New Materials For Aeroengines. Let's Comb Through Them

Jun 15, 2020

Aviation manufacturing is the most concentrated field of high and new technology in manufacturing industry, which belongs to advanced manufacturing technology. The F119 engine developed by HP in the US, the F120 engine by GE, the M88-2 engine by SNECMA in France, and the EJ200 engine jointly developed by Britain, Germany, Italy and Spain. The common feature of these high performance aero-engines, which represent the world's advanced level, is the widespread adoption of new materials, new processes and new technologies. Today we're going to look at some new materials for high-performance aeroengines.

High temperature alloy

Superalloys are developed to meet the demanding requirements of jet engines on materials. So far, superalloys have become a kind of key materials that cannot be replaced for hot-end components of military and civil gas turbine engines. At present, in the advanced aeroengine, the amount of superalloy has accounted for more than 50%.

The development of superalloy is closely related to the technical progress of aero-engine, especially the turbine disk, turbine blade material and manufacturing process of engine hot-end components are important symbols of engine development. Due to the high requirements on the high-temperature resistance and stress tolerance of materials, Nimonic80 alloy strengthened by Ni3 (Al, Ti) was developed in Britain at the early stage to be used as turbine blade material for turbojet engines. Meanwhile, Nimonic series alloys were successively developed. In the United States, dispersion strengthened nickel-based alloys containing aluminum and titanium have been developed, such as Inconel, Mar-M and Udmit alloy series developed by Pratt & Whitney, GE and Special Metal companies respectively.

In the development of superalloys, the manufacturing process plays a great role in promoting the development of alloys. Due to the appearance of vacuum smelting technology, the removal of harmful impurities and gases in the alloy, especially the accurate control of the alloy composition, the performance of superalloy is constantly improved. Subsequently, the successful research of new technologies such as directional solidification, single crystal growth, powder metallurgy, mechanical alloying, ceramic core, ceramic filtration, isothermal forging and so on has promoted the rapid development of superalloys. Among them, directional solidification technology is the most prominent. Directional and single-crystal alloy produced by the directional solidification process is used at a temperature close to 90% of the initial melting point. Therefore, all the advanced aero-engine blades in the world are made of directional, single crystal alloy. From the international point of view, nickel-based casting superalloys have formed equiaxial crystal, directionally solidified columnar crystal and single crystal alloy systems. Powder superalloy is also developed from the first generation 650℃ to 750℃, 850℃ powder turbine disk and dual performance powder disk for advanced high performance engines.

China's superalloy development with the development of aircraft engine research and production requirements. High temperature alloy and entrepreneurship in China started in the 1970 s, due to the demand of the first and second generation engine in our country, the research and development of our country the GH series deformation of high temperature alloy and K series of casting high temperature alloy, at the same time developed many new manufacturing technology, such as vacuum melting and casting, the hollow blade casting, isothermal forging, etc.

After the 1970s, in the development of superalloys, China introduced European and American technology, according to foreign technical standards for the development and production, the material purity and comprehensive performance of higher requirements, the development of high-performance deformation superalloys, casting superalloys. In particular, the research and development of DZ series directional solidified columnar alloy and DD series single crystal alloy have brought the production technology and product quality control of China's superalloys to a new level.

Ultra high strength steel

Ultra high strength steel is used as landing gear material in aircraft. The second generation aircraft landing gear material is 30CrMnSiNi2A steel, tensile strength of 1700MPa, this landing gear has a short life, about 2000 flight hours.

The third generation of fighter aircraft is designed to have a landing gear life of more than 5,000 flight hours. At the same time, due to the increase of airborne equipment, the weight coefficient of aircraft structure decreases, which puts forward higher requirements on the material selection and manufacturing technology of landing gear. Both THE US and China's third generation aircraft use the 300M steel (tensile strength 1950MPa) landing gear manufacturing technology.

It should be noted that the improvement of material application technology is also driving the further extension of landing gear life and the expansion of adaptability. For example, airbus A380 landing gear adopts super-large integral forging technology, new atmosphere protection heat treatment technology and high-speed flame spraying technology, so that the landing gear life can meet the design requirements. Thus, advances in new materials and manufacturing technologies have ensured that aircraft are replaced.

AerMet100 steel has the same strength level compared with 300M steel, while the general corrosion resistance and stress corrosion resistance are significantly better than 300M steel. The matching landing gear manufacturing technology has been applied to F/A-18E/F, F-22, F-35 and other advanced aircraft. The higher strength Aermet310 steel has lower fracture toughness and is under study. The damage tolerance ultra-high strength steel AF1410 has a very slow crack growth rate and is used as a b-1 aircraft wing actuator cylinder joint, which is 10.6% lighter than TI-6al-4V, 60% higher in machining performance and 30.3% lower in cost. The Russian MIG-1.42 USES up to 30% high strength stainless steel. Ph13-8mo is the only high-strength martensitic precipitation-hardened stainless steel, which is widely used as corrosion-resistant components. Preliminary results have been obtained by exploring ultra-high strength stainless steel in China.

Ultra-high strength gear (bearing) steels, such as CSS-42L and GearmetC69, have been developed abroad and tested in engines, helicopters and aerospace. The technology of engine and helicopter transmission materials is very backward in China. Beijing Aeronautical Materials Research Institute has independently researched and developed a kind of ultra-high strength bearing gear steel.

Intermetallic compound

The development of high performance and high thrust-to-weight ratio aero-engine promotes the development and application of intermetallic compounds. Today intermetallic compounds have developed into a variety of families, which are generally composed of binary, ternary, or multielement metallic elements. Intermetallic compounds have great potential in high temperature structural applications, with high service temperature, specific strength and thermal conductivity, especially at high temperature, and excellent oxidation resistance, corrosion resistance and high creep strength. In addition, because intermetallic compound is a new material between superalloy and ceramic material, it fills the gap between these two materials, so it becomes one of the ideal materials for aero-engine high-temperature components.

At present, intermetallic compounds such as ti-Al and Ni-Al are mainly devoted to the research and development of aeroengine structures. These titanium-aluminum compounds have roughly the same density as titanium, but have higher temperatures. For example, the operating temperature was 816℃ and 982℃, respectively. Due to the strong binding force between atoms and the complex crystal structure, the intermetallic compound is difficult to deform and appears hard and brittle at room temperature. At present, after many years of experimental research, a new alloy with high temperature strength, room temperature plasticity and toughness has been developed and installed with good effect. For example, the high performance F119 engine in the United States USES intermetallic compounds in its external casing and turbine disk, while the compressor blade and disk of F120 engine in the verification machine adopts new intermetallic compounds in titanium and aluminum.

Ceramic matrix composites

When it comes to ceramics, it is natural to think of brittle features. More than ten years ago, if it is used in the engineering field of bearing parts, is anyone can't accept, until now for the ceramic composite materials, maybe some people don't know, also don't think ceramic and metal was originally two related basic materials, but since the people after the ingenious combination of ceramic and metal, makes people fundamental changes have taken place in the concept of the material, that is the ceramic matrix composites.

Ceramic matrix composites are promising new structural materials in aviation industry, especially in aeroengine manufacturing. Ceramic matrix composites not only have the advantages of light weight and high hardness, but also have excellent high temperature and high temperature corrosion resistance. At present, ceramic matrix composites are more resistant to high temperature than metal materials, and have excellent mechanical properties and chemical stability. They are ideal materials for high temperature zones of high-performance turbine engines.

At present, all countries in the world are focusing on the research of silicon nitride and silicon carbide reinforced ceramic materials according to the material requirements of the next generation of advanced engines, and great progress has been made. For example, the F120 engine of the American verifier, its high-pressure turbine sealing device and some high-temperature parts of the combustion chamber are made of ceramic materials. French M88-2 engine combustion chamber and nozzle are also made of ceramic matrix composites.

Carbon/carbon composites

In recent years, C/C matrix composite is a new material which is more resistant to high temperature. So far, only The C/C composite material is considered as the only successor material for turbine rotor blades with thrust-to-weight ratio above 20 and engine inlet temperature up to 1930-2227℃. It is the high-temperature resistant material developed by the United States in the 21st century and the highest target pursued by advanced industrial countries in the world. C/C matrix composites, carbon fiber reinforced carbon basic composites, combine the fusible properties of carbon with the high strength and rigidity of carbon fiber to present non-brittle failure. Due to its light weight, high strength, superior thermal stability and excellent thermal conductivity, it is the most ideal high-temperature material nowadays. Especially in the high temperature environment of 1000-1300℃, its strength does not decrease, but increases. It retains the strength and grace of a room temperature environment at temperatures below 1,650 degrees Celsius. Therefore, C/C matrix composites have a great future in aerospace manufacturing industry.

The main problem of the application of C/C matrix composite in aero-engine is its poor antioxidant performance. In recent years, the United States has taken a series of technological measures to solve this problem and gradually applied it to new engines. For example, the afterburner nozzle of the F119 engine in the United States, the nozzle and combustion chamber nozzle of the F100 engine, and some parts of the combustion chamber of the F120 verification machine have been made of C/C matrix composite materials. French M88-2 engine, Mirage 2000 engine afterburner fuel rod, heat shield, nozzle and other C/C matrix composite materials are also used.

Resin matrix composites

Resin matrix composites in aerospace applications of turbofan engine research began in the 1950 s, after more than 60 years of development, GE, PW, RR and MTU and SNECMA company invested a lot of energy for resin matrix composite materials research and development, great progress has been made, will have its engineering application to active aircraft turbofan engine, and the further developing tendencies of are.

The service temperature of resin matrix composites generally does not exceed 350℃. Therefore, resin matrix composites are mainly used in the cold end of aeroengines. The main application parts of resin matrix composites in foreign advanced aircraft engines are shown in the figure.

Fan blade: Engine fan blade is the most representative and important part of turbofan engine. The performance of turbofan engine is closely related to its development. Compared with titanium alloy fan blades, resin matrix composite fan blades have obvious weight loss advantages. In addition to the obvious weight loss advantage, resin matrix composite fan blades have less impact on the fan casing after being impacted, which is conducive to improving the inclusiveness of the fan casing.

At present, the main representatives of composite fan blades that have been commercially-applied abroad are GE90 series engines for B777, GEnx engines for B787, and Leap-X engines for ComAC C919. In 1995, the GE90-94B engine with resin matrix composite fan blades was put into commercial operation, marking the formal realization of engineering applications of resin matrix composites in modern high-performance aeroengines. On the basis of comprehensive consideration of aerodynamics, high-low cycle fatigue cycle and other factors, GE developed new composite fan blades for the subsequent GE90-115B engines.

In the 21st century, the aero-engine's strong demand for high-damage tolerance composite material is driving the further development of composite material technology. However, it is difficult to meet the requirement of high-damage tolerance by improving the toughness of carbon fiber/epoxy resin prepreg. Under this background, 3D braided composite fan blade came into being.

Fan casing: Fan casing is the largest static component of an aeroengine, and its weight reduction will directly affect the thrust-weight ratio and efficiency of the aeroengine. Therefore, foreign advanced aircraft engine OEM has been committed to the weight reduction and structural optimization of fan casing. As shown in the figure, the development trend of fan casing of foreign advanced aeroengine is shown.

Fan cap: Because it is not the main load-bearing component, fan cap is one of the first composite components used in aeroengines. Composite fan cap provides lighter weight, simplified anti-ice structure, better corrosion resistance and better fatigue resistance.

At present, resin matrix composites have been used to prepare fan caps for R.R. RB211 engine, PW PW1000G and PW4000.

Compared with the main engine, resin matrix composites have a wider application space in aeroengine nacelles, as shown in the figure. According to the data, foreign manufacturers have used the resin matrix composite material in the nacl inlet, fairing, thrust back device and noise reduction lining.

Other parts according to the data, in the aero engine fan runner plate, bearing seal cover, cover plate and other parts also in different degrees of the application of resin matrix composite.

Metal matrix composites

Compared with resin matrix composites, metal matrix composites have good toughness, no moisture absorption, and can withstand high temperature. The reinforced fibers of metal matrix composites include metal fibers, such as stainless steel, tungsten, quilt, ni, ni-Al intermetallic compounds, etc. Ceramic fiber, such as alumina, silicon oxide, carbon, boron, silicon carbide, boron nitride, etc.

The matrix materials of metal matrix composites are aluminum, aluminum alloy, magnesium, Qinhe Qinhe alloy, heat resistant alloy, drill alloy and so on. Aluminum-keng alloy, Qinhe ferroalloy as the base composite material is the main choice at present. For example, silicon carbide fiber reinforced Alloy matrix composites can be used to manufacture compressor blades. Carbon fiber or alumina fiber reinforced magnesium or magnesium alloy matrix composites can be used to manufacture turbine fan blades. For example, nickel-chrome-aluminum-iridium fiber reinforced nickel-base alloy matrix composites can be used to manufacture sealing elements for turbines and compressors.

Other parts, such as fan casing, rotor, compressor disc, etc., are made of metal matrix composite abroad. However, one of the biggest problems of this kind of composite material is that the reaction between the reinforced fiber and the matrix metal is easy to produce brittle phase, which makes the material performance worse. Especially at higher temperatures for a long time, the interface reaction is more prominent. At present, the solution is to add appropriate coating on the fiber surface according to different fibers and different matrix, and alloying the matrix metal to slow down the interface reaction and maintain the reliability of the composite properties.