TOWARD UNDERSTANDING PORE FORMATION AND MOBILITY DURING
CONTROLLED DIRECTIONAL SOLIDIFICATION IN A MICROGRAVITY ENVIRONMENT (PFMI)
Research Area: Materials Science
Expedition(s): 5, 7, 8, 13
Principal Investigator(s): ● Richard N. Grugel, PhD, Marshall Space Flight Center,
Huntsville, Alabama
RESEARCH OBJECTIVES
Using a transparent model material, this experiment studies the fundamental phenomena
responsible for the formation of certain classes of defects in metal castings. Investigators
examine the physical principles that control the occurrence of defects in manufacturing on
Earth in order to develop methods to reduce flaws, defects, or wasted material.
EARTH BENEFITS
On Earth, materials that contain pores created and trapped during solidification degrade
properties and cause a distinct weakening in the overall structure of the cast product. Examples
of these materials include semiconductors and aircraft turbine blades.
SPACE BENEFITS
PFMI provides insight on how materials solidify in the space environment. Once this process is
understood and improvements are made, future manufacturing processes can take place in the
microgravity environment providing robust products.
RESULTS
Observed bubble migration up through the liquid column indicates that thermocapillary forces
do play a role in bubble removal during solidification in microgravity, thereby providing a
potential mechanism for avoiding porosity in space processing. Direct comparison between the
ground-based thin (2-D) samples and the flight bulk (3-D) samples showed significant
differences in the interface texture. The flight samples achieved planar growth, an emergence
of dendrites (crystallizes in the shape of a tree or branch), in less time than ground-based
samples. When comparing the planar interface recoil, the flight sample was steeper than the
ground-based sample. Additionally, the dendrite spacing in the flight bulk samples were closer
together than the ground-based thin samples. The use of 2-D (thin) samples in one-g for
comparison with theoretical models is not adequate, therefore solidification of bulk samples in
a microgravity environment and in the lab setting is necessary for a suitable comparison. The
bulk solidification samples, which were filled with succinonitrile (SCN), were melted and re-
solidified to observe the bubbles that formed. During controlled re-solidification, aligned tubes
of gas were seen to be growing perpendicular to the planar solid/liquid interface, inferring that
the nitrogen previously dissolved into the liquid SCN was now coming out at the solid/liquid
interface and forming the little-studied liquid=solid+gas type reaction. Researchers expects that
the results will be directly applicable to understanding solidification for materials processing by
providing insights into fundamental behavior of bubbles.