Abstract:
A spark plug assembly ( 22 ) includes a center electrode ( 34 ) having a high performance metal sleeve ( 50 ) attached at its sparking end. The sleeve ( 50 ) is fitted to a tenon on the end of the center electrode ( 34 ) and fixed in place by a weld line ( 58 ) produced by laser beam pulses ( 56 ). The weld line ( 58 ) is applied by overlapping a plurality of spaced-apart beads in a single, continuous circumscribing line. The sleeve ( 50 ) is permitted to expand and contract under the influence of thermal cycling without constraint except for the fixation weld line ( 58 ). Therefore, the sleeve ( 50 ) does not experience stress build-ups resulting from differing rates of thermal expansion relative to the center electrode ( 34 ), which is preferably made from a nickel or other composition dissimilar to that of the high performance metal sleeve ( 50 ). Various ground electrode ( 30, 60 ) configurations are possible.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to U.S. provisional application entitled LASER WELD OF AN IRIDIUM SLEEVE ONTO CENTER ELECTRODE having Ser. No. 60/721,821 and filed on Sep. 29, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The subject invention relates to a spark plug for an internal combustion engine, furnace, or the like wherein the spark plug includes at least one electrode having a wear-resistant sleeve welded thereto for enhanced durability and longevity.  
         [0004]     2. Related Art  
         [0005]     Within the field of spark plugs, there exists a continuing need to improve the erosion and corrosion resistance and reduce the sparking voltage needed to produce the spark in the gap between center and ground electrodes. To this end, various designs have been proposed using noble and/or precious metal firing tips applied to standard metal electrodes. Typically, the firing tip is pre-formed as a pad, rivet or wire which is later welded onto the end of either the center electrode, the ground electrode, or both.  
         [0006]     Platinum and iridium alloys are two of the noble metals commonly used for these firing tips. Platinum-tungsten alloys have also been used, along with platinum-rhodium alloys and platinum-iridium-tungsten alloys. Other metals and/or alloys are also possible.  
         [0007]     While these and various other noble metal systems typically provide acceptable spark plug performance, particularly with respect to controlling the spark performance and providing spark erosion and chemical corrosion protection, current spark plugs utilizing noble metal tips have well-known performance limitations associated with the relatively small sparking surfaces and with the methods which are used to attach the noble metal components, including various forms of welding. In particular, cyclic thermal stresses in the operating environment, such as those resulting from the mismatch in the thermal expansion coefficients between the electrode tip and the dissimilar base electrode, can decrease service life. Typically, the electrode tip will be fabricated from noble metals and the noble metal alloys mentioned above, whereas the base electrode will be made from nickel, nickel alloy, nickel clad copper, or other commonly used metal. The result of these mismatched thermal coefficients is cracking, thermal fatigue, and various other interaction phenomena that can result in the failure of the welds and, ultimately, of the spark plug itself.  
         [0008]     The condition is particularly significant in the field of industrial power generation, wherein a spark plug may be operated for extended durations at a specified setting. In these types of applications, which are cited merely by way of example, it is desirable to very precisely tune the engine and its fuel supply, together with the ignition system, to obtain the highest possible efficiencies and fuel economies. Erosion and corrosion of the center and ground electrodes can have a profound effect on the efficiency and performance characteristics of such an engine. Accordingly, there is a great need in this field to provide a spark plug having improved erosion and corrosion resistance of the sparking surfaces and related components.  
         [0009]     The prior art has long considered this situation and proposed numerous configurations within which to deploy noble metal components in the spark gap. For example, U.S. Pat. No. 4,904,216 to Kagawa discloses a spark plug having a center electrode fitted with a tubular precious metal sleeve that is attached by resistance welding and then afterward drawn and extruded to a final shape. In another example, U.S. Pat. No. 5,557,158 to Kanao et al., discloses a spark plug including a center electrode that is fitted with a tubular precious metal sleeve. The sleeve is captured on a tenon end and then fixed in position via a cap. In yet another example, U.S. Pat. No. 6,064,144 to Knoll et al., discloses a spark plug wherein a tubular sleeve is fitted to a tenon on the center electrode and retained in position by a compressing cinch. This is followed by a welding or soldering operation.  
         [0010]     Accordingly, it is highly desirable to develop a spark plug having a noble metal firing tip in the form of a sleeve or other configuration applied to the sparking end of the center electrode. However, the prior art attempts have failed to account for potential failure mechanisms associated with the attachment of dissimilar materials to one another over a length, and which materials are subjected to intense thermal cycling. Accordingly, there is a need to develop methods of making spark plugs having improved structures so as to improve spark plug performance and reliability, while also sustaining component integrity in extremely harsh operating environments.  
       SUMMARY OF THE INVENTION  
       [0011]     The subject invention comprises a spark plug assembly for a spark ignited engine, furnace, or the like. The assembly comprises a grounded metallic shell, including a ground electrode. An insulator body is disposed at least partially in the shell. The insulator body has an axial length and a central passage extending axially along its length. An electrically conductive center electrode is disposed in the central passage of the insulator body. The center electrode has an exposed length terminating in a distal tip. The center electrode is made from a first predetermined material composition. A sleeve is disposed about the exposed length of the center electrode and is fabricated from a second material, dissimilar to the first material. A fixation weld line is disposed in a single transverse plane, metallurgically joining the sleeve to the center electrode. As the center electrode and sleeve thermally expand and contract, they do so unencumbered relative to one another along their entire interface length except at the fixation weld. Therefore, differing rates of thermal expansion between the center electrode and the sleeve will not constrict the axial movements of either component. According to this invention, there is far less tendency for the center electrode to develop cracks or thermal fatiguing or other deleterious interaction phenomenon.  
         [0012]     The invention also comprises a method for forming an electrode for a spark plug assembly as used in a spark ignited engine, furnace, or the like. The method comprises the steps of providing a center electrode having an axial length terminating in a distal tip. The method also includes forming a tenon on the center electrode adjacent the distal tip, the tenon having an inset shoulder and an axially extending cheek. A sleeve is provided having a base end and a free end. The method includes sliding the sleeve over the tenon and abutting the base end thereof with the shoulder of the tenon. A laser beam is provided. The method includes moving the laser beam in a relative path along the interface between the base end of the sleeve and the shoulder of the tenon to create a fixation weld line. The method further includes placing the center electrode into service, i.e., in a spark ignited engine, furnace, or the like, with only the fixation weld line joining the center electrode and sleeve so that the center electrode and sleeve are free to thermally expand and contract relative to one another along their entire interface length except at the fixation weld line.  
         [0013]     Accordingly, the subject invention defines the novel assembly and method which overcomes the shortcomings and disadvantages inherent in the prior art designs. Specifically, the subject invention enables a spark plug to operate for extended periods without catastrophic failure due to the avoidance of cracking, thermal fatigue, or other deleterious interaction phenomenon between the center electrode and its high-performance sleeve component. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:  
         [0015]      FIG. 1  is a cross-sectional view of a spark plug according to the subject invention including an exemplary four-prong ground electrode such as typically used in industrial engine applications;  
         [0016]      FIG. 2  is a side elevation view in partial cross-section of the center electrode assembly;  
         [0017]      FIG. 3  is an end view of the noble metal sleeve as fitted to the distal end of the center electrode;  
         [0018]      FIG. 4  is a cross-sectional view taken generally along lines  4 - 4  of  FIG. 3 ;  
         [0019]      FIG. 5  is an enlarged view of the distal end region of the center electrode, including the sleeve welded thereto;  
         [0020]      FIG. 6  is an end view of the center electrode assembly as shown in  FIG. 5 ;  
         [0021]      FIG. 7  is a cross-sectional view taken generally along lines  7 - 7  in  FIG. 6  and depicting the weld zone penetration;  
         [0022]      FIG. 8  is a fragmentary cross-sectional view demonstrating the weld formation in which successive, overlapping, and equally spaced beads are placed along the center line which may be set slightly below the sleeve/shoulder interface;  
         [0023]      FIG. 9  depicts a laser welding set-up for attaching the sleeve to the distal tip of the center electrode so as to achieve a desirable weld formation;  
         [0024]      FIG. 10  is a cross-sectional view of a second embodiment of the invention, wherein an alternative annular ground electrode configuration is used instead of the 4-prong type illustrated in  FIG. 1 ;  
         [0025]      FIG. 11  is a bottom end view taken generally along lines  11 - 11  of  FIG. 10 ;  
         [0026]      FIG. 12  is an enlarged view of the alternative annular ground electrode; and  
         [0027]      FIG. 13  is a side elevation view as taken along lines  13 - 13  of  FIG. 12 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]     Referring to the figures, wherein like numerals indicate like or corresponding parts throughout the several views, a spark plug according to an exemplary embodiment of the subject invention is generally shown at  22  in  FIG. 1 . The spark plug  22  has a conductive metal shell  24  that is typically grounded upon attachment to an engine, furnace, or the like. A non-conductive insulator body  26  is disposed, at least partially, in the shell  24 . The insulator body  26  has an axial length as defined by a longitudinally extending central axis A, which forms a vertical center line for the spark plug assembly  22 . A central passage  28  extends axially through the insulator body  26  and is centered along the central axis A. An electrically conductive ground electrode  30  is connected to the shell  24 , having a free end (or ends as the case may be) in the shape of arms or legs presented at a spark gap. In the embodiment of  FIG. 1 , ground electrode  30  is shown as the so-called 4-prong type, which is used chiefly in industrial engine applications. Alternatively, the traditional single ground wire style may be used, as well as any other type of ground configuration. For example,  FIGS. 10-13  illustrate an alternative, full-annular type ground electrode as will be described in greater detail below.  
         [0029]     The spark plug  22  further includes an upper terminal cap  32  fixed or otherwise retained in the central passage  28  at the top end of the spark plug  22 . The opposite or lower end of the insulator body  26  is fitted with a center electrode assembly, generally indicated at  34 . Interconnecting the upper terminal cap  32  and the center electrode assembly  34  is a conductive spring connector  35 . Of course, this is but one exemplary embodiment of the conductive electrical components contained within the insulator body  26 . Those of skill will appreciate other constructions and arrangements of components so as to achieve a suitable high voltage conducting feature contained within the insulator body  26 . Returning to  FIG. 1  in the embodiment as depicted, a glass seal  36  is provided between the center electrode  34  and the insulator  26  to prevent the escape of combustion gases. The glass seal  36  may be modified to include electrical noise suppression features or other attributes.  
         [0030]     In  FIG. 2 , the center electrode assembly  34  is shown in greater detail, having a main body  38  which can be made from any material, but the preferred embodiment is made of nickel or a nickel alloy. A central flange  40  establishes an upper ledge  42  from which a reduced diameter upper post  44  extends. In this embodiment, the upper post  44  passes through the glass seal  36  and makes physical and electrical contact with the spring  35 . The lower or distal end of the body  38  is machined or otherwise formed in the shape of a round tenon, establishing a shoulder  46  and a cheek  48 . An optional undercut is shown at the intersection of the shoulder  46  and cheek  48 . In an alternative configuration (not shown), the upper post  44  is omitted, and the glass seal  36  is replaced with a fired-in suppressor seal (FISS). An alternative FISS design may provide RFI suppression and form a conductive path between the spring  35  and center electrode assembly  34 .  
         [0031]     A tubular, cylindrical noble metal sleeve  50  is shown in detail in  FIGS. 3 and 4 . The sleeve  50  may be made from pure iridium, an iridium alloy containing rhodium and tungsten, or from other alloying elements. Alternatively, the sleeve  50  may be made from any other precious or noble metal, or alloys thereof, to provide high performance and high erosion and corrosion resistance throughout an extended service life. The inner diameter of the sleeve  50  is sized to allow either a clearance fit or slight interference fit onto the tenon cheek  48  when the internal diameter of the sleeve  50  is at the minimum of its dimensional tolerances and the tenon diameter is at the maximum of its dimensional tolerances.  
         [0032]     Referring again to  FIGS. 2 and 3 , the sleeve  50  is shown including a generally consistent wall thickness extended between a base end  52  and free end  54 . The base end  52  abuts the shoulder  46  of the tenon when installed on the end of the center electrode assembly  34 . The undercut between the shoulder  46  and cheek  48 , if used, will facilitate a good, tight fit of the base end  52  against the shoulder  46 . The axial length of the sleeve  50  is generally equal to the axial length of the cheek  48  such that the free end  54  of the sleeve  50  is disposed in a common, generally transverse, plane with the distal tip of the center electrode  34 . As perhaps best shown in  FIG. 2 , the main body  38  of the center electrode  34  has a major diameter which is generally equal to the major diameter of the sleeve  50 . In practice, however, the wall thickness of the sleeve  50  may be sized slightly smaller than the radial width of the shoulder  46  so that a substantially continuous outer wall surface is presented by the body  38  of the center electrode  34  even in the event of a slight concentricity issue in either the sleeve  50  or the formed tenon. The slightly reduced wall thickness in the sleeve  50  thereby anticipates potential alignment issues so that insertion of the center electrode assembly  34  through the central passage  28  of the insulator body  26  is never challenged. In any event, the thickness of the sleeve  50  is optimized to have sufficient thickness to allow for the electrical erosion expected over the life of the spark plug  22 , but to be thin enough to minimize internal stresses and costs. The sleeve  50  can be manufactured by machining from sheet or rod, or by growth on a carbon rod within an electroplating process, or by any other suitable technique.  
         [0033]     Referring now to  FIGS. 5-9 , the method for attaching the sleeve  50  to the body  38  of the center electrode assembly  34  is shown. The sleeve  50  can be attached by any suitable welding operation after it has been placed over the cheek  48  of the tenon and brought into abutting relationship against the shoulder  46 . Suitable welding techniques include, but are not limited to, laser welding, electron beam welding, and TIG welding, to name but a few.  
         [0034]     The following specifications represent a single exemplary embodiment of the invention. Most or all of the specifications are subject to modification, given changes in equipment, materials, preferences, and other factors. Furthermore, these laser weld parameters have been optimized to increase the penetration and strength of the weld and to reduce splatter on the outside of the finished part. The angle of incidence of the laser beam  56  is nominally perpendicular to the electrode surface, as depicted in  FIG. 9 . The laser beam  56  may be directed 0.004 inches onto the body  38  below the interface between the sleeve  50  and the shoulder  46 . In other words, the center line of the laser beam  56  is aimed 0.004 inches below the shoulder  46 , although other displacements may prove preferable in some situations. Satisfactory results have been found using a laser weld process with the following parameters: 
        Weld energy: 1.6 Joules/pulse        
 
         [0036]     As accomplished, the directed beam of laser light  56  results in a single bead of overlapping weld spots targeted to fuse the sleeve  50  to the body  38 , thereby forming a fixation weld line  58 . The fixation weld line  58  in this configuration can be accomplished if the laser beam  56  is held stationary while the electrode body  38  is held vertically in a collet and rotated for one to four revolutions. Of course, the relative motion between the laser beam  56  and electrode body  38  can alternatively be accomplished by moving the laser while holding the electrode body  38  stationary, or perhaps moving both members at the same time. By following the parameters laid out above, a laser weld of numerous overlapping, regularly spaced beads with a weld bead diameter of approximately 0.02 inches and a weld spacing of approximately 0.008 inches or less can be achieved. This is depicted in  FIG. 8 .  
         [0037]     Only the bottom of the sleeve  50  is welded, i.e., at its base end  52 . The free end  54  of the sleeve  50  is not welded or otherwise affixed to the electrode assembly  34 . This results in an accommodation for differing thermal expansion rates between the body  38  and the sleeve  50 . Therefore, the sleeve  50  is not constricted in its axial direction otherwise than by the fixation weld line  58 . In other words, welding at only one end of the sleeve  50  allows its high performance composition to thermally expand and contract at a different rate to the nickel or other dissimilar composition of the electrode assembly body  38  without building stresses within the sleeve  50 . The completed center electrode assembly  34  is then used in one of various spark plug designs where the spark primarily propagates from the edge of the center electrode rather than from its tip, such as in the 4-prong configuration shown in  FIG. 1  and the annular configuration shown in  FIGS. 10-13 .  
         [0038]     In the embodiment shown in  FIGS. 10-13 , the ground electrode, generally indicated at  60 , is fixed in the lower end of the shell  24  by first resistance welding into a pocket formed in the bottom of the shell  24 , followed by a turnover operation to mechanically lock the ground electrode  60  in an inoperative position. The ground electrode  60  has a noble metal ring  62  that encircles the sleeve  50  on the center electrode  34  with a spark gap being formed in the annular space therebetween. The ring  62  is held in a centric position about the sleeve  50  in hub-like fashion by a frame composed of three spokes  64 . Of course, more or fewer spokes  64  may be used and, indeed, it is even conceivable that in some applications, the frame might be fully annular with no discernable gaps or spokes.  
         [0039]     Numerous methods of forming the ground electrode  60  are contemplated. In one embodiment, the spokes  64  are formed in a separate operation, such as by forging, machining, casting, or the like. Nickel would be a suitable material from which to manufacture the spokes  64 . In like manner, the noble metal ring  62 , which is preferably iridium, can also be separately manufactured, and the two components joined in a later operation, such as by laser welding. However, another possible technique for manufacturing the ground electrode  60  is available. According to this alternative technique, a carbon rod (not shown) is placed in an electro-deposition tank containing an iridium rich (or other noble metal or alloy) bath or an iridium anode. An appropriate electrical differential is established between the carbon rod and the bath (or anode), such that elemental iridium (or other noble metal or alloy) is attached to and evenly deposited about the exterior of the carbon rod to form an iridium shell. Once the iridium shell has achieved sufficient thickness, the rod is removed from the bath and transferred to a new electro-deposition tank in which a nickel rich bath or nickel anode is contained. Again, an electrical potential is established between the rod and the bath (or anode), such that elemental nickel (or other chosen metal) deposits itself about the exterior of the iridium shell, forming a nickel shell. Once the nickel shell has achieved an appropriate thickness, it is removed, cleaned, and machined. Finish operations can include forming scallops along the length of the nickel shell. A slicing operation will then yield individual wafers which eventually are transformed into the ground electrode  60 . At an appropriate stage along the processes, the carbon rod can be removed.  
         [0040]     The purpose for using the sleeve  50  and  62  on the center and ground electrode assembly  34  and  64  is to increase the life of these electrode assemblies, and thus the overall life of the spark plug  22 . The disclosed electrode designs seek to maximize the ground electrode surface area while allowing good breathing of the spark gap, and to maintain a constant ground electrode gap with respect to the cylindrical surface of the center electrode  34 . Therefore, if a continuous ring is not used for the ground electrode, the ground electrodes may be formed so as to have arcuate faces and thereby maintain a constant gap spacing across the entire spark gap.  
         [0041]     The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention. Accordingly the scope of legal protection afforded this invention can only be determined by studying the following claims.