P23
浮遊法による TiC 添加 Ti6Al4V 合金の凝固組織微細化に及ぼす対流の影響
Effect of Convection on Microstructure Refinement in
Ti6Al4V Alloys with Additive TiC Solidified via
Levitation Techniques
原田侑直 1，青木祐和 2，馬渕勇司 2，花田知優 2，上田雄翔 2，門井洸衛 2，櫛舎祐太 2，鈴木進補 2，米田香苗 3，左口凌成 3，山田素子 3，佐藤尚 3，渡辺義見 3，白鳥英 4，中野禅 5，佐藤直子 6，小山千尋 7，
織田裕久 7，石川毅彦 7，渡邊勇基 8，島岡太郎 9，清宮優作 1，小澤俊平 1
Yuma HARADA¹, Hirokazu AOKI², Yuji MABUCHI², Chihiro HANADA², Yuto UEDA², Koei KADOI²,Yuta KUSHIYA², Shinsuke SUZUKI², Kanae YONEDA³, Ryosei SAGUCHI³, Motoko YAMADA³,Hisashi SATO³, Yoshimi WATANABE³, Suguru SHIRATORI4, Shizuka NAKANO5, Naoko SATO6,
Chihiro KOYAMA7, Hirohisa ODA7, Takehiko ISHIKAWA7, Yuki WATANABE8, Taro SHIMAOKA9,Yusaku SEIMIYA¹ and Shumpei OZAWA¹
千葉工業大学，Chiba Institute of Technology
早稲田大学，Waseda University
3
名古屋工業大学，Nagoya Institute of Technology
4
東京都市大学，Tokyo City University
5
株式会社 Henry Monitor，Henry Monitor Inc.,
6
産業技術総合研究所，National Institute of Advanced Industrial Science and Technology
7
宇宙航空研究開発機構，Japan Aerospace Exploration Agency (JAXA)
8
株式会社エイ・イー・エス，Advanced Engineering Services Co., Ltd. (AES)9
日本宇宙フォーラム，Japan Space Forum (JSF)121.

Introduction
Additive manufacturing (AM) is attracting attention for achieving in-space and in-orbit manufacturing

due to its ability to produce intricate three-dimensional products near net shape, without joints. However, thesolidification conditions in AM, which include, unidirectional solidification, repeated laser heating, andsubstantial temperature gradients, often induce a formation of large columnar microstructure, resulting inproducts with anisotropic strength of the products. For Ti6Al4V alloy, a leading material for aerospacecomponents, the addition of TiC particles serves as a heterogeneous nucleation site, promoting the formationof fine equiaxed grains. This effectively inhibits large columnar structures during the solidification via AM.
However, the grain refinement mechanism using TiC on the solidified microstructure of Ti6Al4V alloys has
not yet been well clarified due to the presence of container wall and gravitational convection in the terrestrialexperiments. Therefore, our group is conducting research on the solidification of Ti6Al4V alloys with added
TiC, using levitation techniques involving a space experiment with the electrostatic levitation furnace in theInternational Space Station (ISS-ELF), called Hetero-3D mission.
In the present work, we examined the microstructural differences in Ti6Al4V alloys solidified undercontainer-free conditions using electrostatic levitation (ESL) with minimal convection, and electromagnetic
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levitation (EML)1) with large convection due to electromagnetic agitation on the ground. The aim of this study
was to examine the influence of melt convection during solidification on the microstructure of Ti6Al4V alloys
as well as the effect of added TiC, as preliminary ground work for the Hetero-3D mission.
2.

Experiment procedure
Gas-atomized Ti6Al4V powders, with nominal diameters of less than 45 μm, were thoroughly mixed with5 mass % TiC powders, with nominal diameters less than 20 μm, using a three-dimensional motion mixer.
This mixture was then sintered into a cylindrical compact, with 20 mm in diameter and 10 mm in thickness,using spark plasma sintering (SPS). The sintered sample was cut into cubes, with sides of 2 mm forcontainerless solidification experiments using ESL and 6 mm for those using EML.
For the ESL experiment, a cube cut from the sintered compact was melted and solidified into a nearly
spherical shape using an arc furnace, under a high-purity argon gas atmosphere. The spherical sample waselectrostatically levitated and subsequently melted by irradiating it with CO2 laser beams from three directions.
Upon shutting off the laser, the sample was cooled and solidified under a containerless state. The temperature
history of the sample was monitored by monochromatic pyrometers. Details of the ESL facility andexperiments can be found elsewhere2).
In the EML experiment, a piece of the cube sample was placed on a quartz holder and positioned in thecenter of the EML coil. The sample was then electromagnetically levitated and then inductively melted underhigh purity argon gas flowing at 2 L/min. To enhance the sample heating, irradiation from a semiconductor
laser beam was superimposed on the levitated droplet. After turning off the semiconductor laser heating, thelevitated droplet was cooled and solidified by blowing high-purity helium gas over it. The cross-sectional
microstructures of the solidified samples obtained from both the ESL and EML experiments were examinedusing a scanning electron microscopy (SEM) equipped with an electron backscattered diffraction detector(EBSD).
3.

Results and Discussion
Figure 1 shows typical cooling curves of the levitated sample solidified by using ESL and EML. When theheating time of the levitated melt was prolonged or its maximum temperature was high, the melt experienced
a large undercooling followed by recalescence, as shown in (a) and (c), regardless of the levitation techniques
used. It has been reported that TiC dissolves in the molten Ti6Al4V at elevated temperatures. As a result, theadded TiC particles would dissolve before solidification and would serve as heterogeneous nucleation sites inthese samples.
Conversely, when the heating duration of the sample was minimized and was immediately cooled after
melting, solidification began at the equilibrium liquidus temperature, regardless of whether the experiment
was conducted using ESL or EML, as indicated in Figure 1 (b) and (d). This confirms that excessive heatingcauses the TiC particles to dissolve into the molten Ti6Al4V. For the samples subjected to minimal heating, therelease of latent heat during solidification led to a lower cooling rate post nucleation at the equilibrium
liquidus temperature.
Figure 2 exhibits the typical microstructures of the solidified samples, corresponding to the cooling curvesof (a) to (d) of Figure 1 The microstructures of the samples that experienced a large undercooling before
solidification consisted of relatively large grains as displayed in (a) and (c). In contrast, the microstructures of2 of 4the samples that nucleated at the equilibrium liquidus temperature presented smaller grains, as observed in(b) and (d). This result confirms that the TiC particles acted as a heterogeneous nucleation site.
A higher cooling rate of the sample usually results in small grains in the solidified microstructure 3-5). The
cooling rate of the levitated droplet was slower in the EML than in the ESL, because the electromagneticallylevitated sample inevitable undergoes inductive heating, even when it is forcibly cooled by blowing heliumgas. However, the grain size of the sample solidified by the EML was smaller than that of the sample solidified
by the ESL. Strong electromagnetic stirring of the melt in the EML technique may cause fragmentation orseparation of the crystal growing on the melt surface6-8). In addition, the larger number of TiC particles
contained in the sample for the EML experiment may promote nucleation. Although the sample compositionis the same for both the ESL and EML experiments, the number of TiC particles in the sample is greater in theEML experiment than in the ESL experiment due to the larger sample size. It is difficult to distinguish theeffects of cooling rate, convection and number of TiC particles on the solidified microstructure as long as weuse the ESL and EML on the ground. The results of the Hetero-3D project under microgravity conditions on
the ISS will provide further insight into the grain refinement effects of added TiC on solidified Ti6Al4V.
2200

ESL
(a)

Temperature T , K

2100

EML
(c)

(b)

(d)

2000
1900
1800

1700
1600
1500
1400
010

2030

40

50

60

70

80

Time t , sFigure 1. Typical cooling curves of levitated molten samples by ESL and EMLESLEML

(a)

(b)

(c)

(d)

500μm

500μm

500μm

500μm

Figure 2. Typical microstructures of solidified samples by ESL and EML
4.

Summary
Ti6Al4V samples with 5 mass% TiC, prepared from their respective powders by SPS, were melted andsolidified under levitated state using ESL and EML. Due to prolonged heating and reaching higher maximum
temperatures, TiC particles dissolved in the melt. As a result, the molten sample undercooled deeply prior tosolidification. The microstructures of the solidified samples exhibited relatively large grains. In contrast, when
the sample was subjected to minimal heating and was immediately cooled after melting, nucleation occurred
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at the equilibrium liquidus temperature, due to heterogeneous nucleation from residual TiC particles. This
resulted in a microstructure with smaller grains in the solidified sample. Even though the cooling rate at post
nucleation in the sample solidified using EML is lower than that using ESL, the resulting grain size was smaller.
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© 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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