Aircraft piping system
The piping system is the lifeline of the aircraft; its performance directly affects the overall performance of the aircraft. If the engine is compared to the heart of the airplane, then all kinds of pipelines are like the blood vessel network of the aircraft, which constantly delivers all sorts of nutrients to all corners of the airplane. The reliability and durability of the piping system are essential factors that affect flight safety, reduce maintenance costs, and meet airworthiness requirements, so it is very important to improve the piping system’s technical level to improve the airplane’s performance.
As an advanced lightweight structural material, titanium alloy has excellent comprehensive performance with low density, high specific strength, good fatigue strength and crack extension resistance, excellent corrosion resistance, good welding performance, etc. Therefore, it has an increasingly wide range of applications in aviation, aerospace, automotive, shipbuilding, energy and other industries. About 80% of titanium production is used in aviation and aerospace. Due to the development of titanium alloy tubes and processing technology is very difficult, coupled with the expensive titanium alloy materials, so its application in the application of the limitations is currently mainly in the industrial corrosion resistance and relatively wide range of applications in the ship, such as seawater installations, nuclear power, saline, and alkali industries, as well as ships and so on. But abroad, due to the development of titanium alloy tubing and supporting the development of mature application technology, titanium alloy tubing in developed countries, aviation, aerospace, and other fields have specific applications, such as foreign advanced aircraft air intake piping, hydraulic piping, fuel piping, etc. are widely used titanium alloy tubing. Due to the development of titanium alloy tubing in China, bending and tube fitting technology is not yet fully mature, and titanium alloy tubing in civil and military aircraft has not yet been widely used. Since the application of titanium alloy tubing involves tube development, bending and forming, tube end processing and connection, etc., this paper summarizes the current situation and development trend of the development of titanium alloy tubing, titanium alloy tube bending and forming technology, and titanium alloy tube ends processing and connection technology, analyzes the application of titanium alloy tubing in China’s aircraft and the gap between it and the advanced level in the international arena and points out that titanium alloy tubing is widely used in the piping system of China’s aircraft. It also points out the potential application of titanium alloy tubing in China’s aircraft piping system to gradually guide the extensive and mature application of titanium alloy tubing in China’s advanced aircraft.
development of Titanium alloy pipe
About the manufacture of titanium alloy tubes, countries around the world have been committed to improving the reliability and flexibility of titanium alloy tubes and expanding product series. The manufacturing technology of seamless titanium and titanium alloy tubes in foreign developed countries has been relatively mature. For low strength, low alloying of titanium and titanium alloy seamless pipe manufacturing is used in the cold rolling vacuum annealing process. Billet preparation is mainly used for drilling and extrusion and slant rolling and piercing two types of processes, of which drilling and extrusion method of metal consumption is extensive, the process waste is as high as 10% to 15%, but the billet wall thickness uniformity; and the use of slant rolling and piercing method of metal consumption is small, the process waste of 1% to 3%, the rolling deformation of the temperature, Wide range of rolling deformation temperature, speed, road deformation up to 20% to 90%, can reduce the number of heating and oxidation loss of titanium alloy, improve yield, but the disadvantage is that the billet thickness tolerance is slightly larger. At present, oblique rolling perforation is the primary method of seamless tube production. For medium and high-strength titanium alloy seamless pipe, the use of warm rolling technology, that is, the installation of an induction heating device in the rolling mill, the general temperature control in the recrystallisation temperature below 100 ℃ or so. The warm rolling technique can produce medium-sized Gr5 (Ti-6Al-4V) titanium alloy tubes. In addition, K. Srinivasan et al. investigated the feasibility of extruding commercially pure titanium tubes using an open die. The results showed that using open-die reverse extrusion to manufacture pure titanium tubes has higher quality, consumes less lubricant, and has a more straightforward die structure than the extrusion method of tube making. Still, this method is only suitable for manufacturing relatively short fittings.
Ten years ago, R.W. Schutz of RMI, USA, implemented a hot-piercing TLSL process for the efficient and low-cost production of Gr5, Gr23, Gr29, and Gr28 seamless tubes. The seamless titanium tubes produced, with diameters of up to 610mm, wall thicknesses of up to 26mm, and lengths of up to 12m, have been successfully applied to geothermal salt wells in the energy industry and to deepwater offshore drilling for lifting tubes, Drill pipes and so on.
Improving tubes’ performance and load-bearing capacity has always been a hot topic in titanium tube development, and the world’s manufacturers of titanium tubes continue to raise the strength limits of titanium alloy tubes. The manufacture of titanium alloy tube Gr9 (Ti-3Al-2.5V) grade larger than 860 MPa can already be reliably realised through stress relief annealing in the USA. Although high strength helps to improve the tensile and torsional capabilities of titanium tubes, expanding their adaptability to tensile, high-pressure resistance, composite fatigue, etc., the increase in strength also leads to a reduction in plasticity and toughness, increasing crack sensitivity and increasing the difficulty of subsequent bending and tube end forming. Therefore, with the continuous expansion of titanium tube applications, in addition to material strength, we should pay more attention to other aspects of titanium tube performance, including plasticity, toughness, fatigue life, microstructure, and welding performance, to achieve a good match of comprehensive performance.
Due to the many processing procedures, long production cycles, low efficiency, and high cost of titanium seamless pipe, its application is limited. Based on titanium strip rolling, welding process based titanium welded pipe, due to high material utilisation, high production efficiency, and its expansion, bending performance and seamless pipe almost no difference, so the amount of domestic and foreign titanium welded pipe in the increase year by year, each country are also developing their own titanium welded pipe production system, and in the power station condenser is increasingly being used.
Currently, about 90% of the titanium tubes are seamless and welded tubes made of industrially pure titanium (Gr1 and Gr2), followed by seamless titanium alloy tubes such as Gr9 (Ti-3Al-2.5V), Gr5 (Ti-6Al-4V), and ⅡT-7M (Ti-2Al-2.5Zr).
In manufacturing low-strength, low-alloyed titanium alloy seamless tubes, China also adopts the cold rolling vacuum annealing process, and the technology has matured in China. However, due to the limitations of the cold deformation capacity of medium and high-strength titanium alloy tubes, it is necessary to use warm rolling technology. Still, China’s medium- and high-strength titanium alloy tubes’ warm rolling production technology still needs to mature enough. The development of high-strength titanium alloy thin-walled tubes Gr9 (Ti-3Al-2.5V) is still in the exploratory stage of trial production.
FD Titanium has offered Grade 9 Seamless Tubes for over ten years. We have accumulated a lot of experience and have been replacing imported materials.
Titanium alloy pipe bending and bending machinery
The titanium alloy tube bending process, especially the cold bending process, is a very complex process due to the performance of the tube itself, the tube wall thickness factor, the bending process, mold design and manufacturing level, and many other factors.
Since titanium alloy forming at room temperature is characterized by significant anisotropy, considerable deformation resistance, poor plasticity, limited elongation, forming difficulties, and significant rebound, etc., the domestic and foreign grades of titanium alloy conduit materials suitable for cold bending and forming at room temperature are mainly commercially pure titanium with slightly better ductility, and approximate α-titanium such as Ti-3A1-2.5V (Grade 9) and α+β-titanium such as Ti-6Al-4V ( Grade 5) and other α+β titanium. Some people abroad think that the ratio of bending radius to tube diameter is more than 2, and the tube diameter is less than 75mm (wall thickness 0.4mm~0.9mm) of pure titanium tubes that can be bent and formed smoothly at room temperature. In most cases, titanium alloy tubes need to be heated to bend; the heating temperature of 150 ℃ ~ 260 ℃ is appropriate, as the heating method for the heating element heating and thermocouples control the temperature.
So far, the CNC bending technology of titanium alloy tubes has been relatively mature and perfect. Part of the pure titanium tube using CNC cold bending technology and part of the titanium alloy tube using CNC hot bending process have been applied in large quantities in the rear fuselage of the aircraft each piping system, dramatically improving the aircraft’s manoeuvrability. However, because of the strategic titanium tube bending and application issues, foreign countries have strict confidentiality and less open literature. sunmoo et al. Ti-6Al-4V large-diameter tube (62.37mmX4.4mm) for cold bending, pipe fittings inside the addition of filler material, forming the pipe fittings roundness, ellipticity is only 1.28. Still, no filler type, bending mode, bending process or other content. However, the type of filler, bending method and bending process were not reported in detail.
The above bending work is mainly for low and medium-strength titanium alloy tubing. The domestic has yet to be reported for high-strength titanium alloy tube bending aspects of the work. Although the titanium tube under heating conditions will produce a significant elongation, which is conducive to bending and forming, mold expansion, lubrication of the material and mold under heating conditions, cooling of bending mechanical components, and other issues limit the scope of application of hot bending of titanium tubes. Therefore, the development of precise and efficient cold-bending forming of titanium alloys is an urgent problem that needs to be solved.
Domestic induction heating push-bend bending machines are more common, mainly to adapt to the construction of domestic oil pipelines and the development of medium and high-frequency induction heating and bending equipment and process research. For titanium alloy pipe heating and bending of the heating device required for domestic information, literature on titanium alloy conduit hot bending forming was introduced, and a schematic diagram of the heating device is given. As CNC bending has high efficiency, precision, and other characteristics, both domestic and foreign tend to use this bending process to form titanium alloy pipe fittings to effectively avoid wrinkles, rupture, cross-sectional distortion, and other defects and improve the quality of titanium tube forming. With the gradual popularization and application of titanium alloy tubes in the field of aviation, and because of the high-quality requirements for the formation of aviation bending, it can be predicted that shortly, they will be developed with independent intellectual property rights, through the mold heating by bending machine.
hydraulic piping systems - end forming
The hydraulic pipeline is a part of the aircraft pipeline with the highest working pressure and the strictest reliability requirements, and the technical level of the aircraft pipeline system is concentrated in the hydraulic pipeline system. For the hydraulic system, under the same power conditions, the higher the working pressure, the smaller the bottom area of the required cylinder and pump piston, and the smaller the flow of pipeline needed, and thus the overall size and weight of the hydraulic system will be correspondingly smaller and lighter. A lower media flow rate also reduces the power loss of the flow in the pipeline. Therefore, the continuous research of higher working pressure hydraulic systems and their standard parts is the objective need of the development of the aviation industry.
Hydraulic piping systems in the United States began using 28MPa hydraulic systems in the 1960s, but higher-pressure hydraulic systems have since replaced these. A 28MPa hydraulic system has also long been used in Russia.
There are many structural conduit connectors for aircraft conduit end processing and connection. The flare connection structure is simple and is one of the most widely used and mature connection forms; Su27 aircraft on the 28MPa system use this connection structure form. However, its sealing surface area is large, and the production process of shape errors and surface roughness errors caused by the gap is complex to eliminate, so the sealing is poor, and it cannot self-locking, the need for fuse locking. It takes about 10 minutes to play the fuse at a flare connection point, which is very troublesome for field maintenance and impacts the aircraft’s attendance. To fundamentally solve the sealing problem of the flare connection, the world’s developed countries in the aviation industry have researched this, and now there are a variety of sealing performances that are far better than the structure of the flare connection form. For example, the ferrule type without flare connection, extrusion type without flare connection, and lip-type connection. In addition, I.D. roll forming is a precision forming method for flareless conduit connection, which has many incomparable advantages to other forming processes. Titanium alloy tubing I.D. roll forming technology is an advanced tubing connection technology that meets the high-performance standards of modern aircraft, such as weight reduction and high manoeuvrability. Compared with the traditional welding form of titanium alloy pipe fittings connection, has the advantages of low manufacturing cost, high production efficiency, easy to operate, reliable connection, convenient testing, etc., the formed pipe fittings have good airtightness, can withstand high temperature and high pressure. At present, foreign countries have broken through the titanium alloy pipe fittings inside diameter roll forming technology. In the United States in the past few decades, the development of titanium alloy connecting pipe fittings inside diameter roll forming technology has fully realized computer automatic control, forming parts with high precision in the F15, F16, F76, and other aircraft have been widely used, and is carrying out high-temperature titanium alloy connecting pipe fittings roll-forming technology, and gradually expanding the field of aerospace and aircraft engines. Aerospace field expansion. Russia also has a high level of research in this area; titanium alloy inner diameter roll-forming connecting pipe fittings have been widely used in many aircraft types and relevant special moulding equipment.
In terms of using mechanical methods to connect fittings, domestic research is mainly limited to aluminium alloy, stainless steel, and other materials extrusion forming, less involved in titanium alloy materials. Regarding the hydraulic system of aircraft pipeline, the increased working pressure will inevitably bring about the conduit and connectors’ material strength and the connectors’ structural sealing problem. In terms of performance, all kinds of flareless connection forms are better than flare connection forms. Therefore, the development and adoption of flareless connection forms is the development trend of aircraft pipeline connection.
Ideal material for aircraft hydraulic piping systems
To improve the overall flight performance of the aircraft, to meet the requirements of relatively lightweight, long life, good manoeuvrability, etc., the working pressure of the piping system of large passenger aircraft and fighter jets will be gradually increased, and a variety of titanium alloy tubing with excellent comprehensive performance will be steadily gained wide application. Because of the importance of the hydraulic piping system in the aircraft piping system, it is feasible to apply titanium alloy tubing to the hydraulic piping system of the new aircraft. From the specific strength, stiffness, corrosion resistance, cold bending molding ability, and material maturity considerations, the use of Ti-3Al-2.5V titanium alloy tubing is currently an ideal choice.
Ti-3Al-2.5V titanium alloy is a low aluminum-equivalent near-alpha titanium alloy evolved from the Ti-6Al-4V alloy. It was developed as a cold machinable tubing application with good cold forming and welding properties, allowing for the fabrication of a wide range of seamless tubing, welded tubing, and honeycomb structures, and a good match of strength and plasticity can be achieved through heat treatment. The alloy’s room temperature strength is 20% to 50% higher than that of industrially pure titanium. It is insensitive to chipping and has good corrosion resistance in many media. Therefore, it is suitable for the manufacture of conduits in various aircraft.
Due to the small density of Ti-3Al-2.5V titanium alloy can effectively reduce weight compared with stainless steel tubes. What is more valuable is that its good welding performance makes the subsequent tube ends easy to connect, and its excellent matching performance with the strength and stiffness of the composite structure can further obtain a perfect weight reduction effect. Therefore, Ti-3Al-2.5V is ideal for making high-pressure-resistant lightweight conduits for advanced aircraft. The alloy is suitable for manufacturing components such as aircraft and rocket engine propellant storage tanks and conduits, with a maximum operating temperature of 290°C.
In recent years, the development of titanium alloy materials continues to develop in the direction of high performance, functionalization, and low cost. In line with this, the development trend of titanium alloy tube manufacturing, on the one hand, is to reduce the cost of raw materials, that is, the development of the development does not contain or less precious metal elements, instead of adding iron, oxygen, nitrogen and other inexpensive elements of the alloy, or the development of new low-cost titanium sponge production process. On the other hand, it is through a variety of process ways to reduce the processing of titanium alloy tubing manufacturing costs, such as the development of low processing costs can cold deformation of titanium alloy tubing and continue to expand the scope of application of titanium alloy tubing in the aircraft. In order to improve the overall performance of the aircraft, weight reduction, energy saving, and consumption reduction, foreign aircraft hydraulic systems are now commonly used in 28MPa systems and have gradually begun to adopt higher system pressure. For the high-pressure hydraulic piping system supporting the molding and connection technology, titanium alloy tube CNC bending and I.D. rolling without flare connection process has been more mature and perfect and will continue to become the future development and universal application of technology and technology.
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