CA2008371A1 - Method and device for producing dispersion-hardened shaped metal parts - Google Patents

Abstract

ABSTRACT

Method and Device for Producing Dispersion-hardened Shaped Metal Parts Method and device for producing dispersion-hardened shaped metal, e.g. copper parts, wherein a copper melt is produced that is superheated by a maximum 300°C, which melt is super-saturated with boron and one or several boride-forming elements of the groups IVA, VA and VIA in the periodic system and/or aluminium in such a way that a homogeneous, largely stable boride dispersion forms abruptly in the supersaturated melt, and wherein a given quantity of this boride-containing copper melt is either directly prepared in the female die or metered into it and, immediately after having been prepared or metered in, shaped by means of a male die, preferably applying compacting pressure, and caused to solidify before the finished or near-net-shape copper part is removed. The method is efficient and flexible, and it is possible to produce high-quality parts in simple devices that are easy to automate, making optimum use of the material.

Description

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~escription The present invention relates to a method of producing dispersion-hardened shaped copper parts and a device Eor practising the method.

Dispersion-hardened materials based on copper are of great technical interest or applications requiring the combined properties of great high-temperature strength and high electric or heat conduction as, for example, in the areas of electrical - engineering, welding engineering and automotive engine technology. Yet, these materials so far have not gained great significance in practice. The main reason for this is that the powder-metallurgical manufacturing processes that are usually applied to these materials are not efficient enough. For example, producing the oxidic dispersion-hardened material Cu-A1203 by means of the internal oxidation method results in a semi-finished product which, because of its low du~tility, can be processed into shaped parts only by machining. This procedure, however, eliminates the real advantage of powder-metallurgical production, namely near-net-shape manufacture involving no signifiicant losses of material.

More efficient manufacturing methods such as shape castings so far are not known to have been applied to dispersion-hardened i copper materials. This is to be attributed to the fact that it is not possible to suspend dispersoids like A1203 and BeO
uniformly in a copper melt, without gravitational segregation.
Attempts at limiting gravity-induced segregation in the copper melt by ultrasonic means have also been unsuccessful so far.

The present invention is based on the problem to provide an efficient method of producing dispersion-hardened shaped copper parts from a melt as well as a device for practising the method.
Although the invention will be described on the basis of copper, it is also possible to use silver or gold in place of copper.

The invention relates to a method of producing dispersion-hardened shaped metal parts by the melting route. It includes -2- z~83'7~
~ the steps of: producing a melt of a soft metal selected from - copper, silver or gold that is superheated by a maximum of 300C, which melt is supersaturated with boron and at least one boron-forming element of the groups IV~, VA and VIA in the periodic system and/or aluminium in such a way that a homogeneous, largely stable boride dispersion forms abruptly in the superheated melt, and preparing a given quant;ty of this boride-containing melt either directly in a female die or metering it into it, and shaping this melt immediately after it has been prepared or metered in by means of a male die.

The method according to the invention permits shaped parts or near-net-shaped parts to be produced efficiently, and thus with low material losses, by way of melting. Also, the parts produced have good material properties, because the problem of gravitational segregation can be overcome satisEactorily.

The inventors started from the assumption that uniform distribu-tion o~ fine-grained dispersoids in a molten metal matrix is realised most easily if the disperse phase can be produced in situ, via precipitation reactions in the melt. ~Iowever, this requires that the formation of the dispersoid nuclei occurs homogeneously and that the critical radius of the nuclei remains small. It was found that this is true for the supersaturated material sytems mentioned in Claim l, investigations on melts of Cu-Al-B, Cu-Ti-B~ Cu-Zr-B, Cu-H-B, Cu-V-B, Cu-Nb-B and Cu-Cr-B
having demonstrated that said requirements are satisfied to a particularly great extent by the systems Cu-AlB2, Cu-TiB2, Cu-ZrB2, Cu-HfB2, Cu-VB2, Cu-NbB2 and Cu-CrB2. As a result of their very small solubility product in molten copper and their very high melting temperature, the intermediate compounds AlB2, TiB2, ZrB2, HfB~, VB2, NbB2 and CrB2 always separate out from a copper melt under homogeneous nucleation and with very small critical radii of the nuclei, in which process a large variety of dispersoid shapes, e.g., ; rod-shaped, filamentous or cornered dispersoids may develop.

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The Eormation of a stable dispersoid dispersion could be observed in a wide saturation range of said additive elements. ~ue to the low melting temperature and a relatively high degree of supersaturation chosen, gravity-induced segregation can be delayed for quite some time. Due to the small difference in density between the borides and the copper (in the case of Hfs2 there is no significant diEference at all), the tendency to segregate by gravity is already weak in the elements, such as aluminium, titanium, ~irconium, hafnium, vanadium, niobium and chromium, because the borides are more or less floating in the melt.

Since the dispersoids do not agglomerate and hardly coarsen in the melt, the most obvious course to take is in principle to die-cast the melts into shaped parts. However, because of the poor flow behaviour of the melts, which is connec~ed with the high viscosity resulting from major supersaturation and minor overheating, this cannot be done by one of the usual casting methods.

~ccording to the invention~ this problem is solved by shaping the melts between a male and a female die, a good die filling level being achieved. The necessary device is simple to design, and the melt can be solidified at the same time as being shaped by the male die. The solidified shaped parts are at least of near-net shape and can be reworked at will or used as primary material for semi-finished products.

The melt processing method according to the invention can be easily controlled and readily automated. For instance a system may be used in which several die blocks, each including a female die, are arranged in a carrousel and at least one stationary metering device to fill metered portions of melt into the die blocks is arranged above the carrousel, and at least one stationary ejector for ejecting the solidified shaped parts from the die blocks is arranged underneath the carrousel at positions to which the empty die blocks or the die blocks filled with metered portions of melt are moved successively by turning the 3~7~L

carrousel, the male die or dies Eor shaping the melt portions just filled being preferably arranged above the die blocks, in positions corresponding to those of the ejector or ejectors.

When summarising the advantages of the melt composition used with those afEorded by processing the melt into shaped parts, the following benefits result: high productivity, high structural ~uality, accuracy of shape and surface finish, combined with optimum utilisation of the material and flexible manufacture.

Shaped copper parts of high quality have been achieved when using materials such as hot work steels and molybdenum-based and tungsten-based materials or hard metals for the female and male dies.

Particularly fine-grained soldification of the copper matrix is ensured when applying a pressure between 5 bar and 70 bar.
minor stoichiometric excess of bor~de-forming elements has an additional precipitation hardening eEEect.

In many applications it is advisable to meter the melt into the die or provide the melting charge in form of a compact of adequate composition. This permits especially small-sized shaped parts to be produced, and even doing without an additional melting device, as melting, shaping and solidification can be performed in a single die block.
:' According to the invention it is possible to produce parts of extremely different shapes in an efficient way, and it is also possible to use gold or silver instead of copper. Among the possible parts there may be mentioned: caps for spot welding electrodes, valve stem guides and valve seat rings for combustion engines, electrical contacts, shut-off elements and structural - components for chemical apparatus, elements for rocket and jet engines, continuous casting dies, primary material -to make pipes, wires and profiles, transmission elements such as synchronising disks, and blank screws.

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rhe invention is exemplified by the following figures.

- Fig. 1 shows a first embodiment to illustrate ~he device according to the invention and the method according to the invention with its individual process steps.
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Fig. 2 shows an advantageous development of the device described by Fig. 1, which serves to automate the method according to the invention.

Fig. 3 shows three embodiments of a charging device shown in Fig.
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Fig. 4 shows a second embodiment of the device according to the invention, or the method according to the invention, in which the melt is produced in the female die, and Fig. 5A shows another embodiment of the device accordinq to the invention, desiyned for producing pipes, and Fig. 5B a pipe section produced in accordance with Fig~ 5A.

In all embodiments of the method according to the invention and the device for practising it which are explained in the following on the basis of Fi~s. 1 to 5, copper melts are used as melts, which are supersaturated by boron or boride-forming elements of groups IVA, VA and VIA in the periodic system and/or aluminium. One or sev~ral systems of the compositions Cu-AlB2, Cu-TiB2, Cu-ZrB2, Cu-HfB2, Cu-VB2, Cu-NbB2 or Cu-CrB2, for example, are used as melt systems. The supersaturation mixture of the melts, i.e. of the charges of boron, boride ~orming elements or aluminium, is prepared in such a way that a homogeneous boride dispersion forms abruptly in the melt that is overheated by a maximum of 300 ~C.

A stable, homogeneous boride dispersion with various kinds of dispersoid including rod-shaped, filamentous or cornered dispersoids that develop under homogeneous formation of nuclei and with very small critical radii of the nuclei, can form within wide limits of supersaturation.For example, supersaturation with stoichiometric additions of the kind mentioned above can be chosen in such a way that between 1 vol.% and 35 vol.% boride dispersion ;' . .

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Also, to further increase the strength of the shaped parts, the reaction is preferably carried out with a minor excess of boride-forming elements over the stoichiometric composition that is necessary for boride formation, which excess may amount up to 1.5 wt.%.

The boride-forming elements mentioned above and/or the aluminium are preferably added in such a way that between 2 vol.% and 14 vol.% of borides form in the melt.

Furthermore, the copper melts with said boride forming elements are preferably melted at 50 C to 150 C overheating of the melt.

Besides copper melts, melts of the metals silver and gold can also be used, the gold and silver in these melts showing a surpr:isingly similar behaviour towards borides, in particular borides of titanium and zirkonium, as copper.

In the embodiment of the invention shown in Fig. 1, the melting process to produce the melts described above is represented schematically in Fig. la. The material to be melted is contained in a crucible that is surrounded by an induction heating coil and equipped with a sleeve brick 2, the lower discharge opening of which is closed by means of a stopper rod 1. During the melting process in the temperature range mentioned above, the boron reacts with the boride-forming elements in the melt, which is supersaturated with boron and said elements to give intermediate compounds and the desired abrupt formation of dispersoids takes place in the melt.

In the casting process step indicated in Fi~. lb, the stopper rod ` 1 is lifted and until it is lowered into the discharge opening, a metered quantity of the melt flows into a casting die that serves as female die 3 and is arr~nged underneath the sleeve brick. The female die has in its floor an ejector 6 which is designed as a pushing rod in the embodiment and can be moved in vertical direction, and which closes the floor of the female die 3 during casting. The female die is designed as a hollow cylinder in the .~, ,.

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embodiment. Depending on the shaped part to be produced, however, a great variety of hollow forms are conceivable.

In the melt pressing step outlined in Fig. lc, a male die 4 is lowered from above into the female die 3 filled with the melt, a pressure between about 5 bar and 70 bar being applied for this, so that the dispersoid-containing melt is formed into a shaped part 5 by compacting pressure. The shaped part 5 :is caused to solidify by one of the measures that are usual in melt processing, e.g., by enhancing the intrinsically rapid solidification process by water cooling, and ejected from the female die 3 in the working step sketched in Fig. ld, by pushing the ejector 6 upwards after the male die 4 has been lifted.

Hot work steels and preferably powder metallurgically produced materials based on molybdenum or tungsten or materials from carbide hard metals should be used as materials for the male and female dies.

To automate the production process sketched in Fig. 1, the forms or female dies can be accommodated in a carrousel which holds them in order to fill them step by step with melt under a preferably stationary metering device and subsequently compact the melt into shaped parts under an array of male dies that is also preferably stationary. Fig. 2 is a schematic representation of a corresponding carrousel array, which comprises a stationary metering device 8 arranged above the carrousel and is fixed on a revolving axis 9 so that it is turnable. The carrousel accommodates die blocks 10 into which an exactly metered quantity of melt 11 is filled by means of the metering device 8 after an empty die block previously has been approached by turning the carrousel and is located underneath the metering device 8. The latter can correspond, for example, to the device with sleeve brick and stopper rod shown in Fig~ 1.
, As soon as the exactly metered quantity of melt has been filled in, the filled die block 10 is passed underneath the likewise - stationary array of male dies 4, by an appropriate turn of the : carrousel. The male die is then pressed into the melt 11 as shown in Fig. lc, the melt rising up in the cavity between female and male die, thus forming a rapidly solidifying hollow body which, :, .. .

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after it has solidified and the male die ~ has been removed, is taken out as shaped part 5. An ejector 6 and an ejector cam 7 underneath it serve as stationary ejecting device arranged underneath the carrousel and the arra~ of male dies.

After the shaped part has been ejected, the now empty die block 10 is returned underneath the metering device and the production cycle starts anew. When several die blocks are accommodated in the carrousel it is thus possibly to perform two process steps at the same time. Besides, it is feasible to provide several metering devices, male die arrays and ejector systems, all of them stationary and arranged at speci~ic angular distances from the carrousel.

Suitable metering devices are the devices 8a, 8b, 8c shown in Fig.
3~ to 3c, for example. The simplest solution with a stopper rod 1 that is passed through a melting crucible 2' and closes the discharge opening of this crucible during the melting process is sketched in Fig. 3a. An induction coil 12 surrounding the crucible generates the necessary temperature to melt the material to be melted in the crucible.

What has proven more favourable, in particular for manufacturing shaped parts of small dimensions, is to melt exactly definable portions of material, as outlined in Figs. 3b and 3c. To prepare the portions of material to be melted, powders are prepared, for example by separate atomisation of copper-titanium melts or copper-zirconium melts and copper-boron melts, which powders are subsequently mixed and compressed into compacts of the required weight. According to Fig. 3b this kind of powder compact 13 is melted in a crucible 2' surrounded by an induction coil 12 as shown in Fig. 3a, the boron fractions reacting with the fractions of titanium and/or zirconium to give the desired quantities of borides. When the melting process is complete, the melt with the homogeneous boride dispersion flows through the opening of the crucible into a die block 10 (Fig. 2) arranged underneath.

In the case of leviation melting sketched in Fig. 3c, using a leviation melting coil ~2' around the floating melt 13' that results from the compact, the floating melt with the corresponding ., ., , 2~3~7~
~omoyeneous boride dispersions flows into the die block when the current in the coil has been switched off.

Fig. 4 shows a second embodiment of the invention, in which the melt is directly prepared in the die block 10 or female die. This permits fast production of shaped parts by passing the current through directly. As indicated in Fig. 4a, the die block 10 made of a low-electric-conductivity material, is fed with a powder compact 13, which can be prepared as described above.
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An ejector 6, which is arranged in the floor of the die block 10 so as to be movable, at the same time serves as positive current contact, and a current contact 1~ fitted at the protruding end of the male die 4 is negatively poled. As a result of direct current flow through the contacts 6 and 14 and the male die 4, the powder compact 13' is melted and, when the current is switched off and pressure is applied to the male die 4 that is pressed into the melt, processed into the rapidly solidifying shaped part ~ which can be removed by means of the ejector 6. In this connection it is important that the male die 4 is made of a material such as molybdenum or tungsten, with good heat conductivity. To improve heat dissipation from the solidifying melt even further, water cooling should be provided.

Fig. 5 is a scAematic representation of another embodiment in form of a melt pressing device specially designed for making pipes. A
die block 10 contains a cylindrical recess as receptacle for the melt 11. Through the floor of the die block and up to the lower end of the recess a movable ejector pin 6' extends, which can be pretensioned at the end where it protrudes from the die block by an indicated spring which seals the recess for the melt in the die block 10 from below. The male die comprises the round rod 4a to be pressed into the melt 11, which determines the internal diameter of the piece of pipe to be made, and an upper die part 4b. The latter has a cavity in which a spiral spring is accommodated and a guide hole for the round rod 4a underneath the spring. When the male die is lowered into the melt, the lower end of the round rod 4a impacts on the ejector pin 5', which has about the same diameter as the round rod 4a, and presses the pin 6' slightly downward, against the action of its spring. The spring in the upper die part ';
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4b is also compressed by the round rod ~a, and the underside of the male die part 4b, which has the same diameter as the recess in the die block, comes to lie on the surface of the ascending melt, which is thus enclosed between the male die parts 4a and 4b and the recess in the die block When the melt has solidified the male die is moved upwards, while the ejector pin 6' that is pretensioned by its compressed spring, moves up and ejects the finished piece of pipe shown in Fig. 15. The device of Fig~ 5 is suitable for automated production as explained by ~ig. 2 and produces perfect pipe sections rapidly and in a simple way.
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In addition to the described devices for melting and shaping, any ; other kinds of embodiment are feasible, provide they permit metering of the melt or material to be melted as well as pressure-induced shaping.

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Claims (13)

1. Method of producing dispersion-hardened shaped metal parts by the melting route, comprising:
producing a melt of a soft metal selected from copper, silver or gold that is superheated by a maximum of 300°C, which melt is supersaturated with boron and at least one boride-forming element of the groups IVA, VA and VIA in the periodic system and/or aluminium in such a way that a homogeneous, largely stable boride dispersion forms abruptly in the superheated melt, and preparing a given quantity of this boride-containing melt either directly in a female die or metering it into it, and shaping this melt immediately after it has been prepared or metered in by means of a male die.

2. Method as claimed in Claim 1, wherein the metal melt is supersaturated with stoichiometric additions of boron and the boride forming elements so as to form more than 1 vol.% and up to 35 vol.% of boride dispersion in the metal matrix.

3. Method as claimed in Claim 1, wherein the boride-forming elements are added at an excess of up to 1.5 wt% over the stoichiometric composition of the borides forming in the melt.

4. Method as claimed in any of Claims 1 to 3, wherein the boride-forming elements are selected from aluminium, titanium, zirconium, hafnium, vanadium, niobium and chromium.

5. Method as claimed in Claim 1, 2 or 3, wherein the copper melt is superheated by 50°C up to 150°C.

6. Method as claimed in Claim 1, 2 or 3, wherein boron and at least one of the elements aluminium, titanium, zirconium, hafnium, vanadium, niobium and chromium are added to the melt to form from 2 vol.% to 17 vol.% boride dispersion in the melt.

7. Method as claimed in Claim 1, 2 or 3, wherein a compacting pressure between 5 bar and up to 70 bar is applied when shaping the dispersoid-containing melt.

8. Method as claimed in Claim 1, wherein in order to ensure the metered supply of given quantities of the boride containing melt, powder compacts of the described composition and corresponding mass are melted either in a crucible with hole above the female die or inside a levitation melting coil, from which the melt falls down into the female die when the coil current is switched off.

9. Method as claimed in Claim 1, wherein the female die is fed with a weighed powder compact of the described composition, which is melted by current passing directly through it and subsequently shaped by the pressure of the male die.

10. Method as claimed in Claim 8 or 9, wherein the powder compact is prepared by mixing atomised copper-zirconium and/or copper-titanium alloys with copper-boron alloys at the ratio that is necessary for supersaturation and compressing the mixed powders.

11. Method as claimed in Claim 1, 2 or 3, wherein the following finished parts or at least appropriate near-net-shaped parts are produced by the method according to the invention:
caps for spot welding electrodes, valve stem guides and valve seat rings for combustion engines, electrical contacts, shut-off elements and structural components for chemical apparatus, elements for rocket and jet engines, continuous casting dies, primary material to make pipes, wires and profiles, transmission elements such as synchronising disks, and blank screws.

12. A device for producing dispersion-hardened shaped metal parts by the melting route which comprises a female die adapted to hold a boride-containing melt of copper, silver or gold and a male die adapted to shape the melt in the female die, the female die and the male die being made of hot work steel or powder metallurgically produced materials on the basis of molybdenum or tungsten or carbide hard metal.

13. Device as claimed in Claim 12, wherein several die blocks, each including a female die, are arranged in a carrousel and at least one stationary metering device to fill metered portions of melt into the die blocks is arranged above the carrousel, and at least one stationary ejector for ejecting the solidified shaped parts from the die blocks is arranged underneath the carrousel at positions to which the empty die blocks or the die blocks filled with metered portions of melt are moved successively by turning the carrousel, the male die or dies for shaping the melt portions just filled being preferaby arranged above the die blocks, in positions corresponding to those of the ejector or ejectors.