6th International Advanced Technologies Symposium (IATS’11), 16-18 May 2011, Elazığ, Turkey
Effect of Dissolving Agent Shape for the
Microstructural Tailoring of Sdp Processed
Aluminum Foams
A. Yavuz1, İ. Yavuz2, M. S. Başpınar1, H. Bayrakçeken2
1
Afyon Kocatepe University Faculty of Technology, Metallurgy and Materials Engineering., Afyon/TURKEY
2
Afyon Kocatepe University Faculty of Technology, Automotive Engineering Dept., Afyon/TURKEY
[email protected]
Abstract—The effect of dissolving agent morphology on
the production of aluminium foams by SDP (Sintering and
dissolution process) method was investigated. Effect of two
different agents (NaCl and Na2CO3) with equaxed and tabular
shape on the processing of open cell aluminium foam were
studied. Tabular shaped Na2CO3 resulted in much faster and
vigorous dissolution rate than the NaCl. On the other hand,
when Na2CO3 was used, laminations were observed after
pressing and lose in integrity of the samples after dissolution
process were observed. It was concluded that using mixture of
NaCl and Na2CO3 together can improve the dissolution step in
SDP process. Na2CO3 usage was also found to be better
alternative to increase the interconnectivity of the pores where
some foam applications needs.
melt metal around these granules and finally removing the
pattern. Reference technique was first applied in 1966 and
similar processes have been developed in recent years [4–6].
A more advanced version of this process uses a hot-wall
pressure infiltration process: the salt preform is held under
vacuum while a block of aluminium is melted over it and an
inert gas at high pressure is applied during the subsequent
infiltration step.
In the second method, called Sintering and Dissolution
Process (SDP), the powders have been used to produce a
dense two phase precursor where one phase is water soluble.
The powders (usually Al and NaCl) are mixed and
compacted, forming double connected structures of both
phases. After furnace sintering, by dissolving the leachable
phase, a foam of the other phase is produced. This process,
studied at Liverpool University [7–10], belongs to the
processes defined ‗‗space holder techniques‖ giving
structure of a great uniformity [11]. A recent study
compares the sintering of Al–NaCl compact by traditional
electric furnace sintering and by spark plasma sintering that
allows the increase of plateau stress [12]. Moreover Zhao
developed Lost Carbonate Sintering process (LCS) to
manufacture copper foam using potassium carbonate
(K2CO3) as leachable salt with the same reference technique
of SDP [13]. The most important problem of SDP process is
time consuming due to the long dissolving periods. The
second problem is isolated pore formations due to equaxed
dissolving agents. For this reason, dissolving all the additive
is a difficult process.
The aim of this study is to investigate the effect of
different dissolving agent type and morphology on the SDP
process and microstructural tailoring of aluminum foam.
Although use of NaCl and K2CO3 were studied for the SDP
process, little attention was paid for the use of Na 2CO3 as a
dissolving agent.
Keywords—Al foam, SDP process, NaCl, Na2CO3, Particle
morphology
I.
INTRODUCTION
Metal foams have recently attracted considerable
attention in both academia and industry because of their
exceptional mechanical, thermal, acoustic, electrical and
chemical properties. There is a great diversity of cellular
metallic materials that show various structures and
properties, which has been described in detail by Banhart
[1]. According to the connectivity of cells, cellular metals
can be categorised as either closed- or open-celled. Like
natural cellular materials, cellular metals are also
dominantly used for light-weight structural or functional
purposes.
For structural applications, such as energy absorption, the
most important considerations are porosity, specific
strength, ductility in compression and cost. The
overwhelming majority of metal foams in the market are
therefore closed-cell Al foams manufactured by liquid or
semi-liquid foaming technologies. For functional
applications, such as sound absorption, thermal insulation,
heat dissipation and catalyst support, the cells need to be
open and small. Therefore, cellular metallic materials with
open-celled structures have wider applications in functional
structures [2,3].
Generally open cell foams can be obtained by using a
leachable material (e.g. salt) together with metal. Following
this principle there are two different techniques starting
from melt metal or powders. The first method, called
Replication Technique, consists of three basic steps: by
packing a soluble salt in a mould to have a pattern, casting
II. MATERIAL AND METHOD
Aluminum metal powders were supplied from A.Aesar firm
with a particle size range of -40+325 mesh. Coarse culinary type
NaCl and Merck 106392 code Na2CO3 were used in the
experiments. NaCl salt crushed and screened for particle size
classification. After the screening NaCl salt was classified in to
(-2 mm, +1 mm) and (-1 mm, + 500 µm) size ranges. Similarly
Na2CO3 powders were also screened to the (-1 mm, + 500 µm)
particle size range. Total weight of 30 gr is shaped for each
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A. Yavuz, İ. Yavuz, M. S. Başpınar, H. Bayrakçeken
sample. For this purpose certain amount of NaCl and Na2CO3
were mixed with aluminum powder. Low speed elliptical mixer
was used for mixing. A standard mixing time of 2 hour was
used for each series. Mixture ratios of the sample series are
given in Table 1. After mixing, samples were pressed at 1500
bar standard pressures in a steel mould according to dry pressing
method. After shaping, each of the sample weighed in digital
balance.
Shaped aluminum samples were sintered at 630 oC for 3 hour.
Heating rate of 5 oC/min was used. Weight and dimensions of
the samples were measured before and after sintering step. After
sintering, samples were dipped to the hot water which has 70 oC
for the dissolution of NaCl and Na2CO3. Magnetic stirrer
equipped with hot plate was used for the dissolution process.
100 minute standard dissolution period was applied to the
samples. Standard water volume of 600 ml was used. Tap water
is used for the dissolution experiments. Standard mixing speed
of 210 rpm was used in the experiment. Schematic flow chart of
foam production is given on Figure 1.
7 different sample series were prepared for the comparative
study. A series samples were shaped with mixing different
weight ratios of Na2CO3 with aluminum powder. B series
samples were prepared on the base of standard aluminum/NaCl
ratio. However, different particle size NaCl were used. Mixture
of two different salt type was used to produce C series sample
with similar ratio as in B series sample.
Table 1. Sample types and ingredients
Sample
Code
A1
A2
A3
B1
B2
B3
% Aluminum
(wt)
50
60
70
60
60
60
C1
60
%Dissolving
agent (wt)
50 Na2CO3
40 Na2CO3
30 Na2CO3
40 NaCl
40 NaCl
40 NaCl
20 Na2CO3
20 NaCl
Particle size
of the agent
-1 mm, + 500 µm
-1 mm, + 500 µm
-1 mm, + 500 µm
-1 mm, + 500 µm
-2mm, + 1 mm
-1 mm + 500 µm
-2mm, + 1 mm
-1 mm, + 500 µm
-1 mm, + 500 µm
Efficiency of the dissolution process was evaluated simply
by measuring the weight before and after the dissolution
process. Loss in the weight after dissolution assumed to be
originated by dissolution of salt and calculations made.
Result were presented as % dissolution yield.
Figure 1. Production flowchart of aluminum foam by
SDP process.
The particle morphologies of the NaCl and Na2CO3 are
given in Figure 2. NaCl particles have equaxed shape while the
Na2CO3 particles have long hexagonal tabular like shape.
III.
RESULTS AND DISCUSSION
Two important problem were faced when Na2CO3 used
alone in A series sample due to the long tabular shape. First
problem was faced during mixing stage. Since the shape of
aluminum and Na2CO3 are very different, selective phase
separation was observed between aluminum and Na2CO3
powders. Second problem came up after the dissolution
process. Laminations were easily observed after dissolution
step (Figure 2). The long Na2CO3 particles were directed
perpendicular to the pressing direction. As a result partially
texture was observed in the cross section of the samples.
When Na2CO3 was used, loss in sample weight after
dissolution process was increased and therefore higher
dissolution values were calculated (Figure 3). From this
result one can conclude that the Na2CO3 particles dissolved
faster than NaCl particles at similar particle size range. Due
to faster dissolution some powder clusters were also
liberated from the sintered samples.
When NaCl was used more regular samples were
obtained after sintering and dissolution process (Figure 4).
When the particle size of the NaCl increased the pore sizes
increased.
(a)
(b)
Figure 2. Shape difference of two different dissolving
agent (a: NaCl, b: Na2CO3)
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Effect of Dissolving Agent Shape for The Microstructural Tailoring of Sdp Processed Aluminum Foams
Figure 4. State of NaCl containing samples after dissolution
step.
Figure 2. State of Na2CO3 containing samples after
dissolution step.
Secondly, long tabular shape of the Na2CO3 is difficult to be
isolated by aluminum metal phase due to its shape. This may
allow better permeability of water during dissolution process
and therefore increased dissolution yields were measured.
There is a strong relation between the particle size and the
dissolution yields. When the particle size of the salt is
decreased the dissolution yields increased. Lowest
dissolution yield were measured in B2 series where coarse
grain size were used (Figure 3).
Table 3. Solubility of different dissolving agent at different
temperature.
Salt type
NaCl
Na2CO3
Solubility in water (gr/100 ml)
25 oC
100 oC
35,9
38,7
21,6
45
When the two salt type used together with equal weight
ratios, optimized dissolution yield were obtained. When
NaCl and Na2CO3 were used together, the shape and
dissolution problems optimized when compared to using
Na2CO3 alone (Figure 5).
Figure 3. Dissolution yield of different sample series.
In a given standard dissolution time, the dissolution yield
decrease with increase in the salt content. It can be easily
observed in A series sample. When the dissolution
behaviour of two different salt is compared, it can be easily
seen that dissolution yield of Na2CO3 used samples were
higher than NaCl containing samples.
Two different factor are effective for such a behaviour.
First the solubility of two salt in water are too different than
each other. Water solubility‘s of the two salt at room and
boiling temperature is given in Table 3. When the
dissolution temperature increased the solubility of Na2CO3
is become higher than the solubility of NaCl.
Figure 5. State of Na2CO3+ NaCl containing samples after
dissolution step
When the Na2CO3 was used alone in the samples thicker cell
walls were measured as compared to NaCl used samples.
However, texture was observed in Na2CO3 used samples
(Figure 6).
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A. Yavuz, İ. Yavuz, M. S. Başpınar, H. Bayrakçeken
[12] Wen CE, Mabuchi M, Yamada Y, Shimojima K, Chino Y, Hosokawa
H, et al. Processing of fine-grained aluminum foam by spark plasma
sintering. J Mater Sci Lett 2003;22:1407–9.
[13] Zhao YY, Fung T, Zhang LP, Zhang FL. Lost carbonate sintering
process for manufacturing metal foams. Scripta Mater 2005;52:295–8.
Figure 6. Thicker cell wall and texture in Na2CO3 containing
samples.
It was concluded from the study that, long tabular shaped
Na2CO3 usage alone in production of metallic foams by SDP
process is not useful. Such a production approach result in
problems during mixing, pressing and dissolution stage.
However, mixing of Na2CO3 with NaCl in certain amount solve
novel problems in SDP process. Its addition improves the
dissolution yields and speed up the process. Two important way
is seem to be possible to enhance the process. One is higher
solubility of Na2CO3 and second is change in the
interconnectivity of the pore system due to the shape of Na2CO3
which may cause increase penetration of water during
dissolution stage.
ACKNOWLEDGEMENTS
Authors wish to thank to Afyon Kocatepe University
Research Funding for the financial support on the base of
Project no 09.TEF.06.
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