Electron beam powder bed fusion (PBF-EB) is an additivemanufacturing (AM) method based on layer-by-layer melting of apowder bed. The technology is industrialized in certain applicationsbut still considered as immature and is not as widely used as laserbeam-based systems (PBF-LB). PBF-EB can offer several benefits overPBF-LB such as process cleanliness, thermal efficiency, fast beam speed,higher power and energy transfer, low residual stresses in built partsand a good signal environment for process monitoring. This can beadded on top of the general benefits of AM such as geometricalfreedom, manufacturing efficiency, easy design revisions, short leadtimes and so on. This suggests that PBF-EB holds potential as atechnology for the sustainable production of materials andcomponents. This thesis investigates how PBF-EB can be furtherdeveloped to create new and unique materials features. This isachieved by introducing innovative methods for material processingand by further developing the PBF-EB process itself. The thesisintroduces a charge-free heating method for PBF-EB and the resultssuggest an enhanced processability of difficult-to-process materialsand powders. A method for building multi-materials in PBF-EB isintroduced and demonstrated by the manufacturing of direct andlamellar transitions between different alloys. Methods for processmonitoring and powder bed resistivity evaluation are proposed andxiidemonstrated. It is concluded that the results presented in this thesisenabled new PBF-EB processing modes, increased the knowledge ofthe process, and introduced a new material group by demonstratingthat ceramics can be processed at high temperatures (~1600C). / <p>Vid tidpunkten för framläggningen av avhandlingen var följande delarbeten opublicerade: delarbete 2 och delarbete 4 (inskickat).</p><p>At the time of the defence the following papers were unpublished: paper 2 and paper 4 (submitted).</p>

Traditional methods for increasing the energy absorption of a structure involve using a stronger material or increasing the volume of the structure, resulting in a higher cost or additional weight. Additive manufacturing (AM) can be used to maximize the energy absorption of materials with the ability to create complex geometries such as cellular structures. Previous work has shown that the energy absorption of additively manufactured parts can be improved through functionally graded cellular structures; however, this strategy has not been applied to ultra-high strength steel materials. This work characterizes the effect of multiple functional-grading strategies (e.g. uniform, rod-graded, size-graded) on the energy absorption to weight ratio of laser powder bed fusion (L-PBF) produced M300 maraging steel lattice structures. Each structure is designed with the same average relative density to analyze the structures on an equal mass basis, to evaluate manufacturability, mechanical response, and compare experimental results with numerical simulation.

This thesis focuses on the topic of PBF-LB applied to titanium alloys. Of allalloys, an α + β is chosen, named Ti-6Al-4V. The selection of this particular alloy is driven by its current widespread use in many industrial applications where high strength coupled with low density are both desirable properties. For the last 50 years, parts made with this alloy have been cast or forged and then machined to achieve the final geometry. There is now an opportunity totransform this process chain by additive manufacturing, hence reducing material waste and achieving near net shape from powder feedstock. The process is summarised as follows: a laser selectively melts areas on a build plate where powder is pre-placed. Then a successive powder layer is spread and the process is repeated until completion. Upon removal of the part from the build plate, loose powder in the chamber is collected and recycled whenever possible. The design freedom provided by powder bed fusion methods enables production of intricate geometries and added functionality, despite the need for post-build consolidation and/or microstructural adjustments. Today’s fine and narrow powder cuts (e.g., 15-50μm) are designed to be coupled with low layer thicknesses (i.e., 30μm) to achieve smooth surfaces and high resolutions of small features e.g., internal cooling channels. However, costs associated with production of fine and narrow powder cuts are substantial as refinement of batches requires multiple sieving steps. In addition, resulting building times are considerably long (i.e., days), therefore a beneficial alternative could be that of exploring higher layer thicknesses together with wider and coarser powder cuts. The main idea of this work is to investigate the effects of employing a powder with a wider size distribution 15-90μm. The aim is to reduce the sievingrequired and consequently decrease the costs of developing and building parts made by PBF-LB. An extensive microstructural investigation is conducted on single tracks and cubes built with 27 different process parameter combinations, which also attempts to establish correlations between characteristics of tracks and responses measured in cubes. As a second step, the amount of residual porosity of asbuilt cubes is chosen as the discriminant for further mechanical testing of sub and super-β transus high-pressure heat treated material. / Den här avhandlingen fokuserar på additiv tillverkning av titanlegeringar med laser pulverbädd metoden. Den legering som främst är i fokus är Ti-6Al-4Vsom är en α+β legering. Anledningen till valet av denna titanlegering är att det är den vanligast förekommande titanlegeringen och att den används i ett antal olika industriella tillämpningar där hög styrka i kombination med låg vikt är önskvärda egenskaper. Under de senaste 50 åren har komponenter utav denna legering tillverkats med gjutning eller smide, följt av bearbetning till slutlig geometri. Med hjälp av additiv tillverkning finns nu en möjlighet att förändra tillverkningskedjan i vilket minskat materialspill och en mer nära-slutgeometri kan erhållas direkt genom användning av metallpulver som utgångsmaterial. Processen kan summeras enligt följande: en laser smälter ett förbestämt område på en byggplatta som täckts mer pulver. Därefter adderas ytterligare ett lager med metallpulver ovanpå, på vilket samma process sker igen, och igen osv, tills hela detaljen är färdigtillverkad. När detaljen ska tas loss ifrån byggplattan samlas det kvarvarande icke-smälta pulvret upp och återanvänds i så stor utsträckning som möjligt. Frihetsgraderna vid design i processen möjliggör tillverkning av komplexa geometrier och adderade funktionaliteter, även fast efterbehandling och/eller justeringar av mikrostrukturen kan behövas. Dagens smala pulverstorleksfördelning (tex 15-50μm) är avsedd att ge tunna lagertjocklekar (tex 30μm) för att åstadkomma en fin yta och hög upplösning av små geometrier, såsom exempelvis interna kylkanaler. Men kostnaderna som det innebär att framställa och sortera ut fina och smala kornstorleksfördelningarär avsevärd eftersom det innebär flera steg med silning. Vidare leder de tunnalagertjocklekarna till långa byggtider (typiskt dagar). Ett alternativ, som därför vore fördelaktigt, är att undersöka möjligheten med att bygga tjockare lager med en bredare och större pulverstorleksfördelning. Huvudfokuset i detta arbete fokuserar på att undersöka effekterna av att använda en bredare pulverpartikelstorleksfördelning 15-90μm, med syfte at minska silningsbehovet och därmed reducera kostnaden för att utveckla och tillverka detaljer med laser pulverbädd additiv tillverkning. En omfattande mikrostrukturundersökning har gjorts på enkelsträngar och kuber byggda med 27 olika processparameter-kombinationer, vilket samtidigt försöker identifiera korrelationer mellan enkelsträngarnas karaktäristik med resultaten uppmätta hos kuberna. I ett nästa steg har material, som tillverkats med processparametrar som renderade i minst/mest porer hos kuberna, mekaniskt provats efter att det högtrycksvärmebehandlats över- respektive under β-transus. / <p>Paper A is not included due to the copyright.</p><p>Paper B and C are to be submitted.</p>

When skeletal fractures are too extensive for fixation with plates and screws, autografts are the most used technique for treating this. Within the biomedical field the interest in biodegradable implants made from additive manufacturing have increased. Magnesium alloys has also gained interest because of its favorable mechanical properties.. The objective of this project is to report on new knowledge, possibilities and limitations of powder bed fusion-laser beam (PBF-LB) printed magnesium-based alloys for biomedical applications, specifically the mechanical properties of WE43. Before the practical work was carried through, a gathering of literature from scientific papers was put together to a background with information regarding Magnesium, additive manufacturing, microscopic observation methods and mechanical testing.  The practical elements were divided into 4 different categories: printing, sample preparation for observation and testing, microscopic observation, and mechanical testing. All the collected data was observed and discussed, and lastly compiled in to a result with microscopic images, stress-strain curves and data tables. It was discovered that the mechanical properties differed between the two build orientations. The specimen most appropriate for load bearing implants was the horizontal build direction. The differences between 67° and 90° scan strategy were that the 90° scan strategy with horizontal build orientation showed the lowest Young´s modulus which is favorable, whereas the 67° scan strategy showed higher tensile strength and ductility which also is favorable. Thereby no conclusion could be drawn on whether a 67° or 90° scan strategy was preferable. The conclusion was made that a horizontal build orientation had the most optimal mechanical properties, and that more research needs to be conducted on this topic before it can be used for biomedical applications.

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

[ES] La fabricación aditiva, de sus siglas en inglés AM (Additive Manufacturing) es un proceso que construye objetos sólidos tridimensionales mediante la superposicióon de materiales basados en un modelo de diseño asistido por ordenador. La AM está llamada a convertirse en la próxima revolución industrial, transformando el panorama del desarrollo y la producción. La AM ofrece numerosas ventajas, como posibilidades de diseño complejas y flexibles, la eliminación de procesos intermedios como el mecanizado, la independencia de los costes de producción del tamaño de los lotes, la reducción de los residuos de material, las estructuras ligeras, las reparaciones personalizadas de las máquinas y la capacidad de desarrollar nuevos materiales, entre otras ventajas. En las tecnologías de fabricación aditiva que emplean un rayo láser como fuente de energía, la materia prima inicial (en forma de polvo o cable) es fundida por la fuente de calor láser de forma controlada, capa a capa, hasta crear un componente con dimensiones finales o casi finales. Estas tecnologías implican someter el material impreso a un proceso térmico único, en el que el material se funde en un área muy específica y luego se enfría rápidamente a velocidades extremadamente altas de hasta 10^6 K/s. Por lo tanto, las microestructuras que surgen de los procesos de fabricación en AM difieren significativamente de las que se consiguen en los procesos tradicionales. Además, los materiales que se emplean principalmente en la AM no se han diseñado explícitamente para estas tecnologías. Las características específicas de los procesos de AM pueden utilizarse para lograr microestructuras y propiedades distintas en aceros que han sido adaptados para aprovechar las rápidas velocidades de enfriamiento y la historia térmica del proceso, entre otros factores. Por el momento, el número de calidades de acero comerciales disponibles en el mercado de la AM es limitado. Diversas industrias demandan nuevos grados de acero con menor densidad para disminuir el peso sin comprometer las propiedades mecánicas. Los aceros con alto contenido en manganeso se consideran materiales muy prometedores para aplicaciones estructurales debido a su excepcional combinación de resistencia y ductilidad, con una baja densidad. Sin embargo, a pesar de sus excepcionales propiedades, los aceros con alto contenido en manganeso se enfrentan a diversas limitaciones o retos durante las técnicas de procesado convencionales. Afortunadamente, la solidificación rápida puede resolver estos problemas. En este sentido, las tecnologías de AM basadas en láser proporcionan velocidades de enfriamiento rápidas, así como flexibilidad en términos de diseño geométrico. Los nuevos retos de estas tecnologías implicarán la microsegregación y el agrietamiento en caliente o hot cracking en inglés, que se producen durante la solidificación. Esta tesis está dedicada a explotar el método CALPHAD para realizar cálculos termodinámicos con el fin de diseñar varios aceros con alto contenido en manganeso que puedan prevenir eficazmente los problemas de solidificación rápida en AM. Las composiciones de acero diseñadas se produjeron en forma de polvo para AM mediante atomización con gas. Se analizaron los polvos para determinar su microestructura en relación con la química y la velocidad de enfriamiento. Ajustando adecuadamente los parámetros de impresión, estos polvos de acero con alto contenido en manganeso se imprimieron con éxito en AM, dando lugar a densidades relativas superiores al 99.9%. Se analizó la microestructura de estas muestras totalmente densas y se comparó con sus respectivos polvos, con el fin de identificar cualquier diferencia resultante de las variaciones en la velocidad de enfriamiento y los ciclos térmicos. Por último, tras definir el mejor conjunto de condiciones de impresión para cada composición de polvo, se produjeron varias muestras para evaluar las propiedades mecánicas. / [CA] La fabricació additiva, de les seues sigles en anglés AM (Additive Manufacturing) és un procés que construïx objectes sòlids tridimensionals mitjançant la superposició de materials basats en un model de disseny assistit per ordinador. L'AM està cridada a convertir-se en la pròxima revolució industrial, transformant el panorama del desenvolupament i la producció. L'AM oferix nombrosos avantatges, com a possibilitats de disseny complexes i flexibles, l'eliminació de processos intermedis com el mecanitzat, la independència dels costos de producció de la grandària dels lots, la reducció dels residus de material, les estructures lleugeres, les reparacions personalitzades de les màquines i la capacitat de desenvolupar nous materials, entre altres avantatges. En les tecnologies de fabricació additiva que empren un raig làser com a font d'energia, la matèria primera inicial (en forma de pols o filferro) és fosa per la font de calor làser de manera controlada, capa a capa, fins a crear un component amb dimensions finals o quasi finals. Estes tecnologies impliquen sotmetre el material imprés a un procés tèrmic únic, en el qual el material es funde en una àrea molt específica i després es refreda ràpidament a velocitats extremadament altes de fins a 10^6 K/s. Per tant, les microestructures que sorgixen dels processos de fabricació en AM diferixen significativament de les que s'aconseguixen en els processos tradicionals. A més, els materials que s'empren principalment en l'AM no s'han dissenyat explícitament per a estes tecnologies. Les característiques específiques dels processos d'AM poden utilitzar-se per a aconseguir microestructures i propietats diferents en acers que han sigut adaptats per a aprofitar les ràpides velocitats de refredament i la història tèrmica del procés, entre altres factors. De moment, el nombre de qualitats d'acer comercials disponibles en el mercat de l'AM és limitat. Diverses indústries demanden nous graus d'acer amb menor densitat per a disminuir el pes sense comprometre les propietats mecàniques. Els acers amb alt contingut en manganés es consideren materials molt prometedors per a aplicacions estructurals a causa de la seua excepcional combinació de resistència i ductilitat, amb una baixa densitat. No obstant això, malgrat les seues excepcionals propietats, els acers amb alt contingut en manganés s'enfronten a diverses limitacions o reptes durant les tècniques de processament convencionals. Afortunadament, la solidificació ràpida pot resoldre estos problemes. En este sentit, les tecnologies d'AM basades en làser proporcionen velocitats de refredament ràpides, així com flexibilitat en termes de disseny geomètric. Els nous reptes d'estes tecnologies implicaran la microsegregació i l'esquerdament en calent, o hot cracking en anglés, que es produïxen durant la solidificació. Esta tesi està dedicada a explotar el mètode CALPHAD per a realitzar càlculs termodinàmics amb la finalitat de dissenyar diversos acers amb alt contingut en manganés que puguen previndre eficaçment els problemes de solidificació ràpida en AM. Les composicions d'acer dissenyades es van produir en forma de pols per a AM mitjançant atomització amb gas. Es van analitzar les pólvores per a determinar la seua microestructura en relació amb la química i la velocitat de refredament. Ajustant adequadament els paràmetres d'impressió, estes pólvores d'acer amb alt contingut en manganés es van imprimir amb èxit en AM, donant lloc a densitats relatives superiors al 99.9%. Es va analitzar la microestructura d'estes mostres totalment denses i es va comparar amb les seues respectives pólvores, amb la finalitat d'identificar qualsevol diferència resultant de les variacions en la velocitat de refredament i els cicles tèrmics. Finalment, desprès de definir el millor conjunt de condicions d'impressió per a cada composició de pols, es van produir diverses mostres per a avaluar les propietats mecàniques. / [EN] Additive manufacturing (AM) is a process that builds three-dimensional solid objects by layering materials based on a computer-aided design model. AM is set to become the next industrial revolution, transforming the landscape of development and production. AM provides numerous benefits, including complex and flexible design possibilities, the elimination of intermediate processes like machining, production cost independence from batch size, reduced material waste, lightweight structures, customized machine repairs, and the ability to develop new materials, among other advantages. In additive manufacturing technologies that employ a laser beam as an energy source, the initial raw material (in the form of powder or wire) is melted by the laser heat source in a controlled manner, layer by layer, until a component with final or nearly final dimensions is created. These technologies involve subjecting the printed material to a unique thermal process, where the material is melted in a very specific area and then rapidly cooled at extremely high rates of up to 10^6 K/s. Hence, the microstructures that arise from the manufacturing processes in AM differ significantly from those achieved in traditional processes. Moreover, the materials predominantly employed in AM have not been explicitly designed for these technologies. The specific characteristics of AM processes can be utilized to achieve distinct microstructures and properties in steels that have been tailored to take advantage of the rapid cooling rates and thermal history of the process, among other factors. For the moment, the number of commercial steel grades available in the AM market is limited. Various industries are demanding new steel grades with lower density to decrease weight without compromising mechanical properties. High manganese steels are regarded as highly promising materials for structural applications due to their exceptional combination of strength and ductility, with low density. Nevertheless, despite the exceptional properties of high manganese steels, they encounter various limitations or challenges during conventional processing techniques. Fortunately, rapid solidification may solve these issues. In this sense, laser-based AM technologies provide rapid cooling rates, as well as flexibility in terms of geometric design. The new challenges of these technologies will involve micro-segregation and hot cracking occurring during solidification. This thesis is dedicated to exploiting the CALPHAD method to perform thermodynamic calculations in order to design various high manganese steels that can effectively prevent fast solidification issues in AM. The steel compositions designed were produced in the form of powder for AM using gas atomization. Powders were analyzed to determine their microstructure in relation to the chemistry and cooling rate. By adjusting properly, the printing parameters, these high manganese steel powders were successfully printed in AM, resulting in relative densities exceeding 99.9%. The microstructure of these fully dense samples was analyzed and compared to their respective powders, in order to identify any difference resulting from variations in cooling rate and thermal cycling. Lastly, after defining the best set of printing conditions for each powder composition, various samples were produced to evaluate the mechanical properties, to determine the correlation between the composition, microstructure and properties of these steels. In addition, lattice structures that are close to final part geometries were constructed to quantify the energy absorbed during compression by one of these high manganese steels. The results were then compared to those of 316L, revealing that the high manganese steel absorbs roughly twice as much the specific energy in compression. This finding demonstrates the potential of these novel AM steels for use in industrial applications. / Sánchez Poncela, M. (2024). Design of High Mn Fe-Mn-Al-C Low Density Steels for Additive Manufacturing [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/205174

Additive Manufacturing (AM) is on the verge of causing a downfall to conventional manufacturing with its huge potential in part manufacture. With an increase in demand for customized product, on-demand production and sustainable manufacturing, AM is gaining a great deal of attention from different industries in recent years. AM is redefining product design by revolutionizing how products are made. AM is extensively utilized in automotive, aerospace, medical and dental applications for its ability to produce intricate and lightweight structures. Despite their popularity, AM has not fully replaced traditional methods with one of the many reasons being inferior surface quality. Surface texture plays a crucial role in the functionality of a component and can cause serious problems to the manufactured parts if left untreated. Therefore, it is necessary to fully understand the surface behavior concerning the factors affecting it to establish control over the surface quality. The challenge with AM is that it generates surfaces that are different compared to conventional manufacturing techniques and varies with respect to different materials, geometries and process parameters. Therefore, AM surfaces often require novel characterization approaches to fully explain the manufacturing process. Most of the previously published work has been broadly based on two-dimensional parametric measurements. Some researchers have already addressed the AM surfaces with areal surface texture parameters but mostly used average parameters for characterization which is still distant from a full surface and functional interpretation. There has been a continual effort in improving the characterization of AM surfaces using different methods and one such effort is presented in this thesis. The primary focus of this thesis is to get a better understanding of AM surfaces to facilitate process control and optimization. For this purpose, the surface texture of Fused Deposition Modeling (FDM) and Laser-based Powder Bed Fusion of Metals (PBF-LB/M) have been characterized using various tools such as Power Spectral Density (PSD), Scale-sensitive fractal analysis based on area-scale relations, feature-based characterization and quantitative characterization by both profile and areal surface texture parameters. A methodology was developed using a Linear multiple regression and a combination of the above-mentioned characterization techniques to identify the most significant parameters for discriminating different surfaces and also to understand the manufacturing process. The results suggest that the developed approaches can be used as a guideline for AM users who are looking to optimize the process for gaining better surface quality and component functionality, as it works effectively in finding the significant parameters representing the unique signatures of the manufacturing process. Future work involves improving the accuracy of the results by implementing improved statistical models and testing other characterization methods to enhance the quality and function of the parts produced by the AM process.

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.

Purdue School of Engineering and Technology, Indianapolis / Powder Bed Fusion Additive Manufacturing enables the fabrication of metal parts with complex geometry and elaborates internal features, the simplification of the assembly process, and the reduction of development time. However, the lack of consistent quality hinders its tremendous potential for widespread application in industry. This limits its ability as a viable manufacturing process particularly in the aerospace and medical industries where high quality and repeatability are critical. A variety of defects, which may be initiated during the powder-bed fusion additive manufacturing process, compromise the repeatability, precision, and resulting mechanical properties of the final part. The literature review shows that a non-uniform temperature distribution throughout fabricated layers is a significant source of the majority of thermal defects. Therefore, the work introduces an online thermography methodology to study temperature distribution, thermal evolution, and thermal specifications of the fabricated layers in powder-bed fusion process or any other thermal inherent AM process. This methodology utilizes infrared technique and segmentation image processing to extract the required data about temperature distribution and HAZs of the layer under fabrication. We conducted some primary experiments in the FDM process to leverage the thermography technique and achieve a certain insight to be able to propose a technique to generate a more uniform temperature distribution. These experiments lead to proposing an innovative chessboard scanning strategy called tessellation algorithm, which can generate more uniform temperature distribution and diminish the layer warpage consequently especially throughout the layers with either geometry that is more complex or poses relatively longer dimensions. In the next step, this work develops a new technique in ABAQUS to verify the proposed scanning strategy. This technique simulates temperature distribution throughout a layer printed by chessboard printing patterns in powder-bed fusion process in a fraction of the time taken by current methods in the literature. This technique compares the temperature distribution throughout a designed layer printed by three presented chessboard-scanning patterns, namely, rastering pattern, helical pattern, and tessellation pattern. The results confirm that the tessellation pattern generates more uniform temperature distribution compared with the other two patterns. Further research is in progress to leverage the thermography methodology to verify the simulation technique. It is also pursuing a hybrid closed-loop online monitoring and control methodology, which bases on the introduced tessellation algorithm and online thermography in this work and Artificial Neural Networking (ANN) to generate the most possible uniform temperature distribution within a safe temperature range layer-by-layer.

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.