Laser-plume interactions in deep-penetration remote laser welding of stainless steel

In deep-penetration laser welding a gaseous plume of metal vapor is ejected from the keyhole. Cooling of this vapor then leads to condensation and the formation of a particle cloud above and around the welding zone. The vapor plume and particle cloud interact with the incident laser beam by scattering, absorption, and deformation of the phase front. This complex and dynamic interaction can have detrimental effects on the welding result and is not yet fully understood. This is particularly relevant for welding applications where the vapor plume is not removed with shielding gas or a cross-jet. The present paper reports on investigations into the beam-plume interactions by analyzing the thermal emission and scattered laser light from the interaction zone while welding stainless steel. A high-speed camera with optical filters and a spectrometer were used for this analysis. In addition, Schlieren and shadow images were recorded to visualize the beam-plume interaction. Different zones of beam-plume interaction were distinguished. Near the laser beam focus, a negligible amount of condensed or solid material scattering the laser beam was found within the laser beam caustic. Instead, the thermal emission of the hot metal vapor was found to be the main source of emission in this area. At greater distances from the focus, the scattered laser light was the dominant emission source, while the thermal emission became negligible. These two zones were found to be connected by a multi-phase zone, which could be observed by its thermal emission and the scattering of the laser light, which contained both hot metal vapor and particles. The investigations presented here provide valuable insights into the interactions between the vapor plume and the particle cloud. In the future, this knowledge can be used to avoid or compensate for related adverse effects that occur during laser deep penetration welding, and as a basis for simulations of the beam-plume interaction.