Selective laser melting of MoSiBTiC alloy with plasma-spheroidized powder: Microstructure and mechanical property

Highlights: • Plasma spheroidization enables the spherical MoSiBTiC alloy powder fabrication. • Microstructures of selective laser melted MoSiBTiC alloy are investigated. • Primary Mo solid solution dendrites are highly supersaturated by Si and Ti. • Homogeneous microstructures can be obtained by heat treatment. • Effects of microstructures on hardness and crack propagation are elucidated. Spherical MoSiBTiC alloy powder with little internal gas pores and cracks was successfully fabricated using plasma-spheroidization method. This powder fabrication process significantly decreased the oxygen content of the raw crushed powder. Then, MoSiBTiC alloy samples were fabricated via selective laser melting (SLM) with the plasma-spheroidized powder, and the microstructural characteristics and mechanical property were investigated. The as-built MoSiBTiC alloy exhibited a bimodal microstructure; the microstructure inside the melt pool (MP) was different from that at a melt pool boundary (MPB). The microstructure inside the MP consisted of primary Mo solid solution (Mo_ss) dendrites, and other constituent phases, such as Mo₅SiB₂ (T₂) phase, were formed at the inter-dendritic regions. On the other hand, (Ti,Mo)C precipitates around the Mo_ss dendrites were preferentially coarsened at the MPB, forming a heat affected zone (HAZ) at the underside within the MPB. At the upside within the MPB, the microstructure of the previous layer was partially remelted, and ternary eutectic lamellar structure (Mo_ss/T₂/(Mo,Ti)₂C) was newly formed. The as-built samples were subsequently subjected to heat treatment at different temperatures of 1600, 1700, and 1800 °C for 4 h. With increasing in the heat-treatment temperature, the Mo_ss dendritic microstructure decomposed, and other constituent phases were coarsened at the inter-dendritic regions. Through a detail microstructural characterization of both the as-built and heat-treated samples, it was confirmed that any large oxide was not formed in this study. Rapid solidification via SLM resulted in the refinement of microstructure and the formation of a supersaturated Mo_ss phase, which hardened the as-built sample but simultaneously decreased the crack resistance of the Mo_ss phase.