CSWIP 3.
1 Welding Inspection
Tungsten Inert Gas
or
Gas Tungsten Arc Welding
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TWI Training & Examination Services
Course Reference WIS 10
Course notes section reference 12
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 as a slow process compared
to other arc welding processes
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• The arc may be initiated by a high frequency to avoid scratch
starting, which could cause contamination of the tungsten
and weld
GTAW - Principle of operation
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Tungsten Inert Gas Welding
GTAW
Ceramic
Nozzle Current Conductor
Welding Torch
Shielding gas
Tungsten
Electrode
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Contact Tube
Gas Shield
Filler Wire
Arc
Weld Pool
GTAW power source
POWER SOURCE Transformers for AC – aluminium alloys
440v 50Hz 3 phase or Rectifiers for DC - steels
240v single phase input Transformer/rectifier for AC/DC
Inverters for AC/DC – more portable - expensive
TORCH Sizes/types vary depending on current/application
TORCH HOSE Flexible – may carry current, gas, cooling water.
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RETURN LEAD Note that current actually flows from this lead
INERT GAS SUPPLY Correct type for application.(ar, he, ar/he mixture)
(Cylinder & regulator) Argon/hydrogen for austenitic stainless steel
FLOWMETER To deliver correct gas flow (velocity) depending on welding
(graduated in ltr/min) position and joint configuration.
Equipment for TIG
Power control
panel Transformer/
Rectifier
Power return
cable Inverter
power source
Torch
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assemblies Power control
panel
Tungsten
electrodes Power cable
Flow-meter
Equipment for GTAW
Power source control panel
Remote
Ammeter
control
Start method Welding
current
Start/stop current
AC balance Postflow
(Preflow)
Slope up
2/4 stroke
Slope down
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Pulse ON/OFF Background
current
Pulse frequency Peak current
Current type
ON/OFF switch & polarity
Courtesy of Lincoln Electric
ARC CHARACTERISTICS
Constant Current/Amperage Characteristic
Large change in voltage =
OCV Smaller change in amperage
Volts
Large arc gap
Welding Voltage
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Small arc
gap
Amps
GTAW Torch
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Torch types: • Gas cooled: cheap, simple, large size, short life for
component parts
• Water cooled: recommended over 150A, expensive,
complex, small size, longer life for component parts
GTAW Torch
Tungsten
electrode
Torch Electrode
cap/tungsten collet Collet
housing holder
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Torch
body
Ceramic
nozzle
On/off
switch
GTAWTorch
Gas lenses
Stainless steel
wire sieve
Thread for
gas nozzle
Thread for
torch body
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• reduces eddies in the gas flow
• extends the length of the laminar
flow prevents contamination
• highly recommended in case of
reactive metals (e.g. Ti, Al)
Electrode tip for DCEN
Penetration
increase
electrode diameter
2-2,5 times
Increase
Vertex
angle
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Decrease
Bead width
Electrode tip prepared increase Electrode tip prepared for
for low current welding high current welding
Electrode tip for AC
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Electrode tip ground
Electrode tip ground and then conditioned
GTAW 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
AC
DC -ve
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Vetex angle Note: when welding
aluminium with AC
Note: too fine an angle will current, the tungsten end
promote melting of the is chamfered and forms a
electrodes tip ball end when welding
Tungsten Electrodes
Old types:
• 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:
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• Cerium: DC electrode -ve - steels and most metals
• Lanthanum: AC - Aluminum alloys and magnesium
Choosing the proper electrode
Factors to be considered:
Electrode tip Excessive
Too Welding Too
not properly melting or
low current high
heated volatilisation
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Unstable Tungsten
arc Penetration inclusions
Choosing the proper electrode
Current type influence
+ - + - + -
+ - + - + -
+ - + - + -
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Current type & polarity DCEN AC (balanced) DCEP
Heat balance 70% at work 50% at work 35% at work
30% at electrode 50% at electrode 65% at electrode
Penetration Deep, narrow Medium Shallow, wide
Oxide cleaning action No Yes - every half cycle Yes
Electrode capacity Excellent Good Poor
(e.g. 3,2 mm/400A) (e.g. 3,2 mm/225A) (e.g. 6,4 mm/120A)
Shielding gas requirements
Shielding gas flow
Welding current
• Preflow and
postflow
Preflow Postflow
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Flow rate Flow rate
too low too high
Special shielding methods
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Torch trailing shield Welding in protective tent
Special shielding methods
Pipe root run shielding
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GTAW torch set-up
• Electrode extension
Stickout
Electrode
2-3 times
extension electrode
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diameter
Overheating
Low electron Too Electrode Too Tungsten
emission small extension large inclusions
Unstable arc
GTAW - arc initiation methods
Arc initiation
method
Lift arc HF start
• simple method
• tungsten electrode is in contact • need a HF generator (spark-
with the workpiece! gap oscillator) that generates a
• high initial arc current due to high voltage AC output (radio
the short circuit frequency) costly
• impractical to set arc length in • reliable method required on
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advance both DC (for start) and AC (to
• electrode should tap the re-ignite the arc)
workpiece - no scratch!
• can be used remotely
• ineffective in case of AC
• HF produce interference
• used when a high quality is not
essential • requires superior insulation
Types of current
• can be DCEN or DCEP
DC
• DCEN gives deep penetration
• can be sine or square wave
AC
Type of • requires a HF current (continuos or
periodical)
welding
• provide cleaning action
current
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• requires special power source
• low frequency - up to 20 pulses/sec
Pulsed (thermal pulsing)
current • better weld pool control
• weld pool partially solidifies between
pulses
Pulsed current
Pulse Cycle Peak Background
• usually peak current is 2-10
Current (A)
time time current current times background current
• useful on metals sensitive
to high heat input
• reduced distortions
• in case of dissimilar
thicknesses equal
penetration can be
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Average current
achieved
Time
• one set of variables can be used in all positions
• used for bridging gaps in open root joints
• require special power source
GTAW welding parameters
• welding current penetration
• arc voltage weld width
• travel speed weld width + penetration
• tilt angle
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GTAW Welding Variables
Voltage
The voltage of the TIG welding process is variable only by the
type of gas being used, and changes in the arc length
Current
The current is adjusted proportionally to the tungsten
electrodes diameter being used. The higher the current the
deeper the penetration and fusion
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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 aluminium and its alloys AC current is used
GTAW Welding Variables
Gas type and flow rate
Generally two types of gases are used in TIG welding, argon
and helium, though nitrogen may be considered for welding
copper and hydrogen may be added for the welding of
austenitic stainless steels. The gas flow rate is also important.
Argon (Ar) Inert • Helium Argon mixes
• Suitable for welding carbon • Suitable for welding
steel,stainless steel, carbon steel, stainless
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aluminium and magnesium steel, copper, aluminium
and magnesium
• Lower cost, lower flow rates
• High cost, high flow rates
• More suitable for thinner
materials and positional • More suitable for thicker
welding materials and materials of
high thermal conductivity.
GTAW Welding Variables
1. 2. 3. 4. 5.
1. Pre-flow timer control
Adjusts the time the gas and 4. Down-Slope control (Slope-out)
water valves are open Crater fill and controls crater
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2. Up-Slope control (Slope-In) pipe and the possibility of
Prevents burn throughs and crater cracks
the possibility of tungsten 5. Post-flow timer control
inclusions Adjusts the time the gas flows
3. Output control after welding
Welding current control
Checks With TIG Welding
The welding equipment
A visual check should be made to ensure the welding
equipment is in good condition
The torch head assembly
Check the diameter and specification of the tungsten
electrode, the required vertex angle and that a gas lens is
fitted correctly. Check the electrode stick-out length and that
the ceramic is the correct type and in good condition
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Gas type and flow rate
Check the shielding gas is the correct type, or gas mixture and
the flow rate is correct for the given joint design, welding
position as stated in the WPS
Checks With TIG Welding
Current and polarity
Checks should be made to ensure that the type of current and
polarity are correctly set, and the range is within that given on
the procedure.
Other welding parameters
Checks should be made to other parameters such as torch
angle, arc gap distance and travel speed.
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Safety
Check should be made on the current carrying capacity, or
duty cycle of the equipment and all electrical insulation is
sound and in place. Correct extraction systems should be in
use to avoid exposure to toxic fume.
Mechanised TIG
Cold wire
• no current is flowing
through the wire no
preheat
• simple equipment
• special wire feeders can
provide continuos,
intermittent or pulsed
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feed
• widely used on orbital
pipe welding and tube-
to-tube sheet
applications
Mechanised TIG
Hot wire
• current is flowing through the
wire wire is resistance
heated nearly to its melting
temperature when it is in
contact with the weld pool
• higher welding speed than
cold wire
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• high deposition rate
normally used in flat position
• wire is usually fed into the
weld pool behind the arc
GTAW typical defects
Most welding defects with TIG are caused by a lack of welder
skill, or incorrect setting of the equipment. 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
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• Root concavity (excess purge pressure in pipe)
• Lack of penetration/fusion (widely on root runs)
Gas Tungsten Arc Welding
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
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• Autogenous welding • Low productivity
• Can be automated • High ozone levels +HF
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Any Questions
