Page 129 - Welding of Aluminium and its Alloys

114 The welding of aluminium and its alloys

single sided unbacked butt joints to be made. It is recommended that an
electrode with a fully tapered tip is used to concentrate the arc into the root
of the joint when a weld preparation is to be welded.
One feature seen with helium shielded arc welding, which often gives
cause for concern, is the formation of a black ‘soot’ along the heat affected
zones of the weld. This ‘soot’ is not detrimental to the weld quality and can
easily be removed by stainless steel wire brushing. If left in place between
passes it can affect arc stability and is unsightly on a completed weld.

6.3 Mechanised/automatic welding
Automation or mechanisation of the TIG process can have a number of
benefits.These include the ability to use faster travel speeds, resulting in less
distortion and narrower heat affected zones; the better and more consistent
control of the welding parameters enables very thin sheet material to be
welded; there is a greater consistency in the weld quality; and it is possible
to employ operatives with a lesser degree of skill and dexterity than is
required for manual welding.There are, as ever, some drawbacks to the use
of mechanisation,not least of which is the need to provide the welding fixture
with far more accurate and consistent weld preparations than are required
by the manual welder.Accurate joint fit-up and alignment is crucial to achiev-
ing consistently high weld quality. Jigs and fixtures also need to be capable
of holding the components within tight tolerances and of maintaining these
tolerances as welding proceeds.As an example, autogenous welding of thin
(say 3mm) plate requires root gaps to be maintained at 0–0.025mm and plate
edges to be aligned to better than 0.05mm if root penetration problems are
to be avoided. Adding filler wire will assist in increasing the permissible
tolerances but at the expense of welding speed. It is possible to develop
welding procedures that will provide an acceptable unbacked root pass, but
in many welding fixtures a removable backing bar is part of the clamping
system.This greatly simplifies the task of setting up the joints accurately and
in achieving a sound root and is to be recommended.
Although the parameters of welding current and voltage require con-
trolling within small tolerance bands, the parameters of wire feed speed and
travel speed are far more significant.Variations in wire feed speed may lead
to either underfill if the feed speed slows or overfill and lack of fusion or
penetration defects if the wire feed speed increases. Too slow a wire feed
speed can also result in the wire ‘balling back’ and prevent a smooth melting
of wire into the pool.
Automation or mechanisation of both AC-TIG and DCEN helium TIG
welding may be achieved by adapting the manual techniques using con-
ventional manual equipment attached to manipulating equipment such as
crawler tractors. The task of mechanisation is simplified if the weld is