NDTREVIEWPAPER | ME A PERSPECTIVE OF THE NEEDS AND OPPORTUNITIES FOR COUPLING MATERIALS SCIENCE AND NONDESTRUCTIVE EVALUATION FOR METALS-BASED ADDITIVE MANUFACTURING BY M.J. QUINTANA*†, Y. JI*‡, AND P.C. COLLINS*†‡§ ABSTR ACT This paper presents a perspective of the needs and opportunities associated with the multidisciplinary problem of nondestructive evaluation (NDE) of additive manufacturing (AM). Recognizing the multidisciplinary nature of the problem, as well as the need to bridge knowledge between the different communities, the paper is structured to provide brief backgrounds and details relevant to both communities, as well as present an assessment of the state of the art. This paper, in some respects, is meant to be a primer of the different landscapes, as well as a catalyst for making future connections. At the end, it will be clear that there is much more work to be done, but that the work that is ongoing is exciting, and the potential to exploit NDE techniques for metals-based AM is very high. KEYWORDS: additive manufacturing, nondestructive evaluation, materials state, measurement techniques, materials physics Introduction Increasingly, there is an awareness that the paradigm-changing nature of additive manufacturing (AM) requires a reassess- ment of both materials science and nondestructive evaluation (NDE). Traditionally, these technical specialties/disciplines are separated, as their role in the development, manufac- ture, and use of parts and components in advanced technical systems, such as vehicles, aircraft, defense, and energy systems, is notably different. However, it is also becoming clear that there is a significant opportunity if these traditionally separate subject matter experts can collaborate in the area of AM. The causes associated with why these technical experts are separated is worth a brief discussion. First, there is the typical role that these experts play in any organization. A materials scientist plays important roles in the development and opti- mization of new materials, often long before those materials are qualified and become part of the design and manufacturing ecosystem. A materials engineer may then be highly involved in certain aspects of the manufacturing ecosystem, providing subject matter expertise related to process controls and destruc- tive testing to assure specific metrics of quality (for example, mechanical testing or microscopy). The NDE experts often receive a handoff of parts and components, and then apply their skill sets to ensure that the quality of parts is known to an acceptable degree of uncertainty, monitoring parts over their lifetime in service. In certain organizations, the NDE experts can play a role in the design of the parts if philosophies such as design for inspectability are part of the organization’s culture. Second, there are the types of data these different subject matter experts typically manage. For the materials scientist or engineer, the spatial domain dominates the characterization techniques, enabling the direct observation of grains, texture, precipitates, and defects. For the NDE expert, the tools invariably rely upon measurements involving time, and are thus in the frequency domain, which can be converted into the spatial domain using various techniques. Lastly, the NDE experts are trained to use statistics (that is, probability of detection) to pursue rare events and are, by their occupation, risk averse. Conversely, the research of many materials scientists is primarily focused on the initial stages of new materials development, where it is not uncommon to imagine in an almost unbridled sense the possi- bilities of the new materials under study. * Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA † The Center for Advanced Nonferrous Structural Alloys, a joint NSF I/UCRC between Iowa State University and the Colorado School of Mines ‡ The Center for Nondestructive Evaluation, a graduated NSF I/UCRC § Ames Laboratory, Ames, IA 50011, USA pcollins@iastate.edu Materials Evaluation 80 (4): 45–63 https://doi.org/10.32548/2022.me-04256 ©2022 American Society for Nondestructive Testing A P R I L 2 0 2 2 • M A T E R I A L S E V A L U A T I O N 45

AM is, without question, a new manufacturing paradigm. In its most unconstrained, futuristic sense (see Figure 1), AM is capable of producing net or near-net shapes: whose features span across length scales (Zhou et al. 2015 Riveiro et al. 2019 Kumar and Maji 2020 Marini and Corney 2020) whose topology may be topologically optimized or, emer- gently, generatively designed (Meng et al. 2020 Liu et al. 2018) whose local materials state1, and thus properties/perfor- mance, may be controlled spatially by tuning the process, post process, and/or composition spaces (Tammas-Williams and Todd 2017 Li et al. 2020) and where the local materials state may be both designed and measured during component manufacture, providing a digital record/twin that can be used to both verify/validate the process space and predict the properties/performance of the part during service. To ensure that the material is of a sufficient quality with respect to the design metrics (such as dimensions, properties, and performance), it is necessary to develop and bring to bear new advanced metrology and evaluation tools during the man- ufacturing process. Among the most promising techniques are those that are based upon conventional NDE approaches, yet their applicability requires a direct connection with the mate- rials state. Within AM, there are new physics that operate, which sci- entists and engineers (and companies/organizations) need to understand. For example, as most AM techniques are fusion- based processes, the composition of the as-deposited material may be different than the composition of the starting powder or wire, through either the preferential loss of some volatile elements or the gettering of other elements from the surround- ing atmosphere (Carroll et al. 2015 Sato and Kuwana 1995 ME | AMNDEOVERVIEW 1 Materials state includes, but is not limited to: composition, solute distributions, microstructure (phases, their size, distribution, and correlations), crystallographic texture, and the presence of defect structures (e.g., dislocations, porosity, interfaces, cracks), across all length scales. This definition follows from materials state awareness (MSA), which is defined as “digitally enabled reliable nondestructive quantitative materials/damage characterization regardless of scale” (Buynak et al. 2008). The materials state is what a manufacturing process produces (or what evolves during service) and is also what governs the performance of the material (Buynak et al. 2008 Jacobs 2014 Aldrin and Lindgren 2018). Figure 1. Wide variety of applications of AM techniques: (a) additively manufactured bridge using the wire arc additive manufacturing (WAAM) technique (b) hydraulic hand 3D printed by Oak Ridge National Laboratory that houses electric motors and hydraulic components inside (c) 3D-printed metallic “space fabric” designed and manufactured by NASA and (d) AM meso-structures in a turbine blade. (Figure 1a is reprinted with permission from Feucht et al. [2020] Figure 1b is reused from Love et al. [2013] under Creative Commons Attribution License (CC BY) Figure 1c is reused from Good and Landau [2017] under Creative Commons Attribution License (CC BY) and Figure 1d is reprinted courtesy of The University of Sheffield.) 46 M A T E R I A L S E V A L U A T I O N • A P R I L 2 0 2 2