Note: Descriptions are shown in the official language in which they were submitted.
WO 91/01387 P~/NO90/i)Olt5
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A process of manufacturing particle reinforced metal foam and
~roduct thereof
The present invention relates to a process of providing metal
foam and more particularly to a process resulting in provision
of thin wall close cell particle reinforced metal foam.
Foamed metals, as well as foamed ceramics and plastics, due to
their unique combination of properties and light weight are
earning growing attention as engineering materials.
There are several ways to produce foams. Different foaming tech-
niques are known such as incorporating hydrides in the molten
metal or a~ding organic compounds which release gases on heat-
ing. Vapor deposition on polymeric substrates or casting of
metal around granules which are then leached out leaving a
porous --'al structure are other examples of providing metals
with cel ~lar structure.
The process of foam formation using blowing agents is affected
by the surface tension and viscosity of the actual melt. The
viscosity counteracts bur~ting of the cell walls during a pro-
gressive increase in the volume of the formed bubbles, while a
low surface tension will favour formation of thin bubble walls.
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The properties of foams being gas-in-solid dispersions are
largely determined by their density, but the cell size, struc-
ture and their distribution are also important parameters in-
fluencing the properties.
In general such foamed metals are produced by adding a gas
evolving compound to the molten metal followed by heating of the
resultant mixture to decompose the compound and to produce ex-
panding cellulating gases. The foaming compound is usually metal
hydride such as TiH2 or ZrH2, and after the foaming step the
mould is cooled to form a solid foam material. Cells of non-
uniform structure and/or undesirably large size are experienced
due to the difficulties with uniform distribution of the evolv-
ing gas through the whole volume of the foamed metal.
GB patent No. l.287.994 discloses a process for preparation of
metal foams applying a viscosity increasing agent comprising an
inert gas or an oxygen containing material gaseous at the melt
conditions and treating the thus produced viscous melt with a
foaming agent. Air, nitrogen, carbon dioxide, argon and water
are preferably used in the process as viscosity increasing
agents in amounts from l to 6 grams per lO0 grams of metal
alloy. Metal hydrides are used as foaming agents (hafnium,
titanium or zirconium hydrides) in amounts of from 0,5 to l,0
grams per lOO grams of alloy.
Preferably the increase in viscosity is enhanced by the presence
of a promoter metal, e.g. from 4 to 7 weight% magnesium is used
in aluminium alloys. A good mixing technique is required, the
addition of foaming agents is usually carried out at a tempera-
ture lower than addition of the viscosity increasing agent in a
separate second vessel. The disclosed batchwise process, achiev-
ing better foams with regard to uniform size and distribution of
the cells, and claiming a certain reduction in the consumption
of foaming agents, is a rather complicated time consuming and
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WO9l/~1387 PCT/NO~0/00115
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expensi.ve process requiring several process steps and units
based on use of expensive heat decomposible gas evolving com-
pounds (hydrides).
European patent application No. 0 210 803 discloses a similar
batchwise method of producing foamed metals based on use of from
0,2 to 8,0 weight% metallic calcium as viscosity adjusting agent
and titanium hydride in amounts of from 1 to 3 weight% of the
molten melt as foaming agent.
Still another method of producing cellularized metal by decompo-
sition of a heat-decomposable gas evolving compound in molten
metal is disclosed in US patent No. 3.297.431. The improvement
comprises addition of an intimately dispersed, finely divided
powder to the metal prior to decomposition of the gas evolving
compound (carbonates or hydrides), or dissolving of gas in the
melt. The stabilizing powders may be metals or non-metals,
elements or compounds, and two wettable powders are preferen-
tially used where one of which forms a solid alloy with the
metal. Usually the gas is dissolved at one pressure and then
evolved at a second lower pressure.
A drawback in common for the hitherto known processes is that
all of them are batchwise operating processes using either ex-
pensive gas evolving compounds or dissolved gases as cellulating
means and viscosity increasing or stabilizing additives to
achieve quality metal foams.
Furthermore, the prior art processes require a close control
with the temperature and pressure conditions at different steps
of the process. Consequently, so far there is no method operat-
ing on an industrial scale in an economical way offering a low
cost metal foam to compete with other engineering materials.
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Accordingly, it is an object of this invention to provide a
simple low cost method for preparation of quality foams.
Another object of the invention is to provide a method for up-
grading of scrap metal material.
Still another object of the invention is to provide a novel type
of particle reinforced metal foam having improved mechanical
properties.
The invention in its various aspects will be described in
details, and various other objects, advantages and additional
features thereof will become more apparent from the following
description and accompanying patent claims which are to be read
in conjunction with the attached drawings, Fig. 1-4, where
Fig. 1 shows schematically in the form of a flow-sheet
the process of preparation of metal foam accord-
ing to the invention,
Fig. 2 displays a natural size contact print of the
foamed metal sample prepared according to the
invention,
Fig. 3 shows an optical metallograph picture of the
closed cell Al-foam structure,
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Fig. 4 illustrates graphically results from a compres-
sion test conducted on foam samples.
Referring to Fig. 1, illustrating schematically the process of
metal foam preparation, it has been found that a metal foam of
the closed cell type structure having a uniform density and cell
structure can be provided simply by feeding of finely dispersed
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WO91/013X7 PCT/NO~/nOIl~
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cellulating gas into a molten particle reinforced metal matrix
composite material (PMMC). No Cpecial additives adjusting the
viscosity of the melt or particular precautions with regard to
the distribution of the cellulating gas bubbles through the melt
were required. The gas bubbles rise to the top of the melt and
form foam gradually increasing in volume. No tendency to burst-
ing of the foam cells when they reach the melt surface was ob-
served. This indicates a (highly) stabilized surface of the gas
bubbles. The upper portion of the foam cake solidifies and can
be easily removed. Even foam which is not completely solidified
can be removed whithout changing the cell structure due to the
thick consistency of the formed foam. This is a quite important
feature of the method according to the present invention, which
allows to run the process continuously by transfer of semi-
solidified foam to the moulds. There is even a possibility of
subjecting the foam at this stage to certain forming operations,
something which dffers a flexibility with regard to the final
shape of the resultant metal foam semiproducts.
Example 1
30 kg of an eutectic aluminium alloy (Sil2MglNi2,5) was melted
in an open crucible. The molten alloy kept at a temperature of
650C was added silicon carbide particles of an average size of
12/um, and simultaneously CO2 gas was finely dispersed through the
melt by means of a special treatment rotor as disclosed in US
patent No. 4.618.427. During the feeding of a CO2 surplus into
the formed molten composite material bubbles started to rise to
the top of the melt forming a raising foam layer. The upper por-
tions of the foam solidified with no sign of surface burst.
Fig. 2 shows in natural size a photographic picture of the
resultant foam sample removed as the solidified top part of the
foam cake. The cross-section of the sample exhibits a uniform
distribution of cells having a diameter in the range of from 1
to 5 mm. The density of the sample was measured to 0,2 g/cm3.
WO91/01387 PCT/NO90/00115
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Exam~le 2
20 kg of scrap PMNC material (Al203 reinforced Al-alloy) was re-
melted in an open crucible. Pressurized air was applied as
source of cellulating gas in this case, finely dispersed and
distributed as described in Example 1.
Also in this case the resulting bubbles gave rise to a foamed
structure when they reached the top of the melt in the crucible
and were allowed to cool.
The achieved pores (cells) are essentially spherical and closed
providing the foamed metal with isotropic properties in all
directions, especially with regard to energy adsorption.
Metallographic examination of the structure on the samples
achieved from Example 1 reveals an extremely thin walled foam
stxucture, as illustrated in Fig. 3. The wall thickness in this
metallograph picture, magnification of 20, is in order of the
reinforcing SiC particle size approximately 12tum.
The mechanical behaviour of the produced foam is represented in
Fig. 4 illustrating the results from the testing of compressive
stress conducted on the samples from Example 1. The achieved
flat stress/strain curve from the samples having an initial
height of 26 mm applying a crosshead velocity of 2 mm/min. is
typical for this type of material as long as the cell structure
did not collapse completely. The energy absorption of this foam
was determined to be 2 kJ/l foam, which is a very favourable
value compared to the values reported in literature for commer-
cially provided Al-foams. Obviously, the achieved improved
mechanical properties of the resultant foams are a result of a
beneficial influence from the reinforcing particles incoxporated
in the cell walls.
Evidently, the above described novel method of preparation of
foamed metals according to the present invention offers several
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advantages both with regard to the economics of the process and
the characteristics of the resulting foams.
First of all there is an opportunity to run the process con-
tinuously by continuous remelting or feeding of molten article
reinforced metal material using a variety of available gases as
a cellulating gas, e.g. N2, Ar, CO2, He and even pressurized air,
which is normally easily available at low costs.
There are no special requirements to temperatures, pressure or
uniform distribution of gas bubbles during the foaming and
solidification of the resultant foamed metal. The density and to
a certain extent also the cell size are simply controlled by
dispersion of the cellulating gas through the melt, preferen-
tially by applying the above special treatment rotor, but also
other means ensuring finely dispersed bubbles can be applied.
The foam accumulated on the top of the melt can be directly fed
into moulds for solidification in desired shapes and dimensions
or subjected to a certain grade of deformation/reshaping of the
semis~lidified foam.
Furthermore, even if it is possible to prepare the molten par-
ticle reinforced alloy in a separate process step using an
active gas and addition of reinforcing particles prior to apply-
ing of the cellulating gas, the biggest potential of the present
invention is an up-grading of low grade composite scrap
material. This constantly increasing volume of composite scrap
today represents a considerable problem since it can not simply
be remelted or incorporated to the recycled secondary aluminium.
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