Enhancement of OCT signal interpretability in deep penetration laser welding of aluminum

• Hierarchical cluster analysis of reconstructed 3D keyhole shapes • Archetypical keyholes show equivalencies across different experiments • Feature extraction from keyhole geometries and OCT measurements • Feature mapping between keyhole shapes and corresponding OCT measurements • OCT signals contain information about the keyhole shape beyond depth In laser deep penetration welding a constant weld depth is an essential quality measure that is usually obtained by in-line process monitoring techniques such as optical coherence tomography (OCT). However, due to the lack of referential information about the keyhole shape, the extraction of meaningful information about the keyhole from the OCT signal is limited to the keyhole depth which is acquired by means of statistical analyses. In this research on-line X-ray-recordings alongside OCT measurements were conducted during the welding of pure aluminum (EN AW-1050A) in the Rosenthal regime to enable a more profound interpretation of the OCT signal. From the X-ray recordings of the process zone, 3D keyhole shapes were reconstructed and meaningfully reduced to a set of archetypical keyhole geometries by means of a hierarchical agglomerative cluster analysis. It was found that these archetypical keyhole shapes are not unique for individual experiments but represent prevalent keyhole states that largely remain valid even across experiments conducted with different process parameters. Investigating the interconnections between geometric archetypical keyhole features and features of their corresponding OCT data point distributions revealed significant correlations beyond the mere keyhole depth raising the potential of extending the applications of OCT in laser beam welding processes.