Physical Properties Of Aluminium – Aluminium Uses and Application
What is Aluminium :
Aluminium or aluminum is a chemical element with symbol Al and atomic number 13. It is a silvery-white, soft, nonmagnetic and ductile metal in the boron group. By mass, aluminium makes up about 8% of the Earth’s crust; it is the third most abundant element after oxygen and silicon and the most abundant metal in the crust, though it is less common in the mantle below. The chief ore of aluminium is bauxite.
Physical and Mechanical Properties of Aluminium :
(i) Pure aluminium has silvery colour and lustre, while the commercial grades show a characteristic bluish tinge.
(ii) The high purity aluminium has a much greater resistance to corrosion than the ordinary steel.
(iii) It is ductile and malleable.
(iv) Its specific gravity is 2.7.
(v) In proportion to its weight it is quite strong.
(vi) Melting point = 658°C, boiling point = 2057°C.
(vii) Its electrical resistivity is 2.669 micro ohms/cm3 at 20°C.
(viii) Its tensile strength varies from 95 to 157 MN/m2.
(ix) It is a good conductor of heat and electricity.
(x) It forms useful alloys with iron, copper, zinc and other metals.
(xi) It is unaffected by ordinary atmospheric influences but is corroded in sea water. It is soluble in solutions of caustic alkalies and in hydrochloric acid. When there is an excess of silicon present in the metal it does not withstand atmospheric actions.
(xii) Aluminium is electron positive to most other metals e.g., iron, chromium, zinc, copper, nickel, tin, lead etc. Care is necessary therefore, to prevent it from coming into metallic contact with other metals under conditions where moisture is present in order to avoid electrolytic action i.e., corrosion.
Uses of Aluminium / Application of Aluminium :
1. Because of its softness and difficulty of making sound castings, little pure aluminium is used in the cast form. The largest quantity is employed after it has been mechanically worked in some manner, as by rolling, wire drawing, drop forging or extruding. After being rolled into sheets, it may be stamped into a variety of shapes.
2. It is employed, often alloyed with small amounts of other metals, in the manufacture of furniture, rail-road and trolley cars, automobile bodies and pistons, electric cables and bus bars, rivets, kitchen utensils and collapsible tubes for pastes.
3. In a finely divided flake form, aluminium is employed as a pigment in paint. Aluminium paint is used as a priming coat for wood, as a protective coat for metals.
4. Aluminium is used in deoxidizing molten iron and steel, especially in the top of the ingot when steel is poured in the ingot mould. In a similar manner it is used to prepare the metals from their oxides by heating a mixture of powdered aluminium and the oxide of the metal to be reduced. The mixture is known as Thermit.
5. Aluminium foil is used as silver paper for packing chocolates etc.
Annealing of Aluminium:
Aluminium sheet which has been hardened by cold working, such as hammering or rolling can be annealed by heating it to about 350°C and afterwards cooling in air or water. The period of heating need only be for a few minutes. As a rough guide to correct temperature of heating for annealing, the surface may be rubbed, during the heating process, with a dry matchstick from time to time, the heating being stopped when wood begins to char.
The annealing of aluminium wire for electrical purposes require more care, since the aim is to obtain a high electrical conductivity by obtaining the correct structure. For high conductivity a long exposure at a lower temperature, namely 250°C to 300°C appears to give the best results. In passing, it may be remarked that the aluminium alloys require higher annealing temperature, from about 350°C to 400°C.
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