Aluminium Design and Construction John Dwight

CHAPTER 6
Heat-affected zone softening
at welds
6.1 GENERAL DESCRIPTION
An annoying feature in aluminium construction is the weakening of the
metal around welds, known as HAZ (heat-affected zone) softening (Figure
6.1). With the 6xxx-series alloys, the heat of welding can locally reduce
the parent metal strength by nearly half. With 7xxx alloys, the weakening
is less severe, but extends further out from the weld. For work-hardened
material (5xxx, 3xxx series), the metal in the HAZ becomes locally annealed,
with properties falling to the O-condition level. Only for parent metal
supplied in the annealed or T4 condition can HAZ effects be ignored.
The metal in the HAZ may be weaker than the actual weld metal, or
it may be stronger, depending on the combination of parent and filler
materials used. It often pays to locate welds in regions of low stress, i.e.
away from the extreme fibres or at a section of low moment in a beam.
A full-width transverse weld, as used for the attachment of a web stiffener,
brings a high penalty as it causes softening right across the section.
A designer needs data on two aspects of HAZ softening: its severity
and its extent. The severity is largely a function of the parent material
used, while the extent depends on various factors. The ratio of the area of
affected parent metal (transverse to the weld) to the weld deposit area can
vary from 2 or 3 for a large multi-pass weld to 20 or more for single-pass.
It must be emphasized that the subject of HAZ softening is far from
being an exact science. A well-known method for estimating the extent of
Figure 6.1 Zone of HAZ softening at aluminium welds.
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