Summary:
The large-scale use of composite blades is currently the most effective way for the aviation industry to achieve higher bypass ratio and weight reduction of aero engines. With the research progress of manufacturing technology of composite aero engine blades as the theme, the mainstream processing technology of resin-based, metal-based and ceramic matrix composite aero-engine blades at this stage was introducedThe development status and application of key manufacturing technologies are discussed, including the prepreg compression molding process and three-dimensional braided knot reinforced resin transfer molding process of resin-based composite blades, the compression molding process of metal-based composite blades, the pressure casting process and superplastic forming diffusion joining process, and the melt penetration process of ceramic matrix composite bladesThe development trend of composite blades for aero engines is discussed, and the research direction of key manufacturing technologies for composite blades in the future is proposed.
As a complex thin-walled curved component, aero engine blades are a very critical type of typical parts in aero engines. With the development of modern aviation industry, in order to meet the increasingly demanding aerodynamic performance requirements of aero engines, the configuration of various blades is becoming more and more complex, and the working conditions are becoming more and more severe. In order to continuously improve the performance and efficiency of aero engines, especially with the further improvement of engine bypass ratios, more and more composite components are used in the engines of new generation fighters and large civil airliners, and one of the core components is composite blades.
Composite blades made of resin-based, metal-based, and ceramic-based composites can significantly reduce the weight of aero engines while maintaining or even exceeding the strength and performance of alloy blades, thereby significantly improving the overall performance of a new generation of aero engines. Fan blades made of resin matrix composites, fan and compressor blades made of metal matrix composites, and turbine blades made of ceramic matrix composites have become one of the key technical means for reducing weight and increasing thrust of aero engines. How to ensure the machinability of various new composite blades, provide highly reliable processing technology, and ensure high-performance manufacturing is the premise for further improving the large-scale equipment application of new composite blades for aero engines in the future.
1 Current status of composite aero engine blades
In the 60s of the 20th century, the British Rolls-Royc company began to develop composite fan blades and prepared to apply them to the RB211 engine, but due to the failure to meet the requirements of blade stiffness and toughness, the plan was aborted. With the rapid development of the composite materials industry, the composite blades of General Electric (GE) in the United States, Rolls-Royce in the United Kingdom, SNECMA in France and Pratt & Whitney (Pratt & Whitney) in the United States have come out one after another. At present, the international market for composite aero engine blades is still monopolized by the above-mentioned international giants.
At present, the main applications of composite blades in international aero engines are shown in Table 1. It can be seen that composite blades such as fan blades made of resin matrix composites, fan compressor blades made of metal matrix composites, and turbine blades made of ceramic matrix composites have been widely used in commercial aero engines. The successful application of various types of composite vane engines proves that composite fan blades are suitable for the most demanding commercial flight needs.
Table 1 Composite aero engine blades.
With the rapid development of China's aviation industry, many breakthroughs have been made in the manufacturing technology of aero engine blades, but for the development and manufacture of high-end composite blade products, in terms of production cost, development cycle, material preparation, manufacturing technology, airworthiness experience and airworthiness certification, the gap with the world's leading aviation giants is still huge, which seriously restricts the large-scale application of domestic composite blades and reduces the competitiveness with the international market.
2 Manufacturing process of composite aero engine blades
01 Manufacturing process of resin matrix composite blades
Resin matrix composite blades have a series of advantages such as high specific strength, high specific modulus, fatigue resistance, corrosion resistance, etc., and have many applications in advanced aero engines, and are the most widely used composite blades at present. At present, the manufacturing process of resin-based aero engine composite blades mainly includes prepreg autoclave molding process and three-dimensional braided reinforced resin transfer molding process, in which the core technology is to realize the vertical and horizontal continuous change of blade thickness and one-time molding of curing.
Prepreg autoclave molding process
Prepreg The autoclave molding process requires cutting the prepreg into a designed shape in advance, and then the prepreg is laid and cured. Figure 1 illustrates the autoclave molding process for GE aircraft engine fan blades. The basic process is to combine prepreg stacks made from carbon fiber soaked to partially cured resin with other process auxiliary materials to form a vacuum bag combination system, which is given a certain pressure and temperature in an autoclave to complete the curing of the desired blade parts. The composite materials used in the manual layup have high strength and stiffness, and have been developed and have considerable experience in commercial applications.
Fig.1 Molding process of autoclave of GE aero engine fan blades.
The challenges in preparing preforms using the prepreg placement process are the planar-surface transformation cutting of the prepreg, the precise positioning of the prepreg layup, and the puncture strengthening between the prepreg layers. Rolls-Royce has designed an automated wire and tape laying process for its UltraFan blades, supplemented by 3D laser measurement technology to ensure accuracy, as shown in Figure 2. The blade is made of 500 layers of IM7 M91 carbon fiber reinforced high-tenacity epoxy prepreg produced by Hexcel. After the prefabricated blade is prepared on the automatic fiber tow laying equipment, it is cured at high temperature and high pressure in an autoclave and precision machining, and then the surface coating is carried out, and the leading edge of the blade is coated with a titanium alloy protective sheet. Machine weaving is currently costly, but due to its high efficiency, precision and automation, it will become the main method in the future.
Fig.2. Automation process of super-fan blades.
Three-dimensional braided knots are reinforced resin transfer molding process
CFM used "3D Woven Braided Knot + Reinforced Resin Transfer Molding (RTM) Molding" to manufacture the LEAP-X series engine composite fan blades, and the molding process is shown in Figure 3. The blade solves the thermal deformation phenomenon in the autoclave curing process, ensures its shape accuracy, and can strictly control the fiber volume fraction. The technical advantage of the RTM process is that the design of the carbon fiber preform is separated from the molding process of the resin, which can give full play to the designability of the ply material. The carbon fibers are woven into a three-dimensional woven structure before the blades are formed under high pressure, then cut, twisted and laid into a mold for RTM molding. After demoulding and CNC finishing, titanium alloy edging is installed to complete the overall fabrication of the blade. RTM technology can make the prefabricated body design and molding independent of each other, fully meeting the complex structural requirements in the design process of aero engine blades. Durability tests conducted by SNECMA later showed that the application of RTM technology to manufacture blades not only reduced weight and cost, but also strengthened structural properties such as bird strike resistance.
Fig.3. Forming process of composite fan blades for LEAP-X engine.
02 Manufacturing process of metal matrix composite blades
Metal matrix composites have better specific strength, specific stiffness and structural stability than traditional metal materials, and can design product performance according to needs to achieve the integration of structure and function. Figure 4 illustrates the manufacturing process of GE's Al-Li alloy-based composite blades, which are formed by two methods: compression molding and pressure casting.
Fig.4. Manufacturing process of metal-based composite blades.
Starting from the PW4084 engine, Pratt & Whitney used extruded silicon carbide particles reinforced deformed aluminum alloy matrix composites produced by DWA on the fan outlet guide vanes, and successfully developed the hollow fan blades of silicon carbide fiber reinforced titanium matrix composites with the help of superplastic forming diffusion connection process, as shown in Figure 5. The two blades of the preform are diffusion connected to form a cavity structure, and then heated to a superplastic state, and the torsional forming process can reduce the weight of the engine structure by 14%. Rolls-Royce has also successfully developed wide-chord hollow metal matrix composite fan blades.
Fig.5. Superplastic forming diffusion connection manufacturing process.
Metal matrix composites have been or will be used in compressor stator blades, rotor blades, integral blade rings and other parts, but they have not been widely used in the production and manufacturing of aero engines, mainly because of the complex manufacturing and production process, high manufacturing cost and low pass rate of metal matrix composites.
03 Manufacturing process of ceramic matrix composite blades
Ceramic matrix composites are a kind of composite materials that use ceramics as a matrix and various fibers, and the most widely used ceramic matrix composites are carbon fiber toughened silicon carbide and silicon carbide fiber toughened silicon carbide.
The melt permeation process developed by GE is shown in Figure 6. First, the ceramic matrix composite matrix is heated to a high temperature to melt into a melt, and then infiltrated into the prefabricated body of the reinforcement, and then cooled to obtain the required ceramic matrix composite blades.
Fig.6. Schematic diagram of melt permeation process.
CFM's LEAP engine is the first commercial jet engine to feature ceramic matrix composite components and features 18 fixed ceramic matrix composite turbine rings. GE Allison validated a hollow continuous silicon carbide fiber toughened silicon carbide ceramic matrix composite high-pressure turbine stator blade on the XTC77 1.
Ceramic matrix composite blades have strong mechanical properties and high temperature properties, but fatigue and damage will occur under continuous high temperature, high pressure and low-frequency vibration operation, which may lead to toughening failure and greatly reduce their life, so it is still necessary to develop efficient and low-damage processing technology.
3 The development trend of composite aero engine blades
At present, fan blades and compressor low-pressure blades have gradually changed from titanium alloy hollow blades to composite blades, among which a large number of resin matrix composite blades have been used in civil large-bypass ratio turbofan engines, and the use of composite materials is increasing. Therefore, the development and application of lightweight, high-strength and high-temperature resistant composite materials has become an important means to improve the weight reduction efficiency, thrust-to-weight ratio and fuel economy of aero engines, and it is also the current development trend of aero engine blades.
The weight reduction benefit of the aero engine makes the design of the blades need to develop in the direction of greater curvature. As shown in Figure 7, GE designed the same series of composite fan blades with a gradual increase in the curvature of the individual blades, allowing the engine to gradually reduce the number of blades while maintaining the same intake performance. GE's new generation GE9X engine uses a hybrid material fan blade with carbon fiber resin base in the main area, and a metal cladding covering the blade tip and trailing edge, the trailing edge is carbon fiber and glass fiber hybrid reinforced resin base, the inside is carbon fiber, and the outside is glass fiber. From the materials used in GE9X, it can be seen that the material form of composite blades has a clear trend of changing from single material to hybrid material.
Fig.7. Development of fan blades for GE aero engines.
In terms of blade design and strength checking, computer simulation technology with high accuracy and fast calculation speed is expected to realize the simulation and control of defect formation mechanism, curing process, deformation and control, pressure transmission and other first-class analysis, and the application of simulation technology to the precise control process can greatly shorten the design cycle of aero engines. The simulation method can avoid many shortcomings in the traditional process flow, from the 3D prefabricated weaving design to the calculation part of the whole process of blade strength analysis and verification, can be achieved through **.
In terms of manufacturing process, additive manufacturing technology has the advantage of rapid manufacturing in single parts and small batches of complex structures, and the integration of design, materials and manufacturing will be the direction of its future development. At the same time, the liquid molding process such as resin transfer molding process and vacuum infusion molding process has been greatly developed, and with the progress of weaving technology, the damage tolerance and impact toughness of composite materials will continue to improve. The use of low-temperature curing and low-pressure molding technology to reduce the curing temperature, shorten the reaction time, and maximize their life is also one of the important trends in the development of large-scale composite components and composite materials.
In terms of blade health management, aero engine manufacturers such as Rolls-Royce's Ultrafan engine are implementing a full life cycle monitoring system for composite blades based on digital twin technology, and can also provide repair and remanufacturing methods. Its main means is to monitor the load and structural damage information of composite blades throughout the life cycle through embedded sensors**, actively perceive environmental changes during their service, and greatly improve the safety and reliability of composite structures. In addition to the embedding of sensors, it is also a future development trend to embed composite materials such as piezoelectric materials into the blades to form intelligent blades with integrated sensing and control.
4 Outlook
The development and application of lightweight, high-strength and high-temperature resistant composite materials is an important means to improve the weight reduction efficiency, thrust-to-weight ratio and fuel economy of aero engines, and it is also the future development trend of aero engine blades. Referring to the technical development status of composite blades in foreign countries, it is necessary to further improve the technical level and engineering ability of design, materials, manufacturing, testing and engineering in the development of composite blades for aero engines in China.
1) Improve the composite material system. At present, the materials used in foreign aero engine composite blades are developing in the direction of hybridity, so as to have stronger material properties, and the domestic needs to establish a composite material system for different positions such as the cold end and hot end of the blade, integrate existing manufacturing resources and capabilities, solve the problem of ununified manufacturing specifications, technical reserves and equipment capabilities, realize the integration, sharing and optimal configuration of product life cycle data, and start the revision and formulation of relevant standards for aero engine composite blades as soon as possible.
2) Automation and digitalization of manufacturing processes. Foreign advanced automation technology can greatly reduce the deviation caused by human operation, thereby improving the stability of production. In order to achieve overtaking in corners, China's aviation manufacturing industry is bound to integrate digital technology into the whole process of composite material design, manufacturing, processing, testing and service, so as to improve the level of composite material manufacturing and narrow the gap with advanced manufacturing technology.
3) Enhance technological innovation and R&D capabilities. The integrated and efficient molding disruptive technology represented by additive manufacturing technology is expected to promote the rapid development of composite blades for advanced aero engines in China in the future, and further narrow the technological gap between China and Europe and the United States in advanced blade manufacturing. However, at present, there are still many serious shortcomings in these advanced technologies, and further pre-research and technology accumulation are needed.
Article**: Science and Technology Review, authors Yu Ruichen, Jiang Jinhua, Zhu Xiaojin, Zhang Hesheng, Gao Zhiyuan.