PLASMA ARC WELDING
Plasma Arc Welding process is a an arc welding process wherein coalescence is produced by heating by a constricted arc struck between a tungsten/alloy electrode and the job.
In plasma arc welding process, two inert gasses employed ,one form as shielding gas and the second gas forms the arc plasma .
It is a temporary state of ionized gas in Plasma Arc Welding method, charged by electric current and performed as a conductor of electricity . In ionized state the atoms of gas break into electrons and charged as (+)and (-) and system contains a mixture of ions,the degree of ionization may be between 1% to more than 100%
Plasma Arc Welding Operation
The arc is compressed with the help of water cooling and a small diameter nozzle and consequently the pressure and temperature increase.and thus improves arc stability,shape and heat transfer characteristics.
Type of Plasma arc welding process
Basically plasma arc welding process can be deviled into two types.
- Non-transferred arc welding process
- Transferred arc welding process
Non-transferred arc welding process
In this method,arc is formed between the tungsten and the nozzle where tungsten is in(-)polarity and the watercooled constricting nozzle(+).Arc plasma comes out from the nozzle as a flame.in this method arc is free and work piece does not form a part of the electrical circuit.thus it can controlled better by moving one place to other place.
Transferred arc welding process
In this method ,arc is formed between electrode(-_and the work piece (+). To perform the work, the arc is transferred from the electrode to the work piece,with high energy density and plasma jet velocity,that is why it is employed to cut and melt metals.With this process, stainless steel and non-ferrous metal can cut also.
DC power source or rectifier with open circuit voltage of 70 volt or above is suitable for plasma arc welding. The required high voltage can be achieved by operating the series of two power sources or The arc can be started with argon at normal open circuit voltage and then turned to helium.
Plasma arc welding Torch
It may be hand operated or mechanized (for automated system).basically it is water cooled because of using higher current.
High frequency system
H.F.is required for arc ignition. It may be with the system built or separate.
- Voltage controller
- Current and gas decay controller
For plasma arc welding process, two inert gases required . One is to form arc plasma with low pressure and low flow rate, it kept low to avoid weld metal turbulence,this gas can not provide proper shielding to weld metal that is why other gas is used.
Weld joint and Base metal
With plasma arc welding process, up to 25 mm thicker joint can be welded like:-
- Butt joint,
- J – joint,
- V- Joints,with
- square groove,
- single and double U groove,
- single and double V groove,
- Fillet weld,
- T welds,
- Edge-flange weld,
- Carbon and low alloy steels,
- stainless steels,
- copper alloys,
- Nickel and cobalt base alloys,
- Titanium alloys and
- aluminium alloys.
Advantage of Plasma ARC
- Arc stability
- well penetration
- Excellent weld quality
- High speed welding
Disadvantage of Plasma ARC
- Dangerous ultraviolet and Infra-red rays created
- Limited to weld thickness up to 25 mm.
- Complicated as compare to TIG.