Directional solidification has a wide interest due to its importance to the iron and steel industry. Examples of further application can be found in the aerospace industry regarding the manufacture of turbine blades and the semiconductor industry regarding single-crystal growth applications. Solute convection in the solidification process results in channel formation, which has a freckle-like appearance in cross-section and has a critical effect on the mechanical strength of a casting. For a solidification process that occurs via planar solidification from a solid boundary, one may consider the presence of three distinct regions often identified as horizontal layers, i.e. a fluid binary mixture (the melt), the solid layer and a two-phase (fluid-solid) mushy layer, separating the other two. The mushy layer is practically a porous medium consisting of an interconnected solid phase having its voids filled with the melt binary fluid. Channelling in the mushy layer and the creating of freckles are being considered the main reasons for non-homogeneous solidification and production of defects in the resulting solid product. The production of defects adversely affects the mechanical properties of the solid product leading to undesirable constraints on its industrial use. The purpose of this study is to evaluate the effect the vibrations have on the heat transfer during the solidification process as well as on the average density of the solid product and void formation. Experimental as well as theoretical investigations related to the solidification process were undertaken. Two effects that have been observed in previous experimental studies when metals and metal alloys are vibrated during solidification are a decrease in dendritic spacing, which directly affects density, and faster cooling rates and associated solidification times. Because these two effects happen simultaneously during solidification it is challenging to determine the one effect independently from the other. Most previous studies were on metals and metal alloys. In these studies, the one effect, i.e. the decrease in dendritic spacing, might influence the other, i.e. the faster cooling rates, and vice versa. The direct link between vibration and heat transfer has not yet been studied independently. The purpose of this study was to experimentally investigate the effect of vibration only on heat transfer and thus solidification rate. Experiments were conducted on paraffin wax, because it had a clearly defined macroscopic crystal structure consisting of mostly large straight-chain hydrocarbons. The advantage of the large straight-chain hydrocarbons was that the dendritic spacing was not affected by the cooling rate. Experiments were done with paraffin wax inside hollow plastic spheres of 40 mm diameter with 1 mm wall thickness. The paraffin wax was initially in a liquid state at a uniform temperature of 60°C and then submerged into a thermal bath at a uniform constant temperature of 15°C, which was approximately 20°C below the mean solidification temperature of the wax. Experiments were conducted in approximately 300 samples, with and without vibration at frequencies varying from 10 – 300 Hz. The first set of experiments were conducted to determine the solidification times. In the second set of experiments, the mass of wax solidified was determined at discrete time steps, with and without vibration. The results showed that paraffin wax had vibration independent of solid density contrary to other materials, eg. metals and metal alloys. Enhancement of heat transfer resulted in quicker solidification times and possible control over the heat transfer rate. The increase in heat transfer leading to faster solidifcation times was observed to first occur, as frequency increased and then to decrease. Experimental results showed that paraffin wax had vibration independent of solid density contrary to other materials, eg. metals and metal alloys. Enhancement of heat transfer resulted in quicker solidification times and possible control over the heat transfer rate. The increase in heat transfer leading to faster solidifcation times was observed to first occur, as frequency increased and then to decrease. Theoretical results of heat convection in a porous layer heated from below and subject to vibrations are presented by using a truncated spectral method in space. The partial differential equations governing the mass, momentum, heat, and solute transport were tranformed into a set of ordinary differential equations via a truncated modal expansion. Then the resutling equations were solved to identify the variety of regimes, and transitionbetween them, i.e. from steady convection, via periodic and quasi-periodic convection, towards chaotic or weak turbulent convection. The theoretcial results show that the heat convection subject to vibration is generally reduced when compared with the corresponding convection without vibrations. The exception for a certain frequency range shows about a 10% enhancement in the weak turbulent regime of convection, however, a 10% enhancement is still lower than the heat transfer prior to the transition to weak turbulence. Therefore, the heat transfer mechanism can be excluded as the main reason behind the improvement in solidification when vibrations are applied. Both experimental and theoretical results show an enhancement in heat transfer which correlate qualitativally. / Thesis (PhD)--University of Pretoria, 2014. / tm2015 / Mechanical and Aeronautical Engineering / PhD / Unrestricted

L’interface noyau-graine (ICB) est instable et une zone dendritique se forme sous des conditions très particulières, c’est à dire que la cristallisation est très lente par rapport à la convection très vigoureuse du noyau liquide. Afin de reproduire expérimentalement des conditions semblables, nous avons étudié une zone dendritique sous hyper gravite, dans une centrifugeuse. La hauteur de cette zone diminue quand la gravite augmente alors que la fraction solide augmente fortement : similairement, les études sismologiques suggèrent que la fraction solide dans la graine est proche de l’unité a l’ICB. De plus, la sismologie montre une graine très hétérogène en termes d’anisotropie élastique, d’atténuation ou de vitesse des ondes et met en lumière une forte dichotomie Est-Ouest. Celle-ci pourrait être engendrée par une translation de la graine qui provoquerait de la cristallisation sur une face et de la fusion sur l’autre. Cette hypothèse est testée en conduisant des expériences de cristallisation et de fusion d’une zone dendritique. Nous avons utilisé des ultrasons comme analogues aux ondes sismiques pour quantifier les changements de structure dans la zone dendritique à partir des mesures de l’atténuation et la diffraction. Extrapoles a la graine, nos résultats montrent que l’ICB pourrait fondre sur l’hémisphère Ouest et cristalliser sur l’hémisphère Est. D’autre part, avec du gaz xénon en hyper-gravite, nous avons observé un gradient adiabatique, pour la première fois dans un dispositif expérimental. Cette thèse montre la faisabilité de ces expériences et la possibilité de vérifier expérimentalement les approximations utilisées pour la convection compressible. / The inner core boundary (ICB) is unstable, and a mushy layer forms under very particular conditions in which the crystallization is very slow compared to the very vigorous convection of the liquid core. To mimic these conditions, we have investigated a mushy layer under hyper-gravity in a centrifuge. The thickness of a mushy layer decreases with gravity and the solid fraction increases. This is coherent with seismological studies suggesting that the solid fraction at the ICB is close to unity. Moreover, seismology shows that the inner core is very heterogeneous in terms of elastic anisotropy, attenuation or wave velocity and that there exists a strong East-West dichotomy on the ICB. One hypothesis is that the latter is due to a translation of the inner core that would cause crystallization on one hemisphere and melting on the other one. We have tested that hypothesis with experiments of solidification and melting of a mush. We have used ultrasounds as an analogue to the seismic waves to quantify structural changes in the mush from measurements of attenuation and scattering. From our observations, it is plausible that the ICB on the Western hemisphere s melting while it is solidifying on the Eastern hemisphere. In other experiments, using xenon gas under hyper-gravity, we have observed an adiabatic gradient for the first time. This thesis shows the feasibility of these experiments and the possibility to check experimentally the approximations used for compressible convection.

Nous avons étudié les phénomènes convectifs et leur interaction dynamique avec la formation des microstructures pendant la solidification dirigée d’alliages étalliquesbinaires.La méthode post-mortem a été utilisée d’abord pour étudier la Transition olonnaire-Equiaxe pendant la solidification dirigée d’échantillons cylindriques d’Al-3,5wt%Ni non affiné sous la Technique de Rotation Accélérée de Creuset. La simulation numérique a été éffectuée et acquérie les résultats en concordance avec les manipulations.La technique in-situ a été appliquée pour comprendre l’évolution en fonction de temps des grains pendant solidification d’Al-4wt%Cu. La caractéstiques tatistiques des grains ont été discutées.La convection d’instabilité déclenchée par la poussée ou la tension superfaciale sous les gradients thermiques verticale et horizontale dans un système de double couches liquide-zone poreuse ont réspectivement étudié par analysis d’instabilité linéaire.L’inhomogénéité de la perméabilité de zone pateuse dendritique a été tenue en compte afin de comprendre son influence sur le début de convection pendant la solidification dirigée d’Al-3,5wt%Li. / We studied the convective phenomena and their dynamical interaction with the formation of the microstructurs during directional solidification of binary metallic alloys.The post-mortem method was used first to study the Columnar-Equiaxed-Transition during the directional solidification of unrefined Al-3.5wt%Ni in cylindric samples under the Accelerated Crucible Rotation Technique. The numerical imulation was carried out and achieved the results in agreement with experiments.The in-situ technique was applied to understand the evolution of equiaxed grains during solidification of Al-4wt%Cu in function of time. The statistical characteristics of equiaxed grains were discussed.The buoyancy-driven and surface-tension-driven instability convection under vertical and horizontal thermal gradients in a liquid-porous double-layered system were respectively investigated through linear instability analysis.The inhomogeneity of the dendritic mush permeability was taken into account in order to understand its influence on the triggering of convection during the directional solidification of Al-3.5wt%Li.