Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
The control unit which usually contains the non-transferred pilot arc supply,
the gas controls and the arc ignition system may take the form of an ‘add on’
box or may be built into the main power supply.
The torch is slightly more complex than a GTAW torch as shown in Fig.
6.11. It must have provision for cooling of the replaceable copper constricting
orifice, supply of plasma gas, and a separate supply of shielding gas. The
non-consumable electrode material is usually thoriated tungsten. As in GTAW,
the process may be used without added filler (autogenous welding) but, if a
filler is required, this is usually added to the leading edge of the molten pool.
Modes of operation of plasma welding
The plasma welding process is normally operated with direct current electrode
negative (DCEN). Although, as with GTAW, both AC and DCEP (direct
current electrode positive) operation can be used for aluminium and its alloys,
this usually requires special torch designs and larger electrodes. Welding
may be carried out in the ‘melt in’ mode in a similar manner to GTAW, the
only difference being the use of a constricted arc as the heat source.
Alternatively, the keyhole mode may be used; this takes advantage of the
higher energy density and the increased arc force in the plasma and is described
in more detail in Chapter 8.
Features and applications of plasma welding
The features and applications of plasma welding differ according to the
current range used, i.e.:
Plasma gas
Electrode
power
6.11 Plasma welding torch.
Cathode
Water
cooling
Anode
connection
Shielding
gas
Orifice