Page 145 - Welding of Aluminium and its Alloys

130 The welding of aluminium and its alloys

The tip should be recessed in the gas shroud by at least 5mm when
welding in spray transfer. If the tip is too close to the end of the gas shroud
there is an increased risk of spatter damaging the tip. If the tip protrudes
from the shroud then there is a risk of the tip touching and melting into the
weld pool. This will cause weld pool cracking, may give rise to ‘bird’s
nesting’ and will require the tip to be replaced.

7.3 Welding consumables

7.3.1 Shielding gases
The shielding gases, as with TIG welding, are the inert gases argon and
helium or combinations of these two. Other, active, gases such as oxygen or
nitrogen even in small amounts will give porosity and smutting problems.
The most commonly used gas is argon which is used for both manual and
some automatic welding. It is substantially cheaper than helium and pro-
duces a smooth, quiet and stable arc, giving a wide, smooth weld bead with
a finger-like penetration to give a mushroom-shaped weld cross-section.
Argon, however, gives the lowest heat input and therefore the slowest
welding speeds.There is therefore a risk of lack of fusion defects and poros-
ity on thick sections. Argon may also give a black sooty deposit on the
surface of the weld. This can be easily removed by wire brushing. Sections
of 3mm thick butt welds using conventional and pulsed current are illus-
trated in Fig. 7.13.Thicker section butt and fillet welds are illustrated in Fig.
7.14. In these thicker section welds the characteristic finger penetration of
an argon gas shield can be seen.
Helium increases the arc voltage by as much as 20% compared with
argon, resulting in a far hotter arc, increased penetration and wider weld

(b)
(a)
7.13 (a) MIG, argon shielded 0.8mm wire, 3mm thick unbacked plate
butt, flat position. (b) Pulsed MIG, argon shielded, 0.8mm diameter
wire, 3mm thick unbacked plate butt, flat position.