Method and device for laser coating

Abstract

In a method for producing a coated workpiece, a coating is applied to at least one surface (115) of the workpiece (110) using laser radiation. For this purpose, a first laser beam directed onto the surface is generated by guiding laser radiation emitted by a laser source through beam-shaping optics of a laser processing head onto a first impact zone on the surface. A preferably powdered filler material is fed to the first laser beam in such a way that the filler material is heated and / or at least partially melted in an interaction zone by laser radiation of the first laser beam and bonds with the heated material on the surface in the region of the first impact zone (225-1).A relative movement is generated between the workpiece (110) and the laser processing head such that the first impact zone moves in a feed direction at a feed rate along a feed path, leaving a track (228) of coating material behind it. Furthermore, at least one second laser beam directed onto the surface is generated for post-treating the coating material. Laser radiation emitted by a laser source is guided by focusing optics of the beam-shaping optics of the laser processing head onto a second impact zone (225-2) lagging behind the first impact zone in the feed direction, and acts on the track of coating material.

Description

FIELD OF APPLICATION AND PRIOR ARTThe invention relates to a method for producing a coated workpiece, wherein a coating is applied to at least one surface of the workpiece using laser radiation, and to a device suitable for carrying out the method.A preferred field of application is the production of coated brake disks. The manufacturing process of a coated brake disc comprises one or more coating operations for coating the surfaces of the brake section of a brake disc with a functional layer which can have a wear-reducing function due to relatively high mechanical hardness. Alternatively or additionally, a corrosion-inhibiting effect can also be present. Such functional layers often consist essentially of metal or a metal-ceramic composite; they can have a single layer or a plurality of layers of different properties.Nowadays, functional layers or coatings on brake disks and other components are frequently produced by means of laser build-up welding. In laser build-up welding, a surface of a component is melted by means of a laser beam and a preferably pulverulent filler material is supplied to the melt bath formed in the process. The powder is likewise melted in the melt bath, so that after the melting powder material and the surface solidify, a material layer is formed that is bonded in a materially bonded manner, in particular bonded in a melt metallurgical manner. When metallic material is applied, the application method is also referred to as "laser metal deposition" (LMD). Laser build-up welding is also used in the field of repair and bonding techniques.Nowadays, the method "Extreme High-Speed Laser Material Deposition" (EHLA) is frequently used to achieve high application powers. This is a variant of laser build-up welding, in which the melting of the build-up powder does not take place on the surface of the body to be coated (e.g. brake disc), but before the material reaches the surface. As a result, relatively thin layers can be produced at a relatively high speed. This method was developed at the Fraunhofer-Institute for Laser Technology ILT and the Rheinisch-Westfüzisch Technische Hochtschule Aachen (RWTH Aachen) (cf. DE 10 2011 100 456 A1).Depending on the coating process and the coating material, the free surfaces of the finished functional layers may have different properties. The functional layers are generally mechanically relatively hard and relatively rough on the surface after coating. As a rule, a post-treatment is required in order to achieve the surface quality required for the intended use. In the case of brake disks, a sufficiently planar surface optimized for the braking function can be produced on the coating by a subsequent grinding process.It is also known to posttreat coatings produced by laser deposition welding by means of laser radiation. The patent specification DE 10 2020 106 822 B4 describes an apparatus for laser deposition welding having a laser deposition welding unit with a laser deposition welding head arranged thereon for applying material in the form of a deposition welding track to a surface of a component, one or more material sources for supplying the laser deposition welding head with the material to be deposited and a laser beam source for supplying the laser deposition welding head with laser light for carrying out the laser deposition welding. The device further comprises a material smoothing unit, which is provided to smooth structures of the applied material protruding from the surface by means of a laser beam directed onto the surface, wherein the smoothing laser beam has a laser focus and a laser energy, which are set such that the structures evaporate in the laser beam and at the same time a temperature of the surface outside the structures remains below an evaporation temperature of the applied material. This is intended to allow less wear-intensive post-processing effort.OBJECT AND SOLUTIONAgainst this background, it is an object of the invention to improve a method of the type in question and a device of the type in question in such a way that high-quality functional layers or coatings can be produced with relatively little technological outlay.To achieve this object, the invention provides a method having the features of claim 1. There is further provided an apparatus having the features of claim 10.Preferred developments are specified in the dependent claims. The wording of all claims is made the content of the description by reference.According to one aspect of the invention, in a method for producing a coated workpiece, a coating or a functional layer is applied to at least one surface of the workpiece using laser radiation. In this laser-assisted coating process, a first laser beam directed onto the surface is generated by laser radiation emitted by a laser source being guided through a beam shaping optical unit of a laser processing head onto a first impingement zone on the surface. The workpiece material is thereby heated locally limited in the region of the impact zone by the introduced laser energy. The temperature of the workpiece material can remain below its melting temperature, so that the workpiece material does not melt and-unlike traditional laser build-up welding-no melt bath is produced by the first laser beam. However, it is also possible to adjust the energy of the first laser beam in such a way that a locally limited melt pool with workpiece material is thereby produced.The method further comprises the feeding or a feeding of at least one filler material to the first laser beam in such a way that the filler material is heated and / or at least partially melted in an interaction zone by laser radiation of the first laser beam and is joined to the heated workpiece material in the region of the first impact zone. Preferably, a powdered filler material is used. Powder particles pass into the region of the first laser beam and are thereby heated, predominantly up to the melting temperature or even above, so that the powder particles are at least partially transferred into the molten phase.The interaction zone usually comprises the focus region of the first laser beam and / or regions of increased power density of the laser beam in the vicinity of the focus region. The focus region can lie in the region of the surface, similar to traditional laser build-up welding, so that the added filler material melts only in the melt bath to a predominant extent. It is also possible that-similar to the EHLA method-the interaction zone and / or the focus region lies at a certain distance in front of the surface, so that at least a portion of the filler material is at least partially melted already before contact with the workpiece and melts in this melted state onto the workpiece surface and / or-provided a melt bath is present-in the melt bath.In the method, a relative movement between the workpiece and the laser machining head is generated in such a way that the first impingement zone moves in a feed direction at a feed speed along a feed path and a track of coating material is produced behind it. The relative movement can be effected exclusively by movement of the workpiece, for example by rotation about an axis of rotation, when the laser machining head is stationary. It is also possible to stop the workpiece and move the laser machining head. A combination of movements of laser machining head and workpiece is also possible, for example during the coating of rotationally symmetrical surfaces, in that the workpiece rotates about an axis of rotation and the laser machining head is simultaneously moved radially with respect to the axis of rotation in such a way that a spiral course of the track with coating material is produced.In the method, at least one second laser beam directed onto the surface is generated in addition to the first laser beam in order to posttreat the coating material. Such a post-treatment can improve the layer properties due to the energy input of the laser radiation from the second laser beam in comparison to a material application process without laser-supported post-treatment.A device of the generic type for producing a coated workpiece by applying a coating to at least one surface of the workpiece using laser radiation contains corresponding devices for realizing the method steps. This includes at least one workpiece receiving device for receiving a workpiece to be coated and at least one laser processing head having a beam shaping optics for receiving laser radiation from a laser source and for generating a first laser beam directed onto the surface of the received workpiece in such a way that the first laser beam can be guided onto a first impact zone on the surface in order to heat the workpiece in the region of the first impact zone. Furthermore, devices for feeding at least one filler material into the region of the first laser beam are provided, which are designed such that the filler material can be heated in an interaction zone by laser radiation of the first laser beam and can be melted completely or at least partially and impinges on the heated surface in the region of the first impingement zone in order to connect to the material of the surface there. Furthermore, a movement system for generating a relative movement between the laser machining head and the workpiece is provided, which is configured such that the first impingement zone moves along the surface in a feed direction at a feed speed and a track of coating material is formed behind it in the coating operation. Furthermore, the apparatus has devices for generating at least one second laser beam directed onto the surface for the purpose of post-treatment of the coating material.A particular feature of methods and apparatus according to the claimed invention is the manner in which the second laser beam is guided. The at least one second laser beam directed onto the surface is namely guided (during operation of the device) in such a way that laser radiation emitted by a laser source is guided through the focusing optics of the beam shaping optics of the laser processing head onto a second incidence zone, which is trailing in the feed direction with respect to the first incidence zone, and acts on the track with coating material.According to this proposal, therefore, one and the same focusing optics are used to guide both the first laser beam and the second laser beam into the associated impingement zones. The focusing optics is that part of the beam guiding optics of the laser processing head which acts with refractive power and which lies closest to the processing plane of the laser processing head and during operation of the workpiece surface. Optionally, a transparent protective plate can also be connected upstream. The focusing optics may consist of a single focusing lens or a lens group with two or more lenses which have a focusing effect overall. From a constructional point of view, this makes it possible to save a focusing optics of its own for the second laser beam.From a process engineering point of view, it can be achieved in this way that the first impact zone and the second impact zone are very close to one another or are behind one another or can even partially overlap. As a result, the second laser beam can act on the coating material of the coating material track directly upon and / or after the formation of the connection-leading contact between the heated coating material and the workpiece. In this case, since the two laser beams are guided through the same focusing optics of the laser processing head, it is systematically very simple, without special outlay, to ensure that the second region of incidence is always well aligned, for example centered, with respect to the track of the coating material, if required. Due to the small distance between the first and second laser beam or between the centers of the first and second impingement zones, it may be that, given a corresponding expansion of the impingement zones, the heat influence zones of the two impingement zones overlap one another.Among other things, during the coating process it can optionally be achieved that the cooling curve of the coating material runs with a lower cooling rate in the region of the first impingement zone immediately after the beginning of the bond generation than in the case of conventional systems without a lagging second laser beam. This often allows more favorable conditions to be created for the solidification of the coating material. If necessary, it can also be achieved that the coating material remains in the molten state for a longer time compared to conventional methods, so that a more intimate bond to the underlying material results.Thus, the inventors have observed that, under otherwise similar processing conditions in the region of the first laser beam, the post-treatment by the second laser beam can lead to significantly smoother coating surfaces and that fewer cracks and / or only even smaller cracks and / or pores remain in the coating material than in the case of a similar coating track without post-treatment by a second laser beam. In addition, the tendency to delamination seems to be less than in conventional methods. As a result of the heat input by means of the second laser beam immediately after the generation of the coating track, the build-up of layer stresses can also be effectively counteracted according to the findings of the inventors, so that layer separations due to layer stresses could also be observed much less frequently than in the case of similar processing conditions without aftertreatment by means of a second laser beam.A further advantage results with regard to the protection of the coating material against oxidation and / or other disruptive influences of the ambient atmosphere. In many embodiments, a protective gas is conducted for this purpose into the region of the formation of the coating. A protective gas, such as argon, can be conducted, for example, through a housing of the beam guiding optics and emerge through beam exit openings for the laser beams in order to form a protective gas cloud or bell jar which envelopes the hot point of origin of the coating. Because the two laser beams are situated close to one another, the impingement zones can lie within the same protective gas cloud, so that oxidation of the hot coating material is prevented in the time interval between the formation of the coating and the action of the second laser beam. This contributes to an increase in the coating quality.By guiding the first laser beam and the second laser beam through the same focusing optics of the same laser processing head, the first and the second impingement zone can be very close to one another. According to a further development, a center distance between the first impact zone and the second impact zone is less than ten times as large, in particular less than five times as large, as a maximum diameter of the first or the second impact zone. The center distance can optionally also be smaller than the maximum diameter of the first or second impact zone, so that the impact zones optionally partially overlap. In particular, if the first and the second impingement zone partially overlap, a heat influence zone elongated in the feed direction can be produced, which offers particularly favorable cooling conditions for the coating material.According to a further development, the center distance between the first impact zone and the second impact zone can be adjusted over a certain adjustment range, and in particular preferably continuously or continuously. As a result, the cooling behavior can be influenced in a sensitive manner and adapted to the other processing parameters, among other things. This variant thus offers a further degree of freedom of process control. In some variants, the center distance can be reduced to such an extent that the impact zones overlap partially or completely.The size of an impact zone is determined substantially by the cross-sectional size of the laser beam and the position of the focus zone with respect to the workpiece surface or to the surface to be coated. The focus zone or the focus region is a region extending in the beam direction, in which a waist of the beam cross section is present. If this region of minimum diameter of the focus zone or of the focus region is located in the workpiece surface, then minimally large impact zones are produced. If, on the other hand, the laser beams are defocusing, so that, for example, the focus region lies at a distance above the workpiece surface, the impact zones become correspondingly greater and the local power density decreases in the region of the impact zones. The diameters of the impingement zones can be, for example, in the range from 1 mm to 10 mm, in particular in the range from 2 mm to 5 mm.A method variant which in many cases leads to coatings with a high material density and a relatively smooth surface adhering firmly to the workpiece is distinguished in that the second laser beam acts on the coating material of the track in such a way that, essentially without material removal, a diffusion-controlled redistribution of coating material leads to a reduction in surface roughness and / or to the closing of cracks and / or pores. In this method variant, an essential effect of the second laser beam is that it ensures that the temperature in the freshly formed coating is relatively high over a relatively long period of time, so that an efficient redistribution of coating material, which is controlled essentially via surface diffusion, can take place, preferably without the coating material melting up again and / or evaporating.According to a further development, the operating conditions are set such that a maximum temperature of the coating material in the region of the second impingement region is continuously lower than the evaporation temperature of the coating material, so that no coating material is lost and there is no need for a separate extraction of material vapor.According to a further development, a ratio of the power densities of the first and of the second laser beam can be adjusted in the region of the respective impingement zones. A power density ratio LV between a first power density L 1 in the region of the first impact zone and a second power density L 2 in the region of the second impact zone can be, for example, in the range from 0.1 to 10, in particular in the range from 0.3 to 3.Some method variants are distinguished in that a power density of the second laser beam is set such that, as a result of heat input by means of the second laser beam, a cooling rate of the coating material after emergence from the first region of impingement is reduced compared to processing without a second laser beam. Since the second laser beam impinges on the coating material track temporally and spatially immediately after the formation of the coating in the region of the first impingement zone, the essential effect of the second laser beam can be to slow down the cooling of the coating material without the latter first cooling down more strongly and subsequently being heated again by a second laser beam. The process can be run in such a way that the second laser beam does not lead to an increase in the local temperature of the cooling coating track, but only to a slower cooling. If necessary, the coating material can also be kept molten for longer than in conventional methods, as a result of which inter alia the adhesion of the layer to the substrate can be improved.It is possible to use two different laser sources for the generation of the first laser beam and for the generation of the second laser beam. It is thus possible to operate with different laser radiation in the second incidence zone than in the first incidence zone, for example at a different wavelength and / or in pulsed operation instead of continuous line operation or the like. Overall, this results in high flexibility for optimizing the process parameters. However, increased structural complexity for beam guidance up to the coupling into the beam shaping optics of the laser processing head then arises.In other embodiments, the same laser source is used to generate the first and second laser beams. This results in, among other things, a simplified construction of the overall device.According to a development, that portion of the laser radiation which forms the first laser beam and that portion of the laser radiation which forms the second laser beam pass not only through the focusing optics of the laser processing head but through the entire beam shaping optics of the laser processing head. The coupling-in of the laser radiation is thereby particularly simple.There are variants in which a laser beam coming from the laser source is divided into the first and the second laser beam within the beam shaping optics of the laser processing head. A suitable beam splitter can be provided for this purpose. According to a further development, the beam shaping optics has a geometric beam splitter in the region between a collimating optics and a focusing optics, which splits the laser light largely parallelized by the collimating optics into two partial beams, one forming the first laser beam and the other forming the second laser beam, which are then guided by the same focusing optics into the mutually offset impingement zones.Other variants are distinguished in that the second laser beam is coupled in in the region between a collimating optics and the focusing optics of the beam shaping optics, and the beam shaping optics has a coupling element, in particular a deflection mirror, in the region between the collimating optics and the focusing optics, which deflects the second laser beam in the direction of the focusing lens. An adjustment device for, preferably continuously, adjusting a deflection angle can be provided. The distance between the impact zones can thus be changed or adjusted.According to a further development, it is provided that a distribution of the laser energy between the first laser beam and the second laser beam is variably adjustable or is set to a suitable ratio. The adjustment can preferably be effected continuously. This makes it possible to adapt the combined effect of the first and the second laser beam optimally to the other process parameters.BRIEF DESCRIPTION OF THE DRAWINGSFurther advantages and aspects of the invention are evident from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures. FIG. 1 schematically shows an exemplary embodiment of a laser coating machine for coating workpieces in the form of brake disks; FIG. 2 shows a schematic top view of the rotating brake disc during a coating operation; FIG. 3 shows a schematic view of the exit side of the laser machining head facing the workpiece; FIG. 4 shows a schematic side view of the beam guidance in the laser processing head and the formation of the coating during a coating operation; FIG. 5 shows a schematic temperature-time diagram with different cooling curves; FIGS. 6A, 6B show micrographs of coated brake disks after completion of a conventional coating process (FIG. 6A ) and after completion of a coating process according to an exemplary embodiment (FIG. 6A ). FIG. 7 schematically shows components of a beam shaping optical system and devices for continuously adjusting the center distance between impingement zones of the laser beams.DETAILED DESCRIPTION OF THE EMBODIMENTSFIG. 1 schematically shows an exemplary embodiment of a device 100 for producing a coated workpiece by applying a coating to at least one surface of the workpiece using laser radiation. The device 100 is configured as a coating machine for coating workpieces 110 in the form of brake disks.The term "coating" generally denotes some subject matter, namely a functional layer applied by the act of coating (coating), which functional layer can have a single layer or a plurality of layers lying one above the other. The act of coating is sometimes also referred to as coating.A brake disc has a base body 112, which consists for example of gray cast iron, with a central hub section, which serves for fastening the brake disc to a vehicle axle, and an annular brake section 115, which encloses the hub section. The mass distribution of the base body is overall rotationally symmetrical with respect to the axis of rotation 114 of the brake disc. The braking portion has two axially opposed surfaces 116, 117 parallel to each other. These should each be provided with a coating or functional layer which is rotationally symmetrical with respect to the axis of rotation and the free surface of which, after a subsequent grinding operation, is finally intended to serve as the friction surface of the brake disc.In the example, the annular braking surfaces on both sides are to be coated by means of a modification of high-speed laser build-up welding.The device has, for each brake disk, a workpiece receiving device 120 in the form of a workpiece spindle, which can be rotated about a vertical spindle rotation axis 122 by means of a spindle drive. The rotational speed of the workpiece spindle is continuously adjustable. The direction of rotation can also be adjusted via the control.A brake disk is accommodated in a horizontal orientation, i.e. with a vertically oriented rotational axis ("disk player arrangement") on the workpiece spindle and clamped in a rotationally fixed manner such that the rotational axis 114 of the brake disk is coaxial to the spindle rotational axis. There are also embodiments in which the workpiece axis of rotation is oriented horizontally or obliquely to the vertical.The apparatus comprises a laser processing head 200 which contains a focusing beam shaping optical unit 210 which receives laser radiation from a laser source and shapes therefrom laser radiation which impinges on the surface 116 of the picked workpiece in a more or less focused form and locally heats the latter.The laser beam emitted from the laser source is supplied to a light output 216 via an optical fiber cable 211. The laser beam divergently emerging there is parallelized by means of a collimating lens 212. The collimated laser radiation passes through a sectionally sleeve-shaped component 213. Within this component 213, a focusing lens or focusing optics 215 for focusing the laser radiation is arranged. The focusing optics can be designed in the manner of an f-theta objective. After passing through the focusing optics, the laser radiation passes, among other things, through an exit-side section of the component 213. The entire assembly with light outlet, collimating lens 212 and component 213 can be linearly displaced by means of a linear drive using an electric motor (double arrow). The laser machining head can be aligned parallel to the axis of rotation (as in the schematic FIG. 1 ) or can be set at an angle inclined to the axis of rotation by an adjustable angle of inclination.The apparatus 100 further comprises devices 230 for feeding at least one pulverulent filler material into the region of the emerging laser radiation in such a way that the filler material fed in the form of powder jets 233 can be heated and / or at least partially melted by laser radiation in an interaction zone 222 and in this aggregate state impinges on the surface 116 heated by the laser radiation. The devices include a powder conveyor 232 for conveying powdered filler material. In the powder conveyor 232, a gas, in particular an inert gas such as nitrogen or argon, is added to the powder in order to generate a powder gas stream for conveying the powder. In a distributor component 234, the powder gas stream is distributed into a plurality of feed hoses 235, then flows into the outlet-side section of the component. This has a multiplicity of powder guide channels 236, the longitudinal axes of which lie on a common conical surface, the cone tip of which coincides with the optical axis of the beam shaping optical unit 210 (cf. FIG. 3 ). Powder jets 233 emerge there during operation. Alternatively, an annular nozzle can also be provided, which conducts powder from all sides in the direction of the first laser beam.Furthermore, the laser machining head has inert gas supply devices, not shown, for supplying inert gas into the region of the first impingement zone. The protective gas, e.g. argon, can be introduced into the housing of the beam shaping optics, e.g. in the upper region, and emerge downward through the beam outlet openings for the laser beams or a beam outlet opening common to both laser beams, e.g. elongated beam outlet opening, in the direction of the formation of the coating track, in order to form a protective gas bell there, which encloses the hot region of the interaction between the two laser beams and the coating material and prevents oxidation of the coating material.A particular feature of the laser processing head is that it is constructed in such a way that, starting from the laser radiation emerging at the light outlet 216, two spatially separate laser beams can be generated, which are each focused by the same focusing optics 215 and impinge on the plane of the workpiece surface in two impingement zones which are laterally offset with respect to one another. For this purpose, the laser radiation coming from the same laser source is split into a first laser beam 220- 1 and a second laser beam 220- 2. The first laser beam impinges on the workpiece surface 116 in the region of a first impingement zone 225- 1, while the second laser beam 220- 2 impinges on the surface in the region of a second impingement zone 225- 2, laterally offset with respect to the first impingement zone.The powder is supplied via powder jets 233 substantially concentrically to the first laser beam 220- 1, so that the filler material is supplied to the first laser beam in such a way that the filler material is heated and / or partially or completely melted in an interaction zone 222 by the laser radiation of the first laser beam 220- 1 and then connects to the heated material in the surface of the workpiece in the region of the first impingement zone 225- 1. Deviating from the schematic illustration in FIG. 4, the interaction zone is usually located at a distance (for example in the range of a few millimetres) above the surface to be coated.The protective gas supply devices of the laser processing head are designed such that the first and the second impingement zone lie within a common protective gas bell.The laser machining head is oriented with respect to the rotational direction of the workpiece in relation to the latter such that during the rotational relative movement between the workpiece and the laser machining head, the first impingement zone 225- 1 is moved in a feed direction (arrow 237) at a feed speed along a feed path and a track 228 with coating material is produced behind it on the workpiece surface. The second laser beam 220- 2, on the other hand, impinges in the second impingement zone 225- 2, which lags behind the first impingement zone 225- 1 as viewed in the feed direction, so that the second laser beam 220- 2 impinges on the coating material track just formed at a small distance behind the first laser beam 220- 1.The second impact zone 225- 2 heated by the second laser beam 220- 2 is produced at a very short distance behind the first impact zone 225- 1 generated by the first laser beam. Typical average diameters of the impingement zones can be, for example, in the order of 2 mm to 5 mm. A center distance 226 between the two impingement zones measured in the feed direction is generally less than five times as large or less than twice as large as the larger of the two diameters, i.e. the maximum diameter, of the impingement zones.At typical feed speeds along the (substantially spirally extending) relative movement between the workpiece and laser beams, the laser radiation of the second laser beam 220- 2 thus impinges on the still hot and possibly not yet solidified or not completely solidified trace of coating material immediately after the formation of the coating in the region of the first impingement zone 225- 1. At a feed speed of the order of 200 m / min, a center distance of, for example, 4 mm corresponds to a time offset between the incidence of the first and the second laser beam of the order of approximately 1.2 milliseconds (ms).The laser energy density or laser fluence impinging on the coating material track in the region of the second impingement zone 225- 2 is dimensioned such that no coating material is evaporated or removed in some other way by the second laser beam 220- 2. However, the heat input leads to the solidification process of the coating material assuming a different time profile than in conventional coating methods without this after-heating. In particular, the direct after-heating leads to the cooling rates of the coating material formed in the region of the first impingement zone 225- 1 being significantly lower than in the conventional method in comparison with conventional methods.To illustrate the influence of the second laser beam 220- 2 on the cooling rate, FIG. 5 shows a schematic temperature-time diagram in which the temperature T of the coating material is plotted as a function of the time t. The arrows represent the first laser beam 220- 1 and the second laser beam 220- 2, respectively, and on the time axis those times t 1 and t 2, at which the first and the second laser beam, respectively, strike the coating material in their corresponding impingement zones. The spatial distance of the centers of the impingement zones here is a few millimeters, for example 2 mm to 4 mm, and the temporal distance Δt is in the range of one millisecond. In the region of the first impingement zone, i.e. where the first laser beam and the particles melted thereby strike the workpiece surface, the temperature T lies above the melting temperature T s of the metallic portion of the powdered filler material.The dashed curve AK 1 schematically shows the cooling curve of the coating material in the coating track 228 after leaving the first impingement zone in the conventional method without after-heating. In comparison thereto, the solid line AK 2 schematically represents an assumed profile of the cooling curve with after-heating in the second impingement zone. As soon as the surface coated with coating material moves into the region of the second impingement zone 225- 2, the cooling is slowed down by the corresponding heat input, so that the cooling curve AK 2 runs flatter than the cooling curve AK 1 in a conventional method. This has a slowing effect, among other things, on the solidification speed of the melted metallic portion of the coating material, so that a different structure is also formed. Since after the initial phase of cooling the temperature level remains higher for a longer period of time than in conventional methods, an effective redistribution of material by diffusion processes can also take place. The cooling curve AK 3 represents a process control in which the laser energy densities of the two laser beams are set such that the coating material remains in the temperature range above the melting temperature for a longer time compared to the prior art, as a result of which a more solid melt-metallurgical bond is formed between the coating material and the material lying beneath it (base body or previously applied coating).It has been found that, on account of the laser-assisted after-heating, the solidified coating material after cooling has significantly fewer cracks and / or pores than coating material which was applied under otherwise identical process parameters without the direct after-heating. Among other things, in many cases a considerable reduction in the surface roughness of the finished coating could be found in comparison with conventionally produced coatings without this after-heating. FIGS. 6A and 6B show, for the purpose of illustration, two micrographs of coated brake disks, in which the base bodies 112 consisting of gray cast iron were each coated with a two-layer coating 240 which comprises an adhesive layer 241 consisting essentially of stainless steel, on which a wear-resistant wear layer 242 has been applied which has wear-resistant carbide particles in a stainless steel matrix.FIG. 6A shows the micrograph of a conventionally produced coating. In the example, the free surface of the wear layer has an average roughness depth R a( average roughness value) in the range from 6 μm to 10 μm. In addition to a certain macroscopic waviness, there are also small cracks distributed over the surface and / or coating material residues and / or pores protruding beyond the surface.FIG. 6B shows the micrograph of a comparable brake disk which was produced by a coating method according to the invention with a lagging second laser beam. It can be seen with the naked eye that the free surface of the wear protection layer hardly still has the corrugation that can be easily seen in FIG. 5A. In addition, almost no protruding particles are found. Furthermore, larger pores or cracks are not visible. The mean roughness R a or the mean roughness value of the free surfaces is in this example approximately 4 μm to 6 μm, which means a significant reduction in the surface roughness compared to the conventional procedure.The functional layers produced with after-heating assisted by laser radiation also have a lower tendency to delamination than conventionally produced coatings. This results in improved layer adhesion between substrate and coating.There are numerous possibilities for the technical realization of the after-heating with the aid of a trailing second laser beam 220- 2. An example is explained with reference to FIG. 7, with which it is possible, among other things, to continuously change the center distance 226 between the impingement zones of the two laser beams. For reasons of clarity, the same reference numerals are used for the same or similar components as in the other exemplary embodiments.FIG. 7 schematically shows components of a beam shaping optics 210 having a collimating lens 212 and a focusing optics 215 in the form of an f-theta objective, which is arranged at a distance behind it and represents the last optical element having refractive power in front of the processing plane 211 of the laser processing head. The processing plane 211 is the plane in which the effect of the focused laser radiation and the impinging filler material particles is to occur. The workpiece surface 116 is disposed in or near the machining plane for machining. Collimating lens 212 and focusing optics 215 define optical axis 213 of that portion of the beam forming optics. The laser radiation for forming the first laser beam 220- 1 impinges divergently on the collimating lens centered on the optical axis, which generates radiation parallelized therefrom, which radiation is then focused by the focusing lens to a first laser focus which is arranged in the surface or in the vicinity thereof (for example shortly above). There is the first impingement zone 225- 1.Between the collimating lens 212 and the focusing optics 215, an adjustable deflection mirror 230 is arranged in the region of the optical axis, which mirror can be continuously tilted about a tilting axis directed perpendicular to the optical axis via a suitable adjustment direction. In the example, the deflecting mirror serving as the coupling device is oriented such that it deflects the second laser beam 220- 2 irradiated perpendicularly to the optical axis such that it runs obliquely to the optical axis in the direction of the focusing optics and is focused by the latter in the region of the second incidence zone 225- 2. By adjusting the angle of inclination, the angle of incidence W of the oblique beam portion can be adjusted with respect to the optical axis 213, as a result of which the distance (center distance) between the zones of incidence can also be adjusted continuously. At the same time, the adjustable deflection mirror 213 shades a small portion of the radiation propagating in the vicinity of the optical axis, so that, when the first laser beam 220- 1 is slightly defocusing in the region of incidence, the radiation intensity of the laser beam in the central region is reduced in comparison with variants without central shading. This can contribute to the homogenization of the local distribution of the laser energy in the first impingement zone 225- 1 and thus to an improvement in the layer formation.In this variant, the second laser beam 220- 1 may originate from a second laser source provided in addition to a first laser source generating the first laser beam 220- 1. However, it is also possible to use only a single laser source and to branch off a portion of its radiation by suitable beam splitting and then to couple it between collimating lens and focusing optics in the manner shown.Splitting of the incident laser radiation into two partial beams can also be achieved with the aid of at least one diffractive optical element (DOE).It is possible to use the laser processing head for carrying out other method variants of a coating process. For this purpose, for example, the direction of rotation of the coating unit can be reversed via a switch or button on the operating unit of the coating machine. The workpiece material is then heated or heated by means of the second laser beam directly before it enters the region of the first laser beam, as a result of which it can be achieved that the coating material applied later adheres better to the material to be coated than without preheating.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 10 2011 100 456 A1

[0004] DE 10 2020 106 822 B4

[0006]

Claims

Method for producing a coated workpiece, wherein a coating is applied to at least one surface of the workpiece using laser radiation, comprising the following steps: - generating a first laser beam directed onto the surface by laser radiation emitted by a laser source being guided through a beam shaping optics of a laser processing head onto a first impact zone on the surface; - supplying at least one, preferably pulverulent, filler material to the first laser beam in such a way that the filler material is heated and / or at least partially melted in an interaction zone by laser radiation of the first laser beam and is joined to the heated material at the surface in the region of the first impact zone; generating a relative movement between the workpiece and the laser processing head in such a way that the first impingement zone moves in a feed direction at a feed speed along a feed path and a track of coating material is formed behind it, and generating at least one second laser beam directed onto the surface for the subsequent treatment of the coating material, characterized in that - the at least one second laser beam directed onto the surface is generated in that laser radiation emitted by a laser source is guided by a focusing optics of the beam shaping optics of the laser processing head onto a second impingement zone, which is trailing in the feed direction with respect to the first impingement zone, and acts on the track of coating material.Method according to Claim 1, characterized in that the first and the second laser beam are guided in such a way that a centre distance between the first impingement zone and the second impingement zone is less than ten times as large, in particular less than five times as large, as a maximum diameter of the first or second impingement zone, and / or in that a centre distance between the first impingement zone and the second impingement zone is set variably, in particular continuously, within an adjustment range.Method according to one of the preceding claims, characterized in that impingement zones are produced which have maximum diameters in the range from 1 mm to 10 mm, in particular in the range from 2 mm to 5 mm.Method according to one of the preceding claims, characterized in that a protective gas is conducted into the region of the formation of the coating, and in that the first impingement zone and the second impingement zone lie within the same protective gas cloud, so that oxidation of the hot coating material is prevented in the time interval between the formation of the coating and the action of the second laser beam.Method according to one of the preceding claims, characterized in that, by heat input by means of the second laser beam, a cooling rate of the coating material after emergence from the first impingement zone is reduced compared to processing without a second laser beam.Method according to one of the preceding claims, characterized in that the second laser beam acts on the coating material of the track of coating material in such a way that, substantially without material removal, a diffusion-controlled redistribution of coating material leads to a reduction in a surface roughness and / or to the closure of cracks and / or pores.Method according to one of the preceding claims, characterized in that operating conditions are set such that a maximum temperature of the coating material in the region of the second impingement zone is continuously lower than the evaporation temperature of the coating material.Method according to one of the preceding claims, characterized in that a power density ratio LV between a first power density L1 in the region of the first impact zone and a second power density L2 in the region of the second impact zone can be set, wherein the power density ratio LV is preferably in the range from 0.1 to 10, in particular in the range from 0.3 to 3, wherein the condition L1 > L2 preferably applies.Method according to one of the preceding claims, characterized in that the same laser source is used for generating the first laser beam and the second laser beam, wherein a laser beam coming from the laser source is preferably divided into the first and the second laser beam within the beam shaping optics of the laser processing head.Apparatus (100) for producing a coated workpiece by applying a coating (228) to at least one surface of the workpiece (110) using laser radiation, comprising: - a workpiece receiving device (120) for receiving a workpiece (110) to be coated; - a laser processing head (200) having a beam shaping optical unit (210) for receiving laser radiation from a laser source and for generating a first laser beam (220-1) directed onto the surface of the received workpiece such that the first laser beam can be guided onto a first impact zone (225-1) on the surface in order to heat the workpiece in the region of the first impact zone; means (230) for feeding at least one, preferably powder-form, filler material into the region of the first laser beam (220-1) in such a way that the filler material can be heated and / or at least partially melted in an interaction zone (222) by laser radiation of the first laser beam (220-1) and impinges on the heated surface in the region of the first impingement zone (225-1); a movement system for generating a relative movement between the laser processing head (200) and the workpiece (110) in such a way that the first impingement zone (225-1) moves along the surface in a feed direction at a feed speed and a track (228) containing coating material is formed behind the latter, and - devices for generating at least one second laser beam (220-2) directed onto the surface for post-treatment of the coating material, characterized in that - the devices for generating the second laser beam (220-2) are designed in such a way that the second laser beam (220-2) can be guided through a focusing optics (215) of the beam shaping optics (210) of the laser processing head onto a second impingement zone (225-2) which lags behind the first impingement zone (225-1) in the feed direction in order to act on the track (228) containing coating material.Device according to claim 10, characterised in that a centre distance (226) between the first impact zone (225-1) and the second impact zone (225-2) is less than ten times as large, in particular less than five times as large as a maximum diameter of the first or second impact zone and / or that a centre distance (226) between the first impact zone (225-1) and the second impact zone (225-2) is adjustable, in particular continuously adjustable, within an adjustment range.The device according to claim 10 or 11, characterized in that the device is configured such that the same laser source can be used for generating the first laser beam (220-1) and the second laser beam (220-2).Device according to one of Claims 10 to 12, characterized in that the beam shaping optical unit (200) has, in the region between a collimating optical unit (212) and the focusing optical unit (215), an optical element for generating and / or guiding the second laser beam (220-2), wherein the optical element is preferably a beam splitter, a diffractive optical element and / or a deflection mirror.Apparatus according to one of Claims 10 to 13, characterized in that the laser machining head has protective gas feed devices for feeding protective gas into the region of the first impingement zone, wherein the protective gas feed devices are designed in such a way that the first and the second impingement zones lie within a common protective gas bell.Device according to one of Claims 10 to 14, characterized in that the device has a workpiece receiving device (120) in the form of a workpiece spindle which is rotatable about a preferably vertical spindle rotation axis (122) by means of a spindle drive, wherein the rotational speed of the workpiece spindle and / or the rotational direction of the workpiece spindle can preferably be adjusted by means of the control device.Coated workpiece having at least one surface on which a coating is applied using laser radiation, in particular coated brake disc, characterized in that it is obtainable or produced by applying the method according to one of Claims 1 to 9 and / or using the device according to one of Claims 10 to 15.

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