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 200A) 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
- backing system
- protective equipment
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.
The 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.
In 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.
The 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.
To 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 gas
The 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.
The 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 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|
||0.5 to 1
||1 to 2.5
||2.5 to 5
||5 to 10
||10 to 15
||15 to 30
||30 to 60
||less than 150
||60 to 125
||150 to 250
||125 to 225
||225 to 450
See EN 169:2002 for further information on shade numbers.
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This Job Knowledge article was originally published in Connect, June 1996. It has been updated so the web page no longer reflects exactly the printed version.