Method for preparing metal powder for additive manufacturing process, and use of such powder

Abstract

To provide a novel method for preparing a metal powder for preparing a metal which is difficult to be "laser irradiated" in additive manufacturing.SOLUTION: The present invention concerns a method for preparing a metal powder for an additive manufacturing process involving a near-infrared laser beam sweeping of a powder bed, characterized in that the method comprises: an initial step of selecting a powder having an optical reflectivity of more than 70% for a wavelength in a range of 800-1500 nm; then a step of treating the powder, which is different from a grafting of particles and which induces physical and / or chemical surface modification of the particles of the powder so as to reduce its optical reflectivity at a predetermined wavelength. The invention also concerns the use of such a powder, of which the particles (4) after treatment have a median particle size d50 of 5-50 μm.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a process for preparing metal powders intended for additive manufacturing processes, of the type that involves scanning a powder bed with a near-infrared laser beam. The present invention also relates to the use of the metal powder obtained by the preparation method for additive manufacturing operations. The field of the invention is the preparation of metal powders intended for additive manufacturing in all industrial applications, in particular in the automotive, aviation and aerospace sectors. [Background technology]

[0002] Currently, additive manufacturing techniques of the SLM (Selective Laser Melting) type are developing rapidly, especially for the shaping of metal parts. The principle of the SLM process on powder beds is to melt a thin layer of powder (metal, plastic, ceramic, etc.) with a high-power laser.

[0003] These techniques make it possible to manufacture parts with complex shapes that could not be produced using conventional subtractive machining techniques.

[0004] However, the range of metals available for additive manufacturing is relatively limited.

[0005] By volume, the primary metals used in additive manufacturing are superalloys (nickel-based, Co-Cr, etc.), certain types of steel and ferrous metals, titanium alloys, and certain special aluminum alloys (AlS10G). Each alloy family corresponds to one or more preferred applications.

[0006] Other metals, such as copper, precious metals and their alloys, are currently rarely used in additive manufacturing: on the one hand, their high thermal conductivity quickly dissipates the energy delivered by the laser, and on the other hand, their high optical reflectivity dissipates the energy of lasers, such as the YAG lasers conventionally used in machines. For the same reasons, aluminum and many of its alloys also pose problems in additive manufacturing, therefore necessitating the use of very high laser powers (>300W) to shape these metals. In certain alloys, metallurgical reactions produce compounds with structural hardening and cracking phenomena, especially induced by extreme thermal stresses while they are being shaped by laser sintering. Current additive manufacturing processes are not well suited to producing these metallic components. Nevertheless, many industrial sectors are now interested in metal 3D printing to improve their products and / or develop new ones.

[0007] FR3066705 proposes a solution for making highly reflective metal powders compatible with SLM type additive manufacturing. This solution consists in modifying the surface of the particles of the powder by depositing nanoparticles in the particles and possibly forming a nanostructured layer. For example, for pure copper, copper nanoparticles can be grafted onto the copper particles. The technique presented here is a process of grafting nanoparticles onto the surface of the particles. Moreover, it is advantageous for hygiene and safety reasons, as well as due to processing costs, to avoid the use of nanoparticles. In this case, the invention is widely used. Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to propose a novel method for preparing metal powders that overcomes the above mentioned drawbacks, in particular for shaping metals that are difficult to "laser" in additive manufacturing. [Means for solving the problem]

[0009] To this end, the invention relates to a method for preparing a metal powder intended for an additive manufacturing process, of the type comprising a step of scanning a powder layer by a near-infrared laser beam, said method comprising: A first step of selecting a powder having a light reflectance of more than 70% for wavelengths in the range of -800 to 1500 nm; and then - a step for treating the powder, inducing a physical and / or chemical surface modification of the grains of the powder, distinct from particle grafting, making it possible to reduce its light reflectance at a given wavelength, the grains having, after treatment, a median grain size d50 of 5 to 50 μm.

[0010] Thus, according to the invention, a powder can be obtained which is more lasable after treatment than in the initial state. The treated powder consists of functionalized grains or particles. The powder does not contain nanoparticles. By reducing the optical reflectivity of the powder, the energy dispersion of the laser can be reduced, thus facilitating the melting of the powder with reduced laser power.

[0011] Techniques for measuring reflectivity and / or reflectance are well known to those skilled in the art. For example, measurements can be made using a spectrophotometer.

[0012] Other advantageous features of the invention, taken individually or in combination, are set out below.

[0013] According to the first embodiment - in a first step, the selected powder comprises at least support particles of a first material having a first optical reflectance at a given wavelength, The treatment step consists of at least partially functionalizing the surface of the support particles of the first material by diffusion, by germination-growth or by precipitation, by forming at least one surface layer (20) of a second material having a second light reflectivity lower than the first light reflectivity, thereby reducing the light reflectivity of the powder at a predetermined wavelength.

[0014] After the functionalization treatment, the powder consists of composite grains, each composite grain comprising a support particle of a first material and at least one functionalized surface layer of a second material. The functionalization does not result in the deposition of grafted nanoparticles on the support particle, but rather in a modified surface area of ​​the support particle.

[0015] According to an optional feature of the first embodiment: After the functionalization treatment, the composite grains have a core-shell structure, with each grain having a core constituted by the support particle and a shell consisting of a surface layer covering 10-100% of the surface of the support particle. The functionalization comprises the diffusion of at least one second material selected from the family of metals, semimetals, heteroatoms and flux-type compounds. - Functionalization involves a pre-deposition followed by a diffusion heat treatment. The diffusion is actually an additional operation after the deposition. The functionalization is therefore not a form of deposition per se, but a transformation of the surface of the support particles by a diffusion heat treatment. -Functionalization involves the diffusion of metalloid elements by reacting the surface of the particles with a gas. For example, oxidation carried out in air makes it possible to oxidize the particles superficially. This functionalization process is called a thermochemical diffusion process. -Functionalization involves the germination-growth of at least one deposit of a second material on a particle of a first material. Germination is the phenomenon whereby the first shoots of a second material appear. Germination affects the microscopic properties of the second material (size, purity, morphology and crystal structure) in relation to its macroscopic properties. Growth is the phenomenon whereby the second material propagates over the surface of the first material. -Functionalization involves the germination-growth of at least one deposit of several different materials (including a second material) on particles of a first material. - Germination-growth is carried out on the particles of the first material with controlled roughness. -Functionalization involves the precipitation of at least one second material in the form of a compound that ensures the role of flux (chemical etching) during the laser oscillation operation. The term "flux" means antioxidant chemical compounds that make it possible to remove oxides and ensure optimal bonding of the grains under the laser beam. The latter can be chosen, for example, from halogenated compounds, boric acid and organic acids. The flow function of the second material therefore involves very specific structural properties. - Precipitation takes place without germination-growth. - Precipitation is in addition to germination-growth. -All materials used for precipitation are different from those used for germination-growth. The functionalization does not change the composition of the grains by more than 10% by weight. after treatment, the functionalized surface layer of at least a second material has a maximum thickness of 1 μm per grain.

[0016] According to a second embodiment: - in a first step, the selected powder comprises at least particles of a first material having a first optical reflectance at a given wavelength, - The treatment step consists of physical and / or chemical etching which causes an increase in the surface roughness of the etched particles, thus reducing the optical reflectance of the powder at a given wavelength.

[0017] According to a third embodiment, the first mode and the second mode are combined, and the processing steps include: a first functionalization treatment, after which the powder consists of functionalized particles each comprising a support particle of a first material and at least one surface layer made of a second material, then A second treatment, which involves physical and / or chemical etching, which causes an increase in the surface roughness of the etched particles, thus reducing the optical reflectance of the powder at a given wavelength.

[0018] According to an optional feature of the invention, according to one of three embodiments: the particles of a first material are selected from the family of copper and its alloys, aluminium and its alloys, or noble metals and their alloys; After treatment, the surface modification covers between 10 and 100% of the surface of each particle of the first material. After treatment, the surface modification affects the thickness by up to 1 μm per grain. The treatment does not change the composition of the grains by more than 10% by mass. The treatment reduces the light reflectance of the powder for some wavelengths in the range of 800-1500 nm. The treatment reduces the optical reflectance of the powder for all wavelengths in the range of 800-1500 nm.

[0019] The present invention also relates to the use of the metal powder obtained by the above method for additive manufacturing operations.

[0020] The invention will be better understood on reading the following description, given by way of non-limiting example only and made with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a cross-sectional view of an SLS-type additive manufacturing facility. [Diagram 2] FIG. 2 is a cross-sectional view showing the action of a laser on a powder grain. [Diagram 3] FIG. 3 is a graph showing the reflectivity of aluminum, copper, iron, zinc, nickel, and chromium as a function of incident laser wavelength. [Figure 4] FIG. 4 is a radial cross section of a functionalized particle obtained using the powder preparation method according to the first embodiment of the invention. [Diagram 5] FIG. 5 is a scanning electron microscope (SEM) image of a grain consisting of copper particles functionalized with a layer of nickel. [Figure 6] FIG. 6 shows cross-sections at different scales of a grain consisting of a copper particle functionalized with a layer of nickel. [Figure 7]FIG. 7 is an SEM image of aluminum particles before functionalization treatment. [Figure 8] FIG. 8 is an SEM image of a grain of aluminum support particle partially functionalized with deposits of zinc, copper and chromium. [Figure 9] FIG. 9 is a radial cross-section of an etched particle whose surface has been modified by chemical etching using the powder preparation method according to the second embodiment of the present invention. [Figure 10] FIG. 10 shows radial cross sections of different particles to illustrate different embodiments of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] 1 to 10 show the present invention, designed for preparing metal powders (1) intended for additive manufacturing processes, such as scanning a powder bed (2) with a near-infrared laser beam (3).

[0023] Figures 1 and 2 show the steps of manufacturing a 3D part (5) using the SLM additive manufacturing process. 1. A fine layer of powder (1) is spread by roller (6) from plate (7) to piston (8) to form powder layer (2). At the start of production of part (5), piston (7) is in its highest position. 2. This layer is sintered / melted by a high power laser (3) that traces a 2D cross section onto the surface of the powder (1). The powder solidifies (1) immediately after the laser (3) is stopped. 3. The piston (7) supporting the 3D part (5) during production is lowered from the thickness of the produced layer and the level of the powder feed cartridge is adjusted with the level of the plate (7). 4. A new layer of powder is spread and the process is repeated until the 3D part (5) is obtained.

[0024] The powders (1) used with this technique generally have a grain size of less than 50 μm, the distribution depending on the type of machine used. In all cases, the morphology of the powder (1) is preferentially spherical in order to obtain an optimal flowability and a powder layer (2) as dense and homogeneous as possible.

[0025] As mentioned above, certain metals are difficult to irradiate with a laser in additive manufacturing.

[0026] FIG. 3 shows the optical reflectance (R between 0% and 100%) of aluminum (Al), copper (Cu), iron (Fe), zinc (Zn), nickel (Ni) and chromium (Cr) on the vertical axis as a function of the laser wavelength (WL in μm) on the horizontal axis in logarithmic scale. As can be seen on the right side of the graph where the CO2 laser records wavelengths around 10 μm (9.4 to 10.6 μm), the reflectivity (R) of metals is very high. Therefore, CO2 lasers are not suitable for lasing these metals. As can be seen in the center of the graph, where the YAG laser records infrared wavelengths of about 1,064 nm or 1.064 μm (more commonly in the range of 800-1500 nm or 0.8-1.5 μm), the reflectance (R) is low for iron (Fe), zinc (Zn), nickel (Ni) and chromium (Cr), but still high for aluminum (Al) and copper (Cu).

[0027] 4 to 8 show a first embodiment of the powder preparation method (1) according to the present invention. A solution for shaping said metals consists in carrying out a surface functionalization of the grains (4) during the preparation of the powder (1). The powder (1) is functionalized by at least partially functionalizing the surface (11) of the support particles (10) of a first material by forming at least one surface layer (20) of a second material having a second optical reflectivity lower than the first optical reflectivity at the wavelength of the laser (3). This functionalization makes it possible to reduce the optical reflectivity of the grains (4) and therefore of the powder (1) down to a selected wavelength. The functionalization treatment is selected to modify the surface of the support particles without resulting in the deposition of nanoparticles.

[0028] In reality, there are four objectives: - reducing the optical reflectivity of the powder (1) to the laser (3), thereby improving the laser / material interaction, - adding elements to the surface of the grains (4) to form an alloy in situ during forming; -improving the surface condition of the molded component; -Avoiding the grafting of nanoparticles.

[0029] FIG. 4 shows a composite grain (4) comprising a core constituted by a support particle (10) of a first material and a shell consisting of a surface layer (20) of a second material. In this particular example, the shell covers 100% of the surface (11) of the support particle (10). Alternatively, the shell may cover between 10 and 100% of the surface (11). The core of the grain (4) is made of metals with high optical reflectivity (Al, Cu, precious metals) that are difficult to irradiate with a laser, and the shell of the grain (4) contains one or more metals with low optical reflectivity, which can reduce the energy dispersion of the laser (3) acting on the grain (4) and thus promote the melting of the powder (1).

[0030] Functionalization can be carried out using different techniques, for example: by diffusion of at least one second material selected from the family of metals, semimetals and heteroatoms. - by germination-growth of a deposit of one or more materials on a particle of a first material (in a known manner, the expression "germination-growth" indicates a combination of germination and growth in the order of germination followed by growth). - by precipitation of at least one second material in the form of a compound that ensures the role of flux (chemical etching) during the laser oscillation operation. The compound is precipitated on the grain surface without diffusion. Whatever the technique used within the scope of the present invention, the functionalization does not result in the deposition of grafted particles on the support particle, but in a modified surface area of ​​the support particle. In other words, techniques in which particles of a second material are grafted onto support particles of a first material are excluded from the scope of the present invention.

[0031] According to certain embodiments, precipitation can be carried out in addition to germination-growth.

[0032] Figures 5 and 6 show tests carried out on copper powder (1) functionalized with nickel. The grains (4) contain a continuous fine layer (20) of nickel on the surface of the copper particles (10). Nickel has low optical reflectivity and its presence on the surface of the grains (4) allows for a significant improvement in the laser / material interaction with respect to pure copper powder.

[0033] For example, the powder (1) may have the following characteristics: -Theoretical chemical composition: 90% copper and 10% nickel -Apparent density (Hall): 4.57gcm -3 -Malleability (Hall): 13s / 50g -Grain size: 90%<45μm

[0034] According to another example, the powder (1) may have the following characteristics: -Theoretical chemical composition: 90% copper and 10% nickel -Apparent density (Hall): 2.94g.cm-3 -Malleability (Hall): 19s / 50g -Grain size: 100%<50μm

[0035] Next, SLM forming tests were carried out with different copper-based powders using a 400W output laser. Relatively dense and crack-free solid parts were obtained from the functionalized powder (1) according to the invention described above. At the same time, non-functionalized pure copper forming tests were performed under the same conditions with a 400W power laser. Despite optimizing the forming parameters, no usable parts could be obtained from pure copper. In fact, pure copper can be shaped by SLM, but at very high laser powers (>500W). The need for high levels of power increases the processing costs. Also, there is a high risk of laser retroreflection, which induces a risk to the optical equipment.

[0036] Figures 7 and 8 show the evolution of aluminum alloys. The 7000 series is known for the difficulty of forming them, especially by SLM. According to the invention, pure aluminum particles (10) are pre-functionalized with the constituent elements of the alloy, namely copper, zinc, chromium. The functionalization treatment can be carried out using a wet deposition technique applied by immersion (galvanic displacement). After the treatment, the alloy is located at the surface of the aluminum particles (10) and has a discontinuous shell consisting of islands of zinc, copper and chromium.

[0037] For example, the powder (1) may have the following characteristics: -Theoretical chemical composition: - Al: 92.27 - 92.45% - Zn: 5.4 - 5.6% - Cu: 1.5 - 1.7% - Cr: 0.23 - 0.25% -Apparent density (Hall): 1.1gcm -3

[0038] This powder (1) can then be shaped by SLM. The surface functionalization treatment, on the one hand, improved the laser / material interaction by reducing the reflectivity of the powder (1) and, on the other hand, made it possible to generate alloys in situ by diffusion.

[0039] FIG. 9 shows a second embodiment of the powder preparation method (1) according to the invention, comprising at least particles (10) of a first material and having a first optical reflectivity at the wavelength of the laser. In this embodiment, the solution for shaping low laser metal consists of carrying out surface chemical and / or physical etching on the grains (4) during the preparation of the powder (1), which causes an increase in the surface roughness of the particles (10). This etching is, for example, chemical etching of the supporting grains by a wet process with acids or bases, or a thermochemical oxidation / reduction treatment by a gas process in a fluidized bed. This allows to reduce the optical reflectivity of the grains (4) and therefore of the powder (1) to the wavelength of the laser. Thus, the laser / material interaction is improved.

[0040] FIG. 10 shows a different embodiment of the present invention. In the first mode M1, the functionalization is performed by germination and then growth of a surface layer (20) on the support particles (10). Submode M21 shows partial functionalization with a discontinuous layer (20). Submode M22 shows continuous functionalization with large variations in the thickness of the continuous layer (20). Submode M23 shows continuous functionalization with small variations in the thickness of the continuous layer (20). In submodes M22 and M23, the roughness is controlled. In the second mode M2, surface modification is performed by physical and / or chemical etching. In a third mode M3, the particles of a first material (10) are subjected to a functionalization treatment and then the functionalized particles (10+20) are subjected to physical and / or chemical etching.

[0041] Whatever the description of the embodiments of the present invention: - the first selected powder has an optical reflectance of more than 70% for at least one wavelength in the range of 800-1500 nm; - the treatment makes it possible to reduce the optical reflectance of the powder at least over this predetermined wavelength, which is in the range of 800-1500 nm; The treatment is different from the grafting of the particles.

[0042] Powder (1) may be shaped differently from Figures 4-10 without departing from the scope of the invention as defined in the claims. Furthermore, the technical features of the various variants described in the description may be combined in whole or in part only, in order to adapt powder (1) to the intended use.

Claims

1. 1. A method for preparing a metal powder (1) intended for an additive manufacturing process, of the type comprising the step of scanning a powder layer (2) by a near-infrared laser beam (3), comprising: A first step of selecting a powder (1) having a light reflectance of more than 70% for wavelengths in the range of 800 to 1500 nm; and then - a step for treating said powder (1) inducing a physical and / or chemical surface modification of the grains (4) of said powder (1), distinct from the grafting of particles, making it possible to reduce its light reflectance at a given wavelength, said grains (4) having, after the treatment, a median grain size d50 of 5 to 50 μm; A method comprising:

2. 2. The method according to claim 1, wherein in the first step the selected powder (1) comprises at least support particles (10) of a first material having a first light reflectance at the predetermined wavelength, and the treatment step comprises at least partial functionalization of the surface (11) of the support particles (10) of the first material by diffusion, by germination-growth or by precipitation, without resulting in the deposition of nanoparticles, by forming at least one surface layer (20) of a second material having a second light reflectance lower than the first light reflectance, thereby reducing the light reflectance of the powder (1) at the predetermined wavelength.

3. 2. The method according to claim 1, wherein in the first step the selected powder (1) comprises at least particles (10) of a first material having a first optical reflectance at the predetermined wavelength, and the treatment step comprises physical and / or chemical etching causing an increase in the surface roughness of the particles (10), thus reducing the optical reflectance of the powder (1) at the predetermined wavelength.

4. 3. The method according to claim 2, wherein the treatment step comprises a functionalization, after which the powder (1) consists of functionalized particles (10+20) each comprising support particles (10) of the first material and a surface layer (20) of at least a second material, and then a second treatment comprises a physical and / or chemical etching which causes an increase in the surface roughness of the functionalized particles (10+20) and thus reduces the optical reflectivity of the powder (1) at a predetermined wavelength.

5. 5. The method according to claim 2 or 4, wherein the functionalization comprises the diffusion of at least one second material selected from the family of metals, semimetals and heteroatoms.

6. 5. The method of claim 2 or 4, wherein the functionalization comprises the germination-growth of at least one deposit of a second material on the particles of the first material.

7. 6. The method according to claim 2, 4 or 5, wherein the functionalization comprises the precipitation of at least one second material in the form of a compound ensuring the role of flux during the laser oscillation operation.

8. 8. The method according to claim 2, 4, 5, 6 or 7, wherein the functionalization does not change the composition of the grains (4) by more than 10% by weight.

9. 9. The method of claim 2, 4, 5, 6, 7 or 8, wherein the functionalized surface layer of at least the second material has a thickness of up to 1 μm on each grain.

10. 10. The method of claim 1, wherein the treatment does not change the composition of the grain by more than 10% by weight.

11. 11. The method according to any one of claims 1 to 10, wherein the surface modification affects the thickness of each grain up to 1 μm.

12. 12. The method according to any one of the preceding claims, wherein the particles (10) of the first material are selected from the family of copper and its alloys, aluminium and its alloys, or noble metals and their alloys.

13. 13. The method according to any one of claims 1 to 12, wherein after treatment, the surface modification covers 10 to 100% of the surface of each particle (10) of the first material.

14. Use of a metal powder (1) obtained by the method according to any one of claims 1 to 13 for additive manufacturing operations.

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