shed from the remaining liquid fraction, the cooling rate decreases, which often leads to
seeding and growth of equiaxed (having axes of about the same length) grains. This effect is
described as a columnar-to-equiaxed transition (CET) and is very important, and highly studied,
in metal forming processes and metallurgy since it greatly affects the physical properties and
behavior of virtually all metallic products, including high-value parts such as single crystal
turbine blades in aircraft engines. CET experiments to study and control this transitional
process have been successfully performed in the Materials Science Laboratory (MSL) with the
Low-Gradient Furnace (LGF) module onboard the ISS from November 2009 until April 2010.
Turbulent melt flow is minimized in space, which enables growth of equiaxed grains free of
sedimentation and buoyancy effects. The critical phases of each microgravity experiment, i.e.
the homogenization and solidification phases, were performed during sleep periods of the
astronauts to reduce, as well, vibrational disturbances. Gravity sensors data close to the MSL
confirm that a gravity level below ±0.0005 g was achieved during all experiments, g = 9.8m/s²
on Earth. Aluminium-silicon (AlSi) alloys with and without grain refiners (particles added to limit
crystal grain branching) were processed
successfully in the LGF. First analysis shows that
in the non-grain-refined samples columnar
dendritic growth exists, whereas CET is
observed in the grain refined samples. Critical
parameters for the temperature gradient and
the cooling rate describing CET are determined
from analysis of the thermal data and the grain
structure. These data are used for initial
numerical simulations to predict the position of
the columnar-to-equiaxed transition and will
form a unique database for calibration and
further development of numerical CET-
modeling (Zimmermann 2011).
Preliminary results of an AlSi mixture with grain
refiners show that, during solidification, the
columnar crystallization front advances forward
and an undercooled liquid zone develops ahead
the front, thus facilitating equiaxed crystal
formation. Equiaxed nucleation with grain
refiners follows the free growth model in
simulation. In most castings, grain refiner
particles may be engulfed or pushed by the
growing solid liquid interface. So, these grain
refiner particles cannot initiate grains and normally end up in the grain boundaries, thus
general grain refiner efficiency is very low. It was found that the efficiency of the grain refiners
is at a maximum when addition level is low. Experimental CET, in this case, is at a distance of
~128 ± 2 mm versus the simulation distance of 127.5 mm. Hence, the agreement between
ISS026E014918 – NASA astronaut Catherine
(Cady) Coleman, Expedition 26 flight engineer,
removes the Low Gradient Furnace and installs the
Solidification and Quench Furnace in the Material
Science Laboratory in the Destiny laboratory of the
International Space Station.