When welding high integrity components, a shielding gas is used to protect the underside of the weld pool and weld bead from oxidation. To reduce the amount of gas consumed, a localised gas shroud for sheet, dams or plugs for tubular components is used. As little as 5% air can result in a poor weld bead profile and may reduce corrosion resistance in materials like stainless steel. With gas backing systems in pipe welding, pre-weld purge time depends on the diameter and length of the pipe. The flow rate/purge time is set to ensure at least five volume changes before welding. Stick on tapes and ceramic backing bars are also used to protect and support the weld bead. In manual stainless steel welding, a flux-cored wire instead of a solid wire can
be used in the root run. This protects the underbead from oxidation without the need for gas backing.
Inserts
A pre-placed insert can be used to improve the uniformity of the root penetration. Its main use is to prevent suck-back in an autogenous weld, especially in the overhead position. The use of an insert does not make welding any easier and skill is still required to avoid problems of incomplete root fusion and uneven root penetration.
Protective equipment
A slightly darker glass should be used in the head or hand shield than that used for MMA welding.
Recommended shade number of filter for TIG welding:
Shade number Welding current A
9 Less than 20 10 20 to 40 11 40 to 100 12 100 to 175 13 175 to 250 14 250 to 400
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 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.
Torch
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.
Nozzle
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’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.
Electrode set-back
To ensure consistency, it is important to maintain a constant electrode position behind the nozzle; the torch manufacturer provides guidance on electrode setback and a special tool. The maximum current rating of each nozzle has been established for the maximum electrode setback 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 setback distance is reduced.