Al-Bi合金凝固过程及微合金化元素Sn的影响
Solidification of Al-Bi Alloy and Influence of Microalloying Element Sn
通讯作者: 赵九洲,jzzhao@imr.ac.cn,主要从事合金凝固过程研究
责任编辑: 李海兰
收稿日期: 2018-09-25 修回日期: 2019-01-21 网络出版日期: 2019-06-26
基金资助: |
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Corresponding authors: ZHAO Jiuzhou, professor, Tel:
Received: 2018-09-25 Revised: 2019-01-21 Online: 2019-06-26
Fund supported: |
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作者简介 About authors
黎旺,女,1993年生,博士生
实验研究了Al-Bi合金凝固过程及微合金化元素Sn的影响,发现添加微量Sn能有效改变Al-Bi合金的液-液相变过程、细化富Bi相粒子。Sn对富Bi相的细化效果随着Sn添加量的增加而增强,当添加量≥0.10% (质量分数)时即可达到最佳细化效果。建立了Al-Bi合金凝固过程中组织演变的动力学模型,模拟分析了微合金化元素Sn作用下Al-Bi合金凝固组织形成过程。结果表明,微量Sn可有效降低Al-Bi合金两液相间的界面能,提高富Bi相液滴的形核率,促进Al-Bi合金形成弥散型凝固组织。
关键词:
Al-Bi alloy has a low friction coefficient and high wear-resistant properties and is a good self-lubricating material for advanced bearings in automotive applications if the soft Bi-rich phase is dispersedly distributed in the comparatively harder Al-based matrix. However, Al-Bi alloy is a typical immiscible alloy. When cooling a homogeneous single phase liquid of Al-Bi alloy in the miscibility gap, it transforms into two liquids. The liquid-liquid phase transformation generally leads to the formation of a phase segregated microstructure. In the last decades, considerable efforts have been made to study the solidification behavior of Al-Bi alloy. It is demonstrated that the microstructure evolution during the liquid-liquid decomposition is a result of concurrent actions of the nucleation, growth, Ostwald ripening and motions of the Bi-rich droplets. The nucleation and the immigration of the Bi-rich droplets show a dominant influence on the solidification microstructure of Al-Bi alloy. Enhancing the nucleation rate and reducing the Marangoni migration velocity of the Bi-rich droplets promote the formation of a well dispersed microstructure. Considering that addition of surface active element to the alloy may result in a reduction in the liquid-liquid interface energy, and thus reduce the nucleation energy barrier and Marangoni migration velocity of the Bi-rich droplets, the possibility to control the solidification process and microstructure of Al-Bi alloys by adding micro-alloying element Sn was investigated. The experimental results show that microalloying element Sn can cause an effective refinement of the Bi-rich particles. The refining effect increases with the increase of Bi content up to 0.10%Sn (mass fraciton). A model was developed to calculate the microstructure formation. The numerical results demonstrate that Sn can act as an effective surface active element for Al-Bi alloys and promote the formation of a well dispersed microstructure.
Keywords:
本文引用格式
黎旺, 孙倩, 江鸿翔, 赵九洲.
LI Wang, SUN Qian, JIANG Hongxiang, ZHAO Jiuzhou.
1 实验方法
采用纯度为99.99% (质量分数,下同)的Al、Bi和Sn作为原料。将纯Al装入石墨坩埚内加热熔化并升温到1223 K,加入适量的Bi和Sn;加热至1323 K,保温30 min,在保温过程中不断搅拌,确保形成均一熔体;将熔体浇铸到石墨模中冷却凝固,得到直径12 mm、长90 mm的圆柱样品。用直径0.15 mm的镍铬-镍硅热电偶测定合金试样中心位置的冷却曲线。将获得的样品沿纵向和径向剖开,对切面进行研磨、抛光,制备金相试样,用S-2400N扫描电镜(SEM)观察显微组织,采用定量金相图像方法分析少量相粒子的尺寸分布和平均直径。
2 实验结果
图1为实验测定的Al-9.0%Bi (质量分数,下同)合金冷却曲线。在Al-Bi合金难混溶温度区间内的冷却速率为70~130 K/s,α-Al凝固时间为(0.7±0.05) s。
图1
图1
Al-9.0%Bi合金熔体冷却曲线
Fig.1
Temperature curve of Al-9.0%Bi alloy during cooling (t—cooling time, tα-Al—growth time of α-Al)
图2为Al-xBi (x=5.0%、7.0%、9.0%、12.0%)合金试样中心部位的显微组织。图中黑色相和白色相分别为α-Al基体和富Bi少量相粒子。富Bi少量相以球形颗粒形式弥散分布于α-Al基体中。定量金相分析表明,合金中富Bi相粒子尺寸呈双峰分布(图3)。尺寸较大的分布峰对应于液-液相变过程中形成的初生富Bi相液滴/粒子(以下称为“初生液滴/粒子”或“primary droplets/particles”)。这些液滴形核后经历较长时间的长大/粗化才被固/液界面所吞并,尺寸较粗大;尺寸较小的分布峰对应于在固/液界面前沿形成的次生富Bi相液滴/粒子(以下称为“次生液滴/粒子”或“secondary droplets/particles”),由于形成不久即被固/液界面所吞并,尺寸较小。对比不同成分Al-Bi合金中富Bi相粒子的尺寸分布可知,随着Bi含量的增加,富Bi相粒子的尺寸分布宽度明显增大,且初生富Bi相粒子所对应的分布峰向大尺寸方向移动。
图2
图2
Al-xBi合金显微组织
Fig.2
Microstructures of the Al-xBi alloys with x=5.0% (a), x=7.0% (b), x=9.0% (c) and x=12.0% (d)
图3
图3
Al-xBi合金中富Bi相粒子的二维尺寸分布
Fig.3
2D size distributions of the Bi-rich particles in the Al-xBi alloys with x=5.0% (a), x=7.0% (b), x=9.0% (c) and x=12.0% (d)
图4
图4
Al-9.0%Bi-ySn合金显微组织
Fig.4
Microstructures of Al-9.0%Bi-ySn alloys with y=0 (a), y=0.05% (b), y=0.10% (c) and y=0.15% (d)
图6
图6
Al-xBi-0.10%Sn合金显微组织
Fig.6
Microstructures of the Al-xBi-0.10%Sn alloys with x=5.0% (a), x=7.0% (b), x=9.0% (c) and x=12.0% (d)
图7
图7
Al-xBi-ySn (y=0、0.10%)合金内富Bi相粒子二维平均直径随Bi含量的变化
Fig.7
Average 2D diameters of the Bi-rich particles in the Al-xBi-ySn (y=0, 0.10%) alloys vs Bi content
图5
图5
Al-9.0%Bi-ySn合金中富Bi相粒子的二维平均尺寸随Sn添加量的变化
Fig.5
Average 2D diameters of the Bi-rich particles in the Al-9.0%Bi-ySn alloys with different additions of Sn
3 理论模型
图8
图8
实验测定Al-9.0%Bi合金中富Bi相颗粒体积分数(ϕp)沿试样轴向和径向分布
Fig.8
Measured distribution of the volume fraction of minority phase particles (MPPs) (ϕp) in the Al-9.0%Bi alloy along the axial z direction (a) and radial r direction (b)
定义函数
式中,
式中,
式中,
凝固过程中体系中的溶质浓度(
式中,
金属熔体的黏度(
熔体中溶质Bi的扩散系数(
式中,
偏晶合金两液相间界面能计算式为[2]:
式中,
4 分析讨论
4.1 Al-Bi合金凝固组织演变
Al-Bi合金熔体在难混溶区间冷却过程中,初生富Bi相液滴均质形核[26,28];在固/液界面附近,次生富Bi相液滴是在固/液界面上形核还是在母相熔体中形核取决于
根据实验条件对Al-Bi合金凝固组织演变过程开展了模拟研究。不同成分Al-Bi合金中富Bi相粒子尺寸分布的模拟结果示于图3。可见,模拟结果与实验结果相符,表明所建模型能合理描述Al-Bi合金凝固过程。
图9给出Al-9.0%Bi合金
图9
图9
Al-9.0%Bi和Al-9.0%Bi-0.10%Sn合金测量熔体温度(
Fig.9
Measured temperature (
图10
图10
Al-9.0%Bi和Al-9.0%Bi-0.10%Sn合金富Bi相二维平均直径(
Fig.10
2D average diameters (
4.2 微合金化元素Sn对Al-Bi合金凝固过程影响
向Al-Bi合金中添加第三组元Sn可能从3个方面影响合金凝固组织:(1) 改变合金相图;(2) 改变合金凝固过程中原子的扩散行为;(3) 改变富Bi相液滴与基体熔体间的界面能。当Sn添加量很低时,其对Al-Bi合金相图和原子扩散行为的影响可以忽略,主要通过改变富Bi液滴与基体熔体间的界面能来影响合金凝固过程和组织。加入微合金化元素Sn能有效细化弥散相粒子,表明Sn可作为表面活性元素降低两液相间界面能。添加Sn后两液相间界面能由
溶质在熔体相界面处的吸附行为可表示为[40]:
式中,
由式(9)可知,在给定温度下,溶质在相界面处的吸附量
添加微量合金元素Sn不影响Al-Bi合金体系的相图,
图9和10为Al-9.0%Bi和Al-9.0%Bi-0.10%Sn合金凝固组织演变的模拟结果。添加微合金化元素Sn能有效降低富Bi相液滴形核所需的过冷度,大幅度提高富Bi相液滴的形核率和数量密度,显著细化富Bi相液滴/粒子,促进Al-Bi合金形成弥散型复合组织。
5 结论
(1) 微合金化元素Sn能有效改变Al-Bi合金的液-液相变过程,细化富Bi相粒子。Sn对富Bi相的细化效果随着Sn含量的增加而增强,当添加量≥0.10%时即可达到最佳细化效果;
(2) 微合金化元素Sn影响Al-Bi合金凝固过程的机理为降低两液相间的界面能。