Page 99 - Welding of Aluminium and its Alloys

88 The welding of aluminium and its alloys

5.23 Jigging of plate to maintain flatness. Courtesy of TWI Ltd.

sheets this can result in the edges riding up over each other,requiring the
tack welds to be more closely pitched.The length of tack welds should be
in the region of 8 times the component thickness and spaced at intervals
not greater than 35 times the thickness.They must be made with the same
procedure and with as much care as the main weld. Tacks are expected
to carry the assembly stresses and can therefore experience high loads
that may well cause cracking problems. Incorporation of cracked tacks
in the constructional weld will be an expensive problem to eliminate!
• Ensure that the joint fit-up is accurate.This is perhaps one aspect where
the importance cannot be over-emphasised. Large root gaps, for
instance, will always result in large amounts of distortion as the root
weld metal contracts.
• Do not over-weld and avoid the use of wide bevel angles, large root gaps
and large amounts of excess metal. Fillet welds should be as small as
permitted by design – for example, an 8mm leg length fillet weld con-
tains over 80% more weld metal than a 6mm leg length fillet. It is worth
remembering that not only does excess weld metal increase distortion,
it also costs a lot of money to deposit it!
• Use jigs and fixtures (Fig. 5.23), to hold and retain the components in
the correct alignment. The use of rigid restraints will give increased
levels of residual stress and may increase the risk of cracking. Jigs need
to be designed to provide good access for welding,to be rigid and robust,
to be foolproof in use and to be well maintained to ensure that wear is
taken into account and tolerances are achieved.

5.7 Rectification of distortion
If the measures listed above are not effective, remedial measures to rectify
the distortion will be necessary. These may be based upon those used for
steel but great care needs to be exercised if such techniques are used. The
most effective methods are those that use some form of mechanical working
or stretching as these will not significantly affect the mechanical properties