Transition to circular economy requires the production of sustainable and eco-designed materials that help to reduce environmental impacts of metallic components. The development of sensing layers providing luminescent tracking functionalities is a potential method for extending the service life of metallic parts. In this study, incorporation of luminescent Ce3+ doped yttrium aluminum garnet (YAG:Ce) within a stainless steel 316L (SS316L) matrix has been achieved for the first time by laser powder bed fusion (L-PBF). Embedding of phosphor particles was successfully carried out on a selected area of the 3D printed sample. Despite harsh processing conditions of L-PBF, luminescent emission was detected by optical spectroscopy. Microstructure and chemical composition of the incorporation zone were investigated in order to better understand optical properties. The precipitated particles exhibit new optical features, arising from the modification of the luminescent host lattice and the intricate interactions with the metal matrix.

Powder reuse is essential in laser powder bed fusion (L-PBF) to limit material waste and improve the process sustainability. However, complex laser-material interactions result in an inevitable alteration of the attributes of the recovered powder at the end of the fabrications. Powder degradation is influenced by numerous factors including the processing parameters but also the build topology. In this work, various build characteristics were studied to develop a better understanding of the effects on powder degradation, with the aim of minimizing it. LPBF prints with different melted volume fractions, part spacings and fabrication heights were conducted using stainless steel 316L (SS316L) material. Powders recovered on the build platform, as well as in zones of interest directly adjacent to solidified areas were characterized to investigate the changes in particles characteristics, with special regards to powder oxidation. In combination with these geometrical features, the effect of the oxygen concentration in the build chamber was also studied. The results allow to identify the relation between the oxygen content of recovered powder and the volume fraction of printed parts. It also highlights the presence of heat-affected zones in the near vicinity of the parts with a strong oxygen pick-up in a 500 mu m wide zone around the consolidated material. The results also show the significance of the oxygen concentration in the process

Costs and resource efficiency of laser powder bed fusion (L-PBF) are highly dependent on the ability to produce high quality parts with recycled powders. There is a need to control the quality of the material, which has a direct influence on the performance of the printed parts. Particles oxidation is known to increase with repeated powder recycling and can be a good indicator of powder degradation. The characterization of powders oxygen content is time-consuming, expensive, and usually carried out ex-situ on non-reusable quantities that are not necessarily representative of the entire feedstock. In this work, a new methodology was developed to measure the oxygen content of powders by in-line scanning of powder bed layers. The method takes advantage of stainless steel particles coloration related to their oxidation level in order to assess their oxygen concentration as a function of Red, Green and Blue channel values of image scans. The calibration procedure once carried out, several recycled powder samples were scanned and analyzed, and the determined powder beds oxygen contents were demonstrated to be in accordance with ex-situ measurements. The results highlight a new opportunity to monitor and evaluate powder degradation in-situ on powder bed layers by image analysis.

In Laser Powder Bed Fusion (L-PBF) of metallic materials, costs and material yield strongly depend on the ability to reuse powder efficiently, as a significant amount is not solidified as part by the laser beam. However, some of the powder is nevertheless exposed to high temperatures during the manufacturing process resulting in an alteration of the feedstock properties if reused. Therefore, there is a need to study and understand powder degradation during the L-PBF process and its direct effects on the printed parts. In this study, gas-atomized 316 L stainless steel powder was used, recovered, sieved and reused up to 15 times in order to produce successive L-PBF prints without adding any virgin powder. Both recycled powders and elaborated parts were fully characterized at each iteration to investigate changes in particles (morphology, rheology, microstructure and chemical composition) as well as printed parts (porosity, microstructure, microhardness and tensile properties). Recycled powder exhibited larger particle size and an improved flowability. A gradual increase in oxygen content was observed, along with the presence of colored and oxidized particles, as well as magnetic particles. Parts density slightly decreased with powder reuse and their microstructure featured more numerous and finer grains along reuse cycles. On the other hand, no significant difference was found on the microhardness and the tensile properties of the L-PBF components.