Page 88 - Welding of Aluminium and its Alloys

Welding design 77

taper of at
least 4:1

Single or double sided butt weld
5.9 Suggested design for the joining of dissimilar thickness plates.

The effective size of a full penetration butt weld equals the design throat
thickness, essentially the plate/pipe thickness of the thinner component.As
mentioned elsewhere no credit is taken for the weld metal cap height or
root penetration bead. Although not often used in aluminium fabrications
because of the need to match joint strength and base metal strength, in
lightly loaded joints a partial penetration joint (Fig. 5.5b), may be accept-
able. Partial penetration can be achieved by the use of a close square butt
joint or a thick root face. There are cost benefits associated with the partial
penetration joint as little or no edge preparation is required, it is econom-
ical on filler metal and it is easy to assemble since the root gap does not
need to be controlled. The limitations are that radiographic interpretation
is difficult due to the lack of penetration, the fatigue life is compromised
and static mechanical strength is reduced. The effective size in the case of
the partial penetration weld is the throat of the weld minus the cap height.
Where two sections of unequal thickness are to be welded the welder’s
task will be eased and the best properties, particularly fatigue, will be
achieved if the thicker of the two is bevelled or tapered to match the
thinner. The taper on the thicker component to achieve this should be in
the region of 4 to 1 to reduce the stress-raising effects of an abrupt change
in thickness (Fig. 5.9).
The weld preparation shape may be selected to achieve root penetration
and a sound root, to permit the required pass sequence or to control weld
metal dilution from the parent metal. The MIG process can, but with diffi-
culty, be used to produce a sound, defect-free penetration bead – for a
sound joint either a backing bar or strip needs to be used or the weld must
be cut-back and a second side weld made. These techniques are dealt with
in greater detail elsewhere. The TIG process can be used to make a sound,
fully penetrated root bead without a cut back or backing strip. A ‘landed’
bevel joint (Fig. 5.10) is designed to enable the highest quality root pene-
tration bead to be made using the TIG process. This is of use in applica-
tions such as pipe butt welding, where the welds need to be single sided and
to have a smooth root bead that will not hinder flow in the pipe.