Binder jetting is an additive manufacturing process that layer by layer builds a 3D model by selectively binding regions of powder using binder deposited though an inkjet printhead. The process offers several advantages over other additive manufacturing processes including fast build rates, vast material selection, decreased cost, and part resolution. The main disadvantage of binder jetting is poor mechanical properties, stemming from a poor understanding of the process physics. Porosity in final parts is not uncommon, but there is little understanding of where the porosity originates. The purpose of this thesis is to report the investigation of increased powder bed cohesion and its effects on part formation, part properties, and printing parameters in binder jetting. The interaction between binder and powder is complex. Binder exiting the printhead impacts the powder bed at speeds up to 10 m/s. The kinetic energy carried by the droplet disperses into the powder bed on impact, causing some powder particles to eject from the bed and other particles to rearrange within the bed. The particle ejection and rearrangement is theorized to be the physical cause of porous regions in binder jetted parts. This work uses a method called pre-wetting to introduce small amounts of moisture into the powder bed to effectively increase the cohesive forces between powder particles. Increased cohesion makes particle ejection and rearrangement during the powder/binder interaction more difficult. A method of accomplishing pre-wetting was developed and achieved successful moisture delivery using water and a water/tri-ethylene glycol mixture. Printed lines were used to characterize moisture content and study its effects on line formation and saturation levels. Low levels of moisture were shown to perform the best. Particle ejection and rearrangement was shown to decrease with moisture addition. Pre-wetting was shown to eliminate the defect known as balling, increasing the parameters known to successfully print lines. Water was identified as a poor substance for pre-wetting due to rapid evaporation, but tri-ethylene glycol/water solutions succeeded in proper moisture delivery. Saturation levels in lines decrease with added moisture and part dimensions increase. high-speed x-ray imaging verified pre-wetting reduction in particle ejection and rearrangement as well as supply some preliminary understanding of void formation during the printing process. The first few layers of the binder jetting process have been shown to increase in surface roughness values when compared to the undisturbed powder bed. This is likely due to a balling-like effect seen in layers. The effects of pre-wetting on layer and multi-layer formation were studied. Pre-wetting reduced the surface roughness levels in printed layers to the levels near the levels seen in undisturbed powder beds. In contrast, saturation levels in layers and multi-layers increased in value above those found in parts printed into dry powder, giving indication that porous regions within bound parts are being eliminated. Layer and multi-layer parts showed increased part dimensions with the addition of moisture. Overall, pre-wetting was shown to greatly reduce the effects of the binder/powder interaction and results strongly suggest that pre-wetting mitigates defect creation during the printing process. Further research should include testing of thicker multi-layer parts to study how saturation trends continue with increased layer numbers. In-process drying should be used in conjunction with pre-wetting in multi-layer parts to determine its effects on saturation levels and part dimensions. Post processing should be done to partially sinter, or infiltrate multi-layer parts created with and without pre-wetting to analyze porosity.

Master of Science / Material extrusion is a common form of 3D printing that has historically been limited to producing prototypes, models, and low load-bearing parts. This is primarily because parts are manufactured layer-by-layer, resulting in poor adhesion along the build direction, and machines struggle to print with high-strength polymers, which tend to shrink significantly as they cool. However, one way to address these limitations is to use fiber-reinforced materials in combination with multi-axis deposition strategies. In material extrusion, embedded fibers will align themselves along the deposition path, providing structural, thermal, and chemical improvements. Multi-axis toolpathing can enable the deposition of this fiber-filled material in full 3D along a part's expected stress paths. This is possible using a complex kinematic system like an industrial robot arm that can rotate the angle of the tool relative to the part as it is printing. The objective of this work was to develop and test a tool capable of multi-axis continuous carbon fiber reinforcement, which required a dedicated cutting mechanism to shear the fiber at the end of each deposition path, control over the amount of fiber used, and a slender tool profile to avoid collisions during multi-axis printing. The findings of this work revealed that while the use of continuous carbon fiber further reduced the adhesion between deposition paths, it substantially improved the strength of the part along them. To validate the multi-axis capability of the system, a toolpath was generated for a curved tensile bar. The results showed that the continuous carbon fiber multi-axis toolpath resisted a load 820.57% higher than an XY-planar sliced part printed with traditional filament, confirming the effectiveness of the presented approach. Multi-axis motion can also be used for avoiding support material requirements. In traditional 3-axis material extrusion, steep overhanging features often require additional, sacrificial material to be printed underneath. This leads to longer print times, more material waste, and a poor surface finish left behind on the final part. To minimize the amount of support material required, various techniques have been explored, including changing the toolpath, part geometry, or material processing parameters. However, none of these techniques have been successful in eliminating the need for supports entirely. A promising approach to address this issue is multi-axis material extrusion, where the angle of the printing tool and the direction of the layers can be precisely controlled during the printing process. This technique can be used to ensure that the tool is always extruding material onto a well-supported surface, rather than over thin air. However, research to date has not yet fully explored how the range of achievable overhang features changes as the tool is rotated. To address this knowledge gap, this work used an industrial robot arm equipped for material extrusion to investigate the relationship between tool angle, build direction, and achievable overhang threshold. The results showed that the same overhang limitations that exist in the XY plane will rotate with the tool and are unaffected by gravitational forces. These findings provide valuable insights for advancing the use of multi-axis material extrusion in the production of complex and intricate 3D objects without the need of supports.

Doctor of Philosophy / Startups can and do fail. For an entrepreneur, product developer, or researcher with a physical and capital-intensive product idea, this dissertation can serve as a resource to bridge the gaps between business, engineering, and design and reduce the risk of failure when trying to create a startup. The process described in this dissertation describes how to evaluate the key elements of an idea and conduct a series of interviews with potential customers to find evidence that supports pursing that idea further, challenge the startup team to change some aspect of the idea, or drop it altogether. Once the startup team has found a problem, as well as a solution to that problem, this dissertation describes an approach creating that solution. Then this dissertation describes an approach for critically evaluating the foundational elements of the problem and the solution. The goal for a critical evaluation is to identify additional foundational elements which relate to the product that may increase its value or decrease the risk of product failure.

Pure copper is widely employed as the primary metal in thermal management and electromagnetic applications due to its exceptional electrical and thermal conductivity. Laser powder bed fusion (LPBF) is a versatile additive manufacturing technique that utilizes high laser energy to selectively melt and fuse successive layers of metal powder to create metallic components with intricate geometries. Nonetheless, LPBF of pure copper is known as a challenging manufacturing process attributed to low optical absorptivity, rapid dissipation of laser energy, and affinity to oxidation. This thesis focuses on the process development and optimization for LPBF of Cu. Firstly, the Process-structure-property relation was examined by assigning a wide range of process parameters to print Cu-LPBF coupons. The optimum process parameters were defined based on maximum relative density, which was obtained at the full laser power of the EOS M280. The results emphasized the significant impact of laser power and hatch spacing on the part quality. Second, Cu oxide exhibits higher optical absorption than pure copper, as reported in the literature. Therefore, the thin film of oxide that was created either on recycled or intentionally oxidized power particles would be a possible easy way to increase the heat energy absorbed from the laser beam. However, the current work emphasized the adverse effects of oxide presence on part quality, particularly when using a medium laser power machine. In this regard, a new method of in-situ Cu oxide reduction during LPBF was proposed to develop an easy and environment-friendly approach to recover the contaminated powder. Applying laser ablation on the powder surface and the solidified layers results in considerable improvement, where the oxygen content is reduced by 70% in the LPBF samples compared to the initial state of the oxidized powder. Finally, the power density of Cu-LPBF coils was improved by enhancing the filling factor and increasing the electrical conductivity. The dimensional limitation of Cu-LPBF fabricated parts was initially identified. The power of utilizing sample contouring was highlighted to upgrade surface quality. Adjusting beam offset associated with optimum scan track morphology upgraded the minimum feature spacing to 80 um. The electrical impedance of full-size Cu-LPBF coils, newly reported in this study, was measured and compared with solid wire. It can reflect the performance of Cu-LPBF coils (power factor) in high-frequency applications. / Thesis / Doctor of Philosophy (PhD)

Ceramic colloidal processing routes such as slip casting, gelcasting and direct ink writing provide valuable insight into the role of interaction forces between particles, solvents, and polymeric additives in the rheology, particle packing, and strength of a ceramic green body. For difficult-to-densify ceramics such as the UHTCs, which find their place in extreme environment applications, precise control of each step of the manufacturing process is key. In this work, a fundamental study on the interaction between particles in non-aqueous slip casting is performed comparing the rheological behavior and consolidation with current models for interaction potential within a suspension. The advantages and drawbacks of such a model are discussed in relation to formulating a colloidal process for advanced ceramics such as ZrB2, and a case for a cyclohexane slip casting system resulting in low viscosity, shear-thinning behavior and green density of 64%, is made. The focus on non-aqueous colloidal processing is extended to gelcasting, involving three different sets of chemically curable polymer systems: HEMA+MBAM, TMPTA, and PEGDA. Merits of the gelcasting process including homogeneity, green strength, and processing time reduction are discussed, with the HEMA+MBAM system resulting in nearly an order of magnitude increase in green density from slip casting. Gelcast samples were also sintered to a density of 88% and capable of being processed in a variety of complex shapes with fine feature size on the mm scale. The properties examined in slip casting and gelcasting, as well as others pertaining to the setup of an extrusion-based additive manufacturing system, are carefully considered to design an ink that has been used to print ZrB2. The role of each additive as well as the solvent in creating an ink that is not only within the correct viscosity range for extrusion and shape retention, but also produces a strong and densely packed green body, is discussed. Finally, adjustment of printing parameters, and the method of using a low-cost rheology match to tune the settings of a pneumatic screw-extrusion printing setup, are explained. Each of these processes points to new and practical methods of complex shaping ZrB2 that can provide insight into processing of these challenging materials and create new avenues for their use in extreme environment applications, such as thermal protection systems in atmospheric re-entry vehicles. / Doctor of Philosophy / This work examines the use of ultra-high temperature ceramics (UHTCs), which are materials with some of the highest melting points in existence. These are an intriguing option for extreme environment applications. One such application is the protection of rockets, scramjets, and other hypersonic (speed > Mach 5) vehicles from the high temperatures experienced during flight and re-entry. In this work, the UHTC Zirconium diboride (ZrB2) is used as a reference material. For many of the same reasons UHTCs such as ZrB2 have extreme melting points, they can be difficult to manufacture, particularly in complex shapes. Like many ceramics, UHTCs are not melted and cast as metals are, but rather are processed in powder form to a compact known as a green body. The green body is placed in a high-temperature furnace at 2/3 - 3/4 of the melting point, where the powder undergoes sintering, or consolidation into a dense part. The manufacture of a green body that is versatile in its capacity to be molded into any shape, and allows for close packing of the particles in the powder compact to avoid failure-inducing flaws in the final component under intense loads, remains a challenge for UHTCs. Most UHTCs are hot pressed, where the powder alone is consolidated under intense heat and pressure, but this process offers very little complex shaping capacity or control of the uniformity of the part. In this work, three methods for green body manufacture using colloid-based routes, which all have unique capabilities and challenges, are described. The first process is slip casting, which is a centuries-old process that has been used for the manufacture of pottery, whitewares, and art ceramics. When used effectively, slip casting ensures that the forces between ceramic particles in a suspension, or "slip", are well-controlled such that the ceramic particles will not form clumps, or agglomerates, which create non-uniformities that weaken the final component. With information about the powder, solvent, and additives in a slip, the extent to which this will be effective can be predicted with mathematical models. This work compares the results of these models with slip casting suspensions in different solvent environments to gain knowledge about slip casting as an option for complex shaping of ZrB2. The second colloidal process discussed is gelcasting, in which the suspension of ceramic powder can undergo chemical gelation, or a reaction that transitions the suspension from a liquid to a solid, not unlike that of a natural gel such as gelatin, agarose, or albumin (egg white). The gel, which is loaded with ceramic powder, allows for more versatile shaping than slip casting, and shorter processing time; a gelcast ceramic is generally solidified in less than an hour, while a slip cast typically dries overnight. The presence the gel also provides strength to the green body, which is advantageous in handling as well as any machining to adjust the shape that may be necessary prior to sintering. The final process detailed in this work is direct ink writing, a type of additive manufacturing (or 3D printing). Knowledge gained from slip casting and gelcasting was used to carefully design a ceramic colloid that could be deposited in a layer-by-layer fashion to create a complex shape with high uniformity and control, as well as minimal surface cracking. The printed green bodies were compared in strength and sintering behavior to the gelcasts from previous chapters, and the expansion of shaping capacity for each route as it relates to aerospace applications, is described.

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To analyze the impact of different objective functions and additive manufacturing (AM)constraints on structural topology optimization, it is necessary to perform an in-depth comparative study. This analysis should consider specific structural design factors, such as compliance, volume, or stress minimization, and assess their effects on the topology optimization for AM. In addition, the inclusion of AM constraints can have a significant influence on various aspects, including optimal part geometry, part volume, support structure volume, and structural performance. Thus, it is essential to examine and compare these factors to determine the optimal part design for AM. This study focuses on comparing topology optimization results obtained using compliance, stress, or multi-objective minimization, with and without AM constraints. The comparative analysis is conducted in the study, utilizing four structural design examples: cantilever beam, bridge-shaped structure, L-shaped beam and connecting rod. The comparison results provide insights into the effects of build orientation, AM constraints such as overhang, and different design objectives on the structural topology optimization for AM

Thermoelectric generators convert heat energy to electricity and can be used for waste heat recovery, enabling sustainable development. Selective Laser Melting (SLM) based additive manufacturing process is a scalable and flexible method that has shown promising results in manufacturing high ZT Bi2T e3 material and is possible to be extended to other material classes such as M g2Si. The physical phenomena of melting and solidification were investi- gated for SLM-based manufacturing of thermoelectric (M g2Si) powders through comprehen- sive numerical models developed in MATLAB. In this study, Computational Fluid Dynamics (CFD)-based techniques were employed to solve conservation equations, enabling a detailed understanding of thermofluid dynamics, including the temperature evolution and the con- vection currents of the liquid melt within the molten pool. This approach was critical for optimizing processing parameters in our investigation, which were also used for printing the M g2Si powders using SLM. Additionally, a phase field-based model was developed to sim- ulate the directional solidification of the M g2Si in MATLAB. Microstructural parameters like the Secondary and Primary Dendritic Arm Spacing were studied to correlate the effects of processing parameters to the microstructure of M g2Si. / Master of Science / Thermoelectric generators are devices that transform heat energy into electricity, offering a way to capture and utilize waste heat for sustainable purposes. A cutting-edge manufacturing method called Selective Laser Melting (SLM) has shown great potential in creating high-performance materials like Bi2T e3 for thermoelectric applications. Researchers are now exploring the extension of this technique to other materials, such as Mg2Si. This study delves into the intricate process of melting and solidifying Mg2Si powders using SLM. Advanced computer models were created in MATLAB, to simulate these processes in detail. By employing Computational Fluid Dynamics (CFD) techniques, heat and fluid flow within the molten material was also closely examined. These simulations were vital for fine-tuning the printing settings used to fabricate Mg2Si powders via SLM. Moreover, a specialized model based on phase field theory was developed to mimic the solidification of Mg2Si. The effects of changing manufacturing parameters on the microstructure of the final product were examined. Understanding these microstructural aspects is crucial for optimizing the manufacturing process and ultimately enhancing the performance of Mg2Si for thermoelectric applications.

Mechanical strength of polylactic acid (PLA) is increasingly relevant with time because of its attractive mechanical properties and 3D printability. Additive manufacturing (AM) methods, such as fused deposition modeling (FDM), stereolithography (SLA), and selective laser sintering (SLS), serve a vital role in assisting designers with cheap and efficient generation of the desired components. This document presents research to investigate the anisotropic response of multi-oriented PLA subjected to multiple monotonic loading conditions. Although empirical data has previously been captured for multi-oriented PLA under tensile and compressive loading conditions, the data has yet to be applied with regard to a representative component geometry. The tensile and compressive empirical data were ultimately used to develop elastic and yield constitutive models which aided in the characterization of PLA under torsion and bending. This representative component geometry is expected to experience a combined torsion and bending load condition in an effort to address this integral gap in the mechanical properties of multi-oriented PLA. In addition to the acquired empirical data, finite element analysis (FEA) and analytical modeling are employed to supplement the accurate modeling of future component analysis. As a result of the proposed array of experiments, the torsional and bending capabilities of PLA are forecasted to vary based on the print orientation. Lastly, the broader impact of this work is dedicated to addressing the material's capability to operate in environments which possess significant torsion and bending such as model aircraft wings and shafts for remote controlled cars.

This study proposed a utility-driven two-stage stochastic mixed-integer linear programming model to understand how the patient preferences impact the additive manufacturing (AM) supply chain design decisions. The goal of the mathematical model is to maximize the utilities derived from the customer preferences by appropriately allocating the AM facilities in the targeted region under customer decision and demand uncertainty. The mathematical model is visualized and validated by developing a real-life case study that utilizes the biomedical implants data for the state of Mississippi. A number of sensitivity analyses are conducted to understand how the patients' behavioral decisions (e.g., price-centric versus time- or quality-centric customers) to purchase biomedical implants impact the AM supply chain design decisions. The results revealed key managerial insights that could be utilized by healthcare service providers and interested stakeholders to provide quality healthcare services by managing patient-centric AM facility siting decisions.