Aluminium is the third most abundant element in the earth’s crust which contains 8% aluminium. It is a constituent of most rocks and in the form of aluminium silicate it is an important source of clays. Commercially, the most important source of the metal is bauxite, which contains 52% Al203, 27.5% Fe203 and 20.5% H20.
Bauxite is treated with caustic soda and calcined at 1200°C to produce high purity alumina. The alumina is then smelted in an electrolytic cell to produce pure aluminium.
It should be pointed out that availability of wrought material is restrained in Australia due to the lack of demand and the number of alloys produced is limited to the more popular grades. Grades other than those produced locally may be imported from a variety of overseas sources. The price of aluminium is dictated by a supply and demand situation and based on prices established by the London Metal Exchange.
Properties of Aluminium That Make It An Attractive Engineering Material
Light weight is perhaps aluminium’s best known characteristic and with a density of 2.7 x 103 kg/m3 is approximately 35% that of steel. This feature together with other characteristics such as corrosion resistance and tensile strength has led to it replacing steel in many automotive applications in a demand for improved fuel efficiency.
Commercially pure aluminium has a tensile strength of approximately 90MPa and can be improved to around 180MPa by cold working. The heat treatable grades can develop a tensile strength of around 570MPa and even higher in some alloys (7001). This figure compares favourably with mild steel which has a tensile strength of approximately 260MPa.
Reference to Australian Standards AS2848 and AS1874 shows the various mechanical properties which can be delivered with the cast and wrought alloys.
It is interesting to note that aluminium alloys increase in strength without toss of ductility or brittle failure problems at sub zero temperatures and it is particularly useful for low temperature applications including cryogenics.
Aluminium has a good resistance to corrosion. This is attributable to a thin oxide film which forms and protects the metal from further oxidation: unless exposed to some substance or condition which destroys this protective coating the metal remains protected from corrosion.
Aluminium is highly resistant to weathering, even in industrial atmospheres which often corrode other metals. Additionally, it is corrosion resistant to attack by some acids. General direct contact with alkaline substances should be avoided as these attack the oxide skin and are therefore corrosive to aluminium. Some alloys, particularly the high strength alloys, are less resistant to corrosion than others. Such alloys can be further protected by a variety of surface treatments or by cladding the exposed surface with a thin layer of an appropriate aluminium alloy.
Aluminium is one of the common metals having an electrical conductivity high enough for use as an electrical conductor. The conductivity of the electrical conductivity grade is approximately 27mÙ/cm (62% of IACS). Because aluminium has less then one third the density of copper, an aluminium conductor of equal current carrying capacity is only half the mass of a copper conductor.
The high reflectivity of aluminium which is over 80% has led to its wide use in lighting fixtures. These reflectivity characteristics lead to its use as an insulating material. For example, aluminium roofing reflects a high percentage of the sun’s heat so that buildings roofed with aluminium are cooler in summer. In the same way the excellent reflecting properties of aluminium ensure that buildings roofed with this material are warmer in winter.