Aluminum alloys are the most frequently encountered type of sheet metal in aircraft repair. AC 43.13-1 Chapter 4, Metal Structure, Welding, and Brazing; Section 1, Identification of Metals (as revised) provides an in-depth discussion of all metal types. This article describes the aluminum alloys commonly used in aircraft sheet metal forming and repair, along with their characteristics and typical applications.
In its pure state, aluminum is lightweight, lustrous, and corrosion resistant. The thermal conductivity of aluminum is very high. It is ductile, malleable, and nonmagnetic. When aluminum is combined with varying amounts of other metals, such as copper, manganese, and magnesium, it forms alloys used in aircraft construction.
Aluminum alloys are lightweight and strong. They do not possess the corrosion resistance of pure aluminum and are usually treated to prevent deterioration. Alclad™ aluminum is an aluminum alloy with a protective cladding of aluminum to improve its corrosion resistance.
To provide a visual means for identifying the various grades of aluminum and aluminum alloys, aluminum stock is usually marked with symbols such as a Government Specification Number, the temper or condition furnished, or the commercial code marking.
Plate and sheet are usually marked with specification numbers or code markings in rows approximately five inches apart. Tubes, bars, rods, and extruded shapes are marked with specification numbers or code markings at intervals of three to five feet along the length of each piece.
The commercial code marking consists of a number that identifies the particular composition of the alloy. Additionally, letter suffixes designate the basic temper designations and subdivisions of aluminum alloys.
Common Aluminum Alloys Used in Aircraft
Alloy 1100
Alloy 1100 is used where strength is not an important factor, but where weight economy and corrosion resistance are desired. This alloy is commonly used for fuel tanks, cowlings, and oil tanks. It is also used for repairing wingtips and tanks. Alloy 1100 is weldable.
Alloy 3003
Alloy 3003 is similar to 1100 and is generally used for the same purposes. It contains a small percentage of magnesium and is stronger and harder than Alloy 1100.
Alloy 2014
Alloy 2014 is used for heavy-duty forgings, plates, extrusions for aircraft fittings, wheels, and major structural components. It is often selected for applications requiring high strength and hardness, as well as service at elevated temperatures.
Alloy 2017
Alloy 2017 is primarily used for rivets. Today, its use is limited compared to newer alloys.
Alloy 2024
Alloy 2024, with or without an Alclad™ coating, is widely used for aircraft structures, rivets, hardware, machine screw products, and other structural applications. It is also commonly used for heat-treated parts, airfoil and fuselage skins, extrusions, and fittings.
Alloy 2025
Alloy 2025 is used extensively in the manufacture of propeller blades.
Alloy 2219
Alloy 2219 is commonly used for fuel tanks, aircraft skin, and structural components. It offers high fracture toughness, excellent weldability, and outstanding resistance to stress corrosion cracking.
Alloy 5052
Alloy 5052 is used where good workability, excellent corrosion resistance, high fatigue strength, weldability, and moderate static strength are required. It is commonly used for fuel, hydraulic, and oil lines.
Alloy 5056
Alloy 5056 is used for manufacturing rivets and cable sheathing, as well as in applications where aluminum comes into contact with magnesium alloys. It provides good resistance to the most common forms of corrosion.
Cast Aluminum Alloys
Cast aluminum alloys are commonly used for cylinder heads, crankcases, fuel injectors, carburetors, and landing wheels.
Heat-Treatable and Non-Heat-Treatable Alloys
Some aluminum alloys, including 1100, 3003, and 5052, are strengthened by cold working rather than heat treatment. Others, such as 2017 and 2024, can be strengthened by heat treatment, cold working, or a combination of both. Various cast aluminum alloys are also hardened by heat treatment.
Alloy 6061
Alloy 6061 is weldable by all common commercial welding processes and maintains good toughness in many cryogenic applications. It is easily extruded and is widely used for hydraulic and pneumatic tubing.
Alloy 7075
Although stronger than Alloy 2024, Alloy 7075 has lower fracture toughness and is generally used in tension applications where fatigue is not critical. The T6 temper should be avoided in corrosive environments because of its susceptibility to stress corrosion cracking. The T7351 temper provides improved stress corrosion resistance and better fracture toughness than T6, while the T76 temper offers improved resistance to exfoliation corrosion.