Process variables

The main variables in TIG welding are:

 Welding current.

 Current type and polarity.

 Travel speed.

 Shape of tungsten electrode tip and vertex angle.

12.2.1 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.

12.2.2 Current type and polarity

 Best welding results are usually obtained with DC-ve.

 Refractory oxides such as those of aluminium or magnesium can hinder fusion but can be removed by using AC or DC electrode positive.

 With a DC positively connected electrode, heat is concentrated at the electrode tip so the electrode needs to be of greater diameter than when using DC-ve 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 so it is limited to welding sections.

12.2.3 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.

Current

type/polarity DC-ve AC DC+ve

Heat Weld profile Deep, narrow Medium Shallow, wide Cleaning

Ions Electrons _{Ions Electrons Ions Electrons}

(A.C.)

12.2.4 Tungsten electrode types

Different types of tungsten electrodes suit different applications:

 Pure tungsten electrodes are used when welding light metals with AC because they maintain a clean balled end, but possess poor arc initiation and stability in AC mode compared with other types.

 Thoriated electrodes are alloyed with thorium oxide (thoria) to improve arc initiation and have higher current carrying capacity than pure tungsten electrodes and maintain a sharp tip for longer. Unfortunately, thoria is slightly radioactive (emitting  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 and operate successfully with DC or AC and as cerium and lanthanum are not radioactive, they have been used as replacements for thoriated electrodes.

 Zirconiated electrodes are alloyed with zirconium oxide with operating characteristics between the thoriated types and pure tungsten. They are able to retain a balled end during welding, so are recommended for AC welding. They have a high resistance to contamination so are used for high integrity welds where tungsten inclusions must be avoided.

12.2.5 Shape of tungsten electrode tip

 With DC-ve, 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 electrode tip should have a length equal to approximately 2-2.5 times the electrode diameter.

 When using AC the electrode tip is ground flat to minimise the risk of it breaking off when the arc is initiated or during welding (shown on the next page).

 If the vertex angle is increased, the penetration increases.

 If the vertex angle is decreased, bead width increases.

 Pure or zirconiated tungsten electrodes are used for AC welding with a

12.2.6 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 improves penetration and reduces porosity.

Characteristics of argon and helium shielding gases for TIG welding.

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, advantageous 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. 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.

Electrode tip

(or vertex angle). Electrode tip

with flat end. Electrode tip with a balled end.

Shielding gas flow rate

 Too low and the shielding gas cannot remove the air from the weld area resulting in porosity and contamination.

 Too high and 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.

 Typically in the range ~10-~12 l/min.

Back purging

It is necessary to protect the back of the weld from excessive oxidation during TIG welding, achieved by using a purge gas, usually pure argon.

For pipe welding spools 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. For purging large systems soluble dams or bungs are required and can it can be a complex operation.

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 so there is positive flow (typically

~4 l/min).

Back purging should continue until two or more layers of weld have been deposited.

For C and C-Mn steels it is possible to make satisfactory welds without a back purge.

12.2.7 Electrode extension

Flow rate too low Flow rate too high

 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, causing melting and lead to tungsten inclusions.

 As a general rule stickout length should be 2-3 times the electrode diameter.