If high-speed thermal cameras are used, it is possible to closely observe the behavior of the melt pool in a time-based series, which could provide invaluable information that helps to understand the AM process. Recent research (Calta et al. 2019) demonstrates that high-speed camera images can be used to resolve thermal emission from spatter events, fluctua- tions in the melt pool itself, the vapor plume, and the solidified track as it cools, as seen in Figure 12. Conclusions This paper has given an overview of both the materials science aspects of AM, as well as the prospects of NDE techniques to provide key information regarding the process. The fun- damental physics associated with AM are complex, and the relevant length scales range from nanometers to centimeters, while the time scales range from sub-microsecond to many seconds. Relevant velocities include not only the obvious “travel speeds” of the AM process, but also the velocity of the solid-liquid interface and the convective flow within the liquid state. Each of these parameters is associated with details of the process and phase transformations that govern the materials state, including deposited composition, grain structure, texture, defects, and residual stress. While it will be impossible to directly measure all of these parameters, there is the prospect that some multilength scale processes will have measurable signatures that can be probed using NDE techniques. A variety of methods and techniques, ranging from visual, ultrasonic, and radiographic (wave-based methods) to electromagnetic and thermographic (diffusion based) have been presented, and have all been shown to offer some benefit, whether it is to understand process variations or make discrete measurements of the materials state. This field remains active. ACKNOWLEDGMENTS The authors acknowledge the support of the Center for Advanced Non-Ferrous Structural Alloys (CANFSA), an NSF Industry/University Cooperative Research Center (I/UCRC) between Iowa State University and The Colorado School of Mines, as well as the support from Center for Nondestructive Evaluation (CNDE), a graduated NSF I/UCRC. Background expertise and related materials have been developed under multiple programs. Currently ongoing programs include research that is sponsored by the Department of the Navy, Office of Naval Research under ONR award number N---. Any opinions, findings, conclu- sions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Office of Naval Research. In this work and for the generation of specimens, access to the additive manufacturing equipment at Oak Ridge National Laboratory’s Manufacturing Demonstration Facility (MDF) was facilitated by the US Department of Energy’s Strategic Partnership Projects (SPP) mech- anism. More information can be found at https://science.energy.gov/ lp/strategic-partnership-projects. Research was sponsored by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program, under contract DE-AC-OR with UT-Battelle LLC. In addition, materials manufactured using LHW AM materials were produced and tested under ONR contract N--C- (“Robotic Laser Additive Manufacturing System with Comprehensive Quality Assur- ance Framework”). REFERENCES D Systems, , D Printers, Software, Manufacturing & Digital Health- care, https://www.dsystems.com (accessed  August ) Abdelrahman, M., E.W. Reutzel, A.R. Nassar, and T.L. Starr, , “Flaw Detection in Powder Bed Fusion using Optical Imaging,” Addit. Manuf., Vol. , pp. –, https://doi.org/./j.addma... Ahsan, F., and L. Ladani, , “Temperature Profile, Bead Geometry, and Elemental Evaporation in Laser Powder Bed Fusion Additive Manufac- turing Process,” JOM, Vol. , pp. –, https://doi.org/./s --- Aldrin, J.C., and E.A. Lindgren, , “The Need and Approach for Characterization-U.S. Air Force Perspectives on Materials State Awareness,” AIP Conference Proceedings, Vol. , https://doi.org/./. Spatter 0 ms 0.780 ms Scan direction 0.936 ms 1.092 ms 1.248 ms 0.156 ms 0.312 ms 0.468 ms Melt pool Vapor plume Solidified track Figure 12. High-speed thermal images collected by the melt pool monitoring camera at LLNL (reused from Calta et al. [2019] under Creative Commons Attribution License [CC BY]). ME | AMNDEOVERVIEW 58 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

AllDP, , D Printed Iron Man Suit: The Most Incredible Projects | AllDP, https://alldp.com//d-printed-iron-man-suit AlMangour, B., D. Grzesiak, and J.-M. Yang, , “Selective Laser Melting of TiB/H Steel Nanocomposites: Influence of Hot Isostatic Pressing Post-Treatment,” J. Mater. Process. Technol., Vol. , pp. –, https:// doi.org/./j.jmatprotec... Baker, R., , Method of Making Decorative Articles, US Patent ,,, filed  November , and issued  April  Balla, V.K., P.D. DeVasConCellos, W. Xue, S. Bose, and A. Bandyopadhyay, , “Fabrication of Compositionally and Structurally Graded Ti-TiO Structures Using Laser Engineered Net Shaping (LENS),” Acta Biomater., Vol. , No. , pp. –, https://doi.org/./j.actbio... Bandyopadhyay, A., and B. Heer, , “Additive Manufacturing of Multi-Material Structures,” Mater. Sci. Eng.: R: Reports, Vol. , pp. –, https://doi.org/./j.mser... Batista, M., V. Furlanetto, and S.D. Brandi, , “Development of a Resis- tance Spot Eelding Process Using Additive Manufacturing,” Metals (Basel), Vol. , pp. –, https://doi.org/./met Bayat, M., A. Thanki, S. Mohanty, A. Witvrouw, S. Yang, J. Thorborg, N.S. Tiedje, and J.H. Hattel, , “Keyhole-Induced Porosities in Laser-Based Powder Bed Fusion (L-PBF) of TiAlV: High-Fidelity Modelling and Experimental Validation,” Addit. Manuf., Vol. , https://doi.org/./j. addma.. Bermingham, M.J., D. Kent, H. Zhan, D.H. Stjohn, and M.S. Dargusch, , “Controlling the Microstructure and Properties of Wire Arc Additive Manufactured Ti-Al-V with Trace Boron Additions,” Acta Mater., Vol. , pp. –, https://doi.org/./j.actamat... Bi, G., C.-N. Sun, H.-C. Chen, F.L. Ng, and C.C.K. Ma, , “Microstruc- ture and Tensile Properties of Superalloy IN Fabricated by Micro-laser Aided Additive Manufacturing,” Mater. & Des., Vol. , pp. –, https://doi.org/./j.matdes... Blank, M., P. Nair, and T. Pöschel, , “Capillary Viscous Flow and Melting Dynamics: Coupled Simulations for Additive Manufacturing Applications,” Int. J. Heat Mass Transf., Vol. , pp. –, https://doi .org/./j.ijheatmasstransfer... Boettinger, W.J., S.R. Coriell, A.L. Greer, A. Karma, W. Kurz, M. Rappaz, and R. Trivedi, , “Solidification Microstructures: Recent Develop- ments, Future Directions,” Acta Mater., Vol. , pp. –, https://doi. org/./S-()- Brandl, E., A. Schoberth, and C. Leyens, , “Morphology, Microstruc- ture, and Hardness of Titanium (Ti-Al-V) Blocks Deposited by Wire- Feed Additive Layer Manufacturing (ALM),” Mater. Sci. Eng. A., Vol. , pp. –, https://doi.org/./j.msea... Brice, C.A., and W.H. Hofmeister, , “Determination of Bulk Residual Stresses in Electron Beam Additive-Manufactured Aluminum,” Metall. Mater. Trans. A, Vol. , pp. –, https://doi.org/./s- --z Brown, C.O., E.M. Breinan, and B.H. Kear, , Method for Fabricating Articles by Sequential Layer Deposition, US Patent ,,, filed  October , and issued  April  Buynak, C.F., J. Blackshire, E.A. Lindgren, and K.V. Jata, , “Challenges and Opportunities in NDE, ISHM and Material State Awareness for Aircraft Structures: US Air Force Perspective,” AIP Conference Proceedings, Vol. , https://doi.org/./. Calta, N., , Final Technical Report Project Title: In-situ Data Acqui- sition and Tool Development for Additive Manufacturing Metal Powder Systems, pp. – Calta, N., P. Collins, A. Martin, M. Matthews, J.N. Weker, R. Ott, K. Stone, and C. Tassone, , “In-situ Data Acquisition and Tool Development for Additive Manufacturing Metal Powder Systems,” Technical Report, SLAC National Accelerator Lab and Lawrence Livermore National Lab, https://doi.org/./ Carroll, B.E., T.A. Palmer, and A.M. Beese, , “Anisotropic Tensile Behavior of Ti-Al-V Components Fabricated with Directed Energy Deposition Additive Manufacturing,” Acta Materialia, Vol. , pp. –, https://doi.org/./j.actamat... Chalmers, B., , Principles of Solidification, Wiley, New York, NY Chlebus, E., B. Kuznicka, ′ R. Dziedzic, and T. Kurzynowski, , “Titanium Alloyed with Rhenium by Selective Laser Melting,” Mater. Sci. Eng.: A, Vol. , pp. –, https://doi.org/./j.msea... Collins, P.C., , “A Combinatorial Approach to the Development of Composition-Microstructure-Property Relationships in Titanium Alloys Using Directed Laser Deposition,” PhD thesis, Ohio State University, Columbus, OH, Publication No. AAI Collins, P.C., C. V. Haden, I. Ghamarian, B.J. Hayes, T. Ales, G. Penso, V. Dixit, and G. Harlow, , “Progress toward an Integration of Process-Structure-Property-Performance Models for ‘Three-Dimensional (-D) Printing’ of Titanium Alloys,” JOM, Vol. , pp. –, https: //doi.org/./s---y Collins, P.C., D.A. Brice, P. Samimi, I. Ghamarian, and H.L. Fraser, , “Microstructural Control of Additively Manufactured Metallic Materials,” Annu. Rev. Mater. Res., Vol. , pp. –, https://doi.org/./ annurev-matsci-- Cottam, R., J. Wang, and V. Luzin, , “Characterization of Microstruc- ture and Residual Stress in a D H Tool Steel Component Produced by Additive Manufacturing,” J. Mater. Res., Vol. , pp. –, https://doi .org/./jmr.. Cunningham, R., C. Zhao, N. Parab, C. Kantzos, J. Pauza, K. Fezzaa, T. Sun, and A.D. Rollett, , “Keyhole Threshold and Morphology in Laser Melting Revealed by Ultrahigh-Speed X-ray Imaging,” Science, Vol. , No. , pp. –, https://doi.org/./science.aav Daehn, G.S., and A. Taub, , “Metamorphic Manufacturing: The Third Wave in Digital Manufacturing,” Manuf. Lett., Vol. , pp. –, https: //doi.org/./j.mfglet... Davis, J.E., N.W. Klingbeil, and S. Bontha, , “Effect of Free-Edges on Melt Pool Geometry and Solidification Microstructure in Beam-Based Fabrication of Thin-Wall Structures,” th Annual Int. Solid Free. Fabr. Symp. (SFF ), pp. – DeMott, R., N. Haghdadi, X. Liao, S.P. Ringer, and S. Primig, , “D Characterization of Microstructural Evolution and Variant Selection in Additively Manufactured Ti-Al- V,” J. Mater. Sci., Vol. , pp. –, https://doi.org/./s--- Denlinger, E.R., J. Irwin, and P. Michaleris, , “Thermomechanical Modeling of Additive Manufacturing Large Parts,” J. Manuf. Sci. Eng., Vol. , pp. –, https://doi.org/./. Denlinger, E.R., J.C. Heigel, P. Michaleris, and T.A. Palmer, , “Effect of Inter-Layer Dwell Time on Distortion and Residual Stress in Additive Manufacturing of Titanium and Nickel Alloys,” J. Mater. Process. Technol., Vol. , pp. –, https://doi.org/./j.jmatprotec... Dinda, G.P., A.K. Dasgupta, and J. Mazumder, , “Texture Control during Laser Deposition of Nickel-based Superalloy,” Scr. Mater., Vol. , pp. –, https://doi.org/./j.scriptamat... du Plessis, A., S.G. le Roux, J. Els, G. Booysen, and D.C. Blaine, , “Application of microCT to the Non-destructive Testing of an Additive Manufactured Titanium Component,” Case Studies in Nondestruct. Test. and Eval., Vol. , pp. –, https://doi.org/./j.csndt... Du, W., Q. Bai, Y. Wang, and B. Zhang, , “Eddy Current Detection of Subsurface Defects for Additive/Subtractive Hybrid Manufacturing,” Int. J. Adv. Manuf. Technol., Vol. , pp. –, https://doi.org/. /s--- Ehlers, H., M. Pelkner, and R. Thewes, , “Heterodyne Eddy Current Testing Using Magnetoresistive Sensors for Additive Manufacturing Purposes,” IEEE Sensors Journal, Vol. , No. , pp. –, https: //doi.org/./JSEN.. Ferry, M., , Direct Strip Casting of Metals and Alloys: Processing, Microstructure and Properties, Woodhead, Cambridge, UK 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 59