Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02531683 2006-O1-06
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Method for producing fme metal, alloy and composite powders
10331785.
The invention relates to a method for producing metal, alloy or composite
powders
with a mean particle diameter D50 of at most 25 Vim, a starting powder firstly
being
formed into flake-like particles and these then being comminuted in the
presence of
grinding aids, and to metal, alloy or composite powders obtainable thereby.
Numerous metallurgical or chemical methods are known for producing metal and
alloy powders. If fine powders are to be produced, the known methods
frequently
start with the melting of a metal or an alloy.
If the melt is divided by atomisation, the powder particles form directly from
the
produced melt droplets by solidification. A large number of possibilities, but
also
limitations to the process, arise depending on the type of cooling (treatment
with air,
inert gas, water), the process engineering parameters used, for instance the
nozzle
geometry, gas speed, gas temperature or the nozzle material, and the material
parameters of the melt, such as melting and solidification points,
solidification
behaviour, viscosity, chemical composition and reactivity with the process
media
(W. Schatt, K.-P. Wieters in "Powder Metallurgy - Processing and Materials",
EPMA European Powder Metallurgy Association, 1997, 10 to 23).
As powder production by atomisation is of particular industrial and economic
significance, various atomisation strategies have become established. Specific
methods are selected depending on the powder properties required, such as
particle
size, particle size distribution, particle morphology, impurities, and
properties of the
melts to be atomised, such as melting point or reactivity, and the tolerable
costs.
Nevertheless there are often limits from economic and industrial points of
view to
attaining a specific property profile of the powder (particle size
distributions,
impurity contents, "designated size" yield, morphology sintering activity,
etc.) at
justifiable costs (W. Schatt, K.-P. Wieters in "Powder Metallurgy - Processing
and
Materials", EPMA European Powder Metallurgy Association, 1997, 10 to 23).
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Powder production by atomisation primarily has the drawback that large
quantities
of energy and atomising gas have to be used, and this renders the procedure
very
expensive. In particular production of fine powder from high melting alloys
with a
melting point > 1,400°C is not very economical as, on the one hand, the
high melting
point necessitates a very high application of energy for producing the melt
and, on
the other hand, the gas consumption greatly increases as the desired particle
size
decreases. In addition, difficulties often occur if at least one alloy element
has a very
high affinity to oxygen. Cost advantages may be achieved during production of
particularly fine alloy powder by using specially developed nozzles.
In addition to particle production by atomisation, other single-stage melt
metallurgical methods are also frequently used, such as what is known as "melt-
spinning", i.e. pouring a melt onto a cooled roll, whereby a thin, usually
easily
comminutable strip is produced, or what is referred to as "crucible melt
extraction",
i.e. the immersion of a cooled profiled roll rotating at high speed into a
molten
metal, wherein particles or fibres are obtained.
A further important variant of powder production is the chemical method via
reduction of metal oxides or metal salts. However, it is not possible to
obtain alloy
powders in this manner (W. Schatt, K.-P. Wieters in "Powder Metallurgy -
Processing and Materials", EPMA European Powder Metallurgy Association, 1997,
23 to 30).
Extremely fine particles which have particle sizes of less than one micrometre
may
also be produced by the combination of evaporation and condensation of metals
and
alloys and via gas phase reactions (W. Schatt, K.-P. Wieters in "Powder
Metallurgy
- Processing and Materials", EPMA European Powder Metallurgy Association,
1997, 39 to 41). These methods are very expensive, however.
If cooling of the melt takes place in a relatively large volume/block,
mechanical
method steps of coarse, fine and superfine comminution are necessary to
produce
metal or alloy powder that may be processed by powder metallurgy. An overview
of
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mechanical powder production is given by W. Schatt, K.-P. Wieters in "Powder
Metallurgy - Processing and Materials", EPMA European Powder Metallurgy
Association, 1997, 5 to 47.
Mechanical comminution, in particular in mills, as the oldest method of
particle size
adjustment, is very advantageous from an industrial perspective as it
inexpensive
and may be applied to a large number of materials. However, it makes certain
demands on the process material, for example with respect to size of the
pieces and
brittleness of the material. In addition, comminution cannot be pursued as
desired;
rather a grinding equilibrium forms, which also occurs if the grinding process
is
begun with relatively fine powders. The conventional grinding processes are
modified if the physical limits of the capacity for comminution are attained
for the
respective grinding stock and certain phenomena, such as embrittlement at low
temperatures or the effect of grinding aids, improve the grinding behaviour or
the
capacity for comminution.
A method of fine comminution of relatively brittle pre-comminuted material
that is
particularly suitable in many cases involves the concept of gas contra jet
mills of
which there are numerous commercial suppliers, for example Hosokawa-Alpine or
Netzsch-Condux. This method is prevalent and provides, in particular in the
case of
brittle materials, considerable advantages from industrial (low level of
impurities,
autogenic grinding) and financial perspectives compared to conventional mills
using
purely mechanical comminution, such as ball mills or agitated ball mills. Jet
mills
attain their industrial and thus their financial limits with comminution of
ductile
starting powders, in other words materials that are difficult to comminute,
and low
designated particle sizes. This is explained by the decreasing kinetic energy
of the
powder particles being comminuted in the gas jet. As the kinetic energy of the
powder particles is to be applied only via the carrier gas, the specific
energy
requirement during superfine comminution increases to financially
unjustifiable
ranges and in the case of powders with high ductility is practically
inapplicable. The
sintering activity of these powders thus comminuted does not correspond to the
sintering activity of powder particles produced by conventional grinding
either.
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10331785.
Very fine particles may be obtained, for example, by combining grinding steps
with
hydrogenation and dehydrogenation reactions, including the combination of
reaction
products to form the desired phase composition of the powder (LR. Harris, C.
Noble,
T. Bailey, Journal of the Less Common Metals, 106 (1985), L1 to L4). However,
this method is limited to alloys which contain elements that may form stable
hydrides. Mechanical influences on the comminution in the form of lattice
defects or
other defects may thus be substantially avoided. This is particularly
important if the
functional properties of the powder particles, for example the crystallites,
critically
affect the properties of the powder product, such as in NdFeB permanent
magnets.
Said methods always meet their limits if it is a matter of producing very fine
powders of ductile metals or alloys which have both high reactivity to oxygen
and
high sintering activities.
The coldstream process was developed for producing products of this type,
metallic
particles subjected to intense cooling being centrifuged at extremely high
speeds of
up to 1 Mach via a venturi tube onto a cooled panel. It is thus allegedly
possible to
produce a product with a particle size between 5 and 10 ~m (W. Schatt, K.-P.
Wieters in "Powder Metallurgy - Processing and Materials", EPMA European
Powder Metallurgy Association, 1997, 9 to 10). The act of accelerating the
starting
powder to the speed of sound necessitates an extremely high application of
energy in
this method. Furthermore, abrasion problems may occur and, owing to the
interaction between particles and counterplate, critical impurities are
introduced into
the grinding stock.
A further method for producing fine powder from ductile material is mechanical
alloying. In this process agglomerates are obtained by intensive grinding
treatment,
which agglomerates are made up of crystallites that are approximately 10 to
0.01 ~m
in size. The metallic ductile material changes as a result of the high
mechanical
stress in such a way that fine individual particles may possibly form. These
contain
the composition typical of the alloy. However, the drawback of this process is
that
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considerable impurities are sometimes introduced, primarily by abrasion.
Usually,
however, it is precisely the uncontrolled abrasion that is an obstacle to
industrial use.
In addition there is the fact that discrete superfine particles are only
produced after a
very long grinding period. Fine metal and alloy powders therefore cannot be
5 economically produced by mere mechanical alloying.
The object of the present invention therefore consists in providing a process
for
producing fine, in particular ductile, metal, alloy or composite powders, the
method
being particularly suitable for producing alloys, i.e. mufti-component
systems, and
allowing fundamental properties, such as particle size, particle size
distribution,
sintering activity, impurity content or particle morphology to be purposefully
adjusted or influenced.
The object is achieved according to the invention by a two-stage method, a
starting
powder firstly being formed into flake-like particles and these then being
comminuted in the presence of grinding aids.
The invention therefore relates to a method for producing metal, alloy or
composite
powders with a mean particle diameter D50 of at most 25 Vim, determined using
the
particle measuring apparatus Microtrac~ X 100 to ASTM C 1070-Ol, from a
starting powder with a greater mean particle diameter, wherein
a) the particles of the starting powder are processed in a shaping step into
flake-
like particles, of which the particle diameter to particle thickness ratio is
between
10:1 and 10,000:1, and
b) the flake-like particles are subjected to comminution grinding in the
presence
of a grinding aid.
The particle measuring apparatus Microtrac~ X 100 is commercially available
from
Honeywell, U.S.A.
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10331785.
For determining the particle diameter to particle thickness ratio the particle
diameter
and the particle thickness are determined using a Light-optical microscope.
For this
purpose, the flake-like powder particles are firstly mixed with a viscous,
transparent
epoxy resin in a ratio of 2 volume fractions resin and 1 volume fraction
flakes. The
air bubbles introduced during mixing are then expelled by evacuation of this
mixture. The then bubble-free mixture is poured over a planar substrate and
then
rolled out using a roller. The flake-like particles are thus oriented in the
flow field
between roller and substrate. The preferred position manifests itself in that
the
surface normals of the flakes are oriented on average parallel to the surface
normals
of the planar substrate, in other words the flakes are arranged in layers on
average
flat on the substrate. After curing, suitable samples of suitable dimensions
are
worked from the epoxy resin plate on the substrate. The samples are
microscopically
examined perpendicularly and parallel to the substrate. By using a microscope
with
a calibrated lens and by taking into account the adequate particle orientation
at least
50 particles are measured and an average is formed from the measured values.
This
average represents the particle diameter of the flake-like particles.
Following a
perpendicular cut through the substrate and the sample to be examined, the
particle
thicknesses are determined using the microscope with a calibrated lens, which
microscope was also used to determine the particle diameter. Care should be
taken
that only particles located optimally parallel to the substrate are measured.
As the
particles are completely surrounded by the transparent resin, selecting
suitably
oriented particles and reliably assigning the limitations of the particles to
be
evaluated do not present any difficulties. Again at least 50 particles are
measured
and an average formed from the measured values. This average represents the
particle thickness of the flake-like particles. The particle diameter to
particle
thickness ratio is calculated from the previously ascertained values.
In particular fine, ductile metal, alloy or composite powders may be produced
with
the method according to the invention. Ductile metal, alloy or composite
powders
are in this case taken to mean those powders which, in the event of mechanical
stress
until the yield point is reached, undergo plastic expansion or deformation
before
significant material damage (material embrittlement, material rupture) occurs.
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Plastic material changes of this type are dependent on the material and are in
the
range of 0.1 per cent up to several 100 per cent, based on the starting
length.
The degree of ductility, i.e. the capacity of materials to plastically, i.e.
permanently,
deform under the effect of mechanical stress may be determined or described by
mechanical tensile or pressure testing.
For determining the degree of ductility by tensile testing what is referred to
as a
tensile sample is produced from the material to be assessed. This may be, for
example, a cylindrical sample which, halfway along its length, has a reduction
in
diameter of approximately 30 to 50 % over a length of approximately 30 to 50 %
of
the total sample length. The tensile sample is fixed in a fixing device of an
electromechanical or electrohydraulic tensile testing machine. Length sensors
are
installed on average of the sample over a measuring length which is
approximately
10 % of the overall sample length, before actual mechanical testing. These
sensors
allow the increase in the length to be followed in the selected measuring
length
during application of a mechanical tensile stress. The stress is increased
until the
sample fractures and the plastic content of the change in length is evaluated
using
the stress-strain recording. Materials which achieve a plastic change in
length of at
least 0.1 % in an arrangement of this type will be called ductile in the
context of this
specification.
Analogously it is also possible to subject a cylindrical material sample,
which has a
diameter to thickness ratio of approximately 3:1, to mechanical compressive
stress
in a commercially available pressure testing machine. Permanent deformation of
the
cylindrical sample occurs in this case as well after application of an
adequate
mechanical compressive stress. Once the pressure has been relieved and the
sample
removed, an increase in the diameter to thickness ratio of the sample is
determined.
Materials which achieve a plastic change of at least 0.1 % in a test of this
type are
also called ductile in the context of this specification.
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10331785.
Fine ductile alloy powders which have a degree of ductility of at least S %
are
preferably produced by the method according to the invention.
According to the invention the capacity for comminution of alloy or metal
powders
that cannot be comminuted further per se is improved by the use of
mechanically,
mechanochemically and/or chemically acting grinding aids which are
purposefully
added or produced in the grinding process. A fundamental aspect of this
approach is
that the chemical "desired composition" of the powder thus produced cannot be
changed overall or influenced even such that the processing properties, such
as the
sintering behaviour or flowability, are improved.
The method according to the invention is suitable for producing a wide variety
of
fine metal, alloy or composite powders with a mean particle diameter D50 of at
most
25 Vim.
For example metal, alloy or composite powders of a composition corresponding
to
formula I
hA-iB-j C-kD (I)
may be obtained, wherein
A represents one or more of the elements Fe, Co, Ni,
B represents one or more of the elements V, Nb, Ta, Cr, Mo, W, Mn, Re, Ti,
Si, Ge, Be, Au, Ag, Ru, Rh, Pd, Os, Ir, Pt,
C represents one or more of the elements Mg, Al, Sn, Cu, Zn, and
D represents one or more of the elements Zr, Hf, rare-earth metal,
and h, i, j and k indicate the percentages by weight, wherein
h, i, j and k in each case independently of one another represent 0 to 100 %
by
weight,
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with the proviso that the sum of h, i, j and k is 100 % by weight.
In formula I preferably
A represents one or more of the elements Fe, Co, Ni,
B represents one or more of the elements V, Cr, Mo, W, Ti,
C represents one or more of the elements Mg, Al and
D represents one or more of the elements Zr, Hf, Y, La.
10331785.
h preferably represents 50 to 80 % by weight, in particular preferably 60 to
80 % by
weight. i preferably represents 15 to 40 % by weight, in particular preferably
18 to
40 % by weight. j preferably represents 0 to 15 % by weight, in particular
preferably
5 to 10 % by weight. k preferably represents 0 to 5 % by weight, in particular
preferably 0 to 2 % by weight.
The metal, alloy or composite powders produced according to the invention are
distinguished by a small mean particle diameter D50. The mean particle
diameter
D50 is preferably at most 15 Vim, determined to ASTM C 1070-Ol (measuring
apparatus: Microtrac~ X 100).
By way of example powders which already have the composition of the desired
metal, alloy or composite powder may be used as the starting powder. In the
method
according to the invention however, it is also possible to use a mixture of
several
starting powders which only produce the desired composition after suitable
selection
of the mixing ratio. The composition of the produced metal, alloy or composite
powder may also be influenced by the choice of grinding aid, if this remains
in the
product.
Powders with spherical or irregularly shaped particles and a mean particle
diameter
D50, determined to ASTM C 1070-Ol of greater than 25 Vim, preferably 30 to
2,000
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g,m, in particular preferably 30 to 1,000 Vim, are preferably used as the
starting
powders.
The required starting powders may be obtained, for example, by atomisation of
5 molten metals and, if necessary, subsequent screening or sifting.
According to the invention the starting powder is firstly subjected to a
deformation
step. The deformation step may be carried out in known devices, for example in
a
rolling mill, an eddy mill, a high-energy mill or an attritor or an agitated
ball mill.
10 By suitably selecting the process engineering parameters, in particular as
a result of
the effect of mechanical stresses which are sufficient to achieve plastic
deformation
of the material or the powder particles, the individual particles are
deformed, so they
ultimately have a flake-like form, the thickness of the flakes preferably
being 1 to 20
Vim. This may take place, for example, by one-off stressing in a roller or a
hammer
mill, by repeated stressing in "small" deformation steps, for example by
percussive
grinding in an eddy mill or a Simoloyer~, or by a combination of percussive
and
frictional grinding, for example in an attritor or a ball mill. The high
material stress
during this deformation may lead to structural damage and/or material
embrittlement
which may be used in the following steps for comminuting the material.
Known molten metallurgical fast solidification processes may also be used for
producing strips or "flakes". These, like the mechanically produced flakes,
are then
suitable for comminution grinding described below.
The device in which the deformation step is carried out, the grinding media
and the
other grinding conditions are preferably selected such that the impurities as
a result
of abrasion and/or reactions with oxygen or nitrogen are as small as possible
and
below the critical value for the application of the product or are within the
specification relevant to the material.
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10331785.
This is possible for example by suitable selection of the grinding container
and
grinding media materials and/or the use of gases hindering oxidation and
nitriding
and/or the addition of protecting solvents during the deformation step.
In a particular embodiment of the method according to the invention the flake-
like
particles are produced in a fast solidification step, for example by what is
known as
"melt spinning", directly from the melt by cooling on or between one or more,
preferably cooled roller(s), so flakes are directly formed.
According to the invention the flake-like particles obtained in the
deformation step
are subjected to comminution grinding. In the process, on the one hand, the
particle
diameter to particle thickness ratiochanges, primary particles with a particle
diameter to particle thickness ratio of 1:1 to 10:1 usually being obtained
and, on the
other hand, the desired mean particle diameter of at most 25 ~m is adjusted
without
particle agglomerates that are difficult to comminute occurring again.
Comminution grinding may take place for example in a mill, for instance an
eccentric mill, but also in Gutbett rolls, extruders or similar devices which
bring
about material shattering owing to different movement and stress rates in the
flake.
According to the invention comminution grinding is carried in the presence of
a
grinding aid. Liquid grinding aids, waxes and/or brittle powder for example,
may be
used as the grinding aid. In this case the grinding aids may act mechanically,
chemically or mechanochemically.
By way of example the grinding aid may be paraffin oil, paraffin wax, metal
powder, alloy powder, metal sulphides, metal salts, salts of organic acids
and/or hard
material powder.
Brittle powder or phases act as mechanical grinding aids and may be used, for
example, in the form of alloy, element, hard material, carbide, silicide,
oxide, boride,
nitride or salt powder. By way of example pre-comminuted element and/or alloy
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10331785.
powders are used which, together with the starting powder used, which is
difficult to
comminute, produce the desired composition of the product powder.
Brittle powders used are preferably those which comprise binary, ternary
and/or
higher compositions of the elements A, B, C and/or D that occur in the
starting alloy
used, A, B, C and D having the meanings given above.
Liquid and/or easily deformable grinding aids, for example waxes, may also be
used. Other examples include hydrocarbons, such as hexane, alcohols, amines or
aqueous media. These are preferably compounds which may be required for the
following steps of further processing and/or which may be easily removed after
comminution grinding.
It is also possible to use specific organic compounds which are known from
pigment
production where they are used to stabilise non-agglomerating individual
flakes in a
liquid environment.
In a particular embodiment grinding aids are used which enter a targeted
chemical
reaction with the starting powder to achieve the grinding progress andlor for
adjusting a specific chemical composition. These may be, for example,
decomposable chemical compounds, of which only one or more constituents are
required for adjusting a desired composition, it being possible to
substantially
remove at least one component or constituent by a thermal process.
Reducible and/or decomposable compounds, such as hydrides, oxides, sulphides,
salts and sugars are mentioned as examples which are at least partially
removed
from the grinding stock in a subsequent processing step and/or powder
metallurgical
processing of the product powder, the remaining residue chemically
complementing
the powder composition in the desired manner.
It is also possible that the grinding aid is not added separately but is
produced in situ
during comminution grinding. In this case the procedure may, for example, be
such
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10331785.
that the grinding aid is produced by adding a reactive gas which reacts under
the
conditions of comminution grinding with the starting powder while forming a
brittle
phase. Hydrogen is preferably used as the reactive gas.
The brittle phases which are produced during treatment with the reactive gas,
for
example by formation of hydrides and/or oxides, may usually be removed again
by
appropriate method steps after comminution grinding or during processing of
the
fme metal, alloy or composite powder obtained.
If grinding aids are used which are not removed, or are only partially
removed, from
the metal, alloy or composite powder produced according to the invention, they
are
preferably selected such that the remaining constituents affect a property of
the
material in a desired manner, such as improving the mechanical properties,
reducing
the corrodibility, increasing the hardness and improving the abrasion
behaviour or
the frictional and sliding properties. The use of a hard material is mentioned
here by
way of example, which is increased in content in a subsequent step to the
extent that
the hard material may be further processed with the alloy component to form a
hard
metal or a hard material alloy composite.
After the deformation step and comminution grinding the primary particles of
the
metal, alloy or composite powder produced have, according to the invention, a
mean
particle diameter D50, determined to ASTM C 1070-Ol (Microtrac~ X 100), of at
most 25 p.m.
In addition to the formation of fine primary particles, the known interactions
between superfine particles can lead to the formation of relatively coarse
secondary
particles (agglomerates), of which the particle diameter is far greater than
the
desired mean particle diameter of at most 25 Vim, despite the use of grinding
aids.
A deagglomeration step therefore preferably follows comminution grinding,
during
which the agglomerates are broken open and the primary particles liberated.
Deagglomeration may, for example, take place by applying shear forces in the
form
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10331785.
of mechanical and/or thermal stresses and/or by removing separation layers
previously introduced in the process between primary particles. The
deagglomeration methods to be applied in particular are oriented toward the
degree
of agglomeration, the intended used and the susceptibility to oxidation of the
superfine powder and the admissible impurities in the finished product.
Deagglomeration may, for example, take place by mechanical methods, for
instance
by treatment in a gas contrajet mill, screening, sieving or treatment in an
attritor, a
kneader or a rotor-stator dispergator. The use of a stress field, as generated
in
ultrasound treatment, thermal treatment, for example dissolution or conversion
of a
previously introduced separating layer between the primary particles by cryo-
or
high-temperature treatments, or a chemical conversion of introduced or
purposefully
produced phases, is also possible.
Deagglomeration is preferably carried out in the presence of one or more
liquids,
dispersing aids and/or binders. A slurry, a paste, a kneading compound or a
suspension with a solids content between 1 and 95 % by weight may thus be
obtained. Solids contents between 30 and 95 % by weight may be directly
processed
by known powder technological processes such as injection moulding, film
casting,
coating, and hot-moulding, and are then reacted in suitable steps of drying,
releasing
and sintering to form an end product.
A gas contrajet mill, which is operated under inert gases, such as argon or
nitrogen,
is preferably used for deagglomeration of particularly oxygen-sensitive
powders.
The metal, alloy or composite powders produced according to the invention are
distinguished from conventional powders with identical mean particle diameters
and
identical chemical composition which are produced, for example, by
atomisation, by
a range of particular properties.
The invention therefore also relates to metal, alloy or composite powders with
a
mean particle diameter D50 of at most 25 Vim, determined using the particle
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measuring apparatus Microtrac~ X 100 to ASTM C 1070-O1, which are obtainable
by the method according to the invention.
The metal, alloy and composite powders according to the invention exhibit, for
5 example, excellent sintering behaviour. At low sintering temperatures the
same
sintering densities may be attained as in powders produced by atomisation.
Starting
from powder compacts of a defined compressed density, higher sintering
densities
may be achieved at the same sintering temperature. This increased sintering
activity
is also exhibited, for example, in the fact that, until the maximum
contraction is
10 attained, the contraction during the sintering process is greater than in
conventionally produced powders.
The invention therefore also relates to metal, alloy or composite powders with
a
mean particle diameter DSO of at most 25 pm, determined using the particle
15 measuring device Microtrac~ X 100 to ASTM C 1070-O1, wherein, until the
maximum contraction is attained, the contraction, determined using a
dilatometer to
DIN 51045-1 has at least 1.05 times the contraction of a metal, alloy or
composite
powder with identical chemical composition and identical mean particle
diameter
D50, the powder to be investigated being compressed to a compressed density of
50
% of the theoretical density before the contraction is measured.
The powder to be investigated may be compressed by adding conventional
compression-assisting agents, such as paraffin wax or other waxes or salt or
organic
acids, for example zinc stearate.
Metal, alloy or composite powders which are produced by atomisation and by
comparison with which the powders according to the invention have improved
sintering behaviour, are to be taken to mean those powders which are produced
by
conventional atomisation known to the person skilled in the art.
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10331785.
The advantageous sintering behaviour of the metal, alloy or composite powders
according to the invention may also be recognised in the course of sintering
and
contraction curves, as shown, for example, in Fig. 7.
Fig. 7 shows, for a comparison powder (V) and a powder (PZD) according to the
invention, the course of the contraction S or the contraction rate AS in each
case in
relative units as a function of the temperature TN standardised to the
respective
sintering temperature Ts.
The comparison powder (V) is a product produced by atomisation under inert
conditions and with a composition corresponding to that of the material
described in
Example 1 and the morphology of this powder. The particle size distribution
(D50
approximately 8.4 pm) corresponds to that as shown in Fig. 5. The powder (PZD)
according to the invention is a powder produced according to Example 1 with
the
morphology illustrated in Fig. 6 and an oxygen content of 0.4 % by weight.
After mixing with 3 % by weight microwax as the compression-facilitating
additive
powder compacts were produced from the two powders in a compression mould by
applying a single-axle pressure of 400 to 600 mPa. The green density was in
both
cases approx. 40 % of the theoretical density. These compacts were accordingly
sintered individually in a dilatometer to DIN 51045-1 under protective gas
conditions and using argon as the process gas. Heating at a rate of approx. 1
K/min
(corresponding to approx. 6 * 10~ *Ts/min, where Ts: approx. 1,600 K) took
place
in the process. The push rod of the dilatometer does not exert any pressure on
the
sample which supplies a measurable quantity for sintering contraction in the
temperature range that is of interest for sintering (approx. 0.5 Ts to approx.
0.95 Ts).
The organic pressing aid is expelled to a temperature of approximately 0.45 *
Ts.
The actual sintering process takes place thereafter by further heating at the
same
heating rate from approx. 0.5 Ts to approx. 0.99 Ts.
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10331785.
The advantages of the PZD powder lead to the following observations and to
general
rules which are illustrated with the aid of Fig. 7. For this purpose, the
required
terms, which allow a general description of the sintering processes, shall
firstly be
introduced:
vT9o and PzDT90- temperatures (in standardised units according to TN = T/Ts)
at
which the two sintering bodies, at a heating rate of approx 6 * 10~ * Ts, have
attained a contraction of 90 %, based on the same final contraction (= 100)
attained.
vTlo and PzDTlO~ temperatures (in standardised units according to TN = T/Ts)
at
which the two sintering bodies, at a heating rate of approx 6 * 10~ * Ts, have
attained a contraction of 10 %, based on the same final contraction (= 100)
attained.
vTl and PzD.I.~: temperatures (in standardised units according to TN = T/Ts)
at
which the two sintering bodies, at a heating rate of approx 6 * 10~ * Ts, have
attained a contraction of 1 %, based on the same final contraction (= 100)
attained.
Contraction starts at these temperatures.
v.Lm~ ~d PZD.Lm~: temperatures (in standardised units TN = T/Ts) at which the
maximum contraction rate is reached.
vS(TN), PzDS(TN): contraction as a function of the standardised temperature
TN.
vAS(TN), PzDAS(TN): temperature-dependent contraction rate d(S(TN))/dTN,
determined from the contraction curves to be compared vS(TN) and PzoS(TN).
vSm~ ~d PZDsm~: m~imum value of contraction rates, determined from the
contraction curves derived according to temperature vS(TN) and PzDS(TN).
The following general product properties of the powders according to the
invention
are obtained compared to conventionally produced atomised powders:
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(PZD-I. PZD-I- /PZD.I.~ v'I' vT / vT (I)
max - 10) (
-
)
max max
10
max
PZD V (II)
Tmax < Tmax
PzDTIO < vTlo (III)
PZDT1 < ~T1 (IV)
PzDs < vS (V)
max m~
(PZD.Lm~ - PZDT10) ~ (VTmax - vTlO) (VI)
(PZDTmax -PZDT1) > (VTm~-VTI) (VII)
(PZDT90 PZDT10) ~ (VT90 - VT10) (VIII)
-
(PZDT90 PZDTl) ~ (VT90 - VTl) (IX)
-
10331785.
The following conclusions with respect to the different behaviour of powder
(PZD
powder) produced according to the invention and comparison powders produced by
conventional atomisation may be drawn from these inequalities:
- The sintering range for PZD powder is wider.
- The temperature at which contraction begins, at which, based on identical
final contraction, 10 % of this final contraction is attained and at which
contraction attains its maximum, is lower in PZD powders.
- The peaks of the contraction rates obtained from the standardised
illustration
of Fig. 7 mean that PZD powders have a lower contraction rate at PzDTm
than the comparison powder at vTm~.
- The initial temperature range up to the contraction peak is wider for PZD
powders.
- The temperature range from the start of contraction up to the maximum
contraction is greater for PZD powders.
- The temperature range between the temperature at which contraction of 10
was attained up to the temperature at which contraction of 90 % was attained
is greater for PZD powders.
- The temperature range from the start of contraction up to the temperature at
which 90 % of the final contraction is attained is greater for PZD powders.
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10331785.
These statements relate to single-phase starting states of the powders. In the
event
that there are further phases not all of the inequalities (I) to (IX) have to
always be
met together, in particular very high contraction rates may occur locally on
PZD
powder compacts as a result of particular sintering activations of liquid
phases,
which rates constitute a further advantage with respect to processing
capacity.
However, the validity of the inequalities (III), (IV), (VIII) and (IX) is
unaffected in
this case as well.
The metal, alloy and composite powders according to the invention are
distinguished, owing to a particular particle morphology with rough particle
surface,
moreover by outstanding compression behaviour and, owing to a comparatively
broad particle size distribution, by high compressed density. This manifests
itself in
that compacts made of atomised powder have a lower bending strength, under
otherwise identical production conditions, than the compacts made of powders
according to the invention and with the same chemical composition and mean
particle size D50. A further improvement in the compression behaviour may be
achieved if powder mixtures comprising 1 to 95 % by weight metal, alloy or
composite powders according to the invention and 99 to 5 % by weight atomised
powder are used.
The sintering behaviour of powders produced according to the invention may
also be
purposefully influenced by the choice of grinding aid. Thus one or more alloys
which, owing to their low melting point compared to the starting alloy form
liquid
phases during heating which improve the particle rearrangement and the
material
diffusion and thus the sintering behaviour and the contraction behaviour and
thus
allow higher sintering densities to be attained at the same sintering
temperature or at
lower sintering temperatures the same sintering density as may be achieved
with the
comparison powders, may be used as the grinding aid. Chemically decomposable
compounds, of which the decomposition products with the basic material produce
liquid phases or phases with increased diffusion coefficients which facilitate
compression, may also be used.
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10331785.
X-ray analyses of the metal, alloy or composite powders according to the
invention
show a propagation of X-ray reflexes compared with X-ray reflexes that are
obtained for powders with the same mean particle diameter and the same
chemical
composition which were obtained by atomisation. The propagation is
demonstrated
5 by the propagation of half widths. Usually the half widths of the X-ray
reflexes are
propagated by a factor > 1.05. This is caused by the mechanical stressed
stated of
the particles, the existence of a higher dislocation density, i.e.
disturbances to the
solid in the atomic range, and the crystallite size in the particles. In the
case of
composite powders, alloy- and/or process-induced phases occur in the
10 diffractograms in addition to the propagations of the X-ray reflexes of the
main
phase, which phases are significant for the contraction properties.
The method according to the invention allows production of metal, alloy and
composite powders, in which oxygen, nitrogen, carbon, boron and silicon
contents
15 are purposefully adjusted. Oxide and/or nitride phases may form in the case
of
introduction of oxygen or nitrogen as a result of the high application of
energy.
Phases of this type may be desirable for specific applications as they may
lead to
strengthening of material. This effect is known as the "particle dispersion
strengthening" effect (PDS effect). However, the introduction of such phases
is
20 often associated with a deterioration in the processing properties (for
example
compressibility, sintering activity). Owing to the generally inert properties
of the
dispersoids with respect to the alloy components, the latter may therefore
have a
sintering-inhibiting effect.
As a result of the comminution grinding to be carned out according to the
invention,
said phases are immediately superfinely distributed in the produced powder.
The
phases formed (for example oxides, nitrides, carbides, borides) are therefore
much
more finely and homogeneously distributed in the metal, alloy and composite
powders according to the invention than in conventionally produced powders.
This
again leads to increased sintering activity compared with discretely
introduced
phases of the same kind.
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10331785.
The processing properties of the metal, alloy and composite powders according
to
the invention, for example the compression and sintering behaviour, and the
capacity for processing by metal powder injection moulding (MIM), slurry-based
methods or tape casting, may often be improved even further by adding metal,
alloy
or composite powders conventionally produced, in particular by atomisation.
The invention therefore also relates to mixtures containing 1 to 95 % by
weight of a
metal, alloy or composite powder and 99 to S % by weight of a conventionally
produced metal, alloy or composite powder.
The mixtures according to the invention preferably contain 10 to 70 % by
weight of
a metal, alloy or composite powder according to the invention and 90 to 30 %
by
weight of a conventionally produced metal, alloy or composite powder.
The conventionally produced metal, alloy or composite powder according to the
invention is preferably a powder which has been produced by atomisation.
The conventionally produced metal, alloy or composite powder may have the same
chemical composition as the PZD powder contained in the mixture. Mixtures of
this
type are distinguished from pure PZD powders in particular by a further
improvement in compression behaviour.
However, it is also possible that PZD powder and conventionally produced
powder
have a different chemical composition in the mixture. In this case the
composition
may be purposefully changed and as a result specific powder properties and
consequently the material properties may be purposefully adjusted.
The following examples serve to describe the invention in more detail, wherein
the
examples are intended to facilitate understanding of the principle according
to the
invention and are not to be understood as a limitation thereof.
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Examples
10331785.
The mean particle diameters D50 given in the examples were determined using a
Microtrac~ X 100 from Honeywell, U.S.A. to ASTM C 1070-O1.
Example 1
A Nimonic~ 90 type alloy melt atomised by means of argon and with the
composition Ni20Cr16Co2.5Ti1.5A1 was used as the starting powder. The alloy
powder obtained was screened between 53 and 25 pm. The density was approx. 8.2
g/cm3. The starting powder had substantially spherical particles, as may
clearly be
seen in Fig. 1 (scanning electron microscope image (SEM image) magnified 300
times).
The starting powder was subjected to deformation grinding in a vertical
agitated ball
mill (Netzsch Feinmahltechnik; PR 1 S type), so the originally spherical
particles
assumed flake-like forms. The following parameters were used in particular:
~ Grinding container volume: 51
~ Speed of rotation: 400 rpm
~ Circumferential speed: 2.5 m/s
~ Ball filling: 80 vol.% (bulk volume of the balls)
~ Grinding container material: 100Cr6 (DIN 1.3505: approx. 1.5 % by
weight Cr, approx. 1 % by weight C, approx. 0.3 % by weight Si, approx. 0.4
% by weight Mn, < 0.3 % by weight Ni,< 0.3 % by weight Cu, remainder Fe)
~ Ball material: hard metal (WC-l OCo)
~ Ball diameter: approx. 6 mm (total mass: 25 kg)
~ Originally weighed in quantity of powder: 500 g
~ Duration of treatment: 2 h
~ Solvent: ethanol (approx. 2 1).
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10331785.
Fig. 2 is a SEM image magnified 300 times of the flakes produced in the
deformation step. The high degree of material deformation, which was caused by
the
specific grinding treatment, compared with the starting powder may be seen.
Damage to the structure of the material (crack formation) may also clearly be
seen.
Comminution grinding was then carried out. "hat is referred to as an eccentric
vibration grinding mill (Siebtechnik GmbH, ESM 324) with the following process
engineering parameters was used:
~ Grinding container volume: 5 1 operated as a satellite (diameter 20
cm, length approx. 15 cm)
~ Ball filling: 80 vol.% (bulk volume of the balls)
~ Grinding container material: 100 Cr6 (DIN 1.3505: approx. 1.5 % by
weight Cr, approx. 1 % by weight C, approx. 0.3 % by weight Si, approx. 0.4
% by weight Mn, < 0.3 % by weight Ni, < 0.3 % by weight Cu, remainder
Fe)
~ Ball material: 100 Cr6
~ Ball diameter: 10 mm
~ Originally weighed in quantity of powder: 150 g
~ Grinding aid 2 g paraffin
~ Oscillation amplitude: 12 mm
~ Grinding atmosphere: argon (99.998 %).
After a grinding duration of 2 hours superfine particle agglomerates were
obtained.
Fig. 3 is a SEM image magnified 1,000 times of the product obtained. The
cauliflower-like structure of the agglomerate (secondary particle) may be
seen, the
primary particles having particle diameters of much less than 25 Vim.
A sample of the primary particles or superfine particle agglomerates was
subjected
in a third method step to deagglomeration by ultrasound treatment in
isopropanol in
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10331785.
an ultrasonic device TG 400 (Sonic Ultraschallanlagenbau GmbH) lasting 10
minutes at 50 % maximum power to obtain separated primary particles.
The particle size distribution of the deagglomerated sample was determined
using a
Microtrac~ X 100 (manufacturer: Honeywell, U.S.A.) to ASTM C 1070-Ol. Fig. 4
shows the particle size distribution thus obtained. The D50 value of the
starting
powder was 40 ~m and was reduced to approx 15 ~.m by the treatment according
to
the invention.
The remaining quantity of primary particles from comminution grinding were
subjected in an alternative third method step to deagglomeration by treatment
in a
gas contrajet mill and subsequent ultrasound treatment in isopropanol in an
ultrasonic device TG 400 (Sonic Ultraschallanlagenbau GmbH) at 50 % of the
maximum power. The particle size was again determined using a Microtrac~ X
100.
Fig. 5 shows the particle size distribution obtained. The DSO value was then
only 8.4
Vim. This proves the possibility of further increasing the fine fraction in
the powder
produced according to the invention by high-energy post-treatment.
Fig. 6 shows a SEM image (x 600 magnification) of the powder after treatment
in
the gas contrajet mill. By using suitable screening methods it is accordingly
possible
to obtain alloy powders with even narrower particle size distribution. D50
values of
less than approx. 8 ~m may thus be industrially and economically achieved.
The introduced grinding aid paraffin may be removed during powder
metallurgical
further processing of the alloy powder by thermal decomposition and/or
evaporation
and may be used as a compression aid.
Example 2: Production of Fe24Cr 1 OA 11 Y superfine powders using mechanical
grinding aids without changing the composition of the starting
powder
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10331785.
500 g of a spherical starting powder of a Fe24Crl0Al 1Y alloy with a mean
particle
diameter D50 of 40 ~m was processed to form flakes in a deformation stage
under
conditions analogous to those described in Example 1.
5 Comminution grinding was then carried out in an eccentric vibration grinding
mill,
as described in Example 1. A mixture of comminuted brittle Fe70Cr, Fe60A1 and
Fel6Y powders with a mean particle diameter of approx. 40 ~m and fine Fe
powder
with a mean particle diameter D50 of 10 ~,m was added as the grinding aid.
10 15 g grinding aid was used for comminution grinding. The addition of
approx. 10
vol. % of a mechanically acting grinding aid is a typical content for this
step.
Smaller quantities of grinding aids may also be useful as a function of the
proposed
aim. The composition of the grinding aid used is summarised in Table 1. A
mixture
containing 65 % by weight Fe, 24 % by weight Cr, 10 % by weight A1 and 1 % by
15 weight Y was obtained. The chemical composition of the starting powder is
accordingly not altered by the choice of given alloy contents. A specific
distribution
of the components used (starting powder, grinding aid) is present in the
composite
powder obtained as a result of production according to the invention, so the
composite powder undergoes a metallurgical change during further processing,
for
20 example by sintering or another thermal process.
Table 1 Composition of a mechanical grinding aid
Component Quantity Fe [g] Cr [g] AI [g] Y [g]
[g]
Fel6Y 0.93 0.78 0 0 0.15
Fe60Al 2.50 1.0 0 1.5 0
Fe70Cr 5.14 1.54 3.6 0 0
Fe 6.43 6.43 0 0 0
Total 15 9.75 3.6 1.5 0.15
25 A composite powder with a mean particle diameter D50 of 15 ~m was obtained
after
comminution grinding and deagglomeration in an ultrasonic field. It was
possible to
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10331785.
obtain an alloy in the metallurgical sense from a composite powder of this
type by
thermal post-treatment.
Ezample 3 Production of Fe24Cr10Al 1Y superfine powders using mechanical
grinding aids and changing the composition compared with the
starting powder
In contrast to Example 2, a change in the chemical composition was desired or
allowed during the grinding operation. An atomised alloy of composition
Fe25,6Cr10,67A1 with a mean particle diameter D50 of 40 ~,m was subjected to a
deformation step under the conditions described in Example 1. Flake-like
particles
with a mean particle diameter D50 of 70 ~.m were obtained, of which the
appearance
did not significantly differ from that in Example 1.
Comminution grinding was then carried out. The procedure corresponded to that
in
Example 1 but 10 g of a Fel6Y powder with a mean particle diameter D50 of 40
~m
were used as the grinding aid and the grinding lasted 2 hours.
Table 2 gives the composition and quantity of flake-like starting alloy and
the
grinding aid added for comminution grinding.
Table 2 Composition of the flake-like starting alloy and mechanical grinding
aid
used
Component Quantity Fe [g] Cr [g] Al [g] Y [g]
[g]
Fe25,6Cr10, 150 95.6 38.4 16.0 0
67A1
Fel6Y 10 8.4 0 0 1.6
Total 160 104 3 8.4 16.0 1.6
As may be seen from Table 2, the composite powder obtained had the composition
Fe24Cr10A11Y. The composite powder was subjected to an ultrasound treatment
after which a composite powder with a mean particle diameter D50 of 13
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~.m was obtained.
Ezample 4
10331785.
The procedure was as in Example 3, a mixture of a plurality of brittle
materials and
pure iron powder being used as the grinding aid.
Table 3 contains the composition and original weighed in quantities of the
starting
powder and grinding aid. The brittle grinding aids Fe60Al, Fe70Cr and Y2, 2H
were
brought to a mean particle diameter D50 of 40 ~m before use in a separate
grinding
step. The Fe powder used had a mean particle diameter D50 of 10 ~,m.
Table 3 Composition of the flake-like starting alloy and the mechanical
grinding aid
used
Component Quantity Fe [g] Cr [g] Al [g] Y [g]
[g]
Fe25,6Cr10,150.00 95.60 38.40 16.00 0.00
67A1
Fe60A1 1.19 0.48 0.00 0.71 0.00
Fe70Cr 2.35 0.71 1.64 0.00 0.00
Y2,2H 1.66 0.00 0.00 0.00 1.66
Fe 10.00 10.00 0.00 0.00 0.00
Total 165.20 106.79 40.04 16.71 1.66
As may be seen from Table 3, the composite powder obtained had the composition
Fe24Cr10A11Y. The composite powder was subjected to an ultrasound treatment
after which a composite powder with a mean particle diameter D50 of 15 ~m was
obtained.
Example 5 Production of a Fe24Cr10A11Y superfine powder from two FeCrAI
master alloys and Fel6Y as the single brittle mechanical grinding aid
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10331785.
Flakes with mean particle diameters D50 of 70 Vim, of which the appearance did
not
significantly differ from the powder produced in Fig. 2, were produced in
separate
deformation steps analogously to Example 1 from two atomised alloys with the
composition Fe19,9Cr24,8A1 and Fe27,9Cr5A1 with mean particle diameters D50 of
40 ~.m.
The particularly brittle Fel6Y alloy was used as the only grinding aid during
subsequent comminution grinding, which alloy had previously been comminuted to
a mean particle diameter D50 of approx. 40 Vim. The procedure was as in
Example
l, grinding lasting 2.5 hours.
Table 4 contains the composition and original weighed in quantities of the two
flake-like FeCrAI starting alloys and of the brittle grinding aid (Fel6Y).
Table 4 Composition of the flake-like starting alloys and the mechanical
grinding
aid used
Component Quantity Fe [g] Cr [g] A1 [g] Y [g]
[g]
Fe 19,9Cr24,43 23.8 8.6 10.5 0
8A1
Fe27,9Cr5Al107 71.8 29.8 5.5 0
Fel6Y 10 8.4 0 0 1.6
Total 160 104 3 8.4 16 1.6
As may be seen from Table 3, the composite powder obtained had the composition
Fe24Cr10Al1Y. The composite powder was subjected to an ultrasound treatment
after which a composite powder with a mean particle diameter D50 of 12 qm was
obtained.
Ezample 6 In situ production of the grinding aid
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10331785.
29
An atomised NilSColOCr5,5A14,8Ti3MolV alloy, which is commercially available
under the model name IN 100~, was subjected, as described in Example 1, to a
deformation step under an inert atmosphere.
No brittle grinding aid was added during subsequent comminution grinding,
rather it
was formed in situ during the grinding process. For this purpose, the
eccentric
vibration grinding mill was flooded with a gas mixture consisting of 94 vol. %
argon
and 6 vol. % hydrogen. The grinding container was thermally insulated, so a
processing temperature of approx. 300 °C was established during the
grinding
process owing to the application of energy. The remaining grinding conditions
corresponded to the procedure described in Example 1. The elevated temperature
and the hydrogen content of the process gas lead to the formation of brittle
Ti-H and
V-H compounds which acted in the same manner as the grinding aids introduced
in
1 S Examples 1 to 5 and thus led to comminution. After grinding that lasted 3
h under a
hydrogen-containing atmosphere, an alloy powder with a mean particle diameter
D50 of 13 ~,m was achieved.
The chemical composition of the resultant superfine powder differed only
slightly
from that of the starting powder. The hydrogen content rose to < 1,000 ppm.
During
further processing of the alloy powder produced according to the invention the
hydrogen content fell to below approx. 50 ppm again as a result of sintering
under
vacuum.
Ezample 7 Si powder as the mechanical grinding aid
Spherical atomised Ni38Cr8,7A11,09Hf with a mean particle diameter D50 of 40
prn
was subjected, as described in Example 1, to a deformation step.
150 g of the flake-like powder produced in the attritor were subjected, as
described
in Example l, to comminution grinding in an eccentric vibration grinding mill,
13 g
Si powder with a mean particle diameter D50 of 40 ~m being added as the
grinding
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10331785.
aid. After grinding that lasted 2 h an alloy powder with a mean particle
diameter
D50 of 10.5 ~m and the desired composition Ni35Cr8A18Si1Hf was obtained. The
silicon used is desirable or necessary in terms of alloy engineering. Of the
possible
brittle grinding aids Si is particularly suitable owing to its properties.
After treatment
5 the oxygen content was approx. 0.4 % by weight.
Example 8
Spherical atomised Ni38Cr8,7A11,09Hf with a mean particle diameter D50 of 40
N.m
10 was subjected, as described in Example 7 by using an attritor (agitated
ball mill), to
a deformation step.
Subsequent comminution grinding was carned out in the presence of Si powder
(13
g) as the grinding aid, likewise in an agitated ball mill, the following
technical
15 parameters being adjusted:
Grinding container volume:5 1
Ball filling: 80 vol.%
Grinding container material:100 Cr6
20 Ball material: 100 Cr6
Ball diameter: 3.5 mm
Originally weighed in
quantity of powder:
150 g Ni38Cr8,7A11,09Hf
Circumferential speed: 4.2 m/s
Grinding liquid: ethanol
25 Grinding duration: 1.5 h
Grinding aid 13 g Si powder (D50: approx.
40 pm)
After grinding that lasted 1.5 hours and subsequent ultrasonic deagglomeration
an
alloy powder with a mean particle diameter D50 of 13 Vim, measured by
Microtrac~
30 X 100, was obtained. The silicon used in this case was desirable or
necessary in
terms of alloy engineering in order to adjust the end composition
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10331785.
Ni38Cr8,7A11,09Hf and in terms of process engineering for attaining the
desired
grinding effect. Of the elements that may be considered silicon is best suited
as the
grinding aid owing to its brittleness. This grinding process led to an
increase in the
oxygen content in the powder. At the conclusion of the grinding process the
oxygen
content was 0.4 % by weight.
Example 9
A spherical atomised Nil7Mol5Cr6Fe5W1Co alloy with a mean particle diameter
D50 of 40 pm, which is commercially available under the name Hastelloy~, was
subjected, as described in Example l, to a deformation step.
The flake-like particles obtained were comminution ground in an eccentric
vibration
grinding mill in the presence of tungsten carbide as the grinding aid and
under the
following conditions:
Grinding container volume:5 1
Ball filling: 80 vol.%
Grinding container material:100 Cr6
Ball material: WC-l OCo hard metal material
Ball diameter: 6.3 mm
Originally weighed in f powder: 150 g
quantity o
Oscillation amplitude: 12 mm
Grinding atmosphere: argon (99.998 %)
Grinding duration: 90 minutes
Grinding aid 13.5 g WC powder (D50 =
1.8 p.m)
The result of comminution grinding was an alloy hard material composite powder
in
which the alloy components had been comminuted to a mean particle diameter D50
of approx. 5 pm and the hard material component to a mean particle diameter
D50
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10331785.
of approx. 1 gm. The hard material particles were substantially homogenously
distributed in the alloy powder volume.
The alloy hard material composite powder could be processed by conventional
process steps to form a spray powder, For this purpose, 797 g WC with a mean
particle diameter D50 to ASTM B 330 (FSSS) of 1 ~,m, ethanol, PVA (polyvinyl
alcohol) and suspension stabilisers were added to 163 g of the alloy hard
material
composite powder produced according to the invention for dispersing and
generating
a suspension. A suspension was produced which consisted to 25 vol. % of the
metallic binding phase and to 75 vol. % of the WC hard material phase. This
suspension was further processed by spray granulation and classification to
form a
green spray powder with a particle size of 20 to 63 Vim. The organic
auxiliaries were
firstly removed from this green spray powder by gas evolution at 100 to 400
°C and
sintering then took place at approx. 1,300 °C under an inert
atmosphere. In the
process solid bonds were produced in the spray granules and less solid bonds
between the individual granule grains. Deagglomeration and classification into
the
desired grain fraction (for example 1 S to 45 ~,m) finally took place. The
powder thus
obtained could be further processed by thermal injection in a known manner to
form
hard metal or components coated with an alloy-hard material composite.
Ezample 10
Titanium powder with a mean particle diameter DSO of 100 ~.m was processed
according to the invention and analogously to Example 1 to form flakes.
The flakes were then further processed in a comminution step analogously to
Example l, 10 g TiH2 being added as the grinding agent to the Ti flakes used
(original weighed in quantity: 150 g). After comminution grinding there was a
fine
titanium powder with a mean particle diameter D50 of approx. 15 Vim.
The titanium powder produced according to the invention could be further
processed
by conventional process steps to form mould parts. To protect against
oxidation the
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10331785.
titanium powder produced according to the invention was stored in an organic
solvent, for example n-hexane. Long chain hydrocarbons, such as paraffin or
amines, were added prior to powder metallurgical further processing. For this
purpose, the paraffin was dissolved for example in n-hexane, added to the
powder
and the n-hexane was then evaporated with continuous circulating of the
powder. A
superficial seal against uncontrolled absorption of oxygen was obtained
thereby and
the improvement in compressibility achieved. This procedure allows the
titanium
powder to be processed in air.
After processing in terms of powder technology to form mould parts by single-
axle
compression, removal of the organic constituents in a thermal treatment,
thermal
decomposition of the grinding aid and sintering to form substantially dense
mould
parts took place.
Ezample 11
Flakes made of an alloy 17-4 PH~ (Fe17Cr12Ni4Cu2.5Mo0.3Nb) and which had
been produced analogously to Example 1, were treated in a contrajet mill. The
flakes
had a particle diameter to particle thickness ratio of approx. 1,000:1 and a
mean
particle diameter D50 of 150 gm. The contrajet mill was operated with inert
gas.
Atomised spherical material, which had not been pre-treated, of the same alloy
with
a particle diameter between 100 and 63 pm was used as the grinding aid. The
grinding chamber (volume: approx. 5 1) was filled with 2. 5 1 powder bulk
volume
(67 % by weight grinding aid and 33 % by weight flakes) and the grinding
process
initiated. The fine fraction produced was separated at 10 ~m by corresponding
adjustments of a sifter connected downstream of the mill.
In contrast to the earlier examples, comminution grinding and the generally
required
deagglomeration were performed in one step by the procedure described. A
particular feature of this procedure is the use of characteristic or alloy-
like powder
that cannot, or may barely, be comminuted and which leads to an increased
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application of energy in the grinding process and thus to an improved grinding
effect.
Ezample 12
An atomised Ni 17Mo 1 SCr6Fe5 W 1 Co alloy with a mean particle diameter of
100 to
63 pm, which is commercially available under the name Hastelloy~, was
mechanically treated in a high energy mill (eccentric vibration mill) under
the
following conditions:
Grinding container volume:5 1 (diameter 20 cm, length
approx. 15
cm)
Ball filling: 80 vol.%
Grinding container material:100 Cr6
Ball material: WC-Co hard metal
Ball diameter: 10 mm
Originally weighed in
quantity of powder:
300 g
Oscillation amplitude: 12 mm
Grinding atmosphere: argon (99.998 %)
Grinding duration: 2 h
Flakes were produced which had a diameter to thickness ratio of 1:2 and a
flake
thickness of approx. 20 pm.
Comminution grinding then took place in a gas contrajet mill. During
comminution
particles which had a particle diameter of < 20 ~m were removed by suitably
adjusting a sifter connected downstream. A fine alloy powder which, after
ultrasound treatment, had a mean particle diameter D50 of 12 ~m and a D90
value
of 20 Vim, determined using a Microtrac~ X 100, was thus produced.
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