Page 42 - Welding of Aluminium and its Alloys

34 The welding of aluminium and its alloys

Hardness and
weld {
strength of fully
age-hardened
alloy
Hardness and
strength of weld
Hardness Strength
metal

Hardness and
strength in the
HAZ
30 20 10 0 10 20 30
Distance from centre of weld (mm)
2.19 Effect of welding on 6061 T6 age-hardened alloy – as welded.

similar losses in tensile strength can be found. The loss is caused by a dis-
solution of the precipitates in the 2XXX series alloys and a coarsening or
overageing of the precipitates in the 6XXX and 7XXX alloys.These effects
are illustrated in Fig. 2.19. Greater detail on these effects for individual
alloys can be found in Chapter 3.
One last comment is the potential for the loss of alloying elements from
the weld pool that may result in a reduction in strength. It is true that some
elements, mainly magnesium with its low boiling point and lithium which is
highly reactive with oxygen, may be lost or oxidised during welding. There
is, however, a dearth of information quantifying any effects, which suggests
that it is not perceived as being a problem. Loss of magnesium is worst when
MIG welding, resulting in the sooty deposit occasionally seen along the
weld toes but in this case, and in the case of lithium, careful attention to gas
shielding will minimise any problem.