Computational Modeling of Microstructure Evolution in Solidification of Aluminum Alloys

In this article, a front tracking (FT) model and a modified cellular automaton (MCA) model are presented and their capabilities in modeling the microstructure evolution during solidification of aluminum alloys are demonstrated. The FT model is first validated by comparison with the predictions of the Lipton–Glicksman–Kurz (LGK) model. Calculations of the steady-state dendritic tip growth velocity and equilibrium liquid composition as a function of melt undercooling for an Al-4 wt pct Cu alloy exhibit good agreement between the FT simulations and the LGK predictions. The FT model is also used to simulate the secondary dendrite arm spacing as a function of local solidification time. The simulated results agree well with the experimental data. The MCA model is applied to simulate dendritic and nondendritic microstructure evolution in semisolid processing of an Al-Si alloy. The effect of fluid flow on dendritic growth is also examined. The solute profiles in equiaxed dendritic solidification of a ternary aluminum alloy are simulated as a function of cooling rate and compared with the prediction of the Scheil model. The MCA model is extended to the multiphase system for the simulation of eutectic solidification. A particular emphasis is made on the quantitative aspects of simulations. This article is based on a presentation made in the symposium ”Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

Phase-Field Modeling of Microstructure Evolution in the Presence of Bubble During Solidification

Simulation of the solid–liquid–gas interaction during solidification is challenging due to the presence of complex phase interfaces, bubble deformation, and high liquid/gas density ratio. In this work, a hybrid phase-field lattice-Boltzmann (PFLB) approach, together with a parallel and adaptive-mesh-refinement (Para-AMR) algorithm, is developed to model interactions between the gas bubble and solid growth front during solidification. The solid growth and bubble evolution are solved by the phase-field method. Both melt flow and bubble movement are determined by a kinetic-based lattice-Boltzmann model. Bubble dynamics during alloy solidification is modeled and compared with experiments, and a good agreement is achieved for various solid/liquid interfaces including planar, cellular, and dendritic interfaces. Results show that the effect of the bubble on solid array is dependent on the solid/liquid interface morphology, bubble size, and relative position between the bubble center and dendritic tip. Two interaction mechanisms, including engulfment and entrapment, are compared, and the difference is caused mainly by the redistribution of solute. The interaction mechanism between the rising multibubbles with large deformation and the dendritic array is also discussed.     

凝固显微组织数值模拟研究的进展

概述了凝固显微组织数值模拟研究的发展过程,介绍了在晶粒惊讶上进行组织模拟的两类模型：确定论模型和概率论模型,此外,介绍了显微组织模拟研究方面的最新动态,包括相图计算和相场法的应用,最后,在分析现有模型不足的基础上,指出了显微组织模拟研究中亟需解决的问题,并对今后的发展方向作了分析。     

2D70铝合金半连续铸锭组织及热形变对组织和性能的影响

采用金相显微镜(OM)及扫描电镜(SEM)对2D70铝合金的半连续铸锭的不同位置铸态凝固组织及其热形变后的组织进行了观察,并采用X射线衍射物相分析(XRD),能谱分析仪(EDS)对合金在凝固过程中的形成相进行了分析,对经过不同热形变工艺及淬火时效处理后的棒材进行了力学性能检测。结果表明,2D70铝合金凝固组织为典型枝晶结构,铸锭从心部到外部晶粒逐渐变细小,等轴倾向逐渐增加,沿晶界析出物逐渐增加。典型铸态组织为α(Al)、灰色蜂窝状S(Al2CuMg)、褐色针状Al7Cu2Fe和棕色条状或片状Al9FeNi相。与直接挤压工艺相比,采用先锻后挤大变形工艺后的组织均匀,化合物破碎严重,为进一步热处理和压力加工提供了良好的条件。先锻后挤工艺棒材最终抗拉强度和断后延伸率高于直接挤压棒材。屈服强度峰时效前先锻后挤的低于直接挤压的,而峰失效后前者高于后者。两种变形工艺棒材的分别于时效23和20 h达到强度峰值,先锻后挤工艺棒材的峰时效时抗拉强度、屈服强度、延伸率分别为453 MPa,390 MPa和12%。     

Study on Solidification Microstructure of Slab by Numerical Simulation

The solidification structure was simulated by the commercial software CALCOSOFT under the real cooling condition during the continuous casting processes in this paper. The cellular automation-finite element model (CA-FE) was applied to simulate the microstructure of the slab, and the results were compared with the actual macrostructure. The results of microstructure simulated were in good agreement with that detected in the slabs. In addition, the solidification structure under different secondary cooling intensity was simulated. The results show that when specific water flows increase, the average grain surface area reduces, and columnar grains have the tendency to be longer.     

Modelling of Manganese Sulphide Formation during Solidification,Part II: Correlation of Solidification and MnS Formation

The high temperature properties of steels depend on the solidification parameters and the formation parameters of manganese sulphide precipitates. Therefore, the occurrence of MnS precipitations in relation to primary and secondary microstructures was studied for different steel grades with a primary delta‐ferritic solidification or a primary austenitic solidification. The liquidus and solidus temperatures as well as the δ‐γ‐transformation temperature were calculated thermodynamically and measured by a DTA analysis in order to describe the solidification and transformation temperature range. The MnS formation temperature was calculated thermodynamically and compared to the results of SEM/EDX investigations on fracture surfaces of hot tensile specimens torn at different temperatures after in situ melting and controlled solidification. A special focus of these investigations was the location of MnS precipitates in relation to the primary and secondary grain boundaries. To explain the results, calculations were carried out taking into account the supersaturation of manganese and sulphur during the solidification in residual melt on the primary grain boundaries.     

激光熔敷合金层的组织结构和凝固过程

运用ＳＥＭ，Ｘ射线衍射仪，ＴＥＭ等先进设备，对不同成分合金激光熔敷层到基体显微组织形貌的连续变化，熔敷层的相结构，取向关系和微观缺陷进行了深入系统的研究，根据试验结果和凝固理论，探讨了激光熔敷层的凝固过程，研究结果可以作为激光熔敷技术深入研究和发展的参考。     

脉冲电流对铁基合金凝固组织的影响

利用自行设计的脉冲电流发生装置和高温坩埚炉研究了脉冲电流对Fe70Cr18Ni12合金凝固组织的影响.结果表明,脉冲电流可明显细化Fe70Cr18Ni12合金的凝固组织,使该合金的晶粒尺寸由240 μm细化到1.8 μm;同时脉冲电流处理可改变合金凝固组织中奥氏体相和铁素体相的相对含量.     

Solidification Microstructure and Mechanical Properties of Cast Magnesium-Aluminum-Tin Alloys

The solidification microstructure and mechanical properties of as-cast Mg-Al-Sn alloys have been investigated using computational thermodynamics and experiments. The as-cast microstructure of Mg-Al-Sn alloys consists of α-Mg, Mg₁₇Al₁₂, and Mg₂Sn phases. The amount of Mg₁₇Al₁₂ and Mg₂Sn phases formed increases with increasing Al and Sn content and shows good agreement between the experimental results and the Scheil solidification calculations. Generally, the yield strength of as-cast alloys increases with Al and Sn content, whereas the ductility decreases. This study has confirmed an early development of Mg-7Al-2Sn alloy for structural applications and has led to a promising new Mg-7Al-5Sn alloy with significantly improved strength and ductility comparable with commercial AZ91 alloy.     

Modeling Microstructure and Irradiation Effects

This article gives an overview of the strategy followed nowadays to model the evolution of metallic alloy microstructures under irradiation. For this purpose, multiscale approaches are very often used, which rely on modeling techniques appropriate to each time and space scale. The main methods used are ab-initio calculations, classical molecular dynamics (MD), kinetic Monte Carlo (KMC), mean field rate theory (MFRT), and dislocation dynamics (DD). These methods are briefly presented along with some of their typical uses and main drawbacks. Some examples are provided of the typical information obtained with each of the techniques.     

脉冲电场对普碳钢凝固组织的影响

通过改变脉冲电场的电压对普碳钢熔体进行处理,得到了晶粒细化的效果,并针对晶粒细化效果得出了最优的脉冲电压控制参数.利用原位分析仪分析了凝固过程钢中主要元素的分布与偏析情况.在电场对主要元素的分布影响方面做出了探索性的研究,并结合前人的研究成果,浅析了脉冲电场的作用机制.     

The Influence of Composition on the Solidification Path and Microstructure of HP-Nb Alloys

Metallurgical and Materials Transactions A - Austenitic stainless steels such as 25Cr-35Ni-Nb, HP-Nb alloys are commonly used in high-temperature applications above 850 °C. The HP-Nb...     

Pore Formation During Solidification of Aluminum: Reconciliation of Experimental Observations,Modeling Assumptions,and Classical Nucleation Theory

An in-depth discussion of pore formation is presented in this paper by first reinterpreting in situ observations reported in the literature as well as assumptions commonly made to model pore formation in aluminum castings. The physics of pore formation is reviewed through theoretical fracture pressure calculations based on classical nucleation theory for homogeneous and heterogeneous nucleation, with and without dissolved gas, i.e., hydrogen. Based on the fracture pressure for aluminum, critical pore size and the corresponding probability of vacancies clustering to form that size have been calculated using thermodynamic data reported in the literature. Calculations show that it is impossible for a pore to nucleate either homogeneously or heterogeneously in aluminum, even with dissolved hydrogen. The formation of pores in aluminum castings can only be explained by inflation of entrained surface oxide films (bifilms) under reduced pressure and/or with dissolved gas, which involves only growth, avoiding any nucleation problem. This mechanism is consistent with the reinterpretations of in situ observations as well as the assumptions made in the literature to model pore formation.     

The Effect of Rapid Solidification Velocity on the Microstructure of Ag-Cu Alloys

Electron beam solidification passes have been performed on a series of Ag-Cu alloys between 1 wt pct Cu and the eutectic composition (28.1 wt pct Cu) at speeds between 1.5 and 400 cm per second. At low growth rates conventional dendritic or eutectic structures are obtained. The maximum growth rate of eutectic structure is 2.5 cm per second. At high growth rates microsegregation-free single phase structures are obtained for all compositions. The velocity required to produce this structure increases with composition for dilute alloys and agrees with the theory of absolute stability of a planar liquid-solid interface with equilibrium partitioning. For alloys between 15 and 28 wt pct Cu, the velocity required to produce the microsegregation-free extended solid solution decreases with composition and is related to nonequilibrium trapping of solute at the liquid solid interface. At intermediate growth rates for alloys with 9 wt pct Cu or greater, a structure consisting of alternating bands of cellular and cell-free material is obtained. The bands form approximately parallel to the local interface. On leave at the Center for Materials Research, The Johns Hopkins University, Baltimore, MD 21218.     

Influence of Cerium on Solidification Microstructure of M2 High Speed Steel

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Influence of Some Trace Elements on Solidification Path and Microstructure of Al-Si Foundry Alloys

In the present study, Ca, Ni, V, and Zn were added to a high purity binary Al-7wt pct Si and commercial purity A356 foundry alloy in the nominal range of 50 to 600 ppm in order to study their effect on the solidification path and the resultant microstructure. Thermal analysis was used to assess nucleation and growth of the various phases. It was found that Ca and Ni additions suppress characteristic temperatures associated with nucleation and growth of the eutectic by up to 4 and 1.5 K, respectively. Additionally, Ca was observed to modify the eutectic Si and a concentration as low as 39 ppm Ca was sufficient to precipitate the geometrically unfavored polyhedral Al₂Si₂Ca phase. Furthermore, Ni addition resulted in the formation of two intermetallic phases when the Ni concentration exceeded 300 ppm. These phases have been quantified as Al₃Ni and Al₉FeNi by SEM-EDS. V and Zn had no apparent effect on the cooling curve and the microstructure. Even though it could be shown that V accumulates preferably in β-Al₅FeSi particles, V concentrations of 600 ppm were too low to have any influence on the phase’s morphology.     

大方坯凝固过程鼓肚应变及内裂纹形成的研究

通过建立大方坯凝固过程的传热模型,获得大方坯冷却传热过程的主要凝固参数,在此基础上建立了凝固前沿坯壳所承受的应变模型,讨论了大方坯凝固过程的主要应变及其主要影响因素,并针对实际铸机的设备和工艺状况,计算大方坯凝固过程的鼓肚应变,讨论了具体钢种产生裂纹的可能性.