The contamination of 316L stainless steel surfaces within an electrospray ionisation source of a mass spectrometer is investigated. An accelerated method of contamination is used. Following initial test method development and investigation of the contamination resulting on the ion transfer components (sample cone, outer cone and extraction cone), flat samples are employed within the ionisation source. This enables characterisation of the contamination composition, morphology and build-up with time. Blood plasma is introduced into the mass spectrometer as it is a widely analysed substance that is known to result in contamination. The contamination from a mixture of human blood plasma, diluted in methanol, and a water/acetonitrile mobile phase is found to contain inorganic NaCl crystals embedded in a matrix of organic residues. The morphology shows self-organising features as the contamination builds. A model is proposed to explain the morphology, involving rapid evaporation of the droplets that impinge on the stainless steel surface. Two types of surface modification are considered for the stainless steel: electrochemically grown films and coatings deposited by vapour deposition. A method for electrochemical film growth is developed, enabling nanoporous films to be formed on the stainless steel in 5 M sulphuric acid at 60°C by square wave pulse polarisation between active or transpassive and passive potentials. The films are characterised using glow discharge optical emission spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, Rutherford backscattering spectroscopy and nuclear reaction analysis. The films are shown to be chromium- and molybdenum-rich relative to the substrate, and to consist mainly of sulphates, oxides and hydroxides. The morphology and composition of the films are discussed in relation to the polarisation conditions and mechanism of film formation. A range of vapour deposited coatings are considered: TiN, TiC, TiB2, Graphit-iC, and diamond-like carbon coatings with Si and N2 dopants and with varying sp2:sp3 ratios. In addition, a hydrophobic coating is deposited on the stainless steel by immersion, in order to provide a significant variation in surface energy. Surface analysis of the coatings is carried out, considering their sp2:sp3 ratios, their electrical conductivities, their water contact angle, and the various components of the surface energy. The contamination build-up on the surface of uncoated 316L stainless steel is compared with that on stainless steel with the various surface modifications. A method for quantification of the build-up of contamination on flat samples is developed using white light interferometry. The surface modifications which result in the slowest contamination build-up with time are then applied to the ion transfer components of the mass spectrometer. The robustness of the mass spectrometric response for the selected coated surfaces is compared with that of the uncoated stainless steel. The electrochemically grown films and two of the doped diamond-like carbon coatings are found to be successful in reducing the build-up of contamination.

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The challenges in the deposition of dissimilar materials are mainly related to the large differences in the physical and chemical properties of the deposited and substrate materials. These differences readily cause residual stresses and intermetallic phases. This has led to the development of functionally graded materials which exhibit spatial variation in composition. Laser direct metal deposition due to its flexibility, it offers wide variety of dissimilar and functionally graded materials deposition. Despite considerable advances in process optimization, there is a rather limited understanding of the role of metallurgical factors in the laser deposition of dissimilar and functionally graded alloys. The aim of this work is to understand and explain mechanisms occurring in diode laser deposition of dissimilar materials and functionally graded materials. The first part of this work addressed diode laser deposition of Inconel 718 nickel alloy to Ti-6Al-4V titanium alloy. Here, the effect of laser pulse parameters and powder mass flow rates on the stress formation and cracking has evaluated by experiment and numerical techniques. Results showed that the clad thickness was an important factor affecting the cracking behaviour. In the second part of this study, an image analysis technique has been developed to measure the surface disturbance and the melt pool cross section size during laser direct metal deposition of Inconel 718 on a Ti-6Al-4V thin wall. It was noted that under tested conditions the overall melt pool area increased with the increase in powder flow rate; the powder carrier gas flow rates also seemed to play important roles in determining the melt pool size. In the third part of this study, a parametric study on the development of Inconel 718 and Stainless steel 316L continuously graded structure has been carried out. Results suggested that microstructure and other mechanical properties can be selectively controlled across the deposited wall. The results presented in this dissertation can be used as a metallurgical basis for further development of dissimilar and functionally graded manufacturing using LDMD technique, guiding future manufacturing engineers to produce structurally sound and microstructurally desirable laser deposited samples.

Les premiers stades d'endommagement par fatigue au sein des matériaux métalliques polycristallins sont pilotés par les champs mécaniques locaux se développant à l'échelle des grains en surface. La formation de bandes de glissement persistantes est souvent à l'origine des fissures de fatigue. Cette localisation de la plasticité cyclique apparaît au sein de grains d'un polycristal dans lequel les champs de contraintes élastiques peuvent être fortement hétérogènes en fonction de la microstructure granulaire et de l'anisotropie élastique cristalline. La majeure partie de ce travail est consacrée à évaluer puis à analyser statistiquement les champs de contraintes au sein des grains de surface de polycristaux. Ce travail s'est concentré sur la réponse élastique des matériaux considérés afin d'étudier l'activation du glissement plastique et sa variabilité, puis, de façon un peu moins directe, ses conséquences vis-à-vis de la formation des fissures de fatigue. La méthodologie retenue est basée sur la simulation numérique en champs complets par éléments-finis de la réponse élastique d'un ensemble d'agrégats polycristallins dont les orientations cristallographiques sont tirées aléatoirement.Deux matériaux très différents du point de vue de l'élasticité cristalline et des systèmes de glissement ont été choisis comme support de l'étude. Le premier est l'acier inoxydable austénitique 316L à structure cubique à faces centrées et le second l'alliage de titane TA6V à structure hexagonale compacte. Les distributions de la cission résolue au sein d'ensembles de grains de surface, par classe d'orientation, sont analysées en relation avec les configurations cristallographiques locales afin d'identifier celles qui favorisent - ou au contraire inhibent - l'activation du glissement plastique. Les résultats obtenus, dans le cas du TA6V, suggèrent notamment une activation plus précoce et importante du glissement basal devant le glissement prismatique. De plus, la stratégie de simulation a été adaptée pour rendre compte de la présence de zones texturées appelées « macrozones » dont l'influence sur l'activation de plasticité peut être qualitativement prédite. L'activation du glissement dans le TA6V est également étudiée expérimentalement par la réalisation d'un essai in situ sous MEB où la précocité du glissement basal est constatée.Cet essai a également permis de caractériser la cinétique d'activation des différents types de glissements. Ces données, couplées aux statistiques des cissions simulées, permettent une estimation de la cission résolue critique sur les 2 types de glissement. / The first stages of fatigue damage in metallic polycrystalline materials are governed by local mechanical field at the grain scale. Fatigue crack initiation is often related to the emergence of persistent slip band at surface.Localization of cyclic plasticity occurs within grains of polycristals in which elastic stress field can be highly heterogeneous due to the granular microstructure and crystalline anisotropic elasticity. The main goal of this study is to evaluate and analyse statistically the stress fields in surface grains of polycristals. In this work, the elasticregime only is considered in order to study the subsequent activation of plastic slip and its variability. The possibleconsequences regarding crack formation are also addressed but in a lesser extent. The methodology is based on fullfield finite element numerical simulation of the elastic response of a set of polycrystalline aggregates in which grains orientation is chosen randomly.The two materials chosen for this study exhibit different characteristics regarding crystal elasticity and slip systems.The first one is an austenitic stainless steels 316L (face centered cubic) and the second one a titanium alloy TA6V(hexagonal close packed). The distribution of resolved shear stress within several sets of surface grains, for different classes of crystal orientation, are analysed in relation to local crystallographic configurations in order to identify those which promote - or prevent from - plastic slip activation. For TA6V, the results suggest in particular that basal slip is activated earlier than prismatic slip. In addition, the simulation strategy has been modified tomodel the presence of some sharp band-like crystallographic textures named “macrozones” whose influence has been qualitatively predicted. Plastic slip activation in TA6V is also studied experimentally. In situ tensile test using SEM has shown the earlier activation of basal slip. Experimental data has been combined with simulated distribution of resolved shear stress in order to estimate the critical resolved shear stress of basal and prismatic slip systems.

Metal additive manufacturing can be used for producing complex and functional components in the aerospace industry. This thesis deals with the process-structure-property relationships in selective laser melting of three aerospace alloys: Invar 36, stainless steel 316L, and Ti-6Al-4V. These alloys are weldable but hard to machine, which make them good candidates for the selective laser melting process. Invar 36 has a very low coefficient of thermal expansion because of its nickel concentration of 36% and stainless steel 316L contains 16-18% chromium that gives the alloy a corrosion resistance property. Ti-6Al-4V offers high strength-to-weight ratio, high biocompatibility, and outstanding corrosion resistance. Any changes in the chemical composition of these materials could affect their performance during application. In this thesis, a full factorial design of experiments is formulated to study a wide range of laser process parameters. The bulk density, tensile mechanical properties, fractography, microstructure, material composition, material phases, coefficient of thermal expansion, magnetic dipole moments, and residual stresses of the parts produced are experimentally investigated. An optimum process window has been suggested for each material based on experimental work. The thermal cycle, residual stresses, and part distortions are examined using a thermo-mechanical finite element model. The model predicts the residual stress and part distortion after build plate removal. The thesis introduces two laser energy densities for each material: brittle-ductile transition energy density, ET, and critical laser energy density, EC. Below the brittle-ductile transition energy density, the parts exhibited void formation, low density, and brittle fracture. Above the critical energy density, the parts showed vaporization of some alloying elements that have low boiling temperatures. Additionally, real-time measurements were taken using a pyrometer and a high-speed camera during the selective laser melting process. The trends found in the numerical results agree with those found experimentally. / Thesis / Doctor of Philosophy (PhD)

Selective Laser Melting (SLM), a laser-based Additive Manufacturing technique has appealed to the bio-medical, automotive, and aerospace industries due to its ability to fabricate geometrically complex parts with tailored properties and high-precision end-use products. The SLM processing parameters highly influence the part quality, microstructure, and mechanical properties. The process-structure-property relationship of the SLM process is not well-understood. In the process-structure study, a quasi-2D model of Micro-Selective Laser Melting process using molecular dynamics is developed to investigate the localized melting and solidification of a randomly-distributed Aluminum nano-powder bed. The rapid solidification in the meltpool reveals the cooling rate dependent homogeneous nucleation of equiaxed grains at the center of the meltpool. Long columnar grains that spread across three layers, equiaxed grains, nano-pores, twin boundaries, and stacking faults are observed in the final solidified nanostructure obtained after ten passes of the laser beam on three layers of Aluminum nano-powder particles. In the structure-property study, the mechanical deformation behavior of the complex cellular structures observed in the SLM-fabricated 316L Stainless Steel is investigated by performing a series of molecular dynamics simulations of uniaxial tension tests. The effects of compositional segregation of alloying elements, distribution of austenite and ferrite phases in the microstructure, subgranular cell sizes, and pre-existing (grown in) nano-twins on the tensile characteristics of the cellular structures are investigated. The highest yield strength is observed when the Nickel concentration in the cell boundary drops very low to form a ferritic phase in the cell boundary. Additionally, the subgranular cell size has an inverse relationship with mechanical strength, and the nano-twinned cells exhibit higher strength in comparison with twin-free cells. / Master of Science / Additive Manufacturing's (AM) rise as a modern manufacturing paradigm has led to the proliferation in the number of materials that can be processed, reduction in the cost and time of manufacturing, and realization of complicated part geometries that were beyond the capabilities of conventional manufacturing. Selective Laser Melting (SLM) is a laser-based AM technique which can produce metallic parts from the fusion of a powder-bed. The SLM processing parameters greatly influence the part's quality, microstructure, and properties. The process-structure-property relationship of the SLM process is not well-understood. In-situ experimental investigation of the physical phenomena taking place during the SLM process is limited because of the very small length and time scales. Computational methods are cost-effective alternatives to the challenging experimental techniques. But, the continuum-based computational models are ineffective in modeling some of the important physical processes such as melting, nucleation and growth of grains during solidification, and the deformation mechanisms at the atomistic scale. Atomistic simulation is a powerful method that can offset the limitations of the continuum models in elucidating the underlying physics of the SLM process. In this work, the influence of the SLM process parameters on the microstructure of the Aluminum nano-powder particles undergoing μ-SLM processing and the mechanical deformation characteristics of the unique cellular structures observed in the SLM-fabricated 316L stainless steel are studied using molecular dynamics simulations. Ten passes of the laser beam on three layers of Aluminum nano-powder particles have unfolded the formation mechanisms of a complex microstructure associated with the SLM process. The study on the deformation mechanisms of 316L stainless steel has revealed the contribution of the cellular structures to its superior mechanical properties.

Un nouveau concept d'échangeur de chaleur compacte est développé afin d’améliorer les performances du système de conversion d'énergie pour le réacteur ASTRID. La fabrication de géométries complexes (canaux rectangulaires millimétriques) est possible grâce au procédé de soudage diffusion : des tôles rainurées en acier inoxydable 316L sont empilées en conteneur et soudées lors d'un cycle de Compaction Isostatique à Chaud (CIC). La problématique est alors d'obtenir des interfaces résistantes tout en limitant la déformation des canaux nuisible à l’efficacité de l’échangeur. Pour arriver au meilleur compromis, les travaux de cette thèse vont aider à l’optimisation des paramètres pression/temps/température du cycle de CIC.Le premier axe de travail porte sur la simulation numérique de la déformation d’un tel échangeur lors de la CIC. L’influence des paramètres numériques (taille des éléments, critère de convergence) a été étudiée afin d’optimiser la précision et la vitesse des calculs. Les simulations ont mis en évidence l’importance des défauts d’empilements de la structure (glissements et ondulations des tôles) dans l’augmentation de la déformée de l’échangeur. Puis une caractérisation mécanique poussée des tôles laminées a mené à l’identification d’une loi de comportement entre 20°C à 1040°C.Le second axe porte sur la modélisation de l'évolution des interfaces lors du soudage diffusion et la prédiction de la tenue mécanique de ces dernières, l'ensemble pouvant mener à la définition d'un critère de validité des interfaces.Une étude microstructurale et mécaniques des interfaces a permis d’établir une corrélation entre la tenue mécanique d’un joint soudé-diffusé et son taux de surface soudée. En effet, la disparition de la porosité résiduelle est le critère principal pour obtenir de bonnes propriétés mécaniques à l’interface. Toutefois, le franchissement de l’interface par les joints de grains, est nécessaire pour retrouver les propriétés des tôles laminées. Un modèle analytique de fermeture des porosités (Hill et Wallach) est utilisé pour calculer le taux de surface soudée d’une interface en fonction des paramètres du cycle de CIC en modélisant la contribution des mécanismes (visco)plastique et diffusifs (en surface et au joint). Associé à la corrélation entre tenue mécanique et taux de surface soudée, il permet de proposer un outil prédictif pour la tenue mécaniques des interfaces soudé-diffusées. / A new concept of compact plate heat exchanger is developed for the energy conversion system performances of the ASTRID reactor. Manufacturing the complex geometry is possible by a diffusion-welding process: engraved 316L stainless steel plates are stacked and bonded during a Hot Isostatic Pressing cycle (HIP). The problematic is to get strong interfaces without deforming the channels which is harmful for the exchanger efficiency. To reach a good compromise, this thesis work will help to optimize the HIP parameters (pressure/temperature/ time).The first line of work is about the simulation of the heat exchanger deformation along manufacturing process. The influence of numerical parameters (elements size, convergence criterion) was studied to optimize the accuracy and the calculation time. Simulations have shown the importance of structure stack faults (sliding and plate ripples) in the increase of exchanger deformation. Then, a mechanical characterisation of plates was carried out to identify the constitutive equation between 20°C and 1040°C.The second line is about the interface modelling along welding and the prediction of their mechanical strength, as a whole can lead to the definition of an interface acceptability criterion. A microstructural and mechanical study has enabled to correlate the mechanical strength of a diffusion-bonded junction and its bonded area. Indeed, residual porosity disappearance is the main criterion to get good interfaces mechanical strength. However, the grain boundary migration is required to reach the rolled material properties. A void closure analytical model (Hill and Wallach) was used to estimate the bonded area of an interface according to HIP cycle parameters by modelling the contribution of (visco)plastic and diffusion (surface and boundary) mechanisms. Associated with the correlation between mechanical strength and the fraction of bonded area, it enables to propose a predictive tool for the mechanical strength of diffusion-bonded interfaces.