Aluminium Design and Construction John Dwight

With MIG welding, an electrode-positive DC process, a steady twothirds
of the heat input to the arc is used in melting off electrode wire,
while the other third is released at the weld pool. The total heat input
therefore, and hence the extent of the HAZ, is proportional to the size
of the weld deposit. This forms the basis of equation (6.6). With TIG
welding, an AC process, the total arc energy is about equally shared
between the tungsten electrode and the weld pool. The heat released at
the weld pool is thus half the total energy input. However, only part of
this goes into the (hand-held) filler wire and the amount available for
melting off this latter is less than half the total, perhaps only a third or
a quarter, as compared with two-thirds for the MIG process. For a
given size of deposit (equal to the amount of filler consumed), the
extent of the HAZ (a function of the total energy) is therefore considerably
greater for TIG than it is for MIG welding.
A further factor is that the TIG process is more dependent on the
operator’s technique, as compared with the semi-automatic MIG process.
If the torch is allowed to dwell, the heat input per unit length of weld
goes up, causing the extent of the HAZ to increase still further locally.
The result is that the HAZ extent at a TIG weld is (a) greater than for
a MIG weld having the same deposit size, and (b) variable.
Unfortunately, no systematic study appears to have been made of
HAZ effects at TIG welds, and we only have a limited number of hardness
surveys to refer to. These confirm that the extent of the softening is
much more than with MIG. More information is needed.
6.8.2 Severity of softening with TIG welding
With non-heat-treatable material (3xxx, 5xxx series), it is reasonable to
assume that the use of TIG welding will not increase the severity of
HAZ softening, only its extent. Therefore, the kz-values given in Section
6.4.3 remain valid.
With 6xxx-series material, we have seen that the severity of softening
for MIG welds is only slightly affected by the level of thermal control,
and one may reasonably infer that the values in Table 6.1 could also be
used with TIG welding. Data from a very limited range of test results,
however, have been interpreted as showing that such a view is optimistic
and that lower kz-values should be taken.
With 7xxx-series material, it is known that the severity of softening,
especially the dip at the point of minimum strength (Figure 6.2), is
significantly more pronounced with normal thermal control than it is
with strict. This suggests that with the increased heat input of TIG the
softening will be definitely more severe.
Summarizing, we would at present recommend that kz-values be taken
as follows when welds are made by TIG:
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