Investigation of pore formation mechanisms induced by spherical-powder delivery in directed energy deposition using in situ high-speed X-ray imaging

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• Pore formation mechanisms unique to the spherical-powder delivery in laser DED AM were revealed. • The pore formation dynamics were directly observed by high-speed synchrotron X-ray imaging. • A powder delivered to a front laser-beam region or melt pool interacted with melt flow and caused pores. • The delivered particle would change the pore size on its trajectory. • A space induced by the delivered spherical particle led to pore formation. In the laser-based directed energy deposition (DED) process, blown powder additive manufacturing (AM), either spherical or irregular metal particles can be used as the feedstock material to be delivered into a melt pool for fabrication. The delivered particles interact with the melt flow in different regions of melt pool, and the interactions between the liquid melt pool and the spherical particles are different from those between the melt pool and irregular particles. The objective of this investigation is to reveal the mechanisms and dynamics of pore formation unique to spherical-powder delivery in the laser DED AM process. In situ high-speed and high-resolution X-ray imaging showed that delivered spherical particles could induce pore formation mechanisms through the interactions between the melt pool and the particle front surface, the particle side surface, or the particle back surface. These results indicate that spherical particles induce different pore formation mechanisms from irregular particles. This fundamental understanding will benefit further investigations into reducing porosity and improving DED-fabricated part quality.