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Introduction Three-dimensional printing (3D) and additive manufacturing (AM) of metallic materials are witnessing significant advances. Maturation of both research-grade and commercial production 3D printing equipment during the past two decades, along with an increased diversity of feedstock materials, has spurred significant research activities across academic, government, and industrial research institutions worldwide. The most prominent forms of 3D metal printing involve powder beds, streams of gas-propelled powder jets, or wire for feedstock, lasers and electron beams as the energy sources, and precision automation equipment for digitally directing the energy source, the feedstock, or both along the material/energy deposition pathways required to form the desired shapes, layer by layer. The potential for fabricating metal components directly from digital data using a single piece of fully automated equipment and feedstock materials and without additional hard tooling is very significant. Immediate near-term impacts include dramatic reductions in cost and lead time, the ability to produce small-lot or “one-of-a-kind” components on demand, and the ability to prototype and produce advanced, highperformance, and more efficient components that cannot be manufactured through conventional methods due to inherent

limitations on geometry, material, microstructure, and properties. These benefits have been recognized and metal 3D printing is in use today for limited production by industry, R&D institutions, and governments across the globe. While the advantages of 3D metal printing are compelling, there are also significant scientific and technical challenges that confront widespread implementation and adoption of these technologies. First, the microstructures and crystal textures produced through layerwise additive consolidation are quite novel, and rarely resemble those produced through conventional manufacturing methods such as casting or deformation processing. Second, the physical processes associated with incremental consolidation of metals accompanied by diverse variations in feedstock materials, processing parameters, processing protocols, and equipment architectures are inherently complex, leading to a diversity of “as produced” microstructures and properties. Third, comprehensive understanding of structural and microstructural defects present in metals processed through 3D printing/AM techniques and post-processing protocols such as heat treatment and hot-isostatic pressing cycles to alleviate these defects, as well as to transform the microstructures to those acceptable for service conditions, is limited and is a focus of intense investigation across R&D organizations.

Suman Das, Woodruff School of Mechanical Engineering and School of Materials Science and Engineering, Georgia Institute of Technology, USA; [email protected] David L. Bourell, Mechanical Engineering and Materials Science and Engineering, The University of Texas at Austin, USA; [email protected] S.S. Babu, Department of Mechanical, Aerospace