Page 127 - Welding of Aluminium and its Alloys

112 The welding of aluminium and its alloys

bead caused by a large root gap or a high welding current will also con-
tribute. This defect is particularly prevalent in corner joints in thin sheet
assemblies and is caused by a failure to clean the weld preparations and the
filler wire adequately. It has been found helpful to use unbalanced square
wave AC to increase the arc cleaning action and pulsed square wave AC,
with a heavy bias towards DC positive, has been successfully used in
particularly troublesome applications.
Permanent backing strips may be used to simplify root bead control.
These require a very good fit-up between the underside of the plates and
the backing strip to prevent lack of fusion or suck-back type defects. To
achieve good penetration into the backing strip there must be a root gap
of at least 1.5 times the electrode diameter and this gap must be maintained
along the full length of the component. This means that the joints must be
adequately tack welded together.

6.2.5.4 Weld termination
Controlled finishing of a weld pass is important if defects are to be avoided.
Abruptly switching off the welding current can cause craters, piping (elon-
gated pores) and cracks in the finished weld pool. When finishing the weld
it is necessary to reduce the welding current gradually and to decrease the
arc length as the arc fades away, adding filler rod until such times as the arc
is extinguished. If a crater begins to form the arc should be briefly re-
established, additional filler metal added and the arc decayed as before. On
thin material the travel speed may be increased to a point at which it can
be seen that the metal has ceased to melt.

6.2.6 DCEN helium TIG welding

Welding aluminium with the electrode connected to the negative pole can
be carried out using helium as the shield gas. This gives a higher tempera-
ture arc and increased penetration compared with AC-TIG but the oxide
removal action of the positive arc is absent.This means that cleaning of the
item to be welded assumes even more importance than when using AC.The
higher heat input and the deeper penetration means that higher travel
speeds can be used and a wider range of thicknesses may be welded than
with AC-TIG, although the high travel speeds do mean that the process is
rarely used in a manual context but is almost entirely mechanised. Typical
single pass welds using helium as the shield gas are illustrated in Fig. 6.9
(butt welds) and Fig. 6.10 (fillet weld). Note in particular the wider and
more deeply penetrating fillet weld bead compared with argon shielding.
Suggested welding parameters for butt and fillet welding using helium are
given in Table 6.5.