Potentials of Optical Coherence Tomography for Process Monitoring in Laser Welding of Hairpin Windings

Optical coherence tomography (OCT) is increasingly being used to monitor and control laser-based welding processes. With the ability to provide pre-, in-, and post-process data, OCT represents a holistic sensor solution for advanced process monitoring and seam tracking. A prospective field of application is the laser welding of rectangular copper wires for so-called hairpin stators as used in electric traction motors. For each stator, a large number of high-quality contact points must be welded in a short cycle time. The quality of a weld not only depends on the laser welding process itself but also on possible manufacturing deviations from upstream process steps. Although OCT seems promising for pre-, in-, and post-process monitoring, no scientific studies exist to date concerning hairpin welding. Starting from a profound presentation of the necessary basics, this paper outlines the potentials of OCT in laser welding of hairpin windings based on first experiments. Regarding pre-process monitoring, it is shown which deviations can be detected by OCT even before the process. Analogously, the post-process monitoring feature can be used to evaluate the resulting weld shape. Further experiments explore whether in-process OCT data can be used to determine the keyhole depth during welding, providing an estimate of the welding depth. The welding of hairpins requires complex beamlines and high focus speeds, which pose high challenges to existing OCT systems. Therefore, the proof is first provided for a simplified welding task before it is performed for the hairpin use case. From the findings gained, the need for action for future research activities is derived, primarily focusing on enabling the OCT -based in-process monitoring for the hairpin welding.