Materials Sciences and Applications, 2012, 3, 713-718
http://dx.doi.org/10.4236/msa.2012.310104 Published Online October 2012 (http://www.SciRP.org/journal/msa)
Enhancement of TiB Grain Refining Effect on A356
Gravity Die Casting with the Addition of Yttrium
Lim Ying Pio, Wang Chan Chin
Department of Mechanical Engineering, Universiti Tunku Abdul Rahman, Kuala Lumpur, Malaysia.
Received July 9th, 2012; revised August 12th, 2012; accepted September 14th, 2012
ABSTRACT
The present work investigates the effect yttrium on the grain refining efficiency of Al-5Ti-1B in gravity die cast A356
aluminum alloy. A series of casting experiments were carried out in which the Ti and B contents were maintained con-
stantly at 0.1 and 0.02 wt% respectively. The inoculation level of yttrium was manipulated at the amount of 0, 0.1, 0.2,
0.3, 0.4 and 0.5 wt%. Microstructural characterization of the as-cast A356 alloy was investigated by means of optical
microscope and its phases are detected by XRD. XRF is used to determine th e contents of inocu lating elements such as
Ti, B and Y so that their actual optimal ratio in the casting can be approximated. The mechanical properties tested are
tensile strength and hardness. The inoculation of yttrium was found to enhance the grain refinement effect of Al-5Ti-1B
grain refiner and improve the mechanical properties. The optimal weight percentage of yttrium was discovered to be 0.3.
The grain refining efficiency of combining yttrium and Al-5Ti-1B on A356 aluminum alloy was mainly attributed to the
heterogeneous nucleation of TiB2 and TiAl3 particles which were dispersed more evenly in the presence of yttrium and
also as a result of α-Al grain growth restriction by AlY3 compound precipitated at grain boundaries during solidifica-
tion.
Keywords: Al-5Ti-1B Grain Refiner; Gravity Dies Casting; Mechanical Testing; Microstructure; XRD; XRF; Yttrium
1. Introduction
In the past decades, a great deal of work had been done
on improving the mechanical properties and castability of
aluminum alloys in foundry industries through modifica-
tion and grain refinement techniques. Modification of
as-cast Al-Si alloys (eutectic or hypoeutectic) has been a
well-known practice employed for refining the large size
eutectic silicon needles to fine fibrous or lamellar in its
final solidified morphology [1]. The morphology of eu-
tectic Si can be changed by modifier additives from its
original coarse acicular structure to a finer fibrous struc-
ture resulting in significant enhancement of the me-
chanical properties of Al-Si castings like improved ten-
sile strength and fatigue strength [2]. It has been con-
cluded in a number of experimental work of researchers
that modification of eutectic silicon can be achieved by
several methods like faster solidification, mold vibration,
melt agitation in mushy zone by electromagnetic force
and melt inoculation by using some elements like Na, Sr,
Sb etc. Typical modifier such as Sr has efficient modifi-
cation effect but its efficiency drops with longer melt
holding time. On the other hand, Sb demonstrates good
modification on longer melt treatment time [3]. Besides
modification, grain refinement of Al-Si alloys with the
addition of grain refiners into the melt before pouring is
also a major melt treatment encountered in foundries.
The advantages of grain refinement of aluminum alloys
are both technical and economic, which include reduced
ingot cracking, better ingot homogeneity [4], being less
susceptible to hot cracking [5] and mechanical properties
are improved significantly too [6]. Grain refinement im-
proves the quality of castings by reducing the size of
primary α-Al grains nucleated in the as-cast product,
which otherwise will solidify naturally with coarse co-
lumnar grain structure in the absence of grain refiner.
Fine equiaxed grain structure is desired because it comes
along with several benefits such as uniform distribution
of second phases and microporosity to improve homo-
geneity, improved feeding ability to avoid incomplete
filling of mold [7], reduced porosity and the elimination
of hot tearing, high yield strength, high toughness, im-
proved machinability and excellent deep drawability of
the products [8].
Aluminium alloys are the major lightweight metals
used in various industries for applications ranging from
automotive components to aerospace parts etc. [9]. It is a
continuous improvement prog ram in casting industries to
improve the microstructure of casting in terms of reduc-

Enhancement of TiB Grain Refining Effect on A356 Gravity Die Casting with the Addition of Yttrium 714
ing grain size and modification of eutectic phase so that
its inherent mechanical properties of aluminum alloys
castings can be enhanced. This direction is becoming
more important due to the increasing awareness of re-
ducing greenhouse emissions by using lightweight mate-
rials in the automotive in dustries. Aluminium alloys hav e
excellent strength to weight ratio compared with other
conventional metals like steels and A356 is one of the
most widely used aluminum alloys in many industrial
applications because of its excellent castability, corrosion
resistance and good mechanical properties. It has lower
production cost, fast machining rate and good recyclabil-
ity. Typical commercial grain refiners use to refine A356
aluminum castings are Al-Ti-B and Al-Ti-C master al-
loys. The efficiency of these grain refiners can be easily
undermined by the presence of elements like Zr and V
[10]. In recent years, yttrium has arisen as a promising
element in superalloys for its ability to improve creep
property and oxidation resistance of cast stainless steel
[11,12]. A previous study by Song M., Chen K. H. et al.
[13] indicates that rare earth element like yttrium can
improve the strength of Al-Zn-M-Cu alloy at elevated
temperature. However, there is not much work has been
done to investigate the effect of yttrium on the grain re-
fining efficiency of Ti-B based grain refiner in A356
casting. Therefore, the current investigation attempts to
study the effect of yttrium on the grain refinement effi-
ciency of Al-Ti-B master alloy by using gravity die cast-
ing as the casting pro cess.
2. Materials and Methods
In this study, the commercial A356 aluminum alloy was
used as the base metal in all castings. The liquidus and
the solidus temperatures of the alloy were found to be
615˚C and 538.5˚C respectively according to manufac-
turer’s data. The grain refiners used are Al-5Ti-1B mas-
ter alloy supplied by KBM AFFILIPS. The rare earth
used is Yttrium of 99.9% purity. The manufacturer’s data
of the compositions of the A356 alloy, Al-5Ti-1B master
alloy are given in Table 1.
The gravity die casting mold used is designed accord-
ing to JIS H5202 standard which contains two cavities of
cylindrical shape tensile test piece of gage length 50 mm
Table 1. Compositions of A356, Al-5Ti-1B and Tical315.
%wt Si Fe Mn B C Ti
356A 7.22 0.15 0.01 - - 0.13
Al-5Ti-1B 0.1 0.16 - 1.0 - 5.0
%wt Ni Zn
Sr Mg Al
356A 0.016 0.04 0.01 0.45 Bal
Al-5Ti-1B - - - - Bal
and diameter 14 mm. The internal configuration of the
mold is shown in Figure 1.
The surface of the mold was coated with a layer of
mold release agent in order to facilitate casting knock-
out after pouring and solidification. The A356 aluminum
alloy was put into the crucible of a Nabertherm electrical
furnace and melted up to 750˚C ± 5˚C. After complete
melting, calculated fixed quantity of Al-5Ti-1B which
constitutes the weight percentages of 0.1 Ti and 0.02 B
were added into the melt and stirred for 30 seconds be-
fore the addition of yttrium. The quantity of yttrium was
added as 0.1, 0.2, 0.3, 0.4 and 0.5 wt% in separate ex-
periment. The molten alloy was then directly poured into
the gravity die casting mold.
The castings are purposely designed for ultimate ten-
sile strength test. They were subjected to fettling and
cleaning and subsequently machined to a diameter of 20
mm at the gripping ends. The as-cast samples and ma-
chined samples are shown in Figure 2. The tensile test
was done on INSTRON 5582 with a tensile rate of 2
mm/min. The central part of the tensile speci men was cut
Figure 1. Gravity die casting mold.
Figure 2. Gravity die casting samples.

Enhancement of TiB Grain Refining Effect on A356 Gravity Die Casting with the Addition of Yttrium 715
to a thickness of 10 mm and subjected to fine 80 grit-size
grinding on both sides to smoothen the coarse surfaces
for hardness test. The hardness test was done on Indentec
Universal Hardness Tester. The scales of all tests were
set to be HRA 60 kgf.
A sample of size 5 mm × 5 mm was cut from the
transverse plane at the central part of each tensile speci-
men and mounted in resin to prepare for grinding, rough
polishing and finally fine polishing to the fineness of 0.3
micron. The polishing agent was buehler alpha alumina
particles of 0.3 micron. The samples were chemically
treated with etchant consisting of 200 ml distilled water
and 5 ml HF. Microstructural studies were conducted by
using an optical microscope with a maximum magnifica-
tion power of 2000×. Similarly polished samples of 0.3
mm thickness were used for XRD examination. Before
every examination experiment by XRD, the XRD ma-
chine was calibrated to ensure its de tection was correct.
3. Results and Discussions
The main results obtained from this study are the me-
chanical properties of ultimate tensile strength, hardness
and elongation (strain at fracture). The original A356
data is used as benchmark to deter-mine the performance
of yttrium on Al-5Ti-1B grain refinement in A356 grav-
ity dies casting. The microstructures taken by optical
microscope will be compared to analyze the effect of
yttrium on grain size. XRD analysis results is used to
find out is there any special phase formed in the casting
after adding yttrium as inoculants.
3.1. Hardness
The hardness of original A356 is 19.84. The 0.1 wt% Ti
and 0.02 wt% B increases th e hardn ess to 21.62. Keep ing
the conten ts of Ti and B constant, the additio n of yttrium
from 0.1 to 0.3 wt% also shows improvement in hardness
with the values of 21.10, 22.16 and 23.88 respectively.
However, continuous addition of 0.4 and 0.5 wt% does
not further improve hardness but reduces it to 18.02 and
17.08 respectively. The results show that the best hard-
ness value is obtained by inoculating 0.3 wt% of yttrium
in combination with 0.1 wt% Ti and 0.02 wt% B into the
A356 casting. The graph in Figure 3 shows the data of
hardness vs. yttrium wt%. 0 wt% refers to the A356
casting without any grain refiners. This applies to other
graphs in the following sections.
3.2. Tensile Strength
Two tensile test samples for each type of allo y were sub-
jected to test and the averaged values are taken to plot the
ultimate tensile strength chart as shown in Figure 4. It
can be seen that 0.1 wt% yttrium does not improve the
tensile strength too much; it has a tensile strength of
Ha r dness
of
yttrium
grain refined
A356
15.0
16.0
17.0
18.0
19.0
20.0
21.0
22.0
23.0
24.0
25.0
00.1 0.2 0.3 0.4 0.5
Addition level, wt%
HRA (60 kgf)
Yt tr iu
m
Hardness of yttrium grain-refined A356
Yttrium Addition level, wt%
HRA (60 kgf)
Figure 3. Hardness.
UTS
of
yttrium
grain
-
refined
A35 6
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
00.1 0.2 0.3 0.4 0.5
Addition level, %wt
UTS,
MPa
Yttrium
UTS of yttrium grain-refined A356
Yttrium Addition level, wt%
UTS, MPa
Figure 4. Ultimate tensile strength.
122.65 MPa while the original A356 has 123.49 MPa of
tensile strength. The tensile strength increases when 0.2
and 0.3 wt% of yttrium is added, it improves to 148.29
and 173.40 MPa respectively. However, further addition
of yttrium does not improve tensile strength, instead, the
tensile strength drops to 144.52 and 127.92 MPa for 0.4
and 0.5 wt% of yttrium. The A356 containing 0.1 wt% Ti
and 0.02 wt% B has a tensile strength of 153.18 MPa, it
is higher than the original A356 without grain refinement
but lower than that added with 0.3 wt% yttrium. This
indicates that yttrium is effective to improve the tensile
strength of A356 casting by a maximum of 40%. Based
on the microstructures shown in Section 3.4, the addition
of yttrium results in finer dendritic structure and more
fibrous eutectic phase, this contributes to strengthening
effect on the casting. Coarse and elongated silicon parti-
cles in eutectic phase of the unmodified A356 tends to
fracture at lower tensile force while the refined and more
fibrous silicon particles in yttrium-modified A356 are
more resistant to dislocation under tension. The specimen
which is solely grain-refined by Al-5Ti-1B also develops
a better tensile strength as a result of finer dendritic
structure due to the heterogeneous nucleation promoted
by Al3Ti and TiB2 particles exist in the melt during so-
lidification. When >0.3 wt% yttrium is added, the tensile
strength drops. This could be due to the intermediate
compound containing yttrium aggregates and grows, cuts

Enhancement of TiB Grain Refining Effect on A356 Gravity Die Casting with the Addition of Yttrium 716
up the α-Al matrix and weakens the obstruction to the
boundary movement [14].
3.3. Elongation
Ductility of a metal can be measur ed by its elongation or
strain at specific point in the stress-strain curve. In this
study, strain at fracture is taken into consideration to
analyze the effect of yttrium on the ductility of A356.
The original unrefined A356 has a tensile strain of 0.064.
When it is added with TiB grain refiner to contain 0.1
wt% Ti and 0.02 wt% B, the ductility does not improve
significantly; its tensile strain is 0.069. When yttrium is
added from 0.1 to 0.5 wt%, the tensile strain improves
slowly to the maximum value of 0.076 at 0.3 wt% yt-
trium. Similarly to harness and tensile strength, further
addition of yttrium exceeding 0.3 wt% does not improve
ductility continuously. The strain behavior of yttrium-
modified A356 is shown in Figure 5.
3.4. Microstructural Analysis
The microstructures of the A356 and its Y-Ti-B-grain
refined specimens are shown in Figures 6(a)-(g) (all
scale lengths are 0.133 mm). The original A356 gravity
die casting has coarse α-Al dendritic microstructure with
very fine and rod- like eutectic phase. The addition of 0.1
wt% Ti and 0.02 wt% B into the melt is observed to have
refined the dendritic structure. It is still a dendritic struc-
ture but characterized with smaller secondary dendrite
arm spacing. The addition of yttrium from 0.1 to 0.5 wt%
into the melt is seen to have refined the microstructure
potently. Yttrium is found to be able to reduce the sec-
ondary dendrite arm spacing from the unrefined A356
coarse SDAS of 0.037 mm to the finest SDAS of 0.01
mm in 0.3, 0.4 and 0.5 wt% yttrium-refined casting.
Smaller SDAS is known to strengthen the casting and
produce better mechanical properties of tensile strength
and elongation. Increasing amount of yttrium is observed
to decrease the eutectic silicon phase and the grain
boundary becomes narrow and continuous. In this invest-
Elongation
of
grainrefined
A35 6
0.000
0.010
0.020
0.030
0.040
0.050
0.060
0.070
0.080
0.090
00.1 0.20.3 0.4 0.5
Addition level, %wt
Strain
at
UTS,
%
Yttrium
Elongation of grain-refined A356
Yttrium Addition level, wt%
Strain at UTS, %
Figure 5. Elongation at fracture.
tigation, yttrium is found unable to transform the α-Al
phase from dendritic structure into globular structure.
Previous study showed that such a globular transforma-
tion in A356 can only be achieved by T6 heat treatment
[15]. Yttrium is found to be able to modify the silicon
eutectic phase into a more fibrous form as compared with
that of unrefined A356. Adding yttrium into A356 will
produce the high-melting point Al-Y compounds at the
solid/liquid interface and lead to the formation of solute
undercooling layer, suppression of the growth of α-Al
grains and subsequently grain refine the dendritic struc-
ture of α-Al matrix [16].
3.5. XRD and XRF Analysis
A typical XRD diffractogram for 0.3 wt% yttrium inocu-
lated A356 is shown in Figure 7. The search function of
the XRD software does not detect any significant yt-
trium-aluminum based compounds. Based on the search-
ed results for all samples, the following compound s were
found: Al9Si, Al0.86Zn0.14, Al0.95Mg0.05, Al0.86Zn0.14 and Ti.
It is the limitation of the XRD machine used for not be-
ing able to detect the low contents of yttrium and its as-
sociated compound. However, when comparing with the
XRD diffractogram of other researchers [15], the spec-
trum looks similar and it is expected to contain the phas-
es of α-Al, TiAl3, AlY3, AlY2, TiC and TiB2. It has been
reported that the intermediate compound of AlY that
aggregates along grain boundaries and hence enhances
grain boundaries to resist slipping or dislocation [16].
This in turn will improve the tensile strength of the cast-
ing.
The XRF analysis for A356 and its Ti-B and Y inocu-
lated alloys is shown in Table 2. The main elements to
be highlighted are Al, Si, Ti, B and Y. The mechanical
testing results show that the optimal inoculation level to
improve Ti-B grain refinement efficiency is when the
atm% ratio of Y/B is 14.5 and 2.9 for Y/Ti.
4. Conclusions
The combined effect of grain refiners Al-5Ti-1B (fixed at
0.1 wt% Ti and 0.02 wt% B) and yttrium on the me-
chanical properties of A356 gravity die castings has been
studied. Based on the mechanical testing and metal-
lographic examination conducted for the specimens, the
following conclusions can be drawn:
1) 0.3 wt% yttrium renders the highest hardness value
of 23.88 HRA (60 kgf), a 20% improvement compared to
A356
2) Grain refinement with 0.3 wt% addition of yttrium
shows the greatest improvement in tensile strength by
40%. This shows that yttrium can significantly improve
tensile strength of A356.
3) 0.3 wt% yttrium also yields the be st ductility. How-

Enhancement of TiB Grain Refining Effect on A356 Gravity Die Casting with the Addition of Yttrium 717
(a) (b) (c)
(d) (e)
(f) (g)
Figure 6. (a)-(g): Microstructures of test specimens; (a) A356; (b) 0.1 wt% Ti + 0.02 wt% B; (c) 0.1 wt% Y; (d) 0.2 wt% Y; (e)
0.3 wt% Y; (f) 0.4 wt% Y; (g) 0.5 wt% Y.
4000
2000
0
20 25 35 40 45 50 55 60 65 70
(CPS)
(deg)
Figure 7. XRD diffractogram of 0.3 wt% Y.

Enhancement of TiB Grain Refining Effect on A356 Gravity Die Casting with the Addition of Yttrium 718
Table 2. XRF analysis of chemical compositions.
Element Sample
A356 Ti-B 0.1 wt% Y 0.2 wt% Y 0.3 wt% Y 0.4 wt% Y 0.5 wt% Y
Al, atm% 88.53 89.21 88.03 89.78 88.18 89.67 89.47
Si, atm% 10.78 10.82 11.19 10.33 10.93 10.62 10.48
Ti, atm% 0.06 0.10 0.09 0.09 0.10 0.10 0.11
Y, atm% 0.04 0.04 0.11- 0.21 0.29 0.38 0.51
B, atm% 0.02 0.02 0.02 0.02 0.02 0.02 0.02
Y/B ratio 2 2 5.5 10.5 14.5 19 25.5
Y/Ti ratio 0.67 0.4 1.22 2.33 2.9 3.8 4.64
ever the improvement is not significant, it improves only
by 0.012 mm/mm.
4) Microstructural analysis shows that yttrium is able
to refine grain size by reducing the SDAS and produces
more fibrous eutectic silicon phase. However, the den-
dritic structure of α-Al is still unchanged.
5) Addition level of >0.3 wt% yttrium will not further
improve the mechanical properties of A356.
5. Acknowledgements
The author would like to thank UTAR for providing re-
search facilities and financial support for this research.
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