Aluminium Design and Construction John Dwight

Figure 6.5 Pattern of softening at multi-pass weld on thick material.
HAZ curve has a more rounded knee, with a lower proof/ultimate ratio.
Plots such as those in Figures 6.2 and 6.3, based on hardness surveys,
give a visual picture of how the ultimate stress (f u ) is reduced in the
HAZ. The drop in proof stress will be more marked. This is especially so
for non-heat-treatable material (5xxx series) supplied in a hard temper.
6.3.4 Multi-pass welds
Figure 6.5 shows the typical softened zone at a large multi-pass weld.
Regions 1 and 2 can again be identified, analogous to those shown in
Figures 6.2 and 6.3 for a single-pass weld, now extending uniformly
around the edge of the deposit. As we move away from the weld, the
strength varies in the same general way as before, the lines A and B
being metallurgically equivalent to points A and B in Figure 6.2 or 6.3.
6.3.5 Recovery time
With work-hardened alloys, the final HAZ properties are reached as soon
as the metal has cooled after welding. But, with heat-treated material, the
immediate strength in the HAZ is low, the final HAZ properties only
being developed after enough time has elapsed to allow natural ageing to
occur. Providing the component is held at a temperature of at least 10°C
after fabrication, this time (the recovery time) may be roughly taken as:
6xxx-series alloys 3 days
7xxx-series alloys 30 days
If heat-treated material is held significantly below 10°C, the recovery
time will be longer. On the other hand, quicker recovery can be achieved
by post-weld artificial ageing. This involves holding the welded component
at a temperature between 100 and 180°C for up to 24 hours, the exact
procedure depending on the alloy. Such treatment also has a strengthening
effect.
Copyright 1999 by Taylor & Francis Group. All Rights Reserved.