Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-04-07
2001-11-06
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S248000, C419S003000, C419S005000, C419S006000, C419S023000, C419S028000, C419S047000
Reexamination Certificate
active
06312495
ABSTRACT:
Tungsten-silver and molybdenum-silver composites have long been known as contact materials which are subjected to high electrical loads. These sinter materials combine the material consumption resistance of the high-melting refractory components W and Mo and the good electrical and thermal conductivity of the silver used as the matrix component.
Such contact materials are commonly used in low-voltage power engineering as blow-out contacts in power switches and as main contacts in safety switches.
Important properties of these materials include high wear resistance, material consumption resistance and a low tendency to weld. Therefore, these silver materials are suitable for applications in electromechanical switches which require extremely high breaking capacities.
Due to their composite nature (non-alloyability of the components W and Mo with the matrix metal Ag) and the high melting point of the refractory component, these materials can basically only be manufactured by means of a powder-metallurgical process. Since the material consumption resistance, hardness and conductivity directly depend on the number of pores of the material in question, and since furthermore the material consumption resistance and strength can be improved by decreasing the grain size of the refractory component in the composite, it has generally been attempted to produce as non-porous and fine-grained a material as possible.
According to the prior art, two sintering processes are available for producing such materials:
During liquid-phase or solid-phase sintering, a powder mixture which has the same composition as the desired end composition, is pressed into a formed piece (single-piece production) and sintered at temperatures above or below the Ag liquidus.
In the infiltration process, a porous formed piece made of tungsten or molybdenum, which was also pressed individually, is infiltrated with liquid matrix metal in order to obtain as non-porous, i.e. dense, a composite as possible due to the acting capillary forces.
The disadvantage of the first process is a relatively high remaining residual porosity which might necessitate further compression by means of recompacting. However, the degree of reshaping by recompacting is relatively low. As a consequence, a certain residual porosity remains.
In the second process, the residual porosity is lower, however, the excess of the infiltration metal has to be removed in an additional time-consuming metal-cutting process step. Cf. A. Keil et al., Elektrische Kontakte und ihre Werkstoffe, Berlin 1984, pages 192 et seqq.
The production of metallic composites by means of sintering is additionally made more difficult by the endeavor to improve the quality of the contact material by employing fine-grained refractory components. Fine-grained refractory metal powders have a significantly higher oxygen content than coarse-grained powders. This makes the wetting with the matrix metal more difficult which entails an increased formation of pores. Therefore, fine-grained materials tend to have a higher pore content than coarse-grained materials. Another difficulty is the handling of fine refractory metal powders having an average grain size in the range of <1 &mgr;m. These powders become pyrophoric and tend to spontaneously smolder, incinerate or explode when handled in air.
Due to these properties it has become increasingly difficult to improve tungsten or molybdenum composites with respect to a small grain size combined with a high density of the material by means of conventional sintering technology alone.
Even after an aftertreatment consisting of reshaping steps such as recompacting by compression or the like, the residual porosity of the finished contact pieces still lies in the order of magnitude of percents. Fine-grained materials showed the least satisfactory results in this connection.
However, as a result of the pressure to miniaturize switch components while at the same time meeting the increasing demands on performance and lifetime, the quality of presently available contact pieces is no longer considered sufficient.
In the manufacture of metal composites, generally known reshaping processes can be applied after sintering by means of which, due to their large degree of reshaping, the residual porosity of the obtained materials can be brought down to below the known value. For this purpose, the person skilled in the art can choose from e.g. extrusion, rolling and forging processes. By means of these processes dense and high-quality products can be obtained. The starting material is a powder mixture which is isostatically pressed into rods, subsequently sintered and then reshaped by hot extrusion or hot rolling. In the case of extrusion, the obtained semi-finished product is usually formed by rolling. The high degree of reshaping provided by both processes results in a strong compression of the material. Compression and quality of the material are directly linearly dependent on each other (cf. A. Keil, loc. cit., page 188).
From a technological point of view, a material obtained by extrusion has the further advantage over single-piece production that a continuous section is obtained, which in addition can be plated during production with a solder suitable for the technology of joining materials. This continuous tape can then be integrated directly in the product line at the switch maker's. The desired contact coating is cut off, fed to the carrier and connected thereto for example by means of resistance soldering.
The disadvantage of both reshaping processes is that the starting rods which are subjected to the reshaping have to be sufficiently ductile. Otherwise, the pressing or rolling equipment or the sections to be produced could be damaged during reshaping. In the case of flat sections, cracking or chipping at the edges can occur. It might not be possible to extrude workpieces that are too brittle at all, not even in a heated state. In any case, such flaws preclude a high quality of the material.
What additionally complicates the matter is that particularly composites which are of special technical interest require a high amount of refractory component in the material. However, an increased amount of brittle and hard grains in the ductile matrix renders the entire workpiece brittle and thus unsuitable for reshaping.
Furthermore, it is the prevailing opinion among experts that the difficulties in extrusion increase with the decrease of the grain size in the matrix. This opinion makes the extrusion molding method seem hardly suitable for fine-grain materials. The document DE-A-198 28 692 discloses a process to render a commercially available SnO
2
powder more coarse from 0.6 &mgr;m to more than 5 &mgr;m so that it may be reshaped more easily by means of extrusion molding in an Ag matrix as AgSnO
2
composite.
Consequently, according to the prior art, the reshaping technology of WAg or MoAg composites is restricted to a high silver content range which is of secondary technological and economic interest.
Although on page 193, loc. cit., A. Keil also describes the extrusion molding of WAg sinter blocks produced by sintering powder mixtures below the melting point of silver, the extrusion moldability of WAg is considered limited at tungsten contents of ≦30 wt.-%. In Keil's view, due to the high Ag content, no stable W skeleton can be formed which would render the material brittle. The sintered body retains its sufficiently high ductility and can be extrusion molded.
JP-A-55 044558 discloses the extrusion of a heat-resistant, conductive material consisting of a copper oxide or silver oxide alloy in the form of particles and W or Mo in the form of particles which are combined, sintered and extruded. This results in the surface of the W or Mo being coated with Cu or Ag alloy. Neither the production method nor the application of this teaching is aimed at composites suitable for electric contact materials.
EP-A-0 806 489 discloses a process for producing a composite containing copper and a transition metal, said process comprising sintering a compa
Renner Gerd
Siefken Udo
Louis Renner GmbH
Mai Ngoclan
Mueting Raasch & Gebhardt, P.A.
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