Analysis Method for Recalescence Time Obtained Using High-Speed Camera for Solidification Experiments at the Electrostatic Levitation Furnace in the International Space Station

This study was designed as a preliminary experiment for missions at the electrostatic levitation furnace in the International Space Station (ISS-ELF) such as Hetero-3D. The objective of this study was to employ a high-speed camera to determine the recalescence time of undercooled Ti6Al4V alloy with TiC heterogeneous nucleation site particles more accurately compared to measurements using a pyrometer. The sample was melted and solidified in the ground-based electrostatic levitation (ESL) furnace. The changes in the luminescence emitted from the sample surface due to recalescence were recorded using a high-speed camera, and the intensity at the center of each captured image was analyzed with MATLAB® software. As a result, the intensity of the undercooled samples increased significantly during recalescence. The maximum change in the intensity was 101 per 256 gray levels in the recording at 7,200 frames per second (fps) and the noise was at most only 3. Therefore, the recalescence time could be obtained using the high-speed camera. At 15,000 fps, the recalescence time was within 6.7 × 10-5 s, which was much more accurate than the time (0.11 s) measured using a pyrometer at a sampling rate of 120 Hz in the ESL furnace. For enhanced measurement accuracy, it is crucial to establish an appropriate exposure time to prevent the intensity from reaching the lower limit of 0 just before recalescence and the upper limit of 255 immediately after that. This study suggests that the high-speed camera newly installed in the ISS-ELF in 2023 may have the potential to achieve a more accurate recalescence time than the pyrometer at a sampling rate of 100 Hz in the ISS-ELF.