Transcript PLASMA ARC WELDING

PLASMA ARC WELDING
PAW - Principle of operation
Principle of operation
Principle of operation
TIG vs. Plasma welding
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
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
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
Plasma arc modes
 used for cutting and
joining nonconductive
workpiece
 workpiece is not in
the arc circuit
 heat is obtained
from plasma jet only
 low energy
concentration
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
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
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
Plasma process technique
Plasma process techniques
Keyhole plasma welding
 welding currents over 100 Amps
 for welding thick materials (up to 10 mm)
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
Plasma process techniques
MMA
MAG
TIG
PAW
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
Plasma welding equipment
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
Plasma welding torch
Torch
body
Tungsten
electrode
Water cooled
copper nozzle
Shielding
gas cup
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
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)
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
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
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
Plasma cutting
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
• High fume generation
when cutting in air
• Protection required
from the arc glare
• High equipment and
consumable costs
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
Plasma cutting equipment
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
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