Page 130 - Welding of Aluminium and its Alloys

TIG welding 115

autogenous and a wire feed is not required, although this can be easily pro-
vided from a spool of wire fed from a cold wire feed unit. The wire should
be fed into the leading edge of the weld pool at a similar angle to that used
in manual welding. Both the start of the wire feeding and carriage travel
should be delayed until the weld pool is well established. When ending the
weld the current should be tapered down and the wire feed speed adjusted
to provide crater filling.
DCEN helium TIG is ideally suited to mechanisation since full advan-
tage can be taken of the increase in travel speed, which may be up to 10
times that of an argon shielded AC-TIG weld. It is also possible to weld
thick plates, up to 18mm thick, in a single pass, square edge preparation
with no filler metal, making this a very cost-effective method. The high
travel speeds possible with the technique may lead to undercutting, partic-
ularly if the welding current is increased in the expectation that this will
permit even higher travel speeds to be achieved. Short arc lengths are nec-
essary when autogenous welding, typically 0.8–1.5mm, and in some cir-
cumstances the electrode tip may be below the surface of the plate with the
arc force depressing the weld pool surface. Contraction during cooling will
cause upsetting to occur, resulting in a local thickening of the joint and
providing sufficient excess weld metal that the joint is not underfilled.

6.4 TIG spot and plug welding
By overlapping two plates a spot weld can be achieved by using the DCEN
TIG process to fuse through the top plate and melt into the lower plate.
Initial use of the process was carried out without a filler wire but hot crack-
ing problems with the alloys meant that it was confined to pure aluminium
up to 2mm thick. The development of automatic wire feeding systems
capable of feeding wire into the weld pool as the weld is terminated has
helped in extending the range of alloys that could be welded. Even with
this improvement, however, it has been found that the critical nature of the
surface condition causes welding defects such as oxide films.This means that
the process does not find general use because of low strength and poor
quality.
Further work has taken place using fully automated equipment and
helium shield gas and with low-frequency AC. These improvements have
resulted in a wider use of the process but MIG spot welding tends to be
preferred as providing better and more consistent quality.