Effects of metal 3-D printing processes on acoustic metamaterial elements

Abstract

Acoustic metamaterial properties derive from inherently complex, sub-wavelength geometries which aim to control the propagation of sound and elastic waves. The material properties of the unit cell element, the meta-atom, are important when considering either resonant response, or when the transport medium is a high density fluid, such as water. For water applications, this typically means using metals in the design. Metal additive manufacturing (AM) is a logical approach to achieve the needed design complexity. The most common approach in metal AM is selective laser melting. This approach utilizes a high-power laser beam to locally melt powder into a solid. 3D geometries are built through layer-by-layer melting of material. This approach to fabrication incorporates thermodynamic complexities such as extreme temperature gradients, simultaneous multiple material phases, gradient driven convection, rapid solidification, and occasional void formation. We will outline the complications that arise when using this technology to build one of the simplest acoustic meta-atoms: a thin disk. We show that thermal stresses can thermally warp the geometry, and microstructural heterogeneities can locally change material properties. These effects alter both the AM membrane’s macroscale (resonance) and microscale (crystal structure), significantly impacting the acoustic response. [Work Sponsored by the Office Of Naval Research.]