Evolution of Powder Surface Chemistry and Powder Properties during Powder-based Metal Additive Manufacturing

Licentiate thesis, 2025

Powder-based additive manufacturing (AM) is a transformative technology enabling the fabrication of parts to near-net-shape through layer-by-layer deposition and selective consolidation of thin powder layers according to a digital 3D model. Due to their high surface area to volume ratio, powders are susceptible to surface reactions, particularly oxidation upon exposure to atmospheric oxygen and moisture. Complexity of these AM processes, such as printing atmosphere, interactions of the energy source with the powder particles, leading to “spatter” generation, as well as necessary post-printing steps can induce surface oxidation and alter the powder’s physical properties. Even brief exposure during handling, storage, or within the build chamber can lead to the growth of oxide layer thickness on the powder particle surfaces. Different types of degradation phenomena in the powder feedstock can lead to defects in the final components, compromising their properties as well as the process repeatability. Therefore, this thesis aims to study the impact of powder reuse in AM on powder surface chemistry and powder properties.
When it comes to powder degradation in PBF-LB, reuse and recycling of 316L stainless steel powder were studied. The thickness of the oxide layer on the powder surface almost doubled after 140 hours of total in process exposure. Further on, recycling of the powder through atomization of 316L reused powder and AM scrap was successfully performed and allowed to achieve powder with chemistry and properties comparable to virgin 316L powder used in the study. In the second part, copper powder of high purity was employed for printing components for electromagnetic applications using PBF-EB. The powder proved to undergo varying surface chemistry changes depending on the process parameters used, with a transformation of the dominant CuO oxide on the surface of the virgin particles to Cu₂O after PBF-EB processing. Further on, changes in stainless steel 17-4 PH powder properties during reuse in BJT were studied. Conditioning of the powder was applied before the first print, which aided the particles’ spreadability during the printing process. Post-printing steps to ensure the densification of the components, such as curing, led to a rise in oxide layer thickness of the powder, promoting instability of the printing process after 20 prints and multiple curing cycles.