Forming metal powder

Abstract

Provided is forming metal powder that, in additive manufacturing, such as a powder bed fusion method in which metal powder that has been spread out in advance in a layer is locally melted and fused or sintered, by means of a laser, electron beam, or other heat source to yield a shape, or a fused deposition method in which metal powder, while being jetted, is melted and fused by means of a heat source, is capable of maintaining the fluidity of the forming metal powder from when the forming metal powder is manufactured until when the forming metal powder is employed by being spread out in a layer or by being jetted.　In the forming metal powder, 17 or more flow aids having a 70 nm or less primary particle diameter are adhered to the surface per 1 μm2, and preferably, no more than 400 of the adhering flow aids are adhered.

Description

Metal powder for modeling

[0001] The present invention relates to a metal powder for molding that can be applied to, for example, additive manufacturing methods.

[0002] Additive manufacturing (AM) using metal powder as a raw material is known as a method for directly obtaining three-dimensional parts. It can be broadly divided into two types: fused deposition modeling, in which a powder that has been laid out in layers (powder bed) is locally melted, solidified, or sintered using a heat source such as a laser or electron beam to obtain a shape, and fused deposition modeling, in which powder is sprayed onto the material and melted and solidified using a heat source. In either method, a three-dimensional additive object can be formed by melting, solidifying, or sintering the powder.

[0003] In additive manufacturing, the metal powder is required to have good fluidity, as it is spread or sprayed in layers. For example, Patent Document 1 discloses that the fluidity of the powder for additive manufacturing can be improved by including less than 1% of a second powder that has high sphericity and an average diameter that is 1 / 10 or less of the main metal powder.

[0004] WO2016 / 031279 publication

[0005] According to the inventors' investigations, when using a metal powder for molding in which a second powder with high sphericity and an average diameter of 1 / 10 or less is added at a rate of less than 1% to the main metal powder made of the above-mentioned metals as a flow aid and then spread or sprayed in layers, it has been confirmed that the main metal powder, i.e., the metal powder for molding, may not flow.

[0006] An object of the present invention is to provide a metal powder for shaping that can maintain fluidity from the time of production of the metal powder for shaping to the time of use, such as spreading or spraying in layers.

[0007] The present inventors have conducted extensive research into the adhesion state of flow aids to the surface of metal powders for shaping in order to maintain the fluidity of the metal powders for shaping, and have found that fluidity can be maintained by setting the size of the flow aids and the number of particles attached per unit area within specific ranges, thereby arriving at the present invention.

[0008] The metal powder for molding of the present invention is characterized in that the flow aid having a primary particle diameter of 70 nm or less is ２ There are 17 or more attached to the winning surface.

[0009] The metal powder for molding of the present invention is characterized in that the flow aid having a primary particle diameter of 70 nm or less is ２ It is preferable that 400 or less particles are attached to the surface of each particle.

[0010] The present invention can maintain the fluidity of metal powder for shaping, making it a useful technology for additive manufacturing, in which metal powder for shaping is spread or sprayed in layers and melted, solidified, or sintered to obtain a three-dimensional additively shaped object.

[0011] FIG. 2 is a diagram showing an example of a method for evaluating the number of flow aid particles adhering to the surface of a metal powder for molding.

[0012] The metal powder for molding of the present invention is characterized in that the flow aid having a primary particle diameter of 70 nm or less is ２ In this way, the metal powder for molding of the present invention can prevent the flow aids from agglomerating together during handling such as packaging and transportation in the production of the metal powder for molding, thereby maintaining fluidity. In addition, by preventing direct contact between the metal powder particles for molding, frictional resistance can be reduced, and the flowability of the metal powder for molding can be ensured up to the time of use, when the metal powder for molding is laid out in layers or sprayed.

[0013] If the entire surface of the metal powder for molding is covered with the flow aid, the aggregation of the flow aid is promoted during handling such as packaging and transportation during the production of the metal powder for molding, and the metal powder for molding increases the chance of contact with other particles of the metal powder for molding, which may impair the flowability. ２ Preferably, 400 or less particles are attached to the surface per unit area, more preferably 200 or less particles, and even more preferably 100 or less particles. In addition, from the viewpoint of ensuring flowability, the flow aid attached to the surface of the metal powder for molding of the present invention preferably has a primary particle diameter of 1 nm or more.

[0014] The metal powder for molding according to the embodiment of the present invention preferably has a 50% particle size (hereinafter referred to as "D50") of the cumulative particle size distribution on a volume basis of 10 μm or more and 250 μm or less. By setting the D50 to 10 μm or more, the metal powder for molding according to the embodiment of the present invention is useful in that it suppresses the metal powder from rolling up or floating during handling or additive manufacturing, as well as suppressing oxidation reactions on the metal powder surface. Furthermore, by setting the D50 to 250 μm or less, the metal powder for molding according to the embodiment of the present invention is useful in that it facilitates smoothing the surface of the solidified layer formed during additive manufacturing. A smooth solidified layer surface is useful in that it facilitates laying the next metal powder for molding on the solidified layer. Furthermore, by setting the D50 to 250 μm or less, the metal powder for molding according to the embodiment of the present invention can reduce the output of the heating means for melting, making it easier to control the melting rate of the metal powder for molding and the range of the heated area during localized heating, and is useful in that it facilitates ensuring the molding precision and uniformity of the solidified structure of the resulting part. The cumulative particle size distribution of the metal powder for molding of the present invention is expressed as a cumulative volumetric particle size distribution, and its D50 is expressed as a value measured by the laser diffraction scattering method specified in JIS Z 8825.

[0015] The material of the metal powder for molding of the present invention is appropriately selected depending on the characteristics of the component to be obtained and the additive manufacturing method. Examples include alloys containing Al, Co, Cr, Fe, Ni, W, etc., and are not particularly limited as long as they are metallic materials. The metal powder for molding of the present invention can be obtained by atomization, melt spinning, rotating electrode milling, pulverization, reduction, electrolysis, chemical synthesis, or other methods, and can be appropriately selected depending on the material of the component to be obtained, the production volume, etc.

[0016] The flow aid used in the metal powder for shaping of the present invention is preferably in the form of a powder composed of, for example, an inorganic substance, metal, resin material, etc., and adheres to the surface of the metal powder for shaping. The material and primary particle diameter of the flow aid can be appropriately selected depending on the material and D50 of the metal powder for shaping. Among these, it is preferable to use silica or alumina powder, and silica is more preferable from the viewpoints of ease of availability and handling, in addition to its excellent effect of reducing the attractive forces between the metal powders for shaping. To obtain the metal powder for shaping of the present invention, the metal powder and flow aid obtained above are mixed in a ball mill, and a flow aid having a primary particle diameter of 70 nm or less is milled to a diameter of 1 μm. ２ The metal powder for molding and the flow aid are preferably mixed in a container made of, for example, polypropylene, with the metal powder therein at a volume fraction of 20% by volume, and the mixing process is preferably carried out at a rotation speed of 10 to 100 rpm.

[0017] An example of a method for evaluating the number of flow aids adhering to the surface of a metal powder for shaping according to the present invention is described below. First, the metal powder for shaping is photographed using a scanning electron microscope (hereinafter also referred to as "SEM") to capture images of the flow aids adhering to the surface of the metal powder for shaping, thereby obtaining surface morphology images (SEM images). The SEM used can be, for example, a ZEISS Ultra 55. Observations are performed using an upward-looking secondary electron (Inlens SE) detector, an accelerating voltage of 5 kV, and a working distance of 5 mm.

[0018] The SEM observation magnification was determined to be 15,000x based on the particle size of the flow aid and the number of flow aids present per field of view. The surface morphology of several different metal powders for molding was then evaluated. The image size of the surface morphology image (SEM image) per field of view was set to 1,024 x 768 pixels.

[0019] The image processing involves using public domain image processing software (ImageJ) to perform image processing, including binarization, on the multiple (here, three) surface morphology images (SEM images) obtained above to obtain binary image data. More specifically, the process includes a background removal step (S1) to remove the background, a smoothing step (S2) using Gaussian filtering, a histogram adjustment step (S3) to normalize the image brightness to align the brightness of multiple images, a binarization step (S4) to obtain binary image data by binarization, and a flow aid extraction step (S5). Each step is described in detail below with reference to FIG. 1.

[0020] S1: Background Removal Step The background removal step is a step in which background information is removed by utilizing the local contrast difference between the original image [Figure 1(1)] and an image obtained by applying an average filter process to the original image. In other words, by taking the difference between the original image and an image obtained by smoothing the original image, unnecessary background area data, such as information about the surface shape of the metal powder used for molding, contained in the image is removed from the image data, thereby extracting data in which the contrast of the flow aid is emphasized. [Figure 1(2)] In this case, the filter size of the averaging filter is 50 x 50 pixels.

[0021] S2: Smoothing Step The smoothing step is a step in which the image processed in the background removal step (S1) is smoothed by applying Gaussian filter processing using the following formula 1. Images acquired with an SEM are generally known to contain Gaussian noise, and if such images are used for binarization processing, the influence of noise makes it impossible to accurately identify the number of particles. Since the particle signal intensity is greater than the Gaussian noise, the influence of noise is reduced by smoothing the image. [Figure 1 (3)]

[0022]

[0023] S3: Histogram Adjustment Step The histogram adjustment step is a process for adjusting the average brightness and standard deviation of brightness values ​​of the image processed in the smoothing step (S2) to arbitrary values ​​using the following formula 2. When evaluating multiple images with different fields of view, it is necessary to adjust the brightness values. In SEM observation, brightness values ​​are adjusted visually by the operator, which causes variations in the average brightness and variance of brightness values ​​of the images. Therefore, image processing is used to adjust the average brightness and standard deviation of brightness values ​​of multiple images to arbitrary values. [Figure 1 (4)]

[0024]

[0025] In the above formula 2, x ｉｎ , x ｏｕｔ represents the luminance value of each pixel before and after processing by the histogram adjustment step. Also, m represents the average luminance value of the image before processing by the histogram adjustment step. Also, s represents the variance of the luminance values ​​of the image before processing by the histogram adjustment step. And, m ０ represents the average brightness value of the image after being processed by the histogram adjustment step, and in this case, a value of 125 is used. ０ represents the variance of the brightness values, and in this example, a value of 50 is used.

[0026] S4: Binarization Step In the binarization step, a threshold value for the image brightness is set and binarization processing is performed to obtain a binarized image. At this time, a value of 200 is set as the threshold value to obtain an output. [Figure 1 (5)]

[0027] S5: Flow Aid Extraction Step The flow aid extraction step is a step in which a particle size threshold is set for the image processed in the binary data acquisition step (S4) to extract the flow aid. In this flow aid extraction step, a value of 10 nm to 73 nm is set as the threshold for the primary particle diameter of the flow aid, which contributes to fluidity, and the number of flow aids is measured. The total number of particles measured and the measurement results of the primary particle diameter of the flow aid are then output to a spreadsheet program, and the binary image data after image processing is saved. It is preferable to also store an image in which the particle shape outline is superimposed on the original image. [Figure 1 (6)] Furthermore, the number of flow aids, which are the object of detection, can be extracted using an SEM upper secondary electron image. At this time, the number of flow aids with a primary particle diameter of 70 nm or less attached to the surface of the metal powder in each field of view is measured from the surface morphology images (SEM images) obtained above in any three fields of view, and the average value for the three fields of view is calculated.

[0028] First, metal powder having a component composition of Alloy 718 was produced by gas atomization, and then sieved and classified using a mesh to obtain metal powder with a particle size D50 in the range of +10 μm / −56 μm.

[0029] Next, silica powder with a primary particle diameter of 1 nm [Aerosil (registered trademark) 300, manufactured by Nippon Aerosil Co., Ltd.] was added as a flow aid to the metal powder obtained above, and mixed using a ball mill. Here, the metal powder and flow aid were mixed using a polypropylene container, the metal powder was placed in a container volume fraction of 20%, and the rotation speed was set to 60 rpm to perform the mixing process. Then, according to the above-mentioned method for evaluating the adhesion state of the flow aid, the surface of the metal powder obtained above was measured using a backscattered electron image of a scanning electron microscope at an arbitrary 38.93 μm ２Three visual fields were observed, and the number of flow aids with a primary particle diameter of 70 nm or less adhering to the surface of the metal powder in each visual field was counted, and the average value for the three visual fields was calculated. Furthermore, the fluidity of the metal powder obtained above was measured using the flowability measurement method specified in JIS Z 2502. Furthermore, the bulk density of the metal powder obtained above was measured using the apparent density measurement method specified in JIS Z 2504. The results are shown in Table 1.

[0030]

[0031] The metal powders of Comparative Examples 1 to 3 all contained a flow aid having a primary particle diameter of 70 nm or less at a concentration of 1 μm. ２ Only 16 or less particles adhered to the surface of the powder. Therefore, it was confirmed that the powder did not flow in the evaluation of fluidity. All of the metal powders of Comparative Examples 1 to 3 had a bulk density of 4.40 g / cm ３ In contrast, in the metal powders for molding of Examples 1 to 6 of the present invention, the flow aid having a primary particle diameter of 70 nm or less was confirmed to be less than 1 μm ２ It was confirmed that 17 or more particles adhered to the surface per 1000 particles. Furthermore, all of the metal powders for molding of Inventive Examples 1 to 6 had a fluidity of 18.0 s / 50 g or less, demonstrating their usefulness as metal powders for molding with excellent spreading and jetting properties. In particular, several Inventive Examples, including Inventive Example 1, demonstrated excellent fluidity of 16.0 s / 50 g or less or 15.5 s / 50 g or less. Furthermore, all of the metal powders for molding of Inventive Examples 1 to 6 had a bulk density of 4.45 g / cm. ３ In particular, in some of the examples of the present invention, including Example 1, the density was greater than 4.50 g / cm ３ or more than 4.55 g / cm ３ From the above, it was confirmed that the metal powder for molding of the present invention can maintain fluidity from the time of production of the metal powder for molding to the time of use, such as spreading or spraying in layers, and is useful as a metal powder for molding with excellent packing properties.

Claims

1. A flow aid having a primary particle diameter of 70 nm or less is ２ Metal powder for molding with 17 or more particles adhering to the surface of the ball.

2. A flow aid having a primary particle diameter of 70 nm or less is ２ 2. The metal powder for molding according to claim 1, wherein 400 or less particles adhere to the surface of each particle.

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