Page 90 - Welding of Aluminium and its Alloys

Welding design 79

5.11 Typical self-jigging extrusions. Courtesy of ALCAN.

of the risk of contamination. An ungrooved backing bar will need the root
pass to be back-ground and perhaps a sealing pass to be deposited to
produce a sound weld. If TIG welding using a backing bar the weld should
be made with no root gap. This is necessary to prevent the TIG arc acting
directly on and perhaps melting the backing material.
The permanent backing strip is fused into the root pass and care must be
taken to select an alloy that is compatible with the parent metal and the
filler. The strip should be in the region of 4–6mm thick and tacked in posi-
tion. Many extrusions in aluminium, however, can be produced with the
backing strip incorporated and in this way joint set-up is simplified. It is
possible to design the extrusions with both the backing strip built in and of
such a shape that the joint is self-jigging, as illustrated in Fig. 5.11.
The crevices associated with permanent backing strips result in local
stress concentrations. These may reduce both impact and fatigue resistance
if the root is in a highly stressed area. The crevice may also give rise to
localised corrosion although even in marine environments this has not been
reported as a major problem when the correct alloy has been chosen.
Despite these potential drawbacks, permanent backing strips are a common
feature in many structures used in challenging applications.
Inert gas backing can be used for critical applications such as food pro-
cessing or pharmaceutical process pipework or vessels and is very useful
when there is no access to the back of the weld to back-gouge and seal. An
argon purge will prevent oxidation of the root penetration bead and oxide
films being fused into the joint, giving a smooth, even TIG root bead.Typical
designs of backing bars and strips are given in Fig. 5.12.