Analysis of titanium alloy materials for aerospace fasteners
As an important basic part, fasteners are honored with the title of industrial rice, which is enough to show their significance to industrial development. Since the 1950s, countries all over the world have chosen titanium alloy as the main material of fasteners for aerospace. This is because titanium alloy not only has ideal high temperature and corrosion resistance, but also has good strength and elasticity in line with industry requirements, which can effectively solve the problem of bolt loosening and reduce the interference caused by magnetic field. Titanium alloy fasteners will still be the focus of research in the future, The research content will focus on the characteristics of different materials, which should be paid attention to by relevant personnel.
Advantages of titanium alloy materials
Table of Contents
- Advantages of titanium alloy materials
- Analysis of titanium alloy for fasteners
- Research on development trend of relevant materials
Research shows that the advantages of titanium alloy fasteners are mainly reflected in the following aspects:
Titanium alloy has only very small magnetic permeability. All fasteners made of titanium alloy are non-magnetic fasteners, which can minimize the interference caused by magnetic field . Austenitic stainless steel, which is also a non-magnetic material, usually produces certain magnetism after cold processing, and the magnetism of titanium alloy will not be changed by cold processing and hot processing. Therefore, it can be seen that using titanium alloy to manufacture avionics equipment has very prominent advantages.
The lower density determines that the mass of titanium alloy fasteners is lighter than that of fasteners made of other materials.
High melting point
The test shows that titanium alloy has a higher melting point than steel, so its heat resistance often meets the requirements of fasteners.
High yield ratio
The critical strength of fasteners affected by tensile load is yield strength, followed by tensile strength. This is because the emergence of yield deformation will lead to the failure of fasteners to play their due fastening role . Compared with steel materials, one of the characteristics of titanium alloy is that it has similar tensile strength and yield strength, and the high yield ratio determines its safety performance to meet the requirements of aviation industry to the greatest extent.
High specific strength
As a common metal with high specific strength, titanium alloy can be used to replace light aluminum alloy. If the external load remains unchanged, the size of parts made of titanium alloy is usually smaller than that of aluminum alloy, which can make full use of space, which is of great significance for the aviation industry.
Highly matched Composites
Another reason why titanium alloy is frequently used in fastener manufacturing is that its electrode potential is highly matched with carbon fiber material, which can avoid galvanic corrosion and lead to fastener failure .
Small elastic modulus and thermal expansion coefficient
The formula for calculating the thermal stress is:.
In the formula: e is the modulus of elasticity, α Represents the coefficient of thermal expansion, Δ T stands for temperature change. It can be seen from the above formula that in terms of elastic modulus and coefficient of thermal expansion, steel and nickel alloy are greater than titanium alloy. Therefore, under the premise of the same temperature, the thermal fatigue performance of titanium alloy with only small thermal stress is naturally better than that of other materials.
Analysis of titanium alloy for fasteners
Rivet fastener materials
The types of fasteners used in the aviation industry can usually be divided into rivets, bolts and special fasteners. Among them, the requirements of rivets for materials are ideal for cold plasticity, because only rivets with good cold plasticity can meet the conditions of cold riveting installation. At present, titanium alloy rivets are mainly used in parts with ideal corrosion resistance and low strength requirements, and the cold working performance is outstanding β Grain naturally becomes the preferred material for manufacturing rivets. The titanium alloys described below are β Titanium alloy.
TB2 is a typical metastable titanium alloy, and its nominal composition is . TB2 in solid solution state has excellent welding performance and cold forming performance. At this stage, it is mainly used to manufacture bolts, cold heading rivets, star arrow connecting belt and satellite corrugated shell. TB2 will have greater application space in the future. It is necessary to carry out in-depth research based on it.
The nominal composition of TB5 as a metastable titanium alloy is mainly in the . In terms of cold forming performance, TB5 is roughly the same as pure titanium. Relevant personnel can first do solid solution treatment on it, and then obtain different types of fasteners through cold forming . In addition, the tensile strength of TB5 after aging can usually reach about 1000MPa. As early as decades ago, TB5 has been used to manufacture Boeing aircraft.
Ti-45nb is a special stable titanium alloy for rivets. In the early stage, rivets were mostly made of pure titanium, but pure titanium fasteners with low strength could not be used in high bearing parts. The aerospace industry urgently needed materials with both plasticity similar to pure titanium and strength higher than pure titanium. Ti-45nb came into being. In the research and development of ti-45nb, researchers choose to use metastable titanium alloy with large deformation resistance and more room temperature plasticity than pure titanium to synthesize a new material, ti45nb . It has been proved that ti-45nb has many advantages.
Materials of bolts and fasteners
The aviation fasteners with the largest amount of titanium alloy bolts can be divided into three categories based on the purpose, namely ordinary bolts, interference bolts and high locking bolts . Generally speaking, the titanium alloy used to manufacture bolts should have ideal shear strength and tensile strength. One is Tb3. Tb3, which can be strengthened by heat treatment, Its nominal composition is . The research shows that the advantages of Tb3 are mainly reflected in the good cold forming performance. Generally, the cold heading ratio of Tb3 can reach about 2.8. The overall strength can be significantly improved by solid solution treatment. It has been used to manufacture high-strength fasteners. The second is TB8. As a metastable titanium alloy, the nominal composition of TB8 is . The advantages of TB8 are excellent cold and hot working performance, good hardenability, outstanding creep resistance and good corrosion resistance. In addition, with the addition of stable Elements Nb and Mo, its self diffusion coefficient and melting point give TB8 more ideal oxidation resistance than other titanium alloys. At present, TB8 bolts have been widely used in the design and production of key products in the aviation industry . The third is TC4. TC4 fastener types can be subdivided into bolts, screws, pull screws and ring groove rivets. Its application fields mainly include satellite, aircraft and airborne equipment. The research shows that TC4 belongs to two-phase titanium alloy with medium strength. Most of the existing fasteners take TC4 as the main material. It should be noted that the manufacturing process of TC4 fasteners can only be hot upsetting, and corresponding heating and hot upsetting equipment are required. It is difficult to achieve the ideal level in terms of material utilization and production efficiency. In addition, the strength of TC4 fasteners can not meet the requirements of high-strength fasteners. The poor hardenability determines that the cross-section size often will not exceed 19mm during solution aging. Fourth, TC6. TC6, a dual phase titanium alloy with good comprehensive properties, has both α Titanium alloy and β The nominal composition of titanium alloy is . Except for special cases, TC6 should be used after annealing. Of course, relevant personnel can also strengthen its strength with the help of heat treatment process. The research shows that the advantage of TC6 is also reflected in its good oxidation resistance, which is one of the reasons why it is widely used in the field of fastener manufacturing . Fifth, TC16. As a two-phase titanium alloy, TC16 is characterized by solid solution aging strengthening, which is also the difference between TC16 and other titanium alloys. Its nominal composition is . After solution treatment, TC16 can show good room temperature plasticity, which is also the main reason for its excellent cold heading performance. Generally speaking, the pier forging ratio of TC16 can reach about 1 : 4. There are two processes available for fastener manufacturing based on TC16, namely hot upsetting and cold upsetting. Relevant personnel can choose as appropriate to ensure that the manufacturing efficiency and fastener quality can reach the ideal level . At this stage, TC16 is mainly used to manufacture screws, bolts and nuts without extending to other fields, which needs special attention.
Research on development trend of relevant materials
First, high strength titanium alloy. With the rapid development of aviation industry, the requirements for connection technology are different from those in the past, and the original fasteners can play a limited role. In the future, researchers should focus on the research of materials with higher shear strength and tensile grade. The shear strength should be 750MPa and above, and the critical values of tensile grade are 1200Mpa and 1500MPa respectively.
The second is high temperature resistant titanium alloy. At present, the operating ambient temperature of titanium alloy fasteners is generally low, but the increasing flight speed will inevitably increase the service temperature. The research on high temperature resistant materials is the general trend. According to the requirements of titanium alloy in the aerospace field, a new “short-term service in the environment of 600 ℃ – 800 ℃” is added. The empirically effective scheme is to replace the superalloy with Ti2AlNb, which can solve the problem of serious deformation of the material, but can not reduce the weight of the material to the ideal level. In addition, the plasticity and maturity of Ti Al compounds did not meet the expectations. Based on this, the preferred material for fastener manufacturing is still the two-phase titanium alloy described above. The key to improving the creep resistance and strength of titanium alloy under continuous high temperature is Zr / Al / Sn solid solution strengthening. However, due to many factors, the content of the above elements is limited. Based on this, relevant personnel can choose to control the content of the above elements, It is strengthened with the help of alloy to ensure that titanium alloy has more ideal solid solution strengthening effect.
Research shows that, as appropriate, increase β Stable elements, on the one hand, can optimize the solid solution strengthening effect of titanium alloy, on the other hand, can effectively solve the problem of alloy embrittlement. In addition, Si plays a decisive role in the properties of titanium alloy. The addition of 0.2% Si can promote the discontinuous and uniform distribution of ellipsoidal silicide in titanium alloy α With the formation of dispersion strengthening, not only the dislocation movement can be prevented, but also the creep resistance of the alloy is closer to the ideal state. However, it should be noted that silicide will have a certain impact on the thermal stability of the alloy. On the premise of reducing its plasticity, the degree of ordering is enhanced, and a large amount of Ti3Al is generated. In conclusion, relevant personnel shall ensure that the mass fraction of Si does not exceed 0.5%.
It can be seen from the above analysis that the manufacturing time of aviation fasteners with titanium alloy in China is relatively short, which is also the reason why the existing technology is similar to that in developed countries and lacks significant features. In addition, the problem of small proportion of titanium alloy fasteners has not been completely solved. With the continuous development of the aviation industry, aircraft performance needs to meet higher requirements than in the past. In order to make fasteners play their due role, the key is to develop titanium alloys with higher strength, fatigue and fracture toughness, which is also the direction of the industry.
Author: Tan pan
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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