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PLASMA ARC WELDING. PAW - Principle of operation.

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Presentation on theme: "PLASMA ARC WELDING. PAW - Principle of operation."— Presentation transcript:

1 PLASMA ARC WELDING

2 PAW - Principle of operation

3 Principle of operation

4 TIG vs. Plasma welding

5 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 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 Plasma arc modes  generally used for welding  work is part of electrical circuit  heat is obtained from anode spot and from plasma jet  greater energy transfer to the work

8 Plasma arc modes  used for cutting and joining non- conductive workpiece  workpiece is not in the arc circuit  heat is obtained from plasma jet only  low energy concentration

9 Plasma process techniques Microplasma  very low welding currents (0,1-15 Amps)  very stable needle-like stiff arc  minimises arc wander and distortions  for welding thin materials (down to 0,1 mm thick), wire and mesh sections Medium current plasma  higher welding currents ( Amps)  similar to TIG but arc is stiffer  deeper penetration  more control on arc penetration

10 Plasma process techniques Microplasma and medium current plasma advantages  energy concentration is greater  higher welding speed  energy concentration is greater  lower current is needed to produce a given weld  less distortions  improved arc stability  arc column has greater directional stability  narrow bead  less distortions  less need for fixturing  variations 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)

11 Plasma process techniques Microplasma and medium current plasma limitations  narrow constricted arc  little tolerance for joint misalignment  manual torches are heavy and bulky  difficult to manipulate  for consistent quality, constricting nozzle must be well maintained

12 Plasma process technique

13 Plasma process techniques Keyhole plasma welding  welding currents over 100 Amps  for welding thick materials (up to 10 mm)

14 Plasma process techniques Keyhole plasma welding advantages  plasma stream helps remove gases and impurities  narrow fusion zone reduces transverse residual stresses and distortion  a square butt joint configuration is generally used  reduced joint preparation  single pass weld  reduced weld time

15 Plasma process techniques MMAMAG TIG PAW

16 Plasma process techniques Keyhole plasma welding limitations  more process variables and narrow operating windows  fit-up is critical  increased operator skill, particularly on thicker materials  high accuracy for positioning  except for aluminium alloys, keyhole welding is restricted to downhand position  for consistent operation, plasma torch must be well maintained

17 Plasma welding equipment

18  DCEN for most welding applications  AC (usually square wave) for aluminium and magnesium alloys  pulsed current for better profile and weld bead shape  drooping characteristic power source  “pilot” arc is initiated using HF  pilot arc ensures reliable arc starting and it obviates the need for HF  high OCV required ( V)  additional interlocks to detect low gas flow, loss of coolant, etc  no need for arc voltage control

19 Plasma welding torch Tungsten electrode Torch body Shielding gas cup Water cooled copper nozzle

20 Plasma welding torch  operates at very high temperatures  cooling is mandatory  heavy and bulky  limitations on hand held torches  alignment, setting, concentricity of tungsten electrode needs precision

21 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 l/minshielding gas flow rate l/min back purge as for TIG (also for keyhole)back purge as for TIG (also for keyhole)

22 PAW advantages  improved arc stability at very low currents  greater energy concentration  higher welding speed  narrower beads  less distortion (as much as 50%)  tungsten electrode is recessed inside the torch  no danger of tungsten inclusions  increased torch stand-off distance makes the weld pool much easy to control  arc column is cylindrical  easier out-of-position welding  very deep penetration (keyhole)  reduced weld time  square butt joint  reduced machining costs  plasma gas flushing through the open keyhole helps remove gases

23 PAW disadvantages  narrow constricted arc  little tolerance for joint misalignment  manual plasma torches are heavier than TIG torches  difficult to manipulate  more complex equipment than TIG  expensive  except Al alloys, keyhole plasma is restricted to the flat position  torch must be well maintained for consistent operation  costly

24 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

25 Plasma cutting

26 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

27 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

28 Plasma cutting equipment

29 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

30 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


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