Casting is the shortest route from the raw metal to the finished part. As with other industrially used metals, one differentiates between three finished shapes when casting aluminium:
- Ingots (blocks of metal weighing from four to 25 kilograms) and sows (larger blocks) are the starting products for subsequent melting.
- Billets produced by continuous casting are the starting material for extrusion and rolling.
- Mould castings or finished castings (for example parts for mechanical engineering) only require machining and. if necessary, surface treatment at the most.
Benefits and casting conditions
Aluminium castings offer a number of benefits compared with other metals such as iron; age-hardenable alloys are often used.
- Better castability, which is mostly improved by addition of silicon (up to 13 per cent), so that tight cavities in the mould are also completely filled.
- Relatively low solidification temperature, which allows steel moulds to be used.
- Using a method of construction that is suitable for the material involved, a weight saving of up to 50 per cent is possible for comparable strength.
- They have high to very high dimensional accuracy.
- Surfaces are clean and smooth and require little cleaning.
In aluminium smelters, ingots, sows or continuously cast billets are cast immediately following extraction. Special foundries process the ingots and sows to shaped castings. Various types of melting furnace are available for melting and alloying. Before casting, one usually undertakes melt purification. The casting temperature is between 680 and 750 °C (melting point of aluminium 660 °C). The conditions are monitored continuously for quality assurance purposes. Foundry scrap is recycled internally. In the interests of occupational health and to prevent accidents, it is of paramount importance to avoid contact between the melt and water because of the risk of explosion (hydrogen formation).
Purpose-oriented casting processes
One chooses the process that is most economical depending on the product, lot size and required dimensional accuracy:
- The oldest casting process is sand casting. Using a model made of wood, plastic or metal, a hollow shape with "risers" (reservoirs for molten metal) is produced using silica sand with clay as binder. As the mould is complex and is "lost" after a single use, sand casting is expensive and is only considered for complex shapes or large one-off items.
- With permanent mould casting (sometimes called "chill casting"), the mould is pre-heated to 300 °C and more to achieve uniform solidification.
- In die-casting, molten metal is forced into a permanent steel mould under a pressure of between 200 and 3000 bar. Thanks to the high cycle frequency, it is economical for large lot sizes.
- Continuous casting is used for billets, which are processed further by forging, extrusion or rolling.
- With strip casting, a continuous casting process for which there are a number of variations: the molten metal can be cast between two water-cooled steel belts, into steel moulds that rotate like caterpillar tracks, or between rolls, where it solidifies to strip up to 20 millimetres thick at speeds of up to five metres a minute and is immediately rolled out.
- Casting of strands (continuously cast and rolled rod or Properzi wire) for wire drawing.