Metallic Lattice Structures Research Articles

Influence of microstructure on stainless steel 316L lattice structures fabricated by electron beam and laser powder bed fusion

In this work, three-dimensional stainless steel 316L re-entrant lattice structures were fabricated by two mainstream powder bed fusion (PBF) techniques, namely electron beam PBF (EB-PBF) and laser PBF (L-PBF). Different grain morphology and crystallographic textures were found in the EB-PBF and L-PBF samples, which significantly influenced their mechanical properties through microscopic deformation. The EB-PBF and L-PBF samples achieved energy absorption capacities of 627.4 mJ/mm3 and 834.8 mJ/mm3, respectively, at a lattice relative density of ∼24%. The EB-PBF sample exhibited equiaxed and elongated grains, while elongated grains were primarily observed in the L-PBF sample. The dominant deformation mechanism of the EB-PBF sample was obtained through dislocation. In contrast, the dislocations trapped inside the solidification cellular walls and deformation-induced twinning were the dominant deformation mechanisms for the L-PBF sample, which contributed to its superior compressive strength and energy absorption capacities. This work provides insights into the enhancement of the mechanical properties of the additively manufactured metallic lattice structures through microstructural control.

Laser-Based Additive Manufacturing and Characterization of an Open-Porous Ni-Based Metallic Glass Lattice Structure (Ni60Nb20Ta20).

Due to their amorphous structure, metallic glasses exhibit remarkable properties such as high strength, hardness, and elastic strain limit. Conversely, they also exhibit high susceptibility to brittle fracture, making them less qualified for the use as monolithic structural components. Therefore, they may be preferably used as the reinforcing phase in hybrid materials combined with ductile matrix materials. Especially metal matrix composites with interpenetrating structures are suitable. This requires an open-porous structure of the metallic glass. In the study at hand, an open-porous lattice structure was manufactured from metallic glass powder (Ni60Nb20Ta20) by laser powder bed fusion. A parameter study was carried out with various scanning strategies to manufacture a mechanically stable lattice structure while maintaining the amorphous structure of the metallic glass. Thus, X-ray diffraction measurements were conducted to validate the parameter study. A stable lattice structure with a largely amorphous structure was successfully achieved with a scanning strategy of single scanned lines and a rotation of 90° for each layer. However, nanocrystallization of 7% occurred in the heat-affected zones formed between the individual printed layers during reheating. Conducting compression tests, a compressive modulus of 18 GPa and a maximum strength of 90 MPa in 0°-direction were achieved. In 90°-direction, no compressive modulus could be determined but compressive strength resulted in 15 MPa. Performing nanoindentation with a Young's modulus of 195.1 GPa and Vickers hardness of HVIT = 956.1 was achieved for the printed bulk metallic glass alloy. The resulting lattice structure was further characterized by differential scanning calorimetry for thermal behavior.

On the Technological Feasibility of Additively Manufactured Self‐Supporting AlSi10Mg Lattice Structures

The capability to design and manufacture metal lattice structures is today one of the most promising targets of powder bed fusion technologies. Not only additively manufactured lattices offer great lightweighting possibilities but they also open the way to tailored and graded mechanical response. To best capitalize on this opportunity, research effort is first needed to assess the feasibility of reticular structures and to quantify the expected deviations from the nominal geometry, as a function of the cell topology and dimensions. Notwithstanding the inherent suitability of additive processes to complex shapes, this paper proposes a more exact definition of the technological boundaries for body‐centered cubic lattices, showing to what extent specific dimensional ratios, as well as a self‐supporting cell structure, can be favorable to minimize the deviation from the nominal reticulum in terms of dimensions, density, and presence of defects.

Effect of stress state on the mechanical behavior of 3D printed porous Ti6Al4V scaffolds produced by laser powder bed fusion

Metallic lattice structures have interconnected porosity for promoting osseointegration and stiffness comparable to bone ideal for minimizing stress shielding. These benefits have caused a widespread proliferation of porous metal scaffolds in orthopedic implants. Once implanted, these devices experience complex stress states under physiological loading. To design these structures to mimic the mechanical behavior of bone, their performance under these conditions must be understood. This study explores the effects of stress state on the mechanical behavior of three lattice structures across a range of clinically relevant porosities. The scaffolds were fabricated via 3D printing Ti6Al4V using LBPF. The gyroid exhibited more efficient mechanical performance at equivalent porosity than the other structures for all loading modes. Stress state has a statistically significant impact on the strength of the scaffolds. This study provides the first dataset on the impact of stress state on the fracture of 3D printed biomimetic scaffolds for orthopedic applications.

Mechanical characterization of additively-manufactured metallic lattice structures with hollow struts under static and dynamic loadings

Inspired by the natural hollow structures, periodic lattice structures composed of hollow struts and spheres were designed and fabricated by selective laser melting (SLM) process with 316L stainless steel. Two architecture configurations with Body-Centered Cubic (BCC) and Face-Centered Cubic (FCC) symmetry were taken into consideration. Finite element (FE) simulations based on representative volume element (RVE) models and cell-assembly models were conducted to investigate the elastic response and large deformation behavior of the hollow lattice materials, respectively. Afterwards, compression experiments were carried out on an electronic universal machine and a drop hammer (DH) system to explore the quasi-static and dynamic mechanical response of the lattice specimens. The complete deformation evolutions of the lattice samples under different loading velocities were captured through high-resolution photography and inspected by the digital imaging correlation (DIC) analysis. Both the experimental research and numerical simulations demonstrated that the hollow beam cross-sections contributed to the stable crushing response of the proposed lattice structures under either quasi-static or dynamic compression. Accordingly, the post-yield behavior of the tested lattice structures was quite smooth without any fluctuations. Meanwhile, the specific strength and energy absorption of the hollow lattice structures were found to be superior to many existed lattice materials with solid struts, and comparable with the reported triply periodic minimal surface (TPMS) lattice structures. Finally, the effect of the geometric characteristic parameters on the specific mechanical properties of the lattice structures was discussed according to the supplemental analysis by numerical simulations.

Effect of Unit Cell Height on the Ballistic Performance of the Body-Centered Lattice Structures

Lattice structures, produced by metal additive manufacturing technology, can be a potential alternative in armor applications, which are important parts of defense technologies due to their shock wave damping, energy absorption and light-weight properties. Despite the fact that the protection of metal lattice structures against explosives has been frequently investigated in the literature, their perforation performance is rarely studied. In this research, numerical ballistic penetration tests were carried out with Johnson-Cook strength and failure model parameters by using LS-DYNA software. AlSi10Mg alloy was chosen as a lattice material, which has high energy absorption ability. Both width and length were chosen as 4 mm for the body-centered lattice structure, while eight different cell height (3, 4, 5, 6, 7, 8, 9 and 15 mm) were used as unit cell parameters. The results show that the ballistic performance of lattice structures could be improved by optimizing the unit cell height for the body-centered lattice structures.

2D Numerical Simulation of Auxetic Metamaterials Based on Force and Deformation Consistency

This work showcases a novel phenomenological method to create predictive simulations of metallic lattice structures. The samples were manufactured via laser powder bed fusion (LPBF). Simulating LPBF-manufactured metamaterials accurately presents a challenge. The printed geometry is different from the CAD geometry the lattice is based on. The reasons are intrinsic limitations of the printing process, which cause defects such as pores or rough surfaces. These differences result in material behavior that depends on the surface/volume ratio. To create predictive simulations, this work introduces an approach to setup a calibrated simulation based on a combination of experimental force data and local displacements obtained via global Digital Image Correlation (DIC). The displacement fields are measured via Finite Element based DIC and yield the true local deformation of the structure. By exploiting symmetries of the geometry, a simplified parametrized simulation model is created. The simulation is calibrated via Response Surface Methodology based on nodal displacements from FE-DIC combined with the experimental force/displacement data. This method is used to create a simulation of an anti-tetrachiral, auxetic structure. The transferability and accuracy are discussed, as well as the possible extension into 3D space.

Fracture behavior under compression loading of surface-cleaned metallic lattice structures

Fracture behavior under compression loading of surface-cleaned metallic lattice structures

LPBF Manufactured Functionally Graded Lattice Structures Obtained by Graded Density and Hybrid Poisson's Ratio.

The additive manufacturing (AM) of innovative lattice structures with unique mechanical properties has received widespread attention due to the capability of AM processes to fabricate freeform and intricate structures. The most common way to characterize the additively manufactured lattice structures is via the uniaxial compression test. However, although there are many applications for which lattice structures are designed for bending (e.g., sandwich panels cores and some medical implants), limited attention has been paid toward investigating the flexural behavior of metallic AM lattice structures with tunable internal architectures. The purpose of this study was to experimentally investigate the flexural behavior of AM Ti-6Al-4V lattice structures with graded density and hybrid Poisson’s ratio (PR). Four configurations of lattice structure beams with positive, negative, hybrid PR, and a novel hybrid PR with graded density were manufactured via the laser powder bed fusion (LPBF) AM process and tested under four-point bending. The manufacturability, microstructure, micro-hardness, and flexural properties of the lattices were evaluated. During the bending tests, different failure mechanisms were observed, which were highly dependent on the type of lattice geometry. The best response in terms of absorbed energy was obtained for the functionally graded hybrid PR (FGHPR) structure. Both the FGHPR and hybrid PR (HPR) structured showed a 78.7% and 62.9% increase in the absorbed energy, respectively, compared to the positive PR (PPR) structure. This highlights the great potential for FGHPR lattices to be used in protective devices, load-bearing medical implants, and energy-absorbing applications.

Strut formation control and processing time optimization for wire arc additive manufacturing of lattice structures

Lightweight aluminum alloy lattice structures have broad application prospects in energy absorption, heat insulation, vibration isolation, etc. Additive manufacturing (AM) has been increasingly applied to fabricating lattice structures due to its efficient and flexible technical characteristics. Wire arc additive manufacturing (WAAM), featuring high productivity and low cost, is gradually considered a desirable choice for rapid prototyping of medium to large-scale freeform metallic lattice structures. However, due to the higher thermal conductivity of aluminum alloys, the forming of struts is more easily affected by the interlayer temperature. The control and prediction of strut formation during the fabricating process is the challenge facing the current industry. Simultaneously, there is also an urgent need to improve its processing efficiency. Therefore, this paper describes the strut-based finite element model for numerical simulation of the WAAM thermal process of the ER 4043 aluminum alloy lattice structure. The FE model is calibrated and validated through temperature measurements with thermocouples and a thermal imager. The calibrated FE model was employed to simulate the manufacturing process of the lattice structure. A novel fabrication sequence planning strategy was proposed to make the interlayer temperature distribution uniform so as to optimize the processing time while ensuring stable strut formation.

Three-dimensional assembled dual-material lattice with tailorable thermal expansion: Design method, modeling, and testing

The lightweight dual-material metal lattice structure with the tailorable coefficient of thermal expansion (CTE) has attracted increasing attention for aerospace applications. However, previous work on these metal structures was mainly directed to theoretical design with the limited experimental demonstration in planar geometries because it is challenging to fabricate three-dimensional (3D) metal lattices with highly complex geometric connections. In this article, we propose a node assembly dual-material lattice design method enabling the experimentally demonstrated 3D designs with the tailorable CTE. A theoretical model of the thermal expansion and mechanical properties is provided to link the thermo-mechanical properties to the configuration and base materials. Based on the proposed design method and the theoretical model, we present three geometrical layouts with experimentally demonstrated CTEs of 34.77ppm/°C, -16.21ppm/°C, and 0.05ppm/°C, respectively. Notably, the achieved CTE of 0.05ppm/°C has never been reported in previous studies. In addition, the proposed assembled lattice can be extended periodically. Based on the demonstrations, we expect that the proposed method is promising for structurally efficient lattices design with high dimensional stability in aerospace engineering.

Investigation of the relationship between flexural modulus of elasticity and functionally graded porous structures manufactured by AM

There is an increased interest to produce open porous metallic lattice structures with regular unit cell architectures by additive manufacturing processes, which were impossible or difficult to manufacture using conventional techniques. In this study, the physical and mechanical properties of functionally graded porous structures were investigated to establish a relationship between porosity and elastic modulus of the cellular structure. Three different unit cell structures of diamond, cubic, and octahedroid and three different strut thicknesses of 0.3 mm, 0.5 mm, and 0.7 mm were designed for the experimental and theoretical study. A functionally graded porous structure was formed by using three different unit cell sizes (1.8 mm, 2 mm, and 2.2 mm) in each model. The specimens were produced by laser powder bed fusion (L-PBF) using Ti‐6Al‐4V powder and subjected to a three-point bending test after heat treatment. A mathematical model was created to determine the modulus of elasticity of the porous structures. Experimental results were compared with the theoretical calculations. As a result of this study, a correlation was established between the flexural modulus of elasticity and the porosity of the three different cell structures.

Non-linear dynamic finite element analysis of micro-strut lattice structures made by laser powder bed fusion

The development of additive manufacturing technologies enables the production of a new type of porous materials for the absorption of mechanical energy. These are, for example, metallic lattice structures produced by laser powder bed fusion. The structures can be made from a wide range of alloys, achieve high specific energy absorption, and can be manufactured as hybrid parts with conventional bulk components. To effectively develop lattice structures, it is necessary to complement experimental tests with simulations using the finite element method (FEM) performed under conditions of increased loading velocities. Therefore, this study focuses on the development of the FEM modelling strategy that reflects the effect of strain rate sensitivity of the base material (SS316L) and the most significant geometrical imperfections of the manufacturing process. The strain rate is reflected by the Cowper-Symonds constitutive law, which parameters are determined by the dynamic tensile test on Hopkinson split bars. The imperfections are captured by optical digitalization. The significance of the Cowper-Symonds parameters and geometric imperfections are studied independently, whereas agreement with the experiment is observed. Tests are performed for several lattice structures with different strut orientations and velocities to evaluate the versatility of the proposed approaches. A good correlation between computational and experimental results in terms of energy absorption is found for structures with an experimentally determined strut diameter and the proposed Cowper-Symonds input parameters.

A comparative investigation on the mechanical properties and cytotoxicity of Cubic, Octet, and TPMS gyroid structures fabricated by selective laser melting of stainless steel 316L

Metallic lattice structures can be fabricated by selective laser melting (SLM) with purposefully designed pores and controlled pore sizes that can bio mimic the natural bone, providing adequate mechanical and biological support for the patients. Strut-based structures, like Cubic, Octet; and sheet-based structures, like triply periodic minimal surface (TPMS) gyroid, have been studied extensively in the past. However, it lacks enough comparative study on the mechanical properties and cytotoxicity among these structures. Therefore, Cubic, Octet, and TPMS gyroid of Stainless steel 316 L (SS316L) are designed, manufactured, and characterized at 40/50/60% relative densities in this study. Moreover, the flowability, density characteristics, and cytotoxicity of SS316L powder are validated to ascertain its suitability for 3D printing and implant application. Based on refining the Gibson-Ashby model, it is possible to predict or design the mechanical properties via adjusting the relative densities. The results indicate these structures demonstrated appropriate Young's modulus and outstanding biocompatibility.

Optimization of Dual-Fuel Combustion Synthesis for Rapid Formation of Solution-Processed Metal-Oxide Thin-Film Transistors

Solution processing of metal-oxide semiconductors has received significant attention in various fields of electronics owing to its advantages such as simple fabrication process, large-area scalability, and facile stoichiometric tunability. However, the conventional sol–gel route requires a relatively long annealing time to obtain a low-defect film with high density and sufficient amount of metal–oxygen–metal bonding state, which prevents implementation in cost-effective continuous manufacturing. Here, we report rapid formation of solution-processed oxide semiconductors by employing a dual-fuel-based solution combustion synthesis route. In particular, by optimizing the ratio of dual fuels of acetylacetone and 1,1,1-trifluoro-acetylacetone (molar ratio of 7:3), high-performance indium–gallium–zinc oxide (IGZO) thin-film transistors (TFTs) could be fabricated at 350 °C with the annealing time as short as 5 min (In:Ga:Zn = 0.68:0.1:0.22). Based on spectroscopic analysis, it was found that the dual fuels enabled rapid formation of the metal–oxygen–metal lattice structure with low defective oxygen bonding states. The IGZO TFTs fabricated with an optimized fuel ratio exhibited average field-effect mobilities of 1.11 and 3.69 cm2 V–1 s–1 with annealing times of 5 and 20 min, respectively (averaged in 9∼12 devices). Also, in the case of the 5 min annealed device, the threshold voltage was −0.48 ± 1.96 V, showing enhancement-mode operation. Furthermore, the device showed good stability against both positive gate bias stress and negative gate bias stress conditions with small threshold voltage shifts of −1.28 and – 1.28 V in 5760 s, respectively.

Ti-6Al-4V Octet-Truss Lattice Structures under Bending Load Conditions: Numerical and Experimental Results

Metal lattice structures produced by means of additive techniques are attracting increasing attention thanks to the high structural efficiency they can offer. In order to achieve the maximum structural performance, numerical design techniques are used almost exclusively, thus based on CAE-FEM codes. Nevertheless, the current manufacturing facilities do not yet guarantee defect-free components, and, therefore, such imperfections need to be introduced in the numerical models too. The present work aims to describe a FE modelling technique for lattice structures based on the use of beam and shell elements, and therefore with a very reduced computational cost. The main structural parameters, such as weight and stiffness and strength, are used to drive the model calibration. Simple mathematical relationships, based on Experimental-CAD-FEM comparisons, are provided to estimate the error related to the numerical model in a simple and fast way. The validation was performed by three-point bending test on flat specimen with regular octet-truss microstructure both with and without external skin. The test articles were produced in Ti6Al4V and by means of the electron beam melting (EBM) technology. The results obtained are in excellent agreement with the experimental ones, indeed the maximum error is about 3%. All this indicates these methodologies as possible tools for evaluating the performance of such kinds of high-tech structures.

Synchronous involvement of topology and microstructure to design additively manufactured lattice structures

This article presents a methodical approach to optimize microstructure (e.g., the crystallographic texture) and topology (e.g., unit cell and struts) concurrently to improve the mechanical properties of additively manufactured metallic lattice structures (AMLS), i.e., yield strength and plastic flow stress. Full-field elasto-viscoplastic Fast Fourier Transform (EVP-FFT) crystal plasticity (CP) simulations are employed to determine the optimal microstructure. The CP model parameters were calibrated to measured macroscopic stress–strain response and microstructural data for polycrystalline samples of additively manufactured (AM) Inconel 718 with solution treated and aged (STA) microstructure. Since the crystallographic orientation of the constituent single-crystal grains with respect to the loading direction has a significant impact on the mechanical behavior of the material, stress projection factor analysis was used to determine four candidate textures to explore in for a given unit cell topology. Full-field crystal plasticity simulations were used to determine macroscale yield surface parameters for each of the considered textures, thereby enabling macroscale lattice unit cell simulations that account for the underlying microstructure. The calibrated microstructure-dependent yield surfaces are used to investigate the effect of different microstructures on the mechanical response of different LS topologies with the same relative density. The results show that in a texture with <111> crystallographic direction, parallel to the loading direction, the tensile and compressive yield strength are 20% and 58% larger, respectively compared to the AM STA IN718 texture. Furthermore, when this texture is used in conjunction with the Rhoctan topology, the results demonstrate 50% improvement in both the yield strength and modulus of elasticity relative to previously optimized AMLS designs that did not directly account for microstructure. This simultaneous consideration of microstructure and topology during optimization, thus, significantly enhances the structural integrity of the AMLS.

Compressive performance and energy absorption of additively manufactured metallic hybrid lattice structures

A hybrid lattice cell configuration composed of an octet cell and a rhombic dodecahedron (RD) cell was proposed, aiming to combine the advantages of the bending-dominated and stretching-dominated structures. In order to examine the mechanical performance and energy absorption of the hybrid lattice, quasi-static compression experiments were conducted on the selective laser melting (SLM) printed samples made from 316 L stainless steel. The global deformation evolutions of the hybrid lattices were captured by a digital camera. Meanwhile, numerical simulations were carried out based on the cell assembly finite element (FE) models to reveal the localized deformation modes of the specimens tested in the experiments. Subsequently, parametric analysis was conducted according to the validated FE results to discuss how the overall relative density and volume fraction of each sub-cells influence the mechanical properties of the hybrid lattice structures. According to the experimental and numerical results, the hybrid structure exhibits smoother post-yielding response than the octet lattice and higher initial strength than the RD lattice, which contributes to its superior energy absorption capacity. The present study also demonstrates that the energy absorption abilities of the hybrid lattice structures can be further improved without sacrificing its load bearing capacity by tuning their mesoscopic architectures such as the ratio of each individual component. The experimental results also indicate that the hybrid design can be adopted to enhance the resistance of the lattice material to the process-induced geometric imperfections.

Helical and Spiral Antennas as Electric Small Antennas

The simplest form of a classical antenna is the dipole antenna, which is essentially two pieces of wire placed end to end with a feed point in the middle. The length of this antenna is typically half the wavelength of the signal that is being received or transmitted.&nbsp; An “electric small” antenna is defined as an antenna of much shorter dimensions than the wavelength of the signals it is designed for. Small electric antennas have an advantage when space is the most essential factor. Satellites and mobile communication apparatuses for example can use small antennas in order to free up more space for other components. The problem with classical electric small antennas is that their bandwidth and radiation efficiency shrink as they get shorter. Although such antennas have been in use for decades, they remain difficult to design and limited in their applicability. However, the classic approach of studying metal antennas using only Maxwell equations limits the real internal nature of them as metal lattice structures and thus as quantum wells of free electrons. When interpreted this way, free electrons are obeying the Schrodinger wave equation and this view can give new ideas for designing effective curvilinear electric small antennas.&nbsp; Such a class of electric small helical and spiral antennas is proposed in the following paper.

Fatigue failure prediction in lattice structures through numerical method based on de-homogenization process

Metal lattice structures from additive manufacturing (AM) processes are promising solutions for the design of lightweight components and therefore several strategies for their static modeling are available. However, the high concentration of notches, combined to the surface roughness typical of AM as-built conditions, make lattice structures very vulnerable to fatigue failures. Furthermore, the evaluation of stress and strain in cellular materials is very challenging due to the geometrical complexity and the associated computational heaviness of numerical models. In this paper, the authors present a numerical method based on the homogenization and de-homogenization processes to determine the expected fatigue lifetime of the lattice component. The method requires limited computational effort by limiting the estimation to the most loaded cell, which is considered as representative volume element (RVE) to establish the multi-axial fatigue loads.