Towards sustainability in additive manufacturing: material and process aspects

Abstract: The acceptance of additive manufacturing (AM) depends on the quality of final parts and process repeatability. Recently, many studies have been dedicated to the establishment of the relationship between the process behavior and material performance. Phenomena such as laser-material interaction, melt pool dynamics, ejecta formation and particle movement behavior on a powder bed are of a particular interest for the AM community as these events directly influence the outcome of the process. Another aspect, which hinders the adoption of AM, is the need for cost-efficient powder materials, their sustainable processing and recycling. The research work presented in this thesis explores scientific aspects related to the above-mentioned topics, with a particular focus on the material and process behavior phenomena in powder bed fusion-laser melting (PBF-LM) and directed energy deposition (DED) processes. Paper A shows a comparative study of dissimilarly shaped gas and water atomized low alloy steel powders regarding their processability, packing capacities, particle movement behavior and powder performance in PBF-LM. The impact of chemical composition and morphology of the powders on the process behavior was revealed. Powder spattering and melt pool instabilities were discussed in detail. Paper B contains research on the particle movement and denudation behavior on a powder bed when using near-spherical and non-spherical steel powders. The influence of particle morphology on the dynamics of arbitrary-shaped powder particles was studied by applying an analytical correlation formula to calculate the drag force exerted on powder particles of various shape. Particle entrainment of gas and water atomized powders in front of the laser beam was measured, revealing a significant difference in the powder transfer towards the melt pool.Paper C explains the role of ejecta in the recycled powder and the changing behavior of the material due to ejecta pick-up. The impact of multiple powder recycling steps on the degradation of low alloy steel powder in laser powder bed fusion was studied. Oxygen content, particle size and ejecta occurrence gradually increased after each recycling step and were identified as the main contributors to the property alterations observed in the powder during recycling. In addition, a direct correlation between the increase in oxygen and more frequent spatter ejection with repeated recycling was established. Paper D focuses on the impact of powder aging on the degradation of AlSi10Mg powder during processing in PBF-LM. The analysis of the powder properties, affected by laser exposure and the aging procedure, showed a change of chemical and morphological characteristics of the powders in virgin and aged conditions. The oxygen content in the powders appeared to have a significant effect on the powders' surface appearance and light absorbance, gradually deteriorating the processability of the powders with the increase of oxygen level. Porosity occurrence and its influence on the mechanical properties of the powders was also studied, demonstrating a rapid decrease of ultimate tensile strength and elongation from virgin condition to aged.Papers E and F investigate the possibilities of iron ore waste reduction using Al powder as a reducing agent and a laser beam as a heat source. Paper E focuses on the Fe2O3-Al interaction behavior and extent of the iron ore reduction, whereas Paper F reports on the high-speed imaging investigation possibilities of laser beam-material surface interaction when processing Fe2O3-Al powders and an Fe2O3 powder-AlSI5 wire combination in DED. In-situ observation of various melt pool phenomena and exothermic reaction behavior of the material combinations using high-speed imaging was carried out. In addition to that, the influence of feed materials and laser power on the thermite reaction time was discussed in detail, showing their dissimilar behavior.All six papers include research on laser additive manufacturing using powder feedstocks. The papers discuss various phenomena regarding powder processability, recycling and laser beam-material interaction behavior in both PBF-LM and DED. High-speed imaging was used as the main tool to observe and study the above listed topics.