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Additive Manufacturing of metals has become relevant for industrial applications. The near net-shape production of components produced by Laser Powder Bed Fusion (PBF-LB/M) enables new possibilities in component design combined with a reduction of the amount of needed material. Omitting the extra material, that was part of conventionally produced components due to machining constraints, results in components which in consequence lack the inherent additional safety margins provided by the higher material consumption of conventionally produced components. Therefore, to use PBF-LB/M metals in safety critical applications an in-depth understanding of porosity and internal stresses in parts made by PBF-LB/M is needed. Only non-destructive testing methods—such as computed tomography and residual stress analysis using neutrons—enable the assessment of porosity and stresses in the whole part. In this thesis I tackled creep and tensile static deformed specimens to fill research gaps in this field in terms of analysing PBF-LB/M stainless steel AISI 316L: from structural properties to in-situ behaviour. The initial void population of AISI 316L is studied using X-ray and synchrotron micro computed tomography. Specimens produced with different process parameters were analysed to quantify the influence of process parameters on the initial void population. The possibility to close voids using the laser illumination of subsequent layers is discussed by a quantitative study of the ability of the laser to melt different multiples of the applied layer thickness. The formation of internal stress is inherent to components produced by the PBFLB/ M process. These stresses remain in the components after production as residual stresses. In this thesis a study is presented which aims to unravel the mechanisms that define the spatial distribution of the residual stresses, and their magnitude. In the end, the population of internal voids during mechanical testing is studied by X-ray micro computed tomography. The evolution of damage accumulation in creep specimens is studied at different stages of the creep test. Results are compared to a creep tested conventionally made specimen and to a PBF-LB/M specimen from a tensile test. An interconnection between the PBF-LB/M microstructure and the pattern of damage is revealed.
Multidimensional visualization of preferential flow pathways using neutron and x-ray radiation
(2016)
The occurrence of preferential flow of water in soils is rather the rule than the exception particularly in heterogeneous structured soils. The quantitative prediction is difficult because of the spatial as well as temporal variability. One aim of the dissertation was to visualize the soil structure and flow patterns in mine soils using a combination of neutron radiography, neutron tomography and x-ray computed tomography experiments. Both radiation techniques were used to describe preferential pathways as a soil structure with CT and the process of preferential flow of water with NT. For the first time neutron radiation was applied on undisturbed soil cores to visualize water distributions in natural soils on three examples for heterogeneities of different origin.
In a first investigation we used neutron radiography to analyze the moisture distribution in undisturbed slab-type and cylindrical samples of mine soils containing lignitic fragments and inclined soil layers. 2D radiography sequences indicated that flow pathways are highly dependent on the shape, distribution, and internal structure of the lignitic fragments. For 3D experiments, the quantitative analysis of water contents failed due to a pronounced beam hardening effect. The tensiometer data indicated the existence of local non-equilibrium in pressure heads between fragments and matric pore regions.
The objective of the second study was to analyze and visualize the effects of initial soil structure formation at the surface of bare soils in an artificial hydrological catchment that has been left to undirected ecological succession for three years. The three-dimensional (3D) micro-morphology was described using the X-ray computed tomography (CT); two-dimensional (2D) infiltration patterns were obtained from drip infiltration experiments using time-resolved neutron radiography (NT). The analyses of CT data indicate the existence of (i) structural crusts on soil consisting of sandy to loamy sediments and (ii) depositional crusts on sandy sediments. The results of combined observations using CT and NT suggest that the initial soil surface structure alterations of young sandy sediment modify moisture patterns of infiltrating water only slightly.
The aim of the 3rd experiment was to study effect of two ‘ecological engineers’, moss vegetation and beetle larvae on pore structure and infiltration. The soil pore structure was characterized by micro-computed tomography analyses of bare soil, sparse and dense moss-vegetated samples containing a single burrow. Flow patterns of infiltration experiments were visualized using the neutron radiography technique. The results of this study suggest that moss vegetation and burrowing activity of ground beetles strongly control the initial soil development by modification of pore structure and surface water infiltration.