Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
preparation, fit-up and material properties. Their application restores the
welding performance to a predetermined level and reduces the likelihood of
poor joint quality. Taking a pessimistic view, even if the weld is in the right
place, it may still contain metallurgical defects such as porosity and cracking.
Ideally the control system should monitor the quality of the joint in real time
and take corrective measures to ensure that the required standard is achieved.
Some steps towards the automatic control of quality have been taken, but
this area is still the subject of considerable research.
Bead geometry prediction. The finished bead geometry may itself be a
quality criterion or there may be a clear relationship between geometry and
secondary quality considerations. (For example, convex weld bead
reinforcement may lead to stress intensification and subsequent cracking or
high depth-to-width ratios may be responsible for solidification cracking.) If
a sufficiently accurate model for the relationship between the weld variables
and the geometry is obtained the process parameters may be adjusted on-line
to produce the required geometry. Progress in the development of suitable
mathematical models has recently been made but their application in process
control is still at the research stage.
Thermographic sensing. Remote thermographic imaging of the weld pool
has been found to be a practical [261, 262] method of assessing the temperature
profile of the joint in real time and enables the control of penetration, seam
tracking and metallurgical characteristics of the weld. Using an expert system,
the observed temperature measurements may be related to the likelihood of
defects and the probable mechanical properties of the joint. Sensing may be
performed by a fibre optic and remote thermal imaging system or a thermal
line scanner. The system is no more expensive than a laser stripe sensor but
offers the possibility of much more comprehensive control.
Hybrid control systems. It is often possible to combine the information
from several simple sensors to obtain a better indication of process performance
and ensure more effective control. A torch displacement sensor when combined
with through-arc measurements of voltage and current may, for example, be
used to distinguish between wire feed slip and torch height variation. Increasing
use of these hybrid systems combined with computer control should improve
the ability to achieve true on-line quality control.
10.5 Summary and implications
The range of control options for welding varies from the traditional openloop
manual systems based on welding procedures to complex closed-loop
automated techniques. Improved monitoring techniques and a wide range of
sensors make it possible to measure process performance in both manual and
automated applications and should enable more consistent weld quality to be
achieved.