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Abstract: The complex flow within the molten metallic pool, during advanced manufacturing and joining processes such as welding and additive manufacturing, directly affects the performance of the final components; through the generation of various microstructures and strain gradients. Understanding of the flow within such melt pools can enhance the predictability of the final microstructures and therefore component performance. In situ synchrotron X-ray imaging is employed to observe and map the evolving flow patterns within gas tungsten arc weld pools using tracking particles. The experimentally observed flow patterns are compared with numerical simulations to gain additional insights on accurate predictability in relevance to the physical driving forces within the flow. The spatio-temporal distribution of the flow using the mapped data is analyzed by considering the evolution of driving forces acting throughout the weld pool. The results demonstrate quantitative benchmark flow mapping with confirmation of the overall mechanisms of weld pool flow.

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Abstract: Crystallisation is a complex process that significantly affects the rheology of magma, and thus the flow dynamics during a volcanic eruption. For example, the evolution of crystal fraction, size and shape has a strong impact on the surface crust formation of a lava flow, and accessing such information is essential for accurate modelling of lava flow dynamics. To investigate the role of crystallisation kinetics on lava flow behaviour, we performed real-time, in situ synchrotron X-ray microtomography, studying the influence of temperature-time paths on the nucleation and growth of clinopyroxene and plagioclase in an oxidised, nominally anhydrous basaltic magma. Crystallisation experiments were performed at atmospheric pressure in air and temperatures from 1250 °C to 1100 °C, using a bespoke high-temperature resistance furnace. Depending on the cooling regime (single step versus continuous), two different crystal phases (either clinopyroxene or plagioclase) were produced, and we quantified their growth from both global and individual 3D texture analyses. The textural evolution of charges suggests that suppression of crystal nucleation is due to changes in the melt composition with increasing undercooling and time. Using existing viscosity models, we inferred the effect of crystals on the viscosity evolution of our crystal-bearing samples to trace changes in rheological behaviour during lava emplacement. We observe that under continuous cooling, both the onsets of the pāhoehoe-‘a‘ā transition and of non-Newtonian behaviour occur within a shorter time frame. With varying both temperature and time, we also either reproduced or approached the clinopyroxene and plagioclase phenocryst abundances and compositions of the Etna lava used as starting material, demonstrating that real-time synchrotron X-ray tomography is an ideal approach to unravel the final solidification history of basaltic lavas. This imaging technology has indeed the potential to provide input into lava flow models and hence our ability to forecast volcanic hazards.

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Abstract: The addition of nano-fillers has been widely proposed as a method to enhance the dielectric properties of high voltage polymeric insulation, though there are mixed reports in the literature. Here the potential of silica nano-particles to extend the time to failure specifically through resistance to electrical tree growth in epoxy resin is determined. The benefit of silane treating the nano-particles before compounding is clearly established with regard to slowing tree growth and subsequent time to failure. The growth of trees in needle-plane samples is measured in the laboratory with loadings of 1, 3 and 5 wt% nano-filler. In all cases the average times to failure are extended, but silane treatment of the nano-particles prior to compounding yields much superior results. The emergence of a pronounced inception time before tree growth is also noted for the higher-filled, silane-treated cases. The average time to failure of silane-treated 5 wt% filled material was 28 times that of the unfilled resin. The improvement in performance between the nanocomposites with untreated and treated fillers is attributed to fewer agglomerations and improved dispersion of the filler in the treated cases. Measurements of Partial Discharge (PD) indicated significant differences in PD patterns during the growth of trees in the treated and untreated cases. This distinction may provide a quality control method for monitoring materials. In particular, long periods in which PDs were not measured were observed in the silane-treated cases. Visual imaging of tree growth in the unfilled material allowed the changing nature of the tree from fine to tree to dark tree to be observed as it grew. Corresponding PD measurements suggest the dark tree is gradually becoming conductive, and that growth of maximum PD measured is dependent on the relative rates of the growth of the tree and its carbonization. X-ray computer tomography identified significant differences in average tree channel diameters (a reduction from 2.8 µm to 2.0 µm for 1 wt% and 3 wt% cases). This implies that in addition to tree length changes, evaporated tree volumes also change and may explain the change in partial discharge characteristics observed.

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Abstract: Climate change, and the need of recycling, are two interconnected topics that are gaining more and more relevance nowadays. In order to decrease the CO2 emissions and reduce, or contain, the pollution that is having devastating effects on our climate, recycling seems to be a necessity that can not be postpone or diminished anymore. In the effort of reducing pollution, the aluminium world (industry and Academia) is playing an important role. Recycling aluminium, in fact, require only 10% of the energy necessary to extract it form the bauxite ore, leading to a massive energy and cost saving, as well to a reduction on the CO2 emissions equivalent to take off the road 900000 cars for 12 months. Up to now, the only closed loop recycling process is the one for usage beverage cans (UBC): used cans are collected, molten together, and used again to create new cans without the addition of any other material. When it comes to the automotive industry things are a bit more complicated. There are two main ways, adopted by industries, to recycle Al alloys depending on their composition. Wrought alloys are recycled remelting the scrap with primary Al, and cast alloys are obtained remelting wrought alloys and adjusting their composition for the different purposes. Although these two methods meet the requirement so far, they will lead to a nonrecyclable scrap surplus in the near future, consequently, new possibilities are being explored. In this work, a method to recycle aluminium alloys based on fractional solidification is proposed. The technology developed is based on an idea proposed by A.L Lux and M.C Flemings in which a semisolid alloy is isothermally squeezed towards a filter. Lux and Flemings tested their method on Sn-Pb alloys and on a small scale (500 g). The technology developed in this thesis, has been tested first on model aluminium alloys, and once the optimised procedure was established, on real aluminium scrap alloys, and on a bigger scale (up to 3 kg). The effect of several parameters (temperature, squeezing procedure, ultrasound, and grain refinement) on the purification efficiency was investigated and a 60% purification efficiency was achieved. Along with the technology development, a more fundamental study was carried out focusing on the investigation of the semisolid deformation, the liquid migration through the mush and the role of morphology on the permeability of the semisolid alloy. Finally, through the use of a numerical model, the feasibility of the Scheil-Gulliver approach to our case study, was investigated.

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Abstract: Understanding how gas diffusion electrodes are working is crucial to improve their performance and cost efficiency. One key issue is the electrolyte distribution during operation. Here, operando synchrotron imaging of the electrolyte distribution in silver-based gas diffusion electrodes is presented. For this purpose, a half-cell compartment was designed for operando synchrotron imaging of chronoamperometric measurements. For the first time, the electrolyte distribution could be analyzed in real time (1 s time resolution) even in individual pores as small as a few micrometers. The detailed analyses of dynamic filling processes are an important step for understanding and improving electrodes.