17
3.1 Introduction
The Tungsten Inert Gas, or TIG process, uses
the heat generated by an electric arc struck
between a non-consumable tungsten electrode
and the workpiece to fuse metal in the joint
area and produce a molten weld pool. The arc
area is shrouded in an inert or reducing gas
shield to protect the weld pool and the
non-consumable electrode. The process may
be operated autogenously, that is, without filler,
or filler may be added by feeding a consumable
wire or rod into the established weld pool.
3.2 Process
1
2
3
4
5
6
7
1
Shielding gas
2
Arc
3
TIG filler rod
4
Weld pool
5
Collet
6
Tungsten Electrode
7
Workpiece
Schematic of the TIG welding process
Direct or alternating current power sources
with constant current output characteristics
are normally employed to supply the welding
current. For DC operation the tungsten may
be connected to either output terminal, but
is most often connected to the negative pole.
The output characteristics of the power source
can have an effect on the quality of the welds
produced.
Shielding gas is directed into the arc area by the
welding torch and a gas lens within the torch
distributes the shielding gas evenly over the
weld area. In the torch the welding current is
transferred to the tungsten electrode from the
copper conductor. The arc is then initiated by
one of several methods between the tungsten
and the workpiece.
During TIG welding, the arc can be initiate by
several means:
Scratch Start
With this method, the tungsten electrode
is physically scratched on the surface of the
workpiece and the arc is initiated at the full
amperage set by the operator. The incidence
of the tungsten melting at the high initiation
amperage is high and tungsten inclusions in the
weld metal are quite common.
High Frequency Start
During High Frequency start, the arc will ‘jump’
towards the workpiece if a critical distance
is reached. With this method, there is no
incidence of tungsten inclusions happening. High
Frequency is only available on certain types of
machines and it can affect nearby electronic
equipment.
3.0 Gas Tungsten Arc Welding (GTAW/TIG)