Laser welding is increasingly used in industrial applications, because of the advantages it offers, such as high speed, high accuracy, low heat input and low distortion. As for any other fusion welding process, weld imperfections can occur. Because of the small features of a laser weld and thereby of the imperfections that may occur, the industrial requirement is to detect these reliably using real-time monitoring methods. This monitoring must be fast, reliable and cost-effective, as components with defects either have to be rectified or scrapped, both having a considerable impact on the cost of the component.
Most common techniques in use today for process monitoring, employ photo-diode sensors to record electromagnetic (EM) signals arising from the molten pool during welding, with the objective of correlating the output from the sensor to features such as weld penetration, the occurrence of pin holes, or weld shape. These systems have been developed to monitor laser welding in real-time and generally examine the laser-to-metal interactions to infer the quality of the weld itself. By using different types of sensors, responding to different wavelengths of light, different aspects of the process or weld can be monitored, such as the weld pool temperature, the plasma above the weld pool and the level of back reflection, for instance. The output of these sensors is then correlated to the weld geometry and the possible presence of weld imperfections.
Advanced electronics in recent years has resulted in the introduction of both CCD and CMOS cameras as a means of monitoring the welding process, by capturing EM signals emanating from the weld pool. This way, information of the actual keyhole and weld pool can be captured, for instance, weld pool size and shape, and keyhole geometry and stability, and used for inferring the weld quality. This technique is capable of detecting both surface-breaking and, to some extent, sub-surface weld imperfections.
Seam-tracking is another example of in-process monitoring, where in this case, the monitored signal, i.e. the position of the laser beam in relation to the joint line, is used to correct the process (position) if required. Seam-tracking is in other words, an example of both in-process monitoring and control.