alloy grain boundary α phase fully broken into flake structure, the choice of heating temperature before deformation should be able to ensure that the thermal deformation starts in the β phase zone and ends in the two phase zone, that is to say, the heating temperature should be above T β, but it should not be too high. If the temperature is too high, the deformation will end in the β phase zone, and the α phase at the grain boundary cannot be fully broken. The influence of deformation amount on the tissue is also very important. If the deformation amount is too large (≥ 70%), the lamellar tissue is easy to spheroize and lead to uneven deformation. If the deformation is too small (≤ 30%), deformation is difficult to ensure that the tissue is fully refined. The impact of rolling speed on the structure is similar to the deformation. If the speed is too fast, the overheated structure will easily appear; If the speed is too slow, it is unfavorable to the structure refinement. At present, the rolling methods of wire rods mainly include longitudinal rolling and spiral rolling.
It is easy to make the bar form a strong deformation central area when the longitudinal rolling method is adopted. While spiral rolling can not only make the bar produce flowing shear deformation in both the longitudinal direction and the radial direction, which is helpful to obtain uniform structure . Therefore, at present, spiral rolling process is mostly used to obtain wire rods with excellent structure and comprehensive performance.
The manufacturing process of titanium alloy fastener mainly includes the molding of fastener head, the molding of thread and the extrusion of rounded corners below the head, and secondly, hot treatment, non-circular grinding and surface treatment, etc. . First of all, the forming of fastener head needs to be completed by forging and forming equipment, and the forming methods mainly include cold heading and hot heading. Moreover, with the continuous development of forging process, forging equipment has developed to numerical control. For example, the upsetting machine for multi-model forming developed by relevant manufacturers in various countries can realize digital adjustment from feeding and cutting length, heating temperature and adjustment, processing efficiency, mold and push rod position setting, etc, optimize the processing quality and adopt different processing technologies according to different varieties to improve the processing efficiency. Secondly, Aviation fasteners require high precision and strict quality of threads. The forming methods of external thread generally include thread rolling, thread rolling and turning. Because the turning method will cut off the metal streamline, it will reduce the mechanical properties of the fastener. Therefore, currently, the external threads of fasteners are mainly prepared by wire drawing and rolling. For small-size fasteners, CNC temperature rubbing machine is generally used. The warm wire rubbing process involves the softening and hardening of the fasteners, which can better improve the stress state of the wire plate, reduce the possibility of tooth fracture, reduce the folding of the workpiece, and meet the quality requirements of the titanium fasteners. For large-size fasteners, numerical control temperature rolling machine is generally used. In addition to the above advantages, the numerical control temperature rolling machine has the advantages of fast setting speed and easy quality control compared with traditional mechanical rolling machine. The forming of external thread mostly adopts numerical control thread rolling machine to realize the monitoring function of thread rolling quality to meet the needs of efficient production. In addition, titanium alloy material is very sensitive to notch, and there is a large stress concentration in the connection parts of the fastener head rod, which affects the fastener performance. Therefore, the rounded corners under the bolt head need to be strengthened. At present, Most of them use high-efficiency fillet strengthening machine to form a plastic deformation band at the transition of the head rod, which can improve the hardness by generating residual pressure stress, the surface roughness is reduced to improve the mechanical strength and fatigue strength at the joint .
In addition, the preparation process of fasteners also includes turning, surface coating, automatic defect detection, etc. And with the continuous improvement of the performance requirements for fasteners, its processing technology also needs to be continuously improved.
Titanium alloy fasteners are widely used in aerospace field due to their advantages of low density, high strength and corrosion resistance. European and american countries with developed aviation industry start developing titanium alloy fasteners early, and have formed a titanium alloy material system in line with their own technology, which has been widely used in the field of aerospace. However, the development of china’s aviation titanium alloy fasteners started late. The development of fasteners is mostly based on tracking and copying and technology reference, lacking independent intellectual property rights. The development and research of new materials are out of step with the application research. At the same time, the maturity of titanium alloy material and fastener manufacturing technology for high-strength fasteners is low. However, with the continuous development of the aviation and aerospace industry, china’s demand for titanium alloy fasteners, especially ultra-high strength titanium alloy fasteners, will continue to grow. Therefore, the research on titanium alloy materials and application technologies for high strength fasteners will be accelerated, it is urgent to form a titanium alloy fastener material system in china as soon as possible.
Authors: Dong Ruifeng, Li Jinshan, Tang Bin, Sun Zhigang, Kou Hongchao
Source: Network Arrangement – China Titanium Fasteners Manufacturer – Yaang Pipe Industry Co., Limited (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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