In situ measurements of layer roughness during laser powder bed fusion additive manufacturing using low coherence scanning interferometry

Abstract

Layer-to-layer height measurements of additively manufactured 316L stainless steel using high speed spectral-domain optical coherence tomography (SD-OCT) are presented. Layers are built up using an open architecture laser powder bed fusion machine while height measurements are made in-line along the process laser path following each layer print. Printed cubes, with and without an internal ‘overhang’ channel, were built to investigate the effect of scanning parameters on surface structure. Layer-to-layer scan rotation strategy significantly impacts surface roughness between layers which in turn can influence porosity. Spatter particles, which have been correlated with numerous defect modalities, generate high points in the powder bed and can persist on a melted surface for many layers. Laser power significantly affects overhang morphology, as measured by SD-OCT. Large dross occurs in the high energy density regime, while balling and a capillary-driven coalescence of unstable melt pools perpendicular to the scanning direction occurs in the low energy density regime. High fidelity powder-scale simulations of deep powder layers were used to further elucidate the underlying physics revealed by SD-OCT measurements and high speed imaging, yielding insight to defect formation mechanisms which can lead to improved process parameters. Keywords: Additive manufacturing, Laser powder bed fusion, Selective laser melting, In situ process monitoring, Optical coherence tomography, Interferometry