Note: Descriptions are shown in the official language in which they were submitted.
CA 02332674 2001-O1-29
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A METHOD FOR PRODUCTION OF A FOAM METAL
FIELD OF THE INVENTION
The present invention relates to the field of powder metallurgy. More
specifically, it
concerns a method of manufacturing foamable metal bodies and their use.
BACKGROUND OF THE INVENTION
Already known in the prior art, there is German patent no 4101630 (US patent
no
5,151,246) which describes a method for the production of porous semi-finished
products from aluminum and copper-based alloy powders. The method described
therein comprises the steps of mixing of an alloy powder with a foaming agent,
filling
a press container with the mixture, simultaneously heating the filled
container and
applying pressure at which the foaming agent does not decompose,
simultaneously
cooling and removing the pressure, disassembling of the container followed by
pushing of the solid briquette out of it, which is immediately heat treated to
produce
a porous body or is subjected to preliminary hot deformation via extrusion or
rolling
followed by heat treatment. A very narrow range of products in terms of sizes
and
shapes can be produced with such method since the weight of the briquette is 2-
5
kg. In addition, this method demonstrates a very low output because of the
prolonged
heating of the large size press container filled with the powder mixture. Even
in the
case where the powder mixture would be heated in a container having 100 mm in
diameter and 400 mm in height, the heating operation would be economically not
feasible.
Also known, there is a method for the production of porous semi-finished
products
from metallic powders that incorporates different variants.
A first variant includes the steps of coating the bottom floor of a press
container with
a metallic layer free of foaming agent, covering the metallic layer with a
powder
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mixture comprising a foaming agent, and then covering the layer of powder
mixture
with a second metallic layer free of foaming agent. The container is then
heated, and
hot compaction is carried out. This operation is the end of the method. The
shape
of the body produced can be changed via deformation. Then, the body is foamed
for
formation of a new body wherein a high porous foamed metallic layer appears
between two metallic layers.
A second variant includes the steps of disposing a dense metallic disk in an
empty
press container adapted for extrusion and filling the container with a powder
mixture
containing a foaming agent. Then, the container with the powder mixture is
subjected
to heating followed by the application of a pressure of about 60 MPa. Due to
the
pressure, the central part of the hard metallic disk, which blocks the hole of
the press
die, begins flowing through this hole and ensures extrusion process. During
subsequent extrusion stages, the compacted powder mixture plastically deforms
and
flows through the die hole also. In this case, the dense metallic layer covers
the
extruded powder mixture which is ready for foaming. After foaming of this
combined
body the metallic layer covers a core consisting of a high porous foam.
The combined billets produced via both variants can be further rolled in
sheets, and
due to a heat treatment temperature, can be transformed in a porous metallic
body
(US Patent 5,151,246, September, 1992, B 22 F 3/18, B 22 F 3/24).
The disadvantage of this technique is also the limited possibility of
production of
semi-finished products, especially sheets of commercial sizes, low product
yield and
output, high manufacturing cost.
The method of the present invention is distinct from and overcomes several
disadvantages of the prior art, as will be discussed in detail below.
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SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for the production
of a
foamable metal body that overcomes several of the above-mentioned drawbacks.
More particularly, the present invention proposes a method for the production
of
porous powder metallurgy metal products comprising the steps of:
a) providing a mixture of metal alloy powders with a foaming agent;
b) pouring the mixture of step a) in a mould, preferably a reusable mould;
c) pre-compacting the mixture, preferably by vibration;
d) heating the mould with the mixture at a temperature which ensures liquid-
phase
sintering of the mixture in order to produce a liquid phase sintered
briquette;
e) hot compaction of the sintered briquette to achieve a porous and brittle
body
where the foaming agent is embedded in a metal matrix;
f) reducing the porous and brittle body into chips and continuously hot
rolling the
chips to form the desired semi-finished dense product;
g) conventional deformation of the dense product to desired shape
h) foaming of the shaped dense product by heating the same at a temperature
sufficient to allow the foaming agent to decompose, and thereby forming the
foamed
end product.
According to a preferred feature of the invention, the hot rolled dense blanks
are
deformed by conventional methods to the desired shape by installing the same
in a
special mould, which cavity shape is in compliance with the shape of the end
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product, and heated above a temperature of transition from solid to liquid
state of an
alloy to produce a porous net shape product.
Also preferably, prior to step h) of heating, two or more sheet blanks along
any of
their sides are put together with overlap to increase the length or width of
the
combined blank. These blanks are then heated above a temperature of transition
from solid to liquid state of an alloy to form a porous product with doubled
(tripled ... )
width or length.
According to a preferred embodiment of the invention, the method incorporates
mixing of powder aluminum alloys of various systems: AI-Cu-Mg-Si, AI-Mg-Si, AI-
Mg-
Cu-Si (cast alloys), AI-Cu-Mg-Mn, AI-Mg-Cu, AI-Zn-Cu-Mg, AI-Zn-Mg-Cu (wrought
alloys) with a foaming agent having a decomposition temperature exceeding that
of
the solidus of the aluminum alloy powder matrix. The mixture obtained is
filled in a
split reusable mould which is heated with the powder mixture. Heating of the
powder
mixture is carried out at a temperature which ensures liquid-phase sintering
after
cooling to the temperature 10-20°C below the solidus of most fusible
eutectics. As
a result, the powder mixture looses its looseness. After removing the bottom
of the
mould, the hot mould is placed on the container of a vertical press. The ram
of this
press pushes the sintered powder mixture out of the mould into the press
container,
then a dummy-block is placed and hot compaction of the sintered powder mixture
is
carried out at a low specific pressure to produce porous (86-92 % relative
density),
and easy breakable briquettes. Using highly efficient machines, the cooled
briquettes
are reduced to fragment-shaped chips with powder particles of 0.5-5.0 mm in
size,
chemical composition of which conforms to that of initial aluminum alloy
powder with
uniform distribution of the foaming agent. Then the chips produced are heated
with
a high rate to a temperature below the solidus temperature of the lowest
melting
point eutectic by 10-20°C and is fed in the rolls of a powder rolling
mill. Tangential
stress action results in development of active shear strains in the chips in
the
deformation zone. Chips form new strong metallic bonds which sustain
pronounced
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tensile stresses during formation of a continuous dense hot-rolled sheet with
required
thickness. The hot-rolled sheets produced are cut to blanks which are fed to a
heat
treatment. High-temperature heat treatment is carried out by heating of the
blanks
from below on overheated melts of salts or metals above a temperature of phase
5 transition from solid to liquid state and, based on the visual examination,
foaming is
stopped on the required height. The technical result obtained due to
realization of the
invention incorporates high labor productivity, creation of commercial waste-
free
production of porous semi-finished sheet.
The method of the present invention can be used for production of porous metal
bodies for the civil-engineering machinery, automative and aircraft and other
industries wherein combination of such unique properties of this material as
high
specific strength and rigidity, energy absorption, heat insulation and sound-
proofing,
light weight, incombustibility, buoyancy and absolute environmental
acceptability are
required.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the invention will become apparent
upon
reading the detailed description and upon referring to the drawings in which
figures
1 to 7 are schematic representations of the sequence of steps of a method
according
to a preferred embodiment of the invention. The detailed description of each
figure
is as follows:
Figure 1 schematically represents the step of mixing of metal powders with a
gas
propellant powder;
Figure 2 represents the step of pouring the mixture into a reusable mould and
of
precompacting the mixture by vibration;
Figure 3 represents the step of liquid phase sintering;
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Figure 4 represents the step of pushing a liquid phase sintered briquette from
a
reusable can into press mould;
Figure 5 represents the step of compaction of the sintered briquette;
Figure 6 represents the step of reducing the body into chips, and continuously
hot
rolling the chips into semi-finished dense product;
Figure 7 represents the step of foaming;
Figure 8 is a phase diagram of the powder alloy: AI-Zn-Cu-Mg; and
Figure 9 is a phase diagram of the powder alloy: AI-Mg-Cu-Mn.
While the invention will be described in conjunction with an example
embodiment,
it will be understood that it is not intended to limit the scope of the
invention to such
embodiment. On the contrary, it is intended to cover all alternatives,
modifications
and equivalents as may be included as defined by the appended claims.
DESCRIPTION OF A PREFERRED EMBODIMENT
The purpose of the present invention is the production of complex shape
products
out of continuous hot-rolled sheets of commercial sizes made from the chips
produced from porous brittle hot-compacted briquettes. The technical result
obtained
due to realization of the invention incorporates a dramatic increase in
product yield
(creation of a waste-free technology), a reduction in manufacturing cost of
porous
products, a broadening of the range of products in terms of both their
geometrical
sizes and density.
Referring to figures 1 to 7, the method of production of porous products from
aluminum alloys incorporates mixing of an aluminum alloy powder (1) comprising
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metal particles of an aluminum alloy selected from the group consisting of: AI-
Cu-Mg-
Si, AI-Mg-Si, AI-Mg-Cu-Si (cast alloys), AI-Cu-Mg-Mn, AI-Mg-Cu, AI-Zn-Cu-Mg,
AI-Zn-
Mg-Cu (wrought alloys) with a powder of a foaming agent (2), the foaming agent
(2)
having a decomposition temperature exceeding that of solidus of the aluminum
alloy
powder matrix. The mixture (5) obtained is filled in a split reusable mould
(6) which
is heated with the powder mixture (5), as shown in figure 3. Heating of the
powder
mixture (5) is carried out at a temperature which ensures liquid-phase
sintering after
cooling to 10-20°C below solidus temperature of the lowest melting
point eutectic.
As a result, the powder mixture now in the form of liquid phase sintered
briquettes
(12) loses its looseness. After disassembling of the mould (6), the hot mould
is
placed on the container (14) of a vertical press. The ram (13) of this press
(14)
pushes the sintered powder mixture (12) into the press container (14), then
dummy-
block is placed and hot compaction of the sintered powder mixture is carried
out at
a low specific pressure, as shown in figure 5. The hot-compacted briquettes
(15}
produced show a density of 86-92 rel. %. These briquettes (15) compacted at a
low
pressure are porous (8-14 rel. %) and brittle, thus easily breakable.
Referring to
figure 6, using highly efficient machines, the cooled briquettes (15) are
reduced to
fragment-shaped chips (18) with chips particles of 0.5-5.0 mm in size,
chemical
composition of which conforms to that of initial aluminum alloy powder with
uniform
distribution of the foaming agent. Then, the chips (18) produced are heated
with a
high rate to a temperature which is 10- 20°C below the solidus
temperature of the
low-melting point eutectic of the aluminum alloy. Then, the heated chips (18)
are fed
in the rolls (22) of a powder rolling mill. Tangential stress action results
in
development of active shear strains in chips in the deformation zone. Chips
form new
strong metallic bonds which sustain pronounced tensile stresses during
formation of
a continuous dense hot-rolled sheet with required thickness. The hot-rolled
sheets
(23) produced are cut to blanks which are fed to a heat treatment . Referring
to figure
7, high-temperature heat treatment is carried out by heating of the blanks
(24) from
below on overheated melts of salts or metals (27) at a temperature above a
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temperature of phase transition from solid to liquid state and, based on the
visual
examination, foaming is stopped on the required height.
According to a preferred aspect of the invention, the hot-rolled sheet blanks
are
subjected to a high-temperature heat treatment via heating of the blanks above
a
temperature of phase transition from solid to liquid state and the foaming
process is
stopped by a plate fixed at a required height when the foaming process is
examined
visually. Alternatively, the foaming process is stopped by a plate with the
required
technological projections on its internal surface and fixed at the required
height when
the foaming process is examined visually.
According to another preferred aspect of the invention, the hot-rolled sheet
blanks
are used to form a composite block comprising a lower metallic sheet with a
special
coating, the hot-rolled blank placed on the surface of the lower metallic
sheet, and
an upper metallic sheet with the same coating is located above the hot rolled
blank
at a certain height. Connecting cross elements are provided to ensure bracing
of the
upper and lower metallic sheets. The cross elements simultaneously play a role
of
fastening and connecting elements. For foaming the product, the composite
block
is heated from below in a furnace up to a temperature of phase transition from
solid
to liquid state. The foaming is watched visually and when the foaming aluminum
blank reaches the upper second metallic sheet, the composite block is removed
out
of the furnace and cooled.
Preferably, the upper metallic sheet is a volumetric sheet stamping of a
desired
shape.
Also preferably, the chips (18) are classified by grain sizes from 1.5 up to
5.0 mm,
each size fraction is mixed with fine refractory material powders passive to
aluminum,
then the mixture is filled in moulds and heated in a furnace up to a foaming
temperature which exceeds the transition temperature from solid to liquid
state by
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50-70°C; after completion of the foaming process, the mixture is
screened to
separate the refractory material powders from porous granules.
According to another preferred embodiment of the invention, the chips produced
are
classified by grain sizes from 1.5 up to 5.0 mm, each size fraction is heated
up to a
temperature below the solidus point of the alloy by 10-100°C and then
dispersed as
a monolayer on a flat heated surface and then the fragment-shaped chips are
pelletized by circular movements of a heated massive disk-shaped plate; then
each
fraction of the pellets produced is mixed with fine refractory material
powders passive
to aluminum and then the mixture is filled in moulds and heated up to a
foaming
temperature which exceeds transition temperature from solid to liquid state by
50-
70°C; after completion of the foaming process the mixture is screened
to separate
spherical porous granules from the fine refractory material powder.
According to a further preferred embodiment, the chips produced are classified
by
grain sizes from 1.5 up to 5.0 mm, each size fraction is dispersed as a
monolayer on
a special base, heated from below up to a temperature of phase transition to
liquid
state; when it is examined visually that the foaming particles reach the
desired size,
the foamed granules are removed out of the furnace.
The foamed granules may then preferably be mixed with a resin and injected
into the
internal space of any structural element comprising one or more hollow pieces.
The
resin is cured to increase stiffness and energy absorption of the structural
element.
According to another aspect of the invention, the chips which are not screened
to
size fraction are used to form a composite block which contains a flat
metallic sheet
with special coating on the surface of which a layer of chips is dispersed
and, above
this layer, at a certain height, the second metallic sheet with special
coating, stamped
beforehand for the desired component, is located and after this, the composite
block
formed is heated in a furnace up to a foaming temperature which exceeds the
transition from solid to liquid state by 50-70°C and when it is
examined visually that
CA 02332674 2001-O1-29
the foaming particles reach the upper metallic layer, the block with foamed
powders
is removed out of the furnace and cooled. (To provide for heating of the block
in inert
atmosphere).
Preferably in this case, to ensure bracing between the sheets, they are
fastened
5 together by connecting cross-pieces which simultaneously play a role of
fastening
connecting elements.
Also preferably, the chips which are not screened to size fraction can be used
to fill
to the desired volume fraction the internal space of any structural element
comprising
one or more hollow pieces. The whole assembly is heated above a temperature of
10 transition from solid to liquid state of an alloy to form porous filler
(core).
According to a further aspect of the invention, the rolling of the heated
chips is
conducted together and between two or more heated metal sheets to produce a
composite body. The produced composite body is heated above a temperature of
transition from solid to liquid state of an alloy to form a multilayer
structure with
porous core and metallic bonds between core and facings.
The possibility of realization of the invention characterized by the above-
mentioned
set of the signs and the possibility of the realization of the purpose of the
invention
can be corroborated by the description of the following examples.
Example 1
The example of the realization of the method for production of flat porous
semi-
finished products is as follows.
AI-Mg-Cu-Mn aluminum alloy powder (a liquidus temperature of the alloy is 640-
645°C, a temperature of low-melting point eutectic is 505°C) of
300 kg in weight was
mixed with TiH2 foaming agent of 3.25 kg in weight (a decomposition
temperature
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is 690°C and filled in a split mold of 340 mm diameter, 800 mm in
height with internal
space of 290 mm in diameter. Figure 8 shows a vertical cross section of the
phase
diagram of AI-Mg-Cu-Mn alloys. Hatched zone in this figure represents alloys
used
in the process. As can be appreciated, the average solidus temperature is
503°C and
the liquidus temperature is approximately 650°C. The powder mixture was
compacted by vibration to obtain a density of 1.75-1.8 g/cm3. Weight of the
mixture
in each mould was from 97 up to 100 kg. The powder mixture was heated at a
temperature of 510-515°C to ensure liquid-phase sintering after cooling
down to a
temperature of 480-485°C, the powder mixture lost its looseness. After
disassembling of the mould, the hot mould was placed on the container of a 10
MN
or 15 MN capacity vertical press. The diameter and height of the container
were 300
and 800 mm respectively. The ram of the press pushed the sintered powder
mixture
into the press container, then a dummy-block was placed and hot compaction of
the
sintered powder mixture was carried out at a low specific pressure of 140-200
MPa.
The hot-compacted briquettes produced showed a density of 86-92 rel. %. After
cooling the briquettes were reduced on special machines to fragment-shaped
chips.
Then the chips produced was heated at a high rate in a furnace and fed on a
powder
rolling mill on which 290 kg of sheet was continuously rolled. The hot-rolled
sheet
was cut to 1000 x 1200 x 5 mm blanks using shears installed behind the rolling
mill.
The first part of the blanks was used for free foaming without clamping from
above.
High-temperature heat treatment was carried out by heating of the sheet blanks
from
below on overheated melts of salts. Based on the visual examination, the
foaming
process was stopped by quick removal of the foamed sheet from the furnace when
thickness was 24.5 mm. The size of the porous semi-finished products was
1005 x 1210 x 24.5 mm. The lower surface of the porous sheets was smooth,
while
the upper surface was smooth also but had traces of bulgings appeared due to
internal pores. The density of the porous semi-finished products produced was
0.56-
0.58 g/cm3. Porous semi-finished product yield was 95%.
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The second part of the sheet blanks was foamed in the limited space in terms
of
height. Heat treatment of the sheet blanks was carried out from below at a
temperature of overheated salt melt. When a thickness of 24.5 mm was obtained,
the foaming process was stopped by a plate fixed at this height. The
completion of
the foaming process was determined visually also. The foamed block was quickly
removed out of the furnace. The density of the porous semi-finished products
produced was 0.56 g/cm3. The upper surface of the porous sheets was dead and
smooth. Porous semi-finished product yield was 95-97%.
Scrap was insignificant, in the form of side crops formed after trimming of
the hot-
rolled sheets. The production process is developed by such a way, that scrap
is
almost absent and it is reduced into chips and brought back into hot-rolling
process.
Heating of the primary powders above a temperature of appearance of low-
melting
point eutectic by 10-20°C and subsequent cooling below this temperature
by 20-30°C
ensure development of liquid-phase powder sintering. The powder mixture in
this
state loses its looseness and can be easily pushed from the mould into the
press
container. The first source of appearance of extremely low hydrogen amounts is
decomposition of TiH2 at a heating temperature. The second source is surface
hydrogen appeared due to reaction of sorbate (H20 molecules) with aluminum
cations which diffuse through an oxide film. Surface hydrogen and hydrogen
formed
due to decomposition of TiH2 leave the porous briquettes partially, while the
largest
hydrogen amount is capable of dissolving in appeared low-melting point
eutectic.
Then, hot compaction operation at a low pressure of 140 or 200 MPa is carried
out.
Pressure applied to a sintered briquette is able to form only a porous
briquette. The
porous state is necessary only to facilitate production of the chlpS on
special
machines as these chips are the main initial material for the present
invention. It is
clear that the main operation i.e. hot compaction is a waste-free process in
reality.
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Heating and feeding of the heated chips on a powder rolling mill are almost
waste-
free operations also, while insignificant amounts of torned side edges formed
during
hot rolling shall be trimmed after the rolling and recycled in the process.
Mastered
technological process for foaming of dense hot-rolled sheets results in high
product
yield. Therefore, the yield of 95% measures up to real value.
The effect of the heating temperature on the chips should be specially
discussed.
The heating temperature of the chips prior to rolling preferably complies with
hard
high plastic state, that is to say, it should preferably be below a
temperature of low-
melting point eutectic formation by 40-60°C. When the heated chips is
fed on a
powder rolling mill at a temperature at which liquid phase, low-melting point
eutectic
is absent, chips are subjected to sequentially followed actions of the force
field. At
first, intensive compaction of the hot chips is carried out. The compacted
chips which
move to deformation zone come to the force field which works between rolls and
causes appearance of tangential stresses which result in development of shear
strains which destroy initial particles and form new contacts between new
juvenile
surfaces. Combination of a pressure and temperature ensures formation of
strong
metallic bonds on new interaction contacts between particles due to active
diffusion.
The formed strong metallic bonds are capable of sustaining noticeable tensile
stresses in the deformation zone. As a result, when process parameters are
chosen
correctly, dense continuous sheets from 1.5 up to 6.0 mm in thickness can be
formed.
Heating of the chips prior to rolling at a temperature above that of the
appearance
of low-melting point eutectic by 10-20°C results in the formation of
the liquid phase.
The liquid phase works as a lubricant in the zone of active deformation and
appearing tangential stresses and does not result in the appearance of shear
strains.
In the zone of active deformation, wherein extra rise of a temperature due to
friction
forces is observed, the liquid phase in the deformation zone is the weakest
point in
the structure of a forming sheet. Tensile and tangential stresses destroy the
sheet
CA 02332674 2001-O1-29
14
continuum which is not formed. A hot-rolled sheet demonstrates cracks on the
side
edges, separation of sheet pieces, and in some cases, failure of the sheet
takes
place. The main cause of powder rolling process instability of this kind is
formation
of the liquid phase in chips particles during heating prior to rolling.
If the heating of the primary powder mixture is performed at a temperature of
10-
20°C, which is below that of low-melting point eutectic formation, the
sintered powder
mixture obtained will retain its looseness. Transportation of the disassembled
mould
to the press container will be impossible, a briquette structure will be
loose.
Example 2
The example of realization of the method for production of porous semi-
finished
products from the chips is as follows:
AI-Zn-Cu-Mg aluminum alloys powder (a liquidus temperature of the alloy is 630-
640°C, a temperature of low-melting point eutectic formation is
480°C of 210 kg in
weight was mixed with CaC03 foaming agent of 12 kg in weight (a decomposition
temperature is 720°C) and filled in a split mold of 340 mm in diameter,
800 mm in
height with internal space of 290 mm in diameter. Figure 9 shows surfaces of
crystallization (surfaces of liquidus) of the powder alloy AI-Zn-Cu-Mg
containing Zn-
4,5%, Cu 3,5-4,5%, Mg -negligible, AI-balance. The alloys used are in the AL
corner
of the diagram (small hatched zone) and have a liquidus temperature of
650°C.
Solidus of these alloys is in the interval of temperatures of 510-
520°C.The powder
mixture was compacted by vibration to obtain a density of 1.75-1.8 g/cm3.
Weight
of the mixture in each mould was from 97 up to 100 kg. The powder mixture was
heated at a temperature of 490-500°C to ensure liquid-phase sintering
after cooling
down to 450-460°C and the mixture lost its looseness. After
disassembling of the
mould, the hot mould was placed on the container of a 10 MN or 15 MN capacity
vertical press. The diameter and height of the container were 300 and 800 mm
CA 02332674 2001-O1-29
respectively. The ram of the press pushed the sintered powder mixture into the
press
container, then a dummy-block was placed and hot compaction of the sintered
powder mixture was carried out at a low specific pressure of 140-200 MPa. The
hot-
compacted briquettes produced showed a density of 86-92 rel.%. After cooling,
the
5 briquettes were reduced on special machines to fragment-shaped chips.
To realize the method, the chips produced were graded into grain sizes of 2.0,
3.0,
4.0 and 5.0 mm, each size fraction was mixed with fine refractory material
powders
passive to aluminum, the mixture was filled in moulds, heated in a furnace at
a
foaming temperature which exceeds the transition temperature from solid to
liquid
10 state by 50-70°C; after completion of the foaming process, the
mixture was screened
to separate the refractory material powders from porous granules. The porous
granules from 3.0 up to 10.0 mm in size and 0.3 up to 0.9 g/cm3 in density are
a
good filling agent for any shape of cases for energy absorbing components used
in
the automotive industry.
15 An easier technique for realization of the chips of the same alloy, graded
into grain
sizes of 2.0, 3.0, 4.0 and 5.0 mm is discussed below. Each fraction was
dispersed
as a monolayer on a special base, heated in a furnace from below on overheated
melt of salt up to a foaming temperature; when it was examined visually that
the
foamed granules reached the desired size, they were removed out of the furnace
and cooled. The granules had a hemispheric shape with radius from 5.0 up to
20.0
mm and a density from 0.4 up to 1.0 g/cm3.
Porous granules of this size and shape can find application for production of
volumetric noise suppression and fire barrier components and also large-size
shock
absorption elements. Product yield is 100%.
Although preferred embodiments of the present invention have been described in
detail herein and illustrated in the accompanying drawings, it is to be
understood that
the invention is not limited to these precise embodiments and that various
changes
CA 02332674 2001-O1-29
16
and modifications may be effected therein without departing from the scope or
spirit
of the present invention.
