Advanced Welding Processes: Technologies and Process Control

Advanced gas tungsten arc welding 87
Non-transferred arc mode
In the non-transferred arc mode of operation, the power supply is applied
between the electrode and the constricting orifice. The electrode is usually
connected to the negative side of the power supply and, for this reason, the
orifice is referred to as the anode. This process mode is illustrated schematically
in Fig. 6.10. In this mode, very little energy is transferred to the workpiece
and its main use is as a ‘pilot’ arc which enables the main transferred arc to
be established rapidly. The pilot arc may be established by high-frequency
arc starting or a simple touch-starting system within the torch. Once the pilot
arc is established, a main or transferred arc may be started at any time by
completing the circuit between the power source and the workpiece as shown
in the diagrams above.
Equipment requirements
The basic equipment requirements for the plasma welding system are slightly
more complex than either the conventional or dual-gas GTAW system. They
consist of a welding power source, a control unit and a plasma torch.
A power source with a direct current (DC) output is normally used (although
it is possible with suitable equipment to use AC for aluminium alloys and
variable polarity or ‘square wave’ AC units referred to above have been
developed as plasma power supplies). The constant-current static volt–amp
output characteristic used for GTAW power sources provides a suitable main
arc supply.
Water cooled
copper anode
Tungsten
cathode
Arc
Workplace
6.10 Non-transferred arc mode.
Power
source