Laser beam-material interaction in Powder Bed Fusion

Abstract: The acceptance of additive manufacturing (AM) depends on the quality of final parts and the 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 and their sustainable processing and subsequent recycling. The research work presented in this thesis, to a certain degree, covers the above mentioned scientific aspects and focuses on the behavior of gas and water atomized steel powders in laser powder bed fusion (LPBF). Paper I demonstrates 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 LPBF. 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 II 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 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 with repeated recycling and a more frequent spatter ejection after each recycle was established. Paper III is a successor of Paper I and contains a 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.