Metal powder forming method

Abstract

A metal powder forming method with which metal powder is formed by cutting the surface of a metal workpiece, wherein a plurality of quadrangular pyramid-shaped protrusions each having a rhombic bottom surface are formed on the surface of the workpiece by rolling, and the metal powder is formed by cutting the plurality of protrusions along the direction of a long diagonal line among a pair of diagonal lines of the rhombic shape.

Description

Method for forming metal powder

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

[0002] Patent Document 1 discloses a method for forming metal powder by forming a plurality of fine convex portions on the circumferential surface of a rotating metal cylinder with a rolling roller and then cutting the formed convex portions with a cutting tool. In this method, the shape of the metal powder to be formed mainly depends on the shape of the convex portions formed by rolling. The shape of the convex portions formed by rolling is controlled only by the groove pitch on the surface of the rolling roller and the pressing load of the rolling roller.

[0003] International Publication No. 2023 / 148980

[0004] However, the shape of the metal powder formed by cutting the convex portions is affected by the deformation of the convex portions due to the stress acting during cutting. In Patent Document 1, this deformation during cutting is not considered, and the shape of the formed metal powder is not controlled. Therefore, the aspect ratio of the formed metal powder decreases.

[0005] An object of the present invention is to provide a method for forming metal powder capable of improving the shape of the formed metal powder, particularly its aspect ratio.

[0006] A method for forming metal powder according to one aspect of the present invention is a method for forming metal powder by cutting the surface of a metal workpiece, comprising forming a plurality of convex portions having a truncated square pyramid shape with a bottom surface on the surface of the workpiece by rolling, and cutting the plurality of convex portions along the direction of the long diagonal among the pair of diagonals of the truncated square pyramid to form metal powder.

[0007] According to the above method for forming metal powder, the shape of the formed metal powder, particularly its aspect ratio, can be improved.

[0008] Figure 1 is a schematic perspective view of an apparatus for performing a metal powder formation method according to an embodiment. Figure 2 is a schematic side view of the apparatus. Figure 3 is an enlarged side view of the cutting section of the apparatus. Figure 4 is a schematic perspective view showing the surface of the workpiece after rolling by the first rolling roller. Figure 5 is a schematic perspective view showing the surface of the workpiece after rolling by the second rolling roller. Figure 6 is a schematic perspective view showing the surface of the workpiece after cutting by the cutting tool. Figure 7 is a partially unfolded plan view and side view of the rolling surface unfolded into a plane. Figure 8 is a schematic arrangement diagram and side view showing the metal powder formed after cutting the rolling surface. Figure 9 is a graph showing the relationship between the larger interior angle of the rhombus base of the convex portion that is cut to become metal powder and the aspect ratio of the metal powder.

[0009] The average aspect ratio or average particle size of the metal powder M1 formed by the metal powder formation method according to the embodiment was measured using a particle image analyzer (product name: Morphologi 4, manufactured by Malvern Panalogical). More specifically, several thousand to tens of thousands of particulate metal powder M1 particles were dispersed on a glass plate using a powder dispersion unit, photographed with an objective lens of a predetermined magnification, and the average aspect ratio or average particle size was calculated using image analysis software. The magnification of the objective lens can be appropriately set according to the shape of the metal powder M1, etc. The aspect ratio is the ratio of the major axis diameter to the minor axis diameter of the powder or particle in an image taken from a predetermined direction. The particle size is the equivalent circle diameter of the particle. The equivalent circle diameter is the diameter of a circle having an area equal to the projected area of ​​the photographed particle. In the following description, the average aspect ratio and average particle size of the metal powder M1 will be simply referred to as aspect ratio and particle size, respectively.

[0010] In the metal powder formation method according to this embodiment, a metal powder M1 having a particle size of, for example, 1 μm to 500 μm can be formed. The metal powder M1 can also be used as a molding material in a metal 3D printer (not shown). In that case, the particle size of the metal powder M1 is, for example, 1 μm to 100 μm. The mode diameter of the metal powder M1 may also be 25 μm to 75 μm. The mode diameter is the particle size or range of particle size that is most frequently present in the measurement sample. The metal powder M1 can be used, for example, as a powder material supplied to the powder bed in a powder bed type 3D printer. The metal powder M1 may also be used in other types of metal 3D printers. For example, it may be used as a powder material mixed with a thermoplastic resin in a fused deposition modeling (FDM) 3D printer.

[0011] The closer the aspect ratio of the metal powder M1 is to 1, the closer its shape is to a sphere. In other words, a closer aspect ratio of the metal powder M1 is to 1 improves its fluidity. When the fluidity of the metal powder M1 is improved, it can be spread more densely and uniformly in the powder bed of the powder bed type 3D printing described above, thus improving the quality of the 3D printed product. In addition to 3D printing as described above, the metal powder M1 can also be used as a raw material for metal injection molding and thermal spraying. In these cases as well, if the fluidity of the metal powder M1 is good, the supply of the metal powder M1 inside the injection molding machine or thermal spraying machine can be carried out smoothly.

[0012] The metal powder formation method according to the embodiment will be described below with reference to the drawings. Furthermore, components having the same function as those already described will be denoted by the same reference numerals and their descriptions will be omitted.

[0013] The workpiece 1 is a metal component that serves as the material for the metal powder M1, and is composed of, for example, aluminum, aluminum alloy, copper, copper alloy, etc. In the examples shown in Figures 1 to 3, the workpiece 1 extends with a first axis A1 as its central axis and has a substantially circular shape in a cross section perpendicular to the first axis A1. Therefore, the workpiece 1 as a whole has a cylindrical shape. However, the shape of the workpiece 1 is not limited to this, and for example, a part of the workpiece 1 may have a smaller radial dimension than other parts. Also, the workpiece 1 may have shapes such as an elliptical prism or a polygonal prism.

[0014] The workpiece 1 has a first surface P1 in its state before the rolling and cutting processes are performed. The first surface P1 has an axisymmetric shape around the first axis A1. In the example shown in Figure 1, the first surface P1 constitutes the outer circumferential surface of the workpiece 1. That is, the first surface P1 has the shape of a cylindrical surface with the first axis A1 as its central axis.

[0015] The rolling process is a process of forming a plurality of protrusions 30 on the first surface P1 of the workpiece 1. Rolling is a type of plastic deformation. In the following description, the surface of the workpiece 1 on which the plurality of protrusions 30 have been formed by rolling will be referred to as the second surface P2. In the rolling process, the workpiece 1 is plastically deformed by pressing rolling rollers 10A and 10B against the first surface P1, thereby forming a plurality of protrusions 30. The plurality of protrusions 30 are formed in a state where they are aligned in at least one direction. The material of the rolling rollers 10A and 10B should have a higher hardness than the workpiece 1. For example, the rolling rollers 10A and 10B are made of cemented carbide. In addition, a sliding film may be formed on the surfaces of the rolling rollers 10A and 10B. This sliding film may be, for example, a DLC (Diamond-Like Carbon) coating. Furthermore, the sliding film is formed by known film formation methods such as PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition).

[0016] In the rolling process, rolling rollers 10A and 10B, which serve as molds having a rolling pattern, are pressed against the workpiece 1 to plastically deform the first surface P1 of the workpiece 1 and form the rolling pattern. The first surface P1 of the workpiece 1 is plastically deformed without being cut away, and a plurality of protrusions 30 are formed on the second surface P2 after the rolling process. On the other hand, in the cutting process, the plurality of protrusions 30 formed by the rolling process are cut away with a cutting tool 20. When the plurality of protrusions 30 are cut away, metal powder M1 is obtained as cutting dust. In the following description, the surface of the workpiece 1 after the plurality of protrusions 30 have been cut away will be referred to as the third surface P3.

[0017] A pair of rolling rollers 10A and 10B are arranged on one radial side of the workpiece 1, and a cutting tool 20 is arranged on the other side. The cutting tool 20 in this embodiment is a cutting tool. The arrangement of the workpiece 1, the rolling rollers 10A and 10B, and the cutting tool 20 is not limited to the illustrated example and can be appropriately set according to the processing apparatus for carrying out the metal powder formation method according to this embodiment, as well as the shape, dimensions, etc. of the workpiece 1.

[0018] The rolling roller 10A (hereinafter referred to as the first rolling roller 10A) is rotatable around the second axis A2 and has a disc shape when viewed in the axial direction parallel to the second axis A2. The rolling roller 10B (hereinafter referred to as the second rolling roller 10B) is rotatable around the third axis A3 and has a disc shape when viewed in the axial direction parallel to the third axis A3. The second axis A2 and the third axis A3 extend parallel to the first axis A1, but are not limited to this. The second axis A2 or the third axis A3 may extend in a direction inclined with respect to the first axis A1.

[0019] A rolling pattern 11 is formed on the outer circumferential surface of the first rolling roller 10A. The rolling pattern 11 is composed of a plurality of protrusions. Each of these protrusions extends in a direction intersecting the second axis A2 in a radial view of the first rolling roller 10A and is arranged parallel to one another. The rolling pattern 12 formed on the outer circumferential surface of the second rolling roller 10B is formed by extending a plurality of protrusions in a direction intersecting the extending direction of the protrusions of the first rolling roller 10A. The first rolling roller 10A and the second rolling roller 10B form a diagonal rolling pattern on the outer circumferential surface of the workpiece 1, in which a large number of protrusions 30 are arranged in the rotational and axial directions of the workpiece 1.

[0020] In this embodiment, the first rolling roller 10A and the second rolling roller 10B form a diagonal rolling pattern consisting of multiple protrusions 30 on the outer circumferential surface of the workpiece 1. However, the diagonal rolling pattern consisting of multiple protrusions 30 may also be formed on the outer circumferential surface of the workpiece 1 by a single rolling roller having a diagonal rolling pattern. The cutting tool 20 is equipped with a cutting blade 21 at its tip. The cutting blade 21 is a flat blade extending in a direction parallel to the first axis A1. In this embodiment, in a direction parallel to the first axis A1, the width of the cutting blade 21 of the cutting tool 20 and the widths of the first rolling roller 10A and the second rolling roller 10B are approximately equal.

[0021] In the rolling process according to this embodiment, one end of the workpiece 1 is fixed to the chuck of an NC (numerically controlled) lathe, and the rolling process is performed while the workpiece 1 rotates around the first axis A1. During the rolling process, the first rolling roller 10A is pressed with a predetermined load against the first surface P1 of the workpiece 1 as it rotates around the first axis A1. As a result, as shown in Figure 4, a portion of the first surface P1 is crushed and another portion is raised in a shape corresponding to the rolling pattern of the first rolling roller 10A, forming a plurality of parallel ridges 40.

[0022] The circumferential surface of the workpiece 1 on which the protrusions 40 are formed by the first rolling roller 10A is immediately followed by the formation of protrusions intersecting the protrusions 40 by the second rolling roller 10B. Since the protrusions 40 have already been formed, the formation of the protrusions, which are the rolling pattern of the second rolling roller 10B, causes the protrusions 40 to be intermittently crushed, and as shown in Figure 5, convex portions 30 are formed on the second surface P2 after the rolling process. In this embodiment, the convex portions 30 have the shape of a square pyramid with a rhombus-shaped base and a rounded apex. In a rhombus, a pair of diagonals are perpendicular and the lengths of its four sides are equal. The above rhombus does not include a square, and the lengths of the pair of diagonals are different. In a diagonal rolling pattern, these square pyramidal convex portions 30 with rhombus-shaped bases are continuously arranged in two dimensions on the circumferential surface of the workpiece 1. The two pairs of opposite sides of the rhombus are parallel to each other. Therefore, the first rolling roller 10A and the second rolling roller 10B, each having parallel ridges as a rolling pattern, are suitable for forming a twill rolling pattern in which diamonds are arranged.

[0023] When performing the rolling process, the pitch of the multiple protrusions 30 can be adjusted by adjusting the spacing between the ridges of the rolling pattern of the first rolling roller 10A and the second rolling roller 10B. The pitch of the protrusions 30 is the distance between the vertices of adjacent protrusions 30, and as can be seen from Figure 7, it is equal to the length of the sides of the rhombus described above. Figure 7 is a diagram showing a part of the rolling pattern formed on the circumferential surface of the workpiece 1 unfolded on a plane. In this embodiment, there is a circumferential pitch and an axial pitch, and the circumferential pitch is equal to the length L1 of the diagonal DL extending in the circumferential direction of the rhombus described above (the direction of arrow C in Figure 7). The axial pitch is equal to the length L2 of the diagonal DS extending in the axial direction of the rhombus described above (the direction perpendicular to arrow C).

[0024] Furthermore, the height H1 of the formed protrusion 30 can be controlled by the pressing load of the first rolling roller 10A and the second rolling roller 10B in the rolling process. For example, by reducing the pressing load of the first rolling roller 10A and the second rolling roller 10B when performing the rolling process, the height H1 of the protrusion 30 can be made lower.

[0025] Next, the cutting process will be described. The multiple protrusions 30 that have been formed are cut by the cutting tool 20. In the cutting process, as shown in Figure 5, the protrusions 30 are cut by moving the cutting tool 20 relative to the workpiece 1 in the rotational direction of the workpiece 1, i.e., in the circumferential direction. In this embodiment, the cutting tool 20 is fixed, and the protrusions 30 are cut as the workpiece 1 is rotated. Alternatively, the first rolling roller 10A, the second rolling roller 10B, and the cutting tool 20 may be moved relative to the workpiece 1 in the axial direction. In this case, by adjusting the axial movement speed to match the axial pitch described above, cutting can be performed while continuously rolling the aforementioned rolling pattern in the axial direction on the circumferential surface of the workpiece 1.

[0026] In order to obtain a large amount of metal powder M1 by cutting the protrusion 30, the cutting depth H2 (see Figure 7) is less than the height H1 of the protrusion 30, preferably about 75% of the height H1. However, if the cutting depth H2 is made too small relative to the height H1, the depth of the rolling groove 50 remaining on the third surface P3 after cutting, as shown in Figure 6, will become too deep. If the third surface P3 is used as the first surface and the rolling process is carried out again, if the rolling groove 50 is too deep, traces of the rolling groove 50 may remain on the protrusion 30.

[0027] Next, with reference to Figures 7 and 8, the rolling pattern of the protrusions 30 formed on the second surface P2 and the shape of the metal powder M1 obtained by cutting the protrusions 30, particularly the aspect ratio, will be explained.

[0028] Figure 7 shows a portion of the second surface P2 unfolded on a plane, with protrusions 30 formed on the second surface P2. Here, each protrusion 30 has a rhombus-shaped (excluding square) base, and the longer diagonal DL of the pair of diagonals DL and DS of this rhombus coincides with the rotational direction of the workpiece 1. This rotational direction is the circumferential direction as described above, and is also the cutting direction C of the cutting tool 20. In other words, the cutting direction C of each protrusion 30 is the direction of the longer diagonal DL of the rhombus on its base. By cutting the protrusion 30 along this cutting direction C, the aspect ratio of the metal powder M1 formed after cutting is improved. This point will be discussed later with reference to Figure 8. As mentioned above, the length of the longer diagonal DL is L1, and the length of the shorter diagonal DS is S1 (S1 < L1).

[0029] Furthermore, as shown in Figure 7, of the two pairs of interior angles α and β of the rhombus, the larger interior angle is denoted as α and the smaller interior angle as β (α > β). Since it is a rhombus, the angle opposite to interior angle α is also interior angle α, and the angle opposite to interior angle β is also interior angle β. Also, α > 90 degrees and β < 90 degrees. In this embodiment, the larger interior angle α is set to be between 100 degrees and 140 degrees, and this range is preferable to achieve a desirable aspect ratio. This preferred angle will also be discussed later.

[0030] As described above, the cutting depth H2 of the protrusion 30 is less than its height H1 (H1 > H2). In Figure 7, the cutting position by the cutting blade 21 of the cutting tool 20 is shown by a dashed line. When cut, the upper part of the protrusion 30 that is removed as metal powder M1 is deformed, but the rhombus shape of the bottom surface of this removed part is smaller than the rhombus shape of the diagonals DL and DS (each with lengths L1 and S1) because H1 > H2. Let L2 be the length of the longer diagonal and S2 be the length of the shorter diagonal of the rhombus shape of the bottom surface of the metal powder M1 (L2 < L1, S2 < S1).

[0031] When a square pyramidal protrusion 30 with a rhombus-shaped base is cut with a cutting tool 20 parallel to the base surface along a cutting direction C parallel to the long diagonal DL, stress acts on the protrusion 30 during cutting. Due to this stress, the protrusion is crushed in the cutting direction C as it is cut, and the length of the metal powder M1 in the cutting direction C becomes shorter, and its height increases accordingly. Figure 8 illustrates this deformation. However, Figure 8 is a schematic representation, and the base surface of the cut metal powder M1 is depicted as flat. In reality, the base surface of the metal powder M1 becomes curved after cutting, but the metal powder M1 is shown schematically to make the deformation associated with cutting described above easier to understand.

[0032] As can be seen from Figures 7 and 8, the length of the bottom surface in the cutting direction C decreases from L2 to L3 (L3 < L2). On the other hand, the height increases from H2 to H3. Also, the length in the direction perpendicular to the cutting direction C, i.e., the width, changes from S2 to S3. S3 is approximately the same as or slightly larger than S2. The aspect ratio is the ratio of the major axis (major axis diameter) to the minor axis (minor axis diameter) of powder or particles. Due to the deformation described above during cutting, the aspect ratio of the cut metal powder M1 improves.

[0033] If the convex portion 30 deforms during cutting so that the values ​​of L3, S3, and H3 after deformation are close to each other, and metal powder M1 is formed, the aspect ratio of the metal powder M1 will approach 1. However, if L2 before cutting is too large compared to S2, then even after cutting, L3 will be too large relative to S3, and the aspect ratio will not improve. Here, if the large interior angle α takes a large value, L1 and L2 will become longer. Consequently, the relatively small interior angle β will become smaller, and S1 and S2 will become shorter. Therefore, the relationship between the large interior angle α and the aspect ratio of the metal powder M1 after cutting was investigated. The graph showing the results is shown in Figure 9.

[0034] Atomization is a known method for manufacturing metal powders and metal particles used in 3D printing, metal injection molding, or metal spraying. The aspect ratio of metal particles produced by atomization is preferably 0.7 or higher. As can be seen from the graph in Figure 9, a good aspect ratio is obtained when the large internal angle α is between 100 degrees and 140 degrees. As described above, in the embodiment, α > 90 degrees in order to make S1 < L1, but if α is less than 100 degrees, the aspect ratio becomes somewhat low. On the other hand, if α exceeds 140 degrees, as described above, L3 after cutting becomes too large relative to S3, and the aspect ratio does not improve. Note that the peak and range of α in the above graph may vary depending on adjustments to the ductility of the workpiece material 1, the sharpness of the cutting tool 20, and the rotational speed of the workpiece 1. For example, the aspect ratio may improve even if α exceeds 140 degrees.

[0035] Furthermore, the cutting depth H2 of the protrusion 30 was also examined. As a result, it was found that a cutting depth H2 of the protrusion 30 that is 30% to 50% of the length S1 of the short diagonal DS of the rhombus on the bottom surface of the protrusion 30 is suitable for obtaining a good aspect ratio. Note that since S2 < S1, the ratio of H2 to S2 is a little larger than that of S1. As mentioned above, the height H3 of the metal powder M1 after cutting is greater than the cutting depth H2. Also, the width S3 of the metal powder M1 after cutting does not change much compared to S2 before cutting. Therefore, by setting the cutting depth H2 to 30% to 50% of the length S1 of the short diagonal DS before cutting, the height H3 of the metal powder M1 after cutting can be brought closer to the width S3 and length L3 of the metal powder M1. If the cutting depth H2 is less than 30% of the length S1, the height H3 after cutting will be smaller than the width S3 and length L3, and the aspect ratio will not improve. Conversely, if the cutting depth H2 exceeds 50% of the length S1, the height H3 after cutting becomes larger than the width S3 and length L3, and the aspect ratio does not improve. Furthermore, if the cutting depth H2 exceeds 50% of the length S1, H2 may become larger than the height H1 of the protrusion 30, potentially failing to satisfy the condition for obtaining metal powder M1 (H2 < H1).

[0036] As described above, the rotation directions of the first rolling roller 10A and the second rolling roller 10B coincide with the cutting direction C. In this embodiment, the more protrusions 30 arranged perpendicular to the cutting direction C within the rolling width of a single roll by the first rolling roller 10A or the second rolling roller 10B there are, the greater the rolling load. That is, when there are many protrusions 30 arranged perpendicular to the cutting direction C there are also many grooves between the protrusions 30, and the large interior angle α takes on a large value. Consequently, the number of contact points between the ridges on the rolling pattern of the first rolling roller 10A or the second rolling roller 10B, which form grooves on the circumferential surface of the workpiece 1, and the circumferential surface of the workpiece 1 increases. As a result, the stress at each contact point between the workpiece 1 and the first rolling roller 10A or the second rolling roller 10B becomes smaller. Therefore, in this embodiment, the rolling load is increased as the number of protrusions 30 aligned perpendicular to the cutting direction C within a single rolling width by the first rolling roller 10A or the second rolling roller 10B increases. By doing so, the stress at each contact point can be kept constant, and grooves of an appropriate depth can be formed. That is, protrusions 30 having a desired height H1 can be formed.

[0037] (1) The metal powder forming method according to the embodiment forms metal powder M1 by cutting the surface of a metal workpiece 1. In this method, a plurality of convex portions 30 with a rhombus-shaped base and a square pyramidal shape are formed on the surface of the workpiece 1 by rolling, and the metal powder M1 is formed by cutting the plurality of convex portions 30 along the direction of the longer diagonal DL of the pair of diagonals DL and DS of the rhombus. That is, during cutting, the plurality of convex portions 30 are crushed in the cutting direction C as they are cut and become metal powder M1, and their length in the cutting direction C becomes shorter (L2 → L3). As a result, the convex portions 30 with a rhombus-shaped base and a square pyramidal shape are crushed in the direction of the longer diagonal DL, so that the length L3 of the metal powder M1 after cutting becomes close to the width S3, and the aspect ratio of the metal powder M1 is improved. When the aspect ratio of the metal powder M1 is improved, the fluidity of the metal powder M1 is improved, and the quality of the metal powder M1 as a processing raw material is improved.

[0038] (2) In the embodiment, of the two pairs of interior angles α and β of the rhombus described above, the larger interior angle α is 100 degrees or more and 140 degrees or less. By setting the larger interior angle α within the above angular range, a metal powder M1 with a good aspect ratio can be formed.

[0039] (3) Furthermore, in the embodiment, the cutting depth H2 of the multiple protrusions 30 is 30% to 50% of the length S1 of the shorter diagonal DS of the pair of diagonals DL and DS of the rhombus described above. That is, the above method relates to improving the aspect ratio by focusing on the cutting depth H2 related to the height H1 of the multiple square pyramidal protrusions 30. As described above, during cutting, the protrusions 30 are crushed in the cutting direction C as they are cut and become metal powder M1, and the length in the cutting direction C becomes shorter (L2 → L3). Accordingly, during cutting, the height of the protrusions 30 increases as they are cut and become metal powder M1 (H2 → H3). By setting the cutting depth H2 within the above range, the height H3 of the metal powder M1 after cutting becomes close to the length L3 and width S3 after cutting, and the aspect ratio of the metal powder M1 is further improved. When the aspect ratio of the metal powder M1 is improved, the fluidity of the metal powder M1 is further improved, and the quality of the metal powder M1 as a processing raw material is further improved.

[0040] (4) In this embodiment, when forming a plurality of protrusions 30 on the surface of the workpiece 1 using the first rolling roller 10A and the second rolling roller 10B, the rotation direction of the first rolling roller 10A and the second rolling roller 10B is the cutting direction C, and the greater the number of protrusions 30 arranged perpendicular to the cutting direction C within a single rolling width by the first rolling roller 10A and the second rolling roller 10B, the greater the rolling load of the first rolling roller 10A and the second rolling roller 10B. That is, when the number of protrusions 30 arranged perpendicular to the cutting direction C is large, the number of contact points between the ridges on the rolling pattern of the first rolling roller 10A or the second rolling roller 10B and the circumferential surface of the workpiece 1 increases. Therefore, by increasing the rolling load as the number of protrusions 30 aligned perpendicular to the cutting direction C increases, the stress at each contact point between the ridges on the rolling pattern of the rolling roller 10A or 10B and the workpiece 1 can be kept constant, and grooves of an appropriate depth can be formed. In other words, protrusions 30 having a desired height H1 can be appropriately formed.

[0041] The above embodiments are merely illustrative examples provided to facilitate understanding of the invention. The technical scope of the invention is not limited to the specific technical matters disclosed in the above embodiments, but also includes various modifications, changes, and alternative technologies that can be easily derived therefrom. For example, in the above embodiments, as shown in Figure 2, rolling and cutting are continuously performed on the circumferential surface of the rotating cylindrical workpiece 1, so the third surface P3 can become the first surface P1 for the next rolling process. However, a cutting tool may be added between the cutting tool 20 and the first rolling roller 10A to cut the third surface P3 and remove the rolling groove 50 shown in Figure 6 to form a clean first surface P1.

[0042] Furthermore, in the above embodiment, rolling was performed on the circumferential surface of the cylindrical workpiece 1 using the first rolling roller 10A and the second rolling roller 10B. However, rolling may also be performed by pressing a flat rolling die, on which a pattern of image transfer is formed on its surface, onto the circumferential surface of the cylindrical workpiece. In this case, the rolling die slides as the workpiece rotates. Alternatively, rolling may be performed by transferring the pattern of image transfer formed on the surface of a flat rolling die to the surface of a flat workpiece by press working. In any case, the cutting direction is along the direction of the long diagonal of the rhombus.

[0043] 1. Workpiece 10A First rolling roller (rolling roller) 10B Second rolling roller (rolling roller) 20. Cutting tool 30. Protrusion C. Cutting direction DL. Long diagonal DS. Short diagonal H2. Depth of cut M1. Metal powder S1. Length (of the short diagonal DS) α. Large interior angle

Claims

1. A method for forming metal powder by cutting the surface of a metal workpiece, wherein a plurality of convex portions in the shape of a square pyramid with a rhombus base are formed on the surface of the workpiece by rolling, and the metal powder is formed by cutting the plurality of convex portions along the direction of the longer of the pair of diagonals of the rhombus.

2. The metal powder forming method according to claim 1, wherein the larger of the two pairs of interior angles of the rhombus is 100 degrees or more and 140 degrees or less.

3. The metal powder forming method according to claim 1 or 2, wherein the depth of the cuts in the plurality of protrusions is 30% to 50% of the length of the shorter of the pair of diagonals of the rhombus.

4. A method for forming metal powder according to any one of claims 1 to 3, wherein the rotation direction of the rolling roller when forming the plurality of protrusions on the surface of the workpiece using the rolling roller is the cutting direction, and the rolling load of the rolling roller is increased as the number of the plurality of protrusions arranged perpendicular to the cutting direction within a single rolling width by the rolling roller increases.

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