Metal Additive Manufacturing (AM) provides the means to fabricate complex metallic parts with reduced time to market and material waste and improved design freedom. Industries with strict materials qualifications such as aerospace, biomedical, and automotive are increasingly looking to AM to meet their production needs. However, significant materials-related challenges impede the widespread adoption of these technologies for critical components. In particular, fatigue resistance in as-built parts has proven to be inferior and unpredictable due to the large and variable presence of porosity. This presents a challenge for the qualification of any load bearing part without extensive post-processing, such as Hot Isostatic Pressing, and thorough inspection. Improved understanding of the underlying mechanisms behind defect formation will assist in designing process improvements to minimize or eliminate defects without relying entirely on postprocessing. In this work, the effects of powder, processing parameters, and post-processing on porosity formation in powder-bed metal AM processes are investigated using X-ray microtomography and a newly developed in-situ high speed radiography technique, Dynamic Xray Radiography. High resolution X-ray computed tomography is used to characterize defect morphology, size, and spatial distribution as a function of process and material inputs. Dynamic X-ray Radiography, which enables the in-situ observation of the laser-metal interactions at frame rates on the order of 100 kHz (and faster), is utilized to understand the dynamic behavior and transitions that occur in the vapor depression across process space. Experimental validation of previously held assumptions regarding defect formation as well as new insights into the influence of the vapor cavity on defect formation are presented.

Sacrificial metallic coatings have been used in the past, for protecting steel in industrial and urban developments. It has been suggested that thermally sprayed aluminium coatings, immersed in marine environments, are also capable of protecting steel by providing galvanic protection. Researchers have also speculated that the growth of inert oxide products, on top of the coating as well as inside pores and cracks, could enhance even further its protective “shielding” properties. In addition, the self-healing abilities of the coatings, in case of mechanically induce damage, are still a matter of debate. This research project takes a critical look at these assumptions, focusing on the electrochemical response of aluminium metal sprayed (AMS) coatings immersed in 0.6 M NaCl and 0.6 M Na2SO4 solutions, in order to investigate how the presence of chlorides can alter the corrosion behaviour of these coatings. For such, a thin aluminium protective coating was deposited over two different panels (aluminium 1050 and low carbon steel), by the steel making company Fairfield-Mabey, using electric arc thermal spraying (TS). Subsequently, the coated panels were sectioned into smaller specimens, and with the aid of a SEM/EDS, and XRD, observations and analysis were conducted in their surfaces and cross sections, in order to determine the morphology, quality of deposition, and chemical composition of the coatings. Internal porosity/oxide growth after immersion, was studied using 3-D X-ray tomography scanning. Furthermore, the electrochemical behaviour of the coatings (intact and scribed) was also investigated, initially by analyzing the evolution of the open circuit potential in time, when immersed in chloride rich and chloride free environments. Additionally, the anodic polarization behaviour of the substrate and coatings were simultaneously analyzed, by connecting a zero resistance ammeter (ZRA) with the W.E. terminal of a potentiostat. To conclude, EIS and Rp vs. time plots were made in order to corroborate the data obtained from other tests. Results show that although the coating can generate superficial and internal corrosion products, these oxides are not isolating in nature nor will enhance its protective properties. On the other hand, the corrosion potential behaviour of the coatings revealed that AMS coatings have a more active than the substrate; regardless of the environment in which they were immersed or their substrate. Nevertheless, it was also observed that AMS coatings deposited onto steel will corrode faster than their substrates, and for that matter capable of offering corrosion protection, exclusively if chlorides are present in the solution.

Nuclear Reactor Pressure Vessels (RPV) are manufactured from medium strength low alloy ferritic steel, specifically selected for its high toughness and good weldability. The ability of the RPV material to resist crack growth is crucial given that it is one of the fundamental containment safety systems of nuclear power plants. For most of their lifetime, the RPV operates at sufficiently elevated temperatures to ensure the material is ductile. However, the development of ductile damage, in the form of voids, and the ability to predict ductile tearing in RPV materials using a mechanistically-based model remains difficult. The Gurson-Tvergaard-Needleman (GTN) model of ductile tearing provides one such tool for predicting ductile damage development in RPV materials. The difficulty in using the GTN model lies in the ability to calibrate the model parameters in a robust manner. The parameters are typically calibrated data, derived from fracture tests and relying on an iterative “trial and error” procedure of numerical simulations and comparison with test data until the model reproduces the experimental behaviour with sufficient accuracy. This research has addressed the development of a mechanistically-based approach to the calibration of the GTN model by developing a new understanding of the ductile fracture mechanism in RPV material through conventional metallography and 3D X-ray computed tomography to image the initiation, growth and coalescence of ductile voids. The metallographic and tomographic data were analysed in a quantitative manner to establish a direct link between the microstructural features and void evolution and the key parameters of the GTN model. This approach has established a more robust mechanistically based method for the calibration of the GTN model that will enhance the conventional iterative calibration procedure. The calibrated model was applied to predict ductile tearing behaviour in compact-tension and notched-tensile specimens. The results showed good agreement with test data and also reproduced the morphology and branching of crack extension observed in practise. Whilst these observations were due, in part, to the numerical solving procedure, they enabled new insights to be gained regarding the development of non-uniform void volume fraction distributions in tested specimensThe results from this research will strengthen the guidance provided to structural integrity engineers in industry regarding the calibration and application of ductile damage mechanics models such as the GTN model for predicting ductile initiation and growth in RPV materials.

abstract: AA 7XXX alloys are used extensively in aircraft and naval structures due to their excellent strength to weight ratio. These alloys are often exposed to harsh corrosive environments and mechanical stresses that can compromise their reliability in service. They are also coupled with fasteners that are composed of different materials such as Titanium alloys. Such dissimilar metal contact facilitates galvanic and crevice corrosion, which can further reduce their lifetimes. Despite decades of research in the area, the confluence of mechanical, microstructural, and electrochemical aspects of damage is still unclear. Traditionally, 2D and destructive methods have often been employed to study the corrosion and cracking behavior in these systems which can be severely limiting and lead to inaccurate conclusions. This dissertation is aimed at comprehensively studying the corrosion and cracking behavior of these systems using time-dependent 3D microstructural characterization, as well as correlative microscopy. The microstructural evolution of corrosion in AA 7075 was studied using a combination of potentiodynamic polarization, X-ray Computed Tomography (XCT) and Transmission X-ray Microscopy (TXM). In both experiments, a strong emphasis was placed on studying localized corrosion attack at constituent particles and intergranular corrosion. With an understanding of the alloy’s corrosion behavior, a dissimilar alloy couple comprising AA 7075 / Ti-6Al-4V was then investigated. Ex situ and in situ x-ray microtomography was used extensively to investigate the evolution of pitting corrosion and corrosion fatigue in AA 7075 plates fastened separately with Ti-6Al-4V screws and rivets. The 4D tomography combined with the extensive fractography yielded valuable information pertaining the preferred sites of pit initiation, crack initiation and growth in these complex geometries. The use of correlative microscopy-based methodologies yielded multimodal characterization results that provided a unique and seminal insight on corrosion mechanisms in these materials. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

Additive manufacturing can be used to produce complex, custom geometries, consolidating different parts into one. This reduces the required number of assemblies and allows distributed manufacturing with short lead times. Defects, such as porosity and surface roughness, associated with parts manufactured by laser powder bed fusion, can severely limit industrial application. The effect these defects have on corrosion and hence long term structural integrity must also be taken into consideration. This project aimed to characterise porosity in both solid and lattice cube samples produced by laser powder bed fusion, with the differences in porosity induced by changes in the process parameters, and subsequently, characterising the effect porosity has on corrosion. The alloy used in this investigation is AlSi10Mg, which is widely used in the aerospace and automotive industries. Samples were studied before and after corrosion using X-Ray computed tomography (CT scanning), metallographic examination and scanning electron microscopy (SEM), as well as compression testing for the lattice cubes. It was found that higher laser power leads to more porosity and lower surface roughness. CT scanning was a very effective method to study corrosion using aligned CT images of before-after states. Porosity did not have an effect on the corrosion during the early corrosion stages (168 hours). The manufacturing process parameters induced differences in porosity and surface conditions, but did not strongly affect corrosion. It is probable that crack initiation sites such as internal porosity and defects are filled with corrosion product, delaying the onset of cracking and failure, and the corrosion product that fill the voids adding to the full strength of the lattice will also slightly increase the compressive strength of the samples. / Dissertation (MEng (Metallurgical Engineering))--University of Pretoria, 2021. / Materials Science and Metallurgical Engineering / MEng (Metallurgical Engineering) / Unrestricted

No description available.

Pores provide important hotspots for chemical and biological processes in soils. Earthworm burrows affect the macropore structure and their actions may create new preferential pathways for water and gas flow within soils. This, in turn, indirectly affect plants, nutrient cycling, hydraulic conductivity, gas exchange, and soil organisms. While the effects of invasive earthworms on soil properties has been well-documented in temperate and boreal ecosystems, we know little how these organism may affect tundra soils. In this study, I assessed how the three-dimensional network of soil-macropores are affected by earthworm species (Aporrectodea sp. and Lumbricus sp). I hypothesized: i) that earthworms increase the frequency of macropores with a likely biological origin (biopores); ii) effects of biopores are dependent on tundra vegetation type (meadow or heath); and iii) the macropore network properties are altered by earthworms.  The hypotheses were tested using a common garden experiment with 48 mesocosms. The pore structure of each mesocosm was analyzed using X-ray CT tomography. I found that biopores increased in the tundra from on 0.05 ±0.01 % (mean ± standard deviation) in the control to about 0.59 ± 0.07 % in the earthworm treatments. However, in contrast to my second hypothesis, I found no vegetation dependent effect. Interestingly, I found that earthworms decreased the complexity and directionality of macropores. My findings strongly indicate that burrowing can severely impact the pore properties of previously uninhabited subarctic soils.

No description available.

Limestone is a raw material in the cement and quicklime industry and knowledge about limestone characteristics can help improve and optimize production processes. In the end this can lead to a reduction in CO2 emissions from the industry. In this project X-ray tomography (XRT) was used to examine limestone samples. The aim was to determine if XRT, including synchrotron-based XRT, is a reliablemethod to determine porosity, pore structure and internal distributions of pores and pyrite (FeS2) grains in limestone. The aim also included to determine if XRT could be used to resolve material variations, fine-grained and larger crystals in limestone. In total, there were ten limestone samples and the performed XRT was done by Advanced Light Source (ALS) in Berkeley, California and by Luleå University of Technology. A brief comparison between ALS and Luleå was also done by inspectingsamples that have been through XRT at both facilities. The main software used foranalysis was Avizo v.9.2.0. The results showed that XRT is a suitable method for determining porosity and pore distribution. Interactive thresholding was used in Avizo for measuring porosity. The porosity was determined as a single value and as a narrow range, where a narrow range was more reliable. XRT was also found to be a suitable method for visually determining a variety of textures within the samples. Areas with different materials(such as dolomite) and/or newly-formed crystals were visually distinguishable but individual newly-formed crystals were not as clear when compared to scanning electron microscopy. Individual older fine-grained and larger crystals were hard to resolve. Internal distributions in 3D of both pores and pyrite grains were possible to obtain with XRT. The analysis of internal distributions was found to be a clear advantage with the method of XRT. The equivalent diameter of pores and pyrite grains was also measured and plotted in histograms. The XRT performed at ALS had higher resolution than the XRT performed in Luleå (0.65 vs 2 μm). Lower resolution over-estimated the average equivalent diameter of pores, and boundaries of pores and cavities were harder to see. Therefore, the higher resolution from ALS was preferable. These results contribute to understanding limestone characteristics.

With the advent of X-ray computerised tomography (CT), the possibility of capturing precise volumetric 3D images has become an invaluable tool within the material sciences. With tomographs and synchrotrons becoming more readily available to researchers and scanning time decreasing with newer equipment, the amount of data to be analysed is steadily rising. This rapid increase in data acquisition motivates the development of automatic methods to transform CT images from qualitative to quantitative data. In this work, a method to automatically characterise paper fibres from CT images based on their centre-line and mapping their deformation is presented. Methods for fibre characterisation are presented and tested on computer-generated fibres and CT images of paper fibres. / Med tillkomsten av datorbaserd röntgentomografi (CT) har möjligheten att fånga exakta volymetriska 3D-bilder blivit ett ovärderligt verktyg inom materialvetenskapen. Den mängd data som ska analyseras växer hela tiden tack vare ökad tillgänglighet till tomografer och synkrotroner och minskad skanningstid i och med den tekniska utvecklingen av utrustningen som används. Denna snabba ökning av datainsamling motiverar utvecklingen av automatiska metoder för att omvandla CT-bilder från kvalitativa till kvantitativa data. I detta arbete presenteras en metod för att automatiskt karakterisera pappersfibrer från CT-bilder baserad på deras mittlinje och kartlägga deras deformation. Metoder för fiberkarakterisering presenteras och testas på datorgenererade fibrer och CT-bilder av pappersfibrer.