The direct observation of solidification as a function of gravity level

The solidification of a metal-model material, NH₄Cl−H₂O, was directly observed on Earth at 1 g and at 10⁻⁵ g on a suborbital rocket flight. In the 1 g experiments, nucleation started at the cold walls and then dendrites and dendritic debris were swept into the central region by fluid flow. The numerous crystals in the central zone created an equiaxed zone. Secondary dendrite arms were oriented toward the cold wall with suppressed arm growth in the direction of the flow pattern. The necking and fragmentation of secondary arms were observed. The variation in secondary and tertiary arm spacing ranged from 27 to 38 pct. Individual dendrites grew at similar rates to interface fronts. When solidified in low g, only four nuclei grew to form the complete casting. There were no free floating crystals or visible dendrite remelting. Symmetrical dendrite growth into the fluid and some necking of secondary arms occurred but no coarsening or fragmentation resulted. The growth rate of interfaces was less than that of individual dendrites. Total growth was columnar with no equiaxed zone being formed.     

Factors affecting the dispersity of the dendritic structures in steels

The dispersity of the dendrite crystals in steels is shown to depend on two factors, namely, kinetic (cooling rate) and thermodynamic (degree of dendrite coarsening) factors. The kinetic and thermodynamic solidification parameters that affect the dispersity of the dendritic structures in steels are studied. The quantitative contribution of each component to the well-known empirical formula that relates the distance between secondary dendrite arms to the cooling rate is determined. The division of this formula into thermodynamic and kinetic components opens up fresh opportunities for controlling the structure and properties of steels. The possibility of a concentration-induced or a thermodynamic change in the dispersity of the dendrite structure in steels is demonstrated.     

Scaling of intragranuiar dendritic microstructure in ingot solidification

Analytic scaling formulas of complete constitutional generality for forced velocity cells and dendrites were in earlier research perfected forin situ steady-state solidification conditions involving binary organic alloys. As a further test, these were used, given the velocity and gradient control parameters, to predict the primary and secondary dendrite arm spacings of unidirectionally cooled Al-Cu alloys for which a large data set is available. Numerical methods were employed to determine the control parameters that exist under unsteady-state ingot solidification conditions according to the Scheil formulation. Primary and secondary arm spacings, corrected empirically for ripening, that by and large agree with the Al-Cu experimental data were obtained, demonstrating that the formulas are adequate for the prediction of dendrite scales in steady and unsteady-state conditions. The predictions have been incorporated into a computer program that displays the time-dependent columnar microstructure and mushy zone in an ingot cross section of an oriented single crystal together with the thermal and liquid-solid distributions.     

Modeling of casting solidification stochastic or deterministic?

Solidification modeling has had a phenomenal impact on metalcasting in the last decade. Following its initial success in predicting the occurrence of porosity defects, it has grown to be an essential casting engineering tool. As more complex models have been developed (in response to the realization that simple heat flow models were not adequate to solve the shrinkage problem) , they are being used to design more efficient gating and risering systems, which minimize the amount of metal poured to produce a good casting. Models today include predictions of fluid flow during mold filling, casting distortion, mold–metal interface reactions and cast structure.Until a few years ago solidification simulation was based only on deterministic models. As prediction of microstructural evolution became a contemporary problem, the limitation of deterministic models in predicting such features as dendrite coherency, columnar-to-equiaxed transition, dendrite fragmentation and movement of dendrites by the liquid, became evident. Recently developed stochastic models for solidification are capable of simulating and displaying the growth of columnar and equiaxed grains. However, the physics of dendritic growth is rather approximate. The growth of dendrite arms and their branching are ignored, and only a bulk representation of the grain growth is provided.A micro-scale approach for more accurate dendritic growth simulation in casting processes is presented in this paper. The model couples stochastic modeling at a length scale of 10⁻⁶ m, with deterministic modeling at a length scale of 10⁻⁴ m. A deterministic tip-velocity model is used to calculate the advance of the dendrite tip. Arm thickening is also calculated with a deterministic law derived from the dendrite tip velocity law and crystallographic considerations in combination with a deterministic coarsening model. However, the overall growth of dendrite arms is derived from probabilistic calculations. Branching of dendrites arm is allowed to occur based on morphologic instability. Thus the dendrite morphology, rather than the gain structure can be simulated.A discussion on the advantages and limitations of contemporary deterministic and stochastic models is also included.     

The Morphological Evolution of Low Volume Fraction Tin Dendrites During Coarsening

Tin dendrites tend to grow in closely spaced, parallel sheets; thus, within a certain range of solid volume fraction, it is possible to obtain samples with both dense dendritic regions and dendrite-free regions of liquid. Such samples were produced by directionally solidifying Pb-69.1 wt pct Sn, allowing us to compare tin dendrite structure and coarsening in a traditional dense mushy zone with the same dendrites in much lower volume fraction solid regions. The morphology of the dendrites, both in the dense and less-dense regions is analyzed using three-dimensional reconstructions obtained by serial sectioning. Quantitative measurements of these complex structures were obtained by calculating interfacial curvature and interfacial normal distributions, and the spatial correlations of interfacial curvature. We find that the spatial correlation measurement can be used to determine average secondary or tertiary arm length. We find also that coarsening proceeds in this system by both welding of secondary arms and dissolution and growth via long-range diffusional interactions and that the microstructure becomes more morphologically anisotropic as coarsening proceeds.     

Dendrite coarsening and microsegregation in Al-Cu alloys

Solute redistribution and the amount of eutectic are predicted in the solidification structure of a dendritic Al-Cu alloy. The effect of dendrite coarsening on microsegregation during solidification is formulated. Local solidification time and alloy composition are found to change the type of operating dendrite coarsening. While one type of dendrite coarsening operates for a shorter local solidification time and/or lower alloy composition, at longer solidification times and/or higher alloy composition a different type of dendrite coarsening prevails. Local solidification time and alloy composition are the parameters which determine the homogenization and solution treatment, and the structural heterogeneity of the solidified ingot. METIN BASARAN, formerly Associate Professor, Department of Mechanical Engineering, Technical University of Istanbul, Turkey     

On the rate of dendrite arm coarsening

Approximate but simple expressions for the rate of isothermal coarsening of secondary dendrite arms are presented. Their derivation differs from previous work in that (a) focus is placed on growing dendrite arms, which simplifies the derivation, and (b) the effect of a finite volume fraction solid is taken into account. Expressions for isothermal coarsening are then integrated to predict the final dendrite arm spacing in a cast dendritic microstructure. Coarsening rates predicted for Al-Cu alloys, succinonitrile, and Fe-26 wt pct Ni agree with published experimental data within a factor of 2 which is within the range of variation of experimental data. An erratum to this article is available at .     

Effect of dendrite coarsening on the solidification of interdendritic liquid in iron alloys

The solidification of the interdendritic liquid in austenitic 110G13L steel and white cast iron is studied. In the absence of dendrite coarsening, the solidification mechanism of the interdendritic liquid in the manganese steel is shown to change and solidification occurs in the form of polycrystalline aggregates around dendrites from different centers. The relation between the standard solidification of the interdendritic liquid and the dendrite coarsening in iron alloys is grounded.     

电渣重熔GH984G定向凝固组织CAFE法模拟

 电渣重熔凝固组织的控制直接关系到高温合金的品质与实际生产应用。针对电渣重熔GH984G的定向凝固过程,同时考虑传热和溶质扩散,基于CAFE法与C语言结合,建立了三维电渣重熔凝固过程组织演变的CAFE模型,并对凝固过程温度场和凝固组织演变进行模拟预测。结果表明,铸锭温度场和熔池深度都是首先为较浅平状态,然后不断加深至最后稳定;在电极熔化初始,金属熔池浅平,枝晶生长方向是竖直向上,之后金属熔池不断加深,底部竖直向上的柱状晶方向变为斜向上约26°。同时在铸锭的中心线上出现了等轴晶,等轴晶形核长大后与柱状晶镶嵌生长。此外,随着电极熔速变大,渣金界面上涨速度也随之变大,且熔池深度相应变宽变深。模拟结果与试验结果基本吻合,从而验证了模型与形核参数的适用性。     

Interdendritic fluid flow in a lead-tin alloy

Fluid flow through interdendritic channels of a partially remelted Pb-20 pct Sn alloy has been measured. The flow, resulting from gravity forces, was determined for columnar and equiaxed dendritic structures as a function of dendrite arm spacing. The initial interdendritic flow was found to be consistent with Darcy’s law with a tortuosity factor of 4.6. The initial permeability of the dendritic array was found to be a function of the square of the primary dendrite spacing, and a more complex function of secondary dendrite arm spacing. There was little difference in flow rates for columnar and equiaxed structures of similar size. After the initial flow period the flow rate was observed to increase above that defined by Darcy’s law for castings with small dendrite spacings. This was shown to be a result of coarsening of the liquid channels during flow. The observed coarsening process is considered in terms of diffusion controlled ripening.     

薄带连铸AISI304不锈钢中铁素体行为

为了确定薄带连铸AISI304不锈钢凝固过程中残留铁素体的生成及转变行为,采用彩色金相、电解侵蚀、电子背散射衍射分析技术及X射线衍射分析等研究手段对双辊薄带连铸AISI304不锈钢凝固组织及残留铁素体特征进行了研究.结果表明AISI304不锈钢薄带的凝固组织由表层胞状晶区、中间柱状晶区和中心等轴晶区三部分组成.薄带表层胞状晶区内残留铁素体呈棒状,柱状晶区的残留铁素体形态为鱼骨状,中心等轴晶区的残留铁素体呈弯曲的树枝状;薄带的表层胞状晶区残留铁素体的质量分数为4.6%～6.6%,柱状晶区内的残留铁素体质量分数为3.6%～3.7%,中心等轴晶区内的残留铁素体质量分数为11.27%～11.34%;残留铁素体沿着厚度方向呈现"W"状分布.      

Coarsening and Microsegregation during Solidification of Ni-AI-Cr Dendritic Monocrystals

Dendritic monocrystals of nickel-rich Ni-AI-Cr alloys were directionally solidified at rates ranging from 0.05 to 2.00 m per hour, under gradients of 8 × 10³ and 20 × 10³ K per meter. It was found that if growth is cellular dendritic, the cellular dendritic spacing exhibits the same dependence on growth rate or local cooling rate as does the secondary dendrite arm spacing in columnar dendrites. A study of the isothermal coarsening kinetics of the dendritic solid indicated that an increase in chromium or aluminum concentrations slowed down coarsening, yielding finer cast microstructures. At equal atomic percental increase in concentration the effect of chromium was more significant than that of aluminum in refining the dendritic structure. With increasing local cooling rate the maximum solute concentration remained practically constant, the minimum solute concentration slightly decreased, the segregation ratio slightly increased, and the volume fraction of nonequilibrium interdendritic γ′ phase increased substantially. This phase dissolved during crystal pulling faster in crystals that were grown at a higher rate. The homogenization kinetics of aluminum and chromium were established both analytically and experimentally. A longer time was necessary for chromium than for aluminum in order to achieve the same index or residual segregation. J. J. MONTOYA-CRUZ, formerly Graduate Assistant, University of Connecticut.     

Macro- and Microstructure Evolution of 5CrNiMo Steel Ingots during Electroslag Remelting Process

A comprehensive mathematical model was established and used to simulate the macro and microstructure evolution during the production process of 5CrNiMo steel ingot by electroslag remelting （ESR） method. Along the ingot height, the macrostructure distribution characteristics changed from vertical, fine columnar grains to tilted, coarse columnar grains, and this transformation process occurred at the very beginning of ESR. In the cross section of the ingot, there were three grain morphology regions and two grain type transition regions from the outside to the center of the ingot. These regions were the fine columnar grain region, columnar competitive growth transition re gion, coarse columnar grain region, columnar to equiaxed grain transition （CET） region, and coarse equiaxed grain region. The influence of the remelting rate on the macrostructure and mlcrostructure was investigated using a series of experiments and simulations. The results showed that a low remelting rate could produce a small grain growth angle （GGA） ; the average secondary dendrite arm spacing （SDAS） firstly decreased and then increased as the remelting rate increased. An excessively high or low remelting rate can increase the GGA and average SDAS in ingots. Thus, the remelting rate should be controlled within a suitable range to reduce composition microsegregation and microshrinkage in the ingot to produce an ESR ingot with satisfactory hot forging performance.     

Coarsening mechanisms during isothermal holding of a dendritic Al-10wt%Cu alloy

In-situ isothermal coarsening experiments have been carried out by high-resolution tomography on a dendritic Al-10wt%Cu alloy at a constant temperatures. The rate of coarsening and the coarsening mechanisms are investigated. Three coarsening mechanisms are observed to occur in different parts of the sample. These mechanisms are small arm melting (SAM mechanism) to the benefit of larger ones, interdendritic groove advancement (IGA mechanism) towards the dendrite tips and coalescence of the dendrite tips and groove advancement leading to entrapment of liquid droplets and tubes (CGA mechanism). These mechanisms are similar to those reported in the literature, but analysis of the coarsening rate based on 3D observations of the morphology of the dendrite arms shows that short-range diffusion can explain the rapid evolution of the microstructure. Therefore, improved models are required to adequately resemble the observed coarsening rates.     

半固态铝合金的制备工艺研究

采用自制的电磁搅拌装置，制备出具有球形初生相微粒的半固态Ａｌ６．６％Ｓｉ合金。研究了冷却速率和搅拌强度对半固态合金的球形初生相微粒形成的影响。球形初生相颗粒的生成必须具有适当的冷却速率，而高的磁感应强度则有利于初生相颗粒的细化、球化和均匀化。提出了球形初生相微粒演化机理，认为剧烈的搅拌使初生相在长大过程中，不断发生枝晶臂的弯曲融合、熔断和机械断裂。     

The effect of grain refining on macrosegregation and dendrite arm spacing of direct chill cast AA5182

The effect of titanium and titanium diboride inoculation on the spatial variation of local solidification time for direct chill (DC) cast ingots of aluminum alloy 5182 (AA5182) was studied. The results have been compared to those of an ingot cast without grain refining. To accomplish this, the effect of grain refining on a number of ingot characteristics such as grain size, macrosegregation, spatial variation of dendrite arm spacing, and thermal conductivity was investigated. Furthermore, the effect of grain refining on the well-known relationship between dendrite arm spacing and local solidification time had to be established for AA5182. The results indicated that the spatial variation of dendrite arm spacing in the industrial ingots was independent of grain refining, although the nonrefined ingot produced significantly finer dendrite arm spacings in its center. This was attributed to the influence of showering crystals in the nonrefined ingot. The relationship between dendrite arm spacing and local solidification time was also found to be independent of grain refining.     

Microstructural Evolution and Performance of In-situ Ag-Ni Composite after Solidification under Electromagnetic Stirring and Deformation

The effect of electromagnetic stirring (EMS)on microstructure and performance of Ag-8 mass%Ni com-posite was investigated under both solidified and deformed conditions.Without EMS,the Ag matrix formed short, thick dendrites in the ingot;whereas with EMS,dendrites were long and slim.Ni phase mainly formed particles or ribbons,distributed along boundaries between dendrite arms.Cold drawing of the solidified Ag-Ni ingots,both with and without EMS,produced high strength in-situ metal-matrix composite (MMC)consisting of Ag matrix reinforced by Ni ribbons.EMS improved the ductility of the composite,consequently enhancing its drawability and strength. EMS also increased the electrical conductivity in both solidified ingots and deformed in-situ composite wires.In both cases,hardness and tensile strength remained high.A model based on a combination of the modified linear rule of mixtures and the Hall-Petch relationship was used to rationalize the tensile strength and hardness with respect to its fabrication parameters and the microstructure of Ag-Ni in-situ composite.     

Models for the isothermal coarsening of secondary dendrite arms in multicomponent alloys

Models for the isothermal coarsening of secondary dendrite arms for binary alloys are simply expanded to correlate the secondary dendrite arm spacing to local solidification time, melting temperature of the liquid, and the properties of the solute elements in multicomponent alloys. An equation is derived. Calculations using the equation show reasonable qualitative agreement with experiment.     

Laser Repair of Superalloy Single Crystals with Varying Substrate Orientations

The casting and repair of single-crystal gas turbine blades require specific solidification conditions that prevent the formation of new grains, equiaxed or columnar, ahead of the epitaxial columnar dendrites. These conditions are best determined by microstructure modeling. Present day analytical models of the columnar-to-equiaxed transition (CET) relate the microstructure to local solidification conditions (temperature gradient and interface velocity) without taking into account the effects of (1) a preferred growth direction of the columnar dendrites and (2) a growth competition between columnar grains of different orientations. In this article, the infiuence of these effects on the grain structure of nickel-base superalloy single crystals, which have been resolidified after laser treatment or directionally cast, is determined by experiment and by analytical and numerical modeling. It is shown that two effects arise for the case of a nonzero angle between the local heat flux direction and the preferred dendrite growth axis: (1) the regime of equiaxed growth is extended and (2) a loss of the crystal orientation of the substrate often occurs by growth competition of columnar grains leading to an “oriented-to-misoriented transition” (OMT). The results are essential for the definition of the single-crystal processing window and are important for the service life extension of expensive components in land-based or aircraft gas turbines. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunt,” which occurred March 13–15, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.