As the performance requirements for aircraft for transportation, military, production and other purposes increased, the earliest piston engines could no longer meet the needs of high-speed flight. Therefore, since the 1950s, gas turbine engines have gradually become mainstream.
In 1928, Sir Frank Whittle of the United Kingdom pointed out in his graduation thesis "Future Development in Aircraft Design" while studying at the military academy that under the technical knowledge at that time, the future development of propeller engines could not adapt to the needs of high altitude or flight speeds exceeding 800km/h. He first proposed the concept of what is now called a jet engine (motor engine): compressed air is provided to the combustion chamber (combustion) through a traditional piston, and the high-temperature gas generated is directly used to propel flight, which can be regarded as a propeller engine plus a combustion chamber design. In subsequent research, he abandoned the idea of using a heavy and inefficient piston and proposed using a turbine (turbine) to provide compressed air to the combustion chamber, and the power of the turbine was obtained from the high-temperature exhaust gas. In 1930, Whittle applied for a patent, and in 1937, he developed the world's first centrifugal turbojet engine, which was officially used in the Gloster E.28/39 aircraft in 1941. Since then, gas turbine engines have dominated aviation power and are an important symbol of a country's scientific and technological industrial level and comprehensive national strength.
Aircraft engines can be divided into four basic types according to their uses and structural characteristics: turbojet engines, turbofan engines, turboshaft engines, and turboprop engines:
Aviation gas turbine engines are referred to as turbojet engines, which are the earliest gas turbine engines used. From the perspective of the way thrust is generated, turbojet engines are the simplest and most direct engines. The reasoning relies on the reaction force generated by the high-speed injection of the vortex. However, the high-speed airflow takes away a lot of heat and kinetic energy at the same time, causing great energy loss.
The turbofan engine divides the air flowing into the engine into two paths: the inner duct and the outer duct, which increases the total air flow and reduces the exhaust temperature and speed of the inner duct airflow.
Turboshaft and turboprop engines do not generate thrust by airflow injection, so the exhaust temperature and speed are greatly reduced, the thermal efficiency is relatively high, and the engine fuel consumption rate is low, which is suitable for long-range aircraft. The speed of the propeller generally does not change, and different thrusts are obtained by adjusting the blade angle.
The propfan engine is an engine between turboprop and turbofan engines. It can be divided into propfan engines with ducted propeller cases and propfan engines without ducted propeller cases. The propfan engine is the most competitive new energy-saving engine suitable for subsonic flight.
Civil aerospace engines have gone through more than half a century of development. The structure of the engine has evolved from the early centrifugal turbine engine to the single-rotor axial flow engine, from the twin-rotor turbojet engine to the low bypass ratio turbofan engine, and then to the high bypass ratio turbofan engine. The structure has been continuously optimized with the pursuit of efficiency and reliability. The turbine inlet temperature was only 1200-1300K in the first generation of turbojet engines in the 1940s and 1950s. It increased by about 200K with each aircraft upgrade. By the 1980s, the turbine inlet temperature of the fourth generation advanced fighter jets reached 1800-2000K[1].
The principle of centrifugal air compressor is that the impeller drives the gas to rotate at high speed, so that the gas generates centrifugal force. Due to the expansion pressure flow of the gas in the impeller, the flow rate and pressure of the gas after passing through the impeller are increased, and compressed air is continuously produced. It has a short axial dimension and a high single-stage pressure ratio. Axialflow air compressor is a compressor in which the air flow basically flows parallel to the axis of the rotating impeller. The axial flow compressor consists of multiple stages, each stage contains a row of rotor blades and a subsequent row of stator blades. The rotor is the working blades and the wheel, and the stator is the guide. The air is first accelerated by the rotor blades, decelerated and compressed in the stator blade channel, and repeated in the multi-stage blades until the total pressure ratio reaches the required level. The axial flow compressor has a small diameter, which is convenient for multi-stage tandem use to obtain a higher pressure ratio.
Turbofan engines usually use bypass ratio, engine pressure ratio, turbine inlet temperature, and fan pressure ratio as design parameters:
Bypass ratio (BPR): The ratio of the mass of gas flowing through the outlet ducts to the mass of gas flowing through the inner ducts in the engine. The rotor at the front of a turbojet engine is usually called the low-pressure compressor, and the rotor at the front of a turbofan engine is usually called the fan. The pressurized gas passing through the low-pressure compressor passes through all parts of the turbojet engine; the gas passing through the fan is divided into the inner and outer ducts. Since the emergence of turbofan engines, BPR has been increasing, and this trend is particularly evident in civil turbofan engines.
Engine pressure ratio (EPR): The ratio of the total pressure at the nozzle outlet to the total pressure at the compressor inlet.
Turbine inlet emperature: The temperature of the combustion chamber exhaust when it enters the turbine.
Fan compression ratio: Also referred to as compression ratio, the ratio of the gas pressure at the compressor outlet to the gas pressure at the inlet.
Two efficiencies:
Thermal efficiency: A measure of how efficiently an engine converts the heat energy generated by combustion into mechanical energy.
Propulsion efficiency: A measure of the proportion of the mechanical energy generated by the engine that is used to propel the aircraft.
In the 1970s, the United States was the first to use PWA1422 directional solidification blades in military and civilian aircraft engines.
After the 1980s, the thrust-to-weight ratio of the third-generation engine increased to more than 8, and the turbine blades began to use the first-generation SX, PWA1480, RenéN4, CMSX-2 and China's DD3. Its temperature-bearing capacity is 80K higher than that of the best directional solidification casting high-temperature alloy PWA1422. Advantages. Coupled with the film cooling single-channel hollow technology, the operating temperature of the turbine blades reaches 1600-1750K. .
The fourth-generation turbofan engine uses the second-generation SXPWA1484, RenéN5, CMSX-4, and DD6. By adding Re elements and multi-channel high-pressure air cooling technology, the operating temperature of the turbine blades reaches 1800K-2000K. At 2000K and 100h The lasting strength reaches 140MPa.
The third-generation SX developed after the 1990s includes RenéN6, CMRX-10, and DD9, which have very obvious creep strength advantages over the second-generation SX. Under the protection of complex cooling channels and thermal barrier coatings, the turbine inlet temperature it can withstand reaches 3000K. The intermetallic compound alloy used in the blades reaches 2200K, and the 100h lasting strength reaches 100MPa.
Currently under development are the fourth generation SX represented by MC-NG[4], TMS-138, etc., and the fifth generation SX represented by TMS-162, etc. Its composition is characterized by the addition of new rare earth elements such as Ru and Pt, which significantly improves the high-temperature creep performance of SX. The working temperature of the fifth-generation high-temperature alloy has reached 1150°C, which is close to the theoretical limit operating temperature of 1226°C.
3.1 Composition characteristics and phase composition of nickel-based single crystal superalloys
According to the type of matrix elements, high-temperature alloys can be divided into iron-based, nickel-based, and cobalt-based, and further subdivided into casting, forging, and powder metallurgy macrostructures. Nickel-based alloys have better high-temperature performance than the other two types of high-temperature alloys and can work for a long time in harsh high-temperature environments.
Nickel-based high-temperature alloys contain at least 50% Ni. Their FCC structure makes them highly compatible with some alloying elements. The number of alloying elements added during the design process often exceeds 10. The commonality of the added alloying elements is classified as follows: (1) Ni, Co, Fe, Cr, Ru, Re, Mo, and W are first-class elements, which serve as austenite stabilizing elements; (2) Al, Ti, Ta, and Nb have larger atomic radii, which promote the formation of strengthening phases such as compound Ni3 (Al, Ti, Ta, Nb), and are second-class elements; (3) B, C, and Zr are third-class elements. Their atomic size is much smaller than that of Ni atoms, and they are easily segregated to the grain boundaries of the γ phase, playing a role in grain boundary strengthening [14].
The phases of nickel-based single crystal high-temperature alloys are mainly: γ phase, γ' phase, carbide phase, and topological close-packed phase (TCP phase).
γ phase: γ phase is an austenite phase with a crystal structure of FCC, which is a solid solution formed by elements such as Cr, Mo, Co, W, and Re dissolved in nickel.
γ' phase: γ' phase is a Ni3(Al, Ti) intermetallic compound of FCC, which is formed as a precipitation phase and maintains a certain coherence and mismatch with the matrix phase, and is rich in Al, Ti, Ta and other elements.
Carbide phase: Starting from the second generation of nickel-based SX, a small amount of C is added, resulting in the appearance of carbides. A small amount of carbides are dispersed in the matrix, which improves the high-temperature performance of the alloy to a certain extent. It is generally divided into three types: MC, M23C6, and M6C.
TCP phase: In the case of service aging, excessive refractory elements such as Cr, Mo, W, and Re promote the precipitation of TCP phase. TCP is usually formed in the form of a plate. The plate structure has a negative impact on ductility, creep, and fatigue properties. TCP phase is one of the crack sources of creep rupture.
Strengthening Mechanism
The strength of nickel-based superalloys comes from the coupling of multiple hardening mechanisms, including solid solution strengthening, precipitation strengthening, and heat treatment to increase dislocation density and develop dislocation substructure to provide strengthening.
Solid solution hardening is to improve the basic strength by adding different soluble elements, including Cr, W, Co, Mo, Re, and Ru.
The different atomic radii lead to a certain degree of atomic lattice distortion, which inhibits dislocation movement. Solid solution strengthening increases with the increase of atomic size difference.
Solid solution strengthening also has the effect of reducing the stacking fault energy (SFE), mainly inhibiting dislocation cross slip, which is the main deformation mode of non-ideal crystals at high temperatures.
Atomic clusters or short-range order microstructures are another mechanism that helps to obtain strengthening through solid solution. Re atoms in SX segregate in the tensile stress region of the dislocation core at the γ/γ’ interface, forming a "Cottrell atmosphere", which effectively prevents dislocation movement and crack propagation. (Solute atoms are concentrated in the tensile stress area of edge dislocations, reducing lattice distortion, forming a Coriolis gas structure, and producing a strong solid solution strengthening effect. The effect increases with the increase of solute atom concentration and the increase of size difference)
Re, W, Mo, Ru, Cr, and Co effectively strengthen the γ phase. The solid solution strengthening of the γ matrix plays an extremely important role in the creep strength of nickel-based high-temperature alloys.
The precipitation hardening effect is affected by the volume fraction and size of the γ' phase. The purpose of optimizing the composition of high-temperature alloys is mainly to increase the volume fraction of the γ' phase and improve the mechanical properties. SX high-temperature alloys can contain 65%-75% of the γ' phase, resulting in good creep strength. This represents the useful maximum value of the strengthening effect of the γ/γ' interface, and further increase will lead to a significant decrease in strength. The creep strength of high-temperature alloys with a high γ’ phase volume fraction is affected by the size of the γ’ phase particles. When the γ’ phase size is small, dislocations tend to climb around it, resulting in a decrease in creep strength. When dislocations are forced to cut the γ’ phase, the creep strength reaches its maximum. As the γ’ phase particles increase in size, dislocations tend to bend between them, resulting in a decrease in creep strength [14].
There are three main precipitation strengthening mechanisms:
Lattice mismatch strengthening: γ’ phase is dispersed and precipitated in the γ phase matrix in a coherent manner. Both are FCC structures. The lattice mismatch reflects the stability and stress state of the coherent interface between the two phases. The best case is that the matrix and the precipitated phase have the same crystal structure and lattice parameters of the same geometry, so that more precipitated phases can be filled in the γ phase. The mismatch range of nickel-based high-temperature alloys is 0~±1%. Re and Ru are obviously segregated with the γ phase. The increase of Re and Ru increases the lattice mismatch.
Order strengthening: Dislocation cutting will cause disorder between the matrix and the precipitated phase, requiring more energy
Dislocation bypass mechanism: called Orowan mechanism (Orowan bowing), it is a strengthening mechanism in which the precipitated phase in the metal matrix hinders the dislocation in motion from continuing to move. Basic principle: When the moving dislocation encounters a particle, it cannot pass through, resulting in bypassing behavior, dislocation line growth, and the required driving force increases, resulting in strengthening effect.
3.3 Development of high-temperature alloy casting methods
The earliest alloy used in high-temperature environments can be traced back to the invention of Nichrome in 1906. The emergence of turbo compressors and gas turbine engines stimulated the substantial development of high-temperature alloys. The blades of the first generation of gas turbine engines were produced by extrusion and forging, which obviously had the limitations of the times. At present, high-temperature alloy turbine blades are mostly made by investment casting, specifically directional solidification (DS). The DS method was first invented by the Versnyder team of Pratt & Whitney in the United States in the 1970s [3]. In the decades of development, the preferred material for turbine blades has changed from equiaxed crystals to columnar crystals, and then optimized to single crystal high-temperature alloy materials.
DS technology is used to produce columnar core alloy SX components, which significantly improves the ductility and thermal shock resistance of high-temperature alloys. DS technology ensures that the produced columnar crystals have a [001] orientation, which is parallel to the main stress axis of the part, rather than a random crystal orientation. In principle, DS needs to ensure that the solidification of the molten metal in the casting is carried out with the liquid feed metal always in a just-solidified state.
The casting of columnar crystals needs to meet two conditions: (1) One-way heat flow ensures that the solid-liquid interface at the growth point of the grain moves in one direction; (2) There must be no nucleation in front of the moving direction of the solid-liquid interface.
Because the fracture of the blade usually occurs in the high-temperature weak structure of the grain boundary, in order to eliminate the grain boundary, a solidification mold with a "grain selector" structure is used during the directional solidification process. The cross-sectional size of this structure is close to the grain size, so that only a single optimally grown grain enters the mold cavity of the casting, and then continues to grow in the form of a single crystal until the entire blade is composed of only one grain.
The crystal selector can be divided into two parts: the starting block and the spiral:
At the beginning of the DS process, the grains begin to nucleate at the bottom of the starting block. In the early stage of grain growth, the number is large, the size is small, and the orientation difference is large. The competitive growth behavior between the grains dominates, and the geometric blocking effect of the side wall is weak. At this time, the orientation optimization effect is obvious; when the height of the grains in the starting block increases, the number of grains decreases, the size increases, and the orientation is close. The competitive growth behavior between the grains decreases, and the geometric blocking effect of the side wall dominates, ensuring that the crystal direction can be continuously optimized, but the orientation optimization effect is weakened. By reducing the radius of the starting block and increasing the height of the starting block, the orientation of the grains entering the spiral section can be effectively optimized. However, increasing the length of the starting block will shorten the effective growth space of the casting, and give you a production cycle and preparation cost. Therefore, it is necessary to reasonably design the geometric structure of the substrate.
The main function of the spiral is to efficiently select single crystals, and the ability to optimize the grain orientation is weak. When the DS process is carried out in a spiral, the curved channel provides space for dendrite branch growth, and the secondary dendrites of the grains advance in the direction of the liquidus line. The grains have a strong lateral development trend, and the orientation of the grains is in a fluctuating state, with a weak optimization effect. Therefore, the selection of grains in the spiral mainly depends on the geometric restriction advantage, competitive growth advantage, and spatial expansion advantage of the grains in the spiral segment [7], rather than the growth advantage of the preferred orientation of the grains, which has a strong randomness [6]. Therefore, the main reason for the failure of crystal selection is that the spiral does not play the role of single crystal selection. By increasing the outer diameter of the spiral, reducing the pitch, the diameter of the spiral surface, and reducing the starting angle, the crystal selection effect can be significantly improved.
The preparation of hollow single crystal turbine blades requires more than a dozen steps (master alloy smelting, single crystal membrane shell preparation, complex configuration ceramic core preparation, melt casting, directional solidification, heat treatment, surface treatment, thermal barrier coating preparation, etc.). The complex process is prone to various defects, such as stray grains, freckles, small angle grain boundaries, streak crystals, orientation deviation, recrystallization, large angle grain boundaries, and crystal selection failure.
Horké novinky2024-12-31
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