Method for coating metal workpieces

Abstract

The invention relates to a method for coating a metal workpiece (50) by means of laser deposition welding, comprising the following steps: moving (S1), in particular rotating the workpiece (50) to be coated; irradiating (S2) a surface of the workpiece (50) by means of at least one first laser beam (L1) in order to produce at least one first irradiation zone (20) and a second irradiation zone (22) on the workpiece surface, the second irradiation zone (22) being in front of or behind the first irradiation zone (20) in the machining direction (40); and introducing an additional material (P), preferably in the form of powder, into the first irradiation zone (20), the additional material (P) entering the first laser beam (L1) at least partially before impinging on the workpiece surface and thus being at least partially heated. The invention also relates to a device for carrying out the method and to a workpiece (50) which can be produced by means of the method.

Description

Technical Field

[0001] The present invention relates to the field of laser deposition welding. In particular, the present invention relates to a method and a device for coating metal workpieces, in particular brake disks, by means of high-speed laser deposition welding. Background Art

[0002] Methods for high-speed laser deposition welding are known in the prior art and are described, for example, in DE 10 2011 100 456 B4. Compared to conventional laser deposition welding, this method achieves a significant increase in the achievable processing speed by supplying at least one additional material in at least partially molten form to a process zone located on the surface to be processed. To this end, the additional material, initially present, in particular in powder form, is heated by means of a laser beam at a distance from the workpiece surface and supplied to the process zone, which is also heated by the laser beam. In high-speed laser deposition welding, the feed motion—that is, the relative movement between the surface to be coated and the processing beam—is largely achieved by moving, in particular rotating, the workpiece to be coated. High-speed laser deposition welding can significantly increase the processing speed of the laser deposition welding process.

[0003] In high-speed laser deposition welding, also known as high-speed laser metal deposition (HS-LMD), a significant portion of the laser power is absorbed by the powder flow of the added material and the metal vapors produced during the machining process. However, to achieve a sufficient bond between the workpiece and the coating, it is important that the workpiece surface has a minimum temperature. Therefore, it is known to preheat the workpiece over a large area using induction.

[0004] However, heating such large areas is relatively complex and requires a relatively high energy consumption. Furthermore, the preheating temperature cannot be precisely adjusted. Furthermore, when preheating brake discs made of gray cast iron by induction, for example, the preheating temperature is limited because the surface of the brake element oxidizes at temperatures above approximately 300°C. This oxidized surface negatively impacts the coating process and coating quality.

[0005] The invention is based on the object of improving the coating of metal workpieces by means of laser deposition welding. In particular, even at high processing speeds, it should be possible to avoid bonding errors between the coating and the coated workpiece or between adjacent tracks of the coating (known as delamination). Summary of the Invention

[0006] The underlying object of the present invention is achieved by the subject matter of the independent claims. Further possible embodiments of the invention are described in detail in the dependent claims, the description, and the drawings. The features, advantages, and possible embodiments described in the description with respect to one of the objects of an independent claim are to be considered at least analogously to the features, advantages, and possible embodiments of the corresponding object of the other independent claims, as well as any possible combination of the objects of an independent claim, including the features, advantages, and possible embodiments in combination with one or more of the dependent claims.

[0007] According to a first aspect of the present invention, a method for coating a metal workpiece by means of laser deposition welding is provided. The method comprises: in a first step, moving, in particular rotating, the workpiece to be coated. The workpiece may preferably be a brake disc for a motor vehicle. The workpiece may have a base body that essentially consists of cast iron, in particular gray cast iron. An intermediate layer made of a material, preferably also a metallic material, in particular stainless steel, may be applied to the base body on the surface of the workpiece to be coated. For example, the intermediate layer serves to improve the bonding of the wear protection layer to be applied to the workpiece. Furthermore, the intermediate layer may serve to prevent cracks in the wear protection layer to be applied. The composition of the intermediate layer may be selected based on the material of the wear protection layer to be applied.

[0008] The method comprises, in a second step, irradiating the surface of the workpiece with at least one first laser beam to produce at least one first irradiation zone and a second irradiation zone on the workpiece surface, wherein the second irradiation zone precedes or follows the first irradiation zone in the machining direction. The first irradiation zone may also be referred to as a process zone. The second irradiation zone may also be referred to as a preheating zone or a postheating zone, depending on its placement upstream or downstream of the process zone. The machining direction is essentially determined by the motion of the workpiece. For example, when coating a brake disk, in addition to the rotation of the workpiece, the machining beam may also be moved in a radial direction, thereby forming a spiral machining path. In this case, the machining direction may be a tangent to the machining path starting from the center of the first irradiation zone. In addition to spiral machining paths, spiral machining paths or simple linear, circular, or other machining paths may also be achieved. Movement of the machining beam is not always necessary, in addition to movement of the workpiece. The workpiece surface is locally heated by at least the first laser beam in the process zone and the preheating or postheating zones.

[0009] In the third step of the method, an additional material, preferably in powder form, is introduced into the first irradiation zone (i.e., into the process zone). The additional material can preferably be introduced into the process zone in the form of one or more powder jets using a preferably inert carrier gas. Alternatively, the supply of the additional material in linear form is also conceivable. The additional material is supplied to the process such that it at least partially enters the first laser beam, preferably into its entire cross-section, at a predetermined distance from the workpiece surface before it impinges on the workpiece surface, and is thereby at least partially heated, preferably heated throughout the entire cross-section of the first laser beam. In this case, the additional material, or at least a portion of the additional material, at least partially melts, so that when it impinges on the workpiece surface in the also heated process zone, it can quickly bond with the workpiece material and produce a material bond with particularly low mixing of the connecting partner. The focus of the powder jets supplied from multiple directions can be in the region of the surface of the workpiece to be coated (e.g., in the surface plane), preferably above the workpiece surface. Depending on the application, the focus of the additional material supplied in powder form can also be below the workpiece surface, i.e., in or below the workpiece. The additional material preferably consists of a material with a higher wear resistance than the base body of the component. In particular, the additional material can include an iron-containing matrix material, in particular forming a stainless steel matrix in which hard material particles, such as tungsten carbide particles or titanium carbide particles, are embedded. During the coating method, the additional material preferably forms a flat wear protection layer on the workpiece surface. Wear protection can be enhanced by carbide deposits in the coating. In the case of coated brake discs, this method can increase the number of braking cycles and, therefore, the service life of the brake disc (in particular, by reducing wear during braking).

[0010] The method according to the present invention enables precise and efficient preheating of the workpiece surface along the machining path immediately before the additional material impinges on the process zone, or post-heating of the workpiece surface after it passes through the process zone, thereby improving the bonding of the additional material to the workpiece surface and avoiding bonding errors. The formation of other defects in the coating's material structure, such as pores, can also be counteracted. Compared to large-area preheating according to the prior art, the energy input into the workpiece is more targeted. Furthermore, the temperature can be adapted very quickly to changing thermal conditions during the coating process. This also improves the quality of the coating. Compared to large-area heating of the workpiece to be coated, the reduced energy input reduces the impact on the workpiece's metallurgical properties. In the method according to the present invention, preferably no significant molten pool is generated on the surface of the workpiece to be coated. The penetration depth (or impact depth) of at least the first laser beam into the workpiece to be coated can preferably be at most 20 μm, preferably at most 10 μm, and even more preferably at most 5 μm. In this case, preheating of the workpiece surface can in particular contribute to cleaning and / or degassing carbon from the workpiece to be coated, improving wettability of the workpiece surface with the additional material and / or improving the bond between the additional material and the workpiece.

[0011] In the method according to the invention, it can additionally be provided that an inert shielding gas, for example argon, helium or an inert gas mixture, is guided together with at least the first laser beam through the processing nozzle of the processing head onto the first irradiation zone in order to protect the welding process from reactions with oxygen from the ambient air and to prevent corrosion of the coating track or the heated workpiece surface.

[0012] In the coating method according to the invention, the resulting feed rate, ie the speed of the relative movement between the workpiece surface and the processing beam, can preferably be at least 20 m / min.

[0013] In the method according to the invention, it can also be provided that, in addition to the preheating by at least the first laser beam, the workpiece to be coated does not undergo any additional preheating, in particular no large-area preheating by induction. By eliminating the preheating time and the device for inductive preheating, the processing time and costs can be reduced.

[0014] According to a variant, a third irradiation zone can also be generated using at least the first laser beam. The third irradiation zone is arranged so that it is opposite the second processing zone, starting from the first irradiation zone. For simplicity, it is assumed below that the second irradiation zone is the front preheating zone and the third irradiation zone is the rear postheating zone.

[0015] In the method according to the invention, at least two laser beams can be used. Thus, a first irradiation zone can be generated by means of a first laser beam, and a second irradiation zone can be generated by means of a second laser beam. A third irradiation zone can additionally be generated by a third laser beam. The first laser beam, the second laser beam, and (optionally) the third laser beam can also each be constructed as partial beams of a common laser output beam (i.e., a first partial beam, a second partial beam, and (optionally) a third partial beam). Preferably, the intensity of the second laser beam and / or the third laser beam in the irradiation plane (i.e., the plane of the workpiece surface or the surface plane of the applied layer) is different from that of the first laser beam. In particular, the intensity of the second laser beam in the plane of the workpiece surface can be lower than the intensity of the first laser beam.

[0016] The second and / or third laser beams can also be supplied to the process from separate beam sources, for example laterally. In this way, the laser power of the individual laser beams can be easily adjusted independently of one another. This also makes it easier to position the preheating and / or postheating beams on the processing trajectory. Furthermore, laser beams with wavelengths different from those used for the actual coating process can be used for preheating and / or postheating. The wavelengths can be precisely adjusted to the material-specific coupling, and thus the efficiency of the preheating and / or postheating can be increased. Furthermore, the preheating and / or postheating beams can be radiated onto the workpiece surface or weld bead from outside the supply of the additional material and will not be affected, in particular, by the powder jet of the additional material arranged concentrically around the first laser beam.

[0017] As an alternative to using a plurality of individual laser beams or partial laser beams, it is possible to use only one laser beam, preferably with an asymmetrical beam profile (e.g., elliptical or rectangular) that is elongated in the machining direction, wherein a leading partial area of ​​the projection of the laser beam onto the workpiece surface forms the second irradiation zone, and a subsequent partial area of ​​the projection forms the first irradiation zone. The extension of the asymmetrical laser beam can additionally cover a third irradiation zone, which follows the first irradiation zone and post-heats the produced weld bead or the applied additional material, in order to additionally prevent bonding errors or delamination between the individual applied layers.

[0018] When using multiple individual laser beams or partial laser beams, the first laser beam and / or the second laser beam and / or the third laser beam can have a top-hat intensity profile. This intensity profile is particularly present in the beam focus of the respective laser beam. Alternatively, at least one of the laser beams can have a Gaussian intensity profile. A top-hat intensity profile, also known as a top-hat profile, has the advantage over a Gaussian intensity profile that it enables more uniform heating of both the workpiece surface and the additional material across the entire beam cross-section of the laser beam, particularly in its edge regions.

[0019] Alternatively or additionally, the first laser beam and / or the second laser beam and / or the third laser beam may have an intensity profile with an intensity maximum in the edge region of the respective laser beam. In particular, the first laser beam and / or the second laser beam and / or the third laser beam may each have a circular cross-section and an annular intensity profile across the beam cross-section. Consequently, the intensity at the center of the beam is lower than in its peripheral region. Laser beams with an annular intensity profile achieve more uniform heating of the workpiece surface and / or, in particular, powdered additional material during laser deposition welding.

[0020] The energy density applied in the second irradiation zone is preferably 0.01 J / mm 2 Up to 5J / mm 2 Within the specified energy density range, the metallurgical effects of the heating by the laser radiation on the workpiece to be coated can be kept to a minimum due to the low penetration depth. In particular, the penetration depth of the preheating can be in the range of 5 μm to 500 μm from the workpiece surface. Preferably, the irradiation in the second irradiation zone can be adjusted so that the preheating temperature of the workpiece in the second irradiation zone is within a range of 5% of the melting temperature of the workpiece to be coated up to the evaporation temperature.

[0021] The laser power of the second laser beam and / or the laser power of the third laser beam can be varied during the coating process. For example, the laser power of the second laser beam and / or the third laser beam can be varied by changing the resulting feed rate. The laser power of the second laser beam and / or the third laser beam at the start of the coating process can also have a first value, which is reduced according to regulations to compensate for process-related heating of the component during the coating process and to create thermal conditions that are as similar as possible throughout the coating process. Similar to the adjustment of the laser power of the second laser beam and / or the third laser beam, the laser power of the first laser beam can also be varied if necessary. By varying the laser power, in particular the laser power of the second laser beam, the preheating of the workpiece surface can be precisely and flexibly adapted to the external conditions.

[0022] The total laser power of all laser beams used for the coating process may be at least 1 kW, preferably at least 8 kW. The proportion of the laser power of the second laser beam to the total power may be in the range of 0.05% to 75%, in particular in the range of 1% to 50%.

[0023] At least one of the laser beams used can have a wavelength in the range of 0.4 μm to 2 μm. The beam quality of at least one of the laser beams can be in the range of 2 mm*mrad to 500 mm*mrad. A 2-in-1 or n-in-1 optical fiber or a single-core optical fiber can be used to guide at least the first laser beam or the laser output beam underlying the first (partial) laser beam. An n-in-1 optical fiber within the meaning of the present disclosure has at least one central light-guiding core region for transmitting the laser beam and at least one light-guiding annular region surrounding the core region, wherein the light-guiding regions are preferably separated from each other by a cladding. When using an n-in-1 optical fiber, the power portion of the laser beam guided in the central fiber core can be at least 15% of the total power. For the coating process, a device having a welding head can preferably be used, the welding head including welding optics, by means of which the first laser beam and the powder gas jet containing the additional material are focused onto the process zone, and furthermore, at least the beam portion of the first laser beam or the second laser beam is focused onto the preheating zone. The focal diameter of at least one of the laser beams is in the range of 1 mm to 20 mm. The laser beam can be focused onto the surface of the workpiece to be coated so that its focal diameter lies in the surface plane of the workpiece or is offset a few millimeters above or below the surface plane. In order to split the laser output beam into at least a first (partial) laser beam and a second (partial) laser beam, a corresponding optical element (e.g., an optical wedge or a diffractive optical element) can be arranged in the beam path of the collimated laser output beam.

[0024] In the processing direction, the distance of the second irradiation zone (preheating zone) from the first irradiation zone (process zone) and / or the distance of the third irradiation zone (postheating zone) from the first irradiation zone can be at least 0.5 times the focal diameter of the first laser beam and can be up to 5 times the focal diameter of the first laser beam. The first laser beam, the second laser beam, and / or the third laser beam can in particular have the same, in particular circular, outer diameter. In the case of an annular intensity profile of the laser beam (which can be generated, for example, when using 2-in-1 or n-in-1 optical fibers, by using laser segments guided in the annular fiber core of the n-in-1 optical fiber), an overlap of the irradiation zones can be preferred. The distance of the irradiation zones relative to one another is determined by the distance between their centers in a common plane (in the plane of the surface of the workpiece to be coated, in the case of doubt). Preferably, the irradiation zones are spaced apart so that they are directly adjacent to one another without significant overlap or with no significant gap between them.

[0025] Additional material can be supplied to the first irradiation zone so that an irradiation window remains for producing the second irradiation zone and / or for producing the third irradiation zone. In other words, the powder supply can be implemented so that the second laser beam used to produce the preheating zone (second irradiation zone) or the corresponding portion of the first laser beam is not obstructed by the powder flow. The same applies to the third laser beam used to produce the postheating zone (third irradiation zone) or the corresponding portion of the first laser beam. For example, the additional material can be supplied only laterally with respect to the processing direction and / or supplied penetratingly, that is, from the direction after the processing. It should be understood that if the additional material is supplied penetratingly, any postheating zone will be at least partially covered. However, concentric supply of the additional material is also conceivable. A particularly preferred variation is one in which the additional material is supplied to the first irradiation zone from multiple locations distributed around the first laser beam (either via individual nozzles distributed in an annular pattern or via a C-slot nozzle), wherein the powder supply does not occur from the forward direction, allowing the second laser beam or the corresponding portion of the first laser beam to be directed unimpeded onto the workpiece surface through existing grooves in the powder supply. Similar grooves can be provided in the powder supply to produce the postheating zone.

[0026] In principle, it is preferred to blow the additional material in powder form into the process zone with the aid of a carrier gas, in particular an inert carrier gas. The powder mass flow in the method according to the invention can preferably be at least 20 g / min. To prevent oxidation processes during coating, additional gassing can be provided. In particular, an inert gas, such as argon, can be blown in through the process nozzle, through which the laser beam also impinges at least on the process zone.

[0027] The second and / or third irradiation zones can be arranged offset orthogonally to the processing direction relative to the first irradiation zone. This allows the curvature of the processing trajectory to be taken into account for preheating and / or postheating, particularly when applying circular, spiral, or helical materials. The impact of preheating and / or postheating on parts of the process zone or its immediate surroundings can be adjusted by lateral displacement of the preheating and / or postheating zones. This can be necessary when welding with overlapping trajectories, for example, when only the workpiece or only the intermediate layer is to be preheated.

[0028] The second laser beam and / or the third laser beam can each have a rectangular beam cross-section. The rectangular beam profile can be aligned orthogonally to the processing direction. This allows for a uniform energy density distribution in the pre-heating zone and the post-heating zone.

[0029] The second irradiation zone (preheating zone) and / or the third irradiation zone (post-heating zone) can each have a different size from the first irradiation zone (processing zone). For example, for the area A2 of the second irradiation zone, the following applies: 0.1*A1 ≤ A2 < A1 (where A1 = the area of the first irradiation zone). With a smaller preheating spot, the influence of preheating on a part of the processing zone can be adjusted. This can be advantageous when welding in the case of trajectory overlap, for example, when preheating only the workpiece surface or only the intermediate layer of the coating. Conversely, the following can also apply to the area A2 of the second irradiation zone: A1 < A2 ≤ 3*A1. When using a preheating zone that is larger than the processing zone, the previously applied coating segments (especially the turns of a helically applied coating) can be heat-treated while applying the powder. In addition, with a relatively large preheating zone, the effort required for the precise positioning of the second laser beam relative to the first laser beam is reduced.

[0030] However, depending on the application, a configuration in which the irradiation zones have equal sizes can also be desirable. In particular, in some cases, this can optimize the efficiency of the energy input.

[0031] In particular, the size of the projection area of the laser beam on the workpiece surface and thus the size of the irradiation zone can be variably adjusted.

[0032] According to another aspect of the invention, a device for laser deposition welding is provided. The device includes a carrier unit for a metal workpiece to be coated, wherein the carrier unit has a moving unit for moving, especially for rotating, the workpiece. The device further includes a laser beam unit for providing at least one first laser beam and for generating at least one first irradiation zone and a second irradiation zone on the surface of the workpiece to be coated by means of the at least first laser beam, wherein the second irradiation zone is in front of the first irradiation zone in the processing direction. The device also includes a supply unit for supplying additional material, especially powdery additional material, into the first irradiation zone, wherein the additional material can be supplied to the first irradiation zone such that the additional material at least partially enters the first laser beam in the first processing zone before impinging on the workpiece surface and is thus at least partially heated.

[0033] The laser beam unit preferably has an optical device having a collimation unit and a focusing unit and having a beam splitter element. The beam splitter element is arranged between the collimation unit and the focusing unit in the beam path of the optical device and is configured to split the laser output beam into a first laser beam and at least one second laser beam.

[0034] The beam splitter element can be, for example, a wedge, a cylindrical lens, or a diffractive optical element (DOE). Faceted optics or microlens arrays can also be used as beam splitter elements. With the aid of wedges and DOEs, as well as faceted optics or microlens arrays, individual partial beams can be generated, with the aid of which the workpiece surface is impinged in the corresponding irradiation zones. With the aid of cylindrical lenses, an elliptical beam profile of the laser beam can be generated, so that the laser beam irradiates the individual irradiation zones on the workpiece surface with a continuous beam spot.

[0035] The device may further comprise a displacement unit, by means of which the optical element configured as an optical wedge or DOE can be displaced laterally in the beam path of the laser output beam in order to distribute the laser power to the generated laser beam.

[0036] According to a third aspect, a workpiece that can be manufactured using the coating method according to the present invention is provided. The workpiece is, in particular, a brake disk. The workpiece comprises a metal substrate, in particular a disk-shaped substrate. The substrate can in particular be made of cast iron, such as gray cast iron. The workpiece also comprises at least one coating layer, which is preferably applied spirally and in overlapping coating paths on the surface of the substrate and is materially bonded to the substrate. The workpiece has a mixing region at the transition between the substrate and the coating layer, which has a thickness of at most 20 μm, preferably at most 10 μm, and more preferably at most 5 μm. A material bond between the substrate and the coating layer is formed in the mixing region.

[0037] According to a preferred embodiment, the workpiece can have multiple coatings, each with a different material composition. For example, a first coating can be formed as an intermediate layer made of stainless steel, which is applied to the surface of the substrate. The second coating applied to the intermediate layer can consist of a matrix material in which particles of a hard material, such as tungsten carbide or titanium carbide, are embedded in a stainless steel matrix. Between each of the coatings and / or between overlapping turns of the coating itself, a mixing region with a respective maximum thickness of 20 μm, preferably at most 10 μm, and more preferably at most 5 μm, can be formed between adjacent layers / turns. The workpiece is characterized in particular by a strong, defect-free connection between the substrate and the coatings and between adjacent coatings (or winding tracks), wherein the material structure and the properties of the different materials in the joining region are only minimally affected.

[0038] A workpiece can be produced by means of the method according to the invention, preferably according to one of the variants described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The following description of preferred exemplary embodiments serves to explain the present invention in more detail with reference to the accompanying drawings.

[0040] In the attached figure:

[0041] Figure 1 Schematic diagram showing the creation of two irradiation zones during high-speed laser deposition welding;

[0042] Figure 2 Schematically showing the components of the method in high-speed laser deposition welding according to the present invention;

[0043] Figures 3a to 3d Schematic representations of different configurations for pre-heating and / or post-heating during high-speed laser deposition welding are shown;

[0044] Figure 4 shows a schematic representation of different residence times of powder particles in the process zone on the workpiece surface to be coated during high-speed laser deposition welding;

[0045] Figure 5 Schematically shows the optical arrangement of the apparatus for high-speed laser deposition welding according to the present invention;

[0046] Figure 6 shows a diagram illustrating delamination in a wear protection layer of a workpiece coated by high speed laser deposition welding; and

[0047] Figures 7a to 7n Schematic representations of further different configurations for pre-heating and / or post-heating during high-speed laser deposition welding are shown. DETAILED DESCRIPTION

[0048] Figure 1A nozzle 10 is schematically shown, from which a first laser beam L1 and a second laser beam L2 are emitted. When the first laser beam L1 irradiates a workpiece surface (not shown), a first irradiation zone 20 is generated on the workpiece surface, and the second laser beam L2 similarly generates a second irradiation zone 22. By moving the workpiece to be coated and / or the nozzle 10 relative to each other, the laser beams L1 and L2 are moved in a processing direction 40 above the workpiece surface along a predetermined processing trajectory. Powdered additional material P is also irradiated into the first laser beam L1 via the nozzle 10, causing the powder particles to be heated by the first laser beam L1 and impinge on the workpiece surface along the processing trajectory in the first irradiation zone 20 (process zone). Due to the simultaneous heating of the powder particles and the workpiece surface in the process zone 20, a strong connection is formed very quickly when the powder particles impinge on the workpiece surface. Typically, a complete common molten pool is not formed. Subsequently, the partially molten material deposit in the process zone 20 solidifies into a weld bead 30. In order to accelerate the coating process and at the same time minimize the occurrence of bonding errors, the workpiece surface is preheated by the second laser beam L2 in the second irradiation zone 22 (preheating zone) before the process zone 20. In order to further improve the bonding of the material deposit (additional material P) on the workpiece or the bonding of overlapping tracks of material deposits (so-called delamination defect mode), it is also possible to use laser radiation in the third irradiation zone (24, see Figure 2 b and Figure 2 In d), the weld bead 30 is heated after the process zone 20 .

[0049] Figure 2 The method steps of the coating method according to the present invention are schematically illustrated. Step S1 represents the movement, in particular the rotation, of the workpiece to be coated. In step S2, the surface of the workpiece is irradiated with the aid of at least a first laser beam L1 to produce S20 at least a first irradiation zone 20 and S22 a second irradiation zone 22 on the workpiece surface, wherein the second irradiation zone 22 precedes the first irradiation zone 20 in the processing direction 40. Furthermore, in an optional sub-step S24, a third irradiation zone 24 can also be produced with the aid of at least the first laser beam L1. In step S3, additional material P is introduced into the first irradiation zone 20, wherein the additional material P at least partially enters the first laser beam L1 before impinging on the workpiece surface and is thereby at least partially heated.

[0050] Figures 3a to 3d Different configurations of preheating and / or postheating within the framework of the coating process according to the invention are schematically shown. Figure 3a A configuration is shown in which the workpiece surface to be coated is irradiated by two laser beams, wherein the projection of the first laser beam L1 onto the workpiece surface forms a first irradiation zone 20, and the projection of the second laser beam L2 onto the workpiece surface forms a second irradiation zone 22. The second irradiation zone 22 precedes the first irradiation zone 20 in the machining direction 40 during the coating process. Figure 3b In addition to the generation of Figure 3a In addition to the irradiation zones 20 and 22, a third irradiation zone 24 is also generated by means of a third laser beam L3, wherein the third irradiation zone 24 follows the first irradiation zone 20 in the processing direction 40 to controllably post-heat the additional material P applied in the process zone 40. Figure 3d Also shown is a configuration that produces three irradiation zones 20, 22, 24 on the workpiece surface or weld bead 40. However, Figure 3b In contrast to the representation of , the irradiation areas 20, 22, 24 are generated here by means of a continuous beam point (or projection) of a single laser beam. For this purpose, the laser beam used preferably has an asymmetrical beam profile in cross section, in particular an elliptical beam profile, such as Figure 3d shown.

[0051] Figure 3c The irradiation configuration according to the invention is shown in a particularly preferred embodiment of the coating method according to the invention. Figure 3a The irradiation configuration differs in that the laser beams L1 and L2 produce irradiation zones 20, 22 with an annular intensity distribution. In other words, more energy per unit area is introduced into the edge regions of the respective irradiation zones 20, 22 than into the respective core regions. Consequently, compared to irradiation with a top-hat or Gaussian intensity distribution, a more uniform energy distribution occurs across the width of the respective irradiation zones 20, 22 in the processing direction 40, and thus a more uniform heating of the irradiated surface or powder particles of the additional material P occurs. Figure 4 The illustration in further illustrates the advantages of a beam profile with an annular intensity distribution. Here, a process zone 20 is shown, along with three powder particles P1, P2, and P3 distributed across the width of the process zone 20 and arranged at the front end of the process zone 20. As can be inferred from the representation, due to the circular process zone 20, powder particles P1 and P3 impinging on the edges of the process zone have a relatively short residence time Δt1 in the process zone 20 and are therefore heated less intensively. Powder particle P2 impinging on the center travels a greater distance in the process zone 20 in the processing direction 40 during its movement and is therefore heated for a longer period Δt2. The annular intensity distribution of the first laser beam L1 used can compensate for this uneven energy input across the width of the processing path.

[0052] Figure 5The structure of an optical device 100 that can be used in an apparatus for a coating method according to the present invention is schematically illustrated. The optical device 100 can be arranged, in particular, in a processing head of the apparatus. A laser output beam L is collimated via a fiber optic cable 110—for example, using a 2-in-1 fiber—to a collimation unit 120, in particular, a collimating lens. A beam splitter element 130—here in the form of an optical wedge—is arranged in the beam path of the collimated laser output beam L. This beam splitter element is movable transversely to the propagation direction of the laser output beam L and, with the aid of this beam splitter element, splits the laser output beam L into a first (partial) laser beam L1 and a second (partial) laser beam L2. By lateral positioning of the beam splitter element 130 within the laser output beam L, the total power of the laser output beam L can be specifically divided between the laser beams L1 and L2. The laser beams L1 and L2 are then focused onto the surface of the workpiece 50 to be coated via a focusing unit 150—here, a focusing lens—whereby each produces a corresponding irradiation zone 20, 22 on the workpiece surface.

[0053] Figure 5 FIG. 5 shows a cross section of a coating of a workpiece coated by laser deposition welding. The workpiece 50 comprises a substrate 52 and an intermediate layer 54 applied to the substrate 52. A coating 60 is applied to the intermediate layer 54. The coating 60 comprises a plurality of overlapping coating tracks or overlapping turns of continuous coating tracks. Figure 6 The coating 60 shown shows a bonding defect 62 between two adjacent coating tracks. This defect is also called delamination. The coating method proposed here aims to counteract the formation of delamination 62 and the bonding defect between the coating 60 and the workpiece 50.

[0054] exist Figures 7a to 7n In, with Figures 3a to 3d Similarly, further exemplary configurations of preheating and / or postheating within the framework of the coating process according to the invention are schematically shown. Figures 3a to 3d Description, Figures 7a to 7n The representation in is self-explanatory for a person skilled in the art, which is why a detailed description of the individual arrangements of the laser beam projection with different beam cross sections is omitted at this point. However, it should be mentioned that by arranging one or more laser beams in a laterally offset manner or obliquely on the workpiece surface (see Figures 7c to 7k ), the spiral course of the coating track can be taken into account during the preheating and / or postheating of the workpiece and / or at least one preceding revolution of the coating.

Claims

1. A method for coating a metal workpiece (50) by means of laser deposition welding, the method comprising the following steps: Moving (S1), in particular rotating, the workpiece (50) to be coated; irradiating (S2) the surface of the workpiece (50) by means of at least one first laser beam (L1) to produce at least one first irradiation area (20) and a second irradiation area (22) on the workpiece surface, wherein the second irradiation area (22) is before or after the first irradiation area (20) along the machining direction (40); and An additional material (P), preferably in powder form, is introduced into the first irradiation zone (20), wherein the additional material (P) at least partially enters the first laser beam (L1) before impinging on the workpiece surface and is thereby at least partially heated.

2. The method according to claim 1, further comprising: A third irradiation zone (24) is generated by means of at least the first laser beam (L1), the third irradiation zone being formed on the surface to be coated in a direction opposite to the second irradiation zone (22) relative to the first irradiation zone (20).

3. The method according to claim 1 or 2, in, The first irradiation zone (20) is produced by means of a first laser beam (L1), wherein the second irradiation zone (22) is produced by means of a second laser beam (L2), and / or wherein the third irradiation zone (24) is produced by means of a third laser beam; and The intensity of the second laser beam (L2) and / or the third laser beam in the plane of the respective irradiation areas is different from that of the first laser beam (L1).

4. The method according to any one of the preceding claims, in, The first laser beam (L1) and / or the second laser beam (L2) and / or the third laser beam have a flat top-shaped intensity distribution.

5. The method according to any one of the preceding claims, in, The first laser beam (L1) and / or the second laser beam (L2) and / or the third laser beam have an intensity distribution having an intensity maximum in an edge region of the respective laser beam.

6. The method according to any one of the preceding claims, in, The laser power of the second laser beam (L2) and / or the third laser beam is varied during the coating process.

7. The method according to any one of the preceding claims, in, In the processing direction (40), the distance between the second irradiation area (22) and the first irradiation area (20) and / or the distance between the third irradiation area (24) and the first irradiation area (20) corresponds to at least 0.5 times the focal diameter of the first laser beam (L1) and at most 5 times the focal diameter of the first laser beam (L1).

8. The method according to any one of the preceding claims, in, The additional material (P) is supplied to the first irradiation zone (20) such that an irradiation window remains for producing the second irradiation zone (22) and / or for producing the third irradiation zone (24).

9. The method according to any one of the preceding claims, in, The second irradiation zone (22) and / or the third irradiation zone (24) are offset relative to the first irradiation zone (20) orthogonally to the processing direction.

10. The method according to any one of the preceding claims, in, The second laser beam (L2) and / or the third laser beam has a rectangular beam cross section.

11. The method according to any one of the preceding claims, in, The second irradiation area (22) and / or the third irradiation area (24) have different sizes from the first irradiation area (20).

12. A device for laser deposition welding, comprising: a carrying unit for a metal workpiece (50) to be coated, wherein the carrying unit comprises a movement unit for moving, in particular rotating, the workpiece (50); a laser beam unit for providing at least one first laser beam (L1) and for generating at least one first irradiation zone (20) and a second irradiation zone (22) on the surface of the workpiece (50) to be coated by means of at least the first laser beam (L1), wherein the second irradiation zone (22) precedes the first irradiation zone (20) in a machining direction (40); A supply unit for supplying additional material (P), in particular in powder form, into the first irradiation zone (20), wherein the additional material can be supplied to the first irradiation zone (20) in such a way that the additional material already at least partially enters the first laser beam (L1) in the first processing zone (20) before impinging on the workpiece surface and is thereby at least partially heated.

13. The device according to claim 12, in, The laser beam unit comprises an optical device (100) having a collimating unit (120) and a focusing unit (140) and a beam splitter element (130), the beam splitter element being arranged between the collimating unit (120) and the focusing unit (140) in a beam path of the optical device (100) and being configured to split a laser output beam (L) into the first laser beam (L1) and at least one second laser beam (L2).

14. The device according to claim 13, in, The beam splitter element is a wedge, or a cylindrical lens, or a diffractive optical element (DOE).

15. A workpiece, in particular a brake disc, comprising: A metal substrate, in particular a disk-shaped one; at least one coating layer, which is preferably arranged in a spiral manner and in overlapping coating paths on the surface of the base body and is connected to the base body in a material-locking manner; Therein, the mixing region at the transition between the base body and the coating has a thickness of at most 20 μm, preferably at most 10 μm, more preferably at most 5 μm.

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