Tungsten Inert Gas Welding
Tungsten Inert Gas Welding: The TIG welding process was first developed in the USA during the 2nd world War for the welding of aluminum alloys
• The process uses a non-consumable tungsten electrode
• The process requires a high level of welder skill
• The process produces very high-quality welds.
• The TIG process is considered a slow process compared to other arc welding processes
• The arc may be initiated by a high frequency to avoid scratch starting, which could cause contamination of the tungsten and weld
TIG Welding Variables
TIG Welding Voltage
The voltage of the TIG welding process is variable only by the
type of gas being used, and changes in the arc length
TIG Welding Current
The current is adjusted proportionally to the tungsten
electrode diameter being used. The higher the current the
deeper the penetration and fusion
TIG Welding Polarity
The polarity used for steels is always DC –ve as most of the
heat is concentrated at the +ve pole, this is required to keep
the tungsten electrode at the cool end of the arc. When
welding aluminum and its alloys AC is used
Types of current
DC | DCEN or DCEP/DCEN gives deep penetration |
AC | It can be a sine or square wave that requires an HF current (continuous or periodical) to provide cleaning action |
Pulsed Current
• requires a special power source
• low frequency – up to 20 pulses/sec(thermal pulsing)
• better weld pool control
• weld pool partially solidifies between pulses
Choosing the Proper Electrode
TIG WELDING ARC CHARACTERISTICS
TIG – arc initiation methods
Tig Welding Pulsed current
- usually peak current is 2-10 times the background current
• useful on metals sensitive to high heat input
• reduced distortions
• in the case of dissimilar thicknesses equal penetration can be achieved - one set of variables can be used in all positions
- used for bridging gaps in open root joints
- Require special power source
Choosing the proper electrode
Tig Welding Tungsten Electrodes
Old types: (Slightly Radioactive)
• Thoriated: DC electrode -ve – steels and most metals
• 1% thoriated + tungsten for higher current values
• 2% thoriated for lower current values
• Zirconiated: AC – aluminum alloys and magnesium
New types: (Not Radioactive)
• Cerium: DC electrode -ve – steels and most metals
• Lanthanum: AC – Aluminum alloys and magnesium
TIG Torch set-up
Choosing the Correct Electrode
Tungsten Electrodes
Old types: (Slightly Radioactive)
• Thoriated: DC electrode -ve – steels and most metals
• 1% thoriated + tungsten for higher current values
• 2% thoriated for lower current values
• Zirconiated: AC – aluminum alloys and magnesium
New types: (Not Radioactive)
• Cerium: DC electrode -ve – steels and most metals
• Lanthanum: AC – Aluminum alloys and magnesium
Tungsten Electrode Types
Pure tungsten electrodes:
# color code – Green
#no alloy additions
#low current carrying capacity
#maintains a clean balled end
#can be used for AC welding of Al and Mg alloys
#poor arc initiation and arc stability with AC compared
#with other electrode types
#used on less critical applications
#low cost
Thoriated tungsten electrodes:
# color code – yellow/red/violet
#20% higher current carrying capacity compared to
#pure tungsten electrodes
#longer life – greater resistance to contamination
#thermionic – easy arc initiation, more stable arc
#maintain a sharpened tip
#recommended for DCEN seldom used on AC (difficult to maintain a balled tip)
#This slightly radioactive
Created tungsten electrodes:
#Colour code – grey (orange acc. AWS A-5.12)
#Operate successfully with AC or DC
#Ce not radioactive – replacement for thoriated types
Lanthaniated tungsten electrodes:
#Colour code – black/gold/blue
#Operating characteristics similar to the created electrode
Zirconiated tungsten electrodes:
#Colour code – brown/white
#Operating characteristics fall between those of pure and thoriated electrodes
#Retains a balled end during welding – good for AC welding
#High resistance to contamination
#Preferred for radiographic quality welds
Electrode tip for DCEN
Electrode tip for AC
TIG Welding Variables
Tungsten electrodes
The electrode diameter, type, and vertex angle are all critical
factors considered as essential variables.
The vertex angle is as shown
Choosing the Proper Electrode
Shielding gas requirements
Preflow and Postflow
Special shielding methods
Pipe root run shielding – Back Purging to prevent excessive oxidation during welding, Normally argon.
TIG torch set-up
TIG Welding Consumables
Welding consumables for TIG:
•Filler wires, Shielding gases, tungsten electrodes (non-consumable).
•Filler wires of different material compositions and variable diameters available in standard lengths, with applicable code stamped for identification
•Steel Filler wires of very high quality, with a copper coating to resist corrosion.
•shielding gases mainly Argon and Helium, usually of the highest purity (99.9%)
Tungsten Inclusion
This may be caused by the Thermal Shock of
heating too fast and small fragments
break off and enter the weld pool, so a
“slope up” device is normally fitted to
prevent this could be caused by touch
down also.
Most TIG sets these days have slope-up devices that bring the current to
the set level over a short period of
time so the tungsten is heated more
slowly and gently
A Tungsten Inclusion always shows up as bright white on a radiograph
TIGWelding Defects
A lack of welder skill, or incorrect equipment setting causes most welding defects with TIG. i.e. current, torch manipulation, welding speed, gas flow rate, etc.
• Tungsten inclusions (low skill or wrong vertex angle)
• Surface porosity (loss of gas shield mainly on site)
• Crater pipes (bad weld finish technique i.e. slope out)
• Oxidation of S/S weld bead, or root by poor gas cover
• Root concavity (excess purge pressure in a pipe)
• Lack of penetration/fusion (widely on root runs)
Tungsten Inert Gas Welding Advantages And Disadvantages
| Advantages | Disadvantages |
| High quality | High skill factor required |
| Good control | Low deposition rate |
| All positions | Small consumable range |
| Lowest H2 process | High protection required |
| Minimal cleaning | Complex equipment |
| Autogenous welding (No filler material) | Low productivity |
| Can be automated | High ozone levels +HF |
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