Additive Manufacturing of an Ultrahigh Temperature Mo-Si-B Alloy with Reactive Powder Synthesis

Most of the work on additive manufacturing (AM) has focused on readily available alloys in powder form, but these compositions were developed for ingot casting and are not necessarily optimized for AM. To address this limitation, a novel reactive synthesis-based additive manufacturing technique has been developed to fabricate compositionally uniform and dense alloys. The present study demonstrates the reactive additive manufacturing of Mo-4Si-6B (at.%), a high-temperature refractory alloy. A premixed blend of molybdenum, silicon nitride, and boron nitride powder was used to make an alloy with the desired composition. During heating the component powders react according to 5Mo +1/3 Si₃N₄ + 2BN -> Mo₅SiB₂ + 5/3N₂ with N₂ escaping as a gas. With excess Mo a two-phase Mo + Mo₅SiB₂ microstructure develops. By adjusting the relative amount of each component powder any alloy composition can be fabricated. This capability allows for a high throughput alloy synthesis and screening. With laser powder bed fusion processing the easy flow of the reactive powders is important. To achieve the desired powder flow behavior the rough Si₃N₄ and BN powders were spheroidized by thermal spraying in a plasma torch. A dimensionless number was used to design the process parameters and build efficiency. High-throughput synthesis using build height measurements of individual samples validated the predicted process parameters. Microstructural characterization investigations and indentation hardness testing indicated chemically uniform samples with refined microsegregation in samples with high hardness and no cracking, even with a 10-kg force load. The results demonstrate an effective strategy for additively manufacturing refractory alloys.