1. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology PLASMA WELDING AND CUTTING TWI Training & Examinations Services Course in Welding (EWS/IWS diploma)

2. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Principles of operation Applications: weldingwelding cuttingcutting gouginggouging surfacingsurfacing

3. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Principles of operation

4. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Principles of operation TIG vs. Plasma welding

5. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology TIG vs. Plasma welding comparison TIG welding TIG arc is not constricted  relative wide heat pattern on the workpieceTIG arc is not constricted  relative wide heat pattern on the workpiece arc is conical  heated area varies with electrode-to-work distancearc is conical  heated area varies with electrode-to-work distance electrode extends beyond the end of gas nozzle  possible weld contaminationelectrode extends beyond the end of gas nozzle  possible weld contamination electrode is recessed  arc is collimated and focused by the constricting nozzleelectrode is recessed  arc is collimated and focused by the constricting nozzle electrode is recessed  impossible for the electrode to touch the workpieceelectrode is recessed  impossible for the electrode to touch the workpiece arc is essentially cylindrical  very little change in the heated areaarc is essentially cylindrical  very little change in the heated area Plasma welding

6. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Arc constriction Factors affecting intensity of plasma plasma (electrical) current: higher for cutting, lower for weldingplasma (electrical) current: higher for cutting, lower for welding orifice diameter and shape: smaller for cutting, larger for weldingorifice diameter and shape: smaller for cutting, larger for welding type of orifice gastype of orifice gas orifice gas flow rate: higher for cutting, lower for weldingorifice gas flow rate: higher for cutting, lower for welding distance to workpiecedistance to workpiece

7. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma arc modes

8. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Transferred vs. Nontransferred arc work is part of electrical circuitwork is part of electrical circuit heat is obtained from anode spot and from plasma jetheat is obtained from anode spot and from plasma jet greater energy transfer to the workgreater energy transfer to the work generally used for weldinggenerally used for welding workpiece is not in the arc circuitworkpiece is not in the arc circuit heat is obtained from plasma jet onlyheat is obtained from plasma jet only low energy concentrationlow energy concentration used for cutting and joining non- conductive workpieceused for cutting and joining non- conductive workpiece Transferred arc Nontransferred arc

9. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma process techniquesMicroplasma very low welding currents (0,1-15 Amps)very low welding currents (0,1-15 Amps) very stable needle-like stiff arc  minimises arc wander and distortionsvery stable needle-like stiff arc  minimises arc wander and distortions for welding thin materials (down to 0,1 mm thick), wire and mesh sectionsfor welding thin materials (down to 0,1 mm thick), wire and mesh sections Medium current plasma higher welding currents (15-200 Amps)higher welding currents (15-200 Amps) similar to TIG but arc is stiffer  deeper penetrationsimilar to TIG but arc is stiffer  deeper penetration more control on arc penetrationmore control on arc penetration

10. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma process techniques Microplasma and medium current plasma advantages energy concentration is greater  higher welding speedenergy concentration is greater  higher welding speed energy concentration is greater  lower current is needed to produce a given weld  less distortionsenergy concentration is greater  lower current is needed to produce a given weld  less distortions improved arc stabilityimproved arc stability arc column has greater directional stabilityarc column has greater directional stability narrow bead  less distortionsnarrow bead  less distortions less need for fixturingless need for fixturing variations in torch stand-off distance have little effect on bead width or heat concentration  positional weld is much easyvariations in torch stand-off distance have little effect on bead width or heat concentration  positional weld is much easy tungsten electrode is recessed  no tungsten contamination, less time for repointing, greater tolerance to surface contamination (including coatings)tungsten electrode is recessed  no tungsten contamination, less time for repointing, greater tolerance to surface contamination (including coatings)

11. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma process techniques Microplasma and medium current plasma limitations narrow constricted arc  little tolerance for joint misalignmentnarrow constricted arc  little tolerance for joint misalignment manual torches are heavy and bulky  difficult to manipulatemanual torches are heavy and bulky  difficult to manipulate for consistent quality, constricting nozzle must be well maintainedfor consistent quality, constricting nozzle must be well maintained

12. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma process technique

13. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma process techniques Keyhole 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)

14. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma process techniques Keyhole plasma welding advantages plasma stream helps remove gases and impuritiesplasma stream helps remove gases and impurities narrow fusion zone reduces transverse residual stresses and distortionsnarrow fusion zone reduces transverse residual stresses and distortions square butt joints are generally used  reduced joint preparationsquare butt joints are generally used  reduced joint preparation single pass welds  reduced weld timesingle pass welds  reduced weld time

15. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma process techniques Keyhole plasma welding limitations more process variables and narrow operating windowsmore process variables and narrow operating windows fit-up is criticalfit-up is critical increased operator skill, particularly on thicker materials  high accuracy for positioningincreased operator skill, particularly on thicker materials  high accuracy for positioning except for aluminium alloys, keyhole welding is restricted to downhand positionexcept for aluminium alloys, keyhole welding is restricted to downhand position for consistent operation, plasma torch must be well maintainedfor consistent operation, plasma torch must be well maintained

16. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma welding equipment

17. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma welding equipment DCEN for most welding applicationsDCEN for most welding applications AC (usually square wave) for aluminium and magnesium alloysAC (usually square wave) for aluminium and magnesium alloys pulsed current for better profile and weld bead shapepulsed current for better profile and weld bead shape drooping characteristic power sourcedrooping characteristic power source “pilot” arc is initiated using HF“pilot” arc is initiated using HF pilot arc ensures reliable arc starting and it obviates the need for HFpilot arc ensures reliable arc starting and it obviates the need for HF high OCV required (50 - 200 V)high OCV required (50 - 200 V) additional interlocks to detect low gas flow, loss of coolant, etcadditional interlocks to detect low gas flow, loss of coolant, etc no need for arc voltage controlno need for arc voltage control

18. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma welding torches operates at very high temperatures  cooling is mandatoryoperates at very high temperatures  cooling is mandatory heavy and bulky  limitations on hand held torchesheavy and bulky  limitations on hand held torches alignment, setting, concentricity of tungsten electrode needs precisionalignment, setting, concentricity of tungsten electrode needs precision

19. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Gases for plasma welding Argon for carbon steel, titanium, zirconium, etcArgon for carbon steel, titanium, zirconium, etc Hydrogen increase heat  Argon + (5-15%) Hydrogen for stainless steel, Nickel alloys, Copper alloysHydrogen increase heat  Argon + (5-15%) Hydrogen for stainless steel, Nickel alloys, Copper alloys Argon + Helium mixtures (min 40%) give a hotter arc but reduces torch lifeArgon + Helium mixtures (min 40%) give a hotter arc but reduces torch life Shielding gases as for TIGShielding gases as for TIG shielding gas flow rate 10-30 l/minshielding gas flow rate 10-30 l/min back purge as for TIG (also for keyhole)back purge as for TIG (also for keyhole)

20. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma cutting no need to promote oxidation  no preheatno need to promote oxidation  no preheat works by melting and blowing and/or vaporisationworks by melting and blowing and/or vaporisation gases: air, Ar, N 2, O 2, mix of Ar + H 2, N 2 + H 2gases: air, Ar, N 2, O 2, mix of Ar + H 2, N 2 + H 2 air plasma promotes oxidation  increased speed but special electrodes needair plasma promotes oxidation  increased speed but special electrodes need shielding gas - optionalshielding gas - optional applications: stainless steels, aluminium and thin sheet carbon steelapplications: stainless steels, aluminium and thin sheet carbon steel

21. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma cutting

22. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma cutting features Advantages Can be used with a wide range of materialsCan be used with a wide range of materials High quality cut edges can be achieved High quality cut edges can be achieved Narrow HAZ formed Narrow HAZ formed Low gas consumable (air) costs Low gas consumable (air) costs Ideal for thin sheet and stack cutting Ideal for thin sheet and stack cutting Low fume (underwater) process Low fume (underwater) process Limited to 50mm (air plasma) thick plateLimited to 50mm (air plasma) thick plate High noise especially when cutting thick sections in air High noise especially when cutting thick sections in air High fume generation when cutting in air High fume generation when cutting in air Protection required from the arc glare Protection required from the arc glare High equipment and consumable costs High equipment and consumable costs Limitations

23. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma cutting quality tapered cut up to 6°tapered cut up to 6° rounded top edgerounded top edge gas swirl can reduce taper up to 2°gas swirl can reduce taper up to 2° very smooth surface finish except aluminium and thick materialsvery smooth surface finish except aluminium and thick materials 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 crack sensitive at surface2000 and 7000 series aluminium alloys are crack sensitive at surface

24. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma cutting equipment

25. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma cutting equipment manual cutting - limited to drag alongmanual cutting - limited to drag along machine cutting - stand off close tolerancesmachine cutting - stand off close tolerances motion - CNCmotion - CNC power source - cc dropping characteristicpower source - cc dropping characteristic need high OCVneed high OCV problems with bevels and multiheadsproblems with bevels and multiheads easy to perform interrupted cuttingeasy to perform interrupted cutting

26. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology Plasma gouging lower arc stream velocitylower arc stream velocity gouge is bright and cleangouge is bright and clean virtually no post cleaning requiredvirtually no post cleaning required used mainly on stainless steels and non-ferrous materialsused mainly on stainless steels and non-ferrous materials