In
Plasma-arc Welding (PAW) is an arc welding process, a concentrated plasma arc
is produced and directed towards the weld area. The arc is stable and reaches
temperatures as high as 33,000°C. A plasma is an ionised very hot gas composed
of nearly same numbers of electrons and ions. The plasma starts between the
tungsten electrode and the orifice by a low current pilot arc. What makes
plasma-arc welding unlike other processes is that the plasma arc is
concentrated because it is forced through a relatively small orifice. Operating
currents usually are less than 100 A. When a filler metal is used it is fed
into the arc as is done in Gas Tungsten-arc Welding. Arc and weld-zone
shielding is supplied by means of an outer-shielding ring and the use of inert
gases like argon, helium or mixtures.
There are two methods of plasma-arc
welding:
§ In the
transferred-arc method of plasma-arc welding(in pic left side),
the workpiece being welded is part of the electrical
circuit. The arc transfers from the electrode to the work
piece hence the term transferred.
§ In the non transferred-arc
method of plasma-arc welding(in
pic right side), the arc occurs between the electrode and the nozzle and the
heat is carried to the workpiece by the plasma gas. This thermal-transfer
mechanism is similar to that for an oxy-fuel flame.
Two
methods of plasma-arc welding processes (a) transferred and (b) non transferred
Compared
to other arc welding processes, Plasma-arc Welding process has better arc
stability, less thermal distortion and higher energy concentration, thus
permitting deeper and narrower welds. PAW has higher welding speed ranges from
120 mm/min to 1000 mm/min. A variety of metals can be welded with part
thicknesses less than 6 mm.
The high heat concentration can penetrate completely through the joint with
thicknesses as much as 20 mm for some titanium and aluminium alloys. In the
keyhole technique, the force of the plasma arc displaces the molten metal and
produces a hole at the leading edge of the weld pool. Plasma-arc welding often
is used rather than Gas Tungsten-arc welding for butt and lap joints because of
its higher energy concentration, better arc stability and higher speed of
welding. Proper training and skill are required for operator who works on this
equipment.