Dissertationen zum Thema „Cold kinetic deposition (cold spray)“

The purpose of this diploma thesis is to get a better understanding of cold kinetic deposition (cold spray), principles of functioning of this method followed by an evaluation of advantages and disadvantages of cold spray and its comparison to conventional thermal methods and a simple summary of the practical use of cold spray with respect to different materials. Next there is a summary of the properties and uses of frequently applied metals in electrical engineering, aluminium and copper, description of metal corrosion and an understanding of the diagnostic method of acoustic emission. In the practical part, a sample with copper cold spray coating on aluminium substrate was created. Following, this sample was split for corrosion tests, where the split samples were exposed to a corrosive environment for different times of exposure. The extent of corrosion degradation of the samples was evaluated by acoustic emission and metallographic analysis for corrosion-loaded samples for 100, 200 and 300 hours. In the end, an illustrative design of the application of the cold spray technology was created.

Ti2AlC coatings have been fabricated by cold-spray deposition. The microstructure evolution as a function of basic spray parameters temperature and pressure onto AA6060 aluminium alloy and 1.0037 steel substrates has been studied. Adherent and dense 50–80 μm thick Ti2AlC coatings were deposited on soft AA6060 substrates under gas temperature and pressure of 600 °C and 3.4 MPa, respectively, whilst comparable results were obtained on harder 1.0037 steel by using higher temperature (800 °C) and pressure (3.9 MPa).

The main subject of this Thesis is the production of hard, wear and corrosion resistant cermets tungsten carbide and cobalt cermets (WC-Co) with different contents in cobalt matrix, onto low carbon steels and aluminum alloy Al7075-T6 substrates, by means of Cold Gas Spray (CGS). The current state of the art for the deposition of WC-Co uses High Velocity Oxy-Fuel (HVOF) as the main technique. Understanding both techniques was also one of the keys points in this work. A deep theoretical approach about the CGS process, in which no melting of the particles occurs, was made at first to gain a better comprehension about the behaviour of the powder particles when sprayed onto different substrates and therefore being able to produce good quality coatings. The starting purpose of this doctoral Thesis was to produce WC-25, 17 and 12%Co coatings onto low carbon steel and Al7075-T6 substrates. Until the day, using nitrogen as the process gas, such coatings could not be produced with enough adhesion, thickness and wear and corrosion properties. These are the main characteristics sought by the industry in these coatings. In the end of this doctorate WC-Co coatings were obtained with excellent mechanical and electrochemical properties, adhesion to both low carbon steel and Al7075-T6 substrates. Besides, these properties were increased and improved when compared to the same WC-Co coatings obtained by HVOF conventional deposition technique. Initial problems such as flowability of the powders, bad adherence to the substrate, poor coating quality and extremely low deposition efficiencies were resolved during the period of the Thesis. Also, and taking advantage of the novel coatings and excellent properties obtained using the referred feedstock powders and substrates, the knowledge was transferred to the industry as a trade secret.
En primer lugar, el objetivo principal de este trabajo de investigación fue proporcionar un nuevo método de deposición para depositar cermets de WC-Co. Esta nueva tecnología proporcionó nuevos recubrimientos sin ninguna descomposición de la microestructura del polvo inicial y por lo tanto la mejora de las presentes aplicaciones de WC-Co en la gran industria. La deposición de cermets de WC-Co resistentes al desgaste ha sido siempre una de las principales aplicaciones de las técnicas de proyección térmica convencionales como por ejemplo High Velocity Oxy-Fuel (HVOF). Las demandas de la industria en términos de producción y la necesidad y constante búsqueda de mejores propiedades mecánicas y electroquímicas conducen al objetivo principal y la motivación de esta tesis: la producción de nuevos y mejores recubrimientos de WC-Co sobre varios sustratos utilizando una técnica de deposición nueva, Cold Gas Spraying (CGS). El hecho de que antes de la publicación del primer artículo que nació de este trabajo de investigación no se había depositado previamente con éxito este tipo de materiales por CGS fue también uno de los principales puntos de motivación. Por esta razón, el lector encontrará, en la integridad del documento, los trabajos de investigación que fueron publicados durante estos años de programa de doctorado y cumplen los objetivos principales de esta tesis titulada "Deposición de cermets de WC-Co por Proyección Fría".

La projection à froid est une technologie en plein essor pour le dépôt de matériaux à l'état solide. Le procédé de dépôt des particules par pulvérisation à froid est simulé par la modélisation de l'impact à haute vitesse de particules sphériques sur un substrat plat dans diverses conditions. Pour la première fois, nous proposons une approche numérique par couplage Euler-Lagrange (CEL) afin de résoudre ce problème à haute vitesse de déformation. Les capacités de l'approche numérique CEL pour la modélisation du processus de dépôt de projection à froid sont évaluées par une étude paramétrique de : la vitesse d'impact, la température initiale des particules, le coefficient de frottement et le choix des matériaux. Les résultats de la simulation à l'aide de l'approche numérique CEL sont en accord avec les résultats expérimentaux publiés dans la littérature. La méthode CEL est généralement plus précise et plus robuste dans des régimes de déformations élevées. Un nouveau modèle d'empilement de type CFC, inspiré de la structure cristalline, est construit afin d'étudier le taux de porosité des particules déposées et les contraintes résiduelles dans le matériau de substrat pour diverses conditions. Nous pouvons observer non seulement la géométrie 3D de porosités, mais aussi leur répartition et leur évolution pendant les impacts successifs. Pour les particules, une vitesse d'impact et une température initiale élevées, sont des avantages pour produire des revêtements denses par projection à froid. Des contraintes résiduelles de compression existent à l'interface entre les particules et le substrat. Ces dernières sont causées par les grandes amplitudes et vitesses de déformation plastique induites par le procédé. Un second modèle moins complexe pour la modélisation de l'impact multiple oblique a été créé afin de simuler l'érosion de surface. Une forte érosion de surface est le résultat : d'une plus grande vitesse d'impact, d'un coefficient de frottement élevé et d'un angle de contact réduite. Pour un matériau ductile comme le cuivre, il y a deux modes de rupture : le mode 1 de traction et le mode 2 de rupture par cisaillement. Le premier survient principalement en dessous de la surface du substrat et à la périphérie de impacts, tandis que le second intervient de manière prédominante à la surface des impacts. On observe quatre étapes lors de la propagation des fissures : la formation de porosités, de fissures, la croissance de ces dernières, puis une dernière étape de coalescence et rupture. Un critère simple, où la vitesse d'érosion est fonction de l'angle de contact et de la vitesse critique d'érosion lors d'un impact de vitesse normale , est proposé sur la base des résultats des simulations afin de prédire l'initiation de l'endommagement. La déformation plastique équivalente est également un paramètre clef pour identifier l'initiation de l'endommagement, une valeur critique de 1,042 a été trouvée dans notre étude pour le cuivre
Cold spray is a rapidly developing coating technology for depositing materials in the solid state. The cold spray particle deposition process was simulated by modeling the high velocity impacts of spherical particles onto a flat substrate under various conditions. We, for the first time, proposed the Couple Eulerian Lagrangian (CEL) numerical approach to solve the high strain rate deformation problem. The capability of the CEL numerical approach in modeling the Cold Spray deposition process was verified through a systematic parameter study, including impact velocity, initial particle temperature, friction coefficient and materials combination. The simulation results by using the CEL numerical approach agree with the experimental results published in the literature. Comparing with other numerical approaches, which are Lagrangian, ALE and SPH, the CEL analyses are generally more accurate and more robust in higher deformation regimes. Besides simulating the single particle impact problem, we also extended our study into the simulation of multiple impacts. A FCC-like particles arrangement model that inspired by the crystal structure was built to investigate the porosity rate and residual stress of deposited particles under various conditions. We observed not only the 3D profiles of voids, but also their distributions and developments during different procedures. Higher impact velocity and higher initial temperature of particles are both of benefit to produce a denser cold spray coating. The compressive residual stresses existed in the interface between the particle and substrate is mainly caused by the large and fast plastic deformation. Another simplified model for multiple impacts was created for the simulation of surface erosion. A severe surface erosion is the result of a high impact velocity, a high friction coefficient and a low contact angle. Two element failure models suitable for high-strain-rate dynamic problems were introduced in this study. For a ductile material as Copper, it followed two fracture modes in our study, which are tensile failure mode and shear failure mode. The former one mainly occurred beneath the substrate surface and the periphery of substrate craters, nevertheless the latter one was found predominately at the surface of craters. Four steps were found during the propagation of crack: void formation; crack formation; crack growth; coalescence and failure. A simple criterion equation was derived based on the simulation results for predicting the initiation of damage, which the erosion velocity v_{ero} is a function of contact angle and erosion velocity for normal impact v_{pi/2}. The equivalent plastic strain could also be a parameter for identifying the onset of damage, identified as being 1.042 for Copper in our study

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This thesis presents research on the cold gas-dynamic spray process applied to the deposition of aluminum-copper alloy coatings. Cold spray deposition is a process utilized to create corrosion protection coatings and to perform additive repair for aluminum structures. This thesis utilized a series of Al-Cu binary alloy powders, from 2–5 weight percent copper and characterized their chemistry and microstructure. The powders were deposited using the cold spray approach to study the systematic increase of the alloying agent on the deposition process and coating characteristics. Deposition efficiency, critical velocity, coating thickness, hardness, porosity, and microstructure were all characterized as functions of carrier gas pressure, carrier gas temperature and feedstock powder copper composition. This thesis has demonstrated that all of the aluminum copper powders utilized can be successfully deposited via the low-pressure cold spray process with helium as the carrier gas. The copper content of the powders has a direct effect on the volume fraction of Al2Cu intermetallics, and on the coating hardness, while having no measurable effect on critical velocity for deposition or the coating thickness per pass.

Diploma thesis deals in the first part with recent knowledge of cold spray, its mechanism and parameters of deposition, advanced coatings made by cold spray and their applications, knowledge of Ti-6Al-4V coatings and their heat treatment. Deposit of Ti- 6Al-4V powder was made by cold spray process. In experimental part, microstructure and mechanical properties of supplied and its heat treated material were observed and examined. Mechanical properties and microstructure remained unchanged by annealing at 600 °C as in the case of supplied material. Recrystallization occured in microstructure of and phases by annealing at 800 °C. Grains were emerged in microstructure and mechanical properties were decreased. Mechanical properties were improved by annealing at 900 °C due to quenching. Microstructure consists of and ’ phases. Mechanical properties were the worst for annealed material at 1000 °C because of coarsed grains. Material which was annealed at 800 °C, quenched and precipitation hardened had the best microstructure and mechanical properties.

L'objectif principal de cette étude était de réaliser une modélisation du procédé cold spray, fondée sur l'observation expérimentale et sur des modèles physiques capables de prédire la microstructure du dépôt en fonction de la morphologie de la poudre et des paramètres de projection. Pour y arriver, le travaux se sont organisés autour de trois axes principaux de recherche : caractérisation de la poudre en 3D, simulations d'impact par éléments finis et modélisation d'empilement. Un procédé innovant de caractérisation morphologique de la poudre en 3D, utilisant la microtomographie par rayons X, a été développé. Le traitement des images résultantes a permis d'isoler les particules individuelles, regroupées dans une bibliothèque 3D d'environ 18000 objets. Leur taille et forme ont été caractérisées quantitativement. La méthode de partitionnement des données dite « K-means » a été utilisée pour la répartition des particules en 7 classes de forme.Le deuxième axe de recherche portait sur la simulation d'écrasement des particules, par la méthode des éléments finis (logiciel Abaqus, approche lagrangienne). L'utilisation d'outils de maillage adaptés a permis de réaliser des simulations d'écrasement des particules réelles (en provenance de la bibliothèque 3D). L'automatisation de ces simulations visait la possibilité d'en effectuer en grand nombre mais, face aux problèmes de robustesse rencontrés, le nombre de simulations menées à bien fut limité.Le troisième axe de recherche portait sur le développement d'un modèle d'empilement itératif, fondé sur l'utilisation des résultats des simulations d'écrasement. Ce modèle a été mis en place en 2D par simplicité. Différentes implémentations ont été essayées mais leur développement ne fut pas suffisamment abouti pour l'application à des cas pratiques.La validation des modèles s'est limitée aux simulations d'impact par éléments finis. Les deux types de splats (Ta sur Cu et Ta sur Ta), exigeant de méthodes d'observation expérimentale différentes, ont été traités séparément. Les premiers ont pu être directement observés par microtomographie et regroupés dans une bibliothèque 3D des splats Ta sur Cu. Ensuite, ils ont été comparés, de façon statistique mais aussi individuellement, aux correspondants simulés sans qu'aucune divergence évidente n'apparaisse. Le cas des Ta sur Ta est, en revanche, compliqué du fait de l'homogénéité du système qui empêche l'utilisation directe de la microtomographie. Bien que différentes méthodes visant à apporter une couche du contraste entre particule et substrat aient été essayées, la construction d'une bibliothèque 3D des splats Ta sur Ta n'a pas été possible.L'optimisation des poudres (choix de la granulométrie et de la forme, en vue d'une application donnée) est une des utilisations envisagées pour le modèle d'empilement, ainsi que la simulation de la projection de poudres composites (métal et oxyde). L'inclusion dans le modèle des transformations de phase ouvrirait la porte de la famille de la projection plasma ou de la fabrication additive. Plus généralement, la philosophie derrière la modélisation d'empilement développée dans cette thèse peut être appliquée à tout procédé où l'apport de matière est fait à partir d'une « poudre » subissant une certaine transformation. Enfin, le couplage avec un modèle de comportement pourrait permettre l'estimation de certaines propriétés physiques (par exemple, les conductivités thermique et électrique), dépendant de la microstructure du dépôt
This study on the cold spray process aimed at achieving an original coating build-up model, capable of predicting the resulting microstructure as a function of powder morphology and process parameters. The work focused on three main interrelated subjects: 3D powder characterization, simulation of individual impacts on a flat substrate by the finite element method and deposition build-up modeling.An innovative method based on microtomographical observations was used for 3D characterization of the powder. Image analysis allowed to separate single powder particles and to gather them into a 3D collection containing approximatively 18 000 objects. Their size and shape were quantitatively measured. A cluster analysis method (K-means) was then applied to this data set to divide the particles into 7 classes based on their shape.The second main research topic consisted in performing particle impact simulations on a flat substrate by the finite element method (using the commercial software Abaqus). The use of dedicated meshing tools allowed to simulate the impact of real particles, as observed by microtomography. Scripting techniques were used to carry out a large number of these simulations but, due to limited robustness of the procedure, only few of them were successfully conducted.The third research area focused on the development of a deposition build-up model (in 2D to allow a simpler implementation). Data from finite element results were interpolated and used in an iterative simulation, where impacting particles were deposited one by one. Different approaches were tested but the development of the model could not be completed in the framework of this thesis.Model validation could be performed on finite element simulations. The two kinds of splats (Ta on Cu and Ta on Ta) were considered separately. Concerning the first, direct microtomographical imaging could be applied, due to the heterogeneity of materials. Splats were observed, individually separated and gathered in a 3D collection as done before with powder particles. Simulated and observed splats could then be compared on a statistical basis. No particular discrepancy was observed, confirming the impact simulation method used. The second kind of splats (Ta on Ta) was complicated by the homogeneity of the materials, preventing the use of microtomography. The deposition (before spraying) of a contrast layer between Ta substrate and Ta particle was tried by different techniques. The only method giving exploitable results was the chemical vapor deposition of a Fe layer onto the powder particles. However, the small number of adherent particles and the weak contrast obtained in the images prevented the use of the methods already applied to powder particles and Ta splats onto Cu.The optimization of powder granulometry and shape (towards a specific application) is one of the main expected applications of the deposition build-up model, together with the simulation of composite powders (typically, metal and oxide). The involvement of phase transformation phenomena into the model could extend its application to the whole family of thermal spray processes (plasma, HVOF, etc.) or to other additive manufacturing techniques. In general, the philosophy behind our modeling approach could be applied to every manufacturing/coating technique where the supply material is in powder form and undergoes a certain transformation during the process. Finally, the coupling of such a model with homogenization techniques would allow the prediction of macroscopic properties depending on deposit microstructure (e.g. thermal or electrical conductivity)

The objective of this study is to investigate the feasibility of depositing 1.5 mm thick titanium coatings, as a repair method for aerospace Ti-6Al-4V substrates, using two new commercially available processes: Low Pressure Cold Spray (LPCS) and Pulsed Gas Dynamic Spray (PGDS). The coatings produced were examined and characterized by their porosity level, microhardness, adhesion strength, particle flattening ratio, wipe tests, fracture surface type and wear tests. Phases and chemical composition were determined using X-Ray diffraction analysis and energy dispersive spectroscopy, respectively. It was found that both spraying processes are capable of producing dense, hard and oxide-free coatings using specific parameters. Finally, as a first step towards repair implementation of these processes, damages were simulated on Ti-6Al-4V samples, which were successfully repaired with low porosity and high hardness levels. The feasibility of repairs was confirmed, the next step will consist in qualification testing to assess coating performances under real life application.

Particle-laden supersonic jets impinging on a flat surface are of interest to cold gas-dynamic spray technology. Solid particles are propelled to a high velocity through a convergent-divergent nozzle, and upon impact on a substrate surface, they undergo plastic deformation and adhere to the surface. For given particle and substrate materials, particle velocity and temperature at impact are the primary parameters that determine the success of particle deposition. Depending on the particle diameter and density, interactions of particles with the turbulent supersonic jet and the compressed gas region near the substrate surface can have significant effects on particle velocity and temperature. Unlike previous numerical simulations of cold spray, in this dissertation we track solid particles in the instantaneous turbulent fluctuating flow field from the nozzle exit to the substrate surface. Thus, we capture the effects of particle-turbulence interactions on particle velocity and temperature at impact. The flow field is obtained by direct numerical simulations of a supersonic rectangular particle-laden air jet impinging on a flat substrate. An Eulerian-Lagrangian approach with two-way coupling between solid particles and gas phase is used. Unsteady three-dimensional Navier-Stokes equations are solved using a six-order compact scheme with a tenth-order compact filter combined with WENO dissipation, almost everywhere except in a region around the bow shock where a fifth-order WENO scheme is used. A fourth-order low-storage Runge-Kutta scheme is used for time integration of gas dynamics equations simultaneously with solid particles equations of motion and energy equation for particle temperature. Particles are tracked in instantaneous turbulent jet flow rather than in a mean flow that is commonly used in the previous studies. Supersonic jets for air and helium at Mach number 2.5 and 2.8, respectively, are simulated for two cases for the standoff distance between the nozzle exit and the substrate. Flow structures, mean flow properties, particles impact velocity and particles deposition efficiency on a flat substrate surface are presented. Different grid resolutions are tested using 2, 4 and 8 million points. Good agreement between DNS results and experimental data is obtained for the pressure distribution on the wall and the maximum Mach number profile in wall jet. Probability density functions for particle velocity and temperature at impact are presented. Deposition efficiency for aluminum and copper particles of diameter in the range 1 micron to 40 microns is calculated. Instantaneous flow fields for the two standoff distances considered exhibit different flow characteristics. For large standoff distance, the jet is unsteady and flaps both for air (Mach number 2.5) and for helium (Mach number 2.8), in the direction normal to the large cross-section of the jet. Linear stability analysis of the mean jet profile validates the oscillation frequency observed in the present numerical study. Available experimental data also validate oscillation frequency. After impingement, the flow re-expands from the compressed gas region into a supersonic wall jet. The pressure on the wall in the expansion region is locally lower than ambient pressure. Strong bow shock only occurs for small standoff distance. For large standoff distance multiple/oblique shocks are observed due to the flapping of the jet. The one-dimensional model based on isentropic flow calculations produces reliable results for particle velocity and temperature. It is found that the low efficiency in the low-pressure cold spray (LPCS) compared to high-pressure cold spray (HPCS) is mainly due to low temperature of the particles at the exit of the nozzle. Three-dimensional simulations show that small particles are readily influenced by the large-scale turbulent structures developing on jet shear layers, and they drift sideways. However, large particles are less influenced by the turbulent flow. Particles velocity and temperature are affected by the compressed gas layer and remain fairly constant in the jet region. With a small increase in the particles initial temperature, the deposition efficiency in LPCS can be maximized. There is an optimum particle diameter range for maximum deposition efficiency.
Ph. D.

Cold Spray is a solid-state additive manufacturing process that uses metallic feedstock powders to create layers on a substrate through plastic deformation. This process can be used for the repair of mechanical parts in the aerospace industry as well as for structural applications. Aluminum alloy powders, including Al 6061, 7075, 2024, and 5056, are typically used in this process as feedstock material. Since this process takes place all in the solid state, the properties and microstructure of the initial feedstock powder directly influence the properties of the final consolidated Cold Spray part. Given this, it is important to fully understand the internal powder microstructure, specifically the secondary phases as a function of thermal treatment. This work focuses on the understanding of the internal microstructure of Al 6061, 7075, 2024, and 5056 through the use of light microscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, electron backscatter diffraction, and differential scanning calorimetry. Thermodynamic models were used to predict the phase stability in these powders and were calibrated using the experimental results to give a more complete understanding of the phase transformations during thermal processing.

The master´s thesis deals with a process called cold kinetic deposition technology. Using this technology, a copper layer was formed on a sample with an aluminium base by high-pressure cold spraying at a pressure of 25 bar. In the experimental part, the influence of corrosion degradation in the salt chamber was assessed at the time cycles of 100 h, 200 h and 300h, where changes in internal and surface resistances before and after corrosion were measured. Subsequently, the analysis of corrosion products was performed, where the extent of corrosion attack was determined using an electron microscope. These methods led to a final evaluation of the boundary limits of the applied coating layer by cold kinetic deposition, after the effect of corrosion. Finally, a theoretical application of this technology was suggested. It was discovered that the sample placed and left in the corrosion chamber for the longest time was the most affected by corrosion. Finally, a theoretical application of cold kinetic deposition was proposed.

The aim of this master’s thesis was to evaluate cold spray technologies as an alternative method of repairing contact surfaces of parts, especially made of high-alloy manganese steels, to choose a suitable system of materials and to analyze the properties and structure of the produced coating. The thesis presents the basic of the mechanism of cold spray deposition, the described influence of basic process parameters on the structure and properties of metal coatings. Performed analysis of chemical composition, structure, porosity, hardness and strength of AISI H13 steel coating on a high-alloy manganese steel substrate. The described dependence of the structure and properties of a given coating on the heat treatment regime.

Le cold spray est un procédé où des particules de poudre sont projetées à haute vitesse sur un substrat. En se déformant, les particules de poudre y adhèrent, permettant ainsi de créer un dépôt par empilement. Les dépôts ainsi obtenus ont des propriétés mécaniques élevées, sont très denses, peuvent être épais et ont des rendements élevés, ce qui fait du cold spray un procédé idéal pour la fabrication additive. En revanche, les rendements pour les alliages d’aluminium, couramment employés dans différents domaines, sont insuffisants pour que la fabrication additive soit économiquement viable. Dans cette étude, un traitement thermique de la poudre est réalisé afin de modifier les propriétés des particules de poudre pour améliorer le rendement de projection. L’influence de ce type d’équipement ainsi que des paramètres de projection a été étudiée en mesurant la vitesse des particules (DPV2000) et en comparant les propriétés des différents dépôts. Les dépôts réalisés avec de la poudre traitée thermiquement dans les mêmes conditions mais avec de la poudre non traitée ont permis d’évaluer l’influence de la modification des propriétés des particules de poudre en fonction du traitement thermique. La fabrication additive nécessite de comprendre comment les particules de poudre s’empilent afin de réaliser des formes données. Un modèle de construction, à l’échelle macroscopique, fondé sur des données expérimentales a été développé afin de prédire la forme du dépôt. Les résultats de ces simulations ont été comparés aux dépôts obtenus expérimentalement afin de vérifier si l’épaisseur, la forme ainsi que l’état de surface concordaient
Cold spray is a process where powder particles are sprayed at a high speed onto a substrate. From deformation the powder particles adhere to the substrate, which result in deposition build-up. The cold sprayed coatings show high mechanical properties, are very dense, can be thick and have a high deposition efficiency, which makes cold spray an ideal process for additive manufacturing. However, deposition efficiency for aluminum alloys such as those commonly used in different industrial sectors, are insufficient for additive manufacturing to be economically viable. In this study, a heat treatment of the powder is carried out in order to modify the properties of the powder particles to improve the deposition efficiency. The influence of the type of cold spray facilities and of spraying parameters was studied from the measurement of the particle velocity (DPV2000) and from assessing the properties of the various coatings. The coatings made of the heat treated powders compared with those made of untreated powders using similar conditions for both were used to show the influence of the modification of the particles. Additive manufacturing requires the understanding of how powder particle build-up to achieve given shapes. A model of deposition, at a macroscopic scale, based on experimental data was developed to predict the shape of the deposit. The results of these simulations were compared to experimental deposits to check thickness, shape and the surface state

When a solid, ductile particle impacts a substrate at sufficient velocity, the resulting heat, pressure, and plastic deformation can produce bonding at the interface. The use of a supersonic gas flow to accelerate such particles is known as Cold Spray deposition. The Cold Spray process has been commercialized for some metallic materials, but further research is required to unlock the exciting material properties possible with polymeric compounds. In this work, a combined computational and experimental study a) simulated and optimized the nozzle flow conditions necessary to produce bonding in a polyethylene particle, b) developed and fabricated an experimental device, and c) explored temperature-pressure space across a range of substrate materials, resolving a material dependent ‘window of deposition’ where successful coatings form. Insights into bonding mechanisms are discussed, and paths forward proposed.

Abstract The cold spray process is an additive manufacturing technology primarily suited for ductile metals, and mainly utilized in coating surfaces, manufacturing of freeform parts and repair of damaged components. The process involves acceleration of solid micro-particles in a supersonic gas flow and coating build-up by bonding upon high velocity impact onto a substrate. Coating deposition relies on the kinetic energy of the particles. The main objective of this study was to investigate the mechanics of polymer cold spray process and deformation behavior of polymers to improve technological implementation of the process. A finite element model was created to simulate metal particle impact for copper and aluminum. These results were compared to the numerical and experimental results found in the literature to validate the model. This model was then extended to cover a wide range of impact conditions, in order to reveal the governing mechanisms of particle impact and rebound during cold spray. A systematic analysis of a single high-density polyethylene particle impacting on a semi-infinite high density polyethylene substrate was carried out for initial velocities ranging between 150m/s and 250m/s by using the finite element analysis software ABAQUS. A series of numerical simulations were performed to study the effect of a number of key parameters on the particle impact dynamics. These key parameters include: particle impact velocity, particle temperature, particle diameter, and particle density, composition of the polyethylene particle, surface composition and the thickness of a polyethylene film on a hard metal substrate. The effect of these parameter variations were quantified by tracking the particle temperature, deformation, plastic strain and rebound kinetic energy. The variation of these parameters helped define a window of deposition where the particle is mostly likely to adhere to the substrate.

Engineered components can gain desirable properties when coated with surface materials. Wear-resistant coatings can improve the performance of contacting surfaces and allow for an extended life of the parts. Hard chromium has been the plating material of choice for certain wear and corrosion- resistant coatings because of its desirable combination of chemical resistance, adhesion, and mechanical properties. However, hexavalent chromium, a component of the process for applying hard chromium coatings, has been recognized by the EPA as having hazardous health and environmental impacts. Existing and planned environmental regulations restricts the use of process chemicals containing hexavalent chromium ions. This substantiates a need to develop an environmental friendly process for alternative coatings. Praxair has reported that Cr-Ni-C particles have a better corrosion resistance than current chromium carbide and nickel chromium powders. Today, Cr-Ni-C provides great qualities for flame spray and does not contain the toxic compounds used to deposit hard chromium, but is not compatible with application by cold spray. The purpose of this thesis project is to compare two processes for plating metal powder, chromium nickel carbide (Cr-Ni-C, CRC-410-1 from Praxair), with nickel. The particles were encapsulated using three different methods: one electroplating method previously used on particles, and two electroless plating processes using different solutions. The Cr-Ni-C particles were successfully encapsulated with Ni by one of the electroless deposition methods. The electrolytic deposition experiments did not yield the uniformity of coating without agglomeration that is being attained in industrial practice today. Further research on this method is recommended, due to the material operational cost in an industrial setting that is projected to be over 200 times cheaper than electroless deposition method. In the meantime, it should be possible to produce enough coated powder by electroless deposition to validate the utility of this coated powder in depositing wear- and corrosion-resistant coatings of Cr-Ni-C by cold spray.

abstract: Nanomaterials exhibit unique properties that are substantially different from their bulk counterparts. These unique properties have gained recognition and application for various fields and products including sensors, displays, photovoltaics, and energy storage devices. Aerosol Deposition (AD) is a relatively new method for depositing nanomaterials. AD utilizes a nozzle to accelerate the nanomaterial into a deposition chamber under near-vacuum conditions towards a substrate with which the nanomaterial collides and adheres. Traditional methods for designing nozzles at atmospheric conditions are not well suited for nozzle design for AD methods. Computational Fluid Dynamics (CFD) software, ANSYS Fluent, is utilized to simulate two-phase flows consisting of a carrier gas (Helium) and silicon nanoparticles. The Cunningham Correction Factor is used to account for non-continuous effects at the relatively low pressures utilized in AD. The nozzle, referred to herein as a boundary layer compensation (BLC) nozzle, comprises an area-ratio which is larger than traditionally designed nozzles to compensate for the thick boundary layer which forms within the viscosity-affected carrier gas flow. As a result, nanoparticles impact the substrate at velocities up to 300 times faster than the baseline nozzle.
Dissertation/Thesis
Masters Thesis Electrical Engineering 2017