A plasma is a gas which is heated to an extremely high temperature
and ionized so that it becomes electrically conductive. The plasma arc welding
process uses this plasma to transfer an electric arc to a work piece. The metal
to be welded is melted by the intense heat of the arc and fuses together. In
the plasma welding torch a Tungsten electrode is located within a copper nozzle
having a small opening at the tip. A pilot arc is initiated between the torch
electrode and nozzle tip. This arc is then transferred to the metal to be
welded.
The Pilot Arc type uses a two cycle
approach to producing plasma, avoiding the need for initial contact. First, a
high-voltage, low current circuit is used to initialize a very
small high-intensity spark within the torch body, thereby generating a small
pocket of plasma gas. This is referred to as the pilot arc. The pilot arc has a return
electrical path built into the torch head. The pilot arc will maintain itself
until it is brought into proximity of the workpiece where it ignites the main
plasma cutting arc. Plasma arcs are extremely hot and are in the range of 25,000 °C (45,000 °F).
The other method is performed by making work piece as cathode and then the
whole procedure is performed again.
Thermal-plasmas are produced by plasma torches also known as
plasmatrons. Depending on the primary source, which can be direct current,
alternating current at mains frequency,
or at radio frequency, they are known as dc, ac or rf torches. A conventional
dc plasma torch consists of a tungsten rod cathode and a water-cooled copper anode,
shaped in the form of a nozzle (Figure 1). The two electrodes are separated by an insulator,
which also has an inlet for plasma gas. When a gas is introduced in the electrode
gap and a dc arc is established between the electrodes, the arc is pushed
through the nozzle resulting in a high temperature, high velocity flame.
Electromagnetic forces and gas stabilization constrict the arc column and heat
the plasma to nearly 20,000 degrees. The body of the torch consists of cooling
chambers for cathode and anode. The torch is supplied with water and power through
water-cooled cables which are in turn coupled to the main power supply and
water headers. There are several variations of the torch based on differences
in the stabilization of the arc, electrode geometry, plasma gas, electrode
cooling and the type of gas flow. The plasma jet can be operated in a
transferred/non-transferred arc depending
on whether the arc is electrically transferred to the work piece or not. The
arc normally passes from the cathode through the nozzle orifice to the
anode/ground as it represents the path of least resistance.
The normal combination of gases is argon for the plasma gas,
with argon plus 2 to 5% hydrogen for the shielding gas only for austenitic
stainless steels. Helium can be used for plasma gas but because it is hotter
this reduces the current rating of the nozzle. Helium's lower mass can also
make the keyhole mode more difficult .