Thursday, April 29, 2010

Non Ferrous Materials

Aluminium was discovered in 1827 by friedrich Wohler. Its importance was established after the invention of the dynamo (1867), because a great deal of electricity is consumed in its extraction.

Occurrence and extraction
Aluminum does not occur naturally in the pure form, in compounds, it is the commonest metal on the earth (about 8% of the earth’s crust). The mineral richest in aluminium is bauxite. Among the EEC contries, France, Italy and Greece have significant reserves. Corundum is crystalline aluminium oxide. When pure and clear, it takes the form of gemstones (sapphire, ruby, topaz, amethyst).

To begin with, pure aluminium oxide Al2O3 (alumina) is extracted from bauxite. The oxygen is then removed from alumina by electrolysis. Kryolith (Na3ALF6) is added as a fluxing agent, in order to reduce the melting point from 2000oC to 960oC. The end products used in semi-finished goods (sheets, rods, profile, tubes) are super pure aluminium, Al 99,98 R or aluminium, e.g. Al 99.5 (DIN 171213).

Physical: Melting point 658oC, density 2.7 kg/dm3. Best electrical conductor after silver and copper.
Chemical: Corrosion resistant, impermeable oxide layer.
Mechanical: Tensile Stength cast 160 to 320 N/mm2 rolled 150 to 400 N/mm2. Extensibility 2% to 35%.
Technological: Aluminium can be forged, rolled (down to thin foils), stretched, milled, cast, welded and soldered. Thermite, for instance, which is used in welding together bars, is a mixture of aluminium powder and iron oxide. In aluminium coating, a mixture on to steel and then burned by annealing.

Aluminium Alloys
Important alloying elements are copper, silicon, magnesium, manganese and zinc. The element magnesium and manganese yield a mixed crystal formation. The foreign atoms dissolved in this hinder the movement of dislocations, thereby strengthening the aluminium alloys.

Aluminium forms mixed crystals with the elements copper, zinc and silicon at 500oC. If these alloys are cooled rapidly, the mixed crystal structures remain stable at room temperature. Hardening sets in if these alloys are left to stand for a period. They are therefore known as age-hardening aluminium alloys.

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