IGNITION DEVICE ELECTRODE COMPOSITION

RELATED APPLICATIONS

[0001] This patent application claims priority to US Provisional Patent Application

Serial No. 60/823,672, filed August 28, 2006 which is hereby incorporated herein by reference in its entirety.

1. Field of the Invention

[0002] This invention generally relates to ignition devices such as spark plugs for internal combustion engines or igniters in gas turbines and jet engines. More specifically, this invention relates such ignition devices having iron or cobalt based electrodes.

2. Related Art

[0003] Electrodes having noble metal firing tips are widely used in ignition devices.

Platinum and indium based alloys are the materials of choice for such firing tips, while a nickel-based alloy is usually the preferred material for the electrode bodies to which these tips are attached.

[0004] Ideal characteristics for the electrode body material are high temperature corrosion resistance, good high temperature mechanical strength, good heat transfer capability, cold formability, low raw material cost and good dissimilar material welding capability, particularly when they are used with platinum, iridium and other noble or high temperature metal alloy firing tips. The use of nickel-based alloy electrode bodies with platinum, iridium or other noble metal alloy firing tips has some disadvantages. For example, this combination is not optimal for weldability of the tips to the electrodes, formability of the electrodes, and resistance of the electrode material joints to high temperature oxidation. [0005] While nickel-based alloy electrodes may be used for ignition device ground and center electrodes, particularly with platinum, iridium and other noble or high temperature metal alloy firing tips, there remains a need for other alloy compositions suitable for use as ignition device electrodes, having the electrode properties noted above, and which may also be used as electrode bodies in conjunction with platinum, iridium and other noble or high temperature metal alloy firing tips.

SUMMARY OF THE INVENTION

[0006] This invention provides alloy compositions for alternative electrode materials that exhibit superior performance characteristics for use as the body material for spark plug center and ground electrodes. In particular, these alloys provide superior performance at a lower cost as a potential replacement for nickel-based alloys as the body electrode material for spark plug designs, particularly those which incorporate firing tips of noble or other high temperature materials for the sparking surface.

[0007] In one aspect, the invention includes an ignition device having a ground electrode, center electrode, or both, composed of a cobalrtmsed alloy including: Ni in an amount between about 20 and 24% by weight; Cr in an amount between about 20 and 24% by weight; W in an amount between about 10 and 16% by weight; and Co in an amount between about 32 and 47% by weight.

[0008] In another aspect, the invention includes an ignition device having a ground electrode, center electrode, or both, composed of an iron-based alloy comprising: Cr in an amount between about 18 and 24% by weight; Al in an amount between about 4 and 7% by weight; and Fe in an amount between about 67 and 78% by weight. Some of the iron-based alloy compositions may also include a reactive metal selected from the group consisting of Y,

Zr ₅ Nb, La, Hf and Ta.

[0009] In another aspect, either the C-based or Fe-based electrode alloys may also include Zr and B in an amount, by weight, of 0.005-0.5% Zr and 0.001-0.10% B.

[00010] In another aspect, center or ground electrodes of the invention may also include firing tips attached at a sparking end thereof.

[00011] In another aspect, the firing tips may be formed of at least one of gold, a gold alloy, a platinum group metal or a tungsten alloy, particularly platinum-based and iridium- based alloys.

[00012] These and other features and advantages of this invention will become more apparent to those skilled in the art from the detailed description of a preferred embodiment.

The drawings that accompany the detailed description are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[00013] Preferred exemplary embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein:

[00014] FIG. 1 shows a partial fragmentary view of an ignition device having a noble metal tip attached to the center and ground electrodes composed of a cobalt or iron based alloy according to an exemplary embodiment of the present invention; and

[00015] FIGS. 2-5 show alternative firing tip constructions that can be used for the ignition device of FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[00016] Referring to FIG. 1, there is shown the working end of an ignition device in the form of a spark plug 10 that includes an outer metal shell or housing 12, an insulator 14 secured within the housing 12, a center electrode 16, a ground electrode 18, and a pair of firing tips 20, 22 located opposite each other on the center and ground electrodes 16, 18, respectively. Housing 12 can be constructed in a conventional manner and can include standard threads 24 along with an annular lower end 26 to which the ground electrode 18 is welded or otherwise attached. Similarly, all other components of the spark plug 10 (including those not shown) can be constructed using known techniques and materials, excepting of course center electrode 16, or ground electrode 18, (or both of them) that is composed of a particularly advantageous alloy composition, as will be described below. While spark plug 10 is illustrated to include firing tips 20, 22, the alloys described herein may also be used as center electrodes or ground electrodes in spark plugs 10 which do not include such tips, or which includes a firing tip on only one of the center or ground electrodes. Further, they may be used in spark plugs 10 having different arrangements of the center or ground electrodes, such as surface gap and annular gap ground electrodes.

[00017] As is known, the annular end 26 of housing 12 defines an opening 28 through which insulator 14 protrudes. Center electrode 16 is permanently mounted within insulator 14 by a glass seal or using any other suitable technique. It extends out of insulator 14 through an exposed axial end 30. Ground electrode 18 is illustrated in the form of a conventional ninety-

degree elbow that is mechanically and electrically attached to housing 12 at one end 32 and that terminates opposite center electrode 16 at its other end 34. This free end 34 comprises a firing end of the ground electrode 18 that, along with the corresponding firing end of center electrode 16, defines a spark gap 36 therebetween. However, as noted above, many other alternate ground electrode configurations may be used in accordance with the invention. [00018] Moreover, the firing tips 20, 22 are each located at the firing ends of their respective electrodes 16, 18 so that they provide sparking surfaces for the emission and reception of electrons across the spark gap 36. These tips 20, 22 are provided for increasing the operational life of the spark plug and may be composed of platinum-based alloys, iridium- based alloys and other noble metal or high temperature alloys. For example, noble metal firing tips 20, 22 may also be made from gold or gold alloys, including Au-Pd alloys, such as Au-40Pd (in weight percent) alloys or any of the known pure metals or alloys of the platinum group metals, including: platinum, iridium, rhodium, palladium, ruthenium and rhenium, and various alloy combinations thereof in any combination. For purposes of this application, rhenium is also included within the definition of platinum group metals based on its high melting point and other high temperature characteristics similar to those of certain of the platinum group metals. Additional alloying elements for use in platinum group metal firing tips 20,22 may include, but are not limited to, nickel, chromium, iron, manganese, copper, aluminum, cobalt, zirconium, tungsten and rare earth elements including yttrium, hafnium, lanthanum, cerium, and neodymium. In fact, any material that provides suitable spark erosion corrosion performance in the combustion environment may be suitable for use as firing tips 20, 22. Firing tips 20, 22 may also be made from other high temperature alloys, including various tungsten alloys, such as W-Ni, W-Cu and W-Ni-Cu alloys. Firing tips 20, 22, may also comprise a multi-layer composite of a platinum-based alloy, iridium-based alloy or other noble metal or high temperature alloy and an electrode body material, such as nickel or a nickel -base alloy, an iron-nickel chromium-based alloy or the alloys of the invention described hereinbelow.

[00019] The tips 20, 22 are shown in FIG. 1 in cross-section for purposes of illustrating the firing tips which, in this embodiment, comprise cylindrical, rectangular or other shaped pads welded into place on the firing ends. Alternatively, however, the firing tips 20, 22 need

not be pads, but can take the form of a rivet, a wire, a ball, or any other suitable shape. The construction and mounting of these various types of firing tips is known to those skilled in the art. As shown, the firing tips 20, 22 can be welded into partial recesses on each electrode, or welded so as to partially or fully recess into the electrode in conjunction with the welding process. Optionally, one or both of the pads can be fully recessed on its associated electrode or can be welded onto an outer surface of the electrode without being recessed at all. [00020] The firing tips 20, 22 are permanently attached, both mechanically and electrically, to the center electrodes preferably by metallurgical bonding, such as laser welding, laser joining, resistance welding, or by brazing, swaging, or other suitable means. Laser welding can be done according to any of a number of techniques well known to those skilled in the art. Laser joining involves forming a mechanical interlock of the electrode to the firing tip by using laser light to melt the electrode material so that it can flow into a recess or other surface feature of the firing tip, with the electrode thereafter being allowed to solidify and lock the firing tip in place. This laser joining technique is more fully described in US Patent Applications 10/486,962 filed on August 15, 2002 and 10/787,280 filed on February 26, 2004 , the complete disclosure of which is hereby incorporated by reference. In any event, the attachment process results in the electrodes each having an integral firing tip attached thereto that provides an exposed sparking surface for the center electrode. Alternatively, however, the present invention contemplates use of a center electrode or ground electrode not having a firing tip attached thereto. In other words, the present invention encompasses a center electrode or ground electrode having an integral surface used for directly communicating a spark to the ground electrode or firing tip thereof. [00021] According to one exemplary embodiment of the present invention, the center electrode is formed from a cobalt-based alloy containing the following elemental constituents: cobalt (Co), nickel (Ni), chromium (Cr), and tungsten (W). The preferred amounts of each elemental constituent by weight is as follows: 20 to 24% Ni, 20 to 24% Cr, 10 to 16% W, with the balance substantially Co. More preferably, however, the alloy includes 13% W and 32 to 47% Co. An exemplary alloy is available from ThyssenKrupp VDM of Werdohi, Germany and is known as Conicro® 4023 W - alloy 188. Such alloys can be formed by known processes such as by melting the desired amounts of constituent elements together.

After melting, the alloy can be converted into a powdered form by an atomization process, as is known to those skilled in the art. The powdered alloy can then be isostatically pressed into solid form, with secondary shaping operations being used if necessary to achieve the desired final form. Techniques and procedures for accomplishing these steps are known to those skilled in the art. Other alloy formation methods can be used as well. [00022] According to another exemplary embodiment of the present invention, the center electrode is formed from an iron-based alloy containing the following elemental constituents: iron (Fe), chromium (Cr), and aluminum (Al). The preferred amounts of each elemental constituent by weight is as follows: 18 to 24 % Cr, 4 to 7% Al, with the balance substantially Fe. More specifically preferred aspects of this embodiment including three alloy compositions according to the following table of elemental constituents of each alloy by weight percentage:

TABLE 1

[00023] The alloys are exemplified by alloys available from ThyssenKrupp VDM and are known, respectively, as Aluchrom® I, Aluchrom® I SE, and Aluchrom® Y. [00024] The reactive elements described with respect to Alloy 2 may include at least one element selected from the group consisting of Y, Zr, Nb, La, Hf and Ta. While at least

one reactive element is required with regard to this alloy, the elements of this group may be included in any combination within the composition limits described.

[00025] In addition to the alloy compositions described above, it is believed that, as has been demonstrated in solution-strengthened Ni-based nickel-chromium-iron and dilute nickel alloys generally, the use of zirconium in the range of 0.005-0.5% by weight of the alloy and boron in the range of 0.001-0.01% by weight of the alloy is also particularly useful. In the alloy compositions described above, the use of zirconium in the range of 0.005-0.15% by weight of the alloy and boron in the range of 0.001-0.01% by weight of the alloy is known to be particularly useful. Boron and zirconium are known as grain boundary strengtheners . They segregate to the grain boundaries and serve to stabilize them increasing grain boundary strength and ductility, retarding grain boundary diffusion and sliding and delaying intergranular cracking caused by environmental and mechanical factors under the operating conditions of the electrodes, thereby inhibiting high temperature grain growth and enhancing the resistance of these alloys to high temperature creep, deformation, environmental cracking and various fracture phenomena, such as stress rupture. The performance improvements associated with the addition of zirconium and boron are believed to be synergistic, that is they are greater than the improvements that result when either zirconium or boron are added to these alloys separately.

[00026] Center or ground electrode bodies composed of cobalt and iron based alloys have been found to promote ignition device performance and manufacturability. Center or ground electrode bodies having such alloys exhibit good resistance to high temperature oxidation, demonstrate improved formability, and yield improved weldability when attaching firing tips. In the embodiments discussed above and shown in FIG. 1, the center electrode or ground electrode is formed from one of the cobalt-based or iron-based alloys described above. Also, either the center electrode, ground electrode, or both can have a heat conductive core formed from copper, copper alloys, or other suitable thermally conductive materials. [00027] FIGS. 2-5 show alternative firing tip configurations that can be used in lieu of one or both of the firing tip pads 20, 22 shown in FIG. 1. FIG. 2 shows a segment of wire, FIG. 3 shows a rivet with a hemispherical firing tip , FIG. 4 shows a sphere, and FIG. 5 shows a flat headed rivet with a cylindrical firing tip all of which can be attached using various

known methods. In yet another embodiment, the center and/or ground electrode of FIG. 1 can be made of conventional or other metals or alloys, with the cobalt-based or iron-based alloy(s) described above being used instead to form the firing tip, in which case any of the geometries shown in FIGS 1-5, or other shapes not shown for the firing tips can be used. Techniques for welding or otherwise attaching the cobalt-based and/or iron based alloys will be known to those skilled in the art.

[00028J It will thus be apparent that there has been provided in accordance with the present invention an ignition device that achieves the aims and advantages specified herein. It will, of course, be understood that the foregoing description is of preferred exemplary embodiments of the invention and that the invention is not limited to the specific embodiments shown. Various changes and modifications will become apparent to those skilled in the art. For example, although an ignition device in the form of a spark plug has been illustrated, it will be appreciated that the invention can be incorporated into an igniter of the type in which sparking occurs across the surface of a semiconducting material disposed between the center electrode and an annular ground electrode. All such changes and modifications are intended to be within the scope of the present invention. [00029] 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 do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.