Laser powder bed fusion of 316L stainless steel - Microstructure and mechanical properties as a function of process parameters, design and productivity

One of the most common additive manufacturing techniques for fabricating metallic components is laser powder bed fusion, which has demonstrated great potential in fabricating parts with properties exceeding the properties achieved via conventional methods. To fully utilize the process’s potential, a more profound understanding of the microstructure and properties of the laser powder bed fusion processed material is required. This thesis aims to provide new insights into how the microstructure, mechanical properties and productivity are affected by part design and process parameters. The thesis is framed around a detailed investigation of the parts produced in stainless steel 316L.The provided results reveal that producing parts with standard process parameters leads to near-full density with excellent tensile properties and a microstructure consisting of large elongated grains with predominant <101> orientation characterized by a fine submicron cellular structure. It was demonstrated that part thickness does not influence component density, but the grain morphology and texture are affected close to the part edges. Reducing the part thickness to less than 0.5 mm reduced the predominant texture and reducing the part thickness to less than 1 mm reduced the yield strength.Altering the process parameters affected the crystallographic orientation, grain size and cell size and thus the tensile properties. Minor effects of processing gas composition (Ar, N2 or He) on the chemical composition, microstructure, tensile strength and hardness was detected. In addition, it was revealed that a 20% faster build time could be achieved without compromising the static properties by adjusting the scan speed and hatch distance. Increasing the layer thickness to 80 \ub5m allowed for shortening the build time by a factor of four but with a 14% reduction in yield strength and 17% reduction in ductility.The findings provided in this thesis serve as a basis for the development of rules for part design and qualification of mechanical properties for manufacturing parts with fine design features via laser powder bed fusion. The results emphasize the importance of the part design on the microstructure and the properties of the produced component. In addition, the presented results emphasize the potential for significant improvement in build speed in the laser powder bed fusion process, exemplified for 316L

Investigation of the strengthening mechanism in 316L stainless steel produced with laser powder bed fusion

Of the many benefits of the additive manufacturing process, laser powder bed fusion (L-PBF) has specifically been shown to produce hierarchical microstructures that circumvent the common strength-ductility trade-off. Typically, high strength materials have limited ductility, and vice versa. The L-PBF microstructure, consisting of fine cells, is formed during the rapid solidification of the laser powder bed fusion process. The cell boundaries are often characterized by the segregation of alloying elements and a dislocation network. While there are a number of works describing the strengthening mechanisms in L-PBF-produced 316L, there are still some gaps in understanding the effect of stress-relief and annealing at various annealing temperatures (400, 800 and 1200 \ub0C) on the plastic strain accumulation during deformation. In this study, the authors evaluated strain partitioning using electron backscatter diffraction and kernel average misorientation maps. The results show strain partitioning to be dependent on both the annealing temperature and the pre-straining of samples. Further, the results indicated that the dislocation structure was stable until 400 \ub0C, whereas at 800 \ub0C strain was no longer detected at the cell boundaries. Similarly, after the heat treatment at 800 \ub0C, elemental segregation at the cell walls was no longer detectable. Upon straining, the boundaries of as-built and annealed samples at 400 and 800 \ub0C registered accumulation of additional strain as compared to the unstrained states. The results demonstrate that even a weak array of dislocations along the cell walls can successfully pin dislocations, albeit at a reduced capability relative to the co-existent dislocation and segregate structures found in microstructures of the as-built and annealed samples at 400 \ub0C

Increasing the Productivity of Laser Powder Bed Fusion for Stainless Steel 316L through Increased Layer Thickness

Additive manufacturing (AM) is able to generate parts of a quality comparable to those produced through conventional manufacturing, but most of the AM processes are associated with low build speeds, which reduce the overall productivity. This paper evaluates how increasing the powder layer thickness from 20 \ub5m to 80 \ub5m affects the build speed, microstructure and mechanical properties of stainless steel 316L parts that are produced using laser powder bed fusion. A detailed microstructure characterization was performed using scanning electron microscopy, electron backscatter diffraction, and x-ray powder diffraction in conjunction with tensile testing. The results suggest that parts can be fabricated four times faster with tensile strengths comparable to those obtained using standard process parameters. In either case, nominal relative density of > 99.9% is obtained but with the 80 \ub5m layer thickness presenting some lack of fusion defects, which resulted in a reduced elongation to fracture. Still, acceptable yield strength and ultimate tensile strength values of 464 MPa and 605 MPa were obtained, and the average elongation to fracture was 44%, indicating that desirable properties can be achieved

Constraint Replacement-Based Design for Additive Manufacturing of Satellite Components: Ensuring Design Manufacturability through Tailored Test Artefacts

Additive manufacturing (AM) is becoming increasingly attractive for aerospace companies due to the fact of its increased ability to allow design freedom and reduce weight. Despite these benefits, AM comes with manufacturing constraints that limit design freedom and reduce the possibility of achieving advanced geometries that can be produced in a cost-efficient manner. To exploit the design freedom offered by AM while ensuring product manufacturability, a model-based design for an additive manufacturing (DfAM) method is presented. The method is based on the premise that lessons learned from testing and prototyping activities can be systematically captured and organized to support early design activities. To enable this outcome, the DfAM method extends a representation often used in early design, a function-means model, with the introduction of a new model construct-manufacturing constraints (Cm). The method was applied to the redesign, manufacturing, and testing of a flow connector for satellite applications. The results of this application-as well as the reflections of industrial practitioners-point to the benefits of the DfAM method in establishing a systematic, cost-efficient way of challenging the general AM design guidelines found in the literature and a means to redefine and update manufacturing constraints for specific design problems

Effect of the process gas and scan speed on the properties and productivity of thin 316L structures produced by Laser-Powder Bed Fusion

The development of the laser powder bed fusion (L-PBF) process to increase its robustness and productivity is challenged by ambitious design optimizations, such as thin wall structures. In this study, in addition to the effect of commonly used gases as Ar and N2, increased laser scanning speed and new process gases, such as helium, were successfully implemented. This implementation allowed to build 316L stainless steel components with thin walls of 1 mm thickness with an enhanced build rate of 37 pct. The sample size effect and the surface roughness were held responsible for the reduction in strength (YS > 430 MPa) and elongation (EAB > 30 pct) for the 1 mm samples studied. Similar strength was achieved for all process gases. The increased scanning speed was accompanied by a more random texture, smaller cell size, and grain size factor along the building direction when compared to the material built with the standard laser parameters. Stronger preferential orientation 〈101〉 along the building direction was observed for material built with standard parameters. Finally, the use of helium as a process gas was successful and resulted in reduced cell size. This finding is promising for the future development of high strength 316L stainless steel built with high build rates

Driving and Driven Architectures of Directed Small-World Human Brain Functional Networks

Recently, increasing attention has been focused on the investigation of the human brain connectome that describes the patterns of structural and functional connectivity networks of the human brain. Many studies of the human connectome have demonstrated that the brain network follows a small-world topology with an intrinsically cohesive modular structure and includes several network hubs in the medial parietal regions. However, most of these studies have only focused on undirected connections between regions in which the directions of information flow are not taken into account. How the brain regions causally influence each other and how the directed network of human brain is topologically organized remain largely unknown. Here, we applied linear multivariate Granger causality analysis (GCA) and graph theoretical approaches to a resting-state functional MRI dataset with a large cohort of young healthy participants (n = 86) to explore connectivity patterns of the population-based whole-brain functional directed network. This directed brain network exhibited prominent small-world properties, which obviously improved previous results of functional MRI studies showing weak small-world properties in the directed brain networks in terms of a kernel-based GCA and individual analysis. This brain network also showed significant modular structures associated with 5 well known subsystems: fronto-parietal, visual, paralimbic/limbic, subcortical and primary systems. Importantly, we identified several driving hubs predominantly located in the components of the attentional network (e.g., the inferior frontal gyrus, supplementary motor area, insula and fusiform gyrus) and several driven hubs predominantly located in the components of the default mode network (e.g., the precuneus, posterior cingulate gyrus, medial prefrontal cortex and inferior parietal lobule). Further split-half analyses indicated that our results were highly reproducible between two independent subgroups. The current study demonstrated the directions of spontaneous information flow and causal influences in the directed brain networks, thus providing new insights into our understanding of human brain functional connectome

Diverse definitions of the early course of schizophrenia - a targeted literature review

Schizophrenia is a debilitating psychiatric disorder and patients experience significant comorbidity, especially cognitive and psychosocial deficits, already at the onset of disease. Previous research suggests that treatment during the earlier stages of disease reduces disease burden, and that a longer time of untreated psychosis has a negative impact on treatment outcomes. A targeted literature review was conducted to gain insight into the definitions currently used to describe patients with a recent diagnosis of schizophrenia in the early course of disease ('early' schizophrenia). A total of 483 relevant English-language publications of clinical guidelines and studies were identified for inclusion after searches of MEDLINE, MEDLINE In-Process, relevant clinical trial databases and Google for records published between January 2005 and October 2015. The extracted data revealed a wide variety of terminology and definitions used to describe patients with 'early' or 'recent-onset' schizophrenia, with no apparent consensus. The most commonly used criteria to define patients with early schizophrenia included experience of their first episode of schizophrenia or disease duration of less than 1, 2 or 5 years. These varied definitions likely result in substantial disparities of patient populations between studies and variable population heterogeneity. Better agreement on the definition of early schizophrenia could aid interpretation and comparison of studies in this patient population and consensus on definitions should allow for better identification and management of schizophrenia patients in the early course of their disease

Aspects of building geometry and powder characteristics in powder bed fusion

Additive manufacturing (AM) produces near-net-shaped parts directly from a 3D-CAD model in a layer-by-layer manner. One of the most common AM technique for fabricating metallic components is powder bed fusion (PBF). The PBF process has shown great potential in fabricating metallic parts with properties better or comparable to conventional methods. However, there are some challenges in reproducibility, process stability, robustness, etc. This thesis elaborates on several of these challenges and addresses the influences of feedstock material, build orientation and part design on the final outcome. The PBF process uses fine metal powder as feedstock material and in order to have an economically feasible process, powder recycling is a necessity. However, to ensure a robust process and consistent material properties, the feedstock material need to be handled with caution as powder properties will affect the part quality. The obtained results for 316L stainless steel from this study indicate that powder degradation in terms of surface product changes occurs when the powder is recycled. It was revealed that both recycled and virgin powder were covered by a heterogeneous oxide layer, composed by a homogeneous iron oxide layer with the presence of Cr-Mn-rich oxide particulates that were growing during PBF processing. The results showed that the powder degradation was more pronounced when used in the electron beam system compared to a laser based system due to the long exposure at high temperatures. The manufacturing capabilities of the PBF process has enabled the production of lattice structures without extensive tooling. The properties of such lattice will be influenced by the microstructure. Hence, it is of importance to understand how the part geometry would affect the microstructure. This study presents the effect of build geometry, as e.g. wall thickness and build angle on the 316L microstructure. The obtained results indicated that in the center of ribs over 0.6 mm in thickness, large elongated grains with preferential orientation were created. Reducing the part thickness to below 0.6 mm reduced the predominant texture. The increased cooling rate close to the part surface inhibited grain growth and changed the preferential grain orientation. For the process parameters used, the critical part thickness to avoid large elongated grains was found to be about 0.4 mm. The obtained results could be used for further development of design rules and prediction of mechanical properties of AM parts with small wall thicknesses

The search for a democratic aesthetics : Robert Rauschenberg, Walker Evans, William Carlos Williams

If democracy were an artwork, what would it look like? - This project proposes a specific way of conceptualizing the link between democracy and aesthetics. Contrary to previous accounts, which mostly concentrated on understanding democratic as accessible art or on analyzing the role of art in democratic society. suggest that a democratic aesthetics can be properly understood by interpreting specific formal features of artworks as metaphors for particular key elements of democracy as it is framed in important strands of contemporary democratic theory. The project argues, among other things, that a certain kind of loose collage composition stands in a metaphorical relationship with the structure of a pluralist democratic polity. Systematically developed, this account helps better to understand the democratic quality of individual artworks, and also makes plausible how aesthetics can contribute to the discourse of democratic theory.EThOS - Electronic Theses Online ServiceGBUnited Kingdo