WO2009034318A1

WO2009034318A1 - Ignition device electrodes, and manufacture thereof

Info

Publication number: WO2009034318A1
Application number: PCT/GB2008/003067
Authority: WO
Inventors: David Hughes
Original assignee: Spelectrode Limited
Priority date: 2007-09-14
Filing date: 2008-09-11
Publication date: 2009-03-19
Also published as: GB0717959D0; GB2452784A; GB0720107D0

Abstract

Iridium-tipped electrodes (such as spark plug electrodes) are made by providing an ignition device electrode having a free end; abutting the free end of the electrode to a first end of an axially symmetrical iridium or iridium alloy tip aligned with the longitudinal axis; friction welding the tip to the free end of the electrode; and machining away material from the free end so as to produce a tapered portion of the free end, which tapered portion tapers outwardly with spacing from the free end of the electrode.

Description

Ignition device electrodes, and manufacture thereof

The present invention concerns spark plugs and other ignition devices suitable for use in internal combustion engines (such as automotive engines) and, more particularly, to such ignition devices having noble metal firing tips, and also methods of manufacturing the same. As used herein, the term "ignition device" means spark plugs, igniters, and other such devices that are used to initiate the combustion of a gas or fuel.

BACKGROUND OF THE INVENTION

Ignition devices such as spark plugs with noble metal tips, for example of iridium or iridium-based alloys, have been used in the industry, as such tips may have increased erosion resistance relative to conventional tips.

Iridium and iridium-based alloys typically have a relatively high melting point and are resistant to spark erosion relative to the conventional tip. Iridium itself has a melting temperature of about 245O⁰C and a typical hardness of about 500 HV (Vickers hardness scale), compared to about 177O⁰C and 200HV for work-hardened platinum, and about 1450 ⁰C and 150HV for nickel. It is also the densest metal known and highly resistant to corrosion.

Iridium and high temperature iridium-based alloys have therefore relatively recently been developed for use as tips for electrodes of ignition devices in order to increase the erosion resistance of the firing surfaces of the electrodes.

Because, as indicated, iridium is a hard precious metal alloy with very high melting temperature, conventional resistance welding techniques cannot be used to weld iridium tips to ignition device electrodes, which are typically of nickel or nickel alloys.

Processes have therefore been developed in which an iridium tip is welded to a spark plug electrode by laser welding. In these processes an iridium tip is laser welded to the electrode, by focussing a laser beam having a high energy density on a junction of an iridium metal tip and the electrode, the latter as indicated being of nickel or nickel alloy. Both of the iridium and the metal of the electrode are melted by the high density laser beam and make a molten bond at the junction. Such a process of laser welding an iridium-based tip to a spark plug electrode is described in, for example,

US Patent No. 6846214. Iridium or its high temperature alloy is typically used in the form of a pad or rivet that is laser welded or otherwise metallurgically bonded in such known processes to the centre and ground electrodes on either side of a spark gap. There are, however, known disadvantages to the use of certain iridium-based alloys, including the difficulty in bonding that material to the electrodes, and oxidative volatilization of the alloy at higher temperatures.

OBJECT OF THE INVENTION

The present invention seeks to alleviate problems associated with the prior art methods of bonding iridium tips to ignition devices.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a method of manufacturing an iridium-tipped ignition device electrode, which comprises providing an ignition device electrode having a free end; abutting the free end of the electrode to a first end of an axially symmetrical iridium or iridium alloy tip aligned with the longitudinal axis; friction welding the tip to the free end of the electrode; and machining away material from the free end so as to produce a tapered portion of the free end, which tapered portion tapers outwardly with spacing from the free end of the electrode.

The resulting tipped electrode can then be assembled into a housing of a spark plug (or similar ignition device), in generally conventional manner; the present invention therefore encompasses an ignition device such as a spark plug, which device includes an ignition device electrode manufactured according to the first aspect of the invention, or including an ignition device electrode according to the second or third aspect of the invention, as will be described herein.

When a high temperature iridium alloy is used instead of iridium, the alloy should contain in excess of 50% iridium, preferably more than 80% iridium, more preferably at least 90% iridium. When other alloying elements are present, they may, for example, be elements such as rhodium and/or rare earth elements such as yttrium (which provides enhanced corrosion protection).

The ignition device electrode is typically of nickel or nickel alloy, as is known in the art. The iridium-tipped electrode may be incorporated in a manufactured spark plug or other ignition device by methods known in the art for noble metal-tipped spark plug electrodes.

The material of the electrode, which is, as indicated above, generally of nickel or a nickel alloy, is preferably machined away by a conventional grinding process. This may be done in such a way that the tip is untouched by the machining, unless it is also wished to grind down the diameter of the tip (for example, to avoid using smaller diameter wire stock). The machining of the electrode is preferably such as to produce an outwardly tapered frustoconical portion of the electrode.

The outward tapering of the tapered portion is preferably smooth and free of ribs, corrugations, inclusions or other discontinuities. Furthermore the tapered portion is preferably free of any shoulders or other sharp changes of diameter in longitudinal positions between opposed ends of the tip; any such shoulders are preferably circumferential and distal from the free end.

The free end may be a generally planar face; alternatively, it may include a recess shaped and dimensioned to snugly receive a free end of the tip. The end of the tip, and the recess where present, typically each have a diameter of about 1 mm or less, such as about 0.8 mm..

The electrode according to the invention is preferably the centre electrode of the spark plug, sometimes known as the centre wire. It is preferred that when the electrode is to be a centre electrode, that both of the components, namely the electrode and the tip portion, have a longitudinal axis and axial symmetry. In some embodiments of the invention, the ground electrode may also have an iridium or iridium alloy tip friction welded thereto according to the invention.

According to a second aspect of the invention, there is provided an ignition device as nickel or a nickel alloy), and an axially symmetrical tip portion of an iridium or iridium alloy, the tip portion being friction welded to a free end of the end portion, typically with the axis of the tip portion aligned with a longitudinal axis of the end portion, the end portion being machined so as to taper outwardly with spacing from the free end of the end portion.

According to a third aspect of the invention, there is provided an ignition device electrode comprising: at least an end portion of a first metal (such as nickel or a nickel alloy) and a tip portion of iridium or an iridium alloy, the end portion having uniform grains of the first metal in a first zone spaced from the tip portion, and a heat-affected second zone having flow patterns of the first metal between the first zone and the tip portion, and abutting to the tip portion.

The heat-affected zone which has flow patterns of the first metal, and the correspondingly substantially unaffected iridium or iridium alloy in the tip portion, are characteristic of friction welding, such as that achieved in the method according to the first aspect of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The tip portion is preferably friction welded to the end portion of the electrode in a method which comprises effecting rotation of the tip portion relative to end portion, or vice versa. Such relative rotation is about the axis of the tip portion, either by rotating the tip portion or the end portion, or rotating both in opposing directions..

The tip portion may be friction welded to the free end in a process known as continuous drive friction welding. In this process, the welding is effected by pressing the free end of a stationary electrode against a continuously rotating tip member, or vice versa. The sequence of such a continuous drive friction welding process is typically as follows:

The electrode is clamped in a friction welding machine, and the tip portion is clamped in a rotating chuck mounted to an hydraulically, pneumatically or electromechanically driven carriage. The tip portion is rotated to the desired speed, and as the rotational speed is maintained, a slide brings the tip into contact with the free end of the electrode under an applied pressure (friction force) to generate time (or burn-off length) until the interface is sufficiently heated and the two pieces are welded together.

Alternatively, the tip portion may be friction welded to the free end in a process known as inertia friction welding, which uses the stored kinetic energy of a freely- rotating flywheel as the energy source. In this process, the tip portion is typically fixed to the flywheel axis, and is forced under pressure against the free end of the electrode. Heat is generated at the contacting surfaces to cause a welded joint to form. The sequence of operations in such an inertia friction welding process is typically as follows.

The electrode is clamped in the friction welding machine, and the tip portion in a rotating chuck, mounted to an hydraulically actuated tailstock slide; a drive motor accelerates rotation of a flywheel spindle assembly to a predetermined speed, and then the drive motor is disengaged. The tip portion and the electrode are bought together under an axial force (friction force) converting the kinetic energy of the flywheel to frictional heat at the weld interface.

In the friction welding method used according to the invention, it is preferred to effect rotation of the tip portion relative to the end portion under substantially constant pressure; when rotational drive is ceased it is preferred to increase the pressure until such time as the rotation stops. The rotational speed in the friction welding should be at least 30000 rpm, especially more than 40000 rpm. Rotation is such that the electrode is taken very close to the solidus temperature, but not to a temperature which would involve liquefaction of the electrode (which would be disadvantageous as that would stop pressure contact and result in loss of frictional heating.

It is an advantageous feature of the invention that there need be no preheating of either the electrode or the tip prior to friction welding.

After the tip has been welded to the electrode, the electrode is machined away to form a tapered portion of the electrode which surrounds the tip such that the tip has exposed side walls and end face; the end face is in use intended to generate a spark- in the sparking plug. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a spark plug with a tip produced according to the invention;

Figures 2a, b and c show in schematic views stages in a first method according to the invention of providing an iridium tip on a spark plug;

Figures 3a, b and c show in schematic views stages in a second method according to the invention of providing an iridium tip on a spark plug; Figures 4a, b and c show in schematic views stages in a third method according to the invention of providing an iridium tip on a spark plug;

Figure 5 is a flow chart showing the sequence of operations in an exemplary method according to the invention; and

Figure 6 is a micrograph showing metallurgical features of a spark plug electrode having a friction welded iridium tip according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to Figure 1, a spark plug 1 is shown having a metal housing 2 with a threaded external surface 3. An axially extending electrode (typically of nickel or a nickel alloy) has a free end 5 supported by an insulator 4, and a tip 6 welded to the free end 5. A side electrode 7 is electrically coupled to and supported by the metal housing 2, so as to define a gap between the side electrode 7 and the tip 6; this spark plug "gap" should remain essentially constant over the life of the spark plug.

The tip 6 is of iridium or an iridium alloy, which offers high resistance to spark erosion in the presence of combustive gases present in the combustion chamber of an internal combustion engine.

Referring to Figures 2a and 2b, there is shown an electrode 24 having a flat free end 25; an iridium alloy tip 26 is friction welded to the free end in such a way that the pressure imparted during friction welding causes it to slightly penetrate into the body of the electrode. Referring to Figure 2c, the outside of the electrode is machined away to form a frustoconical part 27 with the tip welded to the end thereof. The frustoconical part has a circumferential shoulder 28 distal from the free end of the free end 25 and the tip 26. Figures 3a to 3c are broadly similar, and like parts are denoted by like reference numerals. The major difference between the embodiment of Figures 3a to 3c and the embodiment of Figures 2a to 2c is that the electrode has a recess 29 shaped and dimensioned to snugly receive the tip 26.

Figure 4a to 4c are broadly similar, and like parts are denoted by like reference numerals. The major difference between the embodiment of Figures 4a to 4c and the embodiment of Figures 3a to 3c is that the except that the electrode itself has a stepped end 30 having a further shoulder 32 distal from the tip 26. The shoulder 27 on the frustoconical part substantially surrounds the captive end 34 of the tip 26

Referring to Figure 5, this shows an exemplary sequence of operations in the method according to the invention.

Firstly, a suitable tip portion is selected 40 and the tip portion is placed 42 in a fixture. The tip portion is then aligned 44 with an electrode; the tip portion is caused to rotate 46 relative to the electrode under constant pressure, using a rotational drive. The rotational drive is then stopped 48 and at this point pressure is increased further until rotation stops.

The welded components are then allowed to cool 50, and then machined at 52 to produce a frustoconical external surface of the electrode. The resulting tipped electrode is then assembled 54 into a conventional spark plug housing.

Referring to Figures 6 a and b, which are respective micrographs of sections of the same item on differing scales, there is shown the junction between a tip portion 126 of a spark plug electrode and the body of the electrode 129. The tip portion 126 is of iridium and the body 129 is of nickel. The tip portion shown has a diameter of about 1mm; there is a heat-affected zone A of the nickel about 0.1mm in depth abutting the retained or captive end 130 of tip portion 126. This heat affected zone is characterized by flow patterns indicating partial melting of the nickel. The adjacent zone of the iridium tip portion shows no such flow patterns.

Distal from the tip portion is a zone B of uniform nickel grains, substantially unaffected by heat produced during the welding process.

Claims

1. A method of manufacturing an ignition device electrode tipped with iridium or an iridium alloy, which method comprises providing a spark plug electrode having a free end; abutting the free end of the electrode to a first end of an axially symmetrical tip of iridium or iridium alloy, aligned with the longitudinal axis; friction welding the tip to the free end of the electrode; and machining away material from the free end so as to produce a tapered portion of the free end, which tapered portion tapers outwardly with spacing from the free end of the electrode.

2. A method according to claim 1 , wherein said machining is such as to produce an outwardly tapered frustoconical portion of the electrode.

3. A method according to claim 1 or 2, wherein the iridium alloy contains at least 90% by weight iridium.

4. A method according to any of claims 1 to 3, in which the electrode is of nickel or a nickel alloy.

5. A method according to any of claims 1 to 4, in which the electrode is machined away such that the tip is untouched by the machining.

6. A method according to any of claims 1 to 5, in which the outward tapering of the tapered portion is smooth and free of ribs, corrugations, inclusions or other discontinuities.

7. A method according to claim 6, in which the tapered portion is free of any shoulders or other sharp changes of diameter in longitudinal positions between opposed ends of the tip.

8. A method according to any of claims 1 to 7, in which the free end of the electrode is a generally planar face; or includes a recess shaped and dimensioned to snugly receive an end of the tip.

9. A method according to any of claims 1 to 8, in which the resulting tipped electrode is a centre electrode of a spark plug.

10. An ignition device electrode comprising: at least an end portion of a first metal, and an axially symmetrical tip portion of an iridium or iridium alloy, the tip portion being friction welded to a free end of the end portion, the end portion being machined so as to taper outwardly with spacing from the free end of the end portion.

11. An ignition device electrode comprising: at least an end portion of a first metal and a tip portion of an iridium or iridium alloy, the end portion having uniform grains of the first metal in a first zone spaced from the tip portion, and a heat- affected second zone having flow patterns of the first metal between the first zone and the tip portion, and abutting to the tip portion.

12. An electrode according to claim 10 or 11 , when manufactured according to any of claims 1 to 10.

13. An ignition device such as a spark plug, which includes an electrode according to claim 10 or11, or an electrode manufactured according to any of claims 1 to 10.