Aluminum Alloys in Aviation
Metallurgy has played a key role in the development of aviation. With the discovery of new materials, new applications have been found to apply these too or to fastly improve existing designs.
Aluminum seems to be the king in aircraft construction, although in recent years some new alloys have been applied. These super alloys are still quite expensive for the aircraft homebuilder. With its good strength to weight and cost ratio, aluminum is still used very widely in the industry.
Current developments indicate that more and more manufacturers (Boeing, Airbus) are using carbon fiber and other non-metallic materials in aircraft construction. As time goes by these materials will have to prove themselves to be as reliable as aluminum.
In this part we talk about the aluminum alloys used in aircraft structures like wings, fuselage and engine baffles. A list is given with their properties and characteristics.
Before we start we need some background. 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 also rather malleable. Which is perfect for our applications in aviation.
In order to understand the properties of metals and in particular aluminum, we will discuss the terms used to describe them here:
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.
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).
The ability of a metal to be bent, formed or hammered without cracking or breaking. Normally the harder the metal is the less malleable it is.
Quite similar to malleability, except it primarily refers to the ability of a metal to be permanently deformed or by drawing or pulling it.
This is the opposite of ductibility and malleability. It is the characteristic which causes the metal to shatter when it is bent or deformed.
The change in size with a change in temperature. Metals usually expand when the temperature rises and for certain applications this can be critical. Think pistons diameters and such.
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.
At high temperatures (200-250°C) aluminum alloys tend to loose some of their strength. However, at subzero temperatures its strength increases while retaining their ductility, making aluminum an extremely useful low-temperature alloy, high flying commercial aircraft commonly fly at -50°C so they profit from this property.
It also has good electrical (ground for aircraft antenna's) 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 loose after repeated usage and this can lead to arcing and fire, which requires extra precaution and special design when using aluminum wiring in aircraft.
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 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.
Aluminum 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.