Method for producing a metal powder for an additive manufacturing process and use of such a powder

Abstract

The invention relates to a method for preparing a metal powder intended for an additive manufacturing process of the type involving the scanning of a powder bed by a near-infrared laser beam, characterized in that it comprises: an initial step for selecting a powder having an optical reflectance higher than 70% for wavelengths ranging between 800 nm and 1500 nm; then a step for treating said powder, which is different from particle grafting, and which causes a physical and / or chemical surface modification of the particles (4) of said powder, such that it is possible to reduce their optical reflectance at a given wavelength. The invention also relates to the use of such a powder, the particles (4) having a median size d50 between 5 pm and 50 pm after treatment.

Description

Technical Field

[0001] This invention relates to a process for preparing metal powders intended for use in additive manufacturing processes, wherein the additive manufacturing process involves scanning a powder bed by a near-infrared laser beam.

[0002] The present invention also relates to the use of metal powder obtained by the preparation method for additive manufacturing operations.

[0003] The field of this invention is the preparation of metal powders intended for use in additive manufacturing for all industrial applications, particularly in the automotive, aerospace and space sectors. Background Technology

[0004] Currently, selective laser melting (SLM) additive manufacturing technology is rapidly developing, especially for forming metal parts. The principle of powder bed SLM is to use a high-power laser to melt thin layers of powder (metal, plastic, ceramic, etc.).

[0005] These technologies enable the manufacture of complex-shaped parts that cannot be produced using conventional subtractive processing techniques.

[0006] However, the types of metals that can be used in additive manufacturing are relatively limited.

[0007] By volume, the main metals used in additive manufacturing are superalloys (nickel-based, Co-Cr, etc.), certain types of steel and ferrous metals, titanium alloys, and certain specific aluminum alloys (AlSiOG). Each alloy family corresponds to one or more preferred applications.

[0008] Other metals (such as copper, precious metals, and their alloys) are currently rarely used in additive manufacturing. On the one hand, the high thermal conductivity of these metals rapidly dissipates the energy supplied by the laser. On the other hand, the high optical reflectivity of these metals disperses the energy of the laser (such as the YAG laser commonly used in machines).

[0009] For the same reasons, aluminum and many of its alloys also present problems in additive manufacturing. Therefore, it is necessary to use very high laser power (>300W) to form these metals.

[0010] For some alloys, metallurgical reactions produce compounds with structural hardening, which can lead to cracking, especially when the alloy is formed by laser sintering, caused by extreme thermal stress.

[0011] Current additive manufacturing processes are not well-suited for producing parts from these metals. However, many industrial sectors are now interested in metal 3D printing for improving their products and / or developing new products.

[0012] FR3066705 proposes a solution for manufacturing highly reflective metallic powders compatible with SLM-type additive manufacturing. This solution involves modifying the surface of the powder particles by depositing nanoparticles into the powder particles (and, if possible, by forming a nanostructure layer). For example, for pure copper, copper nanoparticles can be grafted onto copper particles. The technique proposed in this document is a process for grafting nanoparticles onto the particle surface. Furthermore, it is advantageous to avoid the use of nanoparticles for hygiene and safety reasons and due to processing costs. In this case, the present invention has broad applicability. Summary of the Invention

[0013] The purpose of this invention is to propose a new method for preparing metal powders, particularly for shaping metals that are difficult to “laser process” in additive manufacturing, overcoming the aforementioned drawbacks.

[0014] Therefore, the present invention relates to a method for preparing metal powder intended for use in an additive manufacturing process, wherein the additive manufacturing process involves scanning a powder bed by a near-infrared laser beam, characterized in that the method comprises:

[0015] - An initial step for selecting powders that have an optical reflectivity greater than 70% for wavelengths in the range of 800 nm to 1500 nm; then

[0016] - A step for processing the powder, which is different from grafting the particles and causes physical and / or chemical surface modification of the powder particles, such that their optical reflectivity at a given wavelength can be reduced, and the particles (4) have a median particle size d50 between 5 μm and 50 μm after processing.

[0017] Therefore, this invention enables the production of powders that are easier to laser process after processing than in their initial state. The processed powder consists of functionalized particles or functionalized microparticles. 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.

[0018] Techniques for measuring reflectance and / or reflectance ratio are well known to those skilled in the art. For example, measurements can be performed using a spectrophotometer.

[0019] Other advantageous features of the invention, taken individually or in combination, are presented below:

[0020] According to the first embodiment:

[0021] - In the initial step, the selected powder includes at least carrier particles of a first material, which has a first optical reflectivity at a given wavelength.

[0022] The processing steps include functionalizing the surface of the carrier particles of the first material at least partially by forming a surface layer of at least one second material having a second optical reflectance lower than that of the first material through diffusion, germination growth, or precipitation, thereby reducing the optical reflectance of the powder at a given wavelength.

[0023] Following functionalization, the powder consists of composite particles, each comprising a carrier particle of a first material and a functionalized surface layer composed of at least one second material. Functionalization does not result in the deposition of nanoparticles grafted onto the carrier particles, but rather in the modification of the surface area of ​​the carrier particles.

[0024] According to the optional features of the first embodiment:

[0025] - After functionalization, the composite particles have a core-shell structure, with each particle having a core composed of carrier particles and a shell composed of a surface layer covering 10% to 100% of the surface of the carrier particles.

[0026] Functionalization includes diffusion of at least one second material selected from the groups of metals, metalloids, heteroatoms, and flux-type compounds.

[0027] Functionalization involves initial deposition followed by diffusion heat treatment. Diffusion is essentially an additional operation following deposition. Therefore, functionalization is not a form of deposition itself, but rather a transformation of the surface of the carrier particles through diffusion heat treatment.

[0028] Functionalization involves the diffusion of metalloid elements by reacting the particle surface with a gas. For example, oxidation in air allows particles to be oxidized on the surface. This type of functionalization is known as thermochemical diffusion processing.

[0029] Functionalization includes the germination and growth of at least one second material deposit on the particles of the first material. Germination is the phenomenon of the first sprouting of the second material. Germination affects the microscopic properties of the second material (size, purity, morphology, and crystal structure) related to its macroscopic properties. Growth is the phenomenon of the second material multiplying on the surface of the first material.

[0030] Functionalization includes the germination and growth of at least one deposit of several different materials (including the second material) on particles of the first material.

[0031] - Germination and growth occur on the particles of the first material with controlled roughness.

[0032] Functionalization includes at least one second material in the form of a precipitated compound that ensures the role of the flux during laser processing operations (chemical etching). The term "flux" refers to an antioxidant compound that allows for the removal of oxides and ensures optimal bonding of particles under the laser beam. This antioxidant compound can be, for example, selected from halogenated compounds, borax, and organic acids. Therefore, the flow functionality of the second material involves very specific structural features.

[0033] - Sedimentation occurs without any germination or growth.

[0034] -In addition to any germination and growth, sedimentation also occurs.

[0035] - All materials used for sedimentation are different from those used for germination and growth.

[0036] Functionalization will not alter the composition of the particles by more than 10% by mass.

[0037] - After processing, the functionalized surface layer, composed of at least a second material, has a maximum thickness of 1 μm / particle.

[0038] According to the second embodiment:

[0039] - In the initial step, the selected powder comprises at least particles of a first material having a first optical reflectivity at a given wavelength.

[0040] - The processing steps include physical and / or chemical etching, which results in an increase in the surface roughness of the etched particles, thus reducing the optical reflectivity of the powder at a given wavelength.

[0041] According to the third embodiment combining the first and second modes, the processing steps include:

[0042] - A first functionalization process, after which the powder is composed of functionalized particles, each particle comprising a carrier particle of a first material and a surface layer composed of at least one second material, then...

[0043] - A second process, which includes physical and / or chemical etching, which results in an increase in the surface roughness of the etched particles, thereby reducing the optical reflectivity of the powder at a given wavelength.

[0044] According to optional features of the invention, according to one of three embodiments:

[0045] - The particles of the first material are selected from the groups of copper and its alloys, aluminum and its alloys, or precious metals and their alloys.

[0046] - After treatment, surface modification covers 10% to 100% of the surface of each particle of the first material.

[0047] - After treatment, surface modification affects the maximum thickness of 1 μm / particle.

[0048] The treatment will not alter the composition of the particles by more than 10% by mass.

[0049] - The treatment reduces the optical reflectivity of the powder for a portion of the wavelengths in the range of 800 nm to 1500 nm.

[0050] - The treatment reduces the optical reflectivity of the powder for the entire range of wavelengths from 800 nm to 1500 nm.

[0051] The present invention also relates to the use of metal powder obtained by the above method for additive manufacturing operations. Attached Figure Description

[0052] The invention will be better understood by reading the following description, which is given only by way of non-limiting example and with reference to the accompanying drawings, in which:

[0053] Figure 1 This is a cross-sectional view of an SLS (Selective Laser Sintering) additive manufacturing facility.

[0054] Figure 2 This is a cross-sectional view showing the effect of laser on powder particles.

[0055] Figure 3 It is a graph showing the reflectivity of aluminum, copper, iron, zinc, nickel, and chromium as a function of the incident laser wavelength.

[0056] Figure 4 It is the radial cross-section of the functionalized particles obtained by using the powder preparation method according to the first embodiment of the present invention.

[0057] Figure 5 This is a scanning electron microscope (SEM) image of particles composed of copper particles functionalized with nickel layers.

[0058] Figure 6 These are cross-sectional views at different scales of particles composed of copper particles functionalized with nickel layers.

[0059] Figure 7 This is an SEM image of aluminum particles before functionalization.

[0060] Figure 8 This is an SEM image of particles composed of aluminum carrier particles, which are partially functionalized by deposits composed of zinc, copper, and chromium.

[0061] Figure 9It is a radial cross-section of an etched particle obtained using a powder preparation method according to a second embodiment of the present invention, the surface of which is modified by chemical etching.

[0062] Figure 10 This is a diagram showing the radial cross-sections of different particles to illustrate different embodiments of the invention. Detailed Implementation

[0063] Figures 1 to 10 The present invention is illustrated in that it is designed to prepare metal powder (1) intended for use in additive manufacturing processes (e.g., by scanning a powder bed (2) with a near-infrared laser beam (3)).

[0064] Figure 1 and Figure 2 The steps for producing 3D parts (5) using the SLM additive manufacturing process are shown.

[0065] 1. A fine layer of powder (1) is spread from plate (7) to piston (8) by roller (6) to form powder bed (2). At the start of production of part (5), piston (7) is at its highest point.

[0066] 2. The layer is sintered / melted by a high-power laser (3), which traces a 2D cross section onto the surface of the powder (1). The powder (1) solidifies immediately after the laser (3) stops.

[0067] 3. During production, the piston (7) supporting the 3D part (5) descends from the thickness of the generated layer, while the level of the powder supply box is adjusted by using the level of the plate (7).

[0068] 4. Spread a new layer of powder and repeat the process until a 3D part (5) is obtained.

[0069] The powder (1) used with this technology typically has a particle size of less than 50 μm, the distribution of which depends on the type of machine used. In all cases, the morphology of the powder (1) is preferably spherical to obtain optimal flowability and a powder bed (2) that is as dense and uniform as possible.

[0070] As mentioned above, some metals are difficult to laser process in additive manufacturing.

[0071] Figure 3 The optical reflectivity (R, between 0% and 100%) of aluminum (Al), copper (Cu), iron (Fe), zinc (Zn), nickel (Ni), and chromium (Cr) on the vertical axis is shown as a function of laser wavelength (WL, in μm) on the horizontal axis, expressed on a logarithmic scale.

[0072] As can be seen on the right side of the graph, the CO2 laser recorded a wavelength of approximately 10 μm (between 9.4 μm and 10.6 μm), where the metal's reflectivity (R) is very high. Therefore, CO2 lasers are not suitable for laser processing of these metals.

[0073] As can be seen at the center of the graph, the YAG laser recorded an infrared wavelength of approximately 1064 nm (i.e., 1.064 μm (more generally, ranging from 800 nm to 1500 nm, i.e., 0.8 μm to 1.5 μm)). The reflectivity (R) is low for iron (Fe), zinc (Zn), nickel (Ni), and chromium (Cr), but still relatively high for aluminum (Al) and copper (Cu).

[0074] Figures 4 to 8 A first embodiment of the powder preparation method (1) according to the present invention is illustrated.

[0075] The above-mentioned solution for forming metals includes surface functionalization of particles (4) during the preparation of powder (1).

[0076] The powder (1) is functionalized by at least partially functionalizing the surface (11) of the carrier particles (10) of the first material, the surface functionalization being carried out by forming a surface layer (20) of at least one 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 particles (4) to the selected wavelength, and thus reduce the optical reflectivity of the powder (1) to the selected wavelength.

[0077] Functionalization is selected to modify the surface of the carrier particles without causing nanoparticle deposition.

[0078] In fact, the goal can have four parts:

[0079] - Reduce the optical reflectivity of the powder (1) relative to the laser (3), thereby improving the laser / material interaction;

[0080] - Add elements to the surface of the particles (4) to generate an alloy in situ during forming;

[0081] - Improve the surface condition of the formed parts;

[0082] - Avoid grafting nanoparticles.

[0083] Figure 4 A composite particle (4) is shown, comprising a core consisting of a carrier particle (10) of a first material and a shell consisting of a surface layer (20) of a second material.

[0084] In this particular example, the shell covers 100% of the surface (11) of the carrier particle (10). Alternatively, the shell may cover 10% to 100% of the surface (11).

[0085] The core of the particle (4) is made of a metal (Al, Cu, noble metal) with high optical reflectivity, which is difficult to laser process, while the shell of the particle (4) includes one or more metals with low optical reflectivity, which reduces the energy dispersion of the laser (3) striking the particle (4) and thus promotes the melting of the powder (1).

[0086] Functionalization can be achieved using different techniques, such as:

[0087] - By diffusion of at least one second material selected from the groups of metals, metalloids and heteroatoms.

[0088] - By germinating and growing deposits of one or more materials on the particles of a first material (in a known manner, "germination and growth" refers to a combination of germination and growth, in the order of germination followed by growth).

[0089] - By depositing at least one second material in the form of a compound, the flux is ensured to function during laser processing operations (chemical etching). The compound is deposited on the particle surface without diffusion.

[0090] Regardless of the technique used within the scope of this invention, functionalization does not result in the deposition of particles grafted onto the carrier particles, but rather in the modified surface area of ​​the carrier particles.

[0091] In other words, techniques for grafting particles of a second material onto carrier particles of a first material are excluded from the scope of this invention.

[0092] According to a specific embodiment, in addition to germination and growth, sedimentation can also occur.

[0093] Figure 5 and Figure 6 An experiment was conducted on copper powder (1) functionalized with nickel. The particles (4) comprise a continuous thin 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 particles (4) allows for a significant improvement in laser / material interaction compared to pure copper powder.

[0094] For example, powder (1) may have the following properties:

[0095] - Theoretical chemical composition: 90% copper and 10% nickel

[0096] - Apparent density (Hall): 4.57 g / cm³ -3

[0097] - Castability (Hall): 13s / 50g

[0098] -Particle size: 90% <45μm

[0099] According to another example, powder (1) may have the following properties:

[0100] - Theoretical chemical composition: 90% copper and 10% nickel

[0101] - Apparent density (Hall): 2.94 g / cm³ -3

[0102] - Castability (Hall): 19s / 50g

[0103] -Particle size: 100% <50μm

[0104] Then, forming experiments were conducted using different copper-based powders with a 400W power laser via SLM.

[0105] A relatively dense and crack-free solid part is obtained from the functionalized powder (1) according to the present invention.

[0106] Meanwhile, under the same conditions, non-functionalized pure copper forming experiments were conducted using a 400W laser. Despite efforts to optimize the forming parameters, it was impossible to obtain usable parts from pure copper.

[0107] In fact, pure copper can be formed using SLM (Silicon-to-Metal Lamp), but the laser power is very high (>500W). The need for high power levels increases processing costs. Moreover, there is a high risk of laser backlash and it poses a risk to optical facilities.

[0108] Figure 7 and Figure 8 This illustrates the gradual formation of aluminum alloys. The 7000 series is particularly known for these difficulties in SLM forming. According to the invention, pure aluminum particles (10) are prefunctionalized with the constituent elements of the alloy (i.e., copper, zinc, and chromium). The functionalization process can be carried out using a wet deposition technique applied by immersion (galvanic displacement). After the treatment, the alloy is present on the surface of the aluminum particles (10) with a discontinuous shell consisting of zinc, copper, and chromium islands.

[0109] For example, powder (1) may have the following properties:

[0110] -Theoretical chemical composition:

[0111] -Al: 92.27-92.45%

[0112] -Zn: 5.4-5.6%

[0113] -Cu: 1.5-1.7%

[0114] -Cr: 0.23-0.25%

[0115] - Apparent density (Hall): 1.1 g.cm³ -3

[0116] The powder (1) can then be formed by SLM. Surface functionalization allows for the improvement of laser / material interaction by reducing the reflectivity of the powder (1) on the one hand, and the in-situ generation of alloys by diffusion on the other hand.

[0117] Figure 9 A second embodiment of the powder preparation method (1) according to the present invention is illustrated, which includes at least particles (10) of a first material having a first optical reflectivity at a laser wavelength.

[0118] In this embodiment, the solution for forming low-laser metal includes surface chemical and / or physical etching of the particles (4) during the preparation of the powder (1), resulting in an increase in the surface roughness of the particles (10). This etching is, for example, wet chemical etching of the carrier particles using acid or alkali, or thermochemical oxidation / reduction treatment in a fluidized bed via a gaseous method. This allows for a reduction in the optical reflectivity of the particles (4) to the laser wavelength, and thus a reduction in the optical reflectivity of the powder (1) to the laser wavelength. Consequently, the laser / material interaction is improved.

[0119] Figure 10 Different embodiments of the present invention are illustrated.

[0120] In the first mode M1, functionalization is achieved by germinating on the carrier particles (10) and then growing a surface layer (20). Submode M21 shows partial functionalization with discontinuous layers (20). Submode M22 shows continuous functionalization with large variations in the thickness of the continuous layers (20). Submode M23 shows continuous functionalization with small variations in the thickness of the continuous layers (20). In submodes M22 and M23, roughness is controlled.

[0121] In the second mode M2, surface modification is performed by physical and / or chemical etching.

[0122] In the third mode M3, the particles (10) of the first material undergo functionalization, and then the functionalized particles (10+20) undergo physical and / or chemical etching.

[0123] Regardless of how the embodiments of the present invention are described:

[0124] - The initially selected powder has an optical reflectance of greater than 70% for at least one wavelength in the range of 800 nm to 1500 nm;

[0125] - The treatment allows for a reduction in the optical reflectivity of the powder at least at the given wavelength, which is in the range of 800 nm to 1500 nm;

[0126] - The treatment differs from that of grafted particles.

[0127] Without departing from the scope of the invention as defined in the claims, it can be combined with Figures 4 to 10 Different shaped particles (1). Furthermore, the various technical features of the variations mentioned in the specification can be combined entirely or only partially. Therefore, the powder (1) can be adapted to the intended application.

Claims

1. A method for preparing metal powder (1) intended for use in an additive manufacturing process, the additive manufacturing process involving scanning a powder bed (2) by a near-infrared laser beam (3), characterized in that, The method includes: - An initial step for selecting metal powder (1), said powder having an optical reflectance greater than 70% for wavelengths in the range of 800 nm to 1500 nm, and comprising at least carrier particles (10) of a first material having a first optical reflectance at a given wavelength; then - A step for treating the metal powder (1), which differs from grafting particles and causes physical and / or chemical surface modification of the particles (4) of the metal powder (1) to reduce its optical reflectivity at a given wavelength, the particles (4) having a median particle size d50 between 5 μm and 50 μm after treatment, the step for treating the metal powder (1) includes at least partially functionalizing the surface (11) of the carrier particles (10) of the first material by diffusion without causing the deposition of nanoparticles, by forming a surface layer (20) of at least one second material having a second optical reflectivity lower than the first optical reflectivity, thereby reducing the optical reflectivity of the metal powder (1) at the given wavelength. Functionalization is the diffusion of at least one second material selected from the groups of metals, metalloids, and heteroatoms.

2. The method according to claim 1, characterized in that, The steps for processing the metal powder (1) include: after functionalization, the metal powder (1) is composed of functionalized particles (10+20), each particle including a carrier particle (10) of the first material and a surface layer (20) composed of at least one second material; followed by a second processing which includes physical and / or chemical etching, which causes an increase in the surface roughness of the functionalized particles (10+20), thereby reducing the optical reflectivity of the metal powder (1) at the given wavelength.

3. The method according to claim 1 or 2, characterized in that, Functionalization will not change the composition of the particles (4) by more than 10% by mass.

4. The method according to any one of claims 1 to 3, characterized in that, The functionalized surface layer, which is composed of at least the second material, has a maximum thickness of 1 μm on each particle (4).

5. The method according to any one of claims 1 to 4, characterized in that, The treatment will not change the composition of the particles (4) by more than 10% by mass.

6. The method according to any one of claims 1 to 5, characterized in that, Surface modification affects the maximum thickness of each particle (4) 1 μm.

7. The method according to any one of claims 1 to 6, characterized in that, The carrier particles (10) of the first material are selected from the group of copper and its alloys, aluminum and its alloys, or precious metals and their alloys.

8. The method according to any one of claims 1 to 7, characterized in that, After treatment, surface modification covers 10% to 100% of the surface of each carrier particle (10) of the first material.

9. Use of the metal powder (1) obtained by the method according to any one of claims 1 to 8 for additive manufacturing operations.

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