Title of Invention	A PROCESS FOR JOINING AN ELECTRODE OF A SPARK PLUG TO A PRECIOUS METAL
Abstract	ABSTRACT "A PROCESS FOR JOINING AN ELECTRODE OF A SPARK PLUG TO A PRECIOUS METAL" The present invention describes a process for joining an electrode (1,1') of a spark plug to a precious metal (2), in which method the precious metal (2) is locally joined to the electrode (1, 1') by an introduction of heat which is generated by means of a continuously operating laser beam (3).

Full Text

Prior Art The present invention relates to a process for joining an electrode of a spark plug to a precious metal- Spark plugs with an electrode, such as for example a center electrode, in which the front end of the center electrode is provided with a precious metal tip or in which spark plugs a precious metal is arranged in a peripheral region, have long been known from the prior art. For example, EP 0 637 113 Bl describes a spark plug with a center electrode which includes a heat-resistant and erosion-resistant nickel alloy, the front end of the center electrode being formed with a precious metal tip of iridium or ruthenium. The nickel alloy has a thermal conductivity of approximately 30 Wm-1K-1 or greater. It is stated in this document that the precious metal tip is in disk form and is arranged concentrically at the front end of the electrode metal. Laser beams are applied to an interface between the precious metal tip and the front end of the electrode metal by using a YAG laser, for example, the precious metal tip being pressed with a corresponding force onto the front end of the electrode metal to which the precious metal is to be applied. EP 0 400 950 Bl has disclosed a process for producing a spark plug in which an iridium powder compact is produced, which forms the sparking tip of the center electrode of the spark plug. This iridium powder compact is sintered in a vacuum or in an atmosphere which is not oxidizing or reducing, and the sparking tip is metallurgically joined to the front end of the center electrode. The metallurgical joining may be carried out, for example, using electron beam welding or laser welding. US 5,811,915 and DE 196 41 856 Al likewise describe the provision of small precious metal plates on a spark plug electrode, such as for example a ground electrode or a center electrode. According to these documents, the small precious metal plates are applied by laser welding, specifically with the aid of an Nd:YAG laser, EP 0 575 163 Bl also describes the welding of a small precious metal plate onto a center electrode of a spark plug, the weld seam being located at the periphery of the interface between the small precious metal plate and the end face of the center electrode. In this case, a YAG laser is used for welding. US 4,963,112 likewise discloses the attachment of a small precious metal plate to an electrode of a spark plug, the attachment once again being effected by means of laser welding. This document describes that it is perferable to use pulsed lasers. US 3,461,210, EP 0 588 495 Bl and EP 0 587 446 Bl likewise describe the application of small precious metal plates to spark plug electrodes. In this case too, a pulsed laser beam is always used to weld on the small precious metal plates. However, a common factor to all these processes for applying a small precious metal plate or some other form of a precious metal to an electrode of a spark plug which are known from the prior art is that the application takes place with the aid of a pulsed laser. In joining processes of this type which use pulsed laser beam sources, the materials which are to be secured to one another, i.e. the electrode and the precious metal, are discontinuously melted and solidified again. This means that a continuous melt pool is not produced. However, the permanent melting and resolidification of the material which is to be joined only allows intimate mixing of the melting zone, i.e. uniform distribution of the alloy, to be achieved to a limited extent. Therefore, there is a relatively highly likelihood of cracks forming in the joining zone, and consequently the service life of spark plugs of this type, which are used as what are known as 'long life plugs", is ultimately limited by the relatively short durability of the precious metal/electrode alloy join. A nickel alloy is frequently used as material for the electrode. The joining by means of a pulsed laser beam gives rise to the formation of undesirable nickel-rich alloy regions, which are therefore less resistant to erosion and corrosion. If one considers the surface of an electrode which has been welded by means of a pulsed laser beam and provided with a precious metal, this surface is highly irregular, since it is impossible to produce a continuous melting zone region, but rather the material is constantly melted and solidified. Therefore, it may be necessary for the surface to be treated further after the welding. The proposed process for joining an electrode of a spark plug to a precious metal baving the features in which a precious metal is locally melted onto an, electrode using a continuously operating laser beam, by contrast, has the advantage of reducing the nonuniformities on the surface. At the same time, cracks, pores, voids and fluctuations in the respective alloying constituents, which all weaken the precious metal/electrode material join, are avoided within the zone which has been completely or at least partially melted. Therefore, with a process according to the invention it is possible to increase the service life of the component during operation, since weak points of this type can be avoided or at least minimized. The fact that the uniform melting of the joining partners in the contact zone makes it possible to avoid the solidification cracks which are known when pulsed laser beams are used means that corrosive attack along these cracks is also avoided, and therefore premature failure of the join is eliminated. This is particularly important when the spark plugs are used in the region of the engine. Furthermore, the fact that a continuously operating laser is used means that the heating and cooling rate of the melting zone region can be adjusted to the particular materials and the type of join which is desired, with the result that a defined phase composition is also achieved in the joining region. Furthermore, a continuously operating laser also allows a wider variation in terms of the alloying compositions which can be used for the materials. Therefore, the service life can also be optimized by using optimized alloying compositions for the materials and is not, as has hitherto been the case, determined by whether the materials have good or limited weldability for a pulsed laser. A further advantage of the invention which should be mentioned is that the spectrum of melting zone geometries which can be achieved with a continuous laser beam is much wider than with a pulsed laser. Furthermore, higher process speeds can also be achieved, which also leads to cost savings in production and to a reduced thertcial load on the component which is to be produced. To summarize, therefore, the overall result is an improved melting zone between precious metal and electrode, which leads to a longer service life of the electrode and therefore to the spark plug product functioning better. According to a preferred embodiment of the invention, the precious-metal insert is applied in the form of a strip to the periphery of the electrode over a certain width. Electrodes produced in this way are used, for example, in what are known as surface-gap spark plugs or surface-air-gap spark plugs. According to a further preferred embodiment of the invention, the precious-metal insert is applied to an end side of the center electrode. When the precious metal insert is applied to the end side of the center electrode, however, the precious-metal part should preferably not be completely melted, but rather should only be melted in its joining region. This creates a spark plug electrode with a tip made from wear-resistant precious metal. If, in accordance with the invention, the precious metal is applied by means of a continuously operating laser beam, a diode laser can be used as well as an Nd:YAG or CO2 laser. According to the present invention, the precious metal can be applied to an electrode either in such a manner that it is completely melted and is therefore alloyed into the electrode or that the precious metal is not completely melted, but rather is only melted at the edge region, and is joined to the electrode in this edge region. Further advantages and advantageous configurations of the subject matter of the invention will emerge from the description, the appended drawing and the patent claims. Drawing Two exemplary embodiments of a process for joining a spark plug electrode to a precious metal insert are illustrated in simplified diagrammatic form in the drawing and are explained in more detail below in the description, in which: Figure 1 diagrammatically depicts a joining process in which a precious metal is fed onto an electrode as wire material, and Figure 2A and Figure 2B each diagrammatically depict a process step of a further process for joining an electrode to a precious metal insert in the manner of a top electrode. Description of the exemplary embodiments Figure 1 illustrates, by way of example, a single-stage coating process for an electrode, i.e. a process for joining a spark plug electrode 1 to a precious metal 2. In the context of the present invention, the term precious metal 2 can be understood as meaning a pure precious metal or also any precious metal alloy which is suitable for the particular purpose. Figure 1 shows in particular a section through the spark plug electrode 1, which in this case represents a center electrode, which in the region of a prefabricated groove 6 is being filled with a melted precious metal 2 and which is used, by way of example, in a surface-gap spark plug or a surface-air-gap spark plug. The section through the center electrode 1 shown in Figure 1 is taken along the groove 6. According to the invention, a continuous laser 3 is used for melting, it being possible, by way of example, to use an Nd:YAG laser or a CO2 laser or a diode laser. A diode laser is particularly suitable, since nowadays it is considerably more advantageous than an Nd:YAG or CO2 laser in terms of investment and operating costs. According to the preferred embodiment shown, the precious metal 2, which in this case may, for example, be platinum, is supplied permanently as wire material and is melted onto the center electrode 1 in the region of the prefabricated groove 6 using the continuous laser beam 3 and is introduced into the groove 6 in the form of a molten material 5, so that the precious metal 2 is, as it were, wound onto the electrode 1. At the same time, the base material of the center electrode 1 is also partially melted, and an alloy comprising a small proportion of the melted base material of the center electrode 1 and of the material of the precious metal wire 2 is formed. This continuously operating production process for welding on the precious metal 2 results in homogeneous, intimate mixing of the melting zone and therefore in a uniform alloy distribution, which leads to increased service lives and a smooth surface on the center electrode 1. Furthermore, it is possible to achieve higher process speeds, which in turn results in inexpensive processing and a reduced thermal load on the component, i.e. the center electrode 1. The uniform introduction of heat over the melting zone region results in lower thermal stresses than with pulsed laser processes which are known from the prior art and therefore in turn in an increased service life of the center electrode 1. The optimization of the surface quality is clearly optically recognizable on the product. Microsections also clearly illustrate the way in which the welding zone has been improved, in particular with regard to the intimate mixing. The heating and cooling rates can be adjusted, so that the formation of cracks in the melting zone and the electrode base material is further suppressed or minimized. By varying the heating and cooling rates, it is also possible to achieve a broad range of alloying compositions. Figures 2A and 2B illustrate a production process for a further type of electrode, namely what is known as a top electrode. In this case, a continuously operating laser beam (continuous wave or CW laser), preferably an Nd:YAG laser, is used to weld the join between the precious metal 2 and the nickel alloy of the electrode As can be seen from Figure 2A, a part made from precious metal 2, preferably in cylindrical form, is placed onto an end side of the electrode 1' , which in this case represents a center or ground electrode of a spark plug, or is fitted into a recess 6 therein. In this case, the recess 6 in the end side of the electrode 1' is preferably shaped in such a manner that, when the precious metal part 2 is inserted, it is fixedly joined to the electrode 1'- As can be seen from Figure 2B, in a subsequent process step the joining partners which have been brought into contact in this manner, namely precious metal 2 and spark plug electrode 1', rotate in the direction of the arrow 7, at a rotational speed which is matched to the energy introduced from a CW laser. The laser beam 3 is focused onto the rotating zone which is to be welded and is switched on and off according to an energy ramp which is matched to the joining partners with regard to melting point, heat capacity, etc. The result, then, is a spark plug electrode 1', the tip of which consists of a precious metal 2 or a precious metal alloy which is permanently joined to the nickel alloy of the electrode 1 via a uniform melting zone. In the case of an electrode 1' of this type which is provided with a precious metal 2, known as a roof electrode, the precious metal 2 should not be completely melted, but rather should only be melted in its joining region. WE CLAIM: 1. A process for joining an electrode (1, 1') of a spark plug to a precious metal (2), characterized in that the precious metal (2) is locally joined to the electrode (1, 1') by an introduction of heat which is generated by means of a continuously operating laser beam (3), the precious metal (2) is connected to the electrode (1, 1) in a connecting region and that the laser beam (3) is directed to the connecting region, whereby the precious metal (2) is not completely melted but is melted only in the connecting region. 2. The process as claimed in claim I, wherein the introduction of heat takes place by means of the continuously operating laser beam (3), such that melting takes place at least in a boundary region between precious metal (2) and electrode (1, 1'). 3. The process as claimed in claim 1 or 2, wherein the precious metal (2) is completely melted during the introduction of heat. 4. The process as claimed in one of the preceding claims, wherein the precious metal (2) is applied to the periphery of the electrode (1). 5. The process as claimed in claim 1, wherein the precious metal (2) is applied to an end side of the electrode (1'). 6. The process as claimed in one of the preceding claims, wherein the laser beam (3) is formed by an Nd:YAG, CO2 or diode laser. 7. The process as claimed in one of the preceding claims, wherein a precious metal (2) which is in the form of a wire is fed to a groove (6) which has been prefabricated in the electrode (1) and is melted onto the electrode (1) by the laser beam (3). 8. The process as claimed in one of claims 1 to 6, wherein a precious metal (2), which is designed as a cylinder, is placed onto the end side of an electrode (1') and is then melted in an interface region between precious metal (2) and electrode (1') by means of the laser beam (3). 9. A center, ground or top electrode of spark plug produced by the process as claimed in the preceding claims.

Documents:

in-pct-2002-1533-che abstract-duplicate.pdf

in-pct-2002-1533-che abstract.jpg

in-pct-2002-1533-che abstract.pdf

in-pct-2002-1533-che claims-duplicate.pdf

in-pct-2002-1533-che claims.pdf

in-pct-2002-1533-che correspondence-others.pdf

in-pct-2002-1533-che correspondence-po.pdf

in-pct-2002-1533-che description (complete)-duplicate.pdf

in-pct-2002-1533-che description (complete).pdf

in-pct-2002-1533-che drawings.pdf

in-pct-2002-1533-che form-1.pdf

in-pct-2002-1533-che form-18.pdf

in-pct-2002-1533-che form-26.pdf

in-pct-2002-1533-che form-3.pdf

in-pct-2002-1533-che form-5.pdf

in-pct-2002-1533-che pct.pdf

in-pct-2002-1533-che petition.pdf