High temperature alloy is also called heat strength alloy. According to the matrix structure, materials can be divided into three categories: iron-based nickel-based and chromium-based. According to production mode, it can be divided into deformed superalloy and cast superalloy.
It is an indispensable raw material in the aerospace field. It is the key material for the high-temperature part of aerospace and aviation manufacturing engines. It is mainly used for manufacturing combustion chamber, turbine blade, guide blade, compressor and turbine disk, turbine case and other parts. The service temperature range is 600 ℃ - 1200 ℃. The stress and environmental conditions vary with the parts used. There are strict requirements for the mechanical, physical and chemical properties of the alloy. It is the decisive factor for the performance, reliability and life of the engine. Therefore, superalloy is one of the key research projects in the fields of aerospace and national defense in developed countries.
The main applications of superalloys are:
1. High temperature alloy for combustion chamber
The combustion chamber (also known as flame tube) of aviation turbine engine is one of the key high-temperature components. Since fuel atomization, oil and gas mixing and other processes are carried out in the combustion chamber, the maximum temperature in the combustion chamber can reach 1500 ℃ - 2000 ℃, and the wall temperature in the combustion chamber can reach 1100 ℃. At the same time, it also bears thermal stress and gas stress. Most engines with high thrust/weight ratio use annular combustion chambers, which have short length and high heat capacity. The maximum temperature in the combustion chamber reaches 2000 ℃, and the wall temperature reaches 1150 ℃ after gas film or steam cooling. Large temperature gradients between various parts will generate thermal stress, which will rise and fall sharply when the working state changes. The material will be subject to thermal shock and thermal fatigue load, and there will be distortion, cracks and other faults. Generally, the combustion chamber is made of sheet alloy, and the technical requirements are summarized as follows according to the service conditions of specific parts: it has certain oxidation resistance and gas corrosion resistance under the conditions of using high-temperature alloy and gas; It has certain instantaneous and endurance strength, thermal fatigue performance and low expansion coefficient; It has enough plasticity and weld ability to ensure processing, forming and connection; It has good organizational stability under thermal cycle to ensure reliable operation within the service life.
a. MA956 alloy porous laminate
In the early stage, the porous laminate was made of HS-188 alloy sheet by diffusion bonding after being photographed, etched, grooved and punched. The inner layer can be made into an ideal cooling channel according to the design requirements. This structure cooling only needs 30% of the cooling gas of the traditional film cooling, which can improve the thermal cycle efficiency of the engine, reduce the actual heat bearing capacity of the combustion chamber material, reduce the weight, and increase the thrust-weight ratio. At present, it is still necessary to break through the key technology before it can be put into practical use. The porous laminate made of MA956 is a new generation of combustion chamber material introduced by the United States, which can be used at 1300 ℃.
b. Application of ceramic composites in combustion chamber
The United States has begun to verify the feasibility of using ceramics for gas turbines since 1971. In 1983, some groups engaged in the development of advanced materials in the United States have formulated a series of performance indicators for gas turbines used in advanced aircraft. These indicators are: increase the turbine inlet temperature to 2200 ℃; Operate under the combustion state of chemical calculation; Reduce the density applied to these parts from 8g/cm3 to 5g/cm3; Cancel cooling of components. In order to meet these requirements, the materials studied include graphite, metal matrix, ceramic matrix composites and intermetallic compounds in addition to single-phase ceramics. Ceramic matrix composites (CMC) have the following advantages:
The expansion coefficient of ceramic material is much smaller than that of nickel-based alloy, and the coating is easy to peel off. Making ceramic composites with intermediate metal felt can overcome the defect of flaking, which is the development direction of combustion chamber materials. This material can be used with 10% - 20% cooling air, and the temperature of metal back insulation is only about 800 ℃, and the heat bearing temperature is far lower than that of divergent cooling and film cooling. Cast superalloy B1900+ceramic coating protective tile is used in V2500 engine, and the development direction is to replace B1900 (with ceramic coating) tile with SiC-based composite or anti-oxidation C/C composite. Ceramic matrix composite is the development material of engine combustion chamber with a thrust weight ratio of 15-20, and its service temperature is 1538 ℃ - 1650 ℃. It is used for flame tube, floating wall and afterburner.
2. High temperature alloy for turbine
Aero-engine turbine blade is one of the components that bear the most severe temperature load and the worst working environment in the aero-engine. It has to bear very large and complex stress under high temperature, so its material requirements are very strict. The superalloys for aero-engine turbine blades are divided into:
a.High temperature alloy for guide
The deflector is one of the parts of the turbine engine that are most impacted by heat. When uneven combustion occurs in the combustion chamber, the heating load of the first stage guide vane is large, which is the main reason for the damage of the guide vane. Its service temperature is about 100 ℃ higher than that of the turbine blade. The difference is that the static parts are not subject to mechanical load. Usually, it is easy to cause thermal stress, distortion, thermal fatigue crack and local burn caused by rapid temperature change. The guide vane alloy shall have the following properties: sufficient high temperature strength, permanent creep performance and good thermal fatigue performance, high oxidation resistance and thermal corrosion performance, thermal stress and vibration resistance, bending deformation ability, good casting process molding performance and weldability, and coating protection performance.
At present, most advanced engines with high thrust/weight ratio use hollow cast blades, and directional and single crystal nickel-based superalloys are selected. The engine with high thrust-weight ratio has a high temperature of 1650 ℃ - 1930 ℃ and needs to be protected by thermal insulation coating. The service temperature of the blade alloy under cooling and coating protection conditions is more than 1100 ℃, which puts forward new and higher requirements for the temperature density cost of the guide blade material in the future.
b. Superalloys for turbine blades
Turbine blades are the key heat-bearing rotating parts of aero-engines. Their operating temperature is 50 ℃ - 100 ℃ lower than the guide blades. They bear great centrifugal stress, vibration stress, thermal stress, airflow scouring and other effects when rotating, and the working conditions are poor. The service life of the hot end components of the engine with high thrust/weight ratio is more than 2000h. Therefore, the turbine blade alloy shall have high creep resistance and rupture strength at service temperature, good high and medium temperature comprehensive properties, such as high and low cycle fatigue, cold and hot fatigue, sufficient plasticity and impact toughness, and notch sensitivity; High oxidation resistance and corrosion resistance; Good thermal conductivity and low coefficient of linear expansion; Good casting process performance; Long-term structural stability, no TCP phase precipitation at service temperature. The applied alloy goes through four stages; Deformed alloy applications include GH4033, GH4143, GH4118, etc; The application of casting alloy includes K403, K417, K418, K405, directionally solidified gold DZ4, DZ22, single crystal alloy DD3, DD8, PW1484, etc. At present, it has developed to the third generation of single crystal alloys. China's single crystal alloy DD3 and DD8 are respectively used in China's turbines, turbofan engines, helicopters and shipborne engines.
3. High temperature alloy for turbine disk
The turbine disk is the most stressed rotating bearing part of the turbine engine. The working temperature of the wheel flange of the engine with the thrust weight ratio of 8 and 10 reaches 650 ℃ and 750 ℃, and the temperature of the wheel center is about 300 ℃, with a large temperature difference. During normal rotation, it drives the blade to rotate at high speed and bears the maximum centrifugal force, thermal stress and vibration stress. Each start and stop is a cycle, wheel center. The throat, groove bottom and rim all bear different composite stresses. The alloy is required to have the highest yield strength, impact toughness and no notch sensitivity at the service temperature; Low linear expansion coefficient; Certain oxidation and corrosion resistance; Good cutting performance.
4. Aerospace superalloy
The superalloy in the liquid rocket engine is used as the fuel injector panel of the combustion chamber in the thrust chamber; Turbine pump elbow, flange, graphite rudder fastener, etc. High temperature alloy in liquid rocket engine is used as fuel chamber injector panel in thrust chamber; Turbine pump elbow, flange, graphite rudder fastener, etc. GH4169 is used as the material of turbine rotor, shaft, shaft sleeve, fastener and other important bearing parts.
The turbine rotor materials of American liquid rocket engine mainly include intake pipe, turbine blade and disk. GH1131 alloy is mostly used in China, and the turbine blade depends on the working temperature. Inconel x, Alloy713c, Astroloy and Mar-M246 should be used successively; The wheel disc materials include Inconel 718, Waspaloy, etc. GH4169 and GH4141 integral turbines are mostly used, and GH2038A is used for the engine shaft.