Monitoring deep penetration laser welding processes is relevant in order to detect deviations and to achieve defect-free welds. Several available monitoring systems rely on the optical emissions from the welding process or depth measurements using optical coherence tomography. Training these systems with existing data from welds with and without defects works for many applications, but understanding the causal relationships within the welding process remains essential. In deep penetration welding, the shape and stability of the capillary have a significant effect on both defect formation and the monitoring signals. Therefore, this study aims to investigate the influence of the capillary shape on monitoring signals.

X-ray imaging was employed to capture the capillary shape during welding. Simultaneous measurements of the back-reflected laser beam and the thermal emissions from the process as well as depth measurements using optical coherence tomography were conducted. The simultaneous acquisition enabled the monitoring signals to be directly compared with the capillary shapes. Based on the determined capillary shape, ray tracing simulations were performed to investigate the impact of changes in the capillary shape on the monitoring signals. The comparison between simulated and measured monitoring signals contribute to the advancement of monitoring methods in laser welding based on optical information from the process.