Selection of new materials for modern aircraft mad

2022-08-02
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Aluminum lithium alloy: the choice of new materials for modern aircraft

aluminum lithium alloy material is an advanced lightweight structural material that has developed rapidly in aerospace materials in recent years. It has many excellent comprehensive properties, such as low density, high elastic modulus, high specific strength and stiffness, good fatigue performance, good corrosion resistance and welding performance. Using it to replace the conventional high-strength aluminum alloy can reduce the structural mass by 10% - 20% and increase the stiffness by 15% - 20%. Therefore, it shows a broad application prospect in the aerospace field

although Al Li alloys have shown broad application prospects in the aerospace field. However, due to its higher cost than ordinary aluminum alloy, poor room temperature plasticity, high yield strength ratio, obvious anisotropy and easy cracking during cold processing, it is difficult to form. At present, it can only form simple parts, and it is difficult to manufacture complex parts, which limits its application in structural parts. In recent years, the development and forming technology of Al Li alloys abroad is becoming more and more mature, which is not only widely used in military aircraft and spacecraft; In addition, the consumption of aluminum lithium alloy for civil aircraft is also increasing, such as the outer storage tank of space shuttle endeavour and Airbus a330/340/380 series aircraft. In China, due to the melting and casting process of Al Li alloy, the technology of sheet rolling and extrusion is not mature, and the development of new Al Li alloy is relatively backward. At present, it is only used in some types of spacecraft

1. Development status of advanced Al Li alloy

according to the historical process and composition characteristics of Al Li alloy development, it can be divided into three stages

the first stage is the preliminary development stage, and the time span of this stage is about 1950s to early 1960s. Its main representative is 2020 alloy successfully researched by Alcoa company of the United States in 1957. It was applied to the wing skin and tail horizontal stabilizer of the Navy ra-5c military early warning aircraft, and achieved a weight reduction effect of 6%. The former Soviet Union successfully developed baii23 alloy in the 1960s. However, these two alloys have low ductility, high notch sensitivity and difficult processing and production, which can not meet the requirements of aviation production and tensile, contraction, zigzag, low cycle and high cycle fatigue, crack expansion and fracture mechanics test under sine wave, 3-angle wave, square wave, trapezoidal wave, oblique wave and user-defined waveform, and have not been further applied

in the mid-1960s, under the pressure of the energy crisis, Al Li alloy was re valued and entered the second stage of rapid development. During this period, Al Li alloys were rapidly developed and comprehensively studied, including 1420 alloys developed by the former Soviet Union, 2090 alloys of Alcoa company in the United States, 8090 and 8091 alloys of Alcan company in the United Kingdom, etc. These alloys have the advantages of low density and high elastic modulus. They can be used to replace some 2XXX and 7xxx aluminum alloys in aerospace. For example, the former Soviet Union used 1420 alloy components on MiG-29, Su-35 and other fighter aircraft and some long-range missile warhead shells. Although the second generation Al Li alloy has made remarkable research and application achievements, it is difficult to compete with 7xxx aluminum alloy due to its severe anisotropy, low plastic toughness, serious heat exposure, toughness loss and non weldability of most alloys

in the late 1980s, the third generation of high-strength weldable Al Li alloys, represented by weldalite049 series alloys in the United States, were successively developed and successfully applied in aerospace and other fields. At present, the new third generation Al Li alloys are developing towards super strength, super toughness and ultra-low density, among which high-strength weldable alloys and low anisotropic alloys are more studied. In addition, Al Li metal matrix composites with isotropic properties and reinforced by SiC ceramic particles or whiskers have been developed, with an elastic modulus of 130gpa, making them a strong competitor to other composites in the aerospace field

2. Application and development trend of Al Li alloys in aerospace industry

according to statistics, every 1kg of structural weight can be reduced to obtain more than 10 times of economic benefits, so Al Li alloys with low density are widely valued by the aerospace industry. Al Li alloy has replaced the conventional high-strength aluminum alloy in many aerospace components. Among them, the application in the United States is developing very fast, especially in the aerospace industry. Lockheed Martin has used 8090 aluminum lithium alloy to manufacture the payload cabin of the "Hercules" launch vehicle, with a weight reduction of 182kg

1994, in order to solve the overweight problem of the outer storage tank of the space shuttle Endeavour, Lockheed Martin company and Reynolds metal company developed a new type of 2195 material to replace the previous 2219 alloy. The density of the alloy is 5% lighter than that of 2219 alloy, and its strength is 30% higher than that of 2219 alloy. The overall welded structure tank made of 2195 has reduced the weight by 3405kg, including 1907kg for liquid hydrogen tank and 736kg for liquid oxygen tank. The direct economic benefit is nearly 75million dollars. Therefore, it is called superlight weight tank. Russia has also been in a leading position in the research, production and application of aluminum lithium alloy. In order to improve the load capacity, the outer fuel tank of the space shuttle is made of aluminum lithium alloy, and the low-temperature tank of the "energy" carrier rocket is made of 1460 aluminum lithium alloy

in the aviation field, many advanced fighters and civil aircraft use aluminum lithium alloy. In 1988, Lockheed Martin Combat Aircraft Systems Co., Ltd., Aircraft Systems Co., Ltd. and Reynolds Metal Co., Ltd. jointly developed a plan to develop the application of 2197 alloy - to use its thick plate to manufacture fighter bulkhead deck. In 1996, the U.S. Air Force F-16 aircraft began to use this alloy plate to manufacture the rear deck and other parts

in addition to the United States, other countries, such as Russia, Britain and France, are actively promoting the application of Al Li alloy in Aerospace: 25% of the structural parts of Westland eh101 helicopter are made of 8090 alloy, and its total mass has decreased by about 15%; The third generation rafele fighter of France plans to use aluminum lithium alloy to make its structural frame; Russia has a large number of parts made of aluminum lithium alloy in yake-36, Su-27, SU-36, MiG-29, Mig-33 and other fighter planes

in terms of civil aircraft, Airbus Industries' A330, A340 and A380 aircrafts all use aluminum lithium alloy. Among them, about 3T of aluminum lithium alloy is used for fuselage structure, stringer and other components of each A330 and A340 aircraft. At present, the newer A350 aircraft uses the new 2198 aluminum lithium alloy on the fuselage skin for the first time on the original basis. The Boeing 747, 777 airliners and McDonnell Douglas Aircraft in the United States all use aluminum lithium alloy, and its application parts include fuel tank, spacer frame, wing skin, leading edge and trailing edge. Bombardier C series aircraft fuselage will also be made of brand-new aluminum lithium alloy

3. Advanced manufacturing technology and development trend of Al Li alloy

superplastic forming and diffusion bonding technology

superplastic forming and superplastic forming/diffusion bonding technology (SPF and spf/db) is a process of forming thin-walled parts with complex shape and difficult to machine by means of blow molding and bulging, which is a special forming method with almost no allowance, low cost and high efficiency. Like other superplastic materials, Al Li alloys can obtain uniform, fine and equiaxed grains by alloying or mechanical heat treatment to produce superplastic properties. SPF research on Al Li alloy began in 1980. At the Farnborough international aviation exhibition in 1982, British superplastic forming metal company demonstrated the Superplasticity of Al Li alloy and its superplastic f parts for the first time

American weldalite049 alloy has different superplasticity. It is treated by solid solution at 507 ℃ without back pressure. 4 × At strain rate, the elongation can reach 829%. This strain rate is obviously higher than that of other aluminum alloys, which is of great significance to solve the problem of low superplastic process speed. Russia has used SPF process to process many aircraft parts for 1420, some of which are 1200mm in size × 600mm。

domestic aerospace materials and Processes Research Institute, Beijing Aerospace Manufacturing Engineering Research Institute and other scientific research institutions have carried out a lot of pioneering work on the SPF and spf/db combined process for the output of Al Li alloy is about 25% higher than that of the traditional screw extruder, and achieved a lot of results. At present, the superplastic forming of Al Li alloy is developing from secondary load-bearing component to main load-bearing component, and from single superplastic forming to superplastic forming/diffusion connection combination process, which makes the processing cost of Al Li alloy lower, and the structure more integrated and light weight

spin forming

spin forming technology is an advanced process that integrates the characteristics of forging, extrusion, stretching, bending and other processes with little or no cutting. Shear spinning is a new spinning technology developed on the basis of traditional spinning technology in recent years. It does not change the outer diameter of the blank, but changes its thickness to realize the spinning method of manufacturing various axisymmetric thin-walled parts such as cone (taper thinning spinning). The characteristics of this forming method are that the force on the spinning wheel is small, the half cone angle and wall thickness affect each other, the material flow is smooth, the surface roughness is good and the forming precision is high, and the materials that are difficult to form can be easily formed, stretched and spun. Many Al Li alloy components on spacecraft are hollow revolving thin shell structures, which are especially suitable for processing by spinning. The typical part is the dome cover of the launch vehicle cryogenic tank

three weldalite049 plates with a diameter of 0.65M and a thickness of 10.7mm are used to spin the dome cover of the American "Hercules" launch vehicle. Among them, the middle of 175 light partition strip plate is welded by variable polarity plasma arc welding (VPPA), and the stress is removed after 343 ℃/4h. During spinning, all blanks are heated by flame to maintain 317 ℃; After forming, 505 ℃/0.5h solid solution treatment and water quenching; After artificial aging at 177 ℃/18h, the tensile strength at room temperature reached about 600MPa, and increased to 700MPa at -196 ℃, with good fracture toughness. The same spinning technology was used for the outer tank dome of the space shuttle Endeavour, and advanced shear spinning technology was used for the cylinder section of the outer tank

roll forging

al Li alloys, especially weldalite series alloys and 1420 alloys, have good forging properties, and die forgings made with them will not crack, which has been proved by more than 150 forgings. Therefore, its application in aerospace industry has broad prospects. Roll forging is a new near net shape forming technology developed in recent years. It is a plastic forming process that produces plastic deformation of materials under the action of a pair of reverse rotating dies to obtain the required forgings or forgings. The development of roll forging has two important fields

the first is to realize volume distribution and pre forming in the production of long shaft forgings, reduce forming load, form a precision roll precision forging compound production line, and produce complex forgings in large quantities with less investment. The second is precision roll forging technology, including cold precision roll technology. It has a good development prospect in the precision forming of sheet parts, such as blade forming and variable section leaf spring. In recent years, two directions of roll forging have been successfully applied to Al Li alloy ring forgings and sheet metal parts with ribs. Such as the "Y" shaped frame and docking ring of the outer storage tank of the "Endeavour" spacecraft

welding

welding is one of the main processes for manufacturing aluminum lithium alloy aerospace products such as storage tanks and warhead shells. The former Soviet Union studied the welding of 1420 alloy for more than 10 years,

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