Laser system for laser cladding with a powder jet having hard-material particles

Abstract

The present invention relates to a laser system for laser cladding, comprising: a laser source for producing a laser beam (30) having a wavelength in the range between 0.4 µm and 1.5 µm; and a jet nozzle for directing the laser beam (30) at a workpiece surface (12) and for directing a powder jet comprising a pulverulent material (20) at the laser beam (30) and at the workpiece surface (12); wherein the laser beam (30) exiting from the jet nozzle has a reduced intensity in a core region (314) in comparison with an edge region (312a, 312b, 312c), and wherein the pulverulent material (20) comprises hard-material particles. The invention also relates to a method for laser cladding and to a component which can be manufactured by means of the method.

Description

[0001] Laser system for laser cladding with a powder jet containing hard material particles

[0002] Technical area

[0003] The present invention relates to a laser system and a method for laser cladding.

[0004] State of the art

[0005] Laser cladding is used in areas such as repair, coating, and joining technology. A distinction can be made between conventional laser cladding (laser metal deposition (LMD), direct metal deposition (DMD), or direct energy deposition (DED)) and high-speed laser cladding (HS-LMD or extreme high-speed laser cladding (EHLA)).

[0006] In conventional laser material deposition, as schematically shown in Figure 1 a, a molten pool 16 is created on the surface 12 of a workpiece 10 by means of a laser beam 30. A powdered filler material 20 is introduced into the molten pool 16 through a powder nozzle arranged coaxially or laterally to the laser beam 30 with the aid of an inert conveying or carrier gas. Before impacting the molten pool 16, the powder particles 20, or at least some of the powder particles 20, are exposed to laser light in an interaction zone 40 with the laser beam 30. In the LMD process, the energy input into the workpiece 10 by means of the laser beam 30 is generally greater than the energy input into the powder particles 20. The powder particles 20 are therefore generally only melted after impacting the molten pool 16. Once the melt has solidified, a dense, molten-metallurgically bonded layer forms.A coaxial powder nozzle arrangement generates a focused powder gas jet. To produce a defect-free coating, the interaction time with the powder particles 20 in the melt pool 16 must be sufficiently long to allow temperature equalization between the particles 20 and the melt 16, allowing the particles 20 to transition to the liquid state.

[0007] This limits the speed of the LMD process. Due to the large amount of laser radiation hitting the workpiece, a large mixing and heat-affected zone 14 (HAZ) is created. Unlike in the conventional LMD process, in the HS-LMD process (see Figure 1 b), the powdered filler material 20 is specifically heated above the workpiece surface 12 to temperatures around the melting point or higher. Due to a sufficiently large interaction zone 40 between the laser beam 30 and the powder-gas jet, the powder 20 is heated to such an extent that it essentially immediately forms a solid, in particular molten-metallurgical bond with the workpiece 10 on the workpiece surface 12, which is also preheated by the laser beam 10. This allows significantly higher feed rates, up to 500 m / min, than with conventional laser cladding (0.5 m / min to 2 m / min), since the time required for the particles 20 to melt in the molten pool 16 is eliminated.By reducing the energy input into the workpiece 10, the heat-affected zone 14 and the molten pool 16 are significantly reduced. This means that even temperature-sensitive materials such as aluminum and cast alloys can be coated using HS-LMD. HS-LMD is used for coating components, particularly rotationally symmetrical ones, such as brake discs or plain bearings. For material application using HS-LMD, the component is rotated, and the processing head for supplying the laser beam and powder is moved, in particular, in a straight line perpendicular to or parallel to the component's rotation axis. In this way, a spiral or helical bead can be created, which ultimately forms a coating surface.

[0008] HS-LMD processes are described, for example, in DE 10 2011 100 456 B4 or in DE 10 2018 130 798 A1.

[0009] Description of the invention

[0010] Based on the known prior art, it is an object of the present invention to provide an improved laser system for laser cladding and an improved method for laser cladding. In particular, the quality, especially the material properties, of the workpieces processed by laser cladding are to be increased.

[0011] The problem is solved by a laser system, a method, and a component having the features of the independent claims. Advantageous further developments emerge from the dependent claims, the description, and the figures.

[0012] Accordingly, a laser system for laser cladding is proposed with a laser source for generating a laser beam with a wavelength in the range between 0.4 pm and 1.5 pm. A disk laser or a fiber laser can be used as the laser source. A diode laser can also be used. In this way, for example, laser beams with wavelengths of approximately 450 nm, approximately 515 nm, between approximately 800 nm and approximately 1000 nm, or of approximately 1030 nm, 1060 nm or 1070 nm can be generated. The laser beam can be designed such that it can be guided to a processing head by means of an optical fiber. Due to large usable fiber diameters, the laser beam can be satisfactorily coupled into a comparatively large ring and core portion of a multiple-clad fiber, as described in more detail below, for example when the brilliance of the diode emitters or bars or stacks is limited. The laser source can have a laser power of between 2 kW and 24 kW.If the workpiece is a brake disc, the laser power can be between 8 kW and 24 kW, and if it is a plain bearing, the laser power can be 2 kW.

[0013] The laser system further includes a jet nozzle for directing the laser beam toward a workpiece surface and for directing a powder jet containing a powdered material toward the laser beam and the workpiece surface. The laser beam can be directed orthogonally toward the material surface. The powder jet is tilted relative to the laser beam to select an interaction zone between the powder jet and the laser beam above the material surface. Such an interaction zone enables more efficient deposition.

[0014] The laser beam emerging from the jet nozzle has a reduced intensity in a core region compared to an edge region. The core intensity can, for example, be less than 90% of the edge intensity. Thus, at least within the interaction zone, the laser beam has an intensity in an edge region that is higher than an intensity in the core region of the laser beam, so that the powdery material is exposed to the higher intensity of the edge region upon entering the interaction zone. Due to the inclined alignment of the at least one powder jet to the laser beam, the interaction distance with the laser beam varies across the cross-section of the powder jet. Due to the reduced intensity in the core region, the individual powder particles are supplied with essentially homogeneous energy despite the varying interaction distance.In other words, an intensity maximum at the edge of the laser beam leads to a more uniform distribution of fluence per powder particle, thus expanding the process window toward higher laser powers while maintaining stable weld quality. The following applies to the intensity distribution of the laser beam in the focal plane: iEdge.

[0015] — Icenter — 0.

[0016] The powdered material comprises hard material particles, particularly carbides, which do not dissolve in an interaction zone after interaction with the laser beam. The hard material particles can have a powder size in the range between 15 pm and 63 pm, particularly between 15 pm and 45 pm or between 20 pm and 53 pm. The hard material particles can comprise one or more of the following materials:

[0017] The hard material particles have the effect that they do not overheat when a base material, such as an iron base material or a matrix material, is heated and therefore do not dissolve. This prevents the hard material particles from forming an alloy with the base material in a molten pool. In other words, the hard material particles have the effect of forming a material-to-material bond with a molten pool created on the material surface by the laser beam, without dissolving or melting themselves. Due to the reduced core intensity of the laser beam, the hard material particles do not form an alloy with the material or with a matrix, which means that fewer dissolved chemical elements are present. This means that the applied layer is more resistant and fewer or no cracks, especially vertical cracks, occur in it even under stress.This consequently leads to an increase in the ductility of the workpiece and a reduction in its brittleness. Furthermore, mechanical forces can be compensated for by plastic deformation. The hard material particles also have the effect of preventing or at least reducing the occurrence of residual stresses during the cooling phase of the molten pool, i.e. the phase from the melt to the solid phase. In particular, the use of hard material particles can result in 25% less residual stress. This leads to a reduction in the distortion of the workpiece during the cooling phase. The less distorted workpiece is more resilient and fewer or no cracks, especially no vertical cracks, occur even under stress. In addition, the reduced distortion means that the effort required to rework the workpiece is reduced because less material needs to be removed.

[0018] The use of hard material particles thus benefits the material properties of workpieces machined by laser cladding. For example, due to the reduced core intensity of the laser beam, 95% of the powder particles originally present in the powdered material are actually captured as hard material particles in the machined workpiece.

[0019] The workpiece can be a metallic workpiece. The powdered material can in particular comprise a metallic material. The powdered material can be blasted onto the workpiece surface using a conveying gas, in particular argon or helium, and / or using an inert gas mixture as a process shielding gas. The process shielding gas can additionally shield the processing location from the surrounding atmosphere. The focus of the laser beam can be on the workpiece surface or directly above the workpiece surface. The jet nozzle can have a core opening for the laser beam with reduced core intensity and an annular opening for the powdered material. The annular opening can be designed in the manner of an annular gap nozzle or, by means of several nozzles arranged in a ring around the core opening, in the manner of a multi-jet nozzle. Wide-jet nozzles can also be used to create a linear powder focus.The powder can be sprayed onto the machining site at an angle from the front and / or from the back, relative to the feed direction. A powder focus can have a diameter between 0.2 mm and approximately 6 mm. The workpiece can be, for example, a brake disc, a hydraulic cylinder, a printing roller, a plain bearing, or another rotationally symmetrical workpiece.

[0020] Furthermore, a method for laser material deposition welding is proposed. The method comprises the step of directing a laser beam with a wavelength in the range between 0.4 pm and 1.5 pm onto a workpiece surface. It further comprises the step of directing a powder jet comprising a powdered material onto the laser beam and onto the workpiece surface. The laser beam can be directed orthogonally onto the material surface. The powder jet is inclined with respect to the laser beam in order to select an interaction zone between the powder jet and the laser beam above the material surface. Such an interaction zone enables more efficient deposition. The laser beam and the powder jet are aligned using the jet nozzle. The method comprises the step of heating the powdered material in an interaction zone with the laser beam above the workpiece surface.It further comprises the step of applying, in particular welding, the heated powdered material to the workpiece surface along a predetermined contour to form a wear-resistant layer. The wear-resistant layer forms after cooling and is particularly advantageous for reducing particulate matter pollution in brake discs. Within the interaction zone, the laser beam has a reduced intensity in a core region compared to an edge region. Furthermore, the powdered material comprises hard material particles, which are also present in the wear-resistant layer. The configurations, effects, and advantages disclosed in connection with the device are correspondingly applicable to the method.

[0021] Furthermore, a component, in particular a brake disc, a hydraulic cylinder, a pressure roller, a plain bearing, or another, in particular rotationally symmetrical, workpiece, is proposed. The component has a base body made of a base material, in particular a cast alloy or cast iron. The base body can be produced by primary forming, such as casting. Furthermore, the base body can have undergone machining, such as turning or milling, before becoming the base body for laser deposition welding. The component has a wear-resistant layer applied to the base body by laser deposition welding, comprising a plurality of hard material particles according to the disclosure. The wear-resistant layer is applied to the base body, in particular using the method according to the disclosure. The wear-resistant layer can be configured as a two-phase or multi-phase layer system.The hard material particles are embedded in a matrix material. The hard material particles are preferably substantially spherical and each have a mixing zone with the matrix material in an edge region. The mixing zone has a maximum thickness of 10 μm. The reduced core intensity of the laser beam, as described above, enables such a narrow and uniform mixing zone between the hard material particles and the matrix material across the entire wear-resistant layer due to the more uniform energy input compared to the prior art.

[0022] In one embodiment, the component is a brake disc in which the wear protection layer is applied in a braking area that is adapted to be in frictional contact with brake shoes. Due to the high strength of the hard material particles, such a brake disc has little material removal when the brake disc is in frictional contact with the brake shoes. This can reduce the removal of fine dust that occurs during braking. The wear protection layer can have a varying thickness along a surface of the base body, wherein a lateral thickness of the wear protection layer, i.e. a thickness at the outer edge of the component, has a different extent than a medial thickness of the wear protection layer, i.e. a thickness facing the interior of the component. The thickness refers to a dimension orthogonal to the component surface. In the case of the brake disc, the thickness runs orthogonal to the friction surfaces.For example, the lateral thickness of the wear protection layer can be less than the medial thickness of the wear protection layer. The wear protection layer of the secondary component can have a large number of hard material particles embedded in a matrix material. The features and effects according to the disclosure can be combined with the component. The varying thickness can result from a post-processing step after laser deposition welding. For example, due to the high thermal input into the component during laser deposition welding, distortion of the component can occur. This distortion can be more pronounced in a lateral region of the component than in a medial region. Partial removal of the wear protection layer compensates for this distortion. This can result in a lateral thickness of the wear protection layer being less than a medial thickness of the wear protection layer.In one embodiment, a thickness difference between the medial thickness and the lateral thickness can be at most 200 μm, in particular 100 μm, more particularly at most 50 μm. As a percentage, the thickness difference based on the smallest thickness can be no more than 40%, in particular 25%, more particularly at most 10%. Such small thickness differences can result from the disclosed combination of a laser beam with reduced core intensity and the hard material particles. They reduce the required post-processing time for a component and improve the material properties.

[0023] Furthermore, the component can have a buffer layer below the wear protection layer in order to realize a two-layer system.

[0024] In one embodiment, the hard material particles of the powdered material comprise at least one material from the group consisting of tungsten carbide, titanium carbide, niobium-based alloys, and / or chromium carbide. Experimental setups have shown that these materials do not overheat when exposed to the laser beam used and are therefore particularly well suited for increasing ductility, reducing distortion, and preventing crack formation.

[0025] In one embodiment, the powdered material comprises a meltable matrix material in addition to the hard material particles, so that a multiphase layer is formed from the powdered material on the workpiece surface or, in the case of a process step, so that a multiphase layer is applied to the workpiece surface. In the multiphase layer, the hard material particles interact with the matrix material in such a way that the multiphase exhibits superior material properties. This promotes a reduction in brittleness and contributes to the development of fewer residual stresses during the cooling phase.

[0026] In one embodiment, the powdered material consists of 15 to 40, particularly 23 to 30, percent by volume of hard material particles. The remaining portion can be a matrix material, for example. These volume ratios represent an optimal compromise for achieving increased resistance with efficient bonding through laser cladding.

[0027] In one embodiment, the powdered material comprises at least one material from the group of (i) stainless steels, in particular 430L and 316L; (ii) nickel alloys, in particular corrosion-resistant nickel-based alloys; and (iii) alloys or agglomerations or powder mixtures containing at least one of the constituents titanium, titanium carbide, niobium, niobium carbide, molybdenum, chromium, and / or chromium carbide. These materials can be used in any combination with regard to a single- and two-layer system, as well as a single-phase and multi-phase layer system. In a single-layer system, one layer of the powdered material is applied to the workpiece. In a two-layer system, two layers of a powdered material are applied to the workpiece, wherein the two layers are different from one another.For example, a first layer can be applied as a buffer layer and a second layer as a wear-resistant layer. In a single-phase coating system, the layer has one phase, specifically unmelted hard material particles, whereas in a multi-phase coating system, the layer is divided into a matrix and hard material particles.

[0028] In one embodiment, an outer diameter of the core region within an interaction zone is less than or equal to one third, in particular one quarter, more particularly one fifth, eighth or tenth, of an outer diameter of the edge region. In other words, an outer diameter of the ring beam can be, at least at one point, at most 10 times as large, in particular at most 5 times, 4 times or 3 times as large as the diameter of the core beam. The boundaries of the respective beam components can be determined, for example, using the second moment method. With a narrower edge region, a more uniform temperature distribution among the powder particles follows in principle, since the differences in the interaction time with the laser beam are reduced.The outer diameter of the laser beam, in particular the outer diameter of the ring beam according to the variant described above, can be at least 500 pm, preferably at least 1000 pm, and even more preferably at least 2000 pm, at at least one location in the interaction zone. Productivity can be increased by increasing the laser beam diameter in the interaction zone, particularly on the workpiece surface.

[0029] To generate the beam profile of the laser beam with a core region and an edge region, a multi-clad fiber, in particular a 2-in-1 fiber, can be used to enable efficient beam shaping. The intensity components of the core region and the ring region of the laser beam can be controllable. For example, a 2-in-1 optical fiber with a core diameter between 200 pm and 300 pm and a ring outer diameter between 700 pm and 1000 pm can be used. A multi-clad fiber with more than one ring fiber component can also be used, for example, to generate a beam profile with different intensities in the different ring regions. Additionally or alternatively, beam-shaping elements can also be used to generate the described beam profile, in particular a diffractive optical element (DOE) or a multi-lens array. In this way, non-rotationally symmetric beam profiles, e.g.A linear beam profile can be generated. Furthermore, a ring-shaped beam profile can also be generated in this way using a single-core fiber.

[0030] In one embodiment, the method comprises the step of applying, in particular welding, a buffer material to the workpiece surface along a predetermined contour to form a buffer layer on the workpiece surface. The buffer material is applied prior to the powdered material, so that the buffer layer is formed beneath the wear-resistant layer. In this way, a two-layer system is realized, which leads to increased resistance of the workpiece.

[0031] In one embodiment, the method includes the step of partially grinding the applied powdered material to compensate for any distortion that occurred during application. The workpiece may be a brake disc. Due to thermal stresses, distortion may occur during a cooling phase after application of the powdered material. The geometric component changes resulting from this distortion can be compensated for by partial grinding.

[0032] According to the disclosure, the laser beam within the interaction zone can have a beam profile with a substantially ring-shaped intensity maximum. Thus, the beam profile of the laser beam has an edge region surrounding the central core region of the laser beam, in which edge region the laser beam has a higher intensity, preferably at every point, than in the core region. The edge region can also have multiple ring regions, wherein the intensity of the laser beam within the interaction zone is higher in at least one of the ring regions than in the core region. The intensity profile can be either stepped or smooth at the transitions between the regions. The intensity of the laser beam can be substantially constant along the ring shape. Alternatively, the intensity of the laser beam can be variable along the ring shape and, for example, fluctuate by up to approximately 30%.

[0033] Furthermore, according to the disclosure, the laser beam can have a linear beam profile within the interaction zone, oriented substantially transversely to the feed direction of the laser beam, with an intensity maximum leading in the feed direction and / or with an intensity maximum trailing in the feed direction. The feed direction describes the direction in which the laser beam moves relative to the workpiece surface. It can be composed of a comparatively fast, in particular rotational, feed speed of the workpiece and a comparatively slow, lateral feed speed of the processing head guiding the laser beam in order to generate a spiral or helical material deposit on the workpiece surface.In a laser beam with a linear beam profile, the leading intensity maximum and the trailing intensity maximum each extend linearly, essentially transversely to the feed direction, and are spaced from each other by the likewise linear region of lower intensity (core region of the laser beam). According to this variant, it can further be provided that the powdered filler material is directed obliquely from the front and / or from the rear onto the processing location by means of one or more wide-jet nozzles, which are aligned essentially parallel to the linear laser focus. The laser beam can also be composed of several separate laser beams that at least partially overlap in the focal plane.

[0034] According to the disclosure, an intensity distribution of the laser beam can be substantially plateau-shaped at one location. The plateau shape can also be referred to as a top-hat. The plateau- or top-hat-shaped intensity distribution describes a sudden increase in intensity at the edge of the laser beam to the intensity maximum, which is maintained essentially across the entire width of the edge region before the intensity suddenly drops again toward the core region of the laser beam. The plateau- or top-hat-shaped intensity distribution in the edge region of the laser beam promotes a reduction in the roughness of the applied material layer compared to a Gaussian intensity distribution.At at least one point within the interaction zone, the intensity in the core region of the laser beam can be at most 90%, preferably at most 50%, even more preferably at most 10%, of the intensity maximum in the edge region of the laser beam. The intensity distribution with reduced intensity in the core region of the laser beam can increase the process window with regard to the variability of the laser power used. In particular, with the described intensity distribution in the focal plane, laser powers > 4 kW can be used while simultaneously maintaining the weld quality because more laser power is used to preheat and / or melt the powder for coating the workpiece. The power in the core region of the laser beam can, at least at one point within the interaction zone, be, for example, between 7% and 9% of the laser power of the entire laser beam.In the core region, it can also be between 5% and 7%, in particular approximately 6% of the total laser beam power. According to an alternative variant, the power in the core region can be reduced to a minimum, in particular to 0% of the total laser power.

[0035] Short description of the characters

[0036] Preferred further embodiments of the invention are explained in more detail in the following description of the figures. In the figures:

[0037] Figures 1 a, b schematic representations of an LMD process and an HS-LMD process;

[0038] Figures 2a to d are schematic representations of different laser beams and the resulting interaction paths of a powdered material with the respective laser beam;

[0039] Figure 3 shows a process window width in relation to the laser power as a function of the beam profile of the laser beam;

[0040] Figure 4a shows a beam profile with an annular intensity maximum in cross section;

[0041] Figure 4b shows a linear beam profile with a leading and a trailing

[0042] Intensity maximum;

[0043] Figures 5a to c show a workpiece to which a two-layer system has been applied;

[0044] Figure 6 shows a distortion of a cooled workpiece in comparison with conventional

[0045] solutions; and

[0046] Figure 7 shows a schematic cross-section through a brake disc. Detailed description of preferred embodiments

[0047] Preferred embodiments are described below with reference to the figures. Identical, similar, or equivalent elements in the different figures are provided with identical reference numerals, and a repeated description of these elements is partially omitted to avoid redundancies.

[0048] Figures 1a and 1b were described above. In the following, the influence of the intensity distribution of the laser beam on the interaction with the powdered filler material during laser cladding is explained in more detail with reference to Figures 2a to 2d. Figures 2a to 2d schematically show a sectional front view of a workpiece 10 which is locally melted for laser cladding by means of a laser beam 30, so that a melt pool 16 is formed on the workpiece surface 12. While the laser beam 30 is moved perpendicular to the plane of representation over the workpiece 10, a filler material in the form of a powder jet 20 is blasted onto the processing point by means of a preferably inert conveying gas. For the sake of simplicity, only the powder application from one side is shown in Figures 2a-d.It is understood, however, that during laser cladding, the filler material can be directed onto the processing site in several individual beams arranged in a ring around the laser beam, or as a ring beam, or in the case of a linear beam profile of the laser beam, e.g., from the front and / or from the rear as a linear powder jet. Depending on the position of a powder particle within the powder jet 20, the interaction distance within an interaction zone 40, along which the respective powder particle is exposed to the laser radiation, varies in length. Accordingly, the powder particles are heated to varying degrees by the laser beam 30 depending on their trajectory.While powder particles in the center of the powder jet 20 within the interaction zone 40 are melted, for example, powder particles in the edge region of the powder jet 20 can simultaneously evaporate due to their longer or shorter interaction time with the laser beam 30 (cf. right or upper powder particle in Figures 2a-d) or impact the workpiece surface 12 in a solid state (cf. left or lower powder particle in Figures 2a-d). The temperature gradient of the powder particles during laser cladding is particularly large when the laser beam 30 has a Gaussian intensity profile 32a within the interaction zone 40. This case is illustrated in Figure 2a. Powder particles at the outer (or lower) edge of the powder jet 20 are heated particularly weakly. The uneven interaction time of the powder particles with the laser beam 30 can have a negative impact on the welding result.A high-quality weld bead can be ensured within a narrow process window with precisely coordinated process parameters. Changes in laser power can lead to significant fluctuations in the quality of the weld result. An improvement in the temperature gradient or a narrower temperature range of the powder particles can be achieved by using a laser beam 30 with a plateau- or top-hat-shaped intensity profile 32b, as shown in Figure 2b.

[0049] Furthermore, the homogeneity of the powder heating can be improved if a laser beam 30 is used which has an intensity distribution 32c, 32d within the interaction zone 40 as shown in Figures 2c or 2d. Figure 2c shows a laser beam 30 with a concave intensity profile 32c in the interaction zone 40, in which the intensity decreases from an annular maximum to the core region of the laser beam 30. Due to the high intensity in the edge region of the laser beam 30, even powder particles with a short interaction time are still heated comparatively strongly. A particularly uniform temperature distribution of the powder particles can be achieved for coaxial powder feed with an annular intensity profile of the laser beam 30, in which the majority of the laser energy is present in the edge region of the laser beam 30. A plateau-like or top-hat-shaped intensity distribution 32d in the annular outer region of the laser beam 30 (cf.Figure 2d) has proven particularly advantageous in this regard. Using a laser beam 30 with such an intensity distribution can advantageously influence process stability, particularly during high-speed laser cladding. Figures 2c and 2d each refer to variants in which the laser beam 30 has a rotationally symmetrical cross-section. It is understood that the representations in Figures 2c and 2d are analogously applicable to a laser beam 30 with a linear beam profile, with the respective intensity distribution 32c, 32d then only being present transversely to the length of the linear beam profile.

[0050] Figure 3 shows, by way of example, the change in the process window during high-speed laser material deposition as a function of the beam profile of the laser beam used. The laser powers in kW are plotted vertically; these powers enable the process to be carried out, with otherwise identical process parameters, without any significant loss of quality in the welding result. The illustration relates to high-speed laser material deposition on a tubular workpiece made of mild steel, with the outer diameter of the laser beam in the focal plane being 2000 pm and the feed rate being approximately 80 m / min. When using a laser beam with a Gaussian beam profile (cf. Figure 2a), i.e. with a Gaussian intensity distribution of the laser beam in the focal plane, an acceptable welding result can only be achieved in a very narrow power range of 4 kW to approximately 4.6 kW. The process window 52 is therefore very small.For a laser beam with a top-hat-shaped intensity distribution within the interaction zone across its entire cross-section (see Figure 2b), the process window 54 is already significantly larger. Laser powers between 4 kW and 8 kW can be used for the process without significant loss of quality in the welding result. The process windows 56a to 56d each refer to the use of a laser beam with an annular beam profile with a top-hat-shaped intensity distribution in the annular edge region of the laser beam and with different laser power in the core region of the laser beam. With a core power of 9% of the total laser power, the process window 56a essentially corresponds to the process window 54 with the top-hat-shaped intensity profile as shown in Figure 2b.With a relative reduction of the laser power in the core area of ​​the laser beam to 6% of the total power, the laser power can be increased to 9 kW while maintaining good welding quality. This corresponds to an enlargement of the process window 56b by 25% compared to the process window 54 with a top-hat-shaped intensity profile without annular power or intensity distribution. With a further reduction of the core power to 3% of the total power of the laser beam, losses in the energy efficiency of the process can be observed. This means that good welding results can only be achieved from a laser power of approximately 4.6 kW. The process window 56c with regard to the usable laser power is nevertheless 10% larger than the process window 54 when using a conventional laser beam with a top-hat beam profile.The largest possible process window 56d can be achieved with an annular beam profile, as shown in Figure 3, where the entire laser power is present in the annular portion, i.e., the laser power in the core beam is reduced to zero (see also Figure 2d). High-quality welding results can be achieved with this beam profile between 4.6 kW and 10 kW. This corresponds to an enlargement of the process window by 35% compared to the process window 54 when using a conventional top-hat beam profile. The comparison according to Figure 3 shows that the process window can be opened up to higher laser powers in high-speed laser cladding using a laser beam with an annular intensity maximum and coaxial feeding of the powdered filler material in the beam focus.The findings from Figure 3 can be transferred analogously to a laser beam with a line focus, which has a line-shaped intensity maximum within the interaction zone at its front and rear edges in the feed direction, whereby the powdered filler material is directed onto the processing point only from the front and from the rear in a line-shaped powder jet oriented essentially transversely to the feed direction.

[0051] Figures 4a and 4b show different beam profiles 31a, 31b of a laser beam 30, each having a core region 314 and an edge region 312a, 312b, 312c. According to the invention, the illustrated beam profiles 31a, 31b can be present in a projection plane that runs transversely to the propagation direction of the laser beam 30 and lies within the interaction zone 40 (cf. Figures 1 and 2). The laser beam 30 according to Figure 4a has an annular intensity maximum in its annular edge region 312a and a core region 314 with a lower intensity compared to the edge region 312 (cf. also Figure 2d). Figure 4b shows a linear beam profile 31b of a laser beam 30 that is oriented transversely to the feed direction 60.The laser beam 30 according to Figure 4b has a leading intensity maximum in its front edge region 312b in the feed direction 60 and a trailing intensity maximum in its rear edge region 312c. The core region 314 of the laser beam 30, which is also rectilinear, is located between the rectilinear intensity maxima.

[0052] Figures 5a to 5c each show a cross-section of a workpiece section to which a laser deposition welding process was applied. The originally powdered material 20 is applied to a base material 70 of the workpiece by being heated by the laser beam 30, as described above, to form a material bond with the base material 70 and then cooled. The base material 70 can be cast iron or a cast alloy. Figures 5a to 5c show workpiece sections with different layer thicknesses, each implementing a two-layer system: In Figure 5a, a buffer layer 80 was first applied to the base material 70 as the first layer. This layer has a thickness of 90 μm, for example. A wear-resistant layer 90 was applied to the buffer layer 80 as the second layer. This layer contains the hard material particles 100.In the exemplary embodiment from Figure 5a, the thickness of the wear protection layer 90 is 170 pm. Because the wear protection layer 90 with the hard material particles 100 is applied to the base material 70 with the interposition of a buffer layer 80, the material properties of the workpiece are improved compared to the case in which the hard material particles 100 are applied directly to the base material 70. Improved resistance can be achieved by a wear protection layer 90 that is thicker than in the embodiment in Figure 5b. In the exemplary embodiment from Figure 5b, the buffer layer 80 also has a thickness of 90 pm. The wear protection layer 90 arranged thereon has a thickness of 130 pm. Higher feed rates can be achieved by a wear protection layer 90 that is thinner than in the embodiment in Figure 5a.In the exemplary embodiment shown in Figures 5a and b, a laser source with an output power of 8 kW was used, which achieves a feed rate of 145 m / min. The hard material particles 100 have a diameter in the range between 15 pm and 63 pm, in particular between 15 pm and 45 pm or between 20 pm and 53 pm. The hard material particles 100 are embedded in a matrix material. A mixing zone is located between the hard material particles 100 and the matrix material. In Figures 5a and 5b, the mixing zone is visible as a dark edge surrounding the hard material particles 100. The mixing zone is small compared to the diameter of a substantially spherical urea particle 100. For the respective hard material particle 100, it has a thickness of at most 10 pm, in particular 8, 6, 4, or 2 pm.

[0053] Figure 5c shows another two-layer system. The buffer layer 80 is applied to the base material 70, onto which the wear-resistant layer 90 is applied with a thickness in the range of approximately 190 μm to 245 μm. The wear-resistant layer 90 contains the hard material particles 100. In the exemplary embodiment shown in Figure 5c, a laser source with an output power of 10.5 kW was used as an example. The urea particles 100 are formed, for example, from tungsten carbide; alternatively, the materials disclosed above are conceivable. In Figures 5a to c, the two-layer system is crack-free: No inclusions, for example, are visible either between the individual hard material particles 100 or at any other locations. This increases the ductility and reduces the brittleness of the workpiece.For example, due to the reduced core intensity of the laser beam, 95% of the powder particles originally present in the powdery material 20 are actually captured as hard material particles in the machined workpiece.

[0054] The use of hard material particles 100 has the effect that they do not overheat when the base material 70 is heated and thus do not dissolve. This prevents the hard material particles 100 from forming an alloy with the base material 70 or causing air inclusions, which would increase the residual stresses in the base material 70. These residual stresses lead to distortion as the workpiece cools due to a shielding effect. Figure 6 compares the distortion during laser deposition welding according to the disclosure with conventional methods. The use of hard material particles 100 reduces the residual stresses that occur. As a result, the distortion of the workpiece during cooling is reduced, for example, by 24%. This increases the quality of the workpiece and reduces the material removal required after distortion, which favors material usage.

[0055] Figure 7 shows a cross-section through a rotationally symmetrical component, in particular a brake disc, after a wear protection layer 90 has been applied by laser deposition welding and after grinding has been carried out. A medial thickness d1 of the wear protection layer 90 is greater than a lateral thickness d2. More material was therefore removed from the medial region of the wear protection layer 90 than from the lateral region, which had essentially the same thickness as the medial region before grinding. The reason for this is that the shielding effect leads to greater distortion in the lateral region of the component than in the medial region. The brake disc is a rotationally symmetrical component. The medial region is therefore a radially inner region, and the lateral region a radially outer region.The reduced core intensity in combination with the hard material particles results in the difference between the radially inner thickness d1 and the radially outer thickness d2 being at most 200 pm, in particular 100 pm, more particularly at most 50 pm. In percentage terms, the thickness difference based on the smallest thickness may not exceed 40%, in particular 25%, more particularly at most 10%.

[0056] Where applicable, all individual features presented in the embodiments may be combined and / or exchanged without departing from the scope of the invention.

Claims

Claims 1. Laser system for laser cladding, comprising a laser source for generating a laser beam (30) with a wavelength in the range between 0.4 pm and 1.5 pm; and a beam nozzle for directing the laser beam (30) onto a workpiece surface (12) and for directing a powder jet comprising a powdered material (20) onto the laser beam (30) and onto the workpiece surface (12); wherein the laser beam (30) exiting the beam nozzle has a reduced intensity in a core region (314) compared to a peripheral region (312a, 312b, 312c); and wherein the powdered material (20) comprises hard particles (100).

2. Laser system according to claim 1, wherein the hard particles (100) of the powdered material (20) comprise at least one material from the group consisting of tungsten carbide, titanium carbide, alloys based on niobium and / or chromium carbide.

3. Laser system according to one of the preceding claims, wherein the powdered material (20) comprises a meltable matrix material in addition to the hard material particles (100), such that a multiphase layer is formed from the powdered material (20) on the workpiece surface (12).

4. Laser system according to one of the preceding claims, wherein the powdered material (20) consists of 15 to 40, in particular 23 to 30, volume percent of hard particles (100).

5. Laser system according to one of the preceding claims, wherein the powdered material (20) comprises at least one material from the group consisting of stainless steels; Nickel alloys, in particular corrosion-resistant nickel-based alloys; alloys or agglomerations or powder mixtures containing at least one of the components titanium, titanium carbide, niobium, niobium carbide, molybdenum, chromium and / or chromium carbide.

6. Laser system according to one of the preceding claims, wherein an outer diameter of the core region (314) less than or equal to one third, in particular one quarter, further in particular one eighth, of an outer diameter of the edge region (312a, 312b, 312c).

7. Laser cladding process with the following steps: Aligning a laser beam (30) with a wavelength in the range between 0.4 pm and 1.5 pm onto a workpiece surface (12); Aligning a powder jet comprising a powdered material (20) onto the laser beam (30) and onto the workpiece surface (12); Heating the powdered material (20) in an interaction zone (40) with the laser beam (30) at least partially above the workpiece surface (12); Deposition, in particular by cladding, of the heated powdered material (20) onto the workpiece surface (12) along a predetermined contour to form a wear-resistant layer (90); wherein the laser beam (30) has a reduced intensity in a core region (314) within the interaction zone (40) compared to a peripheral region (312a, 312b, 312c); and wherein the powdered material (20) comprises hard particles (100) which are also present in the wear-resistant layer (90).

8. Method according to claim 7, wherein the hard particles (100) of the powdered material (20) comprise at least one material from the group consisting of tungsten carbide, titanium carbide, alloys based on niobium and / or chromium carbide.

9. Method according to one of claims 7 or 8, wherein the powdered material (20) comprises, in addition to the hard particles (100), a meltable matrix material, such that a multiphase layer is applied to the workpiece surface (12).

10. Method according to any one of claims 7 to 9, wherein the powdered material (20) consists of 15 to 40, in particular 23 to 30, volume percent of hard particles (100).

11. Method according to any one of claims 7 to 10, wherein the powdered material (20) comprises at least one material from the group consisting of Stainless steels; Nickel alloys; Alloys or agglomerations or powder mixtures comprising at least one of the constituents titanium, titanium carbide, niobium, niobium carbide, molybdenum, chromium and / or chromium carbide. A method according to any one of claims 7 to 11, wherein the outer diameter of the core region (314) is less than or equal to one-third, in particular one-quarter, and further in particular one-eighth, of the outer diameter of the outer region (312a, 312b, 312c). A method according to any one of claims 7 to 12, comprising the following step: Applying, in particular by weld overlay, a buffer material to the workpiece surface (12) along a predetermined contour to form a buffer layer (80) on the workpiece surface (12); wherein the application of the buffer material occurs prior to the application of the powdered material (20), so that the buffer layer (80) is formed below the wear-resistant layer (90). A method according to any one of claims 7 to 13, comprising the following step: partially grinding off the applied powdered material to compensate for any distortion that may have occurred during application.Component comprising a base body made of a base material (70), in particular a cast alloy or cast iron; and a wear-resistant layer (90) applied to the base body, comprising a plurality of hard particles (100) embedded in a matrix material; wherein the wear-resistant layer (90) is applied, in particular, by a method according to claims 7 to 14; wherein the hard particles (100) each have a mixing zone with the matrix material in an edge region, which has a thickness of at most 10 pm. Component according to claim 15, wherein. The component is a brake disc in which the wear-resistant layer (90) is applied in a braking area adapted to be in frictional contact with brake pads. A component according to claim 15 or 16, wherein the wear-resistant layer (90) has a varying thickness along a surface of the base body, wherein a lateral thickness (d2) of the wear-resistant layer (90) differs from a medial thickness (d2) of the wear-resistant layer (90). A component according to claim 17, wherein the thickness difference between the medial thickness (d1) and the lateral thickness (d2) is at most 200 pm, in particular 100 pm, and more particularly at most 50 pm. A component according to any one of claims 15 to 18, wherein a buffer layer (80) is arranged below the wear-resistant layer (90), which is free of hard particles (100).

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