Arvind Prasad

The Contribution of Constitutional Supercooling to Nucleation and Grain Formation

The concept of constitutional supercooling (CS) including the term itself was first described and discussed qualitatively by Rutter and Chalmers in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller et al. On that basis, Winegard and Chalmers further considered ‘supercooling and dendritic freezing of alloys’ where they described how CS promotes the heterogeneous nucleation of new crystals and the formation of an equiaxed zone. Since then the importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified. This paper describes our current understanding of the role of CS in promoting nucleation and grain formation. It also highlights that CS, on the one hand, develops a nucleation-free zone surrounding each nucleated and growing grain and, on the other hand, protects this grain from readily remelting when temperature fluctuations occur due to convection. Further, due to the importance of the diffusion field that generates CS, recent analytical models are evaluated and compared with a numerical model. A comprehensive description of the mechanisms affecting nucleation and grain formation and the prediction of grain size is presented with reference to the influence of the casting conditions applied during the practical casting of an alloy.

X-RAY TOMOGRAPHY STUDY OF ATOMIZED Al-Cu DROPLETS

Abstract The atomization of metals and alloys results in the rapid solidification of droplets which yields a complex interplay of nucleation undercooling, recalescence and microsegregation. Characterization of the solidified microstructure has been studied using conventional optical and scanning electron microscopy (SEM). These techniques provide limited information as only a finite number of sections of the powder can be examined. X-ray tomography using synchrotron radiation with a 1 μm beam resolution was used to generate a 3D image for the entire volume of a particle and was compared to microscopic images. The tomography experiments were carried out on Al5%Cu and Al17%Cu atomized droplets of 500 μm in diameter. Microstructural features of solidification such as shrinkage, nucleation and recalescence which are apparent in the 3D images are discussed and compared with conventional characterization methods. L’atomisation de métaux et d’alliages résulte en une solidification rapide de gouttelettes qui produit une interaction complexe de la surfusion de nucléation, de la recalescence, de la microségrégation, etc. On a caractérisé la microstructure solidifiée en utilisant la microscopie conventionnelle optique et électronique à balayage. Ces techniques fournissent une information limitée puisque l’on peut examiner seulement un nombre défini de sections de la poudre. On a utilisé la tomographie à rayonnement cyclotron avec résolution du faisceau de 1 μm pour générer une image à 3D du volume entier d’une particule que l’on a comparée aux images obtenues par microscopie. On a effectué les expériences de tomographie avec des gouttelettes atomisées d’Al-5%Cu et d’Al- 17%Cu, ayant 500 μm de diamètre. On discute des caractéristiques de la microstructure de solidification comme la contraction, la nucléation, la recalescence, etc. qui sont apparentes dans les images à 3D et on les compare avec les méthodes conventionnelles de caractérisation.

Experimental Determination of Heat Transfer Across the Metal/Mold Gap in a Direct Chill (DC) Casting Mold—Part I: Effect of Gap Size and Mold Gas Type

An experimental apparatus to determine the heat-transfer coefficient in the gap formed between the cast metal and the mold wall of a vertical direct chill (DC) casting mold is described. The apparatus simulates the conditions existing within the confines of the DC casting mold and measures the heat flux within the gap. Measurements were made under steady-state conditions, simulating the steady-state regime of the DC casting process. A range of casting parameters that may affect the heat transfer was tested using this apparatus. In the current article, the operation of the apparatus is described along with the results for the effect of gas type within the mold, and the size of the metal-mold gap formed during casting. The results show that the gas type and the gap size significantly affect the heat transfer within a DC casting mold. The measured heat fluxes for all the conditions tested were expressed as a linear correlation between the heat-transfer coefficient and the metal-mold gap size, and the fluxes can be used to estimate the heat transfer between the metal and the mold at any gap size. These results are compared to values reported in the literature and recommendations are made for the future reporting of the metal/mold heat-transfer coefficient for DC casting. The results for the effect of the other parameters tested are described in Part II of the article.

Mould slag property measurements to characterize CC mould – shell gap phenomena

Abstract Multi-faceted experiments were conducted to measure the properties of several mould slags needed for the fundamental characterization of heat transfer and friction in the interfacial gap between the shell and mould during the continuous casting (CC) of steel. A novel apparatus was used to measure the friction coefficient between solidified mould flux and copper at elevated temperatures. The measured softening temperature is interpreted as the glass transition temperature and is used to extrapolate the slag viscosity-temperature curves far into the low temperature – high viscosity region. Continuous-cooling transformation curves were extracted from X-ray diffraction (XRD) analysis of differential scanning calorimetry (DSC) test samples and thermocouple dip tests. Time-temperature transformation curves were obtained from a similar analysis of melted mould powder samples that were atomized into droplets, quenched to form glass and then partially devitrified by reheating to different temperatures for different times and quenched. Polarized light microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis revealed distinct crystalline and glassy layers, but no severe macrosegregation in a tail-out slag film taken from an operating caster. The results from these new measurements have important implications for the prediction of interfacial gap phenomena including mould heat transfer, friction, slag layer fracture and steel surface quality. On a effectué des expériences à facettes multiples pour mesurer les propriétés des scories de plusieurs moules nécessaires à la caractérisation fondamentale de la transmission de chaleur et de la friction dans l’intervalle interfacial entre la coquille et le moule pendant la coulée continue (CC) de l’acier. On a utilisé un nouvel appareil pour mesurer le coefficient de friction entre le fondant solidifié du moule et le cuivre à des températures élevées. On interprète la température mesurée du ramollissement comme étant la température de transition vitreuse et on l’utilise pour extrapoler les courbes de viscosité–température de la scorie bien avant dans la région de basse température–haute viscosité. On a extrait les courbes de transformation par refroidissement continu à partir de l’analyse par diffraction des rayons X (XRD) des échantillons d’évaluation de calorimétrie différentielle à compensation de puissance (DSC) et à partir d’évaluation de trempes de thermocouple. Les courbes temps–température– transformation ont été obtenues à partir d’une analyse similaire d’échantillons fondus de poudre de moule que l’on a atomisés en gouttelettes, trempés pour former un verre et ensuite partiellement dévitrifiés en re-chauffant à différentes températures pour des durées variées, et ensuite trempés. La microscopie à lumière polarisée, la microscopie électronique à balayage (SEM) et l’analyse des rayons X à dispersion d’énergie (EDX) ont révélé des couches distinctes, cristalline et vitreuse, mais pas de macro–ségrégation importante dans un film de scorie d’aval pris dans une machine opérationnelle de coulée. Les résultats de ces nouvelles mesures ont des implications importantes dans la prédiction des phénomènes d’intervalle interfacial incluant la transmission de chaleur du moule, la friction, la rupture de la couche de scorie et la qualité de la surface de l’acier.

Real-time synchrotron x-ray observations of equiaxed solidification of aluminium alloys and implications for modelling

Recently, in-situ observations were carried out by synchrotron X-ray radiography to observe the nucleation and growth in Al alloys during solidification. The nucleation and grain formation of a range of Al-Si and Al-Cu binary alloys were studied. When grain refiner was added to the alloys, the location of the nucleation events was readily observed. Once nucleation began it continued to occur in a wave of events with the movement of the temperature gradient across the field of view due to cooling. Other features observed were the settling of the primary phase grains in the Al-Si alloys and floating in the Al-Cu alloys, the effects of convection with marked fluctuation of the growth rate of the solid-liquid interface in the Al-Si alloys, and an absence of fragmentation. The microstructures are typical of those produced in the equiaxed zone of actual castings. These observations are compared with predictions arising from the Interdependence model. The results from this comparison have implications for further refinement of the model and simulation and modelling approaches in general. These implications will be discussed.

The grain refinement of Al-Si alloys and the cause of Si poisoning: Insights revealed by the interdependence model

Recent work has extended the Interdependence Model to the prediction of grain size of aluminium alloys when refined by Al-Ti-B master alloys. The relative contribution of Ti solute and TiB2 particles to the as-cast grain size was determined indicating the optimum level of addition required to provide cost effective use of master alloy. This paper quantifies the effect of Al5Ti1B master alloy additions on the grain size of Al-Si alloys. The Al-Si system is a special case where additions of Si above a few percent poison grain refinement resulting in larger grain sizes. Consideration of the complicating effect of Si poisoning on the prediction of the grain size of these alloys and possible approaches to dealing with these complications are presented.

Improvement of the interdependence analytical model through selection of interfacial growth rates during the initial transient

The Interdependence Theory is based on the concept that grain refinement of an alloy during the initial transient of solidification is governed by the simultaneous occurrence of growth of an already nucleated grain and the associated Constitutional Supercooling (CS) developing ahead of the growing grain. Nucleation of a new grain occurs when the maximum CS attained at the end of the diffusion field of the solute rejected in front of the growing grain, is larger than the nucleation undercooling DTn of an inoculant particle present ahead of this point. The amount of grain growth plus the length of the diffusion field constitute the Nucleation-Free Zone (NFZ). The mathematical form of this theory is an analytical model which has been shown to be sensitive to the value of the growth rate v of the solid-liquid interface chosen to predict the as-cast grain size. This work presents the development and application of a varying growth rate expression rather than a constant growth rate used in the analytical model to improve its agreement with mMatIC, a numerical model of solidification which has been shown to effectively model grain growth and the evolution of the diffusion field. The possible range of values of growth rate that occurs during initial growth of a grain is obtained from numerical simulations. The resulting modified analytical model is compared against the numerical model for assessing the extent of NFZ formed.

The Measurement of Heat Flow Within a DC Casting Mould

The heat flow between the molten metal and the mould-wall in DC casting is often assumed to be negligible compared to that due to the sub-mould water cooling. Furthermore, the entire DC casting process is often descnbed based on this assumption. However, the assumption of negligible heat transfer in the metalmould region, as oompared to the sub-mould region, and its subsequent minimal influence on cast product quality remains unproven. The focus of the present paper is therefore on understanding the heat transfer in the metal/mould wall region. To this end a method for the laboratory measurement of the flow of heat from the metal being cast to the wall of a DC casting mould has been developed. The equipment and methodology are briefly described together with some of the initial results obtained. The implications of this work for use in simulation models and for the design and operation of OC casting moulds are disct1ssed.

Modelling of the Metal/Mould Interactions in the DC Casting Process for Aluminium and Magnesium

The process of direct chill (DC) casting of aluminium and magnesium alloys is regarded as a mature technology. The thrust of more recent work to understand and upgrade the technology has been centred on developing models of the process, the most advanced of which (e.g., Alsim and Calcasoft) have been used to examine what may be considered macro-features of the process (macro-segregation, hot cracking, etc.). These models, being macroscopic, rarely elaborate on the role of mould-wall heat transfer in the DC casting process. As part of the work on DC casting being conducted at CAST, for the investigation of small scale features of the process (e.g. heat extraction through the mould wall), a 2D finite Difference model of the process near the mould-wall region has been developed. The basic features of the model are described and initial results outlined.In particular, the effect of mould-wall heat transfer on the solid shell formed during the steady state regime of DC casting will be presented.

The Challenges Associated with the Formation of Equiaxed Grains during Additive Manufacturing of Titanium Alloys

It is well established that columnar grain structures usually form when metal alloys are used to additively manufacture components. A challenging goal is to produce an equiaxed grain structure throughout the component to remove anisotropy and refine the grain size in order to improve its mechanical performance. The high cooling rates and associated high temperature gradients are the main reasons for the formation of columnar grains via epitaxial growth of each added layer of material. There appear to be limited strategies for promoting equiaxed nucleation of grains. In addition to cooling rate and temperature gradient, we explore other variables such as the potency of natural or added inoculant particles and the composition of the alloy, and their possible impact on the nucleation of equiaxed grains. Although changing the composition can help, finding a suitably potent nucleant particle is a major challenge. Operating parameters can also influence the microstructure and optimization to produce equiaxed grains may be possible. The limitations of these strategies and possible ways to overcome them are evaluated.