<p>This study is focused on synthesizing Nitinol by additive manufacturing that can provide desirable mechanical properties for orthopedic implants and adding functionally gradient coating that can enhance both safety and biocompatibility for orthopedic implant applications.</p> <p><br></p> <p>The characteristics of additively manufactured Nitinol, by using the direct energy deposition (DED) technique, were experimentally studied. Because of a unique layer-by-layer manufacturing scheme, the microstructure and associated properties (mechanical and thermo-mechanical properties) of the DED Nitinol is different compared to conventionally produced Nitinol. Both the feasibility of manufacturing defect-free microstructure and the precise control of chemical composition were demonstrated. Effects of chemical compositions and post heat-treatment conditions on the phase transformation temperatures of the DED Nitinol were systematically analyzed and compared with those of conventional Nitinol. More precise control of phase transformation temperature from DED Nitinol was possible due to incoherent precipitate formation during aging heat treatment. In a similar way, the mechanical properties of the DED Nitinol were less sensitive to its chemical compositions and post heat-treatment conditions. The feasibility of the precise control of both mechanical and thermo-mechanical properties of the DED Nitinol was demonstrated which can broaden its applications. </p> <p><br></p> <p>The bulk polycrystalline properties of the NiTi phase were studied via molecular dynamics (MD) simulations. Thermo-mechanical properties that are highly sensitive to chemical composition were not precisely predicted from previous reports and studies. In this study, realistic boundary conditions were applied to calculate bulk polycrystalline properties. Thermally driven phase transitions of NiTi between martensite and austenite are simulated with external stresses in both normal and shear directions. It is shown that phase transformation temperatures are affected by applied external stresses, and realistic values compared to experimental data are correctly predicted only when external stresses in both normal and shear directions are similar to the experimentally observed values of 0.05 – 0.1 GPa. The experimentally observed grain orientation and grain boundary thickness were applied to simulation domains for the prediction of the elastic moduli. The elastic moduli of polycrystalline NiTi structure was calculated as 52 GPa which is close to the experimentally reported value of 20-40 GPa while other studies predicted over 85 GPa. </p> <p><br></p> <p>Lastly, pure titanium gradient layers were coated on the Nitinol surface for orthopedic implant applications to eliminate potentially toxic Ni ion release. Using the DED technique, both the core Nitinol and titanium gradient layers were manufactured with high purity and without microstructural defects. An additional biomedical coating of Hydroxyapatite (HA) was deposited on the outer surface using the cold spray technique. The resultant bonding strength was determined to be 26 MPa which exceeded the requirement of the ISO-13779 standard (15 MPa). The <em>in vitro</em> test of the Ni release rate from the entire gradient Nitinol structure was very low, which was comparable to drinking water.</p>

A body exposed to hypersonic flow is subjected to extremely high wall heating rates, owing to the conversion of the kinetic energy of the oncoming flow into heat through the formation of shock waves and viscous dissipation in the boundary layer and this is one of the main concerns in the design of any hypersonic vehicle. The conventional way of tackling this problem is to use a blunt fore-body, but it also results in an increase in wave drag and puts the penalty of excessive load on the propulsion system. An alternative approach is to alter the flow field using external means without changing the shape of the body; and several such methods are reported in the literature. The superiority of such methods lie in the fact that the effective shape of the body can be altered to meet the requirements of low wave drag, without having to pay the penalty of an increased wall heat transfer rate. Among these techniques, the use of local energy addition in the freestream to alter the flow field is particularly promising due to the flexibility it offers. By the suitable placement of the energy source relative to the body, this method can be effectively used to reduce the wave drag, to generate control forces and to optimise the performance of inlets. Although substantial number of numerical investigations on this topic is reported in the literature, there is no experimental evidence available, especially under hypersonic flow conditions, to support the feasibility of this concept. The purpose of this thesis is to experimentally investigate the effect of energy deposition on the flow-field of a 120� apex angle blunt cone in a hypersonic shock tunnel. Energy deposition is done using an electric arc discharge generated between two electrodes placed in the free stream and various parameters influencing the effectiveness of this technique are studied. The effect of energy deposition on aerodynamic parameters such as the drag force acting on the model and the wall heat flux has been investigated. In addition, the unsteady flow field is visualised using a standard Z-type schlieren flow visualisation setup. The experimental studies have shown a maximum reduction in drag of 50% and a reduction in stagnation point heating rate of 84% with the deposition of 0.3 kW of energy. The investigations also show that the location of energy deposition has a vital role in determining the flow structure; with no noticeable effects being produced in the flow field when the discharge source is located close to the body (0.416 times body diameter). In addition, the type of the test gas used is also found to have a major influence on the effectiveness of energy deposition, suggesting that thermal effects of energy deposition govern the flow field alteration mechanism. The freestream mass flux is also identified as an important parameter. These findings were also confirmed by surface pressure measurements. The experimental evidence also indicates that relaxation of the internal degrees of freedom play a major role in the determination of the flow structure. For the present experimental conditions, it has been observed that the flow field alteration is a result of the interaction of the heated region behind the energy spot with the blunt body shock wave. In addition to the experimental studies, numerical simulations of the flow field with energy deposition are also carried out and the experimentally measured aerodynamic drag with energy deposition is found to match reasonably well with the computed values.

In recent years, additive manufacturing (AM) of Alloy 718 has received increasing interest in the field of manufacturing engineering owing to its attractive features compared to those of conventional manufacturing methods. The ability to produce complicated geometries, low cost of retooling, and control of the microstructure are some of the advantages of the AM process over traditional manufacturing methods. Nevertheless, during the building process, the build material undergoes complex thermal conditions owing to the inherent nature of the process. This results in phase transformation from liquid to solid and solid state. Thus, it creates microstructural gradients in the built objects, and as a result,heterogeneous material properties. The manufacturing process, including the following heat treatment that is used to minimise the heterogeneity, will cause the additively manufactured material to behave differently when compared to components produced by conventional manufacturing methods. Therefore, understanding the microstructure formation during the building and subsequent post-heat treatment is important, which is the objective of this work. Alloy 718 is a nickel-iron based super alloy that is widely used in the aerospace industry and in the gas turbine power plants for making components subjected tohigh temperatures. Good weldability, good mechanical properties at high temperatures, and high corrosion resistance make this alloy particularly suitablefor these applications. Nevertheless, the manufacturing of Alloy 718 components through traditional manufacturing methods is time-consuming and expensive. For example, machining of Alloy 718 to obtain the desired shape is difficult and resource-consuming, owing to significant material waste. Therefore, the application of novel non-conventional processing methods, such as AM, seems to be a promising technique for manufacturing near-net-shape complex components.In this work, microstructure modelling was carried out by using multiphase-field modelling to model the microstructure evolution in electron beam melting (EBM) and laser metal powder directed energy deposition (LMPDED) of Alloy 718 and x subsequent heat treatments. The thermal conditions that are generated during the building process were used as input to the models to predict the as-built microstructure. This as-built microstructure was then used as an input for the heat treatment simulations to predict the microstructural evolution during heat treatments. The results showed smaller dendrite arm spacing (one order of magnitude smaller than the casting material) in these additive manufactured microstructures, which creates a shorter diffusion length for the elements compared to the cast material. In EBM Alloy 718, this caused the material to have a faster homogenisation during in-situ heat treatment that resulting from the elevated powder bed temperature (&gt; 1000 °C). In addition, the compositional segregation that occurs during solidification was shown to alter the local thermodynamic and kinetic properties of the alloy. This was observed in the predicted TTT and CCT diagrams using the JMat Pro software based on the predicted local segregated compositions from the multiphase-field models. In the LMPDED Alloy 718 samples, this resulted in the formation of δ phase in the interdendritic region during the solution heat treatment. Moreover, this resulted in different-size precipitation of γ'/γ'' in the inter-dendritic region and in the dendrite core. Themicro structure modelling predictions agreed well with the experimental observations. The proposed methodology used in this thesis work can be an appropriate tool to understand how the thermal conditions in AM affect themicro structure formation during the building process and how these as-built microstructures behave under different heat treatments.

The corrosion resistance under elevated temperature of additively manufactured 316L stainless steel made by directed energy deposition was studied. Test samples were prepared in a hybrid additive manufacturing machine using standard deposition parameters recommended by the manufacturer. Control samples were cut from wrought material to compare the results. The test was performed under a corrosive atmosphere with a solution of water with 3.5 % in weight of salt (NaCl). The total duration of the test was 635 hours, divided in five stages of 12, 24, 48, 226, and 325 hours to analyze the samples between each stage. The samples were analyzed quantitatively measuring weight loss and surface topography, and qualitatively by macroscopic inspection with digital photography, and microscopic inspection with optical and scanning electron microscopy. The results show a higher corrosion rate for the additively manufactured samples compared to the control samples. An evident increase in the size of pits initially present on the samples was observed and quantified on the additively manufactured. Although the additively manufactured samples were more aggressively attacked by corrosion, they still presented a shiny surface finish at the end of the test, reinforcing the idea of the formation of a passive oxide layer and suggesting that the corrosion was focalized in the surface defects by pitting and crevice corrosion mechanisms.

No description available.

The betavoltaic battery is a type of micro nuclear battery that harvests beta emitting radioactive decay energy using semiconductors. The literature results suggest that a better model is needed to design a betavoltaic battery. This dissertation creates a comprehensive model that includes all of the important factors that impact betavoltaic battery output and efficiency. Recent advancements in micro electro mechanical systems (MEMS) necessitate an onboard miniaturized power source. As these devices are highly functional, longevity of the power source is also preferred. Betavoltaic batteries are a very promising power source that can fulfill these requirements. They can be miniaturized to the size of a human hair. On the other hand, miniaturization of chemical batteries is restricted by low energy density. That is why betavoltaics are a viable option as a power source for sophisticated MEMS devices. They can also be used for implantable medical devices such as pacemakers; for remote applications such as spacecraft, undersea exploration, polar regions, mountains; military equipment; for sensor networks for environmental monitoring; and for sensors embedded in bridges due to their high energy density and long lifetime (up to 100 years). A betavoltaic battery simulation model was developed using Monte Carlo particle transport codes such as MCNP and PENELOPE whereas many researchers used simple empirical equations. These particle transport codes consider the comprehensive physics theory for electron transport in materials. They are used to estimate the energy deposition and the penetration depth of beta particles in the semiconductors. A full energy spectrum was used in the model to take into account the actual radioactive decay energy of the beta particles. These results were compared to the traditional betavoltaic battery design method of estimating energy deposition and penetration depth using monoenergetic beta average energy. Significant differences in results were observed that have a major impact on betavoltaic battery design. Furthermore, the angular distribution of the beta particles was incorporated in the model in order to take into account the effect of isotropic emission of beta decay. The backscattering of beta particles and loss of energy with angular dependence were analyzed. Then, the drift-diffusion semiconductor model was applied in order to estimate the power outputs for the battery, whereas many researchers used the simple collection probability model neglecting many design parameters. The results showed that an optimum junction depth can maximize the power output. The short circuit current and open circuit voltage of the battery varied with the semiconductor junction depth, angular distribution, and different activities. However, the analysis showed that the analytical results overpredicted the experimental results when self-absorption was not considered. Therefore, the percentage of self-absorption and the source thickness were estimated using a radioisotope source model. It was then validated with the thickness calculated from the specific activity of the radioisotope. As a result, the battery model was improved significantly. Furthermore, different tritiated metal sources were analyzed and the beta fluxes were compared. The optimum source thicknesses were designed to increase the source efficiencies. Both narrow and wide band gap semiconductors for beryllium tritide were analyzed. / PHD

In an effort to make the aerospace industry more sustainable, GKN Aerospace is heavily investing both time and money into additive manufacturing technologies. A challenging aspect with additive manufacturing is finding a set of process parameters that produces high quality parts and components to a standard that the aerospace industry demands. To aid the process development, turning to simulations is a great alternative and with the prospect of adding to the tools available, the work of this thesis has been focused on developing a mathematical model of the cross sectional deposition geometry. Through a literature study, an initial approach to developing such a model, as well as gaps in knowledge was established. Validation data was gathered by laser scanning additively manufactured builds. A model based on a fourth degree polynomial was developed. The fourth degree polynomial model was validated with the laser scan data using the mean squared error value and coefficient of determination as a quantifiable method of determining the goodness of fit. It was found to be an improvement over the common parabolic model found in the literature based on a second degree polynomial. The improved model manages to capture a wide variety of contact angles and the overlap region better follows the smooth transition between beads.

No description available.

Les programmes d’irradiations technologiques menés dans les réacteurs expérimentaux sont d’une importance cruciale pour le soutien du parc électronucléaire actuel en termes d’étude et d’anticipation du comportement sous irradiation des combustibles et des matériaux de structures. Ces programmes permettent d’améliorer la sûreté des réacteurs actuels et également d’étudier les matériaux pour les nouveaux concepts de réacteurs.Les conditions d’irradiations des matériaux dans les réacteurs expérimentaux doivent être représentatives de celles des réacteurs de puissance. Un des principaux intérêts des réacteurs d'irradiations technologiques (Material Testing Reactors, MTRs) est de pouvoir y mener des irradiations instrumentées en ajustant les paramètres expérimentaux, en particulier le flux neutronique et la température. La maîtrise du paramètre température d’un dispositif irradié dans un réacteur expérimental nécessite la connaissance de l'échauffement nucléaire (terme source) dû au dépôt d'énergie des photons et des neutrons interagissant dans le dispositif. La bonne évaluation de cet échauffement est une donnée clé pour les études thermiques de dimensionnement et de sûreté du dispositif.L'objectif de cette thèse est d'améliorer les méthodes d’évaluation de l'échauffement nucléaire en réacteur. Ce travail consiste en l’élaboration d'un schéma de calcul complet innovant, couplé neutron-photon (permettant d’obtenir la contribution des neutrons, des gamma prompts et des gamma de décroissance), fondé principalement sur le code de transport Monte-Carlo TRIPOLI-4 (à 3-dimensions et à énergie continue). Une validation expérimentale du schéma a été effectuée en s’appuyant sur les mesures de calorimétrie réalisées dans le réacteur OSIRIS (CEA Saclay). Des études de sensibilité ont également été menées pour établir l’impact de différents paramètres sur les calculs d’échauffement nucléaire, dont les données nucléaires. Cela a permis de définir le schéma de calcul définitif pour reproduire au plus près la réalité des irradiations technologiques. Le travail de thèse débouche sur un outil opérationnel et prédictif pour l'estimation de l'échauffement nucléaire répondant aux besoins de l’expérimentation en réacteur de recherche et qui peut être étendu plus largement dans des réacteurs de puissance. / Technological irradiation programs carried out in experimental reactors are crucial for the support of the current nuclear fleet in terms of study and anticipation of the behavior under irradiation of fuels and structural materials. These programs make it possible to improve the safety of the current reactors and also to study materials for the new concepts of reactors.Irradiation conditions of materials in experimental reactors must be representative of those of nuclear power plants (NPPs). One of the main advantages of material testing reactors (MTRs) is to be able to carry out instrumented irradiations by adjusting experimental parameters, in particular the neutron flux and the temperature. The control of the parameter temperature of a device irradiated in an experimental reactor requires the knowledge of the nuclear heating (source term) due to the deposition of energy of the photons and the neutrons interacting in the device. A relevant evaluation of this heating is a key data for the thermal studies of design and safety of devices. The objective of this thesis is to improve the methods of the evaluation of nuclear heating in reactors. This work consists of the development of an innovating and complete coupled neutron-photon calculation scheme (allowing to obtain the contribution of neutrons, prompt gamma and decay gamma), mainly based on the TRIPOLI-4 Monte Carlo transport code (with 3-dimensions and continuous energy). An experimental validation of the calculation scheme has been performed, based on calorimetry measurements carried out in the OSIRIS reactor (CEA Saclay). Sensitivity studies have been undertaken to establish the impact of various parameters on nuclear heating calculations (in particular nuclear data) and to fix the final calculation scheme to be closer to the technological irradiation aspects. The thesis work leads to an operational and predictive tool for the nuclear heating estimation, meeting the experimentation needs of research reactors and can be extended more generally to NPPs.

Additive manufacturing (AM) is a quickly rising method of manufacturing due to its ability to increase design freedom. This allows the manufacturing of components not possible by traditional subtractive manufacturing. AM can greatly reduce lead time and material waste, therefore decreasing the cost and environmental impact. The adoption of AM in the aerospace industry requires strict control and predictability of the material deposition to ensure safe flights.  The method of AM for this thesis is Laser Metal Deposition with wire (LMD-w). Using wire as a feedstock introduces a potential problem, the material transfer from the wire to the substrate. This requires all process parameters to be in balance to produce a stable deposition. The first sign of unbalanced process parameters are the material transfer stabilities; stubbing and dripping. Stubbing occurs when the energy to melt the wire is too low and the wire melts slower than required. Dripping occurs when too much energy is applied and the wire melts earlier than required.  These two reduce the predictability and stability that is required for robust manufacturing.  Therefore, the use of thermal simulations to predict the material transfer stability for LMD-w using Waspaloy as the deposition material has been studied.  It has been shown that it is possible to predict the material transfer stability using thermal simulations and criterions based on preexisting experimental data. The criterion for stubbing checks if the completed simulation result produces a wire that ends below the melt pool. For dripping two criterions shows good results, the dilution ratio is a good predictor if the tool elevation remains constant. If there is a change in tool elevation the dimensionless slenderness number is a better predictor.  Using these predictive criterions it is possible to qualitatively map the process window and better understand the influence of tool elevation and the cross-section of the deposited material. / Additiv tillverkning (AT) är en kraftigt växande tillverkningsmetod på grund av sin flexibilitet kring design och möjligheten att skapa komponenter som inte är tillverkningsbara med traditionell avverkande bearbetning.  AT kan kraftigt minska tid- och materialåtgång och på så sett minskas kostnader och miljöpåverkan. Införandet av AT i flyg- och rymdindustrin kräver strikt kontroll och förutsägbarhet av processen för att försäkra sig om säkra flygningar.  Lasermetalldeponering av tråd är den AT metod som hanteras i denna uppsats. Användandet av tråd som tillsatsmaterial skapar ett potentiellt problem, materialöverföringen från tråden till substratet. Detta kräver att alla processparametrar är i balans för att få en jämn materialöverföring. Är processen inte balanserad syns detta genom materialöverföringsstabiliteterna stubbning och droppning. Stubbning uppkommer då energin som tillförs på tråden är för låg och droppning uppkommer då energin som tillförs är för hög jämfört med vad som krävs för en stabil process. Dessa två fenomen minskar möjligheterna för en kontrollerbar och stabil tillverkning.  På grund av detta har användandet utav termiska simuleringar för att prediktera materialöverföringsstabiliteten för lasermetalldeponering av tråd med Waspaloy som deponeringsmaterial undersökts. Det har visat sig vara möjligt att prediktera materialöverföringsstabiliteten med användning av termiska simuleringar och kriterier baserat på tidigare experimentell data. Kriteriet för stubbning kontrolleras om en slutförd simulering resulterar i en tråd som når under smältan.  För droppning finns två fungerande kriterier, förhållandet mellan svetshöjd och penetrationsdjup om verktygshöjden är konstant, sker förändringar i verktygshöjden är det dimensionslös ”slenderness” talet ett bättre kriterium.  Genom att använda dessa kriterier är det möjligt att kvalitativt kartlägga processfönstret och skapa en bättre förståelse för förhållandet mellan verktygshöjden och den deponerade tvärsnittsarean.