In-situ synchrotron imaging of powder consolidation and melt pool dynamics in electron beam powder bed fusion

Electron beam powder bed fusion (PBF-EB) is an additive manufacturing (AM) technology that enables the fabrication of metallic parts with arbitrary geometric complexity within a vacuum environment. Due to its ability to process materials at high temperatures (> 1000 °C), processing of crack and oxidation sensitive materials, as well as refractory alloys is possible. However, due to limited fundamental understanding of the intricate dynamics during powder consolidation and melt pool formation, the development of advanced processing strategies has mainly been limited to experimentally time-consuming parameter studies, as numerical models have mostly been unable to accurately predict processing conditions at the part or even layer scale. In this study, we perform high-speed in-situ X-ray imaging during multi-layer single track powder melting experiments on MiniMelt, a recently developed, custom-built PBF-EB machine for in-situ X-ray investigations. Our experiments reveal several key melt pool formation dynamics, some of which are being identified for the first time. They show how melt pool formation involves the coalescence of molten powder particles into larger droplets and how these droplets either fuse with the melt pool or solidify as balling particles. They also elucidate the origins of melt pool oscillations and spatter formation and demonstrate how the superposition of these mechanisms can lead to chaotic and escalating movement within the melt. We expect our results to improve and extend the phenomenological understanding of the powder consolidation mechanisms during PBF-EB and to aid in the development of new scanning strategies as well as the validation of numerical models.