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金属学报  2015, Vol. 51 Issue (6): 733-744    DOI: 10.11900/0412.1961.2014.00560
  论文 本期目录 | 过刊浏览 |
Al-7Si-Mg合金凝固过程形核模型建立及枝晶生长过程数值模拟*
陈瑞1,许庆彦1(),吴勤芳2,郭会廷2,柳百成1
1 清华大学材料学院先进成形制造教育部重点实验室, 北京 100084
2 明志科技有限公司, 苏州 215006
NUCLEATION MODEL AND DENDRITE GROWTH SIMULATION IN SOLIDIFICATON PROCESS OF Al-7Si-Mg ALLOY
Rui CHEN1,Qingyan XU1(),Qinfang WU2,Huiting GUO2,Baicheng LIU1
1 Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084
2 Mingzhi Technology Co. Limited, Suzhou 215006
引用本文:

陈瑞, 许庆彦, 吴勤芳, 郭会廷, 柳百成. Al-7Si-Mg合金凝固过程形核模型建立及枝晶生长过程数值模拟*[J]. 金属学报, 2015, 51(6): 733-744.
Rui CHEN, Qingyan XU, Qinfang WU, Huiting GUO, Baicheng LIU. NUCLEATION MODEL AND DENDRITE GROWTH SIMULATION IN SOLIDIFICATON PROCESS OF Al-7Si-Mg ALLOY[J]. Acta Metall Sin, 2015, 51(6): 733-744.

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摘要: 

针对铝合金砂型铸造较低冷速特点, 通过实测和分析不同凝固条件下的冷却曲线, 建立了适用于铝合金形核密度随最大形核过冷度呈指数性变化的形核函数. 通过与Pandat软件热力学、动力学、平衡相图数据库相耦合, 并利用空间坐标变化等算法, 建立了适用于三元铝合金二维、三维枝晶生长的CA模型. 在该模型中, 同时考虑了溶质扩散、成分过冷、曲率过冷、晶体择优取向以及不同组元之间相互作用等重要因素的影响. 利用建立的形核和生长模型, 模拟了Al-7Si-0.36Mg合金在不同凝固条件下的二维枝晶演化及形貌特征, 描述了溶质组元的分布特征以及定量地预测了二次枝晶臂间距的变化, 并与实验结果进行了对比. 三维枝晶的模拟结果有效反映了枝晶空间结构复杂性和多样性, 并与实验结果吻合良好.

关键词 三元铝合金形核模型元胞自动机枝晶生长二次枝晶臂间距    
Abstract

Due to the extensive applications in automotive and aerospace industries of Al-7Si-Mg casting alloys, its understanding of the dendrite microstructural formation is of great importance to control the desirable microstructure and thereby to modify the performance of castings. In this work, through analyzing the measured cooling curves in different cooling conditions of Al-7Si-0.36Mg ternary alloy during sand casting, a theoretical nucleation model correlated maximum nucleation undercooling with the nucleation density is proposed. Besides, a 2D and 3D cellular automaton (CA) model allowing for the quantitatively predicting dendrite growth of ternary alloys is presented. This model introduces a new tracking neighboring rule algorithm to eliminate the effect of mesh dependency on dendrite growth. The thermodynamic and kinetic data needed in the simulations is obtained by coupling with Pandat software package in combination with thermodynamic/kinetic/equilibrium phase diagram calculation databases. This model has also taken account the multi-component diffusion, constitutional undercooling, curvature undercooling, dendrite preferential growth angles as well as the effect of interactions between the alloying elements etc. This model is applied to quantitatively simulate the dendrite growth with various crystallographic orientations of Al-7Si-0.36Mg ternary alloy in 2D and 3D during polycrystalline solidification, and the predicted secondary dendrite arm spacing (SDAS) shows a reasonable agreement with the experimental results. The experimental observed complicated and diverse dendrite microstructure occurring in solidification process can be well reproduced by this 3D-CA model which has considered the effects of various preferred growth orientations, the interactions of adjacent dendrites as well as the influence of S/L interface anisotropies. The simulated results effectively demonstrated the abilities of this model in prediction of dendrite microstructure in ternary alloys.

Key wordsternary aluminum alloy    nucleation model    cellular automaton    dendrite growth    secondary dendrite arm spacing
    
基金资助:*国家重点基础研究发展计划项目2011CB706801, 国家自然科学基金项目51374137和51171089, 及国家科技重大专项项目2012ZX04012-011和2011ZX04014-052资助
图1  TiB2有效形核面及a-Al依附在TiB2衬底上的形核过程示意图
图2  形核衬底尺寸分布函数及冷却曲线示意图
图3  不同优先生长角度枝晶示意图及局部坐标系(x0, y0, z0)与世界坐标系(x, y, z)之间的位置关系
图4  实验用阶梯铸件几何尺寸示意图
图5  Al-7Si-0.36Mg合金不同阶梯处测得的冷却曲线
图6  测温点No.1~No.7的微观组织形貌和晶粒尺寸
No. Tmin / ℃ ΔTm / ℃ Te / ℃ Δt / s Rc / (℃?s-1) NS / cm-2 NV / cm-3 l2 / mm
1 611.3 3.3 565.6 257.0 0.18 54.2 226.6 70.1
2 611.0 3.6 563.1 208.0 0.23 64.1 291.5 66.8
3 610.2 4.4 562.0 134.0 0.36 80.2 407.9 60.7
4 608.8 5.8 560.8 83.0 0.58 101.6 581.6 53.2
5 606.0 8.6 559.8 50.0 0.92 135.6 896.8 43.0
6 604.1 10.6 558.7 30.0 1.51 172.1 1282.3 36.7
7 595.0 19.6 557.0 7.5 5.00 418.2 4854.3 24.9
表1  实验测得的凝固参数及形核密度等微观组织数据
图7  Al-7Si-0.36Mg合金的lnNV与ΔTm-1的函数关系
图8  模拟得到No.6测点处的微观组织演化和Si组元溶质场变化过程
Definition and symbol Value Unit
Initial compositions, w S i 0 , w M g 0 w S i 0 =7.0, w M g 0 =0.36 %
Liquidus temperature, T L l i q ( w S i L , w M g L ) Calculated K
Liquidus slope, m S i L , m M g L Calculated K%-1
Partition coefficient, k S i , k M g Calculated
Gibbs-Thomson coefficient, G 2.4×10-7 K m
Anisotropy coefficient, e 0.03
Diffusion coefficient, D i j ? Calculated m2s-1
Timestep, dt Δ x 2 / 6 m a x ( D i j ? ) s
表2  Al-7Si-0.36Mg合金模拟所需的参数
图9  模拟获得的No.2, No.4, No.6和No.7测温点处的最终凝固组织
图10  不同测温点位置的二次枝晶臂间距模拟结果和实验结果对比
图11  Al-7Si-0.36Mg合金在No.7位置处的三维多枝晶组织演变过程
图12  凝固结束时二维截面枝晶形貌和实验结果对比
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