Introduction to the special issue on Additive Manufacturing and Acoustics^a)

Additive manufacturing (AM), also known as three-dimensional (3D) printing, is a manufacturing process by which the material is selectively placed at locations in space to build up the object one wishes to construct. This is achieved by using a wide range of different technologies that exploit various physical mechanisms to deposit and solidify the material. The starting state is often an empty platform, usually referred to as a build tray, where the material is added in two-dimensional (2D) layers, which are slices of the final part at different elevations relative to the build tray, and the part is then built layer by layer to create a solid in three dimensions. The “additive” nature of AM is in contrast to many traditional fabrication techniques, which apply subtractive methods, i.e., subtractive manufacturing, where the starting point is a 3D solid from which the material is selectively removed via processes such as milling, drilling, or cutting. AM offers processing advantages such as reduced waste, design freedom to create complex shapes, and quick production of custom geometries without the need for extensive assembly or post-processing. Whereas AM has a multitude of applications in engineering and science, the versatility of this manufacturing approach has also advanced the field of acoustics. This special issue highlights some of the unique and advantageous ways in which AM can be used to address long-standing problems in acoustics and stimulate new research topics and, conversely, how acoustics can be used to help improve AM processes.

AM is capable of fabricating objects that have intricate geometries using a wide range of material classes—metals, plastics, elastomers, and even active materials such as piezoelectrics. Further, AM techniques enable the fabrication at different length scales, ranging from centimeters to nanometers. Because most AM processes employ layer-by-layer fabrication, one may construct media with complex geometries like periodic lattices, spatially graded density and/or stiffness, and even designer porosity. Complex geometries like these are typically impossible to fabricate using subtractive manufacturing because tool access to small interior portions of the material is prohibitive. The complex geometries made possible with AM have enabled researchers to design new structures and materials, which have acoustic responses that are otherwise not possible to realize.

Materials fabricated using A0000M often have mechanical properties and/or microstructures, which are different from those that are manufactured using traditional methods such as casting or molding. These differences are due to various aspects of the AM process, such as the layer-by-layer nature by which the material is built and the mechanism with which the materials are consolidated (e.g., sintering of metal powders). These aspects can introduce anisotropy in the material properties, surface roughness, porosity, and formation of residual stresses, all of which can cause damage to evolve differently in the AM materials compared to traditionally manufactured materials. In addition, the microstructure and mechanical properties can vary significantly in the AM materials within a printed object and between different objects. Nondestructive evaluation (NDE) of these material properties is, therefore, necessary to determine whether the AM materials are suitable for use in a given application and predict when and where damage will occur. Although such an evaluation can be performed using a wide range of methods, such as optical or mechanical testing, evaluation using ultrasound is particularly useful because of its inherent ability to probe within a material, its nondestructive nature, and its robust history as a characterization tool. Over the past few years, researchers have explored existing ultrasonic techniques and developed new tools to characterize the AM materials. This issue contains representative state-of-the-art works on this topic, which will be of particular interest to researchers and practitioners of ultrasonic NDE.

The articles in this special issue can be generally divided into two categories, those in which acoustics are used to interrogate an additively manufactured material and those in which an additively manufactured object is designed to have a prescribed acoustic effect. In the first category, the well-established acoustic methods, such as NDE using resonant ultrasound (Fisher, 2020; Kube et al., 2021) and classical pulse-echo methods (Bakaric et al., 2021; Gillespie et al., 2021; Bellotti et al., 2021), are used to characterize the different material properties of the AM materials. For example, Kube et al. (2021) studied the influence of the residual stress and texture in the AM metals on their resonant ultrasonic responses. Fisher (2020) applied resonant ultrasound spectroscopy to measure the elastic moduli and Poisson's ratio of the AM metal lattice structures. Bakaric et al. (2021) used the ultrasonic measurements of the phase velocity and attenuation to characterize the mechanical properties of the photopolymer materials fabricated with the different AM processes, whereas Bellotti et al. (2021) used nonlinear ultrasound to define the microstructural changes related to the dislocations in the AM metals and those with subsequent post-fabrication heat treatment. In addition, the researchers used ultrasonic NDE in situ to monitor the AM process and describe the material as it was being consolidated (Sotelo et al., 2021; Gillespie et al., 2021). Specifically, Gillespie et al. (2021) used the in situ ultrasonic pulse-echo measurements combined with high-speed X-ray imaging to monitor the behavior of the melt pool, which formed from the laser sintering of metal powders during the AM process. Sotelo et al. (2021) used contact ultrasonic transducers in the pulse-echo configuration under a build plate to monitor the phase transformation in the directed energy deposition (DED) of Ti6Al4V samples by monitoring the shifts in the time-of-flight of the ultrasonic signals.

The range of geometries enabled by AM provides for new opportunities to manipulate the acoustic waves. Porous media, such as foams, which have a range of acoustic applications, can be fabricated using AM techniques. Konarski et al. (2021) printed variable-porosity aluminum foams using direct metal laser sintering and characterized their acoustic response using an acoustic impedance tube and employed the statistical analysis to extract ranges of the material properties based on an established porous medium model. The field of acoustic metamaterials, which are materials with architected subwavelength features, has benefitted considerably from the expansion of the availability of AM, resulting from the ability to print complex, 3D structures such as lattices (Hyun and Kim, 2021; Smith and Matlack, 2021; Cushing et al., 2022). Hyun and Kim (2021) designed a planar acoustic lens with a complex geometry, enabled by AM, by using a topology optimization approach to maximize the acoustic intensity at a focal position. Smith and Matlack (2021) designed and manufactured a phononic material for ultrasonic filtering, which reduced the extraneous nonlinearities that can be problematic in NDE methods. Cushing et al. (2022) designed, built, and tested an anisotropic acoustic pentamode material (a type of architected solid that only supports the longitudinal wave motion over wide frequency bands), which displays the anisotropic longitudinal sound speed. The geometries, which have complex internal channels, are challenging to fabricate using nonadditive methods, requiring multiple steps such as machining and molding. Kliewer et al. (2021) used AM to design reconfigurable acoustic topological insulators, which break inversion symmetry, and examined the effects of the AM processes, such as surface roughness, on their performance. Zhao et al. (2021) leveraged the ability of AM to design complex acoustic absorbing geometries, which manipulate the reconfigurable tube networks to create low-frequency, broadband absorbing designs. Yves and Alù (2021) also leveraged the unique geometries enabled by the additive methods to design acoustic metasurfaces with subwavelength anisotropy for hyperbolic and elliptical sound propagation.

The additive processes also enable the creation of new materials and structures for use in acoustics, but the artifacts from the printing processes, such as dimensional variation or surface roughness, can strongly influence the desired acoustic behavior. Wiest et al. (2022) designed asymmetric absorbing metasurfaces, which leverage the Willis coupling while taking into account the additive design constraints, such as dimensional variation, across the metasurface. The feasibility of using AM to fabricate ultrasonic surgical devices, including bone-penetrating needles and bone cutting devices, was evaluated by Cleary et al. (2021) by comparing the simple designs using additive and traditional approaches. Their results show a clear difference in the performance when using the AM methods in comparison to the traditional techniques. Di Giulio et al. (2021) designed and fabricated the complex lattices with specified acoustic and thermoacoustic properties based on the finite element numerical simulations and porous medium models, and when compared to the experimentally measured data, a good agreement was found.

Finally, as Guest Editors, we would like to thank the editorial staff for their assistance in creating the special issue in accordance with the standards of the Journal. We would also like to thank the Editor-in-Chief, James Lynch, for his encouragement and advice. Most importantly, we would like to extend a special thanks to all of the contributors and reviewers who provided high quality manuscripts, which clearly displays the expanding breadth of ongoing research in the highly cross-disciplinary intersection of AM and acoustics. We hope that this collection stimulates further research in these areas of study, and we look forward to future contributions to the Journal.