CASTING OF ALUMINIUM

CASTING OF ALUMINIUM:

              Aluminum is considered as one of the more easily cast metals. It requires less energy to bring it to the molten stage. About 12000F (6500C) and it’s easier on the refractory furnace lining. In addition to sand casting permanent metal moulds and die casting are used extensively.

           The low density and low melting point combine to practically eliminate most problems of sand washes that occur when heavier metals are poured. Molten aluminium readily absorbs oxygen and hydrogen. Since most of the pin holes in the aluminium castings are caused by hydrogen, precautions must be taken to minimize the presence of hydrogen. This is done by having the metal no hotter than necessary for pouring and by keeping excess moisture from the moulding sand.

          Alloying elements intentionally added to facilitate aluminium casting and heat treatment are silicon, magnesium and copper. Silicon increases the fluidity of molten aluminum, allowing it to flow farther in thin walls of the mould cavity and produce finerdetail.it also reduces internal shrinkages and reduce coefficient of expansion. Aluminium casting containing more than 8% magnesium will respond to heat treatment however it also makes the metal more difficult to cast. Copper is one of the principal hardening constituents in aluminium. It increases the strength of the aluminium in both the heat and non-heated treated conditions.

Aluminium alloys:

                   Aluminium alloys are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys.

The most important cast aluminium alloy system is Al-Si, where the high levels of silicon (4.0% to 13%) contribute to give good casting characteristics. Aluminium alloys are widely used in engineering structures and components where light weight or corrosion resistance is required. Alloys composed mostly of the two light weight metals aluminium and magnesium have been very important in aerospace manufacturing since somewhat before 1940. Aluminium-magnesium alloys are both lighter than other aluminium alloys and much less flammable than alloys that contain a very high percentage of magnesium.

Aluminium alloy surfaces will keep their apparent shine in a dry environment due to the formation of a clear, protective layer of aluminium oxide. In a wet environment, galvanic corrosion can occur when an aluminium alloy is placed in electrical contact with other metals with more negative corrosion potentials than aluminium.                

Aluminium alloys with a wide range of properties are used in engineering structures. Alloy systems are classified by a number system (ANSI) or by names indicating their main alloying constituents (DIN and ISO).The strength and durability of aluminium alloys vary widely, not only as a result of the components of the specific alloy, but also as a result of heat treatments and manufacturing processes. A lack of knowledge of these aspects has from time to time led to improperly designed structures and gained aluminium a bad reputation.

 Aluminium alloys, like all structural alloys, also are subject to internal stresses following heating operations such as welding and casting. The problem with aluminium alloys in this regard is their low melting point, which make them more susceptible to distortions from thermally induced stress relief. Controlled stress relief can be done during manufacturing by heat-treating the parts in an oven, followed by gradual cooling in effect annealing the stresses.

 The low melting point of aluminium alloys has not precluded their use in rocketry; even for use in constructing combustion chambers where gases can reach 3500 K. The Agene upper stage engine used a regeneratively cooled aluminium design for some parts of the nozzle, including the thermally critical throat region. Aluminium slabs being transported from a smelter.

Some of the uses for aluminium metal are in:

•           Transportation (automobiles, aircraft, trucks, railway cars, marine vessels, bicycles etc.) as sheet, tube, castings etc.

•           Packaging (cans, foil, etc.)

•           Construction (windows, doors, siding, building wire, etc.)

•           A wide range of household items, from cooking utensils to baseball bats, watches.

•           Street lighting poles, sailing ship masts, walking poles etc.

•           Outer shells of consumer electronics, also cases for equipment e.g. photographic equipment.

•           Electrical transmission lines for power distribution

•           MKM steel and Alnico magnets

•           Super purity aluminum (SPA, 99.980% to 99.999% Al), used in electronics and CDs.

•           Heat sinks for electronic appliances such as transistors and CPUs.

•           Substrate material of metal-core copper clad laminates used in high brightness LED lighting.

•           Powdered aluminum is used in paint, and in pyrotechnics such as solid rocket fuels and termite.

A variety of countries, including France, Italy, Poland, Finland, Romania, Israel, and the former Yugoslavia, have issued coins struck in aluminum or aluminum-copper alloys.   Aluminum alloys with great durability and high strength, some with a tensile strength as good as that of constructional steels, are available to the designer in the form of extruded profiles, rolled sheet and plate, castings and forgings. The majority of these alloys consist of aluminium with carefully controlled additions of copper, magnesium, silicon, manganese, zinc and more recently lithium. Aluminum alloys are the dominant materials for airframe structures. There are three main classes of aluminum alloys used in aerospace applications, though only the wrought heat-treated alloys have sufficient strength for structural components.