Method of producing copper-chromium fusion alloys as contact material for vacuum power switches

#The invention relates to a method of producing copper-chromium fusion alloys, having a chromium content of at least 25% and at most 60% by mass with a homogeneous macrostructure without lattice defects and without shrink holes or pipes, for use as contact material for vacuum power switches with breaking currents above 10 kA. A starting material for the fusion process is prepared from the alloy components copper and chromium, and then this starting material is melted to form the fusion alloy.

#Contact materials for vacuum power switches with breaking currents above 10 kA must, besides the reliable current cutoff, fulfill additional requirements. The contact material must manage high permanent currents of several thousands amperes without an undue temperature increase in the switching tube caused by the resulting dissipated heat. In the interests of an easy and silent drive, there must be little tendency to weld. The contact burnoff should be low enough so that at least 100 short-circuit breaks and 10,000 breaks at nominal current are ensured, with the contact exhibiting a smooth-surface burnoff behavior, for dielectric reasons. The texture and composition of the contact material should be such that the breaking current distribution curve is as narrow as possible and the most frequent value is no higher than 4 A. Lastly it must be assured that as the contact material melts and evaporates, the arc will not release gas components which would lead to a critical pressure increase in the switching chamber of above 10.sup.-4 mbar.

#A powder-metallurgically produced sintering/infiltration material which contains chromium and copper as base materials is commonly used as as contact material for vacuum power switches. It is produced from especially pure starting materials under shield gas and/or under vacuum. However, even if it is produced under conditions of high vacuum, perturbation zones and defects occur in the structure which are attributable to the reactivity of the chromium. In the infiltration (impregnation) process, these defects lead to faulty wetting on individual grain surfaces of the skeleton. Such deviations from the ideal structure may affect the burning and movement of the arc on the contact surfaces and may impair the current breaking capacity and voltage strength.

#The invention will now be explained in greater detail with reference to the following examples:

#From a powder mixture of 65% by mass of copper powder and 35% by mass of chromium powder a blank of a diameter of 80 mm and a length of 500 mm is pressed isostatically under a pressure of 3000 bar. It is thereafter sintered under vacuum at temperatures just below or, in the case of the formation of a liquid phase, about 50.degree. C. above the melting point of copper. The sintered blank is used as a fusible electrode in an arc melting furnace and is remelted under helium as a shield gas, to which a few percent hydrogen or carbon monoxide may be admixed as reducing agents. To obtain the required high energy density, the arc current should, at the given dimensions, be at least 1000 A. The fused electrode material solidifies in a water-cooled copper chill mold. The resulting ingot is drawn off downwardly and the fusible electrode is fed from above.

#A thick-walled copper pipe of OFHC copper is filled with chromium powder, so that the masses of the copper pipe and chromium powder are the same. Thereafter the pipe is closed at the ends and tapered in an extrustion molding tool for cross-section reduction and compaction of the chromium powder. This composite body is inserted as a fusible electrode in an arc furnace and remelted by the method described above in example 1.