PLASMA ARC WELDING. PAW - Principle of operation.
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Transcript of PLASMA ARC WELDING. PAW - Principle of operation.
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