Page 76 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Welding Processes

Tungsten inert gas (TIG)/gas tungsten arc
welding (GTAW)
Process description
Figure 5.9 shows the equipment. Fusion is obtained from the
heat of an arc formed between a non-consumable tungsten
electrode and the workpiece. The arc and weld pool are
protected from the atmosphere by a gas shield supplied
through the welding torch. Filler metal can be supplied
separately into the weld pool (Fig. 5.10). If a joint is welded
without the addition of filler it is called an autogenous weld.
Polarities
Steels are TIG welded using direct current with the welding
electrode connected to the negative pole (direct current
electrode negative, or DCEN). The reason for this is to keep
most of the heat in the workpiece and less in the electrode
and so prevent the electrode from overheating and possibly
melting.
Aluminium alloys and magnesium alloys are welded using
alternating current (a.c.). The positive cycle removes the high
temperature oxide layer (known as cathodic cleaning) while
the negative cycle helps to keep the electrode from melting.

Figure 5.9 TIG welding equipment

61

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200905QG Welding chap5.3d Page 61 of 66