Proceedings of the 7th Annual ISC Graduate Research Symposium
ISC-GRS 2013
April 24, 2013, Rolla, Missouri
Aaron Thornton
Department of Mechanical and Aerospace Engineering
Missouri University of Science and Technology, Rolla, MO 65409
Freeze-form Extrusion Fabrication of Functionally Graded Materials
ABSTRACT
This paper discusses the production of functionally graded
material (FGM) parts with a novel additive manufacturing
process called freeze-from extrusion fabrication (FEF). The
goal of this project is to produce FGM parts with good
mechanical properties. After initial trials, poor mechanical
strength and poor density resulted in the final fabricated parts
achieving only 25 – 30% of the desired values. This paper
discusses the study performed to determine why this was
occurring and how to fix the problem(s) associated with the
poor mechanical properties.
1. INTRODUCTION
Freeze-form Extrusion Fabrication (FEF) is an environmentally
friendly additive manufacturing process which builds 3-D parts
layer-by-layer. The build material is in the form of an aqueousbased colloidal paste [1-2]. The build process is computer
controlled based off the desired CAD model of the part being
fabricated. The organic binder content is only 2-4 vol%,
whereas in other similar processes this content is much higher.
Paste solids loadings can achieve up to 60 vol%.
While there are many additive manufacturing processes,
only a few of them are similar to Freeze-form Extrusion
Fabrication. The reason for this is that FEF is capable of
fabricating complex ceramic parts. Robocasting [3-4],
Extrusion Freeform Fabrication [5], 3-D Printing [6], and
Selective Laser Sintering [7-8], are a few additive
manufacturing processes which can also produce ceramic parts.
Robocasting is a very similar process which also uses
colloidal pastes. The most significant difference between
Robocasting and FEF is that Robocasting relies on the extruded
paste to dry, whereas FEF relies on the water in the paste to
freeze after extrusion. Because the water solidifies after
extrusion, much lower amounts of binder content is needed,
making FEF much more environmentally friendly.
3-D Printing is another process which can fabricate
ceramic parts. 3-D printing works rather differently from FEF.
3-D printing is still a layer-by-layer process but it lays down
powder beds and then prints binders to glue different particles
together in a desired shape. FEF extrudes paste out of a needle,
and can only print one bead track at a time. Unlike 3-D
printing, the current FEF machine is capable of printing
functionally graded material (FGM) parts. Since 3-D printing
uses a large amount of resins to glue particles together to form a
green part, it is not as environmentally friendly as FEF.
Typical applications of FGM parts fabricated with FEF are
leading edges for hypersonic vehicles, missile nose cones,
nozzle throat inserts for propulsion systems. Many of these
parts must withstand extreme heat conditions ((> 2000°C), but
must also be able to interface and fasten to a structure which is
usually made of a metal. One ideal way to make these parts is
to grade from a ceramic to a metal. This allows the same part to
exhibit both high heat resistances where it is geometrically
needed, and to be tightly secured to a structure at the same
time. Additive manufacturing is one of the most feasible ways
to fabricate FGM parts.
This paper discusses the testing of Zirconium Carbide and
Tungsten functionally graded material (FGM) test bars. The
resulting mechanical strength of these bars was low. Therefore
a study was and is still being performed to obtain a full
scientific understanding of why these test bars exhibit such low
flexural strength. The study also includes upgrades to
machinery to provide better operating conditions for FGM part
fabrication.
2. FEF EQUIPMENT SETUP
The triple-extruder mechanism was designed using three
stainless steel cylinders, each containing a paste driven by an
individual plunger whose movement is controlled by a DC
servo motor (Kollmorgen AKM23D); see Figure 1. The
encoder signal from the servo amplifier provided a resolution of
0.62 µm for the plunger’s movement. The paste flow rate in
each cylinder was controlled by the plunger’s velocity, and the
force exerted on the plunger was measured by a load cell
(Omega LC-305). The FEF system used a static mixer to blend
the three different pastes and mixed them into a homogeneous
stream as they passed a series of mixing blades positioned at
alternating angles.
The triple-extruder mechanism was mounted on a gantry
system, which consisted of three orthogonal linear drives
(Velmex BiSlide), each with a 508 mm travel range. The Xaxis consisted of two parallel slides and was used to support the
Y-axis. The Z-axis was mounted on the Y-axis, and the
extrusion mechanism was mounted on the Z-axis. Four DC
servo motors (Pacific Scientific PMA22B), each with a resolver
for position feedback at a resolution of 1000 counts per
revolution, drove the various axes. Each motion axis had a
maximum speed of 127 mm/s and a position sensor resolution
of 2.54 μm.
1
Composition
100%vol ZrC
87.5%vol ZrC/12.5%vol W
75%vol ZrC/25%vol W
62.5%vol ZrC/37.5%vol W
50%vol ZrC/50%vol W
Figure 1. Triple FEF system in a temperature-controlled
enclosure. Three servo motors control linear cylinders for
extrusion and a three-axis gantry system controls motion.
The part fabrication process was conducted in a freezing
environment, which could be controlled to as low as -20°C
using a liquid nitrogen injection system. Later in the project,
the liquid nitrogen system was replaced with a freezer system.
The cold temperatures enabled the aqueous paste to solidify at
temperatures below the freezing point of water after it was
extruded to solidify the paste, thus avoiding part deformation
during the fabrication process and enabling fabrication of larger
parts. A heating jacket was used to keep the paste temperature
above the freezing point of water until it was deposited. In the
present study, the freezer’s temperature was kept at -10°C,
while the heating jacket’s temperature was kept at 10°C.
The 3-axis gantry system movement was controlled by a
motion control program with the Delta Tau Turbo PMAC PCI
board. Paste extrusion was controlled with three servo motors
using a National Instruments PXI chassis and LabVIEW Real
Time 8.6.
2.1. Zirconium Carbide and Tungsten FGM Test Bars
Zirconium Carbide and tungsten flexure test bars were made
via the FEF process. These bars were made from two different
sets of paste; 1) 100%vol zirconium carbide and 2) 50%vol
zirconium carbide with 50%vol tungsten. Five compositions
were made by varying the velocities of the two extrusion rams
on the FEF machine. Figure 2 helps explain how this was
achieved. For example in order to achieve 87.5%vol
ZrC/12.5%vol W, ram 1, containing 100%vol ZrC paste, was
set to extrude paste at 75% of the total extrusion velocity. Ram
2, containing 50%volZrC/50%volW paste was set to extrude at
25% total extrusion velocity.
100%vol ZrC
Velocity of Ram 1
100%
75%
50%
25%
0%
50%volZrC/50%volW
Velocity of Ram 2
0%
25%
50%
75%
100%
Figure 2. Material Compositions and their correlating Ram
Velocities.
In order to achieve good statistical analysis of the
mechanical strength, five test specimens were fabricated from
each of the compositions totaling 25 test bars. These test bars
were cut, ground, polished and broken following ASTM
C1161-02b standards.
In order better understand the data from flexural strength
of the FEF bars, they were compared to a conventional
manufacturing process. ZrC and W powders were mixed into
the same ratios as that of the FEF fabricated bars, and were
pressed into bars under 30,000 psi in an Isostatic press. These
bars were also cut, ground, polished and broken following
ASTM C1161-02b standards. The results of these Isostatic
pressed bars are compared to the FEF fabricated bars in Fig. 3.
FEF Fabricated
Composition
Isostatic Pressed
Relative
Density
Flexural
Strength
(MPa)
Relative
Sintered
Density
Flexural
Strength(MPa)
100%ZrC
62.05%
73
98.49%
224
12.5%W+87.5%ZrC
47.89%
25
94.41%
265
25%W+75%ZrC
56.19%
25
97.34%
398
37.5%W+62.5%ZrC
47.28%
28
95.40%
414
50%W+50%ZrC
70.08%
31
99.81%
404
(vol.%)
Figure 3. Flexure test results of FEF printed bars
compared with Isostatic pressed bars.
As can be seen in Fig 3 the overall flexural strength of FEF
fabricated bars is significantly lower than the Isostatic pressed
bars. At this point the challenge became to figure out why the
FEF fabricated bars resulted with such low density and
strength.
2
100% ZrC FEF Bar
100% ZrC Isostatic Pressed Bar
Bar
Figure 4. Microstructure comparison of a FEF fabricated
bar (left) and a iso-pressed bar (right).
Figure 4 is a view of the microsturcure comparing the FEF
fabricated bars to the Isostatic pressed bars. The important thing
to note in Fig. 4 is the large voids on the FEF bar side. It is
hypothesized that these large voids are caused by ice crystals.
In order to reduce the size of these large ice crystals, glycerol
was introduced into the paste used on the FEF machine. The
idea was that the glycerol would increase the number of
nucleation sites for the water, therefore creating more ice
crystals which would keep the overall size of the ice crystals
small.
More test bars were printed from 100%vol ZrC paste
containing glycerol on the FEF machine. These bars were once
again cut, ground, polished, and broken according to ASTM
C1161-02b standards. In order to conserve costly powder, only
tests on 100% ZrC were performed.
After these bars were tested, the resulting flexural strength
did not improve significantly jumping from 73 mPa to 82 mPa
while the density only improved from 62% to 73%. The
glycerol helped improve mechanical properties, but not
significantly.
2.2. Alumina Test Bars
At this point it was decided to go back to previous materials
which had in the past resulted with very good properties such as
density and strength. Further testing was performed with
alumina.
Single composition alumina bars were fabricated on
the FEF machine much in the same way as the ZrC/W bars.
These bars were then put into the freeze-dryer to remove all of
their water, as per usual FEF post processing. After the freezedrying process completed, these bars were cut in half to
examine their cross-sections. Figure 5 is a picture of one of
these cross-sections at 20X magnification.
Figure 5. Top and bottom of the cross-section of an alumina
test bar.
Figure 5 shows the top and bottom cross-sections of an
alumina bar. The bottom side of the bottom picture is the
bottom of the test bar, or in other words it was the first layer
that was printed on the FEF machine. Build direction in terms
of layer sequence is starting at the bottom and going up. If
observed closely, layers and individual bead tracks can be
observed in the cross-section of the bar.
After observing this cross-section it is easy to see why this
bar, and likely the ZrC bars as well, cannot achieve > 95%
density. As can be seen in the bottom picture of Fig. 5 there are
large voids which are right in the center of the bar and some
cracks throughout.
A density test, Archimedes density, was performed on these
bars and despite some of their large pores and cracks they all
reached 85% density. If the pores and cracks can be eliminated
then it is possible to reach > 95% density, as desired. The next
step is to determine why these large pores and cracks occur.
3
Upon further study of the cross-sections, it can be seen that
after about the 7th or 8th layer, no more large pores or cracks
appear. In fact if the bar were solid as these upper layers appear
to be, then the bar would indeed have greater than 95% density.
There are a few reasons this could occur.
The first and most likely cause is that the bars are built
upon an aluminum plate which acts as the substrate for part
fabrication. Since this plate is sitting in the cryogenic
environment, it conducts a lot of heat very rapidly out of the
part being built. If this is the case, the aluminum substrate is
causing the paste to freeze too fast, not allowing it to fill into
the desired shape.
Figure 6 helps to explain what a desired bead shape in this
case should be. The desired bead shape for all bead tracks being
laid down is a perfect rectangle. This prevents any voids from
forming between bead tracks and helps to form a solid green
part.
to overshoot, or miss the desired absolute position along the
toolpath.
A third possible cause is overall poor temperature control
within the FEF cabinet. At the time the alumina bars were
printed the FEF machine used liquid nitrogen sprayed directly
into the box to cool the environment. A commercial PID
controller was purchased to control the environment
temperature at -100C. After connecting a second thermocouple
for comparison, it is easily seen that this is not the case within
the FEF box. Figure 7 shows the temperature recording while
the environmental chamber is being cooled by liquid nitrogen.
(a) Circular bead crosssections
(large pores)
Figure 7. Temperature within the FEF Environmental
Chamber.
(b) Rectangular-like
bead cross-sections
(smaller pores)
(c) Perfect
rectangle bead
cross-sections
(no pores)
Figure 6. Examples of possible bead cross-sections as
printed by the FEF process.
A second possible cause is errors in the gantry system.
Since the gantry setup contains two x-axes, a large amount of
following error has been observed between the two axes. This
along with other observed errors is a likely cause for the gantry
In order to ensure better temperature control inside the FEF
environmental chamber some upgrades were made. The liquid
nitrogen has been replaced with a glycol chiller designed to
cool beverages. A radiator exchanges heat with the FEF
environmental chamber and the poly ethylene glycol. The poly
ethylene glycol is then cooled by the chiller. A large fan blows
air across the radiator inside the environmental chamber in
order to help distribute the cold air.
At the current time a controller is being implemented with
the fan, which blows across the radiator, to turn it on and off. A
compact Rio manufactured by National Instruments along with
labview will control the fan. It will use an on/off control
scheme to turn the fan on and off. When the fan is off no air
will cross the radiator, and it will stop cooling the
environmental chamber. Once this system is set up and working
properly, testing of the alumina bars may continue.
The environmental chamber is made of plywood which is
lined with two inch thick polystyrene foam. There were gaps in
many places in the environmental chamber and overall it was
not serving its purpose well. A layer of transparent plastic drop
cloth was lined all along the inside of the environmental
chamber. This was to help stop air flow in and out of the box,
and to provide a vapor barrier. After this plastic was installed,
more two inch foam was installed inside the box on all surfaces
to help add more insulation to the box.
After this insulation was added and the chiller connected to
the radiator inside the environmental chamber, an initial test
4
was performed to see how the temperature would vary within.
During this test the glycol chiller was set to -230C. Figure 8
shows this data.
Figure 8. Temperature inside the FEF environmental chamber
after the installation of the glycol Chiller.
The desired operating temperature for the FEF
environmental chamber is -100C. This new system which was
implemented held below -170C for a little over 24 hours. The
most important thing which this test shows us is that the
temperature variation within the environmental chamber is
already much more steady than it was with the liquid nitrogen.
The new setup can achieve desired temperatures and hold them
for enough time to fabricate parts.
After the cooling system is fully implemented further
experiments with the alumina test bars can be performed.
Several experiments will be conducted to determine the
cause of poor part density. The first set of tests to be performed
will be to determine if the aluminum substrate really is
affecting the fabricated parts. This will be done by building the
same parts, just much taller. In other word rather than building
a test bar which is 30 layers thick, it will be printed 60 layers
thick. Another test is to replace the substrate with a different
material which is more insulating such as a polymer or ceramic.
In order to test whether the gantry system is causing errors
in the desired toolpath, alumina test bars will be printed in
many different orientations and positions around the build
volume.
8. ACKNOWLEDGMENTS
This project is funded by NSF grant #CMMI-0856419 with
matching support from the Boeing company through the Center
for Aerospace Manufacturing Technologies and the Intelligent
Systems Center at the Missouri University of Science and
Technology, and by the Air Force Research Laboratory contract
#10-S568-0094-01-C1 through the Universal Technology
Corporation.
9. REFERENCES
[1] Huang, T., Mason, M. S., Hilmas, G. E., Leu, M.C., 2006,
“Freeze-form Extrusion Fabrication of Ceramic Parts,”
International Journal of Virtual and Physical Prototyping,
1(2), pp.93-100.
[2] Mason, M. S., Huang, T., Landers, R. G. , Leu, M. C.,
Hilmas, G. E. , 2009, “Aqueous-Based Extrusion of High
Solids Loading Ceramic Pastes: Process Modeling and
Control,” Journal of Materials Processing Technology,
209(6), pp. 2946-2957.
[3] Cesarano III, J., Segalmen, R., Calvert, P., 1998,
“Robocasting Provides Moldless Fabrication from Slurry
Deposition,” Ceramics Industry, 148, pp. 94-102.
[4] He, G., Hirschfeld, D., Cesarano III, J., Stuecker, J., 2000,
“Processing of Silicon Nitride-Tungsten Prototypes,”
Ceramic Transactions, 114, pp. 325-332.
[5] Hilmas, G., Lombardi, J., Hoffman, R., 1996, “Advances
in the Fabrication of Functional Graded Materials Using
Extrusion Freeform Fabrication,” Proceedings of Solid
Freeform Fabrication Symposium.
[6] Cima, M., Oliveira, M., Wang, H., Sachs, E., Holman, R.,
2001, “Slurry-Based 3DP and Fine Ceramic
Components,” Proceedings of Solid Freeform Fabrication
Symposium.
[7] Kruth, J., Mercelis, P., Froyen, L., Rombouts, M., 2004,
“Binding Mechanisms in Selective Laser Sintering and
Selective Laser Melting,” Proceedings of Solid Freeform
Fabrication Symposium.
[8] Leu, M.C., Adamek, E., Huang, T., Hilmas, G. E., Dogan,
F., 2008, “Freeform Fabrication of Zirconium Diboride
Parts Using Selective Laser Sintering,” Proceedings of
Solid Freeform Fabrication Symposium.
7. SUMMARY AND CONCLUSION
Many hypotheses have been formed as to why the FEF
fabricated test bars do not achieve good mechanical strength.
Many of these hypotheses are still being tested. An upgraded
cooling system, which can maintain the environmental chamber
at steady temperatures, has been added to the FEF machine. Its
implementation is being finalized.
5