Early Stages Of Solidification Research Articles

Morphological Instability in Early Stages of Solidification of Pure Metals Part I: A Theoretical Model

A simple one dimensional model has been introduced to investigate the morphological instability observed in many solidification processes. It is shown that the solidified shell material with higher thermal conductivity might result in planar shell growth, whereas the mold material with higher thermal conductivity may cause irregular growth of the shell which, generally, causes cracking near the surface, and the thicker mold causes faster growth of the shell, and the higher thermal contact resistance leads to faster growth of the shell.

Influence of Particle Size on Ice Nucleation and Growth During the Ice‐Templating Process

The solidification behavior of suspensions of alumina particles during directional solidification is investigated here by in situ observations using X‐ray radiography and tomography. The objective of this study was to assess the influence of particle size on the solidification behavior of the suspensions during the early stages of solidification. Four powders with particle size in the range of 0.2–3.4 μm (median size) were investigated. Solidification is obtained by cooling at a constant rate, starting from room temperature. Attention is specifically paid to the nucleation and growth behavior of the ice crystals in these suspensions. We propose that the nucleation of ice crystals is controlled by the particle size, the surface of the particles acting as nucleation sites. Smaller particle size leads to a lower degree of supercooling because nucleation and growth can proceed at a higher temperature than with larger particles. The initial interface velocity is dependent on the degree of supercooling, and controls the extent of the initial structural gradient in the resulting porous materials.

On The Formation of Extruded Surface Segregation Layer in Aluminum Direct Chill Casting Process

The aim of this research work was to develop a mathematical model to study the formation of extruded surface segregation phenomenon during the early stages of solidification in an aluminum direct chill casting process. The mathematical analyses contain a one dimensional mathematical model of heat flow, solidification, interdendritic strain and macrosegregation. Some typical cases in conventional direct chill casting processes related to increase surface segregation and fluctuation in macrosegregation with distance from the ingot surface were simulated. Good agreement was found by previous measurements and simulated results. Model results were discussed to explain the effects of different thermal solidification factors and interdendritic strain on the extruded surface segregation layer phenomenon. The results show that this phenomenon is sensitive to alloy composition and forms in the gap between the ingot surface and mould wall. Also, the model results point out that this phenomenon appears as a direct effect of the extrusion mechanism of interdendritic solute liquid associated with the interdendritic strain distribution in the coherent region in the mushy zone.

Effect of Casting Condition on Density and Hardness Gradients of Al-Al<sub>2</sub>Cu Alloy FGM Fabricated by Centrifugal <i>In Situ</i> Method

Functionally graded material (FGM) is a combined material that has a component gradient from one material at one surface to another material at the opposite surface. As one of the fabrication processes of FGM, centrifugal in-situ method has been proposed. Centrifugal in-situ method is a casting that centrifugal force is applied during solidification to both the primary crystal and the matrix. In a previous study, the density and hardness gradients of Al-3mass%Cu FGM ring fabricated by centrifugal in-situ method have been investigated. According to the study, Cu concentration within the FGM ring monolithically increases towards the ring's inner position, and its density also increases toward inner region. This is because the density of the primary -Al crystal is larger than that of the molten Al-Cu alloy in the early stage of solidification. Based on this solidification process, it is considered that the casting condition and the initial Cu concentration of Al-Cu master alloy affect on the density and hardness gradients in the Al-Cu FGM ring. In this study, effects of the casting condition on the density and hardness gradients of Al-Al2Cu FGM rings fabricated by the centrifugal in-situ method were investigated. It was found that density gradient of the Al-Al2Cu FGM rings increases with increasing Cu concentration of Al-Cu master alloys. Also, processing temperature for Al-Cu master alloy can control density gradient of Al-Al2Cu FGM rings. These phenomena were explained by variation of the densities of primary -Al and the molten Al matrix during the solidification.

In situ analysis of dendritic equiaxed microstructure formation in Al-Cu alloys by synchrotron X-ray radiography

This paper reports on experiments dedicated to equiaxed solidification carried out on Al — 10 wt% Cu alloy at the European Synchrotron Radiation Facility (ESRF) in Grenoble-France. Equiaxed growth was achieved in nearly isothermal conditions and observed continuously in real time from the early stages of solidification to the developed grain structure by X — ray radiography. The length of primary dendrite arms was measured on several growing equiaxed grains as a function of time and it was found that dendrite arm evolution can be well fitted by the Kolmogorov — Johnson — Mehl — Avrami (KJMA) function. The growth of four primary dendrite arms of a single grain was then characterized. The dendrite arms behaved differently depending on their growth directions and the presence of grains in their vicinities. At the beginning, the dendrite arm directed upwards was growing faster than the ones growing below because of “self — poisoning” due to a gravity driven fluid flow. In the late stage of solidification, analysis on several couples of grains growing towards each other confirmed that solutal interaction was the main cause of growth being stopped.

Application of synchrotron X-ray radiography to the study of dendritic equiaxed microstructure formation in Al–Cu alloys

The phenomena involved during equiaxed growth, which is the most common growth morphology in metal casting, are dynamic. Therefore, in situ investigation is necessary to fully analyse the microstructure formation. Synchrotron X-ray radiography has become a major tool for in situ characterization of solidification in metallic systems. This paper reports on dedicated equiaxed solidification experiments carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble-France on Al–10 wt.% Cu alloy. The analysis is based on the recorded images obtained through absorption radiography. Equiaxed growth is achieved in nearly isothermal conditions and observed continuously in real time from the early stages of solidification to the final state. The evolution of the dendrite arm lengths and the corresponding growth rates are analysed for two couples of grains in relation with their environment. This analysis suggests that the solutal interaction is dominant between equiaxed grains.

Dynamic wetting and heat transfer at the initiation of aluminum solidification on copper substrates

Dynamic wetting and heat transfer during the start of solidification were studied with the help of molten aluminum droplets falling from a crucible onto a copper substrate. A high-speed camera captured the change in the spreading droplet’s geometry, while thermocouple, inserted inside the substrate, allowed a heat transfer analysis to be performed. Droplet spreading factors and interfacial heat fluxes were then used to, respectively, characterize dynamic wetting and heat transfer for the various experimental conditions explored. These were: (1) effects of chemical composition of the aluminum alloy, (2) initial temperature of the substrate, (3) surface roughness of the substrate, and (4) composition of the gaseous atmosphere. The experiments were all carried out in gaseous atmospheres containing oxygen in sufficient amount to form oxide skins at the surface of the droplets and the substrates. The results showed instances where an improvement in the dynamic wetting was accompanied by an increase in heat transfer during the early stages of solidification but this was not systematic. In these cases where a positive correlation was not observed, it was postulated this was caused by factors such as variations in the oxidation at the surface of the substrates and the droplets as well as gas trapped at the interface between the droplets and the substrates.

High-Temperature Phase Transitions and Mechanical Properties in Steel of Peritectic Composition

Continuous casting of steels of peritectic composition has proven to be problematic mostly because of surface crack occurrence. We have studied in situ the peritectic reaction as well as the kinetics of the subsequent peritectic transformations using the so-called concentric solidification technique in a high-temperature laser-scanning confocal microscope. We have also assessed grain growth in reheated as well as in situ cast Fe-C specimens in a Gleeble 3500 thermo-mechanical simulator. Our findings will be discussed with reference to the observation in practice that so-called 'blown-out' grains are associated with oscillation marks in continuously cast slabs. Background It is generally accepted that the surface quality of continuously cast steel slabs is a strong function of events occurring during the early stages of solidification in the meniscus region of the mould. Brimacombe and Sorimachi [1] provided a comprehensive classification of the origin of cracks in the continuous caster and McPherson and McLean [2] followed by Wolf [3], summarised much of the work relating to transverse cracking. Peritectic steels are the most difficult to produce with respect to surface quality [2-5], most probably because of the volume change that accompanies this phase transition [5,6]. Ludlow et al [6], using a technique introduced by Hiebler, Bernard and Xia [7,8] , confirmed that the maximum crack susceptibility is encountered between 0.1% and 0.14% carbon equivalent.

Density and Hardness Gradients of Functionally Graded Material Ring Fabricated from Al-3 mass%Cu Alloy by a Centrifugal <I>In-Situ</I> Method

In present study, the Al-Al2Cu functionally graded material (FGM) ring was fabricated from Al-3 mass%Cu initial master alloy by the centrifugal in-situ method. In the case of Al-3 mass%Cu alloy, the density of the primary � -Al crystal is larger than that of the molten Al alloy. Therefore, the solid � -Al phase migrates towards the outer periphery of the ring when the centrifugal force is applied in the early stage of solidification. Consequently, since the Cu concentration within the FGM ring monolithically increases towards the ring’s inner position, the FGM ring, whose density increases toward inner region, can be successfully fabricated by the centrifugal in-situ method from dilute Al-Cu alloy. It is also found that the hardness increases towards the inner region of the ring within the Al-Al2Cu FGM ring. The hardness of the fabricated specimens at the inner region of the ring increases in a large scale by the heat treatments, since Guinier-Preston (GP) zones would be formed by aging. [doi:10.2320/matertrans.MB200710]

Homogeneity and Equiaxed Grain Structure of Billets

The homogeneity of as-cast structures is essential to the application of advanced steel materials. The relationship between as-cast equiaxed grain structure and homogeneity was examined by means of OPA (Original Position Analysis) technology. The results indicate that macrosegregation of the solute elements was obviously suppressed in silicon steel billets with equiaxed grain structure. Furthermore, two methods to achieve an equiaxed grain structure were described. One is to use titanium-based inoculation technology. TiN particles and TiN/ Ti2O3 complex particles can precipitate at the early stage of solidification and then play an important role of heterogeneous nucleation when the solidification condition is carefully controlled. The other method is to maintain a small temperature gradient in the mould to enhance the constitutional undercooling As a result, the as-cast equiaxed grain structure was well developed in the experimental billets.

A coupled thermomechanical, thermal transport and segregation analysis of the solidification of aluminum alloys on molds of uneven topographies

A coupled thermomechanical, thermal transport and segregation analysis of aluminum alloys solidifying on uneven surfaces is presented here. Uneven surfaces are modelled as sinusoids with different wavelengths and amplitudes. Effects of various coupling mechanisms between the solid-shell deformation, air-gap formation, heat transfer, fluid flow and segregation, near the mold-metal interface, are observed for different mold topographies during the early stages of solidification of an aluminum alloy. The role of inverse segregation, arising from shrinkage driven flow in the melt, melt superheat and varying mold surface topography on nucleation of air-gaps and evolution of stresses in the solidifying shell is examined. The numerical model consists of a volume-averaged solidification model coupled with a small-deformation model combining elasto-viscoplastic deformation in the solidifying shell with air-gap nucleation and imperfect contact at the metal/mold interface. Heat transfer at the mold-metal interface is either contact pressure or air-gap dependent and is modelled using the actual contact pressure or air-gap size obtained from the contact sub-problem at the metal-mold interface. Variation in heat transfer leads to variations in fluid flow, segregation and stresses developing in the solid and mushy-zone, which in turn affect the morphology of the growing solid-shell. A wavelength range that leads to a reduction in equivalent stresses, segregation and growth front morphology unevenness, in the evolving solid-shell, is obtained for varying solute concentrations. One of the main objectives of the current analysis is to seek optimal mold surface topographies that minimize surface defects leading to desired cast surface morphologies.

A thermomechanical study of the effects of mold topography on the solidification of Aluminum alloys

A thermomechanical study of the effects of mold topography on the solidification of Aluminum alloys at early times is provided. The various coupling mechanisms between the solid-shell and mold deformation and heat transfer at the mold/solid-shell interface during the early stages of Aluminum solidification on molds with uneven topographies are investigated. The air-gap nucleation time, the stress evolution and the solid-shell growth pattern are examined for different mold topographies to illustrate the potential control of Aluminum cast surface morphologies during the early stages of solidification using proper design of mold topographies. The unstable shell growth pattern in the early solidification stages results mainly from the unevenness of the heat flux between the solid-shell and the mold surface. This heat flux is determined by the size of the air-gaps formed between the solidifying shell and mold surface or from the value of the contact pressure. Simulation results show that a sinusoidal mold surface with a smaller wavelength leads to nucleation of air-gaps at earlier times. In addition, the unevenness in the solid-shell growth pattern decreases faster for a smaller wavelength. Such studies can be used to tune mold surfaces for the control of cast surface morphologies.

Nucleation of bubbles on a solidification front—experiment and analysis

The heterogeneous nucleation of bubbles on an advancing solidification front during the freezing of water containing a dissolved gas has been experimentally and analytically studied. The formation of bubbles resulting from supersaturation of liquids is commonly encountered in different fields such as heat transfer, manufacturing, and bioscience. In this work, the sizes of nucleating bubbles and the concentration profiles of dissolved oxygen and carbon dioxide gases in the water ahead of the solidification front have been measured. From successful comparisons between the measured and predicted critical radii of nucleating bubbles and distributions of dissolved gas content, the phenomena of heterogeneous nucleation in a binary weak solution during the freezing process are quantitatively confirmed. The results show that an increase in gas content at the solidification front in the liquid decreases the free-energy barrier and critical radii of bubbles that are formed on the solidification front. The sizes of the critical radii decrease and the number of nucleating bubbles increase in the early stage of solidification. As the solidification rates decrease at longer times, the content of the dissolved gas in the liquid on the advancing interface decreases and the critical radii of nucleating bubbles increase.

G155 一方向凝固過程におけるソルトフィンガー対流の定量的評価

The present experiment is conducted to examine the evolution of salt-finger convection during unidirectional solidification of NH_4Cl-H_2O system. We perform flow visualization and velocity measurement. The salt-finger convection experiences the dynamical behavior in the early stages of solidification, i.e., the transition of longitudinal motion to lateral motion. The velocity of upward flow is larger than that of downward flow.

モンテカルロ・差分ハイブリッドモデルと計算熱力学連成による凝固組織シミュレーション

In order to predict solidification morphology of multi component materials, hybrid computational modeling, which consist of deterministic finite difference method for the heat transfer and the solutes diffusions and stochastic Monte Carlo method for nucleation and crystal growth, was developed to be closely incorporated with computational thermodynamics. This model was found to be capable of simulating the columnar to equiaxed transition (CET) macrostructures of ternary ferritic stainless steels and predicting the solutes concentration dependency of the morphology vary accurately. By using very small finite difference meshes and Monte Carlo cells this model was also found to be capable of representing the development of the dendritic crystal growth of early stage of solidification. These results demonstrated the wide space scale applicability of this model to solidification phenomena. Furthermore, by incorporating stochastic models for solid state transformation, it was attempted to develop computational modeling for simulating more complex solidification morphology involving δ solidification, γ solidification and δ⁄γ transformation.

Solidification behavior of Sn-15 wt pct Pb alloy under a high shear rate and high intensity of turbulence during semisolid processing

Microstructural evolution under intensive forced convection is of importance to the development of new semisolid processing technologies. Although efforts have been made to understand the nucleation and growth behavior of the primary phase under forced convection, important aspects of microstructural evolution still remain as open questions. Experimental work was undertaken to investigate the effects of a high shear rate and high intensity of turbulence on the solidification behavior of a Sn-15 wt pct Pb alloy using a twin-screw extruder. It was found that increasing the shear rate and intensity of turbulence can give rise to substantial grain refinement, which can be attributed to the increased effective nucleation rate caused by the extremely uniform temperature and composition fields in the bulk liquid at early stages of solidification. It was also found that forced convection increases the growth velocity and stabilizes the solidification interface, resulting in a transition of the growth morphology from rosette to spheroid with increasing shear rate and intensity of turbulence. Discussions are made on the effects of flow conditions on the nucleation behavior, constitutional undercooling, stability of the solidification interface, and growth morphology.

Effects of substrate surface conditions on heat transfer and shell morphology in the solidification of a copper alloy

The effect of substrate surface conditions on the heat transfer and morphology of solidifying shells of a copper alloy has been characterized by a series of dip tests. Results showed that in the early stages of solidification, rough and grooved surface conditions provide a greater heat transfer than polished ones. The shells solidified on rough and grooved surfaces had significant localized variations in thickness, indicating that heat transfer at the substrate-metal interface was not uniform. The implication of these results on strip casting is discussed.

Finite element numerical simulation on thermo-mechanical behavior of steel billet in continuous casting mold

The thermo-mechanical behavior of a thin, growing shell during the early stages of solidification in a continuous casting mold is very important to the ultimate quality of the final billet. A two-dimensional, transient finite element model has been developed to treat the heat flow and deformation of the solidifying shell in the continuous casting billet mold as a coupled phenomena. The major application of the model is to predict the extent of the gap between the mold and the shell and focus on the influence of mold taper on the thermo-mechanical behavior of the steel billet to help to understand the formation of off-corner cracks and break-outs in the solidifying shell. The calculations indicate that the gap is initially formed at the corner of the billet, where heat transfer is greatly reduced. Insufficient mold taper contributes to a hot spot in the off-corner region, which corresponds to the lowest shell thickness. At the same time, the solidifying front on the diagonal of the billet is subjected to an excessive mechanical strain, which causes the off-corner cracks and even the break-outs.

A model for texture development in BSCCO high-Tcsuperconductors

Textured BSCCO superconductors are studied and the variousmechanisms for alignment of BSCCO grains addressed. To date, surface energyeffects leading to texture development of BSCCO superconductors have beenconsidered only with respect to the free surface of melt-processed Bi-2212thick films. However, these previous efforts have not included the surfaceinteractions between BSCCO crystals and other solid surfaces present in theBSCCO system. In the present work, a model based on an interfacial energyrelation is proposed. In order to verify the model a variety of experimentshave been performed. The observation of the microstructure in different BSCCOsamples corroborates this model. The following discussion offers a plausibleexplanation for the various observed phenomena during the partial meltprocess. During the early stages of solidification, when the peritectic liquidis abundant, BSCCO crystals are rather mobile, facilitating their contact andinteraction. As a result, if a crystal has its wide planar c-surface incontact with a foreign surface (e.g. silver substrate, secondary phases, freesurface or another BSCCO crystal), it can minimize its surface energy and islikely adhere to that surface. This mechanism, applied to a system with planarconstraints, such as a 2212-Ag film, will result in a textured sample,depending on the thickness of the superconducting layer. In a bulk samplehowever, BSCCO crystals may only minimize their surface energy by adhering toother BSCCO crystals, which will consequently form clusters of locally alignedcrystals, i.e. colonies, with no long-range texture. In this fashion, we mayalso address the role of silver in promoting texture development in BSCCOsuperconductors.

Wear of Roll Surface in Twin-roll Casting of 4.5% Si Steel Strip.

The rolls are one of the most important components of the twin-roll casting process, determining not only strip quality, but due to their expense, add to the price of the as-cast strip. The potential for reducing operational cost and ensuring required strip quality may be achieved through a reduction in wear of the roll surface and hence reduced frequency of roll surface reconditioning and replacement. To achieve this, it is imperative to understand the mechanisms of roll surface wear which occur during casting operations.The present study was aimed at determining the influence of liquid metal, roll separation force and strip incline from the vertical casting direction on the wear of the roll surface in a twin-roll casting process.It was found that roll wear in the twin-roll casting process is accompanied by removal of material from the roll surface and redistribution of topography of the roll surface. Applying high roll separation forces lead to increase erosion of the roll surface and coarsening of the roll surface topography. Roll surface wear caused by deformation occurs by imprinting of the strip surface on the roll surface as well as grinding of the roll surface by the strip. Even with the application of high roll separation forces the highest contribution to wear of the roll surface is by the interaction of the melt with the roll surface in the very early stages of solidification.