Journal
ACTA MATERIALIA
Volume 166, Issue -, Pages 294-305Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.actamat.2018.12.027
Keywords
Spatter; Porosity; Synchrotron radiography; X-ray imaging; Additive manufacturing; Powder bed fusion; Selective laser melting
Funding
- EPSRC MAPP Future Manufacturing Hub [EP/P006566/1, EP/I02249X/1, EP/M009688/1]
- AMAZE (Additive Manufacturing Aiming towards Zero Waste and Efficient Production of High-Tech Metal Products) project - 7th Framework Programme of the European Commission [FP7-2012-NMP-ICT-FoF-313781]
- Research Complex at Harwell
- ERC Advanced Grant (CORREL-CT Project) [695638]
- UCL [PR16195]
- EPSRC [EP/I02249X/1, EP/P006566/1, EP/M009688/1] Funding Source: UKRI
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Understanding defect formation during laser additive manufacturing (LAM) of virgin, stored, and reused powders is crucial for the production of high quality additively manufactured parts. We investigate the effects of powder oxidation on the molten pool dynamics and defect formation during LAM. We compare virgin and oxidised Invar 36 powder under overhang and layer-by-layer build conditions using in situ and operando X-ray Imaging. The oxygen content of the oxidised powder was found to be ca. 6 times greater (0.343 wt.%) than the virgin powder (0.057 wt.%). During LAM, the powder oxide is entrained into the molten pool, altering the Marangoni convection from an inward centrifugal to an outward centripetal flow. We hypothesise that the oxide promotes pore nucleation, stabilisation, and growth. We observe that spatter occurs more frequently under overhang conditions compared to layer-by-layer conditions. Droplet spatter can be formed by indirect laser-driven gas expansion and by the laser-induced metal vapour at the melt surface. Under layer-by-layer build conditions, laser re-melting reduces the pore size distribution and number density either by promoting gas release from keyholing or by inducing liquid flow, partially or completely filling pre-existing pores. We also observe that pores residing at the track surface can burst during laser re-melting, resulting in either formation of droplet spatter and an open pore or healing of the pore via Marangoni flow. This study confirms that excessive oxygen in the powder feedstock may cause defect formation in LAM. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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