Additive manufacturing and processing of architected materials

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Introduction Architected materials are a class of materials that are rationally designed or naturally inspired, three-dimensional (3D), multiscale, and often incorporating multiple materials. They have the potential to provide breakthrough performance for a variety of applications if they can be realized at large enough overall size scales, with adequate throughput, and from the appropriate constituent materials. For the most part, remarkable properties of these materials have been demonstrated through cumbersome fabrication processes or with small samples in academic settings. Recently, with acceleration in the development of additive manufacturing (AM) techniques, hierarchical, 3D architected materials have become more accessible. Historically, architected materials have been fabricated by machining, folding, brazing, joining, and other more conventional processes that have either not been able to achieve the necessary feature resolution or the 3D complexity. This, combined with the typically slow and sometimes costly nature of these processes, has limited the adoption of architected materials. AM, more colloquially referred to as 3D printing, adds material and builds up structure in a layerby-layer fashion as opposed to material removal methods. As a result, its strength is geometric complexity. This makes printing methods the ideal class of manufacturing techniques for realizing architected materials.

Since their properties arise from geometry and scale as opposed to chemical composition alone, architected materials require accurate and precise physical replication of complex rational designs with minimal defects. Faithful reproduction demands feature sizes ranging from the nano- to the macroscale, complex curvatures, unique material combinations and composites, as well as reproducibility.

Fundamental challenges in fabrication of architected materials The concept of architected materials has existed for decades and has been most commonly applied in the aerospace industry. Typically, this has been in the form of honeycomb sandwich panel structures, due to their relative ease of fabrication and assembly, and somewhat beneficial performance. These structures have been popular due to their light weight and high bending stiffness; they have also been used with composite materials in thermal protection systems. While relatively easy to fabricate, they are far from optimal geometries for their intended use. In some historical examples, these materials were actually quite costly, but ultimately required, such as in NASA’s Apollo space capsule heat shield where “the advantage of having the failsafe features of a monolithic heat shield embedded in a honeycomb matrix overrode the disadvantages of extended

Christopher M. Spadaccini, Lawrence Livermore National Laboratory, USA; [email protected] doi:10.1557/mrs.2019.234

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