Use of Thermophysical Properties to Select and Control Convection During Rapid Solidification of Steel Alloys Using Electromagnetic Levitation on the Space Station
Use of Thermophysical Properties to Select and Control Convection During Rapid Solidification of Steel Alloys Using Electromagnetic Levitation on the Space Station
0 1.-Department of Mechanical Engineering, Tufts University , Medford, MA , USA. 2.-Mechanical and Industrial Engineering Department, University of Massachusetts , Amherst, MA , USA. 3.-Leibniz-Institut fu ̈r Festerko ̈rperund Werkstoffforschung (IFW) , Dresden , Germany. 4.-Institut fu ̈r Materialphysik im Weltraum, Deutsches Zentrum fu ̈r Luftund Raumfahrt (DLR) , Cologne, Germany. 5. -Institut fu ̈r Mikro-und Nanomaterialen, Universita ̈t Ulm , Ulm, Germany. 6.-
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.
Austenitic stainless steels that are specified for
use in welding and casting applications are often
selected based on a desire to limit hot cracking. A
Schaeffler diagram1 or DeLong diagram2 is used to
predict the ferrite content after welding; a Schoefer
diagram3 is used in a similar manner for casting
applications. For the ternary FeCrNi system, the
phase diagram is characterized by a pseudobinary
eutectic line running along a composition ratio (CR)
Cr/Ni ratio of around 1.5 based on solute weight
percent4 that also corresponds to a decreasing
ternary peritectic as the solvent Fe concentration
decreases. The solidification path for a hypoeutectic
alloy adjacent to the peritectic line (1.0 < CR < 1.5)
would involve nucleation of the metastable ferrite
(bcc d-phase) from the liquid with subsequent
conversion to stable austenite (fcc c-phase) within
the metastable mushy zone. This two-step process is
known as double recalescence.5 Transformation of
the skeletal ferrite results in a microstructure
resistant to hot cracking6 and is known as the
ferrite-to-austenite FA-mode behavior.7 Phase
selection in a macroscopic part would be controlled
by the delay, or incubation time, between nucleation
events and by the relative growth velocities of the
two phases.8 It is desirable for this delay to be as
long as possible such that most of the casting has
experienced double recalescence. This nucleation
and growth competition behavior is also observed in
a wide range of commercially important alloy
systems including FeNi,9 FeC,10 FeCo,11 TiAl,12,13
CoSi,14 TiZrNi,15 and NdFeB.16
The results of synchrotron studies10 revealed that
the close-packed (110) plane of the metastable d-phase
is crystographically coincident with the close-packed
(111) plane of the stable c-phase in Fe–C and that the
transformation was massive-like rather than
peritectic. Interfacial energies between phases are a strong
function of misorientation angle and interface
type.17,18 The findings from analyses of surface
energetics suggest that nucleation occurs within the
preexisting metastable FeCrNi solid19 and that
nucleation initiates as a peritectic transformation at slow
cooling rates,20 such as during welding, and as a
massive transformation at high cooling rates, such as
during casting or rapid solidification processing10 in
As the thermal driving force that drives
secondary recalescence events is independent of
stirring or primary undercooling, classic nucleation
theory would suggest that the delay is independent
of each.19 For rapid solidification processing, of
particular interest is the observation that the delay
time is a weak function of primary undercooling and
a strong function of melt stirring,21 and this article
provides a coherent dataset from a single
experimental platform on a single sample to allow new
theories to be tested.
To study recalescence phenomena, containerless
techniques are used to promote deep undercooling
for these highly chemically reactive melts. Two
approaches are commonly employed in
groundbased research: electrostatic levitation (ESL) and
electromagnetic levitation (EML). A key attribute of
ESL testing at the NASA Marshall Space Flight
Center (MSFC), where levitation is caused by
interaction between the sample and a static electric
field, is that internal convection is minimized
during free cooling such that there is no induced
stirring and solidification proceeds on a quiescent
sample. In contrast, EML testing is associated with
significant induced stirring on a highly turbulent
sample caused by the interaction between the
sample and the alternating field responsible for
overcoming gravity. On the ground, only quiescent
or turbulent conditions are accessible. By going to
reduced gravity in space, the forces required to
position the sample are much smaller, and by using
EML, a broad range of convective conditions is
accessible.22 Predicting fluid flow is accomplished
using magnetohydrodynamic (MHD) modeling,
which has been validated by comparing liquid
recirculation predictions to observations on a
levitated CuCo alloy droplet.23
Space testing of double recalescence phenomena
thus has four key steps. First, thermophysical
properties are measured using containerless
techniques on the ground to support thermal cycle
control modeling activities. Second, MHD modeling
is used to design specific run parameters for space
experiments that control convection over a wide
range of operating conditions.24 Third,
thermophysical properties are remeasured on orbit to validate
and confirm conditions for the particular sample
used for space tests. Fourth, recalescence is
observed over a wide range of undercooling and
fluid flow conditions to define the influence of
convection on nucleation phenomena, growth
kinetics, phase selection, and metastable phase
formation. This article presents a summary of how these
steps lead to developing a baseline dataset that may
be used to confirm existing or future solidification
Ground-Based Experiments at NASA MSFC
The temperature dependence of key material
properties was measured in ground-based ESL tests
to facilitate planning of the space processing prior to
on-orbit verification for the specific flight EML
samples. Electrostatic levitation is accomplished
by placing a 40-mg charged spherical sample
between a pair of oppositely charged, vertically
stacked electrodes at ultra-high vacuum (UHV) to
eliminate arcing. A complex control system is used
to maintain sample stability, whereas a 200 W
Nd:YAG laser heating system is used to thermally
condition the sample. ESL positioning and heating
are thus decoupled.
LED lighting is used to back-light a shadow of the
sample into a high-speed camera while temperature
is recorded using a one-color pyrometer calibrated
in situ at the melt plateau. Density and thermal
expansion were measured by superheating a molten
sample of known mass and then turning the laser
off. Calibrated images taken radially were used to
track system volume by assuming axial symmetry.
Viscosity and surface tension were measured by
melting the sample and then reducing the heating
power of the laser to induce an isothermal hold of
sufficient duration to excite surface oscillations
using the electrostatic field. Mode 2 oscillation
amplitude for ESL testing was monitored by
measuring the magnitude of the change in dynamic
polar diameter after cessation of active excitation.
The natural frequency of these oscillations was used
to evaluate the surface tension, whereas the
oscillation decay was used to evaluate the viscosity.25
Ground-Based Experiments at DESY
In situ time-resolved x-ray diffraction
measurements have been performed at the P0726
highenergy Material Science beamline of the third
generation Radiation source PETRA III at the
Deutsches Elektron-Synchrotron (DESY), a member
of the Helmholtz Association (HGF), outside
Hamburg. A total of 1.1-gram samples were processed in
a mobile EML developed by IFW-Dresden. For
levitation experiments, the sample was placed in a
UHV chamber backfilled to 250 mbar with
highpurity helium gas after evacuation. Positioning and
heating of EML samples was realized through use of
a water-cooled copper coil powered by a 10-kW
generator operating at 280 kHz. The sample sits in
a potential well generated by the coil magnetic
fields. Gravity pulls the sample down into the
levitation field causing significant heating and
inducing turbulent convection.27 Unlike ESL, EML
positioning and heating for ground-based testing
are thus coupled.
Active cooling was achieved by pumping
recirculated inert chamber atmosphere across the sample
surface. The sample temperature was measured
with a single-color pyrometer, whereas recalescence
was imaged using a high-speed digital camera. The
structure of the levitated sample was measured in
transmission geometry with monochromatic
radiation of 121.3 keV, or a wavelength of k = 0.1022 A˚ ,
with a beam size of 500 9 500 lm2. The scattered
intensity was acquired using a two-dimensional
flatpanel Perkin Elmer XRD 1621 x-ray detector
configured to provide sufficient counting statistics
within the acquisition rate of 5 Hz. The detector
was mounted symmetrically around the scattering
axis at a 0.8-m sample-to-detector distance. The
total scattering intensity is proportional to the
scattering vector, Q, and was obtained by azimuthal
integration of the calibrated and dark current
corrected two-dimensional intensity data using the
FIT2D software.28,29 The alloy used for synchrotron
EML processing was selected to obtain incubation
delay times compatible with the x-ray detector
acquisition rate. For this work, the ternary
Fe72Cr16Ni12 (wt.%) alloy samples were produced
at the German Space Agency (DLR) by arc-melting
of 99.95% pure elemental feedstock in high-purity
6N argon after preconditioning by high-vacuum
Magnetohydrodynamic Modeling of Induced
Convection inside a EM-levitated molten steel
droplet was characterized using the MHD model
developed and experimentally validated in the
previous research.23 Using a finite volume method,
this model converts a full Navier–Stokes equations
into a system of linear equations, which is then to be
solved by a commercial computational fluid
dynamics package, ANSYS Fluent. The EM force field was
calculated with a subroutine code and implemented
into the solver. For turbulence, the renormalization
group (RNG) k–e model was used as the flow to
include the laminar-turbulent transition regime
and recirculation fields.30 Flow status, flow pattern,
and the range of accessible convection were
predicted under various test scenarios. The material
properties used for the simulations can be found in
Flow pattern varies as a function of both heating
and positioning currents. With the positioning
current fixed at 145 A, the heating current was
varied from 0 to 150 A. With positioning current
alone, two circulations were observed in each
hemisphere—one clockwise circulation near the equator
and the other in counterclockwise near the pole. As
the heating current is increased, the circulation near
the pole grows and eventually swallows the other up.
This is shown in Fig. 1a. A plot of convection velocity
versus heating control voltage can then be developed
based on multiple simulation results as shown in
Fig. 1b. The control of the positioning and heating
currents during space experiments, and the resulting
predictions of melt convection, were meticulously
planned using the results from the MHD simulations
as discussed later.
Rapid Solidification Experiments on the
International Space Station
Undercooling experiments were run over a wide
range of melt convection conditions in space using
the MSL-EML facility as part of an international
collaboration between NASA and ESA. Since
processing is conducted in the reduced gravity of space,
the magnetic fields needed to contain the sample
could be much reduced with a concomitant
reduction in induced fluid flow. Sample positioning was
controlled using a 150-kHz quadrapole field, and
heating was accomplished by overlaying an
independent 350-kHz dipole field onto the same SUPOS
coil system—an acronym for superposition. Thus, in
space, EML positioning and heating were again
Two types of experiments could be run.31 First,
thermophysical properties were spot-checked to
define how the specific flight sample behaves. The
top-view axial camera and pyrometer (ACP)
simultaneously recorded temperature and video images of
the sample surface for facility health monitoring.
Onboard software was subsequently used to trigger
the acquisition of density or viscosity video images;
either dynamic equatorial diameter or integrated
sample area could be used for analysis. Second,
high-speed imaging was used to study solidification
phenomena. The side-view radial camera (RAD) ran
at 30 kHz with a 0.3-s pre-trigger period
programmed prior to recalescence. Note that property
measurement and solidification studies could be
conducted during the same thermal cycle through
independent analyses of the ACP and RAD images.
These two types of experiments are shown in
Fig. 2. In the top part of the image, the time–
temperature profile is displayed in blue on the
graph. Green shows a trace of the positioner, and
red shows a trace of the heater control voltage
setting. During free-cooling, two pulses from the
dipole heating field were used to excite mode 2
surface oscillations by squeezing-in radially at the
equator. Sample area as a function of time is plotted
in the lower-left box in Fig. 2a from analysis of the
ACP images. Temperature was continuously
changing throughout the decay process, and thus,
multiple measurements over small time periods,
and the associated small temperature ranges, could
be accomplished throughout a single run.
Solidification behavior is highlighted in the
lowerright box in Fig. 2b, and a representative RAD
image showing double recalescence is displayed.
The undercooled liquid is self-illuminating, and
with the selection of appropriate aperture settings
appears as a dark gray against a black background.
Near the center of the sample, the light gray
metastable phase has nucleated and grows into
the undercooled liquid. The stable phase appears as
a brighter (and, thus, hotter) region at the center of
this metastable region.
The nominal 1.1-g spherical samples were
produced at the University of Ulm by vacuum casting of
ingot sections taken from a master-alloy produced
from 4 N elemental feedstock. For this work, the
ternary Fe60Cr20Ni20 (wt.%) alloy was selected to
observe fast transformation kinetics compatible
with the high-speed camera acquisition rate. Tests
were conducted at 35,000 Pa (350 millibar)
backpressure to limit any composition shift caused by
the potential for preferential evaporation of
chromium from the alloy. In space, most tests were
pressurized using helium, although a limited set
was tested using argon so that any influence on
solidification phenomena introduced by changing
the cooling rate could be investigated independently
from any influence from convection.
RESULTS AND DISCUSSION
To plan the space experiments, thermophysical
properties were measured on the ground and the
results used to conduct a broad range of MHD
calculations on possible run configurations. From
this broad set of results, key parameters were
identified and simulations were focused to generate
the desired operational conditions for the flight
experiment. On-orbit, property evaluations were
accomplished in situ to confirm the specific
properties of the space sample [Orbit], and MHD models
were reevaluated. Figure 1a shows a typical MHD
result for a given configuration of sample size,
temperature, heater control voltage setting, and
positioner control voltage setting. On the left side of
the spherical sample displayed in the figure, the
force distribution is shown corresponding to a
condition where no excitation pulse is being applied.
On the right, flow patterns show the formation of
two opposing toroidal loops—one in the upper and
one in the lower hemisphere. The graph in Fig. 1b
shows the range of actual run conditions that were
accomplished during the space testing. Note that
the recirculation loop velocities are predicted to be
laminar at 0.02 m/s and turbulent at 0.20 m/s,
respectively, for control voltage settings from 0 to
5 V. The green region shows the convection velocity
at superheated temperatures for both laminar and
turbulent regimes. The blue and red regions
represent undercooled temperatures for laminar and
turbulent regimes, respectively. Convection velocity
increases as the temperature is elevated. The
maximum convection velocity increases as more
heating control voltage is added to the coil. The
trimmed region (bold solid black line) on the curves
represents the limit to convection based on
overheating of the undercooled sample. Figure 1b shows
the heating control voltages used in the space
experiments with the resulting prediction of
recirculation velocities on-orbit; green symbols indicate
laminar conditions, red indicate fully developed
turbulent conditions, and yellow indicate the
transition between these extremes. Liquid shear may
also be evaluated from the MHD simulation results.
For the conditions run, shear was approximately
linearly dependent on the recirculation velocity and
the predicted values for each test were tabulated for
use in predicting the damage parameter for delay
time model evaluations.
Time-resolved x-ray diffraction investigations on
weld solidification show primary ferrite (FA-mode)
or primary austenite (AF-mode) with subsequent
melting of the metastable primary phase; the
solidification path depends on both the weld speed and
composition.32 Rapid solidification can be much
different.33 Figure 3 shows DESY synchrotron
processing such that the equilibrium c-phase fcc-peaks
disappear once the solid is molten. On solidification,
primary d-ferrite forms and then is subsequently
melted during the formation of austenite such that
only austenite then remains (FA-mode); the
bccpeaks have a lifetime of about 1 s. This progression
is also shown as an intensity plot in the second part
of the figure. Note that low-temperature
transformations are not documented here as this article
concentrates on rapid solidification of the
neareutectic stainless steel alloy family.
The high-speed video was used to identify when
and where the two recalescence events initiated to
evaluate the delay time between them. Undercooled
liquid is dark gray, metastable ferrite mid-gray, and
stable austenite light-gray on a black background.
Figure 4 shows two representative images from the
video recordings of the space tests. The first test was
run with minimal stirring (0.02 m/s). Under
laminar conditions, the delay between primary and
secondary recalescence events was long giving
enough time for the primary metastable phase to
grow across the entire surface of the droplet before
nucleation of the stable phase. The second sample
was run with significant stirring (0.12 m/s). Under
turbulent conditions, the delay was short and the
metastable phase had not grown far into the
undercooled liquid before nucleation of the second
phase occurred. Delay results were independent of
test atmosphere where with minimal stirring
cooling rates in argon and helium were 20 and 50 K/s,
The second part of Fig. 4 shows a graph of delay
time as a function of undercooling for various run
scenarios. The orange circles represent ESL
ground-testing with no induced stirring; black
squares represent EML ground-testing with
turbulent flow. The space tests are shown with
diamonds; red represents fully turbulent results
where shear is also on the order of the
groundbased results, yellow represents transitional
conditions, and green represents laminar flow at the
lowest induced flow conditions possible in reduced
gravity. The solid line shows how ESL behavior
deviates significantly from the range of
spaceaccessible conditions shown by the dotted lines.
These results conclusively show that convection
significantly decreases the delay time. Turbulent
space and turbulent ground EML tests show a
similar behavior and laminar space results
approach but do not achieve the delay behavior of
quiescent ground ESL.
Thermophysical properties can be measured on
the ground before a space mission and can be used
to predict the range of potential thermal cycle run
conditions that may be achieved using MHD
techniques. Remeasurement on-orbit provides
validation of actual run conditions. In space, a wide range
of stirring conditions are possible—from laminar to
turbulent—allowing investigation of phase selection
behavior. No difference was seen among vacuum,
Ar, and He environments, indicating that the
cooling rate is not a factor in determination of the
delay between stable and metastable phase
formation. Space testing results support the prediction
that both undercooling and melt stirring will reduce
the observed delay.
Funding supporting this work was provided by
NASA under grant number NNX16AB59G and
NNX16AB40G, by DLR under contract 50WM1170
at Ulm University and PARMAG contract
50WM1546 at IFW Dresden, and by the ESA MAP
Project Thermoprop AO-99-022 and AO-2009-1020
contract number 4200014306. The authors would
also like to thank Olof Gutowski and Dr. Uta Ru¨ tt at
DESY for assistance in using the high-energy
materials science beamline P07 and ESL staff Mike
SanSoucie, Trudy Allen, and Glenn Fountain at
NASA Marshall Space Flight Center in Huntsville,
AL, for ground-based thermophysical property
measurement experiment support.
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