Metallurgical Abstracts on Light Metals and Alloys vol.57
Peculiar microstructural evolution and hardness variation depending on laser powder bed fusion-manufacturing condition in Ti-6Al-2Sn-4Zr-6Mo
Prince Valentine Cobbinah1, Sae Matsunaga1, Yoshiaki Toda2, Ryosuke Ozasa3, Masayuki Okugawa3, Takuya Ishimoto3,4, Yuheng Liu3, Yuichiro Koizumi3,
Pan Wang5, Takayoshi Nakano3* and Yoko Yamabe-Mitarai1
1 Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
2 Research Center for Structural Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan
3 Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
4 Aluminium Research Center, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
5 Singapore Institute of Manufacturing Technology, Singapore City 637662, Singapore
[Published in Smart Materials in Manufacturing., 2 (2024), 100050]
https://doi.org/10.1016/j.smmf.2024.100050
E-mail: p.cobbinah22s[at]ams.k.u-tokyo.ac.jp, mitarai.yoko[at]edu.k.u-tokyo.ac.jp, Nakano[at]mat.eng.osaka-u.ac.jp
Key Words: LPBF; Ti6246; polycrystalline microstructure; metastable phase; rapid cooling; thermal history
The objective of this study was to comprehensively analyze the phase and microstructure evolution and related hardness variations in Ti-6Al-2Sn-4Zr-6Mo wt.% (Ti6246) samples produced by laser powder bed fusion (LPBF) under various laser conditions and to gain insight into the mechanisms of these changes using numerical thermal analysis. Samples fabricated with higher volumetric energy densities (VEDs) exhibited a finer α/α' microstructure, resulting in higher hardness; VED positively correlated with hardness, except under very high-VED conditions. This finer microstructure can be attributed to the solid-phase transformation from β phase to metastable α' martensite during the LPBF process caused by rapid cooling, which generates a higher density of α' phase nucleation sites under high-VED conditions. Despite showing a very fine microstructure, the samples fabricated under very high-VED conditions showed lower hardness, deviating from the overall trend. Sharpening of the X-ray diffraction peaks in the high-VED samples suggested a partial transformation of the α' phase to the α phase due to reheating of the underlying layers by laser irradiation, which was considered to be a factor in the hardness reduction. Numerical analysis showed that the underlying layer is exposed to high temperatures for a relatively long time under high-VED conditions. In conclusion, the hardness of LPBF-fabricated Ti6246 is determined by a combination of two thermal processes unique to LPBF: rapid cooling and reheating of the pre-solidified part. This thermal history results in the formation of a metastable α' phase and partial phase transformation toward the α phase. These findings could be used to fabricate Ti6246 parts with desired physical properties via LPBF.