一.Application of superalloy in aero engine
Turbine engine workflow: When the engine is started, the air enters the compressor by the inlet, is pressurized and enters the combustion chamber, mixes with the fuel emitted by the fuel injection nozzle, forms a uniform mixture and is quickly ignited and burned in the combustion chamber, produces high-temperature gas flowing through the guide into the turbine, and the turbine rotates at a high speed (the normal speed can reach 1100r/min) under the high temperature and high pressure gas flow. The gas from the turbine is expelled from the tail nozzle to generate thrust. Due to vibration, airflow erosion, especially the centrifugal effect caused by rotation, the high temperature parts of the aircraft engine will be subjected to greater stress, gas contains a lot of oxygen, water vapor, and there are corrosive gases such as SO2,H2S, which will play a role in oxidation and corrosion of high temperature parts. Whether it is a military aircraft, civil aircraft, in addition to structural and functional performance, but also require safety and stability, so modern engines in addition to high thrust-to-weight ratio, high temperature, high pressure ratio and other performance, there are strict reliability, durability, maintainability requirements.
Superalloy has high thermal stability and thermal strength, and can have good corrosion resistance and oxidation resistance at high temperatures. It is an essential key material for the manufacture of hot end components of aviation turbine engines, mainly used in the manufacture of turbine hot end components, namely turbine disk, turbine guide blade, turbine working blade, combustion chamber and afterburner components. In modern advanced aircraft engines, the amount of superalloy materials accounts for 40%-60% of the total engine.
The combustion chamber is the highest working temperature area of the engine components, and when the gas temperature in the combustion chamber reaches 1500-2000 ° C, the temperature of the chamber wall alloy can reach 800 ~ 900 ° C, and the local temperature can reach 1100 ° C. The alloy used as the combustion chamber is subjected to thermal stress and gas impact force, especially during takeoff, acceleration and parking, and the temperature changes are more drastic. Due to cyclic heating and cooling, the combustion chamber often appears deformation, warping and thermal fatigue cracks at the edge.
In recent years, most of the superalloys used in the combustion chamber are solid solution strengthened alloys, which contain a large number of W,Mo,Nb and other solid solution strengthened elements, high temperature strength, good forming and welding performance. The representative brands are GH1140, GH3030, GH3039, GH3333, GH3018, GH3022, GH3044, GH3128, GH3170 and so on.
The guide blade is a component that adjusts the direction of gas flow from the combustion chamber, also known as the guide. It is one of the parts on the turbine engine that is subjected to large thermal impact. Especially when the combustion chamber is not uniform and the operation is not good, the guide blade is subjected to greater heat load, and the operating temperature of the guide blade of the advanced turbine engine can reach 1100℃. Distortion caused by thermal stress, thermal fatigue cracks caused by drastic temperature changes and local burns are the main defects of guide blades in operation.
Most of the alloys used as guide blades are produced by precision casting process, and more W, Mo,Nb,Al,Ti and other solid solution strengthening and aging strengthening elements can be added to the alloys, and the content of C and B in the alloys is also higher than that of deformed high-temperature alloys. Some guide blades are also welded directly from age-strengthened sheets. Advanced aero engines mostly use hollow cast blades, which have good cooling effect and can increase the service temperature. The use of domestic guide vane alloy temperature can reach 000 ~ 1050 ℃, representative K214 precision casting alloy, K233, K406, K417, K403, K409, K408, K423B, etc.
With the development of the engine, in order to meet the further increase of the engine turbine disc temperature, the structure of the guide blade has also changed, and GH5605 and GH5188 are tried to be adopted. The welded laminated structure of the deformed superalloy sheet is used as the guide blade.
Turbine blades are the most severe components in aero-engines with high working temperature and great centrifugal stress, vibration stress, thermal stress and airflow erosion force during rotation. The tensile stress of blade body is about 140MPa, and the average stress of blade root is 280-560MPa. The temperature of blade body and root part is about 650-980℃ and 760℃ respectively. The gas inlet temperature of the advanced aero engine has reached 1380℃ and the thrust has reached 226kN. Typical of GH4033, GH4037 GH4143, GH4049, GH4151, GH4118, GH4220 etc., can be used in 750-950 ℃. In the development of new machines and the modification of old machines, casting superalloy is used to manufacture turbine blades. Typical grades of casting alloys are K403,K417, K417G,K418, K403, K405, K4002 and so on.
The turbine disc accounts for the largest mass in the aero engine components, with the single mass of more than 50kg, and the single mass of the large turbine disc reaches hundreds of kilograms. In the turbine disc studio, the general rim temperature can reach 550-650 ° C, while the wheel center temperature is only about 300 ° C, and the temperature difference of the entire turbine disc is very large. Therefore, a large radial thermal stress is generated. The turbine blades rotate at high speed during normal rotation and bear great centrifugal force. The stress on the tenon tooth part is more complex, including tensile stress and torsion stress, which form a high stress and low cycle fatigue during starting and stopping.
Deformed superalloys for turbine disks, one type is iron-nickel based superalloys, typical alloy grades are GH2132, GH2135,GH2901,GH4761, etc., the operating temperature is below 650℃; Another kind of nickel-based superalloy, the typical brand GH4196, GH4133, GH4133B, GH4033A, GH4698 etc., using temperature can reach 700 ^ 800 ℃.
二.Application of superalloy in rocket engine
Carrier rocket is a vehicle to send a variety of spacecraft into space orbit, superalloy in the space field is mainly used in thrust carrier rocket engine. Figure 2 is a schematic diagram of the liquid fuel rocket engine and its structure, which transforms the reactants (propellants) in the propellant reservoir or vehicle into high-speed jets to generate thrust. As can be seen from Figure (b), the air flow at the nozzle of the rocket engine reaches 2500m/s and the temperature is as high as 1350℃.
Rocket engine superalloys can be used in principle with aviation turbine engine alloys, but compared with aviation engines, rocket engine materials have some new characteristics:
Nickel-based deformed superalloys usually add 10%-25% Cr element to ensure that the alloy has good oxidative corrosion resistance, so the nickel-based alloy is actually Ni-Cr as the matrix. In addition, some alloys add the elements Co(15%-20%),Mo (about 15%) or W (about 11%) in Ni-Cr solid solution to form a ternary system deformed superalloy with Ni-Cr-Co,Ni-Cr-Mo,Ni-Cr-W as the matrix, respectively. Table 6 shows the brands, chemical compositions and operating temperatures of nickel-based deformed superalloys commonly used in China. Figure 6 shows the development trend of superalloy application on turbine blades and platters.
The cobalt-based deformation superalloy is essentially based on the Co-Ni-Cr ternary system, and contains W,Mo,Nb,Ta and other solid solution strengthening elements and carbide forming elements. Compared with nickel-based deformed superalloys, the work hardening rate is larger, and the surface quality of the parts after forming is better, but in the forming process, more hot working heating times or cold deformation intermediate annealing times are generally required, and the tonnage of the processing forming equipment is also required. Cobalt-based deformed superalloys have high strength and excellent thermal fatigue resistance, thermal corrosion and wear resistance when they are higher than 980℃. However, cobalt-based deformed superalloys have carbide as the main strengthening phase and lack a homogeneous strengthening phase, and their durable strength is lower than that of nickel-based deformed superalloys in low and medium temperature ranges. Table 9 lists the high temperature mechanical properties of typical cobalt-based deformed superalloys.