Use of Thermophysical Properties to Select and Control Convection During Rapid Solidification of Steel Alloys Using Electromagnetic Levitation on the Space Station

Abstract

A major reason to conduct solidification experiments in space is that the unique conditions accessible in reduced-gravity allow investigation of fundamental questions while limiting the influence of sedimentation or buoyancyinduced convection. When processing metallic alloys using containerless electromagnetic levitation, convection may be controlled over a wide range, spanning the laminar-turbulent transition, by proper selection of facility operating conditions. By measuring key thermophysical properties such as density, viscosity, and electrical resistivity on-orbit, the specific sample being processed may be characterized and the results used to update pre-mission magnetohydrodynamic model predictions of induced stirring within the droplet. Thus, convection becomes a controlled experimental parameter that can be applied to an investigation of how stirring influences the metastable-tostable transformation during rapid solidification of FeCrNi alloys. For these alloys, the incubation or delay time is observed to be a weak function of undercooling and a strong function of applied convection.

Metastable solidification pathways of undercooled eutectic CoSi–CoSi2 alloys

“…In other work[14], the delay time of Co-61.8 at. % Si alloys using EML was about 9.6 s at an undercooling of K. These previous results imply that the transformation delay time depends on the level of melt convention induced by different experimental techniques[24,25]. The intensity of melt flow would also depend on the volume fraction of primary solid.…”

Retained free energy as a driving force for phase transformation during rapid solidification of stainless steel alloys in microgravity

“…A single sample of 60Fe-20Cr-20Ni (wt%) was processed in microgravity aboard the International Space Station using the ISS-EML electromagnetic levitation facility through a collaboration between NASA, ESA and the German Space Agency DLR-Köln. 15 This facility uses a dual-frequency coil that superimposes two RF oscillating currents to independently control positioning and heating. The 6.5 mm diameter sample was processed in 450 mbar helium or argon to vary the conductive cooling rate.…”

“…The transition from laminar to turbulent flow may only be investigated in microgravity and this work is based largely on analysis of experiments run using the ESA Electromagnetic Levitation Furnace (ISS-EML) on the International Space Station. 15 Note that microgravity alone does not influence phase selection. Rather microgravity is enabling in order to conduct controlled experiments beyond what may be accomplished on earth.…”

Fundamentals of Liquid Processing in Low Earth Orbit: From Thermophysical Properties to Microstructure Formation in Metallic Alloys

“…Steel-casting alloys show complex solidification steps with primary metastable ferritic phase formation, rapidly followed by a transformation to the stable austenite. 10 This transformation has been shown to be significantly influenced by liquid convection. Maintaining microstructural control requires development of casting models which include these effects.…”