Method for producing metal powder

Abstract

In order to enhance circularity while suppressing a decrease in average particle diameter, the range of area envelopment degree of a starting raw material in which the degree of decrease in the average particle diameter of a metal powder between before and after spheroidization is within a predetermined allowable range is identified in advance (S1), and the spheroidization is performed using, as the starting raw material, a metal powder having an area envelopment degree within the identified range of area envelopment degree (S2).

Description

Method for producing metal powder

[0001] The present invention relates to a method for producing metal powder.

[0002] As a method capable of economically producing fine metal powder having a particle size of about several micrometers, chemical methods such as coprecipitation method, electrolysis method, reduction method from salt solution, etc. are widely used. However, the metal powder produced by the chemical method has a problem that although the powder particle size (particle size of primary particles) is several micrometers, usually these primary particles aggregate to form large particles (aggregates) in many cases. Therefore, as a method for producing metal powder that solves this problem, an impact force by high-speed collision is repeatedly applied to a metal raw material powder containing isolated particles composed of aggregates of fine primary particles, and granulation including dissociation of this aggregate and bonding of primary particles inside the newly generated isolated particles or between other isolated particles is caused, thereby miniaturizing, densifying, and spheroidizing the isolated particles composed of aggregates (Patent Document 1).

[0003] Japanese Patent Laid-Open No. 8-120309

[0004] By the way, when a metal material is cut to recover metal cutting powder and this cutting powder is spheroidized using a pulverizer such as a jet mill, there is a problem that the circularity of the metal powder increases but the average particle size of the metal powder decreases and fine powder increases.

[0005] The problem to be solved by the present invention is to provide a method for producing metal powder that can increase the circularity while suppressing the decrease in the average particle size.

[0006] According to the present invention, when spheroidizing the starting material metal powder to increase the circularity of the metal powder, the degree of decrease in the particle size of the metal powder before and after the spheroidization process is within a predetermined allowable range in advance, the range of the area envelope degree of the starting material is specified, and the spheroidization process is performed using a metal powder having an area envelope degree within the specified range of the area envelope degree, thereby solving the above problem.

[0007] According to the present invention, it is possible to increase the circularity while suppressing the decrease in the average particle size.

[0008] This is a flowchart illustrating one embodiment of the method for producing metal powder according to the present invention. This is a flowchart illustrating the subroutine for step S1 in Figure 1. This is a diagram showing the relationship between the bulk density of the metal powder and the spheroidizing process. This is a graph showing an example of the degree of increase in circularity before and after the spheroidizing process. This is a graph showing an example of the degree of decrease in average particle diameter before and after the spheroidizing process. This is a diagram to explain one of the reasons why the average particle diameter decreases in the spheroidizing process according to a comparative example of the present invention. This is a diagram to explain the degree of area envelope. This is a graph showing the area envelope of starting materials A, B, and C. This is a graph showing the degree of decrease in average particle diameter for each processing condition of the spheroidizing process according to an example of the present invention. This is a graph showing the degree of increase in circularity for each processing condition of the spheroidizing process according to an example of the present invention.

[0009] Hereinafter, examples of embodiments for carrying out the present invention will be described with reference to the drawings. Figure 1 is a flowchart showing one embodiment of the present invention for manufacturing metal powder, and Figure 2 is a flowchart of the subroutine for step S1 in Figure 1. The present invention's method for manufacturing metal powder is a method for increasing the circularity of metal powder by spheroidizing a starting material metal powder, and as shown in Figure 1, it comprises a step S1 in which a range of area envelope of the starting material is specified in advance so that the degree of decrease in the particle size of the metal powder before and after spheroidizing is within a predetermined allowable range, and a step S2 in which a metal powder having an area envelope within the range of area envelope specified in step S1 is used as the starting material and spheroidized to increase the circularity of the metal powder.

[0010] Here, step S2 is a process in which the metal powder with improved circularity is recovered to become the actual product, whereas step S1 is a preliminary preparation process in which the range of area coverage of the starting material required to carry out the actual step S2 is determined in advance by experiment or simulation.

[0011] Figure 3 shows the relationship between the bulk density of metal powder and spheroidization processing. Generally, metal powder 1 manufactured by cutting or other processes has irregular shapes, resulting in gaps between the powder particles and a low bulk density, as shown in the left diagram of the figure. When such metal powder 1 is used as a raw material for metal 3D printing, there is a problem in that a metal 3D printed product with the desired strength or rigidity cannot be obtained due to the low bulk density before sintering. For example, when the required bulk density of metal powder 1 before sintering for metal 3D printing is 56% or more, the bulk density of the starting material metal powder 1 is often around 30%. Note that bulk density refers to the density when powder is filled into a container of a certain volume and its internal volume is used as the volume, and it is a density that includes pores in the substance itself and voids between substances in the volume used for density calculation.

[0012] In contrast, when the metal powder 1 is spheroidized to bring each powder closer to a sphere, the maximum bulk density theoretically becomes 74%, as shown in the right-hand figure of Figure 3. Using such metal powder 1 for metal 3D printing can increase the strength or rigidity of the product obtained after sintering. Therefore, in the metal powder manufacturing method of this embodiment, the metal powder 1, which is the starting material, is subjected to spheroidization.

[0013] However, when metal powder with a circularity of 0.78, as measured by the image analysis method of the Japanese Industrial Standard JIS Z8827-1, was used as the starting material and subjected to spheroidization processing using a jet mill, it was confirmed that while the circularity of the metal powder increased, the average particle size of the metal powder decreased, and the amount of fine powder increased. Note that circularity is defined as 4πS / L, where S is the projected area of ​​the powder particle and L is the perimeter of the powder particle. ２ Circularity is a physical property value defined by the ratio of the circumference of a circle with the same area as the projected area of ​​the particle to the length of the contour of the particle's projection. Circularity = 1 indicates a perfect circle, and the more complex the shape, the smaller the circularity value becomes. As mentioned above, circularity can be measured by analyzing powder images captured with a microscope using image analysis methods.

[0014] Furthermore, the average particle diameter refers to the size of a hypothetical group of particles with uniform diameters that have exactly the same physical properties (number, length, area, volume, etc.) as the actual group of particles, when considering a group of particles composed of many particles with various particle diameters. In particular, the median diameter D50, which focuses on the number of powder particles, is an index that indicates the particle diameter at which the number of powder particles larger and smaller than that value are halved. The average particle diameter D50 can be measured by analyzing powder images captured with a microscope using image analysis methods.

[0015] Figure 4A is a graph showing an example of the degree of increase in circularity before and after spheroidization using a jet mill grinder, and Figure 4B is a graph showing an example of the degree of decrease in the average particle size (D50) before and after the same spheroidization process. As shown in Figure 4A, the circularity of the starting material before spheroidization was 0.78, while the circularity of the metal powder after spheroidization increased to 0.82-0.86. However, looking at the average particle size (D50) at this time, as shown in Figure 4, the D50 of the starting material before spheroidization was 25 μm, while the D50 of the metal powder after spheroidization decreased to 5-10 μm. While the increase in circularity due to spheroidization is desirable for metal powder used in metal 3D printing, the decrease in average particle size leads to an increase in fine powder, which causes problems such as a decrease in the strength, rigidity, and especially toughness of metal 3D printed products, making them brittle.

[0016] Figure 5 is a diagram illustrating one of the reasons why the average particle size decreases in the spheroidization process according to a comparative example of the present invention, Figure 6 is a diagram illustrating the reason why the degree of decrease in the average particle size is small in the spheroidization process according to an embodiment of the present invention, and Figure 7 is a diagram illustrating the area envelope degree. The inventors considered that in the spheroidization process of metal powder, the circularity increases but the average particle size decreases because, although impact is applied to the metal powder from the pulverizer during the spheroidization process, as shown in Figure 5, depending on the shape of the metal powder, the particles of the metal powder are pulverized by the impact applied from the pulverizer.

[0017] In other words, as shown in the left diagram of Figure 5, if the metal powder (one particle) 1 has a complex outer shape and contour with irregularities, when an impact is applied to the metal powder 1, stress concentrates in the recess 11 shown in the same diagram, and as shown in the right diagram of Figure 5, this recess 11 becomes the starting point for the metal powder 1 to pulverize into two, which we thought might be the reason why the circularity increases but the average particle size decreases. To put it another way, as shown in the left diagram of Figure 6, if the metal powder 1 has an outer shape and contour that is not complex with few irregularities, even if an impact is applied to the metal powder 1 from the pulverizer, there are fewer areas where stress concentrates, and as shown in the right diagram of Figure 6, pulverization does not occur and the circularity increases, and even if the circularity increases, the average particle size does not decrease.

[0018] Therefore, we concluded that introducing the area envelope of metal powder 1 as an indicator for the outer shape and contour of metal powder 1, which increases circularity but decreases average particle diameter due to spheroidization processing, correlates with the complex outer shape and contour with irregularities. Here, the area envelope is defined as the value (ratio, X1 / X2) obtained by dividing the area X1, which is the area around the actual perimeter of metal powder 1, by the area X2, which is the area around the envelope perimeter, as shown in Figure 7. The numerator of the area envelope, "Area X1, which is the area around the actual perimeter," refers to the area shown by the filled parts in the left and right figures of the same figure, and the denominator, "Area X2, which is the area around the envelope perimeter," refers to the area enclosed by the thick line L in the left and right figures of the same figure (envelope perimeter).

[0019] The area envelope is a physical property value that approaches 1 as the outer shape and contour of the metal powder 1 is smooth with fewer irregularities, as shown in the left figure of Figure 7, and becomes smaller than 1 as the outer shape and contour of the metal powder 1 is complex with many irregularities, as shown in the right figure of Figure 7. The area envelope can be measured by analyzing powder images captured with a microscope using image analysis methods.

[0020] As shown in Figure 1, the method for producing metal powder in this embodiment includes a step S1 to identify the range of area envelope of the metal powder to be used as a starting material, and a step S2 to perform a spheroidizing process using the metal powder having an area envelope within the range of area envelope identified in step S1 as a starting material, thereby increasing the circularity of the metal powder.

[0021] Step S1, which identifies the range of area envelope of the metal powder used as a starting material, includes, as shown in Figure 2, a step S11 for selecting processing conditions for spheroidization, a step S12 for measuring the average particle diameter, circularity, and area envelope of the metal powder (i.e., the starting material) before spheroidization, a step S13 for spheroidization according to the processing conditions for spheroidization selected in step S11, a step S14 for measuring the average particle diameter and circularity of the metal powder after spheroidization in step S13, a step S15 for determining the degree of decrease in the average particle diameter after spheroidization relative to the average particle diameter before spheroidization, and the degree of increase in the circularity after spheroidization relative to the circularity before spheroidization, and a step S16 for determining whether the degree of decrease in the average particle diameter and the degree of increase in circularity determined in step S15 are within a predetermined allowable range for each of the processing conditions for spheroidization selected in step S11, and identifying the range of area envelope of the starting material.

[0022] The processing conditions for spheroidization selected in step S11 include the spheroidization method and the conditions of the equipment set for each method. Spheroidization methods include, for example, spheroidization using a jet mill, a high-speed rotary mill (hybridization system, hybridizer), and a mill utilizing the mechanofusion method. A jet mill mill pulverizes and spheroidizes raw materials by colliding them with high-speed jet streams ejected from opposing nozzles, while a high-speed rotary mill pulverizes and spheroidizes raw materials by colliding them with a high-speed rotating rotor. A mill utilizing the mechanofusion method spheroidizes raw materials by repeatedly applying strong compressive and shearing forces to raw materials introduced into a cylindrical mixing container using a press head attached to the tip of the rotor, which is fixed to the inner wall of the container by centrifugal force.

[0023] In step S11, a method corresponding to the spheroidizing process used in step S2 to obtain the actual product is selected, and the conditions of the equipment set for the selected method are selected. For example, examples of processing conditions selected in step S11 include using a jet mill grinder, a high-speed rotary grinder, or a grinder utilizing the mechanofusion method, and a grinding time of 3 to 7 minutes.

[0024] In the subsequent step S12, the average particle size, circularity, and area envelope of the starting materials are measured. Figure 8 is a graph showing the results of measuring the area envelope of three types of starting materials A, B, and C. Although these starting materials A, B, and C are metal powders of the same material, their area envelopes differ from each other. Starting material A had an area envelope of 0.72 to 0.80, starting material B had an area envelope of 0.53 to 0.78, and starting material C had an area envelope of 0.81 to 0.89.

[0025] In the subsequent step S13, spherical processing is performed for each of the spherical processing conditions selected in step S11. Then, in the subsequent step S14, the average particle size and circularity of the metal powder after spherical processing are measured. Here, the permissible range for the degree of increase in circularity and the permissible range for the degree of decrease in average particle size before and after spherical processing are determined in advance. For example, the degree of increase in circularity (= circularity after processing / circularity before processing) is 1.05 or more, and the degree of decrease in average particle size D50 (= D50 after processing / D50 before processing) is 0.9 or more. Alternatively, the absolute value of the circularity after spherical processing may be used as the permissible range instead of the degree of increase in circularity. Similarly, the absolute value of the average particle size after spherical processing may be used as the permissible range instead of the degree of decrease in average particle size.

[0026] In step S15, the degree of increase in circularity and the degree of decrease in average particle size are determined from the average particle size and circularity of the starting material measured in step S12 and the average particle size and circularity of the metal powder after spheroidization measured in S14. Then, in the subsequent step S16, it is determined whether the determined degree of increase in circularity and the degree of decrease in average particle size are within a predetermined tolerance range. If both the degree of increase in circularity and the degree of decrease in average particle size are within the tolerance range, the combination of area envelope measured in step S12 for the processing conditions selected in step S11 becomes the processing conditions and area envelope specified in step S1. On the other hand, if at least one of the degree of increase in circularity and the degree of decrease in average particle size is not within the tolerance range, the combination of area envelope for those processing conditions is outside the range of processing conditions and area envelope specified in step S1.

[0027] For each of the three starting materials A, B, and C shown in Figure 8, spheroidization processing was performed (process S13) using a jet mill, a high-speed rotary mill, and a mechanofusion mill, with processing conditions (process S11) of 3 to 7 minutes of grinding time. The degree of increase in circularity and the degree of decrease in average particle size before and after spheroidization processing were determined, and it was judged whether these were within the predetermined acceptable range. As a result, as shown in Figure 8, regardless of the processing conditions selected in process S11, starting material C, with an area envelopment degree of 0.81 or higher, showed that both the degree of increase in circularity and the degree of decrease in average particle size before and after spheroidization processing were within the acceptable range. In contrast, starting materials A and B, with an area envelopment degree of less than 0.81, did not satisfy the acceptable range for at least one of the degree of increase in circularity and the degree of decrease in average particle size before and after spheroidization processing.

[0028] Next, a starting material having an area envelopment degree within the range specified in step S1 of Figure 1, i.e., an area envelopment degree of 0.81 or higher, was prepared, and the spheroidization process in step S2 was carried out using a jet mill, a high-speed rotary mill, and a mill utilizing the mechanofusion method, under processing conditions for spheroidization with a grinding time of 3 to 7 minutes. Figure 9A is a graph showing the degree of decrease in average particle size under different processing conditions for spheroidization in this embodiment, and Figure 9B is a graph showing the degree of increase in circularity. As shown in Figure 9A, the degree of decrease in average particle size before and after spheroidization fell within the predetermined allowable range, and as shown in Figure 9B, it was confirmed that the degree of increase in circularity before and after spheroidization also fell within the predetermined allowable range.

[0029] As described above, the metal powder manufacturing method of this embodiment is a method for increasing the circularity of metal powder by spheroidizing the starting material metal powder. In this method, the range of area envelope of the starting material is specified in advance so that the degree of decrease in the average particle size of the metal powder before and after spheroidizing is within a predetermined allowable range. The metal powder having an area envelope within the specified range is used as the starting material for the spheroidizing process, thereby suppressing the degree of decrease in the average particle size before and after spheroidizing. As a result, the increase in fine metal powder can be suppressed.

[0030] Furthermore, the metal powder manufacturing method of this embodiment involves further selecting the processing conditions for the spheroidizing process in advance, measuring the average particle diameter and area envelope of the metal powder before spheroidizing and the average particle diameter of the metal powder after spheroidizing based on the selected processing conditions, determining the degree of decrease in the average particle diameter after spheroidizing relative to the average particle diameter before spheroidizing, and identifying the range of area envelope of the starting material within which the determined degree of decrease in average particle diameter falls within a predetermined allowable range. As a result, the degree of decrease in average particle diameter before and after spheroidizing can be suppressed for each spheroidizing processing condition.

[0031] Furthermore, in the metal powder manufacturing method of this embodiment, the circularity before and after spheroidization is further measured according to the selected processing conditions, the degree of increase in circularity after spheroidization relative to the circularity before spheroidization is further determined, the range of area envelope of the starting material and the processing conditions for spheroidization are further specified so that the determined degree of decrease in average particle size and the degree of increase in circularity are within a predetermined allowable range, metal powder having an area envelope within the specified range is used as the starting material, and the spheroidization is performed according to the specified spheroidization processing conditions, thereby ensuring an increase in circularity before and after spheroidization. As a result, circularity can be increased while suppressing a decrease in average particle size.

[0032] Furthermore, in the method for producing metal powder according to this embodiment, which involves spheroidizing a starting metal powder to increase the circularity of the metal powder, the average particle diameter, circularity, and area envelope of the metal powder before spheroidizing and the average particle diameter and circularity of the metal powder after spheroidizing are measured in advance. The degree of decrease in the average particle diameter after spheroidizing relative to the average particle diameter before spheroidizing is determined, and the range of area envelope of the starting material within which the determined degree of decrease in average particle diameter falls within a predetermined allowable range is identified. When producing the target metal powder, the spheroidizing process is performed using metal powder having an area envelope within the identified range as the starting material, and the spheroidized metal powder is recovered. As a result, the degree of decrease in average particle diameter before and after spheroidizing can be suppressed.

[0033] Furthermore, in the metal powder manufacturing method of this embodiment, the processing conditions for the spheroidizing process are selected in advance, the degree of increase in circularity after spheroidizing compared to the circularity before spheroidizing is determined based on the selected processing conditions, the range of area envelope of the starting material and the processing conditions for the spheroidizing process are further specified so that the determined degree of decrease in average particle size and the degree of increase in circularity are within a predetermined allowable range, a metal powder having an area envelope within the specified range is used as the starting material, and the spheroidizing process is performed according to the specified spheroidizing processing conditions, thereby ensuring an increase in circularity before and after spheroidizing. As a result, circularity can be increased while suppressing a decrease in average particle size.

[0034] 1... Metal powder (particles) 11... Recess

Claims

1. A method for increasing the circularity of metal powder by spheroidizing a starting material metal powder, wherein the range of area envelope of the starting material is specified in advance so that the degree of decrease in the average particle size of the metal powder before and after spheroidizing is within a predetermined allowable range, and the metal powder having an area envelope within the specified range is used as the starting material for the spheroidizing process.

2. A method for producing metal powder according to claim 1, comprising: further selecting processing conditions for the spheroidizing process in advance; further measuring the average particle diameter and area envelope of the metal powder before spheroidizing and the average particle diameter of the metal powder after spheroidizing based on the selected processing conditions; further determining the degree of decrease in the average particle diameter after spheroidizing relative to the average particle diameter before spheroidizing; and specifying the range of area envelope of the starting material within which the determined degree of decrease in average particle diameter falls within a predetermined allowable range.

3. A method for producing metal powder according to claim 2, wherein the circularity of the metal powder before spheroidization and the circularity of the metal powder after spheroidization are measured in advance according to the selected processing conditions, the degree of increase in the circularity after spheroidization relative to the circularity before spheroidization is determined, the range of area envelope of the starting material and the processing conditions for spheroidization are further specified such that the determined degree of decrease in average particle size and the degree of increase in circularity are within a predetermined allowable range, and a metal powder having an area envelope within the specified range of area envelope is used as the starting material, and the spheroidization is performed according to the specified processing conditions for spheroidization.

4. A method for increasing the circularity of metal powder by spheroidizing a starting material metal powder, comprising: firstly measuring the average particle diameter, circularity, and area envelope of the metal powder before spheroidizing and the average particle diameter and circularity of the metal powder after spheroidizing; determining the degree of decrease in the average particle diameter after spheroidizing relative to the average particle diameter before spheroidizing; identifying the range of area envelope of the starting material within which the determined degree of decrease in average particle diameter falls within a predetermined allowable range; and, when manufacturing the target metal powder, using a metal powder having an area envelope within the identified area envelope range as the starting material to perform the spheroidizing process and recovering the spheroidized metal powder.

5. A method for producing metal powder according to claim 4, wherein the processing conditions for the spheroidizing process are selected in advance; the degree of increase in circularity after the spheroidizing process relative to the circularity before the spheroidizing process is determined based on the selected processing conditions; the range of area envelope of the starting material and the processing conditions for the spheroidizing process are further specified so that the determined degree of decrease in average particle size and the degree of increase in circularity are within a predetermined allowable range; and a metal powder having an area envelope within the specified range is used as the starting material, and the spheroidizing process is performed according to the specified processing conditions for the spheroidizing process.

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