GB2032317A - Electrode for electrical resistance welding - Google Patents

Abstract

An electrode comprises a sintered body (6) of metallic material of high electrical conductivity in which there is embedded a plurality of elongated members (11) (e.g. of wire, rod or strip form) of a highly heat resistant metallic material of higher melting point than the body material. The body may be subsequently deformed to a desired form. <IMAGE>

Description

SPECIFICATION Electrode for electrical resistance welding This invention relates to an electrode for electrical resistance welding and to a process of producing such an electrode. This invention has particular, but not exclusive, application to electrodes of the type having a body of a metallic material with high electrical conductivity and at least at or adjacent the contact surface of the electrode, a highly heatresistant metallic material, the highly heat-resistant material having a higher melting point than the material of high electrical conductivity.

Such an electrode has been proposed in German Offenlegungsschrift 15 65 605. The material with high electrical conductivity is copper or a copper alloy, while tungsten, molybdenum and tungstencopper impregnting materials are mentioned as the inserted, heat resistant materials. The maufacture of such electrodes is very costly. A ring-shaped recess is worked into one end of a blank having good conducting properties. The heat-resistant material is pressed into this recess and the composite is subsequently formed into shape mechaicallyin an extrusion press. Since the copper-chromium electrodes, presently available commercially, are very moderately priced, such an expensive process has no possibility of establishing itself in the market.

Moreover, it is very difficult to subject materials such as tungsten and molybdenum to more extensive degrees of deformation.

A different, similar electrode has previously been proposed in German Offenlegungsschrift 15 65 318.

In a body consisting essentially of copper, rods are embedded which consist of copper-based alloys of the highest strength, for example, of copperchromium, copper-tungsten, copper-silverziroconium, or copper-cobalt-beryllium. The necessary firm bond is established by subjecting the electrodes to a severe mechanical deformation, which results in a reduction in the diameter by a factor of at least 10. The rods and the electrode body weld together as a result of this severe deformation.

Such a severe deformation naturally necessitates the use of correspondingly ductile materials. Forthis reason, only copper-based alloys are used in the arrangement described in German Offenlegungsschrift 1565318. Many highly heat-resistant and particularly high melting, refractory metals, such as, for example, tungsten and molybdenum, cannot be deformed nearly to such an extent. On the other hand, it is also costly if intermediate annealings are required in the case of so severe a degree of deformation.

A similar electrode has also been proposed in German Patent 625 201. For its preparation, a sheet of tungsten, molybdenum or tantalum is wound into a spiral and the resulting spaces therearound are filled with the material of high conductivity, that is, primarily with copper or with copper alloys. The process also is exceptionally expensive and suitable only for uneconomic, manual single-item manufacture. The teachings of German Patent 625 201 have therefore not found acceptance in industrial practice.

Since then, there has been no absence of attempts to increase the service life of electrodes for electrical resistance welding and to decrease the tendency to adhere without at the same time making the electrodes uneconomically expensive. The service life of a welding electrode is determined by its resistance to combustion and by the stability of its shape. High resistance to combustion presupposes that the large amount of heat, produced on welding, is readily conducted away. It therefore requires an electrical conductivity. Such materials, however, regularly have properties other than those required for achieving good shape stability.Good shape stability presupposes high abrasion resistance, hardness, heat stability and - particularly in the case of electrical resistance welding, where the electrodes are pressed against the work-piece to be welded - also compression strength Accordingly, it has already been proposed to harden the electrode material of high conductivity by adding suitable materials, for example, by a dispersion-hardening process and to increase the service life by these means (German Auslegeschrift 2024 943, German Offenlegungsschrift 25 54990). Electrodes of copper, containing a few per cent of aluminia, have also shown themselves to be particularly promising with respect to abrasion resistance (A.V. Nadkarni and E.P.Wever "A new dimension in resistance welding electrode materials", Welding Research Supplement to the Welding Journal, November 1977, pages 331-388). The gain in service life, achievable by hardening the copper material, is restricted to narrow limits because the hardening does not have a major effect on the heat resistance and on the stability of the shape.

It has also already been proposed (German Offen Iegungsschrift 25 54990) to manufacture the electrode of a copper material, to bore a blind hole parallel to the axis in the tip of the electrode and to embed in this blind hole a pin of a highly heat resistant, optionally also of a high melting point material such as molybdenum and tungsten. The embedding is accomplished by thermal shrinkage, by extrusion or by a similar reduction in the cross section of the electrode or by screwing in the pin.

There are great problems with manufacturing such electrodes, especially when using a pin of a high melting point material such as tungsten or molybdenum. Because of the thermal and mechanical stress reversals on welding, the pin is not-securely retained in its round hole. This is understandable, considering that the copper material is heated up to its softening point during welding. In the German Offenlegungsschrift 25 54990, it is therefore additionally proposed to provide the pin with a exterior thread and to screw it into the blind hole. Considering their hardness however, any cutting work on high-melting point metals such as tungsten and molybdenum is extremely expensive and is of no practical importance so that formation of a screwthread makes sence only for pins of lower melting point materials.However, even in the case of these, the use of a screw-threaded pin creates problems.

On the one hand, such a pin can be used only in the case of thicker electrodes, and on the other, there are contact problems. Because of defective transition resistance between pin and electrode body, there may be undesirable over-heating.

A further disadvantage of the electrode, described in the German Offenlegungsschrift 25 54 990, consists in the fact that a large area in the centre of the electrode tip is occupied by the pin so that, especial ly when using high-melting point materials, the thermal and electrical conductivities of this central region are considerably inferior to those of the edge region. At the same time, the edge region, through which a considerable portion of the welding current passes, continues to have little shape stability, so that a so-called "mushroom formation", which occurs during welding, can be prevented only inadequately.

Electrodes for electrical resistance welding must meet particularly high requirements with respect to shape stability and a lowtendencyto adhere, when sheet metal is to be welded which, as is customary when building automobiles, is provided with a corrosion-inhibiting metallic coating, i,e. galvanized, cadmium plated or aluminized. As a result of the fact that low-melting point alloys are formed between the coating metal and the copper of the welding electrodes, the erosion of material frnm the welding electrodes and the tendency of the welding elec trodesto adhere is increased. In orderto remedy this, attempts have previously been made to remove the coating mechanically at the welding sites, for example, by means of brushes or by rotating welding electrodes.This is, however, expensive and does not bring about the expected success.

It is therefore an object of the present invention to provide an improved construction of electrode of the aforementioned type and by an economic process which results in a significantly increased shape stability and, at the same time, good combustion resistance and a limited tendency to adhere.

In order to solve this task, it is conceivable to manufacture the welding electrodes from composite fibre material, in which the fibres consist of highly -heat-resistant metals. In the case of composite materials of metallic fibres, use is, however, made of the fact that the tensile strength of fibres increases with decreasing fibre diameter and every effort is therefore made to keep the fibres as thin as possible Composite fibre materials have highertensile strength than materials not reinforced with fibres.

With regard to compression strength, however, which is important forweldi ng electrodes, com-posite fibre-materials do not have a corresponding superiority over materials which are not-reinforced with fibres; the bending strength of-a fibre-is namely inversely proportional to the third power of its slenderness ration.

It is well known that composite tungsten-copper fibre materials, contani ng 40 volume per cent of tungsten fibres, have a 30 per cent higher electrical conduritivity in-the fibre direction than corresponding tungsten-copper materials produced by power metallurgical means and are therefore very interest ing for electrical contacts. Nevertheless, because of the high maufacturing costs, such materials have not yetfound-an industrial application ("Metallic Composites", publication of the Rau Company, Pforzheim, 1976, page 167). According to this publication, the relatively expensive and thin tungsten wires are difficult to deform and must therefore be embedded in the copper matrix by melt infiltration of the copper. Any further deformation is then possible only by hot forging.

In order to avoid the difficulties of manufacturing fibre composites containing tungsten fibres, an attempt has already been made to embed tungsten, in the form of tungsten powder, in fibrous fashion in a matrix and to consolidate it by drawing processes (German Offenlegungsschrift 23 51 226). As long as the tungsten however is present in the form of a powder, it cannot reinforce the matrix mechanically.

For this purpose, the tungsten would have to be sintered, so that the powder is converted into solid, uniform fibres. If, however, the matrix consists of a metal with a high electrical conductivity, it is not possible to sinter the tungsten, because the matrix cannot be exposed to the temperatures required for sintering the tungsten.

Fibre composites with fibres of highly heatresistant metals, therefore, cannot be considered for the manufacture of electrodes for electrical resistance welding.

According to the present invention, there is provided an electrode for electrical resistance welding, in which there is embedded in a body of a metallic material with high electrical condustivity and at least at or adjacent the contact surface of the electrode a pluralty of elongated members of a highly heatresistant metallic material, the highly heat-resistant material having a higher melting point than the material of high electrical conductivity, characterized in that the body in which the elongated members are embedded is a sintered body.

As material with a high electrical conductivity, primarily copper and copper alloys, and including especially an alloy of copper with 0.02 - 2% silver; of copper dispersion hardened with aluminium oxide or zirconium oxide; or of copper containing 0.5 - 5 weight per cent of graphite. The selection of the highly heat-resistant material partly depends on the area of application of the electrodes. Primarily tungsten and molybdenum, but also tantalum and similar refractory metals have proved their value.

Also, the materials steel or chromel are worths of particular consideration as the less highly melting, highly heat-resistant materials.

By means of the invention utilising the embedding of wires or rods of a highly heat-resistant material of given dimensions, an electrode is provided with the desired shape stability as well as with good-resist ante so combustion and an insignificant tendency to adhere. The use of a sintered body of a material with high electrical conductivity essentially creates-the preconditions for manufacturing the inventive electrode in an economic manner, as will be explained further below. In German Offenlegungsschrift 1565 318 there is admittedly a disclosure that the processes of powder metallurgy may be used for the manufacture-of electrodes. However, this clearly refers to the manufacture of the tungsten-copper rods employed. It was not however realized that a rigidly bonded electrode is obtainable without a very severe deformation.

Moreover, it is admittedly proposed in German Patent 906835 that, for the manufacture of electrical switching contacts, wires or rods of a high melting point metal may be embedded in a metal which is a good conductor. The manufacture consists however of infiltrating a bundle of rods or wires into the molten, high conductivity metal in a mould and is so expensive that it has not found any industrial application.

The cross section, which is to be selected for the elongated members in the form of rods or wires in any particular case in the present invention, depends on the conditions under which the welding electrode is to be used. Welding electrodes for electrical resistance welding generally have a diameter between 5 and 30 mm. In the case of thinner electrodes, thinner wires or rods are of course embedded and, in the case of thicker electrodes, thicker and/or more wires or rods are embedded than in the case of thinner electrodes. The number of embedded wires, rodes or strips depends on their cross section as well as on the current load, the heat stress and the pressure to which the electrode is to be subjected.

The preferred number of wires, rods, or strips in an electrode can conveniently be between five and thirty. The dimensions for the elongated members may suitably be 0.5 to 3 mm diameter for wires, 0.2 to 10 mm cross-section for rods and 1 to 50 mm cross-section for strips.

Preferably, the wires, rods or strips are appropriately located on one of several concentric circles about the longitudinal axis of the electrode.

Preferably, the elongated members do not extend over the full length of the electrode so that, when using expensive tungsten, molybdenum or the like, this material is used only where it is actually required and not in the rear section of the electrode, which is clamped in a holder and usually cooled with a coolant (e.g. water).

According to a further aspect of the present invention there is provided a process for the manufacture of an electrode for electrical resistance welding comprising forming a material of high electrical conductivity in powder form into a body having a plurality of channels therein, inserting an elongated member of a highly heat resistant metallic material of higher melting point than the material of high electrical conductivity into said channels, sintering the body with the elongated members therein, and deforming he sintered body to a desired form.

Through the powder-metallurgical manufacture of a moulded body, provided with channels for elongated members in the form of highly heat-resistant wires or rods, difficulties, which are inherent to the above-described known processes for embedding highly heat-resistant, especially high melting metals in metals which melt at a lower temperature, are obviated or mitigated. The wires, rods or strips embedded in the channels already find a good support in the moulded body as a result of shrinkage which occurs during the sintering, and because of the diffusion zones of the high conductivity materials which form about the wires, rods or strips.

The mechanical deformation of the sintered moulded body provided by the present invention achieves further consolidation of the structure of the moulded body. At the same time, this deformation is intended to ensure that the dimensions are close to the final shape of the electrode, so that subsequently only simple cutting operations are required in order to confer the final shape of the electrode. The extent of the mechanical deformation is kept small, and in the case of wires or rods or strips of a refractory material, for example, so small that these are stretched no more than 30% of their original length as a result of the deformation.

As previously indicated, the wires or rods may not always fill the whole length of the moulded body. In such a case, the mechanical deformation of the sintered moulded body a!so causes the unfilled sections of the chanels to close, because the sintered, high conductivity material flows into these sections of the channels.

For the manufacture of the sintered moulded body, the high conductivity metal powder may be pressed around wires or rods of the highly heatresistant material. The wires or rods are then cut off above the extruded moulded body, whereupon the extruded moulded body, with rods, wires or strips embedded, is sintered.

Preferably, however, the body is formed by compressing the powder around a plurality of parallel mandrels, and withdrawing the mandrels to define parallel channels in the body. This process is preferred for the manufacture, because it avoids the problems of cutting off the hard wires, rods or strips above the extruded moulded body. The process is development of a well-known process from German Patent 20 25 166 and German Offenlegungsschrift 23 57 309, which was developed for manufacturing contact tips for welding machines having a continuous, cylindrical channel.

An embodiment will now be described, by way of example, with reference to the accompanying drawings in which :- Figures 1 to3 are schematic representations of a longitudinal section through an extrusion matrix and showing the different phases in the manufacture of a moulded body manufactured from powder.

Figure 4 is a schematic representation of a section through a filling station and shows the introduction of highly heat-resistant sections of wires into the moulded body.

Figure 5 is a plan view illustrating the orientation of the moulded body under the filling station.

Figure 6 is a schematic representation of a longitudinal section through an extrusion matrix and shows the process of transforming the sintered moulded body.

Figure 7 is a cross-sectional view on the line A-A of Figure 6.

Figure 8 is a cross-sectional view on the line B-B of Figure 6.

A preferred process for manufacturing the electrodes according to the invention is shown in its various steps in Figures 1 to 8. Figures 1 to 3 show the moulding of metal powder in three steps.

Moulding is carried out between an upper die 2 and - a lower die 3. A series of mandrels 1 is attached to the upper die 2. The depending ends of these mandrels are arranged so that they can be guided into the lower die 3, as to pass parallel to one another through a filling space Sofa press matrix 4.

In Figure 1, the upper die 2 is lifted sufficiently so that the filling space 5 of the matrix can be filled with the powder or with the powder mixture which is of an electrically highly conductive material; at the same time, spaces, which later are to become the channels 7 (Figure 3) into which elongated members in the form of highly heat-resistant sections of wire 11 are received (Figure 4), are occupied by the mandrels 1. In the position shown in Figure 2, the powder, which has been introduced into the filling space 5 of the press matrix 4, is consolidated into a moulded body 6. In the position shown in Figure 3, the upper die 2 with its mandrels 1 is retracted and the moulded body 6 with parallel channels 7 formed therein by the mandrels 1 which have been retracted, is exposed as the lower die 3 travels up or as the press matrix is lowered.

Figure 4 shows how the moulded body 6 with its channels 7 is moved stepwise on a transporting device 8 under a filling station 9 for the introduction of the highly heat-resistant wire sections 11. This filling station 9 is loaded with wire sections 11 of highly heat-resistant material through supply lines 10 from a storage bin, wich is not shown. As soon as a moulded body 6 reaches a stop 12, a gate valve 13 opens automatically and the wire sections 11 fall into the channels 7, this latter process being facilitated significantly by conical flaring 15, formed during the consolidation process by the conical projections 16 of the upper die 2 (Figure 1).With the closing of the gate valve 13, a further set of wire sections 11 slides into the filling station 9 and stop 12 makes way to allow the filled moulded body 6 to be passed in an upright position on the transporting device 8 through a sintering furnace, which is not shown.

During sintering a diffusion zone is formed in the transition area between the material of high electrical conductivity and highly heat-resistant material of higher melting point.

It is advantageous to select a moulded body 6 of hexagonal shape into which the wire sections 11 are introduced. In this manner, the moulded body 6 can be guided on opposite sides by bars 14 of the transporting device 8 and the channels 7 can be aligned underthefilling station 9 (Figure 5).

Figures 6 to 8 show the final extrusion moulding process, in which a sintered body, formed in the sintering furnace, is deformed. A sintered body 17 is placed by hand or by machine in upper portion 18 of an extrusion moulding matrix 19 and then pressed by means of an extrusion mouldng die 20 through the extrusion moulding matrix 19, which tapers towards its lower end. The cross sections of the upper portion 18 of the matrix 19 and of the lower, tapered portion 21 of the matrix are shown separately in Figures 7 and 8 respectively. In the example shown, the upper, wider, receiving portion 18 of the extrusion moulding matrix is haxagonal. It could also, however, be octagonal or any other diffrent suitable shape, which will ensure alignment of the moulded body 6 under the filling station 9 as in Figure 4.The lower, tapered portion 21 of the extrusion moulding matrix (Figure 8) is circular in this example, because resistance welding electrodes are most frequently circular. Basically of course, any other external shape is possible. It is, moreover, possible to push the sintered body through the press matrix not to the extent of its full length, but only part of the way and to then eject it again in the reverse direction. Instead of prismatic shapes, it is possible to obtain with this procedure shapes which have been tapered or reduced as desired; even shapes with additional internal contours at the end of the electrode e.g. for water cooling. It is within the expertise of an extrusion-moulding expert to produce customary shapes of electrodes for electrical resistance welding.

An important advantage of the invention lies in the fact that the length of the introduced sections of wires 11 of highly heat-resistant materials may be shorter than the length of the moulded body 6. As a result, it is possible to keep the rear end of the electrode which is to be formed by an extrusion moulding, i.e. that portion of the electrode is to be inserted into a holder and which contains the usual facilities for water cooling, free from the highly heat-resistant material where it is not required and to restrict this highly heat-resistant material to the compression-resistant front portion of the electrode.

Those sections of the channels 7, which are not filled with wire sections 11, are filled with the highly conductive material during the extrusion moulding process.

By means of the process described above, electrodes can be produced economically, such electrodes being reinforced by a predetermined number of wires, rods or strips which run parallel to the longitudinal axis of the electrodes and perpendicular to their contact surface. Because of this construction, the electrodes have particularly advantageous properties for electrical resistance welding.

In the following, four examples of the operation of the invention are described.

Example 1 Copper powder, having an average particle size of 10ç is moulded into a bolt in which 21 channels of circular cross-section are arranged concentrically and the cross-section of which bolt is a regular hexagon. Rods of tungsten are introduced into the channels, the rods having a diameter which is 0.05 mm less tha the diameter of the channels and which fill the whole length of the channels. The moulded body is then sintered in a protective gas at 1,050 C and subsequently deformed by cold extrusion moulding.

Example 2 Copper powder, having an average particle size of 1 0y and containing 0.5 weight per cent of a finegrained chromium powder (particle size less than 111) and 0.1 weight per cent of zirconium hydride, is moulded into a bolt of hexagonal cross-section and 19 channels are defined therein in hexagonal arrangement. Into the circular channels rods of molybdenum are introduced, the rods having a diameter which is 0.1 mm less that the diameter of the channels and which are only half as long as the channels. Subsequently, sintering is carried out under a protective gas at 800 C. In the section which contains no molybdenum rods, the sintered moulded body is cold extrusion moulded into a shape which externally is hexagonal, a cooling chamber being formed at the same time internally.

On the other hand, the section of the moulded body which contains the molybdenum rods, is deformed into a round shape by hot extrusion moulding.

Example 3 Powder of a copper-aluminium alloy with o.5 weight per cent of aluminium, whose aluminium portion has been transformed by internal oxidation into aluminium oxide (Al203), is moulded into a bolt of hexagonal cross-section having 14 channels of circular cross-section in hexagonal arrangement.

Into each channel a tungsten wire section is introduced, the wire section having a diameter which is 0.1 mm less than the diameter of the channel. The length of the wire sections is identical with the length of the channels. The bolt, containing the wire sections, is sintered under vacuum at 1,000 C and subsequently deformed by hot extrusion moulding to a hexagonal moulded body of lesser dimension.

Example 4 Powder of an internally oxidized copper-tin alloy, containing 5 weight per cent of tin oxide, is moulded into a bolt of hexagonal cross-section having 12 cannels of circular cross-section concentrically arranged in two circles about the bolt axis. Into each channel a wire section of a chrome-nickel steel material No. 4841 is introduced, the diameter of which is 0.05 mm less than the internal diameter of the channel. The length of the wire sections is 70% of the length of the bolt. The bolt, filled with the wire sections, is sintered under vacuum for 30 minutes at a temperature of 1 ,000 C and subsequently deformed by cold extrusion moulding into a round electrode blank, the cross-section of which has been reduced to one half of the initial cross-section. The rear section of the electrode, in which there are no steel wires, is at the same time provided with a cooling chamber by a reverse extrusion moulding process. During welding, cooling water is passed into the cooling chamber.

The process of the invention is suitable for largely automated, mass production of electrodes.

Claims (16)

1. An electrode for electrical resistance welding, in which there is embedded in a body of a metallic material with high electrical conductivity and at least at or adjacent the contact surface of the electrode, a plurality of elongated members of a highly heatresistant metallic material, the highly heat-resistant material having a higher melting point than the material of high electrical conductivity, characterized in that the body in which the elongated members are embedded is a sintered body.

2. An electrode as claimed in claim 1, in which the elongated members are in the form of wires having a diameter between 0.5 and 3 mm.

3. An electrode as claimed in claim 1, in which the elongated members are in the form of rods having a cross-section between 0.2 and 10 mm.

4. An electrode as claimed in claim 1, in which the elongated members are in the form of strips having a cross-section between 1 and 50 mm.

5. An electrode as claimed in any preceding claim, in which the elongated members are of tungsten, molybdenum, tantalum, steel, chromel, or similar, high-melting point material.

6. An electrode as claimed in any preceding claim, in which the body consists of an alloy of copper with 0.02 - 2% silver; of copper dispersion hardened with aluminium oxide or zirconium oxide; or of copper containing 0.5 - 5 weight per cent of graphite.

7. An electrode as claimed in any preceding claim, in which five to thirty elongated members are embedded in the sintered body.

8. An electrode as claimed in any preceding claim, in which the elongated members are arranged symmetrically about the longitudinal axis of the electrode.

9. An electrode according to any preceding claim, in which the elongated members do not extend up to the rear end of the electrode.

10. A process for the manufacture of an electrode for electrical resistance welding comprising forming a material of high electrical conductivity in powder form into a body having a plurality of channels therein, inserting an elongated member of a highly heat resistant metallic material of higher melting pointthatthe material of high electrical conductivity into said channels, sintering the body with the elongated members therein, and deforming the sintered body to a desired form.

11. A process as claimed in claim 10, comprising forming the body by compressing the powder around a plurality of parallel mandrels, and withdrawing the mandrels to define parallel channels in the body.

12. A process as claimed in claim 10 or 11, in which the sintered body is deformed by extrusion.

13. An electrode for electrical resistance welding, substantially as hereinbefore described with reference to the accompanying drawings.

14. An electrode for electrical resistance welding, substantially as hereinbefore described with reference to any one of the foregoing Examples 1 to 4.

15. A process for the maufacture of an electrode for electrical resistance welding, substantially as hereinbefore described with reference to the accompanying drawings.

16. A process for the manufacture of an electrode for electrical resistance welding, substantially as hereinbefore described with reference to any one of the foregoing Examples 1 to 4.