Dissertations / Theses on the topic 'Solidification Processing'

Solidification from the melt is an essential step in nearly all conventional processes to produce bulk materials for industrial applications. Rapid quenching from the liquid state at cooling rates of 102 to 106K/s or higher has developed into a new technology for processing novel materials. InxGa1 - xSb a ternary III-V compound semiconductor was synthesized by using the rapid spinning cup (RSC) technique. Several compositions of these alloys were batched and cast into ingots in evacuated sealed quartz tubes. These ingots were then melted and ejected onto a rapidly rotating copper disk. This resulted in the generation of flakes or powders depending on the rpm of the disk. Microstructural characterization of the flakes and powders was performed using XRD, SEM and TEM. Efforts were also made to measure the bulk resistivity of the annealed flakes to see the effect of annealing on ordering of the phases.

The main aim of this work is to investigate heterogeneous nucleation of solidification of metals and alloys by a combination of differential scanning calorimetry and transmission electron microscopy using a newly modified entrained particle technique. Attention is focused on investigating (a) heterogeneous nucleation of Cd, In and Pb particle solidification by Al in rapidly solidified Al-Cd, Al-In and Al-Pb binary alloys; (b) effects of various ternary additions such as Mg, Ge and Si on heterogenous nucleation of solidification of Cd and Pb solidification by Al; (c) heterogenous nucleation of solidification of Si by solid Al in hypoeutectic Al-Si alloys. In addition, the melting behaviour of Cd, In and Pb particles embedded in an Al matrix is investigated. The rapidly solidified microstructures of melt spun Al-Cd, Al-In and Al-Pb alloys consist of faceted 5-200nm diameter Cd, In and Pb particles homogeneously distributed throughout an Al matrix. Cd particles exhibit an orientation relationship with the Al matrix which can be described as {111}_Al//{0001}_Cd and andlt;110andgt;_Al//andlt;112and#773;0andgt;_Cd, and In and Pb particles exhibit a near cube-cube and cube-cube orientation relationship with the Al matrix respectively. Cd, In and Pb particles embedded in the Al matrix exhibit distorted truncated octahedral or truncated octahedral shapes surrounded by {111}_Al and {100}_Al facets. The solid Al-solid Cd, solid Al-solid In surface energy anisotropies are constant over the temperature range between room temperature and Cd and In melting points respectively. The solid Al-liquid Cd and solid Al-liquid In surface energy anisotropies decrease with increasing temperature above Cd and In melting points. Solidification of Cd, In, Pb particles embedded in an Al matrix is nucleated catalytically by the surrounding Al matrix on the {111}_Al faceted surfaces with an undercooling of 56, 13 and 22K and a contact angle of 42°, 27° and 21° for Cd, In and Pb particles respectively. Addition of Mg to Cd particles embedded in Al increases the lattice disregistry across the nucleating plane, but decreases the undercooling before the onset of Cd(Mg) particle solidification. Addition of Ge to Al decreases the lattice disregistry across the nucleating plane, but increases the undercooling before the onset of Pb particle solidification embedded in the Al(Ge) matrix. These results indicate that chemical interactions dominate over structural factors in determining the catalytic efficiency of nucleation solification in Al-Cd-Mg and Al-Pb-Ge alloys. Contact between Si precipitates and Pb particles embedded in an Al matrix decreases the undercooling before the onset of Pb particle solidification. The equilibrium melting point of Cd particle in the melt spun Al-Cd alloy is depressed because of capillarity, and the depression of equilibrium melting point increases with decreasing particle size. In the melt spun Al-In and Al-Pb alloys, however, most of the In and Pb particles embedded within the Al matrix grains are superheated, and the superheating increases with decreasing particle size. The heterogeneous nucleation temperature for Si solidification by Al depends sensitively on the purity of the Al. Na and Sr additions have different effects on the Si nucleation temperatures. With an Al purity of 99.995%, Na addition increases the Si nucleation temperature, while Sr addition does not affect or decreases the Si nucleation undercooling, depending on the amount of Sr addition. The solidified microstructure of liquid Al-Si eutectic droplets embedded in an Al matrix is affected by the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle, however, with high Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form a large number of non-faceted Si particles embedded in Al.

The shrinkage during solidification of aluminium and iron based alloys has been studied experimentally and theoretically. The determined shrinkage behaviour has been used in theoretical evaluation of shrinkage related phenomena during solidification.

Air gap formation was experimentally studied in cylindrical moulds. Aluminium based alloys were cast in a cast iron mould while iron based alloys were cast in a water-cooled copper mould. Displacements and temperatures were measured throughout the solidification process. The modelling work shows that the effect of vacancy incorporation during the solidification has to be taken into account in order to accurately describe the shrinkage.

Crack formation was studied during continuous casting of steel. A model for prediction of crack locations has been developed and extended to consider non-equilibrium solidification. The model demonstrates that the shrinkage due to vacancy condensation is an important parameter to regard when predicting crack formation.

The centreline segregation was studied, where the contributions from thermal and solidification shrinkage were analysed theoretically and compared with experimental findings. In order to compare macrosegregation in continuous casting and ingot casting, ingots cast with the same steel grade was analysed. However, the macrosegregation due to A-segregation is driven by the density difference due to segregation. This is also analysed experimentally as well as theoretically.

The primary focus of the present work is the development of macro-models for numerical simulation of binary alloy solidification processes, consistent with microscopic phase-change considerations, with a particular emphasis on capturing the effects of non-equilibrium species redistribution on overall macrosegregation behaviour. As a first step, a generalised macroscopic framework is developed for mathematical modelling of the process. The complete set of equivalent single-phase governing equations (mass, momentum, energy and species conservation) are solved following a pressure-based Finite Volume Method according to the SIMPLER algorithm. An algorithm is also developed for the prescription of the coupling between temperature and the melt-fraction. Based on the above unified approach of solidification modelling, a macroscopic numerical model is devised that is capable of capturing the interaction between the double-diffusive convective field and a localised fluid flow on account of solutal undercooling during non-equilibrium solidification of binary alloys. Numerical simulations are performed for the case of two-dimensional transient solidification of Pb-Sn alloys, and the simulation results are also compared with the corresponding experimental results quoted in the literature. It is observed that non-equilibrium effects on account of solutal undercooling result in an enhanced macrosegregation. Next, the model is extended to capture the effects of dendritic arm coarsening on the macroscopic transport phenomena occurring during a binary alloy solidification process. The numerical results are first tested against experimental results quoted in the literature, corresponding to the solidification of an Al-Cu alloy in a bottom-cooled cavity. It is concluded that dendritic arm coarsening leads to an increased effective permeability of the mushy region as well as an enhanced eutectic fraction of the solidified ingot. Consequently, an enhanced macrosegregation can be predicted as compared to that dictated by shrinkage-induced fluid flow alone. For an order-of-magnitude assessment of predictions from the numerical models, a systematic approach is subsequently developed for scaling analysis of momentum, heat and species conservation equations pertaining to the case of solidification of a binary mixture. A characteristic velocity scale inside the mushy region is derived, in terms of the morphological parameters of the two-phase region. A subsequent analysis of the energy equation results in an estimation of the solid layer thickness. It is also shown from scaling principles that non-equilibrium effects result in an enhanced macro-segregation compared to the case of an equilibrium model For the sake of assessment of the scaling analysis, the predictions are validated against computational results corresponding to the simulation of a full set of governing equations, thus confirming the trends suggested by the scale analysis. In order to analytically investigate certain limiting cases of unidirectional alloy solidification, a fully analytical solution technique is established for the solution of unidirectional, conduction-dominated, alloy solidification problems. The results are tested for the problem of solidification of an ammonium chloride-water solution, and are compared with those from existing analytical models as well as with the corresponding results from a fully numerical simulation. The effects of different microscopic models on solidification behaviour are illustrated, and transients in temperature and heat flux distribution are also analysed. An excellent agreement between the present solutions and results from the computational simulation can be observed. The generalised numerical model is subsequently utilised to investigate the effects of laminar double-diffusive Rayleigh-Benard convection on directional solidification of binary fluids, when cooled and solidified from the top. A series of experiments is also performed with ammonium chloride-water solutions of hypoeutectic and hypereutectic composition, so as to facilitate comparisons with numerical predictions. While excellent agreements can be obtained for the first case, the second case results in a peculiar situation, where crystals nucleated on the inner roof of the cavity start descending through the bulk fluid, and finally settle down at the bottom of the cavity in the form of a sedimented solid layer. An eutectic solidification front subsequently progresses from the top surface vertically downwards, and eventually meets the heap of solid crystals collected on the floor of the cavity. However, comparison of experimental observations with corresponding numerical results from the present model is not possible under this situation, since the associated transport process involves a complex combination of a number of closely interconnected physical mechanisms, many of which are yet to be resolved. Subsequent to the development of the mathematical model and experimental arrangements for macroscopic transport processes during an alloy solidification process, some of the important modes of double-diffusive instability are analytically investigated, as a binary alloy of any specified initial composition is directionally solidified from the top. By employing a close-formed solution technique, the critical liquid layer heights corresponding to the onset of direct mode of instability are identified, corresponding two a binary alloy with three different initial compositions. In order to simulate turbulent transport during non-equilibrium solidification processes of binary alloys, a modified k-8 model is subsequently developed. Particular emphasis is given for appropriate modelling of turbulence parameters, so that the model merges with single-phase turbulence closure equations in the pure liquid region in a smooth manner. Laboratory experiments are performed using an ammonium chloride-water solution that is solidified by cooling from the top of a rectangular cavity. A good agreement between numerical and experimental results is observed. Finally, in order to study the effects of three-dimensionality in fluid flow on overall macrosegregation behaviour, the interaction between double-diffusive convection and non-equilibrium solidification of a binary mixture in a cubic enclosure (cooled from a side) is numerically investigated using a three-dimensional transient mathematical model. Investigations are carried out for two separate model systems, one corresponding to a typical metal-ally analogue system and other corresponding to an actual metal-alloy system. As a result of three-dimensional convective flow-patterns, a significant solute macrosegregation is observed in the transverse sections of the cavity, which cannot be captured by two-dimensional simulations.

The primary focus of the present work is the development of macro-models for numerical simulation of binary alloy solidification processes, consistent with microscopic phase-change considerations, with a particular emphasis on capturing the effects of non-equilibrium species redistribution on overall macrosegregation behaviour. As a first step, a generalised macroscopic framework is developed for mathematical modelling of the process. The complete set of equivalent single-phase governing equations (mass, momentum, energy and species conservation) are solved following a pressure-based Finite Volume Method according to the SIMPLER algorithm. An algorithm is also developed for the prescription of the coupling between temperature and the melt-fraction. Based on the above unified approach of solidification modelling, a macroscopic numerical model is devised that is capable of capturing the interaction between the double-diffusive convective field and a localised fluid flow on account of solutal undercooling during non-equilibrium solidification of binary alloys. Numerical simulations are performed for the case of two-dimensional transient solidification of Pb-Sn alloys, and the simulation results are also compared with the corresponding experimental results quoted in the literature. It is observed that non-equilibrium effects on account of solutal undercooling result in an enhanced macrosegregation. Next, the model is extended to capture the effects of dendritic arm coarsening on the macroscopic transport phenomena occurring during a binary alloy solidification process. The numerical results are first tested against experimental results quoted in the literature, corresponding to the solidification of an Al-Cu alloy in a bottom-cooled cavity. It is concluded that dendritic arm coarsening leads to an increased effective permeability of the mushy region as well as an enhanced eutectic fraction of the solidified ingot. Consequently, an enhanced macrosegregation can be predicted as compared to that dictated by shrinkage-induced fluid flow alone. For an order-of-magnitude assessment of predictions from the numerical models, a systematic approach is subsequently developed for scaling analysis of momentum, heat and species conservation equations pertaining to the case of solidification of a binary mixture. A characteristic velocity scale inside the mushy region is derived, in terms of the morphological parameters of the two-phase region. A subsequent analysis of the energy equation results in an estimation of the solid layer thickness. It is also shown from scaling principles that non-equilibrium effects result in an enhanced macro-segregation compared to the case of an equilibrium model For the sake of assessment of the scaling analysis, the predictions are validated against computational results corresponding to the simulation of a full set of governing equations, thus confirming the trends suggested by the scale analysis. In order to analytically investigate certain limiting cases of unidirectional alloy solidification, a fully analytical solution technique is established for the solution of unidirectional, conduction-dominated, alloy solidification problems. The results are tested for the problem of solidification of an ammonium chloride-water solution, and are compared with those from existing analytical models as well as with the corresponding results from a fully numerical simulation. The effects of different microscopic models on solidification behaviour are illustrated, and transients in temperature and heat flux distribution are also analysed. An excellent agreement between the present solutions and results from the computational simulation can be observed. The generalised numerical model is subsequently utilised to investigate the effects of laminar double-diffusive Rayleigh-Benard convection on directional solidification of binary fluids, when cooled and solidified from the top. A series of experiments is also performed with ammonium chloride-water solutions of hypoeutectic and hypereutectic composition, so as to facilitate comparisons with numerical predictions. While excellent agreements can be obtained for the first case, the second case results in a peculiar situation, where crystals nucleated on the inner roof of the cavity start descending through the bulk fluid, and finally settle down at the bottom of the cavity in the form of a sedimented solid layer. An eutectic solidification front subsequently progresses from the top surface vertically downwards, and eventually meets the heap of solid crystals collected on the floor of the cavity. However, comparison of experimental observations with corresponding numerical results from the present model is not possible under this situation, since the associated transport process involves a complex combination of a number of closely interconnected physical mechanisms, many of which are yet to be resolved. Subsequent to the development of the mathematical model and experimental arrangements for macroscopic transport processes during an alloy solidification process, some of the important modes of double-diffusive instability are analytically investigated, as a binary alloy of any specified initial composition is directionally solidified from the top. By employing a close-formed solution technique, the critical liquid layer heights corresponding to the onset of direct mode of instability are identified, corresponding two a binary alloy with three different initial compositions. In order to simulate turbulent transport during non-equilibrium solidification processes of binary alloys, a modified k-8 model is subsequently developed. Particular emphasis is given for appropriate modelling of turbulence parameters, so that the model merges with single-phase turbulence closure equations in the pure liquid region in a smooth manner. Laboratory experiments are performed using an ammonium chloride-water solution that is solidified by cooling from the top of a rectangular cavity. A good agreement between numerical and experimental results is observed. Finally, in order to study the effects of three-dimensionality in fluid flow on overall macrosegregation behaviour, the interaction between double-diffusive convection and non-equilibrium solidification of a binary mixture in a cubic enclosure (cooled from a side) is numerically investigated using a three-dimensional transient mathematical model. Investigations are carried out for two separate model systems, one corresponding to a typical metal-ally analogue system and other corresponding to an actual metal-alloy system. As a result of three-dimensional convective flow-patterns, a significant solute macrosegregation is observed in the transverse sections of the cavity, which cannot be captured by two-dimensional simulations.

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

19 Dec 2004.
Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2447" Amber Lynn Genau. 12/19/2004. Report is also available in paper and microfiche from NTIS.

Shrinkage in a solidifying metal has been studiedexperimentally as well as theoretically. The main focus hasbeen to examine the mechanisms causing an air gap to formbetween a casting and a mould, and hot crack formation to occurin a solidifying metal.

¨The formation of an air gap has been experimentallystudied during solidification of Al- and Cu-based alloys in acylindrical mould. The displacements of the casting and themould causing an air gap have been measured duringsolidification and cooling of the casting. The temperaturedistribution was measured simultaneously. Mathematicalmodelling has been performed to increase the understanding ofthe solidification process and the strains formed in thesolidifying metal contributing to the formation of an air gapbetween casting and mould. Most of the work was dedicated todevelop a new model to describe the strain duringsolidification, but traditional theory was used for themodelling work as well.

The model suggested in this work includes non-equilibriumeffects on the solidification process and the shrinkage. Theformation and condensation of lattice defects formed in thesolid phase during solidification and its effect on thesolidification process as well as on the material shrinkageresulting in air gap formation was considered. The results fromthe modelling work show good agreement with the experimentalresults. The conclusion is that it is important to includethese non-equilibrium effects in modelling of shrinkage duringsolidification.

The same conclusion was drawn from results of experimentalwork with high temperature tensile testing of in situsolidified samples and the development of a new theory for hotcrack formation. It was found that a super saturation oflattice defects formed during the solidification processenhances the nucleation and growth of hot cracks duringcooling.

The current UK strategy for radioactive waste management is to permanently store the waste in an underground repository. Final disposal of the radwaste may then be preceded by chemical conditioning and physical encapsulation. The objective of this work was to determine the extent of actinide immobilisation in cement. Since actinides are hazardous and costly to study directly, a chemical analogue approach to studying actinide immobilisation was adopted. Th(IV), Ce(III, IV) and Eu(III) were chosen as actinide simulants and their suitability assessed by a critical review of the literature. Ca(OH)₂ and C-S-H dominate the observed chemical properties of the aqueous phase in cement. As they are of such importance, it was these cement components which were used to investigate the reaction of the simulant elements with cement. The phases found to be predicted were ThO₂, ThSiO₄, Eu(OH)₃, Ca₂Eu₈(SiO₄)₆O₂, CeO₂, CeSiO₄ and Ca₂(SiO₄)₆O₂. CeSiO₄ and Ca₂Ce₈(SiO₄)O₂ are newly reported phases, produced by hydrothermal synthesis. Rietveld refinement confirmed CeSiO₄ to have the zircon structure, with space group 14₁/amd and cell parameters a = 6.9564(3) A, c = 6.1953 (4) A. Ca₂Ce₈(SiO₄)₆O₂ exhibits the apatite structure, with space group P6₃/m and cell parameters a = 9.4343(3) a, c = 6.8885(4) A. Preliminary solubility studies were carried out on all of the solubility-limiting phases. Phase impurity, poor crystallinity and incongruent solubility of phases hindered the generation of solubility product data. Nevertheless, these phases have naturally occurring analogues which are known to be environmentally stable and have low solubilities. On the basis of the experimental results obtained, it may be concluded that cement has the potential to be a very effective immobilisation matrix for actinide elements. Recommendations for future experiments using active elements are discussed.

Au cours de la dernière décennie, les alliages de fonderie Al-Si ont été utilisés de plus en plus comme une alternative appropriée à la fonte dans la fabrication de composants de moteurs (par exemple les culasses). Les objectifs du projet étaient d'étudier l'effet des éléments tels que le cuivre, le magnésium et le fer sur les défauts de solidification, et sur l'évolution des phases poste-eutectiques les alliages de fonderie Al-Si. Tout d’abord, les travaux antérieurs sont soigneusement examinés afin de mieux comprendre les charges de fatigue thermomécanique, les caractéristiques, les exigences et les matériaux applicables dans les composantes du moteur. Par la suite, les défauts de solidification (tendance de fissuration à chaud (HTS) et microporosité) des alliages à base d’Al-Si ont été évalués. En augmentant la teneur en Cu et en Fe des alliages, la valeur de HTS et de microporosité ont été augmentées. Les indices théoriques de fissuration à chaud ont été simulés avec un modèle de microségrégation multiphasique avec rétrodiffusion dans la phase primaire «multiphase back diffusion model». La corrélation obtenue entre les résultats expérimentaux (HTS) et les résultats simulés est excellente. L’effet de la composition chimique (Cu, Mg et Fe contenu) dans les alliages Al-Si sur l'évolution de la microstructure ont donc été étudiées. Les microstructures à l'état de coulée et à l'état de traitement thermique de mise en solution (SHT) ont été évaluées par les microscopies optique/électronique. Deux intermétalliques contenant du Mg (Q-Al5Cu2Mg8Si6, π-Al8FeMg3Si6) qui apparaissent avec une couleur grise sous le microscope optique ont été discriminés par des attaques chimiques que nous avons développées. L’analyse calorimétrique différentielle à balayage (DSC) a été utilisée pour examiner les transformations de phase survenant au cours du processus de chauffage et de refroidissement. Les calculs thermodynamiques ont été effectués pour évaluer la formation de la phase à l'état d'équilibre et hors-équilibre. Les résultats ont démontré que la séquence de solidification et la stabilité des intermétalliques contenant du Cu/Mg ont été fortement influencée par la composition chimique des alliages. La phase Q-Al5Cu2Mg8Si6 a été solidifiée soit à la même température ou plus tôt que la phase θ-Al2Cu en fonction de la teneur en Cu de l'alliage. Par ailleurs, les phases Q-Al5Cu2Mg8Si6 et π-Al8FeMg3Si6 qui étaient solubles à 505℃ dans l'alliage Al-7Si-1.5Cu-0.4mg, sont restées presque intactes dans l'alliage Al-7Si-1.5Cu-0.8mg wt.-%. Bien que l’intermétallique-AlCuFe a été à peine observé dans la microstructure de coulée, la réaction entre la phase primiare α-Al avec la phase β-Al5FeSi a causé la formation de la phase N-Al7Cu2Fe au cours de la mise en solution. La transformation de phase à l'état solide de la phase β-Al5FeSi à la phase N-Al7Cu2Fe a également été étudiée.
Over the past decade, Al-Si based foundry alloys have increasingly been used as a suitable alternative for cast iron in the fabrication of engine components. This project was aimed to study the effect of Cu, Mg and Fe elements on solidification defects (hot rearing tendency and microporosity), and on evolution of post eutectic phases in the Al-7Si (wt.-%) based alloys. Initially, the previous works and the most pertinent literatures were thoroughly reviewed to elaborate the thermo-mechanical fatigue loads, characteristics, requirements and materials applicable in engine components (mainly cylinder-head). Subsequently, the solidification defects of the Al-Si based alloys were evaluated. By increasing Cu and Fe content of the alloys, the hot tearing sensitivity and the microporosity content of the alloys were both enhanced. Multiphase back diffusion model was utilized to simulate the theoretical hot tearing indices. A very good correlation was obtained between the experimental and the theoretical hot tearing indices. Effect of the chemistry (Cu, Mg and Fe content) on microstructure evolution of the Al-Si foundry alloys was consequently studied. As-cast and solution heat treated (SHT) microstructures of the alloys were evaluated by optical- and electron-microscopy. Two etchants were developed to discriminate the Mg-bearing intermetallics (Q-Al5Cu2Mg8Si6, π- Al8FeMg3Si6) under optical microscope. Differential scanning calorimetry (DSC) was utilized to examine the phase transformations occurring during heating/cooling process. Thermodynamic computations were carried out to assess the phase formation in the equilibrium/non-equilibrium conditions. According to the predicted/experimental results, the solidification sequence and the stability of Cu/Mg bearing intermetallics are strongly influenced by the chemistry of the alloys. Q-Al5Cu2Mg8Si6 phase was solidified either at the same temperature or earlier than θ-Al2Cu phase depending the Cu content of the alloy. Moreover, Q-Al5Cu2Mg8Si6 and π- Al8FeMg3Si6 which were soluble at 505℃ in the alloy Al-7Si-1.5Cu-0.4Mg, remained almost intact in the alloy Al-7Si-1.5Cu-0.8Mg wt.-%. Tough the AlCuFe- intermetallic was barely observed in the as-cast microstructure, the reaction of α-Al with the β-Al5FeSi phase caused the formation of the N-Al7Cu2Fe phase during SHT. The solid state phase transformation (precipitation temperature and mechanism) of β-Al5FeSi to the N-Al7Cu2Fe phase was also investigated.

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

This PhD thesis presents an overview of the most recent industrial applications of the EPM (Electromagnetic Processing of Materials) going into details on the directional solidification processes in TiAl alloys and silicon for photovoltaic applications. The design and realization of an induction DSS prototype is presented.
Dopo un'ampia panoramica sulle applicazioni industriali più recenti dell'EPM (Electromagnetic Processing of Materials) questa tesi di dottorato approfondisce i processi di solidificazione direzionale di leghe di TiAl e del silicio per il fotovoltaico. Una intera sezione è dedicata alla progettazione e realizzazione di un prototipo di forno DSS ad induzione.

Orientadores: Amauri Garcia, Noe Cheung
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
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Resumo: As ligas de solda à base de estanho apresentam excelente fluidez e temperaturas de trabalho ideais para a união de componentes eletrônicos. A solda com ligas do sistema estanho chumbo é a mais comum para soldas em eletrônica. Entretanto, há muitas preocupações com o uso do chumbo, devido aos diversos efeitos adversos na saúde humana e contaminação do meio ambiente. Por essas razões, na maioria dos países o chumbo já é condenado e proibido de ser incorporado em diversos produtos. Neste sentido, a indústria eletrônica está de olho em soldas livres de chumbo que possam substituir a clássica solda estanho-chumbo. É objetivo deste trabalho analisar a solidificação de ligas eutéticas dos sistemas Sn-Ag e Sn-Cu, que são duas ligas com potencial de substituição. Foram desenvolvidos experimentos para determinar a influência do acabamento superficial da chapa molde nos parâmetros térmicos de solidificação durante a solidificação direcional ascendente em regime transitório de extração de calor de ligas eutéticas Sn-Pb, Sn-Cu e Sn-Ag. Foram utilizados dois tipos de acabamentos superficiais na chapa molde: lixado e ranhurado, para investigar as condições de afinidade metal/substrato. Foi desenvolvida uma abordagem teórico-experimental para determinar quantitativamente as variáveis térmicas, tais como: coeficiente de transferência de calor global (hg) e velocidade de deslocamento da frente de solidificação. As micro estruturas de solidificação foram caracterizadas e os espaçamentos dendríticos secundários (?2) foram medidos na direção longitudinal dos lingotes, e correlacionados com as variáveis térmicas que atuaram durante a solidificação.
Abstract: Tin based alloys for welding applications have excellent fluidity and adequate temperature working range to join electronic components. The most used tin alloys for welding is the eutectic Sn-Pb alloy. However, there are some concerns about lead, due to hazardous effects to health and to environment. Due to theses reasons, many countries condemn and prohibit the use of lead in several products. In this sense, the electronic industries are looking for lead-free solder alloys with a view to replace the traditional Sn-Pb eutectic alloy. The aim of this work is to analyze the solidification of Sn-Ag and Sn-Cu eutectic alloys which are potential alloys candidates to replace the eutectic Sn-Pb alloy. Experiments were conducted to determine the influence of the mold wall roughness on the thermal solidification parameters during the upward unsteady-state directional solidification of eutectic Sn-Pb, Sn-Cu and Sn-Ag alloys. Two different kinds of surface mold finishing, sanded and grooved, were used in order to analyze metal/substrate affinity. A combined theoretical and experimental approach has been used to quantitatively determine such thermal variables, i.e., transient global heat transfer coefficient (hg) and solidification growth rates. The microstructures have been characterized and the secondary (?2) dendrite arm spacings were measured along the castings length and correlated to transient solidification thermal variables.
Mestrado
Materiais e Processos de Fabricação
Mestre em Engenharia Mecânica

La présente étude porte sur l'influence de la vitesse de refroidissement et du taux d'inoculation sur la structure de solidification des fontes à graphite sphéroïdal. Des modèles de simulation numérique de la solidification sont proposés. Des pièces comportant 5 cylindres de diamètres différents, instrumentés à l'aide de thermocouples, ont été coulées. Les courbes de refroidissement ont été dépouillées par analyse thermique directe et dérivée. La structure des cylindres a été caractérisée par analyse d'images : on a ainsi déterminé la densité surfacique de nodules, leur densité volumique, le taux de graphite et celui de cémentite. Des corrélations entre ces paramètres ont été établies et chiffrées, de même que des corrélations entre les caractéristiques des courbes de refroidissement et les paramètres structuraux des pièces. L'ensemble des corrélations expérimentales obtenues représente un moyen empirique pour décrire l'effet du taux d'inoculation et de la vitesse de refroidissement sur la structure de solidification des fontes. Les différents aspects de la simulation numérique et de la confrontation entre les résultats des calculs et les données expérimentales sont ensuite successivement abordés. La cinétique de solidification est décrite par un modèle qui comporte des lois de germination et de croissance des sphères eutectiques (nodule de graphite plus coquille d'austénite). On présente enfin un modèle amélioré qui considère le dépôt d'austénite hors des sphères eutectiques. La confrontation simulation-expérience montre que le type de modélisation examine permet de décrire l'effet de l'inoculation et de la vitesse de refroidissement sur la structure des pièces coulées. Cette approche peut aider à la rationalisation des connaissances empiriques que l'on a de ces phénomènes en fonderie

Thesis (PhD) -- Stellenbosch University, 2014.
ENGLISH ABSTRACT: The process route for the production of base and platinum-group metals from natural sulfide ores commonly requires the conversion of high-iron furnace matte into an iron-lean converter matte. This is followed by pre-treatment through cooling of the iron-lean molten matte, physical processing of the solidified matte and hydrometallurgical metal extraction. Lonmin is the third largest producer of platinum-group metals in the world and utilizes Peirce-Smith converters for blowing high-iron furnace matte with air to a final iron concentration or endpoint. The molten matte is water granulated and solidification occurs via fast-cooling. The solidified matte is ground in a closed circuit ball mill with hydrocyclone classification and subjected to first stage atmospheric leaching. The specification of an ideal or desirable converter iron endpoint requires careful consideration. Most importantly, it must ensure the crystallization of converter matte with mineralogical qualities that are within the setpoints of the downstream unit processes and techniques. An additional consideration is for the final blown converter matte to achieve an optimum bulk concentration of the base metals Ni and Cu and platinum-group metals Pt, Pd, Rh, Ru and Ir. Mattes characteristic of variable iron endpoints were regularly produced at the Lonmin converter plant section. Uncertainty by plant metallurgists in knowing the desirable iron endpoint, particularly within the context of the Lonmin base metal refinery, and poor control has had detrimental effects on the mineralogical quality of the final matte and hence on the processing characteristics of the solidified matte particles downstream. A desirable iron endpoint required investigation, selection and implementation at Lonmin. The primary focus of this study was therefore to quantify the effect of a specific iron endpoint on the mineralogy and mineral chemistry of solidified converter matte. A fundamental examination of the solidification process upon cooling was regarded as critical to an in-depth understanding of the attained mineralogy and mineral chemistry as a function of a specific iron endpoint. It became equally important to quantify the effect of the resultant mineralogy, and hence iron endpoint, on the physical property of mineral structures in relation to downstream grinding, liberation and leaching characteristics. Despite considerable industry context, limited in-depth and coherent studies on the effect of a specific iron endpoint on fast-cooled converter matte systems were found in both industrial and scholarly literature. Previous findings in literature offered a limited quantitative understanding of the effect on mineralogy and mineral chemistry. Phase and cooling equilibria of multi-component, iron endpoint specific Ni-Cu-S matte systems were also not fully available. These would have been particularly useful in understanding the complexities of converter matte solidification as a function of iron endpoint. Physical property knowledge of converter matte mineral structures was hardly available and even less so in relation to grinding, liberation and leaching processes. A comprehensive investigation was therefore required to address these extensive knowledge gaps with respect to fastcooled converter matte systems in an industrial framework. Three Peirce-Smith converter production samples, representative of the extent in variability of iron endpoints attained at the converter plant, were used in a systematic investigation coupled to a novel combination of modern analytical techniques, computational thermochemistry and metallurgical testwork. The modern analytical techniques included the application of high resolution transmission electron microscopy and focused ion beam scanning electron microscopy tomography. Computational thermochemistry was applied through the use of MTDATA phase diagram software. Metallurgical testwork involved laboratory batch grinding at various specific energies. Closely associated leach experiments were also considered relevant to this wide-ranging investigation. The Peirce-Smith converter samples investigated were indicative of mattes that attained specific endpoints of 5.17%, 0.99% and 0.15 weight% Fe. The highest combined bulk concentration of the important base and platinum-group metals was achieved in the matte which attained a specific iron endpoint of 0.99%. The mineralogy of all three converter mattes was dominated by nickel sulfide mineral structures matched to the natural mineral of heazlewoodite. Mineral structures of copper sulfide, NiCu-alloy, spinel and OsRu-alloy were also constituents of the different converter mattes. The attainment of a specific iron endpoint was found to result in measurable mineralogical differences with respect to relative mineral abundances, external morphological characteristics and mineral chemistry. The mineralogical differences were particularly distinct between mineral structures of the high (5.17%) and low (0.99% and 0.15%) iron mattes. Subtle mineralogical differences were evident between mineral structures of the low iron mattes. The 0.99% Fe matte was characteristic of a significantly higher NiCu-alloy relative abundance, compared to the 5.17% Fe matte. The NiCu-alloy structures were found to act as the primary collectors of the economically significant platinum-group metals. Mineralogical observations were used to develop an understanding of the underlying mineralization mechanism of NiCu-alloy structures. High-fidelity color and grayscale 3D reconstructions were produced of the resultant mineralized structures. It was shown theoretically that variations in iron endpoint specific starting compositions of oxygen-free liquid matte systems alter the solidification pathway towards the eutectic. Moreover, a quantitative understanding of liquid phase solidification of the high and low iron matte systems, including oxygen, was developed to within ±2.5 oC. Most of the specific energy available for grinding was expended breaking the nickel sulfide matrix, particularly of the high iron matte. The breakage rates of copper sulfide mineral structures in the 5.17% Fe matte were calculated to be higher than in the 0.15% Fe matte at 25kWh/t specific energy. The degree of copper sulfide liberation was shown to be higher for the 5.17% Fe matte than for the 0.15% Fe matte at the same specific energy of grinding. A higher degree of Ni extraction and Cu cementation could be achieved when leaching low iron matte particles. The production of converter matte attaining a specific iron endpoint of 0.99% was found to be the most suitable with respect to endpoint selection criteria. A practical iron endpoint range of 1.6% to 1.0% was recommended for the production of converter matte with a resultant mineralogical quality within the constraints of the Lonmin base metal refinery. This study offers an integrated understanding of base and platinum-group metals production as a function of a desirable iron endpoint at Lonmin. This was not previously available in metal production literature. New technology for the monitoring and consistent control of such a practical iron endpoint range can subsequently be implemented.
AFRIKAANSE OPSOMMING: Die prosesroete vir die produksie van onedel en platinumgroepmetale uit natuurlike swawelertse vereis gewoonlik die omsetting van ’n ysterryke hoogoondmat in ’n ysterarm omsettermat. Hierna volg voorbehandeling deur die afkoeling van die ysterarm gesmelte mat, fisiese verwerking van die soliede mat, en hidrometallurgiese metaalekstraksie. Lonmin is die derde grootste produsent van platinumgroepmetale ter wêreld en gebruik Peirce-Smith-omsetters om ysterryke hoogoondmat met lug te blaas totdat dit ’n finale ysterkonsentrasie- of ystereindpunt bereik. Die gesmelte mat word met water granuleer, en solidifikasie vind deur middel van snelafkoeling plaas. Die soliede mat word in ’n geslotekringbalmeul met hidrosikloonklassifikasie gemaal en aan eerstestadium- atmosferiese loging onderwerp. Die spesifikasie van ’n ideale of gewenste ystereindpunt verg deeglike oorweging. Bowenal moet dit verseker dat die omsettermat kristalliseer met mineralogiese eienskappe wat binne die setpunte van die eenheidsprosesse en - tegnieke verder af in die prosesstroom val. ’n Bykomende oorweging is dat die uiteindelike geblaasde omsettermat ’n optimale massakonsentrasie van die onedel metale Ni en Cu en die platinumgroepmetale Pt, Pd, Rh, Ru en Ir moet bevat. Matte met die kenmerke van wisselende ystereindpunte is gereeld by die Lonminomsetteraanleg geproduseer. Die onsekerheid van metallurge by die aanleg oor die gewenste ystereindpunt – veral binne die konteks van die Lonmin-raffinadery vir onedel metale – sowel as swak beheer het ’n nadelige uitwerking gehad op die mineralogiese gehalte van die uiteindelike mat, en dus ook op die verwerkingskenmerke van die soliede matdeeltjies verder af in die prosesstroom. Die bepaling van die gewenste ystereindpunt het sorgvuldige ondersoek, seleksie en toepassing deur Lonmin vereis. Hierdie studie is dus hoofsaaklik uitgevoer om die uitwerking van ’n spesifieke ystereindpunt op die mineralogie en minerale chemie van soliede omsettermat te kwantifiseer. ’n Grondliggende ondersoek na die solidifikasieproses by afkoeling is as noodsaaklik beskou vir ’n diepgaande begrip van die verworwe mineralogie en minerale chemie as ’n funksie van ’n spesifieke ystereindpunt. Mettertyd het dit egter ewe belangrik geword om die uitwerking van die gevolglike mineralogie, en dus die ystereindpunt, op die fisiese eienskappe van minerale strukture met betrekking tot maling-, vrystellings- en loogprosesse verder af in die prosesstroom te kwantifiseer. Ondanks heelwat bedryfskonteks, het nóg bedryfs- nóg vakkundige literatuur veel diepte- en samehangende studies oor die uitwerking van ’n spesifieke ystereindpunt op snelafgekoelde omsettermatstelsels opgelewer. Vorige bevindinge in die literatuur het boonop ’n beperkte kwantitatiewe begrip van die uitwerking op mineralogie en minerale chemie getoon. Die fase- en afkoelingsekwilibriums van ystereindpuntspesifieke Ni-Cu-S-matstelsels met veelvuldige komponente was ook nie ten volle beskikbaar nie. Dít sou veral goed te pas gekom het om die kompleksiteite van omsettermatsolidifikasie as ’n funksie van ystereindpunt te verstaan. Kennis van die fisiese eienskappe van die minerale strukture van omsettermat was kwalik beskikbaar, terwyl selfs minder inligting oor maling-, vrystellings- en loogprosesse opgespoor kon word. Daarom was ’n omvattende ondersoek nodig om hierdie beduidende kennisleemtes met betrekking tot snelafgekoelde omsettermatstelsels in ’n nywerheidsraamwerk aan te vul. Drie Peirce-Smith-omsetterproduksiemonsters wat die wisselende bestek van ystereindpunte by die omsetteraanleg verteenwoordig, is in ’n stelselmatige ondersoek gebruik, tesame met ’n vernuwende kombinasie van moderne ontledingstegnieke, gerekenariseerde termochemiese bewerkings en metallurgiese toetswerk. Die moderne ontledingstegnieke sluit onder andere in hoëresolusie-transmissie-elektronmikroskopie (HRTEM) en gefokusdeioonstraalskandering-elektron-mikroskopie (FIB SEM) tomografie. Die gerekenariseerde termochemiese bewerkings is met behulp van MTDATAfasediagramsagteware uitgevoer. Metallurgiese toetswerk het die maling van laboratoriumlotte teen verskillende spesifieke energieë behels. Nou verwante loogproefnemings is ook as relevant vir hierdie omvattende studie beskou. Die bestudeerde Peirce-Smith-omsettermonsters het op matte met spesifieke eindpunte van 5.17%, 0.99% en 0.15 gewig% Fe gedui. Die hoogste gekombineerde massakonsentrasie van die belangrike onedel en platinumgroepmetale is in die mat met ’n spesifieke ystereindpunt van 0.99% gevind. Die mineralogie van ál drie omsettermatte is oorheers deur die minerale strukture van nikkelsulfied, wat met die natuurlike mineraal heazlewoodiet ooreenstem. Die verskillende omsettermatte het ook die minerale strukture van kopersulfied, NiCu-allooi, spinel en OsRu-allooi bevat. Daar is bevind dat die verkryging van ’n spesifieke ystereindpunt tot meetbare mineralogiese verskille in die relatiewe volopheid van minerale, die eksterne morfologiese kenmerke sowel as minerale chemie lei. Die mineralogiese verskille was veral duidelik te sien tussen die minerale strukture van die ysterryke (5.17% Fe) en ysterarm (0.99% en 0.15% Fe) matte. Fyn mineralogiese verskille is ook tussen die minerale strukture van die ysterarm matte bespeur. Die 0.99% Fe-mat het tipies beduidend meer NiCu-allooi as die 5.17% Fe-mat bevat. Die NiCu-allooistrukture tree oënskynlik op as die hoofversamelaars van die ekonomies belangrike platinumgroepmetale. Mineralogiese waarnemings is gebruik om ’n begrip te ontwikkel van die onderliggende mineralisasiemeganisme van NiCuallooistrukture. Die gevolglike gemineraliseerde strukture is met behulp van driedimensionele rekonstruksies met hoë kleurgetrouheid sowel as in grysskaal voorgestel. Daar is teoreties aangetoon dat variasies in ystereindpuntspesifieke beginsamestellings van suurstofvrye vloeibare matstelsels die solidifikasieroete na die eutetikum wysig. Daarbenewens is die vloeifasesolidifikasie van die ysterryke en ysterarm matstelsels, wat suurstof insluit, op sowat ±2.5 oC gekwantifiseer. Die meeste van die spesifieke energie wat vir maling beskikbaar was, is gebruik om die nikkelsulfiedmatriks te breek, veral vir die ysterryke mat. Berekeninge toon dat die breektempo’s van die minerale strukture van kopersulfied by die 5.17% Fe-mat hoër was as by die 0.15% Fe-mat teen ’n spesifieke energie van 25 kWh/t. Die mate van kopersulfiedvrystelling was hoër by die 5.17% Fe-mat as by die 0.15% Fe-mat by dieselfde spesifieke energie vir maling. ’n Hoër mate van Ni-ekstraksie en Cu-sementasie is verkry toe ysterarm matdeeltjies geloog is. Wat eindpuntseleksiemaatstawwe betref, is die produksie van ’n omsettermat met ’n spesifieke ystereindpunt van 0,99% as die mees geskikte aangewys. ’n Praktiese ystereindpuntbestek van 1.6% tot 1.0% word aanbeveel vir die produksie van ’n omsettermat met ’n gevolglike mineralogiese gehalte wat binne die perke van die Lonmin-raffinadery vir onedel metale val. Hierdie studie bied ’n geïntegreerde begrip van die produksie van onedel en platinumgroepmetale as ’n funksie van ’n gewenste ystereindpunt by Lonmin. Hierdie inligting was nie voorheen in literatuur oor metaalproduksie beskikbaar nie. Nuwe tegnologie vir die monitering en konsekwente beheer van so ’n praktiese ystereindpuntbestek kan dus op grond hiervan in werking gestel word.

Currently, there is not enough information available on the effect of inclusions on extrusion alloys. Theoretical calculations in the past demonstrated the probable role of oxides in Fe-intermetallic phase selection (Cao et al., 03). However, no concrete evidence can be found in the literature to support this argument. This study investigates the role of in-situ oxides in intermetallic phase selection. Various Mg oxides (spinel and MgO) were formed in-situ by adding different levels of Mg. A special intermetallic extraction process was used for 3D analysis. SEM, EDS and XRD analysis were used for qualitative and quantitative analysis. Dry and wet surfaces of the oxide bi-films were observed with the wet surfaces highly associated with MgO and spinel particles. MgO particles had spherical morphology and there average diameter was observed to be in the range 200nm-400nm, whereas spinel particles had octahedral morphology with average length of the side in the range 1-2μm. MgO was found in locations which appear to be the most probable nucleation points of α-AlFeSi intermetallics and Mg₂Si. These results provide a new and more distinctive perspective on the actual morphology of Fe-rich intermetallics and Mg oxides than the ones that exist in the literature. It also provides direct evidence of the role of inclusions (oxides) in intermetallic phase nucleation. This information can be utilised to improve the surface properties in 6xxx extrusion alloys.

Orientador: Amauri Garcia
Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica
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Resumo: O desenvolvimento de microestruturas otimizadas durante o processo de solidificação são de fundamental importância nas propriedades e desempenho de produtos acabados baseados em ligas metálicas. Neste estudo é analisada a cinética envolvida no processo de solidificação, seus efeitos nos parâmetros macro e microestruturais e a sua conseqüente influência nas propriedades mecânicas. Com esse intuito, ligas hipoeutéticas do sistema binário são estudadas Al-Ni por meio de experimentos de solidificação vertical ascendente sob regime transitório de condução de calor. Os espaçamentos dendríticos primários (?1) e secundários(?2) foram medidos ao longo de todos os lingotes para cada uma das ligas analisadas e correlacionados com as variáveis térmicas de solidificação. Uma abordagem teórico-experimental é utilizada na determinação quantitativa de tais variáveis térmicas: coeficiente de transferência de calor na interface metal/molde, velocidade de deslocamento da isoterma liquidus, gradientes térmicos, taxa de resfriamento e tempo local de solidificação. Os dados experimentais referentes à solidificação das ligas são confrontados com os principais modelos teóricos de crescimento dendrítico da literatura. Este estudo aborda, também, a influência do teor de soluto nos espaçamentos dendríticos para as ligas estudadas. Do ponto de vista macroestrutural, verifica-se que a transição colunar/equiaxial (TCE) ocorre para ligas hipoeutéticas Al-Ni para uma taxa crítica de resfriamento de 0,16 K/s. Por ensaios de tração as propriedades mecânicas das ligas do sistema Al-Ni são correlacionadas com parâmetros da micro-estrutura dendrítica resultante do processo de solidificação. Verifica-se que os limites de escoamento e de resistência à tração crescem com o aumento da concentração de soluto e decrescem com o aumento dos espaçamentos dendríticos, ?1 e ?2. O alongamento específico, por outro lado, mostra-se independente da composição e do arranjo dendrítico. Para a liga Al-5%Ni foi também realizado um estudo de solidificação rápida por refusão da superfície a laser para análise das variações microestruturais e de dureza entre as áreas não tratadas e tratadas superficialmente.
Abstract: The development of optimized microstructures during the solidification stage of processing is of fundamental importance to the mechanical properties and to the performance of finished products of metallic alloys. In this study the kinetics of solidification and its effects on macro and microstructural parameters, as well as the consequent influence on the final mechanical properties are analyzed. Hypoeutectic Al-Ni alloys are studied by upward unidirectional solidification experiments under transient heat flow conditions. Primary (?1) and secondary (?2) dendrite arm spacings are measured along the castings for all alloys analyzed and correlated with transient solidification thermal variables. A combined theoretical/ experimental approach is used to quantitatively determine such thermal variables, i.e., transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time. The experimental data concerning the Al-Ni alloys solidification are compared to the main predictive dendritic models from the literature and the dependence of dendrite arm spacing on the alloy solute content is also analyzed. From the macrostructural point of view, it is found that the CET occurs for a critical value of cooling rate of about 0.16 K/s for hypoeutectic Al-Ni alloys.With a view to correlate mechanical properties to dendrite arm spacings, tensile testings were carried out. It is found that the ultimate tensile strength and the yield strength increase with increasing alloy solute content and with decreasing primary and secondary dendrite arm spacings. In contrast, the elongation is found to be independent of both alloy composition and dendritic arrangement. For the Al 5%Ni alloy a rapid solidification study is carried out by using laser surface remelting in order to permit microstructural and microhardness variations throughout the resulting treated and untreated zones, to be analysed.
Doutorado
Materiais e Processos de Fabricação
Doutor em Engenharia Mecânica

Aluminium alloys, including both foundry and wrought alloys, have been extensively used for light-weight structural and functional applications. A grain refined as-cast microstructure is generally highly desirable for either subsequent processing ability or mechanical properties of the finished components. In this thesis, the grain refined microstructures in Al alloys have been achieved by intensive melt shearing using the melt conditioning by advanced shearing technology (MCAST) without deliberate grain refiner additions. Such grain refinement has been attributed to the enhanced heterogeneous nucleation on the dispersed oxide particles. It has been established that the naturally occurring oxides in molten Al alloys normally have a good crystallographic match with the a-Al phase, indicating the high potency of oxide particles as the nucleation sites of the a-Al phase. The governing factors for these oxide particles to be effective grain refiners in Al alloys have been proposed, including the achievement of good wetting between oxide particles and liquid aluminium, a sufficient number density and uniform spatial distribution of the dispersed oxide particles, and near equilibrium kinetic conditions in liquid alloys. In the present study, near equilibrium kinetic conditions can be achieved by intensive melt shearing using a twin screw mechanism, which has been confirmed by the observed equilibrium a-AlFeSi phase in a cast Al alloy and the transformation from g- to a-Al2O3 at 740±20oC under intensive shearing. For different alloy systems, depending on the alloy system, and melting conditions, due to the particular types of oxide formed and its crystallographic and chemical characteristics, the nucleation site of the nucleated phase is different. Specifically, MgAl2O4 relative to MgO, and a-Al2O3 relative to g-Al2O3, have higher potency as heterogeneous nucleation sites of a-Al phase in Al alloys. In future, the modification of the crystallographic match, and of the other surface characteristics related to the interfacial energy between the specific oxides and nucleated phase by trace alloying addition through segregation to the interface between oxides and nucleated phases combined with physical melt processing (such as intensive shearing in the present study) should be investigated in more detail.

ABSTRACTXIONG, MING. Investigation of Transport Phenomena in the Presence of Interfaces: Forced Convection in Composite Porous/Fluid Domains, Solidification with a Mushy Region, and Meniscus Formation in Dip Coating Processing (Under the direction of Andrey V. Kuznetsov)Transport phenomena play an important role in many practical applications. Every time a new technology is developed, analysis of transport processes is crucial for its success. Numerical and analytical investigations of transport processes in forced convection in composite porous/fluid domains, solidification of binary alloys, and meniscus formation in dip coating process are performed. These processes include mass, momentum, and energy transport across interfaces. For forced convection in composite porous/fluid domains, the validity of single-domain approach is investigated via comparisons between the numerical and exact solutions. An analytical solution for fluid flow described by the Brinkman-Forchheimer-Darcy equation is obtained by utilizing the boundary layer approximation. Solidification of binary alloys is studied by utilizing a porous medium approach for modeling transport processes in the mushy zone. A three-phase model is developed to predict microporosity formation during this process. Solute redistribution during this process is modeled by using the Scheil and lever rules to describe solute transport at the local scale. The investigations show that initial hydrogen concentration is an important factor affecting microporosity formation. Also, some effective ways of controlling microporosity formation are suggested based on these investigations. Another process studied in this dissertation is the dip coating with liquid carbon dioxide used as a solvent. This is a new deposition technique developed in recent years. A model accounting for evaporation during this process is obtained based on the classical free meniscus theory. Numerical results agree well with experimental data. These results show that the dry film thickness increases with the increase of evaporation rate and initial solute concentration.

Thesis (Ph. D.)--Worcester Polytechnic Institute.
Keywords: castability; metal casting; error analysis; casting fluidity; a356; solidification processing; fluidity. Includes bibliographical references (leaves 85-90).

The objective of this research is to increase the understanding of the solidification behaviour of some industrially important wrought aluminium alloys. The investigation methods range from direct investigations of as-cast ingots to laboratory-scale techniques in which ingot casting is simulated. The methods span from directional solidification at different cooling rates to more fundamental and controlled techniques such as DTA and DSC. The microstructure characteristics of the castings have been investigated by optical and Scanning Electron microscopy. Hardness tests were used to evaluate the mechanical properties. The effects of adding alloying elements to 3XXX and 6XXX aluminium alloys have been studied with special focus on the effects of Zn, Cu, Si and Ti. These elements influence the strength and corrosion properties, which are important for the performance of final components of these alloys. Solidification studies of 0-5wt% Zn additions to 3003 alloys showed that the most important effect on the microstructure was noticed at 2.5 wt% Zn, where the structure was fine, and the hardness had a maximum. Si addition to a level of about 2% gave a finer structure, having a relatively large fraction of eutectic structure, however, it also gave a long solidification interval. The addition of small amounts of Cu, 0.35 and 1.0 wt%, showed a beneficial effect on the hardness. Differences have been observed in the ingot surface microstructures of 6xxx billets with different Mg and Si ratios. Excess Si compositions showed a coarser grain structure and more precipitations with possible negative implications for surface defect formation during DC casting. The comparison of alloys of different Ti content showed that the addition of titanium to a level of about 0.15 wt% gave a coarser grain structure than alloys with a normal Ti content for grain refinement, i.e. < 0.02 wt%, although a better corrosion resistance can be obtained at higher Ti contents. The larger grain size results in crack sensitivity during DC casting. A macroscopic etching technique was developed, based on a NaOH solution, and used in inclusion assessment along DC cast billets. Good quantitative data with respect to the size and spatial distribution of inclusions were obtained. The results from studied billets reveal a decreasing number of inclusions going from bottom to top, and the presence of a ring-shaped distribution of a large number of small defects in the beginning of the casting. The present study shows how composition modifications, i.e. additions of certain amounts of alloying elements to the 3xxx and 6xxx Al alloys, significantly change the microstructures of the materials, its castability, and consequently its mechanical properties
QC 20100901

Orientadores: Amauri Garcia, Noé Cheung
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
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Resumo: Neste trabalho, o software ANSYS, baseado no Método dos Elementos Finitos, é adaptado para a simulação tridimensional do fluxo de calor no processo de refusão superficial a laser. A análise numérica é validada com resultados simulados por outros modelos existentes na literatura para casos de refusão superficial a laser de alumínio puro e com resultados simulados e experimentais de uma liga Al-5%Ni. Ensaios experimentais próprios foram realizados em amostras de uma liga Al-1,5%Fe, utilizando um laser à fibra dopado com Itérbio, com potência máxima disponível de 2 kW. Para efeito comparativo, as trilhas foram feitas variando-se valores de velocidade de deslocamento do feixe laser para um mesmo valor de potência. Observou-se que a microestrutura tanto do substrato quanto da zona tratada apresentou morfologia tipicamente celular. As microestruturas resultantes dos tratamentos a laser foram analisadas através de microscopia eletrônica de varredura, sendo observados espaçamentos celulares extremamente refinados na área tratada a laser refletindo no aumento significativo da dureza confirmado por ensaios de microdureza Vickers. Uma técnica de dissolução parcial das amostras tratadas a laser foi aplicada para evidenciar os intermetálicos no substrato e na região tratada a laser, mostrando a modificação da redistribuição dos intermetálicos no interior da poça fundida e dando indicações de aumento da resistência à corrosão na região tratada
Abstract: In this work, the software ANSYS, based on the Finite Element Method, is adapted to simulate the three-dimensional heat flux during the laser remelting surface treatment. The numerical analysis is validated against theoretical results furnished by other models from the literature for laser surface remelting of aluminum and against theoretical and experimental results of Al-5wt%Ni alloy samples. Laser remelting experiments with Al-1,5%wtFe samples have been carried out by using a 2kW Yb fiber laser. For comparative effects, the laser tracks were performed with different laser beam velocities for a fixed value of power. It was observed that both the substrate and the treated region had a typical cellular morphology. The microstructures resulting from the laser treatment were analyzed by using electron scanning microscopy and very refined cell spacing has been observed, which can induce a significant hardness increase confirmed by Vickers microhardness tests. A partial dissolution technique has been performed to foreground the intermetallics at the substrate and at the laser treated zone, showing the intermetallics redistribution inside the molten pool and giving indications of increased corrosion resistance on the treated region
Mestrado
Materiais e Processos de Fabricação
Mestre em Engenharia Mecânica

The influence of processing and process parameters plays a key role for the Aluminium foundry and transport industries as it affects the quality and soundness of the cast products. Particularly, the choice of a process chain in Aluminium foundry, otherwise of process parameters, influences the reject rates, hence casting costs, the process yield and the production rate. The process chain in Aluminium foundry is a complex sequence of processes and the final casting quality depends on many parameters. Several aspects of this subject are still not fully understood. The motivation of the research presented in this doctoral thesis work was, therefore, to fill this gap in knowledge. The study has aimed at understanding the influence of various process and process parameters of foundry on the quality of aluminium alloy castings and, in particular, Al-Si based castings. A literature review and a sufficient background of previously reported results on the influence of processing and process parameters on the quality of aluminium alloy castings, physical fundamentals as well as industrial challenges, motivation and goals were carried out. Special attention in Aluminium process chain has been given to: The modification of aluminium-silicon cast alloys: before casting aluminium alloys, the molten metal can be treated in order to improve the microstructure and properties of alloys by addition of small quantities of certain "modifying" elements. The pouring of molten metal into the mould: this is one of the critical steps in foundry technology, since the behaviour of the liquid and its subsequent solidification and cooling determine whether the cast shape will be properly formed, internally sound and free from defects. The chill casting processes, such as gravity, low-pressure and high-pressure die casting processes: the essential feature of chill casting is the use of permanent metal moulds, into which the molten alloy is either poured directly or injected under pressure, giving rise to the separate processes of gravity and low/high pressure die casting. Permanent moulds offer obvious advantages in terms of simplicity of production for large quantities of parts, but are subject to limitations yet to be discussed. The heat treating process applied to high-pressure die castings: conventional die castings are utilised to produce many products but unfortunately the presence of porosity limits the application. In addition to porosity, the microstructure inherent with conventional die casting could not meet the mechanical requirements needed for many applications. Subsequent heat treating, which can positively alter the microstructure, is rarely possible due to defects that emerge during thermal processing, such as blistering.

Thesis (M.S.)--University of Wisconsin--Madison, 1991.
Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 157-163).

Thesis (M.S.)--University of Wisconsin--Madison, 1986.
Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 125-131).

碩士
國立成功大學
工程科學系專班
101
The casting skill has been developed for several thousand years. With the progress of time, the environment of application becomes more severe and hence the promotion requirements of material properties and mechanical strength increase. In a casting process, the temperature and concentration fields affect the microstructures of materials, which directly influence the mechanical and physical properties. The morphology of solidification microstructure is difficult to control in a casting process, while the grain size can be easily changed. The directional solidification and the growth of single crystal are the advanced casting techniques. In this experimental study of directional solidification, lead-tin alloy is used as the casting material and four experimental models with different heat temperatures, descending speeds of platform, water temperatures of copper chill are utilized to analyze their effects on the directional solidification. The microstructures are observed by using an optical microscope. The macro and micro structures are employed to investigate the influences of the four models on the control of preferred dendritic direction, the grain size, the constraint of dendritic growth, the temperature gradient and the growth rate. From the analysis results, it is expected to help the further grasp of the controlled mechanism of directional solidification.

Yeung Man Hau.
Thesis (M.Phil.)--Chinese University of Hong Kong, 1998.
Includes bibliographical references (leaves 52-53).
Text in English; abstract also in Chinese.
Yeung Man Hau.
Chapter Chapter 1 --- Introduction
Chapter 1. --- Background of solidification --- p.1
Chapter 1.1 --- The driving force for solidification
Chapter 1.2 --- Capillarity effect (or Gibbs-Thomson effect)
Chapter 2. --- Nucleation --- p.3
Chapter 3. --- Growth --- p.4
Chapter 3.1 --- Constrained growth and unconstrained growth
Chapter 3.2 --- Directional solidification
Chapter 4. --- Growth of pure substances --- p.6
Chapter 4.1 --- Metals
Chapter 4.2 --- Stability of planar S/L interface
Chapter 4.3 --- Non-metals
Chapter 5. --- Solidification of single-phase binary alloys --- p.7
Chapter 5.1 --- Equilibrium solidification
Chapter 5.2 --- Constitutional undercooling
Chapter 5.3 --- Stability of planar S/L morphology
Chapter 5.4 --- Minimum scale of perturbation in directional growth
Chapter 5.5 --- Development of growth morphology
Chapter 5.6 --- Growth rate of cell/dendrite tip
Chapter 5.7 --- Arm spacing and coarsening
Chapter 6. --- Solidification of binary eutectic alloys --- p.11
Chapter 6.1 --- Classification
Chapter 6.2 --- Growth of lamellar eutectics
Chapter 6.3 --- Stability of planar morphology
Chapter 6.4 --- Coupled zone (Competitive growth of eutectic and dendrites)
Chapter 6.5 --- Off-eutectic solidification
Chapter 7. --- Solidification of ternary eutectic alloys --- p.14
References --- p.16
Figures --- p.17
Chapter Chapter 2 --- Experimental Methods
Chapter 1. --- Fused silica tube cleaning --- p.37
Chapter 2. --- Alloy preparation --- p.37
Chapter 3. --- Undercooled specimen preparation --- p.38
Chapter 4. --- Specimen examination --- p.38
Chapter 5. --- TEM sample preparation --- p.39
References --- p.40
Figures --- p.41
Chapter Chapter 3 --- Solidification of Undercooled Molten Pd60 .5Cu25Si14.5 Alloy
Chapter 1. --- Introduction --- p.44
Chapter 2. --- Experimental --- p.46
Chapter 3. --- Results --- p.46
Chapter 3.1 --- Thermal profiles
Chapter 3.1.1 --- Temperature-time chart plotter (plotter)
Chapter 3.1.2 --- Differential thermal analysis (D TA)
Chapter 3.2 --- Microstructures
Chapter 3.2.1 --- Effect of undercooling on the microstructure
Chapter 3.2.2 --- Effect of quenching after 1st exothermic peak on the microstructure
Chapter 3.2.3 --- Effect of annealing at the onset temperature of 1st exothermic peak on the microstructure
Chapter 3.2.4 --- Effect of using slower cooling rate on the microstructure
Chapter 4. --- Discussions --- p.50
Chapter 5. --- Conclusion --- p.51
References --- p.52
Figures --- p.54

Melt convection and solid phase movement play an important role in solidification processes, which significantly influence the formation of grain structures and solute segregations. In general, the melt convection and grain movement are a result of buoyancy forces. The densities within melt are different due to the variation of temperature and concentration, leading to thermally and solutally driven melt convection. Similarly, the density differences between the grains and the bulk melt cause the grain movement, leading to solid sedimentation or grain floating, as the case may be. Free, unattached solid grains are produced by partial remelting and fragmentation of dendrites, by mechanical disturbances such as stirring or vibration and by heterogeneous nucleation of grains in solidification of grain-refined alloys. In this way, movement of solid crystals during solidification can be ascertained in the following two cases. In the first case, during columnar solidification of non-grain-refined alloys, solid movement is possible in the form of dendrite fragments detached from the columnar stalks by the process of remelting and fragmentation. Movement of grains during columnar solidification gives rise to altogether different microstructure from columnar to equiaxed. In the second case, during equiaxed solidification of grain-refined alloys, the movement of solid crystals is possible in the form of equiaxed dendrite crystals nucleated due to presence of grain refiners. The rate and manner by which the free solids settle (or float) will influence macrosegregation in metal castings. Control of the solidification process is possible through an understanding of the solid movement and its effect on macrosegregation and microstructure. With this viewpoint, the overall objective of the present thesis is to study, experimentally and numerically, the phenomenon of solid phase movement during solidification. Through this study, deeper insights of the role of solid phase movement in solidification are developed which can be used for possible control of quality in castings. Both columnar and equiaxed solidification are considered. Models for transport phenomena associated with columnar solidification with solid phase movement are rarely found in the literature, because of inherent difficulty associated with consideration of microscopic features such as remelting and fragmentation. To tackle this problem, solidification modules for remelting and fragmentation are developed first, followed by integration of these molecules in a macroscopic solidification model. A Rayleigh number based fragmentation criterion is developed for detachment of dendrite fragments from the developing mushy zone, which determines the conditions favorable for fragmentation of dendrites. The criterion developed is a function of net concentration difference, liquid fraction, permeability, growth rate of mushy layer, and thermophysical properties of the material. The effect of various solidification parameters on fragmentation is highlighted. The integrated continuum model developed is applied to stimulate the solidification of aqua-ammonia system in a side-cooled rectangular cavity. The numerical results are in good qualitative agreement with those of experiments reported in literature. A gentle ramp of the mushy zone due to settling of solid crystals, as also noticed in experimental literature, is observed towards the bottom of the cavity. The influence of various modeling parameters on solid phase movement and resulting macrosegregation is investigated through a parametric study. Movement of grains during columnar solidification gives rise to altogether different microstructure and sometimes may initiate a morphological transition of the microstructure from columnar to equiaxed if the number and size of equiaxed grains ahead of the columnar front become sufficient to arrest the columnar growth. The generalised model developed, considering solid phase movement during columnar solidification is used to predict columnar-to-equiaxed transition (CET) based on a prescribed cooling rate criterion. It is found that presence of convection significantly affects the solidification behaviour. Moreover, the movement of dendrite fragments and their accumulation at the columnar front further trigger the occurrence of CET. Cooling configuration, too significantly affects the nature of CET. In unidirectional solidification cases, the locations of CET are found to be in a plane parallel to the chill face. However, for the case of the non-unidirectional solidification (as in side-cooled cavity), the locations of CET need not be in a plane parallel to the chill face. In contrast to fixed columnar solidification, equiaxed solidification is poorly understood; in particular, the phenomena associated with solid crystal movement. Movement of unattached solid crystals, formed due to heterogeneous nucleation on grain-refiners, is induced by the convective currents as well as by buoyancy effects, causing the solid to sediment or to float, depending on density of solid compared to that of the bulk melt. While moving in the bulk melt these crystals can also remelt or grow. A series of casting experiments with AI-based alloys are performed to investigate the role and influence of movement of solid crystals on macrosegregation and microstructure evolution during equiaxed solidification. Controlled experiments are designed for studying, separately, settling and floatation of equiaxed crystals for different cooling conditions and configurations. Further, these experiments are carried out in convective and non-convective cases to understand the effect of convection on solid phase movement. Temperature measurements are performed at various locations in the mould during the experiments. After the cavity is solidified, microstructural and chemical analyses of the experimental samples are carried out, several notable features are observed in temperature histories, macrosegregation pattern, and microstructures due to settling/flotation phenomenon of solid crystals. It is found that the flow behavior of solid grains has a profound influence on the progress of solidification (in terms of grain size distribution and fraction eutectic) and macrosegregation distribution. In some cases, the induced flow due to solid phase movement can cause a flow reversal. The observations and quantitative data obtained from experiments, with the help of detailed solidification conditions provided, can be used for future validations of models for equiaxed solidification. Subsequently, numerical studies are carried out, using a modified version of the macroscopic model developed for columnar solidification with motion of solid crystals, to predict the transport phenomena during equiaxed solidification. The model is applied to simulate the solidification processes corresponding to each of the experimental cases performed in this study. For a better understanding of the phenomenon of movement of solid crystals, the following two special cases of solidification are also presented: 1) without movement of solid crystals and 2) movement of solid crystals without any relative velocity between solid and liquid phases. The numerical predictions showing nature of flow field and progress of solidification are substantiated by the experimental data for the thermal analysis, qualitative microstructural Images and quantitative microstructural analysis. It is concluded, with the help of various experiments and simulations, that movement of solid crystals influences the casting quality appreciably, in terms of macrosegregation and microstructures. It is expected that the improved understanding of the role and influence of solid phase movement during solidification processes (both columnar and equiaxed) obtained through this thesis will be useful for possible control of quality of as-cast products.

Melt convection and solid phase movement play an important role in solidification processes, which significantly influence the formation of grain structures and solute segregations. In general, the melt convection and grain movement are a result of buoyancy forces. The densities within melt are different due to the variation of temperature and concentration, leading to thermally and solutally driven melt convection. Similarly, the density differences between the grains and the bulk melt cause the grain movement, leading to solid sedimentation or grain floating, as the case may be. Free, unattached solid grains are produced by partial remelting and fragmentation of dendrites, by mechanical disturbances such as stirring or vibration and by heterogeneous nucleation of grains in solidification of grain-refined alloys. In this way, movement of solid crystals during solidification can be ascertained in the following two cases. In the first case, during columnar solidification of non-grain-refined alloys, solid movement is possible in the form of dendrite fragments detached from the columnar stalks by the process of remelting and fragmentation. Movement of grains during columnar solidification gives rise to altogether different microstructure from columnar to equiaxed. In the second case, during equiaxed solidification of grain-refined alloys, the movement of solid crystals is possible in the form of equiaxed dendrite crystals nucleated due to presence of grain refiners. The rate and manner by which the free solids settle (or float) will influence macrosegregation in metal castings. Control of the solidification process is possible through an understanding of the solid movement and its effect on macrosegregation and microstructure. With this viewpoint, the overall objective of the present thesis is to study, experimentally and numerically, the phenomenon of solid phase movement during solidification. Through this study, deeper insights of the role of solid phase movement in solidification are developed which can be used for possible control of quality in castings. Both columnar and equiaxed solidification are considered. Models for transport phenomena associated with columnar solidification with solid phase movement are rarely found in the literature, because of inherent difficulty associated with consideration of microscopic features such as remelting and fragmentation. To tackle this problem, solidification modules for remelting and fragmentation are developed first, followed by integration of these molecules in a macroscopic solidification model. A Rayleigh number based fragmentation criterion is developed for detachment of dendrite fragments from the developing mushy zone, which determines the conditions favorable for fragmentation of dendrites. The criterion developed is a function of net concentration difference, liquid fraction, permeability, growth rate of mushy layer, and thermophysical properties of the material. The effect of various solidification parameters on fragmentation is highlighted. The integrated continuum model developed is applied to stimulate the solidification of aqua-ammonia system in a side-cooled rectangular cavity. The numerical results are in good qualitative agreement with those of experiments reported in literature. A gentle ramp of the mushy zone due to settling of solid crystals, as also noticed in experimental literature, is observed towards the bottom of the cavity. The influence of various modeling parameters on solid phase movement and resulting macrosegregation is investigated through a parametric study. Movement of grains during columnar solidification gives rise to altogether different microstructure and sometimes may initiate a morphological transition of the microstructure from columnar to equiaxed if the number and size of equiaxed grains ahead of the columnar front become sufficient to arrest the columnar growth. The generalised model developed, considering solid phase movement during columnar solidification is used to predict columnar-to-equiaxed transition (CET) based on a prescribed cooling rate criterion. It is found that presence of convection significantly affects the solidification behaviour. Moreover, the movement of dendrite fragments and their accumulation at the columnar front further trigger the occurrence of CET. Cooling configuration, too significantly affects the nature of CET. In unidirectional solidification cases, the locations of CET are found to be in a plane parallel to the chill face. However, for the case of the non-unidirectional solidification (as in side-cooled cavity), the locations of CET need not be in a plane parallel to the chill face. In contrast to fixed columnar solidification, equiaxed solidification is poorly understood; in particular, the phenomena associated with solid crystal movement. Movement of unattached solid crystals, formed due to heterogeneous nucleation on grain-refiners, is induced by the convective currents as well as by buoyancy effects, causing the solid to sediment or to float, depending on density of solid compared to that of the bulk melt. While moving in the bulk melt these crystals can also remelt or grow. A series of casting experiments with AI-based alloys are performed to investigate the role and influence of movement of solid crystals on macrosegregation and microstructure evolution during equiaxed solidification. Controlled experiments are designed for studying, separately, settling and floatation of equiaxed crystals for different cooling conditions and configurations. Further, these experiments are carried out in convective and non-convective cases to understand the effect of convection on solid phase movement. Temperature measurements are performed at various locations in the mould during the experiments. After the cavity is solidified, microstructural and chemical analyses of the experimental samples are carried out, several notable features are observed in temperature histories, macrosegregation pattern, and microstructures due to settling/flotation phenomenon of solid crystals. It is found that the flow behavior of solid grains has a profound influence on the progress of solidification (in terms of grain size distribution and fraction eutectic) and macrosegregation distribution. In some cases, the induced flow due to solid phase movement can cause a flow reversal. The observations and quantitative data obtained from experiments, with the help of detailed solidification conditions provided, can be used for future validations of models for equiaxed solidification. Subsequently, numerical studies are carried out, using a modified version of the macroscopic model developed for columnar solidification with motion of solid crystals, to predict the transport phenomena during equiaxed solidification. The model is applied to simulate the solidification processes corresponding to each of the experimental cases performed in this study. For a better understanding of the phenomenon of movement of solid crystals, the following two special cases of solidification are also presented: 1) without movement of solid crystals and 2) movement of solid crystals without any relative velocity between solid and liquid phases. The numerical predictions showing nature of flow field and progress of solidification are substantiated by the experimental data for the thermal analysis, qualitative microstructural Images and quantitative microstructural analysis. It is concluded, with the help of various experiments and simulations, that movement of solid crystals influences the casting quality appreciably, in terms of macrosegregation and microstructures. It is expected that the improved understanding of the role and influence of solid phase movement during solidification processes (both columnar and equiaxed) obtained through this thesis will be useful for possible control of quality of as-cast products.

In this thesis, we present an experimental and numerical study of globularization during reheating of thixocast billet having non-dendritic microstructure. The process of reheating is an important step in the semisolid processing and is essential to control its microstructure and hence its mechanical properties. Material chosen for this study is Aluminum alloy, A356. The primary focus of this study is the heat treatment below eutectic temperature i.e. transformation in solid phase. It is found that during short duration heat treatment, globularization of primary α grains and spheroidization of eutectic Si flakes take place which improves the mechanical properties of semisolid cast products significantly. A prolonged heat treatment is found to degrade the properties of castings since it enhances the porosity and coarsening of Si. The study suggests that a precise heat treatment practice can be designed to improve the semisolid microstructure. A computational model based on Phase field approach has been proposed to study this phenomena. Predictions based on this model are qualitatively compared with corresponding experimental observations. Since eutectics form an important step in multiphase solidification, an attempt has been made to develop an enthalpy based explicit micro-scale model for eutectic solidification. In this preliminary study, growth of adjacent α and β phases in a two dimensional Eulerian framework has been simulated. The model is qualitatively validated with Jackson Hunt theory. Results show expected eutectic growth. This methodology promises significant saving in computational time compared to existing numerical models.

In this thesis, we present an experimental and numerical study of globularization during reheating of thixocast billet having non-dendritic microstructure. The process of reheating is an important step in the semisolid processing and is essential to control its microstructure and hence its mechanical properties. Material chosen for this study is Aluminum alloy, A356. The primary focus of this study is the heat treatment below eutectic temperature i.e. transformation in solid phase. It is found that during short duration heat treatment, globularization of primary α grains and spheroidization of eutectic Si flakes take place which improves the mechanical properties of semisolid cast products significantly. A prolonged heat treatment is found to degrade the properties of castings since it enhances the porosity and coarsening of Si. The study suggests that a precise heat treatment practice can be designed to improve the semisolid microstructure. A computational model based on Phase field approach has been proposed to study this phenomena. Predictions based on this model are qualitatively compared with corresponding experimental observations. Since eutectics form an important step in multiphase solidification, an attempt has been made to develop an enthalpy based explicit micro-scale model for eutectic solidification. In this preliminary study, growth of adjacent α and β phases in a two dimensional Eulerian framework has been simulated. The model is qualitatively validated with Jackson Hunt theory. Results show expected eutectic growth. This methodology promises significant saving in computational time compared to existing numerical models.

Thesis (M.S.)--University of Wisconsin--Madison, 1994.
Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 40-41).

碩士
元智大學
機械工程學系
93
Surface coating is the best way to protect the metal against corrosion. As the coating on the devices is deteriorated, a paint remover would be applied to strip the coating. Paint-stripped waste contains a lot of hazardous materials, which makes it difficult to handle properly. There is still lack of administrative rules to regulate this kind of waste now, such that seeking for an acceptable disposal way is in a hurry. Therefore, the objectives of this study are to characterize the paint-stripped waste from the National Defense Administrations and to evaluate the optimum process to manage the paint-stripped waste by cement solidification. The paint-stripped waste samples were collected from a National Defense Administration. After the samples were pre-treated to eliminate the impurity, several analyses were employed to determine the properties of paint-stripped waste, including the moisture content, total ash content, organic amount, heating values, pH value, elemental composition and heavy metal levels. The information could be used to evaluate the requirements. The basic compositions of cement solid have cement, sand and water. The air-dried paint-stripped waste was used as the additive of the solid. The water to solid ratio, the cement to paint-stripped waste ratio, and the curing time were considered to understand their effects on the development of compressive strength and leaching levels of heavy metals (after curing 28 days) of the cement solids. The plasticizer or chelating agent (EDTA-4Na or silica gel) was introduced to improve the performance of the cement solids. Results show that the as-received paint-stripped waste contained high concentrations of heavy metals, wherein the chromium (Cr) had the highest level of 203 mg/L. In the cement solids, the highest compressive strength (423 kg/cm2) occurred as the water to solid ratio was 0.3 and the cement to paint-stripped waste ratio was 2. However, the Cr could not meet the regulation standard. This implies that the solidification by use of sole cement was not enough to handle the paint-stripped waste issue effectively. After the introduction of plasticizer, the best water to solid ratio was down to 0.25 and the best cement to paint-stripped waste ratio was 2 again. The highest compressive strength has increased approximately 1.5 times after the addition of plasticizer; unfortunately, the Cr level could not be reduced significantly. Although the best compressive strength was still observed at the water to solid ratio of 0.3 and the cement to paint-stripped waste ratio of 2 after the EDTA-4Na was added, and the strength was similar to the value from the solid without the addition of EDTA-4Na, the leaching concentrations of all heavy metals were lower than the regulation standards. The introduction of silica gels resulted not only in acceptable leaching levels of heavy metals, but also enhanced the compressive strength of the solids.

In several applications of casting, dendritic microstructure is not desirable as it results in poor mechanical properties. Enhancing the fluid flow in the mushy zone by stirring is one of the means to suppress this dendritic growth. The strong fluid flow detaches the dendrites from the solid-liquid interface and carries them into the mold to form slurry. The detached dendrites coarsen in the slurry and form into rosette or globular particles based on processing conditions. This slurry offers less resistance to flow even at a high solid fraction and easily flow into the die-cavity. The above principle is the basis of a new manufacturing technology called “semi-sold forming” (SSF), in which metal alloys are cast in the semi-solid state. This technique has several advantages over other existing commercial casting processes, such as reduction of macrosegregation, reduction of porosity and low forming efforts. A major challenge existing in semisolid manufacturing is the production of metallic slurry in a consistent manner. The main difficulty arises because of the presence of a wide range of process parameters affecting the quality of the final product. An established method of producing slurry is by stirring the alloy using an electromagnetic stirrer. From an elaborate review of literature, it is apparent that solidification in presence of electromagnetic stirring involves a wide range of shear and cooling rates variation. However, the CFD models found in the literature are generally not based on accurate rheological properties, which are known to be functions of the relevant process parameters. Hence, there is a clear need for a comprehensive numerical model for such a solidification process, involving accurate rheological data for the semisolid slurry subjected to a range of processing conditions. The objective of the present work is to develop a numerical model for studying the transport phenomena during solidification with linear electromagnetic stirring. The study is presented in the context of a billet making process in a cylindrical mould using linear electromagnetic stirring. The mould consists of two parts: the upper part of the mould is surrounded by a linear electromagnetic stirrer forming the zone of active stirring, and the lower part of the mould is used to cool the liquid metal. The material chosen for the study is Al-7.32%Si (A356) alloy, commonly used for die casting applications. A complete numerical model will therefore have two major components: one dealing with rheological behavior of the semisolid slurry, and the other involving macroscopic modeling of the process using computational fluid dynamics (CFD) techniques. For the latter part of the model, determination of rheological behavior of the slurry is a pre-requisite. The rheological characteristics of the stirred slurry, as a function of shear rate and cooling rate, is determined experimentally using a concentric cylinder viscometer. Two different series of experiments are performed. In the first series, the liquid metal is cooled at a constant cooling rate and sheared with different shear rates to get the effect of shear rate on viscosity. In the second series of experiments, the liquid metal is cooled at different cooling rates and sheared at a constant shear rate to obtain the effect of cooling rate on viscosity. During all these experiments, the shear rate is calculated from the measured angular velocity of spindle using inductive position sensor; viscosity of the slurry is calculated based on the torque applied to the slurry and angular velocity of the spindle; and the solid fraction is calculated from measured temperature of the slurry based on Schiel equation. From these data, a constitutive relation for variable viscosity is established, which is subsequently used in a numerical model for simulating the transport phenomena associated with the solidification process. The numerical model uses a set of single-phase governing equations of mass, momentum, energy and species conservation. The set of governing equations is solved using a pressure based finite volume technique, along with an enthalpy based phase change algorithm. The numerical simulation of this process also involves modeling of Lorentz force field. The numerical study involves prediction of temperature, velocity, species and solid fraction distribution. First, studies are performed for a base case with a moderate stirring intensity of 250A primary current and 50 Hz frequency. It is found that the electromagnetic forces have maximum values near the mould periphery, which results in an ascending movement of the slurry near the mould periphery. Because of continuity, this slurry comes down along the axis of the mould. Stirring produces a strong fluid flow which results good mixing in the melt. Correspondingly, a homogenized temperature distribution is found in the domain. Because of strong stirring, the solid fraction in the slurry is found to be distributed almost uniformly. It is also found that fragmentation of dendrites increases solid fraction in the slurry with processing time. During processing, the continuous rejection of solute makes the liquid progressively solute enriched. It is predicted from the present study that the remaining liquid surrounding the primary solid phase finally solidifies with a near-eutectic composition, which is desirable from the point of view of semisolid casting. Correspondingly, a set of experiments are performed to validate the numerically predicted results. The numerical predictions of temperature variations are in good agreement with experiments, and the predicted flow field evolution correlate well with the microstructures obtained through experiments at various locations, as observed in the numerical results. Subsequently the study is extended to predict the effect of process parameters such as stirring intensity and cooling rate on the distributions of solid fraction and solute in the domain. It is found, from the simulation, that the solidification process is significantly affected by stirring intensity. At increasing primary excitation current, the magnitude of Lorentz force increases and results in increase of slurry velocity. Correspondingly, the fragmentation of dendrites from the solid/liquid is more during solidification at higher stirring intensity, which increases the fraction of solid in the slurry to a high value. It is also found that the solute and fraction of solid in the liquid mixes well under stirring action. Thus, a near uniform distribution of solute and solid fraction is found in the domain. It is found that stirring at high currents produces high solid fraction in the liquid. Also, at very low cooling rate, the solid fraction in the liquid increases. The present study focuses on the model development and experimental validation for solidification with linear electromagnetic stirring for producing a rheocast billet. Further studies highlighting the effects of various process parameters on the thermal history and microstructure formation are also presented.

In several applications of casting, dendritic microstructure is not desirable as it results in poor mechanical properties. Enhancing the fluid flow in the mushy zone by stirring is one of the means to suppress this dendritic growth. The strong fluid flow detaches the dendrites from the solid-liquid interface and carries them into the mold to form slurry. The detached dendrites coarsen in the slurry and form into rosette or globular particles based on processing conditions. This slurry offers less resistance to flow even at a high solid fraction and easily flow into the die-cavity. The above principle is the basis of a new manufacturing technology called “semi-sold forming” (SSF), in which metal alloys are cast in the semi-solid state. This technique has several advantages over other existing commercial casting processes, such as reduction of macrosegregation, reduction of porosity and low forming efforts. A major challenge existing in semisolid manufacturing is the production of metallic slurry in a consistent manner. The main difficulty arises because of the presence of a wide range of process parameters affecting the quality of the final product. An established method of producing slurry is by stirring the alloy using an electromagnetic stirrer. From an elaborate review of literature, it is apparent that solidification in presence of electromagnetic stirring involves a wide range of shear and cooling rates variation. However, the CFD models found in the literature are generally not based on accurate rheological properties, which are known to be functions of the relevant process parameters. Hence, there is a clear need for a comprehensive numerical model for such a solidification process, involving accurate rheological data for the semisolid slurry subjected to a range of processing conditions. The objective of the present work is to develop a numerical model for studying the transport phenomena during solidification with linear electromagnetic stirring. The study is presented in the context of a billet making process in a cylindrical mould using linear electromagnetic stirring. The mould consists of two parts: the upper part of the mould is surrounded by a linear electromagnetic stirrer forming the zone of active stirring, and the lower part of the mould is used to cool the liquid metal. The material chosen for the study is Al-7.32%Si (A356) alloy, commonly used for die casting applications. A complete numerical model will therefore have two major components: one dealing with rheological behavior of the semisolid slurry, and the other involving macroscopic modeling of the process using computational fluid dynamics (CFD) techniques. For the latter part of the model, determination of rheological behavior of the slurry is a pre-requisite. The rheological characteristics of the stirred slurry, as a function of shear rate and cooling rate, is determined experimentally using a concentric cylinder viscometer. Two different series of experiments are performed. In the first series, the liquid metal is cooled at a constant cooling rate and sheared with different shear rates to get the effect of shear rate on viscosity. In the second series of experiments, the liquid metal is cooled at different cooling rates and sheared at a constant shear rate to obtain the effect of cooling rate on viscosity. During all these experiments, the shear rate is calculated from the measured angular velocity of spindle using inductive position sensor; viscosity of the slurry is calculated based on the torque applied to the slurry and angular velocity of the spindle; and the solid fraction is calculated from measured temperature of the slurry based on Schiel equation. From these data, a constitutive relation for variable viscosity is established, which is subsequently used in a numerical model for simulating the transport phenomena associated with the solidification process. The numerical model uses a set of single-phase governing equations of mass, momentum, energy and species conservation. The set of governing equations is solved using a pressure based finite volume technique, along with an enthalpy based phase change algorithm. The numerical simulation of this process also involves modeling of Lorentz force field. The numerical study involves prediction of temperature, velocity, species and solid fraction distribution. First, studies are performed for a base case with a moderate stirring intensity of 250A primary current and 50 Hz frequency. It is found that the electromagnetic forces have maximum values near the mould periphery, which results in an ascending movement of the slurry near the mould periphery. Because of continuity, this slurry comes down along the axis of the mould. Stirring produces a strong fluid flow which results good mixing in the melt. Correspondingly, a homogenized temperature distribution is found in the domain. Because of strong stirring, the solid fraction in the slurry is found to be distributed almost uniformly. It is also found that fragmentation of dendrites increases solid fraction in the slurry with processing time. During processing, the continuous rejection of solute makes the liquid progressively solute enriched. It is predicted from the present study that the remaining liquid surrounding the primary solid phase finally solidifies with a near-eutectic composition, which is desirable from the point of view of semisolid casting. Correspondingly, a set of experiments are performed to validate the numerically predicted results. The numerical predictions of temperature variations are in good agreement with experiments, and the predicted flow field evolution correlate well with the microstructures obtained through experiments at various locations, as observed in the numerical results. Subsequently the study is extended to predict the effect of process parameters such as stirring intensity and cooling rate on the distributions of solid fraction and solute in the domain. It is found, from the simulation, that the solidification process is significantly affected by stirring intensity. At increasing primary excitation current, the magnitude of Lorentz force increases and results in increase of slurry velocity. Correspondingly, the fragmentation of dendrites from the solid/liquid is more during solidification at higher stirring intensity, which increases the fraction of solid in the slurry to a high value. It is also found that the solute and fraction of solid in the liquid mixes well under stirring action. Thus, a near uniform distribution of solute and solid fraction is found in the domain. It is found that stirring at high currents produces high solid fraction in the liquid. Also, at very low cooling rate, the solid fraction in the liquid increases. The present study focuses on the model development and experimental validation for solidification with linear electromagnetic stirring for producing a rheocast billet. Further studies highlighting the effects of various process parameters on the thermal history and microstructure formation are also presented.

In most casting applications, dendritic microstructure morphology is not desired because it leads to poor mechanical properties. Forced convection causing sufficient shearing in the mushy zone of the partially solidified melt is one of the means to suppress this dendritic growth. The dendrites formed at the solid-liquid interface are detached and carried away due to strong fluid flow to form slurry. This slurry, consisting of rosette or globular particles, provides less resistance to flow even at a high solid fraction and can easily fill the die-cavity. The stated principle is the basis of a new manufacturing technology called “semi-solid forming” (SSF), in which metal alloys are cast in the semi-solid state. This technique has numerous advantages over other existing commercial casting processes, such as reduction of macrosegregation, reduction of porosity and low forming efforts. Among all currently available methods available for large scale production of semisolid slurry, the cooling slope is considered to be a simple but effective method because of its simple design and easy control of process parameters, low equipment and running costs, high production efficiency and reduced inhomogeneity. With this perspective, the primary objective of the present research is to investigate, both experimentally and numerically, convective heat transfer and solidification on a cooling slope, in addition to the study of final microstructure of the cast billets. Some key process parameters are identified, namely pouring temperature, slope angle, slope length, and slope cooling rate. A systematic scaling analysis is performed in order to understand the relative importance of the parameters in influencing the final properties of the slurry and microstructure after solidification. A major part of the present work deals with the development of an experimental set up with careful consideration of the range of process parameters involved by treating the cooling slope as a heat exchanger. Subsequently, a comprehensive numerical model is developed to predict the flow, heat transfer, species concentration solid fraction distribution of aluminum alloy melt while flowing down the cooling slope. The model uses a variable viscosity relation for slurry. The metal-air interface at the top during the melt flow is tracked using a volume of fluid (VOF) method. Solidification is modeled using an enthalpy based approach and a volume averaged technique. The mushy region is modeled as a multi-layered porous medium consisting of fixed columnar dendrites and mobile equiaxed or fragmented grains. In addition, the solidification model also incorporates a fragmentation criterion and solid phase movement. The effects of key process parameters on flow behavior involving velocity distribution, temperature distribution, solid fractions at the slope exit, and macrosegregation, are studied numerically and experimentally for aluminium alloy A356. The resulting microstructures of the cast billets obtained from the experiments are studied and characterized. Finally the experimental results are linked to the model predictions for establishing the relations involving interdependence of the stated key process parameters in determining the quality of the final cast products. This study is aimed towards providing the necessary guidelines for designing a cooling slope and optimizing the process parameters for desirable quality of the solidified product.

In most casting applications, dendritic microstructure morphology is not desired because it leads to poor mechanical properties. Forced convection causing sufficient shearing in the mushy zone of the partially solidified melt is one of the means to suppress this dendritic growth. The dendrites formed at the solid-liquid interface are detached and carried away due to strong fluid flow to form slurry. This slurry, consisting of rosette or globular particles, provides less resistance to flow even at a high solid fraction and can easily fill the die-cavity. The stated principle is the basis of a new manufacturing technology called “semi-solid forming” (SSF), in which metal alloys are cast in the semi-solid state. This technique has numerous advantages over other existing commercial casting processes, such as reduction of macrosegregation, reduction of porosity and low forming efforts. Among all currently available methods available for large scale production of semisolid slurry, the cooling slope is considered to be a simple but effective method because of its simple design and easy control of process parameters, low equipment and running costs, high production efficiency and reduced inhomogeneity. With this perspective, the primary objective of the present research is to investigate, both experimentally and numerically, convective heat transfer and solidification on a cooling slope, in addition to the study of final microstructure of the cast billets. Some key process parameters are identified, namely pouring temperature, slope angle, slope length, and slope cooling rate. A systematic scaling analysis is performed in order to understand the relative importance of the parameters in influencing the final properties of the slurry and microstructure after solidification. A major part of the present work deals with the development of an experimental set up with careful consideration of the range of process parameters involved by treating the cooling slope as a heat exchanger. Subsequently, a comprehensive numerical model is developed to predict the flow, heat transfer, species concentration solid fraction distribution of aluminum alloy melt while flowing down the cooling slope. The model uses a variable viscosity relation for slurry. The metal-air interface at the top during the melt flow is tracked using a volume of fluid (VOF) method. Solidification is modeled using an enthalpy based approach and a volume averaged technique. The mushy region is modeled as a multi-layered porous medium consisting of fixed columnar dendrites and mobile equiaxed or fragmented grains. In addition, the solidification model also incorporates a fragmentation criterion and solid phase movement. The effects of key process parameters on flow behavior involving velocity distribution, temperature distribution, solid fractions at the slope exit, and macrosegregation, are studied numerically and experimentally for aluminium alloy A356. The resulting microstructures of the cast billets obtained from the experiments are studied and characterized. Finally the experimental results are linked to the model predictions for establishing the relations involving interdependence of the stated key process parameters in determining the quality of the final cast products. This study is aimed towards providing the necessary guidelines for designing a cooling slope and optimizing the process parameters for desirable quality of the solidified product.

by Chan Kim Wai.
Thesis (M.Phil.)--Chinese University of Hong Kong, 1997.
Includes bibliographical references.
by Chan Kim Wai.
Chapter Chapter I --- Introduction
Chapter 1.1 --- Rapid solidification
Chapter 1.1.1 --- Rapid quenching --- p.1-1
Chapter 1.1.2 --- Undercooling --- p.1-2
Chapter 1.2 --- Grain refinement
Chapter 1.2.1 --- What is grain refinement? --- p.1-5
Chapter 1.2.2 --- Previous results in grain refinement
Chapter 1.2.2.1 --- Pure metals (or dilute alloys) --- p.1-5
Chapter 1.2.2.2 --- Alloys --- p.1-9
Chapter 1.2.2.3 --- Semiconductor --- p.1-10
Chapter 1.2.3 --- Critical crystal growth velocity V* --- p.1-11
Chapter 1.2.4 --- Proposed models to grain refinement
Chapter 1.2.4.1 --- Dynamic nucleation and cavitation --- p.1-12
Chapter 1.2.4.2 --- Remelting (melt-back) --- p.1-14
Chapter 1.2.4.3 --- Interdendritic fluid flow --- p.1-15
Chapter 1.2.5 --- Volumetric manifestation of grain refinement --- p.1-15
Chapter 1.3 --- Aim of this project --- p.1-16
References
Figures
Chapter Chapter II --- Experimental
Chapter 2.1 --- Pure palladium
Chapter 2.1.1 --- Sample preparation and procedure --- p.2-1
Chapter 2.1.2 --- Limitation and choice of flux --- p.2-2
Chapter 2.1.3 --- High temperature furnace --- p.2-3
Chapter 2.1.4 --- Measurement of specific volume
Chapter 2.1.4.1 --- Theory --- p.2-4
Chapter 2.1.4.2 --- Setup --- p.2-5
Chapter 2.1.5 --- Observing internal morphology --- p.2-5
Chapter 2.2 --- Palladium with insoluble impurity
Chapter 2.2.1 --- Choice of insoluble impurities --- p.2-6
Chapter 2.2.2 --- Sample preparation --- p.2-7
References
Figures
Chapter Chapter III --- Results and Discussion
Results
Chapter 3.1 --- Pure palladium
Chapter 3.1.1 --- Specific volume --- p.3-1
Chapter 3.1.2 --- Grain structure and internal voids --- p.3-2
Chapter 3.2 --- Palladium with insoluble impurity
Chapter 3.2.1 --- Pinning effect of insoluble impurities --- p.3-3
Chapter 3.2.2 --- Pd-Ni-S system
Chapter 3.2.2.1 --- Grain refinement in Pd99.9Ni-S)0.1 --- p.3-4
Chapter 3.2.2.2 --- Change of ΔT* with addition of sulfur --- p.3-5
Chapter 3.2.2.3 --- Internal voids --- p.3-5
Discussion
Chapter 3.3 --- Dynamic nucleation of Pd-Ni-S system --- p.3-6
Chapter 3.4 --- Void formation of pure palladium and Pd-Ni-S --- p.3-6
Chapter 3.5 --- Grain refinement and specific volume --- p.3-7
Reference
Figures