Page 97 - Welding of Aluminium and its Alloys

86 The welding of aluminium and its alloys

5.20 Balanced welding in a butt weld. Courtesy of TWI Ltd.

• Where appropriate use a double-V preparation and balance the welding
about the plate centre line (Fig. 5.20).
• Use the lowest heat input process and welding parameters, consistent
with achieving the required quality. Of the fusion welding processes the
power beam processes – electron beam or laser welding – will give the
least distortion.
• Use the fewest number of weld passes to fill the joint. This implies that
a high heat input process will result in less distortion than a low heat
input process. This may seem to be in conflict with the point above but
it should be remembered that it is the total heat input to the joint that
is significant.The sum of heat inputs from a large number of small passes
will result in a higher total heat input than that from a small number
of large beads for the same volume of weld metal. TIG welding, for
instance, will almost always give more distortion than MIG welding the
same component.
• On long welds, weld from the centre towards the ends. On items such
as beams this will approximately halve the amount of bowing that would
be expected if the beam was welded by starting at one end and welding
through to the opposite end.
• Use a ‘back-step’ sequence, i.e. weld from a cold section of joint towards
a hot section already welded (Fig. 5.21).
• Break the construction down into sub-assemblies, weld the individual
sub-assemblies and assemble the complete item, balancing any distor-
tion from the individual items to minimise the overall distortion.
• Preset the components (Fig. 5.22). If the amount of distortion is known
or can be predicted, the items can be assembled and offset by the
amount of expected distortion. On completion of welding the distortion