Experiments Using a Ground-Based Electrostatic Levitator and Numerical Modeling of Melt Convection for the Iron-Cobalt System in Support of Space Experiments

Abstract

Materials research is being conducted using an electromagnetic levitator installed in the International Space Station. Various metallic alloys were tested to elucidate unknown links among the structures, processes, and properties. To accomplish the mission of these space experiments, several ground-based activities have been carried out. This paper presents some of our ground-based supporting experiments and numerical modeling efforts. Mass evaporation of Fe50Co50, one of the flight compositions, was predicted numerically and validated by the tests using an electrostatic levitator (ESL). The density of various compositions within the Fe-Co system was measured with ESL. These results serve as reference data for the space experiments. The convection inside a electromagneticallylevitated droplet was also modeled to predict the flow status, shear rate, and convection velocity under various process parameters, which is essential information for designing and analyzing the space experiments of some flight compositions influenced by convection. INTRODUCTIONSince 2015, several materials experiments have been performed using the materials science laboratory electromagnetic levitator (MSL-EML) aboard the International Space Station (ISS). More than 50 principal investigators from 13 different countries have been participating in this space program. Thermophysical properties, transport phenomena, and glass/quasicrystal formation of various alloys are being studied using MSL-EML. To increase the chance of success in the space experiments, the team has meticulously planned and prepared for their science experiments. The safety aspects, mass evaporation [1] and thermophysical properties [2-4] of each sample have been measured as part of the ground support program.The measurement of mass evaporation is important. Evaporation often causes composition shifts of tested alloys resulting from preferential evaporation of volatile species. Such composition shifts can be reduced by limiting superheating and time for thermal hold during property measurements. On the other hand, evaporated materials may end up as aerosols, which can be hazardous to the health of the astronauts. Otherwise the evaporated materials will deposit on the optics, obscuring measurements, or on the coils. Excessive coil deposition can cause flaking of deposited materials, cross-contamination, short circuiting, and arcing. Each batch has 18 samples, and the amount of mass evaporation of each sample is different depending on the composition and thermal history. After discussion among scientists, the thermal cycles of each samples are adjusted such that the total amount of mass evaporation does not exceed a designated value.For flight samples whose internal convection is of interest, numerical models were developed [5] and used [6]. The developed numerical models have been used to predict the convection status under an anticipated combination of process parameters. The status of convection affects phase selection during solidification of metal alloys which exhibit ...