Gas turbine engine (GTE) blades are a critical component of aircraft power plants. As Soviet engine designer Nikolai Kuznetsov noted, emphasizing their importance, “An engine is blades and bearings.” A turbojet engine uses various types of blades: fan and compressor blades for air compression, turbine blades for power extraction, and stationary guide vanes. Improvements in aircraft engine efficiency and reliability have mirrored advancements in turbine blade manufacturing technology, progressing from simple stamping to single-crystal casting and the development of sophisticated cooling systems.
Early generations of aircraft engines, with turbine entry temperatures (TET) not exceeding 900-1000 Kelvin (K), used stamped blades. TET is crucial for the efficiency, power, and reliability of a turbojet engine. A higher TET results in a greater amount of energy that can be converted into mechanical work, thus increasing the power plant’s efficiency and thrust.
In gas turbine engine design, it is customary to specify gas temperatures in Kelvin, the SI (International System of Units) unit of thermodynamic temperature. Kelvin begins at absolute zero, the lowest possible temperature, where all thermal motion ceases. Using the Kelvin scale ensures versatility and accuracy in engineering and scientific calculations because it is absolute, independent of the specific substance’s properties, and contains no negative values. This simplifies mathematical operations and process modeling.
In aircraft engine design and thermodynamics, gas and material temperatures often reach very high values, making the Kelvin scale the preferred choice for technical specifications and calculations. The conversion between Celsius and Kelvin is given by: T(K) = T(°C) + 273.15. For instance, 1800 °C = 1800 + 273.15 = 2073.15 K. Therefore, in the mid-1950s, the turbine entry temperature of GTEs was in the range of 627-727 °C.
Why Turbine Blades Are Key to the Aero Engine Efficiency Race
The demand for increased engine power necessitated higher TETs, leading to the development of new heat-resistant alloys that could not be stamped. Investment casting technologies for turbine blades were developed, resulting in increased heat resistance and service life. However, the equiaxed grain structure of cast blades proved susceptible to fatigue failure along grain boundaries oriented perpendicular to the direction of centrifugal loads.
Addressing metal fatigue led to the development of directional solidification (DS) technology. DS eliminates transverse grain boundaries in the metal structure, forming columnar grains aligned along the primary stress direction. These blades exhibited improved fatigue resistance. The next step was the production of single-crystal (SX) blades, consisting of a single crystal without any grain boundaries. Single crystals provide maximum strength and service life, enabling turbine operation at gas temperatures exceeding 2000 K.
“The blade’s directional structure is achieved in special melting furnaces by slowly moving the mold containing the molten metal vertically from the hot zone of the furnace to the cold zone,” the United Engine Corporation (UEC) told the Aviation of Russia website. “This process allows the metal to crystallize and solidify. A metallic single-crystal seed is placed at the base of the mold. A single-crystal structure forms on its surface upon contact with the molten metal in the mold. Further “growth” occurs. As the mold slowly moves to the cold zone of the furnace, the single-crystal structure transfers to the casting, and its height increases along the blade.”
At UEC-Saturn (Rybinsk), a high-rate directional solidification method is employed. The mold containing the molten metal is moved from the hot zone of the furnace and simultaneously immersed in a bath of liquid cooling metal (aluminum), which allows for more effective control of the crystallization process. Blades manufactured using this technology are used in the PD-14 and PD-8 engines.
Engine efficiency is also enhanced by optimizing the external contours of the blades for operation in a high-speed gas flow. Turbine operating temperatures in GTEs reach 1800-2000 °C (3272-3632 °F), exceeding the melting point of the blade materials. To maintain their integrity, a comprehensive set of cooling measures is employed. Alloying elements such as tungsten, cobalt, chromium, molybdenum, rhenium, and ruthenium are added to the alloys, with the total number of these elements potentially reaching 15.
In addition, blades are coated with thermal barrier coatings (TBCs) and wear-resistant coatings, including ceramic coatings, using plasma spraying technology. These coatings reduce thermal load and protect against hot gas corrosion. A plasma spraying system consists of a powder feeder, a plasma torch to heat and accelerate the particles of the sprayed material, a power supply for the plasma torch, a control system that ensures precise adherence to gas, power, and cooling water parameters, and manipulators that synchronize the movement of the plasma torch and the part being coated. During spraying, the material is fed into the hot plasma stream, where it melts, accelerates, and is directed onto the part, forming a coating. This method is also used for component repair.
The primary method of blade protection is convective-film cooling, implemented through a complex system of channels within the blades and the turbine disk. Air bled from the compressor serves as the coolant. It is cooled by mixing with water or fuel and is supplied through internal channels via the engine shaft and channels of a centrifugal compressor integrated into the turbine disk. This convective heat exchange cools the blades from the inside.
Moreover, to prevent deformation, a protective air film is created on the blade surface through groups of holes less than one millimeter in diameter. This film further inhibits overheating. The geometry of the internal channels is complex, and their manufacturing requires high precision. At UEC enterprises, these holes are created using electrical discharge machining (EDM) with “superdrill”-type machines, employing brass electrodes and distilled water for cooling.
Friction Welding: Critical Technologies in Domestic Aero Engine Manufacturing
Additive manufacturing (3D printing) technologies are also being actively implemented. These technologies enable the creation of blades with high precision and complex internal geometries, including optimized cooling channels that are extremely difficult or impossible to manufacture using traditional methods. Power Machines, which is part of UEC, is also conducting pilot projects on 3D printing of blades for GTE-65.1 energy gas turbines. These developments involve the creation of new blade profiles and improved cooling systems aimed at increasing turbine efficiency and equipment life.
In the current environment of production digitalization, UEC enterprises are implementing artificial intelligence (AI) technologies to improve the quality and reliability of gas turbine engine blade manufacturing. For example, UEC-Aviadvigatel (Perm) uses neural network models to analyze the results of fan blade tests, including the determination of mode shapes from visual data. This enables the timely identification of potential defects and the optimization of design solutions in the early stages of development.
In Rybinsk, a robotic system with a machine vision platform and artificial intelligence has been implemented for automatic surface inspection of polished blades. This system detects minute defects that are undetectable by traditional visual inspection methods, significantly increasing the speed and accuracy of quality control.
Furthermore, machine learning and neural network methods are used in the predictive diagnostics of engine component condition during operation, contributing to the timely detection and prevention of failures. The integration of artificial intelligence into the processes of production, inspection, and operation of GTE blades at UEC enterprises significantly enhances the technology readiness level, reliability, and efficiency of domestic aircraft engines.
The technologies for manufacturing single-crystal blades and effective cooling systems are currently only available to a few countries with developed aviation industries, namely the United Kingdom, the United States, France, and Russia. China is also actively developing its own technologies in this area, striving to close the technological gap. For its prospective CJ-1000 engine, Chinese engineers are using modern directional solidification methods and developing turbine blade cooling systems. However, they have not yet achieved the level of materials and technologies required to create blades with a service life and thermal resistance comparable to Russian and Western counterparts. Despite significant efforts, China continues to face challenges in creating heat-resistant alloys and coatings capable of withstanding the extreme gas temperatures in turbines.
Novel Blade Treatment Methods Enhance Aeroengine Reliability
Russian enterprises within the United Engine Corporation are successfully implementing a complete production cycle for such blades, including directional solidification technologies and plasma spraying of thermal barrier coatings. The application of additive manufacturing not only accelerates the production of prototypes but also opens up prospects for the serial production of complex and high-tech parts. This allows Russia to maintain its technological sovereignty and create alternative gas turbine engines for the global energy and civil aviation markets.
Therefore, the development of domestic technologies for the manufacture and protection of GTE blades is a strategically important task aimed at increasing engine efficiency and service life, as well as strengthening Russia’s position among the world leaders in aircraft engine manufacturing.
(No Ratings Yet)
Loading...