Materials Sciences and Applicatio n, 2011, 2, 891-898
doi:10.4236/msa.2011.27119 Published Online July 2011 (
Copyright © 2011 SciRes. MSA
Effects of Using Fins and Carboceramics on the
Solidification Characteristics of Aluminum Casting
Seyyed Mohammad Ali Boutorabi1, Masoud Zandira2
1School of Metallurgy and Materials Science, Center of Excellence for Advanced Materials, Iran University of Science and Tech-
nology, Tehran, Iran; 2Department of Materials and Manufacturing Technology, Chalmers University of Technology, Göteborg,
Received December 15th, 2010; revised May 21st, 2011; accepted May 5th, 2011.
Recent studies have shown that using the cooling fins is one of the most appropriate methods to eliminate the shrinkage
porosities in the castings. In fact, Fins play the role of heat exchangers during the solidification and their usage is the
same as that of chills. The coolin g effect of fins is directly related to their geometry. Also the effect of fin on decreasing
the time of local freezing is a func tion of metal’s thermal conductivity , mould thermal conductio n, superheating and fin
dimensions. In this research, the effect of three types of fins on local solidification time of a casting of A356 and 6069
alloys as well as the effect of moisture content on cooling rate of sand moulds made of carboceramics have been studied.
Results indicate that fin affects severely on solidification time and this influence is more severe in castings produced in
moulding materials such as carboceramics which can absorb more moisture. The results show that carboceramics as a
new refractory material decrease the solidification time significantly. This is particularly evident when the fins were
used. Generally, using fins and carboceramics decrease the solidification time down to 45%. This is good news for
casting industry in which the mechanical properties can be easily improved using simple ways.
Keywords: Fins, Carboceramics, Solidification Time, A356, 6069
1. Introduction
Fin has many definitions, some of which are referred.
Some references define fin as a flash attached to the
casting originated as a result of cope and drag setting [1,
2]. The same definition however can be considered for
flashes. In other words, there is no difference here be-
tween shrinks and fin. In the references related to thermal
exchange, fin is regarded as a broad metallic sheet with
high thermal conductivity and mainly as sheets with thin
and long bars, acting as heat exchangers or thermal sinks
Cooling fins decrease the local solidification time and
remove porosities and improve the mechanical properties
[5,6]. Fins also increase the effective range of feeding or
even may omit the need to use feeder in castings. Hence
fins can be regarded as chills in casting design [7,8].
Inevitable existence of flashes in moulding parts indi-
cates the important role of fin, because flashes can be
regarded as a type of fin attached to the moulding part
originated circumstantially [7,8]. Since the first measure
in fettling the castings is to remove the flashes, no study
has been done on the effect of flashes as fin, considering
its thermal properties.
In general, fins’ properties are as follows [4,7,9-12]:
1) Fin and casting integrity in comparison with chills
set up with gaps in the mould, are important from view-
point of exact comprehensiveness towards the casting.
2) Since fin is a part of the casting, there should be no
air gap in the thermal exchange route so that in addition
to replicability, thermal exchange of fin, remains effec-
3) Fins do not require to be produced independently
like chills.
4) Fins do not need to be separated from the returned
sand for reuse. Fins are not the same as that of chills.
5) Usage of chills and their position in the mould en-
tail limitations in some case particularly in consumable
The problems of using fins can be summarized as fol-
Effects of Using Fins and Carboceramics on the Solidification Characteristics of Aluminum Casting Alloys
1) Removing the fins during fettling would increase
the costs of cleaning [7,9].
2) Fin would decrease the casting efficiency. Of course
it should be stated that this decline in efficiency is negli-
gible because fins increase the solidification rate and
decrease the volume of the feeder.
Studies conducted on cooling fins in moulding parts are
limited. The main studies began since 1985 at Wiscon-
sin-Madison University and continued by other research-
ers [13]. Most studies have been on the effects of fins in
Pour-out test method in different times (Figure 1) [3,4,
9-13]. In this method, the melt is drained from the mould
cavity after a specific time. Hence the effect of fin is
studied by observing different sections of frozen samples
in different times and evaluating the growth of freezing
front at different points of the part, in particular near the
fin. The effect of fin is indicated by Af/S in which Af is
the frozen surface with fin present and S is the frozen
surface without the fin (Figure 2), and is indicated in a
diagram based on the solidification time proportionate
with S/T in which S is the frozen surface without fin and
T is the thickness of the fins (Figure 3) [1,3,9-12].
Superheating is a factor affecting the role of fin [7].
Any change in superheating will change the fin’s effec-
tive range which can be seen in Figure 3.
The effect of fin on castings made of lead and copper
Figure 1 Schematic drawing of some of the results of Pour-
out test, the time of pouring the melt is increased from right
to left respectively [11].
Figure 2. Schematic drawing of the cross section of solidifi-
cation front of the casting with fin, S is the frozen surface
without fin, T is the thickness of the fin, L is its length [1,4,
Figure 3. Diagram of considering the effect of heat conduc-
tion of melt on increasing the effective depth of fin [4], R.S
= ΔT/Tm, (ΔT is superheating and Tm is melting point).
indicates that the ratio Af/S will be maximized as the S/T
ratio increases and then it will decrease. In other words,
one can conclude that if the fin thickness is fixed, its ef-
fectiveness will not exceed than a specific range [12,13].
Sand and Cavity, Carboceramics
The cavity is a place where the molten metal solidifies.
Mechanical properties and metallurgical structure of
castings depend on factors including melt quality, casting
temperature, inoculation efficiency, mould cavity char-
acteristics, heat transfer, etc.
Heat transfer from filled mould depends on different
factors the most important of which is the sand moisture.
Of course type of the refractory materials, type of addi-
tives like bentonite, coal, size and shape of sand granules
are also important. But sand moisture plays a dominant
role in transferring the heat from mould to the environ-
In old fashioned and modern moulding systems, the
moisture content ranges between 3.5 and 5 percent. Ex-
ceeding this limit would decrease the mould characteris-
tics. Originally, in moulding, the ratio of the bentonite
used to the moisture content is defined as an index which
is around 2;
Clay/Water = 2
If the bentonite increases, the sinter point of the mould
will decrease. And if the moisture content decreases, the
mould’s strength will decrease. Also in this case, the gas
porosity will increase. The general behavior of the mould
characteristics due to the bentonite and moisture content
ratios is shown in Figure 4.
Now if the moisture content increases, the cooling rate
will increase. Recently a new type of artificial sand is
presented in European market that in addition to the fact
that sand granules are nearly round, there are porosities
on them. It seems that considering the specific coordi-
Copyright © 2011 SciRes. MSA
Effects of Using Fins and Carboceramics on the Solidification Characteristics of Aluminum Casting Alloys 893
Figure 4. Showing the relation between the quality of the mould cavity and the bentonite (clay) to moisture content.
nates of this sand it is possible to decrease the bentonite
/water ratio without changing the mould cavity properties
and meanwhile increase the cooling rate and conse-
quently change the DAS size and increase the mechani-
cal properties. The holes in the carboceramics can absorb
more moisture and as a consequence the solidification
time will decrease. In this research, dual effects of using
fins and carboceramics on the solidification time of the
castings have been investigated.
2. Experimental Procedure
In the present work, the effect of moisture content on
cooling rate of sand moulds made of carboceramics, and
also the effect of fin on the solidification time have been
investigated. To do this, after designing and constructing
the pattern, the K thermocouple and Labview 2 software
2.1. Moulding and Casting
The pattern used in this research is seen in Figures 5 and
6. The mould cavity was prepared using SiO2 sand with
various percentages of bentonite and moisture. Carboce-
ramic sand was also used similar to SiO2 sand at different
bentonite moisture ratios.
Also, Pep set and Dry Pep set methods were applied to
prepare the mould. In this case, ½% resin and ½% cata-
lyst were added to the sand and mixed for 5 minutes. In
Figure 5. The cross section of the pattern.
Dry Pep set system, the mould prepared in the first step,
was preheated in furnace in 800˚C for two hours. A 25 kg
medium induction furnace was used. The A356 and 6069
alloys were melted and all moulds were cast at 720˚C.
2.2. Thermal Analysis
All castings were thermally analyzed by Lab view II
software. In this case, K type thermocouple was prepared
as seen in Figure 7, and was installed with 25 mm dis-
tance from the casting bottom in the middle of the section.
For any situation, at least two castings were produced and
thermal analysis was carried out by computer.
The cooling curve and first derivative were drawn and
analyzed. The solidification time of both castings were
calculated from the curve of first derivative. The meas-
urement was in a way that the time of beginning of the
solidification in cooling curve is equal to the first jump in
Copyright © 2011 SciRes. MSA
Effects of Using Fins and Carboceramics on the Solidification Characteristics of Aluminum Casting Alloys
Figure 6. Dimensions of the three different used fins.
Figure 7. Schematic of the used K-type thermocouple.
the first derivative curve and the time of end of the so-
lidification is where the base line and curve of eutectic
section converge.
2.3. Cooling Curves and their Derivatives
Figure 8 indicates the curves of the samples of thermal
analysis for alloy A356. As seen in the diagrams, the
curves include two hypoeutectic and eutectic reactions.
The cooling curves and first derivatives in alloys A356
and 6069 can be seen in Figure 9. The general behavior
of alloy 6069 is like pure metal.
The solidification time of all moulds, cast at different
conditions has been calculated from the first derivative.
3. Results and Discussion
3.1. Mould Cavity
The mould cavity is the main base in which the melt is
cast. Because the molten metal flowing into the mould
cavity has high temperature, many considerations should
be taken into account. The most important cases include:
permeability, mould hardness, wet and dry strength,
mould resistance against thermal shocks, resistance
against erosion of floating fluid, etc.
Physical and metallurgical properties of the moulds
prepared in the project have been revised specifically. In
the green sand moulds, by increasing the ratio of ben-
tonite to moisture in the low strength area, low resistance
against erosion will be forced. The only advantage of this
part would be the faster heat transfer and consequently
decreasing the local solidification time.
Because in this area, the main properties required by
the mould namely resistance against erosion are so low,
it is not recommended to work at this area and it is not
possible to achieve the desirable properties through de-
creasing the solidification time. Of course, as is seen this
is evident in mould with SiO2 base. But if carboceramic
sand is used, the added moisture not only moisturizes the
sand surface, penetrates into the sand and therefore,
thermal transference is accelerated. Hence the area for
Copyright © 2011 SciRes. MSA
Effects of Using Fins and Carboceramics on the Solidification Characteristics of Aluminum Casting Alloys895
Figsure 8. Cooling curves of the castings with different wa-
ter content for alloy A356.
Copyright © 2011 SciRes. MSA
Effects of Using Fins and Carboceramics on the Solidification Characteristics of Aluminum Casting Alloys
Figure 9. Cooling curves and first derivatives and the cast-
ings produced in different mould material and different
constituents for alloys A356 and 6069.
using more moisture for carboceramics should be studied
more, because in this mode, it is possible to decrease the
local solidification time and to gain smaller DAS. It is
clear that mechanical properties are increased following
this mode.
In addition to above advantages, higher thermal con-
ductivity of carboceramic containing Al2O3 is also of
The unevenness of the primary surface produced by
carboceramic sand, if its moisture is high, will be lower
than that of moulds casted by SiO sand. To compensate
this, one way is to change the granular distribution of
carboceramic sand. Moulds with resin and catalysts are
of greater strength and thermal conductivity is lower in
these casts than the wet sand, and the solidification time
of the castings is 25% higher than those moulded by wet
When the moulds produced by chemical method are
cooled in the furnace, the chemicals are removed and the
solidification time is lengthened (Figure 10).
3.2. Moulding Materials
The speed of heat transfer from the mould cavity out-
wards is a function of type of moulding material and
mould modulus. As seen, heat transfer is accelerated by
increasing in moisture content and therefore the solidifi-
cation time is decreased. This is due to water which is the
factor for accelerating the heat transfer. Of course as is
evident from Figure 4, moisture can be added only in a
limited percentage. In some cases we may reach a point
where the sand is stuck to the mould and will become
sticky. Meanwhile, in this case, mould strength will de-
crease. Now if the mould thickness is low, we will enter
a point where the sand elasticity, in other words, its
formability will decrease. The reasonable metallurgical
limit is when it is possible to maintain bentonite and wa-
ter ratio around 2. In this case we will have a rigid mould
with high permeability (Figure 4).
In natural washed SiO2 sands, the particles of sand
grains are not porous and the water added to the sand will
wet the surface of the grains. And the adding bentonite to
the humid sand is accompanied with activating the so-
dium ion of the bentonite. Consequently, the moulding
materials with a low compaction exerted on them
through hand, or machine, the desirable rigidity will be
On the other hand, in carboceramic sands, because of
the porosity of the sand granules, the water penetrates
into the porosity of the sand, and will change the heat
transfer and cooling rate of the mould cavity and this is a
very important fact (Figure 11).
As is clear in the diagram, carboceramic in addition to
Figure 10. Variations of solidification time as a function of
moisture content of the mould and the fin size.
Copyright © 2011 SciRes. MSA
Effects of Using Fins and Carboceramics on the Solidification Characteristics of Aluminum Casting Alloys897
Figure 11. Morphology of the carboceramic sand which is
indicated the high porous surface.
maintaining the mould’s rigidity, absorbs moisture up to
5 percent. This is an advantage of this type of sand which
has been less emphasized up to now.
The mould made of Pep Set indicates lower solidifica-
tion time in comparison with wet sand implying the ab-
sence of moisture in that. This is evident also in dried
Pepset mould, because in that, resin and catalyst bands
are transformed and the sand is approximately dried
(Figure 11).
3.3. Fins
Fin decreases the local solidification time. The main
reason for decreasing the solidification time is the fact
that the melt flowing into the mould, also fills the fins
and considering the fact that the fin surface is where the
heat transfers, and this is a broad and considerable sur-
face, the casting solidified more quickly. This will de-
crease the DAS and consequently increases the me-
chanical properties. What is important is that fins move
isotherm temperature surfaces when solidifying and
therefore lead to transfer, decrease, and desirable distri-
bution or elimination of shrinkage porosity and this is so
important in casting industry.
This research indicates that the solidification time will
decrease up to 45% in case fins are used and this is good
news for foundrymen.
4. Conclusions
1) Local solidification time is a function of moulding
materials, casting thermal conductivity and fin dimen-
2) Carboceramics will have good future in casting in-
3) As the casting’s thermal conductivity decreases, the
fin effect increases.
4) The increase in the thermal conductivity of melt
will increase fin effect.
5) The increase in superheating will increase the fin
6) Fin’s optimum thickness in practice is 0.1 half of
the thickness.
7) The most appropriate length of fin is 4 times the
half of casting thickness. In this situation, up to 70 per-
cent of local solidification time has been recorded.
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