Characterization of particles inside the metal vapor plume during laser beam welding in atmosphere and vacuum

During laser beam welding of metals, process emissions are generated above the workpiece surface, through which the incident laser beam passes. One part of the process emissions are particles which arise from the hot metal vapor phase and agglomerate to form larger chains until they have cooled down completely. Depending on the chemical composition and size of this material emissions, the light waves are reflected, absorbed or scattered to varying degrees. The Mie theory comprehensively describes the scattering and absorption behavior of particles and essentially covers the welding plume produced during laser beam welding with a few assumptions. To specify these assumtions in more detail, this study analyzes the nanoparticles deposited on a glass carrier in the beam path of the laser. The deposition shows prismatic and octahedral structures in the scanning electron microscope, besides the predominantly spherical particles. In a vacuum and therefore with a reduced oxygen content, the ammount of emissions of the keyhole decreases and particles collected in the plume are identified as solid solution bcc-type particles containing Cr, Cu, Fe, Mn, and Ni. In atmosphere at 1000 mbar however, complex solid solutions phases with diffraction patterns compatible to spinel-type NiCr2O4 and NiFe2O4 are formed.