Patent Application
20100183469
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 systems 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 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 example, in EP-A-0 327 064, U.S. Pat. No. 4,050,143 or U.S. Pat. No. 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 homogeneously 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 shortened. This applies, in particular, to reactive metallic powders, i.e. to reactive metals 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 penetrate unhindered and with multiple reflection, the entire region of the bulk powder material.
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 receiving antennas or absorbers for the microwave radiation. If the fibre-like components 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 maximum of absorbed energy is produced. The bulk powder material now has high microwave 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 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07 405 206.9 | Jul 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/005489 | 7/4/2008 | WO | 00 | 1/13/2010 |
