DE2436927C2 - Method and device for the powder-metallurgical production of molded parts for electrical contact pieces - Google Patents

Description

The invention relates to a method for powder-metallurgical manufacturing in molded parts a residual porosity of less than 1% and with a uniform pore distribution made of dispersion-hardened Silver-metal oxide (AgMeO) composite powder for electrical contact pieces.

When compacting contact pieces with a predominant silver content and active additives by re-pressing in the sintered state, on the one hand the desired final shape should be obtained and on the other hand the pores should be in? Sintered bodies are largely removed. It is essential that no gases are trapped in the contact material. Whether the last requirement is met, showing a subsequent heat treatment of the contacts, for example, at 1073 ⁰ K. In the case of gas pockets occurs swelling of the contacts increase in volume on.

Dispersion-hardened contact materials with the base metal silver and in it fine and even Distributed metal oxide precipitations have a significantly higher deformation work compared to pure silver. In the powder metallurgical production (pressing, sintering, re-pressing) of contact pieces Dispersion-hardened silver-metal oxide composite powders (AgMeO) are obtained with the usual technology via the steps of pressing, sintering and re-pressing contact pieces that have a residual porosity of 2 to 4%. It was found that the burning behavior such contact materials deteriorate very quickly with a slightly increasing porosity, whereby the service life of the contact pieces in a switching device is reduced.

In the case of difficult-to-form materials and complex molded parts, there are manufacturing conditions the difficulty of uniformly increasing the residual porosity at all 'points of the contact pieces to eliminate a limit value.

It is the object of the invention to develop a method and a device with which it is possible as economical as possible contact pieces as sintered molded parts made of dispersion-hardened silver-metal oxide composite powder to produce with a residual porosity of less than 1% and thus a high erosion resistance of the

ίο to reach contact pieces in the arc.

In order to achieve a higher compaction, the sintering and repressing step is generally repeated applied This technique is very complex. Even if the steps are sintering and repressing

is used twice, a higher one will be applied Compaction achieved, but the residual porosity still remains between 1 and 2%. This has residual porosity however, it still results in too high a burn-up value.

> o Even if the cold re-pressing is carried out twice The aim is to replace the hot re-pressing, which is more complex in terms of production technology, namely one Residual porosity less than 1%, not yet achievable. Although easily plastically uniform metals, such as B. pure silver, by a hot press step high density, yet are the silver-metal oxide systems due to the dispersion hardening, it is difficult to form even at higher temperatures.

The above task is supported by the

jo invention achieved in that the composite powder is first pressed into a preform that the Preform is hot-pressed to a 5 to 15 area percent larger intermediate form and that the intermediate form for 2 to 8 area percent larger

j5 final shape is re-pressed cold.

Contrary to the general opinion that a cold repressing step following the hot pressing step no decisive further compression can be achieved, surprisingly showed that the required small residual porosity of less than 1% of those produced by powder metallurgy Molded parts made from dispersion-hardened silver-metal oxide composite powders just through a cold re-pressing step can be reached.

An advantageous development of the method according to the invention can consist in that the Preform is sintered before hot pressing. Furthermore, it can be advantageous that the intermediate shape before cold re-pressing in an inert atmosphere

ίο is sintered.

In order to be able to carry out the individual process steps economically, a further invention is one Device provided, which consists of a press with a Warmpreß- and a Kaltnachpreß-position, one Heating position before the hot pressing position, from cutting tools to remove the burrs on the End shape and with automatically controlled slides for feeding the moldings into the respective processing position is provided.

M) The shape of the hot pressing tool must be coordinated with the shape of the cold pressing tool (final shape). Both pressing steps take place in the same cycle on the same device. Because of the possible short cycle times is the method according to

bι invention to carry out very economically.

In the case of sintering in an inert atmosphere, e.g. B. in a nitrogen atmosphere, the feed of the moldings takes place for cold re-pressing via a separate

Feeding device.

The invention is explained in more detail with the aid of the drawing and an exemplary embodiment. Show it

F i g. 1 to F i g. 3 the production of a contact piece according to the method according to the invention,

F i g. 4 the specific distance between contact pieces made of AgCdO 12 as a function of the porosity or on the density.

In Fig. 1 shows the preform 11 of a contact piece made of AgCdO 12 for low-voltage switching devices with a silver layer 13 on the soldering side in the pressed state (cross section). The size of the contact piece is 16-15 x 2.5 mm ³ . The maximum height of the preform is denoted by Λΐ. The angle of the inclined surfaces of the pressed preform is denoted by cn. The preform 11 is produced in a known manner by inserting the AgCdOl 2 composite powder with a very fine and uniform CdO distribution in silver in a die made of steel or hard metal, not shown in the figure, and overlying the silver layer as a second filling layer of electrolysis silver powder with a particle size of <0 .06 mm is compressed with a pressure of 4500 bar. Before it is filled, the electrolysis silver powder is granulated into a flowable form. The porosity of the compact is about 12%. Because of the inclined surfaces of the pressing body 11, a subdivided lower punch is used during pressing in order to obtain a more uniform density in the various parts of the pressing body compared to a pressing body compacted with an undivided lower punch.

The pressed body 11 (preform) is then on 1073 K and fed to a hot pressing tool by a pneumatically controlled feed device and compressed with 6000 bar. Pass from feeding to reaching the maximum pressure about 1.5 seconds. The printing time is between 0.5 and 1 second.

The intermediate form 14 of the preform 11, which has been re-pressed once, is shown in FIG. The height Λ _{2 of} the intermediate form 14 and the angle α? of the inclined surfaces are smaller than the height h \ and the angle ct \ of the preform 11. The AgCdO 12 layer is again designated by 12 and the silver layer by 13. the

> Intermediate form 14 is larger than the preform 11 in terms of its area by 5 to 15 area percent. The porosity of the intermediate form is approximately 1.5 to 3%. During the subsequent cold re-pressing, the degree of deformation is 2 to 8 area percent. Accordingly, the shape of the hot pressing tool must be coordinated with the shape of the cold pressing tool, which determines the final shape.

In Figure 3, the end shape 15 of the contact piece is shown. The height in the end shape 15 is smaller than

!> the height Λ2 of the intermediate shape 14. The angle ai of the inclined surfaces is also smaller than the angle 1x7. The porosity of the final shape 15, calculated from the density determination after the buoyancy weighing, is now less than 0.6%. In a cross section in different

- "Zones could practically no more pores be detected. The erosion value in the L> iitbogen with an appearance sirom of 1000 amperes was 40% lower for the contact pieces manufactured according to the invention compared to contact pieces that were cold or cold only once according to the usual manufacturing technique The welding force distribution curve was also lower in the case of the contact pieces produced according to the invention, ie more favorable than the distribution curve with only one hot post-compression.

1 »sealed contact pieces.

In Figure 4 is in a graphical representation Dependence of the specific burn-off of contact pieces made of AgCdOI2 as a function of the Porosity or shown by the density. On the

^Γ) abscissa are plotted the porosity fin% and the density ρ in g / cm ³ and on the ordinate the burn-A in 10- ⁶ cm ³ / As. From the course of the burn curve it can be seen that the porosity of the Koi.takt back is of considerable importance

^{4 (1} to keep as far below 1% as possible.

For this purpose 2 sheets of drawings

Claims (4)

1 claims; 1. Process for the powder-metallurgical production of molded parts with a residual porosity of less than 1% and with a uniform pore distribution made of dispersion-hardened silver-metal oxide (AgMeO) composite powder for electrical contact pieces, characterized in that that the composite powder is first pressed into a preform, that the preform is pressed into a 5 to 15 Area percent larger intermediate form is hot-pressed and that the intermediate form for 2 to 8 Area percent larger final shape is cold re-pressed. 2. The method according to claim 1, characterized in that that the preform is sintered prior to hot pressing. · 3. The method according to claim 1 or 2, characterized in that the intermediate form before Cold pressing is sintered in an inert atmosphere. 4. Device for performing the method according to one or more of claims 1 to 3, characterized by a press with a hot press and a cold press position, with a Heating position in front of the hot pressing station, with cutting tools to remove the burrs on the Final shape and with automatically controlled slides for feeding the moldings into the respective Machining position.