Manufacturing method of sintered body

Abstract

To provide a sintered body that is nearly uniform in strength at every portion even where complicated in shape.SOLUTION: A gear 1, as a sintered body, having a generally disk-like shape having a diameter of 100 mm or smaller, comprises a gear body 2 and a plurality of teeth 3 arranged along a circumferential direction of the gear body 2 and protruding radially outward, in which density falls under a standard deviation of smaller than 0.1 at circumferentially equally-distributed n points (n: integer equal to or greater than 2) and porosity in area falls under a standard deviation of smaller than 1.5 at circumferentially equally-distributed m points (m: integer equal to or greater than 2) of the sintered body.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sintered body obtained by sintering a powder compact. [Background technology]

[0002] Generally, sintered compacts are formed by applying pressure to powder. For example, to form a ring-shaped sintered compact, (1) a pipe-shaped cavity is created using a die, a lower punch, and a core pin, (2) the cavity is filled with mixed powder, (3) the excess mixed powder is removed, (4) the upper punch is set in place, and pressure is applied from above and below, and (5) the pressure is released, and the lower punch is pushed up to remove the molded product from the die. There are two methods for filling powder: "drop-in" and "suction-in," and the filling method for moldings (1) to (5) above is "drop-in." Note that "suction-in" is a method in which the heights of the die and lower punch are aligned, and with sufficient mixed powder placed on top, the die is raised or the lower punch is lowered to fill the mixed powder into the cavity.

[0003] The powder compact formed in this manner is sintered to obtain a sintered body that can be used for various parts, etc. For example, if the sintered body is a sintered oil-impregnated bearing, the powder compact is subjected to degreasing, sintering, sizing, cleaning, oil impregnation, etc. in this order.

[0004] Here, variations in the filling of the sintered body may occur depending on the method of filling the mixed powder. For example, variations in filling in the vertical direction or phase will result in density differences (density unevenness) during molding. This density unevenness may lead to variations in the strength of each part of the sintered body. Furthermore, during sintering, shrinkage will vary, making it difficult to maintain the specified dimensional accuracy and causing warping. Both the "drop-in" and "suction-in" filling methods are prone to density unevenness, with "drop-in" in particular tending to result in greater density unevenness. One method of reducing density unevenness is to vibrate the mold after filling the powder, but there are many areas where the vibration is not transmitted from the mold, so its effectiveness is thought to be limited.

[0005] Conventionally, known techniques for controlling powder density include the molding method described in Patent Document 1. Patent Document 1 describes a molding method in which powder filled in a die is compressed between upper and lower punches to obtain a green compact, in which the powder is leveled off and filled into a die having a large-diameter portion at the upper edge of the inner periphery of the die cavity, the die is temporarily raised, and the powder that was in the large-diameter portion flows down and accumulates inside the die cavity, and then compression begins.

[0006] Another known technique is a method in which air is blown into a powder box to agitate the powder in the powder box and fill the cavity with the air (aerated filling method). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 03-044880 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the molding method described in Patent Document 1 increases the density only in necessary areas of the sintered body, and is thought to be less effective for complex shapes such as gears. Also, the aerated filling method does not actively move the powder within the cavity, so density unevenness still occurs, and with sintered bodies with complex shapes, etc., there is a risk that the strength of each part will vary due to density unevenness.

[0009] The present invention has been made in view of the above circumstances, and has as its object to provide a sintered body in which the strength of each part is approximately constant even if it has a complex shape. [Means for solving the problem]

[0010] The sintered body of the present invention is a sintered body obtained by sintering a powder compact, and is characterized in that the standard deviation of density at n equally distributed locations (n ​​is an integer of 2 or more) around the circumference of the sintered body is less than 0.1.

[0011] The sintered body is generally disk-shaped, and the standard deviation of the density at four equally spaced locations in the circumferential direction is less than 0.1. The sintered body is further characterized in that the diameter of the sintered body is 100 mm or less.

[0012] The sintered body is characterized in that the standard deviation of the pore area ratio at m equally spaced locations (m is an integer of 2 or more) in the circumferential direction is less than 1.5.

[0013] The sintered body is characterized in that it is a gear having a plurality of teeth protruding radially outward. [Effects of the Invention]

[0014] The sintered body of the present invention has a density standard deviation of less than 0.1 at n equally distributed locations in the circumferential direction of the sintered body, so that density unevenness is small, the strength of each part is approximately constant, and strength can be homogenized. Furthermore, since the density unevenness is small, warping (deformation) of the sintered body is suppressed, so a sizing process to correct deformation is not required, leading to cost reduction.

[0015] Furthermore, the standard deviation of the pore area ratio at points evenly distributed m in the circumferential direction of the sintered body is less than 1.5, which further contributes to the homogenization of strength.

[0016] Even for gears with complex shapes for which it is relatively difficult to achieve uniform strength, density variations are small and the strength of each part can be made approximately uniform. [Brief explanation of the drawings]

[0017] [Figure 1] 1A and 1B are schematic plan views of a gear as an example of a sintered body of the present invention. [Figure 2] FIG. 2 is a diagram showing an example of equally spaced locations in the circumferential direction of the sintered gear of FIG. 1. [Figure 3] 1 is a process schematic diagram showing the production of a sintered body of the present invention. FIG. [Figure 4] FIG. 10 is a diagram for explaining the flow of a filling process. [Figure 5]FIG. 10 is a diagram showing an example of a jig used in a filling step. [Figure 6] 1 is a graph showing the gear weight of a powder compact in Test Example 1. [Figure 7] 1 is a graph showing the thickness of the sintered body in the vicinity of the tooth root in Test Example 1. [Figure 8] FIG. 1 is a diagram showing the observation direction of gear teeth. [Figure 9] 1 is an observation image of a tooth tip surface of a sintered body in Test Example 1. [Figure 10] 1 is an observation image of an axial cross section of a sintered body in Test Example 1. [Figure 11] 1 is an observation image of an axial cross section of a sintered body in Test Example 1. [Figure 12] 1 is an observation image of an axial cross section of a sintered body in Test Example 1. [Figure 13] 1 is a graph showing the density of a sintered body in Test Example 1. [Figure 14] FIG. 1 is a schematic diagram of a tooth breakage test in Test Example 1. [Figure 15] 1 is a graph showing the tooth fracture load of the sintered body in Test Example 1. [Figure 16] 1 is a graph showing the spanning tooth thickness of the sintered body in Test Example 1. [Figure 17] FIG. 1 is a graph showing the particle size distribution of the iron powder used in Test Example 2. [Figure 18] 10 is a graph showing gear weights of powder compacts in Test Example 2. [Figure 19] 10 is an observation image of a tooth tip surface of a sintered body in Test Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0018] The sintered body of the present invention will be described below with reference to the drawings. The sintered body of the present invention is used for machine parts and the like that require high strength, hardness, etc. Examples of the sintered body include gears and rolling elements for bearings.

[0019] 1 is a plan view schematically showing a gear as an example of the sintered body of the present invention. The direction parallel to the central axis O of the gear is called the "axial direction," the direction perpendicular to the central axis O is called the "radial direction," and the direction around the axis centered on the central axis O is called the "circumferential direction."

[0020] As shown in Figure 1, the gear 1 has a substantially circular outer shape when viewed in any one direction (for example, in a plan view). The gear 1 is substantially disk-shaped and has a gear body 2 and a plurality of teeth 3 arranged along the circumferential direction of the gear body 2 and protruding radially outward. In Figure 1, there are 28 teeth 3 formed, and these are arranged at equal intervals in the circumferential direction. A mounting hole 4 for a rotating shaft is formed in the center of the gear body 2, and a key groove 5 is formed in part of the circumferential direction.

[0021] According to the present invention, even for gears with complex shapes, density variations in each part can be suppressed and excellent dimensional accuracy can be achieved. The number of teeth on the gear is not particularly limited. Furthermore, the outer shape of the gear is not limited to a substantially circular shape in plan view as shown in FIG. 1, but may also be a substantially elliptical shape, a substantially triangular shape, or a substantially rectangular shape. Furthermore, teeth do not have to be formed along the entire circumference of the gear body; for example, teeth may be formed only along a portion of the entire circumference of the gear body (for example, a portion within a 60° range in the circumferential direction).

[0022] The size of the gear is not particularly limited and is set appropriately depending on the intended use. For example, the gear diameter φ (referring to the tooth tip circle diameter d in FIG. 1) may be 200 mm or less, or may be 100 mm or less. As the gear diameter becomes smaller, the teeth themselves tend to become smaller, resulting in a powder particle size that becomes larger relative to the tooth size, making it more difficult to fill the powder. Even in such cases, according to the present invention, filling unevenness can be reduced. The gear diameter φ may be, for example, 30 mm or more.

[0023] The tooth thickness t (see FIG. 1) is, for example, 1 mm to 20 mm, preferably 3 mm to 10 mm, and the tooth module is, for example, 0.3 mm to 1.5 mm, preferably 0.5 mm to 1.5 mm.

[0024] The sintered body of the present invention has small density variations in each part, and the density [unit: g / cm 3 ] at n equally distributed points in the circumferential direction (n is an integer of 2 or more) 3 ] standard deviation σ d is less than 0.1. In the present invention, the circumferentially uniformly distributed portions refer to portions that are approximately uniformly distributed in a direction around an axis centered on the central position of the outer shape of the sintered body when viewed in any one direction (for example, in a plan view). d is expressed by the following equation (1).

[0025]

number

[0026] In addition, the above standard deviation σ d is preferably less than 0.08, more preferably less than 0.06.

[0027] Using a gear as the sintered body, an example of equally spaced locations in the circumferential direction is shown in Figure 2. Figure 2 shows four equally spaced locations in the circumferential direction (0°, 90°, 180°, 270°) with the center O of the gear 1 as the center. In Figure 2, the position where the key groove is located is set to 0°. In this case, corresponding parts (for example, parts surrounded by dotted lines in Figure 2) are cut out from these locations, and the densities are calculated, and the average value d ave. and standard deviation σ d It should be noted that the corresponding parts need not be identical as long as they have the same shape and volume to some extent.

[0028] Furthermore, the sintered body of the present invention has a standard deviation σ of the pore area ratio (unit: %) at m equally distributed locations in the circumferential direction (m is an integer of 2 or more, and may be the same as or different from n). r It is preferable that the standard deviation σ is less than 1.5. r is expressed by the following equation (2).

[0029]

number

[0030] The pore area ratio can be calculated by image analysis of a photographed image (image taken with a magnifying glass, microscope, or SEM image) of a cross section of a sintered body within a predetermined field of view, as shown in the examples below.

[0031] The above standard deviation σ r is preferably less than 1.2, and may be less than 1.0.

[0032] The configuration of the gear as the sintered body of the present invention is not limited to that shown in Fig. 1. For example, it is not limited to the spur gear shown in Fig. 1, but may be a bevel gear, a screw gear, a worm gear, or the like.

[0033] The production of the sintered body of the present invention will be described below.

[0034] As shown in Figure 3, the sintered body of the present invention is manufactured through (1) a filling step of filling a cavity with raw material powder, (2) a molding step of compression-molding the filled powder, and (3) a sintering step of sintering the green compact obtained by compression molding. The obtained sintered body may be subjected to mechanical processing such as polishing or shot peening, if necessary. Since the manufacturing methods (1) to (3) above suppress deformation after sintering, it is preferable not to perform a sizing step after sintering. Each step will be explained below.

[0035] The main material constituting the sintered body is a metal powder or a ceramic powder. Examples of metal powder include Fe (iron) powder and iron alloy powder containing iron as the main component. The iron alloy containing iron as the main component contains, for example, 50% by mass or more of iron element, preferably 80% by mass or more of iron element, and more preferably 90% by mass or more of iron element. Elements added to the alloy include, for example, at least one of Co (cobalt), Cr (chromium), Cu (copper), Mn (manganese), Mo (molybdenum), Ni (nickel), W (tungsten), V (vanadium), Si (silicon), and C (carbon). Examples of the metal powder that can be used include those produced by water atomization, gas atomization, reduction, carbonyl method, etc.

[0036] The average particle size of the metal powder is not particularly limited, but is, for example, 20 μm to 200 μm. By setting the average particle size to 20 μm or more, it is easy to ensure the fluidity of the raw material powder, and by setting it to 200 μm or less, it is easy to pack the metal powder more densely in the packing step. The average particle size is preferably 50 μm to 150 μm. The average particle size is the particle size (D50) at which the cumulative volume in the volume particle size distribution measured by a laser diffraction particle size distribution analyzer becomes 50%.

[0037] As the ceramic powder, powders such as silicon nitride (Si3N4), silicon carbide (SiC), aluminum oxide (Al2O3), zirconium oxide (ZrO2), yttrium oxide (Y2O3), sialon, etc. may be used alone or in combination of two or more.

[0038] The raw material powder contains metal powder or ceramic powder as a main component, and the sintered body obtained is a metal sintered body or a ceramic sintered body.

[0039] The raw material powder may contain a lubricant to improve fluidity. Examples of lubricants include fatty acids such as stearic acid and metal salts thereof. The content of the lubricant is not particularly limited, but is preferably 0.1% to 3% by mass, and more preferably 0.1% to 2% by mass, based on the total amount of the raw material powder.

[0040] The raw material powder may also contain other materials, such as graphite powder. The amount of graphite powder contained is not particularly limited, but is preferably 0.3% by mass to 1% by mass of the total amount of the raw material powder.

[0041] The raw material powder can be obtained by mixing metal powder, ceramic powder, lubricant, and other materials by a well-known mixing method. The mixing method is not particularly limited, and dry mixing can be performed using, for example, a Henschel mixer, a ball mixer, a ribbon blender, a Loedige mixer, an Ultra Henschel mixer, or the like. Commercially available raw material powders may also be used.

[0042] (1) Filling process This process involves filling the cavity with prepared raw material powder, and includes the steps of (1a) adding the raw material powder to the cavity and (1b) vibrating the added raw material powder using a jig. This filling process will be explained with reference to Figure 4. Note that Figure 4 shows the case of the "drop-in" filling method.

[0043] In FIG. 4, first, in the state (a), the lower punch 6 of the molding die is lowered to form a cavity C of a predetermined depth. The cavity C is formed by the lower punch 6, die 7, etc., and has a shape corresponding to the shape of the sintered body to be manufactured. In the state (a), a plate material 8 for forming a step is placed above the die 7 so that raw material powder can be deposited also on the upper surface of the cavity C. The height of this plate material 8 is not particularly limited, but is, for example, 1 mm to 10 mm.

[0044] Next, raw material powder is added to cavity C and is leveled to the height of plate 8 (b). In this state, jig 9 is inserted into cavity C (c). Jig 9 has multiple protrusions 9a, and the tips of these protrusions 9a are inserted to the deepest part of cavity C (the bottom surface of the cavity in Figure 4(c)). Then, vibration device 10 is attached to jig 9, and vibration is applied for a predetermined period of time (d). After vibration, jig 9 is pulled out from cavity C (e), plate 8 is removed, and the raw material powder is leveled on the top surface of die 7 (f). This fills cavity C with raw material powder.

[0045] When the jig 9 is removed from the state shown in Fig. 4(d), the vibration may be stopped before the jig 9 is removed, or the jig 9 may be removed while the vibration is still applied. The vibration time (including the time when the jig is removed while the vibration is still applied) is not particularly limited, but is preferably 1 second or more, and more preferably 3 seconds or more. The results of the examples described later show that extending the vibration time does not significantly improve the density or strength, so the vibration time may be, for example, 60 seconds or less, or even 30 seconds or less.

[0046] As shown in Figure 4, by inserting the protrusions 9a of the jig 9 into the raw material powder in the cavity C and vibrating it with a vibration device 10, the raw material powder vibrates, moving like a pseudo-fluid, and is packed evenly and closely to the finest details. Furthermore, since even areas where the raw material powder has small bridging during filling are packed closely in the same way, it is possible to reduce uneven filling in areas with complex shapes and in areas such as the top and bottom, and to pack the entire cavity evenly and closely.

[0047] The jig will be explained with reference to Figure 5. Figure 5(a) is a plan view of the jig seen from the protrusion side, Figure 5(b) is a cross-sectional view taken along line BB, and Figure 5(c) is a diagram showing the correspondence between the gear (sintered body) and the jig protrusions. As shown in Figures 5(a) and (b), the jig 9 has a disk-shaped base material 9b and multiple protrusions 9a provided upright on the surface of the base material 9b. The protrusions 9a are formed in a pin-like shape, making the jig 9 a pin-holder-shaped jig. In Figure 5(a), the multiple protrusions 9a are arranged at equal intervals around the circumference.

[0048] In the configuration of Fig. 5, the protrusions 9a of the jig 9 are arranged to correspond to the positions of the teeth 3 of the gear 1. Specifically, as shown in Fig. 5(c), the protrusions 9a are arranged along the reference pitch circumference p of the gear 1 for each tooth 3. In this configuration, the number of protrusions 9a is the same as the number of teeth 3 of the gear 1. The number and arrangement of the protrusions are not limited to this. For example, they may be arranged at a position corresponding to the inner periphery of the gear. By arranging the protrusions on the entire gear other than the teeth, it is expected that shape distortion such as gear warpage can be improved. It is also expected that variation in straddling tooth thickness can be suppressed. Furthermore, the protrusions may be arranged in multiple concentric circles, with the protrusions arranged at equal intervals in the circumferential direction on each circle. Furthermore, in this case, the protrusions may be arranged with a phase shift between the circles. By shifting the phase, it is possible to improve the strength of not only the gear teeth but also the tooth valleys, as in the example.

[0049] The configuration of the jig is not limited to that shown in Fig. 5 and can be changed as appropriate depending on the shape of the sintered body. Because the raw material powder in the cavity is prone to bridging, especially near the mold, it is preferable to arrange the protrusions along the outer shape of the sintered body. It is also preferable to arrange the protrusions at the locations where the strength is weakest.

[0050] The materials of the jig substrate and protrusions are not particularly limited, and examples thereof include martensitic stainless steel such as SUS403, ferritic stainless steel such as SUS430, austenitic stainless steel such as SUS303, and high-speed tool steel such as SKH51. For example, when a magnetic material is used for the protrusions, they may be demagnetized before use, taking into consideration the effect on iron powder.

[0051] (2) Molding process This process is a process of obtaining a powder compact by uniaxially pressing the raw material powder filled in the cavity. This process is performed by pressing with position control, etc. The mold comprises, for example, a die and a pair of punches (upper punch and lower punch) fitted into the upper and lower openings of the die. In the state shown in Figure 4(f), pressure is applied with the upper punch (not shown) and the lower punch 6, and the powder compact is obtained by compressing at a predetermined pressure using a floating die method or the like. The pressure (surface pressure) of the uniaxial pressing is, for example, 400 MPa to 1500 MPa.

[0052] (3) Sintering process In this step, the powder compact is fed into a sintering device and sintered. This sintering process causes diffusion at the interfaces between particles of the metal powder or ceramic powder, resulting in a sintered body. Sintering methods that can be used include atmospheric pressure sintering, atmospheric pressure sintering, and pressure sintering (hot pressing). Sintering conditions are set appropriately depending on the composition of the metal powder or ceramic powder. For example, when the metal powder is iron powder or iron alloy powder, the sintering temperature is, for example, 1100°C to 1300°C. The sintering time is, for example, 0.2 hours to 24 hours.

[0053] After the sintering step, mechanical processing such as polishing is carried out as necessary to produce a sintered body.

[0054] According to the above-mentioned method, the raw powder in the cavity is vibrated using a jig, which turns the powder into a pseudo-fluid, allowing it to uniformly fill complex shapes such as gear teeth and narrow spaces. At the same time, bridging between powder particles and between the powder and the mold is reduced, enabling close packing in the vertical direction. This eliminates density variations during molding and reduces strength variations and deformation of the sintered body. Furthermore, improved strength makes it possible to design thinner sintered bodies. Furthermore, reduced density variations suppress warpage (deformation) of the sintered body, eliminating the need for a sizing process to correct deformation, leading to lower costs.

[0055] The shape of the sintered body of the present invention is not particularly limited, and may be selected appropriately depending on the application, such as a roughly disk shape such as a gear shape, a sphere, a cylinder, a cone, a truncated cone shape, or a rectangular parallelepiped shape. [Example]

[0056] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to the following examples.

[0057] <Test Example 1> As the sintered body, a gear having the following specifications was produced. Gear tooth profile: Standard Tooth profile: straight teeth Module: 0.8 Pressure angle: 20° Number of teeth: 118 Reference pitch diameter: 94.4 mm Tip diameter: 96.0mm Root diameter: 92.3mm Dislocation coefficient: 0 Number of teeth: 14 Spanning tooth thickness: 32.205 mm (measured so that the probe contacts both tooth surfaces near the reference pitch circle) Sintered density: 6.8±0.2g / cm 3

[0058] The raw material powder for the gears was a mixed powder of iron alloy powder (Ni: 3 mass%, Mo: 1 mass%, Cu: 2 mass%, Fe: balance) manufactured by JFE Steel Corporation, 0.5 mass% of lubricant, and 0.6 mass% of graphite.

[0059] Gears were manufactured through the following steps: filling, molding, and sintering. The filling process was carried out using the method shown in Figure 4, and the raw material powder was filled using the "drop-in" method, which is generally difficult to achieve uniform filling. The press used was a position-controlled powder molding press S-400EX manufactured by Mitsubishi Materials Techno Corporation.

[0060] 1. The lower punch was lowered to create a gear-shaped cavity approximately 10 mm deep. 2. A 3mm plate was placed on the die to ensure that the level height of the raw material powder was 3mm above the die. 3. The raw material powder was shifted toward one radial side of the cavity (from the 0° side to the 180° side in this example) and dropped into the cavity. 4. The raw material powder was leveled off with a spatula so that it was piled up 3 mm above the top of the die. 5. With 3 mm of raw powder piled up on top of the die, a pin-frog-shaped jig was inserted into the cavity. The pins of the pin-frog-shaped jig were 0.7 mm in diameter and 16 mm long, for a total of 236 pins. 118 pins were arranged on the gear's reference pitch circumference, corresponding to the position of the gear teeth, and 118 pins were arranged on a circumference 0.95 mm smaller on one side of the tooth root diameter of 92.3 mm toward the inner diameter, with a phase shift of half a pitch (1.525°). The pins arranged on the inner circumference were located near the roots of the gear teeth, and this contributes to improving the strength of the roots, which are subjected to heavy loads when used as a product. 6. The pin-frog-shaped jig was vibrated with an effective acceleration of 1G to 2G. 7. The pin-frog jig was removed from the cavity. 8. The plate above the die installed in step 2 above was removed, and the raw material powder was leveled off above the die with a spatula from the opposite direction to step 3 above (from the 180° side to the 0° side in this example). 9. Pressurization was applied using upper and lower punches and the mixture was compressed using the floating die method. 10. The lower punch was raised and the powder compact was removed. 11. The green compact was sintered at 1200°C using a tray pusher furnace to obtain a sintered body.

[0061] Steps 5 to 7 above were carried out under the following three conditions. Note that each condition is indicated as "3s," "30s," or "standard" in the tables and figures. "3s": The pin-frog-shaped jig was inserted to the deepest part of the cavity and then pulled out over 3 seconds while vibrating. "30s": The pin-frog-shaped jig was inserted to the deepest part of the cavity, and then vibrated for 27 seconds, after which it was pulled out over 3 seconds while still vibrating. "Standard": As a comparative example, the vibration steps 5 to 7 above were omitted.

[0062] For the green compacts and sintered compacts produced under each of the above conditions, the gear weight and the thickness near the base of each gear tooth (four locations in total: 0°, 90°, 180°, and 270°) were measured. The measurement results for the green compacts are shown in Table 1 and Figure 6, and the measurement results for the sintered compacts are shown in Table 2 and Figure 7. The average values ​​in the tables are the average values ​​for 10 gears produced under each condition.

[0063] [Table 1]

[0064] [Table 2]

[0065] As shown in Table 1 and Figure 6, performing the vibration process increased the gear weight of the green compact and the thickness near the tooth roots. Specifically, the gear weight increased by 4.8% for the "3s" and 6.0% for the "30s" compared to the "standard." The thickness near the tooth roots increased by 3.3% for the "3s" and 4.0% for the "30s" compared to the "standard." The vibration process increased the amount of raw material powder in the cavity, increasing the green compact density during press pressure application. As a result, the amount of springback after release of the press pressure increased, which is thought to have increased the green compact thickness. Furthermore, when comparing "3s" and "30s," it was found that the gear weight of the green compact and the thickness near the base of the teeth tended to increase with the extension of the vibration time. The thickness was thickest at the 0° position on the upstream side when dropping the raw powder, and thinnest at the 180° position on the downstream side.

[0066] As shown in Table 2 and Figure 7, the gear weight of the sintered body and the thickness near the tooth root showed results similar to those of the powder compact.

[0067] Next, the tooth tip (appearance) and axial cross section of the sintered compact were observed using a scanning electron microscope (SEM). The tooth tip was observed from the radial outside (direction A in Figure 8). The axial cross section was observed by cutting the tooth at the circumferential center line (direction B in Figure 8).

[0068] SEM images of the tooth tip surfaces at the 0° and 180° positions of the teeth fabricated under each condition are shown in Figure 9. As shown in Figure 9, the gears fabricated under the "3s" and "30s" conditions had smaller and fewer voids than the gear fabricated under the "standard" condition.

[0069] Microscope images (100x magnification) of axial cross sections at the 0° and 180° positions of the teeth fabricated under each condition are shown in Figures 10 to 12. As shown in Figures 10 to 12, similar to the results for the tooth tip, the gears fabricated under the "3s" and "30s" conditions had smaller and fewer voids than the gear fabricated under the "standard" condition.

[0070] 10 to 12, the pore area ratio at each of four equally spaced locations (m=4) in the circumferential direction of the sintered compact was determined using image analysis software (WinRoof2013, manufactured by Mitani Shoji Co., Ltd.) The pore area ratio was calculated using the following formula (3). Pore ​​area ratio [%] = (total pore area ÷ area of ​​evaluation range) × 100 (3) In this example, the evaluation range in the above formula (3) is the range surrounded by the white dotted lines in FIGS. 10 to 12, and its area is 12.36 mm 2 (2.15mm x 5.75mm = 12.36mm 2 The measurement results for each pore area ratio are shown in Table 3.

[0071] [Table 3]

[0072] As shown in Table 3, the average void area ratio of the gears manufactured under the "3s" and "30s" conditions was lower than that of the gears manufactured under the "standard" conditions. Specifically, the void area ratio in the cross-sectional view was reduced from 14.7% for the "standard" to 10.9% for the "3s" and 9.5% for the "30s". The variation in the void area ratio was also reduced, and the standard deviation σ for the "3s" and "30s" conditions was r was less than 1.5.

[0073] Next, the density of the sintered body was measured at four equally spaced locations (n=4) around the circumference. Each location (based on the 0°, 90°, 180°, and 270° positions) was cut along a plane perpendicular to the radial direction, approximately at a point where there are 10 to 12 gear teeth. The measurement results for each density are shown in Figure 13 and Table 4.

[0074] [Table 4]

[0075] As shown in Figure 13 and Table 4, the density variation at each location for "3s" and "30s" is smaller than that for "standard," and the standard deviation σ d The "standard" is 0.109g / cm 3 whereas "3s" is 0.040g / cm 3 , "30s" is 0.035g / cm 3 The density was less than half that of the "standard" in the "3s" and "30s" tests. As shown in Figure 13, the density at each location was generally higher in the "3s" and "30s" tests than in the "standard" test, and the density variation at the measurement position was small. In terms of density variation, the difference in density between the 0° and 180° positions was large under all conditions, but the "3s" and "30s" tests were able to reduce this difference.

[0076] The average density of the entire sintered body shown in Figure 13(a) (average value of eight samples) was 6.58 g / cm for the "standard" 3 whereas "3s" was 6.84g / cm 3 , "30s" is 6.85g / cm3 and high density.

[0077] Next, to evaluate the strength of the sintered gear teeth, a tooth breakage test was conducted using one sample gear for each condition. Figure 14 shows an outline of the test. As shown in Figure 14, multiple gear teeth (15 in this test) with partially cut teeth were placed between a load anvil 11 and a support anvil 12. The gear was set so that only the tip of the test tooth 3A contacted the load anvil 11, and the entire support tooth 3B contacted the support anvil 12. A load was applied in this state, and the load at which the test tooth 3A broke was taken as the tooth breakage load (unit: kN). Because the tooth breakage load is affected by the tooth thickness, the thickness of the test tooth at the reference pitch circle (at 94.4 mm PCD) was measured in advance. The measurement results are shown in Table 5 and Figure 15.

[0078] [Table 5]

[0079] As shown in Figure 15 and Table 5, the tooth breakage load results for "3s" and "30s" were approximately 1.2 times and 1.3 times higher than the "standard", respectively. In addition, the variation in tooth breakage load was also reduced, and the standard deviations σ for "3s" and "30s" were l is less than 0.16, and in particular the standard deviation σ l was less than 0.10. Also, when comparing tooth breakage load per tooth thickness (tooth breakage load / tooth thickness), the "3s" and "30s" were approximately 1.15 times and 1.24 times, respectively, compared to the "standard." As described above, the sintered bodies of "3s" and "30s" have high density in each location, which is thought to have resulted in improved strength in each location, as shown in Figure 15.

[0080] Next, the spanning tooth thickness of the sintered gear was measured using a tooth thickness micrometer. The number of spanning teeth was 14. The measurement results are shown in Table 6 and Figure 16.

[0081] [Table 6]

[0082] As shown in Fig. 16 and Table 6, for the "standard" the spanning tooth thickness at the 180° position was extremely small compared to the spanning tooth thickness at other positions, resulting in a value that deviated significantly. On the other hand, the spanning tooth thickness at "3s" and "30s" showed little difference depending on the position.

[0083] <Test Example 2> As the sintered body, a gear having the following specifications was produced. Gear tooth profile: Standard Tooth profile: straight teeth Module: 0.8 Pressure angle: 20° Number of teeth: 50 Reference pitch diameter: 40 mm Tip diameter: 41.6mm Root diameter: 38.0mm Tooth tip corner R: 0.25 Bottom corner R: 0.25 Dislocation coefficient: 0 Number of teeth: 6 Spanning tooth thickness: 13.55mm

[0084] Two types of iron powder were used as the raw material powder for the gears. The particle size distribution of these iron powders is shown in Figure 17.

[0085] In Test Example 2, a gear was produced by carrying out the filling process, molding process, and sintering process in a manner generally similar to Test Example 1. The pin-frog-shaped jig used had 174 pins with a diameter of 0.7 mm, evenly spaced around the circumference.

[0086] In Test Example 2, a step plate was not installed (step 2 of Test Example 1 was omitted). Because a step plate was not installed, after step 7 above, the raw material powder in the cavity was below the leveling surface due to close packing. Therefore, as a step in place of step 8 above, the raw material powder was added again to the top of the cavity, and then the raw material powder was leveled off with a spatula.

[0087] The above steps 5 to 7 were carried out under the following four conditions. "3s-A": A pin-frog-shaped jig was inserted to the deepest part of the cavity, vibrated for 3 seconds, and then pulled out. "3s-B": A pin-frog-shaped jig was inserted halfway into the deepest part of the cavity, vibrated for 3 seconds, and then pulled out. "3s-C": After inserting the pin-frog-shaped jig to the deepest part of the cavity, it was pulled out over 3 seconds while vibrating. "Standard": A comparative example in which the vibration process was omitted.

[0088] The weight of each gear of the powder compacts produced under the above conditions was measured. The measurement results for the powder compacts are shown in Figure 18. Figure 18(a) shows the results when the raw material powder was filled by the "drop-in" method, and Figure 18(b) shows the results when the raw material powder was filled by the "suction" method.

[0089] As shown in Figure 18(a) and (b), the gear weight increased as a result of the vibration process. It is also preferable to insert the pins of the jig to the deepest point, which allows the mixed powder to be vibrated efficiently and packed closely. Furthermore, it was found that the "suction" method, like the "dropping" method, vibrates the raw material powder, turning it into a pseudo-fluid, and similar effects can be achieved.In addition, under all conditions, the gear weight of the "dropping" method was lower than that of the "suction" method, and the "dropping" method was more difficult to fill with raw material powder than the "suction" method.

[0090] Next, the tooth tips of the powder compact were observed under a microscope. In this test, the observations were made on both the upper punch side and the lower punch side.

[0091] As shown in Figure 19, the "standard" had a large difference in the area of ​​the voids (black areas) between the top and bottom, and large voids were observed on the bottom punch side. On the other hand, under the other conditions in which the vibration process was performed, the difference in the area of ​​the voids between the top and bottom was smaller than that of the "standard," and no large voids were observed.

[0092] Generally, during press molding, it is difficult to fill the mold with raw material powder in the areas corresponding to the gear teeth, which results in a decrease in strength due to low density and an increase in variation in spanning tooth thickness due to increased density variation. In contrast, the results of the above examples show that performing the vibration process increased the gear weight, thickness, density, and strength, even with the same cavity volume. For example, by inserting the needles of a pin-frog-shaped jig into positions corresponding to the gear teeth and vibrating them, the iron alloy powder in the cavity became a pseudo-fluid due to the vibration, which is thought to have improved filling even in the details of complex shapes such as gear teeth without bridging. Furthermore, performing the vibration process not only improved the overall density, but also reduced density variation due to position and improved strength. Good results were also obtained in reducing variation in the straddling tooth thickness. [Industrial Applicability]

[0093] The sintered body of the present invention has a substantially uniform strength in each part, and is therefore suitable for machine parts that require high strength and hardness, and is particularly suitable for use in gears with complex shapes. [Explanation of symbols]

[0094] 1 gear 2 Gear body 3 teeth 4 Mounting holes 5 Keyway 6 Lower punch 7 Die 8 Board material 9 Jig 9a protrusion 9b Base material 10. Vibration device 11 Load anvil 12 Support anvil C cavity

Claims

1. A method for producing a substantially disk-shaped sintered body obtained by sintering a powder compact, comprising the steps of: The manufacturing method includes a filling step of filling a cavity with raw material powder, a molding step of compression-molding the filled raw material powder, and a sintering step of sintering a green compact obtained by compression-molding, the filling step includes a step of adding the raw material powder into the cavity, and a step of using a jig having a plurality of protrusions to vibrate the raw material powder in the cavity with the protrusions inserted therein, A method for producing a sintered body, wherein the standard deviation of density at n equally spaced locations (n ​​is an integer of 2 or more) in the circumferential direction of the sintered body is less than 0.

1.

2. 2. The method for producing a sintered body according to claim 1, wherein the standard deviation of the density at four equally spaced locations in the circumferential direction of the sintered body is less than 0.

1.

3. 3. The method for producing a sintered body according to claim 2, wherein the diameter of the sintered body is 100 mm or less.

4. 4. The method for producing a sintered body according to claim 1, wherein the standard deviation of the pore area ratio at m equally spaced locations (m is an integer of 2 or more) in the circumferential direction of the sintered body is less than 1.

5.

5. 5. The method for producing a sintered body according to claim 1, wherein the sintered body is a gear having a plurality of teeth projecting radially outward.

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