JP7730682B2 - Copper-based powder, its manufacturing method, and manufacturing method of optically modeled object using copper-based powder - Google Patents

Description

The present invention relates to copper-based powder, and more specifically to copper-based powder used in metal laser sintering, a method for producing three-dimensional objects by irradiating them with energy rays such as laser light.

Three-dimensional modeling devices, or 3D printers, that can easily create three-dimensional objects are becoming increasingly popular. Among the methods for manufacturing objects using these three-dimensional modeling devices, powder bed fusion (L-PBF) is known as a method for producing metal objects. In L-PBF, a high-energy laser beam is irradiated onto the surface of a layer of powder to sinter or melt and solidify the powder particles. These layers, each several tens of microns thick, are then stacked and repeatedly bonded to produce three-dimensional objects. L-PBF using metal-based powders is also becoming more practical, and L-PBF using metal-based powders such as Co-Cr alloys, maraging steels, stainless steels, and nickel-based superalloys has achieved high processing precision and a high level of product perfection, leading to its commercialization. However, the current L-PBF method is limited to the metal-based powders that can be used, and the metal products that can be produced are also limited to a certain range.

The main reason for this is the light absorption of the metal-based powder used as the raw material. Specifically, the L-PBF method utilizes the phenomenon in which metal-based powder absorbs the optical energy of a laser beam and heats up, causing the metal-based powder particles to sinter or melt and solidify. Therefore, the metal-based powder must be able to efficiently absorb optical energy. The wavelength of the laser beam typically used in the L-PBF method is in the near-infrared or far-infrared range. Metals with low optical absorption in this wavelength range (e.g., aluminum, gold, silver, copper, etc.) cannot absorb sufficient heat from the laser beam, resulting in low sintered densities of the resulting metal laser-sintered objects. Furthermore, metals with high thermal conductivity quickly dissipate the heat from the laser beam, even if they absorb it as heat, before sufficient sintering or melting and solidification occurs, making it difficult to obtain high-density metal laser-sintered objects. Furthermore, copper's relatively high melting point of approximately 1084°C also makes sintering difficult. For this reason, even though copper is a metal with high thermal and electrical conductivity and excellent workability, it has been considered difficult to apply to the L-PBF method.

For this reason, various studies have been conducted to apply copper-based powder to the L-PBF method. For example, proposals have been made to form a specific metal coating, such as gadolinium (Gd), on the surface of copper particles to increase the laser light absorption rate (Patent Document 1), to form a copper oxide coating (Patent Documents 2 and 3), and even to form a carbon coating derived from an organic compound on the surface of copper or aluminum particles (Patent Document 4).

As described above, metal laser-sintered objects obtained using copper-based powder in which the surface of the copper powder is coated with a metal or metal oxide with high laser light absorption contain substances (coatings) other than copper, and therefore can be endowed with properties not found in pure copper in addition to the various properties inherent to copper (e.g., thermal conductivity, electrical conductivity, and processability). For example, Patent Document 5 proposes that by using copper alloy-based powder containing a predetermined amount of chromium in copper, it is possible to obtain copper alloy-sintered objects with excellent thermal conductivity, electrical conductivity, and strength while increasing the laser light absorption rate.

International Publication No. 2019/064745 International Publication No. 2019/017467 Japanese Patent Application Laid-Open No. 2020-94271 Japanese Patent Application Laid-Open No. 2018-199862 Japanese Patent Application Laid-Open No. 2019-70169

As mentioned above, copper is a metal with excellent thermal conductivity, electrical conductivity, and workability, and is used in a variety of industrial applications. Among these, there are industrial applications that require hardness, such as electrode material for resistance welding and injection molding dies, and there is a potential demand for applying copper molded objects made using the L-PBF method to such applications.

Therefore, the object of the present invention is to provide a copper-based powder that has laser light absorption and fluidity suitable for the L-PBF method, and that can be used to produce shaped objects with high hardness.

The inventors of the present invention have discovered that by adhering carbide to the surfaces of the copper particles (primary particles) that make up the copper powder, it is possible to obtain a molded object that has high hardness and laser light absorption properties suitable for the L-PBF method. The present invention is based on this discovery.

The copper-based powder according to the present invention comprises:
A copper-based powder consisting of particles having an average particle size of 1 μm or more and 100 μm or less,
The particles are
Copper particles,
carbide adhered to at least a portion of the surface of the copper particles;
The reflectance at a wavelength of 1070 nm is 60% or less.

According to the present invention, by adhering carbide to the surfaces of the copper particles that make up the copper-based powder to the extent that the laser light reflectance is 60% or less, not only is the powder provided with laser light absorption properties suitable for the L-PBF method, but it is also possible to obtain highly hard shaped objects while improving the fluidity of the copper-based powder, which is necessary when carrying out the L-PBF method.

[Definition]
In this specification, "copper particles" refers to particles made of copper or a copper alloy, "copper powder" refers to a collection of multiple copper particles, and "copper-based powder" refers to a collection of multiple copper particles having carbides attached to at least a portion of their surfaces.
In addition, "adhesion" refers not only to a state in which the carbide is physically in contact with or fixed to the surface of the copper particle, but also to a state in which the carbide is chemically bonded to the surface, or a state in which part of the carbide is embedded in the copper particle and integrated (alloyed).

[Copper-based powder of the present invention]
The copper-based powder of the present invention comprises copper particles and carbide adhered to at least a portion of the surface of the copper particles, the particles having an average particle size of 1 μm or more and 100 μm or less, and a reflectance of 60% or less at a wavelength of 1070 nm. In the present invention, carbide is adhered to the surfaces of the particles constituting the copper-based powder so that the reflectance of the copper-based powder at a wavelength of 1070 nm is 60% or less, thereby providing laser light absorption suitable for the L-PBF method. Specifically, the light absorption of copper is on the order of a few percent in the wavelength range (1030 nm or more and 1070 nm or less) of Yb fiber laser light typically used in metal laser sintering, making it a metal that does not easily absorb laser light. Furthermore, its thermal conductivity is also significantly higher than that of titanium, iron, nickel, etc., making it difficult to heat by laser light irradiation as is. In contrast, in the present invention, carbide is adhered to the surfaces of the copper particles so that the reflectance at a wavelength of 1070 nm is 60% or less, thereby improving laser light absorption. In the present invention, the upper limit of the reflectance is 60%, preferably 55% or less, more preferably 50% or less, and even more preferably 45% or less. On the other hand, the lower limit of the reflectance is not particularly determined, and a lower value is preferable from the viewpoint of laser light absorption, but from the viewpoint of the feasibility of producing a copper-based powder having carbide adhered thereto, the lower limit is typically 10% or more, more typically 15% or more, and even more typically 20% or more.

In this specification, "reflectance" refers to the spectral reflectance measured using a spectrophotometer equipped with an integrating sphere unit, and refers to the ratio calculated based on the amount of total reflected light measured on the surface being measured (copper-based powder) for light of a specific wavelength, using as a reference the amount of total reflected light in a specific wavelength range of a standard reflector (such as a barium sulfate standard reflector) with known spectral reflectance. The reflectance of commonly used base powders made from copper particles at a wavelength of 1070 nm is approximately 70% to 80%.

Furthermore, with the copper-based powder of the present invention, carbides are attached to the surfaces of the copper particles, making it possible to obtain shaped articles that are harder than those made using copper particles. Furthermore, because the copper shaped articles contain trace amounts of carbides, it is believed that the loss of strength when the shaped articles are placed in high-temperature environments can be suppressed. Therefore, it is possible to obtain shaped articles that are particularly suitable for applications such as electrode materials for resistance welding and injection molding dies.

Furthermore, according to the present invention, the fluidity of the powder is improved by adhering carbide to the surfaces of the copper particles. While the reason for this is unclear, it is believed that the adhesion of carbide to the surfaces of the copper particles results in the formation of minute irregularities on the particle surfaces, improving fluidity. The excellent fluidity of copper-based powders facilitates squeegeeing during stereolithography using the L-PBF method, enabling the preparation of a uniform, flat powder bed. While base powders suitable for the L-PBF method are required to have a fluidity of 5 seconds/50 g or more and 30 seconds/50 g or less, the copper-based powder of the present invention can achieve a fluidity of 30 seconds/50 g or less. Note that fluidity refers to a value measured in accordance with JIS Z 2502.

The copper particles that make up the copper-based powder can be any primary particle with an average particle size of 1 μm or more and 100 μm or less. For example, copper particles can be obtained by wet reduction of copper compounds such as copper acetate or copper sulfate using various reducing agents such as hydrazine. Copper particles can also be obtained by atomization using molten copper.

There are no particular restrictions on the shape of the copper particles, but when used in metal laser sintering processes such as the L-PBF method, a nearly spherical shape is preferable from the perspective of forming a base powder bed with a high powder packing density by squeegeeing. Therefore, it is preferable to use copper particles obtained by atomization. Examples of atomization methods include gas atomization and water atomization, but gas atomization is preferred for the ease of making copper particles more nearly spherical. High-pressure gas atomization makes it possible to obtain copper particles with an even more uniform particle shape that is closer to a sphere.

The copper particles obtained as described above can be classified as needed to ensure uniform particle size. This classification can be easily carried out by using an appropriate classification device to separate coarse and fine particles from the obtained copper powder to achieve the desired average particle size.

In the present invention, by using copper particles having an average primary particle size of 1 μm or more and 100 μm or less as described above, a copper-based powder having an average primary particle size of 1 μm or more and 100 μm or less can be obtained. Using a copper-based powder having an average primary particle size within the above range makes it possible to form a powder bed with a high packing density in a metal laser-sintering method such as the L-PBF method, and also to increase the sintered density of a molded product obtained by sintering or melt-solidifying the copper-based powder. Note that, in this specification, the term "average particle size" refers to the volume-cumulative particle size (D ₅₀ ) at 50% by volume of the cumulative volume as measured by a laser diffraction/scattering particle size distribution measurement method. From the viewpoint of obtaining a fine and dense stereolithographic product and the flowability of the copper-based powder, the average particle size of the copper particles used is preferably 2 μm or more and 80 μm or less, and particularly preferably 3 μm or more and 64 μm or less.

Next, a method for adhering carbide to the surface of the copper particles will be described. The carbide used in the copper-based powder of the present invention is not particularly limited, as long as it is a metal carbide with high light absorption at a wavelength of 1070 nm. However, from the perspective of achieving the effects of the present invention, boron carbide, titanium carbide, vanadium carbide, chromium carbide, manganese carbide, cobalt carbide, nickel carbide, zirconium carbide, niobium carbide, molybdenum carbide, hafnium carbide, tantalum carbide, and tungsten carbide are preferred. Chromium carbide is particularly preferred from the perspective of achieving the effects of the present invention while maintaining the electrical conductivity of copper.

Carbide deposition is achieved by contacting the copper particles with the carbide. Specifically, carbide can be deposited on the surfaces of copper particles by various methods, including chemical vapor deposition methods such as plasma CVD and atomic layer deposition (ALD), physical deposition methods such as sputtering, coating methods in which a carbide-containing paste is applied to the surfaces of copper particles and then fired, and hybridization methods. Among these methods, hybridization is preferred for depositing carbide, as it produces copper-based powder with high sphericity and excellent fluidity. The hybridization method, also known as high-velocity airflow impact deposition, is a technique in which multiple types of powders are dry-dispersed in a high-velocity airflow, and the surfaces of the particles are modified or composited with other types of particles primarily through impact forces. In the present invention, carbide can be deposited on at least a portion of the surfaces of the copper particles by mixing copper particles and carbide particles using the high-velocity airflow impact deposition method.

When using the high-velocity airflow impact method, the carbide preferably has a particulate form. From the perspective of carbide adhesion and obtaining copper-based powder with high sphericity, the carbide particles preferably have an average particle size of 1 nm or more and 300 nm or less, and more preferably 50 nm or more and 150 nm or less. When carbide having such an average particle size is adhered to the surface of copper particles using the high-velocity airflow impact method, it is believed that the presence of the carbide particles will form minute irregularities on the copper surface. Furthermore, the high-velocity airflow impact method also improves the sphericity of the copper particles themselves. As a result, it is believed that the fluidity of the resulting copper-based powder is improved. The carbide particles described above can be obtained by conventional methods, such as static synthesis, dynamic synthesis, and vapor-phase synthesis. In this specification, the average particle size of carbide particles is defined as the average value of particle sizes measured for 20 randomly selected particles using a transmission electron microscope.

From the perspective of improving the fluidity of the copper-based powder, the particle size ratio between the copper particles and carbide particles used (the ratio of the average particle size of the copper particles to the average particle size of the carbide particles) is preferably 100 to 5,000, and more preferably 200 to 3,000.

The carbide is attached to the surface of the copper particles so that the reflectance of the copper-based powder at a wavelength of 1070 nm is 60% or less. While the greater the amount of carbide attached, the greater the reduction in reflectance. From the perspective of improving the hardness of the molded product while maintaining the inherent properties of copper, the proportion of carbide relative to the total copper-based powder is preferably 0.2% by mass or more and 5.0% by mass or less, and more preferably 0.4% by mass or more and 2.0% by mass or less. The amount of carbide attached can be adjusted by the ratio of copper particles to carbide particles when mixing them and the hybridization time. The presence or absence of carbide attachment can be confirmed using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM/EDX) or X-ray diffraction (XRD), among other methods. Depending on the particle size and amount of carbide attached, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), and other methods may be used. Quantitative measurement of the amount of carbide adhesion can be performed using known means such as an X-ray photoelectron spectrometer (XPS), an inductively coupled plasma (ICP) emission spectrometer, or a carbon analyzer.

Furthermore, in the present invention, the adhesion of carbide to the surface of the copper particles improves the stability of the copper particles over time. It is known that metallic copper is extremely susceptible to oxidation by air and moisture, and that the powder properties are prone to change during storage. Even when using copper powder from the same lot, the properties of the copper powder change over time during storage, which can potentially result in changes in the properties of the stereolithography product. According to the present invention, the adhesion of carbide to the surface of the copper particles is thought to inhibit the progression of oxidation reactions with air and moisture, and as a result, it is expected that more stable stereolithography products can be obtained.

[Method of manufacturing a stereolithography object using copper-based powder]
A method for producing a stereolithography object using the copper-based powder described above will now be described.
First, copper base powder is supplied to a modeling stage, and a squeegee blade is used to squeeze the powder surface to form a powder bed of a predetermined thickness (Step 1). Note that, in the present invention, squeegeeing refers to the metal laser sintering method in which a blade, spatula, roller, or the like is brought into contact with and moved against the surface of a powder bed made of base powder to smooth the surface of the powder bed and remove excess base powder.

Next, a light beam, such as a laser beam, is irradiated at any position on the surface of the powder bed. This irradiation position can be determined from a cross-sectional plan view created based on 3D CAD data of the product to be manufactured. Multiple copper particles at the position irradiated with the light beam sinter or melt and solidify together, forming a first layer (Step 2).

Next, the position of the modeling stage is moved by a distance equivalent to the thickness of the first layer (Step 3). Steps 1 to 3 are repeated, and multiple layers are stacked on top of the first layer, in order, with the second and third layers stacked on top, to produce a copper stereolithography object.

Such metal laser sintering devices are typically equipped with an infrared laser as the light beam, and include solid-state lasers with wavelengths in a wavelength band including infrared light at 1064 nm, fiber lasers with wavelengths in the range of 950 nm to 1900 nm, and _CO2 lasers with wavelengths in the range of 10.6 μm. Rare earth elements such as Yb (1030 nm to 1070 nm), Nd (approximately 950 nm), Tm (approximately 1900 nm), and Er (approximately 1550 nm) are commonly used as amplification media for the glass core of fiber lasers. Since the copper-based powder of the present invention has a reflectance of 60% or less at a wavelength of 1070 nm, it is preferable to use a Yb-doped fiber laser with a central wavelength of 1070 nm. The laser irradiation mode may be either single-mode or multimode, although differences in beam quality and focusing ability exist. Furthermore, the above-mentioned sintering method is merely an example of a case in which sintering is used, and is not limited thereto.

The stereolithography product obtained from the copper-based powder in the above manner possesses electrical and thermal conductivity similar to that of a stereolithography product made from copper powder, but is dense and has excellent mechanical strength due to sufficient sintering or melt-solidification caused by laser light absorption. Furthermore, the product has high hardness due to the inclusion of carbides. Specifically, the stereolithography product obtained using the copper-based powder of the present invention can achieve a Vickers hardness (Hv) of 40 Hv or more as measured in accordance with JIS Z 2244. Furthermore, the stereolithography product obtained in this manner is applicable to a variety of applications, but is particularly suitable for applications used in high-temperature environments (e.g., electrode materials for resistance welding, injection molding dies, etc.). It is known that the hardness of a pure copper product decreases due to the growth of copper crystal grains during heat treatment. However, in the present invention, the copper contains carbides, which inhibits crystal grain growth. As a result, the hardness of the product is less likely to decrease even when used in high-temperature environments.

Next, embodiments of the present invention will be described in detail with reference to the following examples, but the present invention is not limited to these examples.

<Preparing copper powder>
The following two types of gas atomized copper powder were prepared.
Copper powder 1: MA-CH-S, manufactured by Mitsui Mining & Smelting Co., Ltd. (pure copper, average primary particle diameter D ₅₀ : 33 μm)
Copper powder 2: MA-CCR25L, manufactured by Mitsui Mining & Smelting Co., Ltd. (copper containing 1.2 wt % Cr, average primary particle diameter D ₅₀ : 28 μm)

<Preparation of carbide>
The following four types of carbides were prepared:
Carbide 1: NP-SIC-8, manufactured by EM Japan Co., Ltd. (silicon carbide (SiC) 99% or more, particle size: <80 nm)
Carbide 2: NP-CR3C2, manufactured by EM Japan Co., Ltd. (chromium carbide (Cr ₃ C ₂ ) 99.7% or more, particle size: 30 to 120 nm)
Carbide 3: NP-ZRC-3, manufactured by EM Japan Co., Ltd. (zirconium carbide (ZrC) 99% or more, particle size: 80 nm)
Carbide 4: NP-WC-1, manufactured by EM Japan Co., Ltd. (tungsten carbide (WC) 99.9% or more, particle size: 55 nm)

<Preparation of copper-based powder>
The copper powder and carbide were weighed out so that the total amount was 1.5 kg, and the two were stirred at 1000 rpm for 10 minutes using a mixer (OMO-3, manufactured by Nara Machinery Works, Ltd.). Next, 150 g of the mixed and stirred product was weighed out and circulated at 6000 rpm for 5 minutes using a hybridizer (HYB-1, manufactured by Nara Machinery Works, Ltd.), thereby obtaining a copper-based powder.
Furthermore, since no carbide was visually observed remaining on the mixer and hybridizer after the copper-based powder was removed, it can be assumed that almost all of the added carbide adhered to the copper powder.

<Measurement of average particle size>
The average particle size (D50) of the obtained copper-based powder was measured using a laser diffraction/scattering particle size distribution analyzer (Microtrac Bell Co., Ltd., MT3300EXII). The measurement results are shown in Table 1 below.

<Reflectance Measurement>
The reflectance of the copper powder and each copper-based powder was measured using a spectrophotometer (U-4100, manufactured by Hitachi High-Technologies Corporation) by filling the copper powder into a concave holder, sealing it with a quartz cover glass, and using the integrating sphere method at a wavelength of 1070 nm. The measurement results are shown in Table 1 below.

<Measurement of liquidity>
Using a flowability measuring device (Tsutsui Scientific Instruments Co., Ltd.), 50 g of each of copper powder and copper-based powder was placed in a funnel, and the flowability (seconds) of the powder was measured according to JIS Z 2502. The measurement results are shown in Table 1 below. Note that "NG" in Table 1 indicates that the powder got stuck when falling from the funnel, and not all of the powder (copper powder or copper-based powder) placed in the funnel fell. Furthermore, with the copper-based powders of Examples 5 and 6, the powder sometimes got stuck in the funnel and stopped midway.

As is clear from Table 1 above, copper-based powder with carbide attached to copper powder has improved fluidity compared to copper powder.

<Production of optically modeled objects>
The copper powder and copper-based powder were each subjected to laser lithography using a metal laser lithography machine (Concept Laser M2, manufactured by GE Additive) at an output of 370 W and a layer pitch of 0.03 mm to produce 15 mm x 15 mm x 10 mm objects. The energy density per unit volume of the laser beam was 600 J/ ^mm3 when copper powder 1 (pure copper) was used, and 160 J/ ^mm3 when copper powder 2 (copper-chromium alloy) was used.
The resulting shaped object was then heat treated in a nitrogen stream at 700° C. for 3 hours.

<Evaluation of characteristics of stereolithography objects>
(1) Vickers Hardness The Vickers hardness (Hv) of the stereolithography objects before and after heat treatment was measured in accordance with JIS Z 2244. In addition, the degree of increase in hardness of the stereolithography objects (after heat treatment) using each copper-based powder compared to the stereolithography object (after heat treatment) using copper powder without carbide adhesion was calculated using the following formula.
Hardness increase rate (%) = (hardness of stereolithography object using copper-based powder - hardness of stereolithography object using copper powder) / hardness of stereolithography object using copper powder × 100
The measurement results are shown in the following Table 2. Note that for Example 5 and Comparative Example 2, only the Vickers hardness (Hv) after the heat treatment was measured.

(2) Relative Density For each stereolithography product after heat treatment, the theoretical density of the product was calculated from the densities of the copper powder and carbide, the bulk density was calculated by Archimedes' method in accordance with JIS-R1634, and the relative density was calculated from the ratio of the bulk density to the theoretical density. The results are shown in Table 2 below.

(3) Electrical Conductivity The electrical conductivity (100% IACS) of each of the stereolithographic objects after the heat treatment was measured using an eddy current phase displacement sensitive measuring device (SIGMASCOPE SMP350, manufactured by Fischer Instruments Co., Ltd.) The results are shown in Table 2 below.

As is clear from Table 2, the hardness of the resulting stereolithography product increases when carbide is attached to the copper powder. Furthermore, although the hardness of the stereolithography product decreases upon heat treatment, the stereolithography product obtained from the copper powder to which carbide is attached (i.e., the copper-based powder) has a higher hardness retention rate than the stereolithography product obtained from the copper powder to which carbide is not attached (Comparative Examples 1 and 2).
Furthermore, it can be seen that even stereolithography objects obtained from copper powder with carbide attached (i.e., copper-based powder) have electrical conductivity equivalent to that of stereolithography objects obtained from copper powder without carbide attached.

Claims (9)

A copper-based powder consisting of particles having an average particle size of 1 μm or more and 100 μm or less,
The particles are
Copper particles,
and a metal carbide adhered to at least a portion of the surface of the copper particle,
A copper-based powder having a reflectance of 60% or less at a wavelength of 1070 nm. 2. The copper-based powder according to claim 1, wherein the proportion of the metal carbide relative to the total copper-based powder is 0.2 mass % or more and 5.0 mass % or less. 3. The copper-based powder according to claim 1, wherein the metal carbide is at least one selected from the group consisting of boron carbide, titanium carbide, vanadium carbide, chromium carbide, manganese carbide, cobalt carbide, nickel carbide, zirconium carbide, niobium carbide, molybdenum carbide, hafnium carbide, tantalum carbide, and tungsten carbide. 4. The copper-based powder according to claim 1, wherein particles made of the metal carbide adhere to the surfaces of the copper particles, so that at least a portion of the surfaces are coated with the metal carbide. 5. The copper-based powder according to claim 4, wherein the metal carbide particles have an average particle size of 1 nm or more and 300 nm or less. A method for producing the copper-based powder according to any one of claims 1 to 5, comprising:
bringing a metal carbide into contact with copper particles to cause the metal carbide to adhere to the surfaces of the copper particles;
A method comprising: The contact of the metal carbide with the copper particles is
mixing the copper particles and the metal carbide particles by impacting them in a high-velocity air stream;
sputtering the metal carbide onto the copper particles; or applying the metal carbide-containing paste onto the surfaces of the copper particles;
The method of claim 6, which is carried out by The method of claim 6 or 7, wherein the copper particles have an average particle size of 1 μm or more and 100 μm or less. A method for manufacturing a stereolithography object using copper-based powder, comprising a step of sintering or melting and solidifying the copper-based powder according to any one of claims 1 to 5 using laser light.