Aviation aluminum alloy has many excellent characteristics such as high toughness, corrosion resistance and fatigue resistance. Since the early 20th century, it has been widely used in the aviation field as the basic framework material of aircraft. The skin, beam, stringer and other equipment components on the aircraft are made of aluminum alloy materials. According to the differences of aircraft service performance and use, the amount and specification of aluminum alloy on the aircraft also exist
Quite different. For example, the aerospace aluminum alloy material of Boeing 767 aircraft accounts for about 80% of the total weight of the aircraft, while the aluminum alloy material consumption of military F-15 fighter aircraft is only about 35% to ensure flight speed and combat performance. Therefore, the quality of aluminum alloy material greatly affects the aircraft aviation performance.
Adding zinc, magnesium, copper, lithium, manganese and other metal elements into the simple aluminum through a series of processes to obtain aluminum alloy material, which has the excellent characteristics of low density, high toughness, corrosion resistance and so on, and is especially suitable for the aviation field. For example, Wright brothers used Al Cu alloy to manufacture the crank box of aircraft engine in the early 20th century. It can be said that the development of aluminum alloy materials
The exhibition is affected not only by the material synthesis technology, but also by the aircraft aviation performance. Under the interaction of the two, the development process of aluminum alloy materials is mainly divided into the following four stages.
1.1 high static strength aluminum alloy
In 1906, a.wilm discovered the age hardening phenomenon in Al Cu alloys, which made the aluminum alloy materials begin to be concerned by the aviation field. At the early stage of development in the aviation field, aircraft design only considers the static strength during flight, and the main body and parts are processed with aluminum alloy materials with high static strength, so as to reduce the dead weight of the aircraft and improve the load capacity and mileage of the aircraft. In 1932, Sande and Meissner
It is found that the age hardening effect of Al Zn Mg alloys after quenching artificial aging heat treatment is very significant, and the physical properties are much better than the traditional Al Cu Mg alloys. Based on this discovery, the aircraft skeleton materials in this period are usually Al Zn Mg alloys, and aerospace aluminum alloy materials such as 2014 and 2024-T3 have been developed successively. During World War II, in order to meet the service requirements of military aircraft, the United States and Germany began to focus on developing aluminum alloy materials with higher static strength. They discussed and analyzed Al Cu alloys and Al Zn Mg alloys, and conducted a large number of experiments. Finally, they developed Al Zn mg Cu 7075-T6 alloys with high static strength, and then developed 7178-t6 alloys with higher static strength on this basis. These alloys were widely used in civil and military aircraft in the 1940s and 1960s.
1.2 corrosion resistant aluminum alloy
In the early stage of flight design, the aircraft usually adopts the structure of thick and large cross-section, which significantly increases the tensile stress on the body. At the same time, the body operates in a corrosive environment, and the stress corrosion is very significant, which is very likely to lead to catastrophic accidents. For example, in 1985, a Boeing 747 passenger plane crashed due to stress corrosion fracture, resulting in more than 500 deaths. Aluminum alloy material design in this period
Instead of simply considering the static strength, the corrosion resistance should be considered additionally to reduce stress corrosion. Based on a large number of experimental data and theoretical analysis, 7075-T73 with stress corrosion resistance was successfully developed by aging heat treatment process on the basis of 7075-T6. Although this material sacrifices some static strength, its corrosion resistance has been significantly improved, and the safety performance and service life of the aircraft have been improved
Greatly improved. Later, 7075-t76 was further developed, which not only obtained stronger corrosion resistance, but also sacrificed lower static strength than 7075-T73, which improved the comprehensive performance of aluminum alloy materials and became a typical representative of corrosion-resistant aluminum alloys.
1.3 high strength and high toughness aluminum alloy is based on the requirements of aircraft failure safety design. Aircraft design should not only consider the static strength and corrosion stress of materials, but also put forward higher requirements for fracture toughness. Under the trend of this demand condition, the United States took the lead in using high-purity aluminum ingots to process and prepare aluminum alloy materials. During the processing, the content of Fe, Si and other impurities was strictly controlled, and the high-strength 7050 aluminum alloy was successfully developed. After these high-strength aluminum alloys were further processed by aging heat treatment technology, the aluminum alloy with high strength, high toughness and stress corrosion resistance could be obtained. The alloy has been successfully applied to the wing structure of Boeing 757 aircraft. 1.4 high comprehensive performance aluminum alloy since the 1980s to 1990s, based on the design safety, aircraft material design should not only consider the static strength, stress corrosion and toughness, but also include the residual strength after the main structure is damaged in the design requirements, which puts forward higher requirements for the comprehensive performance of aluminum alloy materials. Alcoa first carried out T77 heat treatment on 7150 alloy to obtain ultra-high strength 7150-t77 alloy. Under the condition of ensuring stress corrosion, its alloy strength has also been greatly improved. On this basis, Alcoa has successfully developed 7055-t77 aluminum alloy with high strength and 2524-t3 aluminum alloy with high damage resistance. Compared with 7150-t77, 7055-t77 has equivalent stress corrosion resistance, but its strength has been greatly improved, which is the highest strength among aluminum alloy materials at present. These comprehensive aluminum alloy materials take into account all aspects of aircraft design, promote the rapid development of the entire aerospace industry, and are applied to F-35, B777 and other advanced aircraft, which will be an important trend in the development of aviation aluminum alloy materials in the future.
Aviation aluminum alloy its processing is a tedious and refined process, involving many key control points, mainly including billet forming processing, heat treatment processing and integrated processing.
2.1 billet forming and processing as the aircraft has higher and higher requirements for the comprehensive properties of materials, high-quality billet processing has become a crucial step in the whole process. For high-quality billets, the content of impurities such as Fe, Si and Na shall be as low as possible, and the content of hydrogen and oxidation inclusions shall be strictly controlled to prevent porosity or loose structure in the billets. In order to meet this requirement, a series of melting and casting technologies have been developed, such as melt electromagnetic stirring (EMS) and ultrasonic casting. EMS uses the force generated by the electromagnetic field to stir the aluminum melt, which reduces the possibility of Fe content brought in by manual stirring and effectively prevents the damage of the oxide film on the melt surface. At the same time, the introduction of ultrasonic vibration in the ingot casting process can further refine the ingot grain and make it more uniform, so as to reduce the impurity content in the ingot casting process and improve the ingot processing quality. The content of alloying elements in high-strength aluminum alloy ingots is relatively large, and it is easy to form high supersaturation, non-equilibrium crystallization and low melting point phase in the ingot casting process. Therefore, it is necessary to homogenize it in the ingot casting process, and the two-stage homogenization process from low temperature to high temperature can be used to ensure the uniformity of precipitation of dispersed phase particles. For example, due to uneven particle distribution of Zr element in 7050 Alloy after solidification, in the process of single-stage homogenization, crystal nucleus cannot be formed in the region with relatively low Zr element content (as shown in Figure 1). The two-stage homogenization process can better improve the crystallization effect of 7050 Alloy and make the ingot more uniform by dissolving the second phase of Zr at high temperature and precipitation at low temperature
2.2 heat treatment and processing for aviation aluminum alloy materials, people only considered its maximum static strength at first. However, it was found that stress corrosion limited the aircraft service performance during the actual flight process. Therefore, the material had to improve the stress corrosion resistance on the basis of sacrificing a certain static strength, so they began to heat treat the aluminum alloy materials, and successfully developed 7075-T73 alloy with stress corrosion resistance. Heat treatment processing mainly includes three processes: solid solution, quenching and aging. 2.21 solid solution solid solution is an indispensable heat treatment process in aluminum alloy processing. The purpose is to add other alloy elements to aluminum matrix to form alloy materials with special properties. The temperature and time of solid solution are two key factors to determine the properties of the alloy, which have a very significant effect on the aging precipitation after quenching. When the solid solution temperature and time of the alloy increase, the solute and vacancy concentration after quenching can be effectively increased, making the aging precipitation easier to precipitate, so as to improve the strength and hardness of the alloy. It is worth mentioning that the higher the solution temperature is, the better. Excessive temperature is easy to cause local overburning and alloy quality defects. Therefore, the solution temperature must be strictly controlled in the actual operation process. For 7xxx series alloys, temperature classification is usually used to achieve the effect of full solid solution. Graded solid solution can make the alloy elements completely dissolve in the matrix to a large extent, increase the concentration of solute atoms, and delay the recrystallization trend of alloy elements. In the subsequent quenching operation, the depth of the hardenable layer is greatly improved, thus making the toughness and strength of the alloy higher. In addition, the effect of solid solution time on the processing quality of the alloy can not be underestimated. Too short time will lead to the alloy elements not fully dissolved in the matrix, and the solute concentration will be reduced, which will seriously affect the properties of the aluminum alloy; However, if the time is too long, the degree of crystallization of solute atoms in the alloy will increase, and the grain growth will be too large, which is not conducive to the homogenization and growth of aluminum alloy. In serious cases, it may also cause the alloy to be oxidized and bubbles, and greatly reduce the alloy properties.
2.2.2 quenching is a crucial step in the aluminum alloy processing process. After solid solution, the alloy is transferred to the quenching tank for quenching. The selection of quenching process is usually based on the time temperature performance curve (TTP) of the alloy. The quenching sensitive temperature range of 7xxx series alloy is 200 ℃ ~ 420 ℃, and the nose tip temperature is 300 ℃ ~ 350 ℃. Therefore, during the quenching operation, it is necessary to quickly pass through the sensitive temperature area to prevent the alloy from warping and deformation during the quenching process. Before quenching, the higher the solution temperature is, the lower the yield strength of the alloy will be. At this time, once the alloy is subjected to thermal stress, it is very easy to produce plastic deformation. Therefore, in the process of quenching operation, appropriate quenching medium and quenching rate are usually selected to control it, so as to obtain high supersaturation solid solution, and reduce the thermal stress as much as possible, so as to make the plate surface more uniform and greatly improve the overall quality.
2.2.3 aging is the last heat treatment process in the alloy processing process, and it is also an important step affecting the performance of aluminum alloy. Aluminum alloy can not be strengthened immediately after quenching, but a supersaturated solid solution mixture is formed. This mixture is unstable and prone to desolvation, which greatly improves the hardness and strength of aluminum alloy. This heat treatment process is called aging. Aging of aluminum alloy is a very complex phase change process, and there are many theories about its phase transformation mechanism, such as the theory of dispersed phase hardening, the theory of enriched hardening zone and the theory of slip interference. At present, the theory of enrichment hardening zone is generally accepted. This theory holds that during the rapid cooling time of supersaturated solid solution mixture, a large number of "vacancies" appear in the alloy crystal, and these vacancies accelerate the diffusion rate of solute atoms, making the atoms form enrichment in the grain. With the continuous increase of solid solution temperature, the enrichment degree of atoms becomes higher and higher within a certain time, and the atoms are automatically arranged in a certain order, Thus ensuring the hardness and strength of the aluminum alloy.
From the perspective of the development history of aluminum alloy, the development of
aviation aluminum alloy is not only affected by material technology, but also constrained by aircraft service performance. Therefore, in the future, aluminum alloy must develop in the direction of improving comprehensive performance to meet the material requirements of aviation industry in an all-round way.
