Microstructure evolution of FD-POEM powders during high-temperature plasma spheroidization

Abstract

The freeze–dry pulsated orifice ejection method (FD-POEM) can be coupled with plasma spheroidization (PS) as an advanced approach for fabricating spherical refractory powders used in laser additive manufacturing. However, the microstructure evolution of FD-POEM powders, which include components of various melting points, during ultrahigh-temperature PS treatment remains unclear. In this work, the morphology, chemical composition, and microstructure of MoSiBTiC alloy powders, as representative PS powders, were systematically investigated. The mesh-structured FD-POEM powders underwent complete densification, resulting in a 52 % reduction in particle size. The PS powders were composed of coarse solid solution Mo (Moss) and fine eutectic Mo5SiB2/TiC phases, consistent with the composition of additively manufactured MoSiBTiC alloys. With an increase in Si content, the volume fraction of fine Mo5SiB2 phases increased, resulting in increased Vickers hardness. Notably, the oxygen content of FD-POEM powders decreased by > 99 %, owing to the effective elimination of surface oxides on the initial elemental powders during ultrahigh-temperature PS treatment. This research provides an innovative approach for reducing the oxygen content in refractory powders to be used for additive manufacturing. Enhanced understanding of the microstructure evolution of PS powders can facilitate the composition optimization of refractory powders and development of a microstructure–property database for additively manufactured materials.