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

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。

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 liquidliquid 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.