Understanding the coaxial optical coherence tomography signal during the laser welding of hidden T-joints

In the case of "hidden" T-joints, the nonvisible web sheet is welded through the face sheet. Positioning tolerances and movement due to the distortion of the web sheet represent the main challenges to maintain on track during laser beam welding. This study investigated the assumption that keyhole monitoring based on optical coherence tomography (OCT) could be utilized for seam tracking by detecting critical misalignments between the laser beam and the web sheet position. It hereby analyzed whether an emerging misalignment between the laser beam and the web sheet could be discerned in the OCT data before the laser beam misses the web sheet and penetrated only the face sheet. This prediction represents a key requirement for the industrial use of this approach. To this end, this study investigated the welding of hidden T-joints with angular and parallel offsets of the laser path relative to the web sheet centerline while recording with OCT. The interface zone between the web and face sheets was recorded by a high-speed camera to detect the transmission of laser radiation through a lower keyhole opening. It was shown that a lower keyhole opening, in combination with transmitted radiation, was advantageous for the reliable detection of an emerging misalignment. This lower keyhole opening can occur lateral to the web sheet prior to a misalignment, therefore enabling a prediction of spot misalignments via OCT. The welding parameter dependent maximum distance of the laser spot edge to the web sheet edge at detection was 0.1 mm in this study.