METALLIC HOLLOW SPHERE STRUCTURES MULTIFUNCTIONAL MATERIALS FOR
LIGHTWEIGHT APPLICATIONS: TYPES, PROPERTIES AND
CASE STUDIES
Andreas Öchsner1,2,a, Thomas Fiedler3,b, Christian Augustin4,c
1
Department of Applied Mechanics, Faculty of Mechanical Engineering,
Technical University of Malaysia, Skudai, Johor, Malaysia
2,3
University Centre for Mass and Thermal Transport in Engineering Materials,
School of Engineering, The University of Newcastle, Callaghan, New South Wales
2308, Australia
4
a
Glatt GmbH, Binzen, Germany
Andreas.Oechsner@gmail.com; bThomas.Fiedler@newcastle.edu.au;
c
Christian.Augustin@glatt.com
Abstract: Metallic hollow sphere structures (MHSS) are a new group of cellular
metals characterised by easily reproducible geometry and therefore consistent
mechanical and physical properties. A new powder metallurgy based manufacturing
process enables the production of metallic hollow spheres of defined geometry. This
technology brings a significant reduction in costs in comparison to earlier applied
galvanic methods and all materials suitable for sintering can be applied. The paper
presents exemplarily mechanical and thermal properties of hollow sphere structures in
different configurations and shows several practical applications of this new
engineering material.
1. INTRODUCTION
The concept of porous and cellular metals first emerged in the beginning of the 1970s
[1–3]. The basic idea seeks to imitate the cellular structure of high-performance
lightweight structures in nature such as the human osseous structure and can therefore
be related to the field of bio-inspired research. A closely related approach has already
been successfully brought into application in both the aviation industry [4,5] and the
space industry [6,7] through the use of hexagonal honeycomb structures in sandwich
cores.
Nowadays, foams made of polymeric materials are widely used in all fields of
technology. For example, Styrofoam® and hard polyurethane foams are widely used
as packaging materials. Other typical application areas of polymeric foams are the
fields of heat insulation and sound absorption. During the last few years, techniques
for the manufacturing of novel cellular and porous metals have been developed [8,9].
These materials exhibit a high potential for future oriented applications due to their
specific properties.
By definition, cellular metals are materials with high porosity which are divided into
distinct cells. The boundaries of these cells are made of solid metal, while the internal
regions are air cavities. Cellular metals therefore exhibit densities which are typically
below 10% of their corresponding base metal. This porosity can be quantified by the
relative volume which is the volume occupied by the base material divided by the
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total volume of the structure. Two special cases of cellular metals are metallic foams
and sponges. Metallic foams like Alporas® (Fig. 1 a) originate from a liquid and are
characterised by closed cells.
Fig. 1. Cellular metals: (a) Alporas® aluminum foam; (b) M-Pore® aluminum sponge.
Metallic sponges such as M-Pore® (Fig. 1 b) are characterised by an interconnected
porosity of open cells. In contrast to cellular metals, porous metals contain a multitude
of microscopic pores and the densities of these materials are in the range of their base
material(s). It should be mentioned here that the definitions of metallic foam, sponge
and porous metals are not mutually exclusive.
Well-known advantages of cellular metals are their high ability for energy adsorption
[10–12], good damping behaviour [13–15], sound absorption [16], excellent heat
insulation [17–19] and a high specific stiffness [20–22]. The combination of these
properties opens a wide field of potential applications, e.g. in automotive, aviation or
the space industry [23–25]. However, despite more than 30 years of intensive
scientific research, few industrial applications of these technologies can be found.
Essential limiting factors for their utilization are unevenly distributed material
parameters [11,12] and relatively high production costs. Less variation in the physical
properties can be achieved with lattice block materials [26,27]. These structures are
manufactured by investment casting and therefore exhibit a well-defined, reproducible
geometry (periodic 3D truss structure). However, this manufacturing technology is
only able to produce open-celled structures. Further limitations are high costs and
anisotropic properties caused by the mesostructure orientation.
Fig. 2. Manufacturing process of metallic hollow-sphere structures.
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Metallic hollow-sphere structures (MHSS) are a new group of cellular metals
characterized by easily reproducible geometry and therefore consistent mechanical
and physical properties. A new powder metallurgy-based manufacturing process
enables the production of metallic hollow spheres of defined geometry [28]. This
technology brings a significant reduction in costs in comparison to earlier applied
galvanic methods and all materials suitable for sintering can be applied. EPS
(expanded polystyrol) spheres are coated with a metal powder–binder suspension by
turbulence coating. The green spheres produced can either be sintered separately to
manufacture single hollow spheres or be pre-compacted and sintered in bulk (Fig. 2)
thus creating sintering necks between adjacent spheres [29]. Depending on the
parameters of the sintering process the microporosity of the sintered cell wall can be
adjusted. In a subsequent debindering process, the EPS spheres are pyrolysed. The
increase of the carbon content of the sintered metal by the diffusion of the incinerated
binder and polymer causes degradation of mechanical properties and corrosion
resistance. Special reducing processes are required to reduce this effect [30].
Examples of single hollow spheres are shown in Fig. 3.
Fig. 3. Single hollow spheres of different diameter.
Fig. 4. Adhesively bonded metallic MHSS: (a) Cross-section of a syntactic MHSS;
(b) partial morphology.
Various joining technologies such as sintering, soldering and adhering can be used to
assemble the single hollow spheres to interdependent structures [31,32]. Adhering is
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an economic way of joining and therefore is attractive for a wide range of potential
applications. Another important advantage is the possible utilization of the
mechanical behaviour and morphology of the adhesive layer as a further design
parameter for the optimization of the structure’s mechanical properties for specific
applications. Figure 4 shows two different types of adhesively bonded MHSS.
In the case of syntactic morphology, the hollow spheres are completely embedded
within the adhesive matrix as in the case of classical fiber-reinforced plastics. In
contrast, the adhesive is concentrated at the contact points of neighboring spheres for
partial MHSS. Consequently, partial MHSS also exhibit interconnected porosity.
3. MECHANICAL AND THERMAL PROPERTIES
Hollow sphere structures show a different physical behaviour compared to classical
solid materials since the macroscopic behaviour is determined by the cell wall
material and the cellular structure (i.e. sphere dimensions, spatial arrangement and
joining technology). As example, mechanical and thermal properties obtained from
finite element simulations will be presented in the section [33].
3.1 Mechanical Properties
The stress-strain relations of partial and syntactic metallic hollow sphere structures
are plotted in Fig. 5. It can be observed that the syntactic MHSS exhibit significant
higher stresses in comparison to the partial structures. Furthermore, the stresses
increase with the density of the structure.
Fig. 5. Stress-strain relations of adhesively joined (heterogeneous) MHSS.
The linear-elastic range of the stress-strain curve (Fig. 5) can be characterised by the
material parameter Young's modulus E which is plotted in Fig. 6 versus the average
density of the structures. The stiffness of partial MHSS increases linearly with the
density, whereas the curve of syntactic MHSS exhibits a parabolic shape. In the field
of lightweight technology, high specific material parameters are of great interest. If a
material parameter is plotted versus the density, high specific values are located in the
top left area of the graph. Identical specific values lie on straight lines that intersect
the origin of the coordinate system. Accordingly, it can be concluded that syntactic
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MHSS exhibit the higher specific stiffness for average densities larger than 0.52
g/cm3.
Fig. 6. The dependence of Young's modulus E of adhesively bonded (heterogeneous)
MHSS on the average density.
3.2 Thermal Properties
Figure 7 shows the influence of different morphologies for homogeneous steel
structures. The effective thermal conductivity is plotted versus the sphere wall
thickness t. As a result of the higher volume fraction of the matrix, the syntactic
MHSS exhibit significant higher thermal conductivity. In contrast to this behaviour,
the partially bonded structures show only low values, especially for a small sphere
wall thickness t.
Fig. 7. Influence of the morphology of homogeneous steel MHSS on the effective
thermal conductivity.
In Figure 8, the influence of the morphology on the thermal properties of adhesively
bonded structures is visualized. Due to the higher volume fraction of the epoxy matrix
(Ep), the thermal conductivity of syntactic MHSS exceeds the values of the partial
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morphology. However, the deviation is smaller in comparison to the homogeneous
structures. This phenomenon can be explained by the low thermal conductivity of the
adhesive matrix. Increase of the volume fraction of the matrix therefore only slightly
increases the effective conductivity of the structure.
Fig. 8. Influence of the morphology of heterogeneous MHSS on the effective thermal
conductivity.
Further information on the thermal properties of cellular materials can be found in the
monograph [34].
4. APPLICATIONS
Cellular metals as well as hollow sphere structures exhibit a multitude of interesting
properties. Based on these characteristics, highly integrated applications can be
created. First, due to the high porosity of MHSS, the material is able to compress at
high strains. The typical stress–strain diagram of a MHSS reveals a stress plateau
which characterizes the ability of the structure to absorb energy at a low and constant
stress level even at high strains.
Fig. 9. Hollow spheres filled in metallic tubes to serve as crash absorber.
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This property enables the application of MHSS in energy-absorbing structures (cf.
Fig. 9), e.g. crash elements in the automotive industry [24].
Fig. 10. Advanced sandwich structures with cellular core material (HSS): a) Carbon
fibre reinforced plates as face sheets; b) steel plates as face sheets.
Sandwich structures are essential machine elements in lightweight construction and a
typical field of (possible) industrial application of cellular metals. In particular,
aluminium sandwich constructions have been recognised as a promising concept for
structural design of lightweight transportation systems such as aircraft [35], highspeed
trains [36] and fast ships [37]. The classic definition of a sandwich structure in
lightweight applications is a composite where a light core material is enclosed by two
or more layers of a fully dense material. The basic principle of sandwich structures is
for two strong face sheets to bear the applied loads while the core acts as a spacer
which retains the face sheets in position by carrying shear stresses. Therefore, the core
must be stiff enough in the direction perpendicular to the faces to ensure that they
remain the correct distance apart and exhibit sufficient shear stiffness to ensure that
when the panel is bent the faces do not slide over each other. The core material does
not need to reach the mechanical performance of the face sheets and typically lowdensity materials are applied [38]. Nowadays, the industrial standards for sandwich
cores are honeycomb structures which exhibit excellent stiffness at very low densities.
However, the processing of honeycomb structures, especially as a core between
curved face sheets, is complex and therefore increases the manufacturing costs.
Furthermore, honeycomb structures possess poor resistance to contact and impact
loads [39,40] and exhibit anisotropic properties. This shows that there is a necessity
for new innovative core materials. In Fig. 10 hollow spheres are applied as core
material. For industrial applications these core materials have to be able to compete
with classic honeycomb structures. The benefits of cellular metal as sandwich cores,
relative to competing concepts, arise primarily in curved configurations where the
isotropy of the material is advantageous [10,20,41]. Due to the multifunctional
properties of cellular metals the selection of the optimum core material requires the
consideration of all characteristics with relevance to the intended application.
Another clear attribute of cellular structures is the damping of mechanic and acoustic
oscillations. In conjunction with low density, this property suggests the utilisation in
parts where high accelerations occur. Structural oscillations can be damped and
energy consumption can be reduced due to the small amount of accelerated mass. The
significant potential of hollow sphere composite structures in machine tools has
already been demonstrated [42]. Furthermore, cellular metals can act as sound
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suppressors and acoustic insulators. Cellular metals also exhibit a low thermal
conductivity in comparison to their metallic base materials. In particular, adhesively
bonded MHSS show very low thermal conductivities, due to the insulating effect of
the adhesive matrix between the metallic shells of the spheres. Consequently, MHSS
can be applied as thermal insulators [43,44]. Other approaches aim for the utilization
of metallic sponges inside heat exchangers [45,46] or heat sinks. However, since
MHSS exhibit only low (partial morphology) or no (syntactic morphology)
interconnected porosity, they possess a high flow resistance and are not suitable for
such applications.
Fig. 11. Hollow sphere structures in machine parts: a) robot arm; b) exhaust silencer.
Many other possible applications of hollow sphere structures have been tested (e.g.
ballistic protection) and should give rise to new engineering structures.
5. OUTLOOK
Hollow sphere structures are a new type of cellular materials which opens the
possibility for novel integrated engineering constructions. Compared to classical
cellular metals such as metal foams, hollow sphere structures reveal a much lower
scattering in the physical properties. Several fields of possible applications have been
presented within this article. The industrial application in series production of
advanced structures is the next challenging step of this material development.
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