INE SKYLINE HFP3000 Manuel D'instructions page 25

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Pulsed current, direct polarity
The principle on which this procedure is based shares
the same features of the former one. The only thing to
be added is that the use of a pulsed current allows
better control of the weld pool in particularly difficult
conditions and especially when working with thin
materials.
The improvement introduced by this technique consists
in a reduced area of thermal alteration and fewer
deformations, cracks and gas bubbles inside the
melting area.
TIG welding is particularly suitable for those welds
which require high quality without even backwelding. It
is also used in those cases which require a good weld
bead without further processing (e.g. grinding). Since
TIG welding is more complex than other welding
procedures, the edges should be carefully cleaned and
prepared: a single-Vee caulking is suggested in case of
thickness above 3 mm.
When welding copper and aluminium, due to the
flowability of these metals, the use of a metal support
(e.g. a stainless steel support) is suggested when
backwelding.
Electrodes should be sharpened by means of a specific
grinder before being used in welding with currents on
direct polarity.
As shown in the figure above, the angle may be very
acute in the case of low currents (30° up to 30-40 A),
whereas it should be obtuse in the case of high currents
(over 90° for currents above 200A).
The electrode should be secured into
the torch so that its maximum
protrusion from the torch tip is 6÷9
mm, as shown in the figure (longer
protrusion only in the case of interior
angle welds).
The best results with this welding technique are
achieved by holding the torch and the filler metal stick
as shown in the following figure.
FILLER
MATERIAL
ß<30°
WORKPIECE
30°÷120°
TORCH
60°÷80°
Page 23
When either variable or reverse polarity is used, the
electrode tip should be round instead of sharp as in the
former case, due to the extreme heat developed on it. If
the electrode melts during the welding (its tip looks
drop-like), it should be replaced with a thicker one or, if
welding with variable polarity, the wave should be
adjusted so as to reduce the current positive
polarisation down to 20%.
As regards the material to be welded, the use of the
following electrodes is suggested:
•
2%- thorium tungsten (red-coloured) for steel, steel
alloys, nickel, copper and titanium
•
pure tungsten (green-coloured) or tungsten with
zirconium (white-coloured) for aluminium and
magnesium
The table below shows the range of amperage used
according to the electrode diameter and the current
polarity used.
Electrode
Direct current
diameter
Direct polarity
(mm)
1 10÷70
1.6 60÷150
2.4 100÷250
3.2 200÷400
4.8 350÷800
The filler metal sticks to be used are those commonly
sold for the purpose. These sticks are made of the
same base material as the workpiece and, in the case
of copper and aluminium, contain a small percentage
(lower than 10%) of de-oxidising agents such as silicon
or magnesium.
The shielding gas commonly used due to its low cost is
argon. Helium or argon/helium mixtures can be used
especially when welding thick materials in order to
improve penetration and increase welding speed.
Gas flow rates normally vary, as current increases, from
8 to 12 l/min in the case of argon and from 14 to 24 l/min
in the case of helium.
In order to prevent oxidation the post-gas flow should
be adjusted so that the weld and the electrode have
time to cool before being exposed to the atmosphere
oxygen. This time should be around a few seconds.
Alternate
Direct current
current
Reverse
Variable
polarity
polarity
10÷15 10÷50
10÷20 40÷100
15÷30 80÷150
25÷50 130÷230
45÷80 200÷320