In-line monitoring of powder layer quality in powder bed fusion processes

• Main Author: Le, Tan Phuc
• Other Authors: Matteo Seita
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering; Engineering::Manufacturing
• Online Access: https://hdl.handle.net/10356/152755
• Institution: Nanyang Technological University

The quality and uniformity of the powder layer have a direct impact on the performance of parts produced via powder bed fusion (PBF). Because powder layer properties depend on many powders- and recoating-specific variables, it is difficult to accurately predict powder bed quality across the variety of PBF processes and powders currently available. In this work, a novel method to assess the powder bed quality as a function of both powder conditions and recoating strategies is proposed. The method relies on the powder bed scanner technology, which provides particle-level resolution images of the entire powder layer as it is recoated. Through numerical analysis of the acquired images, three new metrics to assess powder bed quality, namely the powder layer thickness uniformity, surface area roughness, and surface particle density are defined. The efficacy of these metrics in capturing differences in powder layers across a matrix of recoating experiments using different batches of stainless steel 316L powder is confirmed. The obtained results show how powders with different particle surface conditions, morphology, and moisture content respond to various recoating velocities and re-coater blade types, resulting in layers with different qualities. Based on that, I correlate the layer quality as a function of different recoating velocities with the microstructure and mechanical properties of additive manufactured parts obtained via sample characterization and high-throughput tensile testing. The results indicate a strong influence of recoating velocity on the microstructure, which alters the tensile behaviour of the additive manufactured parts. Owing to the high measurement throughput and versatility, our method offers the opportunity to perform systematic spreadability studies of different powdered materials to optimize PBF processes, as well as provide in situ powder bed quality assessment during part production.