• In TIG welding an arc is maintained between a non-consumable tungsten electrode and the work-piece, in inert gas medium, and is used as a heat source. Filler metal is fed from outside. The principle of operation of the process is shown in Fig. 2.6.

• Direct current is normally used with electrode negative polarity for welding most metals except aluminium, magnesium and their alloys, because of the refractory oxide film on the surface which persists even when the metal beneath melts. With electrode positive, cathode spots form on aluminium surface and remove oxide film due to ionic bombardment, but excessive heat generates at the electrode.

• Welding aluminium is best achieved by using alternating current. Large heat input to the workpiece is supplied during the electrode negative half of the cycle. During electrode positive half cycle the oxide film is removed. Since a high reignition voltage is required when the work is negative various means are used to compensate for this effect. Oxide fails to disperse if such means are not used.

• Electrode material could be pure tungsten for d. c. s. p. Thoriated tungsten or zirconated tungsten can work with a. c. as well as with d. c. welding. In a. c. welding, heat input to the electrode is higher, the tip invariably melts. Electrodes containing thoria or zirconia give steadier arc due to their higher thermionic emissivity compared to the pure tungsten electrode.

• Shielding gases used are: argon, helium, and argon helium mixtrure. For very reac­tive metals welding should be done in an argon filled chamber to obtain ductile welds. In open-air welding with normal equipment some contamination with argon always occurs. Deoxidants are added to the filler metal as a consequence when welding rim­ming or semi-skilled carbon steel, monel metal, copper, cupro-nickel and nickel.

• Copper can be welded with nitrogen as a shielding gas. Nitrogen reacts with liquid tungsten and not with copper. Thoriated tungsten electrode with straight polarity should be employed. With nitrogen atmosphere anode heat input per ampere is higher compared to argon atmosphere. It is good for high conductivity metal as copper.

• The process is costly and is used only where there is a definite technical advantage e. g. welding copper, aluminium, magnesium and their alloys up to 6 mm thick; alloy steels, nickel and its alloys up to 2.5 mm thick, and for the reactive metals.

• Argon spot welds could be made with a torch having the nozzle projecting beyond the electrode tip; it is held against the work, arc is struck and maintained for a preset time and argon is cut-off after a delay. A molten pool forms on the top sheet and fuses into the sheet underneath, producing a plug/spot weld. This welding is ideal for situations having access to one side of the joint only. The equipment required is light

and portable. Process is slow and not adaptable to fully mechanised control as spot welding.