GB2234920A

GB2234920A - Forming an erosion resistant tip on an electrode

Info

Publication number: GB2234920A
Application number: GB8918361A
Authority: GB
Inventors: Terence John Eves; Jeffrey William Margetts
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1989-08-11
Filing date: 1989-08-11
Publication date: 1991-02-20
Also published as: EP0491732A1; US5179313A; DE69024496T2; EP0491732B1; GB8918361D0; WO1991002393A1; DE69024496D1

Description

1 1 Forming an erosion resistant tip on an electrode This invention

relates to the formation of an erosion resistant tip on an electrode, for example on one or both electrodes of a spark plug for use in an internal combustion engine; to an electrode having a tip formed by this process and to a spark plug in which either one or both electrodes are provided with tips formed in accordance with this process.

In order to achieve greater spark plug life and/or to allow the area of the electrode to be reduced to produce more efficient ignition, it is desirable to place at the electrode tips an erosion resistant material. One suitable material is platinum, but a-ny material which is more resistant to chemical erosion than Inconel may be used.

Inconel is the conventional material used for spark plug electrodes and is a Registered Trade mark.

It is known to apply the platinum or other erosion resistant material using a disc which is welded onto the respective base electrodes. However these discs are necessarily of small size and therefore difficult to handle. Furthermore it has proved difficult to ensure the necessary penetration of the weld metal into the substrate to ensure that the tip remains in place under service conditions.

According to the present 'invention, there is provided a process for applying an erosion resistant tip to an electrode, wherein the erosion resistant material is supplied in the form of a continuous wire, the end of the wire is welded to the electrode, the wire is severed at a predetermined distance from the weld to leave a pellet welded to the electrode, a coining operation is carried out to flatten the pellet onto the electrode and finally a second welding operation is carried out during which pressure is applied to the pellet to force the pellet into the material of the electrode.

The welding operations are preferably carried out by electrical resistance welding. The first welding operation is preferably carried out under conditions such that the weld formed is just sufficient to retain the wire secured to the electrode whilst the end of the wire is severed to separate the pellet f rom the remainder of the wire and during a subsequent coining operation.

The leading end of the wire is preferably pointed or tapered so as to f orm a contact area with the electrode which is smaller than the crosssectional area of the wire. This can normally be accomplished by leaving the end of the wire "as shearedll.

It is advantageous for the f irst welding operation to produce a minimum welded area, because when the second welding operation takes place the flow path for the welding current will be smaller, thus producing a higher temperature which allows some melting of the electrode so that the pellet can be pressed into the material of the electrode.

Where the electrode is the centre electrode of a spark plug, the electrode is preferably machined in a subsequent stage so that the exposed end of the electrode is entirely covered by the erosion resistant material.

Where the electrode is the side electrode of a spark plug, the tip material preferably lies flush with the electrode material following the second welding operation.

j According to a second aspect of the invention, there is provided an electrode to which an erosion resistant tip has been applied by the method set forth above. The electrode may be the centre electrode or the side electrode of a spark plug, and the invention extends to a spark plug which has a so- formed electrode tip on either or both electrodes.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 to 5 show sequential stages in the process in accordance with the invention; Figures 6 and 7 show the f inal products in the form of, respectively, a centre electrode and a side electrode of a spark plug for an internal combustion engine; and Figure 8 is a photographic representation of a centre electrode in accordance with the invention, before machining.

Figure 1 shows an electrode 10 of Inconel material and a p] atinum, wire 12 which is fed onto the end of the electrode 10, as indicated by an arrow 14, by a welding head 16. The leading tip of the wire 12 is of a somewhat pointed bliape, resulting from a previous cropping operation.

Once the tip of the wire 12 is in contact with the electrode 10, a resistance welding operation takes place to reach the position of Figure 2, where the pointed end of the wire has spread on and welded to the electrode. At this point all that is necessary is that the wire be "tacked" to the electrode and the strength of the connection between the wire and the electrode need only be "A sufficient to withstand any forces applied as a result of the two following steps-shown in Figures 3 and 4.

Figure 3 shows the wire 12 being cropped above the electrode so as to leave a predetermined amount of material in the form of a pellet 20 attached to the electrode 10.

In the next stage shown in Figure 4, a coining operation is carried out in which the platinum pellet 20 is hit by a tool 22 so that it is flattened against the electrode. It will be seen from Figure 4 that the platinum remains welded 10 to the electrode at a centre portion 24, but that the outer diameter of the pellet is not welded. In practical terms however the outer parts of the pellet will be in close contact with the electrode, and in this respect the drawing is shown with the annular gap between the electrode and the is platinum somewhat exaggerated.

In a final stage shown in Figure 5, a second resistance welding operation takes place accompanied by the use of a substantial downward pressure on the pellet 20 which produces some local melting of the electrode to produce a 20 structure as shown in Figures 5 and 7.

When the electrode is to be used as a centre electrode. a subsequent machining operation takes place in which the shoulders of the Inconel electrode which have not been covered with platinum are removed to leave an electrode 25 which has a tip entirely of platinum, as shown in Figure 6.

Where the electrode is to be used as a side electrode as shown in Figure 7, then it is desirable that the pellet of platinum material be pressed into the electrode material so that the surf ace 26 of the pellet is f lush with the surf ace 30 of the electrode.

n The process described and shown here provides all the known advantages of the use of an erosion resistant tip on a spark plug electrode, but also provides a relatively simple but effective manufacturing process by which the tip can be reliably and permanently attached to the end of the electrode.

In the second welding operation which is where the permanent attachment of the platinum to the electrode takes place the current applied may be 1600 amps for two to three 10 cycles. The pressure applied by the coining tool 22 may be 40 to 50 psi. These figures are for a wire diameter of 0. 9mm which has been cropped to give a platinum pellet weighing from 8 to 12 mg.

After the welding current has ceased, a weld force continues to be imposed for a certain length of time, and this weld force can amount to about 60 lbs.

A

Claims

1. A process for applying an erosion resistant tip to an electrode, wherein the erosion resistant material is supplied in the form of a continuous wire, the end of the wire is welded to the electrode, the wire is severed at a predetermined distance from the weld to leave a pellet welded to the electrode, a coining operation is carried out to flatten the pellet onto the electrode and finally a second welding operation is carried out during which pressure is applied to the pellet to force the pellet into the material of the electrode.

2. A process as claimed in Claim 1, wherein the welding operations are carried out by electrical resistance welding.

3. A process as claimed in Claim 1 or Claim 2, wherein the first welding operation is carried out under conditions such that the weld formed is just sufficient to retain the wire secured to the electrode whilst the end of the wire is severed to separate the pellet from the remainder of the wire and during a subsequent coining operation.

4. A process as claimed in any preceding claim, wherein the leading end of the wire is pointed or tapered so as to form a contact area with the electrode which is smaller than the cross-sectional area of the wire.

5. A process as claimed in any preceding claim, wherein the electrode is the centre electrode of a spark plug, and the electrode is machined in a subsequent stage so that the exposed end of the electrode is entirely covered by the erosion resistant material.

i i 1

6. A process as claimed in any one of Claims 1 to 4, wherein the electrode is the side electrode of a spark plug, the erosion resistant material lies flush with the electrode material following the second welding operation.

7. An electrode to which an erosion resistant tip has been applied by a process as claimed in any preceding claim.

8. A spark plug wherein either or both of the centre electrode or the side electrode have an erosion resistant tip which has been applied by the process as claimed in any one of Claims 1 to 6.

9. A process for applying an erosion resistant tip to an electrode, substantially as herein described with reference to the accompanying drawings.

is

10. A spark plug substantially as herein described with reference to the accompanying drawings.

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