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Application and Development of Aeronautical Aluminum Alloy

March 13, 2020

Abstract: the development of aluminum alloy is closely related to the development of aviation industry.

Aluminum alloy has a series of advantages, such as low density, high strength, good corrosion resistance and formability, low cost and so on. It has a wide application prospect and irreplaceable position in aviation, aerospace, shipbuilding, nuclear industry and weapons industry. In aviation, aluminum alloy is the main material for aircraft body structure.

Although the density of aluminum alloy is low, but its strength is relatively high, close to or higher than high-quality steel, good plasticity, can be processed into a variety of profiles, with excellent electrical conductivity, thermal conductivity and corrosion resistance, widely used in industry, second only to steel.

According to the processing method, aluminum alloy can be divided into deformed aluminum alloy and cast aluminum alloy. Cast aluminum alloy, used as cast; deformed aluminum alloy, can withstand pressure processing, mechanical properties are higher than as-cast. Can be processed into various forms and specifications of aluminum alloy. Mainly used in the manufacture of aviation equipment, daily necessities, building doors and windows.

Aluminum alloy can be heat treated to obtain good mechanical properties, physical properties and corrosion resistance. According to the chemical composition, cast aluminum alloy can be divided into Al-Si alloy, Al-Cu alloy, Al-mg alloy and Al-Zn alloy.

The development of aluminum alloy is closely related to the development of aviation industry. The crank box of the aircraft engine made by A1-Cu-Mn was used on the aircraft built by the Wright brothers at the beginning of the 20th century. In 1906, A.Wilm discovered the phenomenon of age hardening in AI-Cu-Mg series alloy, which made it possible for aluminum alloy to be the main structural material of aircraft. Since then, aluminum alloy as the main structural material of aircraft body has stepped onto the stage of history.

Summary of the development of aluminum alloy can be divided into the following five stages, in different periods due to different needs, scientists have been studying the aviation aluminum alloy, step by step to make the aviation aluminum alloy really used in the development of aircraft.

Static strength demand stage.

The period of this stage is probably from the beginning of the 20th century to the mid-1950s, when the performance of aircraft is relatively simple and backward, and the demand for aircraft material performance only requires it to have high static strength. In this way, the structural weight of the aircraft can be reduced while the deadweight and range of the aircraft can be increased, and the performance of the aircraft can be improved from these aspects. From the point of view of our thinking today, this seems very simple and simple, but it is really the only way to do so in that kind of environment at that time. It is under the stimulation of this demand that scientists have developed 2014 and 2017 aluminum alloys.

2014 aluminum alloy belongs to both hard aluminum alloy and forged aluminum alloy because of its high copper content, so its strength is high and its thermal strength is good, but its plasticity in hot state is relatively poor. 2014 aluminum alloy has good machinability, good performance of contact welding, spot welding and roll welding, poor performance of arc welding and gas welding, and can be strengthened by heat treatment and has extrusion effect.

Then 2024-T3 alloy was developed. by the 1930s, 2024 alloy had been widely used in aircraft. During World War II, in order to obtain higher strength of body materials, a high strength A1-Zn-Mg-Cu alloy 7075 T6 was developed, and then a higher strength 7178-T6 alloy was developed. These alloys were widely used in civil and military aircraft before the 1960s.

Corrosion resistance requirement phase.

This demand phase began in the 1960s, when the aircraft began to use thick cross-section structures, which led to the problem of stress corrosion of the airframe mechanism. The new aluminum alloy material should not only meet the static strength requirements, but also meet the corrosion stress requirements of the body. Later, a 7XXX series aluminum alloy which can meet the requirement of corrosion stress strength was developed, but 15% of the static strength was sacrificed at the same time. In the 1960s, 7075-T73 and 7075-T76 alloys were widely used in aircraft.

Comprehensive performance requirements phase.

During the period from the late 1960s to the early 1970s, the development of aluminum alloy was affected by failure-safety design, and the requirement for high fracture toughness of materials was gradually increased. 7475 alloy was first developed in the United States. In order to further meet the requirements of structural strength and stress corrosion resistance of thick cross-section structure, Alcoa Company developed 705 small T74 alloy. In the late 1970s, aircraft design proposed that the structural materials of the airframe should have high fracture toughness and excellent fatigue resistance while having high strength.

Strong demand for weight loss and high reliability.

In the early 1980s, due to the energy crisis and in order to improve the combat effectiveness of military aircraft, strong weight reduction requirements were put forward for aircraft design. in addition, with the gradual rise of Qin alloy and resin matrix composites, aluminum alloy was faced with great challenges. all these have greatly stimulated the development of non-traditional aluminum alloys and tapping the potential of traditional aluminum alloys. Then it is more difficult for the development of aluminum alloy, in order to meet the needs of aircraft performance, we need to start from the aspect of specific strength, focusing on the study.

Alcoa first successfully developed the T77 heat treatment state of 7150 alloy, which achieved the goal of satisfying the corrosion property without sacrificing the alloy strength in aluminum alloy for the first time. Then Alcoa successfully developed ultra-high strength 7055-T77 alloy and 2524-T3 alloy with excellent fatigue properties. The emergence of these high-performance aluminum alloys has greatly promoted the application and development of aviation aluminum alloys.

The demand phase of reducing manufacturing costs.

From the early 1990s to the present, because of the end of the Cold War, both military and civilian aircraft need to reduce costs. The initial goal is to reduce the structural cost by 25% without changing the existing aircraft structural design. To achieve this goal, The main work carried out in western countries is as follows: structural components assembled by integral machined workpieces instead of forgings or multi-parts, high temperature aging forming of wings and rapid superplastic forming of high strength aluminum alloy, research on welding technology of weldable aluminum alloy and aluminum alloy, manufacture of complex components with high quality castings to reduce structural weight and manufacturing cost, Carry out the research of low-cost and high-performance aluminum file alloy and aluminum matrix composites.

Looking back, the development of aviation aluminum alloy in China obviously lags behind that of foreign countries. From 1950s to 1970s, we began to study the static strength of the structure; from the end of 1970s to the middle of 1980s, we began to study the corrosion resistance; from the end of 1980 to the middle of 1990s, we began to pursue the comprehensive properties of aluminum alloy and the gap between China and foreign countries. From the mid-1990s to the present, we have been studying high purity aluminum alloy.

There is a reason for backwardness. To sum up, the principles are as follows:

(1) start relatively late.

(2) the basis of material research, the lack of innovation in alloy development.

(3) the transition from research to application is slow.

(4) the material system of aviation aluminum alloy is incomplete and the material standard is not perfect.

However, we still have to develop in the future, and we can't stop at our backward state all the time. The development trend of aircraft is high speed, long life, safety and reliability, low structure weight coefficient, low cost and comprehensive stealth. With the increase of aircraft speed, the aerodynamic heating and overload problems of aircraft will become prominent. When flying at Mach 2.0, the standing point temperature of Fhand A-22 is 121 degrees Celsius, while when the aircraft is diving and maneuvering, the standing point temperature can reach M2.5, when the standing point temperature is more than 150 degrees Celsius. This temperature is the limit temperature for the aluminum alloy skin currently used, and there is a hidden danger of unsafety.

In the aspect of low-cost manufacturing technology of aluminum alloy, the main work carried out at present is as follows: firstly, aging forming and superplastic forming of aluminum alloy. Aging forming, also known as creep forming or creep aging forming, is a technology developed to greatly reduce the manufacturing cost of aircraft. Secondly, the welding performance of aluminum alloy is generally poor, so a large number of riveted structures are used in aircraft. Finally, new welding methods are developed, such as stir friction welding technology and so on.