# Morphological Investigation of Foamed Aluminum Parts Produced by Melt Gas Injection

Authors: R. Surace; L. A. C. De Filippis; E. Niini; A. D. Ludovico; J. Orkas
Journal: Advances in Materials Science and Engineering
 (2009)

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## Abstract

Porous metal materials are a new class of materials with low densities, large specific surface, and novel physical and mechanical properties. Their applications are extremely varied: for light weight structural components, for filters and electrodes, and for shock or sound absorbing products. Recently, interesting foaming technology developments have proposed metallic foams as a valid commercial chance; foam manufacturing techniques include solid, liquid, or vapor state methods. The foams presented in this study are produced by Melt Gas Injection (MGI) process starting from melt aluminum. The aim of this investigation is to obtain complex foamed aluminum parts in order to make the MGI more flexible. This new method, called MGI-mould process, makes possible to produce 3D-shaped parts with complicated shape or configuration using some moulds obtained by traditional investment casting process.


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## Body

## 1. Introduction

The  Cymat/Alcan  and Norsk Hydro
melt-foaming process, called Melt Gas Injection (MGI), is a continuous, gas
injection method developed simultaneously and independently by Alcan [1] and
Norsk Hydro [2] in the late 1990s. The starting material, usually a metal
matrix composite, is molten with conventional foundry equipment and transferred
to a tundish where gas is injected via a nozzle incorporated into a rotating
impeller, thus forming a dispersion of small bubbles. The reinforcing particles
are typically SiC or Al2O3, and their volume fraction ranges
from 10% to 20% and the mean size from 5 to 20 μm. The ceramic particles trap
gas bubbles owing to the favorable interface energy and serve as stabilizer of
the cell walls and delay their coalescence. They also reduce the velocity of
the rising bubbles by increasing the viscosity of the melt [3–5]. The bubbles
size can be controlled by adjusting the gas flow rate, the propeller design
(number of nozzles and their size), and its spin speed. Usually, the resulting
foam is carried away by means of conveyor belt where it solidifies and cools. Figure
1(a) shows
a sketch of the process [2].Apparatus for MGI [2–6].
(a)(b)The foam, obtained by MGI, usually presents
a gradient in density and pores elongation as a natural consequence of the gravitationally
induced drainage and the shearing forces of the conveyor belt that lead to
distorted cells in the final product. This obviously has a pronounced effect on
the mechanical properties which become anisotropic [6]. The situation could be
improved by pulling off the foam vertically [7]. Moreover, obtaining shaped
parts by this process is very difficult for the ceramic particles that make difficult
the cutting. Attempts for making shaped parts have  been undertaken by casting
the semiliquid foam into moulds or by shaping the emerging foam with rolls,
thus trying to eliminate this disadvantage. The first reference technique for
producing shaped parts is from Kenny and Thomas [8]; Figure 1(b) shows a
sectional view of the metal foam generating box and the mould for forming
shaped parts. The freshly formed stabilized liquid foam is drawn above a
preheated mould mounted on a support. A platen moved downwardly, pushing the
foam into the mould to form a shaped product with a densified flange area. Kleinheyer
and Bilz [9] proposed the use of a mould on the top of aluminum melt to
collect foam and of a suction device. Nichol [10] patented a metal foam casting
apparatus to obtain formed articles with a ladle, within the heated chamber,
for drawing a foam sample and for transporting to a mould.The aim of this investigation is to
obtain complex foamed aluminum parts in order to make the MGI more flexible.
This new method, called MGI-mould process, makes possible to produce 3D-shaped
parts with complicated shape or configuration. These parts are expected to be
utilized as filling material and for encasing in components without machining. In
the designed equipment, the generated bubbles float to the surface as in the
traditional MGI where they are guided directly into a ceramic mould put above
the furnace crucible. The foaming propeller is inserted through the mould in a
suitable place. The moulds are prepared by traditional investment casting
process, also known as the lost wax process [11], and different shapes are
realized functionally to study foaming in complex parts. After foaming, the
ceramic shell material is mechanically removed from foam and sometimes by
high-pressure water blasting. Figure 2 shows the work flow of MGI-mould process where,
differently from literature previously cited, the mould is placed directly on
the crucible with a fair simplification of equipments.Figure 2
Work flow of MGI-mould process.Foam parts (straight rectangular and
cylindrical bars) show a closed surface skin with a thickness comparable to the
cell-wall thickness (Figure3), and this is a prerequisite for encasing especially
by casting.  Korner et al. [12] reported
that two production routes result in a solid surface skin: the powder compact
foaming, like Alulight, and the Formgrip process. Both processes comprise baking
of a foamable precursor material in a mould. They differ in the way the
precursor material is produced. Instead, in this investigation, aluminum foam-shaped parts with dense skin are realized by MGI-mould process, and they could
be  very suitable for encasing and for filling components.Figure 3
Shaped foam part with dense skin (19 × 19 × 80 mm).


## 2. Experimental Setup

Several parameters influence the
quality of the final foam product:(i)
matrix alloy composition, particles (composition, shape, size, volume
fraction);(ii)
particle-surface interactions;(iii)
gas (composition and purity, flow rate, pressure);(iv)
foaming temperature;(v)
thermal processing conditions (holding time, cooling medium);(vi)
crucible (size and shape);(vii)
propeller (size and shape, direction of rotation, size of the panel,
spin speed);(viii)
inclination of the system during foaming;(ix)
nozzle design.
Some of them were set following previous
results of authors reported elsewhere [13] where a description of equipment was
also given.Duralcan
metal matrix composite (A356/SiC/20p), with a particle size of approximately 12μm, was used; flow rate was fixed at 4 L/min and temperature in the range 670–700°C. In order to measure
the relative stability of the Al foam in the liquid state, two different
foaming gases (nitrogen and air) were performed. During experimental tests, a
suction device has also been used. By the creation of a region of low pressure (0.8 bar), the gradient
between this region and the ambient will propel matter toward the low-pressure
area filling the moulds as desired. Thus, parts were manufactured at ambient pressure
or in suction condition (suitable especially for high parts). As explained, the
main difference with previous authors experimentation and literature reports is
the use of ceramic moulds to collect the foam and to obtain shaped parts. The
starting point of mould manufacturing is the production of a wax pattern, engineered
as an inverted “mushroom” with a variable upper part to shape foam and with a
large lower part to cover the crucible opening. The ceramic coating is built up
through successive stages of dipping and stuccoing to build up a shell of a
thickness sufficient to provide the strength required to process the mould successfully.
The primary dip is usually different to the secondary or backing dip, and the
grade of the used stucco particles may be coarser as the shell thickness
increases. After drying, dewaxing, and sintering, the moulds were prepared for
foaming by opening the upper part to ambient pressure and in other cases by
fixing the connection for the suction device.Following this procedure, three different
geometries were realized: Simple Mould (SM), Mould with Extensions (ME), and Bars
Mould (BM) showed in Figure4. Table 1 reports the mould geometrical dimensions.Table 1
Mould geometrical dimensions.
Mould typeShapeDimension [mm]Height [mm]SMCylindrical∅ 64100MEParallelepiped19 × 198032 × 3280BMCylindrical∅ 24220∅ 28220∅ 33220SM (a), ME (b) and BM (c).
(a)(b)(c)


## 3. Morphological Characterization

In
this section some fundamental investigations are described to clarify the
foamed products quality and to optimize the process parameters. The obtained samples were cut
with metallographic saw to minimize cell damage because, owing to the
high content of ceramic particles, machining of MMC foams is sometimes a
problem. The morphological
parameters were measured in a 2D cross section by an image software tool, and cell walls were observed by a Scanning Electron Microscope
Philips XL 20. The measured parameters are(i)
mould filling ability;(ii)
relative density;(iii)
relative density gradient;(iv)
cells area;(v)
cells shape.


### 3.1. Mould Filling Ability

The mould filling ability was
calculated by the ratio of final foam product height and the initial height of
the moulds (Table1) obtaining the following results (showed in Table 2).Table 2
Mould filling ability.
Mould typePressureGasDimension
(mm)Sample
height (mm)FillingSMAmbientNitrogen∅ 6473.0173.0%SMAmbientAir∅ 6486.7686.8%SMSuctionNitrogen∅ 6462.9562.9%MEAmbientNitrogenL 1966.5083.1%L 3261.3176.6%MEAmbientAirL 3255.2169.0%MESuctionNitrogenL 1972.6590.8%L 3272.7991.0%BMSuctionNitrogen∅ 24161.073.2%∅ 28122.855.8%∅ 32199.590.7%BMSuctionAir∅ 24111.050.4%∅ 2877.5535.2%∅ 32212.096.4%(i)
Simple moulds (SM) are relatively simple to fill with suction but at ambient pressure. The best result is an 86% filling, and it is due to the large
diameter that favors the foam rising.(ii)
On the contrary, the filling of bars mould (BM) at ambient pressure is
not possible for the narrow diameter of the bars, and no results have been
obtained. Differently in suction condition, good results are possible (best
filling 96.4%).(iii)
The foam shows good filling ability in mould with extensions (ME) having
a parallelepiped shape. As expected in suction condition, the filling is better
(about 90%) than at ambient pressure (about 70%).(iv)
The type of injected gas do not show a clear influence on filling ability.


### 3.2. Relative Density

The density of the cellular metal
depends on the volume fraction of metal and gas. Consequently, cellular metals
are characterized by the density compared to parent metal: the relative
density. The used density of parent
material (AlSi7 + 20 SiC) is 2.779 g/cm3. The density for each specimen was determined from the sample weights
and physical dimensions.The average relative density is 0.201
with standard deviation of 0.0367 (Table3). The values are comprised in the
ranges reported from literature. Referring to the data, the factors influence is plotted using a
statistical software (Minitab) evidencing the effects on relative density. The Main
Effect Plot (Figure 5) can help the researcher to determine which factors affect
the process from the line inclination of results with respect to the mean line; a
higher deviation from the mean horizontal line represents a higher influence
of the factor on the process results.Table 3
Process parameters and relative density.
Mould typePressureGasRelative densitySMAmbientNitrogen0.141SMAmbientAir0.179SMSuctionNitrogen0.142MEAmbientNitrogen0.224MEAmbientAir0.227MESuctionNitrogen0.229BMSuctionNitrogen0.249BMSuctionAir0.218Figure 5
Main effect plot for relative density.It is possible to consider that(i)
the type of injected gas is not crucial for relative density;(ii)
the pressure used during the test is not an influential factor on the
relative density;(iii)
the mould shape reveals the most influencing factor. In fact the
relative density is affected by the flow of the material in the shape; for
large circular shape there is low resistance and high foam flow rate (low
relative density for large circular shapes).


### 3.3. Relative Density Gradient

During solidification, molten metal
drains through the cell membranes due to gravity. As a result of drainage, there
is a thick layer of metal at the bottom, and the foam part shows a density
gradient through the thickness. To avoid this defect, it is possible to set
temperature as low as possible, so keeping viscosity high, or by adding
viscosity enhancing substances. This problem was present during experimentation
due to the height of the mould. Thus image analysis of the material was
performed to document the extent of the density gradient existing in the top,
middle, and bottom sectioned specimens. Figure6 shows an uneven density
distribution within the specimen cross section, and two regions of different
cellular structures are marked as example: the upper with elongated pores and
the lower with big pores. Each of them can be regarded as a different cell
architecture that can define a sub-domain. This problem exists strictly for
casting in a similar way, where material properties depend on the
solidification process (especially from the solidification rate) and vary
across the thickness of the part. Thus, for example, the rate of cooling and
thus solidification of particular molten metals can affect the microstructure
of the finished metal structure. The inhomogeneities of cellular metal have to
be quantified in their range of variation and with respect to their location
within the samples.Cross section of an irregular cellular structure (SM, air, ambient).
(a)(b)Cylindrical samples, obtained by
simple mould, have been sectioned in three slices, each of thickness of about 20 mm. A density gradient
diagram of one sample is reported in Figure7. Only one sample shows an anomalous
behavior with an inverted density gradient, because the lower slice is completely
empty, and thus it is lighter than the upper part. The samples obtained by bars
mould have been sectioned in 4 layers, each of thickness of 20 mm (Figure 8). Only one
sample shows an anomaly because the relative density figure (average value of
0.4) is higher than the average value of the other samples (usually 0.2), and
this problem is probably due to a casual error in the test execution. The sample
presents a little number of bubbles inside even if the external part seems
normal, and it is showed in Figure 9.Relative density gradient for the sample
SM, N2, ambient sectioned in 3 slices.
(a)(b)Relative density gradient for the sample
BM, air, suction sectioned in four slices.
(a)(b)Figure 9
Sample BM, (relative density
0.4).Excluding from the following
consideration the strange cases previously presented, the course about the
density gradient is as expected. Nevertheless, it should be noted that a
uniform structure is not always necessary but it depends on the application. For
instance, natural load bearing structures, as bones, are not also uniform and
isotropic, because they have an optimum distribution of the cell material
according to the loading requirements. Therefore, the challenge is not always
to get uniform structures but to achieve the reproducible properties with a
predetermined nonuniform structure [14]. Figure 10 reports the relative density
gradient data showing variation in thickness direction of foam slab. More than
80% of the samples are in the range of relative density 0.1–0.3, and only a few
of them overcome this range.Figure 10
Relative density gradient data.Obtained cellular metal parts are
usually surrounded by a metal skin. The oxide content is much higher on
interior cell surfaces, because that skin was exposed directly to the
atmosphere in the liquid state and during solidification and cooling. This skin
region can be deduced from Figure11 where the local density is different from
other parts.Figure 11
Cross section of sample showing skin
region (SM, nitrogen, ambient).


### 3.4. Cells Area and Shape

The significant structural features of
foam parts are the shape and size of pores. But even if the cell size is
uniform and the pores are arranged regularly, this information is not
sufficient to describe the cellular metal. Also pores can take preferred
orientations. By visual inspection, all samples were evaluated, and the pores
size was measured by software tool.Relative to the bubbles shape, the
presence of irregular structures was observed depending on the dimension of
the cells apparently positioned in preferential zones. The cells spherical and
of medium dimensions are localized in proximity of the base of specimen. Cells
of polyhedral shape and showing bigger areas are in proximity of the top and in
some case in the middle section, and that is due to the coalescence effects.Figures12(a)-12(b) and 12(c)-12(d) show two
samples realized with simple mould and using nitrogen as foaming gas. The
samples to the left are realized at ambient pressure: (a) the upper and (b) the lower
slice. The sample to the right are realized with suction, and (c) and (d) are the
upper and lower slice. The foam specimens, realized at ambient pressure,
present more regular pores distribution and slightly small bubbles.Figure 12
Different pore shape using different pressure
and at different thickness position: (a) upper and (b) lower slice at ambient
pressure; (c) upper and (d) lower slice with suction (∅ 64).The pure air, as foaming gas, led to
more frequent corrugated bubbles, contrary to nitrogen foamed material (Figure13),
differently from other investigation results [15]. Corrugated cell walls are
typically found in metal foams which have rather thin cell walls compared to
the cell size. Curved or corrugated cell walls occur during forming or on
solidification. The face corrugations observed in the cell walls are possibly
caused either by shrinkage of the wall solid during solidification or by the
partial collapse of the cell walls under an applied  force (i.e., manipulation
during production or the weight of the foam itself). In each case, both the
axial stiffness and the flexural rigidity of the curved or corrugated structural
member are reduced [16]. Literature reports that the curvature and corrugations
may account for up to a 70% drop in the modulus, and strength of foams below the
values is estimated for planar cell walls [17]. The face wrinkling can be avoided
by reducing the stress applied to the foam immediately after solidification and
by reducing the rate of cooling [18].Corrugated cell walls due to different
foaming gas (BM, suction): (a) nitrogen and (b) air.
(a)(b)It was also observed that the samples
produced by air show more elongated bubbles than the sample foamed by blowing
nitrogen that are circular (Figure14). Moreover nitrogen leads to bubbles with
dark inner wall compared with that of bubbles obtained by air. An investigation
by SEM of inner surface of the foam bubbles manufactured with nitrogen revealed
that the SiC particles are relatively dense on the surface (Figure 15). The SiC
particle concentration in the cell walls of the foam bubbles is higher in
samples foamed by nitrogen than in those by air. Moreover, no surface layer covering the
SiC particles is formed by nitrogen. Previous analysis of the concentration
depth profiles of cell walls verifies the increase of the thickness of the
oxide layer with increasing oxygen concentration in the blowing gas. The oxide
layer increases bubble stability more than the higher particle concentration in
the cell walls of foam samples produced with nitrogen [19].Different foaming gas (SM, ambient,∅ 64): (a) nitrogen and (b) air.
(a)(b)Figure 15
SEM image of SiC particles at the
surface of N2 foamed wall bubble (SM, nitrogen, ambient).Measurements of the pores size for
samples were realised by a software image tool, implemented by authors in a commercial
software, that can define the pore boundaries by means of pixel contrast. Each sample picture was imported in the
software, and the cell geometry was reconstructed (Figure16). The results obtained
from measurements are reported in Figures 17 and 18. In Figure 17, each
value of the abscissa represents the upper limit of measured area range
(inferior limit of the range is the previous abscissa value), and the ordinate
represents the number of cells included in this range.Geometrical structure of
foam (l = 32 mm).
(a)(b)Figure 17
Pore size distribution (SM,
nitrogen).Area, perimeter, and
wall thickness of cells versus process parameters.
(a)(b)(c)Observing all the results, it is
possible to consider that(i)
the cells, whose area is lower than the average total value of 10 mm2,
are 75% of the total cells number, while 23% oscillate up to the limit of 24 mm2;
the remaining 2% of cells present area wider than 30 mm2;(ii)
50% of cell areas result in the range of 3.1–28.3 mm2 comparable
to literature values [20];(iii)
the variability of the perimeters is wide, and this demonstrates a
certain irregularity of cells shape. Approximately 54% of the measured perimeters
are lower than the average value of 11.75 mm;(iv)
relatively to the cell walls, it is observed that the frequent thicknesses
are in the range 0.1–0.6 mm.
90% of cell wall thickness results in the range of 0.48–1.44 mm as presented in
literature [20].
The cells sphericity evaluation has
been carried out by calculating the circularity parameter [21]. Estimating
the results emerges that values over 0.9 are caught up using nitrogen with suction.
The obtained figures are high,
and this result implies that there are many cells with regular shapes and
contours (Table 4).Table 4
Circularity
parameter.
Mould typePressureGasC rangeC average valueSMAmbientNitrogen0.33–0.950.799SMSuctionNitrogen0.43–0.940.912MEAmbientAir0.28–0.920.663MESuctionNitrogen0.57–0.960.947BMSuctionNitrogen0.52–0.930.903BMSuctionAir0.57–0.950.862


## 3.1. Mould Filling Ability

The mould filling ability was
calculated by the ratio of final foam product height and the initial height of
the moulds (Table1) obtaining the following results (showed in Table 2).Table 2
Mould filling ability.
Mould typePressureGasDimension
(mm)Sample
height (mm)FillingSMAmbientNitrogen∅ 6473.0173.0%SMAmbientAir∅ 6486.7686.8%SMSuctionNitrogen∅ 6462.9562.9%MEAmbientNitrogenL 1966.5083.1%L 3261.3176.6%MEAmbientAirL 3255.2169.0%MESuctionNitrogenL 1972.6590.8%L 3272.7991.0%BMSuctionNitrogen∅ 24161.073.2%∅ 28122.855.8%∅ 32199.590.7%BMSuctionAir∅ 24111.050.4%∅ 2877.5535.2%∅ 32212.096.4%(i)
Simple moulds (SM) are relatively simple to fill with suction but at ambient pressure. The best result is an 86% filling, and it is due to the large
diameter that favors the foam rising.(ii)
On the contrary, the filling of bars mould (BM) at ambient pressure is
not possible for the narrow diameter of the bars, and no results have been
obtained. Differently in suction condition, good results are possible (best
filling 96.4%).(iii)
The foam shows good filling ability in mould with extensions (ME) having
a parallelepiped shape. As expected in suction condition, the filling is better
(about 90%) than at ambient pressure (about 70%).(iv)
The type of injected gas do not show a clear influence on filling ability.


## 3.2. Relative Density

The density of the cellular metal
depends on the volume fraction of metal and gas. Consequently, cellular metals
are characterized by the density compared to parent metal: the relative
density. The used density of parent
material (AlSi7 + 20 SiC) is 2.779 g/cm3. The density for each specimen was determined from the sample weights
and physical dimensions.The average relative density is 0.201
with standard deviation of 0.0367 (Table3). The values are comprised in the
ranges reported from literature. Referring to the data, the factors influence is plotted using a
statistical software (Minitab) evidencing the effects on relative density. The Main
Effect Plot (Figure 5) can help the researcher to determine which factors affect
the process from the line inclination of results with respect to the mean line; a
higher deviation from the mean horizontal line represents a higher influence
of the factor on the process results.Table 3
Process parameters and relative density.
Mould typePressureGasRelative densitySMAmbientNitrogen0.141SMAmbientAir0.179SMSuctionNitrogen0.142MEAmbientNitrogen0.224MEAmbientAir0.227MESuctionNitrogen0.229BMSuctionNitrogen0.249BMSuctionAir0.218Figure 5
Main effect plot for relative density.It is possible to consider that(i)
the type of injected gas is not crucial for relative density;(ii)
the pressure used during the test is not an influential factor on the
relative density;(iii)
the mould shape reveals the most influencing factor. In fact the
relative density is affected by the flow of the material in the shape; for
large circular shape there is low resistance and high foam flow rate (low
relative density for large circular shapes).


## 3.3. Relative Density Gradient

During solidification, molten metal
drains through the cell membranes due to gravity. As a result of drainage, there
is a thick layer of metal at the bottom, and the foam part shows a density
gradient through the thickness. To avoid this defect, it is possible to set
temperature as low as possible, so keeping viscosity high, or by adding
viscosity enhancing substances. This problem was present during experimentation
due to the height of the mould. Thus image analysis of the material was
performed to document the extent of the density gradient existing in the top,
middle, and bottom sectioned specimens. Figure6 shows an uneven density
distribution within the specimen cross section, and two regions of different
cellular structures are marked as example: the upper with elongated pores and
the lower with big pores. Each of them can be regarded as a different cell
architecture that can define a sub-domain. This problem exists strictly for
casting in a similar way, where material properties depend on the
solidification process (especially from the solidification rate) and vary
across the thickness of the part. Thus, for example, the rate of cooling and
thus solidification of particular molten metals can affect the microstructure
of the finished metal structure. The inhomogeneities of cellular metal have to
be quantified in their range of variation and with respect to their location
within the samples.Cross section of an irregular cellular structure (SM, air, ambient).
(a)(b)Cylindrical samples, obtained by
simple mould, have been sectioned in three slices, each of thickness of about 20 mm. A density gradient
diagram of one sample is reported in Figure7. Only one sample shows an anomalous
behavior with an inverted density gradient, because the lower slice is completely
empty, and thus it is lighter than the upper part. The samples obtained by bars
mould have been sectioned in 4 layers, each of thickness of 20 mm (Figure 8). Only one
sample shows an anomaly because the relative density figure (average value of
0.4) is higher than the average value of the other samples (usually 0.2), and
this problem is probably due to a casual error in the test execution. The sample
presents a little number of bubbles inside even if the external part seems
normal, and it is showed in Figure 9.Relative density gradient for the sample
SM, N2, ambient sectioned in 3 slices.
(a)(b)Relative density gradient for the sample
BM, air, suction sectioned in four slices.
(a)(b)Figure 9
Sample BM, (relative density
0.4).Excluding from the following
consideration the strange cases previously presented, the course about the
density gradient is as expected. Nevertheless, it should be noted that a
uniform structure is not always necessary but it depends on the application. For
instance, natural load bearing structures, as bones, are not also uniform and
isotropic, because they have an optimum distribution of the cell material
according to the loading requirements. Therefore, the challenge is not always
to get uniform structures but to achieve the reproducible properties with a
predetermined nonuniform structure [14]. Figure 10 reports the relative density
gradient data showing variation in thickness direction of foam slab. More than
80% of the samples are in the range of relative density 0.1–0.3, and only a few
of them overcome this range.Figure 10
Relative density gradient data.Obtained cellular metal parts are
usually surrounded by a metal skin. The oxide content is much higher on
interior cell surfaces, because that skin was exposed directly to the
atmosphere in the liquid state and during solidification and cooling. This skin
region can be deduced from Figure11 where the local density is different from
other parts.Figure 11
Cross section of sample showing skin
region (SM, nitrogen, ambient).


## 3.4. Cells Area and Shape

The significant structural features of
foam parts are the shape and size of pores. But even if the cell size is
uniform and the pores are arranged regularly, this information is not
sufficient to describe the cellular metal. Also pores can take preferred
orientations. By visual inspection, all samples were evaluated, and the pores
size was measured by software tool.Relative to the bubbles shape, the
presence of irregular structures was observed depending on the dimension of
the cells apparently positioned in preferential zones. The cells spherical and
of medium dimensions are localized in proximity of the base of specimen. Cells
of polyhedral shape and showing bigger areas are in proximity of the top and in
some case in the middle section, and that is due to the coalescence effects.Figures12(a)-12(b) and 12(c)-12(d) show two
samples realized with simple mould and using nitrogen as foaming gas. The
samples to the left are realized at ambient pressure: (a) the upper and (b) the lower
slice. The sample to the right are realized with suction, and (c) and (d) are the
upper and lower slice. The foam specimens, realized at ambient pressure,
present more regular pores distribution and slightly small bubbles.Figure 12
Different pore shape using different pressure
and at different thickness position: (a) upper and (b) lower slice at ambient
pressure; (c) upper and (d) lower slice with suction (∅ 64).The pure air, as foaming gas, led to
more frequent corrugated bubbles, contrary to nitrogen foamed material (Figure13),
differently from other investigation results [15]. Corrugated cell walls are
typically found in metal foams which have rather thin cell walls compared to
the cell size. Curved or corrugated cell walls occur during forming or on
solidification. The face corrugations observed in the cell walls are possibly
caused either by shrinkage of the wall solid during solidification or by the
partial collapse of the cell walls under an applied  force (i.e., manipulation
during production or the weight of the foam itself). In each case, both the
axial stiffness and the flexural rigidity of the curved or corrugated structural
member are reduced [16]. Literature reports that the curvature and corrugations
may account for up to a 70% drop in the modulus, and strength of foams below the
values is estimated for planar cell walls [17]. The face wrinkling can be avoided
by reducing the stress applied to the foam immediately after solidification and
by reducing the rate of cooling [18].Corrugated cell walls due to different
foaming gas (BM, suction): (a) nitrogen and (b) air.
(a)(b)It was also observed that the samples
produced by air show more elongated bubbles than the sample foamed by blowing
nitrogen that are circular (Figure14). Moreover nitrogen leads to bubbles with
dark inner wall compared with that of bubbles obtained by air. An investigation
by SEM of inner surface of the foam bubbles manufactured with nitrogen revealed
that the SiC particles are relatively dense on the surface (Figure 15). The SiC
particle concentration in the cell walls of the foam bubbles is higher in
samples foamed by nitrogen than in those by air. Moreover, no surface layer covering the
SiC particles is formed by nitrogen. Previous analysis of the concentration
depth profiles of cell walls verifies the increase of the thickness of the
oxide layer with increasing oxygen concentration in the blowing gas. The oxide
layer increases bubble stability more than the higher particle concentration in
the cell walls of foam samples produced with nitrogen [19].Different foaming gas (SM, ambient,∅ 64): (a) nitrogen and (b) air.
(a)(b)Figure 15
SEM image of SiC particles at the
surface of N2 foamed wall bubble (SM, nitrogen, ambient).Measurements of the pores size for
samples were realised by a software image tool, implemented by authors in a commercial
software, that can define the pore boundaries by means of pixel contrast. Each sample picture was imported in the
software, and the cell geometry was reconstructed (Figure16). The results obtained
from measurements are reported in Figures 17 and 18. In Figure 17, each
value of the abscissa represents the upper limit of measured area range
(inferior limit of the range is the previous abscissa value), and the ordinate
represents the number of cells included in this range.Geometrical structure of
foam (l = 32 mm).
(a)(b)Figure 17
Pore size distribution (SM,
nitrogen).Area, perimeter, and
wall thickness of cells versus process parameters.
(a)(b)(c)Observing all the results, it is
possible to consider that(i)
the cells, whose area is lower than the average total value of 10 mm2,
are 75% of the total cells number, while 23% oscillate up to the limit of 24 mm2;
the remaining 2% of cells present area wider than 30 mm2;(ii)
50% of cell areas result in the range of 3.1–28.3 mm2 comparable
to literature values [20];(iii)
the variability of the perimeters is wide, and this demonstrates a
certain irregularity of cells shape. Approximately 54% of the measured perimeters
are lower than the average value of 11.75 mm;(iv)
relatively to the cell walls, it is observed that the frequent thicknesses
are in the range 0.1–0.6 mm.
90% of cell wall thickness results in the range of 0.48–1.44 mm as presented in
literature [20].
The cells sphericity evaluation has
been carried out by calculating the circularity parameter [21]. Estimating
the results emerges that values over 0.9 are caught up using nitrogen with suction.
The obtained figures are high,
and this result implies that there are many cells with regular shapes and
contours (Table 4).Table 4
Circularity
parameter.
Mould typePressureGasC rangeC average valueSMAmbientNitrogen0.33–0.950.799SMSuctionNitrogen0.43–0.940.912MEAmbientAir0.28–0.920.663MESuctionNitrogen0.57–0.960.947BMSuctionNitrogen0.52–0.930.903BMSuctionAir0.57–0.950.862


## 4. Conclusions

The
present study investigated aluminum foams manufactured by the new MGI-mould
process. Several conclusions can be drawn as follows.(1)
The new MGI-mould
process was capable of producing the near net shape aluminum foam parts with
length of about 200 mm and different shapes with skin surface and good internal quality especially for the
core of automotive articles.(2)
The disadvantage of
the traditional MGI process (the brittleness due to the reinforcing particles
contained in the cell walls) could be overcome, as presented in this
investigation, with the production of near net shape foam.(3)
Large volume of foam can
be produced, and low densities can be achieved. Density gradient is often evident.
The circularity parameter can reach the value of 0.95.(4)
The results and
distribution of properties depend mainly on the mould shape and foaming
gas.



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*Source: 506024-2009-04-16.xml*