GB2239205A

GB2239205A - A method of electrically spot welding metal filter elements

Info

Publication number: GB2239205A
Application number: GB9027704A
Authority: GB
Inventors: Denis Louis Baralle
Original assignee: Societe Europeenne de Propulsion SEP SA
Current assignee: Societe Europeenne de Propulsion SEP SA
Priority date: 1989-12-22
Filing date: 1990-12-20
Publication date: 1991-06-26
Anticipated expiration: 2010-12-20
Also published as: DE4041041A1; FR2656246B1; FR2656246A1; GB9027704D0; GB2239205B

Abstract

In a method of electrically spot welding a metal filter element to a metal support they are interposed between two electrodes each having an active surface area of not less than ten times the mesh area of the metal filter element. Electrical pulses of lower current than the current used for conventional spot welding are applied and successive weld spots overlap in such a manner that the overlap area between them lies in the range 50% to 60% of the area of each weld spot. <IMAGE>

Description

1 A METHOD OF ELECTRICALLY SPOT WELDING METAL FILTER ELEMENTS The present

invention relates to a method of electrical spot welding, and is intended more particularly for connecting a filter element of the metal gauze type to its metal support, which items are interposed between two electrodes during welding.

The invention is thus applicable, In particular to manufacturing filters suitable for use in space launchers, satellites, or shuttles, or in the chemical industry or in the nuclear industry. Similarly, the invention is applicable to manufacturing the sieves that constitute capillary elements as used in "surface tension" fuel tanks mounted on board space satellites, shuttles, or platforms, said capillary elements then acting as gas barriers. At present, various different techniques are used for making connections of this type. For example, crimping can be used to hold components together by deforming them somewhat. However, under difficult operating conditions, this method gives rise to problems with sealing. 20 It is also possible to use "roller" welding in which the pieces to be welded together are interposed between two rollers which transmit force continuously while transmitting current throughout the entire welding operation. However this technique suffers from the drawback of pushing back the metal in front of the roller(s) so that when welding is finished, a fold is created which is prejudicial to the mechanical strength of the structure made in this way.

Another method of assembling together two elements such as those defined above consists in brazing or soldering using a metal whose melting point is lower than that of the two elements to be bonded together. This method suffers from the a drawback of adding/second or a third metal which is often incompatible with the liquids involved such as nitrogen peroxide (N204), monomethyl hydrazine (MM1H), hydrazine (N2H4), -... In addition, the low melting point metal may be corroded in the long term by electrochemical effects.

2 It is also possible to bond a filter elenent on a support by brazing or soldering in combination with diffusion, however this method suffers from the same drawbacks as brazing or soldering on its own.

Electrical spot welding techniques are also known.

However these conventional techniques are applicable only to welding together pieces made of metals or alloys that are identical or very similar, and they are unsuited for welding together certain materials such as stainless steel on titanium.

Given the values of electrical current conventionally used in electrical spot welding techniques, a metal gauze welded by such a conventional method is subjected to burning that makes it too brittle. The gauze also runs the risk of being dirtied by drops of metal.

The present invention seeks to remedy the above-mentioned drawbacks and to make it possible to weld a metal filter element on a metal support with excellent mechanical strength while fully conserving the integrity of the filter element. These aims are achieved by a method of electrically spot welding a metal filter element to a metal support therefor, the filter element and the support being interposed between two electrodes during welding, the method consisting in using electrodes each having an active surface area of not less than ten times the mesh area of the metal filter element, in applying electrical pulses of lower current than the current used for conventional spot welding, and in ensuring that successive weld spots overlap in such a manner that the overlap area between successive weld spots lies in the range 50% to 60% of the area of each weld spot.

By using the method of the invention, the adhesion obtained by surface diffusion establishes mechanical adhesion which is effective even when the pieces to be assembled are made of metals or alloys that are very different from each other, and which is sufficient to guarantee reliable operation of the assembled filter element in applications such as those mentioned at the beginning of the description.

3 The method of the invention also makes it possible to obtain a weld fillet which is leakproof to better than the limit imposed by the bubble point of the metal gauze.

The welding is preferably performed by discharging a capacitor, thereby ensuring a bond by mechanical adhesion.

The weld fillet made in this way may naturally be of arbitrary shape.

The filter element and/or its support may be made of common steels, of stainless steel, of a copper-based alloy, or of titanium or a titaniumbased alloy, or of aluminium or an aluminium-based alloy. All combinations can be envisaged without going beyond the scope of the invention.

In order to improve the method of the invention, it is possible to cool the top electrode by an external f low of distilled water, or to cool one or both electrodes by an internal flow of water.

An implementation of the invention is described by way of example with reference to the accompanying drawing, in which:

Figure 1 is a section through the essential items required by the method of the invention; and Figure 2 shows a weld fillet obtained by the method of the invention.

As shown diagrammatically in Figure 1, two electrodes 3 and 4 are placed respectively above and below the parts to be welded together which are constituted by a filter element such as a metal gauze 1, and its support 2 which is likewise made of metal. The electrodes may be cylindrical or conical. Once the items are in place, welding is performed by discharging a capacitor, for example. This makes it possible to achieve a bond by surface diffusion, or mechanical adhesion between the two metals. The parameters of the welding machine (not shown) are adjusted so as to obtain assembly of the surface type, i. e. without deep welding of the metals. 35 Several types of material can be welded together. Thus, a metal gauze 1 and a support 2 may each be made from a material selected from the following materials any one 4 of which may be assembled to any other one by the method of the invention: common steels, stainless steels, copper-based alloys, aluminium, aluminium alloys, titanium, and titanium alloys.

It may be observed that when welding a stainless steel gauze on a titanium support, titanium rises into the mesh of the gauze, thereby ensuring not only the mechanical adhesion inherent to the welding method of the invention, but also mutual mechanical interfitting between the two materials.

Unlike conventional methods, when a bond is made using the method of the invention, no weld ingot is formed capable of making the bond brittle.

The active surface area of each of the electrodes 3 and 4 is preferably equal to not less than ten times the area of a mesh in the filter element 1, where mesh size is defined in accordance with French standards NF XII-506 and NF XII-515.

By overlapping weld points so that their mutual overlap areas occupy between 50% and 60% of the area of a single weld point, i. e. as shown in Figure 2, it is possible to make a high quality weld fillet 5 which is leakproof to better than the limit that corresponds to the "bubble point" of the gauze.

The weld fillet may be arbitrary in shape (circular, square, rectangular, oval) providing the above-mentioned overlap condition is satisfied. Similarly, the gauze may be woven in numerous different ways.

As already mentioned, this weld has an effect only on a very limited surface zone in each of the materials. It can be assimilated to mechanical adhesion, and it makes it possible to avoid the risks of brittleness associated with conventional welding, in particular since welding of the present invention does not give rise to an ingot.

In order to improve the technical performance of the method of the invention, the top electrode may be cooled, e.g. by spraying it with a film 6 of distilled.water (Figure 1). In this case, welding takes place under a film of water. Cooling may also be achieved by an internal flow of water inside one of the electrodes 3 or 4, or inside both of them. Figure 1 shows a flow of water 7 inside the electrode 4.

The method of the invention makes it possible to weld together metals such as stainless steel and titanium, and this is novel compared with conventional methods.

The items that can be made using the method of the 5 invention are numerous and varied.

By way of example, mention may be made of filters for use in space shuttles, satellites, or launchers, or in the chemical industry or in the nuclear industry. Similarly, the invention is applicable to making the sieves that constitute the capillary elements used in "surface tension" fuel tanks mounted on board space shuttles, platforms, or satellites, said capillary elements acting as gas barriers.

The present invention is naturally not limited to the particular applications mentioned.

6

Claims

CLAIMS l/ A method of electrically spot welding a metal filter element to

a metal support therefor, the filter element and the support being interposed between two electrodes during welding, the method consisting in using electrodes each having an active surface area of not less than ten times the mesh area of the metal filter element, in applying electrical pulses of lower current than the current used for conventional spot welding, and in ensuring that successive weld spots overlap in such a manner that the overlap area between successive weld spots lies in the range 50% to 60% of the area of each weld spot.

2/ A welding method according to claim 1, in which the welding is performed by discharging a capacitor to ensure bonding by mechanical adhesion.

3/ A welding method according to claim 1 or claim 2, in which the support is made of a common steel and the filter element is made of one of the following materials: common steels, stainless steels, copper-based alloys, aluminium, aluminium alloys, titanium, and titanium alloys.

4/ A welding method according to claim 1 or claim 2, in which the support is made of a copper-based alloy and the filter element is made of one of the following materials: common steels, stainless steels, copper-based alloys, aluminium, aluminium alloys, titanium, and titanium alloys.

5/ A welding method according to claim 1 or claim 2, in which the support is made of stainless steel and the filter element is made of one of the following materials: stainless steels, titanium, and titanium alloys.

6/ A welding method according to claim 1 or claim 2, characterized in that the support is made of stainless steel and the filter element is made of one of the following materials: camxDn steels, copper-based alloys, aluminium, and aluminium alloys.

7 7/ A method according to claim 1 or claim 2, i n which the support is made of aluminium or an aluminium alloy, and in which the filter element is made of one of the following materials: common steels, stainless steels, copper-based alloys, aluminium, aluminium alloys, titanium, and titanium alloys.

8/ A welding method according to claim 1 or claim 2, in which the support is made of titanium or titanium alloy and the 10 filter element is made of stainless steel.

9/ A welding method according to claim 1 or claim 2, in which the support is made of titanium or titanium alloy, and the filter element is made of one of the following materials: common steels, copper-based alloys, aluminium, and aluminium alloys.

10/ A welding method according to claim 1 or claim 2, in which the support and the filter element are made of titanium or 20 titanium alloy.

11/ A welding method according to any one of claims 1 to 10, in which the top electrode is cooled during the welding operation by an external flow of distilled water so that the welding 25 operation takes place under a film of the water.

12/ A welding method according to any one of claims 1 to 10, characterized in that at least one of the top and bottom electrodes is cooled by a flow of water inside said electrode.

13/ A method of electrically spot welding a metal filter element to a metal support therefor, the method being substantially as herein described with reference to the accompanying drawing.

Published 1991 atThe Patent Office, State House. 66171 High Holborn. London WC I R 47P. Further copies rrlaybC obtained from Sales Branch. Unit 6, Nine Mile Point- Cwmfelinfach, Cross Keys. Newport NPI 7H.Z. Printed by Multiplex techniques lid, St Mary Cray. Kent.