Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
Electrode composition
The tungsten electrodes used in GTAW are usually alloyed with a small
amount of thoria or zirconia in order to improve arc starting, by reducing the
work function of the tungsten and improving its emission characteristics. 1
Thoriated electrodes give very good striking and DC running characteristics,
but it has been demonstrated that the consistency of performance is closely
related to the homogeneity of the electrode and, in particular, the regularity
of the thoria distribution. [85] In this study, it was shown that the stable arc
operating time (continuous arc operation) may be extended by up to 100%
when an electrode with a fine, homogeneous distribution of thoria particles
(70 h stable arc operation at 125 A) is substituted for an electrode with a less
regular composition (35 h stable operation), whilst re-ignition delays can be
reduced from 4% of the total number of arc starts to 1% with the betterquality
electrode.
Although thoria (ThO2) is effective in improving arc striking and tip
shape retention, it is naturally radioactive. Concern about potential safety
implications, in particular in electrode manufacture, has led to the investigation
of alternative alloying additions. Oxides of the rare earth elements lanthanum,
yttrium and cerium appear to offer similar characteristics to thoria. Laboratory
investigations [86] indicate that electrodes doped with these substances may
perform better than conventional thoriated types. In these tests the number of
successful arc initiations using high-frequency (HF) arc starting and an opencircuit
voltage (OCV) range of 18 to 36 V was assessed. The results are
summarized below and in Fig. 6.2.
Follow-on current was set to 20–30 A. The electrode vertex angle was
45∞, and the total number of attempts at each open circuit voltage was 30. At
30 V OCV, the performance of ThO2, La2O3, CeO2, and Y2O3, is very similar,
whereas at 24 V OCV, the lanthanum oxide gave the best results. Measurements
of electrode temperature indicated that La2O3, Y2O3 and CeO2 gave lower
operating temperatures than those for pure tungsten and zirconiated tungsten,
and in addition, the amount of electrode melting and tip shape deterioration
was much less.
Rim formation and weight loss
A rim of tungsten ‘whiskers’ forms on the upper vertex of the electrode,
particularly if there is more than 0.05% oxygen present in the shielding gas.
This effect is thought to be associated with the volatilization of tungsten
oxide and the condensation and growth of pure tungsten crystals on the
1The work function of pure tungsten is around 4.54 eV, whereas that of a 2% thoriated
electrode is around 2.63 eV.