Collaborative Research: ISS: Segregation of Inhomogeneous Alloy Melt Driven by Capillary Flow in Microgravity

NON-TECHNICAL SUMMARY

To advance the exploration of space, learning how to control liquids in the absence of gravity is essential. On Earth, the flow of liquid is mostly controlled by gravity. In the absence of gravity, capillary forces (surface tension) govern flow.

In space, water in a glass would not remain conformed to the shape of the glass. In space, water’s shape is driven by capillary forces, and would cover all sides of the glass and form a blob over all surfaces. The absence of gravity also means the absence of buoyancy.

On Earth, a closed bottle of water with some air would have the air on top, but in space, the air bubbles could be anywhere inside the bottle and have any size. This project focuses on molten metallic alloys, which are a liquid useful for brazing and soldering in space for repair of damaged surfaces (due to impact of micrometeoroids and space debris), as well as for construction in space.

Such alloys melt and solidify gradually. The goal is to control the concurrent melting and capillary flow of a molten alloy in microgravity and predict the resulting microstructure of the bond. This project is ensuring durable bonding for repair and construction in space man-made habitats and impacting the ability to control other liquids in space and on Earth.

TECHNICAL SUMMARY

It is observed that near-eutectic binary alloys, subject to concurrent melting and capillary/gravitational flow, are prone to flow-induced segregation whereupon it solidifies into two very different microstructures. The extent of segregation varies, apparently depending on the processing conditions as well as on the geometry of the capillary flow. This project consists of: (i) a series of experiments performed on the U.S.

International Space Station with simultaneous ground-based experiments, and, since the melting/flow solidification process cannot be observed directly – (ii) a detailed mesoscale modelling program of the process (phase field computations). The objectives of the project are to: (i) understand the detailed physical mechanisms of segregation during capillary flow under both microgravity and terrestrial conditions.

Focus is being afforded to the effects of gravity, temperature gradients, peak temperature, and interaction of diffusion-controlled melting and flow of the melt. This activity also allows for the formulation of the mathematical/computational theory needed to operationalize this understanding. (ii) apply this theory to the design of methodologies for brazing and soldering in both space and terrestrial materials bonding.The new theory and predictive models being produced in this work utilize a phase field formulation of the capillary flow under conditions of void formation and is being verified through the experiments capturing the differences in microstructure of the re-solidified melts obtained in microgravity and under terrestrial conditions.

In addition to brazing metals, the results will be relevant for capillary phenomena involving low temperature soldering as well as processes related to bonding of ceramics and metals at high temperatures. Furthermore, the findings of this project are leading to better understanding of capillary phenomena involved with multilayer metal deposition in advanced technologies such as additive manufacturing via selective laser melting.

Educational broader impacts include the addition of new modules to the existing graduate courses at Washington State University and the University of Kentucky, research experience for undergraduate students, and outreach to high school students.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.