SPARK PLUG ELECTRODE AND ITS METHOD OF MANUFACTURE

Abstract: In a method of making a spark plug centre electrode, a billet (12) of a first metal which has good thermal conductiv­ ity is placed within a cup-shaped billet (10) of a second metal having good corrosion resistance, the first metal being melted within the cup-shaped billet of the second metal and, subsequent to cooling, the composite billet is extruded through an extrusion die (17) to form a composite centre electrode in which a core of the first metal is enclosed substantially entirely within a sheath of the second metal.

Description: Spark Plug Electrode and its Method of Manufacture This invention relates to a method of making a spark plug centre electrode having an inner core of a metal which has good thermal conductivity relative to the metal of an outer sheath fully enclosing the inner core, the outer sheath being formed of a corrosion resistant material. It is known to provide a spark plug electrode in which an inner core of a metal having good thermal conductivity, for example copper, is partly encased within a sheath of a metal having good corrosion resistance properties, such as for example nickel or a nickel alloy. The purpose of using a composite electrode of this type is that, while good heat conduction is required of the electrode in order to prevent undue "burning", it is also necessary to provide for a hardwearing portion of the electrode at and around the point where sparking occurs. Thus, the advantage of a composite electrode as described is that a material suitable for good heat conduction is used in conjunction with a material having suitably hard-wearing properties. For example, British Patent 754,591 discloses a method of making a spark plug electrode in which a billet of copper is placed on top of a billet of nickel, the composite billet then being extruded through a suitable die to form a spark plug electrode having at one end a core of metal having good thermal conductivity surrounded by a sleeve of the corrosion resistant metal. In our earlier British patent application number 7942617 there is shown a method of making a composite centre electrode for a spark plug in which an outer sheath of a nickel alloy has an inner core of copper, the improvement being in the method of bonding the copper to thy nickel alloy prior to extrusion. It is an object of the present invention to provide a composite centre electrode for a spark plug and a method of making such, which can be easily connected to a wire extending through the spark plug body by relatively simple welding techniques of known type, and in which the volume of copper within the electrode is carefully controlled. A further object of the invention is to provide a centre electrode for a spark plug which can be easily formed at one end to provide a shoulder for mounting in the body of a spark plug. According to one aspect of the invention there is provided a method of making a centre electrode for a spark plug of the type having a core of a first metal within a sheath of a second metal, the first metal having a thermal conductivity higher than that of the second metal and a melting point lower than that of the second metal, the second metal being resistive to the corrosive environment in which the electrode works, characterised by the steps of inserting a billet of the first metal within a cup-shaped billet of the second metal; heating the two billets together to a temperature at or above the melting point of the first metal but below the melting point of the second metal; cooling the two metals to a temperature below the melting point of the first metal to form a composite billet wherein the first and second metals are united at their interfaces and extruding the composite billet so as to form an elongate rod-shaped extrusion comprising a core of the first metal enclosed substantially entirely within a sheath of the second metal. The heating of the first and second metals may conveniently take place in a reducing atmosphere within a radiant furnace, the cup-shaped billet of the second metal being held in a suitably shaped black body so that conduction of heat from the radiant furnace occurs through the black body in order to melt the billet of said first contained within the cup-shaped billet of the second metal. The melting of the billet of said first metal may be just sufficient to wet the interfaces thereof with the cup-shaped second metal or the whole of the first metal may be melted but, in any event, the first and second metals become united at their interfaces to form a unitary composite billet after cooling. The extrusion of the composite billet can occur with a single blow to form an elongate rod-shaped extrusion having a core of the first metal enclosed substantially entirely within a sheath of the second metal, the side-wall of which is generally thicker in the regions above the end of the copper core than in the region adjacent to this core to provide a suitable volume of the nickel alloy at one end for forming a shoulder thereon. The composite extrusion may be cropped off at one end and the shoulder formed at that end in order to provide mounting means for mounting in the ceramic insulator of a spark plug. The composite electrode may conveniently be welded to a wire connector by electric butt welding in a known manner, or may be friction welded. The invention also provides a centre electrode for a spark plug of the type having a core of a first metal within a sheath of a second metal, the first metal having a thermal conductivity higher than that of the second metal and a melting point lower than that of the second metal, the second metal being resistive to the corrosive environment in which the electrode works, whenever produced by the method described in any of the preceding four paragraphs. These and other embodiments of the invention will become apparent with reference to the accompanying drawings in which, Figure 1 is a cross-section through the cup-shaped billet and a further billet contained within the cup. Figure 2 shows a part-sectional view of the cup-shaped billet contained within a black body, Figure 3 shows a cross-section of the cup-shaped billet with the copper billet melted inside. Figure 4 is a cross-sectional view of the composite billet contained within an extrusion die. Figure 5 is a sectional view of the extruded composite billet and, Figure 6 is a sectional view of the composite billet having a shoulder portion, and Figure 7 is a view of the composite electrode of Figure 6 welded to a wire element. Referring to the drawings, in Figure 1 a cup shaped billet 10 of a heat resistant nickel alloy is of substantially cylindrical shape and has a cylindrical cavity 11 in which has been inserted a copper billet 12, the copper billet having been cropped-off from a length of copper wire (not shown). The cup-shaped billet 10 has a frusto-conical portion 13 at one end thereof comprised solely of nickel alloy. In Figure 2 is shown a part-sectional view of a rectangular black body 14 of carbon, having a plurality of apertures 15 of cross-sectional shape corresponding substantially to the cross-sectional shape of billet 10. The copper billet 12 is shown contained within a cup-shaped billet 10. In Figure 3, the copper billet 12 has been melted within the cylindrical part 11 of cup-shaped billet 10 by conduction of heat from a radiant furnace (not shown). The meniscus 16 is of concave section, the peripheral edge of which is joined to the inner surface of billet 10. Referring to Figure 4, the cup-shaped billet 10 is shown contained within an extrusion die 17 having a cylindrical section 18 of internal diameter corresponding substantially to the external diameter of the billet 10. A converging portion 19 corresponds substantially to the frusto-conical portion of billet 10. An extrusion punch 20 having a frusto-. conical end 21 is capable of axial movement downwardly in order to extrude the billet 10 containing the melted copper billet 12 through an extrusion nozzle 22 provided with a clearance 23 of cylindrical shape. In Figure 5 is shown the extruded billet having an inner core 12 of copper and an outer sheath 10 of nickel alloy substantially entirely enclosing the inner core 12. A flanged head 24 comprised solely of the nickel alloy is formed at one end of the extruded billet 10 which meets at junction 25 to substantially enclose the core 12. The other end of the extruded billet 10 is comprised solely of the nickel alloy, the nickel alloy of this end corresponding generally to the nickel alloy contained within the frustoconical portion 19 of the billet 10 prior to extrusion. In Figure 6 the flanged portion 24 of the extrusion has been cropped-off and a shoulder 26 has been formed at one end in order to facilitate mounting within the ceramic insulator of a spark plug. The copper core 12 is still contained substantially entirely within the nickel alloy of billet 10. In Figure 7 a wire element 27 has been connected at 28 to the extrusion adjacent shoulder portion 26, which is comprised entirely of nickel alloy, by electric butt welding in a known way. In a method of making a centre electrode for a spark plug according to the invention, the cup-shaped billet 10 containing the copper billet 12 is placed within a correspondingly shaped recess in the black body 14, which is of carbon, and heated in a radiant furnace having a reducing atmosphere to a temperature at or above the melting point of the copper billet so as to cause melting of the copper billet 12 within the cup-shaped billet 10 to form a composite billet as shown in Figure 3. The composite billet 10, 12 is then placed in the extrusion press as shown in Figure 4 and extruded, by pressure exerted on the billet via punch 20, through nozzle 22 to the shape of a composite electrode as shown in Figure 5. It will be noted that this composite electrode consists of a cigar-shaped inner core 12 of copper and an outer sheath 10 of nickel alloy which substantially entirely encloses the inner core 12 by virtue of part of the metal of cup-shaped billet 10 meeting at 25 subsequent to extrusion. The flanged portion 24 is cropped-off by a cropping machine and a shoulder portion 26 is then formed on this cropped-off end of the billet 10, the shoulder 26 being comprised solely of the nickel alloy. Since the copper core 12 is substantially entirely enclosed within the nickel alloy sheath 10 a wire element 27 can then be butt welded to the electrode in any known manner, for example by electric butt welding or friction welding to form a welded junction at 28. It will be apparent from the above that close control of the amount of the first metal contained within the cup-shaped billet of the second metal can be achieved since none of the first metal is extruded out of the sheath of second metal. Thus, a centre electrode is provided according to the invention having closely controlled thermal characteristics which can be simply manufactured and yet is suitable for connection with a wire element as described above. It will be further apparent that electrodes of different lengths and with different amounts of copper core can be produced by appropriate variations in the volume of copper placed in the cup; and the ratio of cross section of the copper and nickel alloy composite is determined by the ratio of cylindrical bore to outside diameter of the nickel alloy cup shaped billet.

Claims: CLAIMS 1. A method of making a centre electrode for a spark plug of the type having a core of a first metal within a sheath of a second metal, the first metal having a thermal conductivity higher than that of the second metal and a melting point lower than that of the second metal, the second metal being resistive to the corrosive environment in which the electrode works, characterised by the steps of inserting a billet (12) of the first metal within a cup-shaped billet(10) of the second metal; heating the two billets (10 and 12) together to a temperature at or above the melting point of the first metal but below the melting point of the second metal; cooling the two metals to a temperature below the melting point of the first metal to form a composite billet wherein the first and second metals are united at their interfaces and extruding the composite billet so as to form an elongate rod-shaped extrusion comprising a core of the first metal enclosed substantially entirely within a sheath of the second metal. 2. A method according to Claim 1 further characterised in that the heating step comprises heating the two billets (10 and 12) in a reducing atmosphere within a radiant furnace, the cup-shaped billet (10) of the second metal being held in a suitably shaped black body (14) so that conduction of heat from the radiant furnace occurs through the black body in order to melt the billet (12) of the first metal contained within the cup-shaped billet (10) of the second metal. 3. A method according to Claim 2 further characterised in that the melting of the billet (12) of the first metal is just sufficient to wet its interfaces with the cup-shaped billet (10) of the second metal. 4. A method according to Claim 2 further characterised in that the whole of the billet (12) of the first metal is melted within the cup-shaped billet (10) of the second metal. 5. A method according to any one of the preceding claims further characterised in that the extrusion step comprises extruding the composite billet of the first and second metals with a single blow to form the said elongate rod-shaped extrusion having a core of the first metal enclosed substantially entirely within a sheath of the second metal. 6. A method according to any one of the preceding claims further characterised in that the rod-shaped extrusion is cropped off at one end and a shoulder (26) is formed at that end in order to provide mounting means for mounting in the body of a spark plug. 7. A method according to any one of the preceding claims further characterised in that the first metal is copper and that the second metal is a nickel alloy. 8. A centre electrode for a spark plug of the type having a core of a first metal within a sheath of a second metal, the first metal having a thermal conductivity higher than that of the second metal and a melting point lower than that of the second metal, the second metal being resistive to the corrosive environment in which the electrode works characterised in that it is produced by a method according to any one of the preceding claims.

Inventors: HOWARD, KENNETH
Applicants: FLOFORM LIMITED
Application year: 1981
Main classification: B21C23
IPC codes: H01T13, B21C23, H01T21