Note: Descriptions are shown in the official language in which they were submitted.
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Metal powder mixture and the use of the same
The invention relates to a metal powder mixture and particularly advantageous
uses of
such a metal powder mixture.
It is common to use such metal powder mixtures made of metal and metal alloy
powders
in order to be able to produce materials with a certain alloy composition.
In doing so, metal alloy powder mixtures containing a selection of alloy
elements and at
least one metal corresponding to the metal powder used are also used for such
metal
powder mixtures. These are heterogeneous metal powder mixtures consisting of
at least
two chemically and/or morphologically different components constituting.only
an
intermediate phase along the way to the formation of a desired metal alloy
respectively of
a requirement-compliant material to be produced during heat treatment.
Metal powder mixtures consisting of a fine fraction (e.g. 10% < 10pm)
occurring naturally
during production and a residual gross fraction (e.g. 90% < 45pm and 10% >
10Nm).
Such mixtures have the advantage of allowing for an improved filling density
on the basis
of the favourable space utilisation with adapted particle size distribution.
On the basis of metallurgical reasons metal powders and metal alloy powders
normally
are characterised by different fusing and phase conversion behaviour. Thus,
there are
temperatures respectively temperature ranges where there are solid and liquid
(e.g.
eutectic) phase fractions at the same time respectively a
modification/increase of the
mass transfer, e.g. on the basis of an increased diffusion coefficient of the
element, can
be observed on the basis of phase conversions in the solid phase. However, as
the
mentioned metal powder mixtures normally are not used for the metallurgical
melting
production of alloyed materials, the aforementioned plays a significant role
for the final
material production.
Thus, during the heat treatment that has to be implemented for the formation
of the
desired metal alloy or a desired metal, the atomic and microscopic transfer
properties
(e.g. diffusion, relocation, grain growth) efficient in the contact area of
the powder
particles form the basis for the compression and homogenisation of the
initially
heterogeneous mixture of metal and metal alloy powder or a metal powder. The
term
heterogeneous within this meaning also comprises the difference as regards to
the
particle size distribution (coarse, fine fraction) causing the mass transfer
on the basis of
the high levels of activity of the fine particle fraction, as opposed to a
(mostly) monomodal
distribution of the particle size. In this, the effects to be demonstrated by
means of caloric
methods for the individual powders or the powder mixture (phase conversions,
melting
and solidification ranges) and the effects to be utilised via the chemical
composition of
the alloy, e.g. chemically graded between adjacent powder particles, play a
central role
during alloy or metal formation and representation of the material.
The formation of the desired metal alloy in forrn of a fine powder (d50 <
10pm) for the
powder metallurgical production of corresponding materials/components is
difficult from a
technical and economic point of view and only has a low commercial importance
when
processing such powders on the basis of the occurring issues. During the
required heat
treatment (sintering) and on the basis of the high required temperatures,
there are losses
of essential components on the basis of their high steam pressures during
powder
metallurgical processing, at which the same result in modifications to the
alloy
composition aimed at and cause technical problems within the thermo processing
systems on the basis of material accumulation.
The corresponding powder mixtures are known from DE 10331785 Al and DE 10 2005
001198 Al.
These metal powder mixtures are produced from at least two or even three
different
powders. In this, the individual powders are to be formed from different metal
alloys and
are to be characterised by a narrow particle size distribution.
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However, the metal powder mixtures described within the framework of this
state-of-the-
art have to be produced in a very costly manner, in order to be able to reach
the desired
very low particle size in particular, which is difficult with some metal
alloys, e.g. ductile
metal alloys.
Problems also arise on the basis of the fact that a homogeneous distribution
of the
individual metals that are to be used to form the desired metal alloy can be
achieved to a
very limited extent only within the finally produced material. Thus, it is
hardly possible to
form fine structures as the same are required for cellular materials for
example. Applying
this concept it remains difficult to achieve a homogeneous distribution of the
alloy
components. This, in turn, results in the fact that the material properties
will deteriorate,
particularly the tendency for corrosion and the mechanical strength.
Producing metallic micro-powders (d50 < 10pm) from metal compounds to be
reduced
easily is possible without any efforts in accordance with the state-of-the-
art. Metal alloy
powders designed to form a part of a desired metal powder mixture can be
produced in a
cost-efficient manner by atomising (gas or water atomisation) an alloy melt or
conventionally by means of melting and crushing, if particle sizes d50 < 45Nm
are aimed
at. Producing metal powders that are coarser is also possible by atomisation.
Thus, it is the assignment of the invention to provide a metal powder mixture
that can be
used to obtain a material composed of a metal alloy in a cost-efficient manner
after the
implementation of a heat treatment, at which the material has to be
characterised by a
more homogeneous distribution of the individual alloy- and metal-forming
components
(alloy and element powders). In a second aspect it has to be able to reduce
the maximum
temperature required within the framework of the heat treatment for the
production of the
material.
In accordance with the invention this assignment can be solved by means of a
metal
powder mixture characterised by the features of claim 1. Suitable applications
of such a
metal powder mixture can be found in claim 16.
Advantageous embodiments and further development of the invention can be
obtained
with the features mentioned in inferior claims.
In this, the metal powder mixture in accordance with the invention is composed
of at least
two different powder fractions. For the first powder fraction, a metal powder
is used that is
composed of a metal alloy, which contains a first metal, as regards to which
in connection
with the other alloy components of the first powder fraction contained in the
metal alloy
the beginning of a phase conversion takes place at a temperature that is at
least 200 K
lower than the beginning of the melting of a material to be formed from the
metal powder
mixture by means of heat treatment.
The first powder fraction has an average particle size d50 < 45pm.
The second powder fraction contained within the metal powder mixture in
accordance
with the invention is preferably composed of a single second metal that is
part of the
metal alloy of the first powder fraction. However, it may also be composed of
a mixture of
at least two metals. This powder fraction has an average particle size d50 <
10Nm. In this,
for the second powder fraction the term "single metaP" means a metal that
consists nearly
completely of the single metal or the mixture and at which only very small
alloy additions
or contaminations are admissible, up to a maximum of 3 weight %. In case of a
second
powder fraction that is composed of a mixture of at least two metals, one of
the metals
should be present with a significantly higher share than a further metal
contained therein.
In this, the share should be at least 75 weight % for a metal. This metal is
called second
metal in the following.
The average particle size of the first powder fraction should be at least
three times higher
than the average particle size of the second powder fraction. The second
powder fraction
should be contained with a share of at least 1 weight %.
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As regards to the first powder fraction, a binary metal alloy can be used,
i.e. a metal alloy
consisting of two components. However, it is rnore cost-efficient to use a
metal alloy
composed of at least three different metals for the first powder fraction.
In this, at least one metal is to be contained in the first powder fraction in
the
corresponding metal alloy with a share corresponding to the double value of
the share
that is supposed to be contained in a material formed with the metal powder
mixture upon
implementation of a heat treatment. The share of the second metal in the metal
alloy of
the material produced after the'heat treatment should be at least 10 weight %.
A metal, the phase conversion of which is implemented at the lower temperature
already
mentioned above, can be used within the metal alloy of the first powder
fraction, at which
the same is selected from aluminium, magnesium, zinc, tin, and copper. In
connection
with other alloy components of the first powder fraction, these metals have
the property of
lowering melting temperatures of the metal alloy or reaching partial volume
phase
conversions, including molten conditions.
Powdery iron, nickel, cobalt, and copper can be used as metal for the second
powder
fraction. In this, one of these metals can be contained alone within the
second powder
fraction. However, the second powder fraction can also be composed of at least
two of
these metals as powder mixture.
One option of producing a metal powder mixture in accordance with the
invention is to
use a metal alloy for the first powder fraction ttiat is characterised by a
general
composition M1M2CrR. In this, metal Ml is selected from aluminium, magnesium,
tin,
zinc, and copper. The metal M2 is selected from iron, nickel, and cobalt. R is
selected
from yttrium, molybdenum, tungsten, vanadium, manganese, a rare earth metal, a
lanthanide, rhenium, hafnium, tantalum, niobium, carbon, boron, phosphor, and
silicon.
In such a metal alloy, metal Ml can be contained with a share of 1-70 weight
%, metal
M2 can be contained with a share of 1-60 weight %, Cr can be contained with a
share of
0-80 weight %, and R can be contained with a share of 0-70 weight %.
Furthermore, it is advantageous to design aluminium with a share of at least
15 weight %
in the metal alloy for the first powder fraction. This way, a share of
aluminium of at least 3
weight %, preferably at least 15 weight %, can be contained in a material
obtained after a
heat treatment to be implemented, at which the material has been produced from
the
metal powder mixture in accordance with the invention.
If, within a metal powder mixture in accordance with the invention, a powder
is used in a
first powder fraction that is formed with an alloy containing iron, chrome,
and aluminium,
together with a second powder fraction composed of powdery iron, for example,
a
material can be produced upon implementation of the heat treatment
characterised by a
share of chrome of 15 to 30 weight % and a share of aluminium of 5 to 20
weight %,
along with predominantly iron.
In a metal powder mixture in accordance with the invention, a second powder
fraction
should comprise at least 10 weight %, preferably at least 30 weight %, and-
particularly
preferably at least 50 weight % of the entire mass.
Within the powder the first powder fraction is composed of, the metal
achieving a phase
conversion temperature in connection with the other alloy components that is
at least 200
K lower than the temperature of the beginning of melting of the material to be
produced
(transition temperature) should be contained with a share of at least 10
weight %.
Using the metal powder mixture in accordance with the invention, materials can
produced
upon implementation of a heat treatment, in which all metal components.are
present in a
significantly more homogeneous distribution within the material volume than is
the case
with metal powder mixtures used traditionally. In this, the heat treatment can
be
implemented at temperatures that are at least '10 K lower than the temperature
required
in accordance with the state-of-the-art.
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In this, the combination of the correspondingly selected two powder fractions
in
accordance with the invention and moreover the use of a significantly finer
powder for a
second powder fraction with the substantially smaller particle sizes than is
the case for
the first powder fraction mentioned above is advantageous.
During heat treatment, it is possible to achieve a significantly increased
mass transfer on
the basis of diffusion, relocation, and grain growth using the invention. This
way, it is also
possible to achieve a reduced maximum temperature required for the heat
treatment
during the production of the material from the metal powder mixture, along
with the
homogeneous distribution of the individual metal alloy components.
The heat treatment can be implemented by means of using known sintering
technologies.
However, the same should be suitable for the desired sintering atmospheres and
temperatures.
Using a metal powder mixture in accordance with the invention, an improved
sintering
behaviour of a formed body obtained on the basis of slurry or by pressing.
On the basis of an improved shrinking behaviour, the metal powder mixture in
accordance with the invention can be used to improve the properties of a
component
produced thereof or of a corresponding protective layer applied to a
component.
Furthermore, the corrosion resistance may be improved as well. This way, the
corrosion
protection can be improved on the basis of the targeted formation of a
corresponding
oxide layer on the surface, at which the same is normally characterised by a
layer
thickness of 0.1 - 10pm, if the metal powder mixture in accordance with the
invention
contains aluminium.
A material produced by using the metal powder mixture in accordance with the
invention
may be characterised by an improved pitting corrosion potential when compared
to high-
alloy corrosion resistant steels.
Within the framework of a further alternative embodiment of the invention, the
metal
powder mixture may also contain a further fraction composed of a metal. This
preferably
can be iron, at which the same should contain contaminations and trace
elements with a
share of less than 3 weight %, if containing the same at all. In this, the
further fraction can
also be powdery and significantly more coarse-grained than the two powder
fractions
described above. This way, the average particle size may be higher than 150pm
and
significantly above the aforementioned value. However, the further fraction
can also be
composed exclusively of fibres or contain fibres along with particles.
The fibres may be characterised by diameters in the range of around 1 mm and
length
values of several millimetres.
If the metal powder mixture in accordance with the invention contains a
further fraction,
the share of the second powder fraction may be very low, i.e. below 5 weight
%.
The application of layers onto surfaces of components can be implemented with
the
technologies known per se, such as thermal injection or deposition welding.
Components or parts thereof may also be produced using the so-called rapid
prototyping
procedure. In this, it is recommendable to implement a heat treatment in
addition upon
completion of the production, in order to be able to achieve an even higher
density and
homogeneity.
As regards to a material produced by using a metal powder mixture in
accordance with
the invention, at which the same is composed of a metal alloy NiCrAl, chrome
evaporation can be avoided or at least reduced significantly as opposed to the
solutions
known in accordance with the state-of-the-art, because the temperatures during
the heat
treatment are lower. This is particularly applicable to the comparison to
metal powder
mixtures already characterised by the corresponding alloy composition of the
material to
be produced before the heat treatment is implemented.
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In the following, the invention is to be explained in more detail in an
exemplary manner.
In this:
Figure 1 shows a part of a shell of a hollow ball consisting of an FeCrAI
alloy with five
positions where the chemical analysis of the elements Fe, Cr, and Al has been
implemented;
Figure 2 shows a part of a hollow ball consisting of an FeCrAI alloy with a
lower porosity
of the shell;
Figure 3 shows a graph demonstrating the mass increase during the removal of
FeCrAI
hollow balls; and
Figure 4 shows a cross-section polish with chemical point analysis of an
FeCrAI fibre.
Example 1
In this, a first powder fraction having an average particle size d50 of 25pm
and a
composition of the alloy Ni-50Cr-25AI-0, 125Hf, as well as a second powder
fraction
having an average particle size d50 of 5pm mainly consisting of nickel (99.9
weight %) are
to be used within the framework of the production of metallic hollow balls of
a material Ni-
20Cr-10AI-0.05Hf.
The share of the first powder fraction is 40 weight % and the share of the
second powder
fraction is 60 weight %.
100g of the metal powder mixture are dispersed with 100g of water, 3g of
polyvinylalcohol, and 0.5g Dolapix in a disperser for a period of 1 h at a
speed of
3000RPM, in order to obtain a homogeneous <iistribution of the particles in a
suspension.
The suspension obtained by means of the aforementioned does not sediment when
stirring intensively.
The obtained low-viscosity metal powder suspension is applied to spherical
particles
made of polystyrene as coating and dried. After the coating on the polystyrene
particles
has reached a layer thickness of 100pm, the heat treatment may be implemented.
In this, the works are implemented in an atmosphere with flowing hydrogen
(30L/min).
Initially, the organic components are decomposed thermally, at which the
heating
procedure takes place at a heating rate of 1 K/rnin until a temperature of 600
C is
reached. Afterwards, the temperature is increased to up to 1280 C, at which a
heating
rate of 5K/min is maintained. Upon expiration of the holding time of 2h at
maximum
temperature, a cool-down procedure to room temperature has been implemented at
5K/min.
The metallic hollow balls produced by means of the aforementioned were
characterised
by external diameters of approx. 2mm and a wall thickness of approx. 70Nm. The
bulk
density is 450g/L. The shell material of the metallic hollow balls that has
been produced
with this example of a metal powder mixture in accordance with the invention
is
composed of 20 weight % chrome, 10 weight % aluminium, and 0.05 weight %
hafnium,
along with nickel.
Within the framework of a parallel test example, an alloy of Ni-20Cr-10-AI-
0.05Hf is
processed to a fine alloy powder with a particle size d50 of 10pm by means of
inert gas
atomising a metal alloy. At an analogue approach as with the example in
accordance with
the invention, this powder is subjected to the steps suspension production,
coating,
drying of polystyrene particles, and heat treatment. The metallic hollow balls
produced by
means of the aforementioned achieved significantly higher ultimate strength
values than
the balls produced with the metal powder mixture in accordance with the
invention,
measured on the basis of the deformation until rupture.
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Example 2
A metal powder mixture containing a first powder fraction characterised by an
average
particle size d50 of 15pm and a second powder fraction characterised by an
average
particle size d50 of 3pm was used. For the first powder fraction an Fe-49Cr-
23AL alloy
was selected and the second powder fraction was mainly composed of iron (99.5
weight
%)
In analogy to example 1, 43.5% of the first and 56.5% of the second powder
fraction have
been processed. Within the framework of figures 1 and 2, the homogeneity of
the shell
material becomes obvious by means of a cross-section through the shell of a
hollow ball
produced by means of the aforementioned. The shell material produced with the
metal
powder mixture in accordance with this example was an Fe-23Cr-10AI alloy.
The sintering procedure in a hydrogen atmosphere was implemented at 1240 C
during a
period of 2 hours.
Upon completion of the sintering procedure, an apparent density (FD) in
accordance with
ASTM D212/417 of approx. 0.4g/cm3, a carbon content of 70ppm, and a share of
oxygen
of 0.24% could be achieved with the hollow balls produced by means of the
aforementioned. Using table 1 shown in figure 3, the oxidation properties of
the material
obtained by means of the aforementioned can be demonstrated on the basis of
the mass
increase after 700 hours at holding temperatures between 900 C and 1000 C
without
and with previous removal to air at 1100 C for 2 hours.
Example 3
Metallic fibres of the composition Fe-20-Cr-9AI were produced from a metal
powder
mixture containing a first powder fraction having an average particle size d50
of 8pm of an
Fe-49Cr-23AI alloy with a weight of 2kg, a second powder fraction having an
average
particle size d50 of 3pm and a weight of 0.1 kg consisting of pure iron, and
3kg of a further
fraction consisting of metallic fibres, the average outer diameter of which
was 150pm and
the average length of which was 5mm.
In this, the metallic fibres that have been provided by milling from a block
of the purest
iron (99.9% iron) were circulated in a 5 litres Eurich mixer at a speed of
20RPM. The
temperature of the mixing container was maintained at 50 C 10K by blowing at
the
same with heated air.
A dispersion was made from 2kg of the first and 0.1 kg of the second powder
fraction, as
well as 2kg acetone and 0.2kg polyvinylalcohol (PVA). This dispersion was
sprayed into
the mixer in a centralised manner during the circulation procedure until the
entire powder
mass had been applied to the surface of the fibres. In doing so, the required
safety
regulations on the basis of the organic flammable components have been
complied with.
The fibres coated by means of the aforementioned were placed into a pan
consisting of
A1203 and subjected to a heat treatment durinq a period of 2 hours at 1240 C
in a
hydrogen atmosphere. In this, PVA was expelled, as described in example 1.
Figure 4 shows a cross-section polish through a fibre produced by means of the
aforementioned. On the basis of chemical poirit analyses, the composition of
the fibre
material upon completion of the heat treatment could be determined as
homogeneous
Fe-19Cr-9AI alloy.
Example 4
In this, a metal powder mixture with a first powder fraction characterised by
an average
particle size d50 of 4.4pm and a second powder fraction characterised by an
average
particle size d50 of 3.Opm was used. For the first powder fraction an Fe-49Cr-
23AI alloy
was selected and the second powder fraction mainly consisted of iron (99.5
weight %).
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100g of this metal powder mixture (45g of the first powder fraction and 55g of
the second
powder fraction) are dispersed with 100g of water, 3g of polyvinylalcohol, and
0.5g
Dolapix in a disperser for a period of 2h at a speed of 3000RPM, in order to
obtain a
homogeneous distribution of the particles in the suspension.
In accordance with the so-called Schwartzwatder procedure, as described
amongst
others in US 3,090,094 for example,'an open-cell porous metal foam is
produced. In this,
a reticulated polyurethane foam cut into individual pieces and having a
porosity of 80ppi
and dimensions of 40*40*10mm of the pieces is coated with the metal powder
binder
suspension. In this, the polymer foam structure is to be coated with the
suspension as
completely as possible. The coated pieces then were dried for a period of 2h
at a
temperature of 60 C.
Afterwards, a heat treatment in a hydrogen atrnosphere was implemented. In
this, a
heating rate of 1 K/min was used to increase the temperature to a value of 600
C, in order
to remove the organic components. Afterwards, the temperature was increased
further to
1280 C maintaining a heating rate of 5K/min, at which this temperature was
maintained
for 2 hours. Within the framework of the cool-down procedure to room
temperature, a rate
of 5K/min was maintained as well.
Upon completion of the heat treatment and cool-down an open-cell foam
structure with a
physical density of 0.8g/cm3 was obtained, at which the webs of the porous
structure
upon heat treatment were composed of Fe23Cr10AI alloy.
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