Process for producing CuCr contact pieces for vacuum switches as

Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation

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419 47, 419 49, 419 54, 419 55, 419 58, B22F 324

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active

053307020

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a process for producing a contact piece using copper and chromium for applications in vacuum-switch tubes, in which a powder blank is compacted from the starting components right down to a residual porosity of <1%, as well as to a contact piece produced in such a way.
Composite materials consisting of a conductive component and at least one high-melting component and, if necessary, also containing additives that lower welding force or reduce chopping current have proven their worth as contact materials for vacuum-switch tubes. The widely used CuCr materials are a typical example of this.
Since a high-melting component such as chromium only has a low solubility in the electrically conductive main component such as copper, powder-metallurgical processes are highly considered for manufacturing CuCr contact materials.
A process that is often applied to produce such contact materials is the sintering of a Cr skeleton and the subsequent infiltration of the sintered skeleton with Cu. This is described, for example, in the German patent applications DE-A-25 21 504 or the DE-B-25 36 153. The result in this case is that qualitatively high-grade materials with good switching properties can be obtained.
However, this process is susceptible to defects and requires considerable expenditure for quality assurance. Since a liquid phase is used, the blanks that are formed are clearly oversized and require machining to obtain the final form. Moreover, the requirement for a self-supporting skeleton means that the concentration range available to the high-melting component is restricted.
The last mentioned disadvantages can be avoided by using another widespread process, in which a powder mixture of the components is pressed or sintered and then still cold or hot afterpressed. This is described, for example, in the applications DE-A-29 14 186, the DE-A-34 06 535 and the EP-A-0 184 854. In this process, the concentration of the components can be selected within broad limits and the contour of the blanks can be set close to the final form, since extended cavity systems, such as those that can develop in poor impregnation materials, do not occur in the material. However, such materials have a residual porosity, usually between 4% and 8%, which has a disadvantageous effect on their application as a contact material for contact pieces of vacuum-switch tubes. The reason for this is that with increasing porosity, the danger of later breakdowns escalates and the breaking-capacity limit diminishes, whereby the welding tendency goes up.
From the EP-A-0 184 854, it is already known to compact the powder bodies not by means of solid-phase sintering, but rather by hot-pressing them. As a result, materials are produced which have a negligibly low residual porosity and which avoid the above-mentioned disadvantages. However, this manufacturing method must take place under a vacuum or in highly purified protective gas and is therefore cost intensive and thus relatively uneconomical.
DE-A-37 29 033 discloses another manufacturing method for CuCr contact materials, in which a solid-phase sintering step is combined with a hot-isostatic liquid-phase compressing step (HIP). One starts out from sintered bodies that have a relatively low compression ratio - whereby 80% of the theoretical density was already indicated as sufficient and these sintered bodies are isostatically hot-pressed at temperatures of about 200.degree. C. above the melting point of the conductive component, copper. For this, the sintered bodies must be encapsulated under a vacuum to prevent air or gas from being occluded in the material pores and to prevent the chromium from being oxidized by the residual oxygen component in the pressure gas. Moreover, the encapsulation can prevent the internal armature of the pressing device from being contaminated by an overflowing liquid phase.
As is true of single-axial hot-pressing, negligibly low residual porosities are also produced with isostatic hot-pressing (HIP). However,

REFERENCES:
patent: 3703278 (1972-11-01), Isaksson
patent: 4067379 (1978-01-01), Hassler et al.
patent: 4190753 (1980-02-01), Gainer
patent: 4677264 (1987-06-01), Okumura et al.
patent: 4836978 (1989-06-01), Watanabe et al.

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