Materials Transactions, Vol. 55, No. 12 (2014) pp. 1820 to 1826
© 2014 Japan Foundry Engineering Society
Effects of Casting Design and Reduced Pressure on Mold Filling
of Molten Aluminum Alloy in Expendable Pattern Casting Process+1
Sadatoshi Koroyasu+2
Department of Mechanical and Precision Systems, School of Science and Engineering, Teikyo University,
Utsunomiya 320-8551, Japan
The effects of the casting design and reduced pressure in a flask on the mold filling of a molten aluminum alloy in the expendable pattern
casting (EPC) process were investigated experimentally. An aluminum alloy plate was cast using the EPC process, and the arrival times of the
molten metal were measured for three coats with different permeabilities. The use of a high permeability coat, or the condition of applying
reduced pressure, led to a higher melt velocity. The melt velocity did not increase in proportion to the coat permeability. The experimental values
for the arrival time of molten metal were compared with the calculated values based on the mold filling model used in the previous study. The
values were in relatively good agreement except for when the coat permeability was very low. The arrival time in top pouring was almost the
same as that in bottom pouring, except for when the coat permeability was very low. The melt velocity in the top pouring did not drop, even
when the coat permeability was low. [doi:10.2320/matertrans.F-M2014834]
(Received June 26, 2014; Accepted September 5, 2014; Published November 8, 2014)
Keywords: expendable pattern casting, aluminum alloy, mold filling, coat permeability, reduced pressure, casting design
1.
Introduction
The expendable pattern casting (EPC) process is very
attractive particularly for complex-shaped casting, because
near net shape castings are obtained without having to divide
and assemble the mold and core. Therefore in this process,
it is possible to eliminate not only machining steps but also
the burr finishing process. Furthermore, because a molding
binder is not added to the mold in the EPC process, the load
on the environment can be reduced.
However, in the EPC process, the molten metal is poured
into the cavity produced by the thermal decomposition and
liquefaction of the expendable polystyrene (EPS) pattern, and
the thermal decomposition gas and liquid resin discharge into
the dry sand through the coat layer. Therefore, the mold
filling mechanism in the EPC process is more complicated
than when using a cavity mold.1) Furthermore, when the coat
permeability changes, the thickness of the thermal decomposition gas layer between the melt surface and the nondecomposition EPS pattern changes. Therefore, it seems that
the coat permeability and melt velocity do not have a simple
relationship.2­4) In addition, when the pressure in the flask is
reduced, because the thickness of the thermal decomposition
gas layer changes, the melt velocity also changes. Moreover,
the mold filling situation is affected by whether the casting
design uses bottom pouring or top pouring.1) Especially when
using the EPC process for aluminum alloy, when the melt
velocity is very low, a misrun due to a temperature drop at the
melt surface5,6) can easily occur. On the other hand, when the
melt velocity is too high, it seems that internal residue resin
defects can easily occur because of non-decomposed parts of
the pattern trapped within the molten metal.1)
Therefore it is important to estimate the melt velocity in
the EPC process. Recently, a relatively large number of
investigations have been conducted on the mold filling for the
+1
This Paper was Originally Published in Japanese in J. JFS 86 (2014) 447­
453.
+2
Corresponding author, E-mail: koroyasu@mps.teikyo-u.ac.jp
EPC process, such as the research on molten cast iron
by Maruyama et al.7,8) However, few studies have been
performed on the of the mold filling mechanism in the EPC
process, such as on the quantitative effect of the process
parameters on the melt velocity.2,9) Moreover, there are few
analysis systems for mold filling that can exactly simulate
the EPC process.10,11) Therefore, there are examples of the
direct observation of a mold filling by using a heat-resistant
glass4,12) or an X-ray image.1,13)
In the present work, to clarify the mold filling mechanism
for a molten aluminum alloy in the EPC process, the effects
of the casting design and reduced pressure condition on the
melt velocity were examined experimentally for three coats
with different permeabilities. Additionally, the experimental
values of the melt velocity were compared with the calculated
values based on the mold filling model from the previous
study.2)
2.
Experimental Procedure
Figure 1 shows a schematic of the casting apparatus used
in experiments, which was similar to that used in a previous
study.5,6) The steel molding flask is a cylindrical vessel with
an inside diameter of 200 mm and depth of 300 mm. It has a
suction port with an inner diameter of 40 mm to reduce the
pressure in the flask. The EPS pattern has a plate shape that
is 70 mm wide, 200 mm high, and 10 mm thick, with an
expansion ratio of 50 (density = 21.2 kg·m¹3). As shown in
Fig. 1, the cluster of the bottom pouring system is assembled
with the EPS pattern which has a perpendicular length of
70 mm to a figure. The ceramic tube (25 mm inside diameter,
35 mm outside diameter, and 300 mm length) is used as a
sprue from the pouring basin to the runner. The pouring basin
is the insulating sand mold, which has an inner diameter of
80 mm, depth of 80 mm, and thickness of 15 mm. The runner
is a plate EPS with a cross section of 35 © 10 mm and a
length of 135 mm. The cross section of the ingate is 35 © 10
mm. The cluster for a top pouring system using the EPS
pattern with the same size was also assembled, as shown in
Effects of Casting Design and Reduced Pressure on Mold Filling of Molten Aluminum Alloy in Expendable Pattern Casting Process
to recorder
Sprue
(Ceramic
tube)
to suction
pump
200mm
10mm
300mm
Dry
battery
10mm
Flask
190mm
EPS
pattern
145mm
Tungsten
wire
100mm
Hg
manometer
Diode
Screen
55mm
Output voltage
Pouring basin Plastic film
Touch sensor
10mm
Expendable pattern
Dry sand
φ200mm
Fig. 1 Schematic of casting apparatus for measurement of mold filling in
bottom pouring.
V
Runner
1kΩ
to recorder
Fig. 3
Hg
manometer
to suction
pump
200mm
Flask
10mm
300mm
Sprue
(Ceramic
tube)
Screen
Test
Coat
Time of arrival
(b) Output voltage
Thermal
Coat
Main Component conductivity
Permeability
/W·m¹1·K¹1
Low permeability, Alumina, Silica,
High insulating
Mica
0.18
0.13
B
Standard
permeability
Silica
0.53
1.7
C
High permeability
Silica
(Large particle)
0.48
10
Expendable pattern
φ200mm
Coat type
Test coat used in experiments.
A
Touch sensor
Dry sand
2.4 V
Schematic of touch sensor of molten metal.
Table 1
10mm
3.9 V
0.9 V(1.5 V-0.6 V)
(a) Wiring diagram
Pouring basin Plastic film
1821
Fig. 2 Schematic of casting apparatus for measurement of mold filling in
top pouring.
Fig. 2. In this case, the cross section of the ingate is similar to
that for bottom pouring, but the length of the ceramic tube is
90 mm. Silica sand with a mode diameter of approximately
0.15 mm (AFS grain fineness number 62) was poured into the
flask.
Touch sensors were inserted into the EPS pattern at 10, 55,
100, 145, and 190 mm from the ingate, as shown in Fig. 3(a),
for measuring the arrival time of the molten metal for the
flow direction distance y.8) The touch sensors were tungsten
wires with a diameter of 0.5 mm. They passed through the
center of the 70 mm wide EPS pattern. As shown in Fig. 3(b),
the voltage across the resistance is increased in a stepwise
fashion, every time the molten metal reaches a touch sensor.
The arrival time ta was defined from the increasing time in the
voltage change chart.8) In order to determine the arrival time
from the ingate, the time when the molten metal reached the
ingate was required. This time was determined by extrapolation from the times when the molten metal reached
y = 10 mm and y = 55 mm.
Three kinds of commercial coats for the EPC process with
different permeabilities were used in this study. Table 1 lists
the main physical properties of those coats. Coat A was a
mica base coat with a high void fraction and high heat
insulation for light metals such as an aluminum alloy.
Although the thermal conductivity value of a coat is not
mentioned in detail in this paper, the thermal conductivities
of the coats in Table 1 were measured using the transient hot
wire method14) based on JIS R2618. The thermal conductivity of coat A was approximately 1/3 that of the silica base
coat, as shown below. Coat B was a silica base coat with a
middle permeability, which is generally used for cast iron,
aluminum alloys, etc. Coat C was a silica base coat with a
high permeability, which was composed of large particles.
Each coat permeability in Table 1 was measured using a
method15) in according with JIS Z2603. This coat permeability is an absolute number with the dimension of
(cm2·cmH2O¹1·min¹1) defined in the following equation
using the coat thickness ¤ (cm), the volumetric gas flow rate
per unit area v (cm·min¹1), and the inter-coat differential
pressure ¦P (cmH2O).
K¼
¤v
P
ð1Þ
The coat permeability of coat A was approximately 1/10 that
of coat B, and was very low. The coat permeability of coat C
was approximately 6 times that of coat B, and was very high.
The EPS pattern was coated using a dipping method. The
coated pattern was then dried for 24 h in a drying furnace at
1822
S. Koroyasu
40
10
0.1
Distance from ingate, y/m
0.2
Fig. 4 Arrival time in bottom pouring without reduced pressure.
40
Results and Discussion
Experimental results for arrival time of molten
metal
Figure 4 shows the experimental data for the arrival time
of the molten metal as a function of the distance from the
ingate at the pattern bottom y for three different values of coat
permeability, in the case of bottom pouring and without
reduced pressure. Figure 5 shows a plot similar to that in
Fig. 4, for the case with reduced pressure. In the order of
coat A, coat B, and coat C, with increasing coat permeability
K, the arrival time decreases. In addition, the arrival time in
the case with reduced pressure is shorter than that in the case
without reduced pressure. In the case in which the coat
permeability is large or the case under reduced pressure, the
thickness of the thermal decomposition gas layer is thin. As a
result, it seems that the thermal decomposition of the EPS
pattern was enhanced by an increase in the heat transfer from
the molten metal to the EPS pattern.2) For coat A, as the
distance from ingate y increases, the gradient of the arrival
time increases, and the melt velocity decreases. This can
be explained as follows. For coat A, which has very low
permeability, because the gas layer thickness increases as a
result of a decrease in the discharge rate of the decomposition
gas, the heat transfer from the molten metal to the EPS
pattern decreases, and the thermal decomposition rate
decreases. As a result, the melt velocity also decreases. As
the distance from the ingate increases, especially at the melt
surface where the heat radiation time is long, the effect of the
semi-solidification due to the melt temperature drop appears.
In the case without reduced pressure, the above mentioned
effect is more remarkable, the molten metal does not reach a
distance of 190 mm from the ingate, and a misrun occurs. In
addition, coat A is an insulating coat, which has a thermal
conductivity that is approximately 1/3 that of coat B or
coat C, as listed in Table 1. The insulating characteristic of
coat A rather reduces the temperature drop at the melt surface
to approximately 1/2.6) On the other hand, from the results of
Fig. 4, the melt velocity up to the mold filling distance of
55 mm for coat A is approximately 1/5 than that for coat B.
Bottom pouring
without reduced pressure
30 Key Coat Permeability K
0.13
A
1.7
B
20
C 10
0
0
Arriving time, ta /s
3.
Arriving time, ta /s
323 K. Each coat was coated so that the thickness of coat
after drying was approximately 1 mm.
The aluminum alloy JIS AC2A (A319 equivalent) as a
casting material was melted in a high frequency electric
induction furnace and directly cast from the furnace. The
pouring temperature was set at approximately 1073 K. In the
pouring operation, the melt surface from the bottom of the
pouring basin was maintained at approximately 50 mm. As a
result, the melt head at the melt surface during mold filling
decreased from approximately 350 mmAl to 150 mmAl in the
case of a bottom pouring, and increased from approximately
150 mmAl to 350 mmAl in the case of top pouring.
Atmospheric pressure and a reduced pressure of 13.3 kPa
(differential pressure with atmospheric pressure) were applied
as the pressure conditions in the flask during the pouring
process. The pressure in the flask was reduced by aspiration
from a suction port on the flask side. A plastic film was used
to cover the top of the flask and maintain the reduced
pressure in the flask.
3.1
Bottom pouring
under reduced pressure
30 Key Coat Permeability K
0.13
A
1.7
B
20
C 10
10
0
0
0.1
Distance from ingate, y/m
0.2
Fig. 5 Arrival time in bottom pouring under reduced pressure.
In addition, the temperature drop at the melt surface in the
case of coat A is estimated to be approximately 4 times larger
than that in the case of coat B.5) From these results, for
coat A, the effect of the temperature drop at the melt surface
by the low melt velocity due to the low coat permeability is
larger than the insulating effect of the coat. In addition, under
the conditions in this work for coat B or coat C, it is
estimated that the range of the melt temperature change is
higher than the solidus temperature,5,6) and that the velocity
drop of the melt due to the semi-solidification has not
occurred.
Figures 6 and 7 show plots similar to those in Figs. 4 and
5, for the case of the top pouring. As in the cases shown
in Figs. 4 and 5 for the bottom pouring, the arrival time
becomes shorter, as the coat permeability becomes larger.
Moreover, the arrival time in the case with reduced pressure
is shorter. Even in the case of coat A, the melt velocity does
not decrease with an increase in the distance from the ingate
y, and no misrun occurs.
3.2
Effect of coat permeability on arrival time of molten
metal
A quantitative consideration of the effect of the coat
permeability on the arrival time is examined in the following.
Effects of Casting Design and Reduced Pressure on Mold Filling of Molten Aluminum Alloy in Expendable Pattern Casting Process
Top pouring
without reduced pressure
Key Coat Permeability K
A
0.13
B
1.7
5
C 10
Bottom pouring
Key
Reduced
pressure, kPa
0
13.3
4
6
Arriving time, ta /s
Arriving time, ta /s
10
1823
K=1.7
2
K=10
0
0
0.1
Distance from ingate, y/m
0
0
0.2
Fig. 6 Arrival time in top pouring without reduced pressure.
Fig. 8
0.1
Distance from ingate, y/m
Top pouring
Key
Reduced
pressure, kPa
0
13.3
4
K=1.7
6
Arriving time, ta /s
Arriving time, ta /s
0
0
0.2
Effect of coat permeability on arrival time in bottom pouring.
10
Top pouring
under reduced pressure
Key Coat Permeability K
A
0.13
B
1.7
5
C 10
0.1
Distance from ingate, y/m
0
0
0.2
Fig. 9
Fig. 7 Arrival time in top pouring under reduced pressure.
Figure 8 shows the replotting of experimental results of the
arrival time for the bottom pouring cases shown in Figs. 4
and 5, in order to show a comparison between the coat B of
permeability K = 1.7 and coat C of permeability K = 10. As
can be seen in Fig. 8, even if the coat permeability increases
approximately 6 times, the arrival time only decreases to
approximately 1/2, with no dependence on the reduced
condition in the flask. Therefore the coat permeability ratio
does not necessarily agree with the arrival time ratio. The
reason for this may be as follows. Although the coat
permeability increases, the discharge area of the thermal
decomposition gas decreases by a decrease in the thickness of
the gas layer.2) Thus, the mold filling is considered to be in
dynamic equilibrium, under which the gas discharge rate
reversely decreases. Therefore, it seems to be difficult to
predict the melt velocity from the coat permeability. Thus, it
is necessary to measure the velocities of melt for wide range
of coat permeabilities.
Figure 9 shows a plot similar to that in Fig. 8 in order to
consider the effect of the coat permeability for top pouring.
In this case also, even if the coat permeability increases
approximately 6 times, the arrival time only decreases
approximately 1/2. This result is similar to the result in the
case of the bottom pouring in Fig. 8.
2
K=10
0.1
Distance from ingate, y/m
0.2
Effect of coat permeability on arrival time in top pouring.
Numerical calculation method based on mold filling
model
The experimental results of the arrival time of the molten
metal were compared with the calculated values using the
mold filling model reported previously.2) A summary of this
mold filling model is given as follows. In the EPC process for
the bottom pouring as shown in Fig. 10, the case in which the
molten metal was poured into the cavity formed by the
thermal decomposition of the EPS pattern, and the case in
which the decomposition gas discharges into the dry sand
through the coat, were considered. Furthermore, the thermal
decomposition and gas discharge were considered to proceed
under a dynamic equilibrium state, where a condition in
which the pressure in the gas layer was equal to the melt
head, was maintained.8) In the case of top pouring, when the
thickness of the gas layer that existed under the molten metal
is sufficiently small, and the gas does not rise in the molten
metal by buoyancy, it is seemed that a model similar to that
for bottom pouring can be adopted. In this case, it seems that
the heat transfer from the melt surface to the EPS pattern
through the decomposition gas layer is due to the radiation
and heat conduction, and heat flux q can be expressed as
follows:
3.3
1824
S. Koroyasu
Coat
40
Radiation
+
Conduction
Thermal
decomposition
gas
Molten Metal
B/2
Arriving time, ta /s
EPS
pattern
Center of
plate pattern
Sand
mold
4
Tm
100
4 #
­
þ ðTa Tm Þ ð2Þ
¤
where Ta and Tm are the temperature at the melt surface and
liquid EPS surface, respectively, ¾a and ¾m are the emissivity
values16) at the same places, and ­ and ¤ are the thermal
conductivity and thickness of the thermal decomposition gas
layer, respectively. The thermal decomposition rate of the
EPS pattern is determined using eq. (2), and the melt velocity
can be determined. The generation rate of the thermal
decomposition gas can also be calculated from the decomposition rate of the model. On the other hand, because the
thermal decomposition gas is discharged into the dry sand
through the coat, the discharge rate of the gas can be obtained
from eq. (1) based on the coat permeability K. As the value
of ¦P in eq. (1), the melt head at the melt surface was used
for the case of a non reduced pressure condition in the flask,
and the sum of the degree of reduced pressure and the melt
head was used for the case of the reduced pressure condition.
In addition, if the mold filling is in a dynamic equilibrium
state, by the equivalence of the generation rate from eq. (1)
and the discharge rate of the thermal decomposition gas from
eq. (2), the melt velocity can be determined, and the arrival
times shown in following figures can also be determined.
3.4
Effect of casting design on arrival time of molten
metal
Figures 11, 12, and 13 show the experimental results for
the arrival time without reduced pressure for the three coats
with different permeability. The same data in Figs. 4 and 5
are shown to illustrate the difference between the casting
designs. In these figures, the solid and broken lines represent
the calculated values based on the mold filling model from
the previous study, as mentioned above. As can be seen in
these figures, except for coat A, which has a very low
coat permeability K, there is almost no difference in the
experimental values of the arrival time by the difference in
the casting design, and the experimental values of the arrival
time almost agree with the calculated values. In the middle
region of the mold filling distance, except for coat A, the
arrival times for the top pouring are slightly longer than
those for the bottom pouring. The reason for this may be
as follows. As the inter-coat differential pressure, which
0.1
Distance from ingate, y/m
0.2
Fig. 11 Effect of casting design on arrival time without reduced pressure
for coat permeability K = 0.13.
10
Arriving time, ta /s
Ta
100
without reduced pressure
Coat B (K=1.7)
Key Line Casting
(Exp.) (Calc.) design
Bottom
5
Top
0
0
0.1
Distance from ingate, y/m
0.2
Fig. 12 Effect of casting design on arrival time without reduced pressure
for coat permeability K = 1.7.
5
Arriving time, ta /s
"
10
0
0
Fig. 10 Mold filling model in EPC process.
5:67
q¼
1
1
þ
1
¾a ¾m
without reduced pressure
Coat A (K=0.13)
30 Key Line Casting
(Exp.) (Calc.) design
Bottom
20
Top
without reduced pressure
Coat C (K=10)
4
Key Line Casting
(Exp.) (Calc.) design
3
Bottom
Top
2
1
0
0
0.1
Distance from ingate, y/m
0.2
Fig. 13 Effect of casting design on arrival time without reduced pressure
for coat permeability K = 10.
corresponds to the melt head, becomes larger, the discharge
rate of the decomposition gas becomes larger, and the gas
layer thickness becomes smaller. Thus, because of the
increase in the melt head, the heat flux between the melt
Effects of Casting Design and Reduced Pressure on Mold Filling of Molten Aluminum Alloy in Expendable Pattern Casting Process
4.
Conclusion
In order to examine the effects of the casting design and
reduced pressure condition on the mold filling of a molten
aluminum alloy in the EPC process, aluminum alloy plates
were cast by the EPC process using three coats with different
permeabilities, and the arrival times of the molten metal were
measured. The following conclusions were obtained under
the conditions of this work.
(1) The use of a high permeability coat or the application of
the reduced pressure condition led to a short arrival time
of the molten metal.
(2) Even if the coat permeability increased, the arrival time
of the molten metal did not decrease in proportion to the
coat permeability.
Arriving time, ta /s
40
under reduced pressure
Coat A (K=0.13)
30 Key Line Casting
(Exp.) (Calc.) design
Bottom
20
Top
10
0
0
0.1
Distance from ingate, y/m
0.2
Fig. 14 Effect of casting design on arrival time under reduced pressure for
coat permeability K = 0.13.
Arriving time, ta /s
10
under reduced pressure
Coat B (K=1.7)
Key Line Casting
(Exp.) (Calc.) design
Bottom
5
Top
0
0
0.1
Distance from ingate, y/m
0.2
Fig. 15 Effect of casting design on arrival time under reduced pressure for
coat permeability K = 1.7.
5
Arriving time, ta /s
surface and the EPS pattern, the thermal decomposition rate,
and the melt velocity all increase. Therefore, the reason for
the difference in the arrival time at the middle region of the
mold filling distance by the difference in the casting design,
may be the gradual decrease in the melt head for bottom
pouring, compared to the gradual increase for top pouring, as
the molten metal fills. In the case of coat A, which has a very
low permeability K, the experimental value of the arrival time
for the bottom pouring is considerably larger than the top
pouring and calculated values. This may be an effect of the
semi-solidification due to the increase in the arrival time of
the melt. On the other hand, for the top pouring, there is no
decrease in the melt velocity in the flow direction, and the
arrival time is slightly lower than the calculated value. For the
top pouring, when the decomposition gas thickness increases,
it seems that the gas rises in the molten metal because of
buoyancy. As a result, it is thought that the thickness of the
thermal decomposition gas layer decreases, and the decrease
in the heat transfer rate from the molten metal to the EPS
pattern is controlled.
Figures 14, 15, and 16 show plots similar to those in
Figs. 11, 12, and 13, for the arrival times under reduced
pressure. As in the cases shown in Figs. 11, 12, and 13
without reduced pressure, with the except of coat A, which
has a very low coat permeability K, there is no significant
differences in the experimental and calculated values for the
arrival time by the difference in the casting design. In the
middle region of the mold filling distance, the difference in
the calculated values of the arrival time for different casting
designs is smaller than that in the case without reduced
pressure. The reason for this may be considered as follows.
The inter-coat differential pressure ¦P under reduced
pressure is given by the sum of the melt head and degree
of reduced pressure. Therefore, in the case of reduced
pressure, the rate of change in the inter-coat differential
pressure from the start to the end of mold filling is smaller
than that in the case without reduced pressure. In addition,
the experimental values for the arrival time almost agree with
the calculated values. In Fig. 14 for coat A, which has a very
low coat permeability K, the experimental results for the
arrival time are almost similar to those in the case without
reduced pressure, and the experimental result for the bottom
pouring is considerably larger than that for the top pouring,
and is also larger than the calculated value.
1825
under reduced pressure
Coat C (K=10)
4
Key Line Casting
(Exp.) (Calc.) design
3
Bottom
Top
2
1
0
0
0.1
Distance from ingate, y/m
0.2
Fig. 16 Effect of casting design on arrival time under reduced pressure for
coat permeability K = 10.
(3) The experimental values for the arrival time of the
molten metal were compared with the calculated values
based on the mold filling model from the previous
1826
S. Koroyasu
study, and were found to be in relatively good
agreement with these calculated values, except for the
case in which the coat permeability is very low. When
the coat permeability was very low, the experimental
value of the arrival time for the bottom pouring was
longer, and that for the top pouring was shorter, than
that of the calculated value.
(4) Except for the case in which the coat permeability was
very low, the arrival times in the case of top pouring
were almost the same as those for bottom pouring. The
melt velocity for top pouring did not decrease, even
when the coat permeability was low.
Acknowledgment
This study was carried out by the young member research
encouragement fund of the Japan Foundry Engineering
Society. I wish to express my deepest gratitude for the
support.
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