Additively produced magnetic sheet, method for producing a magnetic sheet, sheet package and electric machine

Description

[0001] The invention relates to an additively manufactured magnetic sheet according to claim 1, a method for manufacturing a magnetic sheet according to claim 11, a sheet stack according to claim 14 and an electrical machine according to claim 15.

[0002] A new process for producing magnetic sheets for electrical machines is screen or stencil printing. Here, a printing paste is printed onto a substrate using a stencil, the open areas of which may also be provided with a screen. This is referred to as stencil printing or screen printing, with stencil printing being a generic term for screen printing. In addition to solvents and / or binders, the printing paste contains metal powder that has a functional role in the final component. The screen printing technique produces a green body which, after further thermal treatment, is usually first debindered and then subjected to a sintering process at a higher temperature. During this process, the metallic powder grains sinter together to create a structured sheet, the magnetic sheet.

[0003] To achieve higher mechanical strength in such sheets, especially at high rotational speeds of electric machine rotors, the manufacturability of two-component magnetic sheets is becoming increasingly important. An example of this is described in EP 3723249 A1. A fundamental problem with such two-component magnetic sheets, which are produced using stencil printing, is that the different materials, while possessing good magnetic properties and high strength, generally have different coefficients of thermal expansion. For example, a high-strength steel, which is a component contributing to the mechanical strength of the magnetic sheet, has a coefficient of thermal expansion of 16 x 10⁻⁶ < K⁻¹ < .Another soft magnetic material has a coefficient of thermal expansion between 10 and 12 x 10⁻⁶ < K⁻¹ within a temperature range of 0 to 100°C. When such different material components or material pairings are sintered together in a green body to form a magnetic sheet, these differing coefficients of thermal expansion lead to the development of high mechanical stresses in the joints or seams between these material components, particularly during cooling from sintering temperatures to room temperature. This can result in warping, cracking, or overall mechanical stresses in the magnetic sheet, which can significantly impair its mechanical or magnetic properties.

[0004] The object of the invention is to provide a magnetic sheet and a method for manufacturing a magnetic sheet, and to generate a sheet stack and an electrical machine from it, in which the magnetic sheet has a higher strength and less intrinsic stress compared to the prior art when using at least two material components within the magnetic sheet.

[0005] The solution to the problem consists in a magnetic sheet having the features of claim 1, in a method for producing a magnetic sheet having the features of patent claim 11, in a laminated core having the features of patent claim 14 and in an electrical machine having the features of patent claim 15.

[0006] The additively manufactured magnetic sheet for a laminated core of an electrical machine according to claim 1 comprises at least three layers bonded by sintering, wherein each layer has at least two material components separated from one another in a planar extent. The invention is characterized in that the arrangement of the material components in their planar extent in a respective layer is symmetrical to a center plane of the magnetic sheet, wherein a ratio of material proportions of the first material component and the second material component changes over the layer sequence. The first material component is arranged in a radially inner region of the respective layer and the second material component is arranged in a radially outer region of a respective layer. The first material component comprises an iron alloy which has an austenitic structure of at least 25 vol. % in the magnetic sheet.

[0007] The advantage of the described magnetic sheet compared to the prior art lies in the fact that the magnetic sheet itself is constructed as a laminate from several varying layers, whereby the layers are firmly bonded together by a sintered bond and the material proportions of the components vary in each layer. This allows stresses arising from the different coefficients of thermal expansion of the individual material components to be compensated.

[0008] The magnetic sheet produced in this way exhibits higher mechanical stability, partly because the radially inner material component has an austenitic structure and therefore possesses greater mechanical strength. The symmetry of the individual layers is also important for reducing stresses caused by differing coefficients of thermal expansion. A plane of symmetry is defined in the sheet's plane, running precisely halfway along its height. The material fraction of each component above and below this plane of symmetry is equal. This serves primarily to reduce stresses due to the different coefficients of thermal expansion and also minimizes the magnetic sheet's tendency to warp.

[0009] In a further embodiment of the invention, the second material component is an iron alloy containing at least 95 wt.%, in particular at least 97 wt.%, and most preferably at least 99 wt.% iron. Ideally, this is pure iron, since pure iron has the best ferritic or martensitic microstructure and exhibits very good soft magnetic properties, which are advantageous for the respective magnetization reversal in an electrical machine.

[0010] The first material component in the radially inner region of the magnetic sheet should preferably be designed to exhibit the highest possible strength and withstand the mechanical stress acting on the magnetic sheet. A chromium-nickel alloy is particularly suitable as the starting material for this first component. This chromium-nickel alloy preferably has a chromium content of between 22 wt.% and 28 wt.%, and particularly between 24 wt.% and 26 wt.%.

[0011] In a suitable embodiment of the invention, the nickel content is between 4 wt.% and 10 wt.%, in particular between 6 wt.% and 8 wt.%, whereby the extreme values ​​in the specified range are included.

[0012] In a further advantageous embodiment of the invention, at least one layer has a transition region arranged between the radial inner region and the radial outer region. In this transition region, both the first and second material components are present in discrete sub-regions. Discrete sub-regions are understood to mean that the individual material components are not a mixture in the transition region, but rather that sections of the transition region contain the first and second material components separately from one another, i.e., discretely.

[0013] The individual material components in the transition zone can be arranged either contiguously on the inner or outer surface, or as islands within the respective other material component. The transition zone, with its discrete sub-regions of the individual material components, is particularly well-suited for matching the different coefficients of thermal expansion. In the smaller, discrete sub-regions described, the absolute expansion during heating in operation or during sintering shrinkage is smaller than in larger regions. By calculations using finite elements, for example, a suitable structure for the discrete sub-regions in the transition zone can be determined, thus minimizing the stresses within the magnetic sheet even further by applying the described finite element method.

[0014] When using this transition region, however, it is advisable for an upper cover layer of the magnetic sheet and a lower cover layer of the magnetic sheet to be designed such that they have no transition region and are formed only by the radially inner and outer regions. This offers the advantage of ensuring magnetic shielding for the transition region of the inner layers, so that magnetic regions in the inherently non-magnetic radially inner region do not lead to eddy current losses.

[0015] To better match the coefficients of thermal expansion within the magnetic sheet, it can also be advantageous for the individual layers to have different thicknesses. Depending on the number of layers forming a magnetic sheet, which is typically advantageously between three and nine layers, the thicknesses of the individual layers can range from 10 µm to 80 µm, and in particular from 20 µm to 50 µm.

[0016] Another part of the invention consists of a method for manufacturing a rotationally symmetrical magnetic sheet for a laminated core of an electric machine and comprises the following steps: a) Applying a first printing paste to a substrate using a stencil printing process to form a first region of a first layer, b) Applying a second region of the first layer to the substrate using a second printing paste, such that a radially inner printing region and a radially outer printing region are formed in the layer, c) Repeating steps a) - c) to form at least three layers that constitute a green body of the magnetic sheet, wherein d) the first printing paste and the second printing paste comprise different functional material pre-components and the functional material pre-components of both printing pastes comprise iron particles in different alloy compositions, and e) the arrangement of the printing regions in each step is designed such that the regions are symmetryd with respect to a median plane that runs parallel to the substrate through the center of the green body.results in and the material composition of the printing paste varies between the individual layers, f) Performing a sintering process of the green body to form the magnetic sheet, such that radially inner printing areas of the green body are transformed into an iron-based material component in the magnetic sheet, which at least partially exhibits an austenitic microstructure.

[0017] As a rule, it is advisable, but not necessary in all cases and depends on the viscosity and moisture content of the printing paste, to insert a drying step between steps b) and c).

[0018] The described method and the magnetic sheet produced therefrom exhibit the same advantageous properties already described for the magnetic sheet according to claim 1. To analyze phase fractions, particularly to determine the austenitic microstructure fraction, a planar section of the material is prepared and evaluated using a reflected-light microscope and image processing software. For image processing, a microscope from Zeiss, for example, or the ZEN software can be used. An advantageous austenitic microstructure fraction of at least 25 vol.% is considered to be present. For microstructure fractions, volume fractions are used as the measured quantity due to the evaluation via a planar section and a two-dimensional image. When specifying alloying elements, wt.% is used, as the admixture of the individual components can be advantageously measured in weight fractions.

[0019] In a further advantageous embodiment of the method, in addition to the radially inner pressure zone and the radially outer pressure zone, transition pressure zones are created in at least two layers, in which both the first material pre-component and the second material pre-component are present in discrete sub-regions. This advantageous embodiment has already been described with regard to the magnetic sheet; this further reduces the effects of the different expansion coefficients of the individual material components.

[0020] Particularly when representing island-shaped discrete sub-areas in the transition printing zones, it can be technically complex to print a material component into an already printed material component. In this case, it can be advantageous to use a pressure-free additive manufacturing process, especially a spraying process, for representing the second printing material pre-component, particularly in island areas.

[0021] Another component of the invention is a laminated core for a rotor of an electric machine, comprising a plurality of magnetic laminations according to any one of claims 1-10 or manufactured according to a method according to any one of claims 11-13. Furthermore, an electric machine comprising such a laminated core according to claim 14 is a component of the invention.

[0022] Further embodiments of the invention and additional features are described in more detail with reference to the following figures. These are purely schematic representations and do not constitute a limitation of the scope of protection. For features that have a different embodiment but bear the same designation, the same reference numeral is used in each figure. The figures show: Figure 1 a schematic exploded view of an electrical machine, in particular an electric motor, comprising a housing, a stator and a rotor mounted on a shaft, Figure 2 a typical laminated core for a rotor of an electrical machine, Figure 3 an example of a layer structure of individual layers of a magnetic sheet, Figure 4 a layer structure of a magnetic sheet in an alternative embodiment Figure 3 , Figure 5a layered structure of a magnetic sheet with an alternative design form Figure 3 and 4 , Figure 6 a top view of a magnetic sheet or a section through a magnetic sheet showing three areas of the material components, Figure 7 a cut through a magnetic sheet as in Figure 6 with alternative design form, Figure 8 a process flow for the production of a magnetic sheet.

[0023] In Figure 1 Figure 1 is an exploded view of an electric machine 6, shown here in the form of an electric motor, for example for an electric vehicle. The individual components of the electric machine 6 will not be discussed further here. However, a rotor 48, which is mounted on a shaft 50, is shown. The rotor 48 comprises a laminated core 4, which is in Figure 2The lamination stack 4 is shown in more detail below, where it is composed of individual stacked magnetic laminations 2. When installed in the rotor 48, the lamination stack 4 is surrounded by windings not shown here.

[0024] In Figure 2 It can be seen that the magnetic sheets 2 are structured planar structures, whereby the term "planar" is understood to mean that they are planar, whereby the extension of the magnetic sheet in the sheet plane is much greater than a height 64 of the magnetic sheet 2. The planar extension, i.e. the diameter of the magnetic sheet 2, is preferably at least five times greater than the height 64.

[0025] In Figure 3 Figure 1 is a schematic representation of a cross-section of a magnetic sheet 2. Only one side of the rotationally symmetrical magnetic sheet 2 is shown.

[0026] In the following, reference is made to the finished magnetic sheet 2, as it is used, for example, in Figure 2 is shown, as well as the manufacturing process of the magnetic sheet 2, which is in Figure 8 is illustrated. The characteristic features of the magnetic sheet 2, in particular its layered structure and the material components used, are described in the Figures 3 - 7 explained in more detail. Regarding the Figures 3 - 7 A finished magnetic sheet 2 is assumed, however, there are also corresponding features of layers and individual material components during production, which only become the final features during the manufacturing process. Different designations are used for this. Figures 3-5The designations relevant to the magnetic sheet are used, and the reference symbols in parentheses are used that correspond to these final features in the green body during the production of the magnetic sheet 2 as preliminary stages. Thus, the magnetic sheet 2 has Figure 3 The magnetic sheet consists of a total of seven layers, 8-1 to 8-7. These layers 8-1 to 8-7 correspond to layers during the manufacturing of the magnetic sheet 2, which are designated 32-1 to 32-7 in parentheses. The bottom layer, 8-1, has a radially inner region 10-1, which corresponds to a radially inner printing area 36 during the stencil printing process 28. Furthermore, layer 8-1 has a second material component 10-2, which corresponds to a radially outer printing area 38. Layer 8-1 has a thickness 24 of approximately 30 µm. Typically, layer thicknesses 24 range between 10 µm and 80 µm.

[0027] The second layer 8-2 again has a first material component 10-1 and a second material component 10-2. This applies continuously to the subsequent layers 8-3 to 8-7. A plane of symmetry 12 is defined, which runs parallel to the sheet plane in the center of the magnetic sheet 2. "In the center of the magnetic sheet 2" means that it runs parallel to the sheet plane, i.e., the sheet surface or a substrate surface (see Figure 8), at half the height 64 of the magnetic sheet 2. The arrangement of the first material component 10-1 and the second material component 10-2 is symmetrical above and below the plane of symmetry 12. Furthermore, the proportions of the first material component 10-1 and 10-2 vary in each layer 8-1 to 8-7. These two measures—the symmetry along the axis of symmetry 12 and the variation in each layer 8-1 to 8—achieve a symmetrical arrangement of the magnetic sheet.7, the individual coefficients of thermal expansion exhibited by the individual material components 10-1 and 10-2 are adjusted to each other in such a way that the material stresses occurring, in particular during sintering shrinkage, are significantly reduced. Furthermore, due to the symmetrical arrangement of the layers 8-1 to 8-7, the magnetic sheet hardly tends to warp and results in a higher degree of planarity of the magnetic sheet 2.

[0028] Furthermore, a transition zone 18 (designated as the transition pressure zone in the green body) is provided in which both the first material component 10-1 and the second material component 10-2 occur across the entire magnetic sheet 2. As already described, this transition zone also serves to cushion and minimize the effect of the different coefficients of thermal expansion exhibited by the individual material components 10-1 and 10-2.

[0029] The material component 10-1 is preferably, but not necessarily, a chromium-nickel alloy, for example, containing 25 wt.% chromium and 7 wt.% nickel. Such an alloy is a stainless steel alloy with very high strength. This material component 10-1, which forms the radially inner region 14 of the magnetic sheet 2, is responsible, in particular, for the strength of the magnetic sheet 2 as a supporting structure, especially at high rotational speeds. In contrast, the material component 10-2 is designed as a magnetic material component, in particular as a soft magnetic component. It has a very high iron content, preferably above 95 wt.%, and most preferably above 99 wt.%. Such a high iron content, ideally pure iron, results in a ferritic material structure, which exhibits particularly good soft magnetic properties.This pure iron structure can be remagnetized very easily, whereby the hysteresis losses during remagnetization of the material are very low.

[0030] In the Figures 4 and 5 are alternative designs of the magnetic sheet to the detailed described structure of the magnetic sheet 2 in Figure 3 as shown. The essential difference of the Figures 4 and 5 compared to the structural design of the magnetic sheet 2 according to Figure 3 is that the transition area 18 is wider than in Figure 3 and that it comprises several island-shaped discrete sub-regions 20, wherein the second material component 10-2 with the soft magnetic properties is embedded in material regions of the material component 10-1. This is also shown in another representation in the Figures 6 and 7 , where each is a top view of the magnetic sheet 2. In particular in Figure 7Discrete subregions 20 of material component 10-2 are embedded in the material component 10-1 in an island-like manner. These discrete subregions 20, which are located in the transition region 18, allow for the different thermal expansion coefficients, particularly in the transition region 18. During the design of the magnetic sheet 2, a corresponding distribution of the discrete subregions 20 in the transition region 18 can be calculated using finite element methods, thus optimizing the matching of the thermal expansion coefficients between the material components 10-1 and 10-2.

[0031] The representation of the magnetic sheet 2 in Figure 5 differs from the representation in Figure 4In particular, the design consists of a lower cover layer 22-1 and an upper cover layer 22-2, in which only the radially inner region 14 and the radially outer region 16 are present, and the transition region 18, which is present in the layers between, i.e., in this case, layers 8-2 to 8-8, is covered. It is covered by the first material component 10-1 and the soft magnetic material component 10-2, resulting in magnetic shielding. This reduces eddy current losses.

[0032] As already described, the Figures 6 and 7 Illustrations of the magnetic sheet 2 in a top view, showing in particular the transition area 18 between the two Figures 6 and 7 varies. The magnetic sheet 2 has a bore 52, which is located in the center of the magnetic sheet 2 and through which the shaft 50 is guided in one installation position.

[0033] In Figure 8The manufacturing process, or rather one method for producing the magnetic sheet 2 with the described layer structure, is now illustrated. These are highly simplified schematic diagrams intended only to better illustrate the individual process steps. Starting from the top left, the diagram begins with... Figure 8A first layer 32-1 of a green body 40 to be produced is created using a stencil printing process 28. In this process, a first printing paste 26-1 is printed onto a substrate 30 using a stencil 56, which in this example is designed as a screen printing stencil 56, and a squeegee 54. The upper right section shows how the first printing paste 26-1 on the substrate 30 is dried in a drying device 58. The drying device 58 can be a drying oven, for example, a roller-guided continuous oven, or it can be a chamber in which drying takes place by infrared illumination.

[0034] In a further step, a second printing paste 26-2 is applied to the same substrate using a different stencil 56'. Both printed printing pastes 26-1 and 26-2 each form a first printing area 34-1 and a second printing area 34-2. Together, these printing areas 34-1 and 34-2 form a first layer 32-1 of a green body 40. These printing areas 34-1 and 34-2 are directly adjacent to each other without overlapping, except for process inaccuracies. In the purely schematic representation according to Figure 8 The detailed structure of layers 32-1 to 32-7 is not discussed here, as is described in the Figures 3 to 5 is described. Here, only a first pressure area 34-1 and a second pressure area 34-2 are assumed. The radially inner pressure areas 36 and radially outer pressure areas 38 are only indicated in this simplified representation. The following are described in the Figure 8Further analogous processes in which multiple layers 32-1 through 32-5 are superimposed using this printing process. The drying process in the drying device 58 can take place after each individual partial print of an area 34-1 or 34-2, or after the double printing of a respective layer 32-n, for example, layer 32-1. This reduces the process complexity, since drying does not have to take place after each individual print.

[0035] The green body 40, shown here, for example, with five layers 32-1 to 32-5, is now removed from the substrate 30 and subjected to further thermal treatment. This initially involves a debinding step, wherein, in a debinding device 60, in particular, organic binders are thermally removed from the printing paste 26-1 and 26-2. The thus prepared green body 40 is sintered in a further process step in a sintering device 62 to form the final magnetic sheet 2 at temperatures between 1000 °C and 1350 °C. During sintering, the individual grains of the iron alloys (not shown here) are bonded together in the grain boundary regions by diffusion processes and, to a lesser extent, by melting processes, so that a monolithic structure is created. The magnetic sheet 2 produced in this way is Figures 3 - 7described and has the corresponding characteristic layer sequence and the resulting mechanical advantages. List of reference symbols

[0036] 2 Magnetic sheet 4 Sheet package 6 Electric machine 8-1 - 8-9 layers 10-1, 10-2 Material components 12 Middle plane = plane of symmetry 14 Radial inner area 16 Radial outer area 18 Transition area 20 Discrete sub-areas 22-1,2 lower, upper cover layer 24 Layer thickness 26-1 First printing paste 26-2 Second printing paste 28 Stencil printing process 30 Substrate 32-1 - 32-9 layers Green body 34-1,2 First printing area, second printing area 36 Radial inner printing area 38 Radial outer printing area 40 Green body 42 Sintering process 44 Transition area 46 Discrete sub-areas Green body 48 Rotor 50 Shaft 52 Bore 54 Squeegee 56 Printing stencil 58 Drying device 60Debinding device 62Sintering device 64Height of magnetic sheets

Claims

1. Screen-printed, rotationally symmetrical magnetic lamination (2) for a laminated core (4) of an electric machine (6), said magnetic lamination comprising at least three layers (8-1, 8-2, 8-3 ... 8-n) connected by sintering, wherein each layer (8-1, 8-2, 8-3 ... 8-n) contains at least two material components (10-1), (10-2) separate from one another in planar extent, characterized in that - the arrangement of the material components (10-1), (10-2) in their planar extent in a respective layer (8-1, 8-2, 8-3 ... 8-n) is symmetrical with respect to a centre plane (12) of the magnetic lamination (2), wherein - a ratio of material proportions of the first material component and of the second material component changes over the layer sequence and - the first material component (10-1) is arranged in a radially inner region (14) of the respective layer (8-1, 8-2, 8-3 ... 8-n) and the second material component (10-2) is arranged in a radially outer region (16) of a respective layer (8-1, 8-2, 8-3 ... 8-n) and - the first material component (10-1) comprises an iron alloy which has an austenitic microstructure in a proportion of up to at least 25% by volume.

2. Magnetic lamination according to Claim 1, characterized in that the second material component (10-2) comprises an iron alloy which comprises at least 95% by weight, in particular at least 97% by weight of iron.

3. Magnetic lamination according to Claim 2, characterized in that the second material component (10-2) has a ferritic or martensitic microstructure.

4. Magnetic lamination according to one of Claims 1 to 3, characterized in that the first material component (10-1) is a chromium-nickel alloy.

5. Magnetic lamination according to Claim 4, characterized in that the first material component (10-1) is an iron-chromium alloy which has a chromium proportion of between 22% by weight and 28% by weight, in particular between 24% by weight and 26% by weight.

6. Magnetic lamination according to Claim 4 or 5, characterized in that the first material component (10-1) has a nickel proportion of between 4% by weight and 10% by weight, in particular between 6% by weight and 8% by weight.

7. Magnetic lamination according to one of the preceding claims, characterized in that at least one layer (8-1, 8-2, 8-3 ... 8-n) has a transition region (18) between the radially inner region (14) and the radially outer region (16), said transition region containing both the first material component (10-1) and the second material component (10-2) in discrete sub-regions (20).

8. Magnetic lamination according to Claim 7, characterized in that a lower outer layer (22-1) and an upper outer layer (22-2) of the magnetic lamination are each formed only by the radially inner region (14) and by the radially outer region (16).

9. Magnetic lamination according to one of the preceding claims, characterized in that the individual layers (8-1, 8-2, 8-3 ... 8-n) of the magnetic lamination (2) have different layer thicknesses (24).

10. Magnetic lamination according to one of the preceding claims, characterized in that individual layers (8-1, 8-2, 8-3 ... 8-n) have layer thicknesses (24) of between 10 µm and 80 µm.

11. Method for producing a rotationally symmetrical magnetic lamination (2) for a laminated core (4) of an electric machine (6), comprising the following steps: a) applying a first printing paste (26-1) by a screen printing method (28) to a substrate (30) to form a first region (34-1) of a first layer (32-1), b) applying a second region (34-2) of the first layer (32-1) to the substrate (30) using a second printing paste (26-2), with the result that a radially inner print region (36) and a radially outer print region (38) are formed in the layer, c) repeating steps a) to c), with the result that at least three layers (32-1, 32-2, 32-3, ..., 32-n) that form a green body (40) of the magnetic lamination (2) are produced, wherein d) the first printing paste (26-1) and the second printing paste (26-2) comprise different functional material starting components and the functional material starting components of the two printing pastes comprise iron particles in different alloy compositions and e) the arrangement of the print regions (34-1, 34-2) in the individual layers (32-1, 32-2, 32-3, ..., 32-n) is configured such that there is a symmetry of the regions (34-1, 34-2) with respect to a centre plane (12), which runs parallel to the substrate (30) through the middle of the green body (40), and the material proportions of the printing pastes vary between the individual layers, f) carrying out a sintering process (42) on the green body (40) to form the magnetic lamination (2), with the result that radially inner print regions (36) of the green body (40) which at least partially have an austenitic microstructure are converted into an iron-based material component (10-1) in the magnetic lamination (2).

12. Method according to Claim 11, characterized in that, in addition to the radially inner print region (36) and the radially outer print regions (38), transition print regions containing both the first material starting component and the second material starting component in discrete subregions (46) are created in at least two layers (32-1, 32-2, 32-3, ..., 32-n).

13. Method according to Claim 11 or 12, characterized in that at least one material component is applied in the transition print region by a printing-free additive manufacturing method, in particular a spraying method.

14. Laminated core for a rotor (48) of an electric machine (6), comprising a plurality of magnetic laminations (2) according to one of Claims 1 to 10.

15. Electric machine comprising a laminated core (4) according to Claim 14.