Arc welding process that produces coalescence of metals by heating them with a constricted arc between an electrode and the work piece (transferred arc) or between the electrode and the water-cooled constricting nozzle (non transferred arc) . Plasma: A gaseous mixture of positive ions, electrons and neutral gas molecules

1. Plasma ARC Welding
(PAW)
Vivek

2. Content
OIntroduction
OHow Plasma Welding works?
OEquipment
OWelding Modes in PAW
ODifference between PAW and TIG
OControl System
OAdvantages
ODisadvantages
OApplications

4. PAWWelding

5. How Plasma Welding Works
Plasma:
O Gas which is heated to an extremely high temperature and
ionized so that it becomes electrically conductive.
O PAW process uses this plasma to transfer an electric arc to the
work piece.
O The metal to be welded is melted by the intense heat of the arc
and fuses together.
Objective of PAW:
O To increase the energy level of the arc plasma in a controlled
manner.
O This is achieved by providing a gas nozzle around a tungsten
electrode operating on DCEN.

6. 2 Variants of PAW:
Transferred arc mode:
O Arc is struck between the electrode(-) and the work piece(+)
O Used for high speed welding and
O Used to weld Ceramics, steels, Aluminum alloys, Copper alloys,
Titanium alloys, Nickel alloys.
Non-transferred mode:
O Arc is struck between the electrode(-) and the nozzle(+), thus
eliminating the necessity to have the work as a part of the
electrical system.
O Arc process produces plasma of relatively low energy density.
O Since the work piece in non-transferred plasma arc welding is
not a part of electric circuit, the plasma arc torch may move from
one work piece to other without extinguishing the arc.

7. Fig. Arc in Plasma Arc Welding

8. Equipment
Power Supply
O A DC power source (generator or rectifier) having drooping
characteristics and open circuit voltage of 70 volts or above is
suitable for PAW.
O Rectifiers are generally preferred over DC generators.
O Working with He as an inert gas needs open circuit voltage
above 70 volts. This voltage can be obtained by series operation
of two power sources; or the arc can be initiated with argon at
normal open circuit voltage and then helium can be switched on.
High frequency generator and current limiting resistors
O Used for arc ignition.
Plasma Torch
O Either transferred arc or non transferred arc typed

9. Shielding gases
O Shields the molten weld from the atmosphere.
O Two inert gases or gas mixtures are employed.
O Argon(commonly used), Helium, Argon+Hydrogen and
Argon+Helium, as inert gases or gas mixtures.
O Helium is preferred where a broad heat input pattern and flatter
cover pass is desired.
O A mixture of argon and hydrogen supplies heat energy higher
than when only argon is used and thus permits higher arc alloys
and stainless steels.
O For cutting purposes a mixture of argon and hydrogen (10-
30%) or that of nitrogen may be used.
O Hydrogen, because of its dissociation into atomic form and
thereafter recombination generates temperatures above those
attained by using argon or helium alone.

10. Welding Parameters:
O Current 50 to 350 Amps,
O Voltage 27 to 31 Volts,
O Gas flow rates 2 to 40 liters/min. (lower range for orifice
gas and higher range for outer shielding gas),
O DCSP is normally employed except for the welding of Al in
which cases water cooled copper anode and DCSP are
preferred.
O Temp of Jet 50000°F (28000°C)
Current and gas decay control
O To close the key hole properly while terminating the weld
in the structure.
Fixture
O To avoid atmospheric contamination of the molten metal
under bead.

11. Modes in paw
Micro-plasma welding
O Welding Current from 0.1A to 15A.
O Arc Length is varied up to 20mm
O Used for welding thin sheets (0.1mm thick), and wire and
mesh sections.
Medium-plasma welding
O Welding current from 15A to 100A.
Keyhole welding
O Welding Current above 100A, where the plasma arc
penetrates the wall thickness.
O Widely used for high-quality joints in aircraft/space,
chemical industries to weld thicker material (up to 10mm
of stainless steel) in a single pass.

12. Quality and Common Faults
 Sunken Bead Undercut too much penetration
 Welding current is too high
 Travel Speed is too low
 Bead too small, Irregular Little Penetration
 Welding Current and Plasma Gas Flow is too
low
 Travel is too fast
 Undercut and Irregular Edges
 Plasma Gas Flow is too high
 Proper Size Bead, Even Ripple and Good
Penetration
 Correct Current, Even torch movement, Proper
Arc Voltage and Plasma Gas Flow

13. Difference between PAW and TIG
Plasma Arc Welding Tungsten Inert Gas Welding
 Two gases are used, One for Plasma Gas and
other for Shielding Gas.
 Only one gas used, which forms plasma
as well as shields the arc and molten
weld pool.
 Uses Constricted Arc.  Uses Non-Constricted Arc.
 Temp. of about 11000°C is achieved.  Temp. of about 4000°C is achieved.
 Deep Penetration is achieved.  Penetration obtained is not so deeper.
 No Filler Material is required.  More Filler Material is required.
 Fast Metal Deposition Rate.  Metal Deposition Rate is not so faster.
 Inert Gas Consumption is very high.  Inert Gas Consumption is very low.
 Costly welding equipment.  Less costly welding equipment.
 Cutting of Hard and Brittle Material is
possible.
 Cutting of Hard and Brittle Material is
not possible.

14. Control System

15. Advantages
O Permits faster metal deposition rate and high arc travel
speed as compared to TIG
O Uniform penetration with high welding rate is possible
O Stability of arc and Excellent weld quality
O Can produce radiographic quality weld at high speed
O Can weld steel pieces up to about half inch thick, square
butt joint
O Useful for semi automatic and automatic processes.
O Process is very fast and clean
O Requires less operator skill due to good tolerance of arc to
misalignments;
O High penetrating capability (keyhole effect);

16. Disadvantages
O Special protection is required as Infrared and UV
Radiations is produced
O Consumption of Inert Gas is high
O Needs high power electrical equipment.
O Gives out ultraviolet and infrared radiation.
O Operation produces a high noise of the order of 100dB.
O Expensive equipment;
O Can weld only upto 25mm thickness.
O High distortions and wide welds as a result of high heat
input (in transferred arc process).
O More chances of Electrical hazards.

18. Base Metal weldable by Plasma Arc Process:
Easily Weldable:
O Al, Cu-base alloys (Cu, Cu-Ni), Magnesium, Ni-base
alloys (Inconel, Nickel, Monel), Precious Metals, Steels
(Low Carbon Steel, Low Alloy Steel, Stainless Steel, Tool
Steels), Titanium (upto 8mm thick) etc...
Acceptable but weldable with care:
O Cast, Malleable, Nodular Iron, Wrought Iron, Lead,
Tungsten etc...
Possible but not Popular:
O Bronzes, Brass, Nickel Silver, Lead, Zinc etc...

19. P Protected electrode, offers long times before electrode maintenance
(usually one 8 Hr Shift)
L Low amperage welding capability (as low as 0.05 amp)
A Arc consistency and gentle arc starting produce consistent welds,
time after time
S Stable arc in arc starting and low amperage welding
M Minimal high frequency noise issues, HF only in pilot arc start, not
for each weld
A Arc energy density reaches 3 times that of GTAW. Higher weld
speeds possible
W Weld times as short as 5 millisec (.005 sec)
E Energy density reduces heat affected zone, improves weld quality
L Length of arc benefit due to arc shape and even heat distribution
D Diameter of arc chosen via nozzle orifice