V. THULASIKANTH, R. PADMANABHAN: A STUDY ON SOUND-ABSORPTION ABILITY OF CLOSED-CELL ALUMINIUM ...
423–431
A STUDY ON SOUND-ABSORPTION ABILITY OF CLOSED-CELL
ALUMINIUM FOAMS
[TUDIJA SPOSOBNOSTI ABSORPCIJE ZVOKA KOVINSKIH PEN
Z ZAPRTIMI CELICAMI NA OSNOVI ALUMINIJA
Vaddi Thulasikanth, R. Padmanabhan
*
School of Mechanical Engineering, Vellore Institute of Technology, Chennai, India
Prejem rokopisa – received: 2023-01-12; sprejem za objavo – accepted for publication: 2023-05-08
doi:10.17222/mit.2023.741
Metallic foams have gained popularity in industrial applications and research due to their unique mechanical properties com-
bined with light weight, vibroacoustic damping and high-temperature resistance. Generally, closed-cell foams have poor sound
absorption properties due to their closed cellular structure when compared to open-cell foams. This paper investigates a new
composite closed-cell foam for its acoustic properties. The new closed-cell aluminium fly-ash foam was made with liquid pro-
cessing, using calcium carbonate (CaCO3) as the blowing agent. The effects of the fly-ash content on the cell morphology and
mechanical properties were analyzed. Acoustic studies were conducted on the prepared closed-cell foam using the impedance
tube method. Samples were prepared in different ways to determine the effects of the changing parameters. The results of
quasi-static compression and microstructural analyses of two different combinations of the foam revealed small pore sizes with
varying relative densities and compressive strengths. It is evident from the acoustic studies that the foams are capable of im-
proved sound absorption of medium and high frequencies.
Keywords: aluminium foams, stir casting, sound absorption, CaCO
3
Kovinske pene so zaradi enkratnih mehanskih lastnosti, majhne specifi~ne gostote, sposobnosti vibroakusti~nega du{enja in
odpornosti proti povi{anim temperaturam zelo popularne v novej{ih raziskavah za vrsto razli~nih industrijskih aplikacij . V
splo{nem imajo pene z zaprto poroznostjo (celice oz. pore so med seboj nepovezane) slab{o sposobnost absorpcije zvoka kot
pene z odprto poroznostjo. V ~lanku je opisana raziskave akusti~nih lastnosti novega kompozita z zaprto poroznostjo. Novi
kompozit z zaprto poroznostjo so izdelali z livarskim me{alno-vrtilnim postopkom pri katerem so v staljeno zlitino Al7Si2Mg
dodajali pepel (2 in 5) w/%, ki se nabira v filtrih med livarsko proizvodno aluminijevih zlitin. Kot penilno sredstvo so pri tem
uporabljali kalcijev karbonat (CaCO3) in dodatek magnezija za izbolj{anje omo~ljivosti delcev pepela s talino. Analizirali so
vpliv koli~ine dodanega filtrnega pepela na morfologijo celic in mehanske lastnosti izdelanih pen. Akusti~ne lastnosti izdelanih
pen so dolo~ili z impedan~no cevno metodo. Vzorce pen so izdelali na razli~ne na~ine tako, da so lahko ugotavljali vpliv
spremembe procesnih parametrov na lastnosti pen. Rezultati kvazistati~nih tla~nih preizkusov in mikrostrukturne analize pen so
pokazale, da imajo izdelane pene majhno velikost por in sprememnjeno specifi~no gostoto glede na koli~ino dodanega filtrnega
pepela. Akusti~ne raziskave so pokazale, da je pri penah z zaprto poroznostjo mo`no dose~i izbolj{anje sposobnosti za
absorpcijo zvoka pri srednjih in visokih frekvencah.
Klju~ne besede: kovinske pene na osnovi aluminija, me{alno-vrtilno litje, absorpcija zvoka, kalcijev karbonat
1 INTRODUCTION
Noise is an unpleasant sound that upsets people and
other creatures both mentally and physically. Because of
the rapid urbanization, various sources of noise from in-
dustries, transportation vehicles (both conventional and
electric),
1
construction and building sites
2
and even un-
der-water vehicles
3
are regarded as a serious environ-
mental problem all around the world. New low-cost
lightweight materials with improved mechanical strength
and high-efficiency noise-absorbing properties in a wide
frequency range are greatly needed. Chang et al. pre-
sented several effective strategies from the previous liter-
ature, used to attenuate the acoustic waves with acoustic
metamaterials and intelligent materials, especially for
low-frequency acoustic absorption.
4
Cellular materials,
due to their unique structures composed of solid areas
and vacant pores, have gained a significant role in dissi-
pating sound energy with various absorbing mechanisms
like viscous shear, friction, Helmholtz resonance, struc-
tural vibration, etc.
5,6
Several advanced materials based
on polymers,
7
metals,
8,9
ceramics,
10
elastomers
11
and their
composites coupled with precision manufacturing have
been developed so far.
However, aluminium foam panels are widely used in
heavy machinery industries, transportation and building
sectors, for sound absorbing because of their unique cel-
lular structures with the combination of excellent proper-
ties like light weight, durability, recyclability,
workability, resistance to humidity and fire.
12–15
En-
hanced sound absorption capacity was achieved with
Alporas (trade name) foam panels by creating cracks
through rolling or compression, attributable to a viscous
loss and thermoelastic damping.
12
The sound absorption
coefficient increases as the pore opening diameter de-
creases, while the pore size has a marginal effect on the
sound absorbing capacity.
13
With a higher surface pore
opening density, the sound-absorption performance in-
Materiali in tehnologije / Materials and technology 57 (2023) 5, 423–431 423
UDK 996.715:534:539.217 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(5)423(2023)
*Corresponding author's e-mail:
padmanabhan.r@vit.ac.in (R. Padmanabhan)

creases due to an increase in the thermoelastic dissipa-
tion and viscous loss. Nosko and Kovacik
15
investigated
ACCESS, ALPORAS, ALULIGHT (trade names) to de-
termine the influence of the foam preparation method on
the sound absorption capacity. An ideal porosity, of
around 75 %, results in superior sound absorption. Sur-
face opening of the pores is important for the sound
wave to be absorbed.
15
Aluminium foams are classified
into two types based on pore connectivity: i) open-cell
foams (pores are interconnected with each other) and
ii) closed-cell foams (pores are independent of each
other). Previous research studies proved that open-cell
foams are better sound absorbers than closed-cell foams
due to their interconnected pore structure.
16,17
Because of
the gas-filled individual cells in the metal matrix,
closed-cell aluminium foams are better sound insulators
and have superior mechanical properties than open-cell
foams.
18,19
Keeping their superior mechanical properties
in mind, several researchers were focused on improving
the sound absorption property of closed-cell foams by al-
tering their surface and cell structure. According to ear-
lier studies, the post-foaming procedures, such as drill-
ing,
20
machining,
18
rolling, compression,
21
heat treatment
(heating and quenching)
22
and others, improve the sound
absorption capacity by introducing pore connectivity to
closed-cell foams.
Though closed-cell foams have great potential, their
widely spread utilization is still limited by the need for
expensive additives, delicate process conditions and dif-
ficulties in mass production at an affordable price.
Among several manufacturing routes, liquid state pro-
cessing at minimal costs is better for mass production.
23
In this work, aluminium foams are produced via the melt
route by admixing the gas-releasing foaming agent in to
the aluminium melt. Foams produced with the dissolved
gas method make use of hydrides (TiH
2
, ZrH
2
, CaH
2
etc.)
or carbonates (CaCO
3
, MgCO
3
, CaMg (CO
3
)
2
, etc.) as
the foaming agents.
24,25
During foaming the evolved gas
bubbles are moving upward due to the buoyancy force.
An addition of ex-situ (SiC, Al
2
O
3
, Al, etc.)
26–28
or cre-
ation of in-situ (TiB
2
, MgAl
2
O
4
, etc.)
29–30
oxides particles
in the melt will improve the viscosity, thereby bubbles
will be entrapped. Hence, there is a strong need to inves-
tigate the economic feasibility of producing aluminium
foams without sacrificing the performance by employing
appropriate processing techniques and using less expen-
sive raw materials (thickening and foaming agents).
Class F fly ash, which is produced at thermal power
plants by the combustion of bituminous and anthracite
coals, was used as a low-cost reinforcement to create
non-ferrous composites with excellent mechanical and
tribological properties. Previous research studies demon-
strated that fly ash can also be used as an effective thick-
ening agent in the production of aluminium foams.
31–33
Many alternative foaming agents were proposed for the
processing of aluminium foams, one of which is CaCO
3
,
a low-cost foaming agent with excellent mechanical
properties.
33–35
In this work, closed-cell aluminium foams are pro-
duced through the melt route with an inexpensive thick-
ening agent (class F fly ash, an industrial waste) and cal-
cium carbonate (CaCO
3
) as the foaming agent. This
material combination with the resulting foam forms
green technology and is little investigated; hence, this
study focuses on the preparation and investigation on the
acoustic characteristics. In addition, the focus is placed
on the impact of two post-foaming processes, hacksaw
machining and drilling holes, on the sound absorption
ability of the prepared aluminium fly-ash closed-cell
foams. Section 2 presents the processing of aluminium
V. THULASIKANTH, R. PADMANABHAN: A STUDY ON SOUND-ABSORPTION ABILITY OF CLOSED-CELL ALUMINIUM ...
424 Materiali in tehnologije / Materials and technology 57 (2023) 5, 423–431
Figure 1: Process flow for composite foams Al-2F and Al-5F

foams and characterization of compression and acoustic
properties. The ex-situ characterization of the foams is
carried out as per standards and the correlations between
the structure and properties are established in Section 3,
followed by conclusions.
2 EXPERIMENTAL PART
2.1 Processing of the composite and foams
Closed-cell foams were made in-house with
Al7Si2Mg alloy and fly ash (2 w/% and 5 w/%) by stir
casting, using CaCO
3
as the foaming agent. Preheated fly
ash (800 °C for 3 hr) with an average particle size of
2–3 μm was used as the reinforcement and 1 w/% mag-
nesium (Mg) was added to the alloy to improve the wet-
ting between fly-ash particles and aluminium melt. Two
composite combinations were prepared in-house during a
stir casting process. We produced 1kg of each composite
by varying the fly ash (Al-2F and Al-5F). For the Al-2F
composite, first the Al7Si2Mg alloy was melted in a fur-
nace at 850 °C and 1 w/% of magnesium was immersed
into the melt to improve the wettability of fly-ash parti-
cles with the molten alloy. The stirring of melt was car-
ried out at 350 min
–1
and the preheated fly ash (1 w/%)
was added gradually to the melt over a period of 20 min.
Finally, the composite mixture was poured in a preheated
metal die and the melt was allowed to solidify at room
temperature.
33
A similar procedure was used when pro-
cessing 5 w/% of fly ash for the Al-5F composite. These
are the steps used for preparing the foams, included in
the process illustrated in Figure 1: i) the Al-2F compos-
ite in the amount of 100 g was placed in a clay graphite
crucible and melted in the furnace at 680 °C; ii) pre-
heated CaCO
3
(a particle size of  50 μm) in the amount
of 3 w/% was added to the composite slurry and stirred at
a 1200 min
–1
speed for 120 s; iii) the foaming mixture
was allowed to expand in the crucible for 180 s; iv) fi-
nally, the crucible containing the foam was taken out and
allowed to solidify under rapid cooling, using com-
pressed air. A similar procedure was used for the Al-5F
composite foam as well.
As-cast foams (the second column in the figure) and
wire cut EDM-sectioned cubic samples of Al-2F and
Al-5F foams (the third column in the figure) are shown
in Figure 1. Scanned images of cut foam sections were
used to analyse the pore structure of the foams.
2.2 Quasi-static compression test
Cellular metals exhibit a unique stress-strain response
under compression with a plateau region, in which the
stress is nearly constant over a wide range of strain. This
behaviour makes cellular metals appealing for energy ab-
sorption applications where a significant amount of de-
formation can be absorbed at a relatively low constant
stress.
36
In general, the compressive strength of alu-
minium foams can vary depending on several factors
such as the type of the alloy used, stabilizing and foam-
ing agents used, manufacturing process, relative density
of the foam, loading rate and the direction of loading rel-
ative to the foam’s cellular structure.
24
Because of their
porous nature, aluminum foams have lower compressive
strengths than solid aluminum. They can, however, have
a good strength-to-weight ratio and high energy absorp-
tion capability, making them useful in applications re-
quiring weight reduction and impact resistance.
36
In or-
der to find the mechanical strength of the foams under
compression loads, typically observed in a vehicle upon
impact, a uniaxial compression test was carried out at
room temperature in the same direction as the principal
foam growth, as per ISO 13314:2011 standard. The test
specimens were sliced to form cubic samples of (30 × 30
× 30) mm using wire cut EDM (shown in Figure 1). The
minimum dimensions of a sample are seven times higher
than the cell size to avoid the size effect. The tests were
carried out in-house using a universal testing machine
(Shimadzu, AG-X plus 50KN) at a cross-head speed of
V. THULASIKANTH, R. PADMANABHAN: A STUDY ON SOUND-ABSORPTION ABILITY OF CLOSED-CELL ALUMINIUM ...
Materiali in tehnologije / Materials and technology 57 (2023) 5, 423–431 425
Figure 2: Acoustic testing set-up with Al-2F perforated foam

2 mm/min. The contact surface between the platens and
foam was lubricated with molybdenum disulphide
(MoS
2
) in order to reduce friction.
37
The foam exhibited
three-stage compression characteristics, which are fur-
ther described and discussed in Section 3.2.
2.3 Sound-absorption ability testing
Sound-absorption ability tests were carried out
in-house using an impedance tube set-up as per ASTM E
1050-98 standard, as shown in Figure 2. The test set-up
consists of a power amplifier, sound source, impedance
tube, two microphones and a digital frequency analysis
system to determine the sound-absorption coefficient of
the prepared foam at normal incidence, that is, 0°. An
acoustic drive (loudspeaker) is mounted at one end of the
impedance tube and a foam sample at the other end, with
an air gap of 10 mm before the end of the tube. Plane
waves are typically generated in the tube using a broad
band signal from a sound source, which propagate
through the tube, impact the sample normally and are re-
flected back, resulting in a standing wave interference
pattern within the tube. Near the sample end of the tube,
a pair of microphones positioned parallel to the inner
wall are used to measure the sound pressure, and a multi-
channel spectrum analyser is used to determine the com-
plex transfer function between the two readings. The mi-
crophone kept closer to the source is taken as the
reference channel. The incident and reflected energy are
separated from the measured transfer function, and the
required acoustic characteristics of the test sample are
estimated.
The influence of the foam surface and structure on
the sound absorption is investigated. The tests were car-
ried out at a frequency range of 500–4000 Hz. The opti-
cal microscopy and SEM analysis of the foams were car-
ried out and all the experimental findings are explained
in following section.
3 RESULTS AND DISCUSSION
3.1 Structure of the foams
The cut sections of the foam samples were scanned at
a resolution of 600 dpi and the images obtained were
analysed in the Image-J software to find the average cell
size. The average cell diameter observed in the Al-2F
foam is 1.24 mm, while that observed in the Al-5F foam
is 1.87 mm as shown in Figures 3a and 3b. Al-2F has a
small cell size and most of its cells are separated by cell
walls when compared to Al-5F. As depicted in Figure 1,
the cells of both Al-2F and Al-5F are close to a spherical
shape and only few are of an irregular shape. However,
Al-2F has a larger proportion of smaller cells than Al-5F.
Foam densities were calculated from the cubic sec-
tions and relative densities ( r
) were determined as
0.20 and 0.17 for the Al-2F foam and Al-5F foam, re-
spectively. The relative density is the ratio of the density
of foam to the density of matrix material at the cell wall.
The porosity percentage (P) of the foam was calculated
with Equation (1).
38
P =( 1– r
) × 100 (1)
The percentage of porosity obtained for Al-2F is
80 % and for Al-5F it is 87 %. The Al-5F foam shows a
larger expansion than Al-2F, which is evident from the
obtained values.
Figure 4 shows SEM micrographs and EDS spectra
of the Al-2F and Al-5F foams at the gas-solid interface.
It is evident from Figures 4a and 4c that the stabilizing
particles formed as a result of the reaction are evenly dis-
tributed along the plateau border and cell walls, which is
confirmed by the presence of elements such as O, Al, Si,
Mg and Ca in Figures 4b and 4d, respectively. In order to
improve the wetting characteristics and chemical reac-
tion between the alloy and fly ash, Mg was added and its
presence is confirmed with the EDS results. Moreover,
compared to Al, Mg has a higher affinity for the oxygen
present in the fly ash. Figure 4e represents the reaction
products formed in the Al-5F composite such as Al
2
O
3
,
Mg
2
Si, MgO and MgAl
2
O
4
. Similar oxides are also
formed in the Al-2F composite.
33, 39
According to the
V. THULASIKANTH, R. PADMANABHAN: A STUDY ON SOUND-ABSORPTION ABILITY OF CLOSED-CELL ALUMINIUM ...
426 Materiali in tehnologije / Materials and technology 57 (2023) 5, 423–431
Figure 3: Pore-size distributions of: a) AL-2F foam and b) Al-5F foam

EDS results, there is a significant decrease in the Mg
content in Al-5F with the increasing fly-ash weight frac-
tion and these results match well with an earlier re -
search.
40
All these reaction products contribute to the sta-
bilizing of the foam; in addition, a thin layer of solid re-
action occurs early in the foaming process as a result of
the CaCO
3
and melt reaction, stabilizing the cells by
modifying the surface tension and preventing the coales-
cence and coarsening of the cells. Furthermore, the solid
particles formed due to the thermal breakdown of car-
bonate (CaO) increase the melt viscosity, enhancing the
foam stabilization.
34
The presence of more oxide parti-
V. THULASIKANTH, R. PADMANABHAN: A STUDY ON SOUND-ABSORPTION ABILITY OF CLOSED-CELL ALUMINIUM ...
Materiali in tehnologije / Materials and technology 57 (2023) 5, 423–431 427
Figure 4: a) SE SEM image of Al-2F foam at the gas-solid interface, b) EDS spectra of Al-2F foam at the gas-solid interface, c) SE SEM image
of Al-5F foam at the gas-solid interface, d) EDS spectra of Al-5F foam at the gas-solid interface and e) X-ray diffraction of Al-5F composite

cles in the Al-2F foam contributes to the creation of a
sufficient surface tension, which enhances the formation
of smaller cells with a high relative density. From the re-
sults it is evident that industrial-waste fly-ash particles
can be used in the production of aluminium foams, al-
lowing the control of the melt viscosity, decreasing the
melt flowability in all directions and contributing to the
stability of cells.
3.2 Compressive behaviour of the foams
Porous metals should have sufficient structural dura-
bility in order to use it for any real-time acoustic and
other engineering applications, such as anti-collision en-
ergy-absorbing bars in an automobile. The compression
strength of the two foams were evaluated using a uniax-
ial compression test at a strain rate of 1.1 × 10
–3
s
–1
and
characteristic stress-strain curves are shown in Figure 5.
Both foams exhibited well-known three-stage defor-
mation characteristics when subjected to quasi-static
compression: the initial linear elastic region (R1), length-
ened plateau region when the stress slowly increases due
to cell plastic deformation (R2) and final densification
region (R3) when collapsed cells are compacted together.
The results show that the compressive yield stress is
2.49 MPa for Al-2F and 1.86 MPa for AL-5F, and these
results are in good agreement with the reported val -
ues.
31,32
Al-2F exhibits an increased compressive
strength, higher than that of Al-5F due to a higher rela-
tive density (0.20), lower pore size (1.24 mm) and lower
porosity (80 %). A greater volume fraction of the solid
phase in Al-2F is indicated by a higher relative density
and lower porosity, which results in an earlier
densification stage during the compression test. The
compressive-strength range obtained for both foams are
suitable for medium impact and energy-absorption appli-
cations.
36
Additionally, the foams obtained will give
some structural support and bear compressive load when
used as sound-absorbing panels or slabs in automobiles,
large conference rooms and factory workshops.
3.3 Acoustic behaviour of the foams
An individual anisotropic closed-cell network will re-
strict the permeability of incident sound waves. In order
to alter these features, the permeability should be in-
creased to enable the acoustic waves to penetrate through
the cells. Earlier researches indicate that modifying the
surface and structure of closed-cell aluminium foams can
improve their sound-absorption capacities.
18,20–21
In this
study, both Al-2F and Al-5F foams were sliced with a
hacksaw and holes were drilled in order to change the
surface and structure as shown in Figures 6a, 6c, 6d and
6f. The foam samples were cut with the hacksaw blade to
obtain a thickness of 10 mm and a diameter of Ø33 mm
using wire cut EDM. Al-2F is shown in Figure 6a and
Al-5F is shown in Figure 6d. Figures 6b and 6e show
the samples at a higher magnification of the incidence
surface formed by the hacksaw and the difference in the
pore size. Perforations of Ø1.5 mm to a depth of 5 mm
were made randomly on another set of acoustic samples,
denoted as Al-2F (H) in Figure 6c and Al-5F (H) in Fig-
ure 6f.
The sound absorption coefficient results for the
foams were plotted in the graph for up to 2000 Hz, as
shown in Figure 6g. It is observed that all the foam sam-
ples show a two-peak wave pattern shifting the peak
from low to high frequencies with minimum sound ab-
sorption coefficient values of 0.3 to 0.4 at 500–600 Hz
and maximum values of 0.78 to 1.0 at 1400–1600 Hz.
The minimum peak at 500–600 Hz for all the foam sam-
ples is due to the effect of the membrane resonator
mechanism by the air gap left in between the sample and
rigid end of the impedance tube.
41,42
According to the
pore structure analysis discussed in Section 3.1, the
Al-2F foam has a higher relative density and smaller av-
erage pore size and porosity than the Al-5F foam. The
higher relative density and smaller pore size of Al-2F in-
dicate that the solid-medium amount is higher than the
fluid-medium amount, lowering the permeability of
sound waves and leading to a decreased sound absorp-
tion. For an improved sound absorption, the estimated
pore connectivity should be at least 0.3 mm and the pore
size should not be less than 1 mm because the values
lower than this cause the material to reflect rather than
absorb the sound.
12
In the first case, the incidence surface finish of both
Al-2F and Al-5F foams was altered by the shearing ac-
tion of the hacksaw. The microscopic images of the
foams from Figures 6b and 6e reveal that cell faces and
cell walls have rough surface profiles with a few
microcracks, their edges being bent and fractured by cut-
ting. This increases the permeability of incident sound
waves in closed-cell foams, further increasing the wave
energy dissipation. As the incidence surface finish was
V. THULASIKANTH, R. PADMANABHAN: A STUDY ON SOUND-ABSORPTION ABILITY OF CLOSED-CELL ALUMINIUM ...
428 Materiali in tehnologije / Materials and technology 57 (2023) 5, 423–431
Figure 5: Quasi-static compression results for foams

altered by the hacksaw, sound waves propagate through
tortuous porous tunnels formed by induced cracks and
bends, and some energy is lost due to the viscous shear
of air molecule against the boundary cell walls. The re-
sults obtained are in agreement with the observations
made in earlier research works on different closed-cell
foams,
12
Al-SiC composites with TiH
2
and CaCO
3
.
18
In general, the viscous resistance mechanism is lim-
ited for smaller material thicknesses as with this mecha-
nism the permeability of sound occurs only via micro
pores or cracks on cell faces.
19
Another easy approach to
opening a closed-cell structure is drilling holes, which
will lead to an absorption via the cavity resonator mecha-
nism. In our case, holes of Ø1.5 mm were drilled and
V. THULASIKANTH, R. PADMANABHAN: A STUDY ON SOUND-ABSORPTION ABILITY OF CLOSED-CELL ALUMINIUM ...
Materiali in tehnologije / Materials and technology 57 (2023) 5, 423–431 429
Figure 6: a) Hacksaw cut Al-2F foam without holes, b) microscopic image of the hacksaw cut Al-2F foam, c) Al-2F (H) foam with holes, d)
hacksaw cut Al-5F foam without holes, e) microscopic image of the hacksaw cut Al-5F foam, f) Al-5F (H) foam with holes, and g) sound absorp-
tion coefficient results for the foams

both foams showed sound absorption coefficient values
that were almost 1.0. It is evident that there is not much
deviation in the peak of the AL-5F foam with or without
the drilled holes. In Al-2F, a significant increase in the
peak is observed after drilling even though it has a higher
relative density, and lower values of pore size and poros-
ity percentage when compared to Al-5F. The increase is
due to the holes drilled to a depth of 5 mm, interconnect-
ing the closed-cells and cavities formed in the foams and
finally leading to a dissipation of sound energy due to
the cavity and resonance sound absorption mechanism. It
can be observed from the results that the sound absorp-
tion performance of the closed-cell composite foams can
be improved by altering the surface and pore geometry
with two techniques used in the current work, i.e.,
low-cost machining using a hacksaw and conventional
drilling. Additionally, it is evident from the SEM micro-
structures shown in Figures 4a and 4c that prior to hack-
saw machining and perforation of the foams, some small
micropores formed in-situ in cell walls and faces, also
augmenting the pore connectivity.
4 CONCLUSIONS
Closed-cell foams made by stir casting using
Al7Si2Mg alloy, low-cost raw materials like fly ash and
calcium carbonate as the foaming agent demonstrated
good mechanical properties. The post-casting treatment
additionally improved their sound-absorption capacity. It
is observed that the amount of fly ash plays a significant
role in stabilizing the foams by creating in-situ oxide
particles during its reaction with the aluminium alloy
used. It is observed from the results that an increase in
the fly-ash weight contributed to an increase in the po-
rosity and cell size, and a decrease in the relative density
of the foams. Both Al-2F and Al-5F foams show an ap-
preciable compressive strength. Our experimental inves-
tigation indicates that the post-foaming casting process
(hacksaw machining) improved the sound-absorption ca-
pacity by altering the surface finish and cell structure
with the viscous drag mechanism and cavity resonator
mechanism. The foams subjected to hacksaw machining
obtained a good sound absorption capacity due to the
viscous drag mechanism. However, the sound-energy
dissipation due to viscous drag is insufficient and can be
further improved by drilling holes in the foams. The re-
sults for the Al-2F (H) and Al-5F (H) foams show that
drilling holes on the hacksaw cut surface changed the
cell structure and enhanced the sound-absorption ability
via the cavity resonator mechanism. According to the
microstructure analysis of the foam structure, the Al-5F
foam with bigger cells has a tendency to absorb more
sound waves because the surfaces of the cell walls are
twisted and there are more cracks and pores, resulting in
increased sound energy absorption. The sound absorp-
tion of the Al-2F and Al-5F foams is most promising for
medium and high-frequency performance. A good sound
absorption capacity, combined with better mechanical
properties, makes the developed foams, Al-2F and Al-5F,
ideal for applications such as enclosures in heavy ma-
chinery, hybrid vehicles, public buildings, etc.
Acknowledgment
We are grateful to the Vellore Institute of Technology
(Chennai and Vellore Campus) for letting us use the pro-
cessing and characterization facilities needed for our
work. We are also thankful to Dr. S. Jeyanthi, Associate
Professor, and Mr. S. Vinoth Kumar, Research Scholar,
Vellore Institute of Technology (Chennai) for their sup-
port in using the acoustic testing facility, granted under
the Seed Fund, School of Mechanical Engineering, VIT
Chennai.
Authors’ Declarations
No Funding Involved
The authors, V . Thulasikanth and R. Padmanabhan,
declare that this research does not involve any source of
funding, financial or non-financial interests, nor does it
need any specific committee (ethics) approval as it does
not involve human/animal participation.
Compliance with Ethical Standards
The two authors, V. Thulasikanth and R. Padma-
nabhan, have no conflict of interest and the research does
not involve any human/animal participation.
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