Plasma Arc Welding

Metallurgy and Materials Engineering Department, University of Punjab Lahore

2. Group # 7
Malik M. Wasim M14-300
M. Awais Raza M14-316
Muzammil Abbas M14-337
Rauf Amjid M14-332

3. Plasma Arc Welding(PAW)

4. Introduced to welding industry in 1964
Method of bringing better control to arc welding process in lower
current ranges
PAW brought to industry by providing an advanced level of control
and accuracy to produce high quality welds.
Provide long electrode life for high production requirements
Used in a variety of operations ranging from high volume welding of
strip metal.
Introduction

5. Principle of PAW
Plasma arc welding is an arc welding process similar to tungsten arc
welding process
The electric arc is formed between an electrode and the work
piece
The key difference from TIG is that in PAW by positioning electrode
within the body of torch ,the plasma arc can be separated from the
shielded gas envelope
The plasma is then forced through a fine-bore copper nozzle which
constricts the arc and the plasma exists the orifice at high velocities
and temperatures.

6. 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 Helium as an inert gas needs open circuit
voltage above 70 volts. This voltage can be obtained by
series operation of two power sources.
High frequency generator and current limiting resistors
used for arc ignition. The arc starting system may be
separate or built into the system
.
Equipment

7. Shields the molten weld from the atmosphere.
Argon, Helium, Argon + Hydrogen and Argon + Helium as inert
gases or gas mixtures
Helium is preferred where a broad of heat input pattern and flatter
cover pass is desired.
For cutting purposes a mixture of argon and hydrogen or that of
nitrogen may be used.
Shielding Gases

8. Electrode
Non consumable tungsten electrode is used.
Cooling system
Water recirculates in order to keep the equipment cool
Plasma torch:
Either transferred arc or non transferred arc typed
It is hand operated or mechanized.

10. Welding Parameters
Current:
Direct current electrode negative (DCEN) is normally employed for plasma
arc welding except for the welding of aluminum in which cases water cooled
electrode is preferable for reverse polarity welding, i.e. direct current
electrode positive (DCEP). Current is adjusted between 50 to 350 amperes.
Voltage
In normal key hole welding a variation in arc length up to 1.5mm does not
affect weld bead penetration or bead shape to any significant extent and thus
a voltage control is not considered essential. So the voltage can be kept in
between 27 and 31 V.

11. Welding Parameters
Gas flow rates :
Two inert gases or gas mixtures are employed. The orifice gas at lower
pressure and flow rate forms the plasma arc. So the inert gas must have
a pressure of 30-50 Liters/min and that of the orifice gas may be 2-40
liters/min.
Temperature:
Atypicalvalueoftemperatureobtainedinaplasmajettorchmaybeof
theorderof28000°C(50000°F)againstabout5500°C(10000°F)in
ordinaryelectricweldingarc.

12. Modes in PAW
Micro plasma welding
Welding current from 0.1A to 15A
Used for welding thin sheets.
Welding current from 15A to 100A.
Welding current above 1000A where the plasma arc penetrates the wall thickness.
Used for high quality joints in aircraft industries to weld thicker materials.
Medium plasma welding
Keyhole welding

13. Types of PAW
Transferred PAW
Arc is established between electrode and work piece.
The work piece is part of the electrical circuit and heat
is obtained from the anode spot and plasma jet.
Therefore higher amount of energy is transferred to
work.
Higher penetration is obtained so thicker sheets can be
welded.
Higher process efficiency.

14. Non transferred PAW
Arc is established between electrode and nozzle.
The work piece is not part of the electrical circuit and heat
is obtained from the plasma jet. Therefore less energy is
transferred to work.
Less penetration is obtained so thin sheets can be welded.
Less process efficiency.

15. Safety Parameters:
1. Maintain a safe workplace.
If possible, move the material to be welded or cut to a safe location designated specifically for welding and
cutting.
2. Keep fire hazards away.
Areas for welding and cutting must be kept clear and free of flammable liquids, such as gasoline, paints, and
solvents; combustible solids, such as paper, packing material, and wood; and flammable gases, such as
oxygen, acetylene and hydrogen.
3. Provide fire barriers.
If welding or cutting cannot be performed in a designated location or away from combustible materials,
provide metal sheets or fire-resistant blankets to prevent heat, sparks, and slag from reaching these materials.
4. Be alert for cracks or crevices.
Sparks and slag can travel long distances. They can start a fire at a location not apparent to the welder. Look
for holes or openings in the floor, crevices around pipes, and other openings which can hide a smoldering fire.

16. Safety Parameters:
5. Provide fire extinguishing equipment.
Be prepared to put out fires. Serviceable fire extinguishers, fire hoses, or sand buckets should be on
hand. Their use depends on the quantity and type of combustible material which may be present.
6. Consider the need for a fire watcher.
Welders may not become aware of a fire starting while welding. Their vision is seriously hampered by
the welding helmet and dark lenses. Depending upon the circumstances of welding location, it may be
advisable to have a fire watcher to man an extinguisher and sound a fire alarm in case of a fire.
7. Know the local fire codes for welding and cutting.
Follow the information on fire protection during welding or cutting operations in National Fire
Protection Association Standard NFPA No. 51B, “Fire Protection in Use of Cutting and Welding
Processes.

17. Features of PAW:
Protected electrode
Low amperage welding capability
Welding time as short as 0.005 seconds
Gentle arc starting and arc consistency
Diameter of arc chosen via nozzle orifice
Length of arc benefit due to arc shape and even heat
treatment

18. Benefits of PAW
ControlandQuality:
Torchdesignallowsforbettercontrolofthearc.
Fasttravelspeed:
Higherheatconcentrationandtheplasmanozzle
allowforhighertravelspeed.
Penetration:
Uniformpenetrationwithhighweldingrateis
possible
ArcStability:
Stabilityofarcandexcellentweldquality

19. Angulardistortion:
Heataffectedzoneissmallerascomparedto
GTAW.
Penetratingcapability:
Highpenetratingcapabilityascomparedother
weldingprocesses.
Usefulforautomaticprocess
Processisveryfastandclean
Benefits of PAW

20. Drawbacks of PAW
Consumption of inert gas is high
Needs high power electrical equipment
Expensive equipment
Can weld only upto 25mm thickness
More chances of electrical hazards

21. Application of PAW
Small part welding:
PAW consistently starts an arc to the small components and
make repeatable weld with very short weld time periods. Such
as needle wires, light blub filaments etc.
Sealed components:
Medical and electronic components are sealed via this
welding.
Tool die and mold repair:
Micro-plasma Welding process are used for tool die and
mold repair.

22. Application of PAW
Aerospace industries
This technique of welding is used in the aerospace industries for high quality joints.
Foodstuff and chemical industries
This method is preferred in the above descried industries as it is less preferable for
corrosion.
Machine and plant construction
For the welding of intricate structures in the machine, no other technique can displace this
technique.
Automobiles and railways