Process for producing a valve seat

Abstract

The invention relates to a process for producing a valve seat for a cylinder head of an internal combustion engine, in which an additional material is fused to the cylinder head, through introduction of energy, at the location at which the valve seat is to be formed, wherein the additional material used is a copper alloy which, in addition to copper, comprises the following elements: iron less than 5% by weight,manganese10% by weight-20% by weight,cobalt 5% by weight-10% by weight,molybdenum 5% by weight, 9% by weight, 5% byweight,boron 1% by weight-3% by weight,chromium less than 3% by weight.

Description

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a process for producing a valve seat for a cylinder head in accordance with the preamble of claim 1.

2. Related Art of the Invention

EP 1 120 472 A1 describes a process for producing a valve seat for a cylinder head. In this process, an additional material is applied around a valve opening through a nozzle, with energy additionally being introduced along the valve opening by a separate laser element and the pulverulent additional material and the material of the cylinder head being partially melted at this location so that they are fused together. The additional material then forms an alloy, therefore, with the material of the cylinder head, and consequently forms a local reinforcement in this region, which in turn forms the valve seat.

The material which is used in this process consists of a copper alloy which contains 6-9% by weight of nickel, 1-5% by weight of silicon, 1-5% by weight of molybdenum and also tungsten, tantalum and niobium.

The valve seat at the cylinder head of internal combustion engines serves the purpose of sealing off the combustion chamber with respect to the environment. The valve presses onto the valve seat, producing a high level of wear at the valve seat, which means that it is necessary to use highly wear-resistant materials in this region. At the same time, the maximum possible quantity of heat should be dissipated from the region of the sealing surface and the combustion chamber to the water jacket.

The process and alloy described in EP 1 120 472 A2 satisfy these demands by using a copper alloy so that at least a good thermal conductivity is achieved, but the wear resistance at the valve seat is still inadequate.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a process for producing a valve seat, the valve seat having a high thermal conductivity and also having a significantly increased wear resistance compared to the prior art.

The solution to the object consists in a process for producing a valve seat in accordance with the features of patent claim 1.

In the process according to the invention for producing a valve seat for a cylinder head of an internal combustion engine, an additional material is applied to the cylinder head and fused to the material of the cylinder head at the location at which the valve seat is to be formed, as a result of energy being introduced. According to the invention, the additional material used in this process is a copper alloy, the alloy comprising, in addition to copper, the following elements:

iron less than       5% by weight,
manganese            10% by weight-20% by weight,
cobalt               5% by weight-10% by weight,
molybdenum less than 5% by weight,
nickel less than     9% by weight,
silicon less than    5% by weight,
boron                1% by weight-3% by weight,
chromium less than   3% by weight,
and inevitable impurities.

With the alloy composition according to the invention, the principal element copper substantially has the effect of producing a good thermal conductivity and bonding to the base material. The alloying element iron has the effect of increasing strength, but a maximum concentration (solubility limit) of 5% for iron in copper should not be exceeded. The alloying element manganese is particularly good at increasing the strength of the alloy. If manganese forms from 10-20% of the alloy, superstructures which have a positive influence on the hot strength and moreover lead to a higher wear resistance are formed.

The alloying element cobalt likewise produces a higher resistance to wear and, moreover, contributes to grain refining. Molybdenum as an alloying element acts as a solid lubricant. The lubricating action is ensured by the molybdenum compounds MoS₂ and MoO₃. At the same time, the molybdenum interacts with the silicon, leading to the formation of advantageous silicides. The alloying element nickel also forms silicides, and moreover leads to the formation of a solid solution with the copper, based on the nickel being completely soluble in the copper matrix.

As has already been mentioned, the alloying element silicon forms compounds with the alloying elements nickel and molybdenum, producing what are known as silicides. The silicon improves the wettability of the additional material with the partially melted base material. The alloying element boron also contributes to improving the wettability.

It should be noted that additional aluminum is supplied to the system from the aluminum-silicon alloy of the base material as a result of the partial melting of the latter.

The alloying element chromium is only slightly soluble in copper. It forms hard materials, such as Cr₂C₃ and silicides, in the alloy. These compounds make a contribution to the hardness of the valve seat. However, the chromium content must be stoichiometrically matched to the silicon content.

In one advantageous embodiment of the invention, the additional material, as well as copper, includes the following elements:

iron       2% by weight-4% by weight,
manganese  11.5% by weight-14% by weight,
cobalt     5% by weight-10% by weight,
molybdenum 2% by weight-4% by weight,
nickel     3% by weight-6% by weight,
silicon    2% by weight-4% by weight,
boron      1.5% by weight-2.5% by weight,
chromium   1% by weight-2% by weight,
and inevitable impurities.

It has emerged that a manganese content of between 11.5% by weight and 14% by weight makes a particularly good contribution to increasing the strength and to the wear resistance without having any adverse effect on the action of the other alloying elements.

In a further embodiment of the invention, the additional material, as well as copper, includes the following elements:

iron       3% by weight,
manganese  12% by weight,
cobalt     5% by weight,
molybdenum 3% by weight,
nickel     3% by weight,
silicon    2% by weight,
boron      1% by weight,
chromium   1% by weight,
and inevitable impurities.

The percentages indicated for the alloying elements are in each case to be understood as being within the context of manufacturing accuracy. When producing the alloy, an inaccuracy of ±0.5% by weight per alloying element is in each case assumed.

In the context of the invention, it may be expedient for the additional material to contain up to 15% by weight of tin (Sn), which forms a CuSn matrix with the copper and in this way increases the basic strength of the material. The addition of phosphorus (P) also contributes to the formation of CuFeP solid solutions and therefore to an increase in the hot strength.

In one embodiment of the invention, the energy is introduced by a laser beam. The laser beam and the additional material are fed to the location of action through a common unit comprising focusing optics and a coaxial nozzle. This ensures that the introduction of energy and the supply of material are always locally matched to one another.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are explained in more detail on the basis of the following drawings, in which:

FIG. 1 shows an excerpt from a cylinder head in the region of the valve seat,

FIG. 2 shows an enlarged excerpt from the valve seat with valves,

FIG. 3 shows the application of the additional material and of the laser beam through a coaxial nozzle,

FIG. 4 shows a cross section through a melt track at a cylinder head surface.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows part of a cylinder head 1 of an internal combustion engine, which is not illustrated overall. The cylinder head 1 has an intake duct 2, in a manner which is known per se, which in the present case could, of course, also be formed as an exhaust duct. The intake duct 2 is closed and opened by a gas exchange valve 3, referred to below as valve 3 for the sake of simplicity, so that air or a fuel/air mix can enter a combustion chamber 4 of the cylinder head 1 from the intake duct 2.

The cylinder head 1 is provided with a valve seat 5 against which the valve 3 bears in its closed position, so as to disconnect the intake duct 2 from the combustion chamber 4.

The valve seat 5 is arranged annularly around a valve opening 6 in the cylinder head.

To produce the valve seat 5, the laser light from an Nd:YAG laser is passed through a glass fiber to focusing optics (not shown in more detail here). The use of a glass fiber makes it easy to guide the laser light to the processing location, with the result that the process costs and the systems outlay can be reduced.

The focusing optics of the laser beam are connected to a coaxial nozzle 8, as illustrated in FIG. 3. The coaxial nozzle 8 comprises two outlets, an inner opening 12 and an outer opening 14. The laser beam 10 is guided through the inner opening 12. The additional material 16 is guided through the outer opening 14. This arrangement has the advantage that the energy source, the laser beam 10, and the additional material, which forms the valve seat 5, are always guided onto the same location. This makes the processing operation direction-independent, so that additional outlay on equipment if the laser beam and additional material were to form a preferred direction can be saved.

The focusing optics shape the laser beam 10 in such a way that on the cylinder head 1 it has a focal point with a diameter of approximately 2-5 mm. The quantity of energy which acts on the cylinder head 1 at this focal point is preferably metered in such a way that the cylinder head material is partially melted at this location. The width of the focal point is designed in such a way that the entire width of the valve seat can be formed by one track of the laser. In this case, the process parameters energy density, diameter of the focal point, advance rate of the laser or the coaxial nozzle around the valve opening 6 and the delivery quantity of additional material have a combined effect. These parameters have to be set in such a manner that the desired melting can be achieved. The coaxial nozzle 8 containing the laser beam 10 and the additional material 16 is therefore moved along the valve opening 6 in the shape of a circle. The advance is in this case between 300 mm per minute and 1000 mm per minute. In the process, the surface of the cylinder head 1 is partially melted in the region of the valve opening 6. The additional material 16, which is supplied from the outer opening 14 of the coaxial nozzle 8, is likewise partially melted by the laser energy. The molten cylinder head material and additional material form an alloy. FIG. 4 shows a cross section through a melt track, with line 20 marking the surface of the cylinder head 1.

The advance rate of the processing, on a production engineering scale, is between 300 mm per minute and 1000 mm per minute. The advance rate is in this case dependent on the geometry of the cylinder head 1 at the valve opening 6.

The mixing depth of the cylinder head surface is defined in the following way by the features of FIG. 4:

• • mixing depth=(cross section 24 beneath the cylinder head surface 20/overall cross section 26 of the melt track 18)×100%.

The molten material of the cylinder head and the molten additional material are combined with one another in a transition layer (mixed layer). The thickness of the transition layer is usually less than 1000 μm. The transition layer, which is not illustrated in FIG. 4, does not necessarily have to coincide with the cylinder head surface 20; it may be at a higher or lower level. Accordingly, only the additional material forms the surface of the melt track 18 and therefore the surface of the valve seat 5. The functional properties of the valve seat 5 are therefore produced exclusively by the additional material 16. The partial melting of the cylinder head surface 20 serves substantially to ensure metallurgical bonding (fixed, non-brittle bonding) between the additional material and the cylinder head 1.

To avoid changes in the properties of the cylinder head material, in particular in the case of very thin webs, as also occur in the region of the valve opening 6, excessively deep mixing is to be avoided. According to the relationship given above, the mixing depth should be less than 30%, preferably less than 20%.

Claims

1. A process for producing a valve seat for a cylinder head of an internal combustion engine, in which an additional material is fused to the cylinder head, through introduction of energy, at the location at which the valve seat is to be formed, wherein the additional material used is a copper alloy which, in addition to copper, comprises the following elements:

2. The process according to claim 1, wherein the additional material, as well as copper, comprises the following elements:

3. The process according to claim 1, wherein the additional material, as well as copper, comprises the following elements:

4. The process according to claim 1, wherein the additional material contains between 0% by weight and 15% by weight of tin.

5. The process according to claim 1, wherein the additional material contains between 0.05% by weight and 0.3% by weight of phosphorus.

6. The process according to claim 1, wherein the energy is introduced by means of a laser beam, and the laser beam and, the additional material pass onto the valve seat together through a coaxial nozzle.