METHOD FOR MACHINING A SURFACE OF A METAL COMPONENT, AND METAL COMPONENT

Abstract

A method for machining a surface of a metal component, in particular a connecting rod or a cam for a motor vehicle, including the following steps: providing a metal component which has a surface to be machined; premachining the surface to be machined; structuring the premachined surface by means of a laser beam in such a way that elevations but no depressions are formed as laser structures on the premachined surface with respect to the level thereof.

Claims

1. A method for machining a surface of a metal component, comprising: premachining a surface to be machined on a metal component, and structuring the premachined surface by a laser beam, wherein the premachined surface is laser-structured in such a way that elevations but no depressions are formed as laser structures with respect to a level of said surface.

2. The method as claimed in claim 1, wherein the surface to be machined is an inner surface of an aperture in the metal component, wherein the laser beam is guided in a direction orthogonal to a circumferential direction of the inner surface.

3. The method as claimed in claim 1, wherein the laser beam is guided in a pulsed manner along at least one machining track over the surface to be machined.

4. The method as claimed in claim 3, wherein the elevations which are formed within a machining track are formed in an overlapping manner on the surface to be machined and have an overlap of greater than or equal to 60 percent.

5. The method as claimed in claim 3, wherein the elevations are generated in a plurality of machining tracks of respectively overlapping elevations on the surface to be machined, wherein the machining tracks are spaced apart from one another free of overlap and have a spacing of at least 50 percent of a track width of the machining tracks.

6. The method as claimed in claim 3, wherein the metal component is a steel component, the premachined steel surface of which is laser-structured with an average beam power of 9±0.5 watt.

7. The method as claimed in claim 3, wherein the metal component is a titanium component, the premachined titanium surface of which is laser-structured with an average beam power of 8±0.5 watt.

8. The method as claimed in claim 1, wherein the elevations have a substantially round shape.

9. The method as claimed in claim 1, wherein the elevations have a width of less than 100 μm.

10. The method as claimed in claim 1, wherein the elevations have a height of less than 20 μm.

11. The method as claimed in claim 1, wherein the surface is an inner circumferential surface and the premachining on the surface to be machined generates a surface structure which is oriented in a circumferential direction of the inner circumferential surface.

12. The method as claimed in claim 11, wherein the inner circumferential surface is laser-structured in a plurality of spaced-apart angular sections.

13. The method as claimed in claim 1, wherein the metal component is a connecting rod or a cam for a motor vehicle.

14. A metal component having a premachined surface which is laser-structured by a laser beam, wherein the premachined surface is laser-structured in such a way that elevations but no depressions are formed as laser structures with respect to a level of said surface.

15. The metal component as claimed in claim 14, wherein the elevations are formed in a plurality of tracks of elevations on the surface to be machined, wherein the elevations have an overlap within a track and wherein directly adjacent tracks of elevations are spaced apart free of overlap.

16. The metal component as claimed in claim 14, wherein the metal component is a titanium component.

17. The metal component as claimed in claim 14, wherein the metal component is a connecting rod or a cam for a motor vehicle.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0026] Exemplary embodiments of the invention are illustrated in the drawing and are explained in more detail in the description which follows. In the drawing:

[0027] FIG. 1 shows a schematic view of a connecting rod having a surface to be machined;

[0028] FIG. 2 shows a schematic sectional view of the connecting rod to explain the laser structuring;

[0029] FIG. 3 shows a schematic sectional view of the surface after laser structuring;

[0030] FIG. 4a shows an SEM image of a surface after laser structuring;

[0031] FIG. 4b shows a schematic illustration of the surface from FIG. 4a; and

[0032] FIGS. 5a and 5b show a schematic sectional view of the structured surface to explain a frictional connection with a second component.

DETAILED DESCRIPTION OF THE INVENTION

[0033] A connecting rod of a motor vehicle is schematically illustrated in FIG. 1 and generally designated by 10. The connecting rod 10 is preferably formed from titanium or a titanium alloy. However, it can also be formed from steel. The connecting rod 10 has an aperture 12 which forms a large connecting rod eye. On the aperture 12 there is formed an inner surface 14 which forms a contact surface for a bearing shell of the connecting rod 10. The inner surface 14 forms an inner circumferential surface of the connecting rod eye.

[0034] The inner surface 14 is, like the connecting rod 10 itself, preferably formed form titanium or else from steel, with a frictional connection customarily being formed between a bearing shell (not shown here) and the inner surface 14. The bearing shell customarily forms a sliding bearing with a crankpin of a crankshaft.

[0035] In order to form a frictional connection between the inner surface 14 and the bearing shell, the inner surface 14 is customarily first of all premachined by means of fine boring so as to grind the aperture 12 to a predefined diameter.

[0036] Since this step of fine boring forms rectilinear structures on the inner surface 14 in the circumferential direction and the surface has a low coefficient of friction, according to the prior art the inner surface 14 must be roughened or be structured after the step of fine boring in order to achieve a defined and sufficient roughness.

[0037] For the purposes of the invention present here, after the premachining step, which is preferably one of fine boring, the inner surface 14, which preferably forms a titanium surface to be machined, is structured in a defined manner or roughened in a defined manner by means of a laser beam in order to achieve a corresponding roughness value for a frictional connection with the bearing shell. Here, the laser beam structuring takes place in such a way that elevations but no depressions are formed as laser structures on the premachined surface 14 with respect to the level thereof.

[0038] FIG. 2 shows a schematic sectional view of the connecting rod 10 to explain the laser structuring. The inner surface 14 is irradiated by means of a laser beam 16 during the laser structuring such that corresponding laser structures are obtained by melting the surface consisting preferably of titanium and thus a structuring and a corresponding surface roughness can be achieved. Here, the laser beam 16 is a pulsed laser beam and preferably an Nd YAG laser beam. The laser beam 16 is preferably guided or moved along the surface 14 to be machined by a Galvo scanner with two mirrors.

[0039] The laser beam has a defined spot width. The inner circumferential surface 14 is preferably irradiated at different angular sections by the laser beam 16, which is moved in machining tracks along the circumferential surface 14 with a defined rate of advance, with individual spaced-apart structuring regions 18 or areas being generated on the inner surface 14. The track width of a machining track corresponds to the spot width of the laser.

[0040] Two structuring regions 18 are preferably generated on the inner surface 14, said structuring regions each spanning an angular section of 150° and each being separated from one another by a non-structured region 19 of 2°. Overall, there result four structuring regions 18 or structuring fields 18 which each cover an angular section of 74°, as is schematically illustrated in FIG. 1. The structuring regions are centrally oriented in the aperture 12, preferably in the axial direction of the aperture 12, in order to form a particularly symmetrical friction value.

[0041] In order, during laser beam structuring, to form exclusively elevations but no depressions as laser structures on the premachined surface 14 with respect to the level thereof, the preferably pulsed laser beam is operated, in the case of a steel component, with an average beam power of 9±0.5 watt. During the laser beam structuring of a steel component, the rate of advance of the pulsed laser beam is 500 to 700 mm/s, the pulse frequency of the pulsed laser beam is 40±3 kHz, and the pulse duration is approximately 5 μs. The track width of a machining track of the laser beam is 50±20 μm.

[0042] In the case of a titanium component, the preferably pulsed laser beam is operated with an average beam power of 8±0.5 watt. During the laser beam structuring of a titanium component, the rate of advance of the pulsed laser beam is 250 to 350 mm/s, the pulse frequency of the pulsed laser beam is 60±3 kHz, and the pulse duration is approximately 5 μs. The track width of a machining track of the laser beam is 50±20 μm.

[0043] Given such a material-dependent, average beam power, rate of advance, pulse frequency and pulse duration, a local melt occurs on the surface with the topography of the premachining where the laser beam inputs energy.

[0044] On cooling the melt, the topography of the premachining is not maintained, but, on account of the thermal expansion, there result raised structures and thus elevations or upward protuberances but no depressions with respect to the surface level of the premachined surface 14.

[0045] The raised structures fundamentally arise during the transition from the melt into the vapor phase. Owing to the cooling gradients which as it were freeze the instantaneous transition structure, there result fine upwardly directed ridges. In this context, the melt puddle is unsettled and gas inclusions occur which generate a lower density in the solidified melt, with the result that the volume balance is equalized and thus no depressions arise. Different parameterization can also result in depressions, but these are not relevant tribologically for the purpose of increasing the friction.

[0046] FIG. 3 illustrates a schematic sectional view of the inner surface 14. The inner surface 14 has a uniform surface structure which is generated by the premachining step of fine boring and is oriented in the circumferential direction of the aperture 12. The surface structure has in general a height R, as is schematically illustrated in FIG. 3.

[0047] The inner surface 14 or the surface 14 of the aperture 12 also has a plurality of elevations or melt ridges 20 which have been generated by the laser structuring on the surface consisting preferably of titanium.

[0048] The elevations or melt ridges 20 have a height h which is up to 10 μm, in particular up to 5 μm. The elevations or melt ridges 20 can significantly increase the roughness of the surface consisting preferably of titanium, with the result that a frictional connection with another component, such as, for example, the bearing shell, is possible.

[0049] FIG. 4a illustrates a scanning electron microscope (SEM) image of the inner surface 14 and FIG. 4b illustrates a schematic view of this inner surface 14.

[0050] The surface structure which has been generated by fine boring is formed by parallel strips which are oriented in the circumferential direction of the aperture 12. The circumferential direction is schematically illustrated in FIG. 4b by an arrow 22.

[0051] The laser structures, namely elevations or melt ridges 20, which have been generated by the laser beam 16 on the inner surface 14 along the machining tracks of the laser beam 16, are formed as overlapping circular structures which are preferably formed in parallel rectilinear lines of respectively overlapping circular structures on the inner surface 14. The machining tracks or the rectilinear parallel lines are oriented orthogonally to the circumferential direction 22 and thus loading direction of the connecting rod in operation, as is shown by an arrow 24 in FIG. 4b.

[0052] The orientation of the laser structures on the inner surface 14 makes it possible to achieve an increased coefficient of friction in the circumferential direction 22, with the result that a rotationally fixed connection can be formed between the inner surface 14 and the bearing shell.

[0053] The circular laser structures, namely the elevations or melt ridges 20, or the parallel lines of respectively overlapping circular structures have a structure width b of 50±20 μm, preferably 65 μm.

[0054] The laser structures, namely the elevations or melt ridges 20, also have a spacing a of at least 50% of the track width of the machining tracks and thus of the structure width b, preferably a spacing a of 55 μm. The degree of overlap of the individual circular structures within each machining track is preferably 60% or 60-80%.

[0055] By virtue of the thus formed elevations or melt ridges 20 in circular laser structures which are arranged in an overlapping manner in rectilinear rows on the inner surface 14, the coefficient of friction of the surface of the aperture 12 can be significantly increased, with the result that a frictional connection with a second component, such as, for example, the bearing shell, can be formed.

[0056] FIGS. 5a and 5b illustrate the inner surface 14 in a schematic sectional view together with the bearing shell 26. The bearing shell 26 is brought into contact with the inner surface 14, as is schematically indicated by arrows 28 in FIG. 5a, with the inner surface 14 forming a frictional connection with the bearing shell 26 by means of the laser ridges 20, as is shown in FIG. 5b. It is thereby possible for a rotationally fixed connection to be formed between the bearing shell 26 and the inner surface 14 with technically little effort, to be precise by the structuring by means of the laser beam 16.

[0057] It will be understood that the method for structuring the surface is also applicable for other surfaces, for example for cams which are intended to form a frictional connection with a shaft tube and together form a camshaft of a motor vehicle.

[0058] A metal component 10 according to aspects of the invention is particularly embodied as a connecting rod or cam of a motor vehicle. Said component has a premachined surface 14 which is laser-structured by means of a laser beam 16. The premachined surface 14 is laser-structured in such a way that elevations but no depressions are formed as laser structures with respect to the level of said surface.

[0059] The elevations are formed in a plurality of tracks of elevations on the surface 14 to be machined, said elevations being formed along the machining tracks of the laser beam. The elevations have an overlap of preferably 60-80% within a track, with directly adjacent tracks of elevations being spaced apart free of overlap.

[0060] The metal component is preferably a titanium component, in particular a titanium connecting rod of an internal combustion engine.