Advanced Welding Processes: Technologies and Process Control

Monitoring and control of welding processes 213
metallic ions in the arc. Improved signals are obtained by selectively monitoring
the arc emission spectrum and, in particular, the intense argon lines at
wavelengths of 7504, 7514, 8006 and 8015 Å. This technique avoids problems
with arc voltage fluctuations unrelated to arc length and may utilize a relatively
simple sensor that is placed at a safe distance from the arc.
Penetration control
Penetration control techniques have been developed mainly for situations
where a full-penetration butt weld is required and welding is to be carried
out from one side of the joint. These techniques may also be applied to
critical root beads, for example in circumferential welding of pipe.
Back face sensing. It has been shown that the energy of certain wavelengths
of radiation emitted from the back of the weld pool is a function of the weld
bead area or width. [253] The radiation from the weld pool may be distinguished
from that emitted by the parent metal by restricting the bandwidth of the
sensor system to 450–550 nm, i.e. in the green part of the visible spectrum.
In practice, the radiation sensor may be a photodiode or phototransistor that
is sensitive to visible radiation; this is mounted beneath the joint with a
suitable filter and protective shield (Fig. 10.18), and the signal from the
sensor is passed to the control unit, where it is compared with a preset
reference value. The error signal may be used to control a range of process
parameters, the most common of which is pulse duration in pulsed GTAW.
In this case, the pulse is maintained at the high level until penetration is
achieved, and the feedback signal reaches the reference level. When this
occurs, the current is reduced to its background level for a fixed time and the
torch is indexed along the seam. The pulse duration varies to suit the joint
conditions, decreasing if there is a tendency for heat build-up and increasing
if the thickness of the plate increases.
For industrial applications the radiation may be conducted to the
Power
source
Control
Filter
Photo-transistor
10.18 Principle of back face penetration control system.