Powder densification behavior and microstructure formation mechanism of W-Ni alloy processed by selective laser melting

Abstract

W-Ni alloy has been an important engineering material in military, nuclear industry and many other fields because of its excellent properties such as high strength and superior heat/corrosion/oxidation resistances. As a mainstream additive manufacturing technology based on powder-bed-fusion principle, selective laser melting (SLM) is suitable for the rapid fabrication of very complex W-Ni alloy components. However, researches on the powder densification behavior and microstructure formation mechanism of SLM W-Ni alloy are not thorough enough. In this work, a series of W-xNi (x = 16, 23, 30 wt.%) binary alloy samples were fabricated by SLM, using W/Ni mixed powders as the raw materials. The densification behaviors of the W/Ni mixed powders and the microstructure formation mechanisms of the different SLM W-Ni samples were studied. The results showed that when the W/Ni mixed powder bed was scanned by the laser, Ni powders with a relatively lower melting point would melt completely whereas W powders with a relatively higher melting point would rearrange themselves in the liquid Ni and only part of them could be dissolved into the liquid Ni. As the initial content of Ni powders increased, the rearrangement and dissolution of W powders proceed more sufficiently, thus leading to the improvement in densification degree. The relative densities of the W-16Ni, W-23Ni and W-30Ni SLM samples are 92.87 ± 0.67%, 98.66 ± 0.26% and 99.67 ± 0.18%, respectively. During the solidification processes of W-Ni molten pools, the precipitation of W grains from liquid Ni, the transformation of liquid Ni to γ-Ni solid solution and the precipitation of Ni2W from γ-Ni solid solution would occur in sequence. For the W-16Ni and W-23Ni samples, the grain morphology of the W precipitations is dendritic. For the W-30Ni samples, the W precipitations are globular. As a result of the unique melting- solidification behaviors, all the SLM samples possessed a tri-modal microstructure composed of unmelted W particles, dendritic/globular W grains, and γ-Ni solid solution with Ni2W intragranular precipitation. The average microhardness of the SLM W-Ni alloy samples decreased with the increase of Ni content, which could be explained by the variation in defect and microstructure features.