Particle Size Effect on Powder Packing Properties and Molten Pool Dimensions in Laser Powder Bed Fusion Simulation

Various defects are produced during laser powder bed fusion (L-PBF) process, and can affect the quality of the fabricated part. Past studies have revealed that the defects formed are correlated with molten pool dimensions. Powder particles are thinly spread on a substrate during L-PBF process; hence, powder packing properties should influence the molten pool dimensions. This study has evaluated influence of particle size on powder packing properties and molten pool dimension obtained from numerical simulation. Using particles with different average diameter (Dav) of 24, 28, 32, 36, and 40 $\mu$m, a series of discrete-element method (DEM) simulations were performed. Packing fraction obtained from DEM simulations became high as Dav became small. Several particles could piled up for small Dav, whereas particles spread with almost one-particle diameter thickness for large Dav. Moreover, packing structure was inhomogeneous and sparse for large Dav. Then we performed multiphysics computational fluid dynamics (CFD) simulations incorporating particles' position as initial solid metal volume. Molten pool width obtained from the simulations was hardly dependent on the Dav and was roughly equivalent to the laser diameter used in the simulations. In contrast, molten pool depth became small as Dav became small. The results suggest that importance of specific surface area, that is, the powder bed with smaller Dav can absorb larger thermal energy because of the larger specific surface area due to the large powder bed thickness and piling up multiple particles.