World Journal of Engineering and Technology, 2013, 1, 9-16
http://dx.doi.org/10.4236/wjet.2013.12002 Published Online August 2013 (http://www.scirp.org/journal/wjet)
Structural Modification of Sand Cast Eutectic Al-Si Alloys
with Sulfur/Sodium and Its Effect on Mechanical
Properties
Chikezie W. Onyia1, Boniface A. Okorie2, Simeon I. Neife2, Camillus S. Obayi2
1Department of Metallurgical & Materials Engineering, Enugu State University of Science and Technology, Enugu, Nigeria;
2Department of Metallurgical & Materials Engineering, University of Nigeria, Nsukka, Nigeria.
Received May 13th, 2013; revised June 12th, 2013; accepted July 23rd, 2013
Copyright © 2013 C. W. Onyia et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
In the present study, the structural modification of sand cast Al-12wt%Si alloy with sulfur/sodium and its effect on me-
chanical properties were investigated. Different addition levels of sulfur and sodium were used to modify and produce
castings of the same shape and size from the alloy. The results indicated that the addition of sodium or sulfur to eutectic
Al-Si alloy can modify the Al-Si eutectic morphology from needle-like eutectic silicon structure to fine-scale eutectic
silicon structure with significant improvement in mechanical properties of the alloy. The optimum levels of modifica-
tion by sodium flux (60% NaF and 40% NaCl) and sulfur were found to be 0.6% - 1.0% and 0.02% - 0.05% of the
weight of the alloy respectively. The alloy modified with 0.6% Na flux had the best mechanical properties closely fol-
lowed by the one modified with 0.02% sulfur. Over modification of the alloy with sodium produced over modification
band which consisted of aluminum dendrites and coarse silicon particles in the microstructure of the alloy. Increase in
concentration of sulfur decreased the degree of fineness of the eutectic silicon structure with significant decrease in
mechanical properties of the alloy and this is suggested to be as a result of the presence of a brittle sulfur compound at
the grain interfaces of the alloy.
Keywords: Aluminum-Silicon Alloy; Sand Casting; Modification; Morphology; Mechanical Properties
1. Introduction
Pacz [1] discovered modification of Al-Si alloys and
since then, experimental studies on modification of Al-Si
alloys are still going on till today. This is due to the huge
importance of modification in the engineering field of
alloy design and production of cast Al-Si alloy compo-
nents with consistent and superior quality.
The binary Al-Si system is a simple eutectic system
with about 12% silicon being the eutectic composition at
577˚C. Al-Si alloys are termed eutectic alloys when sili-
con content is 11% - 13% of the weight of the alloys.
Eutectic Al-Si alloys are used in automobile and aero-
nautical industries for pistons, cylinders, etc.
The composition of most Al-Si foundry alloys is in the
vicinity of the eutectic point in order to take full advan-
tage of its excellent castability, and these alloys normally
contain about 50 - 90 vol% (Al-Si) eutectic. Hence, the
(Al-Si) eutectic is important in determining the me-
chanical properties of Al-Si foundry alloys. There is need
to understand eutectic solidification and modification
mechanism in Al-Si alloys.
In untreated Al-Si alloys, the eutectic silicon phase has
brittle, coarse and plate-like (flake-like) morphology.
The sharp ends of this plate-like silicon phase serve as
stress concentrators and promote crack initiation and
propagation, and ultimately result in poor mechanical
properties of the alloys. Through the process of modifi-
cation, this brittle, coarse and plate-like eutectic silicon
structure can be transformed to fine fibrous eutectic sili-
con structure with much enhanced mechanical properties
[2-5]. Modification is the process of adding trace amounts
of certain elements to an alloy to change the morphology
of the phase(s) present and ultimately improve the me-
chanical properties of the alloy.
Eutectic modification which is modification of a
eutectic alloy resulting in morphological changes in the
eutectic phase(s) present can be explained by the fol-

Structural Modification of Sand Cast Eutectic Al-Si Alloys with Sulfur/Sodium
and Its Effect on Mechanical Properties
10
lowing theories.
Several theories have been put forward to explain
eutectic solidification and modification mechanism in
Al-Si alloys. Liu Qiyang et al. [6] and Dahle and Hillert
[7] suggested that AlP present in Al-Si alloys serve as
potent nucleation sites for eutectic silicon. This is
thought to be as a result of the similarities between the
crystal structures of silicon and AlP. Shankar et al. [8]
proposed that β-(Al, Si, Fe) intermetallic phases present
in the alloys also serve as nucleation sites for the eutectic
silicon during eutectic solidification. Crosley and Mon-
dolfo [9] also reported that AlP, existing aluminum den-
drites and other unidentified particles present in hypoeu-
tectic Al-Si alloys serve as nucleation sites for eutectic
silicon at different degrees of undercooling during eutec-
tic solidification.
Basically there are two classes of theories on the
mechanism of modification [6]. They are restricted nu-
cleation theory and restricted growth theory. According
to restricted nucleation theory, modifier neutralizes the
heterogeneous nuclei of AlP which is a nucleant for
eutectic silicon or it reduces the diffusion coefficient of
silicon in the melt. This enhances undercooling of the
melt before eutectic silicon solidification can take place,
thus refining of the eutectic silicon structure occurs. Ac-
cording to restricted growth theories, the adsorption of
modifier preferentially takes place on twin re-entrant
grooves or growing surfaces of the silicon phase, re-
stricting the silicon growth during eutectic silicon growth,
and thus requiring eutectic solidification to occur at large
undercooling. This causes the eutectic silicon phase to
grow isotropically with a fine fibrous structure rather
than anisotropically with a faceted (plate-like) structure.
Therefore, modification is usually accompanied by a
depression of the eutectic temperature. This morpho-
logical transformation to fine fibrous eutectic silicon
structure is eutectic modification which is accompanied
by enhanced mechanical properties.
An important change that takes place upon addition of
modifier to an Al-Si alloy is with the number of twins.
Lu and Hellawell [10,11] reported that twin density in
unmodified silicon flakes is very low and twin spacing is
around 0.4 - 1.0 mm on a typical cross section while twin
density is much higher in the modified silicon with twin
spacing between 0.005 μm and 0.1 μm, which allows
silicon to bend and branch more easily to form a fibrous
structure.
Research works have been done on eutectic modifica-
tion of Al-Si alloys with sodium [12-15] but studies have
scarcely been done on eutectic modification of Al-Si
alloys with sulfur. In this work sodium in the form of
sodium flux (60% NaF and 40% NaCl) and elemental
sulfur were used to modify eutectic Al-Si alloys and their
individual effectiveness as eutectic modifier analyzed
and compared.
2. Materials and Procedures
In the present study, the charge materials consisted of
commercial purity aluminum and silicon. Tabl e 1 shows
their individual chemical compositions. Degasification of
the melt was done with MnCl2. Sodium flux (60% NaF
and 40% NaCl) and elemental sulfur in powder form
were separately used to modify the alloys.
Al-12wt%Si alloy was prepared from the above charge
materials in a clay graphite crucible in an induction fur-
nace and the melt was held at 750˚C. After degassing
with manganese chloride (MnCl2), sodium flux (60%
NaF and 40% NaCl) or sulfur powder duly wrapped in
aluminum foil was added to the melt for modification.
The melt was gently stirred for 30 sec with an alumina
plunger after addition of modifier to ensure effective
mixing and maximize modification. Melts were held for
5 min and then cast into cylindrical test bars of dimen-
sion 30 mm diameter × 175 mm length in a sand mould.
Sodium flux (60% NaF and 40% NaCl) additions of
0.2%, 0.6%, 1.0% and 1.4% of the weight of the alloy
were made to four sets of castings respectively, and sul-
fur additions of 0.02%, 0.05%, 0.08% and 0.1% of the
weight of the alloy were made to another four sets of
castings respectively. One set of castings was not modi-
fied which served as the control.
Tensile properties of the alloys were determined from
ASTM standard tensile test bars machined from the cast
cylindrical bars, in as cast condition using a Universal
Testing Machine. Hardness test was carried out on 15
mm diameter × 10 mm long cylindrical test bars ma-
chined from the cast cylindrical bars in the as-cast condi-
tion using a Rockwell hardness tester. An OLYMPUS
optical microscope was used to conduct microstructural
analysis on the specimens prepared from the broken ten-
sile test bars to examine the effect of modifier additions
on the morphology of the eutectic silicon phase. The
surfaces of the specimens were ground with different
Table 1. Chemical composition of the charge materials.
Element Al Si Ca Fe Cu Zn Mn Mg Cr B
Aluminum 99.71 0.045 - 0.23 0.002 0.006 0.001 0.001 0.001 0.004
Silicon 0.185 99.341 0.082 0.392 - - - - - -

Structural Modification of Sand Cast Eutectic Al-Si Alloys with Sulfur/Sodium
and Its Effect on Mechanical Properties
11
grades of emery paper from rough to fine grades. The
final polishing was done on a Struers Rotopol-V polish-
ing machine using diamond paste and polishing cloth.
The samples were etched with caustic soda solution (1 g
sodium hydroxide; 99 cm3 water) [16].
3. Results and Discussion
3.1. Microstructural Analysis
Figures 1(a) and (b) are micrographs of unmodified
Al-12wt%Si alloy casting in as cast condition showing
microstructures in which the eutectic silicon phase is
dispersed in the aluminum matrix with needle-like mor-
phology which is actually plate or flake-like in three di-
mension.
Addition of 0.2% Na flux to the alloy casting results in
slight changes in the microstructure of the alloy (Figure
2). It can be observed that most of the eutectic silicon is
not able to grow into large plates but seems to have been
stunted or broken down into smaller sized particles.
Though few still exist as large plates, they are not as
(a)
(b)
Figure 1. (a) Micrograph of unmodified Al-12wt%Si alloy
casting (200×); (b) Micrograph of unmodified Al-12wt%Si
alloy casting (400×).
many and randomly dispersed as those of the unmodified
alloy signifying partial modification of the alloy. Addi-
tion of 0.02% sulfur to the alloy produces fine eutectic
silicon morphology (Figure 3). There seems to be the
presence of few coarse silicon particles sparsely dis-
persed in the aluminum matrix, otherwise the needle-like
eutectic silicon has been transformed to fine fibrous
structure.
Figure 4 which is the micrograph of Al-12wt%Si +
0.6% Na flux alloy casting shows that the microstructure
of the eutectic silicon is fully modified. The otherwise
flake-like (needle-like) eutectic silicon morphology has
been completely transformed to fine-scale eutectic sili-
con morphology. This observed effect of modification is
consistent with the findings of other researchers [2,3,
17-19]. Addition of 0.05% sulfur to the alloy results in a
microstructure of fine fibrous eutectic silicon morphol-
ogy devoid of any needle-like silicon structure (Figure
5). The microstructure is comprised of fine fibrous eutec-
tic silicon of fairly uniform size and distribution in the
aluminum matrix.
Figure 2. Micrograph of Al-12wt%Si + 0.2% Na flux alloy
casting (200×).
Figure 3. Micrograph of Al-12wt%Si + 0.02% S alloy
casting (200×).

Structural Modification of Sand Cast Eutectic Al-Si Alloys with Sulfur/Sodium
and Its Effect on Mechanical Properties
12
Figure 4. Micrograph of Al-12wt%Si + 0.6% Na flux alloy
casting (200×).
Figure 5. Micrograph of Al-12wt%Si + 0.05% S alloy
casting (200×).
Addition of 1.0% Na flux to the alloy also produces
fine eutectic silicon structure (Figure 6). Modifying the
alloy with 0.08% sulfur results in a microstructure of
refined eutectic silicon structure though the silicon struc-
ture is not as fine as the one obtained for the 0.05% sul-
fur-modified alloy (Figure 7).
Over modification band consisting of aluminum den-
drites with coarse silicon particles can be observed on the
micrograph of the alloy modified with 1.4% Na flux,
implying that the Al-Si eutectic growth has changed to a
dendritic growth of aluminum and coarsening of the sili-
con particles (Figures 8(a) and (b)). This observation is
in agreement with the findings of Kobayashi et al. [20]
who suggested that the over modification band which
appears in the microstructure of eutectic Al-Si alloy
modified with sodium consists of aluminum dendrites,
coarse silicon particles and at times a ternary compound
having chemical composition close to AlSiNa. The mi-
crograph of the alloy modified with 0.1% sulfur shows
Figure 6. Micrograph of Al-12wt%Si + 1.0% Na flux alloy
casting (400×).
Figure 7. Micrograph of Al-12wt%Si + 0.08% S alloy
casting (200×).
the presence of fine eutectic silicon and coarse particles
that may be silicon particles or other unidentified parti-
cles unevenly dispersed in the aluminum matrix (Figure
9). This suggests that the optimal modification level of
the alloy with sulfur has been exceeded.
3.2. Effect of Modifier Addition on the
Mechanical Properties of Al-12wt%Si Alloy
The results of the mechanical properties of the experi-
mental alloy castings in as cast condition are presented in
Table 2, and Figures 10-13 show the effect of modifier
addition on the mechanical properties (UTS, 0.2% proof
stress, percentage elongation and hardness) of the alloy.
Modifying the alloy with 0.6% and 1.0% sodium flux of
the alloy weight yields the highest UTS of 182 and 176
MPa respectively for the sodium-modified alloys com-
pared to 154.4 MPa obtained for the unmodified alloy.
Adding 0.02% and 0.05% sulfur to the alloy produces the

Structural Modification of Sand Cast Eutectic Al-Si Alloys with Sulfur/Sodium
and Its Effect on Mechanical Properties
13
(a)
(b)
Figure 8. (a) Micrograph of Al-12wt%Si + 1.4% Na flux
alloy casting (200×); (b) Micrograph of Al-12wt%Si + 1.4%
Na flux alloy casting (400×).
Figure 9. Micrograph of Al-12wt%Si + 0.1% S alloy casting
(200×).
Table 2. Mechanical properties of modified and unmodified
sand-cast Al-12wt%Si alloy in as cast cond it ion.
Alloy UTS
(MPa)
0.2% Proof
Stress (MPa)
%
Elongation
Hardness
(HRA)
unmodified
Al-12wt%Si 154.4119 3.2 37
Al-12wt%Si +
0.2% Na flux 165 121 4.4 39
Al-12wt%Si +
0.6% Na flux 182 126 10.3 48
Al-12wt%Si +
1.0% Na flux 176 124 8.5 44
Al-12wt%Si +
1.4% Na flux 169 124 6.9 41
Al-12wt%Si +
0.02% S 177.9126 9.2 47
Al-12wt%Si +
0.05% S 169.4124 7.1 43
Al-12wt%Si +
0.08% S 161 123 5.2 41
Al-12wt%Si +
0.1% S 160 121 4.6 39
N
a-modified Al-12wt%Si alloy casting
185
Level of Modification (% of the alloy weight)
0.0% Na flux
0.0% S
0.2% Na flux
0.02% S
0.6% Na flux
0.05% S
1.0% Na flux
0.08% S
1.4% Na flux
0.1% S
180
175
170
165
160
155
150
145
140
S-modified Al-12wt%Si alloy casting
Ultimate Tensile Strength (MPa)
Figure 10. Effect of modification level on the ultimate
tensile strength of Al-12wt%Si alloy casting.
highest UTS of approximately 178 and 169.4 MPa re-
spectively for the sulfur-modified alloys.
Comparing the UTS of the sodium-modified and sul-
fur-modified alloys, it is evident that the 0.6% sodium-
modified alloy has the highest UTS closely followed by
the 0.02% sulfur-modified alloy. Addition of 0.6% so-
dium flux and 0.02% sulfur yields the same 0.2% proof
stress of 126 MPa while the unmodified alloy has 0.2%
proof stress of 119 MPa. Modifying the alloy with 0.6%

Structural Modification of Sand Cast Eutectic Al-Si Alloys with Sulfur/Sodium
and Its Effect on Mechanical Properties
14
N
a-modified Al-12wt%Si alloy casting
Level of Modification (% of the alloy weight)
0.0% Na flux
0.0% S
0.2% Na flux
0.02% S
0.6% Na flux
0.05% S
1.0% Na flux
0.08% S
1.4% Na flux
0.1% S
114
S-modified Al-12wt%Si alloy casting
0.2% Proof Stress (MPa)
116
118
120
122
124
126
128
Figure 11. Effect of modification level on the 0.2% proof
stress of Al-12wt%Si alloy casting.
N
a-modified Al-12wt%Si alloy casting
Level of Modification (% of the alloy weight)
0.0% Na flux
0.0% S
0.2% Na flux
0.02% S
0.6% Na flux
0.05% S
1.0% Na flux
0.08% S
1.4% Na flux
0.1% S
0
S-modified Al-12wt%Si alloy casting
2
4
6
8
10
12
Percentage Elongation
Figure 12. Effect of modification level on the percentage
elongation of Al-12wt%Si alloy casting.
sodium flux significantly improves the percentage elon-
gation of the alloy from 3.2% for the unmodified alloy to
10.3%. Addition of 0.02% sulfur to the alloy also pro-
duces enhanced percentage elongation of 9.2%. The alloy
modified with 0.6% sodium flux has the highest hardness
of HRA 48 closely followed by the 0.02% sulfur-modi-
fied alloy.
It can be seen from this experimental study that modi-
fication of Al-12wt%Si sand-cast alloy with sodium or
sulfur improves the mechanical properties of the alloy in
N
a-modified Al-12wt%Si alloy casting
Level of Modification (% of the alloy weight)
0.0% Na flux
0.0% S
0.2% Na flux
0.02% S
0.6% Na flux
0.05% S
1.0% Na flux
0.08% S
1.4% Na flux
0.1% S
0
S-modified Al-12wt%Si alloy casting
Rockwell Hardness (HRA)
10
20
30
40
50
60
Figure 13. Effect of modification level on the hardness of
Al-12wt%Si alloy casting.
terms of the ultimate tensile strength, 0.2% proof stress,
percentage elongation and hardness, which is due to the
modified eutectic silicon morphology obtained in the
microstructure of the alloy. This observation is in agree-
ment with previous reports [2-5]. The reduced mechani-
cal properties of the alloy modified with high sodium
concentration of 1.4% Na flux is suggested to be as a re-
sult of the over modification band in the microstructure
of the alloy which signifies that the optimal level of addi-
tion of sodium has been exceeded. It can also be ob-
served that very slight coarsening of the fine eutectic
silicon structure occurs as the concentration of the sulfur
increases beyond a certain level with attendant signifi-
cant decrease in otherwise well improved mechanical
properties though the mechanical properties still remain
better than those of the unmodified alloy. The cause of
this significant decrease in mechanical properties despite
the still refined eutectic morphology may be the forma-
tion of brittle sulfur compound in the alloys which, due
to its brittle nature, reduces the mechanical properties of
the alloys. This possible explanation could be said to be
similar by analogy to that found in steel where the pres-
ence of brittle sulphides at the grain boundary interfaces
decrease the mechanical properties of the steel [21]. Fur-
ther studies to support this theory should be considered.
4. Conclusions
The structural modification of sand cast Al-12wt%Si
alloy with sulfur/sodium and its effect on mechanical
properties have been investigated. The following conclu-
sions can be made from the experimental results and

Structural Modification of Sand Cast Eutectic Al-Si Alloys with Sulfur/Sodium
and Its Effect on Mechanical Properties
15
theoretical analysis.
Sodium Modification:
 Sodium can successfully modify Al-12wt%Si sand
cast alloy giving it fine fibrous eutectic silicon mor-
phology with significantly enhanced mechanical pro-
perties.
 Addition of 0.6% Na flux of the weight of the alloy
produced the best eutectic Si morphology and me-
chanical properties closely followed by addition of
1.0% Na flux to the alloy.
 The optimum level of modification of Al-12wt%Si
sand-cast alloy with sodium was found to be 0.6% -
1.0% Na flux of the weight of the alloy.
 Addition of 1.4% Na flux of the weight of the alloy
produced over modification band in the microstruc-
ture of the alloy that consisted of Al dendrites and
coarse silicon particles.
Sulfur Modification:
 Sulfur can successfully modify Al-12wt%Si sand cast
alloy and significantly improve its mechanical prop-
erties.
 Addition of 0.02% sulfur of the weight of the alloy
most significantly improved its mechanical properties
closely followed by addition of 0.05% sulfur to the
alloy.
 The optimum level of modification of Al-12wt%Si
sand cast alloy with sulfur was found to be 0.02% -
0.05% sulfur of the weight of the alloy.
 Increasing the concentration of sulfur beyond the op-
timal level moderately decreased the degree of fine-
ness of the eutectic silicon morphology but signifi-
cantly reduced the mechanical properties of the alloy.
The cause of this significant decrease in mechanical
properties despite the still refined eutectic morphol-
ogy is suggested to be the presence of brittle sulfur
compound at the grain interfaces of the alloy.
Sodium Modification vs Sulfur Modification:
Sodium was found to modify Al-12wt%Si sand cast
alloy better than sulfur, producing a more refined eutectic
morphology and enhanced mechanical properties.
The optimum levels of modification of Al-12wt%Si
sand cast alloys with sulfur and sodium were determined,
with attendant improvement in mechanical properties of
the alloys modified within these levels. This improve-
ment in mechanical properties of the alloys modified
within their optimum levels shows that the overall per-
formance of engineering components cast from such al-
loys will be significantly enhanced compared to those
cast from unmodified alloys.
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