Transcript ELECTROTECHNICS

PLASMA WELDING AND
CUTTING
TWI Training & Examinations
Services
Course in Welding
(EWS/IWS diploma)
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Principles of operation
Applications:
•welding
•cutting
•gouging
•surfacing
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World Centre for Materials Joining Technology
Principles of operation
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World Centre for Materials Joining Technology
Principles of operation
TIG vs. Plasma welding
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TIG vs. Plasma welding comparison
Plasma welding
TIG welding
•TIG arc is not
constricted  relative
wide heat pattern on the
workpiece
•electrode is recessed 
arc is collimated and
focused by the
constricting nozzle
•arc is conical  heated
area varies with
electrode-to-work
distance
•electrode is recessed 
impossible for the
electrode to touch the
workpiece
•electrode extends
beyond the end of gas
nozzle possible weld
contamination
•arc is essentially
cylindrical  very little
change in the heated
area
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Arc constriction
Factors affecting intensity of plasma
•plasma (electrical) current: higher for cutting,
lower for welding
•orifice diameter and shape: smaller for cutting,
larger for welding
•type of orifice gas
•orifice gas flow rate: higher for cutting, lower
for welding
•distance to workpiece
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Plasma arc modes
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Transferred vs. Nontransferred arc
Transferred arc
• work is part of
electrical circuit
• heat is obtained from
anode spot and from
plasma jet
• greater energy
transfer to the work
• generally used for
welding
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Nontransferred arc
• workpiece is not in
the arc circuit
• heat is obtained from
plasma jet only
• low energy
concentration
• used for cutting and
joining nonconductive workpiece
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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 (15-200 Amps)
• similar to TIG but arc is stiffer  deeper
penetration
• more control on arc penetration
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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)
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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
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Plasma process technique
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Plasma process techniques
Keyhole plasma welding
• welding currents over 100 Amps
• for welding thick materials (up to 10 mm)
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Plasma process techniques
Keyhole plasma welding advantages
• plasma stream helps remove gases and
impurities
• narrow fusion zone reduces transverse
residual stresses and distortions
• square butt joints are generally used 
reduced joint preparation
• single pass welds  reduced weld time
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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
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Plasma welding equipment
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Plasma welding equipment
• 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 (50 - 200 V)
• additional interlocks to detect low gas flow,
loss of coolant, etc
• no need for arc voltage control
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Plasma welding torches
• operates at very
high temperatures
 cooling is
mandatory
• heavy and bulky 
limitations on hand
held torches
• alignment, setting,
concentricity of
tungsten electrode
needs precision
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Gases for plasma welding
•Argon for carbon steel, titanium, zirconium, etc
•Hydrogen 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 life
•Shielding gases as for
TIG
•shielding gas flow
rate 10-30 l/min
•back purge as for TIG
(also for keyhole)
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Plasma cutting
•no need to promote
oxidation no preheat
•works by melting and
blowing and/or
vaporisation
•gases: air, Ar, N2, O2, mix
of Ar + H2, N2 + H2
•air plasma promotes
oxidation  increased
speed but special
electrodes need
•shielding gas - optional
•applications: stainless
steels, aluminium and thin
sheet carbon steel
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Plasma cutting
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Plasma cutting features
Advantages
Limitations
•Can be used with a wide
range of materials
•Limited to 50mm (air
plasma) thick plate
• High quality cut edges
can be achieved
• High noise especially
when cutting thick
sections in air
• Narrow HAZ formed
• Low gas consumable
(air) costs
• Ideal for thin sheet and
stack cutting
• Low fume (underwater)
process
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• High fume generation
when cutting in air
• Protection required
from the arc glare
• High equipment and
consumable costs
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Plasma cutting quality
•tapered cut up to 6°
•rounded top edge
•gas swirl can reduce taper up to 2°
•very smooth surface finish except
aluminium and thick materials
•dross is minimal
•kerf width wider than oxy fuel cutting
•HAZ width inverse to cutting speed
•no time for chromium carbides to form
•2000 and 7000 series aluminium alloys are crack
sensitive at surface
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Plasma cutting equipment
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Plasma cutting equipment
•manual cutting - limited
to drag along
•machine cutting - stand
off close tolerances
•motion - CNC
•power source - cc
dropping characteristic
•need high OCV
•problems with bevels and
multiheads
•easy to perform
interrupted cutting
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Plasma gouging
•lower arc stream
velocity
•gouge is bright and
clean
•virtually no post
cleaning required
•used mainly on
stainless steels and
non-ferrous materials
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