PLASMA ARC WELDINGPLASMA ARC WELDING

PAW - Principle of operationPAW - Principle of operation

Principle of operationPrinciple of operation

Principle of operationPrinciple of operationTIG vs. Plasma TIG vs. Plasma weldingwelding

TIG vs. Plasma welding TIG vs. Plasma welding comparisoncomparisonTIG 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 nozzle nozzle possible weld possible weld contaminationcontamination

•electrode is recessed electrode is recessed arc is collimated arc is collimated and focused by the and 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 constrictionArc constrictionFactors 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 modesPlasma arc modes

generally used for generally used for weldingwelding

work is part of work is part of electrical circuitelectrical circuit

heat is obtained from heat is obtained from anode spot and from anode spot and from plasma jetplasma jet

greater energy greater energy transfer to the worktransfer to the work

Plasma arc modesPlasma arc modes

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

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

Plasma process techniquesPlasma process techniquesMicroplasmaMicroplasma very low welding currents (0,1-15 Amps)very low welding currents (0,1-15 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 0,1 mm for welding thin materials (down to 0,1 mm

thick), wire and mesh sectionsthick), wire and mesh sectionsMedium current plasmaMedium current plasma higher welding currents (15-200 Amps)higher welding currents (15-200 Amps) similar to TIG but arc is stiffer similar to TIG but arc is stiffer deeper deeper

penetrationpenetration more control on arc penetrationmore control on arc penetration

Plasma process techniquesPlasma process techniquesMicroplasma and medium current plasma Microplasma and medium current plasma

advantagesadvantages 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 effect variations in torch stand-off distance have little effect

on bead width or heat concentration on bead width or heat concentration positional positional weld is much easyweld 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)

Plasma process techniquesPlasma process techniquesMicroplasma and medium current plasma Microplasma and medium current plasma

limitationslimitations

narrow constricted arc narrow constricted arc little tolerance little tolerance for joint misalignmentfor joint misalignment

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

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

Plasma process techniquePlasma process technique

Plasma process techniquesPlasma process techniquesKeyhole plasma weldingKeyhole plasma welding

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

Plasma process techniquesPlasma process techniques

Keyhole plasma welding advantagesKeyhole plasma welding advantages plasma stream helps remove gases and plasma stream helps remove gases and

impuritiesimpurities narrow fusion zone reduces transverse narrow fusion zone reduces transverse

residual stresses and distortionresidual stresses and distortion a square butt joint configuration is a square butt joint configuration is

generally used generally used reduced joint reduced joint preparationpreparation

single pass weld single pass weld reduced weld time reduced weld time

Plasma process techniquesPlasma process techniques

MMA MAG TIG PAW

Plasma process techniquesPlasma 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 on increased operator skill, particularly on

thicker materials thicker materials high accuracy for high accuracy for positioningpositioning

except for aluminium alloys, keyhole except for aluminium alloys, keyhole welding is restricted to downhand positionwelding is restricted to downhand position

for consistent operation, plasma torch for consistent operation, plasma torch must be well maintainedmust be well maintained

Plasma welding equipmentPlasma welding equipment

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

magnesium alloysmagnesium alloys pulsed current for better profile and weld bead pulsed current for better profile and weld bead

shapeshape 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 it obviates pilot arc ensures reliable arc starting and it obviates

the need for HFthe need for HF high OCV required (50 - 200 V)high OCV required (50 - 200 V) additional interlocks to detect low gas flow, loss of additional interlocks to detect low gas flow, loss of

coolant, etccoolant, etc no need for arc voltage controlno need for arc voltage control

Plasma welding torchPlasma welding torch

Tungsten Tungsten electrodeelectrode

Torch Torch bodybody

Shielding Shielding gas cupgas cup

Water cooled Water cooled copper nozzlecopper nozzle

Plasma welding torchPlasma welding torch

operates at very operates at very high temperatures high temperatures cooling is mandatorycooling is mandatory

heavy and bulky heavy and bulky limitations on hand limitations on hand held torchesheld torches

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

Gases for plasma weldingGases 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)

PAW advantagesPAW advantages improved arc stability at very low currentsimproved arc stability at very low currents greater energy concentration greater energy concentration higher welding speed higher welding speed narrower beads narrower beads less distortion (as much as 50%) less distortion (as much as 50%) tungsten electrode is recessed inside the torch tungsten electrode is recessed inside the torch no no

danger of tungsten inclusionsdanger of tungsten inclusions increased torch stand-off distance makes the weld pool increased torch stand-off distance makes the weld pool

much easy to controlmuch easy to control arc column is cylindrical arc column is cylindrical easier out-of-position welding easier out-of-position welding very deep penetration (keyhole) very deep penetration (keyhole) reduced weld time reduced weld time square butt joint square butt joint reduced machining costs reduced machining costs plasma gas flushing through the open keyhole helps plasma gas flushing through the open keyhole helps

remove gasesremove gases

PAW disadvantagesPAW disadvantages narrow constricted arcnarrow constricted arc little tolerance for joint little tolerance for joint

misalignmentmisalignment manual plasma torches are heavier than TIG manual plasma torches are heavier than TIG

torches torches difficult to manipulate difficult to manipulate more complex equipment than TIG more complex equipment than TIG expensive expensive except Al alloys, keyhole plasma is restricted to except Al alloys, keyhole plasma is restricted to

the flat positionthe flat position torch must be well maintained for consistent torch must be well maintained for consistent

operation operation costly costly

Plasma cuttingPlasma cutting•no need to promote no need to promote oxidation oxidation 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 cuttingPlasma cutting

Plasma cutting featuresPlasma 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 qualityPlasma 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 equipmentPlasma cutting equipment

Plasma cutting equipmentPlasma 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 gougingPlasma 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