This investigation was concerned with developing a procedure for using nonexpendable pattern materials in ceramic shell piece molds. Literature relating to this study indicated that nonexpendable materials, used in whole ceramic shell molds, had been limited to frozen mercury.

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In the power generation sector, more specifically, the gas turbine industry, competition has forced the lead time-to-market for product advancements to be more important than ever. For design engineers, this means that product design iterations and final product development must be completed within both critical time windows and budgetary constraints. Therefore, two areas that have received significant attention in the research and in practice are: (1) rapid prototyping technology development, and (2) rapid prototyping technology selection. Rapid prototyping technology selection is the focus of this research. In practice, selecting the rapid prototyping method that is acceptable for a specific design application is a daunting task. With technological advancements in both rapid prototyping and conventional machining methods, it is difficult for both a novice design engineer as well as an experienced design engineer to decide not only what rapid prototyping method could be applicable, but also if a rapid prototyping method would even be advantageous over a more conventional machining method and where in the manufacturing process any of these processes would be utilized. This research proposes an expert system that assists a design engineer through the decision process relating to the investment casting of a superalloy gas turbine engine component. Investment casting is a well-known technique for the production of many superalloy gas turbine parts such as gas turbine blades and vanes. In fact, investment-cast turbine blades remain the state of the art in gas turbine blade design. The proposed automated expert system allows the engineer to effectively assess rapid prototyping iii opportunities for desired gas turbine blade application. The system serves as a starting point in presenting an engineer with commercially-available state-of-the-art rapid prototyping options, brief explanations of each option and the advantages and disadvantages of each option. It is not intended to suggest an optimal solution as there is not only one unique answer. For instance, cost and time factors vary depending upon the individual needs of a company at any particular time as well as existing strategic partnerships with particular foundries and vendors. The performance of the proposed expert system is assessed using two real-world case studies. The first case study shows how the expert system can advise the design engineer when suggesting rapid manufacturing in place of investment casting. The second case study shows how rapid prototyping can be used for creating part patterns for use within the investment casting process. The results from these case studies are telling in that their implementations potentially result in an 82 to 94% reduction in design decision lead time and a 92 to 97% cost savings.

<p>In the last forty years, most of researches in casting fields especially in semi-solid metal state were dedicated to find new ways to enable near net shaped casting of Al alloys to improve the product properties and decreases the product cost. The thixoforming and rheocasting processes are presented as a ways by which the microstructure of the alloys can be changed to non-dendritic microstructure leading to improve the mechanical properties by mitigating the defect associated with the dendritic microstructure. Unfortunately, these processes have proved to be capital cost prohibitive and complicated for commercial production. Further, near net shaped casting of Al wrought alloys along with the superior properties and performance of these alloys have been a challenge for conventional casting routes due to the main disadvantage of hot tearing or hot cracking during solidification, which renders the cast component ineffective. To overcome the disadvantages of thixoforming and rheocasting processes, Controlled diffusion solidification (CDS) process was innovated to enable casting aluminum alloys with a non-dendritic morphology of the primary Al phase in the resultant cast microstructure and thus alleviating the problem of hot tearing and obtaining a cost effective product with improved mechanical properties. The CDS is a simple process involving mixing of two precursor alloys of different thermal masses (temperature and solute) and subsequently cast the resultant mixture of the desired solute composition and temperature as a near net shaped cast product. The process lends itself to easy commercialization with a marginal capital cost required for set up such as the addition of an extra holding furnace. Further, the CDS process would prove itself to be unique in its ability to cast Al based wrought alloys into near net shaped components without additional processes and cost.</p><p>The CDS process has been proven to yield a cast product with a non-dendritic Al phase morphology and this dissertation presents the in-depth details and analysis of the various events occurring during the process to obtain a successful cast part. The process involves various inter-related events such as mixing, re-distribution of thermal field, redistribution of solute field, three types of nucleation events and growth of these different nuclei. Further the dissertation aims to present a study of the critical parameters such as temperatures of the two pre-cursor alloys, initial mass ratio of these alloys and the rate of mixing them on the effectiveness of the CDS process.</p> <p>The results from this study shows that mixing two precursor alloys to form the final desired alloy presents a natural environment for copious nucleation events aided by distribution of these nuclei by forced convection followed by the formation of unique cells in the resultant mixture (micro-scale) with significant thermal and solute gradients. The solidification in the CDS process is unique and different from conventional casting process in that initial growth of the nuclei takes place with the solute diffusing towards and temperature diffusing away from the solid/liquid interface which presents a favorable environment for a stable unperturbed growth of the solid/liquid interface resulting in a non-dendritic morphology of the primary AI phase.</p><p>The proposed events in the CDS process has been verified with a few Al based wrought alloys and organic alloy systems.</p> / Thesis / Doctor of Philosophy (PhD)

Experiments were conducted to characterize and compare the localized corrosion susceptibility of the granular microstructure of aluminum (Al) alloy AA7050 in the peak aged T6 condition cast by the novel controlled diffusion solidification (CDS) process against the conventional wrought plate counterpart. CDS is a casting technique that involves mixing two precursor melts at specific temperatures and compositions before solidification. This process avoids the problem of hot tearing by causing copious nucleation of the solid phase within the melt before solidification, reducing the amount of solute segregation and, thus dendritic growth creating a consistently equiaxed microstructure. The effect of the CDS processing route on its relative localized corrosion susceptibility was elucidated by making links between the microstructure heterogeneities and the localized corrosion susceptibility as evaluated in aqueous saline solutions. Microstructures were characterized and compared with the use of the following techniques: 1. Scanning electron microscopy (SEM) to characterize grain size, shape and distribution. 2. Electron backscattered diffraction (EBSD) in SEM mode to characterize grain misorientation and the associated distribution. 3. Auger electron spectroscopy (AES) to characterize the composition of the grain boundary region including the precipitate free zone (PFZ) and the grain boundary precipitates. 4. Atom probe tomography (APT) to define the size, distribution, and composition of the strengthening matrix precipitates along with the grain boundary region (PFZ and grain boundary precipitates). Electrochemical experiments were conducted to characterize and compare the localized corrosion susceptibility of the two materials (CDS and conventional wrought) exhibited in aqueous saline solutions. Specific techniques include the following: i. Potentiodynamic polarization measurements of mechanically-abraded surfaces to determine the corrosion potential (Ecorr) and breakdown potential (Eb). ii. Potentiostatic anodic polarization of mechanically-abraded surfaces to observe the mode and extent of localized corrosion. iii. Open-circuit potential (OCP) measurements of fracture surfaces to determine the OCP of a surface with a significantly higher grain boundary area fraction relative to the bulk material. iv. Cyclic acidified salt (sodium chloride (NaCl)) fog testing (ASTM-G85-Annex 2) to validate the relative localized corrosion susceptibility under more realistic atmospheric corrosion exposure conductions. The CDS casting technique resulted in an entirely equiaxed microstructure. The microstructure was isotropic with an average grain size of 25±1 µm and an aspect ratio of around 1. This grain structure was in stark contrast with the wrought material, which exhibited a granular structure elongated along the rolling direction. The wrought material had a cord length of 56±3.2 µm in the rolling direction, 51±3 µm in the traverse direction and 13.3±1.6 µm in the short traverse direction. The wrought material had an aspect ratio of around 4 in the longitudinal plane (LS), 2.6 in the short transverse plane (ST) and 1.2 in the rolling plane (LT). AES and APT revealed that the CDS material had a higher amount of copper (Cu) segregation into the grain boundary precipitates. Electrochemical testing showed that the wrought material had a Eb of −750 ± 3 mV while the CDS had a higher Eb of −697 ± 4 mV. The Cu segregation into the grain boundary precipitates yielded more electrochemically active grain boundaries, as revealed by the OCP measurements. Despite this fact, localized corrosion of the CDS material initiated as pitting and propagated as a mixed mode involving intergranular corrosion (IGC) and pitting. The localized corrosion mode exhibited by the wrought material was purely IGC: both in initiation and propagation. The difference in corrosion mode was found to be due to the differences in the size of the Fe-based IMPs and the distribution of the Cu secondary phase precipitates: The CDS had large Fe IMP trapped at the grain boundary triple points and clustering of Cu secondary phase precipitates. Conversely, the wrought material had finely dispersed Fe IMP of significantly smaller size than those found in the CDS, and its Cu secondary phase precipitates are evenly distributed along the grain boundaries. These differences in precipitate distribution enhanced susceptibility for pitting in the CDS and reduced the driving force for IGC. The propagation of localized corrosion was markedly reduced in the CDS material: about half of that exhibited by the wrought material (under identical exposure conditions). Cyclic acidified salt fog testing revealed industry acceptable levels of localized corrosion susceptibility in-line with the benchmark alloys that are currently used in automotive applications. / Thesis / Master of Applied Science (MASc)

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Every year, accidents, falls, sport injuries and other incidents cause thousands of people to suffer spinal cord injury (SCI). In the United States alone, it is estimated that the number of Americans that live with SCI is around 259,000, with 12,000 new cases that happen annually (1). These injuries lead to spinal cord damages expressed by massive nerve tract degeneration followed by neurological loss, paralysis and disabilities. Therapy of SCI patients with non-steroidal anti-inflammatory drugs (NSAIDs) help in diminishing secondary injury and lessen pain and swelling. However these drugs do not promote tissue repair. Therefore there is an unmet clinical need to develop technologies and therapeutic strategies that compensate loss of neuronal tissue, support and facilitate reestablishment of nerve tracks connectivity in the injured spinal cord. Recent progress in nerve regeneration indicates that a tissue engineering approach using soft tissue scaffolds, stem cells and neurotrophins, can lead to a partial therapy in animal models of SCI. Bioengineered scaffolds prepared by freeze casting technology provide an experimental tool for guidance of regenerating neuronal tracts and/or axons and therefore are useful for regeneration of injured spinal cord. In this engineering approach for scaffold preparation, temperature controlled directional solidification of an aqueous polymer(s) solution creates channels of different diameters that can direct axonal outgrowth of neurons populating the scaffold. In a previous study from our laboratory, such scaffolds promoted differentiation of neurons, a process facilitated by co-population of the scaffold’s channels with endothelial cells. “Green” plant proteins, such as soybean proteins, are becoming an attractive alternative source of natural polymers for a variety of biomedical applications including scaffold fabrication for neuronal tissue regeneration. In the present study, we developed a second generation of improved, microchanneled composite scaffolds from gelatin and soy protein isolate cross-linked with genipin (2 w/v %, 0.5 w/v %, 1 w/v %, respectively). The fabrication of these scaffolds by a controlled freeze drying technique, their mechanical properties (stiffness, ~3-4 kPa) as well as their uniform longitudinal channels of a diameter of ~30-55 µm is described. Preliminary biocompatibility experiments in 2D and 3D using the above mentioned scaffolds populated with either undifferentiated PC12 cells or nerve growth factor differentiated PC12 cells indicated partial biocompatibility of the scaffolds for neuronal growth. Improving the biocompatibility of these composite scaffolds is under investigation in our laboratory. / Bioengineering