Next to rolling, extrusion is the most important process for forming aluminium. Billets produced by the continuous casting of unalloyed aluminium or wrought alloys are first heat treated to distribute precipitates and alloying elements uniformly throughout the grain structure. The billets, which are up to 100 centimetres long and 80 centimetres in diameter, are heated to 400-500 °C before they are pushed into a steel receptacle (container). The container is sealed with a die, which is a disc made of high-strength steel (the tool) with an opening in its centre. This opening can have almost any cross section, from circular to complex patterns with several cavities. A ram then presses the billet through the opening with a pressure of 1000 newtons per square centimetre or more; this causes plastic deformation of the aluminium at these temperatures and leads to the formation of a strand that adopts the cross section of the opening. Depending on the cross section and extrudability of the alloy, the strand grows at a rate of up to 40 metres a minute. After exiting the die, the strand is cooled with blowers to increase its strength and prevent it deforming under its own weight when hot.
Waviness and small kinks are subsequently removed by stretching.
The process is economical (up to 40,000 metres of profiles can be made using the same die) and is used in several different variations, with automated machines being used to a large extent:
- In the most commonly used process, direct extrusion, the die and billet container are fixed.
- In the indirect process, the billet and billet container are moved against the die so that there is no friction between the billet and the container, which reduces the extrusion pressure needed by 20 to 30 per cent.
- One uses steel mandrels to extrude tubes or cavities; the mandrels are either inserted though holes in the billet, stationary or move with the strand. Moving mandrels have to be slightly conical so that they can be removed from the finished strand. In porthole dies, the mandrel is supported in the die opening, the aluminium flows around these supports but welds to the strand again immediately afterwards under the pressure applied.
- With continuous extrusion, for example cable sheathing for electrical engineering, one inserts so many billets one after the other until the desired total length is reached and the strand is then cut off.
Products manufactured by extrusion are rod and bar, profiles and tube. As tube can only be extruded to a certain wall thickness, which is not particularly thin, it is subsequently subjected to drawing to obtain the desired dimensions.