Depth monitoring protocol for OCT in laser welding with single-mode core and ring beam configurations

Demand for precise monitoring of high-speed laser welding depth is increasing due to growing electric mobility applications. Optical coherence tomography (OCT) can monitor keyhole depth in real time. Newer fiber laser generations, ideal for such applications, offer single-mode core/ring configurations. However, use of OCT with small keyhole apertures may cause measurement inconsistencies. This work proposes a systematic analysis of signal behavior using a contemporary fiber laser with a novel single-mode core and a ring with separate power control, producing focal core and ring sizes of 40 µm and 285 µm, respectively. Bead-on-plate experiments were conducted on 5 mm thick EN AW-1050 Al-alloy. Core and ring power levels were systematically analyzed along with scan speeds. The OCT beam of 34 µm was aligned with the laser beam using different laser process parameters. Keyhole depth was compared to seam depth from metallographic cross-sections. The study involved systematic development of an alignment protocol to highlight the sensitivity of OCT measurements. The results revealed that a fine alignment of the OCT beam with respect to the laser beam is crucial for reliable acquisitions. The procedure was able to monitor keyhole depths for weld speeds of up to 800 mm/s using only the single-mode beam.