Page 29 - Welding of Aluminium and its Alloys

Welding metallurgy 21

Table 2.2 Moisture permeability of gas hoses

Permeability Common name Hose composition

Highest Natural rubber Isoprene
Neoprene Polychloroprene
Ø Low-density polyethylene
PVC
Polyvinylchloride
Polypropylene
Polytetrafluoroethylene
Teflon High-density polyethylene
Lowest Polytrifluoro-chloroethylene

Of the plastic tubing the most porous is neoprene rubber, the least porous
polytrifluoro-chloroethylene. The best of all is an all-metal system. Any
plastic hoses should be kept as short and as small a diameter as possible
consistent with the application.
Also important is the fact that the moisture collects in the tube over a
period of time when no gas is flowing. The implication of this is that if
welding equipment is left idle for long periods of time the first few welds
to be made on recommencing welding may contain unacceptable porosity.
A systematic porosity problem always occurring, for example, at the com-
mencement of the first shift after a weekend break may be an indication of
this problem. Flushing the hoses through for a short time by operating the
torch trigger may help to reduce the amount of porosity. If this is done with
the MIG (GMAW, gas metal-arc welding) torch do not forget to slacken off
the wire drive rolls!
TIG welding wire should be cleaned with a lint-free cloth and a good
degreasant before use. Once the wire has been cleaned do not handle the
wire with bare hands but use a clean pair of gloves, store the wire in clean
conditions and weld within a short time of cleaning. For the MIG process
there are devices available that can be fitted around the wire where it enters
the torch liner in the wire feed unit and that will clean the wire as it passes
through. Best of all the wire should be shaved to remove any contaminants
and oxides that may have been pressed into the surface during the wire
drawing operation.
Cleanliness of the parent metal is also extremely important in achieving
low levels of porosity – it cannot be emphasised too strongly how impor-
tant this is. Thorough degreasing is essential, followed by a mechanical
cleaning such as stainless steel wire brushing to remove the oxide layer
which may be hydrated. Once degreased and wire brushed the parent mate-
rial should be welded within a short period of time, a period of four hours
frequently being regarded as acceptable. Further details of mechanical
cleaning, degreasing and workshop conditions are given in Chapter 4.