Chapter 8 - Effects of powder characteristics and flow behavior on fluid thermodynamics and processability of metallic powder by laser powder bed fusion

Abstract

Combining the discrete element method (DEM) and finite volume method (FVM), this chapter proposed a newly developed three-dimensional DEM physical model coupling with the mesoscopic FVM model to give a unique insight into the powder paving behavior and resultant melting/solidification characteristics of laser powder bed fusion (LPBF) process. The contact force among powder particles, solid/liquid transition, and surface tension of melt were considered in the model. By characterizing the packing profiles, velocity vectors, and packed state of particles, the simulation results and corresponding experimental validation revealed that the powder exhibited an elevated flowability with increasing the average diameter of powder below 25μm due to the alleviation of the domination of adhesive force among neighboring particles. The discontinuous and loose powder bed with a low packing density of particles had a high tendency to produce the top surface with higher surface roughness, resulting in the formation of a number of defects in LPBF-processed parts such as cave-like porosity, balling, and discontinuous laser-melted tracks. The results also showed that a low laser power could not melt the powder completely, and a high laser power caused excessive liquid formation that was developed into spheroidization. Pore removal and densification behavior of multilayer powder were illustrated. When an appropriate scan speed was applied, the pores remaining in the preceding layer were eliminated entirely due to mass transfer in the molten pool. Reasonable laser linear energy density and irradiation time tended to enhance the laser penetration depth in the powder bed and accordingly decrease the porosity in as-fabricated layers.