Papers by Keyword: Macrosegregation

Abstract: Macrosegregation predictions have been performed for the alloy solidification in regular and irregular geometries. The continuum model is used to describe the mass, momentum, species, and energy transportation. Based on the same conservation equations, different discretization forms are derived for the orthogonal grids and non-orthogonal grids. Comparisons are made between the results with orthogonal grids and non-orthogonal grids for different cases. Firstly, simulations are conducted for the Pb-48%Sn alloy solidified in a rectangle cavity, i.e., the H-H benchmark. Then, simulations are applied to a 53t steel ingot. Prediction results are compared with concentration measurements along different transverse sections. Although both prediction results are in good agreements with the measurements or results in references, some differences can be observed due to the different boundary fitness. It is advised to adopt the non-orthogonal grids in macrosegregation simulations instead of the orthogonal grids for complex computation domains.

Abstract: The peritectic alloys, such as some types of steel, Ni-Al, Fe-Ni, Ti-Al, Cu-Sn, are commercially important. In contrast to other types of alloys, many unique structures (e.g. banded or island ones) can form when peritectic alloys are directionally solidified under various solidification conditions. It can be observed in the course of the directional solidification experiments performed in a rotating magnetic field (RMF) that the melt flow has a significant effect on the solidified structure of Sn-Cd alloys. This effect was investigated experimentally for the case of Sn1.6 wt% Cd peritectic alloy. For this purpose, a Bridgman-type gradient furnace was equipped with an inductor, which generates a rotating magnetic field in order to induce a flow in the melt. As a result, the forced melt flow substantially changes the solidified cellular microstructure. The cell size and the volume fraction of the primary tin phase were measured by an image analyzer on the longitudinal polished sections along the entire length of the samples. The microstructure was investigated by scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS).

Abstract: Cylindrical Pb-Sn alloy samples (diameter: 8 mm, length: 120 mm) of different compositions (30, 40 and 50 wt.% of Sn) were prepared from high pure (4N) components. The unidirectional solidification experiments have been performed according to the upward vertical Bridgman-method by using a rotating magnetic field (RMF) with a magnetic induction of 150 mT and with a frequency of 50 Hz. The sample-movement velocity was constant (0.05 mm/s) and the temperature gradient changed from 7 to 3 K/mm during the solidification process. The first half of samples was solidified without using the magnetic field and the second half was solidified by using the magnetic field. Under the influence of this strong flow induced by the magnetic field, the columnar microstructure of the first part decomposed and a characteristic "Christmas tree"- like macrosegregated structure with equiaxed Pb-dendrites was developed. The secondary dendrite arm spacing (SDAS) and the volume percent of primary Pb-phase (dendrite) were measured by an automatic image analyser on the longitudinal polished sections along the whole length of the samples. The effect of the forced melt flow on the micro-and macrostructure was studied in case of the different sample compositions.

Abstract: There is a continuously developing need for benchmarking of solidification simulations - from the theoretical [1] as well as from the applied [2] points of view. The history of related benchmarking shows differences of the results between different numerical methods, and differences in comparison with the experiments when solving even quite simple solidification situations. The present benchmark test proposes macrosegregation [3] upgrades to the verification benchmark for continuous casting of steel, first presented in [2]. The paper represents guidelines for the presentation of the numerical method, discretisation and results and shows a reasonable comparison between a commercial finite volume based code and our in-house developed meshless method based code.

Abstract: The simulation of macrosegregation in a 2.45-ton steel ingot with the three-phase mixed columnar-equiaxed model was presented previously. The results showed an overestimation of the intensity of bottom negative segregation. The reason is due to the assumed globular morphology for the equiaxed crystal. Therefore, in this paper a simple approach is suggested to treat the dendritic morphology of equiaxed crystals. Three aspects are improved: the drag force between the moving equiaxed crystals and the surrounding melt, the mechanism of the columnar-to-equiaxed transition, the packing limit of the equiaxed crystals. The modified model is used to calculate the macrosegregation of the same ingot. It is found that the modified model predicts less severe negative segregation in the bottom equiaxed zone than the previous globular equiaxed model does, i.e. it agrees better to the experiment. The model considering simplified-dendritic morphology improves the calculation accuracy.

Abstract: The convection pattern and the evolution of mushy zone, temperature and solidification structure are measured during solidification of NH₄Cl-70%H₂O solution in a water-cooled copper mold with transparent sidewalls. The natural convection and crystal sedimentation are measured via Particle Image Velocimetry (PIV) technique. This experiment is simulated using a 5-phase mixed columnar-equiaxed solidification model proposed by current authors [Comp. Mater. Sci. 50 (2010) 32-4]. The 5 phases comprise the extradendritic melt, the solid dendrite and interdendritic melt inside the equiaxed grains, the solid dendrite and interdendritic melt inside the columnar grains. Melt convection and crystal sedimentation are considered. It is demonstrated that the experimentally observed flow patterns and the solidification structure can be qualitatively reproduced. Reasons for the quantitative deviation between the simulation and experiment are discussed. Analysis of the modeling results in details and improvement of the calculation accuracy will be in a subsequent step.

Abstract: Experimental evidence [Ohno 1987] revealed the influence of some pouring techniques on the as-cast structure. In the current work the process of pouring of the molten Al-4.0 wt.%Cu via one or multiple streams into a graphite mold is studied using a 3-phase model by considering the nucleation, the initial growth and transport of globular equiaxed crystals. The three phases are the melt, air and globular equiaxed crystals. Results showed that pouring via multiple streams increases the volume fraction and number density of crystals in the as-filled state. The subsequent solidification is calculated using a 5-phase mixed columnar-equiaxed solidification model. The five phases are the extradendritic melt, the solid dendrite and interdendritic melt inside the equiaxed grains, the solid dendrite and interdendritic melt inside the columnar grains. As final result the as-cast structure including the distinct columnar and equiaxed zones, columnar-to-equiaxed transition (CET), grain size, macrosegregation, and rest eutectic is predicted. Effect of melt convection and crystal sedimentation during the pouring and solidification is taken into account. The predicted as-cast structure, under the influence of single/multiple jet pouring, is evaluated bycomparison with the available experiments of Ohno.

Abstract: This article is to assess the modeling treatment of the growth kinetics (finite or infinite diffusion in liquid and solid phases) during solidification and its influence on the calculation of macrosegregation. A model of diffusion-governed growth kinetic for ternary alloy is developed and used for this assessment. Solidification of a 2D casting (50 x 50 mm²) of a ternary alloy (Fe-0.45 wt.% C-1.06 wt.%Mn) is considered. The result shows that finite diffusion in liquid, important for the initial stage of solidification, plays very important role in the formation of macrosegregation. Moreover, the role of the finite diffusion kinetics in the formation of macrosegregation shows differently in the two extreme cases of solidification structures (columnar or equiaxed).

Abstract: In static castings of multi-component alloys, visually observable bands of channels, with high solute concentration, can form in the final solidified product. The phenomenological explanation for these formations is that perturbations during the solidification process lead to preferred flow paths in the solid-liquid mushy region. Once these flow paths are initiated, the higher solute liquid that flows in them suppresses the solidification rate and thus provides a mechanism through which the preferred paths can evolve into high concentration channels. Models of solidification that couple heat transfer, fluid and flow and mass transport appear able to predict the formation of these channels. In many cases, however, the formation of these numerical channels is highly dependent on the nature of the numerical calculation. In particular, geometric attributes of the channels is a strong function of the size of the computational grid and in some cases the particular method (code) used. In this work, after discussing what might drive the observed discrepancies in predictions, a grid convergence study is undertaken. This study shows that for the case of a side cooled solidification of a binary (Al-4.5wt%Cu) in a square (40mm x 40mm) domain, it is possible to approach grid converged results of the solution of the standard mixture model for macrosegregation. Achieving this level of convergence requires the use of an explicit time stepping scheme to couple the thermal and solute fields along with a Carman-Kozeny permeability and lever rule microsegregation models. The results indicate that to reach grid convergence the size of a grid cell has to be on the order ~0.25-0.5 mm.

Abstract: Mathematical model was developed to estimate the flow rate and direction and of the expected porosity level in the centre part of a slab. Calculations show that centreline segregation is basically affected, at a given composition and cooling technology by the setting, deformation and eccentricity of the supporting rolls. Bulging of the strand between the supporting rolls can also play role.