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