Novel approach for weld depth determination using optical coherence tomography measurement in laser deep penetration welding of aluminum and steel

The optical coherence tomography (OCT) is a measuring technology which is well-established in medical engineering since the early 1990s. More recently, the technology found its way into laser materials processing where it is used for seam tracking and inspection and also for monitoring and control of deep-penetration laser welding. In this work, deep-penetration laser welding of aluminum and steel using an OCT-system for in-process monitoring of the weld depth was investigated. It is shown that statistical data processing is mandatory to extract the actual keyhole depth. Therefore, two different measures, percentile filtering and considering the frequency distribution of the OCT-data, were considered. Thereby, it is demonstrated that the frequency distribution of the OCT-data has a specific pattern with a local maximum which correlates with the keyhole depth. Moreover, this feature is more significant for welding aluminum and therefore enables one to detect the weld depth more independently and accurately compared to currently applied measures.