The research on metamaterials has made significant progresses thanks to hierarchical and multi-porous architectures [1]. These architectures opened up new perspectives in terms of the mechanical properties (materials with negative Poisson’s ratio) and the control of acoustic wave propagation (negative refraction, perfect absorption). The aim of this study is to build Ni-20wt.%Cr architectural structures by laser powder bed fusion (L-PBF) additive manufacturing in order to obtain innovative mechanical and acoustic properties.

The first axis of this study is devoted to build architectural metal diffusers for acoustic absorption by metal L-PBF. The diffusers are spheres with cylindrical pores parallel to the build direction.
The second axis focuses on the study of the acoustic and mechanical properties of the metamaterial. The acoustic properties of the diffuser are determined by diffusion and absorption measurements (Figure 1(a)). Regarding the macroscopic mechanical properties, the mechanical strength of the structure is assessed using compression tests.

A third axis is devoted to the elementary deformation mechanisms of the thin metallic parts of the diffuser, in relation to the manufacturing conditions and the resulting microstructure. Thin walls (range thickness from 0.2 mm (Figure 1(b)) to 2 mm (Figure 1(c))) are produced by L-PBF for various processing parameters. The mechanical behaviour of thin walls is studied by tensile tests. The main mechanical properties of the alloy are next correlated to microstructure [2] (grain size and dendritic cells, crystallographic texture, inter-dendritic precipitation (Figure 1(d)), residual stresses) which can evolve with the build thickness. The aim of this work is to observe if a size effect exists concerning the mechanical properties of thin parts, and its potential consequences on the mechanical integrity of the metamaterial.

References
[1] C. Lagarrigue, J.-P. Groby, O. Dazel, V. Tournat, Applied Acoustics, 2016, 102, 49-54.
[2] E. Hug, M. Lelièvre, C. Folton, A. Ribet, M. Martinez-Celis, C. Keller Materials Sciences & Engineering A, 2022, 834, 142625.