1. PLASMA ARC WELDING

2. PAW - Principle of operation

3. Principle of operation

4. Principle of operation
TIG vs. Plasma welding

5. TIG vs. Plasma welding comparison
TIG welding
Plasma welding
electrode is recessed  arc is collimated and focused by the constricting nozzle
electrode is recessed  impossible for the electrode to touch the workpiece
arc is essentially cylindrical  very little change in the heated area
TIG arc is not constricted  relative wide heat pattern on the workpiece
arc is conical  heated area varies with electrode-to-work distance
electrode extends beyond the end of gas nozzle  possible weld contamination

6. 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

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
MMA
MAG
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. 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 ( V)
additional interlocks to detect low gas flow, loss of coolant, etc
no need for arc voltage control

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

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, 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 l/min
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 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

25. Plasma cutting

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

27. 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

28. Plasma cutting equipment

29. 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

30. 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