OR3-6
静電浮遊，電磁浮遊，ガス浮遊におけるヘテロ凝固核 TiC 添加 Ti6Al4V の結晶粒微細化
Grain Refinement of Ti6Al4V with Heterogeneous
Nucleation Site Particles TiC in Electrostatic Levitation,
Electromagnetic Levitation, and Aerodynamic Levitation
花田知優 1, 青木祐和 1, 馬渕勇司 1, 上田雄翔 1, 門井洸衛 1, 櫛舎祐太 1, 米田香苗 2, 左口凌成 2,
山田素子 2, 佐藤尚 2, 渡辺義見 2, 原田侑直 3, 小澤俊平 3, 中野禅 4, 小山千尋 5, 織田裕久 5, 石川毅彦 5,
渡邊勇基 6, 鈴木進補 1
Chihiro HANADA1, Hirokazu AOKI1, Yuji MABUCHI1, Yuto UEDA1, Koei KADOI1, Yuta KUSHIYA1,Kanae YONEDA2, Ryosei SAGUCHI2, Motoko YAMADA2, Hisashi SATO2, Yoshimi WATANABE2,Yuma HARADA3, Shumpei OZAWA3, Shizuka NAKANO4, Chihiro KOYAMA5, Hirohisa ODA5,Takehiko ISHIKAWA5, Yuki WATANABE6, and Shinsuke SUZUKI1早稲田大学，Waseda University,
名古屋工業大学，Nagoya Institute of Technology,
3 千葉工業大学，Chiba Institute of Technology,
4 株式会社 Henry Monitor，Henry Monitor Inc.,
5 宇宙航空研究開発機構，Japan Aerospace Exploration Agency (JAXA),
6 株式会社エイ・イー・エス，Advanced Engineering Services (AES)
121.

Introduction
To clarify the grain refinement mechanism in Ti6Al4V alloy with heterogeneous nucleation site particles

TiC, a space mission named Hetero-3D was carried out using the electrostatic levitation furnace in theInternational Space Station (ISS-ELF). Meanwhile, melting and solidification experiments with the same
samples in electrostatic levitation (ESL), electromagnetic levitation (EML) and aerodynamic levitation (ADL)
were necessary as ground-based reference experiments. According to the previous research1), ESL, EML, andADL provide different levitation environments such as the Reynolds numbers of 15, 2.4 × 104 and 923 indroplets, respectively. Thus, the objective of this study was to reveal the solidification behavior of the samples
and the grain refinement effects of heterogeneous nucleation site particles in these levitation methods.
2.

Experimental Procedures
Ti6Al4V powder with 5 mass% TiC was sintered using the spark plasma sintering method and cut intocubes of approximately 30 mg for ESL, 700 mg for EML, and 20 mg for ADL. The samples for ESL and ADL
were solidified into spheres in an arc furnace for stable levitations. The samples were melted using lasers andsolidified spontaneously in all levitation methods. The numbers of prior-β grains per 1 mm2 were counted ongrain maps of cross-sections of the samples obtained by SEM-EBSD after the levitation experiments.
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3.

Results
The numbers of prior-β grains in the cross-sections of the samples for ESL, EML (surface), EML (inside),and ADL were 6, 23, 15, and 272 /mm2, respectively. In the ADL sample, larger grains (A in Fig. 1(c)) arelocated on the lower side, while extremely fine grains (B in Fig. 1(c)) are situated in the middle area.

Figure 1. Grain maps of prior-β in the samples in (a) electrostatic levitation (ESL), (b) electromagneticlevitation (EML), and (c) aerodynamic levitation (ADL). Corresponding counts of grains per
1 mm2 are also provided. The arrow in (c) indicates the direction of gravity. Due to a sample
rotation, the gravitational direction could not be identified in ESL and EML.

4.

Discussion
In an ADL sample, a large temperature gradient exists in the vertical direction due to gas jet and laser

heating1). This leads to nucleation starting from the lower side of a sample and subsequent columnar grain
growth. However, as columnar grains grow, equiaxed grains also form due to a decrease in the temperature
gradient caused by latent heat and solute concentration. It is referred to as columnar equiaxed transition.
In ESL and EML samples, most nucleation occurs near a sample surface, where temperature begins todrop first due to contact with the atmosphere during cooling. As a result, most grains in the ESL sample, as
shown in Fig. 1(a), were in contact with the sample surface. In contrast, in a molten sample that experiencessignificant convection, it is hypothesized that growing crystals do not stay in their nucleation locations but
move along the flow. Therefore, a greater number of grains with nearly the same size were formed throughout
the EML sample, which was significantly different from the ESL and ADL samples, as shown in Fig. 1(b).
5.

Conclusions
In EML, the significant convection in a molten sample with heterogeneous nucleation site particles TiC

results in a greater number of prior-β grains with more uniform grain sizes than those in ESL. An ADL sample
with temperature gradient solidifies in vertical direction, which generates both columnar and equiaxed grains.
Reference1)

T. Usui, S. Shiratori, K. Tanimoto, S. Ozawa, T. Ishikawa, S. Suzuki, H. Nagano, and K. Shimano: Surrogate models
for the magnitude of convection in droplets levitated through EML, ADL, and ESL methods. Int. J. Microgravity Sci.
Appl., 40 (2023) 400302, DOI: 10.15011/jasma.40.400302.

© 2023 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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