Page 148 - Welding of Aluminium and its Alloys

MIG welding 133

For manual welding and some mechanised applications mixtures of argon
and helium give good results with characteristics intermediate between the
two gases. These mixtures are useful on thicker materials because they
increase the heat input and provide a wider tolerance box of acceptable
welding parameters than pure argon. They will also improve productivity
by enabling faster travel speeds to be used.The most popular combinations
are 50% and 75% of helium in argon.Typical welds using 50% helium/50%
argon are illustrated in Fig. 7.16. These show weld bead shapes intermedi-
ate between the pure argon and pure helium welds in Figs. 7.14 and 7.15.
The last point to be made concerning gases is purity, already covered in
Chapter 3, but worth re-emphasising because of the major effect that this
has on weld quality. Shielding gases must have a minimum purity of
99.998% and low moisture levels, ideally with a dew point less than -50°C
(less than 39ppm H 2 O) – do not forget that this is at the torch, not at the
outlet of the cylinder regulator!

7.3.2 Welding filler wire

The wire acts as both the filler metal and the anode in the welding arc.
In order to do this the wire picks up the welding current by a rubbing
contact between the wire and the bore of the contact tip. Filler wire diam-
eters vary from 0.8mm to 3.2mm which results in a high surface area to
volume ratio. This relatively large surface area requires the wire to be kept
scrupulously clean since surface contamination will give rise to porosity.
Wires should be stored in clean, dry conditions in their unopened packag-
ing where possible. Wires that have been in store for a substantial period
of time, e.g. 6 months or more, even when stored in their original packag-
ing can deteriorate and give rise to porosity. If left on the welding machine
overnight or over weekends they should be protected from contamination
by covering the reel with a plastic bag. In critical applications it may be nec-
essary to remove the reel from the machine and store it in a steel can
between periods of use.
Condensation can form on the wire if it is brought into a warm fabrica-
tion shop from a cold store, and in conditions of high humidity moisture
may once again form on the wire. Some power sources incorporate heaters
in the wire feeder to prevent this from happening. If condensation is trou-
blesome and this facility is not available, a 40 watt light bulb installed in the
wire feeder cabinet provides sufficient heat to maintain the wire in a dry
state.
It is possible to obtain wire cleaning devices that clip on the wire at the
point where it enters the wire feed cable.These devices consist of a felt pad
carrying a cleaning fluid which removes contaminants as the wire passes