PROCEDE RELEVANT DE LA METALLURGIE DES POUDRES POUR PRODUIRE UN PROFILE EXTRUDE

POWDER METALLURGY METHOD FOR PRODUCING AN EXTRUDED PROFILE

Abrégé français

L'invention concerne un procédé permettant de produire un profilé par extrusion de poudre de métal et/ou d'alliages métalliques, selon lequel de la poudre en vrac est portée à une température d'extrusion inférieure à la température de fusion de la poudre et est comprimée sous pression à travers l'ouverture d'une matrice pour former un profilé. Au moins un métal ou un alliage métallique de la poudre est un métal réactif formant spontanément une couche d'oxyde protectrice naturelle au niveau d'une surface libre et/ou la poudre contient des particules fibreuses absorbant le rayonnement micro-onde et réparties de manière homogène dans la poudre en vrac. La poudre en vrac est portée à température d'extrusion par rayonnement micro-onde. Le procédé permet de chauffer de manière homogène toutes les zones de la poudre en vrac.

Abrégé anglais

In a method for producing a profile by extruding powdered metal and/or
powdered
metal alloys, a bulk powder material is heated to an extrusion temperature
below
the melting temperature of the powder and pressed under pressure through an
opening of a die to form the profile. At least one metal or a metal alloy of
the
powder is a reactive metal spontaneously forming a natural oxide protective
layer
on a free surface and/or the powder contains fibre-like particles distributed
homogeneously in the bulk powder material and absorbing microwave radiation.
The bulk powder material is heated by microwave irradiation to extrusion
temperature. The method achieves rapid and uniform heating in all regions of
the
bulk powder material.

Revendications

Claims
1. Method for producing a profile by extruding powdered metal and/or
powdered metal alloys, in which method a bulk powder material is heated to an
extrusion temperature below the melting temperature of the powder and pressed
under pressure through an opening of a die to form the profile, characterised
in
that at least one metal or a metal alloy of the powder is a reactive metal
spontaneously forming a natural oxide protective layer on a free surface
and/or
the powder contains fibre-like particles distributed homogeneously in the bulk
powder material and absorbing microwave radiation, and in that the bulk powder
material is heated by microwave irradiation to extrusion temperature.
2. Method according to claim 1, characterised in that the density of the bulk
powder material or the dimension of the hollow spaces between the powder
particles including the oxide layers, is matched to the wavelength of the
microwave radiation.
3. Method according to claim 1 or 2, characterised in that the reactive metal
spontaneously forming a natural oxide protective layer on a free surface is
aluminium, magnesium, titanium, tantalum or zirconium.
4. Method according to claim 1, characterised in that the length of the fibre-
like particles is matched to the wavelength of the microwave radiation.
5. Method according to claim 1 or 4, characterised in that the fibre-like
particles are at least partially integrated in the metallic powder particles.
6. Method according to any one of claims 1, 4 or 5, characterised in that the
bulk powder material contains carbon nanotubes (CNTs) in a homogeneous
distribution.
7. Method according to any one of claims 1 to 6, characterised in that the
bulk powder material on heating to extrusion temperature firstly has low

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microwave energy radiated through it at a changing frequency, the absorbed
energy is measured as a function of the frequency and on occurrence of a
maximum of the absorbed energy, the resonance frequency is determined and in
that the bulk powder material then has high microwave energy radiated through
it
at the resonance frequency.
8. Method according to claim 7, characterised in that the determination of the
resonance frequency of the bulk powder material and the following radiation
with
high microwave energy at the resonance frequency to heat the bulk powder
material to extrusion temperature are carried out fully automatically by means
of
control electronics.
9. Method according to claim 7 or 8, characterised in that the bulk powder
material is pre-compacted in an intermediate container, the pre-compacted bulk
powder material is radiated through in the intermediate container at the
resonance
frequency and heated to extrusion temperature and then pressed from the
intermediate container through the die opening by means of a ram.
10. Method according to claim 9, characterised in that the pre-compaction of
the bulk powder material in the intermediate container is carried out with a
screw
conveyor.

Description

CA 02692925 2010-01-07
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Powder metallurgy method for producing an extruded profile
The invention relates to a method for producing a profile by extruding
powdered
metal and/or powdered metal alloys, in which method a bulk powder material is
heated to an extrusion temperature below the melting temperature of the powder
and is pressed under pressure through an opening of a die to form the profile.
In the prior art, an extrusion billet is normally pressed as a metallic block
material
through the opening of a die in an extrusion system. When extruding powdery
materials, the bulk powder materials are generally encapsulated in a container
before extrusion because of their low heat conduction and generally compacted,
for example by cold-isostatic pressing. The poor heat conduction of the bulk
powder materials is made still more difficult by the oxide layers working as
an
insulator on the metal particles. Because of the higher density and
encapsulation
during pressing, the heat transport is improved and the entire bulk powder
material can thus be heated homogeneously by the external supply of heat to
the
desired extrusion temperature, although the time period until a uniform
temperature distribution has been established by heat conduction in the bulk
powder material is comparatively long. For this reason, the direct processing
of
metallic powders in extrusion systerns has not hitherto proven successful.
The bulk powder material provided for extrusion has to be brought as
homogeneously as possible to the desired extrusion temperature. For this
purpose, the bulk powder material according to the prior art is heated in a
suitable
container, either inductively or in a convection oven. Care has to be taken
here
that the heating process lasts long enough to ensure a temperature
distribution
that is as uniform as possible within the bulk powder material. As a
consequence
of this long waiting time to ensure the temperature homogeneity, an undesired
delay in the production process occurs. The risk of too high a heating in the
outer
edge layers of the bulk material and/or too long a heat treatment time is also
increased. This is significant, in particular, if powders consisting of at
least two
different components, so-called composite powders, the components of which

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tend, at an elevated temperature, either individually, for example by
oxidation or
together, to react in an undesirable manner, are to be processed.
The above described methods according to the prior art are disclosed, for
exampie, in EP-A-0 327 064, US-A-4 050 143 or US-A-4 699 657.
The invention is based on the object of providing a method of the type
mentioned
at the outset, with which a rapid and uniform heating can be achieved in all
regions of the bulk powder material.
Leading to the achievement of the object according to the invention is the
fact that
at least one metal or a metal alloy of the powder is a reactive metal
spontaneously
forming a natural oxide protective layer on a free surface and/or the powder
contains fibre-like particles homogerieously distributed in the bulk powder
material
and absorbing microwave radiation;, and that the bulk powder material is
heated
by microwave irradiation to extrusion temperature.
By using the microwave technique to heat the bulk powder material, because of
its deep action, a very rapid and very uniform heating is achieved in all the
regions
of the bulk powder material. As a result, the waiting time to reach
temperature
homogeneity is drastically shorteried. This applies, in particular, to
reactive
metallic powders, i.e. to reactive rnetals spontaneously forming a natural
oxide
protective layer on a free surface, such as aluminium, magnesium, titanium,
tantalum or zirconium. These metallic powders basically have on their surface
an
oxide layer, even though it may be very thin, which, on the one hand, acts as
an
insulator on contact heat transfer, on the other hand, however, assists the
heating
process by the microwaves. This is to be attributed to the fact that the
hollow
spaces between the powder particles including the oxide layers act as so-
called
"wave guides" for the microwaves,, as they correspond with respect to
dimension
to the wavelength of the microwave radiation. As a result, the microwave
radiation can homogeneously perietrate unhindered and with multiple
reflection,
the entire region of the bulk powder material.

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To optimise the penetration of the bulk powder material by the microwave
radiation, the density of the bulk powder material or the dimension of the
hollow
spaces between the powder particles, including the oxide layers, can
additionally
be matched by corresponding compaction of the bulk powder material to the
wavelength of the microwave radiation.
If the powder, apart from the metal particles, also contains microwave
radiation
energy-absorbing, fibre-like components, such as, for example, carbon
nanotubes
(CNTs) these act locally as receivirig antennas or absorbers for the microwave
radiation. If the fibre-like componerits are homogeneously distributed in the
bulk
powder material or, in the optimal case, are even integrated at least
partially in the
metallic powder particles, a very effective and homogeneous heating of the
total
bulk material can thus be achieved. This effect can be further reinforced by
as
precisely as possible matching thE: length of the fibre-like components to the
wavelength of the microwave radiation.
In a preferred embodiment of the method according to the invention, the bulk
powder material on heating to extrusion temperature, firstly has low microwave
energy radiated though it at a changing frequency and the absorbed energy is
measured as a function of the frequency. At a specific frequency, the so-
called
resonance frequency, a maximurri of absorbed energy is produced. The bulk
powder material now has high rnicrowave energy radiated through it at this
frequency, so an effective energy coupling is produced.
The frequency matching process (sweep) with low microwave energy and the
following radiation with high microwave energy at the resonance frequency to
heat
the bulk powder material to extrusion temperature, can also be carried out
fully
automatically by means of control electronics, so the optimum frequency of the
coupled microwave energy is always adjusted for various bulk powder material
quantities and powder compositions.
In a further embodiment of the method according to the invention, the bulk
powder
material may, for example, firstly be pre-compacted with a screw conveyor in
an

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intermediate container. The bulk powder material thus pre-compacted is then
radiated through at the resonance frequency in the intermediate container and
thereby heated rapidly and uniformly to extrusion temperature. By means of a
ram, the pre-compacted bulk powder material which is heated to extrusion
temperature is pressed out of the intermediate container though the die
opening.
In this manner, a continuous extrusion of metallic powder material can be
implemented.