Plasma ARC Welding(PAW)

Vivek

ContentO IntroductionOHow Plasma Welding works?OEquipmentOWelding Modes in PAWODifference between PAW and TIGOControl SystemOAdvantagesODisadvantagesOApplications

PAW Welding

How Plasma Welding WorksPlasma: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.

2 Variants of PAW:Transferred arc mode:O Arc is struck between the electrode(-) and the work piece(+)O Used for high speed welding andO 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 theelectrical 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 fromone work piece to other without extinguishing the arc.

Fig. Arc in Plasma Arc Welding

EquipmentPower Supply

O A DC power source (generator or rectifier) having droopingcharacteristics and open circuit voltage of 70 volts or above issuitable 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 operationof two power sources; or the arc can be initiated with argon atnormal 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

Shielding gasesO 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 alloysand 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 andthereafter recombination generates temperatures above thoseattained by using argon or helium alone.

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 arepreferred.

O Temp of Jet 50000°F (28000°C)Current and gas decay controlO To close the key hole properly while terminating the weld

in the structure.FixtureO To avoid atmospheric contamination of the molten metal

under bead.

Modes in pawMicro-plasma weldingO Welding Current from 0.1A to 15A.O Arc Length is varied up to 20mmO Used for welding thin sheets (0.1mm thick), and wire and

mesh sections.Medium-plasma weldingO Welding current from 15A to 100A.Keyhole weldingO 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 10mmof stainless steel) in a single pass.

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

Difference between PAW and TIGPlasma 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 ispossible.

Cutting of Hard and Brittle Material isnot possible.

Control System

AdvantagesO Permits faster metal deposition rate and high arc travel

speed as compared to TIGO Uniform penetration with high welding rate is possibleO Stability of arc and Excellent weld qualityO Can produce radiographic quality weld at high speedO Can weld steel pieces up to about half inch thick, square

butt jointO Useful for semi automatic and automatic processes.O Process is very fast and cleanO Requires less operator skill due to good tolerance of arc to

misalignments;O High penetrating capability (keyhole effect);

Disadvantages

O Special protection is required as Infrared and UVRadiations is produced

O Consumption of Inert Gas is highO 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.

Base Metal weldable by Plasma Arc Process:

Easily Weldable:

O Al, Cu-base alloys (Cu, Cu-Ni), Magnesium, Ni-basealloys (Inconel, Nickel, Monel), Precious Metals, Steels(Low Carbon Steel, Low Alloy Steel, Stainless Steel, ToolSteels), 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...

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