Note: Descriptions are shown in the official language in which they were submitted.
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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.
