GB2068812A

GB2068812A - Resistance spot welding

Info

Publication number: GB2068812A
Application number: GB8003242A
Authority: GB
Original assignee: British Aluminum Co Ltd
Current assignee: British Aluminum Co Ltd
Priority date: 1980-01-31
Filing date: 1980-01-31
Publication date: 1981-08-19
Also published as: GB2068812B

Abstract

A method of resistance spot welding together workpieces of readily oxidisable light metals or light metal alloys using internally liquid cooled copper or copper alloy electrodes comprising immersing in a quantity of continuously changing non-inflammable cooling liquid each electrode tip and the adjacent workpiece surface during passage of welding current between the electrodes. The cooling liquid is supplied from the interior of the electrode through plugs 11 or through tubes externally of the electrodes. <IMAGE>

Description

SPECIFICATION Improvements relating to a method of and apparatus for resistance spot welding This invention relates to a method of resistance spot welding and to resistance spot welding apparatus.

Resistance spot welding is conventionally carried out by squeezing metal workpieces usually in sheet form, between two electrodes, passing a current between the electrodes and applying forging pressure between the electrodes to form a fused joint at the interface of the workpieces.

Spot welding electrodes are usually of copper or a copper alloy and are internally water cooled to reduce their temperature at the point of welding and increase their useful life.

However when used to weld readily oxidisable metals or alloys such as aluminium magnesium, titanium and tantalum and their alloys such electrodes have a very short useful life primarily because the build up of a surface oxide layer causes an increase in heat production at the point of electrode contact. This results in sticking between the electrodes and the workpieces and the transfer of metal from the workpieces to the operative surfaces of the electrodes. It has been found that conventional internal water cooling of the electrodes is insufficient to overcome this problem.

It is an object of the present invention to provide an improved method and apparatus for resistance spot welding readily oxidisable light metals and their alloys which provides enhanced electrode life.

According to one aspect of the present invention there is provided a method of resistance spot welding together workpieces of readily oxidisable light metals or light metal alloys using internally liquid cooled copper or copper alloy electrodes comprising immersing in a quantity of continuously changing noninflammable cooling liquid the regions including both electrode/workpiece interfaces during passage of welding current between the electrodes.

The invention also provides resistance spot welding apparatus having an opposed pair of internally water cooled electrodes of copper or a copper alloy and means for dispensing a liquid at or adjacent the operative tip of each electrode. Preferably each electrode includes a passage communicating between an internally water cooled part thereof and the exterior of the electrode adjacent its operative tip.

One embodiment of the invention will now be described by way of example with reference to the single figure of the accompanying drawing which shows two electrode tips embracing a pair of aluminium sheet work parts.

Referring to the drawing a pair of copper or copper alloy electrodes 1 and 2 are assumed to be mounted in the jaws of a spot welding apparatus (not shown) to be movable towards and away from each other in conventional manner and to squeeze together two workpieces 3 and 4 of sheet aluminium or aluminium alloy at positions it is desired to spot weld these work parts together. The electrodes are internally bored at 5 and 6 to be connected in known manner with means for circulating cooling water 7 under pressure therethrough.

Each electrode is shown tapered at 8 to provide a reduced diameter tip 9 in contact with the sheets 3 and 4 and the electrodes are formed with a bore 10 parallel with their axes and communicating between the bores 5 and 6 and their tapered faces 8. Tubular brass plugs 11 are disposed in the bores 10 as a press fit with their ends short of the ends of the bores 10. If desired one or both of the electrodes could have rounded tips, for example, of 2 inch diameter spherical radius for an electrode of 3/4 inch diameter.

In a typical arrangement the bores 10 may be 3.2 mm in diameter and the internal diameter of the plugs 11 may be 0.3 mm.

In operation a fine jet of water is emitted adjacent each electrode tip 9 so that when the tips are squeezing the workpieces 3 and 4 together at say 700p.s.i as shown in the drawing, and welding current is passed between the electrodes, and forging pressure at say 2000p.s.i. is applied between the electrodes, globules of water 12 will surround these electrode tips. The globules continuously disperse over the outer surfaces of the workpieces but are constantly replenished through the plugs 11.

Thus during the welding operation each electrode tip 9 and the regions of the workpieces 3 and 4 adjacent these tips are immersed in a quantity of continuously changing cooling water.

It has been found that the water cooling of the electrode tip workpiece interfaces 13 and the regions adjacent thereto enables a considerable increase in the number of welds to be made with a pair of electrodes. Typically this increase may be from about 800 to about 2000 welds although in certain adverse conditions an increase from 100 to 2000 welds has been achieved. This result is believed to be directly attributable to a reduction in temperature of the electrode tip/workpiece interfaces. The mean temperature from a number of tests has been found to be 1 70 C compared with 265 C with uncooled electrode tip/workpiece interfaces.

Although it is convenient to provide the cooling water from the interior of each electrode through the plugs 11 it will be understood that suitable tubing located externally of the electrodes and terminating in the region of the electrode tips could be used. Such tubing could be connected to the same source of water supply as the interior of the electrodes and would enable conventional electrodes to be employed while retaining the benefit of the present invention. Water flow rates have not been found to be critical so long as the globules 12 are maintained. However flow rates of 50 to 250 ml/min. have been found suitable using truncated cone electrode tips (as shown in the drawing) of 5mm diameter.

The desired flow rate may be obtained by suitably choosing the internal diameter of the plugs 11 relative to the water pressure used.

It is normally preferred to use water as the cooling liquid but other non-inflammable liquids may also be used. Small amounts of various ingredients may be added to the cooling liquid for example degreasing agents, wetting agents corrosion inhibitors and soluble oils. The liquid will normally be used at ambient temperature but it may be heated or cooled before use according to the nature of the workpieces being welded together.

It is preferred that the cooling liquid is arranged to flow only when each spot welding operation is being performed. Thus the flow of liquid may begin after the electrodes have been positioned at a predetermined distance apart before current flow begins and may end when the forging pressure between the electrodes is relieved. Control of liquid flow in the case where it is supplied through a tube externally of the electrode may be by a simple switched valve and in the case of the construction described in connection with the drawing relief of pressure within the electrodes will be sufficient to prevent significant passage of liquid through the plugs 11.

Claims

1. A method of resistance spot welding together workpieces of readily oxidisable light metals or light metal alloys using internally liquid cooled copper or copper alloy electrodes comprising immersing in a quantity of continuously changing non-inflammable cooling liquid each electrode tip and the adjacent workpiece surface during passage of welding current between the electrodes.

2. A method according to claim 1 in which said immersion occurs throughout the period of engagement between the electrodes and the workpieces.

3. A method according to claim 1 (or claim 2) in which said liquid is supplied from the interior of each electrode.

4. Resistance spot welding apparatus having an opposed pair of internally liquid cooled electrodes of copper or a copper alloy and means for dispensing cooling liquid at or adjacent the operative tip of each electrode.

5. Apparatus according to claim 4 in which each electrode includes a passage communicating between an internally liquid cooled part thereof and the exterior of the electrode adjacent its operative tip.

6. Apparatus according to claim 3 or claim 4 comprising means for interrupting or reducing the flow of cooling liquid except during a welding operation.

7. Resistance spot welding apparatus substantially as herein described with reference to a single figure of the accompanying drawing.

8. A method of resistance spot welding substantially as herein described.