Aluminium Design and Construction John Dwight

supplied in a coil and of typical diameter about 1.5 mm. It is fed
automatically through the torch and into the arc while the weld is
being made. The torch, which is water-cooled, is much more complicated
than that used for ordinary stick welding, since the supply tube has
to convey four things simultaneously: current, wire, argon and water.
The MIG process is suitable for welds in material down to a minimum
thickness of 4 or 5 mm.
Normally the electrode wire is pushed down the tube leading to the
torch. This can occasionally cause problems with the wire kinking and
jamming in the tube. The equipment for making the smallest MIG welds,
using 0.8 mm wire, avoids this problem by pulling the wire through the
tube. This adds further to the complexity of the torch.
The MIG process has two special attributes. First, it is suitable for
posi-tional welding, including overhead. Second, the arc-length is selfadjust-ing.
In other words, when the welder accidentally moves the
torch nearer or further away from the job, the burn-off rate momentarily
changes until the right amount of wire is sticking out again, thus
maintaining the correct arc-length automatically. This ‘semi-automatic’
feature makes it a relatively easy process to use.
When welding in the downhand position, the section area of the
maximum possible size of MIG deposit is some 40 mm 2 per pass. Much
higher welding speeds are possible than with stick welding of steel. For
a weld on 6 mm plate, a speed of 1.5 m/s would be reasonable, compared
with, say, 0.5 m/s on steel.
3.3.3 TIG welding
TIG is an alternating current (AC) process. The water-cooled torch has
a non-consumable tungsten electrode, and as with MIG it delivers a
flow of argon. The filler wire is held in the other hand and fed in
separately. TIG requires more skill than MIG, both to control the arc
length and to feed in the filler. One disadvantage is that the operator
may inadvertently let the torch dwell in one position, causing a local
build-up of heat and hence an enlarged HAZ.
The TIG process is suitable for the welding of sheet thicknesses,
rather than plate. At the top end of its thickness range (say 6 mm), the
penetration can be improved by using a helium shield instead of argon.
It is possible to adapt the TIG process to autogenous welding, i.e.
dispense with the use of filler wire. The necessary filler metal is provided
by ribs on the parts being joined, which melt into the pool. Such a
technique enables TIG to be set up for automatic welding, with machinecontrolled
traversing of the torch.
A useful variant of TIG is the pulsed-arc process. This employs a DC
supply with current modulation, causing the weld to be produced as a
series of nuggets, which fuse together if the parameters are suitably
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