No description available.

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.

Additive manufacturing (AM) is a promising manufacturing process that provides that ability to fabricate components with complex geometries with relatively low lead times compared to other manufacturing processes. This allows for more freedom of design, as prototypes can easily be produced throughout the development process. AM is also especially beneficial in tooling applications, where internal geometries such as cooling channels are required in order to improve the quality of the manufactured parts. These geometries are more difficult to produce with more conventional manufacturing methods such as forging or casting. Currently, Laser Powder Bed Fusion (LPBF) shows the most promise in the field of Additive Manufacturing (AM) of metals, as it offers the freedom to produce complex components with little post processing required. Additionally, post processing with Hot Isostatic Pressing (HIP) can be implemented to significantly enhance the final properties of the material.  The LPBF-process can produce many different defects within the parts such as: part porosity and lack of fusion. This is mainly due to the layer-by-layer configuration of the process. Parts can also experience large thermal fluctuations and rapid cooling rates which can generate large residual stresses. This can result in significant cracking in certain high alloyed materials which can impact part quality and  material properties. If the cracking is severe enough, it will result in failure of the entire component and render the entire parts completely useless. Post processing with HIP may remove some of these defects and reduce the residual stresses in the material and thus produce a material with properties that are satisfactory. The purpose of this thesis is to investigate the processability of a high alloy cold work tool steel with LPBF. The main focus is the influence of the processing parameters and part geometry on the quality of the produced parts. Furthermore, the influence of the processing parameters on defects and microstructure will also be investigated. The aim is to produce parts that can be enhanced with HIP as a post processing treatment. Additionally, the impact of HIP on the properties of the part will also be investigated in order to determine if the there are any improvements in terms ofreduction in part defects and the removal of any undesired microstructural features which are produced from the process. The experimental results showed that the processability of the tool steel is difficult. Several sample volumes were produced with varying processing parameters and scanning strategies, and all the specimens from all sample volumes exhibited some cracking. Parts produced with a combination of contouring and hatching strategy, where there is an internal structure showed the most promise, as these parts exhibited the least amount of severe cracking. However, additional research of the processing parameters and scanning strategies is required in order to reduce the amount of cracking of the external shell structure and thus, achieve proper densification of the parts when post processing with HIP. / Additiv tillverkning (AM) är en lovande tillverkningsprocess som ger möjligheten att tillverka komponenter med komplexa geometrier med relativt låga ledtider jämfört med andra tillverkningsprocesser. Detta ger större frihet i under designprocessen eftersom prototyper enkelt kan produceras under hela utvecklingsprocessen. AM är också särskilt fördelaktigt i verktygstillämpningar, där interna geometrier såsom kylkanaler krävs för att förbättra kvaliteten på de tillverkade delarna. Dessa geometrier är svårare att tillverka med mer konventionella tillverkningsmetoder som smidning eller gjutning. För närvarande visar det sig att Laser Powder Bed Fusion (LPBF) är det mest lovande inom området additiv tillverkning av metaller, eftersom processen erbjuder friheten att producera komplexa komponenter samt att efterbearbetning som krävs blir mindre. Dessutom kan efterbearbetning med Hot Isostatic Pressing (HIP) implementeras för att avsevärt förbättra materialets slutliga egenskaper. LPBF-processen kan ge upphov till många olika defekter i delarna såsom: delporositet och lack of fusion. Detta beror främst på att processen sker lagervis vilket kan ge upphov att många småfel. Delar kan också uppleva stora termiska fluktuationer och snabba kylningshastigheter som kan generera stora restspänningar. Det kan resultera i stor sprickbildning i vissa höglegerade material vilket kan påverka delarnas kvalitet och materialegenskaper. Om sprickorna som bildas är stora eller djupa nog kommer detta att resultera i att hela komponenten blir oanvändbar. Efterbearbetning med HIP kan ta bort en del av dessa defekter och minska restspänningarna i materialet och därmed producera ett material med goda egenskaper. Syftet med detta arbete är att undersöka bearbetbarheten hos ett höglegerat kallbearbetningsstål med som produceras med LPBF. Huvudfokus är påverkan av processparametrar och detaljgeometrin på kvaliteten på de producerade delarna. Vidare kommer också processparametrarnas inverkan på defekter och mikrostruktur att undersökas. Syftet är att producera delar som kan förbättras med HIP som efterbehandlingsbehandling. Dessutom kommer effekterna av HIP på delens egenskaper också att undersökas för att avgöra om det finns några förbättringar i termer av minskning av deldefekter och avlägsnande av alla oönskade mikrostrukturella egenskaper som produceras från processen. De experimentella resultaten visade att verktygsstålets bearbetbarhet är svår. Flera provvolymer producerades med varierande processparametrar och skanningsstrategier, och alla prover från alla provvolymer uppvisade viss sprickbildning. Delar som tillverkats med en kombination av kontur och hatch, där det finns en inre struktur visade sig mest lovande, eftersom dessa delar uppvisade minst sprickbildning. Ytterligare arbete av processparametrarna och skanningsstrategier krävs dock för att minska mängden sprickbildning i den yttre skalstrukturen och därmed uppnå korrekt förtätning av delarna vid efterbearbetning med HIP.

No description available.

No description available.

No description available.

No description available.

Laser-based additive manufacturing offers a high degree of thermokinetic flexibility that has implications on the structure and properties of the fabricated component. However, to exploit the flexibility of this process, it is imperative to understand the process-inherent thermokinetic evolution and its effect on the material characteristics. In view of this, the present work establishes a fundamental understanding of the spatiotemporal variation of thermokinetics during the fabrication of the non-ferrous alloys using the laser powder bed fusion process. Due to existing limitations of experimental techniques to probe such thermokinetics, a finite element method-based computational model is developed to predict the thermokinetic variations during the process. With the computational approach coupled with experimental techniques, the current work presents the solidification behavior influenced by spatially varying thermokinetics. In addition, it uniquely predicts the process-inherent multi-track multi-layer evolution of thermal cycles as well as thermal stress cycles and identifies their influence on the post-solidification microstructural evolution involving solid-state phase transformation. Lastly, the response of the material with a unique microstructure is recorded under various conditions (static and dynamic), which is again compared with the same set properties obtained for the same material processed via conventional routes.

Metallic biomaterials have an essential portion of uses in biomedical applications. Their properties can be tuned by many factors resulting in their process tuneability. Among metallic biomaterials for biomedical and specifically orthopedic applications, titanium and its alloys exhibit the most suitable characteristics as compared to stainless steels and Co-Cr alloys because of their high biocompatibility, specific strength (strength to density ratio), and corrosion resistance. According to their phase constitution, Ti-alloys are classified into three main groups, namely alpha, beta, and alpha+beta alloys. Depending on the degree of alloying and thermomechanical processing path, it is possible to tune the balance of α and β phases, which permits to tailor properties like strength, toughness, and fatigue resistance. (alpha+beta) Ti alloys, especially Ti-6Al-4V, are widely used alloys in biomedical applications; however, they have some drawbacks like the presence of toxic elements i.e., V and relatively high elastic modulus to that of bones. In view of the lower elastic modulus of body center cubic beta phase (50GPa<100GPa) compared to the alpha+beta, as well as due to their good mechanical properties, excellent corrosion resistance, and biocompatibility, beta-Ti alloys have been recently proposed as a valid alternative to alpha+beta ones. The growing interest in additive manufacturing (AM) techniques opens new and very interesting perspectives to the production of biomedical prosthetic implants. AM will prospectively allow implant customization to the patient and produce it on demand, with large savings on times and costs. Moreover, AM is gaining increasing interest due to the possibility of producing orthopedic implants with functionally graded open-cell porous metals. The main advantages of porous materials are the reduction of the elastic modulus mismatch between bone and implant alloy reducing the stress shielding effect and improving implant morphology providing biological anchorage for tissue in-growth. In this scenario, the first goal of the present PhD thesis work was to identify a high-performance β-Ti alloy formulation suitable to Laser-Powder Bed Fusion (L-PBF) additive manufacturing. In particular, it explores the potential use of a β-metastable Ti alloy, namely Ti-15Mo-2.7Nb-3Al-0.2Si (Beta Ti21S, 21 wt.% of alloying additions, including Silicon) for biomedical applications. Through microstructural, mechanical, and cytotoxicity analyses, it could be shown that this alloy grade exhibits i) an unprecedented ultra-low elastic modulus, ii) improved cytocompatibility due to the lack of Vanadium, and iii) no martensitic transformation responsible for hard and brittle solidification structures. A second goal was to assess the manufacturability of metamaterials made of β-Ti21S via L-PBF. For this purpose, cubic cellular lattice structures of different unit cell sizes (and therefore different strut thickness) have been fabricated and characterized through microstructural analysis using different techniques, and computed tomography combined with linear elastic finite element simulations to identify the minimum cell size that can be printed with adequate dimensional and geometrical accuracy. Samples of the selected unit cell size were also tested to determine their static and fatigue properties. The main results show that i) the suitable manufacturing quality is obtained for strut thickness above 0.5 mm, ii) the mechanical tests place the present cellular structures among the best stretching dominated cellular lattice materials investigated to date in the literature, and iii) the fatigue tests showed a normalized fatigue strength at 107 cycles of close to 0.8, similar to cubic lattices made of Ti-6Al-4V, and higher than most cellular structures in the literature. In the last part of the thesis, a more complex octet truss structure was fabricated in the manufacturable cell size, and its mechanical properties were investigated. The octet truss topology can be beneficial both in terms of mechanical properties and biocompatibility by providing the different types of porosity suitable for bone in-growth.

This study focuses on the laser powder bed fusion processing of NiTi alloys and the feasibility of fabricating very thin stent structures for biomedical applications. A comprehensive correlation between the process and the material’s-structure and properties is established to facilitate the fabrication of NiTi alloys with tailored properties. In the first step, the impact of LPBF processing parameters and post-treatments on evolving the microstructure, texture, superelasticity, and asymmetry is examined. Subsequently, the feasibility of manufacturing very thin mesh structured stents is scrutinized followed by in-depth investigations into differently designed stents considering properties such as surface characteristics, mechanical properties, superelasticity, and recoverability. The obtained results and the represented discussions offer imperative insights, helping to better understand the complexity of the LPBF process and the present challenging aspects. Moreover, detailed contributions are made with the goal of paving the road ahead for the production of patient-specific NiTi stents with enhanced properties. / Thesis / Doctor of Philosophy (PhD)