Ablation of Hybrid Metal Matrix Composites

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Paper 11-057.pdf, Page 1 of 7
AFS Proceedings 2011 © American Foundry Society, Schaumburg, IL USA
Ablation of Hybrid Metal Matrix Composites
D. Weiss
Eck Industries, Manitowoc, WI
J. Grassi
Alotech LLC Ltd., Brooklyn, OH
B. Schultz, P. Rohatgi
University of Wisconsin-Milwaukee, Milwaukee, WI
Copyright 2011 American Foundry Society
ABSTRACT
Ablation is a process in which an alloy is solidified using
a core package that the liquid alloy resides or is
“suspended” within, while a liquid or gas and
combinations thereof remove the core while
simultaneously facilitating a rapid heat removal to cause
solidification of the alloy. This method has been
successfully used for monolithic alloys to get higher
solidification rates to improve mechanical properties. This
paper describes the early development of ablation casting
for MMC’s and the mechanical properties of an Al-SiC-C
hybrid composite alloy. The microstructures and
mechanical properties of ablation cast Al-10SiC-4Ni
graphite have been characterized and compared with
conventionally sand cast composites. The effects of
ablation casting on dendrite size and particle distribution
have been characterized along with its effect on tensile
strength.
graphite in the Al-SiC alloy made it much more
machinable and wear resistant due to the presence of the
graphite. Moreover, in addition to improved strength and
wear behavior the Al-SiC-C (graphite) castings have a
lower density than Al-SiC castings due to the presence of
graphite.
In the ablation process3, 4 the core package is made using
a proprietary binder and, shortly after the alloy is poured,
an ablation media is directed to the core surface. The
ablation media dissociates the binder and “erodes” the
silica sand cores enabling the ablation media solvent to
come in direct contact with the liquid alloy thereby
removing the latent heat of solidification with high
cooling rates. Figure 1 illustrates the process in the
casting of an aluminum A356 steering knuckle.4.
Key Words: Ablation Casting, Hybrid Metal Matrix
Composite, Aluminum, Silicon Carbide, Graphite
INTRODUCTION
Al-SiC alloys have been used as brake rotors in several
automobiles including the Lotus Elise, the Chrysler
Prowler, the General Motors EV-1, and several other cars
made in Japan and Europe. The Al-SiC composite brake
rotors have also been investigated for use in railway
applications.1 However, the manufacture of Al-SiC brake
rotors for high volume applications using conventional
silica sand casting may present problems due to the
settling of the silicon carbide particles. The particles are
heavier than molten aluminum, and there are difficulties
in machining due to high hardness of the silicon carbide
particles. In view of this, Rohatgi et al.2 have developed
hybrid composites incorporating both silicon carbide and
graphite particles in the matrix of aluminum alloys. The
simultaneous presence of heavier SiC particles and lighter
graphite particles resulted in a buoyancy neutral
composite melt where the SiC and graphite particles tend
to remain in suspension during casting. This is due to the
mutual hinderance they place on their respective
movement through the melt. In addition, the presence of
(a)
(b)
Fig. 1 a) Beginning of ablation casting process, where
ablation solvent erodes the surface of the core
package b) Solidified stearing knuckle with sand
cores removed by the ablation solvent.4
The ablation solvent progressively moves from one end of
the casting to the other resulting in rapid unidirectional
solidification. Heavy sections, where solidification is
slower, typically show higher rates of cooling when
ablated. These characteristics of ablation are expected to
lead to a much more refined microstructure, especially in
the case of the interdendritic eutectic3, and a uniform
distribution of reinforcement particles. The degree to
which the ablation casting process is able to reduce the
dendrite cell size and to increase solidification rate in
relation to other casting processes is presented
schematically in Figure 2. A comparison of relative
properties of monolithic A356 alloy resulting from the
various casting methods is given in Table 1.
the trade as 10S4G, and consists of 10 vol% SiC and 4
vol% Ni coated graphite in AA359 alloy2.
EXPERIMENTAL METHODS
A plate shaped pattern was constructed suitable for both
conventional sand casting and ablation. The dimensions of the
cast plate were 20.32 cm (8 in) wide by 40.64 cm (16 in)
long. The section thickness on the left side of the plate casting
shown in Figure 3 is 6.35 cm (2.5 in) and the right side is 3.81
cm (1.5 in). The total casting weight was 11.6 kg (25.6 lbs)
The model, showing placement and scale of the risers and
runners is shown in Figure 3. The gating for the conventional
casting was fully modeled using MAGMA software so that
the effects of turbulence could be minimized. The results of
modeling indicated that the gating and riser system were
adequate to prevent shrinkage defects in the casting, as shown
in Figure 3, and, therefore, the same riser and gating system
was used for both conventional and the ablation processes. It
is possible that the increased cooling rates and thermal
gradients realized through the ablation process may lead to
changes in the gating/risering requirements, and this may be
explored in future work.
Fig. 2 Ablation cooling rates and dendrite cell size
compared to other conventional casting methods.
Table 1 Comparison of Mechanical Properties of A356
Alloy Aluminum Cast by Various Methods 3
Property
Sand
Squeeze
cast
312 (45)
Ablation
228 (33)
Perm.
Mold
262 (38)
UTS MPa
(ksi)
Y.S. MPa
(ksi)
%El
179 (26)
207 (30)
243 (35)
261 (38)
3.5
4
11.0
12.5
325 (47)
During the solidification of a discontinuously reinforced
metal matrix composite, the solidifying alloy will interact
with the particulate reinforcement to either push the
particles into the last freezing, interdendritic regions, or to
engulf them in the solidified dendrites. This tendency is a
natural hindrance to the goal of achieving a uniform
dispersion of reinforcements in cast MMCs. In order to
improve the dispersion of micron size particles in the
MMC, the solidification rate must be high enough either
to promote engulfment of the particles or to refine the
dendritic microstructure or both. Ablation has the
potential to increase the solidification rate and to,
therefore, improve the dispersion of the particulate
reinforcement in the finished casting. In this work, hybrid
Al-SiC-graphite composites were conventionally cast and
then produced by ablation to study the effect of this
process on the matrix microstructure and dispersion of the
reinforcement in the composite as well as the resulting
mechanical properties. The composite used is known in
Fig. 3. Results of casting simulation, showing the
location of shrinkage in the casting with the current
gating and riser system.
The initial work involved the molding and pouring of the
composite in conventional sand molds.
Mechanical
properties were developed for these pieces followed by
pouring and mechanical property determination of the ablated
components. After mixing the alloy for 30 minutes, the melt
was hand poured into the static molds using steel ladle(s) at
the same rate and at a pouring temperature of 732 C (1350 F).
Figure 4 shows the assembled mold for the conventionally
cast plate. The height of the pouring cup was increased to
compensate for the increased viscosity of the composite melt.
Cast iron chills were used at select locations to ensure a sound
casting and polyurethane no bake binder was used in
preparation of the mold.
(a)
(b)
Fig. 4. a) Conventional sand cast mold ready for pouring.
b) Pouring of the first casting.
Ablation was carried out using identical molds in appearance
but using the Alotech/DeVenne inorganic binder formulated
for ablation. The overall dimensions of the mold between the
conventional cast and ablation component were the same;
however, the cope portion of the mold for the ablation process
was modified as follows:
1) Approximately 6 in. of sand was removed directly above
the casting to increase the rate of ablation at this location,
2) A channel was cut at the edge of the cope to allow water
to drain from the mold cavity.
Use of the soluble binder allowed for ablation of the mold via
solvent jets.
The solvent jets were turned on sequentially from the thinnest
sections to the thickest sections of the casting (or from the
right side to the left side of the piece shown in Figure 3). The
temperature of the solvent ablating the mold was maintained
at ambient conditions.
Figure 5 shows the final cast product produced by
conventional casting. Note the evidence of high melt
turbulence shown near the ingate of the casting.
Fig. 5. Conventionally cast product
The conventionally cast and ablated castings were
solutionized at 538 C (1000 F) for 12 hours followed by
hot water quenching at 60C (140F) and then aged at 155
C (310 F) for 5 hours. Samples sectioned from the thin
walled section of the castings were sent to Rio Tinto
Alcan and to the University of Wisconsin-Milwaukee for
evaluation. At least eight samples from each casting were
tensile tested according to ASTM Standard B557.
Sections from the conventional casting and ablated
component were cold mounted in epoxy resin and
metallographically polished. Optical characterization was
carried out using an optical microscope coupled with an
image analysis system. The volume percentage of phases
was determined using an image analysis routine to
determine the area percentage of phases relative to the
field area based on the size, shape and gray threshold of
the respective phases over at least 10 fields. It should be
noted that the phases are assumed equiaxed in this
analysis, which may not be the case for all phases present.
The polished specimens, as well as of the fracture
surfaces examined via scanning electron microscope with
Energy dispersive X-ray spectroscopy (EDS) capability.
RESULTS
The microstructure of the conventionally cast and ablation
product (hereafter referred to as “conventional” and “ablated”
respectively) hybrid 10S4G MMC were characterized as
containing a dispersion of ~84 µm (0.003 in) graphite
platelets, and ~15 µm (0.0006 in) SiC particles. The particles
were found only at the boundaries of dendrites within
interdendritic/intercellular regions. In addition, present in the
cast microstructure were silicon and nickel intermetallics that
were identified by SEM/EDS. In addition to these general
features, the samples from both conventional casting and
ablation casting techniques had porosity defects.
The microstructure of the composite was significantly
affected by the increased solidification rate achieved by the
ablation. Figure 6 shows the microstructure of the
conventional casting and ablated component. It is clear from
the figures that the graphite and SiC particles (labeled in
Figure 6a) in the ablated sample have a higher volume
fraction and are more evenly dispersed than those found in the
conventional casting. The volume percent of the graphite in
the conventional sample and ablated sample was ~ 4 v% and
~ 7 v% respectively. Likewise, the volume percent of SiC
found in the conventional sample and ablated sample was 11
v% and 25 v% respectively. It should be noted that the
graphite particles are in the form of flakes and thus the total
volume percent of graphite will be lower than reported.
The dendrite arm spacing (DAS) was reduced via the ablation
process. In the case of the ablated sample, the DAS/cell size
was 50.3 µm (0.00198 in), compared to the DAS of the
conventionally cast specimen of 62.5 µm (0.00246 in).
Assuming that the dendrite arm spacing is proportional to the
square root of the solidification time, the ablation casting
process resulted in a 35% decrease in solidification time
compared to the conventional sand casting process in the
location where microscopy was performed.
SiC
particles
SiC
particles
α-Aluminum
dendrite
Reinforcement
poor region
(a)
Graphite
(a)
Intermetallics
Porosity
(b)
(b)
Fig. 6. a) Conventionally cast composite. SiC is found
in large clusters among in the interdendritic regions.
Large graphite flakes are also visible b) Ablation
composite sample. The SiC and graphite particles are
more evenly distributed in the microstructure (with a
higher volume fraction than found in the
conventionally cast sample).
Both the conventionally cast and ablation specimens
exhibited a significant amount of porosity due to shrinkage
and entrapment of gas. Figure 7 a-c show microstructures
containing porosity in the conventional and ablated samples.
Close examination, of the gas pores shows that they are lined
with SiC particles. The increased viscosity of the metal matrix
composite melt often prevents entrapped gas bubbles from
leaving the liquid melt, and thus the particles can stabilize the
bubble within the suspension. Also evident from Figure 7(a)
is a large, reinforcement poor region where few, if any,
reinforcement particles are present due to the movement of
particles during low solidification rates. The presence of such
regions can reduce the overall strength of the composite
material. Such reinforcement poor regions were not observed
in the ablated samples.
(c)
Fig. 7. a) Microstructure of the conventionally cast
composite showing shrinkage type pores in lower left
hand side of the micrograph, b) microstructure of the
ablation specimen showing round gas pores, c) Gas
bubble in the modified sample that has been stabilized by
SiC particles.
Chemical analysis of the surface by means of EDS showed
bright intermetallics among the SiC particles as can be seen in
Figure 8. Dot mapping of the surface shown reveals the
presence of Ni in the blocky intermetallic, and not at the
interface between the large graphite particle centered in the
view. This is as a result of the dissolution of Ni from the
surface of the graphite particles into the alloy melt and its
reaction with molten aluminum to form intermetallics. Also,
present were needle like eutectic Si. It was observed that the
intermetallics in the ablated specimen were finer in size than
those found in the conventionally cast specimen.
Table 2. Comparison between the Properties of the
Ablated Component and Conventional Casting.
SiC
particles
Intermetallics
(a)
C
(b)
Al
(c)
Si
Ni
(d)
(e)
Fig. 8 a) SEM micrograph of conventionally cast sample
and EDS dot mapping showing presence of b) graphite, c)
Al, d) Si and e) Ni.
Tensile testing was performed on samples taken from the thin
section of the cast plate, and Table 2 presents the ultimate
tensile strength and percentage elongation for the
conventional casting and ablated component. The tensile
strength of the conventionally cast specimen was 217 MPa
(31.5 ksi), and the tensile strength of the ablated specimen
was on average 249 MPa (36.1 ksi). The average percentage
elongation of the conventional and ablated specimens was .38
and .33% respectively.
Conventional casting
Tensile
#
30.6
1
30.7
2
30.3
3
31.2
4
31.2
5
34.1
6
32.5
7
33.2
8
30
9
31.2
10
Mean
31.5
St Dev.
1.34
%E
0.35
0.35
0.25
0.4
0.45
0.45
0.4
0.4
0.35
0.4
0.38%
0.06
Ablation component
Tensile
#
38.4
1
41
2
31.6
3
37
4
35.7
5
36
6
33.2
7
35.6
8
Mean
36.1
%E
0.35
0.4
0.3
0.25
0.4
0.3
0.3
0.3
0.33%
St. Dev.
2.91
0.05
The fracture surfaces are shown in Figure 10. The surfaces
were very similar in character, with evidence of brittle
fracture at intermetallics, and particulate reinforcement
exposed and protruding from the surface. Shrinkage pores
were found in the fracture surface of the conventionally cast
specimen, and likewise, gas pores were found in the surface
of the ablated specimen that likely contributed to failure.
ablation component compared to the thin section of the
conventional casting could be from the casting process, if
metal is ladled from the top, middle, or bottom of the melt
charge. The increased percentage of reinforcement in the thin
section of the ablation component may have partly
contributed to the higher strength of the ablated samples.
Future work will need to be undertaken in ablation to
examine the effect of solidification in varying thicknesses,
cooling rates, particle dispersion, and mixing/holding time of
the melt on the distribution of particles in the solidification
microstructure.
(a)
(b)
Fig. 9 a) Shrinkage porosity (dendrite) in fracture
surface of the conventionally cast sample, b) gas pore
found in the fracture surface of the ablated cast
sample. Note: interior wall of the gas pore has
features corresponding to the SiC particles that
stabilized it in the casting.
DISCUSSION
Examination of the microstructure of both the conventional
casting and the ablated component revealed the presence of
SiC and graphite particles at cell or dendrite boundaries. This
is evidence that both the particulate reinforcements including
silicon carbide and graphite were pushed by growing αAluminum dendrites into the solute rich last freezing zones.
The increased volume percentage of reinforcement in the
ablated sample was likely due to the increased solidification
rate, and decreased floatation or settling of the reinforcement.
As the microstructure is more cellular in the ablated
specimen, reinforcement particles were trapped in a tight
network of cell boundaries thus increasing the local
concentrations of reinforcement, and reducing the average
interparticle spacing.
In addition to this, it is known that although the combination
of SiC and graphite particles in the liquid aluminum results in
a near neutrally buoyant suspension, there is some settling
and flotation that occurs over time when melting a large
batch. If the melt is insufficiently mixed, the reinforcements
can migrate resulting in areas that are rich in either SiC or
graphite. This can result in rather large differences in the
volume percentage of reinforcement in the solidified
microstructure2.
It is possible that the increase in
reinforcement volume percent in the thin section of the
In addition to improving the distribution of particles in the
casting, ablation has appeared to refine the dendritic cell size.
This is in contrast to published earlier reports in a monolithic
A356 alloy, 3,4 where the microstructural refinement was
process limited only in the eutectic silicon formation. The
particulate reinforcement within the liquid melt during
solidification may act to restrict the dendrite cell size while
they are pushed by the solidifying α-Aluminum. With the
increased cooling rates caused by the impingement of solvent
on the solidifying surface, the particles are entrapped more
rapidly without time to migrate substantially, resulting in
increased branching of the dendrites and a finer cell size over
all. It should be noted that finer intermetallics due to the
solidification of interdendritic solute rich liquid were also
found in the ablated sample. Nickel intermetallics, not
commonly found in the unreinforced alloy, were formed due
to the dissolution of the nickel coating on the graphite into the
matrix.
The samples showed relatively high levels of porosity, even
in the case of the ablation component. In the case of the
ablation component, gas porosity was observed which is
likely due to poor gating design and filling system that was
not designed or optimized for ablation, but rather, the
conventional casting, which aided feeding for the cast
product. The pouring technique and the mold geometry
should be modified significantly for ablation in the future
work since the time delay to remove the mold was rather
significant in duration. Despite this porosity, the strength of
the ablated sample was higher than that of the conventionally
cast sample, and further improvement in the mechanical
properties may be realized by improving the pouring
technique to reduce turbulence in the melt.
CONCLUSION
1.
2.
Ablation of composites has shown to increase the yield
strength of hybrid Al-SiC-graphite composite by 20%
over conventional heavily chilled sand casting. Further
improvements are possible, through higher cooling rates
and a reduction in porosity. The percent elongations of
ablated components were similar to conventional
castings despite the higher strength of ablated castings.
The microstructure of ablated samples exhibit four
distinct phases: SiC particles, graphite platelets, αaluminum dendrites, and Si eutectic second phase. In
addition, some porosity was also present.
3.
4.
5.
6.
The average particle size of the SiC was 15 microns.
The graphite platelet size was 84 microns. The SiC
and graphite particles were present in the
interdendritic regions along with eutectic silicon.
The dispersion of reinforcements is more uniform in
the ablated sample than in the conventionally cast
sample.
The DAS/cell size of the ablated specimen was 50.3
µm (0.00198 in), compared to the DAS of the
conventionally cast specimen of 62.5 µm (0.00246
in). In addition to the refinement of the α-Aluminum
phase, the eutectic silicon and nickel intermetallics
also were refined by the ablation process.
The fracture surface of each sample exhibits
predominantly brittle characteristics, with evidence
of graphite flakes, shrinkage pores and gas pores at
the surface, which may have contributed to the
initiation of failure.
ACKNOWLEDGMENTS
The work presented in this publication was funded
through the SBIR program: W56HZV-08-0067. Work at
UW-Milwaukee is funded through the US-Army Tank
and Automotive Command (TACOM): W56HZV-08-C0716.
REFERENCES
1.
2.
3.
4.
http://mmc-assess.tuwien.ac.at/mmc/Article296.html
Rohatgi, P.K., “Casting Characteristics of Hybrid
(Al/SiC/Gr) Composites,” AFS Transactions, vol. 19,
pp.191-197 (1998).
Grassi, J., U.S. Patent No. US 2008/0041499A1 (Feb.
21, 2008).
Grassi, J., “Ablation Casting,” Aluminum Alloys:
Fabrication, Characterization and Applications,
TMS, (2008).

Ablation of Hybrid Metal Matrix Composites

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