Formation of hypereutectic silicon particles in hypoeutectic Al

Foundry
CHINA FOUNDRY Overseas
Vol.10 No.2 March 2013
Formation of hypereutectic silicon particles in
hypoeutectic Al-Si alloys under the influence of
high-intensity ultrasonic vibration
Xiaogang Jian1 and *Qingyou Han2
1. Fuel Cell Energy, Danbury, CT 06810, USA;
2. MET Department, Purdue University, West Lafayette, IN 47906, USA
Abstract: The modification of eutectic silicon is of general interest since fine eutectic silicon along with fine
primary aluminum grains improves mechanical properties and ductilities. In this study, high intensity ultrasonic
vibration was used to modify the complex microstructure of aluminum hypoeutectic alloys. The ultrasonic
vibrator was placed at the bottom of a copper mold with molten aluminum. Hypoeutectic Al-Si alloy specimens
with a unique in-depth profile of microstructure distribution were obtained. Polyhedral silicon particles, which
should form in a hypereutectic alloy, were obtained in a hypoeutectic Al-Si alloy near the ultrasonic radiator
where the silicon concentration was higher than the eutectic composition. The formation of hypereutectic
silicon near the radiator surface indicates that high-intensity ultrasonic vibration can be used to influence the
phase transformation process of metals and alloys. The size and morphology of both the silicon phase and
the aluminum phase varies with increasing distance from the ultrasonic probe/radiator. Silicon morphology
develops into three zones. Polyhedral primary silicon particles present in zone I, within 15 mm from the
ultrasonic probe/radiator. Transition from hypereutectic silicon to eutectic silicon occurs in zone II about 15
to 20 µm from the ultrasonic probe/radiator. The bulk of the ingot is in zone III and is hypoeutectic Al-Si alloy
containing fine lamellar and fibrous eutectic silicon. The grain size is about 15 to 25 µm in zone I, 25 to 35 µm in
zone II, and 25 to 55 µm in zone III. The morphology of the primary α-Al phase is also changed from dendritic
(in untreated samples) to globular. Phase evolution during the solidification process of the alloy subjected to
ultrasonic vibration is described.
Key words: eutectic solidification; aluminum alloys; microstructure; and ultrasonic vibration
CLC numbers: TG146.21
Document code: A
A
luminum-silicon (Al-Si) alloys are highly versatile
materials, comprising 85% to 90% of the total of
all aluminum cast parts produced for the automotive
industry. Depending on the silicon concentration
in weight percent, Al-Si alloys fall into three major
categories: hypoeutectic (<12% Si), eutectic (12%-13%
*Qingyou Han
Dr. Qingyou Han is a full Processor of Mechanical Engineering Technology and
is the Foundry Education Foundation (FEF) Key Professor at Purdue University.
Before joining Purdue University in 2007, Dr. Han was a research scientist
(1999-2007) at Oak Ridge National Laboratory (ORNL), and a program manager
(2003-2006) in charge of Metalcasting activities under the Energy Efficiency and
Renewable Energy (EERE) program at ORNL. His research interests include
solidification and casting processing of metals, alloys, and composite materials.
He has over 160 scientific publications, 5 patents, and numerous presentations
to his credit, and has organized or co-organized several symposia in his areas of
research.
E-mail: hanq@purdue.edu
Received: 2012-11-02 Accepted: 2013-02-14
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Article ID: 1672-6421(2013)02-118-06
Si), and hypereutectic (14%-25% Si). The majority of
these alloys for industrial applications are hypoeutectic
alloys. The hypereutectic alloys are more wear-resistant
but less ductile than the hypoeutectic alloys. Bi-metal
casting is a technique of casting the hypereutectic
alloy to form the surface layer where wear resistance is
required and then casting the hypoeutectic alloy to form
the bulk of a component [1]. Limited casting geometries
can be made using such a bi-metal casting process. The
bulk aluminum alloy is usually A356 alloy.
Aluminum A356 alloy is one of the most widely used
cast aluminum alloys because of its good mechanical
strength, ductility, hardness, fatigue strength, pressure
tightness, fluidity, and machinability [2]. The A356
alloy contains 50vol% eutectic phases. The final
microstructure is largely determined by eutectic
reaction. Due to its diamond cubic crystal structure
which predominantly grows in <112> direction on
(111) planes, silicon is a faceted phase with strongly
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anisotropic growth thus it is difficult to change the growth
direction [3]. In unmodified A356 alloy, the main eutectic reaction
occurs at 574 ℃ as a binary reaction, which results in coarse
irregular plate-like silicon. The modification of eutectic silicon
is of general interest since fine eutectic silicon along with fine
primary aluminum grains improves mechanical properties and
ductilities. Chemical and physical means have been used for
modifying the morphology of the eutectic silicon. Three wellknown eutectic modification methods have been developed
thus far, namely (1) chemical modification [4-6], (2) quench
modification [3, 7], and (3) superheating modification [8]. These
methods are capable of producing a fine fibrous silicon
structure. However, the formation of polyhedral silicon
particles in A356 alloy has not been much reported in open
literature.
For the hypereutectic alloys, polyhedral silicon displays a
large range of growth morphologies, including flake, blocky or
spherical, skeletal and star-like, depending on the solidification
conditions.
In a recent project investigating the effect of high-intensity
ultrasonic vibration on the microstructure of A356 alloy, we
found that high-intensity ultrasonic vibration has a significant
effect on refining both the primary aluminum phase and the
eutectic silicon phase [9]. Hypereutectic silicon is formed
near the ultrasonic radiator in A356 alloy ingot. This article
describes experimental results obtained using high-intensity
ultrasonic vibration during the solidification of A356 alloy to
induce the formation of hypereutectic silicon particles at the
surface near the ultrasonic radiator.
1 Experimental method
In the present study, the experimental setup, as shown in Fig.
1, consists of an ultrasonic generator, a transducer made of
piezoelectric lead zirconate titanate crystals (PZT), an ultrasonic
horn, and an ultrasonic radiator/probe. The horn and the radiator
were made of Ti-6Al-4V alloy. The unit worked at a frequency
of 20 kHz with a variable power output up to 1,500 W by
adjusting the output acoustic amplitude from 24.3 to 81 mm, or
1 - Sample, 2 - Copper mold, 3 - Probe/Radiator,
4 - Ultrasonic horn, 5 - Transducer and booster,
6 - Inlet of compressed air for cooling, 7 - Ultrasonic power cable,
and 8 - Ultrasonic generator
Fig. 1: Schematic diagram of experimental setup
30% to 100% of the unit’s upper limit. The ultrasonic radiator
was 1.9 cm in diameter and was placed at the bottom of a
copper mold which holds up to 250 g molten aluminum.
This study used A356 alloy. The alloy is a hypoeutectic alloy
containing 7wt.% Si and 0.5wt.% Mg with trace amounts of
Cu, Zn, Ti, and Fe. In the study, the alloy was used to produce a
surface layer with enriched silicon particles of the hypereutectic
phase. The eutectic composition of the Al-Si alloy occurs at
~12wt.% silicon. The as-received commercial aluminum A356
alloy was cut, melted and held in the furnace for half an hour at
700 ± 5 ℃, about 86 ℃ higher than the liquidus temperature of
the alloy, to allow the complete dissolution of silicon particles
in the melt. The molten alloy was then poured into the mold at
a pouring temperature of about 630 ℃.
Ultrasonic vibration at amplitude of 56.7 mm was started
just before the melt was poured into the copper mold, and the
vibration was applied throughout the solidification process
of the ingot. For comparison, the samples without ultrasonic
vibration were cast at the same conditions.
Each sample was 200 g in weight. The shape of the sample
is illustrated in Figs. 1 and 2. The largest diameter of the
conical shaped sample was about 5 cm and height of the
conical shaped section was about 2.54 cm. Below the conical
shaped sample is the cylindrical part of about 10 mm in height.
Zone
Zone
Zone
Fig. 2: Illustration of locations of three zones formed in
sample with ultrasonic vibration
2 Results and discussion
Figure 3 shows a SEM micrograph of as-cast eutectic silicon
located between aluminum dendrites in A356 alloy under
normal solidification conditions (without ultrasonic treatment).
The eutectic silicon shows a typical coarse acicular or plate-
Fig. 3: Silicon morphology in sample without
ultrasonic vibration
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like structure with a length up to 100 mm. Under normal casting
conditions, the microstructure of A356 alloy consists of primary
aluminum dendrites surrounded by eutectic structure [3, 10].
While in the A356 alloy treated by ultrasonic vibration under
the experimental conditions, silicon morphology develops
into three successive different form zones (I, II, and III) along
with the increasing distance from the ultrasonic radiator, as
illustrated in Fig. 2. It can be observed that zone I and II are
quite small with respective volumes less than 10% of the
whole volume of the ingot while zone III has the prevailing
volume. In fact, zone I existed within about 15 mm from
(a)
(b)
the ultrasonic probe/radiator, and zone II existed in a range
between 15 to 20 mm from the ultrasonic probe/radiator.
Figure 4 demonstrates the silicon morphology in the three
zones, which differs greatly from that of untreated A356
alloy. In zone I, polyhedral silicon is the main form presented;
in zone II, the silicon morphology is a motley collection of
polyhedral silicon form together with fine lamellar and fibrous
form; while in zone III, finely dispersed lamellar and fibrous
eutectic silicon are appears. The size of the polyhedral silicon
particles in Zone I is much larger than the lamellar and fibrous
eutectic silicon in Zone III.
(c)
Fig. 4: Silicon morphology in the three zones in a sample with high-intensity ultrasonic vibration during
its solidification process. (a) Zone I; (b) Zone II; (c) Zone III
Quantitative metallographic analysis of silicon morphology
in A356 alloy without and with ultrasonic treatment was
performed using Image-Pro. The results are illustrated in Fig. 5.
Without ultrasonic treatment, the silicon is plate-like [10] and the
average eutectic silicon length is about 26 mm, and the average
width is 2.7 mm. The aspect ratio is slightly less than 10. While
with ultrasonic treatment, in zone I the average length and
width are about 7 mm and 4 mm, respectively, with an aspect
ratio of about 1.7; in zone III, the average length and width are
about 2 mm and 0.6 mm, respectively, with an aspect ratio of
slightly less than 3; in zone II, the polyhedral silicon is similar
to that in zone I and the fine fibrous and lamellar eutectic
silicon is similar to that in zone III. By comparing the aspect
ratios of the untreated (about 9.8) and the zone III (about 2.8)
treated A356 alloy, it may indicate that the silicon morphology
in ultrasonically modified A356 alloy is not just an extra-fine
Fig. 5: Silicon morphology analysis of samples
without and with ultrasonic vibration
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form of the silicon in unmodified alloy [11]. This is different from
the previously discussed quenching modification [3, 7, 12]. The
formation of such small silicon particles in Zone III has been
reported and discussed in our previous study [11].
Figure 6 displays the normalized EDS X–ray analysis
results of aluminum – silicon in A356 alloy without and with
ultrasonic treatment. It is obvious that the silicon content
is higher than average (i.e. 7wt.%Si in A357 alloy with no
vibration) in zone I and II, and lower than average in zone III.
Silicon conentration was calculated by comparing the Ka peak
of both aluminum and silicon, based on the fact that these
contents are approximately linear to their Ka counts. These
results are also revealed in Fig. 7. The silicon distribution
along with the distance from the ultrasonic radiator in the
untreated A356 alloy shows almost a level line at the height
of about 7wt.%. However, in the ultrasonically treated A356
Fig. 6: EDS analysis of silicon particles in samples
without and with ultrasonic vibration
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Fig. 7: Silicon concentration distribution measured
using EDS and optical metallurgical microscope
alloy, the silicon content keeps a high level of about 13wt.% in
zone I and the main part of zone II. And in zone III it decreases
to a level of about 4.5wt.%, which is less than average value
(about 7wt.%).
In addition to a change in silicon morphology, the
primary α -Al grains have also been drastically modified by
ultrasonic treatment. Figure 8 shows typical as-cast primary
α -Al dendrites of A356 alloy without ultrasonic treatment.
Aluminum dendrites were fully developed. One branch of a
primary dendrite shown in the middle of figure is about 800
mm in length, indicating that the grain size is in the range of a
few millimeters since one equiaxed grain usually contains six
primary dendrite arms.
Figure 9 shows the primary α-Al grains presented in the
three zones and their transition regions of A356 alloy with
ultrasonic treatment. In zone I [Fig. 9(a)], the primary α-Al
(a)
(c)
Fig. 8: Primary α-Al dendrites in sample without
ultrasonic treatment.
grains are mostly very fine equiaxed globular grains with
primary polyhedral silicon strewed at boundaries. Eutectic
or independent colonies are rarely seen in this region. In the
area between zone I and II [Fig. 9(b)], a clear, continuous
interface is observed. Zone II differs from zone I mainly in
the presence of eutectic or independent eutectic colonies, and
less primary polyhedral silicon, which can be seen in Fig.
9(c) and the right side of Fig. 9(b). In zone III [Fig. 9(e)],
however, the primary α-Al grains are basically fine globular
grains encompassed by continuous eutectics. Primary
polyhedral silicon no longer exists in this region. Figures
9(b) and (d) reveals that the interface between zone III and
II is less clear than that between zone II and I, because less
(b)
(d)
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(e)
Fig. 9: Primary α-Al grains in various locations in
a specimen with ultrasonic vibration: Zone
I (a), Transition between zone I and II (b),
Zone II (c), Transition between zone II and III
(d), and Zone III (e)
primary polyhedral silicon presents in the region close to the
interface than in the central region of zone II. Accordingly,
the mean size and size distribution of primary α-Al grains in
the three zones are shown in Figs. 10 and 11. In the untreated
A356 alloy, the mean length of the dendritic branches is
about 600 mm, and the mean secondary dendrite arm space
(SDAS) about 45 mm. In the ultrasonically treated A356
alloy, the grains mainly distribute in the range of 25 – 55
mm in zone III, and 15 – 35 mm in zone II, and 15 – 25 mm
in zone I. The average size (mean diameter) decreases from
about 40 mm in zone III to about 28 mm in zone II, then to
about 20 mm in zone I. It is evident that ultrasonic treatment
Fig. 10: Comparison of primary α-Al grain size in
samples without and with ultrasonic vibration
Fig. 11: Size distribution of primary α-Al grains in three
zones of the sample with ultrasonic vibration
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not only changes the nature of primary α-Al grains, but also
refines the said grains. The refinement is better in the area
closer to the ultrasonic radiator.
It is well known that the application of high-intensity
ultrasonic vibration in the liquid gives rise to nonlinear
effects such as cavitation, acoustic streaming, and radiation
pressure [13-14]. Cavitation, or the formation of small cavities
in the liquid, occurs as a result of the tensile stress produced
by an acoustic wave in the rarefaction phase. These cavities
continue to grow by inertia until they collapse under the
action of compressing stresses during the compression halfperiod. Acoustic streaming is a kind of turbulent flow that is
developed near various obstacles (interfaces) due to energy
loss in the sound wave. These nonlinear effects are bound to
interfere with the solidification process of the alloy.
The solidification process of A356 has been extensively
studied [10, 15]. On cooling, primary aluminum dendrites form
from the liquid as the alloy temperature drops below the liquidus
temperature. Further cooling leads to the growth of dendrites
and the enrichment of silicon in the remaining liquid. Eutectic
structure starts to form when the fraction of solid dendrites
reaches about 0.5, and the composition of the remaining liquid
is enriched to the eutectic composition, about 12wt.% of Si [16].
When high-intensity ultrasonic vibration is applied to an
A356 alloy, the growth or expansion of the cavitation bubbles
leads to a large undercooling at the bubble/melt interfaces. This
undercooling may encourage the nucleation of the aluminum
phase near the liquidus temperature of the alloy [16-17]. The
collapsing of the cavitation bubbles during the compressive
phase of the sound wave tends to spread the newly formed
nuclei throughout the solidifying alloy by acoustic streaming,
leading to the formation of small globular grains of the
primary aluminum phase. As the solid fraction increases during
further cooling, the viscosity of the alloy increases. As a result,
the attenuation of ultrasonic vibration is increased and the
ultrasonic streaming is gradually confined to the regions near
the ultrasonic probe/radiator by the growing aluminum grains.
Still cavitation is induced near the ultrasonic probe/radiator in
the remaining liquid which contains an increasing amount of
silicon. When the temperature in the sample is approaching
the eutectic temperature, undercooling in the cavitation bubble
is likely to nucleate the silicon particles since the melting
temperature of the silicon particles is much higher than the
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eutectic aluminum phase. These silicon nuclei grow into
particles of hypereutectic morphology and consume silicon in
the remaining liquid. The attenuation of the acoustic energy
and the confinement of acoustic streaming near the ultrasonic
probe ensure that the hypereutectic silicon (or silicon particles
having the morphology of hypereutectic silicon phase) forms
in the region near the ultrasonic probe.
The formation of the silicon morphology and the sizes of
both silicon phase and the primary aluminum phase are resulted
from the interplay of ultrasonic vibration and the evolving
microstructure. In zone III where ultrasonic vibration is much
weaker than that in zone I, finely dispersed lamellar and fibrous
eutectic silicon occur as previously reported [11]. Spherical
primary aluminum phase is also formed in zone III but its size
is larger than those found in zone I.
3 Conclusions
High-intensity ultrasonic vibration at 20 kHz was applied at
the bottom of A356 alloy ingots throughout the solidification
process of these ingots. The results showed that:
(1) Silicon morphology develops into three zones according
to the increasing distance from the ultrasonic radiator.
Polyhedral primary silicon particles appear in zone I, which is
a thin layer within 15 mm from the ultrasonic probe/radiator.
The silicon concentration in zone I is about 13wt.% and
the polyhedral primary silicon is hypereutectic silicon. The
average length and width of the hypereutectic silicon particles
are about 7 µm and 4 µm, respectively.
(2) Transition from hypereutectic silicon to eutectic silicon
occurs in zone II about 15 to 20 µm from the ultrasonic probe/
radiator. The bulk of the ingot is in zone III and is hypoeutectic
Al-Si alloy containing fine lamellar and fibrous eutectic silicon.
(3) An composition profile is induced under the influence of
high-intensity ultrasonic vibration. The silicon concentration is
about 13wt.% in zone I, much higher than 7wt.% in the A356
alloy, and is about 4.5wt.% in zone III, much lower than that of
the A356 alloy.
(4) The morphology of the primary α -Al phase is also
changed from dendritic (in untreated samples) to globular.
Without being subjected to ultrasonic vibration, the primary
dendrite arms are about 800 µm. When subject to ultrasonic
vibration, the size of the globular grains decreases with
increasing distance from the ultrasonic probe/radiator. The
grain size was about 15 to 25 µm in zone I, 25 to 35 µm in
zone II, and 25 to 55 µm in zone III.
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This work was supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy,
Industrial Technologies Program, Industrial Materials for the Future (IMF), under Contractor No. DE-PS0702ID14270 with UT-Battelle, LLC.
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