Plasma arc welding

1. Plasma ARC Welding
(PAW)

2. Content
1. Introduction
2. How plasma welding works
3. Equipment
4. Welding modes in PAW
5. Advantages
6. Disadvantages
7. Applications

3. 1 - Introduction
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.

4. Plasma ARC Welding

5. 2 - How Plasma Welding Works
Plasma
• Gas which is heated to an extremely high temperature and
ionized so that it becomes electrically conductive
• PAW process uses this plasma to transfer an electric arc to the
work piece
• The metal to be welded is melted by the intense heat of the arc
and fuses together
Objective of PAW
• To increase the energy level of the arc plasma in a controlled
manner
• This is achieved by providing a gas nozzle around a tungsten
electrode operating on direct current electrode negativity

6. Variants of PAW
Transferred arc mode
• Arc is struck between the electrode(-) and the work piece(+)
• Used for high speed welding
• Used to weld Ceramics, steels, Aluminum alloys, Copper alloys,
Titanium alloys, Nickel alloys
Non-transferred mode
• Arc is struck between the electrode(-) and the nozzle(+), thus
eliminating the necessity to have the work as a part of the
electrical system
• Arc process produces plasma of relatively low energy density
• 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. Transferred and Non-Transferred ARC
Welding

8. 3 - Equipment
Power Supply
• A DC power source (generator or rectifier) having drooping
characteristics and open circuit voltage of 70 volts or above is suitable for
PAW
• Rectifiers are generally preferred over DC generators
• 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 used for arc
ignition

9. Shielding gases
• Shields the molten weld from the atmosphere.
• Two inert gases or gas mixtures are employed.
• Argon(commonly used), Helium, Argon+Hydrogen and Argon+Helium, as
inert gases or gas mixtures.
• Helium is preferred where a broad heat input pattern and flatter cover pass is
desired.
• 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.
• For cutting purposes a mixture of argon and hydrogen (10¬
30%) or that of nitrogen may be used.
• 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
• Current 50 to 350 Amps,
• Voltage 27 to 31 Volts,
• Gas flow rates 2 to 40 liters/min. (lower range for orifice gas and higher
range for outer shielding gas),
• Temp of Jet 50000°F (28000°C)

11. 4 - Modes in PAW
Micro-plasma welding
sheets (0.1mm thick), and wire and
• Welding Current from 0.1A to 15A
• Arc Length is varied up to 20mm
• Used for welding thin
mesh sections
Medium-plasma welding
• Welding current from 15A to 100A.
Keyhole welding
• Welding Current above 100A, where the plasma arc penetrates the wall
thickness.
• 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. 5 - Advantages
• Permits faster metal deposition rate and high arc travel
speed as compared to Gas tungsten arc welding
• Uniform penetration with high welding rate is possible
• Stability of arc and Excellent weld quality
• Can produce radiographic quality weld at high speed
• Can weld steel pieces up to about half inch thick, square
butt joint
• Useful for semi automatic and automatic processes.
• Process is very fast and clean
• Requires less operator skill due to good tolerance of arc to
misalignments
• High penetrating capability (keyhole effect)

13. 6 - Disadvantages
• Special protection is required as Infrared and UV
Radiations is produced
• Needs high power electrical equipment
• Gives out ultraviolet and infrared radiation.
• Operation produces a high noise of the order of 100dB
• Expensive equipment;
• Can weld only up to 25mm thickness.
• High distortions and wide welds as a result of high heat
input (in transferred arc process).
• More chances of Electrical hazards.

14. 7 - Applications
• Aerospace Industries
• Foodstuff and Chemical
Industries
• Machine and Plant
Construction
• Automobiles and Railways
• Ship Construction
• Tank Equipment and
Pipeline Construction etc.