The starting material for making aluminium extrusions is the aluminium log. These are cast in lengths of up to 7 metres and available in a wide range of alloys and diameters. After casting they are further heat-treated , (homogenised), before being supplied to the extrusion plant.
In most extrusion plants today the first operation is to cut the long extrusion log into suitable lengths for extrusion based on press capacity and the final extruded length. (Typically 400 mm to 1000 mm) The cut log or billet is fed into the billet heater where it is heated typically to 400 – 500 deg.C. The exact temperature will depend on the alloy, shape complexity and other process parameters.
Extrusion involves pressing a preheated aluminium ingot (450-500°C) under high pressure (1600-6500 tons depending on the size of the press) through a die the opening of which corresponds to the cross-section of the extrusion. The extrusion press speed (normally 5-80 m/min) depends on the alloy and the complexity of the die’s opening.
There are two basic types of extrusion process – direct and indirect. In the traditional direct method the die is stationary.
The pre-heated billet is loaded into the container and squeezed through the die opening, (which corresponds to the cross-section of the extrusion). Typical press sizes range from 1,600 tonnes to 6,500 tonnes – sufficient force to cause the aluminium to flow plastically and exit the die at extrusion speeds in the range of 5-80 m/min. At the exit of the die the temperature of the extrusion will be in excess of 500 deg. C. The extruded length may be as long as 50 metres and may consist of one strand or multiple strands of the same section. With most commercial alloys the extrusion will be cooled or quenched as it exits the press.
Cooling is carried out for two main reasons:
To enable the subsequent stretching and handling of the extruded lengths.
As the first part of the heat treatment of age-hardening alloys – these form the bulk of commercial alloys. This “solution cooling” operation may be achieved with air, water or a mixture of the two depending on the alloy and the final mechanical property requirements. (The use of solution in this context is a metallurgical term referring to what is happening in the aluminium alloy and does not describe the medium used for cooling).
Stretching and Cutting to Length
The long lengths are transferred across the press table into the stretcher. Here, stretcher jaws at either end grip the metal and give it a controlled stretch. This straightens the long lengths and can also have a minor effect on subsequent mechanical properties.
The up to 50 metre long extrusions are then fed to the saw conveyor where they are cut to the customer’s required length. Typical customer cut lengths range from 2-6 meters, but other lengths can be accommodated. The ability to supply longer lengths will depend on the logistics of the individual plants and shorter lengths are often achieved by an additional re-cut operation off-line.
This is a specific form of heat treatment and typically involves heating the extrusions in ovens to a temperature in the range of 170-190 deg. C for 4 to 8 hours. This develops the mechanical properties of the extrusions, particularly their strength. The minimum mechanical properties that must be achieved after ageing are specified in the relevant EN/British Standards for different alloys and tempers.