Acta Metall Sin  2019, Vol. 55 Issue (7): 831-839    DOI: 10.11900/0412.1961.2018.00450
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Solidification of Al-Bi Alloy and Influence of Microalloying Element Sn
Wang LI1,2,Qian SUN1,2,Hongxiang JIANG1,Jiuzhou ZHAO1,2()
1. 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
Abstract

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.

 ZTFLH: TG111.4，TG27
Fund: National Natural Science Foundation of China(Nos.51471173);National Natural Science Foundation of China(51771210);National Natural Science Foundation of China(51501207);National Natural Science Foundation of China(China Manned Space Engineering Project)
Corresponding Authors:  Jiuzhou ZHAO     E-mail:  jzzhao@imr.ac.cn
 Fig.1  Temperature curve of Al-9.0%Bi alloy during cooling (t—cooling time, tα-Al—growth time of α-Al) 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) 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) 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) 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) Fig.7  Average 2D diameters of the Bi-rich particles in the Al-xBi-ySn (y=0, 0.10%) alloys vs Bi content Fig.5  Average 2D diameters of the Bi-rich particles in the Al-9.0%Bi-ySn alloys with different additions of Sn Fig.8  Measured distribution of the volume fraction of minority phase particles (MPPs) (?p) in the Al-9.0%Bi alloy along the axial z direction (a) and radial r direction (b) Fig.9  Measured temperature ($Tmelt$), equilibrium bimodal line temperature ($Tb$), undercooling ($ΔTd=Tb-Tmelt$) of the matrix melt, nucleation rate of the Bi-rich droplets ($Id$) and the volume fraction of α-Al ($ξ$α-Al) as a function of time during a cooling of the Al-9.0%Bi alloy (dash line) and Al-9.0%Bi-0.10%Sn alloy (solid line). The inserted figure is the enlargement of the microstructure evolution during the period from 3.47 s to 3.55 s. The subscripts 1 and 2 represent the primary droplets and the secondary droplets, respectively Fig.10  2D average diameters ($$), number density ($Nd$) and $Id$ of the Bi-rich droplets as a function of time during a cooling of the Al-9.0%Bi alloy (dash line) and Al-9.0%Bi-0.10%Sn alloy (solid line) (The subscripts 1 and 2 represent the primary droplets and the secondary droplets, respectively)