The interaction of volatile metal coatings during the laser powder bed fusion of copper

Speidel, Alistair ORCID: https://orcid.org/0000-0002-8209-9176, Wadge, Matthew D ORCID: https://orcid.org/0000-0002-5157-507X, Gargalis, Leonidas ORCID: https://orcid.org/0000-0002-2739-1115, Cooper, Timothy P ORCID: https://orcid.org/0000-0002-0254-6109, Reynolds, William ORCID: https://orcid.org/0000-0002-6472-5110, Grant, David, Hague, Richard, Clare, Adam T and Murray, James W (2022) The interaction of volatile metal coatings during the laser powder bed fusion of copper. Journal of Materials Processing Technology, 299. 117332. ISSN 0924-0136

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Abstract

The high optical reflectance of Cu at near-infrared wavelengths narrows the process window to fabricate Cu parts by laser powder bed fusion (LPBF). Coating powders with optically absorptive materials has been investigated to improve processability and enhance part properties. However, given the intense heat localization and thin coating layers relative to the powder, the mechanisms of thin film coating interaction in LPBF remain unclear, despite recent work showing the importance of the near-track environment in deposition behavior. In this study, optically absorptive Zn-coated Cu powders were prepared by physical vapor deposition and characterized. Single LPBF tracks were fabricated to elucidate material incorporation phenomena influenced by the volatile Zn coating. It is shown that Zn-coated powder enhances accretion at fastest effective scan speed tested (100 mm/s), where mean track volumes are increased from 0.72 ± 0.05 mm3 (as-received) to 0.91 ± 0.01 mm3 (Zn-coated). This has been correlated to the stronger vapor jet from the volatile Zn-coating, which denudes the surrounding powder bed. This exhausts the powder bed at slower effective scan speeds, causing instability and balling when compared to the as-received powder. It is shown that Zn is localized at the track surface and is undetectable in the track bulk, indicating Zn vaporization on interaction with the incident beam. Zn present mainly occurs through secondary deposition mechanisms like spatter and condensation, rather than in-process alloying. Coating powder feedstocks for use in LPBF therefore affects composition, laser beam absorptivity, and the near-track vapor environment that is known to influence material incorporation behavior.