Emerging technologies in the healthcare industry encompass revolutionary devices or drugs that have the potential to change how healthcare will be practiced in the future. While there are several emerging healthcare technologies in the pipeline, a few key innovations are slated to be implemented clinically sooner based on their mass appeal and potential for healthcare breakthroughs. This thesis will focus on specific topics in the emerging technological fields of nanotechnology for photothermal cancer therapy, 3D printing for irreversible electroporation applications, and imageable biomaterials. While these general areas are receiving significant attention, we highlight the potential opportunities and limitations presented by our select efforts in these fields. First, in the realm of nanomedicine, we discuss the optimization and characterization of sodium thiosulfate facilitated gold nanoparticle synthesis. While many nanoparticles have been examined as agents for photothermal cancer therapy, we closely examine the structure and composition of these specific nanomaterials and discuss key findings that not only impact their future clinical use, but elucidate the importance of characterization prior to preclinical testing. Next, we examine the potential use of 3D printing to generate unprecedented multimodal medical devices for local pancreatic cancer therapy. This additive manufacturing technique offers exquisite design detail control, facilitating tools that would otherwise be difficult to fabricate by any other means. Lastly, in the field of imageable biomaterials, we demonstrate the development of composite catheters that can be visualized with near infrared imaging. This new biomaterial allows visualization with near infrared imaging, offering potentially new medical device opportunities that alleviate the use of ionizing radiation. This collective work emphasizes the need to thoroughly optimize and characterize emerging technologies prior to preclinical testing in order to facilitate rapid translation. / Master of Science

Vat photopolymerization (VP) is an additive manufacturing (AM) technology that permits the fabrication of parts with complex geometries and feature sizes as small as a few microns. These attributes make VP an attractive option for the fabrication of scaffolds for tissue engineering. However, there are few printable materials with low cytotoxicity that encourage cellular adhesion. In addition, these resins are not readily available and must be synthesized. A novel resin based on 2-acrylamido-2-methyl-1-propanesulfonic acid (NaAMPS) and poly(ethylene glycol) diacrylate (PEGDA) was formulated and printed using VP. The mechanical properties, water content, and high fidelity of the scaffold indicated promise for use in tissue engineering applications. Murine fibroblasts were observed to successfully adhere and proliferate on the scaffolds. The growth, migration, and differentiation of a cell is known to dependent heavily on its microenvironment. In engineered constructs, much of this microenvironment is provided by the tissue scaffold. The physical environment results from the scaffold's geometrical features, including pore shape and size, porosity, and overall dimensions. Each of these parameters are known to affect cell viability and proliferation, but due to the difficulty of isolating each parameter when using scaffold fabrication techniques such as porogen leaching and gas foaming, conflicting results have been reported. Scaffolds with pore sizes ranging from 200 to 600 μm were fabricated and seeded with murine fibroblasts. Other geometric parameters (e.g., pore shape) remained consistent between scaffold designs. Inhomogeneous cell distributions and fewer total cells were observed in scaffolds with smaller pore sizes (200-400 μm). Scaffolds with larger pores had higher cell densities that were homogeneously distributed. These data suggest that tissue scaffolds intended to promote fibroblast proliferation should be designed to have pore at least 500 μm in diameter. Techniques developed for selective placement of dissimilar materials within a single VP scaffold enabled spatial control over cellular adhesion and proliferation. The multi-material scaffolds were fabricated using an unmodified and commercially available VP system. The material preferences of murine fibroblasts which resulted in their inhomogeneous distribution within multi-material scaffolds were confirmed with multiple resins and geometries. These results suggest that multi-material tissue scaffolds fabricated with VP could enable multiscale organization of cells and material into engineered constructs that would mimic the function of native tissue. / Doctor of Philosophy / Vat photopolymerization (VP) is a 3D printing (or additive manufacturing) technology that is capable of fabricating parts with complex geometries with very high resolution. These features make VP an attractive option for the fabrication of scaffolds that have applications in tissue engineering. However, there are few printable materials that are biocompatible and allow cells attachment. In addition, those that have been reported cannot be obtained commercially and their synthesis requires substantial resources and expertise. A novel resin composition formulated from commercially available components was developed, characterized, and printed. Scaffolds were printed with high fidelity. The scaffolds had mechanical properties and water contents that suggested they might be suitable for use in tissue engineering. Fibroblast cells were seeded on the scaffolds and successfully adhered and proliferated on the scaffolds. The growth, migration, and differentiation of cells is influenced by the environmental stimuli they experience. In engineered constructs, the scaffold provides many of stimuli. The geometrical features of scaffolds, including how porous they are, the size and shape of their pores, and their overall size are known to affect cell growth. However, scaffolds that have a variety of pore sizes but identical pore shapes, porosities, and other geometric parameters cannot be fabricated with techniques such as porogen leaching and gas foaming. This has resulted in conflicting reports of optimal pore sizes. In this work, several scaffolds with identical pore shapes and porosities but pore sizes ranging from 200 μm to 600 μm were designed and printed using VP. After seeding with cells, scaffolds with large pores (500-600 μm) had a large number of evenly distributed cells while smaller pores resulted in fewer cells that were unevenly distributed. These results suggest that larger pore sizes are most beneficial for culturing fibroblasts. Multi-material tissue scaffolds were fabricated with VP by selectively photocuring two materials into a single part. The scaffolds, which were printed on an unmodified and commercially available VP system, were seeded with cells. The cells were observed to have attached and grown in much larger numbers in certain regions of the scaffolds which corresponded to regions built from a particular resin. By selectively patterning more than one material in the scaffold, cells could be directed towards certain regions and away from others. The ability to control the location of cells suggests that these printing techniques could be used to organize cells and materials in complex ways reminiscent of native tissue. The organization of these cells might then allow the engineered construct to mimic the function of a native tissue.

Step-growth polymerization facilitates the synthesis of a wide range of industrially applicable polymers, such as polyesters and polysulfones. The choice of backbone and end group structure within these polymers drastically impacts the final material properties and processability emphasizing the necessity for thorough understanding of structure-property relationships. Seemingly simple changes, such as exchanging a monomer for its regioisomer, affects the polymers fundamental packing structure triggering a domino effect ultimately influencing the morphological, thermal, mechanical and barrier properties. In conjunction, end groups provide a means by which tunable mechanical properties and application into unique processing methods can be achieved. Synthesizing polyesters with bibenzoate based monomers generates a large range of morphologies. Linear, 4,4' bibenzoate (4,4'BB), is widely considered a mesogenic monomer due to its ability to impart a liquid crystalline (LC) morphology on semi-aromatic polyesters with linear aliphatic spacers. In this body of work, semi-aromatic polyesters using one of 4,4'BB's regioisomers, either 3,4'BB or 3,3'BB, largely resulted in amorphous or semi-crystalline polymers depending on the selection of aliphatic diol. Incorporation of the meta isomer (3,4'BB) into traditionally LC polymers, such as poly(diethylene glycol 4,4'-bibenzoate) and poly(butylene 4,4'-bibenzoate), through copolymerization afforded two polymer series with tunable LC properties. The 3,4'BB exhibited selective disruption of crystalline domains over the LC phase generating a number of polymers with LC glass morphologies. The application of 3,4'BB to a fully-aromatic polyester enabled the synthesis of a novel melt-processable homopolyester with high thermal stability, poly(p-phenylene 3,4' bibenzoate). This structure afforded a nematic LC morphology which revealed beneficial shear-thinning properties similar to industrial standards. The unique LC morphology of this homopolyester inspired further characterization of the range of achievable properties using the basic structure, poly(phenylene bibenzoate), with all the possible regioisomers. This study afforded six polymers systematically varied in chain linearity from a completely meta to a completely para backbone configuration. A range of morphologies were achieved from high Tg amorphous polymers for the meta configurations to semi-crystalline or LC in the polymers with greater linearity. End group functionalization generates influence on polymer properties while limiting the impact on beneficial properties achieved through the backbone structure and packing. Post-polymerization reactions or the addition of a monofunctional endcapper to the polymerization both achieve end group control. In this dissertation, the addition of a monofunctional diester with a sulfonate moiety to a semi-aromatic LC polyester synthesis resulted in a telechelic ionomer. The non-covalent interaction of the ionic groups will hopefully improve the compression and transverse mechanical properties of the LCP. In contrast, post-polymerization functionalization incorporated acrylate groups onto the ends of a basic polysulfones. These reactive groups provided a handle for photo-curing which enabled the 3D printing of the polysulfones using vat photopolymerization. / Doctor of Philosophy / The research within this dissertation encompasses the design of new plastics for consumer and high-performance applications. Since the emergence of synthetic plastics in the 1920’s, these materials have become a necessity in our everyday life with a range of applications in food packaging, microelectronics, architecture, medical devices, automotive, and aerospace. Benefits over metals and glass primarily result from their light weight and wide range of mechanical properties which allow a range of material properties from soft and flexible plastic grocery bags to tough car parts. Different classes of plastics (polymers) are based primarily on the chemicals used to produce the materials, for example polyesters and polysulfones. The chemical structure of these core materials drastically impacts the final properties of the polymers, which in turn influences their application space. This work focused on how subtle changes to these starting chemical structures allows us to tune the final polymer properties. Within the class of polyesters, a focus was placed on materials known as liquid crystalline (LC) polyesters. A liquid crystalline polymer can achieve a physical state between a solid and a liquid which imparts many beneficial properties on the material processing. Liquid-crystal television displays utilized these properties to provide drastically thinner TV’s with higher resolution. Alternatively, LC polyesters find applications traditionally as high-performance fibers, insulators in microelectronics, and stainless-steel replacements in medical applications. Studying the role of chemical structure on the properties of LC polyester enabled the design of materials which improve upon the current technological standards. These changes enabled the design of LC polyesters with lower processing temperatures and the use of fewer starting materials which will inevitably save energy and money during their production. In the case of polysulfones, changing the chemical structure at the end of the polymer chain facilitated the application of novel processing methods, such as 3D printing. The ability to process using this method reduces the amount of material waste during production and provides an opportunity to design novel parts with intricate structures, inaccessible through traditional means.

Polymer synthesis and functionalization enabled the tailoring of polymer functionality for additive manufacturing (AM), elastomer, and biological applications. Inspiration from academic and patent literature prompted an emphasis on polymer functionality and its implications on diverse applications. Critical analysis of existing elastomers for AM aided the synthesis and characterization of novel photopolymer systems for lithography-based 3D printing. Emphasis on structure-processing-property relationships facilitated the attainment of success in proposed applications and prompted further fundamental understanding for systems that leveraged poly(dimethyl siloxane)s (PDMS), aliphatic polyesters, polyamides, and polyethers for emerging applications. The thiol-ene reaction possesses many desirable traits for vat photopolymerization (VP) AM, namely that it proceeds rapidly to high yield, does not undergo significant side reactions, remains tolerant of the presence of water or oxygen, and remains regiospecific. Leveraging these traits, a novel PDMS-based photopolymer system was synthesized and designed that underwent simultaneous chain extension and crosslinking, affording relatively low viscosity prior to photocuring but the modulus and tensile strain at break properties of higher molecular weight precursors upon photocuring. A monomeric competition study confirmed chemical preference for the chain-extension reaction in the absence of diffusion. Photocalorimetry, photorheology, and soxhlet extraction measured photocuring kinetics and demonstrated high gel fractions upon photocuring. A further improvement on the low-temperature elastomeric behavior occurred via introduction of a small amount of diphenylsiloxane or diethylsiloxane repeating units, which successfully suppressed crystallization and extended the rubbery plateau close to the glass transition temperature (Tg) for these elastomers. Finally, a melt polymerization of PDMS diamines in the presence of a disiloxane diamine chain extender and urea afforded isocyanate-free polyureas in the absence of solvent and catalyst. Dynamic mechanical analysis (DMA) measured multiple, distinct α-relaxations that suggested microphase separation. This work leverages the unique properties of PDMS and provides multiple chemistries that achieve elastomeric properties for a variety of applications. Similar work of new polymers for VP AM was performed that leveraged the low Tg poly(propylene glycol) (PPG) and poly(tri(ethylene glycol) adipate) (PTEGA) for use in tissue scaffolding, footwear, and improved glove grip performance applications. The double endcapping of a PPG diamine with a diisocyanate and then hydroxyethyl acrylate provided a urethane/urea-containing, photocurable oligomer. Supercritical fluid chromatography with evaporative light scattering detection elucidated oligomer molecular weight distributions with repeat unit resolution, while the combination of these PPG-containing oligomers with various reactive diluents prior to photocuring yielded highly tunable and efficiently crosslinked networks with wide-ranging thermomechanical properties. Functionalization of the PTEGA diol with isocyanatoethyl methacrylate yielded a photocurable polyester for tissue scaffolding applications without the production of acidic byproducts that might induce polymer backbone scission. Initial VP AM, cell viability experiments, and modulus measurements indicate promise for use of these PTEGA oligomers for the 3D production of vascularized tissue scaffolds. Similar review of powder bed fusion (PBF) patent literature revealed a polyamide 12 (PA12) composition that remained melt stable during PBF processing, unlike alternative commercial products. Further investigation revealed a fundamental difference in polymer backbone and endgroup chemical structure between these products, yielding profound differences for powder recyclability after printing. An anionic dispersion polymerization of laurolactam in the presence of a steric stabilizer and initiator yielded PA12 microparticles with high sphericity directly from the polymerization without significant post-processing requirements. Steric stabilizer concentration and stirring rate remained the most important variables for the control of PA12 powder particle size and melt viscosity. Finally, preliminary fusion of single-layered PA12 structures demonstrated promise and provided insight into powder particle size and melt viscosity requirements. / PHD / Additive manufacturing (AM) enables the creation of unique geometries not accessible with alternative manufacturing techniques such as injection molding, while also reducing the waste associated with subtractive manufacturing (e.g. machining). However, AM currently suffers from a lack of commercially-available polymers that provide elastomeric properties after processing. Poly(dimethyl siloxane)s (PDMS) possess distinctive properties due to their organosilicon polymer backbone that include chemical inertness, non-flammability, high gas permeability, and low surface energy. For these reasons, siloxanes enjoy wide-ranging applications from personal care products, contact lenses, elastomeric sealants, and medical devices. This dissertation focuses on the synthesis and functionalization of novel PDMS-, polyether-, polyester-, and polyamide-containing photopolymers or powders for improved performance in diverse applications that employ processing via vat photopolymerization (VP) or powder bed fusion (PBF) AM. Examples from this work include a novel photopolymer composition that undergoes simultaneous chain extension and crosslinking, affording low molecular weight and low viscosity precursors prior to VP-AM but the properties of higher molecular weight precursors, once photocured. Related work involved the characterization and VP-AM of siloxane terpolymers that suppress crystallization normally observed in PDMS, resulting in 3D printed objects that retain their elastomeric properties close to the glass transition temperature (Tg). Separate work leveraged the unique PDMS backbone for the melt polymerization of PDMS diamines in the presence of a chain extender and urea, yielding isocyanate-free PDMS polyureas in the absence of solvent or catalyst. This reaction creates ammonia as the only by-product and avoids the use of isocyanates, as well as their highly toxic precursors, phosgene. Finally, another research direction facilitates the understanding of observed differences in melt stability between commercially-available grades of polyamide 12 (PA12) powders for powder bed fusion. An anionic dispersion polymerization based in the patent literature facilitated further understanding of the polymerization process and produced melt-stable PA12 microparticles directly from the polymerization process, without requiring additional post-processing grinding or precipitation steps for powder production.

Heat pipes are passive heat transfer devices in which a working fluid is sealed inside a metal enclosure. Properly designed wick structures on the inner surface of the heat pipe are critical as the wick aids in the return of the condensed liquid from the cold end back to the hot end where the vaporization-condensation cycle begins again. Additive manufacturing techniques allow for manufacturing complex parts that are typically not feasible with conventional manufacturing methods. Thus, additive manufacturing opens the possibility to develop high performance heat pipes with complex shapes. In this study, an additive manufacturing technique called Binder Jetting is used to fabricate a fully operational compact (78 mm x 48 mm x 8 mm) flat plate heat pipe. Rectangular grooves with converging cross section along the length act as the wicking structure. A converging cross section was designed to enhance the capillary force and to demonstrate the capability of additive manufacturing to manufacture complex shapes. This work describes the challenges associated with the development of heat pipes using additive manufacturing such as de-powdering and sintering. Multiple de-powdering holes and internal support pillars to improve the structural strength of the heat pipe were provided in order to overcome the manufacturing constraints. The heat pipe was experimentally characterized for thermal performance with acetone as the working fluid for two different power inputs. The heat pipe operated successfully with a 25% increase in effective thermal conductivity when compared to solid copper. / Master of Science / The number of transistors in electronic packages has been on an increasing trend in recent decades. Simultaneously there has been a push to package electronics into smaller regions. This increase in transistor density has resulted in thermal management changes of increased heat flux and localization of hotspots. Heat pipes are being used to overcome these challenges. Heat pipes are passive heat transfer devices in which a working fluid is sealed inside a metal enclosure. The fluid is vaporized at one end and condensed at the other end in order to efficiently move heat through the pipe by taking advantage of the latent heats of vaporization and condensation of the fluid. Properly designed wick structures on the inner surface of the heat pipe are used to move the condensed fluid from the cold end back to the hot end, and the wick is a critical component in a heat pipe. Additive manufacturing techniques offer the opportunity to manufacture complex parts that are typically not feasible with conventional manufacturing methods. Thus, additive manufacturing opens the possibility to develop high performance heat pipes with complex shapes as well as the ability to integrate heat exchangers with the heat source. In this study, an additive manufacturing technique called Binder Jetting is used to fabricate a fully operational compact (78 mm x 48 mm x 8 mm) flat plate heat pipe. Rectangular grooves with converging cross section along the length act as the wicking structure. This work describes the challenges associated with the development of heat pipes using additive manufacturing such as depowdering and sintering. The heat pipe was experimentally characterized for thermal performance with acetone as the working fluid for two different power inputs. The heat pipe was found to operate successfully with a 25% increase in effective thermal conductivity when compared with solid copper.

Master of Science / As parts in any device age, something inevitably breaks. When dealing with a broken metallic part, one can either replace it or repair it. Repairing is generally preferred so long as it is not too costly. Unfortunately, repairing a component is often more expensive due to the material being difficult to work with or the geometry being too intricate to fix. Additive manufacturing, commonly known as 3D printing, allows precise placement of material to build a part and has allowed the repair of complex parts. However, some materials are severely weakened using traditional additive manufacturing technologies, which melt small amounts of material and force it to cool in place quickly. To combat this, methods that do not require the material to melt could be used. Currently, these methods place a large amount of material at once, causing significant waste if the excess needs to be removed. Therefore, this work aims to create a small-scale device using a traditional milling machine. It was shown to be capable of placing small amounts of material while offering the advantage of not melting the metal. In the future, it could provide an avenue to repair previously unreachable.

Impedance-based Non-Destructive Evaluation for Additive Manufacturing (INDEAM) is rooted in the field of Structural Health Monitoring (SHM). INDEAM generalizes the structure-to-itself comparisons characteristic of the SHM process through introduction of inter-part comparisons: instead of comparing a structure to itself over time, potentially-damaged structures are compared to known-healthy reference structures. The purpose of INDEAM is to provide an alternative to conventional nondestructive evaluation (NDE) techniques for additively manufactured (AM) parts. In essence, the geometrical complexity characteristic of AM processes combined with a phase-change of the feedstock during fabrication complicate the application of conventional NDE techniques by limiting direct access for measurement probes to surfaces and permitting the introduction of internal defects that are not present in the feedstock, respectively. NDE approaches that are capable of surmounting these challenges are typically highly expensive. In the first portion of this work, the procedure for impedance-based NDE is examined in the context of INDEAM. In consideration of the additional variability inherent in inter-part comparisons - as opposed to part-to-itself comparisons - the metrics used to quantify damage or change to a structure are evaluated. Novel methods of assessing damage through impedance-based evaluation are proposed and compared to existing techniques. In the second portion of this work, the INDEAM process is applied to a wide variety of test objects. This portion considers how the sensitivity of the INDEAM process is affected by defect type, defect size, defect location, part material, and excitation frequency. Additionally, a procedure for studying the variance introduced during the process of instrumenting a structure is presented and demonstrated. / Doctor of Philosophy / Impedance-based Non-Destructive Evaluation for Additive Manufacturing (INDEAM) is a quality control approach for detecting defects in structures. As indicated by the name, impedance-based evaluation is discussed in this work in the context of qualifying additively manufactured (3D printed) structures. INDEAM fills a niche in the wider world of nondestructive evaluation techniques by providing a less expensive means to qualify structures with complex geometry. Complex geometry complicates inspection by preventing direct, physical access to all the surfaces of a part. Inspection approaches for parts with complex geometry suffuse a structure with energy and measure how the energy propagates through the structure. A prominent technique in this space is CT scanning, which measures how a structure attentuates x-rays passing through it. INDEAM uses piezoelectric materials to both vibrate a structure and measure its response, not unlike listening for the dull tone of a cracked bell. By applying voltage across a piezoelectric patch glued to a structure, the piezoelectric deforms itself and the bonded structure. By monitoring the electrical current needed to produce that voltage, the ratio of applied voltage to current draw---impedance---can be calculated, which can be thought of as a measure of how a system stores and dissipates energy. When the applied voltage oscillates near a resonant frequency of a structure (the pitch of a rung bell, for example) the structure vibrates much more intensely, and that additional movement dissipates more energy due to viscosity, friction, and transmitting sound into the air. This phenomenon is reflected in the measured impedance, so by calculating the impedance value over a large range of frequencies, it is possible to identify many resonances of the structure. So, the impedance value is tied to the vibrational properties of the structure, and the vibration of the structure is tied to its geometry and material properties. One application of this relationship is called impedance-based structural health monitoring: taking measurements of a structure when it is first built as a reference, then measuring it again later to watch for changes that indicate emerging damage. In this work, the reference measurement is established by measuring a group of control structures that are known to be free of defects. Then, every time a new part is fabricated, its impedance measurements will be compared to the reference. If it matches closely enough, it is assumed good. In both cases, impedance values don't indicate what the change is, just that there was a change. A large portion of this work is devoted to determining the types and sizes of defects that can be reliably detected through INDEAM, what effect the part material plays, and how and where the piezoelectric should be mounted to the part. The remainder of this work discusses new methods for conducting impedance-based evaluation. In particular, overcoming the extra uncertainty introduced by moving from part-to-itself structural health monitoring comparisons to the part-to-part quality control comparisons discussed in this work. A new method for mathematically comparing impedance values is introduced which involves extracting the resonant properties of the structure rather than using statistical tools on the raw impedance values. Additionally, a new method for assessing the influence of piezoelectric mounting conditions on the measured impedance values is demonstrated.

In recent years, there has been a growing concern over the adverse effects of traditional fossil fuels on the environment and health. Therefore, there is an increased interest in hydrogen as a fossil-free fuel source, making the need for hydrogen solutions apparent. This supports the purpose and research questions of this study, which aim to determine the suitable materials for handling hydrogen and the necessary design for structural integrity to withstand pressure. This will be achieved through additive manufacturing using polymers. The study also considers the potential of additive manufacturing for large-scale production. After conducting literature studies, polymers are of special interest due to their different structural build compared to metals. Metals do not handle hydrogen well because of the phenomenon known as hydrogen embrittlement. The preferred material properties in polymers are a crystalline structure, high density, and strong mechanical properties. The design and production are conducted using SolidWorks, with simulations of pressure and topology optimization, making it possible to create a part ready for 3-D printing after slicing. The results provide insights into the effects of parameter adjustments on the structure of the parts and the feasibility of large-scale production through additive manufacturing. By analysing the slicer program, conclusions can be made that additive manufacturing is a viable option for large-scale production, given the availability of multiple printers. However, the conclusion regarding the optimal design for handling pressurized hydrogen could not be made due to a lack of time for testing.

<p dir="ltr">Repeated loading and unloading cycles lead to the formation of strain in the material which causes initiation of the crack formation this phenomenon is called fatigue. Fatigue properties are critical for structures subject to cyclic load; hence fatigue analysis is used to predict the life of the material. Fatigue analysis plays an important role in optimizing the design of the 3D printed material and predicting the fatigue life of the 3D printed component.</p><p><br></p><p dir="ltr">The main objective of this thesis is to predict the fatigue behavior of different microstructures of Ti-64 titanium alloy by using the PRISMS-Fatigue open-source framework. To achieve this goal Ti-64 microstructure models were created using programming scripts, then the structures were exported to a finite element visualization software package, with all the required properties embedded in the pipeline. The PRISMS-Fatigue framework is used to conduct a fatigue analysis on 3D printed materials, using the Fatigue Indicator Parameters (FIP), which measure the driving force of fatigue crack formation in the microstructurally small crack growth.</p><p><br></p><p dir="ltr">Three different microstructures, i.e., cubic equiaxed, random equiaxed, and rolled equiaxed microstructures, are analyzed. The FIP results show that the cubic equiaxed grains have the best fatigue resistance due to their isotropic structural characteristics. Additionally, the grain size effect using 1 and 10 micrometers is investigated. The results show that the 1 micrometer grain size cubic equiaxed microstructure has a better fatigue resistance because as grains are small and they have a higher mechanical strength.</p>

Doctor of Philosophy / The long-term research goal of this dissertation is to enable flaw-free production of metal parts using the laser powder bed fusion (LPBF) additive manufacturing (AM) process. As a step towards this long-term goal, the goal of this work is to predict and control the thermal history of an LPBF part. The thermal history is the spatiotemporal distribution of temperature in an LPBF part as it is created layer by layer. Thermal history is the primary cause of flaw formation in LPBF. To realize this goal, the objective of this dissertation is to establish and advance a novel thermal modeling method based on the concept of spectral graph theory, which is more than 10 times faster than existing finite element-based methods for the same level of accuracy. The central hypothesis is that physics-guided prediction, optimization, and control of thermal history mitigates flaw formation and enhances functionally critical properties of LPBF-processed parts when compared to parts produced without control of thermal history. The practical rationale and need for this work are as follows. LPBF is becoming increasingly prevalent due to its ability to fabricate complex structures that would otherwise be impossible with traditional subtractive and formative manufacturing processes. The freedom of geometry afforded by AM processes such as LPBF enables designers to place a stronger emphasis on design efficiency rather than the manufacturability of components. It also facilitates greater supply chain flexibility, reducing part lead times and costs. For example, making an aerospace part weighing just one kilogram with traditional subtractive and formative techniques requires processing 20 kilograms of raw material—a buy-to-fly ratio of 20:1—and lead times for new parts are often several months long. LPBF reduces the buy-to-fly ratio to less than 5:1, and the lead time is just a few weeks. Despite these advantages, LPBF has seen limited industry adoption and use, especially in safety-critical applications, due to the tendency of the process to form flaws. Approximately one in three parts are affected by flaws. Flaw formation leads to inconsistent part properties and can cause catastrophic failures in safety-critical aerospace, defense, and biomedical applications. Flaw formation in LPBF parts is mainly attributed to the thermal history. Thermal history, in turn, is influenced by complex design-process-material-machine interactions that require mathematical modeling. Rapid and accurate prediction of the thermal history can enable practitioners to avoid flaw formation and achieve desired part properties by optimizing the part design and process parameters before the part is printed. This dissertation leverages the graph theory modeling to address the burgeoning practical need for a rapid, accurate, and experimentally validated physics-based approach for mitigating flaw formation and ensuring part quality in LPBF