The Behaviour of Bifilm Defects in Cast Al-7Si-Mg Alloy

Double oxide films (bifilms) are significant defects in the casting of light alloys, and have been shown to decrease tensile and fatigue properties, and also to increase their scatter, making casting properties unreproducible and unreliable. A bifilm consists of doubled-over oxide films containing a gas-filled crevice and is formed due to surface turbulence of the liquid metal during handling and/or pouring. Previous studies has shown that the nature of oxide film defects may change with time, as the atmosphere inside the bifilm could be consumed by reaction with the surrounding melt, which may enhance the mechanical properties of Al alloy castings. As a proxy for a bifilm, an air bubble was trapped within an Al-7wt.%Si-0.3wt.%Mg (2L99) alloy melt, subjected to stirring. The effect of different parameters such as the holding time, stirring velocity and melt temperature on the change in gas composition of the bubble was investigated, using a design of experiments (DoE) approach. Also, the solid species inside the bubbles solidified in the melt were examined using SEM. The results suggested that both oxygen and nitrogen inside the bifilm would be consumed by reaction with the surrounding melt producing MgAl2O4 and AlN, respectively. Also, hydrogen was suggested to consistently diffuse into the defect. The reaction rates and the rate of H diffusion were shown to increase upon increasing the holding time and temperature, and stirring velocity. Such significant effect of the process parameters studied on the gaseous content of the bubble suggesting that a careful control of such parameters might lead to the deactivation of bifilm defects, or at least elimination of their deteriorous effect in light alloy castings.双氧化膜（bifilms）是轻合金铸造过程中的一类重要缺陷，研究表明其会降低合金的拉伸与疲劳性能，同时增大性能离散度，导致铸造性能无法复现且可靠性下降。双氧化膜由折叠的氧化膜构成，内部带有充气缝隙，其形成源于液态金属在转运和/或浇注过程中发生的表面紊流。已有研究表明，双氧化膜缺陷的特性会随时间发生变化：双氧化膜内部的气氛可通过与周围熔体发生反应而被消耗，这一过程可改善铝合金铸件的力学性能。本研究以捕捉于Al-7wt.%Si-0.3wt.%Mg (2L99)合金熔体中的气泡作为双氧化膜的替代模型，并对其施加搅拌作用；采用实验设计（Design of Experiments，DoE）方法，研究了保温时间、搅拌速度、熔体温度等不同参数对气泡内气体成分变化的影响。同时，借助扫描电镜（Scanning Electron Microscopy，SEM）对熔体中凝固气泡内部的固相产物进行了表征。结果显示，双氧化膜内的氧和氮会分别与周围熔体反应，生成MgAl₂O₄和AlN而被消耗；此外，氢会持续扩散进入该缺陷内部。研究还发现，提升保温时间、熔体温度与搅拌速度可加快反应速率与氢的扩散速率。上述工艺参数对气泡内气体含量的显著影响表明，对这些参数进行精准调控，有望实现双氧化膜缺陷的钝化，或至少消除其在轻合金铸件中的有害影响。