PS28
電磁浮遊法を用いた Al および Al-Si 融体の表面張力測定
Surface Tension Measurement of Molten Al and Al-SiAlloys Using Electromagnetic Levitation
米山陽人, 清宮優作, 岩野貴哉, 小澤俊平
Haruto YONEYAMA, Yusaku SEIMIYA, Takaya IWANO and Shunpei OZAWA
千葉工業大学， Chiba Institute of Technology

1. Introduction
Al-Si alloys are utilized in various products manufactured through high-temperature melt processes suchas casting, additive manufacturing, and brazing, due to their unique properties including low density, highthermal conductivity, low thermal expansion, and excellent heat and wear resistance. Despite the critical roleof surface tension in optimizing these processes, the relationship between the surface tension of molten Al-Si
alloys, temperature, and composition remains poorly understood. Even for pure aluminum and silicon, thefundamental components of these alloys, significant discrepancies are observed in reported surface tension
values. Notably, our group has recently reported accurate and precise surface tension data for liquid silicon
by completely preventing sample contamination from the container and considering the effect of oxygen
adsorption from atmosphere. The large discrepancies in the reported data for the surface tension of liquidaluminum may be attributed to challenges in suppressing oxidation in the easily oxidizable aluminum andcontamination from containers at high temperatures.
In this study, we measured the surface tension of molten Al-Si alloys, ensuring free of sample sample
contamination from the container, using the oscillating droplet method with electromagnetic levitation (EML).
This approach accounted for the effects of sample evaporation and oxygen adsorption. The aim of this study
was to elucidate the relationship between surface tension, temperature, and alloy composition.
2. Experimental procedure
Al-Si alloys were prepared through arc melting using high-purity raw materials of 99.99 mass%
aluminum and semiconductor-grade silicon with an electrical resistance of 3200 Ω cm, under high-purity
argon gas at an oxygen partial pressure (Po2) of 10-7 atm. The resultant Al-Si alloy was cut into rectangularsamples measuring 5x5x6 mm and each weighed approximately 400 mg. These samples were chemicallycleaned in an aqueous solution of nitric fluoride and subsequently cleaned with acetone using an ultrasoniccleaner. A piece of the sample was then placed on a quartz holder inside the electromagnetic levitation (EML)chamber. After the atmosphere within the chamber was replaced with a high-purity Ar-He gas mixture at Po2of 10-7 atm, the sample was electromagnetically levitated and melted under the gas flowing at a rate of 2 L/min.
Semiconductor laser heating was superimposed on the levitated sample to enhance heating. The temperatureof the levitated droplet was controlled by varying the output power of the laser, and the partial pressure ofargon and helium gases. A monochromatic pyrometer was used to measure the temperature of the levitateddroplet. The emissivity setting of the pyrometer was adjusted so that the plateau temperature observed during
remelting the sample in repeated melting and solidification cycles matches the equilibrium liquidus
temperature of the alloy. The oscillation behavior of the droplet was monitored from above at 500 fps for 16
sec using a high- speed camera. The frequencies of the surface oscillations of m = 0, ±1, and ±2 for the l = 2
mode, and those of the center of gravity were analyzed from time-sequential data of the HSV images. The1 of 3

surface tension of the droplet was calculated from the frequencies using the Rayleigh equation1) and theCummings and Blackburn calibration2).
3. Results and Discussion
Figure 1 illustrates the typical temperature history of a pure aluminum sample that was repeatedly melted
and solidified using EML. The pyrometer signal becomes corrupted at around 55 seconds as the sample begins
to melt. The detected temperature is higher than that of the equilibrium melting point of aluminum (933 K)
due to the presence of an oxide layer with higher emissivity. Once all the solid aluminum has melted, thetemperature readings stabilizes with reduced signal noise. However, at around 95 seconds, the signal becomeslargely corrupted again as the Al2O3 oxide layer gradually disappear through the high temperature reaction:
Al2O3 →Al2O(g)+O2(g).
When the molten sample cools, it experiences a large undercooling followed by recalescence, observed
around 180 seconds. Upon subsequent remelting of the solidified sample, the pyrometer signal reflects theequilibrium melting point of aluminum, closely matching the maximum temperature noted after recalescence.
These observations underscore the importance of removing the oxide layer for accurate and precise
temperature measurements of molten aluminum alloys using a pyrometer.
Figure 2 shows the surface tension of molten Al-Si alloys as a function of temperature and sample
composition. The surface tension of liquid aluminum decreases linearly with increasing temperature. Theabsolute values and the temperature coefficient of surface tension measured in this study align well with thosereported by Pottlacher’s group.
The surface tension molten Al-Si alloys also decrease as the sample temperature rises. An increase in siliconcomposition lowers the surface tension and the absolute value of the temperature coefficient. This reduction
in surface tension, observed with increasing Si composition, exhibits additive behavior across the measured
temperature range.

Figure 1

Typical temperature history of a pure aluminum sample that was repeatedly melted andsolidified using EML.

Figure 2 Surface tension of molten Al-Si alloys as a function of temperature and sample composition.
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Acknowledgements
This research was partially funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI
under Grant No. P20H02453 and by the Light Metal Education Foundation, Inc.
References1)
2)

Lord Rayleigh: On the Capillary Phenomena of Jets. R. Soc. Lond, 29 (1879) 71, DOI:
https://doi.org/10.1098/rspl.1879.0015
D.L. Cummings, D.A. Blackburn: Oscillations of magnetically levitated aspherical droplets. J. Fluid Mech, 224(1991) 395, DOI: https://doi.org/10.1017/S0022112091001817© 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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