The main variables in TIG welding are:
• Welding current
• Current type and polarity
• Travel speed
• Shape of tungsten electrode tip and vertex angle
• Shielding gas flow rate
2.1 Welding current
• Weld penetration is directly related to welding current
• If the welding current is too low, the electrode tip will not be properly heated and an unstable arc may result
• If the welding current is too high, the electrode tip might overheat and melt, leading to tungsten inclusions
2.2 C
urrenttype and polarity
• The best welding results are usually obtained with DC electrode negative
• Refractory oxides such as those of aluminium or magnesium can hinder fusion but these can be removed by using AC or DC electrode positive
• With a DC positively connected electrode, heat is concentrated at the electrode tip and therefore the electrode needs to be of greater diameter than when using DC negative if overheating of the tungsten is to be avoided. A water cooled torch is recommended if DC positive is used
• The current carrying capacity of a DC positive electrode is about one tenth that of a negative one and it is therefore limited to welding sections
Ions Electrons Ions Electrons Ions Electrons
2.3 Travel speed
• Travel speed affects both weld width and penetration but the effect on width is more pronounced.
• Increasing the travel speed reduces the penetration and width
• Reducing the travel speed increases the penetration and width Current Weld profile Deep, narrow Medium Shallow, wide Cleaning action No Yes – every half
2.4 Tungsten electrode types
Different types of tungsten electrodes can be used to suit different applications:
• Pure tungsten electrodes are used when welding light metals with AC because of their ability to maintain a clean balled end. However they possess poor arc initiation and arc stability in AC mode compared to other types
• Thoriated electrodes are alloyed with thorium oxide (thoria) to improve arc initiation. They have higher current carrying capacity than pure tungsten electrodes and maintain a sharp tip for longer. Unfortunately, thoria is slightly radioactive (emitting α radiation) and the dust generated during tip grinding should not be inhaled. Electrode grinding machines used for thoriated tungsten grinding should be fitted with a dust extraction system.
• Ceriated and lanthaniated electrodes are alloyed with cerium and lanthanum oxides, for the same reason as thoriated electrodes. They operate successfully with DC or AC but since cerium and lanthanum are not radioactive, these types have been used as replacements for thoriated electrodes
• Zirconiated electrodes are alloyed with zirconium oxide. Operating characteristics of these electrodes fall between the thoriated types and pure tungsten. However, since they are able to retain a balled end during welding, they are recommended for AC welding. Also, they have a high resistance to contamination and so they are used for high integrity welds where tungsten inclusions must be avoided.
2.5 Shape of tungsten electrode tip
• With DC electrode negative, thoriated, ceriated or lanthanated tungsten electrodes are used with the end ground to a specific angle (the electrode tip or vertex angle – shown below).
• As a general rule, the length of the ground portion of the tip of the electrode should have a length equal to approximately 2 to 2.5 times the electrode diameter.
• The tip of the electrode is ground flat to minimise the risk of it breaking off when the arc is initiated or during welding (shown below).
• If the vertex angle is increased, the penetration increases.
• If the vertex angle is decreased, bead width increases for AC welding, pure or zirconiated tungsten electrodes are used.
2.6 Shielding gases
The following inert gases can be used as shielding gases for TIG welding:
• Argon
• Helium
• Mixtures of argon and helium
Note: For austenitic stainless steels and some cupro-nickel alloys, argon with up to ~5% hydrogen may be used to improve penetration and reduce porosity
Argon Performance item Helium
Lower than with helium, which can be helpful when welding thin sections. Less change in arc voltage with variations in arc length.
Arc voltage Higher than with argon. Arc is hotter which is helpful in welding thick sections and viscous metals (eg nickel).
Lower than with helium, which gives reduced penetration.
Heating power of the arc
High, which can be of advantage when welding metals with high thermal conductivity and thick materials.
Argon is heavier than air, so requires less gas to shield in the flat and horizontal positions. Also, better draught resistance.
Protection of weld Helium is lighter than air and requires more gas to properly shield the weld. Exception:
overhead welding.
Obtained from the atmosphere by the
separation of liquefied air – lower cost and greater availability.
Availability and cost
Obtained by separation from natural gas – lower availability and higher cost.
Characteristics of argon and helium shielding gases for TIG welding Electrode tip angle
(or vertex angle)
Electrode tip with flat end
Electrode tip with a
‘balled’ end
2.6.1 Shielding gas flow rate
• If the gas flow rate is too-low, the shielding gas cannot remove the air from the weld area and this may result in porosity and contamination.
• If the gas flow rate is too high, turbulence occurs at the base of the shielding gas column. Air tends to be sucked in from the surrounding atmosphere and this may also lead to porosity and contamination.
• Shielding gas flow rates are typically in the range ~10 to ~12 l/min
rate too low
2.6.2 Back purging
It is necessary to protect the back of the weld from excessive oxidation during TIG welding and this is achieved by the use a purge gas – usually pure argon.
For pipe welding, it is relatively easy to purge the pipe bore, but for plate/sheet welding it is necessary to use a purge channel or sometimes another operator positions and moves a back purge nozzle as the weld progresses.
The initial stage of back purging is to exclude all the air at the back of the weld and having allowed sufficient time for this the flow rate should be reduced prior to starting to weld such that there is positive flow (typically
~4l/min).
Back purging should continue until two or more weld layers of weld have been deposited.
For C and C-Mn steels it is possible to make satisfactory welds without a back purge.
2.7 Electrode extension
Flow rate too low Flow rate toohigh
• If the electrode extension is too short, the electrode tip will not be adequately heated leading to an unstable arc
• If the electrode extension is too long, the electrode tip might overheat, cause melting and lead to tungsten inclusions
• As a general rule, stickout length should be 2 to 3 times the electrode diameter