Figure 1 Applications of superalloys in modern civil aviation engines
Aero engines work for a long time at high temperature, high pressure and high speed, and superalloy is the preferred material for hot parts, mainly used in combustion chamber, turbine, nozzle and other hot parts and compressor rear parts, as shown in Figure 1.
1 Application and demand of compressor components
The function of the compressor is to further compress air, provide air source for the combustion of fuel, and also provide high-speed air flow for the cooling of hot parts, the heating of cold parts and the sealing of cold parts. The average temperature of the compressor inlet and outlet air flow is 300~600 degrees Celsius, the peak temperature is more than 700 degrees Celsius, and the total pressure ratio is more than 20.
There are many materials used in compressor parts, and superalloy is one of the key materials. The rear section of the compressor has a high temperature, and the rotor parts (blades, roulette, etc.) and stator parts (blades, casing, etc.) are made of deformed alloys such as GH4169. Better performance powder discs, GH4169G, 718Puls have been used in advanced engines. In order to meet the demand of advanced high-performance engines, lightweight materials such as TiAl intermetallic alloys and Ti matrix composites (MMC) are being studied at home and abroad.
2 Application and requirements of combustion chamber components
The function of the combustion chamber is to release the chemical energy of the fuel into heat energy, which is the source of engine heat energy. The average gas flow temperature at the inlet and outlet of the combustion chamber is 600~2000 degrees Celsius, and the average gas temperature at the core is 1500~2100 degrees Celsius. There are two streams of air that cool the combustion chamber and lower the wall temperature. The combustion chamber is the thermal component with the highest temperature and the greatest temperature change.
There are many types of superalloys used in combustion chamber components, mainly deformation alloys, mainly used in internal and external cartridges, flame tubes and other parts. The traditional superalloy plate is limited by the melting point of the alloy and has basically reached the allowable limit temperature, which is difficult to further develop, and new materials and preparation technology must be studied to meet the needs of use. At present, the most popular new materials in the international research are ceramic matrix composite (CMC), MMC, mechanical alloying alloy (ODS), TiAl intermetallic compounds and porous laminates, etc. CMC used for flame tubes has been able to withstand the high temperature of 1670 degrees Celsius.
3. Application and requirements of turbine components
The function of the turbine is to convert the thermal energy of the gas into mechanical energy, drive the rotor of the compression system, and provide mechanical energy for the engine and the aircraft. The turbine is the thermal component with the largest thermal load and mechanical load. The turbine disc and blade are typical representatives of the technical difficulty of the engine, and can best reflect the changes of "one generation engine, one generation material, one generation process" and the development of superalloys.
The guide blade adjusts the direction of gas flow at the outlet of the combustion chamber, which is the part with the largest thermal impact and the highest temperature. In general, the average temperature of the guide blade in the same state is about 100 degrees Celsius higher than that of the working blade, but the stress is lower.
The working blade is the part with the highest centrifugal load and the worst working condition. The centrifugal force of the blade rotating at high speed at high temperature is equivalent to 20,000 times its own weight, which is like "ice cake spoon stirring hot soup", which is a severe test for materials, technology and design.
After the breakthrough of investment casting technology, the allowable temperature of directional and single crystal alloy prepared by directional solidification is close to 90% of its initial melting point, and advanced aero engines in various countries use directional and single crystal alloy to make turbine blades to improve the working temperature. The super cooled cast cold vortex blade made of second-generation single crystal PWA1484 for the F119 engine has an operating temperature of 1621~1677 degrees Celsius (200 degrees higher than F100).
The international development of single crystal alloy is rapid, and has now developed to the fifth generation. The melting point of single crystal alloy is limited, and the cooling technology is the main method to improve the operating temperature of blade. The higher the cooling effect, the more complex the structure of the blade cavity, the more difficult it is to manufacture, and the need to develop manufacturing processes and new materials. At present, split single crystal blades, porous lamellar blades and Nb-Mo alloys with higher temperature resistance, CMC blades, carbon/carbon composite integral turbines have been studied internationally.
The turbine disc is the most important part of the four thermal components in terms of quality and load. The rim temperature is 550 ~ 750 degrees Celsius, the hub is about 300 degrees Celsius, the temperature difference stress and centrifugal stress are very large, and it is also necessary to withstand the large stress alternating fatigue of starting and stopping. GH4169 turbine disk consumption is the largest, better performance of the powder disk has become the first choice of turbine disk, the second generation of powder disk, dual performance powder disk has been applied in advanced engines. The third generation of powder disk, fiber reinforced metal base turbine disk, single crystal blade + powder disk integral turbine are currently hot research topics.
4 Application and requirements of afterburner and nozzle components
A second combustion is organized in the afterburner to further heat up the gas, and the thrust is generated by rapid discharge through the nozzle according to the set mode. The afterburner and nozzle working conditions are harsh, the gas temperature of the tail nozzle is 727~1877 degrees Celsius, the gas speed is between subsonic and supersonic, and the inner wall is mainly subjected to gas erosion, thermal stress and vibration stress.
There are many deformed superalloys used in afterburner and nozzle components, and casting alloys such as K24 are mainly used to manufacture thin-walled parts such as nozzle adjusting plates. The difficulty of adjusting the difference in length, width and thickness is no less than that of turbine blades. More temperature-resistant CMC regulator has been applied in advanced engines, and the weight reduction effect is remarkable.