Aluminium Design and Construction John Dwight

the softening is the famous ‘one-inch rule’ which often proves adequate,
but not always [18]. This simple method is explained in Section 6.5.3,
after which we go on to present a more scientific treatment (RD method),
which may be used to replace the one-inch rule in situations that demand
a more accurate estimate of the HAZ extent (Sections 6.5.4–6.5.11).
Many people think that HAZ softening is such a minor effect that a
very rough estimate of its extent is all that is needed. It is true that with
large multi-pass welds, as used in massive members, the softened area
only extends a short distance in relation to the size of the weld. For
these, almost any extent-rule will usually do. But with smaller welds,
as used in thin members, the extent of the HAZ is relatively much
greater and a better approach is desirable. For these, the one-inch rule
can lead to an estimate of member resistance that is unacceptably low.
The BS.8118 procedure for predicting HAZ extent gets the worst of
both worlds, since it is more awkward to apply than the one-inch rule
and often inaccurate. Our proposed method is more realistic, although
still fairly approximate compared to most structural calculations. In
recent years, special computer programs have become available, which
accurately model the temperature changes during welding and the
resulting metallurgical effect [19]. For a mass-produced component, it
may be sensible to employ one of these. Alternatively, the HAZ pattern
can be found experimentally by making a hardness survey on a prototype.
Designers should also be aware of the locked-in (‘residual’) stresses
in welded components, even though these are not directly considered
in the design process. As with steel, there is a region of locked-in
longitudinal tensile stress at any weld, balanced by compressive stresses
elsewhere in the section. But compared to steel, where the tensile stress
at the weld is invariably up to yield, the stress levels in aluminium are
relatively low. The zone of locked-in longitudinal tension at an aluminium
weld is generally narrower than the HAZ. Mazzolani provides interesting
plots of residual stress in welded aluminium members [26].
Most of this chapter specifically covers HAZ softening at welds
made by the MIG process. TIG welds, for which the HAZ effects are
much less predictable, are considered in Section 6.8. The friction-stir
process is still very new, but some preliminary results suggest that the
softening at FS welds will tend to be less extensive than that at arc
welds (Section 6.9).
6.2 THERMAL CONTROL
The extent of the HAZ can be critically affected by the control of
temperature during fabrication. In a large multi-pass joint, if no such
control were exercised, the temperature of the surrounding metal would
just keep on rising as more passes were laid, leading to a vastly enlarged
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