In situ morphology-based defect detection of selective laser melting through inline coherent imaging

Additive manufacturing techniques such as selective laser melting offer unique capabilities in the manufacture of geometrically-complex metal components, but online quality assurance and process control still remain major challenges. In this paper, a low-coherence interferometric imaging technique, termed inline coherent imaging (ICI), is coaxially integrated into the selective laser melting process to monitor melt pool morphology changes and stability at 200 kHz. Single track processing and online monitoring of 316 L stainless steel powder were carried out under a range of process parameters: 40-320 W laser powers; 50-600 mm/s scan speeds; 50-500 μm layer thicknesses; and in supported and unsupported local build environments. Galvanometer-based scanning of the imaging spot relative to the melt pool allowed capture of the geometrical profile of the melt pool and environment undetectable by existing thermal-based imaging systems. Exploiting the high-speed imaging rate, time-resolved ICI measurements demonstrate that melt pool fluctuations strongly influence final track quality. Process defects resulting from poor parameter regimes are detected and characteristic fault signatures are identified.