Micro-mechanical analysis on the spreading of tungsten powder in electron beam powder bed fusion additive manufacturing

Abstract

Electron beam powder bed fusion (EB-PBF) enables the additive manufacturing of high-melting-point, reactive metals like tungsten. However, the quality of the powder bed is governed by the micro-mechanics of powder spreading, which remain unclear. In this work, the spreading of tungsten powder during EB-PBF process was numerically reproduced by three-dimensional discrete element method. Micro-mechanics (particle motion behaviors, evolution of contact forces and formation of force arches) of the powder spreading were analyzed under varying operating parameters (spreading velocity (V), spreading height (H_set)) and particle size distribution (PSD). Additionally, powder bed density ρ and surface roughness Ra were also evaluated. Results indicate that low H_set facilitates the formation of short-length, stable force arches in front of the recoater, hindering powder fall onto the substrate. At low V, the force arches undergo partial collapse and are subsequently restored by surrounding particles, enabling high-frequency, small-quantity powder deposition, which results in higher ρ and lower Ra. Conversely, at high V, force arches collapse completely and require longer rebuilding periods, leading to periodic powder deposition and large voids in the powder bed. Increasing PSD standard deviation facilitates the stable force arches by large particles, permitting small particle percolation, which reduces ρ and Ra.