INE Skyline 1500 Bedienungsanleitung Seite 16

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Direct current, reverse polarity
By this procedure the torch is connected to the positive
socket of the power source and the work return lead to
the negative one. Most of the heat is directed to the
electrode which, even if a thick one, reaches a very high
temperature at low amperage; as a consequence, the
electrode will very soon wear out (NB: if the adequate
amperage is exceeded, the electrode will melt due to
the extreme heat).
This type of polarity offers good cleaning action, but
shallow penetration. It is suggested only for welding
alloys covered with a layer of refractory oxide at melting
temperature above that of the metal.
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.
Alternate current, variable polarity
The torch may be connected either to the positive or to
the negative socket. This technique is a combination, at
successive intervals, of the direct and reverse polarity
procedures.
In the interval in which the electrode is positively
polarised the deoxidising action, i.e. the cleaning of the
metal prevails. In the interval in which the electrode is
negatively polarised, the welding of the joint prevails.
By adjusting the wave balance, the one action is
favoured against the other.
It is necessary to point out that, in order to obtain a
stable arc, welding current shall be a square wave, not
a sine wave (e.g. the current supplied by a
non-professional welding machine for welding with acid
or rutile electrodes), since the polarity reversal shall be
instant, not gradual (as in the case of power sources
generating sine waves), otherwise the arc will blow out.
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.
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
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