Overview and properties of aluminium and aluminium alloys

Overview and properties of aluminium and aluminium alloys

Aluminium is a ductile, silvery-white metal, usually with a dull lustre due to a layer of aluminium oxide on its surface. Aluminium is light, weighing about a third of the equivalent in copper or steel. It is corrosion resistant, an excellent conductor of heat and electricity, reflects light and radiant heat, is non-magnetic, does not easily absorb neutrons and is easy to work with.

Aluminium can be joined by welding, brazing, gluing, riveting, and by many mechanical components such as nuts and bolts, screws and nails. It can be ground, polished and sandblasted to obtain a wide variety of finishes. Various chemical polishes can be used, such as alkaline or acid etching, bright dipping (these leave the metal surface very bright), chemical grinding and dip plating. It is suitable for the electrochemical process known as anodising. Plating can also be carried out with other metals or organic coatings such as paints, lacquers and plastic films. Aluminium can be finished by enamelling or metallising.

High purity aluminium (99.9%) is relatively soft and has a fairly low tensile strength of about 50 MPa (500 kg/cm2, or 7000 psi) under annealed conditions. (Annealing involves heating followed by slow cooling to reduce the brittleness of the metal.) However, through alloying and appropriate heat and mechanical treatments it can be made harder and stronger, with tensile strengths of up to 700 MPa. Unlike some other metals, aluminium's strength and ductility increase at very low temperatures. When melted, the solid metal expands by about 7% and solidifies and shrinks to 6.6% of its liquid volume. Hydrogen is the only gas known to be significantly soluble in molten aluminium; its solubility increases with temperature, but is almost zero when the metal freezes.