Coaxial laser-induced breakdown spectroscopy aligned with the welding laser for in-process assessment of Cu–steel laser welding

In this study, laser-induced breakdown spectroscopy (LIBS) was employed using a coaxial optical configuration aligned with a high-power continuous-wave (CW) welding laser to enable real-time acquisition of plasma emission signals during Cu–steel dissimilar metal laser welding. Welding was performed while systematically varying laser power and scanning speed, and the Cu/Fe emission intensity ratio extracted from the coaxially collected LIBS signals was proposed as a criterion for welding quality assessment. The proposed Cu/Fe-based criterion was validated through mechanical joint strength evaluation, demonstrating that LIBS enables in-process, real-time assessment of welding quality. These results indicate that the proposed coaxial, single-laser LIBS approach can be readily integrated into existing laser welding systems without additional excitation sources or complex instrumentation, while enabling accurate and repeatable probing of the weld zone through coaxial signal collection.