Plasma arc welding

The key difference from GTAW is that in PAW, the electrode is positioned within the body of the torch, so the plasma arc is separated from the shielding gas envelope.

A typical value of temperature obtained in a plasma jet torch is on the order of 28,000 °C (50,400 °F), compared to about 5,500 °C (9,930 °F) in ordinary electric welding arc.

The plasma arc welding and cutting process was invented by Robert M. Gage in 1953 and patented in 1957.

A transferred arc possesses high energy density and plasma jet velocity.

Medium plasma uses current between 10 and 100 amps and is used for higher-thickness plate welding with filler wire or autogenous up to 6 mm (0.24 in) plates and metal deposition (hardfacing) using specialised torches and powder feeders (PTA) using metal powders.

High-current plasma above 100 amps is used with filler wires welding at high travel speeds.

Other applications of plasma are plasma-cutting, heating, deposition of diamond films (Kurihara et al. 1989), material processing, metallurgy (production of metals and ceramics), plasma-spraying, and underwater cutting.

Hydrogen, because of its dissociation into atomic form and thereafter recombination generates temperatures above those attained by using argon or helium alone.

A keyhole effect is achieved through right selection of current, nozzle-orifice diameter and travel speed, which create a forceful plasma jet to penetrate completely through the work piece.

The major advantages of the keyhole technique are the ability to penetrate rapidly through relatively thick root sections and to produces a uniform under bead without mechanical backing.

While welding thicker pieces, in laying others than root run, and using filler metal, the force of plasma jet is reduced by suitably controlling the amount of orifice gas.

This process uses a non-consumable tungsten electrode and an arc constricted through a fine-bore copper nozzle.

Since PAC was introduced by Praxair Inc. at the American Welding Society show in 1954, many process refinements, gas developments, and equipment improvements have occurred.