Advanced Welding Processes: Technologies and Process Control

Monitoring and control of welding processes 203
exceeds the control limit at subgroup 5. At this stage, the gas flow had been
reduced from its original value of 12 l min –1 to around 7–8 l min –1 and,
although the effect was just discernible by visual inspection of the finished
welds, the welds were still acceptable for the intended application. By the
time the gas flow had been reduced to 2 l min –1 (subgroup 9) the welds were
100% POOR and visually unacceptable. The charts do provide early warning
of unacceptable performance and alert the user to adverse trends before the
weld quality reaches an unacceptable level.
Visible spectrum monitoring. The use of a spectrographic technique for
real-time monitoring and control of weld quality has been reported. [231]
The system can detect changes in the chemical composition of the arc and,
in particular, the loss of shielding gas, the increase in hydrogen in the arc
atmosphere and changes in the flux composition during the FCAW process.
Although the equipment used in the feasibility study was costly and complex,
it is envisaged that, once the specific monitoring requirements are identified
simple band-pass filters could be used to produce a compact welding monitor.
10.3.2 Summary: process monitoring
Monitoring techniques are available for calibration and troubleshooting in
welding processes; these vary from simple meters to on-line computer-based
systems which collect and report the status, trends and production statistics
on a large number of welding cells.
Regardless of the type of monitoring system used, it is important to ensure
that the correct measuring techniques are applied and the procedure and type
of instrumentation are recorded.
Computer-based monitors provide facilities for on-line quality assurance
which can make a significant contribution to the cost and reliability of the
welding operation and reduce the need for post-weld testing. [232, 233] It is
reported, for example, that one manufacturer has used on-line data logging to
reduce post-weld destructive testing by 50% and saved over $90 000 per year.
[234] The data obtained may also be used for optimizing the welding process
and preventative maintenance programmes. Significantly, many quality systems
standards recognize the use of continuous monitoring as an important control
technique; BS 5750, [235] for example, stated that ‘Continuous monitoring and/
or compliance with the documented procedures’ are required to ensure that the
specified requirements are met. It is important to note that, although these
systems may be relatively sophisticated and provide clear indications of process
deviations, they require manual intervention to correct the process performance.
10.4 Automated control techniques
Using suitable monitoring systems and mechanized welding, it is possible to
correct process deviations automatically without the need for manual