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電磁浮遊炉を用いた直接観察による Al-Mg 合金の酸化皮膜挙動の解明
In-situ Observation of the Destruction of Oxide Layer on
Al-Mg Alloys Using Electromagnetic Levitation
小関みち香, 王維瑞, 市川拓弥, 清宮優作, 菅洋志, 小澤俊平,Michika KOSEKI, Weirui WANG, Takumi ICHIKAWA, Yusaku SEIMIYA, Hiroshi SUGA, andShumpei OZAWA
千葉工業大学，Chiba Institute of Technology

1. Introduction
In brazing aluminum alloys, maintaining an oxide-free surface is crucial for optimal process performance.
Although fluoride-based fluxes can achieve such a surface, the post-braze removal of residual flux is inefficient,consuming excessive time and resources. It has been reported that adding a small amount of magnesium to
Al-Si brazing alloys enables flux-free brazing, though not optimally. This improvement is believed to result
from the reduction of the Al2O3 oxide layer formed on the surface of molten brazing filler alloys by the additionof magnesium. Our group recently proposed a novel perspective on the role of added magnesium: it leads tothe formation of MgAl2O4(s) at the surface of the filler alloys, which induces shrinkage stress that allows theoxide layer to crack and fragment due to the different densities of MgAl2O4(s) and Al2O3(s).
In this study, we employed electromagnetic levitation (EML) to melt aluminum alloys with added
magnesium, facilitating in-situ observation of oxide layer destruction. The primary objective was to validate
our hypothesis regarding the mechanism of oxide layer destruction and to further investigate the effects ofmagnesium addition.

2. Experimental procedure
A cube of Al-Mg alloy sample was chemically cleaned using an aqueous solution of fluoride nitrate in an
ultrasonic cleaner and then placed on a quartz holder inside the EML chamber. The atmosphere within thechamber was replaced with a high-purity Ar-He gas mixture, maintaining an oxygen partial pressure of
approximately 10-2 Pa. The sample was electromagnetically levitated and melted under the gas flowing at
2L/min. The behavior of the oxide layer forming on the molten droplet was monitored using a high-speed
video camera operating at the frame rate of 500 FPS. Some samples were quickly solidified and collected before
the oxide layer could be destructed.
The collected sample was maintained in an iodine-methanol solution heated to 60 °C for two hours. During
this period, the solution was stirred continuously to ensure the selective dissolution of the metal potion only.
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The phase constitution of the residual oxides was then then analyzed using transmission electron microscope(TEM).

3. Results and Discussion
Figure 1 shows typical time-lapse images that capturing the progress of the destruction of the oxide layeron the surface of a molten Al-Mg alloy sample levitated by EML. The levitated sample melted into a sphericalshape, still covered by the oxide layer formed during heating. (Fig. 1a). A white phase is observed in several
locations on the surface of the gray oxide layer. Furthermore, some white phases expand instantaneously
during the heating of the sample, as indicated by the dashed circle in Fig. 1b. A crack appears from this whitephase, followed by rapid disappearance of the oxide layer (Fig. 1c).
Electron diffraction analysis using TEM confirmed the presence of MgAl2O4(s) together with Al2O3(s) in theoxide layer. Given the higher emissivity of MgAl2O4(s) compared to that of Al2O3(s), it is reasonable to supposethat the white phase is MgAl2O4(s). This behavior aligns with our hypothesis that the addition of magnesium
induces shrinkage stress, causing the oxide layer to crack and fragment due to the different densities ofMgAl2O4(s) and Al2O3(s).
This destruction behavior of the oxide layer from MgAl2O4(s) is in consistent with our hypothesis that
adding magnesium to the aluminum alloys induces shrinkage stress that allows the oxide layer to crack andfragment due to the different densities of MgAl2O4(s) and Al2O3(s).
To explain the rapid disappearance of the oxide layer after cracking, we delve into the possibility of gas
formation reactions from the melt. When the oxide layer completely covers the melt surface, gas phaseformation reactions are kinetically inhibited. However, once a gas-liquid interface appears due to cracking,these reactions become possible. The high vapor pressure of magnesium in the alloy may lead to its
evaporation, followed by the formation of MgO(s) with oxygen gas present as impurities in the atmosphere.
This process reduces the oxygen partial pressure in the vicinity of the melt surface, facilitating the reductionof the oxide layer. The formation of Al2O(g) through the following reaction further promotes thedisappearance of the oxide layer.
4Al(l)+Al2O3(s)→3Al2O(g)(1)(a) 0s(b) 0.1s(c) 0.4s(d) 1.0s(e) 1.4s(f) 2.4s

1mmFigure 1. Time-lapse images of oxide layer destruction on molten Al-Mg Alloy sample levitated by EML.

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4. Summary
We observed the destruction process of the oxide layer on an Al-Mg alloy sample over using EML. The rapid
expansion of white phases during heating led to cracking in the gray oxide layer, followed by its swift
disappearance. Electron diffraction analysis confirmed the formation of MgAl2O4(s) and Al2O3(s) within theoxide layer. This research supports the mechanism by which the addition of magnesium contributes to theformation and destruction of cracks in the oxide layer.

Acknowledgements
This research was partially funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI underGrant No. P20H02453 and by the Light Metal Education Foundation, Inc.

References
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H. Miyake and M. Edo: J. JILM 65 (2015) 396-402.

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T Shinoda, S Ozawa, K Kawashima, K Kuribayashi, T Yamayoshi, and Y Itoh, J. Japan Inst. Metals 85 (2021) 352358.

© 2024 by the authors. Submitted for possible open access publication under theterms and conditions of the Creative Commons Attribution (CC BY) license(http://creativecommons.org/li censes/by/4.0/).

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