METHOD FOR THE GENERATIVE MANUFACTURING OF A THREE-DIMENSIONAL METAL PART

Description

GENERAL TECHNICAL FIELD

[0001] The invention is in the field of additive manufacturing.

[0002] The present invention relates more specifically to a method for additive manufacturing of a three-dimensional metal part, having at least one solid part, such as a strip for forming the grooves of the tread of a tire, intended to be inserted into a baking mold for said tire. STATE OF THE ART

[0003] Additive manufacturing makes it possible to manufacture metal parts by repeating a cycle comprising a step of depositing a layer of metal powder, followed by a step of melting this layer by electromagnetic radiation, for example using a laser beam.

[0004] This type of process generates high stresses that deform the parts produced due to significant temperature gradients. These deformations are even greater when the part to be manufactured has significant variations in thickness. These deformations can cause tears.

[0005] As an example, we have shown on the figure 1A attached, a perspective view of an example of the embodiment of a strip as mentioned above and on the figure 1B a diagram of the vertical cross-section of the lamella of the figure 1A . This sipe A, called the "water drop" type, comprises two parts, respectively a body B and a solid part or "water drop" C. The free end of the body B is intended to be embedded in the mass of the tire curing mold, while the opposite end of the body B and the water drop C protrude from the bottom of this mold. Thus, when the rubber is molded, such a sipe makes it possible to create in the depth of the tire a large, fairly wide cavity corresponding to the part C, this cavity extending towards the outside of the tire surface by a fine groove corresponding to a portion of the tongue of the thin body B. Such a sipe A thus makes it possible to create grooves of a particular shape on a tire.

[0006] It is easy to understand that at the transition zone D between B and C, the stresses generated by the additive manufacturing process may lead to the weakening of the lamella A, or even to its rupture, which can be particularly troublesome during the molding of a tire. EP3219468 A1 relates to a method for manufacturing an integral tire mold formed by powder bed additive manufacturing in which the different parts of the metal mold have different properties and densities. This document discloses that to obtain an object having different densities the irradiation conditions can be changed and that the density can be controlled by the overlap distance. PRESENTATION OF THE INVENTION

[0007] The aim of the invention is to provide a method for the additive manufacturing of a three-dimensional metal part having at least one solid part, which makes it possible to significantly reduce the stresses exerted on this part, cracks and deformations, while guaranteeing good mechanical resistance of the part as a whole.

[0008] By “solid” part is meant in the remainder of the description and claims a solid part, preferably having a cross-section with a width greater than 2 mm.

[0009] The invention also aims to provide a method as mentioned above which can essentially be applied to parts having large variations in thickness.

[0010] To this end, the invention relates to a method for additive manufacturing a three-dimensional metal part having at least one solid part and having a variation in thickness, such as a lamella for forming the grooves of the tread of a tire intended to be inserted into a mold for curing said tire, this solid part of the metal part comprising an inner part, called "core" surrounded by an outer part called "shell", this method consisting of successively solidifying by fusion using a laser beam, layers of metal powder deposited successively and defining in each of these different successive layers of powder, either at least one shell region, which constitutes a portion of said shell of the metal part to be manufactured, or at least one core region surrounded by a shell region, said core region constituting a portion of said core of the metal part to be manufactured.

[0011] According to the invention, the fusion of said core regions is carried out by scanning with said laser beam, so as to form longitudinal weld beads of the same widths, parallel to each other, and which are either juxtaposed, or are spaced apart from each other by a distance less than X% of their width, or overlap over a distance less than X% of their width and in that the fusion of said shell regions is carried out by scanning with said laser beam, so as to form longitudinal weld beads of the same widths and of the same width as the weld beads of the core, parallel to each other and which overlap over a distance greater than X% of their width, X being equal to 30.

[0012] Thanks to these features of the invention, it is possible to reduce mechanical stresses while ensuring sufficient density of the shell and core. The resulting part has good mechanical properties (increased strength, absence of tears or fracture initiation zones).

[0013] According to other advantageous and non-limiting characteristics of the invention, taken alone or in combination: the fusion of said core regions is carried out by scanning with said laser beam, so as to form neighboring weld beads spaced apart by a distance of less than 20%, preferably less than 10%, of the width of these beads; the fusion of said core regions is carried out by scanning with said laser beam, so as to form weld beads which overlap over a distance of less than 20%, preferably less than 10%, of the width of these weld beads; the fusion of said shell regions is carried out by scanning with said laser beam so as to form weld beads overlapping over a distance greater than 30% and less than or equal to 60%, preferably greater than or equal to 40% and less than or equal to 50% of the width of these beads; the core and the shell overlap over a width of between 0.01 mm and 0.40 mm, preferably over a width of 0.10 mm;at least one core region of a layer of metal powder comprises at least two zones and among these zones, the scanning with said laser beam of at least one given zone is carried out so that the displacement vectors of said laser beam have an orientation angle relative to a trigonometric reference frame which is different from the orientation angle of the displacement vectors of said laser beam in a zone contiguous to said given zone; at least one shell region of a layer of metal powder comprises at least two zones and among these zones, the scanning with said laser beam of at least one given zone is carried out so that the displacement vectors of said laser beam have an orientation angle relative to a trigonometric reference frame which is different from the orientation angle of the displacement vectors of said laser beam in a zone contiguous to said given zone;said zones are square and arranged in a checkerboard pattern, this checkerboard pattern comprising at least two lines called "even" and "odd"; in each core region and / or in each shell region, the melting of a given layer of metal powder is carried out by scanning, using said laser beam, the zones of the odd lines and odd columns, called "first zones", according to a first orientation angle, the zones of the odd lines and even columns, called "second zones", according to a second orientation angle, the zones of the even lines and odd columns, called "third zones", according to a third orientation angle and the zones of the even lines and even columns, called "fourth zones", according to a fourth orientation angle, these four orientation angles relative to a trigonometric reference frame being different;the metal part comprises n successive superimposed layers, in that the orientation angles of the laser beam displacement vectors respectively of the first, second, third and fourth zones of the core region of a layer have an angular variation with the orientation angles of the laser beam displacement vectors respectively of the first, second, third and fourth zones of the core region of the layer located above, the orientation angles of the laser beam displacement vectors respectively of the first, second, third and fourth zones of the shell region of a layer have an angular variation with the orientation angles of the laser beam displacement vectors respectively of the first, second, third and fourth zones of the shell region of the layer located above, and these two angular variations are different;the core of the part comprises n successive superimposed layers and in that the square core areas of a layer are offset by 1 / q steps, q being an integer, preferably equal to 2, laterally and / or longitudinally with respect to the square core areas of the layer located immediately above; the shell of the part comprises n successive superimposed layers and in that the square shell areas of a layer are offset by 1 / q steps, q being an integer, preferably equal to 2, laterally and / or longitudinally with respect to the square shell areas of the layer located immediately above. ; PRESENTATION OF FIGURES

[0014] Other characteristics and advantages of the invention will appear from the description which will now be given, with reference to the appended drawings, which represent, for informational but non-limiting purposes, different possible embodiments.

[0015] On these drawings: there figure 1A is a perspective view of an exemplary embodiment of a strip for forming the grooves of the tread of a tire, intended to be inserted into a baking mold for said tire, the figure 1B is a diagram of a cross-sectional view of a lamella, the figure 2 is a longitudinal sectional view of the solid part of a groove-forming lamella, the latter having been obtained by the method according to the invention, the figure 3 is a schematic view representing an example of an additive manufacturing installation allowing the implementation of the additive manufacturing method in accordance with the invention, the figure 4 is a schematic and detailed view of a stack of successive layers of metal powder, layers on which different zones presenting different fusion parameters have been represented, the figure 5 is a schematic view in vertical section of a metal part manufactured by the process according to the invention, the shape of which does not correspond to a lamella but which explains the principle of the invention, the figure 6 is an exploded schematic view showing different layers of metal powder used in the manufacture of the part of the figure 5 , there figure 7 is a schematic view of a layer of metal powder of the core, part of which has been fused in the form of weld beads, the figures 8 et 9 are diagrams representing two variants of the fusion of a layer of powder of the core of a metal part to be manufactured, the figure 10 is a diagram showing the melting of a layer of powder from the shell of a metal part to be manufactured, and the figures 11 et 12 are schematic top views showing an example of the orientation of the scanning vectors by the laser beam during the fusion of different areas of the shell region and the core region of two successive layers of metal powder. DETAILED DESCRIPTION OF THE INVENTION

[0016] An example of an embodiment of an additive manufacturing installation allowing the implementation of the method according to the invention will now be described in connection with the figure 3 .

[0017] This installation 1 comprises a support 10, a device 11 for distributing metal powder and a laser 12.

[0018] The distribution device 11 makes it possible to deposit a first layer C1 of metal powder on the support 10, this layer being retained by the edges 100 of the support 10, shown only partially on the figure 3 The device 11 is controlled by a central unit 13, such as a computer, so as to ensure its movement in two perpendicular directions x and y (i.e. in the direction of width, respectively of depth on the figure 3 ) and this, in both directions for each of these directions (arrows F and G). Other devices 11 can move differently, for example only in the x direction but distributing the powder over the entire depth in a single pass.

[0019] Once the first layer C1 has been deposited, the laser 12, controlled by the central unit 13, is moved above the powder layer C1, also in the x and y directions and in both directions for each direction (arrows H and I), so as to fuse the metal powder layer C1, but only within the contour 20 of the three-dimensional part 2 to be manufactured.

[0020] Once this step is completed, a second layer of metal powder C2 is formed above the first C1, using the distribution device 11, and again the laser 12 is operated to fuse the powder at the area of ​​the part 2.

[0021] The process is thus repeated until the n layers necessary for the manufacture of product 2 are deposited. On the figure 3 , layers Cn-1 and Cn-2 correspond respectively to the penultimate and the penultimate penultimate of the n deposited layers.

[0022] The parameters of the laser 12, such as its power, its travel speed and its focal length are adapted according to the nature of the metal powder to be melted.

[0023] The part 2 to be manufactured is modeled in three dimensions, so as to define an interior region 21, called the “core”, surrounded by an exterior region 22, called the “shell”, which constitutes a sort of superficial “skin” covering the entirety of the part 2.

[0024] 3D modeling techniques are known to those skilled in the art and will not be described in detail here.

[0025] An example of a part 2 in section (here a lamella as mentioned above), having a core region 21 and a shell region 22, is shown in the figure 2 .

[0026] The shell 22 preferably has a thickness e of between 0.1 mm and 4 mm, more preferably equal to 0.4 mm.

[0027] It should be noted, however, that for reasons related to problems of mechanical resistance or the use of certain 3D modeling software, certain protrusions of part 2 can be modeled as shell regions 22 and will then be of a thickness greater than 4 mm.

[0028] In order to understand the method according to the invention, we will now refer to the figures 5 et 6 .

[0029] On the figure 5 , we can see a vertical cross-sectional view of a three-dimensional metal part 2, obtained by additive manufacturing. This part here has a schematic arched shape.

[0030] On the left of the figure 5 , some of the n layers of powder that made this part possible. These n layers are referenced C1, Cx, Cy and Cn, x, y and n corresponding to the layer numbers and x and y being between 1 and n.

[0031] These different layers C1 to Cn are then represented on the exploded view of the figure 6 . We note that for each layer of powder C1, Cx, Cy and Cn, we define either at least one shell region, or at least one core region surrounded by a shell region.

[0032] The first layer of powder C1 comprises two shell regions 220, while the last layer Cn only comprises one, taking into account the shape of the part 2.

[0033] At the intermediate layer Cx, there are two core regions 210, each surrounded by a shell region 220, while on the layer Cy, there is a single core region 210, surrounded by a single shell region 220. The set of shell regions 220 defines the shell 22 of the part 2. Similarly, the set of core regions 210 defines the core 210 of the part 2.

[0034] As described below, the displacement parameters of the laser beam are different, depending on whether the core 21 or the shell 22 of the part 2 is to be melted.

[0035] Referring to the figure 7 , it can be seen that the melting of a region of a layer Cx of metal powder is carried out by scanning using the beam 120 of the laser 12, above this layer Cx, so as to form parallel weld beads. In this figure which represents a core region 21, the weld beads are referenced 211.

[0036] The laser 12 is moved so that its beam 120 follows displacement vectors 121 parallel to each other, these vectors being spaced apart by a spacing E.

[0037] Preferably, the laser 12 is moved along a back-and-forth trajectory and from one side to the other of the layer to be treated, for example here from left to right in the figure.

[0038] According to the invention, the formation of these weld beads is done differently in the shell region and in the core region.

[0039] In accordance with the invention, the fusion of each region 210 of core 21 is carried out by scanning using the laser beam 120, according to the displacement vectors 121, so as to form longitudinal weld beads 211, parallel to each other and of the same widths L (the width L being parallel to the spacing E between two displacement vectors of the beam and perpendicular to the vectors 121).

[0040] Furthermore, this fusion is carried out so that the weld beads 211 formed are juxtaposed, as shown in the figure 7 , (and in this case the spacing E is equal to the width L), and the beads touch each other or in such a way that the weld beads 211 partially overlap over a distance L1 less than X% of the width L of the bead 211, as shown in the figure 8 , (and in this case the spacing E is less than the width L) or else so that the weld beads 211 are spaced apart from each other by a distance L2 less than X% of the width L of the bead 211, as shown in fig 9 (and in this case the spacing E is greater than the width L). X is greater than 0 and less than 100. X is equal to 30.

[0041] More preferably, two contiguous core weld beads 211 overlap over a distance L1 less than 20%, or even less than 10% of the width L of these weld beads 211.

[0042] More preferably, two adjacent adjacent core weld beads 211 are spaced apart from each other by a distance L2 less than 20%, or even less than 10% of the width L of these beads 211.

[0043] According to the invention, the fusion of each shell region 220 is carried out by scanning with the laser beam 120, according to the displacement vectors 121, so as to form longitudinal weld beads 221, parallel to each other and of the same widths L as each other (L being parallel to the spacing E and perpendicular to the vectors 121). In addition, the shell weld beads 221 are of the same width as the core weld beads 211.

[0044] Furthermore, this fusion is carried out so that the weld beads 221 overlap, as shown in the figure 10 over a distance L3 greater than X% of the width L of the cord 221, as shown in the figure 10 (and in this case the spacing E is less than the width L).

[0045] More preferably, two contiguous hull weld beads 221 overlap over a distance L3 greater than 30% and less than or equal to 60% of the width L of the bead 221, or better still over a distance L3 greater than or equal to 40% and less than or equal to 50% of the width L.

[0046] Thus, by using different overlap parameters between the shell and core regions and in particular greater overlap in the shell regions than in the core regions, the stresses inside the part 2 thus manufactured are reduced, while ensuring sufficient density of both the shell and the core and obtaining a part with good mechanical strength properties.

[0047] This gives a shell 22 with a density greater than 99% and a core with a density between 95% and 99%.

[0048] One method for determining the density of the resulting shell or core can be as follows. This method involves cutting the shell or core into different sections, for example, about ten sections in planes perpendicular to the weld bead. Then, each face of the sections formed by the cut is polished, for example using an abrasive disc. This polishing step reduces the surface roughness of the face, making the porous parts opening onto this face more visible. The face is then photographed and this photograph is processed using image processing software. The software produces a processed photo, on which appear dark areas corresponding to porous parts and light areas corresponding to non-porous parts, i.e. polished parts.The ratio of the area of ​​the light areas to the total area of ​​the face thus makes it possible to determine the density of the face of this section. By repeating these polishing, photography and image processing operations for different sections and averaging the results obtained, it is thus possible to determine the overall density of the resulting part.

[0049] Depending on the shape of part 2, it is possible to have several massive core regions 210, greater than 2mm wide, not contiguous, see for example the Cx layer on the figure 6 .

[0050] Preferably, a core region 210 and a shell region 220 overlap (overlap) by a width of between 0.01 mm and 0.40 mm, preferably 0.10 mm on the same layer, to ensure good bonding between the two regions. In this case, the shell / core overlap area will be merged twice with the two aforementioned modes of overlapping the vectors according to the shell and according to the core.

[0051] Another parameter of the fusion on which it is possible to act in the method according to the invention concerns the orientation of the vectors 121 of displacement of the laser beam 120.

[0052] So, as shown in the figure 11 , it is possible, while retaining the aforementioned overlap or spacing parameters for the different core 210 or shell 220 regions, to define in a given layer Cx and inside a core region (respectively a shell region), at least two zones in which the scanning with the laser beam 120 is carried out according to two different orientation angles, relative to a trigonometric reference frame.

[0053] Preferably, and as shown in the figure 11 , the different zones are presented in the form of a checkerboard with at least two rows and at least two columns of several square zones.

[0054] Odd rows are referenced 3i and even rows 3p, odd columns 4i and even columns 4p.

[0055] The zones could also be hexagonal for example.

[0056] In the example shown in the figure 11 , and in the core region 210, the displacement vectors 121 of the laser beam 120 of the zones (3i-4i) located at the intersection of the odd columns 4i and the odd rows 3i and hereinafter referred to as "first zones" are oriented according to a first orientation angle α1, those of the zones (3i-4p) located at the intersection of the even columns 4p and the odd rows 3i, hereinafter referred to as "second zones", according to a second orientation angle α2, those of the zones (3p-4i) located at the intersection of the odd columns 4i and the even rows 3p, hereinafter referred to as "third zones", according to a third orientation angle α3 and those of the zones (3p-4p), hereinafter referred to as "fourth zones", located at the intersection of the even columns 4p and the even rows 3p according to a fourth orientation angle α4. There could also be more than four different orientation angles on the checkerboard.

[0057] The same is true in the shell region 220 with first, second, third and fourth orientation angles β1, β2, β3 and β4.

[0058] Still according to this same example, in the core region 210, α1 is oriented at 45°, α2 at 315°, α3 at 225° and α4 at 135°. These orientation angles could be different. We see that for example for α1 (45°) and α3 (225°) that the angular orientations are the same but that the orientation angles of the laser beam are opposite from the point of view of a trigonometric reference frame. In the example of the figure 11 , the angles β1, β2, β3 and B4 are respectively equal to 45°, 270°, 135° and 0°.

[0059] Preferably, the orientation angle α1 of the first zones 3i-4i of the core region 210 of a Cx layer (see figure 11 ) presents an angular variation (called "core") with the orientation angle α'1 of the first zones 3i-4i of the core region 210 of the Cx+1 layer located above (see figure 12 ). The same applies respectively to the different angles of two successive layers. In the example of the figure 12 , the angles α'1, α'2, α'3 and α'4 are respectively equal to 10°, 280°, 190° and 100°. There is therefore an angular variation of 35 degrees between α1 and α'1, respectively between α2 and α'2, α3 and α'3, α4 and α'4.

[0060] Similarly, preferably, the orientation angle β1 of the first zones 3i-4i of the shell region 220 of a Cx layer (see figure 11 ) presents an angular variation (called "shell") with the orientation angle B'1 of the first zones 3i-4i of the shell region 220 of the Cx+1 layer located above (see figure 12 ). The same applies respectively to the different angles of two successive layers. In the example of the figure 12 , the angles β'1, β'2, β'3 and β'4 are respectively equal to 225°, 90°, 315° and 180°. There is therefore an angular variation of 180 degrees between β1 and β'1, respectively between β2 and β'2, β3 and β'3, B4 and B'4.

[0061] The core angular variation could be the same as the shell angular variation, however, it is preferably different.

[0062] Furthermore, and as shown in the figure 4 , it is also possible to have an offset between the zones of two successive layers, for example layers Cn-1 and Cn-2, of 1 / q step P, for example in this figure of a half-step (q=2) in an x ​​direction and / or in a y direction.

[0063] In the example shown, the areas 3i-4i, 3i-4p, 3p-4i, 3p-4p of the layer Cn-2 of a core region 210 are thus offset by half a step P laterally (x direction) and longitudinally (y direction) relative to the areas 3i-4i, 3i-4p, 3p-4i, 3p-4p of the layer Cn-1 located immediately above. The same could be true in a shell region 220.

Claims

1. Process for the additive manufacturing of a three-dimensional metal part (2) of a thin blade for forming furrows in the tread of a tyre, said thin blade being intended to be inserted into a curing mould of said tyre, having at least a monolithic part and having large variations in thickness, comprising an inner part (21), referred to as a "core", surrounded by an outer part (22), referred to as a "shell", this process consisting in successively solidifying successively deposited layers of metal powder (C1, C2, Cx, Cy, Cn) by melting with the aid of a laser beam (120), and in defining in each of these different successive layers of powder either at least one shell region (220), which constitutes a portion of said shell (22) of the metal part to be manufactured, or at least one core region (210), surrounded by a shell region (220), said core region (210) constituting a portion of said core (21) of the metal part to be manufactured, characterized in that the melting of said core regions (210) is effected by scanning with said laser beam (120) so as to form longitudinal weld beads (211) that have identical widths (L), are mutually parallel and that are either juxtaposed or spaced apart from one another by a distance (L2) less than X% of their width (L), or overlap over a distance (L1) less than X% of their width (L), and in that the melting of said shell regions (220) is effected by scanning with said laser beam (120) so as to form longitudinal weld beads (221) that have identical widths (L) and an identical width to the weld beads (211) of the core, are mutually parallel and that overlap over a distance (L3) greater than X% of their width (L), X being equal to 30.

2. Process according to Claim 1, characterized in that the melting of said core regions (210) is effected by scanning with said laser beam (120) so as to form adjacent weld beads (211) that are spaced apart from one another by a distance (L2) less than 20%, preferably less than 10%, of the width (L) of these beads.

3. Process according to Claim 1, characterized in that the melting of said core regions (210) is effected by scanning with said laser beam (120) so as to form weld beads (211) that overlap over a distance (L1) less than 20%, preferably less than 10%, of the width (L) of these weld beads (211).

4. Process according to one of Claims 1 to 3, characterized in that the melting of said shell regions (220) is effected by scanning with said laser beam (120) so as to form weld beads (221) that overlap over a distance (L3) greater than 30% and less than or equal to 60%, preferably greater than or equal to 40% and less than or equal to 50% of the width (L) of these beads (211).

5. Process according to one of the preceding claims, characterized in that the core (21) and the shell (22) overlap over a width of between 0.01 mm and 0.40 mm, preferably over a width of 0.10 mm.

6. Process according to one of the preceding claims, characterized in that at least one core region (210) of a layer of metal powder (C1, C2, Cx, Cy, Cn) comprises at least two zones (3i-4i, 3i-4p, 3p-4i, 3p-4p), and in that, among these zones, the scanning of at least one given zone with said laser beam (120) is effected such that the displacement vectors (121) of said laser beam (120) have an orientation angle with respect to a trigonometric coordinate system that is different from the orientation angle of the displacement vectors (121) of said laser beam (120) in a zone contiguous with said given zone.

7. Process according to one of the preceding claims, characterized in that at least one shell region (220) of a layer of metal powder (C1, C2, Cx, Cy, Cn) comprises at least two zones (3i-4i, 3i-4p, 3p-4i, 3p-4p), and in that, among these zones, the scanning of at least one given zone with said laser beam (120) is effected such that the displacement vectors (121) of said laser beam (120) have an orientation angle with respect to a trigonometric coordinate system that is different from the orientation angle of the displacement vectors (121) of said laser beam (120) in a zone contiguous with said given zone.

8. Process according to Claim 6 or 7, characterized in that said zones (3i-4i, 3i-4p, 3p-4i, 3p-4p) are square and disposed in the form of a chequerboard, this chequerboard comprising at least two rows referred to as "even" and "odd".

9. Process according to Claim 8, characterized in that, in each core region (210) and / or in each shell region (220), the melting of a given layer of metal powder (C1, C2, Cx, Cy, Cn) is effected by scanning, with the aid of said laser beam (120), the zones (3i, 4i) of the odd rows and of the odd columns, referred to as "first zones", at a first orientation angle (α1, α'1, β1, β'1), the zones (3i, 4p) of the odd rows and of the even columns, referred to as "second zones", at a second orientation angle (α2, α'2, β2, β'2), the zones (3p-4i) of the even rows and of the odd columns, referred to as "third zones", at a third orientation angle (α3, α'3, β3, β'3), and the zones (3p-4p) of the even rows and of the even columns, referred to as "fourth zones", at a fourth orientation angle (α4, α'4, β4, β'4), these four orientation angles with respect to a trigonometric coordinate system being different.

10. Process according to Claim 9, characterized in that the metal part (2) comprises n successive superposed layers (C1, C2, Cx, Cx+1, Cy, Cn), in that the orientation angles (α1, α2, α3, α4) of the displacement vectors of the laser beam (120) in the first, second, third and fourth zones, respectively, of the core region (210) of one layer (Cx) exhibit an angular variation with the orientation angles (α'1, α'2, α'3, α'4) of the displacement vectors of the laser beam in the first, second, third and fourth zones, respectively, of the core region (210) of the layer (Cx+1) situated thereabove, in that the orientation angles (β1, β2, β3, β4) of the displacement vectors of the laser beam in the first, second, third and fourth zones, respectively, of the shell region (220) of one layer (Cx) exhibit an angular variation with the orientation angles (β'1, β'2, β'3, β'4) of the displacement vectors of the laser beam in the first, second, third and fourth zones, respectively, of the shell region (220) of the layer (Cx+1) situated thereabove, and in that these two angular variations are different.

11. Process according to one of Claims 8 to 10, characterized in that the core (21) of the part (2) comprises n successive superposed layers (C1, C2, Cx, Cy, Cn), and in that the square core zones of one layer are offset by 1 / q step (P), q being an integer, preferably equal to 2, laterally and / or longitudinally with respect to the square core zones of the layer situated immediately thereabove.

12. Process according to one of Claims 8 to 11, characterized in that the shell (22) of the part (2) comprises n successive superposed layers (C1, C2, Cx, Cy, Cn), and in that the square shell zones of one layer are offset by 1 / q step (P), q being an integer, preferably equal to 2, laterally and / or longitudinally with respect to the square shell zones of the layer situated immediately thereabove.