metals Article Relative Density Measurement of PBF-Manufactured 316L and AlSi10Mg Samples via Eddy Current Testing Marvin Aaron Spurek 1,2, * , Viet Hiep Luong 2 , Adriaan Bernardus Spierings 1 , Marc Lany 3 , Gilles Santi 3 , Bernard Revaz 3 and Konrad Wegener 2   Citation: Spurek, M.A.; Luong, V.H.; Spierings, A.B.; Lany, M.; Santi, G.; Revaz, B.; Wegener, K. Relative Density Measurement of PBF-Manufactured 316L and AlSi10Mg Samples via Eddy Current Testing. Metals 2021, 11, 1376. https://doi.org/10.3390/met11091376 Academic Editor: Giovanni Bruno Received: 2 August 2021 Accepted: 26 August 2021 Published: 31 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Inspire AG, Innovation Center for Additive Manufacturing Switzerland (ICAMS), Fürstenlandstrasse 122, 9014 St. Gallen, Switzerland; spierings@inspire.ethz.ch 2 Swiss Federal Institute of Technology, ETH Zurich, Institute of Machine Tools and Manufacturing (IWF), Leonhardstrasse 21, 8092 Zurich, Switzerland; viet.hiep.luong@alumni.ethz.ch (V.H.L.); wegener@iwf.mavt.ethz.ch (K.W.) 3 Sensima Inspection SARL, 2 Route Cité Ouest, 1196 Gland, Switzerland; marc.lany@sensima.ch (M.L.); gilles.santi@sensima.ch (G.S.); bernard.revaz@sensima.ch (B.R.) * Correspondence: spurek@inspire.ethz.ch Abstract: Powder bed fusion (PBF) is the most commonly used additive manufacturing process for fabricating complex metal parts via the layer-wise melting of powder. Despite the tremendous recent technological development of PBF, manufactured parts still lack consistent quality in terms of part properties such as dimensional accuracy, surface roughness, or relative density. In addition to process-inherent variability, this is mainly owing to a knowledge gap in the understanding of process influences and the inability to adequately control them during part production. Eddy current testing (ECT) is a well-established nondestructive testing technique primarily used to detect near-surface defects and measure material properties such as electrical conductivity in metal parts. Hence, it is an appropriate technology for the layer-wise measuring of the material properties of the fused material in PBF. This study evaluates ECT’s potential as a novel in situ monitoring technology for relative part density in PBF. Parts made from SS316L and AlSi10Mg with different densities are manufactured on a PBF machine. These parts are subsequently measured using ECT, as well as the resulting signals correlated with the relative part density. The results indicate a statistically significant and strong correlation (316L: r(8)= 0.998, p < 0.001, AlSi10Mg: r(8)= 0.992, p < 0.001) between relative part density and the ECT signal component, which is mainly affected by the electrical conductivity of the part. The results indicate that ECT has the potential to evolve into an effective technology for the layer-wise measuring of relative part density during the PBF process. Keywords: powder bed fusion (PBF); eddy current testing (ECT); part quality; in situ relative part density measurement; quality management 1. Introduction In the last decade, additive manufacturing technologies have evolved from rapid pro- totyping to established manufacturing technologies that are increasingly used in industrial production. Powder bed fusion (PBF) is the most commonly used additive manufacturing process for fabricating metal parts, and has evolved to a state-of-the-art technology adopted in various industrial fields such as aerospace, medical, defence, as well as tool, and mould making [1,2]. The PBF process is characterized by the layer-wise melting of a powder bed using a laser beam; hence it enables the direct manufacturing of complex-shaped parts. De- spite the tremendous recent technological development of PBF, Debroy et al. [3] identified the lack of consistent part quality as a major challenge impeding the wider commercial adoption of PBF. Part quality is mainly characterized by mechanical properties such as Young’s modulus, and part properties such as dimensional accuracy, surface roughness, Metals 2021, 11, 1376. https://doi.org/10.3390/met11091376 https://www.mdpi.com/journal/metals