EP2018445B1 - Method for the production of copper-chromium contacts for vacuum switches - Google Patents

Abstract

Homogeneous copper-chromium alloys can be produced at best by rapidly supercooling alloys molten at elevated temperatures because of the miscibility gap in the copper-chromium phase diagram. Great efforts are therefore required to produce contacts from said alloys. According to the invention, copper-chromium contacts for vacuum switches are manufactured by producing a thin copper-chromium sheet as a starting material for the contacts, said thin copper-chromium sheet being produced by means of a casting or spraying process, followed by rapid quenching. Concentration profiles in the strip (50) can be adjusted perpendicular to the direction of the strip by adequately conducting the production process because particularly the chromium diffuses to the surface in the form of Cr particles or droplets (Cr).

Description

The invention relates to a method for the production of copper-chrome contacts for vacuum switches.

Vacuum interrupters for the power supply and distribution require electrical switching contacts made of an arc-resistant material, which the high thermal loads of z. T. over 5 MW / cm ² , no gaseous or other harmful impurities is released and in particular can be produced economically.

Most copper-containing materials are used for switching contacts in vacuum interrupters, predominantly a mixture of copper (Cu) and chromium (Cr), where i. a. the chromium content is between 20% -m (mass%) and 50% (mass%). The special contact shapes used in vacuum switching technology (button or disc contact, spiral contact, pot contact, or axial magnetic field contact) require that the contact area must be machined so that material thicknesses of at least 2 to 3 mm are required.

For the preparation of materials made of copper (Cu) and chromium (Cr) the state diagram is to be used. This is for example from the manual of M. Hansen and K. Anderko "Constitution of Binary Alloys", McGraw-Hill Book Company, Inc. (1958), page 524 known. The state diagram Cu-Cr has thermodynamic peculiarities, in particular a eutectic and a monotectic, which will be discussed below.

Since the two metals copper (Cu) and chromium (Cr) differ significantly in their density, the direct production of homogeneous melting materials is not possible because the heavier component (Cu) settles. In general, so-called remelting materials are used for the highest quality contact materials.

According to the EP 0 115 292 B1 For example, remelting materials can be produced by remelting a coarsely pre-sintered cylinder made of CuCr to a high-density, homogeneous, fine-grained material using an electric arc in a noble gas atmosphere. Slices are then sawn from the cylindrical blanks obtained therefrom, which are subsequently machined again in order to obtain suitable end contours, slots and / or surfaces of the contacts. In particular, the necessary elimination of burrs which are produced during machining forms leads to high costs of contacts made in this way.

Due to the high chromium content, a CuCr material is only suitable for the simplest forms of contact for contouring through punching. When machining the hard material, high tool wear with correspondingly high tool costs is unavoidable.

Other manufacturing methods for copper-chrome switch contacts, such. As the production of densely sintered blanks in near net shape, generally also require an exciting post-processing with the disadvantages described above. In addition, for sintering materials for manufacturing reasons, the particle size distribution is shifted to significantly larger grain sizes, resulting in less favorable switching and Abbrandeigenschaften compared to remelting materials result.

Specific sintered metallurgically produced copper-chromium contact materials are in the DE 38 29 250 A1 and the DE 38 42 919 A1 described.

For high demands on the quality of CuCr switching contacts, therefore, further process steps for grain refinement in the near-surface area considered. For example, a remelting by the use of laser or arcs offers.

The US 4,780,582 discloses a switching contact of copper and chrome for vacuum interrupters. Furthermore, be on the JP 10-287939 which discloses a method of making contacts of copper, such as electronic components such as switches or vacuum switches.

Starting from the above prior art, it is an object of the invention to provide a simplified manufacturing method for copper-chromium contacts.

The object is achieved by a manufacturing method according to claim 1. Further developments of the manufacturing method according to the invention are specified in the dependent claims.

The inventive method is based on the processing of a melt of CuCr by rapid solidification to thin, typically 1 to 2 mm thick strips or sheets, which can be set by the cooling rate of the melt, a Cr concentration profile perpendicular to the sheet surface targeted. It is made use of the technology of manufacturing amorphous metals, in which by rapid solidification, the metal is converted into a thermodynamic non-equilibrium. Such amorphous metals are also referred to as metal glasses.

The technology of rapid solidification of metals is well known in the art, including on the DE 694 18 938 T2 , the DE 35 28 891 C2 and the DE 36 17 608 C2 is referenced. With such methods, so-called amorphous metals are produced with the finest precipitates of high-melting components, such components, for example Chrome can be. Not known for the rapid solidification technique, however, the system copper-chromium with copper as a matrix and chromium as a high-melting component.

In the invention advantageously due to the density difference, the chromium (Cr) naturally accumulates on the surface and the resulting density profile is "frozen" during solidification.

By punching with appropriately shaped punching tools to get the desired contact shape. Post-processing of the switching contacts is advantageously only in the region of the connection points to the contact carriers, i. in the solder range, necessary to produce the necessary tolerances and surface qualities.

The inexpensive punched contacts thus produced can be connected as contact pads with contact-bearing structures, for. B. by brazing to achieve the required mechanical strength.

Alternatively, it is also possible to produce thicker sheets of several mm thickness using the method described, in which fine finely dispersed Cr is only present in some 1/10 mm near the surface. Due to the greater material thickness, which can be generated directly by the cost-effective raw material copper, a mechanically loadable contact piece can be produced in only a single step, which can also be used for molding because of the favorable material properties of copper punching technology. This leads to particularly cost contact discs for vacuum switching contacts.

The main advantages of the described method are on the one hand in the material savings by eliminating machining (sawing, turning, milling) as well as by targeted adjustment of electrical engineering necessary material thickness. On the other hand one achieves a greater profitability by a significantly reduced demand for chromium, since the expensive component chromium is only used in the contact surface area. Furthermore, the punching process results in shorter production times and lower operating costs, since punching is easier to automate instead of turning and milling.

The decisive advantage of the invention on the electro-technical side is a considerably improved switching behavior on the one hand by the natural setting of a fine-grained structure near the surface, on the other hand also by the improved heat conduction through the positive Cu gradient to the contact bottom.

Further details and advantages of the invention will become apparent from the following description of exemplary embodiments. Reference is made in particular to the drawing according to the appendix.

Figur 1

das Zustandsdiagramm von Chrom (Cr) - Kupfer (Cu) und

Figur 2

ein Gefügebild eines Kupfer-Chrom-Werkstoffes mit 20 % Chromanteil (CuCr20),

Figur 3

eine Vorrichtung zur Herstellung von dünnen Werks- stoffbändern nach Art der Herstellung von amorphen Metallen,

Figur 4

ein Gefügebild im Schnitt durch ein mit einer Ein- richtung gemäß Figur 3 hergestelltes Cu-Cr-Band,

Figur 5

die Draufsicht auf ein Legierungsband mit daraus aus- gestanzten Kontaktschreiben und

Figur 6

einen Vakuumschalterkontakt mit geschlitztem Kontakt- topf und einer Kontaktscheibe gemäß Figur 5.

Show it

FIG. 1

the state diagram of chromium (Cr) - copper (Cu) and

FIG. 2

a microstructure of a copper-chromium material with 20% chromium content (CuCr20),

FIG. 3

an apparatus for producing thin strips of material such as amorphous metals,

FIG. 4

a microstructure in section through a with a device according to FIG. 3 manufactured Cu-Cr band,

FIG. 5

the top view of an alloy strip with punched out contact letters and

FIG. 6

a vacuum switch contact with slotted Kontakt- pot and a contact disc according to FIG. 5 ,

In FIG. 1 For example, the chrome-copper state diagram shows 100% chrome on the left side and 100% copper on the right side. The chromium is known to have a comparatively high Melting point, namely 1550 ° C. In contrast, copper has a comparatively low melting point, namely 1083 ° C. At a temperature of 1075 °, a eutectic is formed at a copper content of 98.2. Below the melting point of copper, there is a narrow range of solubility for chromium. Otherwise, in the solid state below 700 ° C copper and chromium are not soluble in each other.

The state diagram chromium-copper furthermore exhibits the peculiarity of a miscibility gap in the liquid state, whereby a monotectic is formed: above the monotectic temperature of 1470 ° C., the range is between about 6% copper and 58% copper up to a temperature of about 2000 ° C two different CuCr melts, which are not miscible with each other.

For the practical production of homogeneous copper-chromium materials in the solid state, the latter means that first of all a homogeneous melt of more than 2000 ° C. has to be produced, which is then cooled rapidly in order to obtain the "homogeneous state"("freeze"). This can, for example, by arc remelting according to the EP 0 115 292 B1 be achieved.

A microstructure of a copper-chromium material produced by the arc remelting process is shown in FIG FIG. 2 shown. Specifically, in FIG. 2 Reference numeral 21 is a copper matrix in which chromium particles 22 are precipitated. Overall, the predetermined chromium content results in a largely isotropic size and concentration distribution of the chromium particles 22 in the copper matrix 21.

In the FIG. 3 an arrangement is shown as it is commonly known for the production of amorphous metal films ("supercooled glasses"). Specifically, in FIG. 2 the reference numeral 31 a rotatable about an axis perpendicular to the paper plane I rotatable copper wheel, 32 a trough with a cooling bath for the copper wheel 31 and 33 a reservoir for a CuCr melt. The electric heater and other control means are in FIG. 3 not shown.

By spraying the homogeneous melt with temperatures T> 2000 ° C from the reservoir 33 onto the rotating wheel 31, i. on the rotating roller, CuCr material of the same composition is deposited on the surface, wherein specific parameters can be specified by cooling and solidification of the melt with a defined cooling rate dT / dt.

In particular, with optimum cooling rate, the solidification process, in contrast to the process used in the production of metallic glasses, so led that the resulting CuCr layer is not homogeneous over the thickness of the resulting metal strip, but a concentration gradient of the excreted chromium is such that Preferably, the lighter chromium accumulates on the top surface of the belt. This is facilitated by the fact that the underside first solidifies, the upper portion of the resulting strip (sheet) but remains liquid longer and thus migrate the deposited Cr particles by their buoyancy in the heavier liquid copper to the top, where a concentration of chromium takes place.

Depending on the peripheral speed of the copper wheel 31, a thin strip 50 or a sheet of predetermined thickness thus results with a copper and chromium concentration corresponding to the melt. The width of the band 50 is predetermined by the transverse extent of the copper roller 31. With appropriate dimensioning can also produce sheets of greater width.

From the strip 50 or sheet produced in this way, it is possible to punch out virtually any desired contact shapes which can be applied to predetermined contact carriers by known means, for example by brazing.

Due to the specific boundary conditions of the cooling of the melt on the copper wheel 31 can be achieved that in the CuCr band 50 is perpendicular to the tape direction an anisotropic distribution of chromium concentration.

The particular advantage of the specified production method is that segregation of the constituents can be predetermined according to their specific weight of the components during the cooling process. This means that the lighter components, in this case the chrome particles or droplets, diffuse to the surface.

A microstructure of such a band-shaped contact material is in FIG. 4 shown. The grinding is done in the direction perpendicular to the band 50 FIG. 3 ,

In detail are in FIG. 4 51 denotes the copper matrix and 52 denotes the chromium particles present therein. A now anisotropic concentration distribution of the chromium portion perpendicular to the surface of the strip 50 is recognized. On the surface of the strip 50 results a high chromium concentration and a finely dispersed distribution of the chromium particles. On the underside of the band 50, on the other hand, a low chromium concentration is present. As a result, the overall thermal conductivity perpendicular to the contact surface is greatly positively influenced, resulting in an improved switching behavior, especially in terms of a higher switching capacity, compared to homogeneous, the prior art corresponding contacts.

The latter facilitates in particular the connection of the contact pad to contact carriers, which usually consist of copper. For the intended use of copper-chrome contacts in vacuum switches while the chromium content is of particular importance. This is now concentrated on the surface of the contact pads.

Overall, this results in considerable advantages for the erosion resistance and other switching properties of the switching contacts of vacuum switches. In particular, in this method of production, the chrome necessary for the switching properties is provided only in the region in which it is physically required, namely in the surface area of the contacts facing the contact gap. This allows, in addition to the achievable higher fine grain of the chromium, to be used with significantly lower total chromium amounts, resulting in significant cost advantages over conventional manufacturing processes.

In the plan view according to FIG. 5 50 means the alloy band out FIG. 3 , which for example has a thickness of 2 mm. From the belt 50, disks 60 with, for example, four radial slots 61 to 64 can be punched out with a tool (not shown in detail). It is essential that only a single appropriately trained tool is needed and that in particular a subsequent machining is no longer required. This achieves a further reduction of the manufacturing costs compared to the prior art.

A complete vacuum switch contact 100 for use as a radial field or axial field contact in vacuum switching devices is in FIG. 6 shown. The vacuum switch contact 100 consists of a contact pin 110 for current conduction and a contact pot 120 with slits 121 to 124 in the pot wall. On the upper edge of the contact pot 120, the contact disk 60 is made FIG. 5 fixed by brazing in such a way that the slots 61 to 64 connect to the slots 121 to 124 in the wall of the contact pot 120.

Through the slits 121 to 124 in the contact pot 120, the current is conducted in the areas formed between the slits 12 to 124. This creates a magnetic field that affects the arc formed during switching. In each case two pot contacts accordingly FIG. 6 form the complete contact arrangement for a vacuum switch. Depending on whether the slits in the contact wells in the same direction or against each other, a total of a radial or axially extending to the contact arrangement magnetic field is generated, which has an effect on the switching behavior.

It can thus be manufactured at low cost vacuum switch contacts with alternatively radial or axial magnetic field. The concentration gradient of the chrome, which is perpendicular to the button, ensures a minimized contact erosion with optimum switching behavior.

Claims (7)

1. Method for producing contacts of copper (Cu) and chromium (Cr) as switching contacts for vacuum switches, a predetermined chromium concentration being set in non-thermodynamic equilibrium from a CuCr melt material by rapid cooling to room temperature, comprising the following method steps:

   - a strip or a thin sheet of copper (Cu) and chromium (Cr) is produced as the starting material for the switching contacts by means of a casting process with rapid cooling, wherein

   - to produce the strip or sheet, liquid copper-chromium of a predetermined concentration is sprayed or poured at high temperature onto a rotating roller, the roller being assigned cooling means, so that the liquid metal alloy is cooled,

   - a contact facing is punched out from the strip or sheet
   and

   - the punched part is fastened on a contact carrier.
2. Production method according to Claim 1, characterized in that the strip or sheet has a thickness of 1 to 2 mm.
3. Production method according to Claim 1 or 2, characterized in that the cooling rate is set in a defined manner.
4. Production method according to one of the preceding claims, characterized in that a predetermined chromium concentration profile is specifically set perpendicularly to the strip/sheet surface by the cooling rate set in a defined manner.
5. Production method according to Claim 4, characterized in that chromium becomes concentrated on the surface of the strip or sheet as a result of the difference in density of chromium and copper.
6. Production method according to Claim 1, characterized in that the fastening of the punched part on the contact carrier is performed by hard soldering or brazing.
7. Production method according to Claim 1, characterized in that a contact facing is formed on the contact carrier by the punched part.

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