Influence of Particle Size on Laser Absorption and Scanning Track Formation Mechanisms of Pure Tungsten Powder During Selective Laser Melting

A three-dimensional laser absorption model based on ray tracing was established to describe the coupled interaction of a laser beam with particles in the powder layers of pure tungsten (W) material processed by selective laser melting (SLM). The influence of particle size and its distribution on the powder-to-laser absorptivity and underlying absorption behavior was investigated. An intrinsic relationship between the absorption, distribution of absorbed irradiance within the powder layers, and surface morphology and geometric characteristics (e.g., contact angle, width and height of tracks, and remelted depth) of the laser scanning tracks is presented here. Simulation conclusions indicate that the absorptivity of the powder layers considerably exceeds the single powder particle value or the dense solid material value. With an increase in particle size, the powder layer absorbs less laser energy. The maximum absorptivity of the W powder layers reached 0.6030 at the particle size of 5 μm. The distribution of laser irradiance on the particle surface was sensitive to particle size, azimuthal angle, and the position of the powder particles on the substrate. The maximum irradiance in the powder layers decreased from 1.117 × 10 –3 to 0.85 × 10 –3 W·μm –2 and the contour of the irradiance distribution in the center of the irradiated area gradually contracted when the particle size increased from 5 to 45 μm. An experimental study on the surface morphologies and cross-sectional geometric characteristics of SLM-fabricated W material was performed, and the experimental results validated the mechanisms of the powder-to-laser-absorption behavior that were obtained in simulations. This work provides a scientific basis for the application of the ray-tracing model to predict the wetting and spreading ability of melted tracks during SLM additive manufacturing in order to yield a sound laser processability.