Metallurgy has a key role in the development of aviation. With the discovery of new materials, new applications have been found to apply these to. In this part eight we talk about aluminum alloys used in aircraft structures like wings, fuselage and engine baffles. A list is given with the common alloys.

Aluminum seems to be the king in aircraft construction, though in recent years some new alloys have been applied. These super alloys are quite expensive. With its good strength to weight and cost ratio, aluminum is still is used very widely in the industry.

General definitions

In order to understand the properties of metals and in particular aluminum, we discuss the terms used to describe them here:

Strength
The ability for a metal to withstand structural stress without failing. For most metals, strength is thought of in terms of tensile strength and is expressed in thousands of pounds per square inch or KPSI.

Hardness
The ability of a metal to resist cutting, penetration or abrasion. A metal may be softened by annealing and hardened by quenching type heat treatments or by working (shaped by pounding or bending).

Malleability
The ability of a metal to be bent, formed or hammered without cracking or breaking. Normally the harder the metal the less malleable.

Ductibility
Similar to malleability, except it primarily refers to the ability of a metal to be permanently deformed or by drawing or pulling it.

Brittleness
Opposite of ductibility and malleability. It is the characteristic which causes the metal to shatter when it is bent or deformed.

Thermal Expansion
The change in size with a change in temperature.

Aluminum

Aluminum is the third most abundant element in the Earth's crust after oxygen and silicon. It appears as a silverish white metal that has a strong resistance to corrosion and like gold, it is rather malleable. A relatively light metal compared to steel, nickel, brass, and copper with a specific gravity of 2.7. Aluminum is easily machinable and can have a wide variety of surface finishes.

Characteristics
At high temperatures (200-250°C) aluminum alloys tend to loose some of their strength. However, at subzero temperatures strength increases while retaining their ductility, making aluminum an extremely useful low-temperature alloy, high flying aircraft commonly fly at -50°C so they profit from this property.

It also has good electrical (ground for aircraft antenna) and thermal conductivities and is highly reflective to heat and light. Copper is the more widely used conductor (copper bus bars and wiring among other things), having a conductivity of approximately 160% that of aluminum. Aluminum connectors have a tendency to become loosened after repeated usage and this can lead to arcing and fire, which requires extra precaution and special design when using aluminum wiring in buildings.

Aluminum alloys also have a good strong resistance to corrosion which is a result of an oxide skin that forms as a result of reactions with the atmosphere. These reactions occur very quickly, usually within 30 minutes. This corrosive skin protects aluminum from most chemicals, weathering conditions, and even many acids, however alkaline substances are known to penetrate the protective skin and corrode the metal.

It is a very versatile metal and can be cast in any form. It can be rolled, stamped, drawn, spun, roll-formed, hammered and forged. The metal can be extruded into a variety of shapes and can be turned, milled, and bored in the machining process. Aluminum can riveted, welded, brazed, or resin bonded (aluminum/composite aircraft are a good example). For most applications, aluminum needs no protective coating as it can be finished to look good, however it is often anodized to improve color and strength.

Aluminum Alloys

Aluminum alloys are identified by a four digit numbering system. The first digit gives the alloy group and the others the alloys present. Below a list of the most commonly used aircraft aluminum alloys and their respective properties.

Heat treatment
The use of Copper and Zinc in an alloy with aluminum changes its properties in such a way that the alloy can be hardened as well as softened by the use of heat. The resulting temper designation is indicated by a letter number combination following the alloy numbers.

Temper designation
Most common temper designations found are T3 and T6. T3 is solution heat-treated and cold-worked by the flattening process. T6 is solution heat-treated and artificially aged.

Composition
Every alloys has its own properties depending on the composition of the alloying elements, for example 2024 alloy has 4.5% Copper, 0.6% Manganese and 1.5% Magnesium added. 6061 has 0.25% Copper, 0.6% Silicon, 1.0% Magnesium and 0.25% Chromium added and 5052 is 2.5% Magnesium and 0.25% Chromium. The 3003 alloy has only 1.2% Manganese added.

Mechanical properties
The mechanical properties of an alloy are an important factor in the decision where to use it. In the list below we give some typical properties:

2024-T3
This is the most common of the the high-strength aluminum alloys. It is aircraft quality. 2024-T3 aluminum sheet is thought of as the aircraft alloy because of it's strength. It has excellent fatigue resistance. Welding is generally not recommended. Typical uses for 2024-T3 Alclad aluminum sheet are aircraft skins, cowls, aircraft structures, and also for repair and restoration because of it's shiny finish (2024-T3 Alclad).
Ultimate strength is 62000 PSI with a shearing strength of 40000 PSI.

6061-T6
This alloy has a very good corrosion resistance and finishing ability, welding goes good too. The strength level of 6061-T6 aluminum sheet is about that of mild steel. 6061-T6 aluminum sheet can be fabricated by most of the commonly used techniques. Typical uses are aircraft landing mats, truck bodies and frames, structural components and more.
Ultimate strength is 45000 PSI with a shearing strength of 30000 PSI.

5052-H32
This one has the highest strength in the NON-heatable alloys. It is not structural. 5052 aluminum sheet has higher fatigue strength than most alloys. 5052 aluminum sheet has excellent corrosion resistance, particularly in marine applications and has excellent workability. 5052 aluminum sheet is commonly used to make fuel tanks.

3003-H14
Most widely used of aluminum alloys, pure aluminum with manganese added for strength, approx 20% stronger than the 100 series. 3003-H14 aluminum sheet has great workability and may be deep drawn, spun, welded or brazed. 3003 aluminum sheet is NON-heat treatable. This aluminum sheet is widely used for cowls and baffle plating.
Ultimate strength is 21000 PSI with a shearing strength of 14000 PSI.

7075
Aircraft manufacturers use high-strength alloys (principally alloy 7075) to strengthen aluminum aircraft structures. Aluminum alloy 7075 has Copper (1.6%), Magnesium (2.5%) and zinc (5.6%) added for ultimate strength, but the copper content makes it very difficult to weld. On the other hand it anodizes really beautifully. 7075 has the best machinability and it will result in a very nice finish.
Ultimate strength is 33000 PSI (-0) and 82000 PSI (-T6) with a shearing strength of 22000 PSI (-0) and 49000 PSI (-T6).