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Equipment for Plasma Welding Plasma welding derives its unique operating characteristics from the
torch design. As in TIG welding, the arc is formed between the end of a small
diameter tungsten electrode and the workpiece. However, in the plasma torch, the
electrode is positioned behind a fine bore copper nozzle. By forcing the arc to
pass through the nozzle, the characteristic columnar jet, or plasma, is formed.
As described in Job Knowledge for Welders, No 7, three different operating
modes can be produced by the choice of the nozzle bore diameter, current level
and plasma gas flow rate:
- Microplasma (0.1 to 15A) is equivalent to microTIG but
the columnar arc allows the welder to operate with a much longer arc length.
The arc is stable at low welding current levels producing a 'pencil-like' beam
which is suitable for welding very thin section material.
- Medium current plasma (15 to 100A) similar to
conventional TIG, is also used for precision welding operations and when a
high level of weld quality is demanded.
- Keyhole plasma (over 100A) produced by increasing the
current level and the plasma gas flow. It generates a very powerful arc
plasma, similar to a laser beam. During welding, the plasma arc slices through
the metal producing a keyhole, with the molten weld pool flowing around the
keyhole to form the weld. Deep penetration and high welding speeds can be
achieved with this operating mode.
 As the plasma arc is generated by the special torch arrangement and
system controller, the equipment can be obtained as an add-on unit to
conventional TIG equipment to provide additional pilot arc and separate plasma
and shielding gases. Alternatively, purpose-built plasma equipment is available.
Despite similarities in plasma and TIG equipment, there are several important
differences in the following components:
- power source
- torch
- backing system
- protective equipment
Power source The power source for plasma welding is almost exclusively
DC and, as in TIG, the drooping, or constant current, output characteristic will
deliver essentially constant current for a given power source setting. The power
source is ideal for mechanised welding as it maintains the current setting even
when arc length varies and, in manual welding, it can accommodate the natural
variations of the welder.
The plasma process is normally operated with electrode negative polarity to
minimise heat produced in the electrode (approximately 1/3rd of the heat
generated by the arc is produced at the cathode with 2/3rds at the anode).
Special torches are available, however, for operating with electrode positive
polarity which rely on efficient cooling to prevent melting of the electrode.
The positive electrode torch is used for welding aluminium which requires the
cathode to be on the material to remove the oxide film.
AC is not normally used in the plasma process because it is difficult to
stabilise the AC arc. Problems in reigniting the arc are associated with
constriction by the nozzle, the long electrode to workpiece distance and balling
of the electrode caused by the alternate periods of electrode positive polarity.
The square wave AC (inverter, switched DC) power source, with an efficiently
cooled torch, makes the use of the AC plasma process easier; rapid current
switching promotes arc reignition and, by operating with very short periods of
electrode positive polarity, electrode heating is reduced so a pointed electrode
can be maintained.
The plasma system has a unique arc starting system in which HF is only used
to ignite a pilot arc held within the body of the torch. The pilot arc formed
between the electrode and copper nozzle is automatically transferred to the
workpiece when it is required for welding. This starting system is very reliable
and eliminates the risk of electrical interference through HF.
TorchThe torch for the plasma process is considerably more complex than
the TIG torch and attention must be paid, not only to initial set up, but also
to inspection and maintenance during production.
NozzleIn the conventional torch arrangement, the electrode is
positioned behind the water cooled copper nozzle. As the power of the plasma arc
is determined by the degree of nozzle constriction, consideration must be given
to the choice of bore diameter in relation to the current level and plasma gas
flow rate. For a 'soft' plasma, normally used for micro and medium current
operating modes, a relatively large diameter bore is recommended to minimise
nozzle erosion.
In high current keyhole plasma mode, the nozzle bore diameter, plasma gas
flow rate and current level are selected to produce a highly constricted arc
which has sufficient power to cut through the material. The plasma gas flow rate
is crucial in generating the deeply penetrating plasma arc and in preventing
nozzle erosion; too low a gas flow rate for the bore diameter and current level
will result in double arcing in the torch and the nozzle melting.
The suggested starting point for setting the plasma gas flow rate and the
current level for a range of the bore diameters and the various operating modes
is given.
ElectrodeThe electrode is tungsten with an addition of between 2 and 5%
thoria to aid arc initiation. Normally, the electrode tip is ground to an angle
of 15 degrees for microplasma welding. The tip angle increases with current
level and for high current, keyhole plasma welding, an angle of 60 degrees to 90
degrees is recommended. For high current levels, the tip is also blunted to
approximately 1mm diameter. The tip angle is not usually critical for manual
welding. However, for mechanised applications, the condition of the tip and the
nozzle will determine the shape of the arc and penetration profile of the weld
pool penetration, so particular attention must be paid to grinding the tip. It
is also necessary to check periodically the condition of the tip and nozzle and,
for critical components, it is recommended the torch condition is checked
between welds.
Electrode set-backTo ensure consistency, it is important to maintain a
constant electrode position behind the nozzle; guidance on electrode set-back
and a special tool is provided by the torch manufacturer. The maximum current
rating of each nozzle has been established for the maximum electrode set-back
position and the maximum plasma gas flow rate. Lower plasma gas flow rates can
be used to soften the plasma arc with the maximum current rating of the nozzle
providing electrode set-back distance is reduced.
Plasma and shielding gasThe usual gas combination is argon for the
plasma gas and argon-2 to 8% H2 for the shielding gas. Irrespective of the
material being welded, using argon for the plasma gas produces the lowest rate
of electrode and nozzle erosion. Argon - H2 gas mixture for shielding produces a
slightly reducing atmosphere and cleaner welds. Helium gives a hotter arc;
however, its use for the plasma gas reduces the current carrying capacity of the
nozzle and makes formation of the keyhole more difficult. Helium - argon
mixtures, e.g. 75% helium - 25% argon, are used as the shielding gas for
materials such as copper.
Plasma gas flow rate must be set accurately as it controls the penetration of
the weld pool but the shielding gas flow rate is not critical.
Backing systemThe normal TIG range of backing bar designs or shielding
gas techniques can be employed when using micro and medium current techniques.
When applying the keyhole mode a grooved backing bar must be used, with or
without gas shielding or total shielding of the underside of the joint. Because
the efflux plasma normally extends about 10mm below the back face of the joint,
the groove must be deep enough to avoid disturbance of the arc jet; if the
efflux plasma hits the backing bar, arc instability will disturb the weld pool,
causing porosity.
Protective equipmentProtective equipment for plasma welding is as
described for TIG in Job Knowledge for Welders No 17. Regarding protection from
arc light, a similar Shade number to TIG at the same welding current level
should be used in head or hand shield. The glass will be slightly darker than
that used for MMA welding at the same current level.
Recommended shade number of filter for plasma welding:
| Shade Nunber |
Welding Current, A |
| Micro Plasma |
Plasma |
| 5 |
0.5 to 1 |
| 6 |
1 to 2.5 |
| 7 |
2.5 to 5 |
| 8 |
5 to 10 |
| 9 |
10 to 15 |
| 10 |
15 to 30 |
| 11 |
30 to 60 |
less than 150 |
| 12 |
60 to 125 |
150 to 250 |
| 13 |
125 to 225 |
above 250 |
| 14 |
225 to 450 |
See BS 639:1989 for further information on shade numbers.
Copyright by TWI, 1999
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