Advanced Welding Processes: Technologies and Process Control

Number of arc starts
35
30
25
20
15
10
5
0
Advanced gas tungsten arc welding 79
OCV 18 22 24 36 18 22 24 36 18 22 24 36 18 22 24 36 18 22 24 36
Pure W 1% La 2O 3 2% Y 2O 3 1% CaO 2 2% ThO 2
Successful Unstable Failed
6.2 Electrode performance (arc start rate versus OCV) for various
GTAW electrodes with rare earth additions. [86]
cooler part of the electrode. This rim can lead to arc asymmetry and instability.
The effect is not directly affected by the alloying addition, although weight
loss of the electrode was less significant with the rare-earth-doped electrodes.
The improvements in electrode life and striking performance obtained with
rare-earth-doped electrodes are relatively small, but, where reliable and
consistent operation is required, for example in automated welding, these
marginal improvements may prove beneficial. Alternatively, high-quality
thoriated electrodes may be specified in situations where consistency is
important.
6.3 Process variants
6.3.1 Pulsed GTAW
Low-frequency (1–10 Hz) modulation of the current in the GTAW process
has been used [87] to provide the following process characteristics:
∑ reduced distortion;
∑ improved tolerance to dissimilar thicknesses;
∑ improved tolerance to dissimilar materials;
∑ reduced thermal build-up;
∑ improved tolerance to cast-to-cast variation.
Using electronic power sources, it is possible to generate a range of
alternative pulse profiles, although in practice nominally square wave pulses
are usually used as illustrated diagrammatically in Fig. 6.3. The low current
or background level is set at a value just sufficient to maintain an arc without