TIG Welding

Advantages of TIG Welding

High quality welds, low distortion
Easy to mechanize and automate
Flexibility and ease of heat control
All metals, all positions

Limitations of TIG Welding
Lower deposition rates
Good skills required
Sensitive to cleanliness and contamination
Arc blow sensitive

Polarity
The Tungsten Electrode in Gas Tungsten Arc Welding (GTAW or Tig) can make contact with a Direct Current (DC) power supply either at the positive (+) or at the negative (-) terminal. In the first case the connection is for DCEP (electrode positive) or reverse polarity. In the second one for DCEN (electrode negative) or straight polarity.

DCEP:
30% of the heat is concentrated in the workpiece
70% of the heat in concentrated in the tungsten electrode
Shallow penetration
Wide weld area

DEN:
70% of the heat is concentrated in the workpiece
30% of the heat is concentrated in the tungsten electrode
Deep penetration
Narrow weld area

Tungsten Electrodes
A few tungsten alloys are used as electrodes for GTAW (tig welding). They are non consumable as they do not enter, intentionally, in the composition of the weld pool. Their function is limited to carry the currents employed, to stand the heat of the arc without melting and to provide electrons by thermionic emission.

Filler Wire
Manual GTAW is often performed autogenously (without added filler metal) when the joint requires only fusion of the abutting ends. However when needed, filler material can be added manually from the side, right in the weld puddle, by alternatively introducing the wire tip into the arc and withdrawing it.

Shielding Gases
The shielding gas is provided continuously through the torch to protect the molten weld metal, the tungsten electrode and the filler wire tip, if used, from air contamination. Backup shielding gas may be provided if necessary on the underside of the weld to purge and displace surrounding air.

Shielding is always needed but it may be critical especially for highly reactive metals like titanium that would be promptly oxidized and ruined if heated in air. To weld titanium by GTAW use is made of glove boxes purged with shielding gas of the highest purity before weld start or of trailing shields, specially designed accessories that provide a sufficient gas trail to protect the welded surface until cool enough.

The gases uses are mostly Argon and Helium or mixtures of the two. Argon is the standard shielding gas. Helium, which is lighter than air, must be supplied with a higher flow than argon because it tends to rise. It transfers more heat than argon for the same current and voltage. Thus it may be needed to join metals of higher conductivity or heavy plates.

When extinguishing the arc at the end of the weld, it is important to let the gas flow for a certain time to shield the hot metal from air. Equipment provides a delay before shutting off the gas flow, but the welder must linger with the torch upon the weld as long as needed.

Materials
Most weldable metals can be welded by the GTAW process. It is adaptable to thin and delicate joints but also to thick and demanding applications, always of good quality although possibly not the most fast and economic process.

Aluminum alloys of the weldable kinds are among those for which GTAW is most suited. One should always remember that not all aluminum alloys can be welded successfully or with the same ease. For a reminder of the reasons making some of the strongest alloys non readily weldable by fusion processes see Aluminum Welding.

Joining is made difficult by the surface formation of tenacious refractory aluminum oxides of melting point much higher than that of aluminum metal. The oxides though are broken up by the cathodic cleaning action of the Electrode Positive part of the alternating current cycle. Once broken they may float upon the molten metal and they no longer interfere with the welding process. Therefore high frequency stabilized alternating current power supply is the most used GTAW process for aluminum with pure tungsten electrodes. Exceptionally, for thin sections, DCEP can be used with thoriated tungsten. Argon is the most used shielding gas.

By: PWL