In the aero-engine, the turbine blade is listed as the first key part because it is in the part with the highest temperature, the most complicated stress and the most severe environment. The performance level of the turbine blade, especially the ability to withstand temperature, has become an important symbol of the advanced degree of a type of engine, in a certain sense, but also a country's aviation industry level of significance.
Aero-engine is known as “the crown jewel of modern industry”, is an extremely complex system, is an extremely harsh conditions (high temperature, high pressure, high stress) in the pursuit of long-term, stable, safe and extreme performance components, difficult to do nature is certain. Turbine blade is the core component of aero-engine, and it is also the component with the harshest working environment in the aero-engine.
When we were kids, we all played with balloons. After blowing up the balloon, once you let go of your finger, the balloon will fly up and around. This is because the gas in the balloon leaks outward while giving the balloon a forward reaction force.
Rockets use this principle to carry large amounts of fuel to create gas, which is then quickly expelled backwards thus gaining thrust.
Aero-engines, on the other hand, use this same principle to make airplanes fly. The difference is that airplanes fly under the atmosphere and do not need to carry their own fuel to make air.
To say that the principle of the aircraft engine is also very simple, ingenious, the engine first fan inhalation of air, and then the compressor will compress the air to increase the pressure, and then the compressed air into the combustion chamber and fuel mixing ignition, and finally the high-speed gas injection to promote the rotation of the turbine; turbine and the compressor and the fan is connected to one or two shafts, the turbine rotates and will drive the fan and the compressor to rotate, and constantly compress the air to produce part of the thrust! The gas drives the turbine to rotate and then is ejected through the tail nozzle, which in turn generates part of the thrust. These two parts of the thrust together to provide power for the aircraft.
Higher performance aircraft engines require higher performance turbine blades.
To obtain high performance with small size and light weight, which is often referred to as increasing the thrust-to-weight ratio (the ratio of the engine's thrust to its own weight), the main measure is to increase the gas temperature. Because the thrust of the engine is mainly generated by the hot gas produced by combustion and cold air mixed with the air pressure difference. The greater the gas pressure difference, the greater the thrust. To increase the gas pressure difference, the combustion temperature has to be higher.
The data show that for every 100 degree increase in the turbine front inlet temperature, the thrust-to-weight ratio can be increased by 10% under the condition that the engine size remains unchanged.
As early as 1928, British engineers came up with the idea of an aero engine. However, it was not until 1939 that the first engine was introduced. Because as envisioned at the time, there was no material to carry its high temperatures.
Now, the mainstream aviation equipment is used on the thrust-to-weight ratio is 7 ~ 8 of the third generation of aviation engines, such as the United States F-15, F-16, F-18, China's J-10, J-11 and other warplanes, civil aviation of the various series such as the Airbus A320, Boeing 737 and so on. Turbine front inlet temperature is about 1680 ~ 1750 K. The United States Pratt & Whitney developed the F119 engine (ready for the U.S. Army F-22 fighter, the world's first five-generation aircraft), thrust-to-weight ratio of 10.8, the turbine front inlet temperature of 1,900 K. This temperature has exceeded the melting point of carbon steel on the Baidu.
In addition to high temperatures, the turbine blades are also rotating at very high speeds. High-speed rotation leads to the turbine blade must bear very high centrifugal force. And at high temperatures, high loading, metal materials will creep. Simply put, creep is the deformation of a material at a certain temperature over a long period of time in response to a small external force. Creep is not good for the blade, it can make the blade radial elongation, twisting and bending, affecting the service life of the blade.
In addition, the material will be fatigued during use, which may lead to fatigue fracture and jeopardize safety
Therefore, the turbine blade material should not only high temperature, but also low creep, fatigue resistance, mechanical properties but also good. To say that it is good to build, then I am afraid that there is nothing else difficult to build now.
Usually cast metal materials are polycrystalline, so why use single crystals or oriented alloys?
For polycrystalline materials, there are grain boundaries between grains. In some cases, grain boundaries can be beneficial for improved material properties. But in the 1960s, Hewlett-Packard found that grain boundaries were a “bad boy” for turbine blades. Ordinary cast polycrystalline high-temperature alloys in the perpendicular to the stress axis of the grain boundary is a high-temperature stress cracks under the “source”, so it must reduce the grain boundary, and thus the development of high-temperature alloys directional solidification technology.
Directional solidification of high-temperature alloys by controlling the growth rate of crystallization, so that the grain according to the main bearing direction of optimal growth, can improve the strength of the alloy, plasticity, thermal fatigue, so that the use of turbine blade temperature reached 1273K.
Further, the aviation industry has developed single-crystal alloy turbine blade, its temperature resistance, creep strength, thermal fatigue strength, oxidation resistance and corrosion resistance characteristics than the directional solidification column crystal alloy has improved significantly.
However, the manufacturing process of single crystal alloy turbine blades can be much more difficult. At present, the most commonly used for the manufacture of single-crystal blade process method is the spiral crystal method, the basic principle is to use the selector of this narrow interface, only allows a grain to grow out of the top of it, and then the grain grows full of the entire cavity, so as to get a single crystal. The crystal competition growth mechanism is the spiral structure of the total climb towards the opposite direction of heat dissipation, resulting in uniform heat dissipation in the spiral body, so in the entire spiral growth process, the most suitable for the growth of that grain will be the other numerous primary grain one by one eliminated, and continue to grow out of the dendritic crystal and ultimately into the specimen body into a single-crystal castings. Unidirectional heat dissipation seems simple, in fact, this process is extremely difficult to control, which involves the material itself and the thermophysical properties of the casting mold, and consider the manufacturing process of heat dissipation conditions and other factors such as the influence of the growth rate of crystals, etc., which need to be rigorously experimental design as well as a large amount of experimental data, after calculations to be able to come up with the accurate results, the difficulty of the very big.
In addition, high-temperature alloys also add trace elements to improve the use of temperature. The second generation of single-crystal alloys and the first generation of single-crystal alloys, compared with the addition of 3% of rhenium, cobalt and molybdenum content increased appropriately, so that its operating temperature increased by 30 K. The third generation of single-crystal alloys, Rene N6 and CMSX-10 to add as much as 5 wt% of rhenium to significantly improve the high-temperature creep strength. The fourth generation of single crystal alloys, by adding ruthenium, further improves the stability of the microstructure of the alloy, the service temperature has reached 1473K.
However, the use of single crystal high temperature alloy materials does not solve the problem. In fact, it is impractical to rely solely on the search for more heat-resistant materials to meet higher pre-turbine temperatures. At present, the turbine before the inlet temperature has reached about 2000K, than the melting point of high-pressure turbine blade metal materials 400 K. In recent years, the turbine before the temperature to an average annual increase of 20K rate, while the degree of metal temperature resistance is only about 8K per year rate of increase.
The gap between the two must rely on the air development of advanced cooling system to realize, such as air film cooling technology. Through the injection of cold gas, with the help of high temperature gas pressure and friction so that it adheres to the wall near the formation of a lower film of cold gas, the wall with high temperature gas isolation, and take away part of the high temperature gas or bright flame on the wall of the radiant heat, so as to play a good role in the protection of the wall.
According to data records, the current cooling technology can realize the temperature drop has reached 400K ~ 600K.
With the aviation engine turbine front temperature continues to rise, the original single-channel hollow cooling blade cooling effect has not been able to meet the reality of demand, the development of more advanced and more complex multi-channel multi-way cooling program has become the next generation of aircraft development of key technologies. The optimization of each cooling scheme is a great test for the design and manufacture of turbine blades!
The development of cooling technology still can not completely solve, or meet the gap between the material bearing temperature and the turbine front inlet temperature. Therefore, it is also necessary to spray thermal barrier coatings on the surface of the gas flow path of the turbine blade.
Thermal barrier coating (referred to as TBC) is deposited on the surface of the parts bonded layer of low thermal conductivity materials, the use of its low thermal conductivity characteristics, the formation of the temperature drop on the inner and outer surfaces, to reduce the surface temperature of the parts (or to improve the temperature-bearing capacity of the parts) of the method. According to the information, the thermal barrier coating can achieve 50K ~ 150K heat insulation effect.
Currently in the turbine engine to obtain practical application of the thermal barrier coating are two-layer structure: the surface layer for the ceramic layer, mainly play a role in thermal insulation, but also play a role in corrosion resistance, scouring and erosion; the inner layer of the metal bonding layer, mainly to improve the physical compatibility between the metal substrate and ceramic layer, to enhance the coating of anti-temperature oxidation performance of the role.
As the coating is coated on the metal substrate, need to consider the adhesion of the coating, the impact of the stability of the microstructure of the metal substrate as well as the coating and metal due to the difference in the coefficient of thermal expansion may lead to spalling problems, etc., which requires the hollow blade metal materials disciplines in conjunction with the joint efforts of many other areas of relevant experts in the public relations, joint research and development.
So far, a piece of monocrystalline hollow blade real cost is comparable to the same weight of gold may not be too much. Even high-performance blades are priced out of the market.
