Method for producing a rotationally symmetrical magnetic sheet, magnetic sheet, laminated core, and electric machine

Abstract

The invention relates to a method for producing a rotationally symmetrical magnetic sheet (2) of a laminated core (4) of an electric machine (6), said magnetic sheet having a plurality of radially running winding teeth (8). The method has the following steps: - producing a main part (10) by applying a printing paste (12) onto a substrate (14) using a pattern printing method (16), wherein the printing paste (12) comprises binders and iron particles, and the iron particles comprise more than 95 wt.% of iron, - carrying out a first thermal treatment (18), in which the binders are removed from the main part (10), thus producing a preform (20), - carrying out a second thermal treatment (22) of the preform (20) in the form of a sintering process in order to produce the magnetic sheet (2), and subsequently - pressing the magnetic sheet (2) with a pressure of more than 5 MPa in a uniaxial manner on the sheet plane (26) thereof by means of a pressing ram (28).

Description

[0001] Description

[0002] Method for producing a rotationally symmetrical magnetic sheet, magnetic sheet, laminated core and electrical machine

[0003] The invention relates to a method for positioning a magnetic sheet according to claim 1, a magnetic sheet according to claim 11 and a laminated core according to claim 12 and an electrical machine according to claim 13.

[0004] Electrical machines, especially electric motors, have a rotor that is mounted on a shaft and has a laminated core with corresponding windings wound around it. The laminated core consists or comprises primarily individual soft magnetic sheets, which in conventional motors are generally punched out of sheet metal strips and stacked to form a laminated core. For modern high-performance motors, additively manufactured sheets, which are produced using stencil printing processes, for example, are sintered and also stacked to form a laminated core. Printed sheets of this type have the advantage that advantageous soft magnetic materials can be used in terms of material pairing in order to generate very good soft magnetic properties. However, the sintering processes used for this purpose lead to isotropic crystal growth, which in turn has a negative effect on the coercive field strength in the preferred directions of the sheet plane.For low frequencies, the magnetic sheet must have the lowest possible magnetic hysteresis losses and thus a low coercive field strength in the direction of flux. The object of the invention is to provide a method for producing a magnetic sheet, as well as a magnetic sheet per se, a laminated core formed therefrom, and an electrical machine which, compared to the prior art, in particular punched magnetic sheets and printed and sintered magnetic sheets, on the one hand have very good soft magnetic properties and, in particular, in the regions where a magnetic flux occurs, have a very low coercive field strength.

[0005] The solution to the problem consists in a method for producing a magnetic sheet according to claim 1, as well as in a magnetic sheet having the features of claim 11, in a laminated core having the features of claim 12 and in an electrical machine having the features of claim 13.

[0006] The described method for producing a rotationally symmetrical magnetic sheet according to claim 1 serves to produce a magnetic sheet for a laminated core of an electrical machine. The magnetic sheet has a plurality of radially extending winding teeth. The method comprises the following steps:

[0007] Producing a green body by applying a printing paste to a substrate by means of a stencil printing process, wherein the printing paste comprises binder and iron particles and the iron particles comprise more than 95 wt . % iron.

[0008] This is followed by an initial thermal treatment, during which the binders are removed from the green body and a preform is created.

[0009] A second thermal treatment of the preform takes place in the form of a sintering process to produce the magnetic sheet.

[0010] The magnetic sheet is then pressed uniaxially in its sheet plane using a press die with a pressure of more than 5 MPa.

[0011] The advantage of the described process is that, thanks to the stencil printing process and the subsequent debinding or sintering process, a magnetic sheet is produced which, compared to conventional punched magnetic sheets of the state of the art, uses materials with particularly good soft magnetic properties and, if possible, a high iron content, comprising at least 95% iron. Ideally, pure iron with only very low levels of contamination is used for this purpose. This printed and sintered magnetic sheet thus differs from the state of the art in terms of its soft magnetic properties.It has surprisingly been found that uniaxial pressing with a pressure of at least 5 MPa in a preferred pressure range up to about 50 MPa, particularly in the winding teeth in the radial direction, creates a crystal structure that positively influences the magnetic flux along this radial direction of the winding tooth, where very low hysteresis losses occur and thus also a low coercive field strength. Furthermore, uniaxial pressing improves the planarity of the magnetic sheet, which, when stacked in a sheet stack, leads to a denser packing and thus also to a higher iron content per volume of the unit sheet stack.

[0012] A green body is understood to be a shaped body for later heat treatment, which usually contains binders, which also include solvents, to improve the shape. A green body is usually mechanically self-supporting and has limited mechanical load-bearing capacity. A green body freed from these binders and / or solvents is referred to in this context as a preform. The shaped body contains only the functional components of the final body, which also actively participate in the sintering process. The preform is converted into a sintered body by heat treatment in the form of a sintering process, in which, unlike a melting process, individual grains in the preform form a coherent, materially bonded structure via diffusion processes. After the sintering process has been carried out, the resulting body is referred to in this context as a magnetic sheet.However, this magnetic sheet can still be subjected to various post-processing processes.

[0013] A stencil printing process is a method in which a printing paste is applied to a substrate, for example by means of a squeegee, to create a layer that is usually between 70 pm and 300 pm (in particular between 150 pm and 250 pm). The paste is only printed on certain uncovered areas of the substrate that are kept free by a stencil. The paste is usually printed through a screen, but this is not absolutely necessary. Stencil printing using a screen is called a screen printing process, which is why the screen printing process is subsumed under the term stencil printing process.

[0014] Furthermore, it is also expedient to carry out a third thermal treatment in the form of recrystallization annealing after the described pressing process. This is preferably carried out in a temperature range between 600 °C and 800 °C. The recrystallization annealing process should be carried out at a constant temperature for at least 8 minutes to about 30 minutes. As a rule, the recrystallization annealing is completed after 60 minutes at a constant temperature within the described temperature window. This annealing process promotes the formation of a magnetically effective texture.

[0015] This is preferably an anisotropic magnetic texture along a radial extent of the winding teeth. This radial extent of the winding teeth is also the magnetic flux path in which the magnetic flux occurs and the soft magnetic properties are intended to be present with the lowest possible hysteresis losses. A so-called Goss texture has proven particularly advantageous as a texture. However, a so-called cube texture is also expedient. The grains of the texture 34 advantageously have an average diameter in an image plane of 80 pm to 400 pm.

[0016] In crystallography, texture is understood to mean the totality of the orientations of the crystallites of a polycrystalline solid. The orientation of a crystal can be determined by the diffraction of an incident wave at the lattice planes. According to the Bragg equation, an angle can be assigned to incident monochromatic radiation and the lattice parameter of the lattice plane. For example, the texture is made visible using polarized light in a Kerr microscope. This produces a micrograph of the magnetic sheet surface and evaluates it under the microscope using image processing software. For image processing, for example, a Zeiss microscope or the associated ZEN software can be used. In the texture described here, more than 50% of the magnetically active domains have a preferred direction that deviates by less than 36° from the radial direction.

[0017] The Goss texture is a special type of texture for magnetic sheets. In this texture, the cubic crystals of the iron crystal lattice lie in the (110) direction in the sheet plane. The cube edges of the cubic lattice, as defined above, essentially follow the radial extension of the winding teeth. In the ideal case, which cannot be represented, the cube edges are oriented along the radius of the magnetic sheet. A so-called cube texture is also useful. In the ideal case, the cube edges also run along the radius but are oriented in the (100) direction.

[0018] Furthermore, it has been found that the grain texture has an average diameter in an image plane of 100 pm to 400 pm. The grains have a preferred thickness of between 40 and 80 pm, in particular around 60 pm.

[0019] Using the stencil printing process, the thickness of the green body to be produced can be varied using parameters such as the squeegee pressure and the amount of paste. The thickness of the green body is preferably between 100 pm and 400 pm, which, after the second thermal treatment, results in a thickness of a magnetic sheet that is in a range between 100 pm and 300 pm. The thickness of the magnetic sheet is particularly preferably between 150 pm and 250 pm. This makes the magnetic sheets significantly thinner than conventionally punched sheets; punched sheets usually have a minimum thickness of 330 pm. It should be noted that eddy current losses during operation of the electrical machine decrease quadratically with smaller thicknesses of the magnetic sheet. Thus, thinner sheets lead to lower eddy current losses in the electrical machine. Sheet thicknesses below 330 pm cannot be achieved with reproducibly high quality using punched sheets using the state of the art.

[0020] In a further embodiment of the invention, the proportion of iron in the iron particles is as high as possible. It should be at least 95%, preferably 97% by weight, and particularly preferably more than 99% by weight. The higher the iron content, the better the magnetizability of the magnetic sheet. In an electric motor, this increases the torque with low eddy current losses, thereby giving the electric motor or electrical machine better efficiency and energy efficiency.

[0021] In a further embodiment of the invention, the pressing ram is rotated during the pressing process by an angle a of less than 10°, in particular less than 3°, at least 0.5°. In addition to the uniaxial pressure on the magnetic sheet, a shearing force is also applied, so that a texture is also generated in the circumferential direction of the magnetic sheet outside the winding teeth enclosed by the rotating yoke.

[0022] A further component of the invention is a rotationally symmetrical magnetic sheet for a laminated core of an electrical machine. The magnetic sheet is distinguished in that it has a thickness of between 100 pm and 250 pm and has a sintered metallic phase which comprises at least 95 wt. % iron. Furthermore, the magnetic sheet has a plurality of radially extending winding teeth. Furthermore, a plurality of radial winding teeth are provided and an anisotropic magnetic texture is present along the radial extent. The magnetic sheet described has the same advantages which have already been described with regard to the magnetic sheet which can be produced using the method according to patent claim 1. The anisotropic magnetic texture which is achieved in particular by the uniaxial pressing of the magnetic sheet leads to particularly good soft magnetic properties of the magnetic sheet.

[0023] A further component of the invention is a laminated core of an electrical machine having a plurality of stacked magnetic laminations, which in turn are produced by a method according to one of claims 1 to 10. Furthermore, part of the invention is an electrical machine comprising a laminated core according to claim 12, wherein the electrical machine is preferably an electric motor.

[0024] Further embodiments of the invention and further features are described in the following specific description with reference to the figures. These are purely exemplary, schematically illustrated embodiments that do not represent any limitations on the scope of protection.

[0025] Showing:

[0026] Figure 1 is an exploded view of a conventional electrical machine with a rotor and laminated core,

[0027] Figure 2 shows a conventional laminated core consisting of several magnetic sheets with a plurality of winding teeth,

[0028] Figure 3 shows a laminated core with magnetic sheets in an alternative design,

[0029] Figure 4 is a plan view of a magnetic sheet,

[0030] Figure 5 is a schematic representation of the manufacturing process of a magnetic sheet, Figure 6 is a representation of the magnetization when applying different textures,

[0031] Figure 7 is a schematic representation of a microstructure of a texture with direction indicators for different magnetic domains.

[0032] Figure 1 shows an overview of a typical electrical machine 6 in the form of an electric motor 46 as an exploded view. The illustration serves to better locate the components described below. The electrical machine 6 has a rotor 48 which is mounted on a shaft 50. Figure 2 shows a laminated core 4 which is composed of a large number of stacked magnetic sheets 2. This laminated core 4 is a component of the rotor 48 according to Figure 1. The individual magnetic sheets 2 of the laminated core 4 have winding teeth 8 around which the electrical lines, which are not shown here, are wound. The winding teeth 8 in the laminated core 4 thus serve as a yoke for an electromagnet.

[0033] Another embodiment of a laminated core 4 is shown in Figure 3; here only four magnetic sheets 2 are stacked on top of one another, although there are usually considerably more magnetic sheets 2, as shown in Figure 2. The magnetic sheets 2 according to Figure 3 also have winding teeth 8, with the magnetic sheet 2 also having a circumferential ring 9 into which the winding teeth 8 merge. The circumferential ring 9 in Figure 3 is arranged in the outer region of the magnetic sheet 2; in the magnetic sheet 2 according to Figure 2, the circumferential ring 9 is arranged in the interior of the magnetic sheet 2.

[0034] Figure 4 shows a plan view of a magnetic sheet 2 which corresponds approximately to that in Figure 3. This shows a radial extension 44 which corresponds to a radius around the center of the rotationally symmetrical magnetic sheet 2. The winding teeth 8 are also arranged along a radius line. Furthermore, the sheet plane is shown in dashed lines in Figure 4 and coincides with the image plane in this illustration. A second arrow in the circumferential ring 9 of the magnetic sheet 2 is designated by a; this angle will be discussed further below.

[0035] Figure 5 schematically illustrates the production of a magnetic sheet 2 and subsequently of a laminated core 4, as described with reference to Figures 1 to 4. The description begins top left with the first part of Figure a, in which a stencil printing process 16 is illustrated. Here, a printing paste 12 is printed onto a substrate 14 using a stencil 54, which is generally shown as a screen stencil, and a squeegee 52. This creates a green body 10. The green body 10 usually has a thickness 40, which in this state is approximately 250 to 300 pm. The green body 10 comprises the components of the printing paste 12. These are solvents, polymeric binders and iron powder. The iron powder has the highest possible iron content, which should be over 95 wt.%. In particular, the iron content of the iron particles in the printing paste is over 99 wt.% , which is why we can already speak of pure iron here .

[0036] The green body 10 is in itself, particularly when it has been dried from solvents, mechanically self-supporting and stable enough that it can be subjected to a heat treatment process, as shown in Figure 5. In this process, it is detached from the substrate 14 or, if the substrate 14 is a flexible substrate, it is fed to the first heat treatment step 18 directly on the substrate for heat treatment. The first heat treatment step 18 is a debinding process in which the green body 10 is optionally dried, i.e. freed from solvents, and is then heated to such an extent that thermal decomposition of the organic binders also takes place. In Figure 5b this process is shown schematically as a continuous process in a tunnel furnace 56. In principle, it is also possible to carry out the individual processes in charging furnaces in individual batches.However, in order to achieve the highest possible efficiency in production, the use of a tunnel kiln, as shown here, is expedient. During the first thermal treatment 18, i.e. the debinding process, temperatures of approximately 250°C, for example, are applied, preferably under inert gas. The second part of the continuous kiln 56 is characterized by the second thermal treatment 22, which is carried out in the form of a sintering process 24. Here, the temperature is significantly higher than during the first thermal treatment; for sintering iron particles, it is approximately 1000°C to 1350°C.

[0037] With regard to terminology, it should be noted that the green body 10 is still referred to as green body 10 upon entering the furnace 56 or during the first thermal treatment. After the first thermal treatment, it is referred to as a preform, which is fed to the sintering process 24. After leaving the continuous furnace or after the sintering process in general, the preform 20 is referred to as magnetic sheet 2. The magnetic sheet 2 has a thickness 42 which lies between 150 pm and 250 pm.

[0038] Subsequently, according to Figure 5 c, the magnetic sheet 2 is uniaxially pressed perpendicular to the sheet plane 26. The effect of this uniaxial pressing process in the uniaxial press 30 by means of the press ram 28 will be discussed later. It should be noted that the press ram 28 is pressed onto the magnetic sheet 2 with a force F which should be at least 5 MPa and generally does not need to be more than 50 MPa. Multiple uniaxial pressing, e.g. up to five repetitions of the pressing process, can be expedient in this case and in order to form the desired texturing particularly well. Tempering of the magnetic sheet 2 during uniaxial pressing is also expedient. This can be done, for example, in the form of hot pressing, for example at a temperature between 100 °C and 300 °C.

[0039] If necessary, the punch can be rotated by an angle a which is less than 5°, for example 1°, in order to introduce shear forces into the magnetic sheet 2 in addition to the uniaxial forces F. It has been found that the described forces have a comparable effect on the crystal structure of the magnetic sheet to a rolling process, which was not to be expected due to the rather static nature of the force application. A particular advantage here is that the uniaxial pressing also places fewer shear forces on the magnetic sheet 2 than is the case with rolling.

[0040] In a further optional step, recrystallization annealing is carried out in a third thermal treatment 32 according to Figure 5d. This third thermal treatment 32 is shown schematically here in the form of a charging furnace, but it can just as easily be carried out in a continuous furnace 56, as in Figure 5b. For the recrystallization annealing 32, a temperature of 600°C to 800°C, for example 620°C or 720°C, can usually be applied constantly for at least 8 minutes up to about 30 minutes. This temperature treatment in this range leads to a further intensified formation of an anisotropic texture in certain material regions of the magnetic sheet 2. The temperature is a factor influencing the size of the crystal grains that form in the texture 34. The higher the recrystallization temperature, the larger the grains formed.It is advantageous to form larger grains (80-150 pm) in the interior of the sheet and smaller grains (50-100 pm) on the surfaces (to suppress anomalous eddy current losses).

[0041] The thus heat-treated magnetic sheets 2 are then stacked to form a laminated core 4, with a robot arm 60, for example, being used to automate the process. The laminated core 4 can be bonded in such a way that the individual magnetic sheets 2 are provided with, in particular, organic coatings and, if necessary, are immersed in a bath containing organic binders. This process will not be discussed further here.

[0042] As already mentioned, an anisotropic texture 34 is produced by means of uniaxial pressing, in particular in the winding teeth 8 in the radial extension 44. This texture 34 is also reinforced by the recrystallization annealing, i.e. the third thermal treatment 32. In Figure 6, the losses Pi,s at 50 Hz in the sheet plane 26 are plotted based on the extension along the winding teeth 8, i.e. on the one hand in the radial extension 44 and transversely thereto, i.e. also in the circumferential direction 64. The first texture 34 is a uniaxial Goss texture 36; if a second texture direction at 90° is added, this is referred to as a cube texture 38. The circle 39 in Figure 6 represents a purely isotropic texture 39. It can be seen that the losses in radial extension 44 for a Goss texture 36 and a cube texture 38 are significantly lower than for an isotropic material.It is therefore extremely advantageous to introduce these textures 34 , 36 , 38 into the magnetic sheet 2 by the described uniaxial pressing and to reinforce them if necessary by the third thermal treatment 32 , i.e. the recrystallization annealing .

[0043] Textures 34 generally do not naturally lead to precisely aligned magnetic domains; rather, they are preferred directions which, as shown in Figure 7, are largely aligned with the radial extension 44. At least 50% of the individual grains should not deviate by more than 36° from the preferred radial and tangential directions 44, as shown in Figure 7 with the arrows and the angle information. This would be referred to as an isotropic texture, which, as shown in Figure 6, causes higher magnetic losses with respect to the preferred directions. List of reference symbols

[0044] 2 magnetic sheets

[0045] 4 sheet package

[0046] 6 electric machine

[0047] 8 winding teeth

[0048] 9 circumferential ring

[0049] 10 green bodies

[0050] 12 printing paste

[0051] 14 Substrat

[0052] 16 stencil printing processes

[0053] 18 first thermal treatment

[0054] 20 pre-bodies

[0055] 22 second thermal treatment

[0056] 24 Sintering process

[0057] 26 sheet level

[0058] 28 press stamps

[0059] 30 uniaxial press

[0060] 32 third thermal treatment

[0061] 34 Textur

[0062] 36 Goss texture

[0063] 38 cube texture

[0064] 39 I isotropic texture

[0065] 40 thick green bodies

[0066] 42 Thickness magnetic sheet a angle

[0067] 44 radial extension

[0068] 46 electric motor

[0069] 48 Rotor

[0070] 50 wave

[0071] 52 squeegees

[0072] 54 Stencil

[0073] 56 runs of en

[0074] 58 Conveyor belt

[0075] 60 robot arm

[0076] 64 Circumferential direction

Claims

Patent claims 1. Method for producing a rotationally symmetrical magnetic sheet (2) of a laminated core (4) of an electric machine (6) having a plurality of radially extending winding teeth (8), comprising the following steps: - Producing a green body (10) by applying a printing paste (12) to a substrate (14) using a stencil printing process (16), wherein the printing paste (12) comprises binder and iron particles and the iron particles comprise more than 95 wt. % iron, - a first thermal treatment (18) , whereby the binders are removed from the green body (10) and a pre-body (20) is formed, - a second thermal treatment (22) of the preform (20) in the form of a sintering process, to produce the magnetic sheet (2) , after which - the magnetic sheet (2) is pressed uniaxially in its sheet plane (26) by means of a press punch (28) with a pressure of more than 5 MPa.

2. Method according to claim 1, characterized in that a third thermal treatment (32) is carried out after uniaxial pressing in the form of recrystallization annealing.

3. Method according to claim 2, characterized in that a temperature between 600 °C and 800 °C is kept constant for a period of at least 8 min during recrystallization annealing.

4. Method according to one of claims 1 to 3, characterized in that an anisotropic magnetic texture (34) is generated along the radial extent of the winding teeth.

5. Method according to claim 4, characterized in that a Goss texture is produced along the radial extent of the winding teeth.

6. Method according to one of claims 4 or 5, characterized in that the grains of the texture (34) have a mean diameter in an image plane of 80 pm to 400 pm .

7. Method according to one of the preceding claims, characterized in that the thickness of the green body (10) is set in a range of 100 pm to 400 pm by means of the stencil printing method (16), resulting in a thickness of the magnetic sheet (2) which is in a range of 100 pm to 300 pm.

8. Method according to claim 7, characterized in that the thickness (42) of the magnetic sheet is set between 150 pm and 250 pm.

9. Method according to one of the preceding claims, characterized in that the iron particles have an iron content of more than 97 wt. %, preferably more than 99 wt. %, .

10. Method according to one of the preceding claims, characterized in that the press punch (28) is rotated during the pressing process by an angle α of less than 10°, in particular less than 3°, at least 0.5°.

11. Rotationally symmetrical magnetic sheet for a laminated core (4) of an electric machine (6) , characterized in that the magnetic sheet (2) has a thickness (42) which is between 100 pm and 250 pm and has a sintered metallic phase which comprises at least an ice content of 95 wt. %, wherein a plurality of radially extending winding teeth (8) are provided and an anisotropic magnetic texture (34) is present along the radial extent (44) of the winding teeth (8).

12. Laminated core of an electric machine comprising a plurality of stacked magnetic laminations (2) manufactured by a method according to any one of claims 1 to 10.

13. Electric machine comprising a laminated core (4) according to Claim 12.

14. Electric machine according to claim 13, characterized in that the electric machine (6) is an electric motor.

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