After iron, aluminium is now the second most widely used metal in the world. This is because aluminium has a unique combination of attractive properties. Low weight, high strength, superior malleability, easy machining, excellent corrosion resistance and good thermal and electrical conductivity are amongst aluminium’s most important properties. Aluminium is also very easy to recycle.
Aluminium is light with a density one third that of steel, 2.700 kg/m3.
Aluminium alloys commonly have tensile strengths of between 70 and 700 MPa. The range for alloys used in extrusion is 150 – 300 MPa. Unlike most steel grades, aluminium does not become brittle at low temperatures. Instead, its strength increases. At high temperatures, aluminium’s strength decreases. At temperatures continuously above 100°C, strength is affected to the extent that the weakening must be taken into account.
Compared with other metals, aluminium has a relatively large coefficient of linear expansion. This has to be taken into account in some designs.
Aluminium is easily worked using most machining methods – milling, drilling, cutting, punching, bending, etc. Furthermore, the energy input during machining is low.
Aluminium’s superior malleability is essential for extrusion. With the metal either hot or cold, this property is also exploited in the rolling of strips and foils, as well as in bending and other forming operations.
Aluminium is an excellent conductor of heat and electricity. An aluminium conductor weighs approximately half as much as a copper conductor having the same conductivity.
Features facilitating easy jointing are often incorporated into profile design. Fusion welding, Friction Stir Welding, bonding and taping are also used for joining.
Aluminium is a good reflector of both visible light and radiated heat.
Tight aluminium boxes can effectively exclude or screen off electromagnetic radiation. The better the conductivity of a material, the better the shielding qualities.
Aluminium reacts with the oxygen in the air to form an extremely thin layer of oxide. Though it is only some hundredths of a (my)m thick (1 (my)m is one thousandth of a millimetre), this layer is dense and provides excellent corrosion protection. The layer is self-repairing if damaged.
Anodising increases the thickness of the oxide layer and thus improves the strength of the natural corrosion protection. Where aluminium is used outdoors, thicknesses of between 15 and 25 ¥ìm (depending on wear and risk of corrosion) are common.
Aluminium is extremely durable in neutral and slightly acid environments.
In environments characterised by high acidity or high basicity, corrosion is rapid.
Further details are given in Corrosion Resistance.
Aluminium is a non-magnetic (actually paramagnetic) material. To avoid interference of magnetic fields aluminium is often used in magnet X-ray devices.
After oxygen and silicon, aluminium is the most common element in the Earth’s crust. Aluminium compounds also occur naturally in our food.