Page 126 - Welding of Aluminium and its Alloys

TIG welding 111

6.15 Oxide intrusion and cracking associated with suck-back.

6.2.5.3 Root bead penetration
It is possible to produce fully fused root beads without backing using AC-
TIG. Up to 3mm thickness the weld can be made without a weld prepara-
tion but above this a V- or U-preparation will be necessary to achieve full
fusion. Root gaps should be avoided. A skilled welder will use the appear-
ance of the weld pool to judge when a fully penetrating root bead has been
produced. When full penetration has been achieved the weld pool will sink
and will have a bright shiny surface. A U-preparation will make it easier
for the welder to judge when this happens. In thicker material when using
a V-preparation the arc tends to favour the side walls instead of acting
directly on the root, sometimes leading to lack of fusion type defects.
An alternative to a U-preparation that avoids this problem is to use a V-
preparation with an included angle of at least 90°.When welding root passes
in position, particularly overhead, root concavity or ‘suck-back’ is a
problem. This requires the filler rod to be pushed into the weld pool to
disrupt the oxide film and to form a convex bead.
The oxide film that causes suck-back is also responsible for a feature
known colloquially as a ‘baby’s bottom’, a very accurate description of
the appearance of this root defect (Fig. 6.15). Oxides tend to migrate to
the centre of the root penetration bead. When these become excessive the
centre of the bead sinks to produce a deep groove along the centre line that
may also be associated with hot cracking. In butt welds a very wide weld