Material-specific beam-plume interactions during deep-penetration laser welding of stainless steel, aluminum, and copper

During deep-penetration laser welding, a hot vapor plume is emitted from the keyhole which, on cooling, condenses into a particle cloud that surrounds the weld zone. This vapor plume and associated particle cloud interact with the incident laser beam through scattering, absorption, and phase front distortion, dynamically altering the beam caustic and potentially affecting weld quality. In this study, the mechanisms governing the beam-plume interaction are investigated by observation of the thermal emission and scattered laser light from the interaction zone during the welding of stainless steel, aluminum, and copper. For this analysis, a spectrometer and a high-speed camera equipped with optical filters were used. The results revealed significant material-specific differences in thermal emission and scattered laser light from the plume, indicating variations in absorption and scattering behavior and thus beam attenuation. Re-heating of plume material until evaporation took place for all three materials. Stainless steel exhibited the strongest thermal emission, while aluminum and copper showed significantly weaker emission. In contrast, the aluminum plume displayed the highest level of laser light scattering. This is attributed to the presence of liquid and solid particles rather than purely vaporized material, even close to the laser beam focus. Distinct interaction zones within the laser beam caustic were identified, each corresponding to specific aggregate states and characteristic laser-plume interactions. For stainless steel and copper, a zone forms close to the keyhole which is primarily composed of vaporized material. Beyond this there is a multi-phase zone containing both vapor and liquid or solid matter. Further from the keyhole, a particle zone with no detectable vapor appears as re-heating becomes insufficient for evaporation. In aluminum, no distinct vapor zone was detected. Instead, strong scattering near the keyhole indicates the presence of particles even at high laser intensities. Thus, only a multi-phase and a particle zone appear to form for aluminum under the welding parameters used.