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Open Pore Metal Foam pore


									                                Open Pore Metal Foam
                                             Dieter Girlich,
                            m.pore GmbH, Enderstraße 94, 01277 Dresden


Open pore metal foams are a new class of structural materials with low density and novel physical
properties. The interaction of physical characteristics (rel. density, material,…) and material properties
must be understood for comparison with other materials and the correct use in applications.
Open pore metal foam offer a high potential in systems with thermal management, catalyst support,
filtration, steam generation, mixing and energy absorption. They also hold particular promise in
applications were several of these features can be exploited simultaneously.
An overview of same applications will be presented.

1 Introduction
Open pore metal foams are a new class of materials with low densities and novel physical properties.
They posses a set of unusual properties compared with bulk materials and offer a unique combination
of several properties that cannot obtained in one conventional material at the same time. The
understanding of their properties depend on the structure and the volumetric effects which are
unfamiliar to the understanding of solid materials.

1.1 Production of open pore metal foams
Open pore metal foams which are descripted in this paper are made by casting. A open pore polymer
foam is used as template to create investment casting molds into which a variety of metals and their
alloys can be cast. For example Aluminium, Copper, Steel and their alloys.
The desired cell size, relative density and geometry is selected by the polymer foam which is the
model for the casting process.. The polymer model is filled with a mold casting slurry which is then
baked to harden the casting material and to decompose the model. The mold is filled with a metal and
cooled. The mold materials are removed to leaving behind the metal equivalent of the original polymer
foam. This method can be used to manufacture foams from 10 to 45ppi with a size of 480 x 480 x
40mm (10ppi) in Aluminium and 200 x 200 x 40 (10ppi) for some copper and steel alloys.
During production, the void fraction and pore size can be controlled independently of one another, and
virtually any shape can be manufactured. The pore size can varied from about 5mm in the 10 ppi foam
to about 1mm in the 40ppi, the void fraction can range from about 95% to 80%. About in this context
means, that there is a statistical distribution of the pore size in the foam.
The main difference of the casting process to other procedures making open pore metals is the
treatmet of the struts, which affect the mechanical and thermal properties of the foam. Also a wide
range of base materials can be used, which guarantees a product that can fit almost any application.
If data of different open pore materials, produced by different procedures but same materials, are
compared they lead to different values of compared parameters because the structre and
microstructure are eifferent.
The metal foam structure consits of ligamnets forming a network of interconnected dodecahedral like
cells. The cells are randomly orientated and mostly homogeneous in size and shape. The triangular-
cylindrical shaped edges of each cell is filled with metall which is a result of the manufacturing process
(Fig 1.).
   Figure 1. Unit cell, foam and strut structure

1.2 Characterisation
A open pore metal foam is characterized by the material, its cell topology, relative density, cell size
(ppi: pores per inch) and the geometry. The properties of the open pore metal foams depend directly
by the structure like the cell size and topology, the relative density (struts of the foam) and the material
from which they are made.

     Figure 2. Relation between structure and physical properties
It is necessary to understand the structure of open pore foams in order to predict its properties. The
key issue for foam properties centers on material distribution within the cell structure which leads to a
volumetric effect of the whole structure.
Most of the properties are dominated by the structure wihich is not yet describted in a way to explain
all the effects which arise. Small pore size has a larger surface area but comes with the cost of higher
pressure drop. A foam structure with a high relative density has better conductive heat and electrical
transfer properties as well as greater strength. The increase in the relative density means a heavier
component with a slower time response in thermal applications. A material is selected for a given
component because its property profil matches the demanded by the application.
In general open pore foams are random polydisperse materials that contain cells of different sizes and
shapes. The volume fraction of solid increases a broad range of microstructures are possible. The
material can be distributed between cell struts or nodes in many ways.
The physical properties of the foams depend on the pore structure, a uniform structure is not
necessary for obtaining reproducible properties and what is more important that porperties can
reproduced without reproducing the structure.

2 Flow
2.1 Describtion
The flow through porous media is described by the pressure gradient as the driving force which is
                                                                                                                    ∆p 1
proportional to a fluid volume passsing through a layer with a length L and an area A (                               = ⋅ v or
                                                                                                                    L  k
∆p dV    1
  =    ⋅     ) and found the pressure drop proportional to the length of the layer.
L   dt k ⋅ Á
The measured pressure drop has to be described by a superposition of a linear and a quadratic term
to fit the data well.

                               ∆p = A ⋅ v + B ⋅ v 2
A stands for the viscous friction of the fluid and B summarices the nonlinear effects. A and B are not
independent. They are correlated and come together as a quotient in the Reynolds number to
describe the heat flow. The linear regime of the pressure drop was seperated by a plot of
∆p                                                                           m
    = A + B ⋅ v and can be shown that laminar flow is in the order of v ≤ 0.1 for all porosities.
L⋅v                                                                          s

                                          Parameter: Porendichte (pore per inch)
                                       Probe 4 (10 ppi)

                  1000                 Probe 6 (20 ppi)

                                       Probe 7 (30 ppi)

                  800          l = 0,196 m
                               Meß fehler von ∆ p: +/- 2,5 Pa
      ∆ p in Pa

                               max. Meß fehler von c: +/- 0,021 m/s



                                                                                         Stegform: Zylinder
                         0.0        0.5       1.0         1.5     2.0       2.5   3.0   3.5      4.0          4.5
                                                                      c in m/s

 Figure 3. Pressure drop, Samples 10, 20 and 30ppi, Length: 200mm
Alternatively, the pressure drop can be described by a power law:

∆p = A ⋅ v B
which is used to describe phase transitions. For low velocities the energy is taken from the potential,
for higher velocities the workingdifferenz ( ( p1 − p 2 ) ⋅ ∆V ) is used to higher the kinetic energy of the
mass ρ ⋅ ∆V . The energy over the proportional part can be used for the fluid moving around the
struts, with the consequence of touching the surface of the struts for many times. This kind of cross-
moving makes metal foams worth for heat exchange, filter, mixer and velocity averager.

2.2 Applications
2.2.1 Average
The velocity distribution of air flow in a vent pipe with the dimension of 400mm x 400mm is shown by
the blue coloured line. A 20mm thick 10ppi metal foam was fitted into the pipe. The velocity distribution
is reduced to the red line.

                                                                                    Geschwindigkeitsverteilung über den
                                                                             Messebene 1 - 10 mm nach Prallplatte
                                                                  75         Messebene 2 - 200 mm nach Metallschaum


                                                        f in %

                                                                        P1   P2          P3         P4          P5     P6   P7   P8   P9





Figure 4. Smoothing the velocity distribution of the air flow in a vent pipe

2.2.2 Turbine
                                                    The high pressure drop of a fluid moving through a
                                                    foam and the independency of the flow direction
                                                    leads to a application showing that the foam can be
                                                    used in a dynamic application. The foam is used as
                                                    a turbine wheel transforming the kinetic energy of
                                                    the input fluid to a rotation movement to generate
                                                    electricity with low power steam.

                                                    Figure 5. Turbine wheel
3 Heat transfer
3.1 Describtion
Open pore metal foams can be used to enhence heat transfer in applications such as heat exchanger,
heat storage, condensors or even heat shields.
For a good heat transfer, the heat source has to be bonded to a layer of open celled foam. A fluid is
pumped through the foam, entering at temperatur T0 and exiting at temperature Te .

Figure 6. Heat distribution around a tube with diameter 10mm and a steel plate heated with a burner
In a foam based heat exchanger a number of transitions can be distinguished. From the heat source
to a plate or tube to spread the heat and bring it into the foam (Transition 1). The transition in the plate
or tubewall is neglected, because it is no limiting factor in the heat exchange performance. The
metallic conduction inside the metal foam, which is limited to a few pore diameters as can be seen in
Fig 6 (Transition 2). The heat transfer from the foam to a fluid (Transition 3). The bonding between the
plate and the foam is a very important step, because the circuit is dominated by the weakest link.
A fine structure exhibit a large internal surface area for local heat exchange, but may lead to a low
permeability, no increase of metalic conduction and hence to slow the gas flow rate. For a design
criterium, the foam always contribute to flow resistence but not necesserely to the transfer of the heat.
A complex interaction between pore size and topology, fluid permeation, convective heat flow, thermal
conduction and radiative heat transfer takes place in the foam.

                                 mittlerere W ärmeübergangskoeffizient und Druckverlust
                                       Parameter Stegdicke, Stegform, Porendichte
                     90                                                     900
                                                                                              Alfa, 10 ppi; 6,9 %;
                                                                                              Stegform Zylinder
                     80                                                     800
                                                                                              Alfa; 10 ppi; 6,7% ;
                                                                                              Stegform: Kreuz
                     70                                                     700
                                                                                              Alfa; 20 ppi; 10,2 %;
                                                                                              Stegform: Zylinder
                     60                                                     600
     α in W/(K m²)

                                                                                              Druckverl.; 10 ppi; 6,9% ;
                                                                                              Stegform: Zylinder
                                                                                  ∆ p in Pa

                     50                                                     500
                                                                                              Druckverl.; 10 ppi; 6,7% ;
                     40                                                     400               Stegform: Kreuz

                                                                                              Druckverl.; 20 ppi; 10,2 %;
                     30                                                     300               Stegform: Zylinder

                     20                                                     200

                     10                                                     100

                      0                                                     0
                          0.0   1.0      2.0              3.0   4.0   5.0
                                               c in m/s

Figure 7. Heat transfer coefficient and pressure drop for 10ppi, 20ppi and a new structure with
different strutsize
3.2 Application

The main objective is to heat, cool a component or to dissipate excess heat. In the case of a heat sink
for power electronics, the metal foam cooler is compared to different cooling systems.


                          90         Metallschaum-Messung 1
    Fußtemperatur in °C

                                     Metallschaum Messung 2




                                30       40             50          60           70   80   90

                                                             Heizleistung in W

   Figure 8. Cooling power of different heat exchangers

The main objective is to exchange heat rapidly with a fluid, usually a gas, flowing through a highly
porous and permeable material. The large internal surface area is the place for local heat transfer.
Heat transfer depends on both the cellular structure and the material of the foam. Higher material
conductivity is associated with higher density materials and a significant increase in thermal
conductivity results from an increase in foam density. A thicker strut can easily be made by the casting
process using more wax in the modelling process.

4 Mechanical
4.1 Properties
In compression, cellular solids can withstand large strains at nearly constant stress, allowing them to
absorb kinetic energy of an impact without generating high peak stresses. The force transmitted
remains below a critica level that upon impact might otherwise cause structural demage.
For that reason, metal foams can be used in energy absorbtion devices such as helmets or bumpers.
The mechanical response of open pore metal foam in compression is characterized by three different
regimes of behavior:
I) initial linear region associated with bending edges
II) roughly constant plateau correponding to cell collapse by buckling, yielding or fracture and
extending up to large strains (10ppi 80%, 40ppi 50%).
III) final sharp increase in stress under with further strain, corresponding to densification of the open
Scale effects occure when the sample size is of the order of the cell size. Reducing size leads to
material weakening. Cell size becomes an length scale, making the mechanical properties scale


         Druckspannung [Mpa]


                                2                                                                                20ppi

                               1.5                                                                                             10ppi



                                 0%          10%        20%        30%             40%              50%        60%   70%            80%        90%            100%
                                         I                    II                                                              III
                                                                                             Dehnung [%]

 Figure 9. Compression of                               AlSi7 Mg - foam with different pore sizes

 Figure 10. Crash test with go carts, foam bumper compared with solid metall bumper

 4.2 description
 For higher energy input or a custom made stress level, open pore materials can be filled with all kind
 of materials.
                                                                          pr e s s ur e de pe nde nc e




                                                                                                                                                 A l Si 7+s chw P U
                                                                                                                                                 A l Si 7+gel b P U

10                                                                                                                                               A l Si 7 st ar k e St ege

                                                                                                                                                 A l Si 7 nor mal
                                                                                                                                                 St ahl D2



     0                               5             10              15                   20                25             30               35

                                                                         x [ mm]

 Figure 11. Open pore foams made of different materials compared with polymer
 filled foams
Aluminium foams filled with polymer can absorb higher quantities of energy than steel foams. Nearly
every absorbtion profil for different applications can be made.


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