METHOD FOR PREPARING A METAL POWDER FOR AN ADDITIVE MANUFACTURING PROCESS AND USE OF SUCH POWDER

Abstract

The present invention relates to a method for preparing a metal powder intended for an additive manufacturing process, of the type that involves scanning a bed of powder by a near-infrared laser beam, characterized in that the method comprises: an initial step for selecting a powder, which has an optical reflectivity of higher than 70% for a wavelength ranging between 800 and 1500 nm; then a step for treating said powder, which is different from particle grafting, and which induces a physical and/or chemical surface modification of the grains of said powder, making it possible to lower its optical reflectivity, at the given wavelength. The invention also relates to the use of such a powder, the grains having, after treatment, a median grain size d50 of between 5 and 50 μm.

Claims

1. A method for preparing a metal powder intended for an additive manufacturing process, of the type that involves scanning a bed of powder by a near-infrared laser beam, characterized in that the method comprises: an initial step for selecting a powder, that has an optical reflectivity of more than 70% for a wavelength ranging between 800 and 1500 nm; then a step for treating said powder, which is different from grafting particles, and which induces a physical and/or chemical surface modification of the grains of said powder, making it possible to lower its optical reflectivity, at the given wavelength, the grains having, after treatment, a median grain size d50 of between 5 and 50 μm.

2. The method according to claim 1, wherein in the initial step, the powder selected comprises at least support particles of a first material, having a first optical reflectivity at the given wavelength, and in that the treatment step comprises at least partial functionalization of the surface of the support particles of the first material, by diffusion, by germination-growth or by precipitation, without resulting in deposition of nanoparticles, by forming a surface layer of at least one second material having a second optical reflectivity, lower than the first optical reflectivity, thus lowering the optical reflectivity of the powder, at the given wavelength.

3. The method according to claim 1, wherein in the initial step, the powder selected comprises at least particles of a first material, having a first optical reflectivity at the given wavelength, and in that the treatment step comprises physical and/or chemical etching causing an increase in the surface roughness of the particles, thus lowering the optical reflectivity of the powder, at the given wavelength.

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

5. The method according to claim 2, wherein that functionalization comprises diffusion of at least one second material selected from the families of metals, metalloids and heteroatoms.

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

7. The method according to claim 2, wherein that functionalization comprises precipitation of at least one second material, in the form of a compound ensuring the role of flux during the lasing operation.

8. The method according to claim 2, wherein that functionalization does not modify the composition of the grains by more than 10% by mass.

9. The method according to claim 2, wherein that the functionalization surface layer consisting of at least the second material has a maximum thickness of 1 μm on each grain.

10. The method according to claim 1, wherein that treatment does not modify the composition of the grains by more than 10% by mass.

11. The method according to claim 1, wherein that surface modification affects a maximum thickness of 1 μm of each grain.

12. The method according to claim 1, wherein the particles of the first material are chosen from the families of copper and its alloys, aluminum and its alloys, or precious metals and their alloys.

13. The method according to claim 1, wherein after treatment, surface modification covers between 10 and 100% of the surface of each particle of the first material.

14. A method of additive manufacturing comprising using metal powder obtained by the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

[0053] FIG. 1 is a transversal cross-sectional view of an SLS type additive manufacturing facility.

[0054] FIG. 2 is a transversal cross-sectional view showing the action of the laser on the powder grains.

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

[0056] FIG. 4 is a radial cross-section of a functionalized particle, obtained by using the powder preparation method in accordance with a first embodiment of the invention.

[0057] FIG. 5 is a scanning electron microscope (SEM) image of grains consisting of copper particles functionalized by a layer of nickel.

[0058] FIG. 6 is a cross-sectional view, on a different scale, of grains consisting of copper particles functionalized by a layer of nickel

[0059] FIG. 7 is an SEM image of aluminum particles prior to functionalization treatment.

[0060] FIG. 8 is an SEM image of grains consisting of aluminum support particles partially functionalized by a deposit consisting of zinc, copper and chrome.

[0061] FIG. 9 is a radial cross-section of an etched particle, the surface of which has been modified by chemical etching, using the powder preparation method in accordance with a second embodiment of the invention.

[0062] FIG. 10 is a diagram showing radial cross-sections of different particles, to illustrate the different embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0063] FIGS. 1 to 10 illustrate the invention, designed for preparing a metal powder (1) intended for an additive manufacturing process, such as scanning a bed of powder (2) by a near-infrared laser beam (3).

[0064] FIGS. 1 and 2 show the steps for producing a 3D part (5), by using an SLM additive manufacturing process.

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

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

[0067] 3. The piston (7) supporting the 3D part (5) during production descends from the thickness of the layer produced, while the level of the powder feed cartridges is adjusted with that of the plate (7).

[0068] 4. A new layer of powder is spread out and the process is repeated until the 3D part (5) is obtained.

[0069] The powders (1) used alongside this technology generally have a grain size of less than 50 μm, with a distribution depending on the type of machine used. In all cases, the morphology of the powders (1) is more preferentially spherical to obtain optimum flowability and a bed of powder (2) which is as dense and homogeneous as possible.

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

[0071] FIG. 3 shows the optical reflectivities (R between 0% and 100%) of aluminum (Al), copper (Cu), iron (Fe), zinc (Zn), nickel (Ni) and chrome (Cr), represented on the ordinate, as a function of the wavelength of the laser (WL in μm), represented on the abscissa with a logarithmic scale.

[0072] As can be seen on the right side of the graph, in which a CO.sup.2 laser has recorded a wavelength of about 10 μm (between 9.4 and 10.6 μm), the reflectivity (R) of the metals is very high. The CO.sub.2 laser is therefore not suitable for lasing these metals.

[0073] As can be seen in the center of the graph, in which a YAG laser has recorded an infrared wavelength of about 1,064 nm i.e. 1.064 μm (more generally, ranging between 800 and 1500 nm, i.e. between 0.8 and 1.5 μm), the reflectivity (R) is lower for iron (Fe), zinc (Zn), nickel (Ni) and chrome (Cr), but still high for aluminum (Al) and copper (Cu).

[0074] FIGS. 4 to 8 illustrate a first embodiment of the powder preparation method (1) in accordance with the invention.

[0075] The solution for shaping the metals mentioned above consists of carrying out surface functionalization of the grains (4) during the preparation of the powder (1).

[0076] The powder (1) is functionalized by at least partially functionalizing the surface (11) of the support particles (10) of the first material by forming a surface layer (20) of at least one second material, having a second optical reflectivity which is lower than the first optical reflectivity at the wavelength of the laser (3). This functionalization makes it possible to lower the optical reflectivity of the grains (4), and therefore that of the powder (1), to the chosen wavelength.

[0077] Functionalization treatment is chosen to modify the surface of the support particle, without resulting in deposition of nanoparticles.

[0078] In practice, the objective may be fourfold: [0079] decreasing the optical reflectivity of the powder (1) with respect to the laser (3), and thus improve the laser/material interaction, [0080] adding elements to the surface of the grains (4) to generate an alloy in situ during shaping, [0081] improving the surface state of shaped components; [0082] avoiding grafting nanoparticles.

[0083] FIG. 4 shows a composite grain (4), comprising a core constituted by a support particle (10) of the first material, and a shell consisting of a surface layer (20) of the 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).

[0084] The core of the grain (4) is made of a metal with high optical reflectivity (Al, Cu, precious metal), which is difficult to laser, while the shell of the grain (4) comprises one or more metals with low optical reflectivity, making it possible to reduce the energy dispersion of the laser (3) impinging on the grain (4), and thus to facilitate the melting of the powder (1).

[0085] Functionalization may be carried out by using different techniques, for example: [0086] By diffusion of at least one second material selected from the families of metals, metalloids, and heteroatoms. [0087] By germination-growth of a deposit of one or more materials on the particles of the first material (in a known manner, the expression “germination-growth” designates the combination of germination and growth, in the order of germination and then growth). [0088] By precipitation of at least one second material, in the form of a compound ensuring the role of flux (chemical etching) during the lasing operation. The compound is deposited onto the surface of the grain without diffusion.

[0089] Whatever the technique used in the scope of the invention, functionalization does not result in deposition of particles grafted onto the support particle, but in a modified surface area of the support particle.

[0090] In other words, the technique, grafting particles of the second material onto the support particles of the first material is excluded from the scope of the invention.

[0091] According to a particular embodiment, precipitation may be carried out in addition to germination-growth.

[0092] FIGS. 5 and 6 illustrate tests carried out on a copper powder (1) functionalized with nickel. The grains (4) comprise 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) makes it possible to considerably improve the laser/material interaction with respect to a pure copper powder.

[0093] For example, the powder (1) may have the following characteristics: [0094] Theoretical chemical composition: 90% Copper and 10% Nickel [0095] Apparent density (Hall): 4.57 g cm.sup.−3 [0096] Castability (Hall): 13 s/50 g [0097] grain size: 90%<45 μm

[0098] According to another example, the powder (1) may have the following characteristics: [0099] Theoretical chemical composition: 90% Copper and 10% Nickel [0100] Apparent density (Hall): 2.94 g.Math.cm.sup.−3 [0101] Castability (Hall): 19 s/50 g [0102] grain size: 100%<50 μm

[0103] Shaping tests by SLM were then carried out with different copper-based powders, by using a 400 W power laser.

[0104] Solid parts that are relatively dense and without cracks were obtained from the functionalized powders (1) according to the invention, described above.

[0105] At the same time, non-functionalized pure copper shaping tests were carried out under the same conditions, by using a 400 W power laser. Despite a run to optimize the shaping parameters, it was not possible to obtain utilizable parts from pure copper.

[0106] In practice, pure copper may be shaped by SLM, but at very high laser powers (>500 W). The need for a high level of power increases the cost of treatment. Also, the risk laser retroreflection is high and induces a risk for the optical installation.

[0107] FIGS. 7 and 8 illustrate the development of aluminum alloys. The 7000 series is known particularly for these difficulties of shaping by SLM. According to the invention, pure aluminum particles (10) are pre-functionalized with the constituent elements of the alloy, i.e., copper, zinc, and chromium. Functionalization treatment may be carried out using a wet deposition technique applied by immersion (galvanic displacement). Following the treatment, the alloy is located on the surface of the aluminum particles (10), with a discontinuous shell composed of zinc, copper and chrome islands.

[0108] For example, the powder (1) may have the following characteristics: [0109] Theoretical chemical composition: [0110] Al: 92.27-92.45% [0111] Zn: 5.4-5.6% [0112] Cu: 1.5-1.7% [0113] Cr: 0.23-0.25% [0114] Apparent density (Hall): 1.1 g cm.sup.−3

[0115] This powder (1) may then be shaped by SLM. The surface functionalization treatment makes it possible, on the one hand, to improve the laser/material interaction by decreasing the reflectivity of the powder (1), and, on the other hand, to create the alloy in situ by diffusion.

[0116] FIG. 9 illustrates a second embodiment of the powder preparation method (1) in accordance with the invention, comprising at least particles (10) of a first material, having a first optical reflectivity at the wavelength of the laser.

[0117] In this embodiment, the solution for shaping the low-laser metals consists of carrying out a surface chemical and/or physical etching on the grains (4) during the preparation of the powder (1), causing an increase in the surface roughness of the particles (10). This etching is, for example, chemical etching of the support grains in a wet process, using an acid or a base, or thermochemical oxidation/reduction treatments by a gaseous process in a fluidized bed. This makes it possible to lower the optical reflectivity of the grains (4), and therefore that of the powder (1), to the wavelength of the laser. The laser/material interaction thus is improved.

[0118] FIG. 10 illustrate different embodiments of the invention.

[0119] In the first mode M1, functionalization is carried out by germination and then growth of a surface layer (20) on a support particle (10). Sub-mode M21 displays partial functionalization, with a discontinuous layer (20). The sub-mode M22 displays continuous functionalization, with large variations in the thickness of the continuous layer (20). The M23 sub-mode displays continuous functionalization, with small variations in the thickness of the continuous layer (20). In sub-modes M22 and M23, the roughness is controlled.

[0120] In the second mode M2, surface modification is carried out by physical and/or chemical etching.

[0121] In the third mode M3, the particles (10) of the first material undergo functionalization treatment and then the functionalized particles (10+20) undergo physical and/or chemical etching.

[0122] Whatever the description of embodiments of the invention: [0123] the powder initially selected has an optical reflectivity of more than 70% for at least one wavelength ranging between 800 and 1500 nm; [0124] the treatment makes it possible to lower the optical reflectivity of the powder at least over this given wavelength, ranging between 800 and 1500 nm; [0125] the treatment is different from grafting particles.

[0126] The powder (1) may be shaped differently from FIGS. 4 to 10 without moving away from the scope of the invention which is defined in the claims. Furthermore, the technical characteristics of the various variants mentioned in the description may be combined in their entirety or only in part. Thus, the powder (1) may be adapted to the intended applications.