Casting can refer to the production of moulded products which require further fabrication (the solid aluminium output of a smelter), as well as near-net shape products, that require limited subsequent processing.
Molten aluminium tapped from reduction cells is weighed, sampled and analysed before it is poured into a holding furnace and heated to approximately 750°C using natural gas as a fuel.
Alloying elements, such as magnesium, silicon and manganese, for additional strength, corrosion resistance and other properties, are added to the aluminium. Clean, sorted scrap can also be added at this stage – primary aluminium smelters recycle mainly run-around scrap (the scrap generated in the primary aluminium production process) and new scrap (the cut offs generated when turning primary products into semi fabricated products – e.g. billet into profiles – or semi fabricated products into fabricated products – e.g. profiles into window frames). During furnace charging and preparation, aluminium dross (a thick liquid or solid phase) forms at the surface of molten aluminium. This mixture of aluminium oxides is also remelted to recover the aluminium that would otherwise be lost.
Metallurgical analysis verifies that the metal meets customer specifications before the molten, alloyed (or pure, unalloyed) aluminium is cast into products of specific dimensions, before being weighed, bundled and strapped ready for transport.
The major products of an aluminium smelter are billets, remelt ingots, slabs, rods and liquid aluminium.
Ingot These products are used as inputs to casting processes, where they will be remelted, before potentially being further alloyed, and cast into near net shape products. They come in a range of shapes and sizes (standard, small, T-bar, sow & properzi ingots) that are designed to be transported easily and that meet customer requirements. High purity ingots can also be produced, for specialist applications in the electronics and aerospace industries. They are produced using a launder system that continuously casts molten metal in a series of horizontal moulds.
BilletIn the direct chill process a fixed water-cooled mould is situated above a deep casting pit. Molten aluminium is bought to the top of the mould by a launder and a floating valve controls the flow of molten aluminium. When the molten aluminium surface in the mould reaches the required level a hydraulic ram pulls the cooling billet downwards, spraying water directly on to it and cooling it further. Below the level of the mould the cast billet consists of a solidified outer shell with a centre that still contains molten aluminium. This central molten aluminium will solidify well below the mould level, particularly for large cross-section castings. The final length of the cast billet or rolling slab is dependent only on the depth of the pit and the sufficient supply of molten aluminium.
These log-shaped castings are produced in various diameters and lengths using a vertical direct chill process. They are used for producing extrusions, also known as profiles, that find major end use in construction, industrial and transportation purposes, as well as for forging purposes in automotive industries.
These cuboid shaped ingots are the input to the rolling process and are produced using a similar technique to billet.
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