Aircraft development trends are high speed, long service life, safety and reliability, low structural weight factor, low cost and low maintenance workload and integrated stealth. The aluminum industry must provide new materials for the replacement of aircraft, and it must be ahead of schedule. The current development of aerospace aluminum alloys is generally based on strength, stiffness, heat resistance, corrosion resistance, long service life, and low-cost manufacturing of forming technology.
1 strength and stiffness
At present, the main measures taken by material workers are: increasing the content of alloying elements, but increasing the content of alloying elements will bring a series of difficulties to the melting, casting, processing and forming of the alloy, and the corrosion resistance, fracture toughness and fatigue of the alloy. Performance, etc. may have adverse effects.
Microalloying alloys is considered to be one of the most effective measures, such as adding Zr and or Sc elements with a mass fraction of less than 0.5% to Al-Mg and Al-Zn-Mg-Cu alloys. Zr, Sc, Zn, Ag, or the like is added to the Al-Li alloy.
The heat treatment is mainly carried out by multi-stage, step-by-step solid solution and aging treatment, and the strength properties of the alloy are improved as much as possible under the premise of meeting the requirements of corrosion resistance.
The new manufacturing technology, spray deposition technology and powder metallurgy, is used to prepare alloy ingots. The latter process can obtain high strength alloys, but the cost is high, and it is difficult to prepare large components. The spray deposition method is considered to be a better technology for developing the next generation of high-strength aluminum alloys, and can produce large ingots at relatively low preparation costs. The content of w(Zn+Mg) of the 7xxx series alloy can exceed the 14% limit for the purpose of improving strength. At present, 7xxx series aviation aluminum alloys with tensile strength exceeding 700N/mm2 can be stably produced. Some research institutes in China have made such materials and obtained a number of patents.
2 heat resistance
The increase in flight speed will inevitably make the aircraft's aerodynamic heating problem and overload problem prominent. The F/A-22's stagnation point temperature during the M2.0 flight reaches 121 °C. When the aircraft is swooping, it can reach M2.5 when maneuvering. At that time, the stagnation temperature can exceed 150 °C, which is the extreme temperature that the aluminum alloy skin can withstand. At this temperature, the safety hazard is prominent, so the aluminum must be raised. The heat resistance of the alloy. Today's research work in this area is mainly focused on:
A small amount of Fe and Ni is added to the low alloyed 2xxx series alloy, such as the newly developed 2650 aluminum alloy, which is a heat resistant alloy that can be used to make the skin of a supersonic aircraft.
A special amount of special alloying elements such as Ag is added to the conventional 2xxx alloy, and the C415 aluminum alloy of the United States is an alloy developed by Alcoa.
The preparation of heat-resistant alloys by powder metallurgy and spray deposition has made great progress and may become the main process for preparing high-temperature aluminum alloys in the future. The 8019 aluminum alloy successfully prepared by these processes in the United States, registered in 1990, is a high-temperature aluminum alloy with w(Fe) = 7.3% to 9.3%; and a heat-resistant aluminum alloy of grade 8009 is also produced. China has also prepared a spray-deposited aluminum alloy that can be used at 350 °C.
3 reliability and long-term service
In order to have high reliability and longest service life, the material requires high fracture toughness, fatigue strength and corrosion resistance. The main measures taken are:
Improve the purity of the alloy, that is, reduce the content of Fe and Si, which are inherent impurities in aluminum. Now almost all aerospace aluminum alloys have corresponding high-purity alloys.
Strictly control the alloy composition, reduce the number of refractory phases and excess phase. For example, the composition range of 2524 aluminum alloy is much narrower than that of 2024 aluminum alloy. Their strength levels are the same, but the fracture toughness and fatigue performance of the former are better than those of the latter. Much higher.
Changing the current heat treatment system parameters and developing a new heat treatment process has become one of the research priorities. For example, T73, T74, T76 states and the recently developed tertiary aging T77 state have been developed to improve the corrosion resistance and fracture toughness of 7xxx aluminum alloys. The materials in these states can meet the requirements of the latest military and civil aircraft design in terms of strength, fracture toughness, corrosion resistance and fatigue performance.
In order to improve the corrosion resistance of aluminum alloys, in addition to the aluminum coating of the traditional 2xxx and 7xxx aluminum alloy sheets, the 6013 and 6056 aluminum alloys with high corrosion resistance have been successfully developed abroad, and the 7A33 alloy has been successfully developed in China; In addition to the aging treatment of 7xxx series aluminum alloys, the overaging treatment of aluminum alloys such as 6013 and 6056, such as T78 heat treatment technology, and the overage system and characteristics of some 2xxx series aluminum alloys, such as Russia's AK4-1, are now under study. The aluminum alloy is subjected to T2 treatment. It seems that multi-stage aging is an effective measure to improve the comprehensive performance of aluminum alloy corrosion resistance, strength, toughness and fatigue performance. In addition to the above methods, it is also possible to carry out surface protection by anodizing treatment, painting, dusting, filming and the like to improve the corrosion resistance of the aluminum alloy.
4 weight loss
Aluminum alloy has two meanings for aircraft weight loss: one is to reduce the absolute quality of aluminum alloy itself; the other is to increase the specific strength of aluminum alloy. Common weight loss measures:
Improve the specific strength of the alloy, develop high-strength alloys, such as 7055 aluminum alloy in the United States, B96 aluminum alloy in the Soviet Union, and some high-component aluminum alloys produced by spray deposition.
The development of Al-Li alloys has led to the development of a series of 2xxx series, 5xxx series, 8xxx series Al-Li alloys, which have been used in aerospace manufacturing. The structural part of the China Tiangong No. 1 resource cabin section replaces the traditional materials with Al-Li alloy material, and the weight loss of the cabin is more than 10%. The straight section of the C919 passenger aircraft fuselage is made of Al-Li alloy. Although the strength of the 1420 series Al-Li alloy developed by the Soviet Union is not high, its density is low, and it is widely used in the manufacture of bearing parts and non-load-bearing parts. A very low density Al-Li alloy such as Al905XL can be prepared by powder metallurgy. In the future, the development trend of Al-Li alloys is to increase their specific strength, reduce production costs, and strengthen waste recycling and utilization.
Honeycomb aluminum structure and foam aluminum components. Honeycomb aluminum structure has been successfully applied in aircraft manufacturing, is made of aluminum foil, and has high production cost. Currently, research work is being carried out to replace honeycomb structure with foamed aluminum.
5 New alloys adapted to new forming technologies
In the low-cost manufacturing technology of aircraft aluminum alloys, the main work currently carried out is:
Ageing and superplastic forming, the former is also known as creep deformation or creep ageing. It is a new forming technology developed to greatly reduce the manufacturing cost of aircraft parts. The United States is researching new aluminum alloys to adapt to this method. The upper and lower airfoil of the wing. The strength, corrosion resistance and heat resistance of the aluminum alloy wing formed by this method are all improved, but the damage tolerance of the alloy is decreased. Therefore, the creep-formed aluminum alloy has become a research topic. Superplastic forming of aluminum alloys has been widely used, but the disadvantage is that the forming rate is low, and it is also necessary to study a new superplastic aluminum alloy with high forming rate and various performances to fully meet the design requirements.
The connection of aluminum alloy components plays an important role in aircraft manufacturing, and riveting is still the main process. Generally speaking, the weldability of aluminum alloys is not satisfactory, and welding is not used much in the connection of aircraft parts and structures. The welding efficiency is much higher than the riveting, and the connection cost is low. Therefore, we should vigorously develop aluminum alloys with good solderability; adopt new welding processes such as friction stir welding (FSW).
Keywords: aviation aluminum alloy, aluminum alloy sheet