Inline Coherent Imaging of Laser Additive Manufacturing

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Kanko, Jordan
inline coherent imaging , laser processing , ICI , powder bed fusion , 3D printing , selective laser melting , laser melting , process monitoring , melt pool morphology , additive manufacturing , laser materials processing , laser additive manufacturing
Metal-based laser additive manufacturing techniques such as selective laser melting offer unique capabilities in the manufacture of geometrically complex metal components. The ability of these techniques to fabricate unique part designs of high-strength metals is of great interest to advanced engineering sectors such as aerospace, automotive, biomedical, dental, and defense. Despite the advantages offered by these processes, online quality assurance and process control still remain major challenges limiting the widespread adoption of these techniques. In this thesis a low-coherence interferometric imaging technique, termed inline coherent imaging (ICI), is integrated into the selective laser melting process to monitor melt pool morphology changes and stability at 200 kHz with axial and transverse resolutions reaching 6 µm and 35 µm respectively. Powder layer, processed track, and layer quality are assessed in situ using ICI. ICI-based imaging of single track processing of 316L stainless steel powder was investigated under a range of process parameters to determine the influence of processing conditions on melt pool morphological stability. Process parameter spaces involving 40-320 W laser powers; 50-600 mm/s scan speeds; 50-500 µm layer thicknesses; and supported and unsupported build environments were investigated. Galvanometer-based scanning of the imaging spot relative to the melt pool presented a unique perspective of the melt pool local geometry, undetectable by existing thermal-based imaging techniques. High-speed, time-resolved ICI measurements demonstrated that melt pool fluctuations and morphological stability strongly influence final track quality. Process defects resulting from poor process parameter regimes were detected and characteristic fault signatures were identified. Building on the success of ICI in monitoring single track processing, an ICI-integrated 3D-capable selective laser melting system was designed and constructed. System imaging and processing capabilities are characterized, and layer-wise ICI measurements of powder bed and processed layers taken during a 3D build are presented. Results presented throughout this work demonstrate the potential benefits ICI offers laser additive processes: layer-wise quality assurance measurements, morphology-based feedback control signals, improved parameter space optimization processes, and reductions in overall build times.
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