JP7540256B2 - W powder - Google Patents

Description

The present invention relates to W (tungsten) powder suitable for near-net molding, such as additive manufacturing (AM) and injection molding (MIM).

W, a high melting point metal, has a low thermal expansion coefficient and high thermal conductivity, and is therefore used in electrodes of electronic devices, heating elements and reflectors of high-temperature furnaces, etc. In addition, W has better radiation shielding properties than Pb (lead) for the same volume, and is therefore also used in radiation shielding materials and components of X-ray inspection equipment, such as collimators that absorb scattered radiation.
Furthermore, since W has poor machinability, it is known that for complex shaped products such as the above-mentioned collimator, in addition to a sintering method using W powder, a molded object is produced by near-net molding such as the AM method or MIM method, and then this molded object is subjected to cutting processing or the like to obtain a product with the final shape.

Here, since W has a high melting point, it is difficult to obtain the W powder used in the near-net molding by gas atomization or water atomization. For this reason, for example, Patent Document 1 proposes a method for producing W powder using thermal plasma. Patent Document 1 describes a method for generating high-temperature plasma using nitrogen gas under reduced pressure, melting the W raw material with this plasma, and continuously collecting and recovering ultrafine W particles suspended in the ambient gas flow. This is a useful technology in that it can be produced on an industrial scale, i.e., in large quantities and at low cost.

Japanese Patent Application Publication No. 2-6339

The inventors have confirmed through their research that when attempting to obtain W powder for use in the above-mentioned near-net molding using the method of Patent Document 1, the W powder may aggregate. This problem of W powder aggregation reduces the fluidity that affects the supply and filling properties of the W powder during near-net molding, which stably supplies the required amount of W powder to the molding device and molding location, and prevents the resulting product from having high strength.

The object of the present invention is to provide a W powder for molding suitable for near-net molding, which has excellent flowability that affects supplyability and filling properties and is suppressed from agglomeration.

The inventors focused on the surface properties to address the problem of W powder agglomeration, and discovered that by setting the reflectance measured at a specific wavelength within a specified range, it is possible to significantly improve the fluidity that affects the supply and filling properties of W powder, thus arriving at the present invention.

The W powder of the present invention has a composition consisting of W and inevitable impurities, a minimum reflectance measured in the wavelength range of 360 to 740 nm of 10% or more, and a flowability measured in accordance with JIS Z 2502 of 15 s/50 g or less.

The W powder of the present invention preferably has a circularity of 0.85 or more.

The W powder of the present invention preferably contains oxygen in the range of 0.001 to 0.020 mass%.

The W powder of the present invention preferably has a lightness L* of 40 or more as measured in accordance with JIS Z 8781-4:2013.

The present invention can provide W powder suitable for near-net molding, with suppressed aggregation and excellent fluidity that affects supplyability and filling properties, making it a useful technology for, for example, the manufacture of X-ray inspection equipment.

1 is a scanning electron microscope photograph showing the appearance of W powder according to an embodiment of the present invention and a comparative example.

The W powder of the present invention has a minimum reflectance of 10% or more measured in the wavelength range of 360 to 740 nm. In near-net molding such as the AM method and MIM method described above, a powder aggregate having a 50% particle size (hereinafter referred to as "D50") of the cumulative particle size distribution based on volume in the range of 1 to 250 μm is used. Here, an aggregate of W powder for molding containing ultrafine particles with a D50 smaller than 1 μm is prone to adhesion of ultrafine particles with a particle size of 0.3 μm or less or aggregates of ultrafine particles with a particle size of 0.3 μm or less to the surface of W powder (hereinafter also referred to as "W powder for molding") that contributes to molding and has a particle size of 1 to 250 μm. If a large amount of ultrafine particles adhere to the surface of the W powder for molding, many fine irregularities are formed on the surface of the W powder for molding.

In an aggregate of W powder for molding having a large amount of ultrafine particles attached to the surface, the ultrafine particles cause diffuse reflection and absorption of light, resulting in a large drop in reflectance. For this reason, the W powder of the present invention has a minimum reflectance value measured in the wavelength range of 360 to 740 nm of 10% or more. This suppresses the formation of fine irregularities due to the ultrafine particles attached to the surface of each W powder for molding. For this reason, the W powder of the present invention is suppressed from agglomerating, and can improve the flowability that affects the supplyability and filling properties. For the same reasons as above, the minimum reflectance value of the W powder according to the embodiment of the present invention is preferably 13% or more, more preferably 13.5% or more.
In addition, the reflectance measured in the wavelength range of 360 to 740 nm in the present invention can be measured in accordance with JIS Z 8781-4:2013 using, for example, a spectrophotometer (CM-2500d) manufactured by Konica Minolta.

The W powder of the present invention has a flowability of 15 s/50 g or less as measured in accordance with JIS Z 2502. The flowability is expressed as the time it takes for 50 g of powder to pass through a funnel equipped with an orifice having a diameter of 2.5 mm. The W powder according to the embodiment of the present invention is assumed to flow in view of its intended use, and from the viewpoint of stably supplying a required amount of powder to a molding apparatus during molding, the flowability is preferably 14 s/50 g or less, more preferably 12 s/50 g or less, and even more preferably 10 s/50 g or less.
In measuring the flow rate, as specified in JIS Z 2502, if the powder does not flow out even when the orifice is opened, the funnel may be lightly tapped once to make the powder flow out.

The W powder according to the embodiment of the present invention may have any shape as long as it can obtain the desired supplyability and filling properties, but it is preferable that the powder be spherical. Specifically, the circularity of the powder is preferably 0.85 or more, and more preferably 0.95 or more.
The circularity in the present invention is calculated by the following formula, and if the powder is truly spherical, the circularity is 1.0. From the viewpoint of improving feedability and packing property, the higher the circularity, the better, but from the viewpoint of manufacturability such as an increase in the number of steps due to classification and a decrease in yield, the circularity is preferably 0.99 or less, and more preferably 0.98 or less.
[Circularity] = 4π [projected area of particle] / [perimeter of particle] ²
Here, the projected area and perimeter of the particle can be measured using, for example, an image analyzer.

The W powder according to the embodiment of the present invention preferably contains 0.001 to 0.020 mass % of oxygen. Oxygen may combine with impurities contained in the W powder or with the W of the base material to form oxides. These oxides are taken into the molded product and remain as defects, reducing the mechanical properties of the molded product. For this reason, the W powder according to the embodiment of the present invention preferably has an oxygen content of 0.020 mass % or less.
On the other hand, if the oxygen concentration of the W powder is lower than 0.001% by mass, there is a risk of heat generation due to rapid oxidation when exposed to the air. Therefore, the W powder according to the embodiment of the present invention preferably has an oxygen content of 0.001% by mass or more.

The W powder according to the embodiment of the present invention preferably has a lightness L* of 40 or more as measured in accordance with JIS Z8781-4:2013.
In an aggregate of W powder for molding having a large amount of ultrafine particles attached to the surface, the ultrafine particles cause diffuse reflection and absorption of light, resulting in a significant decrease in lightness L*. For this reason, the W powder of the present invention has a lightness L* of 40 or more. This suppresses the formation of fine irregularities due to the ultrafine particles attached to the surface of each W powder, suppresses aggregation of the W powder, and improves supplyability and filling properties. For the same reasons as above, the lightness L* of the W powder according to the embodiment of the present invention is preferably 45 or more.
In addition, the lightness L* in the present invention can be measured in accordance with JIS Z 8781-4:2013 using, for example, a spectrophotometer (CM-2500d) manufactured by Konica Minolta.

The W powder of the present invention can be, for example, W raw material powder obtained by reducing W oxide powder, or W raw material powder obtained by cutting and crushing an ingot material and classifying it into an appropriate D50 range of 1 to 250 μm using a near-net molding method. In the present invention, in order to obtain W powder with a minimum reflectance of 10% or more measured in the wavelength range of 360 to 740 nm, it is preferable to add and mix a flow aid as necessary. The flow aid is preferably alumina or silica powder with a D50 of 1 μm or less, and is preferably added in the range of 50 mass ppm or less.

Moreover, by subjecting the above-mentioned W raw material powder to a spheroidizing treatment using thermal plasma, it is possible to obtain a W powder with high fluidity and suppressed aggregation. Here, the spheroidizing treatment is a treatment method in which the above-mentioned W raw material powder is passed through a thermal plasma flame, and droplets that are heated and melted are solidified to obtain spherical W powder. The molten droplets are solidified in a spherical state due to the action of surface tension, so it is possible to obtain W powder that is close to a perfect sphere and has few satellites.
Here, it is preferable to use a mixture of hydrogen gas and an inert gas as the operating gas for the thermal plasma flame. By using hydrogen gas as the operating gas, a thermal plasma flame with a higher energy density can be generated, and the melting and spheroidization of the W raw material powder can be promoted. In addition, the oxygen content of the W powder can be reduced by the reduction action of hydrogen.

The thermal plasma flame has a temperature distribution, and in the high temperature region, it reaches nearly 10,000°C. The W raw material powder passing through such a high temperature region may not only melt, but also partially evaporate and volatilize to gasify, and may generate highly adhesive ultrafine particles with a particle size of 0.3 μm or less when solidified. The amount of ultrafine particles generated is affected by the state of the thermal plasma flame during the spheroidizing treatment, pressure fluctuations in the reaction furnace, etc.
In particular, when the internal pressure of the reactor is reduced during the spheroidizing treatment, the vapor pressure temperature is reduced, promoting the vaporization of the W raw material powder and increasing the amount of ultrafine particles adhering to the obtained W powder. On the other hand, when the internal pressure of the reactor is pressurized during the spheroidizing treatment, a thermal plasma flame with a higher energy density is formed, promoting the spheroidization of the W raw material powder while also promoting the vaporization of W, increasing the amount of ultrafine particles adhering to the obtained W powder. For this reason, when the W powder of the present invention is obtained by a spheroidizing treatment using thermal plasma, it is preferable to adjust the pressure fluctuation (variation) [(maximum value - minimum value) / (maximum value + minimum value)] x 100 (%) in the reactor during the spheroidizing treatment to 5.0% or less.

Furthermore, W powder with a large amount of ultrafine particles attached thereto has fine irregularities on its surface, which makes it more likely to aggregate and reduces its flowability, which affects its supplyability and filling ability. Therefore, in order to obtain the W powder of the present invention, it is preferable to use W powder that has been subjected to a spheroidizing treatment as W elemental powder and to remove the ultrafine particles attached to the surface.
In order to reduce the ultrafine particles adhering to the surface, it is preferable to carry out classification processing using, for example, an air classifier. This air classifier can reduce the ultrafine particles by, for example, using a precision air classifier and setting the rotor rotation speed to 2000 rpm or more. The rotor rotation speed is preferably 3000 rpm or more, and more preferably 4000 rpm or more.

To reduce the amount of ultrafine particles adhering to the surface, the W powder may be immersed in pure water or alcohol and washed by shaking or applying ultrasonic waves. Since ultrafine particles have a large specific surface area and a high oxygen concentration, they may be heat-treated in a reducing atmosphere containing hydrogen.
In order to obtain the W powder of the present invention, it is not necessary to remove all of the ultrafine particles from the surface of the W powder. It is sufficient to reduce the amount of attached ultrafine particles so that the minimum reflectance measured in the wavelength range of 360 to 740 nm is 10% or more.

A commercially available W powder was classified and adjusted so that the aggregate had a D50 of 11 μm to prepare a W raw material powder. A total of 30 mass ppm of silica powder with a D50 of 1 μm or less was added as a flow aid to this W raw material powder and mixed until the flow rate measured in accordance with JIS Z5052 was 15 s/50 g or less, thereby obtaining the W powder of Example 1 of the present invention.

A commercially available W powder was classified and adjusted so that the aggregate had a D50 of 11 μm to prepare a W raw material powder. This W raw material powder was subjected to a spheroidizing treatment using a high-frequency induction thermal plasma device to obtain a W powder having a D50 of 11 μm, which is an example 2 of the present invention.
The operating conditions were as follows: plasma power was 120 kW, argon (flow rate = 260 L/min) and hydrogen (flow rate = 30 L/min) were adjusted as operating gases, and argon (flow rate = 8 L/min) was used as supply gas. At this time, the pressure fluctuation in the reactor during the spheroidization treatment [(maximum value - minimum value) / (maximum value + minimum value)] x 100 (%) was adjusted to be 2.0%.

A commercially available W powder was classified and adjusted so that the aggregate had a D50 of 11 μm to prepare a W raw material powder. This W raw material powder was subjected to a spheroidizing treatment using a high-frequency induction thermal plasma device to obtain W elementary powder.
The operating conditions were as follows: plasma power was 120 kW, argon (flow rate = 260 L/min) and hydrogen (flow rate = 30 L/min) were adjusted as operating gases, and argon (flow rate = 8 L/min) was used as supply gas. At this time, the pressure fluctuation in the reactor during the spheroidization treatment [(maximum value - minimum value) / (maximum value + minimum value)] x 100 (%) was adjusted to be 5.6%.
Next, the spheroidized W powder was classified using a precision air classifier (TC-25) manufactured by Nisshin Engineering Co., Ltd., with the rotor rotation speed set to 4000 rpm, to obtain W powder of Example 3 of the present invention having a D50 of 11 μm.

A commercially available W powder was classified and adjusted so that the aggregate had a D50 of 11 μm to prepare a W raw material powder. This W raw material powder was subjected to a spheroidizing treatment using a high-frequency induction thermal plasma device to obtain W elementary powder.
The operating conditions were as follows: plasma power was 120 kW, argon (flow rate = 260 L/min) and hydrogen (flow rate = 30 L/min) were adjusted as operating gases, and argon (flow rate = 8 L/min) was used as supply gas. At this time, the pressure fluctuation in the reactor during the spheroidization treatment [(maximum value - minimum value) / (maximum value + minimum value)] x 100 (%) was adjusted to be 2.0%.
Next, the spheroidized W powder was classified using a precision air classifier (TC-25) manufactured by Nisshin Engineering Co., Ltd., with the rotor rotation speed set to 4000 rpm, to obtain W powder having a D50 of 11 μm, which is Example 4 of the present invention.

A commercially available W powder was classified and adjusted so that the aggregate had a D50 of 11 μm to prepare a W raw material powder. This W raw material powder was subjected to a spheroidizing treatment using a high-frequency induction thermal plasma device to obtain a W powder having a D50 of 11 μm as a comparative example.
The operating conditions were as follows: plasma power was 120 kW, argon (flow rate = 260 L/min) and hydrogen (flow rate = 30 L/min) were adjusted as operating gases, and argon (flow rate = 8 L/min) was used as supply gas. At this time, the pressure fluctuation in the reactor during the spheroidization treatment [(maximum value - minimum value) / (maximum value + minimum value)] x 100 (%) was adjusted to be 5.6%.

For each of the W powders obtained above, (1) reflectance, (2) fluidity, (3) circularity, (4) oxygen content, and (5) lightness L* were measured. The results are shown in Table 1.
(1) Reflectance and (5) Lightness L*
Measurements were performed using a Konica Minolta spectrophotometer (CM-2500d) in accordance with JIS Z 8781-4:2013. The reflectance and lightness L* were measured by placing a spacer having a thickness of 1 mm between the detector and the powder sample to prevent the powder from adhering to the detector. The reflectance was determined as the minimum value of the reflectance measured in the wavelength range of 360 to 740 nm.
(2) Flowability: Measured according to a method in accordance with JIS Z 2502.
In addition, in the case of the W powder of the invention examples 2 to 4 and the comparative example, the powder did not flow out even when the orifice was opened, so the funnel was lightly tapped once to make the powder flow out before the measurement.
(3) Circularity The circularity was determined by measuring 20,000 particles using a particle image analyzer (Morphologi G3) manufactured by Malvern Instruments.
(4) Oxygen Content The oxygen content was analyzed by inert gas fusion-infrared absorption method.

As shown in Figure 1, the W powder used as the comparative example had a large number of ultrafine particles (light-colored distribution) with a particle diameter of 0.3 μm or less observed on the surface. In addition, the W powder used as the comparative example also had areas where the W powder for molding had aggregated together. Even though the fluidity of this W powder was 15 s/50 g or less, the reflectance was below 10%. Furthermore, the W powder used as the comparative example had an oxygen content of more than 0.020 mass% and a lightness L* of less than 40.

On the other hand, it was confirmed that the adhesion of ultrafine particles having a particle diameter of 0.3 μm or less was suppressed in all of the W powders of the present invention, as shown in Figure 1. The W powders of the present invention all had a reflectance of 10% or more and a flowability of 15s/50g or less. It was also confirmed that the W powders of the present invention all had an oxygen content of 0.020 mass% or less, a lightness L* of 40 or more, and were W powders suitable for near-net molding, with excellent handling properties such as supplyability and filling properties, and with suppressed aggregation of the W powders for molding.

Claims (3)

A W powder having a composition consisting of W and inevitable impurities, the W powder having a minimum reflectance of 10% or more as measured in a wavelength range of 360 to 740 nm, a lightness L* of 40 or more as measured in accordance with JIS Z 8781-4:2013, and a flowability of 14 s/50 g or less as measured in accordance with JIS Z 2502. The W powder according to claim 1, having a circularity of 0.85 or more. The W powder according to claim 1 or 2 contains 0.001 to 0.020 mass% oxygen.