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.
Primary Aluminium Products
Near Net Shapes
Primary Aluminium Products
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.
Remelt
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.-
Billet
In 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.
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Slab
These cuboid shaped ingots are the input to the rolling process and are produced using a similar technique to billet.
Near Net Shapes
Remelt ingots, or even molten aluminium from the primary production process, are the input material for aluminium castings. Aluminium castings have the advantage of being produced to near net shape, maximising the yield and minimising further machining. They have many end uses but the transport industry is the predominant market, consuming around 70% of aluminium castings, with machinery and construction applications making up the remaining 30%.
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Pressure Diecasting
The pressure diecasting processes consume almost twice as many tonnes of aluminium alloys as all other casting processes combined. These processes are especially suited to the production of large quantities of relatively small parts up to around 5 kilograms in weight.
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High Pressure Diecasting
High pressure diecastings are made by injecting molten aluminium alloy into a metal mould under substantial pressure. Rapid injection and rapid solidification under high pressure combine to produce a dense, fine grained surface structure, which results in excellent wear and fatigue properties. Diecastings maintain close tolerances and produce good surface finishes. Diecastings are not easily welded or heat treated, although special techniques, such as vacuum diecasting may reduce the trapped gases that limit these processes. Approximately 85% of aluminium alloy diecastings are produced in aluminium-silicon-copper alloys. This family of alloys provides a good combination of strength and castability at reasonable cost.
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Low Pressure Diecasting
Molten metal is introduced into metal moulds at pressures up to 170 kPa. Thinner walls can be cast by low pressure diecasting than can be cast by permanent mould castings. This process can be highly automated.
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Gravity Diecasting & Permanent Mould Casting
Permanent mould castings are suited to high production runs, and are normally larger than pressure diecastings, the maximum weight of these castings is usually about 10 kilograms. Tooling costs are high, but less than those for pressure diecasting. Destructible cores may be used and internal cavities can be fairly complex. Permanent mould castings are gravity fed with a relatively low pouring rate, either by hand or by robot. The metal mould produces rapid solidification. Castings produced by the permanent mould method have excellent mechanical properties, are generally sound and hold good dimensional tolerances. Permanent mould castings may be heat treated, which further enhances their mechanical properties. For maximum properties the full heat treatment cycle of a solution treatment, followed by a quench, and natural or artificial ageing is used.
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Sand Casting
Moulds are formed by ramming sand onto a pattern. The pattern is removed leaving a cavity in the sand. Internal cavities for the casting may be made with sand cores. Molten metal is poured into the mould and after it has solidified the mould is broken to remove the casting. Sand casting is a versatile and low cost process using a wide range of alloy types. Sand castings do not have the dimensional accuracy of other casting processes nd have a relatively poor surface finish. They have the advantage of flexibility of numbers of castings produced, it may be very few or many.
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Shell Mould Casting
A mould is made of a resin-bonded sand, in the form of a shell from 10 – 20mm thick, much thinner than the massive moulds used in sand casting. Shell mould castings produce finer surface finishes than sand casting and give greater dimensional accuracy. Equipment and production costs are relatively high and the size and complexity of the castings are limited.
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Plaster Casting
In this process the moulds are made of plaster. The plaster slurry is poured around a pattern, the plaster is baked, and the pattern removed leaving a mould cavity. Plaster moulds have high reproducibility, permitting castings to be made with fine detail and close tolerances. The surface finish of plaster casting is good. Although the costs for basic equipment for plaster casting are low, the operating costs are high.
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Investment Casting
This process uses refractory moulds formed over expendable wax or thermoplastic patterns. A refractory slurry is invested around an arrangement of patterns and the refractory is dried and the pattern is melted out, leaving a cavity. The molten metal is then cast into the fired mould.
Investment casting produces components that require almost no further machining. The advantages of the process, to produce thin walls, good tolerances and fine surface finishes, lend it to the production of precision engineered parts.
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Centrifugal Casting
The centrifugal casting method forces metal into spinning multiple moulds arranged around a central pouring sprue. Mould materials may be of steel, baked sand, plaster, cast iron or graphite. Cores may be used in these moulds. There is a limitation on the shape and size of the castings available by this method and the casting cost is high, though the integrity of the castings comes closer to that of wrought products and equates well with permanent mould castings.
Casthouse Controller
RUSAL’s Volgograd Aluminium Smelter
“Innovation – in our processes and in the alloys we produce – is what is leading the industry to grow in new markets and cutting edge applications.”
