6.
METALLIC POROUS MATERIALS
Matej Vesenjak* and Zoran Ren
FACULTY OF MECHANICAL ENGINEERING, UNIVERSITY OF MARIBOR, SMETANOVA 17, 2000 MARIBOR,
SLOVENIA
*
Tel.: +386 2 220 7717, E-mail: [email protected]
6.1 Introduction
A porous material is made of an interconnected network of solid struts or plates which form
the cell’s edges and faces [1]. Porous materials are classified in two groups: (i) open-cell and
(ii) closed-cell. In the open-cell porous structures beams form the cell struts and in closed-cell
porous structure the closed cells are distributed in the base materials enclosed by cell surfaces.
Their mechanical behaviour mainly depends on the relative density (the density of the porous
structure, divided by the base material density) and the base material which can vary between
metals, plastics, glass and ceramics. The other important parameters of the porous structures
are morphology (open or closed cell), the geometry and topology (regular or irregular
structure) and possible filler type. To achieve adequate properties of the porous material, the
base material has to be carefully chosen in regard to its mechanical (strength, stiffness) and
thermal properties (thermal conductivity). The advantages of porous materials in general are
low density (light-weight structures), high acoustic isolation and damping, high energy
absorption capabilities, durability at dynamic loadings and recyclability.
Porous materials have a characteristic stress-strain relationship in compression, which is in
plastic region characterised by large strains at almost constant stress (stress plateau) until the
cells completely collapse (densification). With the strain increase, the cells become oriented
with the loading direction, increasing the stiffness of the porous material until the tensile
failure. The mechanism of cell deformation and collapse also depends on the relative density
of the porous structure.
The micro- and macroscopic properties of porous materials make them very attractive for use
in the automotive, rail, naval and aerospace industries as heat exchangers, filters, bearings,
acoustic dampers, core material in sandwich structures, bio-medical implants and elements for
energy absorption. One of the most important areas for the future application of porous
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Trajnostne tehnologije kovinskih materialov – Sustainable Technologies of Metallic Materials
materials is the automotive industry, where their high impact energy absorption through
deformation is of crucial importance for increasing passive safety of vehicles.
To further expand their application potentials porous materials are often used as parts or cores
in composite structures, e.g. sandwich structures. Sandwich structures are usually comprised
of two face sheets with porous material as a core in between. Such sandwich structures have
an increasing relevance in various engineering applications due to their high stiffness, strength
and reduced mass (light-weight). In some cases single porous material elements (metallic
hollow sphere structures, advanced pore morphology foam) are joined together using different
technologies such as sintering, soldering and adhering. Adhering provides the most
economical way of joining and allows for further cost reduction and therefore the expansion
of potential applications.
The understanding of porous material behaviour under quasi-static and dynamic loading is
valuable for engineering applications such as those related to mechanical energy absorption
through deformation [2, 3]. Proper characterisation of all influential parameters is particularly
important and can be best achieved through combination of dedicated experimental testing
programme and computational simulations. However, the structure of industrial porous
materials in terms of shape, size and distribution of pores cannot be fully controlled with
existing mass production technologies. This results in a certain scatter of mechanical and
thermal characteristics of these materials and their components. Some recently developed
fabrication methods of porous metals result in more homogeneous pore structures [4-7]. The
rest of the presentation focuses on characterisation of mechanical properties of some currently
available metallic porous materials by means of experimental tests and computational
simulations using ABAQUS and LS-DYNA.
6.2 Unidirectional porous structure - UniPore
Some innovative manufacturing approaches have been investigated recently in search of
porous materials with more regular distribution of pores, constant wall thickness and pore
sizes. One such approach is explosive compaction of thin-walled tubes, which after treatment
form a porous structure with straight unidirectional pores – the UniPore structure. The
advanced geometrical properties of the UniPore structure assure wide opportunities for its
application due to its particular and unique mechanical and thermal properties. Mechanical
behaviour of the UniPore structure with unidirectional pores under dynamic loading can be
influenced by size, thickness and base material of original tubes.
a)
b)
Figure 1. UniPore sample (a) and its typical cross-section (b)
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Figure 2. Influence of the inner pipe thickness on the compressive behaviour of
UniPore structure at strain rate of 100 s-1
This manufacturing procedure results in making a porous material with perfectly parallel
unidirectional pores (Fig. 1a). The outer and inner pipes were in analysed case made of
phosphorus deoxidized copper (Cu 99.98 % and P 0.02 %). The compressive mechanical
properties of UniPore structure have been investigated by means of experimental and
parametric computational simulations considering various materials and geometrical
parameters using the LS-DYNA [8].
The simulations have shown that the UniPore structures exhibit characteristic Porous material
behaviour, i.e. onset of yielding after the initial elastic response which is then manifested in
typical stress plateau followed by the final densification (Fig. 2).
6.3 Open-cell aluminium foam - m.pore
The open-cell foam specimens are manufactured using an investment casting process. Starting
point is a porous polymer precursor. The pores are filled with a fire-resistant slurry that is
dried and burned. During the burning the precursor pyrolises while the slurry hardens and
forms the mould for subsequent investment casting of the metallic matrix. The final step is the
removal of the moulding material resulting in a porous metallic geometry that closely
resembles the polymer precursor [9]. The base material of the open-cell foam is Al99.7% and
the relative density of the structure is 6.1 % (porosity is 93.9 %). Their average pore size is 20
pores per inch (ppi).
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Trajnostne tehnologije kovinskih materialov – Sustainable Technologies of Metallic Materials
a)
b)
c)
d)
Figure 3. Aluminium porous material: (a) sample; (b) CT scan; (c) virtual reconstruction; (d) solid CAD model
The cube shaped aluminium foam samples (Fig. 3a) with dimensions 40 x 40 x 40 mm, were
CT scanned at the Department of Radiology, University Medical Centre Maribor. The CT
images were then used for appropriate geometrical statistical analysis and virtual
reconstruction (Fig. 3) [2, 10]. The reconstructed models have been discretised with solid
finite elements and used for explicit dynamic finite element analysis using the engineering
code LS-DYNA. The results of computer simulations are shown in Fig. 4, in which the local
stress concentrations during loading can be clearly observed. The results of the structural
analysis of this irregular porous material indicate a typical compressive stress-strain behaviour
of the porous structures.
Figure 4. Stress concentrations by structural computer analysis of the aluminium porous material
6.4 Advanced Pore Morphology (APM) Foam
Advanced pore morphology (APM) foam consists of sphere-like metallic foam elements (Fig.
5) of sizes ranging from 5 to 15 mm in diameter. The APM spheres can be used individually
or bonded with a matrix as fillers of engineering parts, as core layers for sandwich structures
or other composite materials [11-14]. They have a characteristic stress-strain behaviour under
compressive loading (Fig. 6) [16]. Since the APM foam manufacturing procedure has been
developed only recently, the mechanical characterization of these materials is still limited.
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Figure 5. APM foam element (left) and the CT scan (right)
Figure 6. Behaviour of APM foam elements under compressive loading [16]
Single APM elements have a characteristic cellular material behaviour where the larger foam
elements experience lower densification strain, which corresponds to their observed higher
inner porosity. The use of IR thermography has demonstrated the importance of studying also
the heat generation due to fast plastic deformation during dynamic loading. The yielding starts
at the contact between the loading/support plate and the APM element and then progress
through the sphere in a shear band, finally resulting in a fully plastically compressed APM
foam element. The study of single APM elements provided valuable mechanical properties
and the basic knowledge for an efficient composition of composite APM structures [15, 16].
A recent structural analysis of APM foam element using micro computed tomography
revealed different levels of porosity (Fig. 7). Pore size from micro level (several micrometres
in diameter) up to macro level (several millimetres in diameter) can be found resulting in high
total number of pores in only one APM foam element.
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Trajnostne tehnologije kovinskih materialov – Sustainable Technologies of Metallic Materials
a)
b)
c)
d)
Figure 6. Pore search algorithm based on the µCT figures
6.5 Conclusions
Novel metallic porous materials are directly applicable in contemporary industry. One of the
most important areas for future application of porous materials is automotive industry, where
the increased capability of mechanical energy absorption is of a crucial importance from the
vehicle safety perspective. Properties of porous structures allow their wide use in hollow
automotive parts with the purpose of increasing the stiffness of the vehicle framework and at
the same time to improve the level of passive vehicle safety through increased mechanical
energy absorption during vehicle impact. Experiments have shown that filling the hollow
spaces in vehicle with foams or porous materials leads up to 100% increase of impact energy
absorption at minimal added mass. This consequently lowers the decelerations of vehicle
occupants and increases their safety. New computational models and carefully characterised
properties of porous materials allow parametric computational simulations, with the aim to
define the most suitable design parameters of parts made of porous materials for a given
application.
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6.6 References
[1]
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foams: a design guide, Burlington, Massachusetts, Elsevier Science.
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Vesenjak, M., Veyhl, C. and Fiedler, T. (2012) Analysis of anisotropy and strain rate sensitivity of
open-cell metal foam, Mater. Sci. Eng. A, 541, 105-109.
[3]
Vesenjak, M., Krstulović-Opara, L. and Ren, Z. (2012) Characterization of photopolymer Porous
structure with silicone pore filler, Polymer Testing, 31(5), 705-709.
[4]
Körner, C. and Singer, R.F. (2000) Processing of Metal Foams - Challenges and Opportunities, Adv.
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Sphere Structures under Dynamic Loading, Adv. Eng. Mater., 10(3), 185-191.
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anisotropy of lotus-type porous copper, Adv. Eng. Mater., 14(3), 144-152.
[8]
Hallquist, J.O. (2006), LS-DYNA theoretical manual, Livermore, California, Livermore Software
Technology Corporation.
[9]
Hipke, T., Lange, G. and Poss, R. (2007), Taschenbuch für Aluminiumschäume, Düsseldorf, Alu Media.
[10]
Vesenjak, M., Matela, J., Young, P. and Said, R. (2009) Imaging, virtual reconstruction and
computational material (tissue) testing, Acta medico-biotechnica, 2, 19-30.
[11]
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Adhesive Effects, Adv. Eng. Mater., 12(7), 596-603.
[13]
Avalle, M., Lehmhus, D., Peroni, L., Pleteit, H., Schmiechen, P., Belingardi, G., et al. (2009) AlSi7
metallic foams – aspects of material modelling for crash analysis, Int. J. Crashworthiness, 14(3), 269 - 285.
[14]
Stöbener, K., Lehmhus, D., Avalle, M., Peroni, L. and Busse, M. (2008) Aluminum foam-polymer
hybrid structures (APM aluminum foam) in compression testing, Int. J. Solids Struct., 45(21), 5627-5641.
[15]
Vesenjak, M., Gačnik, F., Krstulović-Opara, L. and Ren, Z. (2012) Mechanical Properties of Advanced
Pore Morphology Foam Elements, Mech. Adv. Mater. Struc., accepted for publication.
[16]
Vesenjak, M., Gačnik, F., Krstulović-Opara, L. and Ren, Z. (2011) Behavior of composite advanced
pore morphology foam, J. Compos. Mater., 45(26), 2823-2831.
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