Al-Cu-Mg三元合金的溶质分凝与凝固进程的数值模拟研究

基于相图计算方法,耦合微观凝固模型,研究了Al-Cu-Mg合金溶质分配因数在凝固过程中的变化规律,并获得了分凝特性与固相体积分数变化的定量关系.以Al-3.9%Cu-0.9%Mg为例计算表明,Cu与Mg的溶质分配因数在凝固过程中变化较大,并且,冷却速率的变化对Cu和Mg的分配因数有较大的影响.以溶质分凝规律为基础,定量预测了Al-15.0%Cu-4.0%Mg合金在不同冷却速率下的固相体积分数随温度的变化曲线.计算表明,随着冷却速率的增大,该曲线向低固相体积分数方向移动,并且非平衡共晶的体积分数增大.其中,在6K/min下的计算结果与DTA分析结果基本一致.     

Study of Microsegregation In Al--Cu—Zn Ternary Alloys by Experiment and Scheil Model

应用相图计算耦合Scheil模型、简单Scheil方程和实验比较研究了Al-Cu-Zn合金单相凝固过程的凝固路径和微观偏析，相图计算耦合Scheil模型的计算结果显示，Al-Cu-Zn合金单相凝固结束点的溶质含量与凝固初始点的成分同步变化，溶质含量和溶质分凝系数随凝固过程的进行不断增加；在热力学计算出合金分凝系数的基础上，简单Scheil方程可以计算初生相含量，从不同溶质得到的计算结果是自洽的，并与相图计算耦合Scheil模型的预测值一致；实验测定了不同成分铝合金的共晶含量和枝晶间距，实验数据与模型结果基本一致。     

Al-Cu-Zn合金溶质分凝的热力学模型

提出了计算多元合金溶质分凝行为的热力学模型，分析了活度法和浓度法两种计算平衡分凝因数方法的异同．应用提出的模型系统地研究了Al—Cu—Zn合金的平衡分凝因数，计算表明，活度法和浓度法的计算结果接近，溶质Cu和Zn的平衡分凝因数在凝固温度范围内随温度增加下降10％左右，Zn和Cu任何一种溶质元素的浓度增加都会降低另一种元素的平衡分凝因数，同时研究了杂质元素对三元Al—Cu基合金中Cu的平衡分凝因数的影响．     

铝基四元合金枝晶组织及微观偏析的数值模拟

建立了基于元胞自动机(CA)和热力学相平衡计算引擎(PanEngine)的二维耦合模型,并应用于铝基四元合金枝晶组织和微观偏析的数值模拟.在该耦合模型中,采用CA方法模拟枝晶组织的演变.以固/液界面的平衡液相线温度和实际温度的差值作为枝晶生长的驱动力,同时考虑了固/液界面曲率的Gibbs-Thomson效应.通过求解溶质传输方程获得固/液界面处三种溶质的液相成分,耦合PanEngine获得固/液界面处的平衡液相线温度及三种溶质的平衡固相成分,为提高计算效率,采用预制数据表格的优化策略将CA与PanEngine进行耦合.将Al-4.5Cu-0.5Mg-1Si(质量分数,%)四元合金凝固时的固相分数随温度的变化以及固相分数和固相成分分布关系的模拟结果与Scheil模型和平衡凝固模型的预测结果进行了对比,结果表明,该模型不仅可应用于模拟多元合金中的枝晶生长形貌,而且能对铝基四元合金系凝固的微观偏析进行定量预测.     

MODELING OF DENDRITIC STRUCTURE AND MICROSEGREGATION IN SOLIDIFICATION OF AL-RICH QUATERNARY ALLOYS

建立了基于元胞自动机(CA)和热力学相平衡计算引擎(PanEngine)的二维耦合模型, 并应用于铝 基四元合金枝晶组织和微观偏析的数值模拟. 在该耦合模型中, 采用CA方法模拟枝晶组织的演变. 以固/液界面的平衡液相线温度和实际温度的差值作为枝晶生长的驱动力, 同时考虑了固/液界面曲率的Gibbs-Thomson效应. 通过求解溶质传输方程获得固/液界面处三种溶质的液相成分, 耦合PanEngine获得固/液界面处的平衡液相线温度及三种溶质的平衡固相成分. 为提高计算效率, 采用预制数据表格的优化策略将CA与PanEngine进行耦合. 将Al-4.5Cu-0.5Mg-1Si(质量分数, %)四元合金凝固时的固相分数随温度的变化以及固相分数和固相成分分布关系的模拟结果与Scheil模型和平衡凝固模型的预测结果进行了对比, 结果表明, 该模型不仅可应用于模拟多元合金中的枝晶生长形貌, 而且能对铝基四元合金系凝固的微观偏 析进行定量预测.     

热溶质对流对定向凝固Al-Al2Cu过共晶合金组织的影响

采用垂直向下的高梯度定向凝固装置，研究了有热溶质对流的Al-38．5％Cu（质量分数，下同）过共晶合金中定向凝固组织变化。结果表明：热溶质对流造成界面前沿的溶质成分（Cu元素）沿轴向减少。在定向凝固速率为5μm／s，合金溶质成分减少到37％Cu以下时，合金定向凝固组织中，初生θ-Al2Cu相会消失，组织变为全耦合生长的共晶组织。合金凝固的固相分数（fs）≥0．49时，组织变为全耦合生长共晶组织所对应的溶质成分的理论计算结果与实验结果相吻合。     

Mg-Zn-Gd三元铸造镁合金的自由凝固路径选择

采用实验和数值计算方法,研究了Mg-4.58Zn-2.6Gd三元铸造镁合金的自由凝固路径选择.实验结果表明,当铸型冷却速率≤0.75 K/s时,合金首先生成的共晶为a(Mg)+W(Mg3Zn3Gd2);当铸型冷却速率≥7.71 K/s时,合金首先生成的共晶为a(Mg)+I(Mg3Zn6Gd).建立了综合考虑合金液相扩散和冷却速率因素的多元合金初生相凝固路径计算模型.耦合热力学计算软件Thermo-Calc及其数据库获得了Mg-4.58Zn-2.6Gd合金凝固路径计算所需的热力学数据,发现计算结果与实验结果吻合良好.     

基于微观偏析统一模型及Thermo-Calc的三元合金凝固路径耦合计算

将合金凝固微观偏析统一模型推广到三元合金凝同微观偏析的预测,并提出了能够计算三元匀晶与共晶合金凝固路径的数值计算方法.实现了在源代码层次上与热力学计算软件Thermo-Calc及其数据库的耦合,以获取多元合金凝固路径计算所需的凝固热力学数据.通过Fe-40V-40Cr,Al-4.5Cu-1.0Si,Al-10Cu一2.5Mg和Al-1.49Si-0.64Mg(质量分数,%)等多元/多相合金在小同冷速下凝固路径的实例计算,以及与相应的凝固组织定量金相实验结果对比,验证了本文多元/多相凝固模型和算法的正确性.     

由实验相图和数值模型对Al-1.5Cu-3Zn合金凝固路径的预测

使用DTA和LMC定向凝固淬火技术分别确定了Al-Cu-Zn合金富铝角的液相面和固／液界面溶质分凝因数。基于一维平板模型的枝晶几何形状描述，使用数值模型计算了Al-1．5Cu-3Zn合金凝固过程中的液相浓度和平衡温度。发现在平衡凝固条件下，液相在α相区内结束凝固，Scheil非平衡凝固条件下的凝固过程在单变量线上结束，样品中初生晶相的含量为97．4％，非平衡共晶含量为2．6％；同时分析了液相混合条件对凝固路径的影响。     

凝固速率跃迁对定向凝固Al-40%Cu过共晶合金初生Al_2Cu相的影响

采用高温度梯度定向凝固装置进行了Al-40%Cu(质量分数)过共晶合金的定向凝固实验,研究了凝固速率跃迁过程中的凝固组织演变.结果表明,当定向凝固速率从10μm/s跃迁减速到2μm/s时,由于固/液界面附近的液相成分向共晶点成分变化以及耦合共晶组织的界面生长温度高于初生Al_2Cu相的界面生长温度,合金凝固组织从初生Al_2Cu枝晶和Al/Al_2Cu共晶组织转变为全耦合层片共晶组织.组织转变过程中,板条状的初生Al_2Cu相先分解成小尺寸的初生相,然后小的初生相逐渐被共晶组织所取代,这种组织转变是凝固界面前沿液相中溶质扩散不足造成的,而不是由合金中存在的热及溶质对流引起的.在初生相生长形态中,由于凝固速率跃迁引起的界面前沿液相中Cu成分富集,造成凝固界面生长温度升高,Jackson因子α变小,Al_2Cu初生相由小平面相向非小平面相转变.     

Influences of Initial Compositions,Dendrite Morphologies and Solid-Back Diffusion on Solidification Path of Al-Si-Mg Alloys

The solidification paths of ternary eutectic Al-Si-Mg alloys in the Al-rich corner were investigated by numerical calculation. The thermodynamic data needed in the calculation were obtained by direct coupling simulation program with a CALPHAD software Thermo-Calc via its TQ6-interface and COST2 database. The influences of the initial compositions, dendrite morphologies and the solid diffusion coefficients on the solidification path were analyzed. The solidification paths were calculated with assumed different solid diffusion coefficients varying in eight orders of magnitude and different dendrite morphologies including the five basic geometrical shapes of spherical, cylindrical, plate-like, inward cylindrical, inward spherical and the assumed equiaxed dendrites. The calculated results comparisons showed that initial compositions, dendrite morphologies and solid-back diffusion can significantly influence the solidification path and the type and amounts of eutectic of Al-Si-Mg alloys. The predicted solidification paths and the eutectic fraction of selected Al-Si-Mg alloys calculated with assumption of equiaxed dendrite morphology agreed well with the experimental results.     

Solidification path calculations of Al-Zn-Mg alloys in Al-rich corner

The solidification paths of Al-Zn-Mg alloys in the Al-rich corner were investigated. The thermodynamic data for the calculation are obtained by direct coupling with the CALPHAD software Thermo-Calc via its TQ6-interface and the COST2 database. The influences of the initial compositions and the extent of solid back diffusion on the solidification path were numerically investigated by sample calculation of the ternary Al-Zn-Mg alloys. The calculation results of solidification paths of the selected alloys: Al-Zn-3 Mg(in wt.%), Al-5 Zn-10 Mg, Al-2.5Zn-15Mg, Al-10Zn-20.5 Mg, Al-8Zn-25 Mg, were: L→(L+α-Al), L→(L+α-Al)→(L+α-Al+TAU), L→(L+α-Al)→(L+α-Al+Al Mg_β), L→(L+α-Al)→(L+α-Al+TAU)→(L+α-Al+TAU+Al Mg_β), L→(L+α-Al)→(L+α-Al+Al Mg_β)→(L+α-Al+TAU+Al Mg_β), respectively. The results show that the initial compositions and the extent of solid back diffusion have a great influence on solidification path, and the amounts of eutectic phase increase with the decrease of the solid back diffusion coefficient. The equilibrium solute partition coefficients for Zn and Mg in alloys are also calculated and their influence on micro-segregation in the primary solidification of Al-5Zn-10 Mg alloy is analyzed.     

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.     

Microstructure design and heat treatment selection for casting alloys using the quality index

The ductility of casting alloys is usually low and it is thus important to simultaneously assess the effect of changes to the microstructure and heat treatment on both ductility and strength of the material. The use for this purpose of the quality index charts* is common in the casting industry with regard to the Al-Si-Mg casting alloys A356 and A357. An analytical method of generating quality index charts for any alloy system is presented. Applications of the method are illustrated with case studies involving Al-Si-Mg, Mg-Al-Zn, and Al-Si-Cu-Mg casting alloys. The analytically determined charts indicate the limits to microstructural improvement available for each material. The possibility of using the charts to optimize the relation between mechanical performance, chemical composition, solidification conditions, and temper is discussed.     

A corrosion investigation of solder candidates for high-temperature applications

The step-soldering approach is being employed in the multi-chip module technology. High-lead-containing alloys are among the solders currently being used in this approach. Au-Sn and Au-Ge based candidate alloys have been proposed as alternative solders for this application. In this work, a corrosion investigation was carried out on potential ternary lead-free candidate alloys based on these binary alloys for high-temperature applications. These promising ternary candidate alloys were determined by the CALPHAD approach based on the solidification criterion and the nature of the phases predicted in the bulk solder. This work reveals that the Au-Sn-based candidate alloys close to the eutectic composition (20 wt.% Sn) are more corrosion resistant than the Au-Ge-based ones.     

二元合金激光快速熔凝过程的非Fourier模型

基于非Fourier定律，建立了脉冲激光加热条件下单相二元合金表面快速重熔和凝固过程的非平衡传热传质模型，并根据碰撞理论和Aziz的连续生长模型处理固／液界面，以解释快速熔凝过程界面动力学的非平衡效应，快速熔凝问题是涉及热质传输的移动界面问题，通过二阶精度的Von Neumann隐式差分格式和界面跟踪方法进行过程模型的数值求解。应用该模型，分析了Al-Cu二元合金的激光表面熔凝过程，结果表明，激光的高能量密度和非平衡界面动力学所引起的熔化过程和凝固过冷对于快速熔凝过程的影响很大，在快速熔凝过程中，界面速度的变化很大，且因基底材料和热流大小而不同，通过计算获得了界面温度，速度、溶质浓度及非平衡分配系数随界面位置的变化，结果显示，在凝固过程中界面速度和界面溶质浓度都存在着很大的波动。     

新型半固态加工专用铝合金的成分设计

采用合金热力学计算方法对Al-Si—Mg系适合半固态加工的合金成分进行了优化设计，并进行了实验验证研究。结果表明：采用国际通用的ThermoCalc软件能够成功地对Al-Si-Mg系合金成分进行优化设计，计算结果与实验结果具有较好地一致性；优化后的Al-6Si-2Mg合金在各工序中都表现出良好的半固态加工性能，连铸和二次加工过程的稳定性和可控性大大提高，坯料的凝固组织更加细化、球化和均质化；通过优化工艺，最终获得了Al-6Si-2Mg合金半固态触变成形的最佳工艺参数和外观完粘、性能优良的半同态压铸件．为半固态触变成形的工业应用打好了基础。     

Numerical simulation on rapid melting and nonequilibrium solidification of pure metals and binary alloys

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Solidification behaviour of Al-7 %Si-Mg casting alloys

The effects of Mg content and cooling rate on the solidification behaviour of Al-7% Si-Mg(mass fraction)casting alloys have been investigated using differential scanning calorimetry, differential thermal analysis and microscopy. The Mg contents were selected as respectively 0.00%, 0.35% and 0. 70% (mass fraction). DTA curves of Al-7%Si-0.55%Mg(mass fraction) alloy at various cooling rates were accomplished and the alloy melt was cast in different cooling rates. The results indicate that increasing Mg content can lower the liquidus and binary Al-Si eutectic transformation temperatures. Large Fe-rich π-phases (AlsFeMg3Si6) are found in the 0.70% Mg alloys together with some small β-phases (Al5FeSi) ; in contrast, only β-phases are observed in the 0.35% Mg alloys. The test results of the Al-7%Si-0.55% Mg alloys identify that the liquidus and binary Al-Si eutectic transformation temperatures decrease, and the quantity of ternary Al-Si-Mg2 Si eutectic phase decreases as the cooling rate increases.