Page 161 - Welding of Aluminium and its Alloys

144 The welding of aluminium and its alloys

operates in the space above the weld pool. The pool fills the cavity below
the arc, solidifying as the torch is traversed vertically up the joint line. The
molten pool is retained in position and moulded to shape by a graphite shoe
attached to and following immediately behind the welding torch.
The process utilises a drooping characteristic power source capable of
providing 600 A at 100% duty cycle coupled to a water-cooled machine
torch. The torch is mounted on a vertical travelling carriage at an angle of
15° from the horizontal. The gas shroud should be at least 25mm in diam-
eter and the tip of the contact tube should be flush with the shroud.
For butt welding the graphite shoe is made from a flat plate shaped with
a groove to mould the cap, flared out towards the top of the shoe where the
weld pool is formed.The fillet weld mould is provided with a pair of ‘wings’
set back to press against the plates to form the fillet. In both cases the shoe
is held against the plates by spring pressure. The shoe must be long enough
to hold the molten metal in place until it has solidified – in the region of
100mm may be regarded as sufficient. It has been found that heating the
shoe to 350°C before commencing welding assists in preventing fouling of
the shoe with parent metal.
During welding the arc must be prevented from arcing onto the weld pool
or the graphite shoe. This requires careful control of the wire position and
the wire feed speed, as a balance must be achieved between the volume of
metal being fed into the pool, the volume of the mould and the traverse
speed.

7.7 MIG spot welding

MIG spot welding may be used to lap weld sheets together by melting
through the top sheet and fusing into the bottom sheet without moving the
torch. The equipment used for spot welding is essentially the same as that
used for conventional MIG, using the same power source, wire feeder and
welding torch. The torch, however, is equipped with a modified gas shroud
that enables the shroud to be positioned directly on the surface to be
welded (Fig. 7.20). The shroud is designed to hold the torch at the correct
arc length and is castellated such that the shield gas may escape.The power
source is provided with a timer so that when the torch trigger is pulled a
pre-weld purge gas flow is established, the arc burns for a pre-set time and
there is a timed and controlled weld termination. The pressure applied by
positioning the torch assists in bringing the two plate surfaces together.
Because of this degree of control the process may be used by semi-skilled
personnel with an appropriate amount of training.
The process may be operated in two modes: (a) by spot welding with the
weld pool penetrating through the top plate and fusing into the lower one
or (b) by plug welding where a hole is drilled in the upper plate to enable