Multiscale modelling of the influence of convection on dendrite formation and freckle initiation during vacuum arc remelting

Vacuum Arc Remelting (VAR) is employed to produce homogeneous ingots with a controlled, fine, microstructure. It is applied to reactive and segregation prone alloys where convection can influence microstructure and defect formation. In this study, a microscopic solidification model was extended to incorporate both forced and natural convection. The Navier-Stokes equations were solved for liquid and mushy zones using a modified projection method. The energy conservation and solute diffusion equations were solved via a combined stochastic nucleation approach along with a finite difference solution to simulate dendritic growth. This microscopic model was coupled to a 3D transient VAR model which was developed by using a multi-physics modelling software package, PHYSICA. The multiscale model enables simulations covering the range from dendrites (in microns) to the complete process (in meters). These numerical models were used to investigate: (i) the formation of dendritic microstructures under natural and forced convections; (ii) initiation of solute channels (freckles) in directional solidification in terms of interdendritic thermosolutal convection; and (iii) the macroscopic physical dynamics in VAR and their influence on freckle formation. 2D and 3D dendritic microstructure were simulated by taking into account both solutal and thermal diffusion for both constrained and unconstrained growth using the solidification model. For unconstrained equiaxed dendritic growth, forced convection was found to enhance dendritic growth in the upstream region while retarding downstream growth. In terms of dimensionality, dendritic growth in 3D is faster than 2D and convection promotes the coarsening of perpendicular arms and side branching in 3D. For constrained columnar dendritic growth, downward interdendritic convection is stopped by primary dendritic arms in 2D; this was not the case in 3D. Consequently, 3D simulations must be used when studying thermosolutal convection during solidification, since 2D simulations lead to inappropriate results. The microscopic model was also used to study the initiation of freckles for Pb-Sn alloys, predicting solute channel formation during directional solidification at a microstructural level for the first time. These simulations show that the local remelting due to high solute concentrations and continuous upward convection of segregated liquid result in the formation of sustained open solute channels. High initial Sn compositions, low casting speeds and low temperature gradients, all promote the initiation of these solute channels and hence freckles. to study the initiation of freckles for Pb-Sn alloys, predicting solute channel formation during directional solidification at a microstructural level for the first time. These simulations show that the local remelting due to high solute concentrations and continuous upward convection of segregated liquid result in the formation of sustained open solute channels. High initial Sn compositions, low casting speeds and low temperature gradients, all promote the initiation of these solute channels and hence freckles

Liquid metal flows in continuous casting molds: A numerical study of electromagnetic flow control, turbulence and multiphase phenomena

Der Effekt eines externen Magnetfeldes auf die mehrphasige und turbulente Strömung in Stranggußkokillen und deren Wechelspiel führt in den wissenschaftlichen Arbeiten zu widersprüchlichen Aussagen. Die verschiedenen Prozessparameter können innerhalb eines kleinen Varianzbereichs entscheidenden Einfluss auf die Aussage haben, ob ein Magnetfeld begünstigend oder schädigend auf die Qualität des Produkts wirkt. Um wichtige Einflussfaktoren zu identifizieren, werden daher numerische Strömungssimulationen des Prozesses durchgeführt. Dazu wird zunächst ein mehrphasiger und inkompressibler Mehrregionen-CFD-Löser für magnetohydrodynamische Strömungen entwickelt und validiert, um die komplexe Strömung in einer Stranggußkokille mit hoher Genauigkeit simulieren zu können. Darauf aufbauend wird das numerische Setup anhand einer Modellkokille mit aktuellen Messdaten validiert. Durch die neuartige Kombination Lagrange'scher Lösungsmethoden mit angepassten Termen für die Magnetohydrodynamik sowie der skalenaufgelösten magnetohydrodynamischen Turbulenz, können erstmals Aussagen zur optimalen Magnetfeldverteilung im Hinblick auf Strömungsstabilität, Turbulenzmodulation und Blasenverteilung getroffen werden. Mit Hilfe dieses Wissens können neuartige Konzepte elektromagnetischer Bremssysteme für den Stranggußprozess entwickelt werden

The effect of electrostatic charges on particle-laden duct flows

We report on direct numerical simulations of the effect of electrostatic charges on particle-laden duct flows. The corresponding electrostatic forces are known to affect particle dynamics at small scales and the associated turbophoretic drift. Our simulations, however, predicted that electrostatic forces also dominate the vortical motion of the particles, induced by the secondary flows of Prandtl's second kind of the carrier fluid. Herein we treated flows at two frictional Reynolds numbers ($Re_\mathrm{\tau}=$ 300 and~600), two particle-to-gas density ratios ($\rho_\mathrm{p}/\rho=$ 1000 and 7500), and three Coulombic-to-gravitational force ratios ($F_\mathrm{el}/F_\mathrm{g}=$ 0, 0.004, and 0.026). In flows with a high density ratio at $Re_\mathrm{\tau}=$ 600 and $F_\mathrm{el}/F_\mathrm{g}=$ 0.004, the particles tend to accumulate at the walls. On the other hand, at a lower density ratio, respectively a higher $F_\mathrm{el}/F_\mathrm{g}$ of 0.026, the charged particles still follow the secondary flow structures that are developed in the duct. However, even in this case, the electrostatic forces counteract the particles' inward flux from the wall and, as a result, their vortical motion in these secondary structures is significantly attenuated. This change in the flow pattern results in an increase of the particle number density at the bisectors of the walls by a factor of five compared to the corresponding flow with uncharged particles. Finally, at $Re_\mathrm{\tau}=$ 300, $\rho_\mathrm{p}/\rho=$ 1000, and $F_\mathrm{el}/F_\mathrm{g}=$ 0.026 the electrostatic forces dominate over the aerodynamic forces and gravity and, consequently the particles no longer follow the streamlines of the carrier gas

Numerical simulation of the shear stress produced by the hot metal jet on the blast furnace runner

During steel casting process a jet of molten metal runs out of the blast furnace hearth and strikes the runner. The continuous impact of hot fluids causes significant damage to its surface, which is made of refractory concrete. In particular, the initial impact on the dry runner is expected to be critical. This work deals with the analysis of the mechanical impact on the runner through the numerical simulation of the process. We propose an incompressible turbulent isothermal Navier-Stokes model, where turbulence is modelled considering two models (standard and SST). The interface dynamics is described by applying the Volume of Fluid (VOF) method, while the surface tension vector is provided by the Continuum Surface model (CSF). Their numerical results are performed in 2D. A comparative analysis of the most suitable transient turbulent multiphase model is presented by simulating benchmark physical experiments. The shear stress arising from the impact of the jet on the runner is also analyzed. An improvement of the classical analytical expression given in [1] is proposed. Both, the chosen turbulence model, and the formulas to compute the shear stress are validated using two benchmark laboratory tests and three numerical experiments. Numerical results are given for the impact of the jet on the dry runner of the blast furnaceThis work was supported by FEDER and Xunta de Galicia [grant number ED431C 2017/60, ED431C 2021/15], the Ministry of Economy, Industry and Competitiveness through the Plan Nacional de I+D+i [grant number MTM2015-68275-R], Agencia Estatal de Investigación [PID2019-105615RB-I00/AEI/10.13039/501100011033] and by the Vicerreitoría de Investigación e Innovación da Universidade de Santiago de Compostela via the Programa de Becas de Colaboración en Investigación 2016S

Modelling of defects in aluminium cast products

Over the last 4 decades, remarkable progress has been made in the modelling of casting processes. The development of casting models is well reflected in the proceedings of the 15 Modelling of Casting, Welding and Advanced Solidification Processes (MCWASP) conferences that have been held since 1980. Computer simulations have enabled a better understanding of the physical phenomena involved during solidification. Modelling gives the opportunity to uncouple the physical processes. Furthermore, quantities that are difficult or impossible to measure experimentally can be calculated using computer simulations e.g. flow patterns and recalescence. However, when it comes to accurately predicting casting performance and in particular, the occurrence of defects like cracks, segregation and porosity there is certainly some way to go. In this paper, the current understanding of the main mechanisms of defect formation during shape and DC casting processes will be reviewed and requirements will be discussed to give a direction to making casting models more predictive and quantitative

Exploring semi-solid alloy deformation with discrete element method simulations and synchrotron radiography

Semi-solid alloys are deformed in a wide range of pressurised casting processes; an improved understanding and modelling capability are required to minimise defect formation and optimise productivity. This thesis combines thin-sample in-situ X-ray radiography of semisolid Al-Cu alloy deformation with 2D coupled lattice Boltzmann method – discrete element method (LBM-DEM) simulations. Mechanisms of strain heterogeneity and localisation are identified during semi-solid deformation in globular Al-Cu alloys with various combinations of initial solid fraction and strain rates. The calibrated set of LBM-DEM simulations is then used to obtain information that is not available in X-ray imaging to extract deeper insights into the semi-solid deformation behaviours observed in the experiments. It is found that the local contraction and dilation of the percolating grain assembly are highly influenced by the initial solid fraction. When deforming a low solid fraction alloy, macroscopic contraction due to grains being pushed together increases the local liquid pressure and expels liquid from the sample surface. In contrast, deforming high solid fraction alloys leads to macroscopic shear-induced dilation by grains pushing each other apart and surface menisci are sucked-in due to the decrease in interstitial liquid pressure. It is also shown that the macroscopic behaviour of semi-solid alloy deformation undergoes a range of rheological transitions with increasing solid fraction. First from a suspension to a percolating solid network and, later, from net dilation to shear cracking. These transitions are investigated with LBM-DEM simulations, and the transition to shear cracking is shown to be related to the local decrease in interstitial liquid pressure caused by shear-induced dilation. The verified coupled LBM-DEM simulation is shown to exhibit a load:deformation response consistent with the critical state framework of soil mechanics, indicating that this approach can be useful for modelling thermomechanics in casting processes.Open Acces

Modeling and Simulation of Metallurgical Processes in Ironmaking and Steelmaking

In recent years, improving the sustainability of the steel industry and reducing its CO2 emissions has become a global focus. To achieve this goal, further process optimization in terms of energy and resource efficiency and the development of new processes and process routes are necessary. Modeling and simulation have established themselves as invaluable sources of information for otherwise unknown process parameters and as an alternative to plant trials that involves lower costs, risks, and time. Models also open up new possibilities for model-based control of metallurgical processes. This Special Issue focuses on recent advances in the modeling and simulation of unit processes in iron and steelmaking. It includes reviews on the fundamentals of modeling and simulation of metallurgical processes, as well as contributions from the areas of iron reduction/ironmaking, steelmaking via the primary and secondary route, and continuous casting

New insights on the fundamentals and modeling of the external sulfate attack in concrete structures

The external sulfate attack (ESA) is a complex degradation process typically compromising the durability of underground foundations, nuclear or industrial waste containments and tunnel linings exposed to sulfate solutions. The structures affected usually remain covered its entire service life, which compromises the detection of this phenomenon before severe material degradation has occurred. Once diagnosed, the large size and criticality of the typical structures affected greatly limit the efficiency of the remedial actions. Consequently, monitoring of the evolution of the structural behavior is often the only applicable measure. This scenario places the development of reliable tools to assist the design of sulfate-resisting concrete structures and assess the risk of ESA in existing properties as key challenges for structural durability. The present thesis aims to advance knowledge in this field by presenting important contributions in three different research lines: numerical modeling of the ESA, role of porosity during the attack and the relevance of reproducing field-like conditions on ESA assessments. Advances on the ESA numerical modelization led to the development of a chemo-transport-mechanical model and a simplified assessment methodology. The former simulates the effects of ionic transport, chemical reactions, degradation mechanisms and the mechanical response of the structure. The validations performed indicate that the model captures the importance of the location of the ettringite formed within the pore network and provides a fair quantification of the overall expansions. The simplified assessment methodology evaluates the risk of failure during the ESA based on the aggressiveness of the media, the reactivity and mechanical properties of the material and the geometric characteristics and service life of the element under attack, without resorting to complex iterative algorithms. Unlike current design guidelines, the application of this simplified procedure allows the definition of flexible and optimized precautionary measures for each application. The second research line involved an extensive experimental program that led to the formulation of a conceptual model to explain the role of porosity during the ESA. The results obtained indicate that high durability against the attack might be achieved by limiting the penetration of sulfates or increasing the capacity of the matrix to accommodate expansive products. Both approaches correspond to opposing pore characteristics of the matrix: the former is usually associated with low porosities while the latter requires matrices with high porosities. These results question the common perception that high porosities are always negative for ESA durability and open up the possibility to design sulfate-resisting materials by increasing the capacity of the matrix to accommodate expansive phases. The third research line evaluates the influence of early sulfate exposure and the effects of confinement on the ESA by two experimental programs. The first study suggests that the delayed exposition to sulfates commonly adopted in accelerated laboratory tests might lead to imprecise damage estimations for structures cast in situ. In these cases, it is recommended to expose the samples to sulfates shortly after casting. The second study suggests that assessing sulfate resistance on specimens in free-expanding conditions might not be representative of the behavior of real structures where the attack is developed in combination with confining conditions. Results indicate that compressive stresses generated by confinement interact with the normal development of the attack by limiting or delaying the appearance of micro-cracks and reducing the amount of ettringite crystals exerting expansive pressures.El ataque sulfático externo (ASE) es un proceso de degradación complejo que afecta principalmente la durabilidad de estructuras de cimentación, contenedores de residuos nucleares o industriales y revestimientos de túneles. Dichas estructuras suelen permanecer enterradas toda su vida útil, lo que compromete la detección del fenómeno antes de que se hayan desarrollado altos niveles de degradación. Una vez detectado, el tamaño e importancia estratégica de las estructuras afectadas limitan las opciones de reparación y su eficacia. Debido a ello, habitualmente la única acción posible consiste en la monitorización de la evolución del comportamiento estructural. Este escenario sitúa el desarrollo de herramientas para el diseño de estructuras resistentes a sulfatos y la evaluación del fenómeno en propiedades existentes como desafíos clave para la durabilidad de estructuras enterradas. Esta tesis doctoral aspira a profundizar el conocimiento en torno a esta temática mediante contribuciones relevantes en tres líneas de investigación: la modelización numérica del ASE, el rol de la porosidad durante el ataque y la relevancia de reproducir condiciones de campo en la evaluación del ASE. Avances en el campo de la modelización numérica han dado lugar a un modelo avanzado y una metodología de evaluación simplificada del ASE. El primero se basa en la simulación de procesos de transporte iónico, reacciones químicas, mecanismos de degradación y respuesta mecánica de la estructura. Las validaciones realizadas indican que el modelo refleja la importancia de la localización de la etringita dentro de la red porosa y proporciona estimaciones ajustadas de las expansiones generadas. La metodología de evaluación simplificada mide el riesgo de fallo estructural basándose en la agresividad del medio, la reactividad y propiedades mecánicas del material y las características geométricas y vida útil del elemento atacado, sin recurrir a algoritmos iterativos. A diferencia de las guías de diseño actuales, su aplicación permite la definición de medidas preventivas ajustadas a cada aplicación. La segunda línea de investigación ha dado lugar a un modelo conceptual que explica el rol de la porosidad durante el ASE. Los resultados obtenidos indican que se pueden alcanzar altas resistencias al ataque mediante la limitación de la penetración de sulfatos en la estructura o incrementando la capacidad de la matriz de acomodar fases expansivas. Los dos enfoques se corresponden a características porosas opuestas de la matriz. La primera se suele asociar a bajas porosidades mientras que la segunda se maximiza en porosidades altas. Los resultados obtenidos cuestionan la idea de que porosidades altas siempre son negativas para la durabilidad ante el ASE y abre la posibilidad de diseñar materiales resistentes a sulfatos incrementando la capacidad de acomodar fases expansivas. La tercera línea de investigación evalúa la influencia de la exposición temprana a sulfatos y los efectos del confinamiento en el ASE. Resultados referentes al primer estudio indican que una exposición tardía a los sulfatos como la empleada en la mayoría de ensayos acelerados puede conllevar a estimaciones incorrectas del daño generado en estructuras fabricadas in situ. El segundo estudio, referente a los efectos del confinamiento, sugiere que el uso de probetas en condiciones de expansión libre puede no ser adecuado para reproducir los efectos del ASE en estructuras donde el ataque se desarrolla en condiciones confinadas. Las tensiones de compresión generadas interactúan con el desarrollo normal del ataque limitando o retardando la aparición de micro-fisuras y reduciendo la cantidad de cristales de etringita creciendo en condiciones confinadas y por ende, ejerciendo presiones expansivas.Postprint (published version