Hollow Struts Research Articles

Characterization of the structural features of Ti-6Al-4V hollow-strut lattices fabricated by laser powder bed fusion

Hollow-strut metal lattices are novel cellular materials. Compared to their solid-strut counterparts, their powder bed fusion additive manufacturing (PBF-AM) features remain largely uninvestigated. This work focuses on characterizing the hollow-strut internal channel and nodal profiles, the defects and microstructures of the hollow-strut thin walls, and the inner surface conditions of the LPBF-manufactured body-centred cubic (BCC) Ti-6Al-4V hollow-strut lattices with different relative densities. BCC lattices are selected because of the low inclination angle (35.26°) of their constituent struts. These low-inclination hollow struts are designed using a recent model developed for PBF of inclined solid struts, together with considerations to prevent powder occlusion and ensure easy removal of powder particles. Detailed characterization indicates that our design considerations resulted in high-quality hollow-strut BCC Ti-6Al-4V lattices, which provide useful design insights for PBF-AM of hollow-strut metal lattices. In terms of microstructure, the Ti-6Al-4V hollow-strut thin walls (≤ 0.5 mm thick) exhibited different microstructures compared with Ti-6Al-4V solid struts, due to the heat accumulation effect in the inner channels. The implications are discussed for in-situ microstructure control.

Improving mechanical properties of lattice structures using nonuniform hollow struts

In contrast to constant-strut lattices, the introduction of innovative strut designs has the potential to enhance the mechanical properties of lattice structures. This study presents a Bézier curve-based nonuniform section design for body-centered cubic lattices with hollow struts (BCCH). Periodic boundary conditions are applied to the unit cells, and a finite element (FE) numerical homogenization method is employed to assess their elastic properties. A comprehensive dataset is generated through the FE model, which is subsequently divided into training, validation, and testing sets. The coordinates of the Bézier curve control points serve as inputs to deep learning networks, which are trained on the training dataset. Relative density and elastic properties are treated as two distinct networks, with the validation set utilized to prevent overfitting. The objective function consists of two weighted components: one aims to maximize the relative Young's modulus, while the other ensures that the relative density achieves a specified value. An evolutionary algorithm is employed to optimize the objective function, with variations in the control point coordinates constrained to specific ranges. By leveraging the fast inference ability of the deep learning model, the stiffness and orientation-dependent mechanical properties can be efficiently tailored. Our results demonstrate that the optimized structures demonstrate superior stiffness (+92.8 %) and distributed stress field compared to the benchmark lattice. The design method also enables tailoring of specific mechanical properties, including isotropic elasticity. 3D-printed lattice designs were fabricated and compression tests confirmed agreement with simulation results in terms of stiffness. Additionally, the optimized designs exhibit superior strength (+99.6 %) and toughness compared to the benchmark lattices.

Fabrication of SiC reticulated porous ceramics with dense struts by in-situ generation of SiC

SiC reticulated porous ceramics have been widely utilized as filters in the smelting industry. To address the problem of hollow ceramic struts and improve the strength and thermal shock resistance of SiC reticulated porous ceramics, a one-step coating process was carried out using slurries with SiC, metallic Si and various carbon sources (carbon spheres, carbon black, and flake graphite). The effect of the carbon sources on the rheological behavior of the coating slurries was investigated, as well as the relationship between the rheological behavior of the slurries and the structure of the green bodies. The hollow struts and flaws resulting from the combustion of polymer templates were then filled with SiC whiskers formed by the reaction between Si and carbon sources during heating. Among the carbon sources, the carbon spheres significantly stimulated the growth of the SiC whiskers, which filled the voids and flaws. After heating at 1550 °C, the SiC reticulated porous ceramics prepared with Si and carbon spheres reached a porosity of 80%, compressive strength of 1.85 MPa, and residual compressive strength rate of 81%. In industrial tests, the fabricated SiC reticulated porous ceramics were used as filters to effectively remove inclusions in molten copper. Importantly, the structure of the SiC filters remained intact after use.

Tridimensional characterization of open cells and hollow strut cavities from SiC and ZrO2 foams: A study accomplished with open-source software tools

The characterization of ceramic foams via X-ray microtomography imaging method is often restricted to a general overview of samples, either to perform qualitative or quantitative analyses. To assess the foam with a focus on some specific components of its structure, such as hollow struts’ cavities and open cells, the generalized characterizations must be overcome. This work presents image analysis methodologies, based on open-source software, tools, and plugins, to achieve the aforementioned characterizations. SiC and ZrO2 foams samples were analyzed and some of the results were compared with those accomplished with pieces of commercial software. The results show good agreement between open-source and commercial software applications, indicating that the presented methodologies can be freely applied by any researcher to analyze their foams.

Production of replicated porous materials based on SiC: Microstructure, mechanical behavior & thermal shock resistance

High strength, thermal shock resistance, and suitable thermal conductivity of silicon carbide foams has led to their widespread applications. Doing that, in this research, SiC foams were produced using the slurry contains SiC as the main material with bentonite, alumina, zirconia, and silica sol. This slurry covered the PU foam one to five times during the replication process. Afterward, samples were sintered at three low sintering temperatures, i.e. 900, 1000, and 1100 °C. The microstructure, phases, compressive strength, and thermal shock resistance of these multi-layered porous structures were studied. It was found that the number of coating layers and sintering temperature influenced the type, extent, and size of defects. Applying multi-layer coating changed the morphology of the worst defect of the replication method, i.e. triangular hollow struts, to circular ones, which impressively enhanced the mechanical strength and thermal shock resistivity. Comparing nine types of samples indicated that the maximum strength was obtained in a five-layer coated sample sintered at 1000 °C, and not at the maximum sintering temperature. Notably, burning off bentonite and silica soles results in blister generation at 1100 °C, which reduced strength, and thermal shock resistance. The optimum strength of the porous sample after exposure to 11 sequences of thermal shock test was obtained in a four-layer-coated sample sintered at 1000 °C, which highlighted the critical strut thickness in porous structures.

In-situ formed SiC whisker strengthening of silicon carbide reticulated porous ceramics based on 3D printed lattices template

The SiC reticulated porous ceramics (RPCs) fabricated via sponge replica method usually showed a poor thermal shock resistance due to their randomly distributed skeleton structure and hollow struts. The present work reports a simple approach to prepare SiC RPCs with 3D lattices structure using the replica of printed resin, followed by pre-heated in coke bed and vacuum infiltration. The increased Si content and flake graphite addition promoted the formation of SiC whiskers network structure on the surface and matrix of SiC preform after pre-heated at 1400 °C. As SiC preforms were applied by vacuum infiltration and sintering, the three-layer struts included dense coating, SiC skeleton layer and filling layer were generated. Furthermore, in-situ SiC whiskers eliminated the coating cracks, also optimized the microstructure of the SiC matrix, thereby enhancing the thermal shock resistance and strength of SiC RCPs, whose strength increased from 0.36 MPa to 1.53 MPa, also the residual strength retention rate increased from 0 % to 75.2 % after three cycles of water- quenching.

Interpenetrating a Hollow Microlattice Metamaterial Enables Efficient Sound-Absorptive and Deformation-Recoverable Capabilities.

Owing to the pervasive noise and crash hazards, tough microlattices with sound absorption capabilities are sought-after. However, typical truss microlattices are unable to fulfill this requirement. To overcome this, herein, we report a new design strategy for truss microlattices via introducing the concept of interpenetrating hollow struts, which thereby constitutes a novel interpenetrating hollow microlattice metamaterial (IHMM). The design is based on interweaved unit cells of a hollow octet-truss (HOT) and a hollow rhombic dodecahedron-like (HRDL) truss. Experimentally demonstrated, the IHMM displays a synergistic gain in both sound absorption and mechanical properties that substantially surpass that of its constituent lattices. High sound absorption coefficients (α > 0.99) and broad half-absorption (3.2 kHz) are observed, with the average α being 110.6 and 85.3% higher than those of the HOT and HRDL, respectively. The sound absorption mechanism is attributed to the presence of cascaded Helmholtz resonance, which is then fully elucidated by impedance and damping analyses. The IHMM also outperforms its constituents in specific strength. A simultaneous high strength (4 MPa) and recoverability (80% strain) and pseudo-reusability are also observed. The mechanisms behind the mechanical reinforcement and exceptional robustness are thoroughly revealed. Overall, this work offers insights into developing multifunctional engineering materials.

Additive manufacturing and performance of bioceramic scaffolds with different hollow strut geometries

Additively manufactured hollow-strut bioceramic scaffolds present a promising strategy towards enhanced performance in patient-tailored bone tissue engineering. The channels in such scaffolds offer pathways for nutrient and cell transport and facilitate effective osseointegration and vascularization. In this study, we report an approach for the slurry based additive manufacturing of modified diopside bioceramics that enables the production of hollow-strut scaffolds with diverse cross-sectional forms, distinguished by different configurations of channel and strut geometries. The prepared scaffolds exhibit levels of porosity and mechanical strength that are well suited for osteoporotic bone repair. Mechanical characterization in orthogonal orientations revealed that a square outer cross-section for hollow struts in woodpile scaffolds gives rise to levels of compressive strength that are higher than those of conventional solid cylindrical strut scaffolds despite a significantly lower density. Finite element analysis confirms that this improved strength arises from lower stress concentration in such geometries. It was shown that hollow struts in bioceramic scaffolds dramatically increase cell attachment and proliferation, potentially promoting new bone tissue formation within the scaffold channel. This work provides an easily controlled method for the extrusion-based 3D printing of hollow strut scaffolds. We show here how the production of hollow struts with controllable geometry can serve to enhance both the functional and mechanical performance of porous structures, with particular relevance for bone tissue engineering scaffolds.

Geometry effect on mechanical properties and elastic isotropy optimization of bamboo-inspired lattice structures

Inspired by the geometry of bamboo, this study proposes a novel bamboo-inspired body-centered cubic (B-BCC) lattice structure consisting of tapered and hollow struts. Using representative volume elements applied with periodic boundary conditions, the mechanical properties and deformation behaviors of the B-BCC lattice structures are thoroughly evaluated by considering a large number of combinations of geometric parameters and volume fractions. Results reveal that the geometric parameters highly influence the deformation behavior of the B-BCC lattice structures under uniaxial compression (e.g, from bending- to stretching-dominated) but little under shear load. For this reason, tunable elastic modulus across a broad range can be realized via adjusting the geometric parameters and elastic isotropy can be obtained across all volume fractions. On this basis, a combination of artificial neural network and elastic isotropy optimization is proposed to obtain the isotropic B-BCC lattice structures with superior elastic modulus. The optimization results show that the elastic modulus of the isotropic B-BCC lattice structures increased by 271.24–1335 % and 17.72–43.63 %, as compared to the original BCC and isotropic hollow BCC lattice structures, respectively. Finally, the multi-layer simulation and compression experiments are applied to validate the optimization results. Good agreements are observed comparing the numerical and experimental results, demonstrating the effectiveness of the proposed bamboo-inspired design and optimization method for lightweight applications with desired properties.

Architected lightweight, sound-absorbing, and mechanically efficient microlattice metamaterials by digital light processing 3D printing

ABSTRACT It is of significance, but still remains a key challenge, to attain excellent sound-absorbing and mechanical properties in a material simultaneously. To overcome this challenge, herein, novel multifunctional microlattice metamaterials based on a hollow truss-plate hybrid design are proposed and then realised by digital light processing 3D printing. Quasi-perfect sound absorption ( > 0.999) and broadband half-absorption have been measured. The sound-absorbing capacity is verified to be based on the designed cascaded Helmholtz-like resonators. Physical mechanisms behind the absorptive behaviours are fully revealed by numerical analyses. The present microlattices also display superior modulus and strength to the conventional cellular materials and modified microlattices, which is attributed to their near-membrane stress state of the plate architecture. The mechanically robust behaviour of the present microlattices in turn derives from the hollow struts. This work represents an effective approach for the design and engineering of multifunctional metamaterials through 3D printing.

Macroscale Fabrication of Lightweight and Strong Porous Carbon Foams through Template-Coating Pair Design.

Manufacturing of low-density-high-strength carbon foams can benefit the construction, transportation, and packaging industries. One successful route to lightweight and mechanically strong carbon foams involves pyrolysis of polymeric architectures, which is inevitably accompanied by drastic volumetric shrinkage (usually >98%). As such, a challenge of these materials lies in maintaining bulk dimensions of building struts that span orders of magnitude difference in length scale from centimeters to nanometers. This work demonstrates fabrication of macroscale low-density-high-strength carbon foams that feature exceptional dimensional stability through pyrolysis of robust template-coating pairs. The template serves as the architectural blueprint and contains strength-imparting properties (e.g., high node density and small strut dimensions); it is composed of a low char-yielding porous polystyrene backbone with a high carbonization-onset temperature. The coating serves to imprint and transcribe the template architecture into pyrolytic carbon; it is composed of a high char-yielding conjugated polymer with a relatively low carbonization-onset temperature. The designed carbonization mismatch enables structural inheritance, while the decomposition mismatch affords hollow struts, minimizing density. The carbons synthesized through this new framework exhibit remarkable dimensional stability (≈80% dimension retention; ≈50% volume retention) and some of the highest specific strengths (≈0.13GPa g-1 cm3 ) among reported carbon foams derived from porous polymer templates.

Microstructure and mechanical properties of open-cell Ni-foams with hollow struts and NiO oxide layers

Based on electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS), open-cell Ni foams fabricated by electroplating on a removable template were investigated. Hollow struts or cavities inside the Ni foams were observed, implying traces of a removable template. The crystallographic orientations and grain structures of the Ni foam struts were also examined. Individual grains of Ni foams covered the struts completely through the thickness direction. The crystallographic orientations of the Ni foam struts display a random distribution. Σ3 twinning boundaries were also observed in the Ni matrix. During a two-hour heat treatment at 1273 K under atmospheric condition, refined NiO layers clearly formed on the surface of the Ni matrix. The overall grain sizes of the NiO are smaller than those of the Ni matrix. The obvious crystallographic relationship between the Ni matrix and the NiO layers was observed at the high misorientation angles. Ni foams with high porosity revealed a stress drop and NiO layers that formed during atmospheric annealing caused a predominant stress drop. Young’s modulus and the hardness of the Ni matrix and NiO layers were measured from a Ni foam cell structure consisting of nodes, by means of nano-indentation tests.

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.

Additive manufacturing of Ti-6Al-4V horizontal hollow struts with submillimetre wall thickness by laser powder bed fusion

Efficient lattice unit cell topologies often necessitate the manufacture of unsupported horizontal struts. The fabrication of unsupported horizontal struts by laser powder bed fusion (LPBF) is inherently challenging, where the associated research data is limited for solid or dense cross-sections while no data exists for hollow struts. By examining LPBF-fabricated Ti-6Al-4V hollow struts of various diameters and wall thicknesses over increasing span lengths, a robust LPBF additive manufacturability range has been established for Ti-6Al-4V hollow struts. To achieve successful fabrication, three laser scan strategies were explored, proposed, and systematically compared. Horizontal solid struts and vertical hollow struts of equivalent diameters (and wall thickness) were similarly assessed over the same span lengths as comparators to quantify the manufacturability. This research provides previously unavailable design data, required for LPBF additive manufacturing of lattices with horizontal strut elements.

The compressive response of additively-manufactured hollow truss lattices: an experimental investigation

The mechanical response of additively-manufactured hollow truss lattices is experimentally investigated under quasi-static compression testing. Exploiting the recent developments in the Fusing Deposition Modelling (FDM) technique, two families of lattices have been fabricated, obtained as tessellation in space of octet-truss and diamond unit cells. Four specimens for each family of lattices have been designed with prescribed relative density, selecting different inner-to-outer radius ratios r/R of their hollow struts. Compression experiments prove that mechanical properties and failure mechanisms of hollow truss lattices are significantly dependent on the r/R ratio. In particular, a shift from quasi-brittle to ductile mechanical response at increasing r/R values has been revealed for the octet-truss lattice, leading to a stable collapse mechanism and increased energy absorption capacity. On the other hand, a more compliant behaviour has been observed in the diamond lattice response, with a monotonic improvement of mechanical properties as a function of the r/R ratio. Such results substantiate the potentialities of additively-manufactured hollow lattice structures as an attractive solution when lightweight, resistant and efficient energy absorption materials are required.Graphic

Competent F18 bioglass-Biosilicate® bone graft scaffold substitutes

Biosilicate® glass-ceramic (BioS) offers an attractive choice for the manufacturing of scaffolds because of their high bioactivity, non-toxicity, bactericidal activity and biodegradability. Despite these positive properties, Biosilicate® scaffolds that have been developed so far show low fracture strength, limiting their clinical application. For this reason, our aim was to increase their strength through vacuum infiltration of F18 bioactive glass. First, we calculated the maximum attainable theoretical compressive strength of a scaffold using the Ryshkewitch and Ashby-Gibson models. We show that for a total porosity of 80 %, σ0 = 250 MPa, and n = 5 the compressive strength estimated by both models is approximately 4.5 MPa. Afterward, the Biosilicate scaffolds were prepared using the foam replica technique and recoated several times with a F18 glass slurry to eliminate surface defects. Scanning Electron Microscopy examination showed that the F18 indeed helped to remove surface defects and partially infiltrated the hollow struts, significantly increasing their mechanical integrity. The F18-BioS scaffolds exhibited a total porosity of 82 %, an average cell size of 525 μm, and compressive strength of 3.3 ± 0.3 MPa, which is close to the predicted value and significantly higher than those of sole BioS scaffolds of a similar structure (< 0.1 MPa). These values are within the range of commercial scaffolds based on Hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP), having a considerable advantage of being more osteoinductive, angiogenic, and highly bactericidal. The F18-BioS scaffolds developed in this work thus have high potential for odontology or craniofacial surgeries that do not involve high load–bearing conditions.

Computer modeling of systematic processing defects on the thermal and elastic properties of open Kelvin-cell metamaterials

Open-cell metamaterials prepared by additive manufacturing or replica techniques are typically prone to processing defects resulting from limited resolution, strut cross-section variations or internal strut porosity. These defects are expected to cause deviations from the ideal (CAD-based or template-based) target microstructures and thus from the envisaged properties. This paper investigates some of these effects in a quantitative manner. Based on computer-generated open Kelvin-cell (tetrakaidecahedral) alumina-based metamaterials, the effective thermal conductivity and elastic constants, mainly Young’s modulus, are calculated in dependence of the voxel size, strut thinning and strut wall thickness. It is shown that the porosity dependence of smooth, straight and full struts agrees closely to the Gibson-Ashby prediction for open-cell foams, while limited resolution and strut thinning leads to property values that tend to be lower and hollow struts lead to higher property values. The Pabst-Gregorová cross-property relation gives an excellent prediction of the conductivity-modulus correlation in all cases.

Post-corrosion mechanical properties of absorbable open cell iron foams with hollow struts

In-depth analyses of post-corrosion mechanical properties and architecture of open cell iron foams with hollow struts as absorbable bone scaffolds were carried out. Variations in the architectural features of the foams after 14 days of immersion in a Hanks’ solution were investigated using micro-computed tomography and scanning electron microscope images. Finite element Kelvin foam model was developed, and the numerical modeling and experimental results were compared against each other. It was observed that the iron foam samples were mostly corroded in the periphery regions. Except for quasi-elastic gradient, other mechanical properties (i.e. compressive strength, yield strength and energy absorbability) decreased monotonically with immersion time. Presence of adherent corrosion products enhanced the load-bearing capacity of the open cell iron foams at small strains. The finite element prediction for the quasi-elastic response of the 14-day corroded foam was in an agreement with the experimental results. This study highlights the importance of considering corrosion mechanism when designing absorbable scaffolds; this is indispensable to offer desirable mechanical properties in porous materials during degradation in a biological environment.

Hybrid Auxetic Structures: Structural Optimization and Mechanical Characterization

With their increased energy absorption capacity, auxetic materials are perfectly fit to develop new, enhanced lightweight crash absorbers for cars. Herein, the mass distribution along the struts is optimized via finite element analysis with a parameterized optimization. Four different auxetic unit cells are taken from the literature and their struts parameterize, the models simulate, and the mass specific energy absorption capacity optimizes. The two models with the highest energy absorption capacity are then selected for experimental investigation and produced by additive manufacturing from a polymer. To further enhance the mechanical properties, the specimens are electrochemically coated with nickel and the polymer molten out by pyrolysis. Those Ni/polymer hybrids are subjected to quasistatic and dynamic impact experiments. Only a small strain rate sensitivity can be detected under dynamic loading, namely, a higher plastic collapse and higher plateau stress. The hollow struts are folding instead of bending, which render them much weaker than predicted by the simulation. In conclusion, it is possible to improve existing crash absorber elements with tailored auxetic hybrid structures. They absorb higher amounts of energy without changing their stiffness under dynamic loading while saving mass and cost.

Using ductile cores for enhancing the mechanical performance of hollow strut β-TCP scaffolds fabricated by digital light processing

Hybrid scaffolds consisting of a three-dimensional network of hollow struts of β-tricalcium phosphate (β-TCP) with cores filled with polycaprolactone (PCL) were fabricated. For the development of the ceramic structures the additive manufacturing technique known as Digital Light Processing (DLP) was employed. After sintering, cores were infiltrated with molten PCL by suction. The enhancement of the scaffolds’ compressive strength and toughness upon infiltration was evaluated through uniaxial compression tests. The effect of the core diameter on the mechanical performance was also analyzed. Moreover, a comprehensive finite element analysis was carried out to broaden the study on the effect of core/shell respective diameters and to analyze the role of the modulus of the core material on the strength of the hybrid structure.