Pure aluminium is relatively soft. Its strength can be increased by alloying or cold working. Most of the aluminium that is commercially available is therefore alloyed with one or more other elements.
The resulting alloy has different properties depending on which alloying elements are added. This makes the choice of alloy an important matter.
Sapa uses a long-established international system for identifying aluminium alloys, in which the first digit in a four-digit alloy code identifies the major alloying element. The European standard uses the same codes. The general system is outlined in the table below
The non-hardenable alloys are mostly used for rolling, since the only way to increase their strength is by cold working. In extrusion, on the other hand, hardenable alloys are the most commonly used. some alloys use manganese or chrome to increase toughness. Iron, which is found in all commercial aluminium, can have a negative effect on factors such as productivity and finish if present in high concentrations. The 6000 series, which is alloyed with silicon and magnesium, is by far the most widely used in extrusion and can be used in most applications.
Most of sapa’s alloys are hardenable, which means that they achieve their final strength as a result of solution heat treatment and ageing (precipitation hardening). Solution heat treatment is carried out by heating to high temperature. This can be done as a separate process, but is normally done during extrusion. This requires that the starting temperature of the blanks and the temperature of the emerging profiles are carefully controlled.
Îmbatrânirea se realizeaza apoi în cuptoare electrice speciale timp de câteva ore. Totusi, în cazuri speciale clientul poate dori sa realizeze independent tratamentul termic.
Ageing is then carried out in special furnaces over a period of a few hours.In special cases the customer may however wish to carry out heat treatment independently.
In natural ageing the alloy is allowed to harden spontaneously at room temperature
Choosing the right alloy
A number of requirements have to be considered when choosing an alloy for extruded products, including:
– surface finish.
– suitability for decorative anodising.
– Corrosion resistance.
– Machinability (cutting or plastic forming).
– Production economics.
The simple guide (table 5.1) should only be used for rough guidance. If you are in any doubt, please ask sapa’s sales staff for advice and instructions. sometimes it may be wise to choose an alloy with slightly lower strength but better formability to optimise costs.
Quick guide to alloy choice
Relative grading. Highest score = 3.
Heat treatment recommendations
a) Coarse grain structure may form (a coarse-grained structure decreases strength and gives a poorer finish
b) sapa 6463 should not be annealed and solution heat-treated, as this reduces the material’s suitability for
c) should be cooled quickly (usually in water). During cooling the alloy must be transferred quickly from furnace
to water (approx. 10 sec.).