The tool-less additive manufacturing (AM) or 3D printing processes (3DP) use incremental consolidation of feed-stock materials to construct part. The layer by layer AM processes can achieve spatial material distribution and desired microstructure pattern with high resolution. This unique characteristics of AM can bring custom-made form and tailored functionality within the same object. However, incorporating form and functionality has their own challenge in both design and manufacturing domain. This research focuses on designing manufacturable topology by marrying form and functionality in additively manufactured part using infill structure. To realize the goal, this thesis presents a systematic design framework that focuses on reducing the gap between design and manufacturing of complex architecture. The objective is to develop a design methodology of lattice infill and thin shell structure suitable for additive manufacturing processes. Particularly, custom algorithmic approaches have been developed to adapt the existing porous structural patterns for both interior and exterior of objects considering application specific functionality requirements. The object segmentation and shell perforation methodology proposed in this work ensures manufacturability of large scale thin shell or hollowed objects and incorporates tailored part functionality. Furthermore, a computational design framework developed for tissue scaffold structures incorporates the actual structural heterogeneity of natural bones obtained from their medical images to facilitate the tissue regeneration process. The manufacturability is considered in the design process and the performances are measured after their fabrication. Thus, the present thesis demonstrates how the form of porous structures can be adapted to mingle with functionality requirements of the application as well as fabrication constraints. Also, this work bridges the design framework (virtual) and the manufacturing platform (realization) through intelligent data management which facilitates smooth transition of information between the two ends. / National Science Foundation #OIA-1355466 / National Science Foundation-DMR- MRI #1625704 / National Institute of Health - COBRE: CDTSPC; Grant # P20GM109024 / US-DOT # 693JK31850009CAAP / Dept. of Commerce Research-ND, Award # 17-08-G-191 / CSMS, NDEPSCoR / NDSU Grand Challenge and Development Foundation

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The application of lattice structures have become increasingly popular as additive manufacturing (AM) opens up the possibility to manufacture complex configurations. However, modelling such structures can be computationally expensive. The following thesis has been conducted in order for the department of Structural Analysis, at SAAB in Järfälla, to converge with the future use of AM and lattice structures. An approach to model lattice structures using homogenization is presented where three similar methods involving representative volume element (RVE) have been developed and evaluated. The stiffness matrices, of the RVEs, for different sizes of lattice structures, comprising of BCC strut-based units, have been obtained. The stiffness matrices were compared and analysed on a larger solid structure in order to see the deformational predictability of a lattice-based structure of the same size. The results showed that all methods were good approximations with slight differences in terms of boundary conditions (BCs) at the outer edge. The comparative analyses showed that two of the three methods matches the deformational predictability. The BCs in all methods have different influences which makes it pivotal to establish the BCs of the structure before using the approach presented in this thesis. / Ökad implementering av gitterstrukturer i komponenter är ett resultat av utvecklingen inom additiv tillverkning. Metoden öppnar upp för tillverkning av komplexa strukturer med färre delmoment. Dock så uppkommer det svårigheter vid simulering av dessa komplexa strukturer då beräkningar snabbt tyngs ner med ökad komplexitet. Följande examensarbete har utförts hos avdelningen Strukturanalys, på SAAB i Järfälla, för att de ska kunna möta upp det framtida behovet av beräkningar på additivt tillverkade gitterstrukturer. I det här arbetet presenteras ett tillvägagångsätt för modellering av gitterstrukturer med hjälp av represantiva volymselement. Styvhetsmatriser har räknats fram, för en vald gitterkonfiguration, som sedan viktats mot tre snarlika representativa volymselement. En jämförelseanalys mellan de olika styvhetsmatriserna har sedan gjorts på en större och solid modell för att se hur väl metoderna förutsett deformationen av en gitterstruktur i samma storlek. Resultaten har visat att samtliga metoder är bra approximationer med tämligen små skillnader från randeffekterna. Vid jämförelseanalysen simulerades gitterstrukturen bäst med två av de tre metoder. En av slutsatserna är att det är viktigt att förstå inverkan av randvillkoren hos gitterstrukturer innan implementering görs med det tillvägagångssätt som presenterats i det här examensarbetet.

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The main aim of this thesis is to investigate the manufacturability of different gyroid designs using Selective laser melting (SLM) process . This study paves the way for a better understanding of design aspects, process optimization, and characterization of titanium alloy (Ti6Al4V) gyroid lattice structures for bone implant applications. First, A MATLAB® code was developed to create various gyroid designs and understand the relationship between the implicit equation parameters and the measurable outputs of gyroid unit cells. A novel gyroid lattice structure is proposed, where the porosity is graded in a radial direction. Second, gyroid designs were investigated by developing a permissible design map to help choose the right gyroid parameters for bone implants. Third, response surface methodology was used to study the process-structure-property relationship and understand the effect of SLM process parameters on the manufacturability of Ti6Al4V gyroid lattice structures. Laser power was found to be the most significant factor affecting the errors in relative density and strut size of gyroid structures. A volumetric energy density between 85 and 103 J/mm3 induces the least errors in the gyroid’s relative density. Fourth, the quasi-static properties of the novel designs were compared to uniform gyroids. The proposed novel gyroids had the highest compressive strength reaching 160 MPa. Numerical simulations were studied to give insight into how manufacturing irregularities can affect the mechanical properties of gyroids. Last, an in-depth defect analysis was conducted to understand how SLM defects may influence the fatigue properties of different Ti6Al4V gyroids. Thin struts have less internal defects than thick ones; thus, they show less crack propagation rate and higher normalized fatigue life. These favorable findings contributed to scientific knowledge of manufacturability of Ti6Al4V porosity graded gyroids and determined the influence of SLM defects on the mechanical properties of gyroid designs for bone implants. / Thesis / Doctor of Philosophy (PhD) / This thesis studies the integration of design aspects, SLM manufacturability, and mechanical characterization of Ti6Al4V gyroid lattice structures used for bone implants. A MATLAB® code was developed to design novel porosity graded gyroids, and develop permissible design map to aid the choice of different gyroid designs for bone implants.. Process maps were also developed to investigate the relationship among laser power, scan speed, and the errors in the relative density of lattice structures. Moreover, the normalized fatigue strength of thin struts gyoid was found to be higher than that of thicker struts.Analytical models and finite element analysis (FEA) models were compared to experimental results. The variation of the results gives a better understanding of the effect of manufacturing defects. An improved insight of gyroids manufacturability has been obtained by integrating the permissible design space with the process-structure-property relationship, and the defect analysis of porosity graded gyroids.

In this work, we sought to provide a deeper understanding of the energy-absorbing capabilities of cast lattice structures. These structures absorb large amounts of energy via plastic deformation, but their most attractive characteristic from a structural standpoint is the favorable energy absorption-to-weight ratio. Conventional machining techniques are not well suited for manufacturing such complex features; therefore, we combined additive manufacturing (AM) with a well-known understanding of the metalcasting process. We used AM to produce sand molds in different sizes and with additional features for various applications — these molds were then filled with molten metal. Current literature suggests that this when appropriately applied, this methodology results in complex geometries castings comparable properties to parts made with traditionally produced sand molds. We chose to study periodic lattice structures for their repeatability and subsequent ease of making predictions via computer simulations. We first produced lightweight cast metal-ceramic composite panels of 225 x 225 x 60 mm. Our AM molds included provisions to install ceramic or hard metal tiles before pouring the molten metal. The tiles were encapsulated in the final casting to yield a composite structure. The initial material selection consisted of an aluminum A356-T6 alloy matrix with silicon carbide tiles. The composite lattice structures were tested against high-velocity projectiles — 0.30 caliber armor-piercing (AP M2) and NATO 7.62 mm ball rounds. We anticipated that the metal matrix alone would not be able to defeat these threats. However, the panels did reduce the striking velocity by approximately 20%. The thickness of the ceramic tiles varied from 4 mm to 8 mm at 2 mm increments. As expected, the hard ceramic tiles proved effective at improving the penetration resistance of the composite lattice structures — the impacts on regions with 4 mm thick tiles resulted in the reduction of striking velocity up to 49%; moreover, as the thickness was increased to 8 mm, the panels defeated the projectiles. We used these results to produce and validate a finite element (FE) model capable of replicating the experimental data within 5%. This model was later used to study how the ceramic material interacts with the lattice to absorb large amounts of kinetic energy from incident projectiles. Following, we manufactured smaller versions of these panels—50 x 50 x 90 mm test specimens for uniaxial compression testing for this instance. Once again, we relied on the capabilities of the FE method to replicate the test results within 10% for peak load and maximum displacement. We utilized computer simulations to optimize the design of the lattice structure. Its energy-absorbing capabilities were improved significantly — in this case, a 30% increase in the specific internal energy (internal energy per unit mass) as the evaluating criteria. The FE model was also used to study the performance of several other truss topologies. Lastly, we used computer simulations to evaluate the feasibility of making these cast lattice structures with ferrous alloys. We chose to work with Fe30Mn4Al0.9C due to its lower density and higher toughness than other steel grades. The first challenge was the lack of thermophysical property data in the literature for this alloy system. Hence, we used the CALPHAD method to calculate all the datasets needed to perform the mold filling and solidification simulation. Several of these calculations were validated experimentally. The location and severity of porosity between the model and the casting were in good agreement. / Doctor of Philosophy / The advent of additive manufacturing (AM), commonly known as 3D printing is a group of different digital-era technologies that has facilitated the production of complex designs that are not feasible to manufacture using conventional techniques. In the realm of metallic components one such technique involves the use of a laser beam to consolidate metallic powders via a layer-by-layer deposition process. Despite its advantages, this process has unique challenges, such as limited material selection and relatively small part volume. In this work, we have employed a hybrid approach that combines the use of AM with expertise in metalcasting to produce lightweight components with complex geometries. We used 3D printed sand molds that are then filled with molten metal of different alloy systems such as aluminum and steel. At first, we incorporate hard ceramic materials to increase the performance of the final parts under ballistics testing. Our aim is to upscale the size of current designs such that these devices can be used in various applications that require high absorption of kinetic energy, and that are lightweight and easy to replace.

<p dir="ltr">The natural honeycomb made from beeswax is an engineering marvel. Modern-day engineering has taken several inspirations from it in the form of hexagonal panels and cells made of various materials such as polymers, ceramics, and metals for light-weighting without compromising on its mechanical properties. Previously, characterizing this structure has relied on two-dimensional (2D) surface observations on the macroscale which have an inherently limited scope in understanding complex three-dimensional (3D) structures. As a result, several seminal features of the honeycomb that would have shed light on how it is constructed and what makes it so mechanically robust are left out of reach and overlooked. X-ray microscopy (XRM) is a powerful tool to characterize these complex structures non-destructively, yielding insights that are not possible without three-dimensional (3D) datasets. Further, when a time-resolved approach is adopted, where an external stimulus is interrupted for an XRM scan, one can obtain four-dimensional (4D) datasets. This provides unrivaled information on how complex 3D structures evolve over time when a stimulus is applied.</p><p dir="ltr">In this work, a time-resolved approach towards understanding how bees build out their hexagonal cells, both under normal and abnormal conditions was developed. Several previously unreported, but seminal features of the honeycomb such as the “coping” and porosity at well-defined locations yielded insights into how the comb is constructed. The corrugated spine is seen to be the foundation on which all hexagonal cells are built on. Additionally, this work also explores how bees accommodate distortions within the ordered lattice during the merger of two combs. Behaviorally they are seen to reduce the distortion within cells to minimize the wastage of wax and to keep the cells usable. A 3D parameter using automated image processing was developed to quantify how distortions are accommodated in an ordered lattice.</p><p dir="ltr">This work will further shed light on the mechanical behavior of the natural honeycomb arising from the corrugated nature of the spine and the gradient in its wall thickness which plays a role in crack deflection when the honeycomb is loaded under tension. When loaded under compression, the honeycomb lattice crumples in a manner to limit the damage to very local regions thereby forming a damage-tolerant crumple zone.</p>