CURRENT TYPE
DCEN
DCEP
Electrode polarity
Negative
Positive
Electron & ion flow
Penetration characteristics
Oxide cleaning action
No
Yes
Heat balance in the
70% at work end
30% at work end
arc (approx.)
30% at electrode end
70% at electrode end
Penetration
Deep, narrow
Shallow, wide
Electrode capacity
Excellent e.g. 3.2mm, 400A
Poor e.g. 6.4mm, 120A
3.3 Electrode Types and
Preparation
Originally, pure tungsten was used as the
electrode material. Tungsten is a refractory
metal, with a high melting point (3380°C)
and has relatively good electrical and thermal
conductivity. It was discovered, however,
that arc stability, tip shape retention and arc
initiation could be improved by adding small
amounts of thorium oxide, thoria, or zirconium
oxide, zirconia, to the electrode.
More recently, it has been found that improved
performance can be obtained by alloying the
electrodes with oxides of lanthanum, yttrium
or cerium, particularly in automatic or orbital
TIG welding where consistency of operation is
important.
Traditionally, ‘thoriated’ electrodes have tended
to be used for DC operation and ‘zirconiated’
electrodes for AC. However, scares regarding
the potential health effects due to inhalation of
radioactive thorium oxide dust have resulted
in some users changing to ‘ceriated’ or
‘lanthaniated’ electrodes in place of ‘thoriated’.
Whilst not ‘consumed’ to form weld metal,
tungsten electrodes are used up over a period
of time due to repeated grinding of tips and due
to some erosion by the arc during welding. They
may also be accidentally consumed by being
dipped into the weld pool and melting into it.
The electrode diameter is determined by the
current and polarity. Recommended diameters
are given in the table below.
Recommended diameter for electrodes
ELECTRODE
CURRENT
DIAMETER (mm)
DC –
DC +
0.5
0 – 20
1.0
21 – 80
1.6
81 – 150
0 – 17
2.4
151 – 230
18 – 25
3.2
231 – 350
26 – 35
4.0
351 – 475
36 – 50
4.8
476 – 650
51 – 67
6.3
651 – 950
68 – 100
8.0
10
Figure 3. Effects of current type.
