Design for additive manufacturing and post processing of cellular lattice structure

Traditional manufacturing which is subtractive in nature generally has been associated with more material and energy wastage than required. Final products manufactured exceeds the level of performance as materials are added in similar concentration even in non-stress region. Hence human made load bearing structures are dense solids like steel, concreate, glass, etc., but nature design its structure with cellular solids like cedarwood, cork, trabecular bone, etc. This is difficult to achieve through traditional manufacturing techniques due to manufacturing constraint associated with subtractive manufacturing. Hence, a very promising technology to fabricate and replicate the natural cellular structure in design is Additive Manufacturing (AM). Design for additive manufacturing and post processing (DfAM&PP) is an important concept for designing a product with additive manufacturing. This study is on design for additive manufacturing and post processing with lattice structure and classification of this lattice structure. Material extrusion process is used for additive manufacturing of lattice structure. Presently two most often used AM post processing steps are support removal and surface finish which accounts most of the AM technology. Support removal process is most tedious and challenging process which accounts more than half of the total process time in additive manufacturing. Further during support removing the printed parts often gets damaged and further surface finish operation is employed to smoothen the surface which was earlier supported by the support structure. Design of supportless lattice structure could solve the post processing challenge and bring high speed printing concept. Scanning electron microscope (SEM) is done to analyse the defect and material testing is done with uniaxial compression test for strength analysis

Energy Absorption and Stiffness of Thin and Thick-Walled Closed-Cell 3D-Printed Structures Fabricated from a Hyperelastic Soft Polymer

This study analyses the energy absorption and stiffness behaviour of 3D-printed supportless, closed-cell lattice structures. The unit cell design is bioinspired by the sea urchin morphology having organism-level biomimicry. This gives rise to an open-cell lattice structure that can be used to produce two different closed-cell structures by closing the openings with thin or thick walls, respectively. In the design phase, the focus is placed on obtaining the same relative density with all structures. The present study demonstrates that closure of the open-cell lattice structure enhances the mechanical properties without affecting the functional requirements. Thermoplastic polyurethane (TPU) is used to produce the structures via additive manufacturing (AM) using fused filament fabrication (FFF). Uniaxial compression tests are performed to understand the mechanical and functional properties of the structures. Numerical models are developed adopting an advanced material model aimed at studying the hysteretic behaviour of the hyperelastic polymer. The study strengthens design principles for closed-cell lattice structures, highlighting the fact that a thin membrane is the best morphology to enhance structural properties. The results of this study can be generalised and easily applied to applications where functional requirements are of key importance, such as in the production of lightweight midsole shoes

Design and additive manufacturing of closed cells from supportless lattice structure

In nature, mesoscopic or microscopic cellular structures like trabecular bone, wood, shell, and sea urchin, can have high load-carrying capacity. These cellular structures with diverse shapes, forms and designs can be mainly classified into open and closed cell cellular structures. It is difficult to replicate these natural complex lattice structures with traditional manufacturing, but additive manufacturing (AM) technology development has allowed engineers and scientists to mimic these natural structures. Fabricating close cell lattice structures is still considered difficult due to the support structure within the lattices. This paper evaluates a novel way of fabricating a close cell lattice structure with a material extrusion process. The design eliminates the need for support structures and the subsequent post-processing required to remove them. A shell-shaped close cell lattice structure bio-mimicking a sea urchin shape was introduced for the load-bearing structure application. The mechanical properties of the proposed structure, including stiffness, deformation behavior and energy absorption, were compared with those of benchmarked honeycomb and open cell sea urchin (SU) lattice structures of the same density. SU lattice structures and honeycomb periodic lattice structures with varied sizes but the same morphology and fixed density were designed and printed in polylactic acid material (PLA). Their physical characteristics, deformation behavior, and compressive properties were investigated experimentally and via finite element analysis. The effect of the unit cell size on mechanical properties was studied and discussed, and the rankings of better performances were drawn. A possible application of the closed cell is for fabricating the load bearing structure; it can also be encapsulated within a fluid to impart strength and damping characteristics

In situ metal organic framework (ZIF-8) and mechanofusion-assisted MWCNT coating of LiFePO/C composite material for lithium-ion batteries

LiFePO4 is one of the industrial, scalable cathode materials in lithium-ion battery production, due to its cost-effectiveness and environmental friendliness. However, the electrochemical performance of LiFePO4 in high current rate operation is still limited, due to its poor ionic- and electron-conductive properties. In this study, a zeolitic imidazolate framework (ZIF-8) and multiwalled carbon nanotubes (MWCNT) modified LiFePO4/C (LFP) composite cathode materials were developed and investigated in detail. The ZIF-8 and MWCNT can be used as ionic- and electron-conductive materials, respectively. The surface modification of LFP by ZIF-8 and MWCNT was carried out through in situ wet chemical and mechanical alloy coating. The as-synthesized materials were scrutinized via various characterization methods, such as XRD, SEM, EDX, etc., to determine the material microstructure, morphology, phase, chemical composition, etc. The uniform and stable spherical morphology of LFP composites was obtained when the ZIF-8 coating was processed by the agitator [A], instead of the magnetic stirrer [MS], condition. It was found that the (optimum of) 2 wt.% ZIF-8@LFP [A]/MWCNT composite cathode material exhibited outstanding improvement in high-rate performance; it maintained the discharge capacities of 125 mAh g−1 at 1C, 110 mAh g−1 at 3C, 103 mAh g−1 at 5C, and 91 mAh g−1 at 10C. Better cycling stability with capacity retention of 75.82% at 1C for 100 cycles, as compared to other electrodes prepared in this study, was also revealed. These excellent results were mainly obtained because of the improvement of lithium-ion transport properties, less polarization effect, and interfacial impedance of the LFP composite cathode materials derived from the synergistic effect of both ZIF-8 and MWCNT coating materials

MoO3 nanoparticle coatings on high-voltage 5 V LiNi0.5 Mn1.5 O4 cathode materials for improving lithium-ion battery performance

To reduce surface contamination and increase battery life, MoO3 nanoparticles were coated with a high-voltage (5 V) LiNi0.5 Mn1.5 O4 cathode material by in-situ method during the high-temperature annealing process. To avoid charging by more than 5 V, we also developed a system based on anode-limited full-cell with a negative/positive electrode (N/P) ratio of 0.9. The pristine LiNi0.5 Mn1.5 O4 was initially prepared by high-energy ball-mill with a solid-state reaction, followed by a precipitation reaction with a molybdenum precursor for the MoO3 coating. The typical structural and electrochemical behaviors of the materials were clearly investigated and reported. The results revealed that a sample of 2 wt.% MoO3-coated LiNi0.5 Mn1.5 O4 electrode exhibited an optimal electrochemical activity, indicating that the MoO3 nanoparticle coating layers considerably enhanced the high-rate charge–discharge profiles and cycle life performance of LiNi0.5 Mn1.5 O4 with a negligible capacity decay. The 2 wt.% MoO3-coated LiNi0.5 Mn1.5 O4 electrode could achieve high specific discharge capacities of 131 and 124 mAh g−1 at the rates of 1 and 10 C, respectively. In particular, the 2 wt.% MoO3-coated LiNi0.5 Mn1.5 O4 electrode retained its specific capacity (87 mAh g−1) of 80.1% after 500 cycles at a rate of 10 C. The Li4 Ti5 O12 /LiNi0.5 Mn1.5 O4 full cell based on the electrochemical-cell (EL-cell) configuration was successfully assembled and tested, exhibiting excellent cycling retention of 93.4% at a 1 C rate for 100 cycles. The results suggest that the MoO3 nano-coating layer could effectively reduce side reactions at the interface of the LiNi0.5 Mn1.5 O4 cathode and the electrolyte, thus improving the electrochemical performance of the battery system

Effect of single-walled carbon nanotube sub-carbon additives and graphene oxide coating for enhancing the 5 V LiNi0.5Mn1.5O4 cathode material performance in lithium-ion batteries

High-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is a promising cathode material for next-generation lithium-ion batteries (LIBs), but its poor cycle performance has impeded its commercialization. In this study, we developed highly stable LNMO cathode materials having an octahedral morphology through a solid-state high-energy ball-mill–cum–spray-drying method. We also developed a novel strategy for modifying this cathode material with two kinds of carbon materials, thereby improving the electrochemical cycling performance. Introducing single-walled carbon nanotubes (SWCNTs) as a sub-carbon conductive additive during the slurry preparation process improved the conductivity of electrons between the particles of the cathode material. The LNMO electrode modified with the SWCNT sub-carbon additives exhibited an average Coulombic efficiency of 99.4% after 500 cycles at 1C, compared with 98.9% for the pristine LNMO-based electrode. Furthermore, we used a wet-chemical method to coat graphene oxide (GO) onto the post-sintered LNMO cathode material to act as a protective layer, preventing corrosion induced by HF in the electrolyte. The capacity retention of the GO-coated LNMO electrode after 500 cycles at 1C (91.8%) was higher than that of the pristine LNMO (52.5%). The corresponding dual-modification strategy, combining the SWCNTs and GO, provided LNMO cathode materials exhibiting superior rate performance and cyclability, with an average Coulombic efficiency of 99.3% and capacity retention of 92.9% after 500 cycles at 1C. Thus, the LNMO cathode materials prepared in this study possessed excellent electrochemical properties favoring their marketability, applicability, and competitiveness for application in high-voltage LIBs

Investigation of Compression and Buckling Properties of a Novel Surface-Based Lattice Structure Manufactured Using Multi Jet Fusion Technology

Lattice structures possess many superior properties over solid materials and conventional structures. Application-oriented lattice structure designs have become a choice in many industries, such as aerospace, automotive applications, construction, biomedical applications, and footwear. However, numerical and empirical analyses are required to predict mechanical behavior under different boundary conditions. In this article, a novel surface-based structure named O-surface structure is designed and inspired by existing Triply Periodic Minimal Surface morphologies in a particular sea urchin structure. For comparison, both structures were designed with two different height configurations and investigated for mechanical performance in terms of compression, local buckling, global buckling, and post-buckling behavior. Both simulation and experimental methods were carried out to reveal these aforementioned properties of samples fabricated by multi jet fusion technology. The sea urchin structure exhibited better mechanical strength than its counterpart, with the same relative density almost two-folds higher in the compressive response. However, the O-surface structure recorded more excellent energy absorption and flexible behavior under compression. Additionally, the compression behavior of the O-surface structure was progressive from top to bottom. In contrast, the sea urchin structure was collapsed randomly due to originated cracks from unit cells’ centers with local buckling effects. Moreover, the buckling direction of structures in long columns was also affected by keeping the relative density constant. Finally, based on specific strength, the O-surface structure exhibited 16-folds higher specific strength than the sea urchin structure

Buckling and Post-Buckling Behavior of Uniform and Variable-Density Lattice Columns Fabricated Using Additive Manufacturing

Lattice structures are known for their high strength-to-weight ratio, multiple functionalities, lightweight, stiffness, and energy absorption capabilities and potential applications in aerospace, automobile, and biomedical industry. To reveal the buckling (global and local) and post-buckling behavior of different lattice morphologies, both experimental and simulation-based studies were carried out. Additionally, a variable-density lattice structure was designed and analyzed to achieve the optimal value of critical buckling load. Latticed columns were fabricated using polyamide 12 material on multi jet fusion 3D printer. The results exhibited that the buckling in lattice columns depends on the distribution of mass, second moment of inertia I, diameter and position of vertical beams, number of horizontal or inclined beams, and location and angle of the beams that support the vertical beams. The number of horizontal and inclined beams and their thickness has an inverse relation with buckling; however, this trend changes after approaching a critical point. It is revealed that vertical beams are more crucial for buckling case, when compared with horizontal or inclined beams; however, material distribution in inclined or horizontal orientation is also critical because they provide support to vertical beams to behave as a single body to bear the buckling load. The results also revealed that the critical buckling load could be increased by designing variable density cellular columns in which the beams at the outer edges of the column are thicker compared with inner beams. However, post-buckling behavior of variable density structures is brittle and local when compared with uniform density lattice structures