Die Implementierung von Laser Powder Bed Fusion bei Aluminiumoxidkeramiken ist aufgrund einer geringen Temperaturwechselbeständigkeit, Bauteilverdichtung, Pulverfließfähigkeit und Lichtabsorption eine große Herausforderung. In dieser Arbeit wurden diese Prob-leme mit unterschiedlichen Ansätzen adressiert. Sprühgetrocknete Aluminiumoxid Granulate wurde zur Verbesserung der Laserabsorption (über 80 % Verbesserung) mit farbigen Nano-Oxidpartikeln dotiert. Es wurden verschiedene Partikelpackungstheorien und Pulverbehand-lungen getestet, um die Pulverbettdichte und damit die Dichte des endgültigen Bauteils (Dichten bis zu 98,6 %) zu erhöhen. Die Pulverqualität wurde durch Schütt und Rütteldichte, Feuchtigkeitsgehalt, Partikelgrößenverteilung, Hausner-Verhältnis, Lawinenwinkel und Oberflächenfraktal charakterisiert. Des Weiteren wurde der Zusatz geeigneter Stoffe zur Verringerung der Rissbildung durch thermische Spannungen getestet. Die In-situ-Bildung von Phasen mit geringer und negativer Wärmeausdehnung reduzierte die Rissbildung in den lasergefertigten Oxidkeramiken stark.:1 Introduction 1 1.1 Motivation 1 1.2 State of the art . 2 1.3 Aim of the project 2 2 Literature review 5 2.1 Additive manufacturing by laser powder bed fusion 5 2.1.1 Classification and process description 5 2.1.2 Advantages against other AM processes 9 2.1.3 Challenges of laser powder bed fusion 12 2.1.4 State of the art of laser powder bed fusion of aluminum oxide based ceramics 13 2.1.4.1 Powder bed preparation and impact on the process 13 2.1.4.2 Critical rating of the powder bed preparation techniques 17 2.1.4.3 Processing methods and properties 19 2.1.4.4 Part properties 26 2.2 Theoretical and experimental considerations for powder bed preparation 35 2.2.1 Spray granulation 35 2.2.2 Particle packing theories 39 2.3 Mechanisms for particle dispersing 41 2.3.1 DLVO-theory 41 2.3.2 Surface charge and electrical double layer 43 2.4 Conceptualization of new ideas for laser powder bed fusion of aluminum oxide 45 2.4.1 Densification, powder flowability and absorption issue 46 2.4.2 Reduction of crack formation 47 3 Doped spray-dried granules to solve densification and absorption issue in laser powder bed fusion of alumina 55 3.1 Dispersing of aluminum oxide, iron oxide and manganese oxide 55 3.1.1 Experimental 55 3.1.2 Particle characterization 57 3.1.3 Saturation amount evaluation of dispersant 59 3.1.4 Particle size distributions after dispersing 62 3.1.4.1 Particle size distributions of alumina powders 62 3.1.4.2 Particle size distribution of dopant 67 3.2 Packing density increase of spray-dried granules 76 3.2.1 Experimental 77 3.2.2 Influence of solid load and particle ratio on granules 83 3.2.3 Influence of dopant shape and multimodal distributions on granules 84 3.2.4 Evolution of pH-value during slurry preparation and slurry stability after mixing of all components 85 3.2.5 Influence of slurry viscosity on yield of granules 88 3.2.6 Addition of coarse alumina to spray-dried granules 89 3.2.7 Application of Andreasen model on mixtures of ceramic particles with spray-dried granules 94 3.2.8 Thermal pre-treatment of granules 98 3.2.9 Influence of surface tension of slurry on granule size and density 110 3.3 Investigation of laser manufactured parts 114 3.3.1 Experimental 115 3.3.2 Influence of different iron oxide dopants and multimodal particle distributions within granules 118 3.3.3 Influence of coarse alumina variation 121 3.3.4 Influence of thermal pre-treatment of powders 127 3.3.5 Grain structure of laser additive manufactured parts 135 3.3.6 Thermal expansion of laser processed parts 137 3.3.7 Influence of thermal pre-treatment and laser processing on manganese amount within granules and laser additive manufactured parts 138 4 Additives to reduce crack formation in selective laser melting and sintering of alumina 143 4.1 Experimental 144 4.2 Additives to reduce thermal stresses 150 4.2.1 Selective laser melting with mullite additives 150 4.2.2 Amorphous alumina formation by rare earth oxide doping 160 4.2.3 Formation of aluminum titanate by use of reduced titanium oxide 169 4.2.3.1 Dispersing of titanium oxide nanoparticles in water 170 4.2.3.2 Thermal treatment of Al2O3/TiO2 granules under argon/hydrogen atmosphere 172 4.2.3.3 Laser manufacturing of parts 178 4.2.4 In-situ formation of negative thermal expansion materials 187 4.2.4.1 Dispersing of zirconia and tungsten oxide nanoparticles 187 4.2.4.2 Influence of spray drying process parameters 191 4.2.4.3 Preparation of final powders for laser powder bed fusion 197 4.2.4.4 Laser manufacturing of layers and parts 200 4.3 Mechanical properties of laser processed parts 205 5 Flowability and inner structure of customized granules 209 5.1 Experimental 209 5.2 Comparison of flowability in terms of Hausner ratio, Avalanche angle and surface fractal measurements 211 5.2.1 Influence of coarse alumina AA18 variation 211 5.2.2 Influence of thermal pre-treatment of powders 213 5.2.3 Influence of dopant content within granules 216 5.2.4 Flowability of zirconia-tungsten oxide granules and alumina granules with mullite or rare earth oxide addition 219 5.2.5 Flowability of titanium oxide doped alumina powders 221 5.3 Cross sections of customized granules to image inner structure 224 6 Summary, conclusions and outlook 233 6.1 Summary and conclusions 233 6.2 Outlook 241 References 245 List of Figures 260 List of Tables 269 / The implementation of laser powder bed fusion of aluminum oxide ceramics is challenging due to a low thermal shock resistance, part densification, powder flowability and light absorptance. In this work, these challenges have been addressed by different approaches. Spray-dried alumina granules were doped with colored oxide nanoparticles to improve the laser absorption (improvement by over 80%). Different particle packing theories and powder treatments were tested to increase the powder bed density and therefore, the final part density (densities up to 98.6%). The powder quality was characterized by apparent and tapped density, moisture content, particle size distribution, Hausner ratio, avalanche angle and sur-face fractal. Furthermore, the addition of suitable was tested to reduce crack formation caused by thermal stresses. The in-situ formation of low and negative thermal expansion phases strongly reduced the crack formation in the laser manufactured oxide ceramic parts.:1 Introduction 1 1.1 Motivation 1 1.2 State of the art . 2 1.3 Aim of the project 2 2 Literature review 5 2.1 Additive manufacturing by laser powder bed fusion 5 2.1.1 Classification and process description 5 2.1.2 Advantages against other AM processes 9 2.1.3 Challenges of laser powder bed fusion 12 2.1.4 State of the art of laser powder bed fusion of aluminum oxide based ceramics 13 2.1.4.1 Powder bed preparation and impact on the process 13 2.1.4.2 Critical rating of the powder bed preparation techniques 17 2.1.4.3 Processing methods and properties 19 2.1.4.4 Part properties 26 2.2 Theoretical and experimental considerations for powder bed preparation 35 2.2.1 Spray granulation 35 2.2.2 Particle packing theories 39 2.3 Mechanisms for particle dispersing 41 2.3.1 DLVO-theory 41 2.3.2 Surface charge and electrical double layer 43 2.4 Conceptualization of new ideas for laser powder bed fusion of aluminum oxide 45 2.4.1 Densification, powder flowability and absorption issue 46 2.4.2 Reduction of crack formation 47 3 Doped spray-dried granules to solve densification and absorption issue in laser powder bed fusion of alumina 55 3.1 Dispersing of aluminum oxide, iron oxide and manganese oxide 55 3.1.1 Experimental 55 3.1.2 Particle characterization 57 3.1.3 Saturation amount evaluation of dispersant 59 3.1.4 Particle size distributions after dispersing 62 3.1.4.1 Particle size distributions of alumina powders 62 3.1.4.2 Particle size distribution of dopant 67 3.2 Packing density increase of spray-dried granules 76 3.2.1 Experimental 77 3.2.2 Influence of solid load and particle ratio on granules 83 3.2.3 Influence of dopant shape and multimodal distributions on granules 84 3.2.4 Evolution of pH-value during slurry preparation and slurry stability after mixing of all components 85 3.2.5 Influence of slurry viscosity on yield of granules 88 3.2.6 Addition of coarse alumina to spray-dried granules 89 3.2.7 Application of Andreasen model on mixtures of ceramic particles with spray-dried granules 94 3.2.8 Thermal pre-treatment of granules 98 3.2.9 Influence of surface tension of slurry on granule size and density 110 3.3 Investigation of laser manufactured parts 114 3.3.1 Experimental 115 3.3.2 Influence of different iron oxide dopants and multimodal particle distributions within granules 118 3.3.3 Influence of coarse alumina variation 121 3.3.4 Influence of thermal pre-treatment of powders 127 3.3.5 Grain structure of laser additive manufactured parts 135 3.3.6 Thermal expansion of laser processed parts 137 3.3.7 Influence of thermal pre-treatment and laser processing on manganese amount within granules and laser additive manufactured parts 138 4 Additives to reduce crack formation in selective laser melting and sintering of alumina 143 4.1 Experimental 144 4.2 Additives to reduce thermal stresses 150 4.2.1 Selective laser melting with mullite additives 150 4.2.2 Amorphous alumina formation by rare earth oxide doping 160 4.2.3 Formation of aluminum titanate by use of reduced titanium oxide 169 4.2.3.1 Dispersing of titanium oxide nanoparticles in water 170 4.2.3.2 Thermal treatment of Al2O3/TiO2 granules under argon/hydrogen atmosphere 172 4.2.3.3 Laser manufacturing of parts 178 4.2.4 In-situ formation of negative thermal expansion materials 187 4.2.4.1 Dispersing of zirconia and tungsten oxide nanoparticles 187 4.2.4.2 Influence of spray drying process parameters 191 4.2.4.3 Preparation of final powders for laser powder bed fusion 197 4.2.4.4 Laser manufacturing of layers and parts 200 4.3 Mechanical properties of laser processed parts 205 5 Flowability and inner structure of customized granules 209 5.1 Experimental 209 5.2 Comparison of flowability in terms of Hausner ratio, Avalanche angle and surface fractal measurements 211 5.2.1 Influence of coarse alumina AA18 variation 211 5.2.2 Influence of thermal pre-treatment of powders 213 5.2.3 Influence of dopant content within granules 216 5.2.4 Flowability of zirconia-tungsten oxide granules and alumina granules with mullite or rare earth oxide addition 219 5.2.5 Flowability of titanium oxide doped alumina powders 221 5.3 Cross sections of customized granules to image inner structure 224 6 Summary, conclusions and outlook 233 6.1 Summary and conclusions 233 6.2 Outlook 241 References 245 List of Figures 260 List of Tables 269

La fusion sélective par laser (Selective Laser Melting, SLM), une des techniques de la fabrication additive (AM), permet la production de pièces en trois dimensions (3D) de formes complexes directement à partir de poudres métalliques. Elle présente de nombreux avantages significatifs par rapport aux méthodes traditionnelles de fabrication mais se heurte encore à une faible disponibilité des matériaux en poudre.Le travail effectué dans cette étude a donc consisté à étudier et à développer un nouveau moyen pour réaliser in situ des pièces en alliages et en composites à partir de mélanges de poudres.Au niveau expérimental le choix s’est porté sur le système Fer-Aluminium et sur un renforcement par des particules de SiC.Les essais ont permis de constater que dans le processus de fabrication de pièces par SLM la puissance du laser et la vitesse de balayage déterminent au premier chef la densité, la microstructure, la composition de phase et les propriétés mécaniques.À partir d’un mélange de poudres, des phases intermétalliques ont été obtenues en contrôlant les paramètres SLM. Un traitement thermique ultérieur influence les paramètres cristallins, le degré d’ordre et les propriétés mécaniques des pièces ainsi formées.Avec l’utilisation de poudres préalliées, un phénomène de texture a été observé prenant la forme de grains allongés/colonnaires orientés dans la direction de construction.Le renforcement de la matrice de fer par des particules de SiC de différentes tailles conduit à une modification structurale avec la formation de produits d’interaction, perlitie et martensite, conduisant à une amélioration de la résistance à la traction par rapport au Fe pur. / Selective laser melting (SLM), as one of the additive manufacturing (AM) technologies, enables the production of three dimensional (3D) complex parts directly from metal powders. It offers many significant advantages compared with traditional manufacturing methods; however it faces a limited availability of powder materials.The work done during this study consisted in investigating and developing a new way of in situ producing alloys and composites from powder mixtures.The iron-aluminum system and reinforcement by SiC particles were considered.Experiments have shown that the laser power and scanning speed primarily determine the density, microstructure, phase composition and mechanical properties in the manufacturing process of SLM parts.Using pre-alloyed powders, a phenomenon of texture was observed in the form of elongated/columnar grains oriented in the building direction.Using powder mixtures, intermetallic phases were obtained by controlling the SLM parameters. A heat treatment influences the crystal parameters, the degree of order and the mechanical properties of the formed parts.The reinforcement of the iron matrix by SiC particles of several sizes leads to a structural change with the formation of interaction products, perlite and martensite, leading to an improvement in tensile strength compared to pure Fe.

This project was conducted as a case study at Öhlins Racing AB, a manufacturer of suspension systems for automotive applications. Öhlins usually manufacture their components by traditional methods such as forging, casting, and machining. The project aimed to investigate how applicable Additive Manufacturing (AM) is to manufacture products for suspension systems to add value to suspension system components. For this, a proof of concept was designed and manufactured. The thesis was conducted at Öhlins in Upplands Väsby via the consultant firm Combitech.  A product catalog was searched, screened, and one part was selected. The selected part was used as a benchmark when a new part was designed for AM, using methods including Topology Optimization (TO) and Design for Additive Manufacturing (DfAM). Product requirements for the chosen part were to reduce weight, add functions, or add value in other ways.  Methods used throughout the project were based on traditional product development and DfAM, and consisted of three steps: Product Screening, Concept Development, and Part Design. The re-designed part is ready to be manufactured in titanium by L-PBF at Amexci in Karlskoga.  The thesis result shows that at least one of Öhlin's components in their product portfolio is suitable to be chosen, re-designed, and manufactured by AM. It is also shown that value can be added to the product by increased performance, in this case mainly by weight reduction. The finished product is a fork bottom, designed with hollow structures, and is ready to print by L-PBF in a titanium alloy.

Das Anwendungsfeld der Luft- und Raumfahrtindustrie kennzeichnet sich durch eine konstante Nachfrage nach Materialien mit einer hohen spezifischen Festigkeit. Die metastabile Beta-Titanlegierung Ti-5553 ist ein aussichtsreicher Werkstoffkandidat, um der Forderung nach geringer Dichte bei gleichzeitig hoher Festigkeit gerecht zu werden. Bereits jetzt findet er Anwendung in strukturell-belasteten Bereichen. Der Herstellungsprozess für die dafür verwendeten Bauteile ist charakterisiert durch einen oftmals materialintensiven Zerspanungsprozess zur Erzeugung der gewünschten Geometrie und eine mehrstufige und zeitintensive Wärmebehandlung zur Einstellung des gewünschten Gefüges sowie den damit verbundenen Eigenschaften. Durch den Einsatz des selektiven Laserschmelzens (SLM) als Vertreter der additiven Fertigungsverfahren sollen diese zwei Problemstellungen adressiert werden. Einerseits wäre es möglich, durch den schichtweisen Aufbau der Bauteile, komplexe Geometrien endkonturnah zu fertigen und damit den Zerspanungsprozess zu minimieren. Andererseits können durch die im SLM-Prozess auftretenden hohen Erstarrungsraten metastabile Gefügezustände geschaffen werden. Daher besteht großes Interesse daran, die Verknüpfung von Prozessbedingungen und dem entstehenden Gefüge zu untersuchen und zu verstehen. Das Prozessieren und Optimieren der Legierung Ti-5553 auf das Verfahren des selektiven Laserschmelzens stellen den Ausgangspunkt dieser Arbeit dar. Durch die anschließende Variation der Belichtungsstrategie beziehungsweise der Anwendung einer Substratheizung konnten Korngröße sowie Textur respektive Phasenbildung im Gefüge beeinflusst werden. Somit war es möglich das Potenzial des selektiven Laserschmelzens aufzuzeigen, indem durch Prozessparameter Bauteileigenschaften während des Herstellungsprozesses aktiv beeinflusst werden. Die Analyse der hergestellten Proben umfasste eine tief greifende Gefügeanalyse sowie die Untersuchung der mechanischen Eigenschaften.

Additive manufacturing with the use of metals have been a steadily increasing field, being able to create products with a higher degree of complexity than traditional processing techniques. SLM is a popular AM process that uses metal powder as feedstock, and one of the key components of this process is the powder rheology. In recent years the use of a powder rheometer has been shown to be a good way of evaluating powder rheology of metal powders used within AM processes, but there is a clear lack of standardised tests and methods. In this study the Compressibility, Permeability, and Aeration test for 316L powders used within SLM processes was evaluated with a FT4 powder rheometer. 15 powders that had undergone printing in SLM processes were studied. This showed that the compressibility test had the best results in differentiating the bad preforming powders, thereafter the Aeration test. The Permeability test wasn’t able to differentiate the bad preforming powders with the settings used. This study demonstrates that some tests with a powder rheometer can evaluate the powder performance in SLM processes, but further research to evaluate the tests and standardise the settings are needed for clearer test results. / Additiv tillverkning med metall är ett område som stadigt ökat i intresse, främst på grund av möjligheten att producera produkter med en mycket högre grad av komplexitet i jämförelse med traditionella processmetoder. SLM är en populär AM process som använder metallpulver som råmaterial, och en av huvudkomponenterna för processen är pulvrets reologi. Under senare år har användningen av en pulver-reometer visat sig ett bra sätt att utvärdera pulver-reologi för metallpulver som används inom AM, men det finns en klar avsaknad av standardiserade test och metoder. I denna studie utvärderas Kompressabilitet, Permeabilitet, och Aerabilitet testen för 316L pulver producerade för SLM processer med en FT4 pulver-reometer. 15 pulver som genomgått SLM printing studerades. Studien visar att kompressabilitets testets utfall bäst överensstämde med det som setts under SLM processen, och bäst urskilde pulvren som fungerat dåligt att printa med, därefter Aerations testet. Permeabilitets testet kunde inte urskilja de sämre pulvren med de inställningarna som användes. Studien demonstrerar att vissa test och index samlade med ett pulver reometer är mer tillförlitliga än andra när det gäller för att utvärdera pulvrets prestanda inom SLM processer, men vidare forskning och studier krävs för att utvärdera testen och standardisera inställningar baserat på pulvret som testas.

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β.

Additive manufacturing (AM) introduces high variability in the microstructure and defect distributions, compared with conventional processing techniques, which introduces greater uncertainty in the resulting fatigue performance of manufactured parts. As a result, qualification of AM parts poses as a problem in continued adoption of these materials in safety-critical components for the aerospace industry. Hence, there is a need to develop precise and accurate, physics-based predictive models to quantify the fatigue performance, as a means to accelerate the qualification of AM parts. The fatigue performance is a critical requirement in the safe-life design philosophy used in the aerospace industry. Fatigue failure is governed by the loading conditions and the attributes of the material microstructure, namely, grain size distribution, texture, and defects. In this work, the crystal plasticity finite element (CPFE) method is employed to model the microstructure-based material response of an additively manufactured Ni-based superalloy, Inconel 718 (IN718). Using CPFE and associated experiments, methodologies were developed to assess multiple aspects of the fatigue behavior of IN718 using four studies. In the first study, a CPFE framework is developed to estimate the critical characteristics of porosity, namely the pore size and proximity that would cause a significant debit in the fatigue life. The second study is performed to evaluate multiple metrics based on plastic strain and local stress in their ability to predict both the modes of failure as seen in fractography experiments and estimate the scatter in fatigue life due to microstructural variability as obtained from fatigue testing. In the third study, a systematic analysis was performed to investigate the role of the simulation volume and the microstructural constraints on the fatigue life predictions to provide informed guidelines for simulation volume selection that is both computationally tractable and results in consistent scatter predictions. In the fourth study, validation of the CPFE results with the experiments were performed to build confidence in the model predictions. To this end, 3D realistic microstructures representative of the test specimen were created based on the multi-modal experimental data obtained from high-energy diffraction experiments and electron backscatter diffraction microscopy. Following this, the location of failure is predicted using the model, which resulted in an unambiguous one to one correlation with the experiment. In summary, the development of microstructure-sensitive predictive methods for fatigue assessment presents a tangible step towards the adoption of model-based approaches that can be used to compliment and reduce the overall number of physical tests necessary to qualify a material for use in application.