Aluminium Design and Construction John Dwight

Figure 6.2 Typical hardness plots at a weld in heat-treated aluminium.
experimentally by conducting a hardness survey, and crudely indicate
the variation in f u (ultimate stress) rather than f o (proof stress), since
indentation hardness relates primarily to f u . The plots in the figure have
not been continued into the actual weld metal, because the strength of
this varies with the filler used.
Two curves are shown for each alloy type, corresponding to welds
made with normal and with strict thermal control. The HAZ can be
divided into two regions (1, 2) as indicated on the plots for normal
control. In region 1, the metal attains solution-treatment temperature
and is thus able to re-age to some extent on cooling. In region 2, this
temperature is not reached, and the metal is over-aged. The hardness
is at a minimum at the boundary between the two regions (point A),
and then rises steadily as we move out to point B. Beyond B, the heat
of welding has negligible effect and full parent properties are assumed
to apply. In region 1, the hardness increases as we move in towards the
weld, although only slightly so for 6xxx material.
It is seen that the use of strict thermal control considerably reduces the
extent of the softening for both alloy types. With 7xxx material, it also
improves the properties in region 1 and reduces the amount of drop at
A. With 6xxx material, the effect of thermal control on severity of softening
is only slight. Very roughly, the relative widths of the two regions (1, 2)
in Figure 6.2 satisfy the expressions below, where x A and x B are the distances
of A and B from the middle of the weld (applicable to MIG welds):
6.3.2 Work-hardened material
6xxx series x A � 0.5x B
7xxx series x A � 0.8x B
(6.1a)
(6.1b)
With the non-heat-treatable alloys, HAZ softening need only be considered
for work-hardened material; it is not a factor with extrusions or
Copyright 1999 by Taylor & Francis Group. All Rights Reserved.