Aerospace manufacturing is the most concentrated high-tech field in manufacturing, belonging to advanced manufacturing technology.
Notable products include the F119 engine developed by Hewlett-Packard in the United States, the F120 engine from General Electric, the M88-2 engine from the French company SNECMA, and the EJ200 engine jointly developed by the United Kingdom, Germany, Italy and Spain. .
These represent the most advanced high-performance aviation engines in the world and have in common the widespread use of new materials, new processes and new technologies. Let's take a look at these new materials used in high-performance aviation engines.
1. High temperature alloys
High-temperature alloys were developed to meet the stringent material requirements of jet engines and have become an irreplaceable class of key materials for hot-end components of military and civilian gas turbine engines.
Currently, in advanced aviation engines, high-temperature alloys represent more than 50% of the material used.
The development of high-temperature alloys is closely related to the technological progress of aviation engines, especially the materials of turbine discs and blades and the manufacturing processes of hot-end engine components, which are important indicators of engine development.
Due to the high demands placed on the material's high temperature resistance and ability to withstand stress, the United Kingdom initially developed the Ni3 (Al, Ti) reinforced Nimonic80 alloy for use as a jet engine turbine blade material. It also sequentially developed the Nimonic alloy series.
The United States has developed dispersion-strengthened nickel-based alloys containing aluminum and titanium, such as the Inconel, Mar-M, and Udmit series of alloys developed by Pratt & Whitney, General Electric, and Special Metals Corporation, respectively.
During the development of high-temperature alloys, manufacturing processes played a significant role in driving alloy development. With the emergence of vacuum melting technology, the removal of impurities and harmful gases from alloys, especially the precise control of alloy composition, has continuously improved the performance of high-temperature alloys.
After that, successful research into new processes such as directional solidification, single crystal growth, powder metallurgy, mechanical alloying, types of ceramic core, ceramic filtration, isothermal forging, etc., drove the rapid development of high-performance alloys. temperature.
Among these, directional solidification technology stands out; The alloys produced by this process – single-crystalline directional alloys – can be used at temperatures close to 90% of their initial melting point.
As a result, the turbine blades of today's advanced aviation engines in many countries are manufactured from single-crystal directional alloys. Globally, equiaxed crystals, directionally solidified columnar crystals, and single-crystalline alloy systems have been formed from nickel-based high-temperature cast alloys.
High-temperature powder alloys have also developed from the first generation 650°C to 750°C and 850°C powder turbine discs and dual-performance powder discs, which are used in advanced high-performance engines .
2. Ultra-high strength steel
Ultra-high-strength steel is used as landing gear material in aircraft. The second generation aircraft used 30CrMnSiNi2A steel in its landing gear, with a tensile strength of 1700 MPa. However, the useful life of this landing gear was relatively short, approximately 2,000 flight hours.
For the third-generation fighter project, the landing gear is expected to have a useful life of more than 5,000 flight hours. Due to the increase in on-board equipment and the decrease in the weight ratio of the aircraft structure, greater demands are placed on landing gear selection and manufacturing technology.
American and Chinese third-generation fighters have adopted the technology of manufacturing landing gears from 300M steel (with a tensile strength of 1950 MPa).
It should be noted that improvement in material application technology is also driving a greater extension of landing gear service life and increasing adaptability.
For example, the landing gear of the Airbus A380 adopted ultra-large integral forging technology, new atmospheric heat treatment technology and high-speed flame spraying technology, ensuring that the service life of the landing gear meets the design requirements. Therefore, the progress of new materials and manufacturing technologies ensures the renewal and modernization of aircraft.
Designing long-life aircraft in corrosive environments requires higher materials standards. Compared to 300M steel, AerMet100 steel provides an equivalent level of strength, but exhibits superior overall corrosion resistance and stress corrosion resistance.
The accompanying landing gear manufacturing technology has been employed in advanced aircraft such as the F/A-18E/F, F-22 and F-35. The higher strength Aermet310 steel, which has lower fracture toughness, is currently under investigation.
Ultra-high-strength AF1410 steel, known for its extremely slow crack propagation rate, is used in the wing actuator joint of the B-1 aircraft, achieving a 10.6% reduction in weight and a 60% improvement in machinability compared to Ti-6Al- 4V, and a cost reduction of 30.3%. The high-strength stainless steel used in the Russian MiG-1.42 represents 30% of the total.
PH13-8Mo is the only high-strength precipitation-hardening martensitic stainless steel widely used in corrosion-resistant components.
Internationally, the development of ultra-high-strength gear (bearing) steels such as CSS-42L and GearmetC69 have been tested in engines, helicopters and aerospace.
3. Intermetallic Compounds
The development of high-performance aeronautical engines with a high thrust-to-weight ratio promoted the development and application of intermetallic compounds. Today, intermetallic compounds have evolved into a diverse family, typically made up of binary, ternary, or multi-elemental metallic compounds.
Intermetallic compounds have significant potential for high-temperature structural applications, offering high use temperatures, high specific strength and thermal conductivity. Especially under high temperature conditions, they have excellent oxidation resistance, high corrosion resistance and high creep resistance.
As intermetallic compounds represent a new material that bridges the gap between high-temperature alloys and ceramic materials, they have become one of the ideal materials for high-temperature components in aircraft engines.
Currently in aircraft engine structure, the main focus of research and development is on intermetallic compounds, with particular emphasis on titanium aluminum and nickel aluminum. These titanium and aluminum compounds share a similar density to titanium but have a significantly higher usage temperature.
For example, their usage temperatures are 816°C and 982°C, respectively. The strong interatomic bonds and complex crystalline structures of intermetallic compounds result in difficult deformation, exhibiting hard and brittle characteristics at room temperature.
After years of experimental research, a new alloy with high temperature resistance and room temperature plasticity and toughness was developed and successfully installed with excellent results. The high-performance F119 engine from the United States, for example, uses intermetallic compounds in the engine casing and turbine disc. The compressor blades and disc of the F120 test engine are made of a new intermetallic compound of titanium and aluminum.
4. Ceramic matrix composites
When you think about ceramics, fragility naturally comes to mind. A few decades ago, using it for load-bearing components in engineering was inconceivable. Even now, when we talk about ceramic composites, some people may not understand, assuming that ceramics and metals are fundamentally unrelated materials. However, the ingenious union of ceramics and metals fundamentally changed our perception of this material, giving rise to ceramic matrix composites.
Ceramic matrix composites are a promising new structural material in the aerospace industry, particularly in aircraft engine manufacturing, where their unique attributes are increasingly evident. In addition to being lightweight and hard, ceramic matrix composites also have exceptional high temperature strength and corrosion resistance at elevated temperatures.
Currently, ceramic matrix composites have surpassed heat-resistant metallic materials in high temperature resistance, demonstrating excellent mechanical properties and chemical stability, making them an ideal material for high-temperature zones of high-performance turbine engines.
Currently, countries around the world are focusing their research on silicon nitride and silicon carbide reinforced ceramics to meet the material requirements of the next generation of advanced engines, and have made significant progress. Some have already begun incorporating these materials into modern aircraft engines.
For example, the United States F120 test engine uses ceramic materials in its high-pressure turbine sealing devices and some high-temperature components of its combustion chamber. France's M88-2 engine also uses ceramic matrix composites in its combustion chamber and nozzle.
5. Carbon/carbon composite materials
Carbon/carbon (C/C) composite materials, emerging as the most notable high-temperature resistant materials in recent years, are currently the only materials considered suitable for use in turbine rotor blades, with a thrust-to-weight ratio exceeding 20 and engine inlet temperatures reaching 1930-2227°C.
These materials are a key area of focus for the United States in the 21st century and a primary objective pursued by advanced industrial nations around the world. C/C composite materials, or carbon fiber reinforced carbon matrix composites, uniquely combine the refractory nature of carbon with the high strength and stiffness of carbon fibers, leading to non-brittle failures.
With their light weight, high strength, superior thermal stability and excellent thermal conductivity, they are today's most ideal high temperature resistant materials. Notably, under high temperature conditions ranging from 1000-1300°C, its resistance does not decrease but rather increases. Even at temperatures below 1650°C, they maintain their strength and shape at room temperature. Consequently, C/C composite materials have significant potential for development in the aerospace manufacturing industry.
The main problem with the application of C/C composite materials in aviation engines is their poor resistance to oxidation. However, in recent years, the United States has gradually resolved this problem through a series of process measures and progressively applied them to new engines.
For example, the afterburner exhaust pipes of the F119 engine, the combustion chamber nozzles and ducts of the F100 engine, and certain parts of the combustion chamber of the F120 validation engine are now made with C/C composites. Similarly, France's M88-2 and Mirage 2000 engines, including their afterburner fuel rods, heat shields, and ducts, also employ C/C composites.
6. Resin-Based Composite Materials
Research into the application of resin-based composite materials in aviation turbofan engines began in the 1950s. After more than 60 years of development, companies such as GE, PW, RR, MTU and SNECMA have invested significant efforts in the research and development of these materials, achieving substantial progress. They have successfully designed these composites for aviation turbofan engines in active service, and there is a trend to further increase their use.
The service temperature of resin-based composite materials generally does not exceed 350°C. Thus, these materials are mainly used in the cold end of aviation engines. The main application areas of resin-based composite materials in advanced foreign aviation engines are illustrated below.
Fan Blade: The engine fan blade is a representative critical component of the turbofan engine, closely related to its performance. Compared with titanium alloy fan blades, resin-based composite fan blades have a very clear advantage in weight reduction. In addition to the clear weight reduction, the impact on the fan casing is less after the resin-based composite fan blade is struck, which is beneficial to increasing the containment capacity of the fan casing.
At present, the main representatives of composite fan blades applied commercially abroad include the GE90 series engines for the B777, the GEnx engines for the B787, and the LEAP-X engines for the C919 of Chinese Commercial Aircraft Corporation.
In 1995, the GE90-94B engine equipped with resin-based composite fan blades officially entered commercial operation, signifying the formal engineering application of resin-based composites in modern high-performance aviation engines. Considering factors such as aerodynamics and high- and low-cycle fatigue, GE developed new composite fan blades for the subsequent GE90-115B engine.
Entering the 21st century, strong demand for high damage tolerance composites in aviation engines has driven the development of composites technology. However, it has become increasingly challenging to meet the requirements of high damage tolerance while continuously improving the strength of carbon fiber/epoxy resin prepreg. In this context, fan blades composed of a 3D woven structure have emerged.
Fan Case: The fan case is the largest stationary part of an aviation engine. Its weight reduction will directly affect the thrust-to-weight ratio and engine efficiency. Therefore, foreign OEMs of advanced aviation engines have always been committed to weight reduction and structural optimization of the fan casing. The development trend of fan casings in advanced foreign aviation engines is shown in the figure.
Fan shroud: Because it is not a primary load-bearing component, the fan shroud was one of the first parts of an aircraft engine to be made from composite materials. Using these materials for fan shrouds allows for lighter weight, a simplified antifreeze structure, superior corrosion resistance and improved fatigue resistance.
Resin-based composite materials are currently used in the construction of fan shrouds on Rolls-Royce's RB211 engines, as well as Pratt & Whitney's PW1000G and PW4000 engines.
Compared with the main body of aircraft engines, resin-based composites have a wider application in short engine fairings, as shown in the figure. According to resources, foreign manufacturers have extensively employed resin-based composites in short air intakes, fairings, thrust reversers and noise-reducing coatings.
Resin-based composites are also applied in varying degrees to other parts of the aircraft engine, such as fan flow guide vanes, bearing seals and cover plates, as per resources.
7. Metal Matrix Compounds
Metal matrix composites, compared to resin-based composites, have excellent toughness, do not absorb moisture and can withstand relatively high temperatures. The reinforcing fibers of metal matrix composites include metal fibers such as stainless steel, tungsten, and nickel and aluminum intermetallic compounds; ceramic fibers such as alumina, silica, carbon, boron and silicon carbide.
The matrix materials of metal matrix composites include aluminum, aluminum alloys, magnesium, titanium and titanium alloys, and heat-resistant alloys. Composites based on aluminum-magnesium alloys, titanium and iron alloys are currently the main choices. For example, silicon carbide fiber reinforced titanium alloy composites can be used to manufacture compressor blades.
Magnesium or magnesium alloy composites reinforced with carbon fiber or alumina fiber can be used to produce turbine fan blades. Nickel-chromium-aluminum-iridium fiber-reinforced nickel alloy composites can be used to manufacture seals for turbines and compressors.
Other parts, such as fan casings, rotors and compressor discs, have production cases with metal matrix composites abroad. However, one of the biggest problems with these composites is that the reinforcing fibers and base metals tend to react and form brittle phases, deteriorating the material's properties.
This is especially pronounced during prolonged use at higher temperatures. Current solutions include applying an appropriate coating to the fiber surface based on different fibers and matrices, as well as metal matrix alloying, to slow down the interface reaction and maintain the reliability of composite materials.
