Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making porous product
Reexamination Certificate
2000-10-05
2003-11-25
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making porous product
C419S007000, C419S009000
Reexamination Certificate
active
06652804
ABSTRACT:
DESCRIPTION
The invention involves a process for manufacturing an openly porous sintered metal film from a metal powder that can be sintered.
In engineering, porous structures, which have a flowing medium flowing through them, are needed for very diverse applications, where either reactive processes are to be supported or solid particles contained in the flowing medium are held back, i.e. filtered out. Filter bodies made of ceramic materials, because of the danger of breaking, must be constructed so that they are relatively thick. Also, filter bodies made of pressed and sintered metal powders are relatively thick for reasons of manufacturing engineering. On account of the thickness that is not to be reduced, correspondingly large flow resistances occur, especially for fine-porous material. The use of plastics as a filter material has limits due to the low solidity and low resistance to temperature. A use of metallic materials as a porous layer is known in the form of fabrics or nonwoven (fleece) fabrics made from metal fibers.
In a porous layer of this type, which has a medium flowing through it, there is the need to minimize undesired flow resistances, so that as thin a layer thickness as possible is desired. From a metal fabric or nonwoven (fleece) fabric, suitably thin layers, for example, in a thickness of approximately 100 &mgr;m, can certainly be manufactured. They are, however, less dimensionally stable, have relatively large pores and in regards to their porosity, have large tolerances. Since in order to manufacture fabrics and fleeces of this type, correspondingly thin and thus also expensive wires must be used, the fabric and fleece made in this way are correspondingly expensive.
From EP-B-0 525 325, a process for manufacturing porous, metallic sintered workpieces is known in which at first, a metallic powder is suspended in a carrier fluid, which consists of a binder dissolved in a solvent and which is adjusted so that the suspension is able to be cast. This suspension is poured into a mold. Then, the solvent is evaporated, so that the metal powder becomes solidified, by the remaining binder, in the geometry given by the mold and forms a green body that can be handled. After separating from the mold, the green body is sintered in the usual manner. This previously known process is provided preferably for the manufacture of relatively thick-walled sintered parts, which can be manufactured because of their geometry better through a molding operation than in the traditional process through pressing a metal powder into a mold. Thin-layered, open, porous parts can not be manufactured with this process.
The purpose of the invention is to improve the previously known process, so that even thin, porous and to the extent necessary, even self-supporting metal films can be manufactured.
This purpose is achieved according to the process according to the invention in that the metal powder is suspended at a prespecified size distribution of powder particles in a carrier fluid, that the suspension is applied in at least a thin layer to a carrier structure, dried, and the green layer formed in this way is sintered, so that the layer thickness of the suspension applied corresponds after sintering at least to the thickness s of the metal film to be created, where s corresponds to at least 3 times the diameter D of the power particles, with D=1 &mgr;m to 50 &mgr;m, where the layer thickness of the finished metal film is at maximum 500 &mgr;m. In this process, the advantage is utilized that during sintering the individual powder particles do indeed connect solidly to each other, however, between the powder particles, open spaces remain, which produce an open porosity relative to the thickness of the metal film, so that the metal film is permeable for a medium flowing through it. The amount of the porosity can be influenced via the particle size of the metal powder used, so that very thin porous metal films with a prespecified pore size can be manufactured. Since non-homogeneities and hollow cavities can occur during manufacturing, the layer thickness must correspond to at least 3-times the diameter D of the powder particles. By the named ratio between the layer thickness s and the particle diameter D it is ensured that several “layers” of powder particles are always arranged over each other and “holes” that go through, which are larger than the desired porosity, are prevented. In this process, it is especially functional if the layer thickness s is 5 to 15 times, preferably 10 to 15 times the diameter D of the powder particles. “Holes that go through” can be avoided by this.
Diameter D is understood to be the average particle diameter of the metal powder used. Metal powders in the context of the invention are not only powders made of pure metal, but also powders made of metal alloys and/or powder mixtures of different metals and metal alloys. Belonging to them are especially steels, preferably chromium-nickel-steels, bronzes, nickel master alloys such as Hastalloy, Inconel or the like, where powder mixtures also contain refractory components such as, for example, platinum or the like. The metal powder to be used and its particle size depend on the respective application.
The consistency of the suspension to be set via the carrier fluid is essentially oriented according to how the suspension is applied to the carrier structure. For casting, if necessary, with subsequent strickling of an excess from the cast suspension layer, the suspension can be adjusted in a somewhat viscous consistency. In a so-called film molding or a spraying on, a low-viscosity consistency must be specified. In order be able to handle the carrier structure with the applied green layer after drying, it is also functional here that the carrier liquid is formed through a binder that is liquefied by a solvent that can be vaporized. In this way, it is ensured that the green layer also has a sufficient solidity as a result of the bonding of the individual powder particles to each other via the binder.
In an especially preferred embodiment form it is provided that the suspension is applied onto the carrier structure in several thin partial layers in sequence. In this process, the individual carrier layers can each be constructed out of an identical suspension. It is also possible, however, in a subsequent embodiment of the invention, for the individual partial layers to each use suspensions with varying size distributions for the metal powder that is used and/or to use different metal powders. This makes it possible, for example, to use metal powders on the one hand, which give an especially good porosity to the completed sintered metal film, and on the other hand, it is also possible to manufacture at least one metal film, which has in its metal composition especially favorable properties for the application purpose, for example, catalytic properties.
It is functional when the respectively applied partial layer is dried on at least prior to the application of the next partial layer. In this way, it is ensured that the first applied partial layer is sufficiently affixed so that it is not deformed by the application process, for example, by a spraying on of the next partial layer. On the other hand, by the remaining solvent portion in the previously applied, dried-on partial layer, it is ensured that also the next partial layer that follows is bonded reliably and with the same packing density and the finished green layer has the desired solidity.
In another embodiment of the invention it is provided that the respective partial layer is sintered prior to the application of the next partial layer. This process is especially advantageous when in a multiple layer construction, different metal powders are applied, which require greatly differing sinter temperatures. In this way, it is possible that at first the partial layer is applied onto the carrier structure, which contains the metal powder with the highest sintering temperature, and after sintering the first metal film, the next following partial layers with the re
Kuhstoss Andreas
Neumann Peter
GKN Sinter Metals GmbH
Jenkins Daniel J.
Woodcock & Washburn LLP
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