WO2008003275A1

WO2008003275A1 - Method for producing molded parts from dispersion-strengthened metal alloys

Info

Publication number: WO2008003275A1
Application number: PCT/DE2007/000949
Authority: WO
Inventors: Markus Hofmockel; Mark Hartmann; Claudia Stadelmann; Markus BÖHM
Original assignee: Ecka Granulate Velden Gmbh
Priority date: 2006-07-06
Filing date: 2007-05-25
Publication date: 2008-01-10
Also published as: DE102006031366B3; DE102006031366C5

Abstract

The invention relates to a method for producing molded parts (rods) from dispersion-strengthened metal alloys, comprising a continuous powder extrusion process of the metal alloy powder at a low degree of deformation to produce molded parts having a grain structure arched in the longitudinal direction, and to the use of the molded parts for producing electrodes and/or electrode caps.

Description

Process for the production of molded parts from dispersion-strengthened

metal alloys

The invention relates to processes for the production of moldings from dispersion-strengthened metal alloys; moldings produced therefrom and their use.

Dispersion-strengthened metal alloys have a wide field of application: u.a. Silver, copper, iron, nickel and aluminum etc. or their alloys. For example, the temperature resistance of Ni-base alloys for use in turbine blades is further improved by dispersion hardening.

The invention will be described below with reference to Cu materials - but the teaching can also be applied to other metals, so that it is by no means limited to copper and its alloys.

In many applications in the electrical, manufacturing and transport industries, there is a great need for molded parts made of Cu materials. Two important properties of copper are its good electrical and thermal conductivity. But its relatively low strength, low hardness and low creep resistance, even at room temperature, limit the applications of pure copper. For many applications, however, high thermal conductivity and high strength are required at the same time, often at elevated temperatures.

Dispersion strengthened copper (DS) extends the range of application of copper. Its properties make it an important construction material for electrical and electronic products / applications, such as resistance welding electrodes, holders for filaments or switch and sliding contacts. The high thermal conductivity and good creep resistance allow the use of dispersion strengthened copper in structural applications in nuclear reactor construction. Oxide dispersion strengthened copper (ODS, oxides dispersion strengthened) can be obtained, for example, as described in DE 19752805 A1. The production of an ODS copper material is also known from US Pat. No. 3,779,714 A1. Electrodes made of ODS copper alloys are disclosed in DE 24 18 686 A1 described. The dispersion strengthened copper is a powder metallurgical material that can not be compacted by pressing and sintering. Instead, densification by hot extrusion is known. By die pressing or cold isostatic pressing the metal powder is precompressed into blocks and then hot extruded. Due to the massive deformation occurs in the forming zone for Reibverschweißen the powder particles against each other and the material is thus completely compressed. A variant is the use of so-called. Cans, where the blocks consist of a kind of can, in which the loose powder bed is filled and sealed.

In extrusion molding, the former powder grains form elongated grains according to the extrusion ratio (fibrous structure) (see Fig. 3). This inevitably creates a aligned in the direction of the rod axis texture. This is due to the relatively brittle material, kerbanfällig and reacts brittle to loads across the fiber orientation. Healing of this grain structure by recrystallization is prevented by the dispersoids.

In order to ensure the formation of a sufficiently large weld nugget in resistance welding, the current density must not fall below a critical value for given contact resistances. By deformation of the electrode cap (= enlargement of the contact surface), the current density decreases continuously. In automated resistance spot welding, the electrode pair is therefore automatically milled after a certain number of welding points in order to restore the original shape of the electrode tip and thus to increase the cycle of use of a pair of electrodes (avoiding expensive downtime for electrode replacement). This so-called "cap milling" is state of the art. Due to the high temperature stability of ODS-copper (oxide dispersion strengthened), the deformation of the electrodes is lower and the milling cycles and thus the lifetime of the electrodes can be significantly increased. Typical such rods of ODS copper are described in US 5030275. Further improvements can be achieved if the fibrous microstructure of ODS copper can be replaced with a more suitable microstructure. The fibrous structure of conventionally extruded ODS copper combined with the very fine-grained microstructure results in that at the working surface of an electrode cap (front tip of the electrode) very many grain boundaries come out. This offers many points of attack for the in-diffusion of zinc alloyed on the electrode tip. At the same time, the resistance spot welding of galvanized sheets leads to the zinc alloying at the tip of the electrode. By diffusion of zinc atoms from the electrode tip inward along the grain boundaries, the undesirable effect of so-called "mushrooming" is promoted and enhanced.

For electrode caps made of conventionally extruded ODS copper rods with a fibrous structure, a rough surface of the electrode tip is observed after cap milling. This, although basically ODS copper is easy to machine because the dispersoids act as chip breakers. However, cap milling is a special case. The low rotational speeds customary here and the sometimes unfavorable cutting angles in combination with high contact pressures of the tools used lead to chattering of the cutting tools in certain geometries of the electrode cap, as a result to material eruptions and thus to unusable electrode surfaces.

The manufacture of components such as e.g. Unfortunately, electrode caps for resistance spot welding using classical P / M (press-sintering-calibrate) technology, which would avoid such a fiber structure, are likely to be eliminated since in sintered copper the sintering activity is severely inhibited.

It is therefore an object of the invention to avoid the disadvantages of the prior art.

The object is achieved by a method having the features of claim 1. This is for the production of moldings with a longitudinally arcuate Komstruktur of dispersion-strengthened metal alloys by continuous powder extrusion of the metal alloy powder with low degree of deformation Further, it relates to the use of such moldings according to claim 8. Advantageous developments emerge from the dependent claims. Due to the fact that according to the invention an arcuate grain structure is present within the components, the service life of electrode caps for resistance spot welding has been considerably extended. It is now offered a little expensive solution for the production of rod-like molded parts made of metal powder. When loaded in the direction of the rod longitudinal shaped parts increases the life of the same.

Preferably, the metal is selected from the group consisting of Ni, Cu, Al, Ag, Fe.

In one embodiment, the metal is an ODS copper alloy, and there is a method of producing moldings having longitudinally arcuate fiber distribution of dispersion strengthened metal alloys by continuously powder extruding the low alloy metal alloy powder at an extrusion ratio of 1.5: 1 to about 4: 1 carried out. Particularly suitable as continuous powder extrusion is a friction wheel method.

It is preferred that the low degree of powder extrusion is a continuous powder extrusion which enables the production of molded articles (rods) in a rapid and cost effective manner.

Extrusion with a low degree of deformation is generally also feasible with conventional extrusion processes, but increases the effort dramatically and therefore does not make sense.

By applying a continuous process, however, this disadvantage is compensated by the time-consuming forming ratio.

Surprisingly, it has been found in the embodiment with ODS copper powder that ODS copper powder can be fully compact even with low degree of deformation during extrusion. Since ODS copper powder is considered relatively hard to deform, it could not be expected that with the low Forming degrees of 1, 5: 1 to 4: 1 (= little internal friction) the compaction by friction welding of the powder particles against each other effectively runs.

In contrast to conventional extrusion, due to the low degree of deformation (NUG), the powder particles do not form into longitudinally stretched but into arcuately arranged fibers (FIGS. 2 and 3). This is accompanied by a special, anisotropic microstructure (see FIGS. 1 and 2).

Very particular preference is therefore the continuous powder extrusion with low degree of deformation - a method with friction wheel, which is easy to install and maintain due to the simple geometry of his system and also arranges the fiber structure in the desired manner by the low extrusion ratio.

The molded parts (rods) made from continuously powder-extruded metal powder of high compressive strength produced according to the invention have a grain structure which is curved in the longitudinal direction, since a low degree of deformation was set during powder extrusion ("NUG structure"). This avoids exposing many grain boundaries when cutting perpendicular to the rod axis, and the resulting surface is less reactive and more diffusive than that of high-strain rods that exhibit a parallel-fiber structure.

Preferably, the molded article is made of an alloy selected from the group consisting of: Cu, Al, Ag, Ni ₁ Fe alloys and DS-reinforced alloys.

Suitable alloys are, for example:

Alloy Density Makes El. Leitfähigkt. Hardness EIc »nga g / cm3 ⁰ C Sm / mm2 HV30%

Cu-Al-B-C-O 8.65 1083 47 144-155 15-20

(C3 / 60)

Cu-Al-B-C-O 8,55 1080 44 150 - 165 8 - 12

(C3 / 80) A preferred molded part consists of continuous powder-extruded ODS copper powder of high compressive strength with a longitudinally arcuate in the molding grain structure, wherein the powder extrusion an extrusion ratio of 1, 5: 1 to about 4: 1 was used with low degree of deformation. These Cu-shaped parts are particularly suitable for the production of electrodes, in particular welding electrodes and especially as an electrode cap for resistance spot welding. A typical use of such electrodes is the resistance spot welding of sheets, in particular galvanized sheets.

In accordance with the invention molded parts made of continuously powder-extruded ODS copper powder have an arcuate grain structure in the pressing direction (see Fig. 1 and Fig. 2) wherein the powder extrusion an extrusion ratio of 1, 5: 1 to about 4: 1 was used with low degree of deformation.

The material made of ODS copper powder with this special grain structure is particularly suitable for the production of electrodes, in particular as electrode caps for resistance spot welding. A common application of these electrode caps is the resistance spot welding of sheets, in particular galvanized sheets.

Materials with anisotropic structure, such as this "NUG structure" are usually not suitable for technical applications. For longitudinally loaded moldings (bars) or moldings whose final shape corresponds approximately to the arcuate structure, such as e.g. However, electrode tips for resistance spot welding offer such arcuate grit bars significant, unexpected benefits:

a) According to the invention Cu electrode caps for resistance spot welding are produced economically in large numbers by continuous powder extrusion with subsequent cold extrusion. The sheet-like orientation of the "NUG structure" (FIG. 1), which is present in the material, already anticipates the later shape of the electrode cap to be produced by the cold forming. In comparison to fibrous texture of conventionally extruded material (Fig. 3) thereby reduces the tendency for cracking (along the grain boundaries) during the extrusion process.

b) by the arcuate grain structure (Figure 1 + 2), in which substantially no grain boundaries on the work surface to the outside, the diffusion-inhibiting effect of DS materials (the dispersoids act as diffusion inhibitors) is enhanced as diffusion through the volume slower runs as along grain boundaries. By diffusion of zinc atoms from the electrode tip inward along the grain boundaries, the undesirable effect of so-called "mushrooming" is promoted and enhanced. The function of the electrode cap limiting effect of mushrooming / mushrooming is thus effectively delayed by the particular structure of the invention and it results in a longer life of the electrode cap to increase the productivity of the resistance welding process, especially in automated welding.

c) the compressive strength, especially in the core, is significantly higher than that of samples of conventionally extruded DS-copper, as was surprisingly found in the investigation of samples of DS-copper with an arcuate grain structure. This increases the life of components made therefrom, e.g. Electrode caps for resistance spot welding. The higher resistance to deformation also counteracts the mentioned "mushrooming".

d) For electrode caps and similar components, the arcuate fiber profile has a more favorable orientation of the microstructure compared to the cutting angles of inserted milling tools, as with conventionally extruded material, which u.a. leads to the formation of a smooth surface during cap milling.

Further features and advantages will become apparent to those skilled in the art from the following description and the accompanying drawings, in which:

1 shows a cross section through a rod made of ODS copper from a continuous extrusion process according to the invention with a low degree of deformation on a scale of 500: 1 Fig. 2 is a fracture surface of the rod of Fig. 1; and

3 shows a cross section through a conventionally extruded ODS copper rod (extrusion ratio 27: 1) on a scale of 500: 1; and

4 is a fracture surface of the rod of Fig. 3rd

In Fig. 1, the microstructure of a rod of an ODS copper alloy is shown in which a copper powder of an average particle size of 300 .mu.m was pressed in a continuously operating extruder with friction wheel without external heating with an extrusion ratio of 2: 1. You can clearly see the arcuate fiber profile in the direction of the rod axis.

In Fig. 2 is a photograph of a fracture surface of the rod of Fig. 1 is shown. Significantly, the arcuate fiber profile in the central region of the rod can be seen, which leads to a very little frayed / cracked fracture surface.

Fig. 3 shows a longitudinal section through a conventionally extruded rod made of an ODS-Cu alloy, from which can be read in the direction of the rod axis extending fiber profile. Rods having this fibrous structure tend to brittle fracture upon bending and are susceptible to cracking when subjected to stress across the fiber texture (e.g., cold extrusion of male electrode caps with female tapered seat).

Production of an electrode cap:

From Cu-Al-BO powder was prepared by continuous powder extrusion with a degree of deformation of 1, 8: 1 rods with a diameter of 15.8mm. By subsequent extrusion on a commercially available multi-stage system, electrode caps of type F16 (DIN ISO 5821) were produced therefrom. These electrode caps were used in a resistance welding machine in automobile body shop (zinc plate). Surprisingly, it has been found that the electrodes only had to be milled after approximately 420 spot welds, whereas DS-Cu electrodes made from bars of the same alloy conventionally produced by discontinuous extrusion with a high degree of deformation (27: 1) had to be reworked after about 380 welds. With a total of 35 milling cycles, this corresponds to a productivity gain of 2450 weld points or 10.5%.

Claims

New claims

A method for producing molded parts (rods) of dispersion-strengthened metal alloys, characterized in that the molded parts are produced by continuous powder extrusion of the metal alloy powder with a low degree of deformation with a longitudinally arcuate grain structure.

2. The method according to claim 1, characterized in that the metal alloy is selected from the group consisting of Ni, Cu, Al, Ag, Fe alloys.

3. The method according to claim 1, characterized in that as the metal alloy, an ODS copper alloy is selected and the extrusion ratio between 1, 5: 1 to 4: 1 is set.

4. The method according to any one of the preceding claims, characterized in that a method with friction wheel is used as a continuous powder extrusion.

5. Moldings of continuously powder-extruded metal alloy powder high compressive strength with an arcuate grain structure in the molding in the longitudinal direction, can be produced by a low degree of deformation during continuous powder extrusion with friction wheel.

6. Molding according to claim 5, prepared from an alloy selected from the group consisting of: Cu, Al, Ag, Ni, Fe alloys.

7. Molding according to claim 6 made of continuously powder-extruded ODS copper alloy powder high compressive strength with a longitudinally arcuate Komstruktur in the molding, made with low degree of deformation at an extrusion ratio of 1.5: 1 to 4: 1.

8. Use of the molded parts according to claim 7 for the production of electrodes and / or electrode caps, welding electrodes and / or Schweißelektrodenkap- pen, electrodes for the resistance spot welding and / or electrode caps for the resistance spot welding of sheets and / or galvanized sheets.