Numerical finite difference model for steady state cellular array growth

An improved finite difference analysis has been used to calculate self-consistent axisymmetric cellular shapes. The results obtained are compared with the analytical models which have been proposed to describe array growth. It is concluded that the analytical models should be used with extreme caution in the cellular region. A preliminary examination of the stability of the steady state shapes has been made numerically. The results are compared with experiment where it is found that neither the minimum undercooling condition nor marginal stability analysis as applied, correctly predicts the growth conditions. The model includes solute diffusion in the solid so that the extent of microsegregation is fully described for the first time.     

Time dependence of tip morphology during cellular/dendritic arrayed growth

Succinonitrile-1.9 wt pct acetone has been directionally solidified in 0.7 X 0.7-cm-square cross section pyrex ampoules in order to observe the cell/dendrite tip morphologies, not influenced by the “wall effects,” which are present during growth in the generally used thin (about 200 μm) crucibles. The tips do not maintain a steady-state shape, as is generally assumed. Instead, they fluctuate within a shape envelope. The extent of fluctuation increases with decreasing growth speed, as the micro structure changes from the dendritic to cellular. The influence of natural convection has been examined by comparing these morphologies with those grown, without convection, in the thin ampoules.     

Natural convective effects in directional dendritic solidification of binary metallic alloys: Dendritic array primary spacing

This paper is concerned with the effect of natural convective patterns evidenced in a previous paper [Acta metall.37, 1143 (1989)], on the dendritic primary spacings. Primary spacings in normal gravity environment have been found to be much smaller than those measured in a microgravity environment, the latter being in good agreement with the diffusion controlled theoretical predictions. The scaling analysis of convective effects developed in the first paper allows us to propose a relationship, which gives the primary spacing as a function of the experimental parameters, in convective transport conditions of the solute in the liquid. This correlation is in good agreement with our experimental results, and those of the literature.     

In situ observation of faceted cellular array growth

Formerly Graduate Student, Department of Materials, University of Oxford     

Experimental and numerical study of pattern formation in faceted cellular array growth

In this article, a review of our recent experimental and numerical study of pattern formation in faceted cellular array growth was presented.In situ observations in transparent faceting organic compounds have revealed the dynamical features of faceted cellular growth. Cell tip splitting and loss of cells in the array have been found to be the main mechanisms for cell spacing adjustment. The time evolution of a faceted cellular array has been followed numerically. It was found that pattern formation was history-dependent and was determined by interactions in the array. These interactions were either transient or persistent, depending on the growth condition. As a result, afinite range of stable cell spacings was found under a given growth condition; that is, there was no sharp selection. The cellular structures were irregular when persistent interactions occurred, whereas relatively regular structures could be formed once the transient interactions had stopped. The implication of these observations for pattern formation in other array growth processes, such as nonfaceted cellular, dendritic, or eutectic growth, will be discussed. This paper is based on a presentation made in the symposium “Growth and Configurations of Faceted Solid-Liquid Interfaces” presented during the TMS annual meeting, New Orleans, Louisiana, February 17–21, 1991, under the auspices of the TMS Solidification Committee. Formerly Assistant Research Engineer, The University of Alabama.     

Macrotransport-solidification kinetics modeling of equiaxed dendritic growth: Part I. Model development and discussion

An analytical model that describes solidification of equiaxed dendrites has been developed for use in solidification kinetics-macrotransport modeling. It relaxes some of the assumptions made in previous models, such as the Dustin-Kurz, Rappaz-Thevoz, and Kanetkar-Stefanescu models. It is assumed that nuclei grow as unperturbed spheres until the radius of the sphere becomes larger than the minimum radius of instability. Then, growth of the dendrites is related to morphological instability and is calculated as a function of melt undercooling around the dendrite tips, which is controlled by the bulk temperature and the intrinsic volume average concentration of the liquid phase. When the general morphology of equiaxed dendrites is considered, the evolution of the fraction of solid is related to the interdendritic branching and dynamic coarsening (through the evolution of the specific interfacial areas) and to the topology and movement of the dendrite envelope (through the tip growth velocity and dendrite shape factor). The particular case of this model is the model for globulitic dendrite. The intrinsic volume average liquid concentration and bulk temperature are obtained from an overall solute and thermal balance around a growing equiaxed dendritic grain within a spherical closed system. Overall solute balance in the integral form is obtained by a complete analytical solution of the diffusion field in both liquid and solid phases. The bulk temperature is obtained from the solution of the macrotrasport-solidification kinetics problem.     

Macrosegregation during dendritic arrayed growth of hypoeutectic pb- sn alloys: Influence of primary arm spacing and mushy zone length

Thermosolutal convection in the dendritic mushy zone occurs during directional solidification of hypoeutectic lead tin alloys in a positive thermal gradient, with the melt on the top and the solid below. This results in macrosegregation along the length of the solidified samples. The extent of macrosegregation increases with increasing primary dendrite spacings for constant mushy zone length. For constant primary spacings, the macrosegregation increases with decreasing mushy zone length. Presence of convection reduces the primary dendrite spacings. However, convection in the interdendritic melt has significantly more influence on the spacings as compared with that in the overlying melt, which is caused by the solutal buildup at the dendrite tips. Formerly Graduate Student, Chemical Engineering Department, Cleveland State University     

The influence of acceleration forces on dendritic growth and grain structure

The results of experiments on the tin-15 wt pct lead system are presented, showing the effects on microstructure of solidification in presence of acceleration forces from 10^-4 to 5g* for three cooling rates. An increase in the acceleration level is shown to drive fluid flow and cause dendrite remelting, fragmentation, and macrosegregation. The cooling rate impacts the final structure through its control of dendrite arm spacings and permeability to fluid flow. At the low (10^-4 g) acceleration, dendrite arm spacings deviated from the predicted relationship to cooling rate. An explanation for this anomaly is given which considers the temperature and concentration gradients in the low-gravity environment.     

Equiaxed dendritic solidification with convection: Part I. Multiscale/multiphase modeling

Equiaxed dendritic solidification in the presence of melt convection and solid-phase transport is investigated in a series of three articles. In part I, a multiphase model is developed to predict com-position and structure evolution in an alloy solidifying with an equiaxed morphology. The model accounts for the transport phenomena occurring on the macroscopic (system) scale, as well as the grain nucleation and growth mechanisms taking place over various microscopic length scales. The present model generalizes a previous multiscale/multiphase model by including liquid melt convec-tion and solid-phase transport. The macroscopic transport equations for the solid and the interdendritic and extradendritic liquid phases are derived using the volume averaging technique and closed by supplementary relations to describe the interfacial transfer terms. In part II, a numerical application of the model to equiaxed dendritic solidification of an Al-Cu alloy in a rectangular cavity is dem-onstrated. Limited experimental validation of the model using a NH₄C1-H₂O transparent model alloy is provided in part III.     

高碳钢小方坯的一次枝晶臂间距的影响因素

高碳钢小方坯显微凝固组织的研究表明:一次枝晶臂在凝固过程中不断粗化变短;碳含量高,容易形成长宽比小的一次枝晶,并且一次枝晶臂间距增宽,容易形成粗大的柱状晶组织;过热度高,易形成粗大的一次枝晶,一次枝晶臂间距增宽;拉速慢,容易形成细长的一次枝晶,一次枝晶臂间距减少;二次冷却水流量比增加,易形成细长的一次枝晶,一次枝晶臂间距降低.结晶器电磁搅拌可显著降低一次枝晶臂长宽比,并减小一次枝晶臂间距;搅拌电流增加,则一次枝晶臂间距减小效果更明显.     

Macrotransport-solidification kinetics modeling of equiaxed dendritic growth: Part II. Computation problems and validation on INCONEL 718 superalloy castings

In Part I of the article, a new analytical model that describes solidification of equiaxed dendrites was presented. In this part of the article, the model is used to simulate the solidification of INCONEL 718 superalloy castings. The model was incorporated into a commercial finite-element code, PROCAST. A special procedure called microlatent heat method (MLHM) was used for coupling between macroscopic heat flow and microscopic growth kinetics. A criterion for time-stepping selection in microscopic modeling has been derived in conjunction with MLHM. Reductions in computational (CPU) time up to 90 pct over the classic latent heat method were found by adopting this coupling. Validation of the model was performed against experimental data for an INCONEL 718 superalloy casting. In the present calculations, the model for globulitic dendrite was used. The evolution of fraction of solid calculated with the present model was compared with Scheil’s model and experiments. An important feature in solidification of INCONEL 718 is the detrimental Laves phase. Laves phase content is directly related to the intensity of microsegregation of niobium, which is very sensitive to the evolution of the fraction of solid. It was found that there is a critical cooling rate at which the amount of Laves phase is maximum. The critical cooling rate is not a function of material parameters (diffusivity, partition coefficient,etc.). It depends only on the grain size and solidification time. The predictions generated with the present model are shown to agree very well with experiments.     

连铸板坯二次枝晶臂间距对中心碳偏析的影响

针对新余钢铁公司生产的1 900 mm×250 mm板坯,通过测量不同拉速、二冷比水量、过热度等工况条件下铸坯的二次枝晶臂间距,确定不同连铸工艺参数条件对二次枝晶臂间距的影响,结果表明二次枝晶臂间距越大,渗透率越高,中心碳偏析越严重.     

Molecular and cellular biology of dendritic epidermal T cells

The function and specificity of gamma delta T cells remain enigmatic. Dendritic epidermal T cells (DETC) expressing an invariant gamma delta TCR represent a well established model system to investigate these key issues. Accumulating evidence supports the initial observation that recognition of a keratinocyte self-antigen by DETC proceeds without a requirement for MHC gene products. In addition, recent data have identified bioactive polypeptides expressed by DETC but not by most other T cells. For example, keratinocyte growth factor appears to be exclusively produced by activated DETC and intestinal intraepithelial gamma delta cells. These findings suggest that DETC may recognize antigen in novel ways as well as perform specialized functions complementary to those normally attributed to T cells.     

A unified microscale parameter approach to solidification-transport process-based macrosegregation modeling for dendritic solidification: Part II. Numerical example computations

This article, as the second part of the present work, applied the unified Φ-parameter-based micro/macrosegregation modeling, which was proposed and described in the first part, to two groups of alloy solidification systems. The first group is on closed simultaneous solidification systems of an Al-4.5 pct Cu alloy and a Fe-0.5 pct C based plain carbon steel to investigate the microscale solute-redistribution behaviors with different influential factors including the solid phase morphologies. A new solute-redistribution equation was derived from the present microscale unified Φ-parameter-based modeling, which includes more microscale dendrite-solidification features while preserving the simple function form equivalent to the well-known lever rule and the Scheil, Flemings-Brody, Clyne-Kurz, and Ohnaka models. In the second example computation group on a directionally solidified, bladelike casting system, a previous continuum model and the related PC-based codes developed by the present authors were extended and modified by incorporating the present microscale Φ-parameter approach to any extent of solid back-diffusion effects. Numerical simulations on the directional-solidification transport phenomena and micro/macrosegregation formations in a bladelike Al-4.5 pct Cu casting show the feasibility and the efficiency of the present solidification-transport process (STP)-based two-scale segregation model and the numerical methods.     

Cellular and dendritic growth: Part II. Theory

A theoretical model is developed to predict the characteristics of cellular or dendritic interface during the controlled solidification of binary alloys. Both heat and mass transport fields near a growing cellular and dendritic front have been considered. A perturbative stability criterion has been developed to determine the dimension and the solute concentration of a tip. It is shown that the fields are composed of two contributions; one characterizes the average field and the other characterizes the cellular and dendritic fields ahead of the growing front. The perturbation in the shape is classified into two categories; one is responsible to the cellular growth and the other to the dendritic growth. Good correlations are obtained between theory and experiments.     

Cellular and dendritic growth: Part I. Experiment

An Al-4.1 mass pct Cu alloy was unidirectionally solidified under various growth rates. Features, such as tip radius of curvature, primary arm spacing, and tip concentration were measured as functions of growth rate. Dependence of tip radius on growth rate was different between cells and dendrites. Measured tip radius and primary arm spacing were maximum at the cellular-dendritic transition. Tip concentration, however, monotonously decreases with growth rate. Linear relationships between tip radius and characteristic dimensions of dendrites like core diameter, half length of tip arc, and the first secondary arm spacing are obtained to determine what affects growth rate, convection, and gravity segregation. Experimental results are compared with current theoretical models for dendrite growth under controlled solidification. It was determined that the measured tip radius is larger than that of theoretical predictions at fast growth rate, but the measured tip concentration is in good agreement with theoretical predictions.     

Monocyte-derived dendritic cells: development of a cellular processor for clinical applications

Since dendritic cells (DCs) are the most professional antigen-presenting cells, (Schuler et al., 1997), increasing interest in their use in clinical approaches has been observed. (Nestle et al., 1998; Murphy G. et al., 1996). We have developed an ex vivo standardized process for the generation of dendritic-like cells (MAC-DCs) from human blood circulating monocytes. Human monocytes can differentiate into very different functional cells according to the conditions of culture, media and cytokines used. In the present study, we demonstrate that both pure monocytes and mononuclear cells differentiate into DCs when they are grown in defined medium AIM-V in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) plus IL13 and in approved biocompatible non-adherent bags. Quality and functional controls of the immature DCs obtained rely on bacterial sterility, viability, morphology and recovery. The MAC-DCs also present an immature DC phenotype with a low expression of CD14 and CD64, and high expression of MHC-I, MHC-II and CD40. They also express B7 costimulatory molecules (CD80, CD86), CD83, and CD1a molecules. They induce strong allogenic T-cell proliferation (mixed lymphocyte reaction as well as proliferation of autologous memory T lymphocytes when incubated in the presence of recall antigens (tuberculosis, Candida albicans, and tetanus toxoid). They also show an increase in phagocytic uptake of yeast, tumour cells and debris. The global closed system which, under reproducible good medical practice (GMP) conditions, enables the production of dendritic cells of clinical quality, has been optimized ("Vac Cell Processor"). It contains all bags, connections, media, reagents, washing solutions, control antibodies, standard operating procedures, data management, traceability and help in the form of dedicated software.