Aluminium Design and Construction John Dwight

heating the metal to a temperature of some 500°C, which causes the
alloying constituents to go into solid solution; and then quenching.
Immediately after the quench these constituents remain dissolved, but
with the passage of time they gradually precipitate out in the form of
small hard clusters, which impede the movement of dislocations under
stress, thereby raising the strength, a process known as age hardening.
Ideally, the solution treatment quench takes place in a tank from a
precisely controlled temperature, which depends on the alloy. However,
with the 6xxx series, the exact quenching temperature is less critical
and for extrusions in these alloys it is usually acceptable simply to
spray quench the metal as it comes out of the die, thus saving cost. With
some 6xxx extrusions, it is even possible to air quench with the spray
turned off, and still obtain useful properties.
After the solution treatment there are two options. Either the material
can be left to age naturally at room temperature over a period of days
(natural ageing), or it can be heated in an oven at a temperature of
about 150–180°C, this being known as artificial ageing or precipitation
treatment. The advantages of the latter are that the material ends up
stronger (but less ductile), and the final properties are achieved in
hours rather than days.
The condition (or temper) of heat-treated material is specified by
means of a T-number. Possible conditions range from T1 to T10, as
defined in BSEN.515. Three are of interest in structural design:
T4 solution treatment, followed by natural ageing;
T5 air quench, followed by artificial ageing (applicable only to some
6xxx-series extruded material);
T6 solution treatment, followed by artificial ageing.
T6 is the fully-heat-treated condition (maximum strength). T4 is more
ductile and is selected when formability is a factor. T5 may be chosen
for very thin extrusions, which would distort excessively if subjected to
a water quench.
Recent specifications distinguish between material that is not subjected
to a straightening process after quenching, and that which is. For plate
or sheet, the latter condition is indicated by adding the digits 51 to the
basic T4 or T6, while for extrusions and drawn tube the additional
digits are 510. In practice, this is a fairly academic distinction, since the
same properties are usually quoted for either condition. Therefore
designers will not go far wrong by simply writing T4 or T6.
4.2 CHARACTERISTICS OF THE DIFFERENT ALLOY TYPES
The general characteristics of the seven alloy series are summarized in
Table 4.4. Those most used in non-aeronautical construction are the
5xxx and 6xxx series.
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