Abstract
Ejecta with a size much larger than the mean particle size of feedstock powder have been observed in powder bed fusion additive manufacturing, both during post-process sieving and embedded within built components. However, their origin has not been adequately explained. Here, we test a hypothesis on the origin of large (much larger than the mass-median-diameter of feedstock powder) ejecta—that, in part, they result from stochastic, inelastic collisions of ejecta and coalescence of partially-sintered agglomerates. The hypothesis is tested using direct observation of ejecta behavior, via high-speed imaging, to identify interactions between ejecta and consequences on melt pool formation. We show that stochastic collisions occur both between particles which are nearly-simultaneously expelled from the laser interaction zone and between particles ejected from distant locations. Ejecta are also shown to perturb melt pool geometry, which is argued to be a potential cause of lack-of-fusion flaws.
Highlights
Laser powder bed fusion additive manufacturing (PBFAM), a subcategory of metal 3D printing technology, is rapidly emerging as an important industrial manufacturing technology for aerospace, medical, and defense applications[1,2]
Bidare, Bitharas, et al.[7,8] used high-speed imaging and modeling to demonstrate the complex nature of plume-powder-melt interactions and argue that sintering and melting of denuded particles contributes to the formation of large ejecta
It has been speculated that ejecta atop the powder bed may contribute to flaws in one of three ways: (1) spatter particles dragged during powder recoating may perturb the powder bed causing height variations[11]; (2) large spatter particles may remain un-melted and become incorporated into the component[12,13]; or, (3) large ejecta may shadow the beam and contribute to lack-of-fusion defects[10]
Summary
Laser powder bed fusion additive manufacturing (PBFAM), a subcategory of metal 3D printing technology, is rapidly emerging as an important industrial manufacturing technology for aerospace, medical, and defense applications[1,2]. It has been speculated that ejecta atop the powder bed may contribute to flaws in one of three ways: (1) spatter particles dragged during powder recoating may perturb the powder bed causing height variations[11]; (2) large spatter particles may remain un-melted and become incorporated into the component[12,13]; or, (3) large ejecta may shadow the beam and contribute to lack-of-fusion defects[10] Among these potential mechanisms, (2), and (3) are more likely since perturbations in powder height are not an uncommon occurrence in PBFAM, typically resulting from interactions of the recoater with elevated or thermally-deformed parts on the build plate[14,15]. It is critical for the formation of defect-free PBFAM components that the mechanism for the formation of large ejecta be understood and, if possible, mitigated
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