while it is
recessed in the orifice gas nozzle in PAW (Fig. 1b). The main difference
is in the construction of the torch. In plasma arc welding,
the plasma arc is tightly constrained as shown in Fig. 1. A small
amount of pure argon gas flow is allowed through the inner orifice
surrounding the tungsten electrode to form the plasma gas. Because
of the squeezing action of the constraining nozzle, the arc in PAW is
concentrated and straight. This constriction increases the heat contained
per unit volume of the arc plasma. Thus, arc temperatures
of the order of 11,000 ◦C are not unusual in PAW.
To initiate the arc in PAW, a low current pilot arc is obtained
between the electrode and the constricting nozzle first, which ionizes
the plasma gas flowing through the nozzle. The plasma gas
flowing through the constriction reaches a very high temperature,
and provides a low resistance path to initiate the plasma arc