A method for manufacturing ferromagnetic parts comprising an overhanging surface and a part obtained by said method

Abstract

A method for manufacturing ferromagnetic parts comprising an overhanging surface and a part obtained by said method. The invention relates to a method for manufacturing a ferromagnetic part, comprising the following successive steps: a) additive manufacturing of an intermediate assembly (1) in a ferromagnetic material comprising a blank (2) of the ferromagnetic part and at least one support (6), the blank comprising at least one overhanging portion (4) supported by at least one support; b) removal of each support by chemical dissolution. Figure for the abstract: Fig. 4

Description

Title of the invention: Method for manufacturing ferromagnetic parts comprising an overhanging surface and part obtained by said method. technical field

[0001] The present invention relates to the field of manufacturing ferromagnetic parts, particularly ferromagnetic parts used in electrical machines, for example rotors or stators. In particular, the present invention relates to the additive manufacturing of ferromagnetic parts. Previous technique

[0002] In many applications using ferromagnetic parts, it is necessary to minimize the eddy currents generated by the presence of an alternating magnetic flux circulating in said ferromagnetic parts. Indeed, these eddy currents are the source of significant magnetic losses, particularly when the ferromagnetic parts are rotors of electric motors operating at high rotational frequencies.

[0003] This is why it is common to manufacture ferromagnetic parts in the form of stacks of insulated rolled sheets. Such ferromagnetic parts thus exhibit alternating ferromagnetic and electrically insulating zones. However, the manufacturing processes for such ferromagnetic parts are limited in terms of the shapes of the ferromagnetic parts they can produce; in particular, they are not suitable for manufacturing ferromagnetic parts adapted for axial magnetic flux applications.

[0004] It may therefore be preferable to use additive manufacturing processes to obtain complex ferromagnetic parts, but these processes are not yet suitable for manufacturing ferromagnetic parts in the form of stacks of alternating ferromagnetic layers with electrically insulating layers.

[0005] In order to reduce eddy currents in ferromagnetic parts obtained by additive manufacturing, it is known to manufacture ferromagnetic parts in the form of thin walls separated by insulating air zones.

[0006] For example, US patent 11,155,903 B2 describes an additive manufacturing process for a ferromagnetic part comprising a continuous thin wall. US patent 11,682,932 A2 describes an additive manufacturing process for a ferromagnetic part comprising spaced air pockets to reduce eddy currents.

[0007] The article by Alexander D. Goodall et al.: “Loss performance of an additively manufactured axial flux machine stator with an eddy-current limiting structure”, Materials Today Communications, Volume 35, 2023, 105978, describes a ferromagnetic part forming a stator obtained by additive manufacturing and comprising insulating air zones.

[0008] The article by C. Klein et al.: “Magnetic Performance of Eddy Current Suppressing Structures in Additive Manufacturing”, Actuators 2024, 13(3), 94, simulates ferromagnetic parts in the form of a continuous wall following the pattern formed by a space-filling curve.

[0009] Although such ferromagnetic parts are theoretically efficient because they reduce losses generated by eddy currents, they are difficult to produce by additive manufacturing. Indeed, these ferromagnetic parts have overhanging sections that can deform or even collapse during their additive manufacturing.

[0010] Patent application EP 3 654 501 A1 describes an additive manufacturing process for a ferromagnetic part comprising a stack of layers separated from each other by insulating air gaps, the layers being supported by supports arranged in the air gaps. However, the supports between the layers form electrical connections between said layers which allow the passage of eddy currents and therefore do not allow a satisfactory reduction of the losses generated by these eddy currents.

[0011] There is therefore a need for an additive manufacturing process suitable for easily manufacturing ferromagnetic parts exhibiting low losses generated by eddy currents.

[0012] The object of the invention is to meet, at least in part, this need(s). Description of the invention

[0013] To this end, the invention relates to a method for manufacturing a ferromagnetic part, the method comprising the following successive steps: a) additive manufacturing of an intermediate assembly in a ferromagnetic material comprising a blank of the ferromagnetic part and at least one support, the blank comprising at least one overhanging part supported by at least one support, b) removal of each support by chemical dissolution.

[0014] By "an overhanging part", we mean here and within the framework of the invention a part having a lower surface forming with the direction of the gravitational force an angle between 45 and 90°, during additive manufacturing.

[0015] Conversely, a "vertical part" is a part that is not overhanging, that is, one that is free of a lower surface forming an angle between 45° and 90° with the direction of the gravitational force during additive manufacturing. Preferably, additive manufacturing is carried out along a manufacturing direction substantially parallel to the direction of the gravitational force.

[0016] According to a first embodiment, the blank may comprise a monolithic wall forming the overhanging part(s), the wall having a grid-like shape when viewed in cross-section of the intermediate assembly, the grid consisting of a main line and a plurality of secondary lines, each secondary line being a branch of the main line extending opposite another secondary line, the secondary lines being spaced apart from each other such that, for each secondary line, the portion of the wall along said secondary line is separated from the portions of the wall along the other secondary lines. Preferably, for each secondary line, the portion of the wall along said secondary line is less than 250 pm from the portion of the wall along the adjacent secondary line opposite said secondary line. The blank may consist of the monolithic wall.

[0017] According to a second embodiment, the blank may comprise a monolithic wall forming the overhanging part(s), the wall having a folded shape such that the wall follows a continuous line when viewed in cross-section of the intermediate assembly, each surface of the wall opposite another surface of the wall being spaced from said other surface. Preferably, the distance between said surfaces is less than 250 pm. Preferably, the continuous line is a space-filling curve of order 2 or higher. Preferably, the space-filling curve is a Peano curve or a Hilbert curve. The blank may consist of the monolithic wall.

[0018] Preferably, the difference between the wall thickness at the end of step a) and the wall thickness at the end of step b) is between 50 and 150 pm, in particular between 50 and 75 pm or between 100 and 150 pm.

[0019] Preferably, the wall has a thickness between 110 and 2000 pm after step a).

[0020] Preferably, the overhanging part is supported by a plurality of supports spaced apart from each other. Preferably, the distance between two adjacent supports is between 100 and 5000 pm.

[0021] Preferably, the support has, at the end of step a), a cylindrical shape, preferably with a diameter between 50 and 200 pm, or the shape of an elongated parallelepiped, preferably with a thickness between 50 and 200 pm and / or a length between 500 and 10000 pm.

[0022] Preferably, the support has, at the end of step a), a height, measured along the direction of the gravitational force during step a), of between 100 and 500 pm.

[0023] Preferably, the ferromagnetic material is iron or an alloy comprising iron and at least one element selected from silicon, cobalt and nickel.

[0024] Preferably, the intermediate assembly has a general toric or parallelepiped shape.

[0025] Preferably, the additive manufacturing of step a) implements a powder bed fusion method, for example laser or electron beam powder bed fusion, or a concentrated energy material deposition method, in particular laser fused material deposition.

[0026] Preferably, step b) includes a substep bi) of immersing the intermediate assembly in a dissolving agent bath and / or circulating a dissolving agent through spaces within the intermediate assembly. Preferably, substep bi) lasts between 0.5 and 5 minutes. Preferably, the bath and / or the circulating dissolving agent is at a temperature between 20 and 100 °C. Preferably, the circulation rate of the dissolving agent through said spaces is between 0.1 and 10 mL / min.

[0027] Preferably, the dissolving agent for the bath and / or circulation comprises an acid selected from nitric acid, hydrochloric acid, or sulfuric acid. Preferably, the dissolving agent comprises a solvent selected from ethyl alcohol or water. Preferably, the acid represents between 1 and 5% by volume of the dissolving agent.

[0028] Preferably, step b) includes a substep b2), subsequent to substep bi), of rinsing the blank, preferably by immersion in deionized water and / or by circulating deionized water through the gaps in the intermediate assembly. Preferably, the deionized water is at a temperature between 10 and 30 °C. Preferably, substep b2) lasts between 5 and 30 minutes.

[0029] Preferably, step b) includes a substep b3), subsequent to substep b2), of drying the blank, preferably with the blank suspended, preferably with the blank rotated on itself at regular intervals. Preferably, substep b3) lasts at least 1 hour.

[0030] Preferably, step b) is followed by a step c) of mechanical machining of the external surfaces of the blank. Mechanical machining makes it possible to achieve the target dimensions for the manufactured part, when they are not achieved at the end of step b), and to obtain a better surface finish. Mechanical machining is particularly advantageous when substep bi) includes the circulation of the dissolving agent and does not involve immersing the intermediate assembly in the dissolving agent bath.

[0031] The invention also relates to a ferromagnetic part obtained from the process according to the present invention.

[0032] The present invention therefore essentially consists of an additive manufacturing process adapted to manufacture ferromagnetic parts having at least one overhanging part.

[0033] The process is simple to implement because the overhanging part is supported by at least one support during its additive manufacturing, which ensures that it does not deform or collapse. For example, in the case of additive manufacturing by laser powder bed fusion, the presence of the support(s) prevents the spreading of a layer of powder on the already built portion of the overhanging part from deforming or even collapsing said already built portion. Thus, the process according to the present invention is suitable for easily manufacturing ferromagnetic parts of complex shapes.

[0034] Furthermore, the ferromagnetic part obtained by the process according to the present invention is free of supports holding the overhanging part(s) of said ferromagnetic part, since each support is eliminated. The absence of supports limits losses caused by eddy currents. The inventors simulated the mass losses when applying an alternating magnetic field at 500 Hz and 0.5 T to a ferromagnetic part obtained according to the process of the present invention and to the intermediate assembly used to obtain said part before the removal of the supports. The inventors measured a 70% reduction in mass losses for the ferromagnetic part obtained according to the process of the present invention compared to the intermediate assembly.

[0035] Furthermore, the use of chemical dissolution in the process according to the present invention makes it easy to remove each support. In particular, it allows the removal of one or more supports arranged in a confined space within the intermediate assembly and which would not be accessible for removal by mechanical machining. The support(s) can be arranged between two opposite surfaces of the blank. The process according to the present invention is thus not limited with respect to the location of each support, which makes it suitable for manufacturing ferromagnetic parts with complex shapes.

[0036] The use of a chemical solvent for the removal of each support in the process according to the present invention also has the advantage of limiting the surface roughness of the manufactured ferromagnetic part and therefore the risk of unwanted electrical contact that may arise between two surfaces of said ferromagnetic part. In particular, reducing the surface roughness of the ferromagnetic part reduces the risk of electric arcing between two opposing surfaces of said part.

[0037] The ferromagnetic part manufactured by the process according to the present invention can be used in a magnetic circuit subjected to an alternating magnetic field. For example, said ferromagnetic part can be a component of an electric motor, such as a rotor or a stator; it can also be a component of an actuator or an axial flux electric machine. Brief description of the drawings

[0038] Other advantages and features will become clearer upon reading the detailed description, given by way of illustration and not limitation, with reference to the following figures:

[0039] [Fig.1] [Fig.1] schematically represents in block diagram the different stages of the process according to the present invention.

[0040] [Fig.2] [Fig.2] is a perspective view of the intermediate assembly obtained at the end of step a) of the process according to the present invention.

[0041] [Fig.3] [Fig.3] is a perspective view of the intermediate assembly of [Fig.2] cut transversely.

[0042] [Fig.4] [Fig.4] is a schematic representation of a cross-section of a first portion of the intermediate assembly of [Fig.3], the first portion comprising supports.

[0043] [Fig.5] [Fig.5] is a schematic representation of a cross-section of a second portion of the intermediate assembly of [Fig.3], the second portion being free of supports.

[0044] [Fig.6] [Fig.6] is a schematic cross-sectional representation of one end of the continuous wall of the blank after step b) of the process according to the present invention.

[0045] [Fig.7] [Fig.7] is a schematic representation of a cross-section of another example of an intermediate assembly at the end of step a) of the process according to the present invention. Detailed description

[0046] Fig. 1 schematically illustrates the different steps a), b) and c) of an example of a manufacturing process for a ferromagnetic part according to the present invention.

[0047] Step a) consists of manufacturing an intermediate assembly by additive manufacturing. The intermediate assembly is made of a ferromagnetic material. The ferromagnetic material may be amorphous or crystalline. The ferromagnetic material may be iron or an alloy comprising iron and at least one element selected from silicon, cobalt, and nickel. The additive manufacturing method is selected from powder bed fusion, in particular laser powder bed fusion or electron beam powder bed fusion, and concentrated energy deposition, in particular laser fused deposition.

[0048] Figures 2 and 3 illustrate an example of an intermediate set 1 obtained at the end of step a). In the example shown here, the intermediate set 1 has a general toroidal shape with a rectangular cross-section, in particular a square cross-section. Other general shapes are obviously possible, in particular a parallelepiped shape.

[0049] The intermediate assembly 1 comprises a roughing 2 of the part to be manufactured. The roughing 2 comprises a continuous wall 3 having a shape folded upon itself so as to form overhanging parts 4 and plumb parts 5 included by the roughing 2.

[0050] According to the definition given above, the overhanging parts 4 and the down-hanging parts 5 are defined as a function of the direction of the gravitational force Fg during the additive manufacturing of the intermediate assembly 1. In the embodiment illustrated in figures 2 and 3, the gravitational force Fg was parallel to the axis of revolution X of the toroidal shape of the intermediate assembly 1. The additive manufacturing was carried out along a manufacturing direction Z parallel to the direction of the gravitational force Fg and in the opposite direction.

[0051] The intermediate assembly 1 also includes a plurality of supports 6 to support the overhanging parts 4. The supports 6 connect the overhanging parts 4 to the plumb parts 5 and thus transmit the weight of the overhanging parts 4 to the plumb parts 5 during step a) of additive manufacturing of the intermediate assembly 1.

[0052] The intermediate assembly 1 is monolithic. The blank 2 and the supports 6 are formed from the same ferromagnetic material. The implementation parameters of the additive manufacturing method for the supports 6 may be different from those for the blank 2. In particular, said parameters for the supports 6 may be chosen so as to promote the chemical dissolution of the supports 6 compared to the blank 2.

[0053] As illustrated in [Fig. 3], the supports 6 are dispersed throughout the intermediate assembly 1 to facilitate their subsequent removal. The intermediate assembly 1 comprises first portions 7 containing the supports 6 and second portions 8 free of supports 6, the first portions 7 alternating with the second portions 8.

[0054] The second portions 8 thus include openings 9 in place of the supports 6 in the first portions 7. As illustrated in [Fig. 3], the supports 6 are in the form of an elongated parallelepiped, i.e., a “wall”. Alternatively, the supports 6 may be cylindrical in shape, i.e., the supports 6 may form columns, spaced apart from one another.

[0055] A section Si of a first portion 7 is illustrated in [Fig.4] and a section S2 of a second portion 8 is illustrated in [Fig.5].

[0056] As can be seen in Figures 4 and 5, the wall 3 has a folded shape along a continuous line when viewed in cross-section of the intermediate assembly 1. The continuous line has two ends 10i and 102 that are spaced apart. The continuous line is therefore an open line. The continuous line is not self-intersecting.

[0057] The continuous line extends throughout the entire section of the intermediate assembly 1 such that the wall 3 covers as much of said section as possible while being free of any points of passage. By "free of points of passage", it is understood that each surface 1h of the wall 3 opposite another surface 112 of the wall 3 is spaced from said other surface 112.

[0058] In particular, the continuous line is a space-filling curve of order higher than 2. In the embodiment illustrated in Figures 2 to 5, the continuous line is a Peano curve of order 2.

[0059] The wall 3 has the same thickness E along its entire length along the continuous line.

[0060] As illustrated in Figures 3 and 4, the supports 6 are arranged along the edges of the overhanging parts 4. Other positionings of the supports 6 are entirely conceivable as long as they support the overhanging parts 4.

[0061] In one embodiment, the second portions 8 may also include supports 6 arranged in a staggered pattern with respect to the supports 6 included by the first portions 7.

[0062] The blank 2 is substantially identical to the part to be manufactured, that is to say that the blank 2 differs from that of the part to be manufactured only in that the wall 3 of the blank 2 is slightly thicker than the wall of the part to be manufactured and in that the external surfaces of the blank 2 can be machined in order to obtain the part to be manufactured with the dimensions sought.

[0063] Step a) is followed by step b) during which the supports 6 are removed by chemical dissolution. Removing the supports 6 eliminates electrical contact between different portions of the wall 3 along the continuous line. Thus, by removing the supports 6, the effective path length of the electric current in the ferromagnetic part to be manufactured is increased, and therefore the electrical resistance encountered by this current is increased.

[0064] Step b) comprises substep bj during which the intermediate assembly 1 is immersed in a bath of dissolving agent and / or during which a dissolving agent is circulated in the intermediate assembly through the interstices 12 of the wall 3. The circulation of the dissolving agent in the interstices 12 is symbolized by the arrows 13 in figures 4 and 5. The interstices 12 are formed by the spaces between the surfaces 111 and 112 opposite the wall 3.

[0065] Circulating the dissolving agent through the spaces 12 of the intermediate assembly 1 allows for the elimination of gases formed by chemical dissolution, particularly by the redox reaction of said chemical dissolution. Furthermore, circulating the dissolving agent helps to control the exothermic effect caused by the chemical dissolution.

[0066] The intermediate assembly 1 can be rotated on itself at regular intervals during its immersion in the bath.

[0067] The intermediate assembly 1 can be suspended, in particular above a container or the bath, during the circulation of the dissolving agent.

[0068] Immersion of the intermediate assembly 1 in the bath and circulation of the dissolving agent can be carried out successively. In particular, circulation of the dissolving agent can be carried out prior to immersion of the intermediate assembly 1 in the bath.

[0069] The dissolving agent of the bath and / or the dissolving agent put into circulation may be a solution comprising between 1 and 5% by volume of nitric acid HNO3 in an ethyl alcohol-based solvent.

[0070] If necessary, circulation can be achieved by means of a pump connected to conduits, for example plastic pipes, opening into the gaps 12. A mask resistant to the dissolving agent, for example protective tape, can be positioned to obstruct the space between the conduits and the wall 3. In particular, said mask is positioned to obstruct the gaps 12, then openings are made in said mask and the conduits are inserted into said openings. In addition to obstructing the space between the conduits and the wall 3, the mask also allows the conduits to be mechanically fixed to the intermediate assembly 1.

[0071] The flow rate of the dissolving agent in the spaces 12 of the intermediate assembly 1 is around 1 mL / min. Thus, for an intermediate assembly in which the largest space 12 has a volume of 0.5 mL, the residence time of the dissolving agent in the intermediate assembly 1 is less than 30 s.

[0072] The dissolving agent in the bath can have a temperature between 20°C and 100°C, for example equal to 50°C. The dissolving agent circulating in the gaps 12 of the intermediate assembly 1 can have a temperature between 20°C and 100°C, for example equal to 50°C, the temperature being controllable by a thermostat.

[0073] Alternatively, the dissolving agent of the bath and / or the dissolving agent put into circulation may be in vapor phase, in particular resulting from the evaporation of a solution comprising between 1 and 5% by volume of nitric acid (HNO3) in an ethyl alcohol-based solvent. For example, intermediate assembly 1 can be housed in a first vacuum chamber during step bi), the first chamber being fluidically connected to a second chamber in which the solution is heated. Initially, the fluidic connection between the second chamber and the first chamber can be closed, and then, once the solution in the second chamber has evaporated, said fluidic connection is opened. A valve can be used to open and close said fluidic connection.

[0074] The rate of material removal by chemical dissolution depends mainly on the dissolving agent and its temperature. For example, for a solution at 50 °C, comprising between 1 and 5% by volume of nitric acid HNO3 in an ethyl alcohol-based solvent, acting as a dissolving agent, the rate of material removal is between 30 and 50 pm / min.

[0075] Thus, for such a dissolving agent and in the case where it is desired to remove between 50 and 75 pm of material, that is to say, the distance between a surface of the intermediate assembly 1 before chemical dissolution and the same surface after chemical dissolution is between 50 and 75 pm, substep bi) must last between 1 and 2.5 min. Obviously, other durations can be considered for substep bi) depending on the dissolving agent used, its temperature, and the quantity of material that it is desired to remove.

[0076] Since the supports 6 have two surfaces, opposite to each other, exposed to the dissolving agent during substep bi), then a material removal of between 50 and 75 pm is sufficient to remove supports 6 of thickness between 100 and 150 pm.

[0077] Thus, the chemical dissolution in step b) results in a loss of thickness in the wall 3 of the blank 2. For example, the difference between the wall thickness after step a) and the wall thickness after step b) is between 50 and 150 pm, specifically between 50 and 75 pm when the wall is attacked on only one side and between 100 and 150 pm when the wall is attacked on both sides. Therefore, the wall 3 of the blank 2 has a thickness E after step a) that is greater than the desired thickness for the ferromagnetic part, the difference between these thicknesses being a function of the amount of material removed by the chemical dissolution.

[0078] Furthermore, the chemical dissolution in step b) also affects the roughness of wall 3 of the blank 2. In particular, the roughness of wall 3 after step b) is less than the roughness of wall 3 before step b). For a substep bi) according to the example described above, the inventors measured that a wall 3 of the blank 2, exhibiting a roughness between 10 and 15 pm before step b), then exhibits a roughness between 6 and 11 pm after step b).

[0079] In addition, the chemical dissolution of step b) also causes a rounding of the edges of the wall 3 of the blank 2. The result of such a rounding for a body 14, the lower part 15 of which has undergone chemical dissolution according to the above and the upper part 16 of which has not undergone such chemical dissolution, has been illustrated in [Fig.6].

[0080] The edges 17 of the lower part 15 were rounded as a result of chemical dissolution. This rounding can be characterized by a rounding coefficient equal to the ratio 1 / L, in which 1 corresponds to the distance between the edge 17 after chemical dissolution and the intersection between the planes containing the faces 18i and 182 forming said edge 17 after chemical dissolution, and L corresponds to the distance between the edge 17 after chemical dissolution and the same edge 17 before chemical dissolution.

[0081] For a substep bi) according to the example described above, the inventors measured a rounding coefficient of approximately 20%.

[0082] Substep bi) is followed by substep b2) during which the intermediate assembly 1, which is now free of support 6, is rinsed. Thus, substep b2) corresponds to rinsing the blank 2. The rinsing is carried out with deionized water at room temperature.

[0083] Preferably, the rinsing of the blank 2 includes immersion in deionized water and / or circulation of the deionized water in the gaps 12 of the blank 2. Immersion in deionized water and circulation of the deionized water can be carried out simultaneously or successively. Circulation can be achieved using a pump connected to conduits in a manner similar to the circulation in substep bi).

[0084] Rinsing is carried out immediately after substep bi) in order to stop the chemical dissolution reaction as quickly as possible.

[0085] The rinsing lasts between 5 and 30 minutes, for example 15 minutes.

[0086] Substep b2) is followed by substep b3) during which draft 2 is dried. During the drying of blank 2, the latter is suspended and rotates a quarter turn on itself at regular intervals.

[0087] Step b) can be followed by a step c) of mechanical machining of the external surfaces 19 of the blank 2. The external surfaces 19 of the blank 2 correspond to the surfaces of the blank 2 which are not opposite other surfaces of the blank 2. The machining of step c) is carried out so that the blank 2 has the dimensions desired for the ferromagnetic part to be manufactured and / or so that the external surfaces 19 have a surface finish desired for the ferromagnetic part to be manufactured.

[0088] Figure 7 illustrates a cross-section S3 of another example of an intermediate assembly 1 obtained at the end of step a). This intermediate assembly 1 differs from that illustrated in Figures 2 to 5 in that the wall 3 has the shape of a grid when viewed in cross-section of the intermediate assembly 1. The intermediate assembly 1 is monolithic and is made of the same ferromagnetic material.

[0089] The grid consists of a main line and a plurality of secondary lines, each secondary line being a branch of the main line extending opposite another secondary line. Each portion of the wall 3 along a secondary line is called a bar 20. The portion of the wall 3 along the main line is called a column 21. Thus, the grid comprises bars 20, extending opposite each other, notably parallel to each other, and a column 21 connecting the bars 20. The bars 20 form the overhanging portions 4 of the blank 2, and the column 21 forms the vertical portions 5 of the blank.

[0090] For each bar 20 comprising an overhanging portion, the intermediate assembly 1 includes at least one support 6 connecting the lower surface of said bar 20 to the upper surface of the adjacent bar 20. The supports 6 thus support the overhanging portions 4 during step a) of additive manufacturing. The removal of the supports 6 during step b) can be carried out similarly to that described above.

[0091] Other variants and improvements may be envisaged without departing from the scope of the invention as defined by the following claims.

Claims

Demands

1. A method for manufacturing a ferromagnetic part, comprising the following successive steps: a) additive manufacturing of an intermediate assembly (1) in a ferromagnetic material comprising a blank (2) of the ferromagnetic part and at least one support (6), the blank comprising at least one overhanging portion (4) supported by the at least one support, b) removal of each support by chemical dissolution.

2. Method according to claim 1, the blank comprising a monolithic wall (3) forming the overhanging part or parts, the wall having the form of a grid, when viewed along a cross-section of the intermediate assembly, the grid being made up of a main line and a plurality of secondary lines, each secondary line being a branch of the main line extending opposite another secondary line, the secondary lines being spaced apart from each other so that, for each secondary line, the part of the wall following said secondary line is spaced apart from the parts of the wall following the other secondary lines.

3. Method according to claim 1, the blank comprising a monolithic wall (3) forming the overhanging part or parts, the wall having a folded shape such that the wall follows a continuous line when viewed from a cross-section of the intermediate assembly, each surface (111) of the wall opposite another surface (112) of the wall being spaced from said other surface, preferably the distance between said surfaces being less than 250 pm.

4. Method according to the preceding claim, the continuous line being a space-filling curve of order greater than or equal to 2, preferably the space-filling curve being a Peano curve or a Hilbert curve.

5. A method according to any one of claims 2 to 4, the difference between the wall thickness (E) at the end of step a) and the wall thickness at the end of step b) being between 50 and 150 pm.

6. Method according to any one of claims 2 to 5, the wall having a thickness (E) between 110 and 2000 pm at the end of step a).

7. A method according to any one of the preceding claims, the overhanging part being supported by a plurality of supports spaced apart from each other, preferably the distance between two adjacent supports being between 100 and 5000 pm.

8. A method according to any one of the preceding claims, the support having, at the end of step a), a cylindrical shape, preferably with a diameter between 50 and 200 pm, or the shape of an elongated parallelepiped, preferably with a thickness between 50 and 200 pm and / or a length between 500 and 10000 pm.

9. Method according to any one of the preceding claims, the support having, at the end of step a), a height, measured along the direction of the gravitational force during step a), of between 100 and 500 pm.

10. A method according to any one of the preceding claims, the ferromagnetic material being iron or an alloy comprising iron and at least one element selected from silicon, cobalt and nickel.

11. A method according to any one of the preceding claims, the intermediate assembly having a general toroidal or parallelepiped shape.

12. A method according to any one of the preceding claims, additive manufacturing in step a) implementing a powder bed fusion method, for example laser or electron beam powder bed fusion, or a concentrated energy material deposition method, in particular laser fused material deposition.

13. A method according to any one of the preceding claims, step b) comprising a substep bj of immersing the intermediate assembly in a dissolving agent bath and / or of circulating a dissolving agent in interstices (12) included by the intermediate assembly, preferably substep bi) for between 0.5 and 5 min, preferably the bath and / or the circulating dissolving agent being at a temperature between 20 and 100 °C, preferably the circulation flow rate of the dissolving agent in said interstices being between 0.1 and 10 mL / min.

14. The method according to the preceding claim, the dissolving agent for the bath and / or the circulation comprising an acid selected from nitric acid, hydrochloric acid or sulfuric acid, preferably the dissolving agent comprising a solvent selected from ethyl alcohol or water, preferably the acid representing between 1 and 5% by volume of the dissolving agent.

15. Method according to claim 13 or 14, step b) comprising a substep b2), subsequent to substep bj, of rinsing the blank, preferably by immersion in deionized water and / or by circulating deionized water in the gaps (12) of the intermediate assembly, preferably the deionized water being at a temperature between 10 and 30 °C, preferably substep b2) lasting between 5 and 30 min.

16. A method according to the preceding claim, step b) comprising a substep b3), subsequent to substep b2), of drying the blank, preferably the blank being suspended, preferably the blank being rotated on itself at regular intervals, preferably substep b3) lasting at least 1 h.

17. A method according to any one of the preceding claims, step b) being followed by a step c) of mechanical machining of the external surfaces (19) of the blank.

18. Ferromagnetic part obtained from the process according to any one of the preceding claims.

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