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The effect of powder oxidation on defect formation in laser additive manufacturing

DOI: 10.1016/j.actamat.2018.12.027 DOI Help

Authors: Chu Lun Alex Leung (University College London; Research Complex at Harwell) , Sebastian Marussi (Research Complex at Harwell; The University of Manchester) , Michael Towrie (Research Complex at Harwell) , Robert C. Atwood (Diamond Light Source) , Philip J. Withers (The University of Manchester) , Peter D. Lee (University College London; Research Complex at Harwell)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acta Materialia

State: Published (Approved)
Published: December 2018
Diamond Proposal Number(s): 13641 , 16214

Abstract: 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. In 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.

Journal Keywords: Spatter; Porosity; Synchrotron radiography; X-ray imaging; Additive manufacturing; Powder bed fusion; Selective laser melting

Subject Areas: Materials

Instruments: I12-JEEP: Joint Engineering, Environmental and Processing

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