Page 112 - Welding of Aluminium and its Alloys

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TIG welding

6.1 Introduction
Tungsten arc inert gas shielded welding, EN process number 144 abbrevi-
ated to TIG, TAGS or GTAW (USA), is an arc welding process that uses a
non-consumable tungsten electrode and an inert gas shield to protect the
electrode, arc column and weld pool, as illustrated in Fig. 6.1. The welding
arc acts as a heat source only and the welding engineer has the choice of
whether or not to add a filler wire. The weld pool is easily controlled such
that unbacked root passes can be made, the arc is stable at very low welding
currents enabling thin components to be welded and the process produces
very good quality weld metal, although highly skilled welders are required
for the best results. It has a lower travel speed and lower filler metal deposi-
tion rate than MIG welding, making it less cost effective in some situations.
TIG tends to be limited to the thinner gauges of aluminium, up to
perhaps 6mm in thickness. It has a shallower penetration into the parent
metal than MIG and difficulty is sometimes encountered penetrating into
corners and into the root of fillet welds. Recommended weld preparations
taken from BS 3019 ‘TIG Welding of Aluminium’ are given in Table 6.1.

6.2 Process principles

The basic equipment for TIG welding comprises a power source, a welding
torch, a supply of an inert shield gas, a supply of filler wire and perhaps
a water cooling system. A typical assembly of equipment is illustrated in
Fig. 6.2.
For welding most materials the TIG process conventionally uses direct
current with the electrode connected to the negative pole of the power
source, DCEN. As discussed in Chapter 3 welding on this polarity does not
give efficient oxide removal. A further feature of the gas shielded arc
welding processes is that the bulk of the heat is generated at the positive
pole.TIG welding with the electrode connected to the positive pole, DCEP,

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