1. Plasma Arc Welding
The objective of the Plasma Arc Welding (PAW) process is to increase the energy level of the arc plasma in a controlled manner. This objective has been achieved by providing a special gas nozzle around a tungsten electrode operating on DCEN. The constricted plasma formed is highly ionized and concentrated.
There are two variants of the Plasma Arc Welding (PAW) process. One is the transferred arc process and the second is the non-transferred arc process. In the transferred arc mode, an arc is struck between the electrode and the workpiece. In the non-transferred mode, the arc is struck between the electrode and the nozzle, thus eliminating the necessity to have the work as a part of the electrical system.
Linnert, Welding Metallurgy,
AWS, 1994

2. A small flow of argon is supplied through the nozzle and its constricting orifice to form the arc plasma. Sometimes diatomic gases like nitrogen or hydrogen are used as plasma gases because of good heat transfer characteristics.
Shielding of the arc and weld zone is provided by a gas flowing through an encircling outer nozzle assembly. The shielding gas can be argon, helium, or mixtures.
Two welding techniques are possible with PAW: melt-in, and keyhole. The Low current melt-in technique is normally used when welding thin sheet sections. Narrower welds can be made with PAW than with GTAW because of the constricted arc. High current Melt-in can be done on butt or lap joints up to about 1/8 inch thick. PAW can also be used on Titanium and other reactive metals.
AWS Welding Handbook
Ed 8 Vol

3. In the keyhole mode, a penetrating hole is formed at the leading edge of the weld pool. The molten weld metal flows around the hole and solidifies behind the keyhole to form the weld bead. Therefore, keyhole welds are complete penetration welds with high depth to width ratios. This results in low weld distortion. With operating currents up to 300 amperes, this mode can be used to weld materials up to about 3/4 inch thick, and to weld titanium and aluminum alloys.
Stinchcomb, Welding Technology
Today, Prentice Hall, 1989

4. Advantages Greater Energy Concentration
Improved arc stability over other processes
Higher heat content, Higher travel speeds
Greater penetration capabilities
Finer sections can be welded with low current PAW than GTAW
Advantages
Greater Energy Concentration
Improved arc stability over other processes
Higher heat content, Higher travel speeds
Greater penetration capabilities
Finer sections can be welded with low current PAW than GTAW
The major advantage of the plasma arc process is the greater energy concentration of the streaming plasma. This offers improved stability of the arc with higher heat content and thus faster travel speeds than other arc processes.
The stiffness of the plasma stream also promotes deeper penetration, and because of the heat concentration, finer sections can be welded.

5. Disadvantages
Operator skill required is slightly greater than for GTAW
Equipment more expensive
Orifice replacement necessary
Disadvantages
Operator skill required is slightly greater than for GTAW
Equipment more expensive
Orifice replacement necessary
The penalties to be paid are that greater operator skill is required, the equipment is more expensive and orifice tips may require frequent replacement.