US3864827A

US3864827A - Method for making an electric contact by powder metallurgy and the resulting contact

Info

Publication number: US3864827A
Application number: US283409A
Authority: US
Inventors: Horst Schreiner; Bernhard Rothkegel
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1971-09-01
Filing date: 1972-08-24
Publication date: 1975-02-11
Anticipated expiration: 1992-02-11
Also published as: DE2143844A1; GB1407253A; FR2151009A1; DE2143844B2; IT964329B; FR2151009B1; CH562649A5; JPS4857165A; JPS5138062B2; DE2143844C3; AT318053B

Abstract

A layer of refractory metal or metal compound powder which is mixed with a low melting temperature metal or metal alloy powder, and a layer of low temperature melting metal or metal alloy powder, are arranged together in the cavity of a compacting die and consolidated together to form an integrated two-layer compact which is thereafter heated above said low temperature but below the refractory powder''s sintering temperature to cause the pores of the compact''s refractory portion to be impregnated with the low temperature melting component while leaving an excess thereof forming a thin layer in the refractory portion. Because the two layers of the compact are consolidated together so that the compact may be moved safely without the layers separating during the heating, the use of a continuous furnace for the heating is commercially practical; and because the refractory portion is not sintered, the finished product''s dimensions can be accurately predetermined by the compacting die cavity dimensions. By heating the compact with its low temperature melting end down and resting on a refractory carrier surface having a plurality of depressions beneath this end, said layer is formed with a smooth exposed surface suitable for soldering or welding to a supporting metal part. Thus, a finished electric contact, for example, may be produced by the two steps of compacting and heating only, having a thick contact layer resistant to arc burning and contact welding trouble and a thin support layer that is solderable or weldable to a metal support or operating element. If the low temperature melting metal powder mixed with refractory powder in the refractory portion of the compact, and the low temperature melting powder in the other portion of the compact, are different metals, the heating of the compact can cause alloying or intersolution of the different metals so that in the final product the low temperature melting component of the product has the same composition throughout.

Description

Schreiner et al.

[ METHOD FOR MAKING AN ELECTRIC I CONTACT BY POWDER METALLURGY AND THE RESULTING CONTACT [75] Inventors: Horst Schreiner; Bernhard Rothkegel, both of Nuremberg, Germany [73] Assignee: Siemens Aktiengellschaft, Munich,

Germany [22] Filed: Aug. 24, 1972 [2]] Appl. No.: 283,409

[30] Foreign Application Priority Data Sept. 1, 1971 Germany 2143844 [52] US. Cl. 29/630 C, 29/630 R, 200/264 [51] Int. Cl H0lr 9/00 [58] Field of Search 29/420, 420.5, DIG. 31, 29/628, 629, 630 R, 630 A, 630 B, 630 C, 630 D; 200/166 C, 166 CM, 166 F [56] References Cited UNITED STATES PATENTS 2,641,670 6/1953 Graves 200/166 C 2,694,126 11/1954 Binstock 200/166 C 3,143,626 8/1964 Schreiner et al 200/166 C 3,226,517 12/1965 Schreiner 200/166 C 3,254,189 5/1966 Evanicsko et al... 200/166 C 3,359,623 12/1967 Gwyn 29/630 C 3,511,953 5/1970 Schmidt 200/166 C 3,610,859 10/1971 Schreiner et al 200/166 C Primary ExaminerC. W. Lanham Assistant Examiner-James R. Duzan Attorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin [57] ABSTRACT A layer of refractory metal or metal compound pow- [451 Feb. 11, 1975 der which is mixed with a low melting temperature metal or metal alloy powder, and a layer of low temperature melting metal or metal alloy powder, are arranged together in the cavity of a compacting die and consolidated together to form an integrated two-layer compact which is thereafter heated above said low temperature but below the refractory powders sintering temperature to cause the pores of the compact's refractory portion to be impregnated with the low temperature melting component while leaving an excess thereof forming a thin layer in the refractory portion. Because the two layers of the compact are consolidated together so that the compact may be moved safely without the layers separating during the heating, the use of a continuous furnace for the heating is commercially practical; and because the refractory portion is not sintered, the finished products dimensions can be accurately predetermined by the compacting die cavity dimensions. By heating the compact with its low temperature melting end down and resting on a refractory carrier surface having a plurality of depressions beneath this end, said layer is formed with a smooth exposed surface suitable for soldering or welding to a supporting metal part. Thus, a finished electric contact, for example, may be produced by the two steps of compacting and heating only, having a thick contact layer resistant to are burning and contact welding trouble and a thin support layer that is solderable or weldable to a metal support or operating element. 1f the low temperature melting metal powder mixed with refractory powder in the refractory portion of the compact, and the low temperature melting powder in the other portion of the compact, are different metals, the heating of the compact can cause alloying or intersolution of the different metals so that in the final product the low temperature melting component of the product has the same composition throughout.

4 Claims, 16 Drawing Figures 1 METHOD FOR MAKING AN ELECTRIC CONTACT BY POWDER METALLURGY AND THE RESULTING CONTACT BACKGROUND OF THE INVENTION This invention relates particularly to electric contacts made by powder metallurgy methods, such as contacts for electric power circuit breakers and the like. Such a contact should resist burning by the electric arc which forms between it and a cooperating contact when the two contacts separate. It should have the lowest possible welding tendency when subjected to such conditions; and when closed with the other contact, it should have a low contact electric resistance to avoid heating while continuously carrying electric power, keeping in mind that such contacts are enclosed by a housing made of insulating material which is not highly heat resistant.

In addition, the contact preferably should be solderable or weldable to the metal part which supports or actuates the contact; and the contact should be capable of being manufactured economically in large quantities by an uncomplicated manufacturing method capable of high production rates.

To achieve a low order of burn-off caused by arcing, the contact should be made of a material of very low or nonexisting porosity.

DESCRIPTION OF THE PRIOR ART It is known that an electric contact may be made by compacting two different metal powder layers and the resulting two-layer compact sintered by using temperatures below the melting temperature of the metal pow der comprising both layers, but high enough to sinter the powder particles together, the result being a twolayer sintered contact. However, the interlocking strength of the two sintered metal powder layers depends on the powder particle sizes involved.

Furthermore, it is common to make a contact by first making a sintered metal powder porous body by sintering a refractory metal or metal compound powder compact; and as a second step, to thereafter impregnate its pores with a molten low temperature melting metal, such as copper or silver, in a separate heating operation. But for impregnation, the sintered body must be balanced on the low temperature melting metal, this resulting in an undesirably large number of faulty contacts when using a continuous furnace through which the two pieces must be moved with some attendant vibration of necessity. Mechanical holding means for the two pieces are not reliable. Also, only a single layer contact can be produced because an excess of the impregnating metal forms an irregular mass which must be removed by machining or the like.

German published Pat. application No. 2,018,642 provides the suggestion that to reduce the labor and time required by the above two-step practice, a twolayer compact may be formed, one layer comprising the refractory powder, and the other the low melting temperature metal powder. This compact is heated to a temperature above the melting temperature of the layer comprising the relatively lower melting temperature metal powder so that the latter becomes molten and impregnates the pores of the other layer. This published technique is inadequate to solve the problem that if an excess of the impregnating metal powder is used, after heating this excess is in the form of an irregularly adhering mass which must be machined or otherwise worked, this representing a postsintering treatment with its incidental time consumption and expense. Therefore, only single-layer products can be produced.

The general art of powder metal and its processing is succinctly outlined by pages 40l to 435 of Vol. 16 of the Encyclopedia of Chemical Technology, Second Edition, published by lnterscience Publishers, N.Y., N.Y.

SUMMARY OF THE INVENTION An object of the present invention is to provide a powder metal method particularly for making an electric contact having contact and support layers and which meets the previously outlined preferred characteristics for an electric contact suitable for electric power circuit breakers and the like, while avoiding the problems of the prior art methods described hereinabove.

According to this invention, a conventionally unidirectional compacting die of the single action or double action type may be used. Two layers of metal powder are formed in the die cavity, one layer comprising a refractory metal powder or compound, such as powdered tungsten, molybdenum, rhenium, or their alloys, or carbides of these metals or alloys, intermixed with a low temperature melting metal or metal alloy powder, this layer ultimately forming the contact layer; the other layer comprising a support or impregnating metal powder, such as powdered silver or copper, or alloys of these metals with other metals. The usual punch or punches are then operated to consolidate a coherent two-layer compact that is entirely self-supporting. This compact is placed with its contact layer uppermost and its support layer down with its end resting on a refractory surface profiled to provide a plurality of depressions throughout the area of the support layers end. This surface may be formed by a ceramic plate which can be moved through a continuous furnace. Thus supported, and while in a non-oxidizing atmosphere or vacuum, the compact is liquid-phase sintered by using a temperature above the melting temperature of the support layer metal and the low temperature melting component of the contact layer, but below the sintering temperature of the contact layer metal powder. The support layer metal powder fuses and moves upwardly through the pores of the contact layer portion of the compact, ultimately filling these pores substantially completely and supporting or integrating the powder particles. Simultaneously, the low temperature component powder of the contact layer previously mixed therewith, also melts, promoting the impregnation by support metal of the other or support metal layer.

When forming the compact, the support metal powder layer volume is proportioned so as to be slightly in excess of the volume required to fill the contact layer pores as described; or in other words, an excess of the support metal powder is deliberately used. Surprisingly, when supported on the refractory surface profiled as indicated, this excess forms a smooth uniformly distributed layer on the bottom of the contact layer, integrally joined to the support metal impregnating the pores of the contact layer and having an external surface with a finish suitable for soldering or welding to another metal part and requiring no postsintering treatment of any kind. Furthermore, the support metal layer melts and because of its surface tension forms a thin layer of its metal on the bottom of the contact layer, even when the support layers initial volume may be less than that required to fully fill all the pores of the contact layer, the refractory particles of the latter being interconnected by the low melting temperature component mixed with its refractory particles. Thus, the Contact's porosity may be adjusted. Surprisingly, if the 'low melting temperature metal powder mixed with the refractory powder in the contact layer, differs from that of the support layer, the heating not only melts both with the support layer metal impregnating the contact layers pores, but with time at the temperature required, the two metals, and more might be involved, alloy together so that the low temperature melting components acquire the same composition or analysis through the product and as to both layers.

This foregoing summary is brief and does not include many details disclosed hereinafter.

DESCRIPTION OF THE DRAWINGS The accompanying drawings are schematic and represent in all instances, excepting for the graphs of FIGS. 6 through 9, vertical sections with the understanding that the parts are circular in cross-section. In these drawings the various figures are as follows:

FIG. 1 shows the compacting die and punches producing the two-layer metal powder contact;

FIG. 2 shows the resulting compact with its support layer side down and resting on the profiled refractory surface during the heating step;

FIG. 3 shows the final contact obtained, the sectional portion of this view, in an entirely schematic manner on a magnified scale, suggesting the appearance of the contact layer;

FIG. 4 is a first modification of the procedure shown by FIGS. 1;

FIG. 5 shows the finished contact obtained from this procedure of FIG. 4;

FIG. 6 is a graphic representation of the interrelationship of the contact metal powder density, the compacting pressure and the filling factor for the contact metal powder layer;

FIGS. 7 to 9 correspond to FIG. 7 but concern various examples described hereinafter;

FIGS. 10 to 13 are like FIG. 4 but show progressive steps involved by another modified practice of the invention;

FIG. 14 shows the compact obtained by the steps represented by FIGS. 10 through 13;

FIG. 15 shows the compact of FIG. 14 ready for the heating step; and

FIG. 16 shows the appearance of this compact shown in FIG. 15 after the heating step.

DESCRIPTION OF THE PREFERRED EMBODIMENT Having reference to FIG. 1, a conventional compacting press may be used having a metal powder compacting die 1 forming a cylindrical cavity in which is first formed a layer 2 of the support metal powder. This may be a copper, silver, or an alloy thereof metal powder. On this layer 2 the metal powder layer 3 is formed for forming the ultimate contact layer, this metal powder being a tungsten, molybdenum or rhenium powder or a mixture of these refractory metal powders, or a refractory alloy of these metals, or a powder carbide, such as tungsten carbide or molybdenum carbide and intermixed therewith one or more of the metal powders referred to as comprising the layer 2. In other words, the layer 3 is a mixture of refractory powder and low temperature meltingmetal powder. The support layer 2 comprises only the low temperature melting metal powder. The layer 3 is, of course, substantially thicker than the layer 2. The metal of the layer 2 and the nonrefractory metal mixed with refractory components of the layer 3 has a relatively low melting temperature as compared to the sintering temperatures of the refractory metals of compounds of the layer 3. With the two layers in place, the compacting

punches

4 and 5 are operated, either single-action by movement of only one punch or double-action by movement of both, to consolidate the metal powder of the two layers into a coherent and firm two-layer compact. The two layers should adhere reliably for handling of the resulting compact and enough compacting pressure should be used to obtain this adherence.

FIG. 2 shows the resulting integrated layer compact 6 with its support metal layer 7 down and its contact layer 8 extending upwardly, the bottom face or end of the layer 7 resting on a profiled plate 9 made of a suitable refractory, such as a ceramic material. The profile of the upper surface of this plate 9 provides a plurality of uniformly arranged depressions 10, the size and spacing of these depressions relative to the diameter of the compact being such as to provide a multiplicity of depressions for the diametrical extents of the compact. The depressions are uniformly distributed in all directions.

With the compact 6 supported as described above, it is subjected to the liquid-phase sintering under nonoxidizing conditions such as by using an inert gas atmosphere or by using a vacuum furnace. The heating temperature is above the melting temperature of the sup port metal and its counterpart in the contact layer 8 and below the sintering temperature of the refractory metal powder of the contact layer 8.

During this sintering the metal powder of the compacted support layer 7 melts and moves upwardly through and more or less filling the open pores of the compacted contact metal powder 8 of the compact 6. By initially proportioning the layer 2 in FIG. 1 so that its volume is in excess of that required to fill the pores of the compacted layer 8, the excess of the support metal forms a layer of the support metal on the underside of the contact layer 8. This layer is formed by surface tension regardless of whether or not the metal of the support layer completely fills the pores of the contact layer 8. If these pores are not filled by the support metal from the layer 7, its refractory and undersintered particles are bonded or supported by the low temperature metal initially included thereby. At the same time this latter metal, when melted, promotes the impregnation by the support metal to the extent possible considering the retention of the support layer caused by the surface tension effect.

FIG. 3 shows the finished contact 11. The contact layer 12 now has all of its pores more or less filled with the support metal. As schematically represented by the section of FIG. 3, if the contact is polished and microscopically examined, it is found that the refractory metal or metal compound particles are surrounded by a matrix of the low melting temperature metals which permanently hold the unsintered particles coherently together. The support metal forms a layer 13 having a smooth bottom surface requiring no special aftertreatment to condition it for soldering or welding to a supporting or actuating metal part. The support metal 13 is, of course, solid and inherently integrally joins with the layer 14 of the same metal.

In the foregoing manner a finished electric contact is formed by only the two steps represented by FIGS. 1 and 2, respectively. No postsintering treatment is required. The smooth as-melted layer 13 can effectively be soldered or welded to another metal part because it is either silver or copper or their alloys which are inherently solderable and weldable, its surface being free from projections, lumps or the like.

If the low temperature melting metal initially mixed with the refractory component of the compact, differs from that of the initial support layer, for example one being silver and the other copper, the heating with time causes the two to alloy together and actually reach the same concentration of the two metals, this occurring throughout the entire final product.

In the above the contact produced has a flat contact surface, the cross-section being circular.

For the manufacture of a contact with a spherical contact layer, as shown schematically in FIG. 4, the cavity of the die 15 has a lower punch 16 with a concave molding surface corresponding inversely to the desired spherical contact shape. Into the cavity is first filled the intermixed powder 17 for the contact layer and this is pre-compressed by the upper punch 18. After withdrawing the upper punch 18, the metal powder 19 for the support layer is filled in and both powder layers are compressed by the upper punch 18 using a pressure no greater than used before. In the heat treatment at above the melting temperature of the low temperature melting metals, the pore space of the contact layer is first filled, completely if desired, and the excess of the melted metal of the support layer remains as a thin layer on the bottom surface of the contact layer. This heat treatment, which is a liquid-phase sintering, the refractory component being unsintered and unmelted, but the low temperature melting components being melted, is conducted as described hereinbefore.

The surface of the press punch 18 can advantageously be profiled. A diamond or honeycomb-shaped profile has been found to be advantageous. In any event, the surface of the punch 18 should have a plurality of projections extending in all diametric directions and which form corresponding recesses in the flat surface of the compacted refractory powder or powders 17. Depending on the amount of low temperature melting metal mixed with the refractory metal powder component and the volume of the layer 19, it is possible to produce a finished contact which is practically free of pores in the contact layer and in which the impregnating excess fills the depressions referred to in the contacts support side. Such a surface needs no postfinishing and provides a very good solder joint with the carrier metal.

In FIG. 5 a spherical surfaced contact is shown schematically in section. The support layer is designated 20 and the spherical contact layer 21.

It is to be understood from the foregoing that in filling the die cavity the support metal powder may be filled either before or after the contact metal powder or metal compound is filled. In the case ofeither of the desired shapes of contact, the punch having the profile surface for making depressions in the contact metal powder layer may be used.

By mixing the low temperature melting metal powder with the refractory powder, the particles of the latter, after the heating described, are bound together or supported and, depending upon the proportioning, the pores or voids between the particles are filled more or less. Depending on the thickness or volume of the support metal layer 19, the latter forms the

support layer

7 or 20 of FIGS. 2 and 5, of necessity, because of its surface tension when molten, and more or less further impregnates the pores of the contact layer. If one support metal, such as silver, and another support metal, such as copper, are involved, with adequate time during the described heating or liquid-phase sintering, the two metals alloy so that the support metal has the same composition throughout the finished contact. The solderable or weldable support layer is ready for use although it may be coined if desired.

SPECIFIC EXAMPLES OF THE INVENTION Example I For the manufacture of a two-layer contact block with a contact layer of tungsten-silver and a support layer of silver as a finished formed piece, one procedure according to the invention is as follows:

The cavity of the die of steel is filled with a layer of electrolytic silver powder of less than 37 microns particle size and on top of it, a layer of a powder mixture of tungsten powder, which was obtained by reduction of tungsten trioxide (particle size less than 45 microns), and of electrolytic silver powder of a particle size of less than 37 microns. The composition of the tungstensilver or WAg powder mixture is chosen according to the desired final composition of the WAg contact layer. In the present example, it consists of 65 percent by weight of tungsten powder and 35 percent by weight of electrolytic silver powder. The filling heights of the two layers are determined by the desired height of the contact layer of the finished formed part. The two powder layers filled in on top of each other are jointly pressed to form a compact with firm edges. With a given composition of the WAg powder mixture, the final composition of the contact layer can be chosen within certain limits via the pressure P used.

As shown by FIG. 6, the density p of a molded body of WAg35 powder mixture as well as the corresponding filling factors for r and for pure tungsten r (ordinate to the right outside, or ordinate to the right, inside, respectively) are given in FIG. 6 as a function of the pressure. Thus, for instance, the density p of the pressed body of WAg35, which was compressed at a pressure of 2 Mp/cm (2,000 kglcm is about 10.0 g/cm This corresponds to a filling factor rwMss of 0.673. Relative to the tungsten component, the filling factor is r 0.337. The heat treatment takes place at l,l00 C. for 1 hour in a hydrogen atmosphere in the manner described. Above 960 C. the silver is present in the liquid phase. During the sintering of the contact layer, impregnation with the liquid silver takes place, so that a practically porefree contact layer may be obtained.

In FIG. 6 the increase of the density of the WAg35 layer is given under the described sintering conditions. The increase in density and therewith the increase in the filling factor may be seen from Curve S. The pore volume of the contact layer in the sintered condition is filled in the example described with the silver of the second layer. After the impregnation, the contact layer consists of 63.4 percent by volume of silver and 36.6 percent by volume of tungsten. This corresponds to 33.3 percent by weight of silver and 66.7 percent by weight of tungsten. The height of the silver layer of the pressed body is apportioned so that the pore volume of the contact layer is completely filled, if necessary, and a residual quantity is left for a silver layer thickness of 0.15 mm to remain.

A condition for economical commercial operation is good flow properties of the metal powders or metal powder mixtures used, to permit rapid die cavity loading. The flow properties of the metal powders should be such that if, using a 60 funnel and a nozzle with a diameter of 4 mm, the flow time is longer than 40 seconds for 100 g of flow, the powders should be converted into a flowable form by the usual methods.

After the heat treatment, a WAg two-layer contact block with a silver layer on the support side is obtained, which can be joined directly with the carrier metal by soldering or welding. A polished section through the contact block shows a structure of WAg practically free of pores and a second layer of practically pore-free pure silver.

The values of the pressure to be used can be taken from FIG. 6 for manufacturing WAg contact layers with a tungsten content higher than in the example discussed. With a pressure of 8 tons/cm and the WAg35 powder mixture used, a contact layer with a tungsten filling factor r,,. of 0.44 is obtained.

Example 2 In FIG. 7 the density and the filling factor are given, similarly as in Example 1, vs. the pressure for a powder mixture WAg20. With this, WAg contact layers with a tungsten filling factor r between 0.4 and 0.58 can be obtained. From powder mixture of reduced tungsten powder of a particle less than 45 microns and electrolytic silver powder of less than 37 microns particle size, with a mixing ratio of 80 percent by weight of tungsten and 20 percent by weight of silver, a pressed two-layer body with a silver layer and a second contact layer is prepared with a pressure P of 4 Mp/cm In the pressed condition the contact layer has a density of 12.05 g/cm. corresponding to a filling factor of r 0.73, and a tungsten filling factor r,,. of 0.5147. The heat treatment is at 1,100 C. for 1 hour in an H atmosphere. During the heat treatment the density of the contact layer remains practically constant, so that the S curve coincides with the P curve. During the heat treatment the pores of the contact layer are filled practically completely with the liquid silver. The result is a contact layer of 51 percent by volume of tungsten and 49 percent by volume of silver. The density p of the contact layer is 14.98 g/cm. The composition of the Contact layer corresponds to 34.4 percent by weight of silver and 65.6 percent by weight of tungsten. The height of the silver layer should be apportioned so that the pore volume of the contact layer is filled and a residual amount remains for a silver layer thickness of Example 3 When a contact layer having an even higher tungsten content is desired, a powder mixture of 90 percent by weight of tungsten powder and 10 percent by weight of silver powder may be used. As in the preceding examples, a molded two-layer body, consisting of a silver layer and a contact layer, is produced from the WAg I 0 powder mixture. FIG. 8 shows the density and the filling factor as a function of the pressure of the contact layer of WAglO. Depending on the choice of the pressure, a tungsten filling factor of between 0.5 and 0.65 can be adjusted. The molded two-layer body is subjected to a heat treatment at l,l0O C. for l hour in an H, atmosphere. The density of a WAgIO contact layer practically does not increase during the heat treatment so that. the sinter density curve S coincides with the pressingdensity curve P. The WAglO layer pressed at 6 Mp/cm shows a density of 13.3 g/cm. After the pore volume of the contact layer is filled with silver, the contact layer consists of 62.5 percent by volume of tungsten and 37.5 percent by volume of silver, corresponding to 75.4 percent by weight of tungsten and 24.6 percent by weight of silver. The density of the contact layer is 15.95 g/cm. This finished molded contact block has also a silver layer of 0.15 mm on the solder side.

The method of this invention offers the possibility to apportion the second layer of pure silver so that it is not sufficient to fill the entire pore volume of the contact layer. In that case, there remains in the contact layer a residual porosity corresponding to the amount of silver chosen, while the remaining silver layer on the solder side results, solely due to the surface tension of the silver, in a very thin layer of less than 50 microns.

It is the generally accepted opinion of the experts in this field that sintered impregnated contacts should be used as free of pores in the contact layer as possible. Measurements of the burn-off in the arc at a maximum currentof 350 A. have shown that with a residual porosity of 5 percent in the contact layer the burn-off is increased only 10 percent over the burn-off value in the poreless contact material. Only with a further increase in the residual porosity to 10 percent is the burn-off doubled.

Example 4 A contact layer of WAgNi, and a second layer of pure silver is made by using for the contact layer a powder mixture of reduced tungsten powder with a particle size of less than 45 microns, electrolytic silver powder with a particle size of less than 37 microns and carbonyl nickel powder with a particle size of less than 10 microns. The composition of the powder mixture is 63.8 percent by weight of tungsten powder, 35 percent by weight of silver powder and 1.2 percent by weight of nickel powder. FIG. 9 shows the density p and the filling factor r for the'WAg35Nil.2 pressed bodies as a function of the pressure. The die cavity is filled, first with a layer of the electrolytic silver powder and on top, a second layer of the powder mixture WAg35Nil.2. The pressure used for compressing is 2 Mplcm a molded body of 10.1 g/cm density being obtained. The heat treatment of the molded two-layer body is at 1,l00 C. for 1 hour in a hydrogen atmosphere. The density of the contact layer rises to 13.1 g/cm (S curve in FIG. 9). The influence of the nickel addition to the contact layer becomes clear by comparing FIG. 9 with FIG. 6. Without the addition of nickel, the density of the body pressed at 2 Mp/cm rises only to 10.9 g/cm (as against 13.1 g/cm in the nickel-containing specimen). The second layer of the pressed body of pure silver is apportioned so that in the heat treatment the liquid phase of the silver impregnates the pore volume of the first layer completely and an excess of a silver layer of about 0.15 mm remains on the solder side. With complete impregnation of the first layer, a contact block is obtained with a contact layer of 58.8 percent by weight of tungsten, 40.1 percent by weight of silver and 1.1 percent by weight of nickel is produced, corresponding to the volume percentages of 43.6 percent W, 54.6 percent Ag and 1.8 percent Ni. Example For a contact layer of WAgCuNi and a second layer on the solder side of AgCu, a powder mixture for the contact layer of reduced tungsten powder with a particle size of less than 45 microns, electroyltic silver powder with a particle size of less than 37 microns and carbonyl nickel powder with a particle size of less than microns is used. The composition of the powder layer is 63.8 percent by weight of tungsten powder, 35 percent by weight of silver powder and 1.2 percent by weight of nickel powder. Into a die cavity is first filled a layer of electrolytic copper powder and on top a second layer of the powder mixture WAg35Nil.2. The pressure used for compressing is 5 Mp/cm? The density of the compressed contact layer of WAg 35 M12 is l 1.8 g/cm. During the heat treatment of the compacted two-layer body at 1,100 C for 1 hour in a hydrogen atmosphere, a liquid silver phase is produced in the contact layer and in the second layer a liquid copper phase. During the heat treatment of 1 hour, an equalization of the concentrations substantially occurs so that one obtains a contact layer with a tungsten skeleton, the pores of which are impregnated with an Ag- CuNi alloy, and a second layer, which also consists of AgCuNi. Example 6 Corresponding to the examples cited above, a compact is prepared from a powder mixture of WAg for the contact layer and of a powder mixture of AgCu or AgCu alloy powder, respectively. In the subsequent heat treatment at above the melting temperature of the silver and the silver-copper alloy, a contact layer of WAgCu is formed, and a second layer on the solder side of AgC u. Example 7 The two-layer compact, in this case, consists of a layer of a powder mixture of tungsten carbide with silver powder, and a second layer of silver powder. 1n the heat treatment at above the melting temperature of silver a contact is produced with a contact layer of WCAg and a silver layer on the solder side. Example 8 Here a two layer compact is used, consisting of a powder mixture which consists of WRelO alloy powder (tungsten with 10 percent by weight of rhenium) and electrolytic copper powder, and of a second or support layer of electrolytic silver powder. In the subsequent heat treatment at above the melting temperature of copper, a two-layer contact block with a contact layer of WReCuAg is produced and a second layer of AgCu on the solder side. Example 9 The molded two-layer body in this instance consists of a first layer of a powder mixture of tungsten powder, tungsten carbide powder and silver powder and a second layer of copper powder. After the heat treatment at above the melting temperature of copper, a twolayer contact is obtained consisting of a contact layer of W,WC, AgCu and a second layer of CuAg.

Throughout the foregoing examples the liquid-phase sintering is conducted as described previously. The contacts may have either flat or other shaped working surfaces or ends. In all cases the low temperature melting metal layer forms the solderable or support side or end of the contact, and this layer is extremely thin as compared to the working contact itself.

In examples which follow, as shown by FIGS. 10 to 13, the manufacture of a two-layer body is involved, with the contact having a spherical end.

Having reference to FlG. 10, a powder mixture layer 22 of at least one high temperature metal or a high temperature melting metal alloy or a high temperature melting metal compound, or in general the refractory component, and metal of high electric conductivity and melting at a low temperature, or a low temperature melting metal alloy of high electric conductivity, is filled into the cylindrical filling space or cavity of the mold or die 15a for forming the contact layer. The upper press punch 23 has a profiled surface 24 and the lower press punch 25 has a concave mold surface.

The profile of the surface 24 should be such as to mold a plurality of uniformly distributed depressions or recesses in the compact on which it consolidates.

As shown in FIG. 11, this powder layer is compressed by the profiled upper punch 23 to form the body 26, the lower punch 25 being held. A metal powder 27 of a metal or a metal alloy with a melting temperature lower than that of the refractory material of the contact layer is filled on the compacted body 26 for forming the support layer (FIG. 12). FIG. 13 shows how both layers are compressed by means of the

punches

23 and 25 to form a two-layer compact 28.

FIG. 14 shows this two-layer compact 28 after ejection from the die cavity. After the second pressing operation, the regular depressions in the contact layer 26 remain intact and are filled in with the metal powder of the low-melting metal of the support layer 29. The compact 28 is now placed, as is shown in FIG. 15, with the support layer 29 on a profiled ceramic plate 30, which has a profiled surface with regular depressions 31. After the heat treatment at above the melting temperature of the low-melting support layer, a two-layer contact is produced as a molded part 32, as shown in FIG. 16, which in its contact layer 33 consists of a highmelting skeleton, the pores of which are predominantly impregnated with the low-melting component and which has on the supporting side of the contact block a second layer of the support metal 34. The profile 31 of the ceramic plate 30 causes the uniform distribution of the support metal, after melting and impregnation of the porous skeleton, on the bottom side of the contact block.

The following example is to illustrate the above method further.

Example 10 For the manufacture of a two-layer contact with a contact layer of tungsten-silver (WAg) and a support layer of silver (Ag) as the finished formed part, in a press mold or die cavity of steel is placed a layer of a powder mixture of tungsten powder, obtained by reduction of tungsten trioxide with a particle size of less than 45 microns, and of electrolytic silver powder with a particle size of less than 37 microns. The composition of the WAg powder mixture is chosen depending on the desired composition of the WAg contact layer. In the present example it consists of 65 percent by weight of tungsten powder and 35 percent by weight of electrolytic silver powder. This powder mixture is compressed by an upper ram with honeycomb-like profile to form a molded body at 2 Mp/cm The density of this molded body of WAg35 is about 10.0 g/cm. This corresponds to a filling factor of 0.673. Relative to the tungsten component, the filling factor is 0.337. The filling space above the pressed WAg35 body is adjusted so that the layer of electrolytic silver powder filled on top of it is sufficient for filling the pores of the contact, and forming a second layer of 0.3 mm thickness. The two layers are compressed together at 2 Mp/cm to make a molded two-layer body. After ejection, the molded body with firm edges is placed, with the silver layer down, on a profiled ceramic plate, whose surface has regular honeycomblike depressions of 2 mm X 2 mm area and 0.5 mm depth. The subsequent heat treatment takes place at l,l C. in a hydrogen atmosphere in a gravity-discharge furnace. The average dwelling time in the constant-temperature zone is about 60 minutes. Above 960 C. the silver is present in the liquid phase. During the heat treatment the impregnation of the contact layer with the liquid silver takes place, so that a practically porefree contact layer is obtained. On the silver support side, the depressions embossed into the contact layer are filled out with silver. The silver layer thickness is between 0.1 and 0.5 mm. The largest layer thickness of 0.5 mm is reached at the deepest points of the depressions. The surface of the contact layer is coated with a thin, shiny silver layer, and specifically with a layer thickness smaller than 5 um.

What is claimed is:

l. A powdered metallurgy method for making a twolayer contact or the like, comprising:

a. providing a first layer of intermixed refractory metal or metal compound powder and a low temperature melting metal powder;

b. providing a second layer of low temperature melting metal powder;

c. compacting said first and second layers together to form a coherent two-layer compact;

d. providing a refractory surface having a plurality of depressions on the surface thereof;

e. supporting said contact with said second layer on said refractory surface with said plurality of depressions therebeneath;

f. heating said compact under non-oxidizing conditions to a temperature above the melting temperature of said low temperature melting metal to impregnate the pores of said first layer with the metal of said second layer; and

g. the stepof providing said second layer including proportioning said second layer to provide said contact with a volume of said low temperature melting metal which will cause the formation on the bottom of said compact of a thin layer of low temperature metal and such that said low temperature melting metal will at least partially fill the pores in said compact.

2. The method of claim 1 in which said compacting comprises first compacting said first-named layer by pressing a plurality of depressions in the surface thereof and applying thereto said second-named layer and compacting both layers together, whereby said thin layer fills said depressions with a majority ofits volume.

3. The method of claim 1 in which said refractory metal or metal compound powder is selected from the class consisting essentially of powdered tungsten, mo-

lybdenum, rhenium, mixtures or alloys thereof, and

carbide compounds thereof, and said low temperature melting metal powder mixed therewith is selected from the class consisting essentially of powdered silver, copper and alloys thereof together and with nickel, iron and cobalt, and said metal powder of said secondnamecl layer is selected from the class consisting essentially of powdered silver, copper and alloys thereof.

4..The method of claim 2 in which said refractory metal or metal compound powder is selected from the class consisting essentially of powdered tungsten, molybdenum, rhenium, mixtures or alloys thereof, and carbide compounds thereof, and said low temperature melting metal powder mixed therewith is selected from the class consisting essentially of powdered silver, copper and alloys thereof together and with nickel, iron and cobalt, and said metal powder of said secondnamed layer is selected from the class consisting essentially of powdered silver, copper and alloys thereof.

Claims

1. A POWDERED METALLURGY METHOD FOR MAKING A TWO-LAYER CONTACT OR THE LIKE, COMPRISING: A. PROVIDING A FIRST LAYER OF INTERMIXED REFRACTORY METAL OR METAL COMPOUND POWDER AND A LOW TEMPERATURE MELTING METAL POWDER; B. PROVIDING A SECOND LAYER OF LOW TEMPERATURE MELTING METAL POWDER; C. COMPACTING SAID FIRST AND SECOND LAYERS TOGETHER TO FORM A COHERENT TWO-LAYER COMPACT; D. PROVIDING A REFRACTORY SURFACE HAVING A PLURALITY OF DEPRESSIONS ON THE SURFACE THEREOF; E. SUPPORTING SAID CONTACT WITH SAID SECOND LAYER ON SAID REFRACTORY SURFACE WITH SAID PLURALITY OF DEPRESSIONS THEREBENEATH; F. HEATING SAID COMPACT UNDER NON-OXIDIZING CONDITIONS TO A TEMPERATURE ABOVE THE MELTING TEMPERATURE OF SAID LOW TEMPERATURE MELTING METAL TO IMPREGNATE THE PORES OF SAID FIRST LAYER WITH THE METAL OF SAID SECOND LAYER; AND G. THE STEP OF PROVIDING SAID SECOND LAYER INCLUDING PROPORTIONING SAID SECOND LAYER TO PROVIDE SAID CONTACT WITH A VOLUME OF SAID LOW TEMPERATURE MELTING METAL WHICH WILL CAUSE THE FORMATION ON THE BOTTOM OF SAID COMPACT OF A TEMPERATURE MELTING METAL WILL AT LEAST PARTIALLY FILL THE THIN LAYER OF LOW TEMPERATURE METAL AND SUCH THAT SAID LOW PORES IN SAID COMPACT.

2. The method of claim 1 in which said compacting comprises first compacting said first-named layer by pressing a plurality of depressions in the surface thereof and applying thereto said second-named layer and compacting both layers together, whereby said thin layer fills said depressions with a majority of its volume.

3. The method of claim 1 in which said refractory metal or metal compound powder is selected from the class consisting essentially of powdered tungsten, molybdenum, rhenium, mixtures or alloys thereof, and carbide compounds thereof, and said low temperature melting metal powder mixed therewith is selected from the class consisting essentially of powdered silver, copper and alloys thereof together and with nickel, iron and cobalt, and said metal powder of said second-named layer is selected from the class consisting essentially of powdered silver, copper and alloys thereof.

4. The method of claim 2 in which said refractory metal or metal compound powder is selected from the class consisting essentially of powdered tungsten, molybdenum, rhenium, mixtures or alloys thereof, and carbide compounds thereof, and said low temperature melting metal powder mixed therewith is selected from the class consisting essentially of powdered silver, copper and alloys thereof together and with nickel, iron and cobalt, and said metal powder of said second-named layer is selected from the class consisting essentially of powdered silver, copper and alloys thereof.