Materials Sciences and Applicatio ns, 2011, 2, 1322-1330
doi:10.4236/msa.2011.29180 Published Online September 2011 (
Copyright © 2011 SciRes. MSA
Characterization of Porosity in a Laser Sintered
MMCp Using X-Ray Synchrotron Phase
Contrast Microtomography
Emmanuelle Girardin1, Chiara Renghini2,3, Jack Dyson1, Vittorio Calbucci1, Francesca Moroncini1,
Gianni Albertini1.4
1Università Politecnica delle Marche, Dipartimento di Fisica e Ingegneria dei Materiali e del Territorio, Via Brecce Bianche, Ancona,
Italy; 2Università Politecnica delle Marche, Dipartimento SAIFET—Sezione di Scienze Fisiche—Via Brecce Bianche, Ancona, Italy;
3Istituto Nazionale di Biostrutture e Biosistemi—Consorzio Interuniversitario—Viale Medaglie d’Oro 305, Roma, Italy; 4CNISM
(Consorzio Nazionale Interuniversitario per le Scienze fisiche della Materia), Italy.
Received April 14th, 2011; revised May 16th, 2011; accepted June 3rd, 2011.
Direct Laser Sintering (DSL), a technology enab ling the production of dense metal components directly from 3D CAD
data, was used for the first time to produce a Metal Matrix Composite (MMCp) based on Al-Si-Cu alloy in view of its
application in different fields, in particular for aeronautics. The porosity of the material obtained so was investigated
by using optical and electron microscopy and, in particular, X-ray computed microtomography techniques. DSL is a
unique technique to produce complex components in an economical way while computed microtomography is a unique
technique to evaluate the porosity and pore and cracks distribution in a not destructive way. A near homogeneous dis-
tribution of the porosity and pore sizes was observed both comparing different regions of the same specimen and also
by comparing different samples obtained by using the same DLS production method. A quantitative analysis of the
damage in the composite is also reported.
Keywords: Metal Matrix Composite, Direct Laser Sintering, X- Ray Synchrotron, Computed Microtomography, Porosity
1. Introduction
Direct Laser Sintering (DLS) is a technology enabling to
produce dense metal components, directly from 3D CAD
data. The quality of the product is comparable to that
from a good investment casting, while the mechanical
properties are comparable to those of a cast or machined
part [1].
The application field of DLS is very wide as the so-ob-
tained components can be used in place of almost any
conventionally manufactured part, either machined or cast.
In addition, the greater component complexity and feature
richness, the greater the economy in production. Current
applications concern subjects so different as Formula 1,
Aerospace, Medical devices and Tooling. In particular,
DLS technology can be used in Formula 1 and Aerospace
industry not only to produce complex and structurally
challenging parts made of various materials, but also it can
offer lead times unachievable with other methods.
DLS enables the production of very strong but light-
weight components, also containing hollows or intelli-
gent internal structures. DLS is also unique to combine
different parts into a single one, thus obtaining complex
components, making them lighter and potentially im-
proving their functionality and strength.
The materials submitted to DLS range from steels to
non-ferrous materials and titanium [1-7]. Some applica-
tions are also reported for composites, although, at our
knowledge, only few cases concern Metal Matrix Com-
posites (MMC). Some examples can be found on WC-
Co/Cu composites [8,9], iron-SiC [10], (Al, Si, Mg)-SiC
[11], Cu based alloy reinforced with Ni particles [12],
(Fe,Ni)-TiC [13].
With the recent development of new three-dimensional
(3D) characterization tools, a clear, accurate, and quanti-
tative analysis of MMCs can be obtained. Several tech-
niques have been used for visualization of microstruc-
tures in 3D. Serial sectioning techniques using mechani-
cal polishing coupled with optical microscopy [14,15] or
Characterization of Porosity in a Laser Sintered MMCp Using X-Ray Synchrotron Phase Contrast Microtomography1323
focused ion beam (FIB) milling [16-18] and image re-
construction have been used.
X-ray microtomography (microCT) is an excellent
technique that eliminates destructive cross-sectioning,
and allows for superior resolution and image quality with
minimal sample preparation [19,20].
MicroCT is a 3D radiographic imaging technique,
similar to conventional CT tomography systems used in
medical and industrial applications. Unlike such systems,
which typically have a maximum spatial resolution of
about 1 mm, micro-CT is capable of achieving a spatial
resolution close to 1 micron. In both conventional tomo-
graphy and microtomography, hundreds of 2D projection
radiographs are taken of a specimen at many different
angles. The information contained in a radiograph is a
projection of the absorption density inside the sample
onto a plane perpendicular to the direction of the X-ray
beam. If the sample is then imaged several times in dif-
ferent orientations, a 3D (volume) information on the
sample structure can be obtained using computer algo-
rithms. This process, referred to as image reconstruction,
allows slices of the investigated object to be observed
without physically cutting it. However, the maximum
power of an X-ray laboratory source is limited, with a
consequent upper limit to the available X-ray flux.
The micro-CT using X-ray synchrotron radiation ex-
ploits the same idea as conventional computer tomogra-
phy, but several advantages come from the use of syn-
chrotron radiation. In particular, synchrotron radiation
offers the possibility of selecting X-rays with a small en-
ergy bandwidth from a wide and continuous energy spec-
trum and, at the same time, it guarantees a high enough
photon flux for efficient imaging [21-23]. Moreover, the
use of synchrotron radiation allows for tuning the selected
photon energy, in order to optimize the contrast of the dif-
ferent phases in the investigated samples. This possibility
is of great interest for micro-CT since it allows high spatial
resolution images (from 10 microns to 1 micron) to be
generated, with high signal-to-noise ratio [24-26].
3D visualization and quantification of heterogeneous
microstructures by X-ray tomography has been success-
fully performed in Sn-rich alloys [27], powder metal-
lurgy steels [28], metal matrix composites [29-32], and
aluminum and copper alloys [33,34]. In addition to visu-
alization, such microstructural data sets can be incorpo-
rated into finite element models to predict the onset of
local damage mechanisms and the macroscopic deforma-
tion behavior [32,35-37]. A sound understanding of
damage in MMCs requires adequate visualization and
quantification of fracture based on the 3D microstructure.
More importantly, it requires a significant amount of
statistical characterization and analysis, particularly of
particle fracture and void growth. The distribution of
defects and voids, before and after deformation, needs to
be quantified.
Aim of the present work is to determine if DLS tech-
nique is suitable to produce an MMC door stop fitting for
aeronautical application, in order to replace the up-to-
now used forged titanium. Good mechanical properties
and a good corrosion resistance are required for such
applications. The microstructure and, in particular, the
porosity play a fundamental role for those aims [38-41].
It is evident that porosity has a very strong effect on
the properties of the composites. In particular, Young’s
modulus, exural strength and thermal conductivity in-
crease with decreasing porosity [38,39]. Porosity appears
to be detrimental for crack propagation.
Our attention was thus focused mainly on porosity: a
volumetric imaging technique to investigate the samples
was chosen and a quantitative analysis of the porosity has
been carried out by using the VGStudio MAX Software
The considered alloy is Al6061-7Si-5.6Cu2O. Its high
density is obtained after an exothermal reaction between
Cu oxide and Al particles, a suitable microstructure and a
good strengthening are also obtained.
2. Materials and Methods
2.1. Samples
The characteristics of the materials used in the sample
preparation [11] are summarized in Table 1.
The Al6061-7Si-5.6Cu2O alloy was prepared by
blending aluminium and silicon powders in the proper
mass fraction in a 2F Turbula shaker/mixer (Basel, Swit-
zerland). DLS was performed in a commercial M250Xtend
machine (Electro Optical Systems GmbH, Munich, Ger-
many) under an argon atmosphere. A CO2 laser intensity
of 8.6 W·mm2, a layer thickness of 0.1 mm and a hatch
spacing of 0.3 mm were used.
Table 1. Characteristics of powders.
Material Mean particle size (m) Particle shape Supplier
Al 30 spherical TLS Tecnik GmbH & Co, Bitterfeld
Si 7 Irregular Saint Gobain, Duisburg
Cu2O 4 Irregular TLS Tecnik GmbH & Co, Bitterfeld
Copyright © 2011 SciRes. MSA
Characterization of Porosity in a Laser Sintered MMCp Using X-Ray Synchrotron Phase Contrast Microtomography
Tensile specimens of 50mm length and 3mm thickness
were prepared (Figure 1). They are referred to as IFAM1,
IFAM2, ···. The density of sintered materials was deter-
mined according to ISO Standard 5017.
The microstructure has been studied by optical and
scanning electron microscopy (FESEM, Zeiss, Germany).
For those observations, the sintered samples were sec-
tioned along the thickness and perpendicular to the scan-
ning line.
The phase analysis was performed by X-ray diffraction
(XRD, Siemens, Germany) using Cu K
Figure 2 shows two examples of surface morphology.
Pores and metal agglomerates are visible, the agglomer-
ates being larger than 100 m in size.
Figure 3 shows an example of XRD pattern obtained
from Al-7Si-5.6Cu2O together with the corresponding
phase recognition.
2.2. X-Ray Computed Microtomography
MicroCT experiments were performed at ELETTRA
(Trieste-Italy) on beamline SYRMEP (Figure 4) with a
monochromatic beam energy of 28 keV and a sam-
ple-to-detector distance of 5 cm. A two-dimensional (2D)
detector recorded projections of the sample at different
angular positions. In the present study 900 projections
were considered within an angular range of 180°. Refer-
ence images without sample have been recorded in order
to eliminate the effects of a non-homogeneous intensity
or spectral composition of the X-ray beam. The exposure
time was 18 seconds per projection. Images were re-
corded on a 2048 × 2024 CCD detector with the pixel
size set to 4.5 μm. The 3D structure was finally recon-
structed from 900 projections using an algorithm imple-
mented at ELETTRA. A volume of interest was recon-
structed for each sample. Each voxel of the reconstructed
image was cubic with a 9 μm size. The reconstructed
values of the linear attenuation coefficient for the em-
ployed X-ray energy ranged between 0 and 9 cm1 and
were distributed in 256 gray levels.
Volume rendering is a 3D visualisation method whe-
rein the data volume is rendered directly without de-
composing it into geometric primitives. A 2 GHz Pen-
tium with 1 Gb RAM and commercial software VGStu-
dio MAX 2.1 were used to generate 3D images and to
illustrate the distribution of phases in 3D. In order to
achieve optimal settings for the image quality, we used
Scatter HQ algorithm with oversampling factor of 5.0
and activated color rendering.
2.3. Extraction of Quantitative Parameters
Quantitative analysis of the 3D architecture can be ob-
tained, based on the structural indices usually considered
for bone samples [44]. A material volume (MV) corre-
Figure 1. Direct Metal Laser Sintered tensile samples of
Al6061 + 7%Si + 5.6%Cu2O.
Figure 2. Surface morphologies of the laser-sintered specimens.
Copyright © 2011 SciRes. MSA
Characterization of Porosity in a Laser Sintered MMCp Using X-Ray Synchrotron Phase Contrast Microtomography1325
Figure 3. XRD pattern of the Al-7Si-5.6Cu2O.
Figure 4. SYRMEP experimental set-up [43].
sponds to the number of voxels with an absorption coef-
ficient in the range defined for a given material. The total
volume (TV) is related to the number of voxels corre-
sponding to all the materials in the data set in question.
The ratio of a material surface (MS) to the material vol-
ume (MV) is approximated using the Cauchy-Crofton
theorem from differential geometry (it is generally not
possible to calculate the material surface from polygons):
the mean number of crossings per unit length of ran-
domly chosen lines through a 3-dimensional structure
approaches half of the true ratio of surface to volume
Porosity is defined as the percentage of void space in a
solid [46] and it is a morphological property independent
of the material. In our case, the porosity depends on the
fabrication process and it is an important parameter in
determining the mechanical properties of the component.
The total porosity (P) was obtained according to the
P = 1 MV/TV
The specific surface available for pore adhesion is
given by the material surface-to-volume ratio (MS/MV).
3D images also enable the direct assessment of metric
indices of feature sizes by actually measuring distances
in the 3D space. Pore wall thickness (P.Th), and pore
separation (P.Sp) or pore diameter can be thus computed.
Copyright © 2011 SciRes. MSA
Characterization of Porosity in a Laser Sintered MMCp Using X-Ray Synchrotron Phase Contrast Microtomography
The pore wall thickness depends on MS/MV and is cal-
culated as P.Th = 2/(MS/MV). The mean number of
pores per unit length (P.N) depends on MV/TV and is
calculated as P.N = (MV/TV)/P.Th. The mean space
(P.Sp) between pores depends on P.N and P.Th. It is
calculated as P.Sp = (1/P.N) P.Th.
3. Results
3.1. Microstructural Characterisation
Three of the samples of Figure 1 were investigated by
using X-ray Computed Microtomography. An example
of the macroporous network and the microstructure of
the investigated samples is reported in Figure 5. No sig-
nificant differences existed in the internal microstructure
among these samples.
3D quantitative parameters were calculated directly
from 3D images to characterize the samples. This quanti-
fication first required segmenting the different phases to
separate them from the background. In these samples,
such segmentation was easily performed by simple thre-
sholding because the gray level histogram was clearly
bimodal with a first peak corresponding to background
and a second peak corresponding to the material of sam-
ple (Figure 6).
The main parameters obtained from those images are
summarised in Table 2.
3.2. Pore Analysis
A detailed examination of the porosity was carried out by
using the spatial computational analysis techniques. The
porosity computation was based on the calculation of a
local pore map and was implemented using 3D Chamfer
discrete distance [42]. The pore value is the diameter of
the largest sphere completely included in a given portion
of each the investigated samples.
Figure 7 shows an example of central slices in the or-
thogonal planes (xy, yz and xz) and a 3D volume recon-
struction of the local pore sizes. Different thicknesses are
plotted with different colors. After obtaining these maps,
a quantitative analysis of the pore size distribution in the
3D microstructure was possible.
In Figure 7, the presence of several cracks with di-
mension in a range between 100 and 700 µm can be ob-
served in addition to pores with diameters in a range be-
tween 9 and 100 µm.
Figure 8 shows the distribution of 3D pore size: these
distributions were obtained by dividing the thickness
Figure 5. 3D reconstruction of sub-volume of sample
IFAM1 showing the microstructure.
Figure 6. Gray Level Histogram of sample 1, showing the
utilized threshold.
Table 2. Summary of structural characteristics of the samples.
Samples MV/TV Porosity (%) MS/MV P.Th (μm) P.Sp (μm)
IFAM1 0.77 0.90 23.2 5.1 0.060 0.005 442.2 2.4 133.8 9.1
IFAM2 0.74 0.90 25.9 4.9 0.070 0.005 405.6 1.1 141.6 6.2
IFAM3 0.74 0.95 26.1 5.4 0.070 0.005 412.9 2.2 145.2 5.4
Copyright © 2011 SciRes. MSA
Characterization of Porosity in a Laser Sintered MMCp Using X-Ray Synchrotron Phase Contrast Microtomography1327
Figure 7. Example of central slices (a, b and c) in the or-
thogonal planes (xy, yz and xz) and 3D volume (d) of the
local pore size. The different thicknesses of pores are plot-
ted with different colors.
map in steps of 10 µm.
From Figure 8 a large amount of micropores with ra-
dius 0 < R < 20 μm is obtained. Lower amounts are ob-
served at higher radius.
The most of the radii are smaller than 55 μm, thus in-
dicating that the sample porosity is mainly related to
small pores.
In order to assess the uniformity of the distribution, the
percentages of holes with different sizes were considered
in three regions of the sample IFAM2. The results are
shown in Table 3, which reports the percent number of
pores in the three most important ranges of radius: <10
μm, 10 - 20 μm, 20 - 30 μm.
The distribution of pores size is uniform within the
sample: in fact for each of the three intervals, the per-
centage does not vary much from one area (down, centre,
top) to another.
The results reported in detail for sample IFAM2 are
also representative of those obtained from the other two
investigated samples.
In this section we also report a quantitative analysis of
damage in the composite. In particular, we have analyzed
the void growth in the Al-Si-Cu alloy matrix in three
dimensions. Figure 9 shows the distribution of 3D cracks
size: these distributions were obtained by dividing the
thickness map in steps of 50 µm.
The distribution of the cracks is not the same in all the
samples. In particular, the sample IFAM1 contains cracks
with larger dimensions.
Figure 8. Pore size distribution in the analyzed samples.
Copyright © 2011 SciRes. MSA
Characterization of Porosity in a Laser Sintered MMCp Using X-Ray Synchrotron Phase Contrast Microtomography
Table 3. Per cent porosity for each radius range.
R < 10 10 < R < 20 20 < R < 30 Total number of pores
Top of the sample 53.6% 24.0% 10.9% 6619
Centre 55.5% 24.3% 9.8% 7661
Down 56.3% 23.8% 10.8% 6850
Average porosity 55.2% 24.0% 10.5% 21130
Figure 9. Crack size distribution in the analyzed samples.
4. Conclusions
For the first time, DLS has been used for the Al-Si-Cu
MMCp alloy. Porosity is an important parameter that
influences the mechanical properties of the material, also
in view of its application as a door stop fitting for aero-
nautical needs.
The results show that the porosity is distributed ho-
mogeneously inside the samples both considering its
global value and also considering the contributions com-
ing from the different classes of voids of different sizes.
In particular, a porosity of 25% ± 5% was obtained for
all samples.
The most of the pores have diameter not larger than
100 μm and the porosity is mostly due to a great quantity
of small pores, with radius between 10 and 20 μm.
The occurrence of cracks and their not homogeneous
distribution in samples nominally submitted to the same
treatments can indicate that the so far obtained structure
is brittle.
5. Acknowledgments
The authors acknowledge the ELETTRA User Office for
kindly providing beam-time, and Dr. L. Rigon and Dr. L.
Paccamiccio for the technical support during the experi-
The authors wish to acknowledge the EU Network of
Excellence project Knowledge based Multicomponent
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Characterization of Porosity in a Laser Sintered MMCp Using X-Ray Synchrotron Phase Contrast Microtomography1329
Materials for Durable and Safe Performance (KMM-NoE)
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