Metal powder for additive manufacturing

Abstract

To provide metal powder with a small content of needle-like particles, the metal powder being suitable for a deposition-type additive manufacturing device that supplies the powder from a nozzle.SOLUTION: The present invention provides metal powder for additive manufacturing, wherein the ratio of the number of powder particles with an aspect ratio of 0.4 or less and a maximum longitudinal diameter of 150 μm or greater is 0.30% or less with respect to the total number of particles constituting the powder.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a metal powder suitable for metal additive manufacturing, and more particularly to a metal powder suitable for metal additive manufacturing that is also suitable for a deposition method. [Background technology]

[0002] (Metal Additive Manufacturing Background) In recent years, additive manufacturing methods have begun to be applied to the production of metal objects. Representative metal additive manufacturing methods include the powder bed method (powder bed fusion method) and the deposition method (directed energy deposition method).

[0003] (Explanation of the powder bed method) In the powder bed method, the irradiated areas of the spread powder are melted and solidified by irradiation with a laser beam or electron beam. This melting and solidification causes the powder particles to bond together. Irradiation is selectively applied to parts of the metal powder, and the unirradiated areas do not melt, forming a bonded layer only in the irradiated areas.

[0004] New metal powder is then laid on top of the bonded layer, and the powder is irradiated with a laser or electron beam. The irradiation melts and solidifies the metal particles, forming a new bonded layer. The new bonded layer also bonds with the existing bonded layer.

[0005] As the melting and solidification processes caused by irradiation are repeated, an aggregate of bonding layers gradually grows. This growth results in a three-dimensional object. Using this type of additive manufacturing method, it is easy to obtain objects with complex shapes. (See Patent Document 1 for an example of an additive manufacturing method using the powder bed method.)

[0006] (Deposition method explanation) On the other hand, in the deposition method, a laser is used as a heat source, and metal powder is sprayed from a nozzle onto the laser's focal point, melting the metal powder and layering it (see Patent Document 2 for an example of an additive manufacturing method using the deposition method).

[0007] In order to develop powders suitable for metal additive manufacturing, efforts are being made to improve powder properties such as laser absorption rate, inclusion concentration, and fluidity. Fluidity is one of the most important properties for spreading powder uniformly on a powder bed and for supplying powder continuously from a nozzle in deposition methods.

[0008] The Japanese Industrial Standards (JIS) Z2502 was established in 2020 to specify a method for measuring the flow rate of metal powders. According to the JIS, the flow rate of metal powders is evaluated by measuring the time (s / 50g) required for 50g of powder to fall through a funnel containing a sample. It is generally known that the more spherical the powder particles, the higher the flow rate. Efforts to increase the sphericity of powder particles as much as possible to improve flow rate have been made for powders intended for metal additive manufacturing. Another proposed method involves mixing nanoparticles to reduce the adhesive force between metal powder particles and improve the flow rate of metal powders (see Patent Document 3). [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 4661842 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-196264 [Patent Document 3] Patent Publication No. 2021-75784 Summary of the Invention [Problem to be solved by the invention]

[0010] Generally, the nominal particle size range of metal powders used in the deposition method is 45 to 150 μm, which is larger than the particle size range (10 to 45 μm) used in the powder bed method.

[0011] In the deposition method, powder is sprayed from a nozzle, so the powder needs to flow by its own weight inside a thin pipe, so powder fluidity is important. However, if the powder fluidity is low, the powder may clog the pipe midway, preventing the powder from being properly supplied to the laser melted area, and the manufacturing process may be interrupted.

[0012] Furthermore, the evaluation of the fluidity of metal powders specified in the aforementioned JIS Z2502 is a method performed by sampling a small amount of 50 g, and therefore is not a method that can actually fully evaluate the fluidity required in the deposition method.

[0013] In order to avoid interrupting the modeling process in a deposition-based modeling device, it is necessary to keep the powder flowing in the nozzle without clogging for several hours. However, when additive manufacturing is actually performed for a long period of time using such a metal additive manufacturing device, even powder with high sphericity and excellent fluidity may no longer flow properly in the conveying nozzle, causing the nozzle to clog and forcing the modeling process to be interrupted.

[0014] Therefore, we investigated the nozzle blockage and found that acicular powder with a major axis of approximately 300-800 μm had accumulated in the blockage, as shown in Figure 1. We found that this acicular powder had accumulated in the powder discharge section over an extended period of powder transport, causing the nozzle blockage. The diameter of the nozzle outlet in the powder discharge section is either the same as the major axis of the acicular powder particles or slightly larger. Thus, when considering the long-term fluidity of actual operations, we found that the problem of blockage due to acicular powder still occurs.

[0015] This acicular powder does not occur every time atomized powder is produced; rather, it occurs unexpectedly and unintentionally. Generally, by classifying powder, most of the acicular powder is sieved out, removing most of it. Therefore, classification was considered sufficient because the proportion of acicular powder is reduced by classification compared to before classification. Once classified, the value does not interfere with the measurement of flowability (s / 50g) specified by JIS. However, since the JIS flowability evaluation uses a small amount of metal powder (50g), it is not suitable for evaluating situations such as the long-term supply of minute amounts of acicular powder from a nozzle. Furthermore, the inclusion of minute amounts of acicular powder that occur unexpectedly is difficult to accurately capture and evaluate using a small amount of powder like the JIS. Therefore, the contamination of extremely small amounts of acicular powder has not been fully recognized or studied until now.

[0016] However, in deposition methods where powder is allowed to flow in a nozzle for a long period of time, even a very small amount of acicular powder can accumulate in the nozzle over long periods of operation, causing blockages. Therefore, there is a need for metal additive manufacturing powders that have as few acicular powder particles as possible removed.

[0017] Therefore, the problem that the present invention aims to solve is to provide a metal powder that has a low content of acicular particles and is suitable for a deposition-type additive manufacturing device that supplies powder from a nozzle. [Means for solving the problem]

[0018] As a result of extensive research, the inventors have developed a metal powder for additive manufacturing that has a low content of acicular particles and is less likely to cause nozzle clogging, based on the following considerations.

[0019] The inventors discovered that one of the reasons why acicular powders pass through sieve meshes with openings (the length of the spaces between the lines that make up the mesh) smaller than their major axis during classification is that the acicular powders in the powder material can change their posture and stand up when subjected to intense vibration.When the acicular powders stand up, their minor axis, which is smaller than the size of the openings, aligns with the direction of the holes in the mesh, causing the acicular powders to slip through the sieve mesh (see Figure 2).

[0020] When classifying powders using a typical vibrating sieve, tapping balls made of urethane rubber are placed on the mesh. When the vibrating sieve is used without the tapping balls, vibration is suppressed, and acicular powder particles do not shake violently compared to when tapping balls are used. Therefore, (1) sieving is performed sequentially while changing the mesh size using tapping balls to obtain powders of a desired mesh size, and then (2) these powders are sieved using a vibrating sieve without tapping balls using a sieve with slightly larger mesh sizes. This prevents the acicular particles from rising due to vibration, so the acicular particles remain horizontal on the sieve with the slightly larger mesh size, and powders of the desired size pass through the sieve without clogging. This allows for efficient removal of only the acicular powder particles.

[0021] In this way, we have discovered that by suppressing the vibrations applied to the powder by eliminating or drastically reducing the number of tapping balls, thereby preventing the acicular particles from rising up, it is possible to preferably obtain metal powder for additive manufacturing with very few acicular particles after classification through a sieve.

[0022] Therefore, the first means for solving the problem is a metal powder for additive manufacturing in which the proportion of powder particles having an aspect ratio of 0.4 or less and a maximum major axis of 150 μm or more is 0.30% or less of the total constituent powder.

[0023] The second aspect is the metal powder for additive manufacturing according to the first aspect, in which the maximum value of the maximum major axis of the powder particles constituting the powder is 1000 μm or less.

[0024] The third means is the metal powder for additive manufacturing according to either the first or second means, in which the proportion of powder particles having an aspect ratio of 0.4 or less and a maximum major axis of 150 μm or more by number is 0.01% or more of the total constituent powder. That is, it is a metal powder in which the proportion by number of acicular powder particles is 0.01 to 0.30%.

[0025] Another method is a method for producing metal powder for additive manufacturing in which metal powder is classified by a permeation sieving device equipped with tapping balls by passing the metal powder sequentially through sieves with different mesh sizes, and then the metal powder is sieved again through a permeation sieving device without tapping balls and with a sieve with larger or larger mesh sizes, thereby producing metal powder in which the proportion of powder particles with an aspect ratio of 0.4 or less and a maximum major axis of 150 μm or more is 0.30% or less of the total constituent powder. By adding a step that utilizes the difference in vibration depending on whether or not tapping balls are used, acicular powder can be efficiently removed. [Effects of the Invention]

[0026] When additive manufacturing is performed using the metal powder for additive manufacturing described in the present invention, the powder has excellent long-term fluidity. Therefore, even if additive manufacturing operations are continued by supplying powder using a nozzle using a deposition method, the additive manufacturing operations can be continued for a long time without the powder clogging the nozzle. Furthermore, additive manufacturing using the metal powder of the present invention produces a densely molded object with low porosity. Furthermore, if the maximum diameter of the acicular powder in the metal powder is set to 1000 μm or less, a densely molded object with even lower porosity can be obtained, ensuring stable fluidity during manufacturing and further reducing nozzle clogging. [Brief explanation of the drawings]

[0027] [Figure 1] This is a secondary electron image of acicular powder of a Ni-based alloy taken with a scanning electron microscope (SEM). [Figure 2]This is a diagram explaining the phenomenon of rising acicular powder passing through the slits of a classification screen. (a) is a schematic diagram viewed from above, and (b) is a schematic diagram viewed from the side. DETAILED DESCRIPTION OF THE INVENTION

[0028] As examples of the alloy component compositions of the metal powder for additive manufacturing of the present invention, Ni-based alloy powder, Co-based alloy powder, Fe-based alloy powder, and Cu-based alloy powder were prepared using metal powder raw materials with the following component compositions, and gas atomized powders were produced and classified. The percentages in the following component compositions are by mass. The component compositions of the Ni-based alloy powder, Co-based alloy powder, Fe-based alloy powder and Cu-based alloy powder in the metal powders of Examples 1 to 8 shown in Table 1 and Comparative Examples 1 to 13 shown in Table 2 are as follows.

[0029] [Ni-based powder] Ni: 52%, Cr: 20%, C: 0.05%, Mn: 0.1%, Si: 0.2%, Mo: 3.0%, Co: 0.3%, Nb: 5.2%, Al: 0.5%, Ti: 0.9%, B: 0.003%, balance: Fe and unavoidable impurities.

[0030] [Co-based powder] Cr: 28.0%, Mo: 6.0%, balance: Co and unavoidable impurities. (This is a powder made of a CoCrMo alloy.)

[0031] [Fe-based powder] Ni: 18.0%, Co: 9.0%, Mo: 4.9%, Ti: 0.7%, Al: 0.1%, Cr: 0.2%, Cu: 0.1%, C: 0.02%, balance: Fe and unavoidable impurities. This powder is made of maraging steel.

[0032] [Cu-based powder] Z: 1.0%, balance: Cu and unavoidable impurities. This powder is made of a CuZr alloy.

[0033] Although various methods other than gas atomization can be used to produce the metal powder for additive manufacturing of the present invention, atomization is preferred. Among atomization methods, gas atomization is preferred. In gas atomization, raw materials are placed in a container (quartz crucible) with a small hole at the bottom, and these raw materials are melted by high-frequency induction heating in an argon gas or nitrogen gas atmosphere. When the molten raw materials flow out of the small hole at the bottom of the crucible, high-speed argon gas or nitrogen gas is sprayed, causing the molten metal to scatter and rapidly cool and solidify into powder.

[0034] (Powder classification) The classification can be carried out, for example, by a dry vibrating sieve. In the examples, the nominal particle size for deposition was adjusted to 45 to 150 μm by classification. A vibrating sieve is a device that efficiently sifts and separates the raw materials on the screen by vibrating the screen using a motor or vibrator, and it uses tapping balls to bounce on the screen to prevent clogging. In this invention, a Dalton vibrating sieve (internal ring type) 502 was used as the vibrating sieve device. Vibration was reduced to prevent acicular powder from rising and passing through the mesh in the direction of its minor axis. Although omitting the urethane tapping balls is generally a difficult procedure to adopt because it reduces the efficiency of classification, intentionally reducing them can reduce the proportion of acicular powder particles.

[0035] As an example of a specific classification procedure, a case where a nominal particle size of -125 μm / +45 μm is obtained will be shown. First, sieving is carried out in a vibrating sieve device using tapping balls in the usual manner, using meshes with nominal openings of 125 μm and 45 μm in succession. Next, without using a tapping ball, the powder is sieved using a vibrating sieve device with a 150 μm mesh to actively remove the needle-like powder that does not pass through the sieve. When the tapping balls are removed and the material is passed through the vibrating sieve, vibrations are more easily suppressed, so the acicular powder particles do not shake as violently as when the tapping balls are used. This prevents the acicular powder from becoming elongated and slipping through the mesh, allowing for selective removal of acicular powder with a longer diameter than the sieve openings. In the procedure where the tapping balls are removed, a mesh one size coarser than the nominal particle size is used. This is to prevent powder particles other than acicular powder from clogging the mesh and increase efficiency.

[0036] Tables 1 and 2 show the average size of metal powder D 50 (μm), the percentage of acicular powder particles (%), the value of the powder with the longest maximum diameter (μm), the fluidity of the metal powder (s), and the porosity of the molded body when additively manufactured (%) are shown.

[0037] [Table 1]

[0038] [Table 2]

[0039] The acicular powder referred to in the present invention refers to powder particles having an aspect ratio of 0.4 or less and a maximum major axis of 150 μm or more. The major axis diameter and minor axis diameter are the lengths of the major and minor axes when a particle is surrounded by two pairs of parallel lines. The aspect ratio is the ratio of the major axis diameter to the minor axis diameter of a particle. The more elongated the powder, the lower the aspect ratio. The maximum major axis diameter is the maximum length measured at two points on the particle's outline.

[0040] It is desirable that the maximum major axis of the metal powder of the present invention does not exceed 1000 μm. This is because an excessively large maximum major axis of the powder is likely to cause nozzle clogging. Even if the number ratio meets the standard, the presence of even one powder particle with a maximum major axis of 1000 μm or more can cause nozzle clogging. Therefore, it is desirable that the maximum value of the maximum major axis of each powder particle in the metal powder of the present invention is 1000 μm or less.

[0041] The flowability of metal powders was confirmed in accordance with JIS Z2502. It is evaluated by opening the orifice at the bottom of a funnel containing 50 g of metal powder sample and measuring the time (s / 50 g) required for 50 g of powder to fall from the funnel. The results are shown in Tables 1 and 2. The ease of handling of the powder under normal conditions while the nozzle is not clogged can be evaluated using this flowability as a guide.

[0042] However, when evaluating fluidity using a small amount of powder (50g), it is only possible to confirm the behavior of the powder for a short period of time (less than 20 seconds). Therefore, to evaluate long-term fluidity, particularly whether practical fluidity is ensured for the deposition method, it is useful to evaluate using a scale other than normal fluidity. Therefore, in the present invention, the number density of acicular powder particles in the metal powder is set to 0.30% or less. Preferably, the number density of acicular powder particles in the metal powder is set to 0.15% or less. On the other hand, if the number density of acicular powder particles is 0.01% or less, classification using a vibrating sieve takes an extremely long time, resulting in a significant drop in productivity. Therefore, the number density of acicular powder particles in the metal powder may be set to 0.01% or more.

[0043] The number of powder particles was counted and the shape was analyzed using a Malvern Morphologi G3 image analyzer. First, the metal powder was dispersed on a glass slide, and the powder shape projected two-dimensionally using an optical microscope was observed for each powder particle, and the equivalent circle diameter was determined from the area of ​​the powder particle. The powders to be counted are determined based on the mesh size of the sieve used for classification, and in the examples, powder particles with a size of 45 μm or more are counted. The procedure of taking images of powder particles one by one and calculating shape parameters through image analysis can be performed automatically using computer software, making it possible to examine the shapes of tens of thousands of powder particles.

[0044] (D 50 (Regarding measurement) Average particle diameter D 50 The particle diameter is determined as a volume average from a cumulative curve when the total volume of the metal powder is 100%. The particle diameter at the point on this curve where the cumulative volume is 50% is called D 50 Particle diameter D 50 is measured by the laser diffraction scattering method. A suitable device for this measurement is the Microtrac MT3000 laser diffraction and scattering particle size distribution analyzer from Nikkiso Co., Ltd. Powder is poured into the cell of this device together with pure water, and the particle size is detected based on the light scattering information of the particles.

[0045] (D 50 setting range) When used in deposition-based additive manufacturing, the D of metal powder 50 is 45 μm <D 50 <150 μm. Preferably, 55 μm <D 50 <120 μm, more preferably 60 μm <D 50 <90 μm.

[0046] (Evaluation of objects created by additive manufacturing) Using the metal powder of the present invention, a 10 mm square block was fabricated using a deposition-type three-dimensional additive manufacturing device (Mitsubishi Heavy Industries Machine Tool Co., Ltd., LAMDA200). The porosity of the resulting molded body was calculated using the following procedure. The relative density was calculated by dividing the density of the 10 mm square block measured using the Archimedes method by the calculated specific gravity obtained from the component analysis values. The porosity was also calculated using the following formula. Porosity (%) = 100 (%) - relative density (%)

[0047] (Relationship between the number of acicular powder particles and porosity) As can be seen from Tables 1 and 2, as the proportion of acicular powder increases, the amount of powder supplied from the powder supply nozzle decreases, resulting in an increase in porosity. When the proportion of acicular powder is particularly high, the nozzle becomes clogged midway, preventing the supply of metal powder and causing the molding to stop midway.

[0048] The metal powders of Examples 1 to 8 had a low proportion of acicular powder particles and little impact on the deterioration of fluidity, so when they were subjected to additive manufacturing, the porosity of the molded body was low and a densely packed molded body was obtained.

[0049] In Comparative Examples 1 to 12, the proportion of acicular powder was high, resulting in higher porosity of the molded bodies compared to Examples 1 and 2. In Comparative Examples 3, 6, 9, and 12, the powder contained powder with a maximum major axis exceeding 1000 μm, which resulted in blockage during molding and made it impossible to obtain a molded body. [Industrial Applicability]

[0050] The metal powder of the present invention is suitable as a powder for additive manufacturing to three-dimensionally form metal parts and the like, and is particularly suitable as a metal powder for deposition-type additive manufacturing. [Explanation of symbols]

[0051] 1. Sieve mesh (screen) 2. Gaps in the mesh 3 Acicular powder

Claims

1. A metal powder for additive manufacturing made of a Ni-based alloy, a Co-based alloy, an Fe-based alloy, or a Cu-based alloy, wherein the number ratio of powder particles having an aspect ratio of 0.4 or less and a maximum major axis of 150 μm or more is 0.01 to 0.30% of the total constituent powder.

2. 2. The metal powder for additive manufacturing according to claim 1, wherein the maximum value of the maximum major axis of the powder particles constituting the powder is 1000 μm or less.

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