Evaluation of laser weld monitoring – A case study

Abstract

On-line monitoring of the quality of laser welding is of interest for many industrial applications. For photodiodes the monitoring strategy usually aims at observing whether the signal exceeds a threshold. This well known technique is mainly based on empirical values and the monitoring system has to be trained for each application. For an improved understanding of the connection between the physics of the welding defect generated and the resulting monitored signal, these experiments were observed by high speed imaging, followed by evaluation and modeling. A commercial system with three detector wavelength windows was studied for nine different industrial welding applications with distinct defects. We present here selected cases for which we try to generalize the findings and to draw conclusions for the applicability of commercial monitoring systems. For example humping was clearly detected by the signal but spatter and crater formation (for overlapping Zn-coated steel) was not. An interesting cause for welding defects is oil, grease or detergent contamination of the joint prior to welding. Monitoring defects caused in this way produced unexpected signal responses, which can be partially explained by our analysis. We summaries that an improved understanding of the signal source facilitates the reliable application of monitoring systems.On-line monitoring of the quality of laser welding is of interest for many industrial applications. For photodiodes the monitoring strategy usually aims at observing whether the signal exceeds a threshold. This well known technique is mainly based on empirical values and the monitoring system has to be trained for each application. For an improved understanding of the connection between the physics of the welding defect generated and the resulting monitored signal, these experiments were observed by high speed imaging, followed by evaluation and modeling. A commercial system with three detector wavelength windows was studied for nine different industrial welding applications with distinct defects. We present here selected cases for which we try to generalize the findings and to draw conclusions for the applicability of commercial monitoring systems. For example humping was clearly detected by the signal but spatter and crater formation (for overlapping Zn-coated steel) was not. An interesting cause for we...