Weight reduction in automobiles is amongst the most economical ways of reducing greenhouse gas emissions and increasing fuel efficiency. The recently invented ablation casting process is a novel technique of producing high performance light weight parts which meet this objective. In this technique a water jet demolds the water soluble sand mold and subsequently impinges upon the solidifying metal, thereby producing high cooling rates in the casting which in turn leads to microstructural refinement and higher mechanical properties. The objective of this study was to develop a comprehensive understanding of the effect of various parameters involved in the casting of a thin walled part using the HiPerMag casting process for the wrought aluminum alloy AA 7050. The study is divided into three major parts that deal with the composition of the sand binder system, optimization of the sand mold thickness, various aspects of the water jet parameters and the desired microstructural parameters which will result in a defect free part. In first phase of the study, sand mold properties such as the green and dry strengths of the water soluble sand binder system used in the study were tested to ensure that they meet the molding requirements. An average green strength of 160 kPa and an average dry strength of 3825 kPa were found for the water soluble sand binder system. These values were similar to those reported in the literature for clay bonded sands and were sufficient to make molds for casting in the current study. Secondly, a heat transfer model was developed to find a minimum mold thickness required to design a mold for the HiPerMag casting process such that the liquid metal remained sufficiently insulated before being quenched. Based on the model, for a mold with a cope thickness of 12.9 cm, the heat flux losses to the surroundings were reduced by up to 90 % versus a case where a thinner mold was used. In addition, an analytical solution was derived for the mold thickness problem from which it was found that at a distance of 10 cm from the mold cavity there was a negligible increase in temperature of the sand at that location at large times. Further, the minimum mold thickness was determined based on the temperature profile in the sand mold during the HiPerMag casting process. This study showed that a thin mold of about 2 cm thickness was sufficient to provide insulation to the hot metal during the HiPerMag casting process. Thirdly, it was found that, based on cooling curve data and microstructural analysis, that a jet spacing of 15.3 cm and a time delay of 7.4 s between successive jet activations starting from the farthest jet (located near the edge of the casting), was necessary to obtain a single solidification front throughout the casting. This also ensured that grain size variation in the casting was less than 10 μm for having uniform mechanical properties. Also, it was found for a thin walled casting, the amount of solid present in the solidifying casting at the time of water jet impingement had a negligible effect on the movement of the solidification interface. Finally, the effect of jet momentum on surface defects was examined. It was determined that the maximum jet momentum resulting in no surface defects at temperatures close to the liquidus for Al AA 6061 alloy was approximately 2 kg.m/s. / Thesis / Master of Applied Science (MASc)

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.

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.

"Research and development of magnesium casting alloys depends largely on the metallurgist’s understanding and ability to control the microstructure of the as-cast part. Currently few sources of magnesium solidification information and as-cast microstructures exist. Therefore, the goal of this research is to increase the general knowledge base of magnesium solidification behavior and to characterize the resultant microstructures. Equipment has been developed and constructed to study the solidification behavior of magnesium-aluminum casting alloys via non-equilibrium thermal analysis and continuous torque dendrite coherency measurements. These analyses have been performed on six magnesium-aluminum alloys, including industry dominant alloys such as AM60 and AZ91E, and experimental alloys which show commercial potential such as AXJ530. The resultant microstructures have been characterized for general microstructure trends and the various phases present were analyzed using optical and scanning electron microscopy, as well as energy dispersive x-ray spectroscopy. The measurements were performed using a cooling rate on the order of 1-2°C/s, and results of these analyses show that in general, magnesium-aluminum casting alloys have relatively large solidification ranges, non-dendritic microstructures, and coherency points that are similar to those of aluminum casting alloys. These results should prove useful for research directed towards development of new magnesium alloys that are targeted for specific applications, as well as for optimizing casting procedures for Mg-Al alloys to obtain defect free cast structures."

No description available.

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.

Thèse (M.Eng.) -- Université du Québec à Chicoutimi, 2008. / La p. de t. porte en outre: Mémoire présenté à l'Université du Québec à Chicoutimi comme exigence partielle de la maîtrise en ingénierie. CaQQUQ Bibliogr.: f. [147]-152. Document électronique également accessible en format PDF. CaQQUQ

Mémoire (M.Eng.)--Université du Québec à Chicoutimi, 2001. / Document électronique également accessible en format PDF. CaQCU

Les alliages Ti-Al sont depuis peu utilisés industriellement dans le domaine aéronautique. Il est nécessaire d’affiner les grains dans ces alliages en évitant la formation des précipités. Une nouvelle méthode d’inoculation appelée inoculation isomorphe a été développée où les particules agissent comme des sites de croissance plutôt que comme des sites de germination, évitant ainsi la barrière d’énergie pour la germination. Trois alliages inoculants ont été développés, les deux premiers, Ti-10Al-25Nb et Ti-25Al-10Ta, allient une bonne cohérence du paramètre de maille avec l’alliage de base et une bonne stabilité dans le liquide. Le troisième, Ti-47Ta, met en avant l’aspect stabilité. Les coulées ont montré que les premiers deux alliages ont affiné les grains sans laisser de particules hétérogènes. L’alliage binaire Ti-Ta a une densité trop importante et les particules ont sédimentées dans le lingot où elles n’ont pas pu jouer leur rôle. L’efficacité des inoculants est supérieure à l’unité, chaque particule étant responsable de la formation de plus d’un nouveau grain. Ce dernier effet est attribué à la polycristallinité des inoculants qui peuvent se fragmenter par dissolution préférentielle aux joints de grains. Les calculs prenant en compte la fragmentation et la dissolution indiquent des efficacités proches de l’unité, ce qui confirme les valeurs expérimentales d’efficacité anormalement élevées / Ti-Al alloys are an important material for aerospace applications. In order to implement them in more applications it is important to develop a method of grain refinement which can avoid precipitates. A new method of inoculation called isomorphic inoculation was developed where inoculant particles act as direct centers of growth rather than nucleation sites, avoiding the energy barrier required for nucleation. Three inoculant alloys were tested, two which balanced lattice matching between the inoculant and bulk alloy and the inoculant stability in the liquid melt, Ti-10Al-25Nb and Ti-25Al-10Ta, and one which prioritized stability, Ti-47Ta. Casting trials were carried out which showed the balanced alloys sucessfully grain refined the as-cast structure without leaving any heterogeneous particles in the structure. The binary Ti-Ta alloy was not successful due to its high density which caused the particles to settle to the bottom of the ingots where they could not participate in solidification. The inoculants were found to have an efficiency greater than one, meaning each particle was responsible for more than one new grain forming in the cast structure. This was attributed to the polycrystalline nature of the partciles which may break up into multiple particles by preferential dissolution or wetting of the grain boundaries during interaction with the melt. Calculations showed that taking into account particle break up and dissolution the efficiencies approached one, indicating this mechanism is responsible for the anomalously high efficiencies observed

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