Suppression of Bubble Formation in Levitated Molten Samples of Ti6Al4V with TiC for Hetero-3D at the International Space Station (ISS)

Abstract

The space mission Hetero-3D is planned to clarify the grain refinement mechanism of Ti6Al4V with titanium carbide (TiC) heterogeneous nucleation site particles; the mission involves melting and solidifying samples in the electrostatic levitation furnace in the International Space Station (ISS-ELF). This study aimed to confirm the optimal sample preparation process for preventing bubble formation in ISS-ELF experiments. Bubble formation can impede the observation of the nucleation behavior of the samples. In this study, TiC particles were prepared using two methods: pulverizing with a mortar and pestle or a ball mill and crusher mill. These methods produce two types of particles that differ in their compositions and distributions of particle diameters. The TiC particles pulverized with the mortar and pestle were smaller and contaminated by the material from which the mortar and pestle were manufactured. Furthermore, Ti6Al4V and TiC particles were sintered under two different conditions. Samples sintered under a lower pressure had higher porosity than those sintered under a higher pressure. Owing to these differences, samples comprising the TiC particles pulverized using the ball mill and crusher mill could prevent bubble formation in ground-based electrostatic levitation experiments; in contrast, samples comprising TiC particles pulverized using the mortar and pestle and sintered under a low pressure formed a bubble. In conclusion, the optimal sample preparation process for suppressing bubble formation involves 1) preventing contamination on the surface of the TiC particles, 2) improving the fluidity of TiC or sintering the particles under a high pressure, and 3) reducing the surface area of TiC by increasing the mean diameter.