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This work seeks to characterize the impact of build interruptions to additively manufactured Al-Si-10-Mg produced by the powder bed fusion (PBF) process. Additive manufacturing represents a significant investment in overhead, machine, and material making an interruption to the process a potential waste of money and time. Interruptions in the form of power outages, lack of powdered feedstock, and/or shielding gas will cause the machine to operate in an unintended manner, potentially even stopping the build process. The process of manufacturing will influence the microstructure, which determine the material’s properties and performance. An interrupted PBF process could exhibit unique microstructural features and reduced mechanical properties that distinguish the resulting material from a continuous PBF process. Experiments were performed to simulate a production interruption with varying time periods of interruption and air exposure. The zone of interruption was characterized using optical micrographs, EDS, and hardness measurements to determine any effects of the interruption.

<p> Rising costs and volatility in aviation fuel and increased regulations resulting from climate change  concerns have driven gas turbine engine manufacturers to focus on reducing fuel consumption.  Implementing centrifugal compressors as the last stage in an axial engine architecture allows for  reduced core diameters and higher fuel efficiencies. However, a centrifugal compressor's  performance relies heavily on its stationary diffusion system. Furthermore, the highly unsteady  and turbulent flow field exhibited in the diffusion system often causes CFD models to fall short of  reality. Therefore, rapid validation is required to match the speed at which engineers can simulate  different diffuser designs utilizing CFD. One avenue for this is through the use of additive  manufacturing in centrifugal compressor experimental research. This study focused on implementing a new generation of the Centrifugal Stage for Aerodynamic  Research (CSTAR) at the Purdue Compressor Research Lab that utilizes an entirely additively  manufactured diffusion system. In addition, the new configuration was used to showcase the  benefits of additive manufacturing (AM) in evaluating diffusion components. Two diffusion  systems were manufactured and assessed. The Build 2 diffusion system introduced significant  modifications to the diffusion system compared to the Build 1 design. The modifications included changes to the diffuser vane geometry, endwall divergence, and increased deswirl pinch and vane  geometries. The Build 2 diffusion system showed performance reductions in total and static  pressure rise, flow range, and efficiencies. These results were primarily attributed to the changes  made to the Build 2 diffuser. The end wall divergence resulted in end wall separation that caused  increased losses. The changes to the diffuser vane resulted in increased throat blockage and lower  pressure rise and mass flow rate. In addition to the experimental portion of this study, a computational study was conducted to study  the design changes made to the Build 2 diffusion system. A speedline at 100% corrected rotational  speed was solved, and the results were compared to experimental data. The simulated data matched  the overall stage and diffusion system performance relatively well, but the internal flow fields of  the diffusion components, namely the diffuser, were not well predicted. This was attributed to  16 using the SST turbulence model over BSL EARSM. The BSL EARSM model more accurately  predicted the diffuser flow field to the SST model.  </p>

Triply Periodic Co-continuous piezocomposites, which consist of a ferroelectric-ceramic phase and an elastic-polymer phase continuously interconnected in three dimensions (3D), are emerging flexible piezoelectric materials with high efficiency in absorbing and converting multi-directional mechanical stimuli into electrical signals. Current co-continuous piezocomposites cannot be achieved with controlled piezoelectric properties due to the limited capability of traditional fabrication methods in carefully controlling the morphology of each phase, additive manufacturing such as Suspension-Enclosing Projection-Stereolithography process thus was selected. Porous ceramic skeleton with randomly distributed grain size is commonly observed in sintered ceramic skeleton fabricated by additive manufacturing. The effective piezoelectric properties of the piezocomposites were thus studied utilizing a two-scale method. Through analyzing the simulated results of different process parameters, optimal parameters of 3D printing processes including post-processes was subsequently suggested.

Composite materials have increasingly been used as an alternative to metals and other isotropic materials for primary structural components in aerospace industries. Unlike traditional isotropic materials, composite materials are known to have complex internal microstructures. Therefore, it is essential to develop methods for the inspection, evaluation, and monitoring of composite materials. Ultrasonic-guided waves and, more precisely, Lamb waves have proven to be an efficient and accurate technique for the non-destructive testing. Since guided waves are dispersive and multimodal, it is important to develop a practical method to manipulate Lamb waves to achieve better structural health monitoring and non-destructive inspection results. There are minimal studies involving manipulating guided waves for the inspection of composite materials. Moreover, the currently proposed methods to manipulate Lamb waves are complex and costly. The objective of this dissertation research is to offer practical and straightforward methods with a simple design to control Lamb waves using additively manufactured lenses used as superstrates on composite plates. This dissertation is organized in three major parts. Part I focuses on the Lamb wave propagation in composite plates with different lay-up and plate orientations. Finite element simulations were performed to investigate the behavior of Lamb wave propagation in different plates. A semi-finite element approach was used to derive the dispersive curves in each plate. In Part II, a lap-joint study was conducted to investigate the interaction of Lamb waves in the lap joint regions. Two different lap joints were considered, composite-aluminum and composite-plastic. In each lap joint the thickness of the top surface (aluminum or plastic) is continuously increased. In Part III, additively manufactured lenses are designed to modulate the wavefront of Lamb waves in thick composite plates. The first design is a prism-shaped lens proposed to steer Lamb waves to a targeted direction. Multiple prism designs are considered to offer a flexible steering direction by either changing the prism thickness or the wedge angle. The second design is a plano-concave shaped lens designed to focus the Lamb wave at a targeted focal point. This dissertation research will provide a clear understanding of Lamb wave propagation in anisotropic material, anisotropic-isotropic lap joints, and wavefront modulation on anisotropic material using additively manufactured lenses. This approach contributes to the development of better quality SHM for online monitoring systems.

A two-scale model consisting of ceramic grain scale and composite scale are developed to systematically evaluate the effects of microstructures (e.g., residual pores, grain size, texture) and geometry on the piezoelectric responses of the polarized triply periodic bi-continuous (TPC) piezocomposites. These TPC piezocomposites were fabricated by a recently developed additive manufacturing (AM) process named suspension-enclosing projection-stereolithography (SEPS) under different process conditions. In the model, the Fourier spectral iterative perturbation method (FSIPM) and the finite element method will be adopted for the calculation at the grain and composite scale, respectively. On the grain scale, a DL approach based on stacked generative adversarial network (StackGAN-v2) is proposed to reconstruct microstructures. The presented modeling approach can reconstruct high-fidelity microstructures of additively manufactured piezoceramics with different resolutions, which are statistically equivalent to original microstructures either experimentally observed or numerically predicted. Design maps for hydrostatic piezoelectric charging coefficients dh show they can achieve optimal performance at wide ranges of micro-porosity and geometry parameter u for the proposed TPC piezocomposites. In addition, geometry parameter u plays a dominant role in determining the intensity of hydrostatic voltage coefficient gh and hydrostatic figure of merit (HFOM) of all the presented TPC piezocomposites in the vicinity of the starting point of three-dimension (3D) interconnectivity. Within this range, these properties would increase first with the increasing of micro-porosity volume fraction (VF) and start to decrease once they reach peak values. The presented TPC piezocomposites exhibit a superb hydrostatic properties, with the same 20% VF of ceramics and 2% VF of micro-porosity with respect to composites and ceramics, respectively, TPC of face center cubic (FCC) demonstrates 327-fold enhancement of HFOM than that of the piezocomposite with three intersecting ceramic cuboids. The piezoelectric properties of FCC are superior to those of body center cubic (BCC) and simple cubic (SC). The calculated piezoelectric charging constants d33 and relative permittivity κ33 were then compared with the data measured from the products fabricated by the SEPS under different process conditions. The calculation results at both grain scale and composite scale were found to agree well with experimental results.

<p>Thesis Abstract: Laser powder bed fusion (LPBF), a specialization within additive manufacturing, is a high precision metal powder processing technique that has gained immense attentions in the past decade. The layer-by-layer densification technique provides a unique set of abilities that permits the large-scale production of geometrically complicated structures with highly tunable microstructures. Alloy 718 (718) is one of the most studied materials within the LPBF field due to its extraordinary printability. Although it has a significant industrial and academic focus, there are consequential questions that still need to be addressed because of the immense LPBF design space.</p><p>Our works demonstrate the multiple pathways that an alloy system like 718 can be optimized for specific applications by altering the processing parameters or by the addition of oxide particles to create a fine dispersion for high temperature capabilities. Room temperature tensile testing revealed that the processing parameters directly controlled the mechanical properties, allowing tailoring of the tensile strength and elongation to the needs of specific applications. Similar experiments were conducted to exhibit the flexibility of LPBF by incorporating a wider, economic, bimodal powder size distribution that maintained similar mechanical properties. Additions of oxide particles enabled the findings of the reactive nature within this welding process, which ultimately led to a refined oxide dispersion strengthened (ODS) 718 matrix with superior mechanical properties up to 900$^\circ$C. This novel metal matrix ceramic was lastly showcased by producing a complex microlattice structure. Detailed in-situ tensile tests in combination with electron backscatter diffraction (EBSD) and finite element modeling revealed that crystallographic reorientation around bending nodes enhanced the global ductility of the material.</p>

<p>Fabricating complex submicron 3D structures can be achieved by multi-photon lithography, especially two-photon lithography is commonly used to obtain precision and flexibility in printing sophisticated sub-micron 3D structures. Several disadvantages stemmed from a two-photon lithography experiment setup, including cost, the necessity of a large laboratory space to use a femtosecond laser and a high-order process. A two-step absorption is chosen instead of two-photon lithography as a primary excitation process achieving the same degree of quadratic optical non- linearity as two-photon lithography at a lower cost with a relatively compact laboratory size. The working mechanism of Two-step absorption is the following. Quadratic nonlinearity comes from radicals from excited triplet states photoinitiators. Ground states of photoinitiators get excited by the incident laser. Those excited singlet photoinitiators go through the intersystem crossing, becoming the ground triplet state of photoinitiators. There are two branches after the ground triplet states, especially for photoinitiator benzil molecules with the incident laser on. Either it becomes a radical without photons received from the incident laser or gets excited again to an excited triplet state by the incident laser. Those excited triplet-state photoinitiator molecules become radicals that occur in polymerization. However, those from the ground triplet states add linearity to polymerization. When it comes to multiple exposures, the linearity becomes problematic, especially outside the region and tails of the voxel. For example, suppose the intensity at two tails of the voxel is 1% relative to the maximum intensity at the focal point. In that case, the absorbed dose will be added up to the maximum intensity at the focal point when it comes to 100 exposures. Quadratic nonlinearity and linearity are jumbled together in the current two-step absorption process. In this work, optimization of photoinitiator concentration was conducted to reduce the linearity. Confined and high throughput 3D structure fabrications are achieved by controlling initiator depletion. Simulations are also developed with multi-physics models to compare with the empirical results.</p>

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