Additive Fertigungsverfahren stellen in den letzten Jahren einen Megatrend dar, da sich mit diesen Verfahren endkonturnahe Werkstücke mit hohem Materialausnutzungsgrad herstellen lassen. Die auch als 3D-Druck bekannt gewordenen additiven Fertigungsverfahren sind jedoch nicht auf die Prototypenfertigung aus Kunststoffen begrenzt. Beim selektiven Laserschmelzen werden metallische Werkstoffe im Pulverbettverfahren mittels Laserstrahl aufgeschmolzen und somit schichtweise aufgebaut. Das Verfahren findet bereits in der Luft- und Raumfahrt, der Medizintechnik, aber auch in der Automobilindustrie und im Maschinenbau Anwendung für Prototypen, Einzelanfertigungen oder Kleinstserien. Des Weiteren ermöglicht es auch die Herstellung von Werkstücken mit besonders hoher Komplexität, die mit spanenden Verfahren nicht herstellbar sind, und bietet dadurch neue Gestaltungsmöglichkeiten bei der Konstruktion. Innerhalb des Vortrags werden nach der Vorstellung des Verfahrens verschiedene Methoden zur numerischen Simulation des selektiven Laserschmelzens von Edelstahl (1.4404) am Beispiel der kommerziellen Software Simufact Additive erläutert. Dazu werden der mechanische und thermo-mechanische Lösungsansatz betrachtet sowie die Methode zur Kalibrierung der Simulationen erläutert. Die Erläuterung eines voll transienten thermo-mechanischen Ansatzes erfolgt unter Verwendung der Software MSC Marc. Des Weiteren wird der Einfluss der Orientierung des Werkstücks im Herstellungsprozess auf resultierende Spannungen, Verzug sowie mechanische Eigenschaften analysiert und mit experimentellen Untersuchungen untersetzt.

Diploma thesis is focused on mechanical properties testing by small punch test and comparison of these properties with mechanical properties obtained by conventional tensile test. Steels P92 and AISI 316L in a shape of discs were tested. There were determined values of mechanical properties, namely yield stress, and ultimate tensile stress, obtained by the small punch test – constant deflection rate in this thesis. Values of initial stress and residual stress were evaluated from relaxation small punch test. Mechanical properties obtained by small punch test – constant deflection rate, and small punch test – relaxation, respectively, are compared with mechanical properties obtained by conventional tensile test, and relaxation tensile test, respectively. There were proposed own empirical relationships for restatement of mechanical properties obtained by small punch test to mechanical properties obtained by conventional tensile test in the thesis. Relatively small agreement of results obtained by small punch test, and conventional tensile test was demonstrated by a comparison. Application of own empirical relationships resulted in better agreement. Very good agreement of results of small punch test – relaxation, and relaxation tensile test was shown by their comparison.

L'objectif de la thèse proposée par AREVA NP porte sur la prédétermination des contraintes résiduelles issues de l'opération de tournage de finition de l'inox 316L. Le modèle numérique simulant la génération des contraintes résiduelles propose une approche originale en appliquant des chargements thermomécaniques équivalents sur la surface finale de la pièce sans recourir à la modélisation de l'enlèvement de matière (formation du copeau, séparation de la matière,...). De ce fait, l'étude se divise en deux parties complémentaires. D'un coté, une campagne de mesures expérimentales a été menée de manière à recueillir un ensemble de données nécessaires à la mise en place des chargements thermomécaniques. Ces expériences sont conduites en fonction des données fournie par AREVA NP. De l'autre, un modèle numérique applique ces chargements et reproduit les phénomènes conduisant à l'apparition des contraintes résiduelles. Ces résultats numériques sont alors confrontés à ceux trouvés dans la biblio et aux mesures réalisées en interne. Il apparaît que cette nouvelle approche fournie des résultats intéressants et qu'elle permet de simuler plusieurs passages d'outils ce qui reste hors d'atteinte des modèles numériques actuelles.

Les stents coronariens sont des dispositifs médicaux utilisés pour soutenir mécaniquement les artères atteintes d'athérosclérose. Cette maladie se caractérise par l'accumulation de dépôts lipidiques à la surface des parois artérielles, ce qui provoque une diminution du diamètre effectif de l'artère et la perturbation du débit sanguin. Les stents sont principalement constitués d'une structure métallique recouverte d'un polymère. Malgré leur succès commercial, plusieurs lacunes au niveau de l'adhérence sont rapportées à la suite de leur expansion, telles que des délaminations et l'apparition de fissures. Ces défaillances affectent notamment l'efficacité des stents. L'objectif de cette thèse est donc d'étudier cette problématique en utilisant des revêtements fluorocarbonés déposés par plasmas. Ces travaux doctoraux proposent une nouvelle technique d'évaluation de l'adhérence appelée, le Small Punch Test. Cette méthode reproduit l'expansion d'un stent sur des échantillons plats ce qui facilite les caractérisations des revêtements après déformation. Des caractérisations au microscope électronique à balayage ainsi qu'au microscope à force atomique ont permis d'évaluer l'influence de l'épaisseur des revêtements sur leur cohésion. Des fissures ont été observées sur les échantillons possédant une épaisseur supérieure à 35 nm. Afin de déposer des revêtements à faible teneur en nano-pores et d'augmenter leur stabilité en milieu aqueux, ces travaux étudient également la croissance et les propriétés des revêtements fluorocarbonés en fonction de différents décapages plasmas. Les décapages d'hydrogène ont notamment conduit à la formation d'hydroxydes à la surface de l'acier qui ont agit comme initiateur à la polymérisation. Le revêtement ainsi formé possède une teneur réduite en nano-pores, est riche en CF2 et est composé de longues chaînes polymères tel qu'observé par XPS, ToF-SIMS et FTIR. Cependant, les décapages plasmas ont modifié et réduit l'épaisseur de la couche d'oxyde de l'acier inoxydable ce qui l'a rendue plus sensible à une post-oxydation

This research studies microdrilling of biocompatible materials including commercially pure titanium, 316L stainless steel, polyether ether ketone (PEEK) and aluminum 6061-T6. A microdrilling technique that uses progressive pecking and micromist coolant is developed that allows drilling of 127 micrometers diameter microholes with an aspect ratio of 10:1. The drilling parameters, dominant wear pattern, hole positioning accuracy and effect of AlTiN tool coating are experimentally determined. The experimental data trend agrees with classical Taylor's machining equation. Despite of fragile and long microdrills, the progressive pecking cycle and micromist allowed deep hole drilling on all the tested materials. Drill wear is more pronounced at outer cutting edge due to higher cutting speeds. However, when drilling 316L stainless steel attrition wear at chisel edge is dominant. Hole quality degradation due to formation of built up edge at the drill tip is observed. Coated drill improves tool life by 122% and enhances hole quality when drilling 316L stainless steel. The hole positioning accuracy is improved by 115% and total hole diameter variation decreased from 0.11% to 0.003% per mm of drilling distance.

This thesis work was done to get a fundamental knowledge of the mechanical and microstructural properties of 316L stainless steel fabricated with the additive manufacturing technique, laser powder-bed fusion (L-PBF). The aims of the thesis were to study the mechanical and microstructural properties in two different building orientations for samples built in two different machines, and to summarize mechanical data from previous research on additive manufactured 316L. Additive manufacturing (AM) or 3D-printing, is a manufacturing technique that in recent years has been adopted by the industry due to the complexity of parts that can be built and the wide range of materials that can be used. This have made it important to understand the behaviour and properties of the material, since the material differs from conventionally produced material. This also adds to 316L, which is an austenitic stainless steel used in corrosive environments. To study the effect of the building orientation, samples of 316L were built in different orientations on the build plate. The density and amount of pores were also measured. Tensile testing and Charpy-V testing were made at room temperature. Vickers hardness was also measured. Microstructure and fracture surfaces were examined using light optical microscope (LOM) and scanning electron microscope (SEM). The microstructure of the 316L made with L-PBF was found to have meltpools with coarser grains inside them, sometime spanning over several meltpools. Inside these coarser grains was a finer cellular/columnar sub-grain structure. The tensile properties were found to be anisotropic with higher strength values in the orientation perpendicular to the building direction. Also high dense samples had higher tensile properties than low dense samples. The impact toughness was found to be influenced negatively by high porosity. Hardness was similar in different orientations, but lower for less dense samples. Defects due to lack of fusing of particles were found on both the microstructure sample surfaces and fracture surfaces. The values from this study compare well with previous reported research findings.

In this study, the fatigue- and fracture properties and microstructure of a marine component of austenitic stainless steel 316L manufactured with the novel method Wire Arc Additive Manufacturing were investigated and compared with data from literature. The purpose was to find a critical flaw size in the material related to its fatigue life. It was done by studying the microstructure and interpreting fatigue- and mechanical data for the marine component in empirical models related to the fatigue- and fracture properties. Fracture properties were approximated to estimate fatigue life and critical flaw size. Fatigue limit and fatigue threshold were based on hardness test data, fracture toughness, and FADs on Charpy-V impact test data. The material manufactured with Wire Arc Additive Manufacturing had superior fatigue properties than cast and rolled equivalents and performed better in the fatigue test than recommendations for austenitic stainless steel in a seawater environment from the British Standard 7910:2019. Due to the conservative model's fatigue limit and fatigue threshold, the results are conservative. The reason for that could be the crack closure properties of the material. The results for fracture toughness are lower than the literature data. This is most likely due to conservative models based on Charpy-V impact test data. The most important properties of the fatigue life are the fatigue limit and the fatigue threshold due to their relationship with crack growth. Testing the lifetime of the component in seawater is complex and time-consuming due to the corrosion and the need for low test frequency.

This thesis details the creation and application of a generalized process plan for the hybrid manufacturing of AISI 316L stainless steel, using direct energy deposition (DED) and ball-nose end-mill machining, that includes the inspection and measurement of objects created by that hybrid manufacturing process plan. The proposed process plan progresses through the selection of substrate thickness, single-track, multi-track, and multi-layer depositions, then on to machining processing. A manufacturers' recommended set and range of DED parameters were used to create a designed experiment that aided in the analysis of objects created in each of the DED process planning steps; those objects were then machined in the same enclosure using a set of machining parameters screened from industry recommendations for ball-nose milling of stainless steel, after which measurements were taken for surface roughness, some material characteristics, and for tool deterioration. The results, analyses, and discussions collected herein show that the proposed process plan can provide models for geometrical outputs for each step in the plan, some improvements in substrate stability, surface roughness, tool deterioration, and material porosity due to voids. Current research in hybrid manufacturing does not show generalized process planning influences. The process plan as demonstrated by the work in this thesis will help operators, designers, and researchers in the future by defining a generalized workflow that can be applied to other materials used in hybrid manufacturing.

No description available.

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)