Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology PLASMA WELDING AND CUTTING TWI Training & Examinations Services Course in Welding.

Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

PLASMA WELDING AND CUTTING

TWI Training & Examinations TWI Training & Examinations ServicesServices

Course in WeldingCourse in Welding

(EWS/IWS diploma)(EWS/IWS diploma)


Principles of operation

ApplicationApplications:s:•weldingwelding

•cuttingcutting

•gouginggouging

•surfacingsurfacing


Principles of operation


Principles of operationTIG vs. Plasma TIG vs. Plasma weldingwelding


TIG vs. Plasma welding comparison

TIG TIG weldingwelding

•TIG arc is not TIG arc is not constricted constricted relative relative wide heat pattern on wide heat pattern on the workpiecethe workpiece

•arc is conical arc is conical heated area varies heated area varies with electrode-to-with electrode-to-work distancework distance

•electrode extends electrode extends beyond the end of gas beyond the end of gas nozzlenozzle possible weld possible weld contaminationcontamination

•electrode is recessed electrode is recessed arc is collimated and arc is collimated and focused by the focused by the constricting nozzleconstricting nozzle

•electrode is recessed electrode is recessed impossible for the impossible for the electrode to touch the electrode to touch the workpieceworkpiece

•arc is essentially arc is essentially cylindrical cylindrical very little very little change in the heated change in the heated areaarea

Plasma weldingPlasma welding


Arc constriction

Factors affecting intensity of Factors affecting intensity of plasmaplasma•plasma (electrical) current: higher for plasma (electrical) current: higher for cutting, lower for weldingcutting, lower for welding

•orifice diameter and shape: smaller for orifice diameter and shape: smaller for cutting, larger for weldingcutting, larger for welding

•type of orifice gastype of orifice gas

•orifice gas flow rate: higher for cutting, orifice gas flow rate: higher for cutting, lower for welding lower for welding

•distance to workpiecedistance to workpiece


Plasma arc modes


Transferred vs. Nontransferred arc

• work is part of work is part of electrical circuitelectrical circuit

• heat is obtained heat is obtained from anode spot from anode spot and from plasma jetand from plasma jet

• greater energy greater energy transfer to the worktransfer to the work

• generally used for generally used for weldingwelding

• workpiece is not in workpiece is not in the arc circuitthe arc circuit

• heat is obtained heat is obtained from plasma jet onlyfrom plasma jet only

• low energy low energy concentrationconcentration

• used for cutting and used for cutting and joining non-joining non-conductive conductive workpieceworkpiece

Transferred arcTransferred arc Nontransferred Nontransferred arcarc


Plasma process techniques

MicroplasmaMicroplasma• very low welding currents (0,1-15 very low welding currents (0,1-15

Amps)Amps)• very stable needle-like stiff arc very stable needle-like stiff arc

minimises arc wander and distortionsminimises arc wander and distortions• for welding thin materials (down to for welding thin materials (down to

0,1 mm thick), wire and mesh 0,1 mm thick), wire and mesh sectionssections

Medium current plasmaMedium current plasma• higher welding currents (15-200 higher welding currents (15-200

Amps)Amps)• similar to TIG but arc is stiffer similar to TIG but arc is stiffer

deeper penetrationdeeper penetration• more control on arc penetrationmore control on arc penetration


Plasma process techniquesMicroplasma and medium current Microplasma and medium current

plasma advantagesplasma advantages• energy concentration is greater energy concentration is greater higher welding higher welding

speedspeed• energy concentration is greater energy concentration is greater lower current is lower current is

needed to produce a given weld needed to produce a given weld less distortions less distortions• improved arc stabilityimproved arc stability• arc column has greater directional stabilityarc column has greater directional stability• narrow bead narrow bead less distortions less distortions• less need for fixturingless need for fixturing• variations in torch stand-off distance have little variations in torch stand-off distance have little

effect on bead width or heat concentration effect on bead width or heat concentration positional weld is much easypositional weld is much easy

• tungsten electrode is recessed tungsten electrode is recessed no tungsten no tungsten contamination, less time for repointing, greater contamination, less time for repointing, greater tolerance to surface contamination (including tolerance to surface contamination (including coatings)coatings)



Microplasma and medium current Microplasma and medium current plasma limitationsplasma limitations

• narrow constricted arc narrow constricted arc little little tolerance for joint misalignmenttolerance for joint misalignment

• manual torches are heavy and manual torches are heavy and bulky bulky difficult to manipulate difficult to manipulate

• for consistent quality, constricting for consistent quality, constricting nozzle must be well maintainednozzle must be well maintained


Plasma process technique



Keyhole plasma weldingKeyhole plasma welding

• welding currents over 100 Ampswelding currents over 100 Amps• for welding thick materials (up to 10 for welding thick materials (up to 10

mm)mm)



Keyhole plasma welding advantagesKeyhole plasma welding advantages

• plasma stream helps remove gases plasma stream helps remove gases and impuritiesand impurities

• narrow fusion zone reduces narrow fusion zone reduces transverse residual stresses and transverse residual stresses and distortionsdistortions

• square butt joints are generally square butt joints are generally used used reduced joint preparation reduced joint preparation

• single pass welds single pass welds reduced weld reduced weld timetime


Plasma process techniquesKeyhole plasma welding limitationsKeyhole plasma welding limitations• more process variables and narrow more process variables and narrow

operating windowsoperating windows• fit-up is criticalfit-up is critical• increased operator skill, particularly increased operator skill, particularly

on thicker materials on thicker materials high accuracy high accuracy for positioningfor positioning

• except for aluminium alloys, keyhole except for aluminium alloys, keyhole welding is restricted to downhand welding is restricted to downhand positionposition

• for consistent operation, plasma for consistent operation, plasma torch must be well maintainedtorch must be well maintained


Plasma welding equipment


Plasma welding equipment• DCEN for most welding applicationsDCEN for most welding applications• AC (usually square wave) for aluminium AC (usually square wave) for aluminium

and magnesium alloysand magnesium alloys• pulsed current for better profile and weld pulsed current for better profile and weld

bead shapebead shape• drooping characteristic power sourcedrooping characteristic power source• ““pilot” arc is initiated using HFpilot” arc is initiated using HF• pilot arc ensures reliable arc starting and pilot arc ensures reliable arc starting and

it obviates the need for HFit obviates the need for HF• high OCV required (50 - 200 V)high OCV required (50 - 200 V)• additional interlocks to detect low gas additional interlocks to detect low gas

flow, loss of coolant, etcflow, loss of coolant, etc• no need for arc voltage controlno need for arc voltage control


Plasma welding torches

• operates at very operates at very high temperatures high temperatures cooling is cooling is mandatorymandatory

• heavy and bulky heavy and bulky limitations on limitations on hand held torcheshand held torches

• alignment, setting, alignment, setting, concentricity of concentricity of tungsten electrode tungsten electrode needs precisionneeds precision


Gases for plasma welding•Argon for carbon steel, titanium, Argon for carbon steel, titanium, zirconium, etczirconium, etc

•Hydrogen increase heat Hydrogen increase heat Argon + (5- Argon + (5-15%) Hydrogen for stainless steel, Nickel 15%) Hydrogen for stainless steel, Nickel alloys, Copper alloysalloys, Copper alloys

•Argon + Helium mixtures (min 40%) give Argon + Helium mixtures (min 40%) give a hotter arc but reduces torch lifea hotter arc but reduces torch life•Shielding gases as Shielding gases as

for TIGfor TIG

•shielding gas flow shielding gas flow rate 10-30 l/minrate 10-30 l/min

•back purge as for back purge as for TIG (also for TIG (also for keyhole)keyhole)


Plasma cutting•no need to promote no need to promote oxidationoxidation no preheat no preheat

•works by melting and works by melting and blowing and/or blowing and/or vaporisationvaporisation

•gases: air, Ar, Ngases: air, Ar, N22, O, O22, , mix of Ar + Hmix of Ar + H22, N, N2 2 + H+ H22

•air plasma promotes air plasma promotes oxidation oxidation increased increased speed but special speed but special electrodes needelectrodes need

•shielding gas - shielding gas - optionaloptional

•applications: stainless applications: stainless steels, aluminium and steels, aluminium and thin sheet carbon steelthin sheet carbon steel


Plasma cutting


Plasma cutting featuresAdvantageAdvantagess

•Can be used with a Can be used with a wide range of wide range of materialsmaterials

• High quality cut High quality cut edges can be achievededges can be achieved

• Narrow HAZ formedNarrow HAZ formed

• Low gas consumable Low gas consumable (air) costs(air) costs

• Ideal for thin sheet Ideal for thin sheet and stack cuttingand stack cutting

• Low fume Low fume (underwater) process(underwater) process

•Limited to 50mm (air Limited to 50mm (air plasma) thick plateplasma) thick plate

• High noise especially High noise especially when cutting thick when cutting thick sections in airsections in air

• High fume High fume generation when generation when cutting in aircutting in air

• Protection required Protection required from the arc glarefrom the arc glare

• High equipment and High equipment and consumable costsconsumable costs

LimitationsLimitations


Plasma cutting quality•tapered cut up to 6°tapered cut up to 6°

•rounded top edgerounded top edge

•gas swirl can reduce taper up gas swirl can reduce taper up to 2°to 2°

•very smooth surface finish very smooth surface finish except aluminium and thick except aluminium and thick materialsmaterials

•dross is minimaldross is minimal•kerf width wider than oxy fuel cuttingkerf width wider than oxy fuel cutting

•HAZ width inverse to cutting speedHAZ width inverse to cutting speed

•no time for chromium carbides to formno time for chromium carbides to form

•2000 and 7000 series aluminium alloys are 2000 and 7000 series aluminium alloys are crack sensitive at surfacecrack sensitive at surface


Plasma cutting equipment


Plasma cutting equipment•manual cutting - manual cutting - limited to drag alonglimited to drag along•machine cutting - machine cutting - stand off close stand off close tolerancestolerances•motion - CNCmotion - CNC

•power source - cc power source - cc dropping characteristicdropping characteristic

•need high OCVneed high OCV

•problems with bevels problems with bevels and multiheadsand multiheads

•easy to perform easy to perform interrupted cuttinginterrupted cutting


Plasma gouging

•lower arc stream lower arc stream velocityvelocity

•gouge is bright gouge is bright and cleanand clean

•virtually no post virtually no post cleaning requiredcleaning required

•used mainly on used mainly on stainless steels stainless steels and non-ferrous and non-ferrous materialsmaterials

Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology PLASMA WELDING AND CUTTING TWI Training & Examinations Services Course in Welding.

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