1.Shi-changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Fund Project:Project supported by the Chinese Academy of Sciences Strategic Priority Program on Space Science, China (Grant No. XDA15013800), the Natural Science Foundation of Liaoning Province, China (Grant No. 2020-MS-005), and the National Natural Science Foundation of China (Grant Nos. 51771210, 51501207, 51971227)
Received Date:29 August 2020
Accepted Date:22 January 2021
Available Online:01 April 2021
Published Online:20 April 2021
Abstract:Solidification microstructure of aluminum alloy has a great influence on the properties of the casting. An aluminum alloy with the structure of fine equiaxed grains has low casting defects and presents excellent mechanical properties. Recently, chemical inoculation by adding grain refiner is the technique most extensively used to achieve a fine, equiaxed grain structure of Al alloy in the industrial production. In order to investigate the detailed solidification microstructure evolution of the alloy, many numerical models have been proposed. Cellular automaton method is one of the powerful tools for simulating the morphology evolution of detailed grains in the solidification process of alloy. However, the present cellular automata model has a shortcoming, that is, its calculation of the nucleation rate is based on the experimental number density of grains. In this work, a population dynamics-cellular automaton model is developed for describing the solidification microstructure evolution of the inoculated aluminum alloy. The model takes account of the heterogeneous nucleation of α-Al nucleus, the initial spherical growth of α-Al grains and the dendritic growth process. The model is used for simulating the solidification microstructure evolution of the commercial-purity aluminum (CP-Al) inoculated by Al-5Ti-1B master alloy. The results indicate that the heterogeneous nucleation process of α-Al can be divided into the two stages. In the early stage of nucleation, there are enough effective particles in the melt. The nucleation rate of α-Al increases with the increase of the undercooling of the melt. After a short time, the nucleation of α-Al is dominated by the number density of the effective particles in the melt. Nucleation process stops when the recalescence takes place. The effects of the additive amount of Al-5Ti-1B master alloy and the cooling rate of the melt on the solidification microstructure of the CP-Al are investigated by using the established model. The final solidification structures of CP-Al are predicted. And a comparison between the predicted results and the experimental ones shows that they are in good agreement with each other. Keywords:aluminum alloys/ nucleation/ grains growth/ modeling
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2.1.α-Al形核及晶粒球形长大的群体动力学模型
定义函数f (r, t)来描述α-Al晶粒的尺寸分布. 则t时刻, 熔体内半径处于r — r + dr范围内α-Al晶粒的数量密度为f (r, t)dr. α-Al晶粒的数量密度n, 平均半径$ \langle r \rangle $, 体积分数φ可以通过下式计算:
$n = \int\nolimits_0^\infty {f(r,t)} {\rm{d}}r,$
$ \langle r \rangle = (1/n)\int\nolimits_0^\infty {rf(r,t)} {\rm{d}}r,$
通过拟合计算确定α-Al与TiB2粒子之间的接触角θ. 即取不同的接触角θ, 模拟0.3%的Al-5Ti-1B中间合金添加到工业纯铝熔体中, 并以3.5 K/s的冷却速度凝固后工业纯铝的最终凝固组织及晶粒尺寸. 对比模拟结果与实验结果, 确定θ为4.8°. 图1为添加0.01% Al-5Ti-1B中间合金后, 熔体以3.5 K/s的速度冷却时, 工业纯铝凝固过程中熔体的过饱和度、液-固相变的形核驱动力以及α-Al形核率与凝固时间的关系. 随着熔体温度的降低, 熔体逐渐变为过饱和状态. 熔体的过饱和度和形核驱动力随着熔体冷却时间的延长逐渐增大. 当熔体的温度降低到某一临界温度时, α-Al在Al熔体中开始形核. 形核过程非常短暂, 大约为0.07 s. 由于受到有效TiB2粒子数量的影响, α-Al的形核过程可分为两个阶段: 阶段I为形核刚开始发生时, 熔体中有效TiB2粒子数量充足, α-Al形核率由(6)式确定; 阶段II为随着熔体过饱和度、α-Al形核驱动力的增大, 有效TiB2粒子数量增加速度不能满足熔体形核时, 有效TiB2粒子数目将限制熔体中α-Al的形核(图1中阶段II). 熔体冷却过程中, 晶粒形核和长大释放凝固潜热趋向于降低熔体过冷度, 熔体与外界热交换引起的熔体温度降低将增加熔体过冷度. 在凝固的初始阶段, 熔体中α-Al晶粒的形核率及数量比较低, 因此熔体过冷度随着凝固时间的增加逐渐增加. 高的过冷度使得熔体内部晶核形核率增加, 形成较多新的α-Al晶粒. 当凝固潜热释放的速度超过熔体与外界换热速度时, 再辉发生, 熔体过冷度减小, 不能进一步提供新的有效异质核心, 形核过程停止. 图 1 0.01% Al-5Ti-1B中间合金细化处理工业纯铝凝固过程中, 熔体过饱和度S = Cl – Ce, α-Al形核驱动力ΔGV, 异质形核率I随时间的变化. 熔体冷速为3.5 K/s Figure1. The supersaturation of the melt S = Cl – Ce (dash line), the driving force of nucleation ΔGV (dot line) and the heterogeneous nucleation rate I (solid line) of α-Al during cooling the CP-Al melt inoculated by 0.01% Al-5Ti-1B master alloy at the cooling rate of 3.5 K/s.
α-Al异质形核期间, 晶粒数量随着凝固时间增加逐渐增加, 尺寸分布变宽(如图2所示). 形成的α-Al晶粒在形核早期以球状方式快速生长, 当晶粒半径超过球状生长的临界值($ 21r^* $)时, 界面失稳, 晶粒开始以树枝状的方式进行生长. 图3给出了两种不同生长方式的晶粒数量密度在α-Al异质形核期间随凝固时间的变化. 可见, 球状α-Al晶核数量在形核初期快速增加, 随后逐渐地演变成为树枝晶. 最终, 晶核都以树枝晶的方式生长. 从图4(a)—图4(c)可以看出, 生长初期, 树枝晶距离较远, 以多重对称的方式沿着各自优先生长取向生长. 随着树枝晶周围溶质富集程度的不断增加, 固-液界面的不稳定性加剧, 一次枝晶臂粗化, 二次枝晶臂开始显现. 当不同的树枝晶逐渐长大并互相接触时, 枝晶周围的溶质场发生碰撞, 枝晶臂生长互相阻碍, 树枝晶逐渐失去对称性. 在凝固末期, 不同的树枝晶相互接触, 枝晶臂不断生长和粗化, 最终形成具有不规则形貌的凝固组织(如图4(d)所示). 图 2 0.01% Al-5Ti-1B中间合金细化处理工业纯铝凝固过程中, α-Al晶核半径r分布随时间的变化. 熔体冷速为3.5 K/s Figure2. The radius distribution of α-Al nucleus at different time during cooling the CP-Al melt inoculated by 0.01% Al-5Ti-1B master alloy at the cooling rate of 3.5 K/s.
图 3 0.01% Al-5Ti-1B中间合金细化处理工业纯铝凝固过程中, α-Al晶核总数量nall, 球状晶nspherical以及树枝晶数目ndendrities随时间的变化. 熔体冷速为3.5 K/s Figure3. The number density of all nucleus nall in the Al melt, the number density of spherical nucleus nspherical and the number density of dendrities ndendrities during cooling the CP-Al melt inoculated by 0.01% Al-5Ti-1B master alloy at the cooling rate of 3.5 K/s.
图 4 0.01% Al-5Ti-1B中间合金细化处理工业纯铝凝固微观组织演变过程 (a) 固相分数 = 0.1%; (b) 固相分数 = 1.0%; (c) 固相分数 = 25.0%; (d) 固相分数 = 70.0%. 熔体冷速为3.5 K/s. 计算区域尺寸为600 μm × 600 μm × 600 μm Figure4. Solidification microstructure evolution during cooling the CP-Al melt inoculated by 0.01% Al-5Ti-1B master alloy at a cooling rate of 3.5 K/s: (a) Solid fraction = 0.1%; (b) solid fraction = 1.0%; (c) solid fraction = 25.0%; (d) solid fraction = 70.0%. The size of computational domain is 600 μm × 600 μm × 600 μm.
23.2.中间合金添加量对铝合金凝固过程的影响 -->
3.2.中间合金添加量对铝合金凝固过程的影响
图5为工业纯铝熔体中添加不同数量Al-5Ti-1B中间合金凝固时, α-Al异质形核率与熔体过冷度随时间的变化关系. 可以看出, 当Al-5Ti-1B中间合金加入量为0.01%时, α-Al在熔体过冷度约为0.4 K时开始形核. 形核过程中, 阶段I的形核主要受熔体过冷度的控制, 形核率随熔体过冷度的增大不断增加. 异质形核过程进入阶段II时, 熔体中有效TiB2粒子数量不足, α-Al形核受到熔体中有效TiB2粒子数量限制. 随着凝固潜热释放速度的增加, 熔体过冷度开始降低, 再辉发生, 形核过程停止. 形核过程中阶段I和II持续时间分别占总形核时间的50%左右. 当Al-5Ti-1B加入量为0.4%和1.0%时, 由于Al-Ti合金体系富铝角液相线斜率为正值, 额外的溶质Ti含量的增加使液相线温度提高, 因此熔体形核时达到的最大形核过冷度较大, 熔体中有效TiB2粒子数量随之增加. 当熔体过冷度分别约为0.6和1.0 K时, α-Al开始形核. 形核过程中, 阶段I发生的时间占整个形核过程的90%左右. 阶段II发生较短时间后, 熔体中形核及晶粒生长带来的固相分数增加释放凝固潜热速率足够快, 熔体过冷度开始降低, 再辉发生, 形核过程停止. 图 5 不同数量Al-5Ti-1B中间合金细化工业纯铝凝固时异质形核率I, 熔体过冷度ΔT随时间的变化. 熔体冷速为3.5 K/s Figure5. Calculated heterogeneous nucleation rate I of α-Al and the undercooling of the melt ΔT for the CP-Al inoculated by different amount of Al-5Ti-1B master alloy at a cooling rate of 3.5 K/s.
图6为模拟计算的工业纯铝最终凝固组织. 可以看到, 随着中间合金添加量的增加, 凝固组织中含有的晶粒数目逐渐增多, 晶粒尺寸逐渐减少. 当凝固组织晶粒尺寸较大时, 枝晶的枝晶臂均较发达, 晶粒大小分布较分散. 这类凝固组织通常具有较差力学能与后续加工性能. 随着晶粒尺寸的降低, 枝晶臂的生长受到限制, 凝固组织主要以均匀细小的等轴晶粒组成. 图7给出了不同数量细化剂细化工业纯铝晶粒尺寸的预测值与实验值. 可以发现, 模拟结果与实验结果符合得较好. 图 6 不同数量Al-5Ti-1B中间合金细化工业纯铝凝固组织模拟结果 (a) 0.005%; (b) 0.01%; (c) 0.4%; (d) 1.0%. 熔体冷速为3.5 K/s. 计算区域尺寸为900 μm × 900 μm × 900 μm Figure6. Simulated solidification microstructure for the CP-Al melt inoculated by different amount of Al-5Ti-1B master alloy at a cooling rate of 3.5 K/s: (a) 0.005%; (b) 0.01%; (c) 0.4%; (d) 1.0%. The size of computational domain is 900 μm × 900 μm × 900 μm.
图 7 不同数量Al-5Ti-1B中间合金细化工业纯铝晶粒尺寸预测结果与实验结果. 熔体冷速为3.5 K/s. 误差棒为标准偏差 Figure7. Predicted and measured grains size vs. the additive amount of Al-5Ti-1B master alloy for the inoculated CP-Al at a cooling rate of 3.5 K/s. The error bars represent the standard deviations.
23.3.冷却速度对细化处理条件下铝合金凝固过程的影响 -->
3.3.冷却速度对细化处理条件下铝合金凝固过程的影响
图8给出了不同冷速条件下α-Al异质形核率随时间的变化. 可以看出, 当熔体的冷却速度为1.0, 2.0和5.5 K/s时, 第I阶段形核占据了α-Al异质形核的大部分, 第II阶段形核只持续很短的时间后, 再辉发生, 形核停止(如图8(a)—图8(c)). 当熔体的冷却速度为10 K/s时, 形核阶段I持续时间所占比例降低, 之后阶段II的形核过程受熔体中异质核心粒子数量的限制, 直到再辉发生, 形核停止, 如图8(d). 图 8 不同冷却速度下经0.5% Al-5Ti-1B中间合金细化处理, 工业纯铝凝固时α-Al形核率随时间的变化 (a) 1.0 K/s; (b) 2.0 K/s; (c) 5.5 K/s; (d) 10.0 K/s Figure8. Calculated heterogeneous nucleation rate of α-Al during cooling the CP-Al inoculated by 0.5% Al-5Ti-1B master alloy at the different rate: (a) 1.0 K/s; (b) 2.0 K/s; (c) 5.5 K/s; (d) 10.0 K/s.
图9为不同冷速条件下, 0.5% Al-5Ti-1B中间合金细化处理工业纯铝凝固组织的模拟结果. 可以看到, 随着熔体冷却速度的增加, 凝固组织中含有的晶粒数目逐渐增多, 晶粒尺寸逐渐减少. 当熔体冷却速度较低时, 凝固过程中形核数目较少, 凝固组织晶粒较大. 当熔体冷却速度较高时, 凝固过程中短时间内产生的晶核数目较多, 晶粒没有充足的时间长大, 枝晶的枝晶臂生长受到周围晶粒的阻碍, 合金最终凝固组织主要以均匀细小的等轴晶粒组成. 图10给出了不同冷速条件下经0.5% Al-5Ti-1B中间合金细化, 工业纯铝晶粒尺寸的预测值与实验值. 可以发现, 模拟结果与实验结果符合得较好. 图 9 0.5%Al-5Ti-1B中间合金细化工业纯铝, 在不同冷却速度凝固后凝固组织的模拟结果 (a) 1.0 K/s; (b) 2.0 K/s; (c) 5.5 K/s; (d) 10 K/s. 计算区域尺寸为900 μm × 900 μm × 900 μm Figure9. Simulated solidification microstructure for the CP-Al melt inoculated by 0.5% Al-5Ti-1B master alloy at the different cooling rate of the melt: (a) 1.0 K/s; (b) 2.0 K/s; (c) 5.5 K/s; (d) 10 K/s. The size of computational domain is 900 μm × 900 μm × 900 μm.
图 10 0.5%Al-5Ti-1B中间合金细化工业纯铝, 在不同冷却速度下凝固后晶粒尺寸预测结果与实验结果. 误差棒为标准偏差 Figure10. Predicted and measured grains size vs. the cooling rate of the melt for the CP-Al inoculated by 0.5%Al-5Ti-1B master alloy. The error bars represent the standard deviations.