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Aerospace materials are all kinds of materials used in aircraft and their power units, accessories and instruments, which are one of the decisive factors in the development of aerospace engineering and technology, and aerospace materials science is also a pioneering branch of materials science.
Aerospace materials have excellent resistance to high and low temperatures as well as aging and corrosion resistance, and can adapt to the space environment.
The various types of materials used in aircraft and their power units, accessories, and instruments are one of the decisive factors in the development of aerospace engineering technology. Aerospace materials science is a pioneering branch of materials science. The design of aircraft constantly put forward new topics to materials science, and promotes the development of aerospace materials science; the emergence of a variety of new materials provides new possibilities for the design of aircraft and greatly promotes the development of aerospace technology.
The progress of aerospace materials depends on the following three factors:
In addition to high stress and inertial forces, aerospace materials are subjected to shock loads and alternating loads caused by factors such as takeoff and landing, engine vibration, high-speed rotation of rotating parts, maneuvering flight, and sudden winds. Engine gas and solar irradiation cause the aircraft to be in a high temperature environment, and as the flight speed increases, the aerodynamic heating effect comes to the fore, resulting in “thermal barriers”. In addition, also subject to alternating temperatures, in the stratosphere at subsonic speeds, the surface temperature will drop to about -50 ℃, the polar circle within the territory of the severe winter environment temperature will be below -40 ℃, metal components or rubber tires prone to embrittlement phenomenon. Gasoline, kerosene and other fuels and various lubricants, hydraulic oil, most of the metal materials to produce corrosion, non-metallic materials to produce swelling, and solar irradiation, wind and rain erosion, underground humid environment for long-term storage of mold will accelerate the aging process of polymer materials.
Aerospace vehicles operate in the atmosphere or outer space for a long time, and serve in extreme environments but also have extremely high reliability and safety, excellent flight and maneuverability, in addition to optimizing the structure to meet the aerodynamic needs, processability requirements and use of maintenance requirements, but also depends on the excellent characteristics and functions of the material.
For reducing the mass of the structure, a 30% reduction in density is more useful than a 50% increase in strength. Aluminum alloys, titanium alloys, and composite materials are the main aerospace structural materials with high specific strength and stiffness, which can improve the payload, maneuverability, and range of the vehicle, while reducing the cost of flight.
Ultra-high strength steel (yield strength >1380 MPa) will not be used in more than 10% of aerospace engineering. For modern aircraft such as supersonic fighters, the amount of ultra-high strength steel is stabilized at 5% to 10%, and its tensile strength is 600 to 1850MPa, sometimes as high as 1950MPa, with fracture toughness KIc = 78 to 91MPa-m1/2. In the active corrosive media used in the airframe load-bearing structural parts, generally to use high-strength corrosion-resistant steel, equipped with hydrogen fuel engines to choose the aircraft Carbonless corrosion-resistant steel as a component material for service in liquid hydrogen and hydrogen medium.
Metal matrix composites, high temperature resin matrix composites, ceramic matrix composites, and carbon/carbon composites have been playing an increasingly important role in the aerospace field. Carbon/carbon composites combine the refractoriness of carbon with the high strength and rigidity of carbon fiber, have superior thermal stability and excellent thermal conductivity, and still have considerable strength and toughness at 2500°C, and the density is only 1/4 of that of high-temperature alloys. hybrid composites have received increasing attention, such as the addition of glass fiber to carbon fiber composites can improve their impact properties, while The addition of carbon fiber to glass fiber reinforced plastic can increase its stiffness.
Most of the structural materials of the manned spacecraft sections are aluminum alloy, titanium alloy, composite materials, such as the space shuttle orbiter mostly made of aluminum alloy, supporting the main engine thrust structure made of Chin alloy, part of the main frame of the middle fuselage using boron fiber reinforced aluminum alloy metal matrix composite materials, cargo bay doors using special paper honeycomb sandwich structure to graphite fiber reinforced epoxy resin composite material for the panel. Missile head, spacecraft re-entry module external surface and rocket engine internal surface, to use ablative materials, under the action of heat flow, ablative materials can be decomposition, melting, evaporation, sublimation, erosion and other physical and chemical changes, the mass consumption of the material surface to take away a lot of heat, in order to achieve the purpose of preventing the re-entry of the atmosphere when the heat flow to the vehicle interior, cooling rocket engine combustion chamber and nozzle. In order to maintain a suitable working temperature inside the cabin, the reman cabin section to take radiation heat protection measures, the outer skin for high temperature resistant nickel-based alloy or beryllium plate, the internal structure of heat-resistant Chin alloy, the outer skin and the internal structure filled with quartz fiber, glass fiber composite ceramics and other materials with good thermal insulation properties.
With the implementation and continuous development of human spaceflight, lunar exploration and deep space exploration, high-resolution satellites, hypervelocity vehicles, reusable launch vehicles, space maneuvering vehicles and other space projects, new and more demanding requirements for materials are put forward, providing new opportunities and impetus for the development of new materials for spaceflight, and the field of materials must be as early as possible in the innovation of material systems, independent protection of key raw materials and engineering applications. The material field must make a major breakthrough in material system innovation, independent guarantee of key raw materials and engineering applications as early as possible.
In addition, the application of laminated composite materials in aerospace engineering is becoming more and more widespread, such as the A380 uses 3% GLARE, a new type of laminate. Laminate is a composite material that makes two different kinds of materials laminated together by pressure, usually composed of upper panel, upper glue layer, core material, lower glue layer, lower panel, whose strength and stiffness is higher than that of separate panel material or core material, and has been applied to transport aircraft and fighter jets. GLARE laminate is made by pressure (or hot press tank) to laminate multiple layers of thin aluminum plates and unidirectional glass fiber prepreg (impregnated with epoxy adhesive) laminated and hot-pressed, as shown in Figure 1. The aluminum sheet is properly pre-treated to make it easier to adhere to the fiber prepreg layer. Table 1 shows the types of GLARE laminates that can be produced commercially, which can be made in different thicknesses according to needs. The fibers can be 2, 3, 4 layers, etc., and the fiber content and orientation can be in accordance with the table, and each type of GLARE laminate can have different forms, which can be adjusted according to specific needs.
Aerospace materials are not only the material guarantee for the development and production of aerospace products but also the technical basis for promoting the renewal of aerospace products. From the nature of the materials themselves, aerospace materials are divided into four categories: metallic materials, inorganic non-metallic materials, polymer materials and advanced composite materials; according to the use of function, they can be divided into structural materials and functional materials 2 categories. For structural materials, the most critical requirements are light weight and high strength and high temperature corrosion resistance; functional materials include microelectronic and optoelectronic materials, sensor sensitive element materials? Functional ceramic materials, optical fiber materials, information display and storage materials, stealth materials, and smart materials.
For aerospace materials, it includes 3 major categories of materials, aircraft body materials, engine materials, and airborne equipment materials. And aerospace materials include launch vehicle arrow body materials, rocket engine materials, spacecraft materials, and aerospace functional materials.
Specifically at the level of materials, aerospace materials cover a wide range, including aluminum alloys, titanium alloys, magnesium alloys, and other light alloys, ultra-high-strength steels, high-temperature titanium alloys, nickel-based high-temperature alloys, intermetallic compounds (titanium-aluminum system, niobium-aluminum system, molybdenum-silicon system). Refractory metals and their alloys and other high-temperature metal structure materials, glass fiber, carbon fiber, aromatic amide fiber, aromatic heterocyclic fiber, ultra-high molecular weight polyethylene fiber and other composite reinforcement materials, epoxy resin, bismaleimide resin, thermosetting polyimide resin, phenolic resin, cyanate resin, polyaryl acetylene resin, and other composite matrix materials, advanced metal-based and inorganic non-metal-based composites, the advanced intermetallic compound-based composites, advanced ceramic materials, advanced carbon/carbon composites, and advanced functional materials.
The prediction of the composition ratio of aircraft fuselage structural materials shows that the dominant material in the early 21st century is aluminum alloy. The first issue to be solved when developing aluminum alloys for aerospace technology is how to reduce structural mass while ensuring high operational reliability and good workmanship. The urgent problem to be solved is the development of high-strength aluminum alloys with good welding properties and their use in the manufacture of monolithic welded structures. The way to increase the payload of the vehicle is to increase the strength or reduce the density (without reducing the strength).
Alloying aluminum with lithium reduces the alloy density and increases the modulus of elasticity. Aluminum-lithium (Al – Li) alloy sheets, including thin sheets less than 0.5 mm thick, have been produced by strip coil rolling.
The use of aluminum base layer composites can significantly improve the reliability, service life and payload of aircraft skins, which are characterized by exceptionally low crack expansion rates (1/20 ~ 1/10 of conventional materials), high strength (50% to 100% increase) and fracture toughness, and low density (10% to 15% reduction). Cracked rivet material is very promising.
In modern aircraft structures, steel usage is stable at 5% to 10%, while in some aircraft, such as supersonic fighters, steel is a purpose-specific material.
High-strength steels are typically used in structural components requiring high stiffness, high specific strength, and high fatigue life, as well as good medium-temperature strength, corrosion resistance, and a range of other parameters. Steel is an irreplaceable material, both in the production of semi-finished products and in the manufacture of complex structural parts, especially in the production of welded structural parts where welding is the final process.
For a long time, the most used steel in the aircraft manufacturing industry is the medium-alloyed high-strength steel with strength level of 1600 ~ 1850MPa and fracture toughness of about 77.5 ~ 91MPa/m2. At present, in maintaining the same fracture toughness index, the minimum strength level of steel has been increased to 1950MPa, but also developed a new economic alloying of high crack resistance, high strength welded structural steel.
The development direction of high-strength steel is to further improve the metallurgical production process, select the best chemical composition and heat treatment specifications to develop a strength performance level of 2100 ~ 2200MPa of high-reliability structural steel.
In the role of active corrosive media used in the body bearing structural parts, especially in all-weather conditions on the use of bearing structural parts, the widespread use of high-strength corrosion-resistant steel, the strength level of this steel is similar to the alloy structural steel, reliability parameters (fracture toughness, corrosion cracking strength, etc.) greatly exceeded the alloy structural steel.
The advantages of high-strength steel are: different welding methods can be used to implement welding, welding load-bearing structural parts, after welding without heat treatment, either in the hot state, or in the cold state, have a good punchability, etc..
The most promising application of high-strength steel materials, is the martensitic type of low carbon dispersion strengthening corrosion-resistant steel and transition type of austenitic – martensitic steel, research shows that in maintaining high reliability and good workmanship conditions, is able to significantly increase the level of strength of high-strength corrosion-resistant steel.
Low-temperature technology and equipment is a special field of application of high-strength corrosion-resistant steel and the direction of development, equipped with hydrogen fuel engines for aircraft with good prospects for development, should be in the liquid hydrogen and hydrogen medium to work in the carbon-free corrosion-resistant steel as the direction of research.
The potential to improve the proportion of titanium alloy in the fuselage parts is quite huge, according to forecasts, the proportion of titanium alloy in passenger aircraft fuselage will reach 20%, while the proportion of military aircraft fuselage applications will increase to 50%? The premise is to ensure that.
Titanium alloy has higher strength and reliability; further improve the use of temperature; with high process performance and good weldability; can produce a variety of semi-finished products; improve the form of the structure, develop new design solutions, as much as possible in the structure of the use of mature alloys and processes.
The use of high-strength titanium alloys can reduce the mass of the structure, while improving the weight efficiency, reliability and processability of the structure. We plan to develop a plate alloy with both high strength (1350 MPa) and high processability, which will be four times stronger than industrial pure iron and have process characteristics similar to industrial pure titanium; we will also develop and use a “near-alpha” thermally strong titanium alloy with higher thermal strength, thermal stability and service life.
One of the development directions of titanium alloys is to develop and use high thermal strength, especially with high stability and long life of the “near-alpha” thermal strength titanium alloy. The 6th generation aero-engine will use solid solution reinforced and intermetallic compound reinforced titanium alloy plates.
Titanium-aluminum compound-based alloy is the future direction of research, “γ” alloy at 700 ~ 900 ℃ temperature specific thermal strength than steel and thermal strength alloy, but plasticity is poor.
The new direction of developing thermally strong titanium alloys is the use of intermetallic compounds reinforced with β solid solution-based alloys. This alloy is characterized by high thermal strength and satisfactory plasticity properties at temperatures of 600 ~ 700°C. Compared with existing titanium alloys, the development of this type of titanium alloy can result in a 25% to 30% increase in strength and thermal strength.
The emphasis needs to be on optimizing the alloy chemistry, casting and deformation processes. The selection of optimal heat treatment specifications and the adoption of new methods of designing parts will allow the use of intermetallic compounds in the structure of aero-engines and aerospace technology equipment, where the increase in service temperature and the reduction of mass are decisive factors.
An important symbol representing the level of aerospace technology development is the number of polymer composites used, polymer composites have very obvious superiority in terms of specific strength and specific stiffness, combined with good structural properties and special properties, in the aviation field has been widely used? Airbus A3XX aircraft will use polymer composites up to 25%.
The use of polymer composites with carbon fiber reinforced plastics as the matrix is one of the effective measures to reduce the structural mass. Polymer composites usually refer to high elastic modulus carbon fiber reinforced plastics, characterized by high stiffness (elastic modulus 196 GPa) and high temperature dimensional stability, while also maintaining a high compressive strength (1000 MPa). The use of carbon fiber reinforced plastics in the new generation of aerospace technology equipment can improve the aerodynamic stiffness of the tail components, especially the tail tip components, reduce the structural mass, and ensure the required flight technology quality. These properties of high modulus carbon fiber reinforced plastics, combined with low density, allow for the manufacture of manipulators for assembly and maintenance of space stations. Problems to be solved in the next few years include: further improvement of structural properties and special properties of carbon fiber reinforced plastics, especially to increase the operating temperature to 400 ℃.
As a structural material, new composite materials – organic plastics will play an increasingly important role. In recent years, the 2nd generation of organic plastics are being developed, the σb (tensile strength) value of single-purpose organic plastics reached 3000 ~ 3200MPa, E value increased to 130G Pa. Experimental research shows that it is possible to obtain the elastic modulus of 200 ~ 250GPa organic plastics, it should be noted that this is actually to expand the working temperature range by a factor of 1 (205 ~ 300 ℃) It is also possible to significantly reduce the water absorption of the composite. In terms of specific strength and modulus of elasticity, modern organic plastics, and especially those of the future, will exceed all known polymer? metal and ceramic matrix composites.
Currently, glass fiber reinforced plastics and carbon fiber reinforced plastic structural parts manufactured by the prepreg process are increasingly being used. When this process is used, parts with normal and complex curvature can be made in a single process. Compared with conventional polymer composites, prepreg-based composites are characterized by a 40% to 50% increase in crack resistance. Shear strength is increased by 20% to 50%? 20% to 35% increase in fatigue strength and lasting strength? With this composite material, the labor and energy consumption can be reduced by 1/2; the structural mass (especially in the case of honeycomb filler) can be reduced by 50%, and the structural sealability can be improved by 5 times.
The development of special alloys with the best alloying and the best organization method can significantly improve the performance of single crystal blade, one of the most promising alloys is the zinc alloying of hot strength nickel alloy.
Nickel-containing alloys have higher operating temperatures and higher endurance strength properties, and record endurance strength values of σ1000100> 300 MPa have been obtained for test alloys containing 6% to 7%, thus ensuring the development of single crystal blades with cooling channels for 6th generation engines. By using nickel-containing alloy, the turbine inlet temperature can be increased to 2000 ~ 2100K, the cooling air consumption can be reduced by 30% ~ 50%, and the blade service life can be extended by 1 ~ 3 times when the cooling air consumption is the same.
The requirements for gas turbine engine disk materials are slightly different from the requirements for blade materials: First, the working temperature of the turbine disk is lower than that of the blade; second, the requirements for material reliability are increased. The above requirements for improving the performance of turbine disc alloys should be solved by a comprehensive approach, such as developing the principle of alloying, improving the strengthening mechanism, and developing new technological methods for melting, deformation and heat treatment.
The special issue facing the aero-engine manufacturing industry today is to develop, for example, welded magazines? fire tube and a range of other hot-field welded structural components. The main problem in developing fire tube materials is to improve their structural stiffness, the solution to this problem also needs to meet a series of demanding and comprehensive process requirements: good weldability, high process plasticity, etc.. The use of the above-mentioned alloys can increase the working temperature of the fire tube by 150 ~ 200℃, extend the reliability and service life by 50% ~ 100%, and substantially increase the specific strength of the welded cassette, while reducing the mass by 15%.
The use of antioxidant protective coating is an important factor in extending the service life of heat-strengthening alloys (firstly, turbine blades).
At present, new processes and coatings with various complex compositions have been introduced as an alternative technology to the process of producing protective coatings by diffusion aluminizing in mixed powders. Researchers have developed a new method of plasma vacuum coating with different elemental ions as a matrix. In the case of approximately the same coating thickness (50 ~ 70μm), the original sprayed alloy with alloying can effectively protect the blade from sulfides. oxide corrosion, and can extend blade life by an order of magnitude compared to mass-produced aluminized coatings.
In the new method of coating multi-component materials with high-energy vacuum plasma process, the action of high-speed plasma flow on the solid surface results in purposeful strengthening of the composition, organization, micro-geometry, and physicochemical properties of the treated surface. The main advantages of the process are: high coating quality, dense and non-porous, good plasticity, strong adhesion (more than 100M P a); good versatility, all types of protective coatings can be applied on an industrial device; high deposition accuracy.
The cost of coating, coating equipment and process is low, using multi-component materials high energy vacuum plasma process to apply coatings, can obtain a variety of coatings, both diffusion coatings, cohesive coatings and cohesive – diffusion coatings?
The urgency to further improve the operating temperature and part life of gas turbine engines has put a demand on the search for a new alloy matrix with better stability than the nickel matrix phase reinforced solid solution. The new alloy matrix is suitable to use Ni3Al type intermetallic compound, and the covalent bonding of intermetallic compound can solve the problem of thermal strength of the alloy more effectively than the ordinary metal bonding of solid solution? The level of thermal strength of these alloys can be adjusted according to the supplementary alloying of the Ni3Al matrix, and the casting organization determined by the casting process. In this case, the thermal strength of the alloy increases as the transition is made from equiaxed to columnar and then to single-crystal organization.
Single-crystal intermetallic alloys have a better overall performance. At the same level of thermal strength (temperature 1100°C), the amount of rare and precious refractory metals such as tungsten (W) and molybdenum (M o) contained in the intermetallic compound alloy is significantly lower.
Intermetallic compound-based alloys can be effectively used to manufacture cooled and uncooled nozzle guide vanes, fire tubes and nozzle parts in the operating temperature range of 900 ~ 1150°C. The latest scientific achievements of the city can increase the thermal strength of the alloy to more than 50 ~ 70 MPa.
Further breakthroughs in the field of thermal strength materials research (working temperature increased to more than 1300 ℃) depend on metal composites to ensure. The matrix of metal composites can be made of different materials, such as titanium, intermetallic compounds, etc., while the reinforcing materials can be made of filamentary crystals, diffuse refractory compound particles including silicon carbide particles, oxide fibers or tungsten fibers.
Special composites are the so-called natural composites, which are made according to a directional crystallization process of eutectic alloys. Each eutectic phase in such alloys grows perpendicular to the crystallization line, so that a fibrous organization with a certain orientation can be obtained by moving the plane crystallization line. The reinforcing agent of this material is a continuous skeleton of filamentary crystals of refractory metal carbide (TaC, NbC) single crystals stirred together with each other. The developed natural composite material can maintain high lasting strength levels (σ1200b> 70 MPa) at high temperatures of 1200°C. It is predicted that the share of composite materials in advanced gas turbine engines will increase significantly (up to 40%).
Various media and atmospheric environments on the role of materials manifested as corrosion and aging. Aerospace materials in contact with the media are the aircraft fuel (such as gasoline, and kerosene), rocket propellants (such as concentrated nitric acid, nitrous oxide, hydrazine), and a variety of lubricants, hydraulic fluid, etc., most of which have a strong corrosive effect or swelling of metal and non-metallic materials, the sun’s irradiation in the atmosphere, the erosion of wind and rain, underground moisture in the long-term storage of mold will accelerate the aging of polymer materials. Process, corrosion resistance, anti-aging performance, and anti-mold performance is aerospace materials that should have good characteristics.
The role of space environment on materials is mainly manifested in the high vacuum (1.33×10-10Pa) and the influence of cosmic ray irradiation. Metal materials in high vacuum contact with each other, because the surface is purified by the high vacuum environment and accelerate the molecular diffusion process, “cold welding” phenomenon; non-metallic materials in high vacuum and cosmic ray irradiation will accelerate volatilization and aging, sometimes this phenomenon will make the optical lens due to volatile deposition and contamination, sealing structure due to aging and Failure. Space materials are generally selected and developed through ground simulation tests in order to adapt to the space environment.
In order to reduce the structural mass of the vehicle, it is considered the goal of the vehicle design to select the smallest possible safety margin and achieve an absolutely reliable safety life. For vehicles that are used once in a short time, such as missiles or launch vehicles, people strive to maximize the material performance to the limit. In order to make full use of material strength and ensure safety, the “damage tolerance design principle” has been used for metallic materials, which requires not only high specific strength but also high fracture toughness. Under simulated conditions of use, the crack initiation life and crack expansion rate of the material are measured, and the allowable crack length and corresponding life are calculated as an important basis for design, production and use. For organic non-metallic materials, natural aging and artificial accelerated aging tests are required to determine the insurance period of their life, the breakage pattern of composite materials, life and safety is also an important research topic.
The progress of aerospace materials depends on the following 3 factors, and only after all 3 have been developed to a mature stage is it possible to apply them to flying vehicles. Therefore, countries all over the world have given priority to the development of aerospace materials.
With the rapid development of the space industry, spacecraft structural materials will also be in long-term continuous development. The proportion of new light alloys used in spacecraft structures is gradually increasing, and the application of composite materials has contributed to the change of materials used in spacecraft structures and is in rapid development. Metal matrix composites, which combine the excellent properties of metals and inorganic/organic materials, have also entered the vision of aerospace structure researchers. In addition, structural materials are inseparable from structural design, and some traditional complex structures are being replaced by new multifunctional structures (MFC) and 3D-printed structures. In the future, the structural materials for spacecraft will show a trend of diversification and high performance.
Traditional light alloys still dominate, new light alloys will be gradually applied in order to adapt to the requirements of modern satellite high-performance, light-structure, alloy materials have the tendency to be gradually replaced by composite materials. Especially when the composite materials in the automotive and aviation field have been a great success, it also began to leap in the aerospace field with higher requirements for material lightness. However, with the deepening of research, it is found that the commonly used resin-based composites have some inherent defects, such as poor toughness, poor secondary processing performance, poor heat and moisture resistance, poor adaptability to the space environment, etc. It is difficult to be applied in a large area on spacecraft in a short time, which provides space and opportunities for the application and development of alloy materials in the aerospace field.
The development momentum of composite materials is good, and the application scope will continue to increase. The development time of composite materials is relatively short, but its rapid development trend is enough to make people believe that it has great application prospects. Composites have always been ahead of aerospace applications in the aviation field. Its application in aircraft has been developed from secondary structural materials to main structural materials. The amount of composite materials for structural parts of large aircraft in the world, such as Boeing 787 and Airbus 380, accounts for 40% to 50%, and the amount of composite materials for structural parts of advanced helicopters even accounts for more than 80%. Boeing and Airbus public research data show that by 2020 their aircraft structural parts will all use composite materials. By analogy, composite materials in the aerospace field will have huge development space and prospects. This is also evidenced by the fact that the aluminum alloy truss joints previously widely used on satellite trusses are being replaced by carbon fiber composite joints. Figure 2 shows a typical carbon fiber composite joint. Up to now, carbon fiber high performance composites are still the focus of composite research and application. In order to narrow the gap with the international advanced level, China now pays great attention to the advance research of composite materials. With the development of low-cost integrated manufacturing technology, the increasing maturity of automated, large-scale and high-precision manufacturing equipment, and the continuous improvement of the performance of matrix resin and carbon fiber, the resistance to moisture and heat and the elongation at break of carbon fiber reinforced resin matrix composites have been significantly improved, and the amount of composite materials on spacecraft structures will certainly increase further.
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