337
ISSN 1392–1320 MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 22, No. 3. 2016
The Microstructure and Compressive Properties of Aluminum Alloy (A356)
Foams with Different Al-Ti-B Additions
Zan ZHANG
1
, Jing WANG
1
, Xingchuan XIA
1
, Weimin ZHAO
1, 2
, Bo LIAO
1
,
Boyoung HUR
3
1
School of Materials Science and Engineering, Hebei University of Technology, Dingzigu, Hongqiao district, Tianjin
300130, China
2
Key Lab for Micro- and Nano-Scale Boron Nitride Materials in Hebei Province
3
School of Nano’Advanced Materials Science and Engineering, Gyeongsang National University of South Korea, Republic
of Korea
http://dx.doi.org/10.5755/j01.ms.22.3.8559
Received 26 October 2014; accepted 31 March 2015
Closed-cell aluminum alloy (A356) foams with different percentages of Al-Ti-B are prepared by melt foaming method,
using Ca and TiH
2
as thickening agent and foaming agent, respectively. SEM and Quasi-static compression tests are
performed to investigate the effect of Al-Ti-B on the microstructure and compressive properties of aluminum alloy
(A356) foams. The results show that foams with Al-Ti-B percentage of 0.3 wt.% possess good combinations of micro
hardness, yield strength, plateau strength, densification strain and energy absorption capacity under the present
conditions. The reasons are mainly due to the foams with Al-Ti-B percentage of 0.3 wt.% possess optimal eutectic Si
morphology (with eutectic Si existing in the forms of particles or short fiber).
Keywords: aluminum foam, melt foaming method, Al-Ti-B, microstructure, compressive property.
1. INTRODUCTION
Metal foams are a relatively new class of materials
showing unique combination properties that cannot be
obtained by any dense metals, even for foamed polymeric
or ceramic materials [1
–
3]. According to the connectivity
of the pores, metal foams can be divided into open-cell or
closed-cell. Open-cell foams are mainly used as functional
materials, such as heat exchangers, catalyst carrier because
of their good permeability for fluids and large surface area.
Closed-cell foams are mainly used in the load bearing and
energy absorbing fields due to their higher yield strength
and plateau stress [2, 3]. Among all of the metallic metal
foams, closed-cell aluminum foams have been utilized
more extensively than other foams in such services as
energy absorber, light-weight structures, in-door sound
absorbers, automobile roofs and crash-energy absorption
elements of space vehicles [4]. In addition, as Si-
containing aluminum alloys possess excellent castability,
good corrosion resistance and weldability, in the last two
decades, more and more researchers have been focusing on
the manufacturing process and characterizing the
properties of Si-containing aluminum alloy foams
[1, 5
–
6].
Yang and Nakae [7] investigated the effect of foaming
agent content and foaming temperature on the
macrostructure of aluminum alloy foam (A356) and the
results showed that a properly controlled holding
temperature of the melt and the titanium hydride content
were good for the acquirement of foamed aluminum.
Fabrizio et al. [8] produced aluminum alloy (AlSi7Mg0.3)
Corresponding author. Tel.: +86 22 60202414; fax: +86 22 60204477.
E-mail address: xc_xia@hebut.edu.cn (X. Xia)
foams with a relative density of ~35 % by replication of
salt precursors and reported that the distribution of grain
size is homogeneous in the foams. Malekjafarian and
Sadrnezhaad [9] studied the effect of density (porosity) on
the mechanical properties of composite foams (with
A356/SiC composite as matrix). Meanwhile, Ravi Kumar,
et al. [10] investigated the effect of SiC content on the
foaming behavior and mechanical properties of A356/SiC
composite foams and the results showed that the presence
of SiC could lead to good foaming behaviors. While, SiC
content had an unconspicuous effect on the compressive
plateau strength of the foams. Schüler, et al. [11] studied
the deformation behavior and mechanical properties of
aluminum alloy (A356) foams under quasi-static and
dynamic loading conditions.
Mechanical properties of Si-containing aluminum
foams are generally dependent on the strength of basic
materials, relative density and morphology of the foams
[12]. In addition, the strength of the basic materials is
mainly due to the microstructure morphologies of the cell
walls. Some of the critical microstructure characteristics
are grain size and the morphology of eutectic silicon [13].
Meanwhile Al-Ti-B ternary master alloys, particularly for
Al-5Ti-1B, have been widely used as aluminum alloys
grain refiners to improve their mechanical properties
[14
–
19]. Though the solidifications of aluminum alloy
(A356) and aluminum alloy (A356) foam are volume
solidification, the cooling rates between the alloy and foam
are quite different [20], which may result in different
strengthen mechanisms of Al-Ti-B in aluminum alloy and
aluminum alloy foams. As described above, though
researchers have focused on the production and mechanical
properties of aluminum alloy (A356) foams, the effect of
the microstructure and micro-morphology on the
338
compressive properties of these foams are seldom involved
in and further research is needed. Therefore, the aim of this
study is to investigate the effect of Al-Ti-B ternary master
alloys on the variation of micro- morphology of A356
aluminum alloy foam, and more importantly to evaluate
the effect of Al-Ti-B on the compressive properties of
A356 aluminum alloy foams.
2. MATERIALS AND EXPERIMENTS
Melt foaming method is applied to fabricate A356
aluminum alloy foams with different percentages of Al-Ti-
B. In the present experiment, commercial A356 aluminum
alloy (Al-7Si-0.4Mg) is used as base material.
Commercially pure Al powders (≥ 99.5 wt.%) and TiH
2
powders (300 ± 20 mesh) are used as thickening agent and
foaming agent, respectively. Al-Ti-B master alloy
(Commercially pure, with 5 wt.% Ti, 1 wt.% B) is used to
modify the morphology. The preparation procedures
mainly include the following steps: (1) melting certain
quality of raw material (~1 kg) in a low carbon steel
crucible to a fixed temperature, boron nitride (nano-scale,
Key Lab for Micro- and Nano-Scale Boron Nitride
Materials in Hebei Province) is applied to prevent the
diffusion of Fe into the melt; (2) adding different contents
of (0 %, 0.2 %, 0.3 %, 0.4 %, 0.6 % and 0.8 wt.%,
hereinafter % refers to wt.%) Al-Ti-B to the melt; (3)
adding certain amount of Al powders (2%) to the melt
accompanied by stirring, with the stirring speed of 800 rpm
for 10 min. The impellor is driven by an electromotor and
the stirring speed is controllable [21]; (4) adding certain
quantity of TiH
2
(1.2 %) to the melt accompanied by
stirring, with the stirring speed of 1200 rpm for 30 s and
then holding the slurry for 2 min; (5) cooling the crucible
in the air after it is foamed (A356 aluminum alloy foam
with Al-Ti-B contents of 0 %, 0.2 %, 0.3 %, 0.4 %, 0.6 %
and 0.8 %, henceforth referred as ATB0, ATB0.2, ATB0.3,
ATB0.4, ATB0.6 and ATB0.8, respectively). During the
whole procedures except for the last step the temperature
of the melt is controlled at 933 ± 5 K. Specimens for
microstructure observation are machined by electro-
discharging machining [22]. Representative metallographic
preparation processes are applied to prepare specimens for
metallographic characterization. Specimens are finally
ground using 2000 grit emery paper, polished using
0.25 μm diamond paste and then etched using 2 vol.%
hydrofluoric acid alcohol. Microstructures of the aluminum
alloy foams are examined by a Hitachi S4800 scanning
electron microscope (SEM) equipped with energy
dispersive X-ray spectrometer (EDX), and the average
grain size is calculated by the linear intercept method. The
phase composition is examined by X-ray diffraction
(XRD) (SmartLab, Rigaku) with CuKa radiation.
Specimens for compression tests are machined into the
size of 25 × 25 × 25 mm
3
by electro-discharging machine
to avoid size effect. The porosity of the foam is deduced
from mass and volume. Analytical balance (with the
precision of 0.0001 g) and caliper are used to measure the
weights and accurate dimensions, respectively. Uniaxial
compression tests are performed by using SUNS Electron
Universal Material Testing Machine, with a maximum load
of 300 KN. All tests are performed under displacement
control, with a displacement rate of 1.5 mm/min (with the
initial strain rate of 0.001/s) at room temperature. In this
paper, the first peak stress of the deformation curve is
defined as yield strength. Vaseline is used to minimize the
friction between sample and plates. Load and displacement
are recorded automatically by a computer, engineering
stress
is defined as the load (KN) on specimen divided
by specimen cross area; engineering strain
is defined as
the displacement (mm) of specimen divided by specimen
height (mm). Extrapolation method is used to determine
the densification strain [23]. Cell wall micro hardness is
tested on a SHIMADZU micro-hardness tester with
Vickers pyramid and load of 10 g. At least 10 points are
selected randomly and the average data are employed.
3. RESULTS AND DISCUSSION
3.1. Specimens of Al alloy (A356) foams
Fig. 1 shows the typical morphology of a specimen
used for compression test. It can be seen that the cell
structure is homogeneous. The pore sizes and porosities of
the foam produced in the present experiment are mainly
distributed in 1 – 3 mm and 75 – 85 %, respectively. It
should be noted that ATB0.6 and ATB0.8 fracture in the
middle when being took out of the crucible, indicating that
the foams are brittle and not easy to keep their integral
structures.
Fig. 1. Optical cross section of compression specimen with Al-
Ti-B percentage of 0.2%
3.2. Micro-morphology of Al alloy (A356) foams
XRD analysis (as shown in Fig. 2) on ATB0.3 is
applied to investigate the evaluation of the phases in the
foams. It shows that α-Al, Si, TiB
2
, TiSi
2
, SiTi, Al
5
Ti
2
and
Al
3
Ti phases exist in the foam. In order to understand the
influence of Al-Ti-B on the micro-morphology of the
foams, SEM and EDS tests (as shown in Fig. 3) are applied
on the cell walls. It can be seen that foam cell walls are
mainly composed of α-Al and eutectic Si. On the whole,
Al-Ti-B has little influence on the grain size of α-Al.
While the grain size is influenced by super-cooling degree
and the number of heterogeneous nucleation particles
[24, 25]. The cooling rate of foams is slow because of the
existence of gas, larger super-cooling degree is needed to
nucleate. In the case of A356 Al alloys, where Si content is
7 wt.%, the refining efficiency of commercial Al-Ti-B
master alloys is relatively poor. This is due to the
interaction of Ti with Si to form titanium silicides (as
shown in Fig. 2) which depletes the melt of Ti preventing
grain refinement of the alloy. This phenomenon has been
5m
m
339
the subject of different studies and it is known as poisoning
effect [26
–
29].
Fig. 2. XRD pattern of ATB0.3
Fig. 3. SEM and EDS results of Al alloy foams with Al-Ti-B
contents of a
–
0 %; b
–
0.2 %; c
–
0.3 %; d
–
0.4 %;
e
–
0.6 %; f
–
0.8 %
In addition, liquid of aluminum alloy with Al-Ti-B
should be used up in a certain time. While in this study Al-
Ti-B is kept in aluminum alloy melt in the processes of
thickening, foaming, holding and cooling, which
accelerates fading of Al-Ti-B, hence decreasing the grain
refine efficiency. Not only heterogeneous nucleation but
also growth restriction impact on effective refinement [30].
The grains may grow up again as the foams with low
thermal conductivity solidify slowly. Therefore, α-Al grain
size is not refined apparently. It is clear that the
distribution of the eutectic Si is along with the grain
boundaries. However, Al-Ti-B has a significant impact on
the morphology of eutectic Si. For ATB0 the eutectic Si is
coarse (as shown in Fig. 3 a). For ATB0.2 part of the
eutectic Si changes into small pieces (as shown in
Fig. 3 b). For ATB0.3 most of the eutectic Si exists in the
forms of fine blocks or short fibers (as observed in
Fig. 3 c). However, further increment of Al-Ti-B causes
the coarsening of eutectic Si that becomes blocky again (as
shown in Fig. 3 d, e and f). This, means that excessive Al-
Ti-B could not refine the morphology of eutectic Si.
3.3. Mechanical properties
Fig. 4 shows the variation tendency of cell walls’
micro-hardness. It should be noted that the micro-hardness
tests are performed on the cell wall matrix rather than on
the cell wall grain boundary. It is clear that Al-Ti-B
additions have an effect on the cell walls’ micro-hardness.
In summary, the micro-hardness of foams increases firstly
and then decreases with the increase of Al-Ti-B content.
The values (HV) of ATB0, ATB0.2, ATB0.3, ATB0.4,
ATB0.6 and ATB0.8 are about 45.1, 45.76, 48.68, 46.71,
40.23 and 36.84, respectively. Compared with ATB0, the
micro-hardness of ATB0.2, ATB0.3 and ATB0.4 increase
slightly and ATB0.3 possesses the highest micro-hardness
under the present conditions. However, for ATB0.6 and
ATB0.8 the values drop significantly compared to ATB0.
Fig. 4. Micro hardness variation tendency of the specimens with
different contents of Al-Ti-B
Quasi-static engineering stress (σ)-strain (ε)
compression curves of aluminum alloy foams with uniform
porosity (~80 %) but different Al-Ti-B contents (0 %,
0.2 %, 0.3 %, 0.4 %, 0.6 % and 0.8 %) are shown in
Fig. 5 a. The curves follow typical behavior of cellular
foams with three deformation stages: (a) an initial elastic-
plastic deformation stage, where partially reversible cell
walls bending occurs [31, 32]; (b) an extended plateau
stage, where cell walls buckle, yield and quantity fracture
and (c) a rapidly increasing stress, where the cell walls
become compacted together and the material attains bulk-
like properties [32]. It can be seen that the additions of Al-
Ti-B have an important effect on the stress-strain curves of
Al alloy foams. Meanwhile, it should also be noted that the
yield strength of the foams increased first and then
decrease with the increase of Al-Ti-B contents: the yield
strength of ATB0, ATB0.2, ATB0.3, ATB0.4, ATB0.6 and
ATB0.8 are about 1.5, 2.1, 3.3, 1.6, 0.9 and 0.7 MPa,
respectively. It is obvious that the yield strength of ATB0.3
is more than twice of ATB0. However, the yield strength
of ATB0.4 decline sharply (as shown in Fig. 5 b) to 1.6
MPa. For ATB0.6 and ATB0.8, the values are about half
of ATB0. All this means that the Al-Ti-B content should
be controlled to get optimal yield strength. In addition, the
strain of yield strength for ATB0.3 is about 0.05, while the
340
values for ATB0, ATB0.2, ATB0.4, ATB0.6 and ATB0.8
are approximately 0.025. In the extended plateau stage, the
mean plateau stresses (defined as the average value of the
stress with the strain between 0.10 and 0.45) increase first
and then decrease with Al-Ti-B content increasing (as
shown in Fig. 5 b). The mean plateau stresses of ATB0,
ATB0.2, ATB0.3, ATB0.4, ATB0.6 and ATB0.8 are about
1.77, 2.60, 2.91, 2.26, 1.53 and 1.33 MPa, respectively. It
is noted that ATB0.3 possesses the highest mean plateau
stress under the present conditions. While, the values for
ATB0.6 and ATB0.8 are lower than ATB0. Fig. 5 b shows
the variation tendency of densification strains of aluminum
alloy foams with different Al-Ti-B additions. It can be seen
that Al-Ti-B additions have a significant influence on the
densification strain. The densification strain for ATB0 is
about 0.57 and the value increases to 0.65 for ATB0.3 and
then decreases rapidly.
a
b
Fig. 5. a
–
quasi-static compressive engineering stress (σ)-
engineering strain (ε) curves of aluminum alloy foams
with different contents of Al-Ti-B; b
–
variation
tendencies of densification strain, mean plateau stress and
yield strength
Up to now, metal foams are mainly applied in energy
absorption fields. The energy absorption capacity of
material can be evaluated by integrating the area under the
stress-strain curve, which can be denoted as Eq. 1:
0
dW
, (1)
where W is the energy absorbed capacity of foams,
is the
stress where the strain is
[33]. As shown in Fig. 6, in all
cases energy absorption capacities increase with strain
increasing. In addition, Al-Ti-B has a significant influence
on the energy absorption capacity. It is observed that the
curves for ATB0.2, ATB0.3 and ATB0.4 approximate to
linearity, which is similar to curve of ATB0. While for the
ATB0.6 and ATB0.8, the curves are obviously different
from the curve of ATB0. In addition, during the
compression test it is important to evaluate the energy
absorbed before the foam is compacted, because if the
foam is compacted the buffering effect will be decreased
seriously [34]. Here available energy absorption capacity
(AEAC, defined as the energy absorbed until the
densification strain during the compression test [34]) is
used to assess the energy absorption capacity of foams
under the present conditions. It can be seen that the AEAC
for ATB0, ATB0.2, ATB0.3, ATB0.4, ATB0.6 and
ATB0.8 are about 1.06, 1.58, 1.90, 1.34, 0.74 and
0.62 MJ/m
3
, respectively. It should also be noted that
ATB0.3 possess the highest AEAC value and excessive
Al-Ti-B additions will deteriorate the AEAC value. It
indicates that appropriate amount of Al-Ti-B additions can
intensify the AEAC.
Fig. 6. Energy absorption capacities of Al alloy foams with
different contents of Al-Ti-B
As described above ATB0.3 possesses good
combination of micro-hardness, yield strength, plateau
stress, densification strain and energy absorption capacity
under the present conditions. Mechanical properties of
metal foams are generally depended on the strength of
basic materials, relative density, macro- and micro-
morphology of the foams [12, 35]. In this paper, the basic
material and relative density of the foams are almost
identical, meaning macro-morphologies of the foams are
similar too. Thus, the mechanical properties should mainly
depend on micro-morphologies of the foams. As described
above, the grain sizes of the foams change little with the
addition of Al-Ti-B. Meanwhile, it has been confirmed that
the most important micro-morphology in A356 aluminum
alloys is the form of eutectic Si, which has a determinative
effect on the alloys mechanical properties [13]. For
ATB0.3 most of eutectic Si exist in the forms of fine
particles or short fibers (as observed in Fig. 3 c). This
indicates that refinement of eutectic Si can strengthen
mechanical properties of aluminum (A356) foam. In
addition, precipitated phases in the foams act as obstacles
to the movement of dislocations during the loading of the
foams [36]. It is likely for dislocation line to loop around
the particles by Orowan looping rather than to cut through
them. According to Orowan strengthen mechanism,
dislocation loops provide more resistance to the movement
of subsequent dislocations and thus increase the strength of
341
the composites [37, 38]. Also, append dislocations are
created due to the difference of the coefficient of thermal
expansion between the matrix and TiB
2
particles, thus
these dislocations make the plastic deformation more
difficult. It is interesting to note that the aluminum alloy
foams with excess Al-Ti-B do not behave the highest
micro-hardness and compressive properties. It is
considered that the increasing Al-Ti-B additions results in
too many inclusions, some of which with big sizes and
agglomerated distribution may act as crack sources during
the compressive deformation and hinder further
improvement of the mechanical properties. All of these
reasons lead to the appropriate compressive properties of
foams with Al-Ti-B content of 0.3 %.
4. CONCLUSIONS
In the study, the microstructure and compressive
properties of aluminum alloy foams with different contents
of Al-Ti-B are investigated. The results are summarized as
follows:
Aluminum alloy (A356) foams with Al-Ti-B content
of 0.3 % possess good combinations of the micro-hardness,
yield strength, plateau strength, densification strain and
energy absorption capacity under the present conditions. In
addition, Al-Ti-B has important effect on the morphology
of eutectic Si, the eutectic Si will change into particles or
short fibers with Al-Ti-B addition of 0.3 %, leading to
optimal mechanical properties under present conditions. In
the case of higher Al-Ti-B content the eutectic Si will
become coarser again, and the increasing Al-Ti-B
additions results in too many inclusions, which hinder
further improvement of the mechanical properties.
Acknowledgements
The present authors wish to thank the financial support
provided by Hebei Province School Cooperation Fund
Projects, National Natural Science Foundation of China
(No.51501053), “863” project of China
(NO.2013AA031002), Major Project of China
(2013ZX04004027), by Program for Changjiang Scholars
and Innovative Research Team in University
(No.IRT13060), Science and Technology Project of Hebei
Province (13211008D).
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