Metallic Lattice Structures Research Articles

Failure analysis of auxetic lattice structures under crush load

Lattice structures are promising materials in the terms of energy absorption, acoustic and vibrational damping, high strength-to-weight ratios and thermal management capabilities. In particular, auxetic lattice structures, among others, show high energy absorption performances due to their characteristic negative Poisson’s ratio. In this study, it is aimed to compare auxetic deformation mechanisms under quasi-static crush loads. Ti6Al4V tensile test specimens were produced with Electron Beam Melting (EBM) Additive Manufacturing (AM) technology. Moreover, a constitutive equation was derived and calibrated according to tensile results. The calibrated data were used to generate non-linear computational crush models including elastoplastic material data, damage initiation criterion, damage evaluation law and element deletion. The computational models are utilized for optimum topology design and mechanical performance prediction of different auxetic cells, including anti-tetrachiral, hexachiral, re-entrant and honeycomb lattice structures that are prone to prematurely fail under crush loading conditions. Consequently, it was found that the chiral auxetic deformation mechanism experienced better energy absorption ability over re-entrant deformation mechanism for metallic lattice structures.

A Method of Improved Faster R-CNN for Detecting Internal Defect of Metal Lattice Structure Based on Super-resolution Reconstruction

A Method of Improved Faster R-CNN for Detecting Internal Defect of Metal Lattice Structure Based on Super-resolution Reconstruction

Incorporating defects into model predictions of metal lattice-structured materials

Metal lattice structures are optimized for high specific strength properties and are now customizable using additive manufacturing methods. However, many of these methods, like selective laser melting, can introduce defects such as porosity, parasitic material, and disconnected struts into the structure, which can negatively affect mechanical behavior. While there are many computational models used to predict the mechanical response of lattice-structured materials, most use an idealized structure and are often not robust enough to account for defects. In this study, metal lattice structures are characterized for defects and mechanically tested in compression. The defect types and distributions are characterized using x-ray micro-tomography and the tomography analyses is used in two different model predictions. First, in an equivalent continuum model, where the defects are used to predict the variability in mechanical properties, and second, in a finite element analysis, where the predicted stress-strain response for both realistic and idealized structures are compared. Investigating this further, a finite element analysis of an octet lattice quantifies the reduction in strength associated with disconnected struts and captures a dependence on the strut's orientation relative to the loading direction. Overall, incorporating defect information gleaned from tomography data improves predictions of mechanical properties by capturing a more realistic deformation response for lattice-structured material.

Compressive and Energy Absorption Properties of Pyramidal Lattice Structures by Various Preparation Methods.

Metallic three-dimensional lattice structures exhibit many favorable mechanical properties including high specific strength, high mechanical efficiency and superior energy absorption capability, being prospective in a variety of engineering fields such as light aerospace and transportation structures as well as impact protection apparatus. In order to further compare the mechanical properties and better understand the energy absorption characteristics of metal lattice structures, enhanced pyramidal lattice structures of three strut materials was prepared by 3D printing combined with investment casting and direct metal additive manufacturing. The compressive behavior and energy absorption property are theoretically analyzed by finite element simulation and verified by experiments. It is shown that the manufacturing method of 3D printing combined with investment casting eliminates stress fluctuations in plateau stages. The relatively ideal structure is given by examination of stress–strain behavior of lattice structures with varied parameters. Moreover, the theoretical equation of compressive strength is established that can predicts equivalent modulus and absorbed energy of lattice structures.

Multiaxial static strength of a 3D printed metallic lattice structure exhibiting brittle behavior

AbstractThis paper focuses on numerical the prediction of multiaxial static strength of lattice structures. We analyze a body‐centered cubic cell printed with Selective Laser Melting in AlSi10Mg aluminum alloy. Parent material is experimentally characterized, and the Gurson‐Tveergard‐Needleman (GTN) damage model is calibrated to predict failure in numerical simulations. The GTN model is used to predict failure of the lattice structures exhibiting brittle localized fracture, and it is validated through static tests. The results of experimental tension/compression monotonic tests on lattice samples are compared with the results of numerical simulations performed on as‐built geometry reconstructed by X‐ray computed tomography, showing a good correlation. Combining the damage model with computational micromechanics, multiaxial loading conditions are simulated to investigate the effective multiaxial strength of the lattice material. Yielding and failure loci are found by fitting a batch of points obtained by some multiaxial loading simulations. A formulation based on the criterion proposed by Tsai and Wu (1971) for anisotropic materials provides a good description of yielding and failure behavior under multiaxial load. Results are discussed, with a specific focus on the effect of as‐built defects on multiaxial strength, by comparing the resistance domains of as‐manufactured and as‐designed lattices.

Compressive Behaviour of Additively Manufactured Lattice Structures: A Review

Additive manufacturing (AM) technology has undergone an evolutionary process from fabricating test products and prototypes to fabricating end-user products—a major contributing factor to this is the continuing research and development in this area. AM offers the unique opportunity to fabricate complex structures with intricate geometry such as the lattice structures. These structures are made up of struts, unit cells, and nodes, and are being used not only in the aerospace industry, but also in the sports technology industry, owing to their superior mechanical properties and performance. This paper provides a comprehensive review of the mechanical properties and performance of both metallic and non-metallic lattice structures, focusing on compressive behaviour. In particular, optimisation techniques utilised to optimise their mechanical performance are examined, as well the primary factors influencing mechanical properties of lattices, and their failure mechanisms/modes. Important AM limitations regarding lattice structure fabrication are identified from this review, while the paucity of literature regarding material extruded metal-based lattice structures is discussed.

Tuning of Static and Dynamic Mechanical Response of Laser Powder Bed Fused AlSi10Mg Lattice Structures through Heat Treatments

The use of additive manufacturing allows the production of complex designs, including metallic lattice structures, which combine lightness and good mechanical properties. Herein, the production of AlSi10Mg lattice structures by laser powder bed fusion is explored and consistent processing parameters are selected. Thereafter, it is demonstrated that heat treatments, specifically designed on the basis of the alloy's metallurgy, can be used to selectively induce different microstructural modifications and, consequently, finely tune the static and dynamic mechanical behavior of the aluminum lattice structures. In particular, removal of residual stresses results to be the dominant factor in allowing a smooth quasistatic compressive behavior and improving the structure's ability to absorb energy during collapse. On the contrary, the increase in ductility connected to the spheroidization of the Si network is shown to be of paramount importance in improving the structure's dynamic damping ability by allowing local plasticization.

Multiscale Analysis of High Damping Composites Using the Finite Cell and the Mortar Method

Metal lattice structures filled with a damping material such as polymer can exhibit high stiffness and good damping properties. Mechanical simulations of parts made from these composites can however require a large modeling and computational effort because relevant features such as complex geometries need to be represented on multiple scales. The finite cell method (FCM) and numerical homogenization are potential remedies for this problem. Moreover, if the microstructures are placed in between the components of assemblies for vibration reduction, a modified mortar technique can further increase the efficiency of the complete simulation process. With this method, it is possible to discretize the components separately and to integrate the viscoelastic behavior of the composite damping layer into their weak coupling. This paper provides a multiscale computational material design framework for such layers, based on FCM and the modified mortar technique. Its efficiency even in the case of complex microstructures is demonstrated in numerical studies. Therein, computational homogenization is first performed on various microstructures before the resulting effective material parameters are used in larger-scale simulation models to investigate their effect and to verify the employed methods.

Precipitation-induced transition in the mechanical behavior of 3D printed Inconel 718 bcc lattices

This work aims to investigate the effect of microstructure on the mechanical behavior of metallic lattice structures. With that purpose, Inconel 718 bcc lattices were manufactured by selective laser melting and their room temperature compressive behavior was compared in the as-built and aged conditions. A distinct transition from bending-dominated to stretch-dominated behavior takes place following the aging treatment as the as-built supersaturated solid solution (containing a minor fraction of nanoparticles) transforms into a microstructure with a comparatively large fraction of precipitates with dimensions spanning several hundred nanometers. Precipitation leads to a stiffer and stronger structure, with a higher capacity to absorb energy. The obtained mechanical property results are compared to the predictions of the Maxwell criterion and the Gibson-Ashby model for bcc lattices. It is shown that microstructure variations explain, at least partially, the low correlation between the bcc experimental data in the literature with the model predictions.

Mechanical interaction between additive-manufactured metal lattice structures and bone in compression: implications for stress shielding of orthopaedic implants

One of the main biomechanical causes for aseptic failure of orthopaedic implants is the stress shielding. This is caused by an uneven load distribution across the bone normally due to a stiff metal prosthesis component, leading to periprosthetic bone resorption and to implant loosening. To reduce the stress shielding and to improve osseointegration, biocompatible porous structures suitable for orthopaedic applications have been developed. Aim of this study was to propose a novel in-vitro model of the mechanical interaction between metal lattice structures and bovine cortical bone in compression. Analysis of the strain distribution between metal structure and bone provides useful information on the potential stress shielding of orthopaedic implants with the same geometry of the porous scaffold. Full density and lattice structures obtained by the repetition of 1.5 mm edge cubic elements via Laser Powder Bed Fusion of CoCrMo powder were characterized for mechanical properties using standard compressive testing. The two porous geometries were characterized by 750 μm and 1000 μm pores resulting in a nominal porosity of 43.5% and 63.2% respectively. Local deformation and strains of metal samples coupled with fresh bovine cortical bone samples were evaluated via Digital Image Correlation analysis up to failure in compression. Visualization and quantification of the local strain gradient across the metal-bone interface was used to assess differences in mechanical behaviour between structures which could be associated to stress-shielding. Overall stiffness and local mechanical properties of lattice and bone were consistent across samples. Full-density metal samples appeared to rigidly transfer the compression force to the bone which was subjected to large deformations (2.2 ± 0.3% at 15 kN). Larger porosity lattice was associated to lower stiffness and compressive modulus, and to a smoother load transfer to the bone. While tested on a limited sample size, the proposed in-vitro model appears robust and repeatable to assess the local mechanical interaction of metal samples suitable for orthopaedic applications with the bone tissue. CoCrMo scaffolds made of 1000 μm pores cubic cells may allow for a smoother load transfer to the bone when used as constitutive material of orthopaedic implants.

A feasibility study on additive manufactured hybrid metal/composite shock absorbers

Commonly adopted shock absorbers and, in general, crashworthy structural components, based on sandwich structural concepts and/or complex dumping mechanisms, are, generally, characterized by high volumes and significant additional mass. This research activity is focused on the investigation of the feasibility and effectiveness of novel thin additive manufactured hybrid metal/composite lattice structures as lightweight shock absorbing devices for application to structural key components in impact events. These hybrid structures would represent a real step beyond the state of the art of shock absorbers being characterized by an additive manufactured metal lattice core, able to maximize the absorbed energy by plastic deformations and, at the same time, by a composite skin/cohesive coating, fully integrated with the internal metal lattice structure, able to lower the global weight and increase the stiffness and strength of the shock absorber. First, an extensive explicit numerical activity has been performed finalised to the assessment of the mechanical behaviour of basic lattice Unit Cells configurations under impact conditions in shock-absorbing panels. The variation of the geometrical characteristics of the lattice cells have been taken into account by adopting a parametric Python routine in ABAQUS with a simplified FEM formulation based on beam and shell elements.Once identified the key features maximizing the energy absorption capabilities of the metallic core, several complex models with 3d solid element formulation have been developed.A final comparison between the hybrid configurations and the state of the art shock absorbing panels, demonstrated the effectiveness of the proposed lightweight hybrid configuration based on additive manufacturing techniques in terms of mass reduction, mechanical and energy absorption performances.

A Novel Rapid Manufacturing Process for Metal Lattice Structure.

A novel lattice structure manufacturing process is proposed in this article, which has the potential to overcome the manufacturing shortcomings of small-scale metal lattice structure. The proposed hierarchical process has four segments: Design, Bending, Dip, and Join (DBDJ). The proposed research use one-dimensional metallic wires/rods instead of powder, two-dimensional sheet, or liquid metal, which is highly transformative than the status quo. The topology-based design technique is focused to construct the lattice structure using a continuous thin rod. The layers are stacked in an additive manner to construct the three-dimensional lattice structure. The dip-coating meditate material transfer facilitates the node joining using transient liquid phase diffusion bonding, and hence, the manufacturing of the complex lattice structure is performed. The research framework provides a unique and holistic approach from design to manufacturing for realizing small-scale metallic lattice structure. A range of multiscale lattice structure is manufactured with the proposed DBDJ process. Very low relative density (∼3.8%) unit cell is achieved, and compressive tests demonstrate no failure at the joining node, which is reported in this article.

Additively manufactured Ti–6Al–4V thin struts via laser powder bed fusion: Effect of building orientation on geometrical accuracy and mechanical properties

Porous metal lattice structures have a very high potential in biomedical applications, setting as innovative new generation prosthetic devices. Laser powder bed fusion (L-PBF) is one of the most widely used additive manufacturing (AM) techniques involved in the production of Ti6Al4V lattice structures. The mechanical and failure behavior of lattice structures is strongly affected by geometrical imperfections and defects occurring during L-PBF process. Due to the influence of multiple process parameters and to their combined effect, the mechanical properties of these structures are not yet properly understood. Despite the major commitment to characterize and better comprehend lattice structures, little attention has been paid to the impact that single struts have on the overall lattice properties.In this work, the authors have investigated the tensile strength and fatigue behavior of thin L-PBF Ti6Al4V lattice struts at different building orientations (0°, 15°, 45°, and 90°). This investigation has been focused on the effect that microstructural defects (particularly porosity) and actual surface geometry (including surface texture and geometrical errors such as varying cross-section shape and size) have on the mechanical performances of the struts in relation to their building direction. The results have shown that there is a tendency, particularly for low printing angles, of fatigue life to decrease with decreasing of the building angle. This is mainly due to the surge in surface texture and loss in cross-sectional regularity. On the other hand, the monotonic tensile test results have shown a low sensitivity to these factors. The strut failure behavior has been examined employing dynamic digital image correlation (DIC) of tensile tests and scanning electron imaging (SEM) of the fracture surfaces.

Image-based numerical characterization and experimental validation of tensile behavior of octet-truss lattice structures

Abstract The production of lightweight metal lattice structures has received much attention due to the recent developments in additive manufacturing (AM). The design flexibility comes, however, with the complexity of the underlying physics. In fact, metal additive manufacturing introduces process-induced geometrical defects that mainly result in deviations of the effective geometry from the nominal one. This change in the final printed shape is the primary cause of the gap between the as-designed and as-manufactured mechanical behavior of AM products. Thus, the possibility to incorporate the precise manufactured geometries into the computational analysis is crucial for the quality and performance assessment of the final parts. Computed tomography (CT) is an accurate method for the acquisition of the manufactured shape. However, it is often not feasible to integrate the CT-based geometrical information into the traditional computational analysis due to the complexity of the meshing procedure for such high-resolution geometrical models and the prohibitive numerical costs. In this work, an embedded numerical framework is applied to efficiently simulate and compare the mechanical behavior of as-designed to as-manufactured octet-truss lattice structures. The parts are produced using laser powder bed fusion (LPBF). Employing an immersed boundary method, namely the Finite Cell Method (FCM), we perform direct numerical simulations (DNS) and first-order numerical homogenization analysis of a tensile test for a 3D printed octet-truss structure. Numerical results based on CT scan (as-manufactured geometry) show an excellent agreement with experimental measurements, whereas both DNS and first-order numerical homogenization performed directly on the 3D virtual model (as-designed geometry) of the structure show a significant deviation from experimental data.

Experimental study on the high-damping properties of metallic lattice structures obtained from SLM

Modern additive manufacturing technologies allow the creation of parts characterized by complex geometries that cannot be created using conventional production techniques. Among them the Selective Laser Melting (SLM) technique is very promising. By using SLM it is possible to create lightweight lattice structures that may fill void regions or partially replace bulk regions of a given mechanical component. As a consequence, the overall mechanical properties of the final component can be greatly enhanced, such as the resistance to weight ratio and its damping capacity against undesired vibrations and acoustic noise. Nevertheless, only a few research works focused on the characterization of the dynamic behavior of lattice structures, that were mainly investigated in the low frequency range or directly tested on some specific applications. In this work the dynamic behavior of lattice structures in the medium-high frequency range was experimentally investigated and then modelled. For this purpose, different types of lattice structures made of AlSi10Mg and AISI 316L were measured. Experimental modal analysis was performed on the obtained specimens in order to assess the influence of lattice material and unit cell geometry on their global dynamic behavior. Experimental results revealed that lattice structures have superior damping characteristics compared to solid materials having an equivalent static stiffness. Eventually, the classic Rayleigh model was found to be adequate - with some approximation - to explain the damping behavior of a generic lattice structure.

Macro-, meso- and microstructural characterization of metallic lattice structures manufactured by additive manufacturing assisted investment casting

Cellular materials are recognized for their high specific mechanical properties, making them desirable in ultra-lightweight applications. Periodic lattices have tunable properties and may be manufactured by metallic additive manufacturing (AM) techniques. However, AM can lead to issues with un-melted powder, macro/micro porosity, dimensional control and heterogeneous microstructures. This study overcomes these problems through a novel technique, combining additive manufacturing and investment casting to produce detailed investment cast lattice structures. Fused filament fabrication is used to fabricate a pattern used as the mold for the investment casting of aluminium A356 alloy into high-conformity thin-ribbed (~ 0.6 mm thickness) scaffolds. X-ray micro-computed tomography (CT) is used to characterize macro- and meso-scale defects. Optical and scanning electron (SEM) microscopies are used to characterize the microstructure of the cast structures. Slight dimensional (macroscale) variations originate from the 3D printing of the pattern. At the mesoscale, the casting process introduces very fine (~ 3 µm) porosity, along with small numbers of (~ 25 µm) gas entrapment defects in the horizontal struts. At a microstructural level, both the (~ 70 μm) globular/dendritic grains and secondary phases show no significant variations across the lattices. This method is a promising alternative means for producing highly detailed non-stochastic metallic cellular lattices and offers scope for further improvement through refinement of filament fabrication.

A Methodology for Computed Tomography-Based Non-Destructive Geometrical Evaluations of Lattice Structures by Holistic Strut Measurement Approach

Abstract The presence of lattice structures is increasing in the manufacturing domain especially in the air/spacecraft and biomedical applications due to their advantages of high strength-to-weight ratios, energy absorption, acoustic and vibrational damping, etc. Dimensional accuracy of a lattice structure is one of the most important requirements to meet the desired functionality as there could be significant deviations in the as-produced part from the designed one. Evidently, an approach (non-destructive) to evaluate the dimensional accuracy of all the elements and eventually the lattice quality is of great significance. X-ray computed tomography (CT) has emerged as a promising solution in the field of industrial quality control over the last few years due to its non-destructive approach. In this work, we propose a methodology for geometrical evaluations of a lattice structure by measuring the deviation in the shape and size of its strut elements holistically. The acquired CT data of the complete lattice are extracted in the form of a point cloud and then segmented and stored as a single strut element with unique identification so that measurements can be performed on the strut individually. As demonstrated with a metallic BCCz-type lattice structure, the methodology helps in critical evaluation of its quality and the correlation with spatial position of the individual struts; e.g., the lattice exhibits large variations of shape among the inclined struts while the vertical struts possess consistency in their shape.

3D metal lattice structure manufacturing with continuous rods

In this paper, a new possibility of fabricating a metal lattice structure with a continuous rod is demonstrated. A multi-layer, periodic, and aperiodic lattice structure can be manufactured with a continuous thin rod by bending it with a repetitive pattern. However, joining their nodes are challenging and an important problem to solve. This paper is investigating the joining of nodes in a loose lattice structure by delivering materials through the dipping process. Both liquid state (epoxy) and solid-state (inorganic particles) joining agents are considered for polymer–metal and metal–metal bonding, respectively. Liquid Carrier Systems (LCS) are designed considering their rheological behavior. We found 40% solid loading with the liquid carrier system provides sufficient solid particles transfer at dipping and join the lattice node using transient liquid phase bonding (TLP). 3D metal lattice structures are constructed, and their mechanical properties are investigated. The lattice structure shows comparable strength even with smaller relative density (< 10%). The strength and elastic modulus of all the fabricated samples decreases with the increase in cell size, which is consistent with the traditional wisdom.

Hybrid materials based on polymers-filled AM steel lattices with energy absorption capabilities

Metal lattice structures fabricated by additive manufacturing (AM) processes combined with polymers provide hybrid materials with unexpected mechanical properties. In particular, this article investigates the increased capability of absorbing elastoplastic energy of the composite material, which is up to 75% higher than the sum of energies absorbed by each constituent material. Furthermore, the fabrication process to obtain reliable and repeatable hybrid components by filling the 316 L metal lattice with polymer is introduced and validated.

Mechanical Properties of Metallic Additive Manufactured Lattice Structures according to Relative Density

Mechanical Properties of Metallic Additive Manufactured Lattice Structures according to Relative Density