Modelling of inoculation of metallic melts: application to grain refinement of aluminium by Al–Ti–B

A numerical model is presented for the prediction of grain size in inoculated castings and is tested against measured grain sizes obtained in standard grain-refiner tests on aluminium alloys. It is shown that for potent nucleants, such as commercial grain refiners for aluminium, the nucleation stage itself is not the controlling factor. The number of grains is determined by a free-growth condition in which a grain grows from a refiner particle at an undercooling inversely proportional to the particle diameter. With measured particle size distributions as input, the model makes quantitatively correct predictions for grain size and its variation with refiner addition level, cooling rate and melt composition. The model can assist in optimizing the use of existing refiners and in developing improved refiners.     

A model of grain refinement incorporating alloy constitution and potency of heterogeneous nucleant particles

A model for the determination of relative grain size is developed based on the assumption that nucleant substrates are activated by constitutional undercooling generated by growth of an adjacent grain. The initial rate of development of constitutional undercooling is shown to be equivalent to the growth restriction factor and is a useful approximation for the effect of solute on grain size when potent nucleant substrates are present. However, when the nucleants are of a poor potency then a calculation of the fraction solid necessary to develop the constitutional undercooling required for nucleation needs to be performed. Using the model, trends in grain size observed experimentally by the addition of titanium to pure aluminium and to a typical casting alloy, AlSi7Mg0.3, can be predicted. It was also found that summing the individual growth restriction factors of each solute element in a multi-component alloy can grossly overestimate the actual value of the growth restriction factor for the alloy.     

Performance comparison of AlTiC and AlTiB master alloys in grain refinement of commercial and high purity aluminum

1 Introduction Since AlTiC master alloys were improved greatly as a new type of master alloy used in grain refinement of aluminum and aluminum alloys in the middle of 1980s[1, 2], there were many important progresses in the preparation techniques and the …     

Columnar to equiaxed transition of eutectic in hypoeutectic aluminium–silicon alloys

Directional solidification of unmodified and strontium modified binary, high-purity, aluminium–7 wt% silicon and commercial A356 alloys has been carried out to investigate the mechanism of eutectic solidification. The microstructure of the eutectic growth interface was investigated with optical microscopy and Electron Backscattering Diffraction (EBSD). In the commercial alloys, the eutectic solidification interface extends in the growth direction and creates a eutectic mushy zone. A planar eutectic growth front is observed in the high-purity alloys. The eutectic aluminium has mainly the same crystallographic orientation as the dendrites in the unmodified alloys and the strontium modified high-purity alloy. A more complex eutectic grain structure is found in the strontium modified commercial alloy. A mechanism involving constitutional undercooling and a columnar to equiaxed transition explains the differences between pure and commercial alloys. It is probably caused by the segregation of iron and magnesium and the activation of nucleants in the commercial alloy.     

Correlation of recalescence with grain refinement of magnesium alloys

The grain refinement of Mg-Al based alloys with carbon inoculation was investigated by a computer-aided cooling curve analysis(CA-CCA) system.The results show that carbon inoculation decreases the main parameters of the recalescence regime during the initial stage of solidification.These parameters include the recalescence undercooling(Δθ_(rec)),duration of recalescence (t_(rec)),and liquid peak parameter(LPP) which is firstly introduced into magnesium alloys.The resultant grain size decreases with incre...     

Solidification of Mg-Zn-Zr Alloys: Grain Growth Restriction,Dendrite Coherency and Grain Size

The solidification characterization of Mg-xZn-0.5Zr (x = 0, 1, 3, 4, 5 wt%) alloys has been extensively investigated through thermal analysis, microstructure characterization and thermodynamic calculations. The impact of Zn content on the grain growth restriction, dendrite coherency and thus the final grain size has been investigated and discussed. Increasing Zn content, the grain size of Mg-xZn-0.5Zr alloy was firstly refined and then coarsened with the finest grain size of ~ 50 μm for the Mg-3Zn-0.5Zr (ZK31) alloy. Significant effects of the grain size on the mechanical properties were observed in the investigated alloys. The combination of growth restriction factor theory and dendrite coherency point provides a reasonable explanation of the grain size results. It helps to further understand the mechanisms of grain refinement and grain coarsening related to solute content, providing reference for alloy design and grain size prediction.     

Simulation of α-Al grain formation in high vacuum die-casting Al-Si-Mg alloys with multi-component quantitative cellular automaton method

Al-Si-Mg alloys are the most commonly used material in high vacuum die-casting(HVDC),in which the morphology and distribution ofα-Al grains have important effect on mechanical properties.A multi-component quantitative cellular automaton(CA)model was developed to simulate the microstructure and microsegregation of HVDC Al-Si-Mg alloys with different Si contents(7%and 10%)and cooling rates during solidification.The grain number and average grain size with electron backscatter diffraction(EBSD)analysis were used to verify the simulation.The relationship between grain size and nucleation order as well as nuclei density was investigated and discussed.It is found that the growth of grains will be restrained in the location with higher nuclei density.The influence of composition and cooling rate on the solute transport reveals that for AlSi7Mg0.3 alloy the concentration of solute Mg in liquid is higher at the beginning of eutectic solidification.The comparison between simulation and experiment results shows that externally solidified crystals(ESCs)have a significant effect for samples with high cooling rate and narrow solidification interval.     

Effect of the alloy composition on the grain refinement of aluminum alloys

Experiments were carried out to evaluate the effect of solute elements on grain refinement in aluminum based hypoeutectic alloys. Grain sizes of the as-cast structures were measured. The measured grain sizes were plotted vs. the undercooling parameter, P, the growth restriction factor, Q, and the solidification interval, ΔT, of the alloys. The results indicated that grain sizes decrease monotonically with increasing ΔT over the whole range of the hypoeutectic compositions whereas the relationships between the grain size and P or Q are “V” type curves.     

Eutectic solidification and its role in casting porosity formation

Understanding and controlling the eutectic solidification process in Al-Si alloys permits prediction of the formation of casting porosity, eventually leading to methods for its control and elimination. In addition, it enables control of eutectic structure, silicon morphology, and eutectic grain size to further improve the alloy properties. This paper presents the current understanding of eutectic solidification in hypoeutectic Al-Si foundry alloys and the relationship between eutectic solidification and porosity formation. New concepts in engineering eutectic solidification are also explored. For more information, contact L. Lu, The University of Queensland, CRC for Cast Metals Manufacturing, Division of Materials Engineering, Brisbane Qld. 4072, Australia; e-mail l.lu@minmet.uq.edu.au.     

评价溶质元素细化晶粒能力的参数

推导了凝固界面前沿液相一侧的液相线温度梯度,并由此梯度提出了评价溶质元素细化晶粒能力大小的参数H。对铝基合金的实验结果表明:H参数数值越大,铸态晶粒尺寸越小。因此,H参数可用于指导晶粒细化剂的选择。     

On dendritic solidification of multicomponent alloys with unequal liquid diffusion coefficients

A model has been developed previously by Rappaz and Thévoz (Acta metall., 1987, 35, 1478; 1987, 35, 2929) for the solidification of binary alloys having an equiaxed dendritic morphology. The extension of this model to multicomponent alloys is straightforward if the diffusion coefficients in the liquid of the various solute elements are equal. When they are different however, it becomes necessary to distinguish the concentrations at the dendrite tip position, governing the growth kinetics, from the average concentrations within the interdendritic liquid region. A model is presented which couples the dendrite growth kinetics with an overall solute balance performed at the scale of the grain. The diffusion profile outside the grain envelope is calculated using a Landau transformation and an implicit scheme. The calculated solidification paths during and after recalescence are compared with the Scheil approximation for various conditions. Additionally, a model predicting the secondary dendrite arm spacing in multicomponent alloys is briefly described.     

Modeling the overall solidification kinetics for undercooled single-phase solid-solution alloys. I. Model derivation

Departing from the volume-averaging method, the equiaxed solidification model was extended to describe the overall solidification kinetics of undercooled single-phase solid-solution alloys. In this model, a single grain, whose size is given assuming site saturation, is divided into three phases, i.e. the solid dendrite, the inter-dendritic liquid and the extra-dendritic liquid. The non-equilibrium solute diffusion in the inter-dendritic liquid and the extra-dendritic liquid, as well as the heat diffusion in the extra-dendritic liquid, is considered. The growth kinetics of the solid/liquid interface is given by the solute or heat balance, where a maximal growth velocity criterion is applied to determine the transition from thermal-controlled growth to solutal-controlled growth. A dendrite growth model, in which the nonlinear liquidus and solidus, the non-equilibrium interface kinetics, and the non-equilibrium solute diffusion in liquid are considered, is applied to describe the growth kinetics of the grain envelope. On this basis, the solidification path is described.     

MODELING OF FERRITE GRAIN GROWTH OF LOW CARBON STEELS DURING HOT ROLLING

1．I尬roductlonThe dewlopment of mathematical model to predict the inal room temperature proper－ties ofhot－rolled steels hajs received consider劝ie attention over the past t侧 decades[1一31．These models contain equations to calculate the mlcrostructure     

稀土铝合金的枝晶间距

研究了加稀土Ｌａ、Ｃｅ或Ｙ的工业纯铝及Ａｌ－Ｍｎ,Ａｌ－Ｍｇ,Ａｌ－Ｃｕ,Ａｌ－Ｌｉ,Ａｌ－Ｃｕ－Ｌｉ,Ａｌ－Ｍｇ－Ｓｉ,Ａｌ－Ｃｕ－Ｍｇ,Ａｌ－Ｚｎ－Ｍｇ等１４种合金的枝晶间距。讨论了微量稀土、原合金元素、冷却速度和有效晶粒细化剂对枝晶的细化作用。结果表明,在各种冷速条件下,微量Ｌａ、Ｃｅ或Ｙ能显著细化工业纯铝和各种铝合金的枝晶;提出的溶质传输特征参数Ｐ０可正确反映稀土对铝合金枝晶的细化能力与原有合金元素的种类和含量的关系;冷却速度越大,稀土。对铝合金枝晶的细化作用越小;有效晶粒细化剂对铝合金枝晶的细化作用远不能与其对铝合金铸态晶粒的有效细化相提并论。     

A quantitative multi-phase field model of polycrystalline alloy solidification

A multi-phase field model for quantitative simulations of polycrystalline solidification of binary alloys is introduced. During the free-growth stage of solidification, the model exploits the thin-interface analysis developed by Karma [3] in order to realistically capture bulk phase diffusion and the sharp interface corrections predicted by traditional models of solidification. During grain boundary coalescence, the model is constructed to reproduce the properties of repulsive grain boundaries described by Rappaz et al. [29]. The model provides a very simple mechanism for decoupling of solute and concentration fields at steady state, an important feature for calculating grain boundary energies.     

Numerical simulaton of cellular/dendritic transition and its growth during unidirectional solidificationin of Ti–Al alloy

采用基于溶质扩散控制模型CA方法对Ti44Al合金定向凝固初始阶段变速冷却过程中胞/枝晶转变过程进行了数值模拟.模拟结果表明,在胞/枝转变过程中,存在胞-枝混合生长区,在一些胞晶相邻面无二次枝晶生长.在给定的冷却速率下,枝晶臂间距大于胞晶臂间距;而在过渡区,枝晶间距达到最大.另外,晶核数量对柱状晶间距产生影响,随着植入晶核数量的增加,柱状晶间距非均匀化程度明显减小.出现过渡区的原因与枝晶生长所引起固/液界面前沿成分波动有关.模拟与实验结果吻合较好.     

Relating Initial Texture to Deformation Behavior During Cold Rolling and Static Recrystallization Upon Subsequent Annealing of an Extruded WE43 Alloy

Deformation behaviors during cold rolling and static recrystallization behaviors upon subsequent annealing of an extruded WE43 alloys with different initial textures were investigated in this study. Three types of differently textured WE43 initial alloys were labeled as samples I, II and III. The results showed that multiple twinning modes and basal slip dominated the deformation of samples during cold rolling. Cold-rolled sample I activated the larger number of double twins with high strain energy, accompanied by the more uniform strain distribution, than cold-rolled samples II and III. During subsequent annealing, recrystallized grains preferentially occurred in double twins, twin-twin intersections and grain boundaries, thereby making cold-rolled sample I have the more rapid recrystallization rate. Similar recrystallization textures formed in three types of cold-rolled samples at the recrystallization nucleation stage, and they never largely changed with the annealing time due to the uniform grain growth induced by the solute drag and the precipitation pinning at grain boundaries. After full recrystallization, the grain growth was controlled by the solute drag, instead of precipitates which were re-dissolved into the matrix. Finally, the nucleation and growth kinetics of static recrystallization were calculated, and the effects of initial texture on activation energies of recrystallization nucleation and growth were discussed.     

Cellular automaton modeling of semisolid microstructure formation

Computer modeling of semi-solid structure formation is of significance in both understanding the mechanisms of globular structure formation and determining the effect of solidification conditions on final microstructure. A modified cellular automaton (mCA) model has been developed, which is coupled with macroscopic models for heat transfer calculation and microscopic models for nucleation and grain growth. The InCA model is applied to A356 Al alloy - one of the most widely used semi-solid alloys, to predict grain morphology and grain size during semi-solid solidification, and determines the effects of pouring temperature on the final microstructure. The modeling results show that the lower the initial temperature, the finer grain size will be obtained. In addition, the model can be used to predict the solutal micro-segregation.     

Inclusions nucleating intragranular polygonal ferrite and acicular ferrite in low alloyed carbon manganese steel welds

The inclusion-assisted formation of two types of intragranular ferrite in low alloyed C-Mn steel welds, intragranular polygonal ferrite (IPF) and acicular ferrite (IAF), was investigated in relation to the inclusion characteristics (mainly size and chemistry) and the welding heat input. For this analysis, inclusions engulfed by one ferrite grain and those shared by two ferrite grains were considered as the IPF nucleants, and the rest of them (in contact with more than three ferrite grains) were considered as the IAF nucleants. All inclusions were multi-component oxides having Ti oxides as the nucleoid. The inclusion size exhibited a log-normal distribution and the average size increased as the heat input increased. Inclusions larger than ∼0.4 μm were found to be effective on IAF nucleation, and the effectiveness increased as the inclusion size increased. Accordingly, the overall IAF nucleant fraction increased as the heat input increased due to the corresponding inclusion size increase. However, the probability of the IAF nucleant in the same inclusion size range was insensitive to the heat input. There was no difference in the elemental distribution between the IAF and IPF nucleants except for Si. The Si distribution was uniform in the IPF nucleants while its content was relatively high at the inclusion periphery in the case of the IAF nucleants regardless of the heat input and the inclusion size.     

INTERACTION OF PARTICLES WITH A SOLIDIFIED INTERFACE DURING SOLIDIFICATION OF METAL MATRIX COMPOSITE REINFORCED WITH PARTICLES

ＩＮＴＥＲＡＣＴＩＯＮＯＦＰＡＲＴＩＣＬＥＳＷＩＴＨＡＳＯＬＩＤＩＦＩＥＤＩＮＴＥＲＦＡＣＥＤＵＲＩＮＧＳＯＬＩＤＩＦＩＣＡＴＩＯＮＯＦＭＥＴＡＬＭＡＴＲＩＸＣＯＭＰＯＳＩＴＥＲＥＩＮＦＯＲＣＥＤＷＩＴＨＰＡＲＴＩＣＬＥＳＣＨＵＳｈｕａｎｇｊｉｅ;Ｗ...