PSI - Issue 53

S. Leonardi et al. / Procedia Structural Integrity 53 (2024) 327–337 S. Leonardi et al. / Structural Integrity Procedia 00 (2023) 000–000

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the past decades, many e ff orts have been made to design advanced metallic materials that combine lightweight with enhanced properties. Examples comprise low-density composites manufactured by liquid metal infiltration (Tarantino and Mortensen (2022)), metallic foams produced by replication (Conde et al. (2006)) as well as microcellular met als fabricated by templating techniques (Malgras et al. (2016)) and by 3D-printing processes (Schaedler and Carter (2016)). In particular, the advent of additive manufacturing technologies - coupled with progress in modern com putational tools - now enables the fabrication of highly porous materials in which the topology of the solid phase is precisely-engineered to achieve key functions and performances. Over the past two decades, architected cellular materials – as these materials are also called - have been the focus of intensive research enabling, e.g., to decouple intrisically correlated material properties such as strength vs. density (Schaedler and Carter (2016)). To yield attractive properties at minimum weigth, architected cellular materials utilize topology-optimized porous designs (Osanov et al. (2016)), which often consist of a periodic pattern of a unit-cell geometry built with strut elements. In work to date, lattice structures have been made out of any consituent material Meza et al. (2015); Zheng et al. (2014) and have been produced at any structural scale, from the centimeter (Gallien et al. (2022)) down to nanometer (Meza et al. (2015); Zheng et al. (2014)). Metallic microlattices are indeed among the lighest and sti ff est metallic materials demonstrated to date. Yet, their mechanical response is often impared by the non-negligible number of defects in the as-built part. Alongside metallurgical defects characterized by the presence of pores or cracks within the metal (Zhang et al. (2019); Zhang et al. (2019)), metallic cellular lattices produced by direct 3 D -printing processes, such as laser powder bed fu sion (LPBF), contain several topological defects associated e.g. with the variation of the strut diameter and waviness (Liu et al. (2017)). Topological defects in turn cause detrimental stress concentrations and eventually lead to a poor structural integrity of the 3 D -printed part (Liu et al. (2017); Latture et al. (2018)). To overcome this issue, architected materials with non-periodic pore features have been recently proposed and studied. By contrast to unit-cell based lat tices, random architected materials contain heterogeneous (i.e. disordered) pore features and are thus less-sensitive to symmetry-breaking topological defects. Examples of random architected porous materials explored to date comprise notably particulate architectures designed by computational homogenization (Tarantino et al. (2019)); (Zerhouni et al. (2019)), spinodoid topologies obtained by inverse-design (Kumar et al. (2020)) as well as Voronoi-like cellular microstructures generated via tessation (Martinez et al. (2022)) and via computational morphogenesis (Hooshmand Ahoor et al. (2022)). This work deals with cellular architectures with heterogeneous, yet precisely controlled, pore features. Notably, the materials of this work contain through-thickness circular pores randomly dispersed into a dense metallic matrix. Their porous architecture is generated numerically (Section 2.1) and then produced by LPBF additive manufacturing process using two di ff erent metallic powders (Section 2.2). Using image analysis, we first examine the influence of the LPBF processing parameters on the pore topology of the 3 D -printed parts (Section 3) and then use image-based Finite-Element (FE) simulations to quantify the role of topological imperfections on the resulting mechanical properties (Section 4).

2. Materials and Methods

This section describes the experimental and numerical methods used throughout the study. Notably, it starts with a brief description of the computer-design protocol used to generate the porous architectures, see Section 2.1. The latter are subsequently fabricated by means of LPBF additive manufacturing technology, whose relevant parameters employed for fabrication are reported in Section 2.2. Finally, the basic features of the image analysis protocol for the quantification of the topological defects together with the FE simulations framework are presented respectively in Section 2.3 and 2.4 .

2.1. Design strategy of the porous architectures

The architected cellular materials of this work contain random distributions of through-thickness pores heteroge neously dispersed into a dense metallic matrix. Their design strategy, extending from the computer generation to the LPBF fabrication, consists of four steps and is illustrated schematically in Figure 1. The design protocol starts with the generation of the random porous architecture. This is achieved by means of a random sequential absorption (RSA) algorithm, which is here applied to construct a two-dimensional unit cell containing random particle inclusions at a fixed surface fraction, Figure 1. To do so, the RSA algorithm relies on a straitforward working principle. The latter