Method for coating metal workpieces

Abstract

The invention relates to 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) a surface of the workpiece (50) by means of at least one first laser beam (L1) to generate at least one first irradiation zone (20) and a second irradiation zone (22) on the workpiece surface, wherein the second irradiation zone (22) is ahead of or behind the first irradiation zone (20) along a machining direction (40); and introducing a preferably powdery additional material (P) into the first irradiation zone (20), wherein the additional material (P) at least partially enters the first laser beam (L1) before impingement on the workpiece surface and as a result is 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

[0001] Process for coating metallic workpieces

[0002] Field of the invention

[0003] The present invention relates to the field of laser cladding. In particular, the invention relates to a method and a device for coating metallic workpieces, in particular brake discs, using high-speed laser cladding.

[0004] State of the art

[0005] Methods for high-speed laser cladding are known from the prior art and are described, for example, in DE 10 2011 100 456 B4. With this method, a significant increase in the achievable processing speed is achieved compared to conventional laser cladding by supplying at least one filler material in at least partially molten form to a process zone on a surface to be processed. For this purpose, the filler material, which is initially in powder form, is heated by a laser beam at a distance from the workpiece surface and supplied to the process zone, which is also heated by the laser beam. The feed movement - i.e. the relative movement between the surface to be coated and the processing beam - in high-speed laser cladding is largely realized by a movement, in particular a rotation, of the workpiece to be coated.By means of high-speed laser cladding, the processing speed of the laser cladding process can be significantly increased.

[0006] In high-speed laser metal deposition (HS-LMD), a significant portion of the laser power is absorbed by the powder flow of the filler material and by the metal vapor generated during the machining process. However, to achieve a sufficient bond between the workpiece and the coating layer, it is important that the workpiece surface has a required minimum temperature. For preheating the workpiece, it is therefore common practice to heat the workpiece over a large area using induction heating.

[0007] However, this large-area heating is comparatively complex and requires a comparatively large amount of energy. Furthermore, the preheating temperature cannot be precisely adjusted. Furthermore, when preheating a gray cast iron brake disc using induction, for example, the preheating temperature is limited because the surface of the brake body oxidizes at temperatures above approximately 300 °C. An oxidized surface would negatively impact the coating process and the quality of the coating.

[0008] The present invention is based on the object of improving the coating of metallic workpieces using laser cladding. In particular, it should make it possible to avoid bonding defects between the coating layer and the coated workpiece, or between adjacent layers of the coating layer (so-called delamination), even at high processing speeds.

[0009] The invention

[0010] The object underlying the invention is achieved by the subject matter of the independent claims. Further possible embodiments of the invention are specified in the subclaims, the description and the drawings. Features, advantages and possible embodiments that are set out in the description for one of the subject matter of the independent claims are to be regarded at least analogously as features, advantages and possible embodiments of the respective subject matter of the other independent claims and of any possible combination of the subject matter of the independent claims, optionally in conjunction with one or more of the subclaims. According to a first aspect of the invention, a method for coating a metallic 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 can preferably be a brake disc for a motor vehicle. The workpiece can have a base body, which can consist essentially of cast iron, in particular of gray cast iron. On the surface of the workpiece to be coated, an intermediate layer made of a material—preferably also a metallic material, in particular of stainless steel—can be applied to the base body. The intermediate layer serves, for example, to improve the bond between the wear-resistant layer to be applied and the workpiece. Furthermore, the intermediate layer can serve to stop cracks in the wear-resistant layer to be applied. The composition of the intermediate layer can be selected depending on the material of the wear-resistant layer to be applied.

[0011] In a second step, the method comprises irradiating a surface of the workpiece using at least one first laser beam to create at least a first irradiation zone and a second irradiation zone on the workpiece surface, wherein the second irradiation zone precedes or lags the first irradiation zone along a processing direction. The first irradiation zone can also be referred to as the process zone. The second irradiation zone can also be referred to as a preheating zone or a postheating zone, depending on its arrangement upstream or downstream of the process zone. The processing direction is essentially determined by the movement of the workpiece. When coating a brake disc, for example, in addition to the rotation of the workpiece, the processing beam is moved in a radial direction, resulting in a spiral-shaped processing path.In this case, the machining direction can be a tangent of the machining path starting from the center of the first irradiation zone. In addition to spiral machining paths, helical machining paths or simple straight, circular, or other machining paths are also possible. A movement of the machining beam in addition to the movement of the workpiece is not always necessary. The workpiece surface is locally heated by at least the first laser beam, both in the process zone and in the preheating or postheating zone.

[0012] In a third step of the method, a preferably powdered filler material is introduced into the first irradiation zone (i.e., the process zone). The filler material can preferably be irradiated into the process zone in the form of one or more powder jets using a preferably inert carrier gas. Alternatively, a supply of a wire-shaped filler material would also be conceivable. The filler material is fed to the process in such a way that, before it impacts the workpiece surface, it enters the first laser beam at least partially, preferably across its entire cross-section, at a predetermined distance from the workpiece surface, and is thereby heated at least partially, preferably across its entire cross-section.The filler material or at least part of the filler material is at least partially melted so that when it hits the workpiece surface in the also heated process zone it can quickly bond with the workpiece material and create a material bond with particularly low mixing of the joining partners. The focus of a powder jet supplied from several directions can be in the area of ​​the surface (e.g. in the surface plane) of the workpiece to be coated, preferably above the workpiece surface. Depending on the application, the focus of a filler material supplied in powder form can also be below the workpiece surface, i.e. in the workpiece or below the workpiece. The filler material is preferably made of a material that has a higher wear resistance than a base body of the component.In particular, the filler material can comprise an iron-containing matrix material, which in particular forms a matrix of stainless steel in which hard material particles, such as tungsten carbide particles or titanium carbide particles, are embedded. In the coating process, the filler material preferably forms a flat wear-resistant layer on the workpiece surface. The carbide inclusions in the coating layer can increase wear protection. When coating a brake disc, this can increase the number of braking cycles and thus the service life of the brake disc (particularly by reducing abrasion during braking).

[0013] The method according to the invention makes it possible to precisely and efficiently preheat the workpiece surface along the machining path immediately before the filler material hits the process zone, or to reheat it after it has passed through the process zone in order to improve the bond between the filler material and the workpiece surface and avoid bonding defects. The formation of other defects - e.g. pores - in the material structure of the coating layer can also be counteracted. Compared to the large-area preheating according to the prior art, the energy is introduced into the workpiece in a much more targeted manner. Furthermore, the temperature can be adapted very quickly to changing thermal conditions during a coating process. In this way, the quality of the coating can also be improved.Compared to large-area heating of the workpiece to be coated, the impact on the metallurgical properties of the workpiece can also be reduced by using less energy. In the method according to the invention, preferably no pronounced molten pool is created on the surface of the workpiece to be coated. A penetration depth (or depth of action) of the at least first laser beam into the workpiece to be coated can preferably be at most 20 pm, preferably at most 10 pm, even more preferably at most 5 pm. Preheating of the workpiece surface in this case can contribute in particular to the cleaning and / or outgassing of carbon from the workpiece to be coated, improved wettability of the workpiece surface with the filler material and / or improved bonding between the filler material and the workpiece.

[0014] In the method according to the invention, it can additionally be provided that an inert protective gas, for example argon, helium or inert gas mixtures, is directed together with the at least first laser beam through the processing nozzle of a processing head onto the first illumination zone in order to shield the welding process from the reaction with oxygen from the ambient air and to prevent corrosion of the coating path or the heated workpiece surface.

[0015] In the coating method according to the invention, a resulting feed rate, i.e. a rate of relative movement between the workpiece surface and the processing beam, can preferably be at least 20 m / min.

[0016] In the method according to the invention, it can also be provided that the workpiece to be coated is not subjected to any additional preheating, in particular no large-area preheating by induction, apart from the preheating by the at least first laser beam. By eliminating the preheating time and the device for inductive preheating, the process time and process costs can be reduced.

[0017] According to a variant, a third irradiation zone can also be generated using the at least first laser beam. The third irradiation zone is arranged such that it is opposite the second processing zone, starting from the first irradiation zone. For the sake of simplicity, it is assumed below that the second irradiation zone is the leading preheating zone and the third irradiation zone is the trailing postheating zone.

[0018] At least two laser beams can be used in the method according to the invention. Accordingly, the first irradiation zone can be generated by means of the first laser beam and the second irradiation zone by means of a second laser beam. The third irradiation zone can additionally be generated by means of a third laser beam. The first laser beam and the second laser beam, and (optionally) also the third laser beam, can each be designed as partial beams (i.e. first partial beam, second partial beam and (optionally) third partial beam) of a common laser output beam. Preferably, the second laser beam and / or the third laser beam can have a different intensity in the irradiation plane (i.e. plane of the workpiece surface or surface plane of the applied layer) than the first laser beam.In particular, the intensity of the second laser beam in the plane of the workpiece surface may be lower than the intensity of the first laser beam.

[0019] The second laser beam and / or the third laser beam can also be fed into the process from a separate beam source, for example laterally. In this way, the laser power of the individual laser beams can be adjusted easily and independently of one another. This also makes it easy to position the preheating beam and / or the postheating beam on the processing path. Furthermore, laser beams with different wavelengths than those used for the actual coating process can be used for preheating and / or postheating. The wavelengths can be precisely matched to a material-specific coupling, thus increasing the efficiency of the preheating and / or postheating. Furthermore, the preheating beam and / or the postheating beam could be directed onto the workpiece surface or the workpiece from outside the feed of the filler material.the weld bead would be blasted and would in particular not be affected by a powder jet of the filler material arranged concentrically around the first laser beam.

[0020] As an alternative to using multiple separate laser beams or partial laser beams, a single laser beam can be used, preferably with an asymmetric beam profile that is elongated in the machining direction (e.g., oval or rectangular). A leading portion of the laser beam projection on the workpiece surface forms the second irradiation zone, and a subsequent portion of the projection forms the first irradiation zone. The extension of the asymmetric laser beam can additionally cover the third irradiation zone, which trails the first irradiation zone and reheats the generated weld bead or the applied filler material to further prevent bonding defects or delamination between individual coating layers.

[0021] When using multiple separate 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 plateau-shaped intensity distribution. The described intensity distribution is present in particular in the beam focus of the respective laser beam. Alternatively, at least one of the laser beams can have a Gaussian intensity distribution. A plateau-shaped intensity distribution, also referred to as a top-hat distribution, has the advantage over a Gaussian intensity distribution that it enables more uniform heating of both the workpiece surface and the filler material across the entire beam cross-section of the laser beam, i.e., particularly in its edge regions.

[0022] Alternatively or additionally, the first laser beam and / or the second laser beam and / or the third laser beam can have an intensity distribution with an intensity maximum in the edge region of the respective laser beam. In particular, the first and / or the second and / or the third laser beam can each have a circular cross-section with an annular intensity distribution across the beam cross-section. The intensity is thus lower in the center of the beam than in its edge region. A laser beam with an annular intensity profile enables even more uniform heating of the workpiece surface and / or the filler material, in particular in powder form, during laser cladding.

[0023] The fluence applied in the second irradiation zone is preferably in the range of 0.01 J / mm 2 up to 5 J / mm 2. In the specified fluence range, the influence of heating by the laser radiation on the metallurgy of the workpiece to be coated can be kept low due to a shallow penetration depth. In particular, the penetration depth of the preheating can be in the range of 5 pm to 500 pm starting from the workpiece surface. Preferably, the irradiation in the second irradiation zone can be adjusted so that a preheating temperature of the workpiece in the second irradiation zone is in a range of 5% of the melting temperature to the evaporation temperature of the workpiece to be coated. A laser power of the second laser beam and / or a laser power of the third laser beam can be changed during a coating process. For example, the laser power of the second laser beam and / or the third laser beam can be changed when the resulting feed rate is changed.The laser power of the second and / or third laser beam can also have an initial value at the beginning of a coating process, which is reduced according to a specified value in order to compensate for process-related heating of the component during the coating process and to create as consistent thermal conditions as possible throughout the entire coating process. Analogous to the adjustment of the laser power of the second and / or third laser beam, the laser power of the first laser beam can also be changed as needed. By changing the laser power, especially of the second laser beam, the preheating of the workpiece surface can be precisely and flexibly adapted to the external conditions.

[0024] The sum of the laser powers of all laser beams used for the coating process can be at least 1 kW, preferably at least 8 kW. The proportion of the laser power for the second laser beam to the total power can be in the range of 0.05% to 75%, in particular in the range of 1% to 50%.

[0025] At least one of the laser beams used can have a wavelength in the range from 0.4 pm to 2 pm. The beam quality of at least one of the laser beams can be in the range from 2 mm*mrad to 500 mm*mrad. A 2-in-1 or an n-in-1 optical fiber or a single-core fiber can be used to guide the at least first laser beam or a 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 and at least one light-guiding ring region surrounding the core region for transporting the laser beam, wherein the light-guiding regions are preferably spaced from one another 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 with a welding head can preferably be used, which includes welding optics by means of which the first laser beam, together with a powder gas jet containing the filler material, is focused onto the process zone. Furthermore, at least a beam component of the first laser beam or a second laser beam is focused onto the preheating zone. The focus diameter of at least one of the laser beams is in the range from 1 mm to 20 mm. The laser beam(s) can be focused onto the workpiece surface to be coated such that their focus diameter lies in the surface plane of the workpiece or is offset by a few mm upwards or downwards from the surface plane.In order to split a laser output beam into at least the first and second laser (partial) beams, a corresponding optical element (for example an optical wedge or a diffractive optical element) can be arranged in the beam path of the collimated laser output beam.

[0026] A distance of the second irradiation zone (preheating zone) from the first irradiation zone (processing zone) and / or a distance of the third irradiation zone (postheating zone) from the first irradiation zone in the processing direction can correspond to at least 0.5 times a focus diameter of the first laser beam and at most 5 times the focus diameter of the first laser beam. The first laser beam and the second laser beam and / or the third laser beam can in particular have an identical, in particular circular, outer diameter. In the case of an annular intensity profile of the laser beams, which can be generated, for example, when using a 2-in-1 or n-in-1 optical fiber by using the laser portion guided in an annular fiber core of the n-in-1 fiber, an overlap of the irradiation zones can be preferred.The distance between the irradiation zones is determined by the distance between the centers of the irradiation zones in a common plane (in case of doubt in the surface plane of the workpiece to be coated). Preferably, the irradiation zones can be spaced apart in such a way that they are just adjacent to one another, without significantly overlapping or without a significant gap between them. The filler material can be fed to the first irradiation zone in such a way that an irradiation window for generating the second irradiation zone and / or for generating the third irradiation zone remains. In other words, the powder feed can be designed in such a way that the second laser beam or a corresponding portion of the first laser beam for generating the preheating zone (second irradiation zone) is not obscured by the powder flow. The same applies to the third laser beam ora corresponding portion of the first laser beam for generating the post-heating zone (third irradiation zone). For example, the filler material can be fed in exclusively laterally with respect to the machining direction and / or in a piercing manner, i.e. from a direction following the process. It is understood that if the filler material is fed in a piercing manner, any post-heating zone would be at least partially concealed. However, concentric feeding of the filler material is also conceivable. A variant in which the filler material is fed into the first irradiation zone from a plurality of positions distributed around the first laser beam (orThe powder is fed through a single nozzle (e.g., via individual nozzles distributed in a ring or via a C-shaped slit nozzle), with no powder feed from the forward direction, so that the second laser beam or a corresponding portion of the first laser beam can be directed unhindered onto the workpiece surface through the existing recess in the powder feed. A similar recess can be provided in the powder feed to create a post-heating zone.

[0027] In principle, it is preferred if the filler material is blown into the process zone in powder form by means of a carrier gas, in particular an inert one. The powder mass flow in the method according to the invention can preferably be at least 20 g / min. To avoid oxidation processes during coating, additional gas supply can be provided. In particular, an inert gas, for example argon, can be blown onto at least the processing zone through a processing nozzle, through which the laser beam(s) also exit. The second irradiation zone and / or the third irradiation zone can be arranged offset from the first irradiation zone, orthogonal to the processing direction. This allows the curvature of the processing path to be taken into account for preheating and / or postheating, particularly in the case of circular, spiral, or helical material deposition.By lateral displacement of the preheating and / or postheating areas, the influence of preheating and / or postheating on a portion of the process zone or its immediate surroundings can be adjusted. This may be necessary for welding with track overlap, for example, if only the workpiece or only the intermediate passes are 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 machining direction. This results in a uniform fluence distribution in the preheating zone and / or the postheating zone, respectively.

[0029] The second irradiation zone (preheating zone) and / or the third irradiation zone (postheating zone) can each have a different size than the first irradiation zone (process zone). For example, the following can apply to the area A2 of the second irradiation zone: 0.1*A1 < A2 < Al (where Al = area of ​​the first irradiation zone). With a smaller preheating spot, the influence of the preheating on a partial area of ​​the process zone can be adjusted. This can be advantageous when welding with track overlap, for example, if only the workpiece surface or only the intermediate layers of the coating are to be preheated. Conversely, the following can also apply to the area A2 of the second irradiation zone: Al < A2 < 3*A1.By using a larger preheating zone compared to the process zone, previously applied coating sections (especially the turns of a spirally applied coating) can be subjected to heat treatment simultaneously during powder deposition. Furthermore, a comparatively large preheating zone reduces the effort required for precise positioning of the second laser beam relative to the first laser beam. Depending on the application, however, a configuration in which the irradiation zones are of equal size may also be desirable. In particular, this can optimize the efficiency of energy input in some cases.

[0030] In particular, the size of the projection area of ​​the laser beam(s) on the workpiece surface, and thus the size of the irradiation zones, can be variably adjusted.

[0031] According to a further aspect of the invention, a device for laser cladding is provided. The device comprises a carrier unit for a metallic workpiece to be coated, wherein the carrier unit has a movement unit for moving, in particular for rotating, the workpiece. Furthermore, the device comprises a laser beam unit for providing at least a first laser beam and for generating, by means of at least the first laser beam, at least one first irradiation zone and a second irradiation zone on a surface of the workpiece to be coated, wherein the second irradiation zone precedes the first irradiation zone along a processing direction.The device also comprises a feed unit for feeding a filler material, in particular a powdered filler material, into the first irradiation zone, wherein the filler material can be fed to the first irradiation zone in such a way that it at least partially enters the first laser beam before it strikes the workpiece surface in the first processing zone and is thereby at least partially heated.

[0032] The laser beam unit preferably has an optical system with a collimation unit and a focusing unit, as well as 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 system and is designed to split a laser output beam into the first laser beam and at least one second laser beam. The beam splitter element can be, for example, an optical wedge, a cylindrical lens, or a diffractive optical element (DOE). A facet optic or a microlens array can also be used as the beam splitter element. By means of the optical wedge and the DOE, as well as by means of a facet optic or a microlens array, separate partial beams can be generated, by means of which the workpiece surface is exposed in the respective irradiation zone.Using the cylindrical lens, an elliptical beam profile of the laser beam can be created so that the laser beam irradiates the respective irradiation zones on the workpiece surface with a continuous beam spot.

[0033] The device may further comprise a displacement unit by means of which the optical element designed as an optical wedge or as a DOE can be displaced laterally in the beam path of the laser output beam in order to distribute the laser power to the resulting laser beams.

[0034] According to a third aspect, a workpiece is provided that can be produced by means of a coating method according to the invention. The workpiece is, in particular, a brake disk. The workpiece comprises a metallic base body, in particular a disk-shaped one. The base body can consist in particular of cast iron, for example, gray cast iron. The workpiece further comprises at least one coating layer, which is arranged, preferably in spiral and overlapping coating paths, on a surface of the base body and is integrally connected to the base body. At a transition between the base body and the coating layer, the workpiece has a mixing region that has a thickness of at most 20 μm, preferably of at most 10 μm, even more preferably of at most 5 μm. The material bond between the base body and the coating layer is formed in the mixing region.

[0035] According to a preferred variant, the workpiece can have a plurality of coating layers, each having a different material composition. For example, a first coating layer can be formed as an intermediate layer made of stainless steel, which is applied to the surface of the base body. A second coating layer applied to the intermediate layer can be composed of a matrix material in which hard material particles, for example tungsten carbide or titanium carbide, are embedded in a matrix of stainless steel. Between each of the coating layers and / or between overlapping turns of a coating layer as such, a mixing region with a respective maximum thickness of 20 pm, preferably of at most 10 pm, even more preferably of at most 5 pm, can be formed between the adjacent layers / turns.The workpiece is particularly characterized by a solid, defect-free connection between the base body and the coating layer, as well as, if applicable, between adjacent coating layers (or winding paths), in which the material structure of the different materials and their properties in the area of ​​the connection are only minimally influenced.

[0036] The workpiece can preferably be produced by means of a method according to the invention according to one of the variants described above.

[0037] Examples of implementation

[0038] The following description of preferred embodiments, in conjunction with the drawings, serves to explain the invention in more detail.

[0039] They show:

[0040] Fig. 1 Schematic of the creation of two irradiation zones during

[0041] High-speed laser cladding;

[0042] Fig. 2 Schematic process components in the inventive

[0043] High-speed laser cladding; Figs. 3a-d: Schematic representations of various configurations for preheating and / or postheating during high-speed laser cladding;

[0044] Fig. 4 Schematically the different residence times of

[0045] Powder particles during high-speed laser cladding in the process zone on a workpiece surface to be coated;

[0046] Fig. 5 Schematically an optical arrangement of an inventive

[0047] Device for high-speed laser cladding;

[0048] Fig. 6 A figure illustrating delamination in the

[0049] Wear protection layer of a workpiece coated by high-speed laser cladding; and

[0050] Figs. 7a-n Schematic representations of further different configurations for preheating and / or postheating during high-speed laser cladding.

[0051] Figure 1 schematically shows a nozzle 10 from which a first laser beam Li and a second laser beam L2 emerge. When irradiating a workpiece surface (not shown), the first laser beam Li creates a first irradiation zone 20 on the workpiece surface, and the second laser beam L2 creates a second irradiation zone 22 in a similar manner. By moving the workpiece to be coated and / or the nozzle 10 relative to one another, the laser beams Li, L2 are moved across the workpiece surface in a processing direction 40 along a predetermined processing path. Furthermore, a powdered filler material P is irradiated into the first laser beam Li via the nozzle 10, so that the powder particles are heated by the first laser beam Li and impinge on the workpiece surface along the processing path in the first irradiation zone 20 (process zone).Due to the simultaneous heating of the powder particles and the workpiece surface in process zone 20, a solid bond is formed very quickly when the powder particles hit the workpiece surface. As a rule, no complete melt pool is formed. The partially molten material deposit in process zone 20 solidifies into a weld bead 30 in the aftermath. In order to accelerate the coating process and at the same time minimize the occurrence of bonding defects, the workpiece surface is already preheated by the second laser beam L2 in the second irradiation zone 22 (preheating zone) in the lead-up to process zone 20. To further improve the bonding of the material deposit (filler material P) to the workpiece or the bonding of overlapping layers of the material deposit (defect pattern of so-called delamination), the weld bead 30 can also be heated in a third irradiation zone (24, see Fig.2b and 2d) are heated by laser radiation.

[0052] Figure 2 schematically illustrates method steps of a coating method according to the invention. Step S1 represents the movement, in particular the rotation, of a workpiece to be coated. In step S2, a surface of the workpiece is irradiated by means of at least the first laser beam Li to generate S20 at least the first irradiation zone 20 and to generate S22 the second irradiation zone 22 on the workpiece surface, wherein the second irradiation zone 22 precedes the first irradiation zone 20 along a processing direction 40. Furthermore, in an optional sub-step S24, the third irradiation zone 24 can also be generated by means of the at least first laser beam Li. In a step S3, the filler material P is introduced into the first irradiation zone 20, wherein the filler material P at least partially enters the first laser beam Li before impinging on the workpiece surface and is thereby at least partially heated.

[0053] Figures 3a to 3d schematically illustrate various configurations for preheating and / or postheating within the framework of a coating process according to the invention. Figure 3a shows a configuration in which the workpiece surface to be coated is irradiated using two laser beams, with a projection of the first laser beam L1 on the workpiece surface forming the first irradiation zone 20 and a projection of the second laser beam L2 on the workpiece surface forming the second irradiation zone 22. The second irradiation zone 22 precedes the first irradiation zone 20 in the coating process along the processing direction 40.Figure 3b shows a configuration in which, in addition to the irradiation zones 20, 22 according to Figure 3a, a third irradiation zone 24 is generated by means of a third laser beam L3, wherein the third irradiation zone 24 follows the first irradiation zone 20 along the processing direction 40 in order to subject the filler material P applied in the process zone 40 to controlled post-heating. Figure 3d also shows a configuration in which three irradiation zones 20, 22, 24 are generated on the workpiece surface or on the weld bead 40. In contrast to the illustration according to Figure 3b, however, the irradiation zones 20, 22, 24 are generated here by means of a contiguous beam spot (or a projection) of a single laser beam. For this purpose, the laser beam used preferably has an asymmetric beam profile in cross-section, in particular an oval beam profile, as shown in Figure 3d.

[0054] Figure 3c shows the irradiation configuration in a particularly preferred embodiment of a coating method according to the invention. This irradiation configuration differs from the irradiation configuration according to Figure 3a in that the laser beams L1 and L2 each generate irradiation zones 20, 22 with an annular intensity distribution. In other words, more energy per unit area is introduced into the edge region of the respective irradiation zone 20, 22 than into the respective core region. This results in a more uniform energy distribution across the width of the respective irradiation zone 20, 22 along the processing direction 40 compared to irradiation with a tophat-shaped or Gaussian-shaped intensity distribution, and thus a more uniform heating of the irradiated surface or the powder particles of the filler material P.The advantage of a beam profile with an annular intensity distribution is further illustrated by the illustration in Figure 4. This shows the process zone 20 and three powder particles Pi, P2, P3 distributed across the width of the process zone 20 and each arranged at the leading end of the process zone 20. The illustration shows that due to the circular shape of the process zone 20, the powder particle P2 striking the center travels a longer distance in the process zone 20 as the process zone 20 moves along the processing direction 40 and is accordingly heated for a longer period of time Δt2 than the powder particles Pi and P3 striking the edge of the process zone, which have a comparatively short residence time Δt1 in the process zone 20 and are correspondingly heated to a lesser extent.With a ring-shaped intensity distribution of the first laser beam Li used, this uneven energy input across the width of the processing track can be counteracted.

[0055] Figure 5 schematically shows the structure of an optics system 100 that can be used for a device for the coating method according to the invention. The optics system 100 can, in particular, be arranged in a processing head of the device. A laser output beam L is collimated onto a collimation unit 120, in particular a collimation lens, via a fiber optic cable 110—for example, with a 2-in-1 fiber. 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 displaceable transversely to the propagation direction of the laser output beam L and by means of which the laser output beam L can be split into a first laser (sub-)beam L1 and a second laser (sub-)beam L2. By lateral positioning of the beam splitter element 130 in the laser output beam L, the total power of the laser output beam L can be specifically divided between the laser sub-beams L1 and L2.The laser beams Li and L2 are then focused onto the surface of a workpiece 50 to be coated via a focusing unit 150 - here a focusing lens - and thereby generate a corresponding irradiation zone 20, 22 on the workpiece surface.

[0056] Figure 5 shows a cross-section of the coating of a workpiece coated by laser deposition welding. The workpiece 50 comprises a base body 52 and an intermediate layer 54 applied to the base body 52. ​​A coating layer 60 is applied to the intermediate layer 54. The coating layer 60 comprises several overlapping coating layers, or overlapping turns of a continuous coating layer. The coating layer 60 as shown in Figure 6 shows bonding defects 62 between two adjacent coating layers. This defect pattern is also referred to as delamination. The coating method proposed here is intended to counteract the formation of delaminations 62 and bonding defects between the coating layer 60 and the workpiece 50.

[0057] In Figures 7a to 7n, analogous to Figures 3a to 3d, further exemplary configurations for preheating and / or postheating within the framework of a coating process according to the invention are schematically illustrated. In connection with the description of Figures 3a to 3d, the illustrations in Figures 7a to 7n are self-explanatory for the person skilled in the art, which is why a detailed description of the individual arrangements of the laser beam projections with the different beam cross-sections is omitted here. It should be mentioned, however, that by arranging one or more laser beams laterally offset or at an angle on the workpiece surface (cf. Figures 7c to 7k), the spiral course of the coating path can be taken into account during preheating and / or postheating of the workpiece and / or at least one previous turn of the coating layer.

Claims

Patent claims 1. A method for coating a metallic workpiece (50) by means of laser deposition welding, the method comprising the steps: Moving (Sl), 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 (Li) to generate at least one first irradiation zone (20) and one second irradiation zone (22) on the workpiece surface, wherein the second irradiation zone (22) precedes or follows the first irradiation zone (20) along a machining direction (40); and Introducing a preferably powdered filler material (P) into the first irradiation zone (20), wherein the filler material (P) already enters at least partially into the first laser beam (Li) before impinging on the workpiece surface and is thereby at least partially heated.

2. The method of claim 1, further comprising: Generating by means of the at least first laser beam (Li) a third irradiation zone (24) which is formed on the surface to be coated in a direction opposite to the second irradiation zone (22) with respect to the first irradiation zone (20).

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

4. Method according to one of the preceding claims, wherein the first laser beam (Li) and / or the second laser beam (L2) and / or the third laser beam has a plateau-shaped intensity distribution.

5. Method according to one of the preceding claims, wherein the first laser beam (Li) and / or the second laser beam (L2) and / or the third laser beam has an intensity distribution with an intensity maximum in the edge region of the respective laser beam.

6. Method according to one of the preceding claims, wherein a laser power of the second laser beam (L2) and / or the third laser beam is changed during a coating process.

7. Method according to one of the preceding claims, wherein a distance of the second irradiation zone (22) from the first irradiation zone (20) and / or a distance of the third irradiation zone (24) from the first irradiation zone (20) in the machining direction (40) corresponds to at least 0.5 times a focus diameter of the first laser beam (Li) and at most 5 times the focus diameter of the first laser beam (Li).

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

9. Method according to one of the preceding claims, wherein the second irradiation zone (22) and / or the third irradiation zone (24) is offset orthogonally to the processing direction with respect to the first irradiation zone (20).

10. Method according to one of the preceding claims, wherein the second laser beam (L2) and / or the third laser beam has a rectangular beam cross-section.

11. Method according to one of the preceding claims, wherein the second irradiation zone (22) and / or the third irradiation zone (24) has a different size than the first irradiation zone (20).

12. Apparatus for laser cladding, the apparatus comprising: A carrier unit for a metallic workpiece (50) to be coated, wherein the carrier unit has a movement unit for moving, in particular for rotating, the workpiece (50); A laser beam unit for providing at least a first laser beam (Li) and for generating, by means of at least the first laser beam (Li), at least a first irradiation zone (20) and a second irradiation zone (22) on a surface of the workpiece (50) to be coated, wherein the second irradiation zone (22) precedes the first irradiation zone (20) along a processing direction (40); A feed unit for feeding a filler material (P), in particular a powdered filler material, into the first irradiation zone (20), wherein the filler material can be fed to the first irradiation zone (20) in such a way that it already enters the first laser beam (Li) at least partially in the first processing zone (20) before it strikes the workpiece surface and is thereby at least partially heated.

13. Device according to claim 12, wherein the laser beam unit has an optic (100) with a collimation unit (120) and with a focusing unit (140), as well as with a beam splitter element (130) which is arranged between the collimation unit (120) and the focusing unit (140) in the beam path of the optic (100) and which is designed to split a laser output beam (L) into the first laser beam (Li) and at least one second laser beam (L2).

14. The device of claim 13, wherein the beam splitter element is an optical wedge, a cylindrical lens, or a diffractive optical element (DOE).

15. Workpiece, in particular brake disc, the workpiece comprising: A metallic base body, in particular a disc-shaped one; At least one coating layer, which is arranged, preferably in spiral and overlapping coating paths, on a surface of the base body and is integrally connected to the base body; Wherein a mixing region at the transition between the base body and the coating layer has a thickness of at most 20 pm, preferably of at most 10 pm, more preferably of at most 5 pm.

Images