Page 89 - Welding of Aluminium and its Alloys

78 The welding of aluminium and its alloys

Bevel angle
15 to 20°
Root
radius 1.5 to For AC-TIG welding the
2mm land should be 4.5mmmin,
for DC -ve TIG and MIG
the land should be 2mmmin

Parallel land
zero gap

5.10 ‘Landed’ single-V butt weld.

The strength of a sound, defect-free butt weld generally matches that of
the filler metal or the annealed strength of the parent metal, as discussed
in Chapter 2. The butt joint is the best in a dynamic loading environment,
particularly if the excess weld metal is dressed flush. To achieve the best
properties the two component parts require accurate alignment, which
implies adequate tacking, jigging and fixturing.

5.5.2 Backing bars and backing strips
Although it is possible to deposit a sealing run on the reverse side of a butt
weld without a back-gouge, this cannot be relied upon to give a sound,
defect-free weld. Single sided joints may be welded by TIG to produce a
sound root pass but the conventional (non-pulsed) MIG process often
requires either permanent or temporary backing on which to deposit the
MIG root pass. The purpose of the backing bar or strip is to support the
root pass where conditions make the control of the bead difficult. Conven-
tionally, a backing bar is temporary and can be lifted away as soon as the
weld has been completed, and a backing strip is a permanent part of the
joint. A backing bar or strip can greatly simplify the task of setting up
the joint – for example, root gap variations are easily coped with and joints
can be self-jigging, good root bead appearance can be achieved and costs
can be reduced.
A grooved temporary backing bar will produce good penetration bead
shape, the groove being used as a mould for the molten weld metal. This
will provide a better dynamic performance than a permanent backing strip.
Backing bar material can be an inexpensive mild steel but a longer life can
be obtained from the bar with less risk of contamination if stainless steel is
used. Ceramic backing, provided as a flexible strip of tiles or on adhesive
tape, can also be used. Copper or copper alloys should be avoided because