Aluminium Design and Construction John Dwight

softening in the heat-affected zone. Broadly they divide into a stronger
type and a weaker type.
The stronger type of 6xxx material in the T6 condition is sometimes
described as the ‘mild steel’ of aluminium, because it is the natural
choice for stressed members. In fact it is a weaker material than mild
steel, with a similar proof or yield stress (250 N/mm 2 ), but a much
lower tensile strength (300 N/mm 2 ). It is also less ductile.
The weaker type of 6xxx alloy, which is not normally offered as sheet or
plate, is the extrusion alloy par excellence. It is more suitable than any other
alloy for the extrusion of thin difficult sections, and is a common choice for
members that operate at a relatively low stress level, especially when good
surface finish is important. Typical examples are when design is governed
by stiffness (rather than strength) as with many architectural members, or
when fatigue is critical, as for example in the structure of railcars.
(c) 7xxx-series alloys
These alloys mainly contain zinc and magnesium. Like the 6xxx-series,
they can be either of a stronger or a weaker type. The stronger type
embraces the strongest of all the aluminium alloys, with tensile strengths
in the T6 condition up to 550 N/mm2 . The application of such alloys is
almost entirely in aircraft. They have inferior corrosion resistance, poor
extrudability and are unsuitable for arc welding. As with 2xxx alloys,
sheet can be supplied in clad form to prevent corrosion (Section 2.2.5).
The weaker type of 7xxx material is a very different proposition and
in the non-aeronautical field is a valid alternative to the stronger type
of 6xxx material, especially for welded construction. It has superior
mechanical properties to 6xxx material, and HAZ softening at welds is
less severe. But corrosion resistance, although much better than for the
stronger kind of 7xxx alloy, is not as good as with 6xxx alloy. Also there
can be a possibility of stress-corrosion. Extrudability is not quite as
good as for the 6xxx series, but hollow (bridge-die) extrusions are still
possible. An important factor, when using 7xxx-series alloy as against
6xxx alloy, is the need for greater expertise in fabrication.
4.3 DATA ON SELECTED WROUGHT ALLOYS
4.3.1 How mechanical properties are specified
Aluminium material standards quote two levels of stress, both of which
must be attained for a batch of material to be accepted:
fo fu minimum value of the 0.2% proof stress (or ‘0.2% offset’);
minimum tensile strength (or ‘ultimate stress’).
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