Design and Development of Cellular Structure for Additive Manufacturing

The demand for shorter product development time has resulted in the introduction of a new paradigm called Additive Manufacturing (AM). Due to its significant advantages in terms of cost effective, lesser build time, elimination of expensive tooling, design flexibility AM is finding applications in many diverse fields of the industry today. One of the recent applications of this technology is for fabrication of cellular structures. Cellular structures are designed to have material where it is needed for specific applications. Compared to solid materials, these structures can provide high strength-to-weight ratio, good energy absorption characteristics and good thermal and acoustic insulation properties to aerospace, medical and engineering products. However, due to inclusion of too many design variables, the design process of these structures is a challenge task. Furthermore, polymer additive manufacturing techniques, such as fused deposition modeling (FDM) process which shows the great capability to fabricate these structures, are still facing certain process limitations in terms of support structure requirement for certain category of cellular structures. Therefore, in this research, a computer-aided design (CAD) based method is proposed to design and develop hexagonal honeycomb structure (self-supporting periodic cellular structure) for FDM process. This novel methodology is found to have potential to create honeycomb cellular structures with different volume fractions successfully without any part distortion. Once designing process is complete, mechanical and microstructure properties of these structures are characterized to investigate effect of volume fraction on compressive strength of the part. Volume fraction can be defined as the volume percentage of the solid material inside the cellular structure and it is varied in this thesis by changing the cell size and wall thickness of honeycombs. Compression strength of the honeycomb structure is observed to increase with the increase in the volume fraction and this behavior is compared with an existing Wierzbicki expression, developed for predicting compression properties. Some differences are noticed in between experimentally tested and Wierzbicki model estimated compressive strength. These differences may be attributed to layer by layer deposition strategy and the residual stress inherent to the FDM-manufacturing process. Finally, as a design case study, resin transfer molding (RTM) mold internally filled with honeycomb is designed and tested instead of the regular FDM mold. Results show that our proposed methodology has the ability to generate honeycomb structures efficiently while reducing the expensive build material (Mold) consumption to near about 50%. However, due to complex geometry of the honeycomb pattern the build time increased about 65% compare to solid FDM mould. In this regard, FDM tool-path can be optimized in future, so that overall product cost will be minimized. As per the author’s knowledge, this design methodology will have a greatest contribution towards creating sustainable and green product development. Using this, in future, expensive build material and production time can also be minimized for some hydroforming and injection molding applications

Investigation of the properties of alkali-activated slag mixes involving the use of nanoclay and nucleation seeds for 3D printing

This study investigated the properties of alkali activated slag (AAS) binders formulated for extrusion‐based 3D printing. The fresh properties of AAS mixes were tailored through the use of nanoclay (NC) and nucleation seeds. The printability criteria employed were the ease of extrusion (extrudability) and the stability of the layered structure (buildability). Introduction of 0.4% NC in AAS mixes led to improved thixotropic properties due to the flocculation effect, which accounted for the extrudability and shape fidelity of the binder. Inclusion of 2% hydromagnesite seeds in this mix design provided additional nucleation sites for the increased precipitation of hydrate phases, resulting in denser microstructures. This enhanced the hydration reaction and improved the structural build-up rate necessary for large-scale 3D printing. The developed AAS mix containing 0.4% NC and 2% hydromagnesite seeds was used in the printing of an actual 3D structure to demonstrate its feasibility to be used in 3D printing applications

A combined experimental-numerical framework for residual energy determination in spent lithium-ion battery packs

The present research proposes a combined framework that evaluates remaining capacity, material behavior, ions concentration of remaining metals, and current rate of chemical reactions of spent Li‐ion batteries accurately. Voltage, temperature, internal resistance, and capacity were studied during charging and discharging cycles. Genetic programming was applied on the obtained data to develop a model to predict remaining capacity. The results of experimental work and those estimated from model were found to be correlated, confirming the validation of model. Materials structure and electrochemical behavior of electrodes during cycles were studied by cyclic voltammetry, scanning electron microscopy, and energy dispersion spectrum

3D printing of high-volume fly ash mixtures for digital concrete construction

Digital concrete construction has recently become the subject of very rapidly growing research activities all over the world. It opens a new horizon and unlimited possibilities for the concrete industry, especially in terms of geometrical flexibility, reduction of manpower and costs, increased productivity, speed of construction, and sustainability. The potential of digital production of concrete is represented by exponentially increasing innovative projects and research, which are often addressed using the generalized term “3D-concrete-printing”. Recent advances in concrete printing have mainly focused on developing Portland cement (PC) based binders that are both extrudable and buildable. The production of PC is associated with many environmental challenges including the air pollution and devastation in land and resources. One of the promising measures is the development of sustainable cements, which can be achieved via two main pathways: (i) partial PC replacement with industrial by‐products and wastes and (ii) new cement formulations based on different chemistry with lower environmental impacts. This thesis is dedicated to material synthesis of two promising sustainable building materials such as (i) high volume fly ash (HVFA) cement and (ii) geopolymer based cement for extrusion-based 3D printing application. Rheology of fresh concrete is critically important in 3D printing process, that determines the material ability to be extrudable and buildable. With an aim of utilizing minimum 60-70% fly ash, HVFA cement was formulated while understanding the key parameters influencing yield stress, viscosity and thixotropy properties. Thixotropy was quantified using shear thinning and viscosity recovery protocols. In addition, structural-build up was measured in the dormant period, to control the buildability of printed structures. Considering the rheological requirements of a large-scale concrete printing, nanoclay was added as viscosity modifying agent (VMA) to improve the control mix performance.The nanoclay was found to improve the thixotropy but had no effect on structural-build up rate which limited the part buildability. To accelerate the build-up rate, micro silica fume was incorporated in the mix design and better buildability was achieved due to significant improvement in the strength development property. The rheology and reaction product of the final mix design were characterized to resolve the underlying mechanisms using laser scattering, isothermal calorimetry, X-ray Powder Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) techniques. In the second part of the thesis, fly ash based geopolymer was synthesized with improved rheological properties for 3D printing process. High volume of fly ash was supplemented with blast furnace slag and micro silica fume to provide adequate microstructural packing required for printability. Shape retention, which is related to viscosity recovery was found to be improved with silica fume addition up to 10-15% without much affecting the extrudability. Solution of potassium silicate was used as activator in different concentration (molar ratio) and solution-to-binder ratio to understand the geopolymer rheology based on yield stress and viscosity values. The activator solution was found to reduce the yield stress and enhance the cohesiveness, similar to the use of superplasticizer in conventional PC system. Therefore, nanoclay was again used to improve the printability of geopolymer while minimizing the viscous effect of activator solution. In contrast, powder (potassium) silicate activated system show rheological responses similar to those of HVFA cement system. XRD and FESEM were used to quantify the resultant geopolymer reaction product and relate it to the 28 days mechanical strength. In the last section, directional properties of both 3D printed HVFA and geopolymer samples were studied in terms of compression, flexural and tensile bond strength. Due to layering effect, 3D printed samples exhibited orthotropic properties with reduced bond strength with increasing inter-layer time gap. Effects of deposition speed and print height were also investigated, which concludes that the material build-up rate plays an important role in determining the final effect along with the printing parameters. Therefore, the build-up rate was accessed via early age mechanical strength testing, where the fresh concrete was uniaxially deformed to obtain green strength and stiffness. Based on the green strength development and deformation behaviour, print path and speed were optimized to build two large-scale 3D printed structures for demonstrating the proposed criteria and mix designs are suitable in practice. The contradictory challenge of 3D printing concrete i.e both pumpable as soft fluids and buildable as solid elements were solved in this thesis by understanding the rheology of two promising sustainable materials before, during and after the printing. Based on experimental observations, it is concluded that PC based plain mortars are deemed suitable for concrete printing while geopolymers need significant material tuning based on the printing process.Doctor of Philosoph

Experimental study on mix proportion and fresh properties of fly ash based geopolymer for 3D concrete printing

This paper presents the material design and fresh properties of geopolymer mortar developed for 3D concrete printing application. Unlike traditional casting, in 3D printing, extruded materials are deposited layer-by-layer to build complex architectural and structural components without the need of any formwork and human intervention. Extrudability, shape retention, buildability and thixotropic open time (TOT) are identified as critical early-age properties to characterize the 3D printable geopolymer material. Five different mix designs of geopolymer are tested in a systematic experimental approach to obtain a best printable mix and later it is used to print a 60-centimeter-tall freeform structure using a concrete gantry printer to validate the formulation.NRF (Natl Research Foundation, S’pore)Accepted versio

In-plane energy absorption characteristics of a modified re-entrant auxetic structure fabricated via 3D printing

Here, we present the in-plane energy absorption characteristics of modified re-entrant auxetic honeycombs realized via fused filament fabrication in conjunction with parametric analysis and geometry optimization. The influence and interaction effects of the geometrical parameters such as strut-length ratio and joint-angles on the stiffness, strength and energy absorption characteristics of modified re-entrant auxetic honeycombs were evaluated. Subsequently, Finite Element results obtained using ABAQUS/Explicit were corroborated with measured data. Deformation mode, stress-strain response and energy absorption behavior of an optimal re-entrant auxetic honeycomb were studied and compared with conventional re-entrant auxetic structure. Our modified auxetic structure reveals an 36% improvement in the specific energy absorption capacity. Our analysis indicates that due to the introduction of more nodes with low rotational stiffness, the failure strain of the modified re-entrant structure has increased resulting in improved energy absorption capacity

Investigation of the rheology and strength of geopolymer mixtures for extrusion-based 3D printing

This study presents the development of fly ash-based geopolymer mixtures for 3D concrete printing. The influence of up to 10% ground granulated blast-furnace slag (GGBS) and silica fume (SF) inclusion within geopolymer blends cured under ambient conditions was investigated in terms of fresh and hardened properties. Evolution of yield stress and thixotropy of the mixtures at different resting times were evaluated. Mechanical performance of the 3D printed components was assessed via compressive strength measurements and compared with casted samples. SF demonstrated a significant influence on fresh properties (e.g. recovery of viscosity), whereas the use of GGBS led to higher early strength development within geopolymer systems. The feasibility of the 3D printing process, during which rheology was controlled, was evaluated by considering extrusion and shape retention parameters. The outcomes of this study led to the printing of a freeform 3D component, shedding light on the 3D printing of sustainable binder systems for various building components

Extrusion and rheology characterization of geopolymer nanocomposites used in 3D printing

This study focused on the suitability of geopolymer mixes for extrusion-based additive manufacturing applications. Geopolymer mixes including alkaline potassium silicate activator with different molar ratios were prepared and evaluated for their rheological responses (e.g. yield stress, viscosity and thixotropy) that influenced their extrudability and buildability. Due to the low yield stress of geopolymers, nanoclay was introduced to enhance their thixotropy, which was assessed by structural breakdown and viscosity recovery tests. An activator-to-binder ratio of 0.35 and water-to-solid ratio of 0.30 were found to work well with a clay addition of 0.5% (i.e. by mass of total binder component). The addition of nanoclay led to an improvement in the rheological properties of geopolymer nanocomposites, which was attributed to the flocculation characteristic of the clay particles. A small-scale bathroom unit was printed by using the developed geopolymer nanocomposite, indicating the suitability of the proposed mix design to be used in 3D printing applications