Manufacturing process for a porous part by laser powder bed fusion

Abstract

Method for manufacturing a porous part by laser powder bed fusion. Additive manufacturing method for a porous part (16), comprising repeating the following steps: a) forming a bed (7) of metallic particles (11) from a powder (3), the tenth percentile (D10) of the equivalent diameter of the particles being greater than 250 µm, b) bonding the particles together by scanning at least a portion (9) of the bed with a laser beam (8) along a path following a first pattern (10) then a second pattern (13), each formed of parallel lines (101, 131) spaced apart from each other, the first and second patterns being interlaced or identical, the laser beam having a diameter (ϕ) less than the median equivalent diameter (D50) of the powder, the ratios of the vector gaps (H, H') of the first and second patterns to the median equivalent diameter of the powder being each between 0.5 and 3.0. Figure for the abridged version: Fig. 5

Description

Title of the invention: Method for manufacturing a porous part by laser fusion on a powder bed Technical field

[0001] The invention relates to the technical field of the manufacture of porous parts for the transport of a fluid, in particular the manufacture of heat exchangers or reactors allowing the implementation of chemical reactions with a porous structure, in particular heat pipes with a porous structure. Prior art

[0002] A heat pipe is a thermal device intended to transfer a flow of thermal energy from a hot source to a cold source, by taking advantage of the successive liquid / gas and gas / liquid phase changes of an exchange fluid which circulates within the heat pipe. The exchange fluid in the gaseous state, containing the thermal energy exchanged with the hot source, is transported in the heat pipe from an evaporator to a condenser. It is thus condensed in contact with the cold source. In liquid form, it is then transported in the opposite direction from the condenser to the evaporator, at least partly under the effect of the capillary forces induced by the contact of the exchange fluid with the porous structure.

[0003] For the heat exchange, and optionally the chemical reaction, to take place optimally, it is necessary for the porous structure to have both a high exchange surface with the fluid and a high permeability to the fluid. High permeability, generally associated with a large pore size, facilitates the circulation of the fluid through the porous structure, but does not a priori promote heat exchanges, while a large exchange surface makes the heat transfer between the fluid and the porous structure more efficient and / or increases the number of sites where the chemical reaction can take place, but to the detriment of the ease of flow of the fluid. The exchange surface with the fluid and the permeability to the fluid are therefore initially antagonistic.

[0004] Various methods for manufacturing porous structures with controlled permeability and exchange surface are known, for example by weaving metal fibers, by extrusion, by sintering metal particles. However, these methods impose numerous geometric limitations for the manufactured parts and / or are not suitable for the manufacture of parts with porosity gradients. Furthermore, in the case of sintering, the temperature holding times for binding the powder particles are generally high and therefore not very compatible with the industrial requirements of high production yield.

[0005] With regard to heat pipes, when the condenser is located lower in gravity than the evaporator, there is competition between the capillary forces and the gravitational and viscous forces linked to the friction of the fluid against the walls of the heat pipe. The return of the liquid from the condenser to the evaporator can therefore only take place if the capillary forces are greater than the gravitational and viscous forces. When the condenser is located higher in gravity than the evaporator, the gravitational force contributes to the pumping of the fluid. In a gravity-free application, only the capillary forces cause the pumping of the fluid from the condenser to the evaporator. It is known that the circulation of the exchange fluid by capillary effect is optimal if the porous structure has pores smaller than 200 μm, particularly in the portion of the heat pipe in contact with the evaporator.

[0006] In order to promote the effect of capillary forces, the production of heat pipes by powder bed additive manufacturing has been proposed to control the pore size of the porous structure, and thus the permeability and the exchange surface. For example, the article D. Jafari et al., Applied Thermal Engineering, vol. 143, 2018, pages 403-314, doi:10.1016 / j.applthermaleng.2018.07.111 and the article M. Ameli et al., Applied Thermal Engineering, vol. 52, 2013, pages 498-504, doi: 10.1016 / j.applthermaleng.2012.12.011 describe porous structures in the form of lattices. The lattices comprise dense bars forming the sides of a polyhedron and which define cellular pores. They are formed by the successive fusion, by localized irradiation using a laser beam, of layers of metal particles during additive manufacturing. The permeability of these lattices can be controlled by modifying the length and diameter of the bars.However, it is difficult to form lattices with pores smaller than 200 pm using these techniques. In particular, the reduction in pore size is accompanied by a densification of the lattice. Furthermore, lattices produced by additive manufacturing on powder beds must be depowdered before use. However, the elimination of residual particles from such a lattice is complex to implement in practice.

[0007] Patent application EP 3 279 597 A1 describes a method for additive manufacturing of a porous part by shaping a powder of particles having a size ranging from 10 pm to 250 pm.

[0008] Unpublished patent application FR 2010978 describes a method for manufacturing a porous structure for transporting a fluid, the manufacturing comprising the shaping by additive manufacturing of a powder of particles whose median equivalent diameter is greater than 100 pm.

[0009] There is therefore a need for a method of manufacturing a porous structure for transporting a fluid, in particular intended to form a heat exchanger and / or in contact with which a chemical reaction can take place via the transported fluid. In particular, there is a need for a method of manufacturing a porous structure with controlled permeability and while comprising pores of a small size, in particular less than 200 pm.

[0010] The aim of the invention is to respond, at least in part, to this need. Statement of the invention

[0011] To this end, the invention relates to a method for additive manufacturing of a porous part, the method comprising the repetition of a cycle comprising the steps of: a) forming a bed of metal particles from a powder of said particles, the tenth percentile Di0 of the equivalent diameter of the particles of the powder being greater than 250 pm, b) bonding the particles together by scanning at least a portion of the bed with a laser beam along a path following a first pattern then a second pattern which are each formed of parallel lines spaced apart from each other, the first and second patterns being interlaced or identical, the laser beam having a diameter less than the median equivalent diameter D50 of the powder, the ratio of the vector deviation of the first pattern to the median equivalent diameter of the powder and the ratio of the vector deviation of the second pattern to the median equivalent diameter of the powder each being between 0.5 and 3.0.

[0012] Known additive manufacturing methods for porous parts use powders having a median equivalent diameter of less than 100 μm, and even more generally less than 50 μm. This is the case, for example, for the production of cellular pore lattices described above. It is known to those skilled in the art that the driving force for the coalescence of neighboring particles during laser scanning depends on the curvature of said particles at their point of contact, that particles of small diameter have smaller curvatures at their point of contact than particles of larger diameter, and therefore, that the coalescence of neighboring particles during laser scanning is easier for particles of small diameter. The use, according to the invention, of a powder of metallic particles having a Di0 greater than 250 μm for additive manufacturing is therefore neither usual nor intuitive for those skilled in the art.

[0013] Furthermore, the method according to the invention does not require the generation of a computer file containing the geometry of a lattice and the complex path of the laser beam, layer by layer, to produce the lattice. It is therefore simple to implement.

[0014] Furthermore, in the methods of the prior art, the diameter of the laser beam is greater than the median equivalent diameter of the powder, ensuring the complete fusion of a high number of particles as the laser beam passes over these particles, and thus promoting the formation of a dense part. According to the invention, scanning in a non- conventional by the laser beam having a small diameter compared to the Di0 of the powder following the first pattern and the second pattern advantageously allows an optimal compromise between diffusion and accumulation of heat at the scale of the part to create bonds, also called bond necks, between the particles and ensure the formation of porosity between the bonded particles, favorable to fluid transfer. In particular, the scanning following the first and second patterns locally heats the particles scanned by the laser beam, which induces an increase in the average temperature of the portion of the bed.

[0015] The “median equivalent diameter”, also called D50, of a powder of particles corresponds to the median of the particle size distribution of the equivalent diameters of the particles of the powder. The particle size distribution of the equivalent diameters of the particles of a powder can be measured by laser particle size measurement, for example with a Mastersizer 2000 particle size analyzer marketed by the company Malvern Pa-nalytical.

[0016] Preferably, the powder has a ninety percentile D90 of the equivalent diameter of the particles of the powder of less than 1200 pm, preferably less than 900 pm, preferably less than 600 pm.

[0017] The powder may have a median equivalent diameter D50 of between 250 pm and 900 pm, preferably between 350 pm and 500 pm.

[0018] The percentile N (DN) of the equivalent diameter of the particles of a powder is the value of this equivalent diameter corresponding to the percentages, by volume, of N% on the cumulative distribution curve of the equivalent diameters of the particles. For example, 10%, by volume, of the particles of the powder have an equivalent diameter less than Di0 and 90%, by volume, of the particles have an equivalent diameter greater than or equal to Di0; 90%, by volume, of the particles of the powder have an equivalent diameter less than D90 and 10%, by volume, of the particles have an equivalent diameter greater than or equal to D90. Thus, the fifty percentile D50 corresponds to the median equivalent diameter of the distribution.

[0019] The powder may have a dispersion index Id of less than 0.5, or even less than 0.3, or even less than 0.1.

[0020] The dispersion index Id of a powder is the ratio of the difference between the ninety percentile D90 and the tenth percentile Di0 of the powder, divided by the median equivalent diameter D50 of the powder.

[0021] Preferably, at least 80%, preferably at least 90%, preferably at least 95%, better still at least 99% of the particles of the powder have a form factor greater than 0.6, preferably greater than 0.9.

[0022] The form factor of a particle is determined as the ratio of the smallest diameter to the largest diameter of the particle, measured on a re- presenting the particle and produced by optical microscopy at a magnification of 20 times.

[0023] Preferably, the sphericity index of the powder is between 0.7 and 1, preferably between 0.9 and 1.

[0024] The sphericity index of a powder can be determined by measuring the shape factors of each particle in a set of particles of the powder, then calculating the arithmetic mean of the shape factors thus measured. For the sphericity index of a powder to be representative, the set of particles may comprise more than 30,000, preferably more than 80,000, particles. The sphericity index of a powder can be measured on Morphology G3S equipment marketed by Malvern Panalytical.

[0025] Preferably, the particles of the powder are obtained by a gas atomization technique or a water atomization technique. In the water atomization method, the powder obtained is irregular, which makes the part manufactured by the method according to the invention more favorable to the rise of liquid by capillarity.

[0026] Preferably, the particles of the powder comprise at least one metal for more than 90.0% of their mass, preferably more than 95.0% of their mass, or even consist solely of metal. Preferably, the metal is chosen from aluminum, copper, nickel, iron and an alloy comprising more than 50% by mass of a metal chosen from aluminum, copper, nickel and iron, for example a stainless steel.

[0027] Preferably, the bed formed in step a) has a thickness of between 400 and 1400 μm.

[0028] Preferably, the formation of the bed of particles comprises spreading a portion of the powder by a roller. Advantageously, the bed thus has a substantially constant thickness and the particles which constitute it are distributed substantially uniformly over its surface.

[0029] In step b), the bonding of the particles together can be achieved by partial fusion and coalescence of the particles of the powder between them.

[0030] Preferably, the ratio of the diameter of the laser beam to the median equivalent diameter of the powder is less than 0.5, preferably less than 0.3, better still less than 0.2. The energy of the laser beam is thus concentrated on an area of ​​a size smaller than the median diameter of the particles of the powder, which facilitates the formation of bonds between the particles while maintaining porosity between the particles.

[0031] The lines of the first and second patterns may be curvilinear or rectilinear. Preferably they are rectilinear, which simplifies the implementation of lasing, i.e. scanning by the laser beam, of the portion of the bed.

[0032] The “vector gap” of a pattern is the distance separating two adjacent lines scanned consecutively by the laser beam, the distance being measured perpendicular to said lines.

[0033] Preferably, the ratio of the vector deviation of the first pattern to the median equivalent diameter of the powder and the ratio of the vector deviation of the second pattern to the median equivalent diameter of the powder are each between 0.6 and 2.0.

[0034] Preferably, the vector deviation of the first pattern is equal to the vector deviation of the second pattern.

[0035] Preferably, the first and second patterns are interlaced.

[0036] Preferably, each of the lines of the second pattern is equidistant from two adjacent lines of the first pattern. Thus, the laser energy is distributed uniformly towards each of the adjacent lines of the first pattern. For example, the second pattern is obtained by translating the first pattern along an axis perpendicular to the lines of the first pattern.

[0037] Preferably, the line of the first pattern and the line of the second pattern which are scanned first by the laser beam are adjacent. Preferably, they are scanned by the laser beam in the same direction of travel.

[0038] Each of the lines of the first pattern can be scanned in the same direction of travel and / or each of the lines of the second pattern can be scanned in the same direction of travel. Alternatively, each of the lines of the first pattern can be scanned in a direction of travel opposite to the direction of travel of the adjacent lines of the first pattern, and / or each of the lines of the second pattern can be scanned in a direction of travel opposite to the direction of travel of the adjacent lines of the second pattern.

[0039] Preferably, in particular when the particles are made of stainless steel, the volume energy density provided by the laser beam during scanning according to the first pattern and / or according to the volume energy density provided by the laser beam during scanning according to the second pattern are between 0.5 J / mm3 and 3.0 J / mm3.

[0040] Such a volumetric energy density results in good bonding of the bed particles both to each other and, where appropriate, to the blank of the part formed during the previous cycles of additive manufacturing. In addition, it makes it possible to avoid excessive densification of the part and to control the interparticle porosity in the part suitable for fluid transfer.

[0041] The volume energy density is expressed by the formula

[0042] [Math.l] p - -JL. “ vh£

[0043] in which: - P is the power of the laser beam, expressed in W, - V is the scanning speed of the laser beam, expressed in mm.s', - H is the vector deviation, expressed in mm and - e is the thickness of the metal particle bed in mm.

[0044] In step b), preferably, the latency time between the start of scanning according to the first pattern and the start of scanning according to the second pattern is between 0.1 and 3 s.

[0045] Preferably, after scanning according to the second pattern, the method may comprise scanning the portion of the bed by a laser beam along a path following a third pattern then a fourth pattern which are each formed of substantially parallel lines, spaced from each other and forming an angle of between 60° and 120°, preferably between 75° and 105°, more preferably between 85° and 95°, or even perpendicular, with the lines of the first pattern, the third and fourth patterns being interlaced or identical. Scanning the portion of the bed according to the third and fourth patterns increases the energy input over the entire portion of the bed. The resulting distribution of the heat accumulation makes it possible to produce a porous part with greater mechanical strength compared to the case where only scanning according to the first and second patterns is implemented.

[0046] The lines of the third and fourth patterns may be curvilinear or, preferably, rectilinear. According to the variant where the lines of the third and fourth patterns are curvilinear, the angle formed between said lines with the lines of the first pattern is the angle formed by the tangents of said lines at the points of intersection with the lines of the first pattern. For example, the third pattern, respectively the fourth pattern, are obtained by rotation of the first pattern, respectively the second pattern, along an axis normal to the bed of particles.

[0047] Preferably, the ratio of the vector deviation of the third pattern to the median equivalent diameter of the powder and the ratio of the vector deviation of the fourth pattern to the median equivalent diameter of the powder are each between 0.5 and 3.0, preferably between 0.6 and 2.0.

[0048] Preferably, the line of the third pattern and the line of the fourth pattern which are scanned first by the laser beam are adjacent, more preferably, said lines are scanned by the laser beam in the same direction of travel.

[0049] Each of the lines of the third pattern can be scanned in the same direction of travel and / or each of the lines of the fourth pattern can be scanned in the same direction of travel. Alternatively, each of the lines of the third pattern can be scanned in a direction of travel opposite to the direction of travel of the adjacent lines of the third pattern, and / or each of the lines of the fourth pattern can be scanned in a direction of travel opposite to the direction of travel of the adjacent lines of the fourth pattern.

[0050] Preferably, the volume energy density during scanning according to the third pattern and / or according to the fourth pattern is between 0.5 J / mm3 and 3 J / mm3.

[0051] Preferably, in step b), the latency time between the start of scanning according to the second pattern and the start of scanning according to the third pattern is between 0.1 and 3 s and / or the latency time between the start of scanning according to the third pattern and the start of scanning according to the fourth pattern is between 0.1 and 3 s.

[0052] In step b), the power of the laser radiation may be between 20 W and 500 W and / or the diameter of the laser beam may be between 50 pm and 150 pm and / or the scanning speed of the laser beam may be between 20 mm / s and 2000 mm / s, preferably between 50 mm / s and 1500 mm / s, preferably between 100 mm / s and 1000 mm / s.

[0053] The area of ​​the portion swept in step b) may be between 0.25 cm2 and 16 cm2. The swept portion may have the shape of a rectangle with a length of between 5 mm and 40 mm, preferably between 8 mm and 20 mm, and a width of between 5 mm and 40 mm, preferably between 8 mm and 20 mm.

[0054] The method may further comprise implementing step b) on another portion of the bed, preferably on several portions of the bed arranged side by side so as to form a checkerboard. Thus, the progressive supply of heat to each of the portions of the bed can be controlled optimally. The implementation of step b) on the different portions of the bed may be carried out by following different first and second patterns on each portion and / or, where appropriate, according to different third and fourth patterns, and / or according to laser beam parameters that are different from each other. A porous part having variations in pore sizes and / or a variation in porosity may thus be manufactured. In particular, the part may have a porosity gradient.The parameters of the laser beam may be the radiation power, the diameter of the laser beam and / or the scanning speed of the laser beam, preferably the radiation power and the scanning speed of the laser beam.

[0055] The invention also relates to a metal part comprising pores distributed randomly over the entire part, and having a ratio of the closed porosity rate to the open porosity rate of less than 0.1, preferably less than 0.05, more preferably less than 0.02, the open porosity rate being between 20% and 65%, the part having a number distribution of pore sizes in which: 5% to 30% of the pores have a size less than M * 0.2; 33% to 60% of pores have a size less than M * 0.4; 66% to 80% of pores have a size less than M * 0.6; 80% to 100% of the pores have a size less than M * 0.8; and 90% to 100% of the pores have a size less than M, M being between 250 pm and 900 pm, preferably between 350 pm and 500 pm.

[0056] The pores are distributed irregularly throughout the part. The porous part according to the invention therefore differs from the porous lattice structures of the prior art in which the lattices are repeated in an orderly manner in the structure.

[0057] The closed porosity rate is the ratio of the total volume occupied by the closed pores on the volume occupied by the porous part. A closed pore is fluidically isolated from other pores. No fluid can penetrate into a closed pore. The closed porosity rate is measured by pycnometry, for example helium pycnometry, or by thrust of Archimedes.

[0058] The open porosity rate is the ratio of the total volume occupied by the pores open to the volume occupied by the porous part. The open pores are in fluid communication with each other. For example, the open porosity rate can be determined by contacting, for example immersion, the porous part with a liquid, for example ethanol, then measuring the mass of the liquid absorbed by the structure.

[0059] Preferably, the pores have an average size Tm of between 100 μm and 300 μm. The average size Tm of the pores is the arithmetic mean in number of the sizes of the pores.

[0060] The size of a pore can be determined by the following method. The porous portion can first be infiltrated with a resin. A slice to be analyzed is cut and then polished to obtain a good surface finish, said polishing being carried out with a small grain polishing paper of at least grade 1200, preferably with a diamond paste. Images of the polished slice are taken using an optical microscope or an electron microscope, for example a scanning electron microscope, preferably in a mode using backscattered electrons in order to obtain a very good contrast between the metallic phase and the resin. The magnification used is such that the width of the image is between 10 times and 50 times the average pore size. A first image can be taken using a visual estimate of the average pore size.The size of a pore is determined by analyzing the images using a thresholding method followed by erosion / dilation and then Watershed segmentation used, for example, by image analysis software such as the ZEN CORE 2© software marketed by the company Zeiss© or the ImageJ software (https: / / imagej.nih.gov / ij / ). The size of each pore is then expressed from the diameter q> defined as follows: .

[0061] [Math.2] / 4'“Apare (P=

[0062] being the area of ​​each pore expressed in pm2.

[0063] Alternatively, the average pore size can be measured by tomography, for example, X-rays.

[0064] Preferably, the part is made of a metal as described above.

[0065] Preferably, the part is manufactured by the method according to the invention.

[0066] Preferably, M is equal to the median equivalent diameter D50 of the powder.

[0067] The invention also relates to a heat exchanger or a reactor, of preferably a heat pipe, comprising, or even consisting of, the part according to the invention. Brief description of the drawings

[0068] Other advantages and characteristics will emerge from reading the detailed description, the examples presented for illustrative and non-limiting purposes, and the attached drawing in which:

[0069] [Fig. 1 A] is a schematic view showing an example of implementation of step a) of forming a bed of metal particles;

[0070] [Fig. 1B] is a schematic view representing an example of implementation of step b) of bonding particles together;

[0071] [Fig.2A] and [Fig.2B] are schematic top views of a scanning of a portion of a particle bed by a laser beam along a path following a first pattern;

[0072] [Fig.3] is an enlargement of the top view of the particle scan according to [Fig.2A] or [Fig.2B];

[0073] [Fig.4A] and [Fig.4B] are schematic top views of a scanning of a portion of a particle bed by a laser beam along a path following a second pattern;

[0074] [Fig.5] is an enlargement of the top view of the particle scan according to [Fig.4A] or [Fig.4B];

[0075] [Fig.6] is a schematic top view of a scanning of a portion of a particle bed by a laser beam along a path following a third pattern;

[0076] [Fig.7] is a schematic top view of a scanning of a portion of a particle bed by a laser beam along a path following a fourth pattern;

[0077] [Fig.8] is a perspective photograph of an example of a porous part manufactured by the method according to the invention;

[0078] [Fig.9] is a curve representing the mass gain by capillary rise of ethanol in the room according to [Fig.8] over time; and

[0079] [Fig. 10] is a photograph taken by optical microscopy of a slice of a porous part manufactured by the method according to the invention, the slice being cut then polished after infiltration of the porous part by a resin. Detailed description

[0080] Figures 1A and 1B illustrate a device for implementing the method according to the invention. The device comprises a spreading system 1 comprising a feed piston 2, supporting a pile of powder 3, and a roller 4 arranged above the pile of powder 3. The device also comprises a laser source 6 and a plate 5.

[0081] [Fig.1A] illustrates an example of implementation of step a) of the method during which a bed 7 of metal particles is formed on the plate 5 from the powder 3, the powder 3 having a median equivalent diameter Di0 greater than 250 pm and a D90 less than 1200 pm. In the example illustrated, step a) comprises the lifting of the pile of powder 3 by the feed piston 2 to the height of the roller 4, followed by the movement of the roller 4 so as to scrape the upper part of the pile of powder 3 until the particles of the powder 3 are spread on the plate 5 and thus form a bed 7 of particles with a thickness of between 400 pm and 1400 pm.

[0082] [Fig. 1B] illustrates the implementation of a step b) during which the laser source 6 emits a laser beam 8 which scans a portion of the bed of particles formed in step a).

[0083] For example, to form a part made of particles of stainless steel, the laser beam 8 has a diameter of 70 pm and the scanning speed of the laser beam 8 is 558 mm / s. With such a diameter and such a scanning speed, the energy input to the powder bed by the laser beam 8 is localized. This allows, in the irradiated zone, the particles to partially melt to bond with their neighbors.

[0084] Figures 2A and 2B illustrate two different modes of scanning a portion 9 of the bed 7 of particles by the laser beam 8 along a path following a first pattern 10. The first pattern 10 is composed of rectilinear lines 101, parallel to each other and spaced apart from each other by a vector spacing H. In the example illustrated by [Fig.2A], each of the lines 101 is traversed by the laser beam 8 in the same direction of travel indicated by the arrow FL. In the example illustrated by [Fig.2B], each of the lines 101 of the first pattern 10 is traversed by the laser beam 8 in a direction of travel opposite to the direction of travel of the adjacent lines 101, as indicated by the arrows F2 and F3.

[0085] [Fig.3] shows an enlarged view of a scan according to [Fig.2A] or 2B.

[0086] The laser beam has a diameter ¢) less than the median equivalent diameter D50 of the powder. Thus, when scanning along a line 101 of the first pattern 10, the energy of the laser beam is concentrated only on a portion of the particles along the line 101.

[0087] Furthermore, in this example, the vector deviation H, which according to the invention is between 0.5 *D5o and 3.0* D5o, is substantially equal to the equivalent diameter of the powder.

[0088] For example, to manufacture a part from stainless steel particles having a median equivalent diameter of 500 pm, the laser beam 8 having a diameter of 70 pm can travel through a first pattern 10 whose lines 101 are 10 mm long with a scanning speed of 558 mm / s, and with a vector spacing H of approximately 400 pm. Thus, scanning by the laser beam 8 of a line 101 heats up slightly, or not at all, the particles which have already undergone irradiation by the laser beam and which are arranged along the other lines 101 of the first pattern 10.

[0089] When scanning particles 11 by the laser beam 8 along a line 101 of the first pattern 10, the energy supplied by the laser beam 8 heats said particles 11. Thus, when the laser beam 8 scanning along a line 101 passes over or near surfaces of two particles 11 in contact, the particles coalesce, for example under the effect of their partial fusion, and become linked to each other. There is then the formation of a connecting neck 12 between said particles 11.

[0090] Although scanning according to the first pattern 10 binds the particles 11 together, the inventors have found that scanning the portion 9 by the laser beam 8 along a path following a second pattern 13 is necessary to form a part having sufficient mechanical strength.

[0091] Figures 4A and 4B illustrate two different embodiments of a scan following the second pattern 13. In each of the scanning modes, the second pattern 13 is composed of rectilinear lines 131, parallel to each other and to the lines 101 of the first pattern 10. The lines 131 are spaced from each other by a vector spacing H' equal to the vector spacing H of the first pattern 10. The lines 131 are interlaced with the lines 101 so that each line 131 is spaced from an adjacent line 101 by a distance equal to H / 2.

[0092] In the scanning mode illustrated by [Fig.4A], each of the lines 131 is traversed by the laser beam 8 in the same direction of travel indicated by the arrow F4. In the scanning mode illustrated by [Fig.4B], each of the lines 131 is traversed by the laser beam 8 in a direction of travel opposite to the direction of travel of the adjacent lines 131, as indicated by the arrows F5 and F6.

[0093] [Fig. 5] shows a close-up view of a scan according to [Fig. 4A] or 4B. When scanning particles 11 by the laser beam 8 along a line 131 of the second pattern 13, the light energy supplied by the laser beam 8 heats said particles 11. Thus, when the laser beam 8 scanning along a line 131 passes close to a connecting neck 12 formed during scanning along the first pattern 10, then heating reinforces said neck 12, for example by partial fusion of the two particles 11 linked by said neck 12. In a similar manner to scanning according to the first pattern 10, when the laser beam 8 scanning along a line 131 passes over or close to surfaces of two particles 11 in contact with each other, these particles 11 coalesce and form a new connecting neck 12'. Furthermore, during scanning following the second pattern 13, the formation and reinforcement of connecting necks 12, 12' are facilitated by the accumulation of heat generated by the sweep following the first pattern 10.

[0094] To further increase the strength of the part, scanning of the portion 9 of the bed 7 by the laser beam 8 along a path following a third pattern 14 and a fourth pattern 15 can be implemented.

[0095] [Fig. 6] illustrates an example of scanning of the portion 9 by the laser beam 8 along a path following a third pattern 14. The third pattern 14 is composed of rectilinear lines 141, parallel to each other and spaced apart from each other. Preferably, the vector spacing between two adjacent lines 141 is the same as the vector spacing H between two lines 101. The lines 141 are perpendicular to the lines 101 of the first pattern 10. For example, each of the lines 141 is traversed by the laser beam 8 in a direction of travel opposite to the direction of travel of the adjacent lines 141, as indicated by the arrows F7 and F8.

[0096] The formation of bonds between the particles 11 during the laser scanning following the third pattern 14 occurs according to physical phenomena similar to those described in [Fig. 5]. The formation is however facilitated by the accumulation of heat generated by the scanning following the first pattern 10 then following the second pattern 13.

[0097] [Fig. 7] illustrates an embodiment of scanning the portion 9 by the laser beam 8 along a path following a fourth pattern 15. The fourth pattern 15 is composed of rectilinear lines 151, parallel to each other and spaced apart from each other. Preferably, the vector spacing between two adjacent lines 151 is the same as the vector spacing H' between two lines 131. The lines 151 are parallel and are interlaced with the lines 141 of the third pattern 14. In the scanning mode illustrated by [Fig. 7], each of the lines 151 is traversed by the laser beam 8 in a direction of travel opposite to the direction of travel of the adjacent lines 151, as indicated by the arrows F9 and F10.

[0098] The formation of bonds between the particles 11 during the laser scanning following the fourth pattern 15 occurs according to physical phenomena similar to those described in [Fig.5] except that the formation is facilitated by the accumulation of heat generated by the scanning following the first pattern 10, the second pattern 13, then following the third pattern 14.

[0099] Of course, step b) described above can be repeated on another portion of the particle bed, in particular adjacent, for example depending on the shape and / or the distribution of the porosity of the desired part.

[0100] Steps a) and b) form a cycle allowing the formation of a layer of the part to be manufactured. This cycle is repeated so as to obtain the porous part by additive manufacturing. Example 1

[0101] By way of illustration, the inventors have implemented the method according to the invention for the production of a porous part 16 as photographed in [Fig.8]. The part 16 is a straight block having a square base of 10 mm on each side and a height of 12 mm. As observed, the part has numerous interstices forming pores in the metal matrix.

[0102] The manufacturing method was carried out with a continuous wave Yb fiber laser emission source of wavelength 1070 nm emitting a laser beam 8 with a radiation power of 268 W and a diameter of 90 pm. The scanning speed of the laser beam 8 was 638 mm / s. The vector deviation H for each of the first 10, second 13, third 14 and fourth 15 patterns was equal to 700 pm. The first 10, second 13, third 14 and fourth 15 patterns were similar to those illustrated in Figures 2B, 4B, 6 and 7, respectively. The volume energy density when scanning according to each of the first 10, second 13, third 14 and fourth 15 patterns was 0.75 J / mm3.

[0103] The powder used for the manufacture of the part 16 was a water atomized 316L stainless steel particle powder, the powder having a tenth percentile Di0 of 350.8 pm, a median equivalent diameter D50 of 546.9 pm and a ninety percentile D9o of 844.7 pm. The thickness of the powder bed 7 formed in each cycle was 800 pm.

[0104] Various tests were carried out to evaluate the capillary performance of the obtained part 16. [Fig.9] represents the mass gain during the capillary rise of ethanol in the part 16. It can be observed that after 2 s, 1 g of ethanol has risen by capillarity in the pores of the part 16.

[0105] The inventors measured an open porosity rate of 56% and a closed porosity rate substantially equal to 0%. They also measured a capillary performance:

[0106] [Math.3] = 1.9 pm

[0107] where, K is the permeability of the part 16 and Rc is the average effective capillary radius of the pores defined as:

[0108] [Math.4] O _ Tm ~ 2^0X6

[0109] with Tm the average pore size and 0 the contact angle between the fluid for capillary rise and the part 16.

[0110] The capillary performance of the part 16 is superior to the capillary performance of prior art porous lattice structures, for example the article D. Jafari et al., Applied Thermal Engineering, vol. 143, 2018, pages 403-314, doi:10.1016 / j.applthermaleng.2018.07.111 describes obtaining a porous lattice structure with a capillary performance equal to 1.04 pm. Example 2

[0111] In another exemplary embodiment, the method according to the invention was implemented for the production of a porous part 17 in the form of a straight block with a length of 10 mm, a width of 5 mm and a height of 7 mm. The polished sectional view of said part 17 is illustrated [Fig. 10].

[0112] The manufacturing method was carried out with a continuous Yb fiber laser emission source with a wavelength of 1070 nm emitting a laser beam 8 with a radiation power of 216 W and a diameter of 90 pm. The scanning speed of the laser beam 8 was 450 mm / s.

[0113] The vector deviation H for each of the first 10, second 13, third 14 and fourth 15 patterns was equal to 700 pm. The first 10, second 13, third 14 and fourth 15 patterns were similar to those shown in Figures 2B, 4B, 6 and 7, respectively. The volume energy density when scanning according to each of the first 10, second 13, third 14 and fourth 15 patterns was 0.85 J / mm3.

[0114] The powder used for the manufacture of this part 17 is the same as the powder used for the manufacture of the part 16 of example 1. The thickness of the powder bed 7 formed at each cycle was 800 μm.

[0115] The part 17 obtained has an open porosity rate of 46% and a closed porosity rate substantially equal to 0%. As can be seen in [Fig. 10], the part 17 comprises particles 18 of the powder linked together by linking necks and separated by pores 19. It also comprises molten agglomerates 20 resulting from the complete fusion of particles and which have a size greater than 1 mm.

[0116] The number distribution of the equivalent diameters of the pores 19 of the part 17 is such that: - 16% of pores 19 have an equivalent diameter less than 110 pm, - 48% of pores 19 have an equivalent diameter less than 220 pm, - 79% of pores 19 have an equivalent diameter less than 330 pm, - 91% of pores 19 have an equivalent diameter less than 440 pm, - 95% of pores 19 have an equivalent diameter less than 550 pm.

[0117] Such a porosity distribution is particularly well suited to fluid transfer.

[0118] Other variants and improvements may be envisaged without departing from the scope of the invention as defined by the claims below.

[0119] For example, the metal particles may comprise a core of a first metal material partially or completely covered by a layer of a second metal material. The second metal material may have a melting temperature lower than the melting temperature of the first metal material. metallic. Thus, the part can be formed by melting the second material so that the particles of the powder form bonds between them without the core of the particles melting. The mechanical strength of the part thus formed is then partly ensured by the necks between the particles generated by the melting of the second material. Alternatively, the second metallic material can be a catalyst for a chemical reaction or a promoter of a phase change of an exchange fluid intended to circulate in the porous portion, for example chosen from platinum, cobalt, nickel, ruthenium and their alloys.

Claims

Claims

1. A method for additively manufacturing a porous part (16), the method comprising repeating a cycle comprising the steps of: a) forming a bed (7) of metal particles from a powder (3) of said particles (11), the tenth percentile (Di0) of the equivalent diameter of the particles of the powder being greater than 250 pm, b) bonding the particles together by scanning at least a portion (9) of the bed with a laser beam (8) along a path following a first pattern (10) then a second pattern (13) which are each formed of lines (101, 131) parallel and spaced apart from each other, the first and second patterns being interlaced or identical, the laser beam having a diameter (¢) less than the median equivalent diameter (D50) of the powder,the ratio of the vector deviation (H) of the first pattern to the median equivalent diameter of the powder and the ratio of the vector deviation (H') of the second pattern to the median equivalent diameter of the powder each being between 0.5 and 3.0.,

2. Method according to claim 1, the ratio of the diameter of the laser beam to the median equivalent diameter of the powder being less than 0.5, preferably less than 0.3, better still less than 0.

2.

3. A method according to any one of claims 1 and 2, the ratio of the vector deviation of the first pattern to the median equivalent diameter of the powder and the ratio of the vector deviation of the second pattern to the median equivalent diameter of the powder each being between 0.6 and 2.

0.

4. A method according to any preceding claim, the vector deviation of the first pattern being equal to the vector deviation of the second pattern.

5. Method according to the preceding claim, the bed formed in step a) having a thickness of between 400 and 1400 μm.

6. A method according to any preceding claim, each of the lines of the second pattern being equidistant from two adjacent lines of the first pattern.

7. A method according to any preceding claim, wherein the latency time between the start of scanning according to the first pattern and the start of scanning according to the second pattern is between 0.1 and 3 s.

8. A method according to any preceding claim, comprising, after scanning according to the second pattern, a scanning the portion of the bed by a laser beam (8) along a path following a third pattern (14) then a fourth pattern (15) which are each formed of lines (141, 151) substantially parallel, spaced from each other and forming an angle of between 60° and 120°, preferably between 75° and 105°, more preferably between 85° and 95°, or even perpendicular, with the lines of the first pattern, the third and fourth patterns being interlaced or identical.

9. Method according to any one of the preceding claims, the particles of the powder comprising at least one metal for more than 90.0% of their mass, preferably more than 95.0%, preferably the metal being chosen from aluminum, copper, nickel, iron and an alloy comprising more than 50% by mass of a metal chosen from aluminum, copper, nickel and iron, for example a stainless steel.

10. Method according to any one of the preceding claims, and in particular according to claim 9, the volume energy density during scanning according to the first pattern and / or according to the second pattern, and / or where appropriate according to the third pattern and / or the fourth pattern, being between 0.5 J / mm3 and 3.0 J / mm3.

11. A method according to any one of the preceding claims, making the swept portion in step b) being between 0.25 and 16 cm2, and / or the swept portion having the shape of a rectangle with a length between 5 mm and 40 mm, preferably between 8 mm and 20 mm, and a width between 5 mm and 40 mm, preferably between 8 mm and 20 mm.

12. Method according to any one of the preceding claims, in step b), the power of the laser radiation being between 20 W and 500 W, and / or the diameter of the laser beam being between 50 pm and 150 pm, and / or the scanning speed of the laser beam being between 20 mm / s and 2000 mm / s, preferably between 50 mm / s and 1500 mm / s, preferably between 100 mm / s and 1000 mm / s.

13. Metal part (16) comprising pores (19) distributed randomly over the entire part, and having a ratio of the closed porosity rate to the open porosity rate of less than 0.1, preferably less than 0.05, more preferably less than 0.02, the open porosity rate being between 20% and 65%, the part further having the following pore size distribution in number: - 5% to 30% of the pores have an equivalent diameter less than M * 0.2; - 33% to 60% of the pores have an equivalent diameter less than M * 0.4; - 66% to 80% of the pores have an equivalent diameter less than M * 0.6; - 80% to 100% of the pores have an equivalent diameter less than M * 0.8; and - 90% to 100% of the pores have an equivalent diameter less than M, M being between 250 pm and 900 pm, preferably between 350 pm and 500 pm, the metal part being manufactured by a method according to one of claims 1 to 12 and in which the bonding of the particles together, in step b), is carried out by partial fusion and coalescence between them of the particles of the powder.

14. Part according to the preceding claim, M being equal to the median equivalent diameter (D50) of the powder.

15. A heat exchanger or reactor, preferably a heat pipe, comprising a part according to any one of claims 13 and 14.

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