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 in particular 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 methods of metal additive manufacturing 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 bonds the powder particles together. Irradiation is selectively performed on a portion of the metal powder, and the non-irradiated areas do not melt, and a bond layer is formed only in the irradiated areas.

[0004] New metal powder is laid on top of the bonded layer and irradiated with a laser or electron beam. The metal particles melt and solidify to form a new bonded layer. The new bonded layer also bonds with the existing bonded layer.

[0005] As the melting and solidification caused by irradiation is repeated in sequence, an aggregate of bonding layers gradually grows. This growth results in a shaped object with a three-dimensional shape. By using this type of additive manufacturing method, objects with complex shapes can be easily obtained. (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, metal powder is sprayed from a nozzle onto the laser focusing area, and the metal powder is melted and layered (see Patent Document 2 for an example of an additive manufacturing method using the deposition method).

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

[0008] The method for measuring the flow rate of metal powders will be specified in the Japanese Industrial Standards (JIS) Z2502 in 2020. The flow rate of metal powders according to the JIS is evaluated by measuring the time (s / 50g) required for 50g of powder to fall from a funnel containing a sample. It is generally known that the closer the powder particles are to a spherical shape, the higher the flow rate. Efforts have been made to increase the sphericity of powder particles as much as possible to improve flow rate for powders for metal additive manufacturing. In addition, a method has been proposed in which nanoparticles are mixed in 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] JP 2015-196264 A [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 must be made to flow by its own weight inside a thin pipe in order to inject the powder from a nozzle, so the fluidity of the powder is important. However, if the powder has low fluidity, the powder may get clogged in the pipe midway, making it impossible to properly supply the powder to the laser melted area, and the modeling process may be interrupted.

[0012] Furthermore, the evaluation of the fluidity of metal powder stipulated in the aforementioned JIS Z 2502 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 modeling operations in a deposition-based modeling device, it is necessary to keep the powder flowing in the nozzle for several hours without clogging. However, when additive modeling is actually performed for a long period of time using such a metal additive modeling device, even powder with high sphericity and excellent fluidity may not flow properly in the conveying nozzle, causing nozzle clogging and forcing the modeling operation to be interrupted.

[0014] Therefore, when the blocked nozzle area was investigated, it was found that acicular powder with a long diameter of about 300-800 μm had accumulated in the blocked area, as shown in Figure 1, and that the cause of nozzle blockage was the accumulation of this acicular powder in the powder discharge area due to long-term powder transport. The diameter of the nozzle outlet in the powder discharge area is equal to or slightly larger than the long diameter of the acicular powder particles. Thus, when considering fluidity over a long period of time, as in actual work, it was 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, but occurs unexpectedly and unintentionally. Basically, by classifying the powder, most of the acicular powder is removed because the majority of the powder is sieved out. Therefore, it was considered that classification is sufficient because the proportion of acicular powder is reduced by classification compared to before classification. If the powder is classified, the value will be one that does not interfere with the measurement of the flow rate (s / 50g) specified by JIS. However, since the JIS flow rate is evaluated using a small amount of metal powder (50g), it is not suitable for evaluating situations such as the supply of a small amount of acicular powder from a nozzle for a long period of time, and it is difficult to accurately capture and evaluate the inclusion of a small amount of acicular powder that occurs unexpectedly using an evaluation using a small amount of powder such as the JIS. Therefore, the inclusion of extremely small amounts of acicular powder has not been fully recognized as an issue and has not been examined until now.

[0016] However, in the deposition method where the powder flows in the nozzle for a long time, even a very small amount of acicular powder like this accumulates in the nozzle during long-term operation, causing blockages. Therefore, there is a need for metal additive manufacturing powder that removes as many acicular powder particles as possible from the nozzle, which are generated unexpectedly and unintentionally.

[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 viewpoints described below.

[0019] The inventors discovered that one of the reasons why acicular powders pass through the mesh of a sieve with openings (the length of the space between the lines that make up the mesh) smaller than the long axis during classification is that the acicular powder in the powder material changes its posture so that it stands up when subjected to intense vibration. When the acicular powder stands up, the short axis, which is smaller than the size of the openings, aligns with the direction of the mesh holes, causing the acicular powder to slip through the sieve (see Figure 2).

[0020] When classifying powders with a general vibrating sieve device, tapping balls made of urethane rubber are placed on the mesh. When the powders are sieved in the vibrating sieve device without the tapping balls, the vibration is suppressed, and the acicular powder particles do not shake violently compared to when the tapping balls are used. Therefore, (1) sieving is performed sequentially while changing the mesh size with the tapping balls, to obtain powders of a predetermined mesh size, and then (2) these powders are sieved with a sieve with a slightly larger mesh size by the vibrating sieve device without the tapping balls. In this way, the acicular particles are prevented from rising up due to vibration, so that the acicular particles remain horizontal on the sieve with the slightly larger mesh size, and the powders of the predetermined size pass through the sieve without clogging. Therefore, only the acicular powder particles can be efficiently removed.

[0021] In this way, it was discovered that by suppressing the vibration applied to the powder by eliminating or drastically reducing the number of tapping balls, and thereby preventing the rising of acicular particles, it is possible to suitably obtain metal powder for additive manufacturing containing 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 in the total constituent powder is 0.30% or less.

[0023] The second aspect of the present invention 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 aspect is the metal powder for additive manufacturing according to either the first or second aspect, 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 0.01% or more of the total constituent powder. In other words, the metal powder has a proportion of acicular powder particles of 0.01 to 0.30% by number.

[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 sequentially passing the metal powder through sieves with different mesh sizes, and then sieved again through a permeation sieving device without tapping balls and a sieve with mesh sizes larger than or equal to large, 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. Effect of the Invention

[0026] When additive manufacturing is performed using the metal powder for additive manufacturing described in the means of the present invention, the powder has excellent long-term fluidity, so even if additive manufacturing work in which powder is supplied using a nozzle in a deposition method is continued, the additive manufacturing work can be continued for a long time without the nozzle being blocked by the powder. In addition, additive manufacturing using the metal powder of the means of the present invention can obtain a dense shaped body with low porosity. Furthermore, if the maximum value of the maximum major axis of the acicular powder in the metal powder is 1000 μm or less, a dense shaped body with even lower porosity can be obtained, and stable fluidity during modeling can be continuously ensured, further reducing nozzle blockage. [Brief description 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). [Diagram 2]This is a diagram explaining the phenomenon in which the raised acicular powder passes through the slits of a classification screen, (a) being a schematic diagram viewed from above, and (b) being a schematic diagram viewed from the side. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] As examples of the alloy component composition of the metal powder for additive manufacturing of the present invention, for Ni-based alloy powder, Co-based alloy powder, Fe-based alloy powder, and Cu-based alloy powder, metal powder raw materials having the following component compositions were prepared, gas atomized powder was produced, and classified. The percentages in the following component compositions are by mass. The components of the Ni-based alloy powder, the Co-based alloy powder, the Fe-based alloy powder and the 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 a maraging steel.

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

[0033] The metal powder for additive manufacturing of the present invention can be produced by various methods other than the gas atomization method, but the metal powder can be obtained by the atomization method. Among the atomization methods, gas atomization is preferable. In gas atomization, raw materials are charged into a container (quartz crucible) having a small hole at the bottom, and these raw materials are melted by high-frequency induction heating in an argon gas atmosphere or a nitrogen gas atmosphere. When the molten raw materials flow out from 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 be rapidly cooled and solidified into powder.

[0034] (Powder classification) The classification can be carried out, for example, by a dry vibrating sieve. In the embodiment, the nominal particle size was adjusted to 45 to 150 μm for deposition by classification. A vibrating sieve is a device that efficiently sifts the materials on the screen by vibrating the screen with a motor or vibrator, and separates the material by particle size. To prevent clogging, a tapping ball is bounced on the screen. In the present invention, a DALTON vibration sieve (inner ring type) 502 was used as the vibrating sieve device. The vibration was reduced to prevent the acicular powder from rising and passing through the screen in the direction of the short diameter. Although omitting the urethane tapping ball is generally a difficult procedure to adopt because it reduces the efficiency of classification, intentionally reducing it can suppress the number ratio of acicular powder.

[0035] As an example of a specific classification procedure, a case in which a nominal particle size of -125 μm / +45 μm is obtained will be shown. First, sieving is carried out in a vibrating sieve apparatus using tapping balls in the usual manner, using meshes with nominal openings of 125 μm and 45 μm in sequence. Next, the mixture is sieved using a vibrating sieve device without using a tapping ball, using a mesh of 150 μm, to actively remove the needle-like powder that does not pass through the sieve. When the tapping ball is removed and the material is put through the vibrating sieve device, the vibration is easily suppressed, so the acicular powder particles do not shake violently compared to when the tapping ball is used. This prevents the acicular powder from turning vertically and slipping through the screen and falling, so that acicular powder with a longer diameter than the sieve mesh can be selectively removed. In the procedure where the tapping ball is removed, a screen one size coarser than the nominal particle size is used. This is to prevent powder particles other than acicular powder from clogging the screen and increase efficiency.

[0036] Tables 1 and 2 show the average size of the metal powder D 50 The figures show the particle size (μm), the number of acicular powder particles (%), the value of the powder with the largest maximum diameter (μm), the fluidity of the metal powder (s), and the porosity (%) of the molded body after additive manufacturing.

[0037] [Table 1]

[0038] [Table 2]

[0039] The acicular powder 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 long axis diameter and short axis diameter are the length of the long axis or short axis when a particle is surrounded by two pairs of parallel lines. The aspect ratio is the ratio of the long axis diameter to the short axis diameter of a particle. The more elongated the powder, the lower the aspect ratio. The maximum long axis diameter is the maximum length at two points on the particle's outline.

[0040] It is preferable that the maximum major axis of the metal powder of the present invention does not exceed 1000 μm. If the maximum major axis of the powder is excessively long, it is likely to cause nozzle clogging. Even if the number ratio meets the standard, if even one powder particle has a maximum major axis of 1000 μm or more, it can cause nozzle clogging. Therefore, it is preferable 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 powder was confirmed based on 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) it takes 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 the fluidity of a small amount of powder (50 g), the behavior of the powder can only be confirmed 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 that of normal fluidity. Therefore, in the present invention, the number density of the acicular powder in the metal powder is set to 0.30% or less. Preferably, the number density of the acicular powder in the metal powder is set to 0.15% or less. On the other hand, if the number density of the acicular powder is 0.01% or less, it takes a very long time to classify the powder using a vibrating sieve, which significantly reduces productivity. Therefore, the number density of the acicular powder 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 image analyzer, Morphologi G3. First, the metal powder was dispersed on a glass slide, and the powder shape projected two-dimensionally was observed for each powder particle using an optical microscope, and the circle equivalent 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, so in the examples, powder particles having a size of 45 μm or more are counted. The process 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 (About the measurement) Average particle diameter D 50 The particle diameter is calculated as the 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 / 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 is 50 is 45μm <D 50 <150 μm. Preferably, 55 μm <D 50 <120 μm, more preferably 60 μm <D 50 <90 μm.

[0046] (Evaluation of objects produced by additive manufacturing) Using the metal powder of the present invention, a 10 mm square block was produced 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 by the Archimedes method by the calculated specific gravity obtained from the component analysis values. The porosity was calculated using the following formula. Porosity (%) = 100 (%) - relative density (%)

[0047] (Relationship between the number of acicular powder particles and porosity) From Tables 1 and 2, it can be seen that as the proportion of acicular powder increases, the amount of powder supplied from the powder supply nozzle decreases, and the porosity increases. When the proportion of acicular powder is particularly large, the nozzle becomes clogged midway, preventing the supply of metal powder, and the molding process stops midway.

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

[0049] In Comparative Examples 1 to 12, the proportion of acicular powder was high, and therefore the porosity of the molded bodies was higher than in Examples. In addition, Comparative Examples 3, 6, 9, and 12 contained powder with a maximum major axis exceeding 1000 μm, and therefore blockage occurred during molding, and a molded body could not be obtained. [Industrial Applicability]

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

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

Claims

1. 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 in the total constituent powder is 0.01 to 0.30%.

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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