Pure aluminum and pure magnesium are completely unsuitable as structural materials for airframes, because they have very low strength. However, when alloyed (chemically mixed) with each other or with other metals, their strength is vastly improved, and they form the most widely used group of airframe materials. Alloying metals include zinc, copper, manganese, silicon and lithium, and may be used singly or in combination.
There are very many different variations, each having different properties and so suited to different uses. Magnesium alloys are very prone to attack by sea water, and their use in carrier-based aircraft is generally avoided. Aluminum alloys, although denser than magnesium alloys, are much less prone to chemical attack, and are cheaper, so are more widely used. 2024 alloy, known as duralumin, consists of 93.5 percent aluminum, 4.4 percent copper, 1.5 percent manganese and 0.6 per cent magnesium, and is the most widely used of all materials in aircraft structures. Aluminum alloys are more prone to corrosion than pure aluminum, so pure aluminum is often rolled onto the surfaces of its alloys to form a protective layer. The process is known as cladding, and sheets of alloy treated like this are known as clad sheets or Al-clad. Another common means of protecting aluminum alloys is anodising – conversion of the surface layer to a form which is more corrosion-resistant by an electro-chemical process. Aluminum-lithium alloys are superior to aluminum-zinc and aluminum-copper alloys in strength and stiffness, so can be used to save weight. Their use is limited because they are around three times as expensive.
An interesting property which certain aluminum alloys share with titanium is that they can be super-plastically formed (SPF). When the material is heated to a certain temperature, far below its melting point, it is capable of being stretched by several times its own length without tearing or local thinning. It can then be deformed, using an inert gas such as argon, to fill a mould and take its shape exactly, with no spring-back when the pressure is released. There are various techniques based on this property, which can be used to make extremely complicated shapes accurately and with minimum weight. The high initial cost of tooling means SPF is limited to certain high-cost items, and it is not yet suited to mass production. Items such as pressure vessels, small tanks and reservoirs may be made using this technique.
Advantages of aluminum and magnesium alloys
1. High strength-to-weight ratios
2. A wide range of different alloys, to suit a range of different uses
3. Low density, so greater bulk for same weight means they can be used in a greater thickness than denser materials, and thus are less prone to local buckling; this applies to magnesium alloys even more than aluminum alloys
4. Available in many standard forms – sheet, plate, tube, bar, extrusions
5. Aluminum alloys are easy to work after simple heat treatment
6. Can be super-plastically formed (certain aluminum alloys only)
1. Prone to corrosion, so need protective finishes, particularly magnesium alloys
2. Many alloys have limited strength, especially at elevated temperatures
3. Magnesium alloys have low strength (but high strength-to-weight ratio)
4. No fatigue limit (see section on fatigue later in this chapter)