ALUMINUM ALLOYS USED IN AVIATION

Although the use of composite materials in aircraft has increased in recent years, aluminum is still used at a rate of 50–60%. Aluminum is extensively employed in the fuselage, general structural components, and in compartments where high strength and corrosion resistance are required.

Aluminum, a ductile metal, is located in group 3A of the periodic table and has an atomic number of 13. Aluminum, an element with a +3 valence and an atomic weight of 26.89, has a density of 2.7 g/cm³ at 20 °C, a melting point of 659.8 °C, a boiling point of 2450 °C, a specific heat of 0.224 Cal/g (at 100 °C), a heat of fusion of 400 Cal/g, electrical conductivity at 20 °C of 65% that of copper, thermal conductivity 0.5, and light reflectivity of 90%, and these properties can be greatly changed by adding alloying elements. It is usually found in nature as bauxite ore (Al₂O₃) and has superior resistance to oxidation. With the development of technology and due to its technical properties, aluminum, which falls into the category of light metals and is one of the youngest members of the global metal world, is widely used in many fields of industry. The strength-to-weight ratio (specific strength) of aluminum is very high, it is soft, and it weighs one-third of steel. As a result of alloying it with alloying elements and increasing its mechanical properties to be comparable to steel, aluminum alloys have expanded their areas of use. Due to their low density and high mechanical properties, their use is rapidly increasing in many important production areas such as medicine, construction, food, automotive, aviation, aerospace, and defense industry. .

Aluminum alloys are numbered using a 4-digit system. The first digit symbolizes the main alloying element. The second digit gives information about the modification number of the alloy. If the second digit is 0, this means there is no modification. The last two digits indicate the alloy designer. In industrial applications, the mechanical properties of aluminum alloys are also increased by heat treatment processes, and their areas of use are expanded. Heat treatment generally covers heating and cooling processes applied to metallic materials to change their mechanical properties. For this purpose, heat treatments such as annealing, solution treatment, and aging are applied to aluminum alloys. .

The 1XXX, 3XXX, 4XXX, and 5XXX series of wrought aluminum alloys are non-heat-treatable alloys. These alloys can only be hardened through shaping. The 2XXX, 6XXX, 7XXX, and 8XXX series alloys, on the other hand, can be hardened by heat treatment. Alloys created by combining aluminum with different materials are used in a wide range in the main structure of aircraft and many of its parts. There are many reasons why aluminum is preferred in aircraft structural parts. The most important reasons for the preference are aluminum’s lightness, corrosion resistance in the atmosphere, thermal and electrical conductivity, and ease of production. Since aluminum is physically soft in its pure form, it does not have sufficient strength for aircraft construction. Commercially obtained aluminum gains some hardness and strength through mechanical processing via the remaining foreign substances. These simple alloys are suitable for the production of secondary aircraft elements, but for the production of primary elements, which carry loads and whose fracture could endanger the aircraft, aluminum alloys with higher strength are used. .



Use of Aluminum in Aviation.

Since the 1915s, aluminum has started to be used instead of heavier steel in the aviation sector. During this period, copper-alloyed 2XXX series aluminum was used as a structural material in wings and fuselages. Aluminum use became widespread during and after the Second World War. During these dates, the 2014–2017–2024 alloys were used, while the need for high tensile strength paved the way for the use of 7000 series aluminum. Currently, 2XXX, 5XXX, 6XXX, and 7XXX series aluminum are used. In addition, 3XXX and 4XXX series aluminum are also used in a limited manner. Lithium-containing 8XXX series aluminum, which provides advantages in terms of lightness and elasticity, is currently used only in military aircraft/helicopters and spacecraft due to its very high price, and cannot be used in civilian passenger/cargo aircraft. Aluminum standards used in the aviation sector are recognized by AMS (Aerospace Materials Specifications). .

Although the use of composite materials in aircraft has increased in recent years, aluminum is still used at a rate of 50–60%. Aluminum is heavily used in the aircraft fuselage, general structural elements, and compartments where durability and high corrosion resistance are required. In military aircraft, the aluminum usage rate rises to 75–80%. Aluminum is used in aircraft wings, flaps, landing gears, main fuselage frames and stringers, hydraulic systems, fuselage, connecting rivets, and engine exhaust sections. .

Aluminum Alloys Used in Aircraft Wings.

Wings are evaluated in two separate structures. Since the upper wing is subjected to load, 7075-T6 / T651 alloy is used. In some aircraft, 7050-T7451, produced in aluminum sheet form, is also used. The lower wing is subjected to bending and tensile loads, and is made from 2024-T3 / T351 material, which has high fatigue strength and high damage tolerance due to its flexibility. However, recently, 2324-T39 (Boeing) and 2124-T3/T351 (Airbus) alloys, which provide more advantage in fatigue and damage tolerance, have started to be used. .

Aluminum Alloys Used in Aircraft Fuselage.

Main fuselage structure frames and stringers: 7050-T7451 and 7050-T7651 (AMS 4201) .

Fuselage attachment stringers: 7075-T6 / T651.

Fuselage skin: 2024-T3 / T351 Alclad (clad on both sides) .

Load-bearing areas: 7075-T6 / T651 plates and sheets.

Aluminum Alloys Used in Wheels.

In this section, to withstand the strength and flexibility encountered during landing in the main load-bearing part of the aircraft landing gear, 7075-T6 / T651, 7050-T7451, and 2024-T3 / T351 are used, while alloys with higher strength and durability such as 2014-T4 / T351 and 2017-T4 are used as connecting elements. .



Aluminum Alloys Used in Other Areas.

5052-H32 / H34 Plate: Used in applications where high fatigue and corrosion resistance and good formability are required. Most intensively used in fuel tanks. .

5052-T0 Drawn Seamless Pipe and Rod: Used in fuel and oil pipes and their connecting parts. .

6061-T4 / T6 Plate/Sheet: Used in regions and systems requiring high strength, good machinability, weldability, and corrosion resistance. Most commonly used in aircraft ladders, service systems, electronic device boxes. .

6061-T0 / T4 / T4511 / T6 / T6511 Extrusion: Used in various connecting parts as different extrusions (profiles). Used in fuselage panels, edge coverings of emergency exit locations in wings, and engine cowlings. Recently, 6013-T6 and 6063-T6 materials are also used instead of 6061 due to better formability and higher corrosion resistance. .

Use of Aluminum in Joining Aircraft Parts.

If permanent and airtight connection against air is desired, welding should be used. In this process, 4XXX series aluminum is used. In other joining methods, mechanical joining (rivets, pins, threaded fasteners, collar components, special connecting elements) is used. The materials used in mechanical joining are manufactured from 5056-H34 / H32, 2017-T3/T351, 2117-T3/T351, 2024-T351 / T3.