Pure aluminium is relatively soft. To overcome this, the metal can be alloyed and/or cold worked. Most of the aluminium reaching the marketplace has been alloyed with at least one other element.
There is a long-established international system for identifying aluminium alloys (see the table below). The first digit in the four-digit alloy code identifies the major alloying element.
The European standard uses the same codes.
The table below gives the broad outline of the systems.
|Alloying element||Alloy code||Alloy type|
|None (pure aluminium)||1000 series||Not hardenable|
|Manganese||3000 series||Not hardenable|
|Silicon||4000 series||Not hardenable|
|Magnesium||5000 series||Not hardenable|
|Magnesium + silicon||6000 series||Hardenable|
The 6000 series is by far the most widely used alloy in aluminium extrusion.
As cold working is the only way to increase the strength of the alloys that cannot be hardened, most of these go for rolling. In extrusion, on the other hand, hardenable alloys are the most commonly used.
The 6000 series, which has silicon and magnesium as the alloying elements, is by far the most widely used in extrusion. In a 7021 alloy, zinc and magnesium are responsible for the hardening effect. Some alloys use manganese, zirconium or chrome to increase toughness.
Iron, which is found in all commercial aluminium, can have a negative effect on toughness and finish (amongst other things) if present in high quantities.
Two hardening methods are used on alloys in the extrusion process. ‘Solution’ heat treatment is carried out during extrusion by carefully controlling the temperature of the emerging profile. ‘Precipitation’ hardening (ageing), which takes a few hours, occurs in special furnaces after the extrusion process.
Amongst the factors affecting the choice of the right alloy for an extruded product are:
– Strength, finish, suitability for decorative anodising, corrosion resistance, suitability for machining and forming, weldability and production costs.