A method for producing a powder metallurgical nut blank and a powder metallurgical nut blank

Abstract

The application provides a powder metallurgy nut blank preparation method and a powder metallurgy nut blank. The method comprises: filling at least three kinds of metal powders with different average particle sizes into an annular cavity; wherein a first assembly is sequentially divided into a large pressure area, a medium pressure area and a small pressure area along the pressing direction, and the large pressure area is filled with metal powder with the largest average particle size, the medium pressure area is filled with metal powder with medium average particle size, and the small pressure area is filled with metal powder with the smallest average particle size. The technical problem solved by the application is that in the process of pressing a workpiece with a large height-diameter ratio, the pressure along the transmission direction will appear significant gradient attenuation, resulting in uneven distribution of powder density in the compact, especially in the area far from the pressing surface or the core of the structure, the effective forming pressure of the powder is insufficient, and the density is low. The density gradient is difficult to completely eliminate in subsequent sintering, which seriously affects the wear resistance and overall service life of the key friction area of parts such as nuts.

Description

Technical Field

[0001] This invention relates to the field of powder metallurgy technology, and more specifically, to a method for preparing powder metallurgy nut blanks and the powder metallurgy nut blanks themselves. Background Technology

[0002] In the field of powder metallurgy, when preparing parts with a low aspect ratio, conventional processes mainly use methods such as unidirectional pressing and bidirectional pressing to press metal powder into shape. These processes apply pressure in a specific direction to make the powder particles dense and form a compact with a certain shape and strength, and then obtain the final part through a sintering process.

[0003] However, the relevant technology has at least one of the following problems: In the process of pressing workpieces with large height and diameter, the pressure will show a significant gradient attenuation along the transmission direction, resulting in uneven powder density distribution inside the compact. Especially in the area far from the pressure surface or the core of the structure, the effective forming pressure of the powder is insufficient, resulting in low density. This density gradient is difficult to completely eliminate in the subsequent sintering, which seriously affects the wear resistance and overall service life of key friction areas of parts such as nuts. Summary of the Invention

[0004] The technical problem solved by this invention is that in the process of pressing workpieces with large height and diameter, the pressure will show a significant gradient attenuation along the transmission direction, resulting in uneven powder density distribution inside the compact. Especially in areas far from the pressure surface or in the core of the structure, the effective forming pressure of the powder is insufficient, resulting in low density. This density gradient is difficult to completely eliminate in subsequent sintering, which seriously affects the wear resistance and overall service life of critical friction areas of parts such as nuts.

[0005] To address the aforementioned problems, this invention provides a method for preparing a powder metallurgy nut blank, comprising: assembling a metal nut substrate and a forming fixture to obtain a first assembly, the first assembly having a pre-set annular cavity for filling with metal powder; filling the annular cavity with at least three metal powders of different average particle sizes; wherein the first assembly is sequentially divided into a high-pressure zone, a medium-pressure zone, and a low-pressure zone along the pressing direction, with the high-pressure zone filled with metal powder of the largest average particle size, the medium-pressure zone filled with metal powder of a medium average particle size, and the low-pressure zone filled with metal powder of the smallest average particle size, such that the average particle size is distributed in a trapezoidal pattern along the pressing direction to obtain a second assembly; pressing the second assembly after filling with metal powder to obtain a blank; and sequentially degreasing and sintering the blank to obtain the powder metallurgy nut blank.

[0006] Compared with existing technologies, the technical effects achieved by this solution are as follows: This invention fundamentally changes the traditional uniform particle size powder filling mode and innovatively introduces a particle size gradient distribution strategy. Based on the pressure decay during the pressing process, a high-pressure zone, a medium-pressure zone, and a low-pressure zone are defined. By placing powders with smaller average particle size and high sintering activity in the low-pressure zone, powders with medium average particle size and high sintering activity in the medium-pressure zone, and powders with larger average particle size and low sintering activity in the high-pressure zone, the differences in powder densification ability in different regions caused by pressure decay are effectively mitigated. This significantly improves the overall density uniformity of the pressed compact and solves problems such as uneven subsequent sintering and poor overall product performance caused by density gradients.

[0007] In one embodiment of the present invention, during the pressing process, the second assembly is subjected to bidirectional pressure from an upper pressure head and a lower pressure head; the upper end of the second assembly is defined as the upper pressing end face that mates with the upper pressure head, and the lower end of the second assembly is defined as the lower pressing end face that mates with the lower pressure head; the second assembly is divided into a high-pressure zone, a medium-pressure zone, a small-pressure zone, a medium-pressure zone, and a high-pressure zone from the upper pressing end face to the lower pressing end face, so that the average particle size of the metal powder gradually increases from the central region of the annular cavity to the upper pressing end face and the lower pressing end face, respectively; wherein, the height difference between the upper pressing end face and the lower pressing end face is H, the maximum radial dimension of the annular cavity is D, and satisfies: 5≤H / D≤10.

[0008] Compared with existing technologies, the technical effects achieved by this solution are as follows: This solution addresses the challenge of uneven axial pressure distribution during bidirectional pressing of annular cavities with a specific aspect ratio (5 ≤ H / D ≤ 10). It designs a powder particle size gradient distribution strategy precisely matched to the pressure field. Due to the large aspect ratio, the attenuation effect of pressure transmission from the upper and lower pressing ends towards the center is more significant, easily leading to low core compact density. Based on the pressure attenuation law, this solution symmetrically divides the annular cavity axially into a composite structure of "high pressure zone - medium pressure zone - small pressure zone - medium pressure zone - high pressure zone," and correspondingly configures powders with particle sizes of "large - medium - small - medium - large." This design achieves active adaptation between local pressure levels and powder sintering activity: the "pressure zone" in the core, where the pressure is lowest, is filled with powders with the smallest particle size, largest specific surface area, and highest surface energy, utilizing their high sintering activity to compensate for insufficient forming pressure; while the "high pressure zone" at the higher pressure ends is filled with larger particle size powders, achieving sufficient densification through higher pressure. By employing a reverse matching mechanism, the densification differences caused by the pressure gradient are offset from the physical nature of the powder raw materials, making the density distribution of the pressed blank more uniform along the axial direction. This not only effectively solves the common technical problems of loose core and large performance gradient after sintering in traditional bidirectional pressed blanks, but is also particularly suitable for the uniform forming of cavities with large aspect ratios, providing a key guarantee for obtaining powder metallurgy nut blanks with consistent structure and excellent performance.

[0009] In one embodiment of the present invention, the average particle size of the metal powder ranges from 3 micrometers to 80 micrometers, and the difference in average particle size between metal powders with different average particle sizes is greater than or equal to 5 micrometers.

[0010] Compared with existing technologies, the technical effects achieved by this solution are as follows: This solution achieves synergistic control of powder filling characteristics and sintering kinetics by precisely limiting the particle size range of metal powder (3-80 micrometers) and the particle size difference (≥5 micrometers). On the one hand, this particle size range ensures that the powder system has good flowability and filling density, providing a foundation for the stable and uniform filling of complex gradient structures within the annular cavity. On the other hand, the forced minimum particle size difference of ≥5 micrometers ensures that there are significant differences in the specific surface area and surface energy of powders in adjacent regions, thereby forming a clear active gradient during sintering. This active gradient is opposite in direction and numerically matched to the pre-designed pressure gradient, allowing highly active fine powder to preferentially densify through strong surface diffusion and grain boundary migration mechanisms in the low-pressure region; while in the high-pressure region, coarse powder achieves densification through plastic deformation under higher mechanical forces. The combination of the two significantly reduces the asynchronous densification caused by pressure decay on a macroscopic level, and promotes coordinated sintering and interface fusion between different particle size layers on a microscopic level. Ultimately, it achieves a more uniform and thorough densification effect on the entire green body, overcoming the shortcomings of insufficient response of traditional uniform particle size or fuzzy graded powders in gradient pressure fields.

[0011] In one embodiment of the invention, the pressing is cold isostatic pressing, the pressure of which is 250 MPa to 300 MPa, and the holding time is 60 to 180 seconds.

[0012] Compared with existing technologies, the technical effects achieved by this technical solution are as follows: cold isostatic pressing provides an isotropic pressing environment, and combined with the aforementioned particle size gradient distribution, it further eliminates the influence of non-uniform pressure fields. Under the set pressure and holding time, powder particles of different sizes can be fully rearranged and plastically deformed to obtain a green body with high uniformity.

[0013] The forming fixture includes an outer fixture. The metal nut base is connected to the outer fixture by threads, and an exhaust gap for venting gas is formed between the metal nut base and the outer fixture.

[0014] Compared with existing technologies, the technical effects achieved by this technical solution are as follows: the threaded connection facilitates assembly and disassembly, and the formed venting micro-slits provide a channel for the volatilization of forming agents during degreasing and the discharge of residual gases during sintering, which helps to reduce internal defects and further improve the density and purity of the final blank.

[0015] In one embodiment of the present invention, sintering is hot pressing sintering; the external tooling is made of ceramic; during the hot pressing sintering process, the difference in thermal expansion coefficients between the metal powder and the ceramic is used to make the metal powder radially constrained by the external tooling when it expands due to heat.

[0016] Compared with existing technologies, the technical effect achieved by this solution is that the ceramic external tooling maintains shape stability at high temperatures. Utilizing the fact that the coefficient of thermal expansion of metal is greater than that of ceramic, the expansion of the metal powder during heating is restricted by the ceramic tooling, applying additional radial pressure. This superimposes a radial densification driving force on top of the axial pressure, significantly improving the overall sintering density.

[0017] In one embodiment of the present invention, the process between degreasing and sintering further includes pressing a ceramic core rod into the central hole of the green body, and the ceramic core rod and the powder layer formed by the metal powder forming an interference fit.

[0018] Compared with existing technologies, the technical effects achieved by this technical solution are as follows: by introducing a ceramic core rod and using an interference fit, active radial support and pre-stress are provided to the inner hole of the powder layer before sintering. This can compensate for the possible radial shrinkage of the powder, constrain its deformation, help maintain the geometric accuracy of the inner hole and obtain a more uniform radial density distribution.

[0019] In one embodiment of the invention, the interference fit between the ceramic core and the powder layer on one side is 0.2 mm to 1 mm.

[0020] Compared with existing technologies, the technical effects achieved by adopting this technical solution are: quantifying the range of interference fit values, ensuring sufficient preload to provide effective support and constraint, while avoiding powder layer damage or assembly difficulties caused by excessive interference.

[0021] In one embodiment of the present invention, the process parameters for hot pressing sintering are: heating rate ≤ 5°C / min, pressure loading rate ≤ 0.5 MPa / min, holding pressure of 80 MPa to 100 MPa, and holding time of 50 to 120 minutes.

[0022] Compared with existing technologies, the technical advantages achieved by this solution are as follows: the slow heating and pressurization rates facilitate the uniform transmission of temperature and stress fields, avoiding thermal stress or localized overpressure caused by rapid changes. Appropriate holding pressure and time provide sufficient conditions for atomic diffusion, particle rearrangement, and pore closure, ensuring the high density and excellent mechanical properties of the final product.

[0023] On the other hand, the present invention also provides a powder metallurgy nut blank, which is prepared by the preparation method as described in any of the above examples.

[0024] Compared with existing technologies, the technical effects achieved by adopting this technical solution are as follows: it can achieve the technical effects corresponding to any of the above examples, which will not be elaborated here.

[0025] By adopting the technical solution of the present invention, the following technical effects can be achieved:

[0026] (1) By matching the powder particle size gradient with the pressing pressure gradient, the insufficient densification in the pressure attenuation region is fundamentally compensated, and a compact with highly uniform density is obtained.

[0027] (2) By combining cold isostatic pressing, ceramic tooling with exhaust micro-slits and optimized hot pressing sintering process, a densified environment that is conducive to exhaust, uniform pressurization and can utilize thermal expansion to generate additional compressive stress was constructed.

[0028] (3) The final powder metallurgy nut blank has a high density and uniform distribution of wear-resistant layer inside, which significantly improves the mechanical properties and wear resistance of the product, and is especially suitable for components such as screw nuts with high requirements for life and precision. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings to be used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the structure of a second assembly provided in an embodiment of the present invention;

[0031] Figure 2 This is a schematic diagram showing the relationship between the second assembly and the upper and lower pressure heads when the assembly is in the pressing stage.

[0032] Figure 3 This is a flowchart of the powder metallurgy nut blank preparation method.

[0033] Explanation of reference numerals in the attached figures:

[0034] 10. Second assembly; 101. Ceramic I; 102. Ceramic II; 103. Ceramic III; 104. Ceramic IV; 200. Metal nut base; 300. Elastic rubber sleeve; 400. Metal powder; 500. Fastening device; 601. Upper pressure head; 602. Lower pressure head. Detailed Implementation

[0035] The following will refer to the appendix to this application. Figures 1 to 3The technical solutions in this application are clearly and completely described herein. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The components of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0036] It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, in the description of this application, terms such as "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0037] Traditional powder metallurgy processes often employ unidirectional pressing or hot isostatic pressing (HIP) when preparing irregularly shaped parts such as lead screw nut blanks. During pressing, these methods exhibit a pressure gradient attenuation along the transmission direction, resulting in uneven powder density distribution within the blank. Particularly in areas far from the pressing surface or the structural core, insufficient powder forming pressure leads to low density. This density gradient is difficult to completely eliminate during subsequent sintering, thus affecting the wear resistance and overall service life of critical friction areas in parts such as nuts.

[0038] To address this, the present invention provides a method for preparing a powder metallurgy nut blank, comprising: assembling a metal nut substrate and a forming fixture to obtain a first assembly, the first assembly having a pre-set annular cavity for filling with metal powder 400; filling the annular cavity with at least three metal powders 400 of different average particle sizes; wherein the first assembly is sequentially divided into a high-pressure zone, a medium-pressure zone, and a low-pressure zone along the pressing direction, and the high-pressure zone is filled with metal powder 400 of the largest average particle size, the medium-pressure zone is filled with metal powder 400 of a medium average particle size, and the low-pressure zone is filled with metal powder 400 of the smallest average particle size, such that the average particle size is distributed in a trapezoidal shape along the pressing direction to obtain a second assembly 10; pressing the second assembly 10 after filling with metal powder 400 to obtain a blank; and sequentially degreasing and sintering the blank to obtain the powder metallurgy nut blank.

[0039] Specifically, the metal nut base 200 refers to the core component constituting the internal structure of the powder metallurgy nut blank, providing support for the subsequent filling and forming of the metal powder 400. The forming fixture refers to the external mold or mold system used to cooperate with the metal nut base 200 to jointly construct the powder forming space. The first assembly refers to the initial structure formed after the metal nut base 200 and the forming fixture are installed, with a pre-reserved annular cavity for filling the metal powder 400. The annular cavity refers to the annular space formed inside the first assembly, used to accommodate the metal powder 400 to be pressed, ultimately forming the main body of the nut blank.

[0040] Furthermore, the gradient distribution of the metal powder 400 refers to a filling method in which at least three metal powders 400 with different average particle sizes are arranged in a gradually varying manner along the pressing direction, aiming to optimize the density of different regions during the pressing process. Regions with lower expected forming pressure refer to the annular cavity interior regions that are expected to withstand lower pressing forces during the pressing process due to pressure transmission attenuation or structural characteristics. Regions with higher expected forming pressure refer to the annular cavity interior regions that are expected to withstand higher pressing forces during the pressing process.

[0041] Preferably, the metal powder 400 is high-speed steel powder, and has 3 to 5 different high-speed steel powders with different average particle sizes.

[0042] Furthermore, a layered filling method can be adopted during the filling process. This involves dividing the internal region of the annular cavity into layers based on different pressures. The region with the lowest pressure is designated as the low-pressure zone, filled with high-speed steel powder with the smallest average particle size. The region with moderate pressure (between the lowest and highest pressures) is designated as the medium-pressure zone, filled with high-speed steel powder with a medium average particle size. The region with the highest pressure is designated as the medium-pressure zone, filled with high-speed steel powder with the largest average particle size. The high-pressure and low-pressure zones are evenly divided according to the types and quantities of high-speed steel powder available, and the powder is sequentially distributed along a gradient of average particle size.

[0043] Furthermore, the first assembly after filling is then fitted with an elastic rubber sleeve 300 and a fastening device 500 for fixing the elastic rubber sleeve 300 to obtain a second assembly 10.

[0044] Furthermore, the second assembly 10 is subjected to cold isostatic pressing, and a blank is obtained after removing the fastening device 500 and the elastic rubber sleeve 300.

[0045] Finally, the blank is degreased and sintered to obtain powder metallurgy nuts.

[0046] In one embodiment of the present invention, during the pressing process, the second assembly 10 is subjected to bidirectional pressure from an upper pressing head 601 above and a lower pressing head 602 below; the upper end of the second assembly 10 is defined as the upper pressing end face that mates with the upper pressing head 601, and the lower end of the second assembly 10 is defined as the lower pressing end face that mates with the lower pressing head 602; the second assembly 10 is divided into a high pressure zone, a medium pressure zone, a small pressure zone, a medium pressure zone, and a high pressure zone from the upper pressing end face to the lower pressing end face, so that the average particle size of the metal powder 400 gradually increases from the central region of the annular cavity to the upper pressing end face and the lower pressing end face respectively; wherein, the height difference between the upper pressing end face and the lower pressing end face is H, the maximum radial dimension of the annular cavity is D, and satisfies: 5≤H / D≤10.

[0047] It should be noted that the powder particle size gradient matching densification method proposed in this invention is applicable to a wide range of powder metallurgy forming scenarios. Especially for annular workpieces with a large aspect ratio, where axial pressure attenuation is more significant during pressing, this method can achieve a particularly outstanding homogenization effect. In a preferred embodiment, this method is particularly suitable for preparing annular parts with an aspect ratio (defined as the ratio of the workpiece's axial height H to its maximum radial dimension D) satisfying 5 ≤ H / D ≤ 10. In such structures, it can fully demonstrate its technical advantages in compensating for pressure gradients and improving overall density.

[0048] Specifically, the upper end of the second assembly 10 is an upper pressing end face, and the lower end of the second assembly 10 is a lower pressing end face. These end faces are pre-designed interfaces for bearing pressing forces. They can be specific surfaces of the forming tooling or surfaces of auxiliary pressing components that cooperate with the forming tooling.

[0049] During the pressing process, the upper pressure head 601 applies pressure downwards from the upper pressing end face, while the lower pressure head 602 applies pressure upwards from the lower pressing end face. This bidirectional pressing mechanism aims to apply pressure to the metal powder 400 simultaneously from two opposing directions. The bidirectional pressing is achieved in a cold isostatic pressing device by, in addition to the overall isostatic pressing effect, by setting movable pistons or pressure plates on the upper and lower end faces of the second assembly 10 and applying additional axial force to them, thereby achieving directional bidirectional pressing.

[0050] Furthermore, the average particle size of the metal powder 400 gradually increases from the central region of the annular cavity towards the upper and lower pressing faces. This means that in the radially central region of the annular cavity, the filled metal powder 400 has a relatively small average particle size, while the average particle size of the metal powder 400 gradually increases closer to the upper and lower pressing faces. This particle size gradient distribution can be achieved by placing the first assembly vertically and dividing the area in advance according to the different average particle size types of high-speed steel powder. For example, if there are high-speed steel powders with small, medium, and large average particle sizes, the annular cavity can be divided into five equally sized areas from top to bottom: large, medium, small, medium, and large. After filling the large area with high-speed steel powder with the largest average particle size, the area is filled with high-speed steel powder with a medium average particle size, followed by high-speed steel powder with the smallest average particle size, then high-speed steel powder with a medium average particle size, and finally high-speed steel powder with the largest average particle size, to complete the filling process.

[0051] Preferably, the average particle size of the metal powder 400 ranges from 3 micrometers to 80 micrometers, and the difference in average particle size between metal powders 400 with different average particle sizes is greater than or equal to 5 micrometers. Specifically, this average particle size refers to the statistical average diameter of the metal powder particles 400, which has a significant impact on the powder's flowability, bulk density, and compression behavior during the pressing process. Limiting the average particle size to the range of 3 to 80 micrometers aims to balance the needs of the powder in different pressing regions. Specifically, setting 3 micrometers as the lower limit of the average particle size is to avoid agglomeration and decreased flowability caused by excessively small powder particle size. When filling an annular cavity, powder with too small a particle size may be difficult to distribute evenly, especially in areas with lower expected forming pressure, thus forming voids or local density inhomogeneities. In addition, the larger specific surface area of ​​excessively fine powder increases interparticle friction during the pressing process, affecting the effective transmission of pressure. On the other hand, setting 80 micrometers as the upper limit of the average particle size is to prevent insufficient compressibility and uneven pressure transmission caused by excessively large powder particle size. During the pressing process, powder with excessively large particle size has relatively few contact points between particles, making it difficult to achieve sufficient density in areas with high expected forming pressure. Furthermore, excessively large particle size can lead to coarse grains during subsequent sintering, thus affecting the mechanical properties of the final powder metallurgy nut blank. By precisely controlling the average particle size within this range, this application ensures that the metal powder 400, when distributed in a gradient, can adapt to the pressing conditions in different regions, achieving better filling and density effects.

[0052] Preferably, the pressing is cold isostatic pressing, with a pressure of 250 MPa to 300 MPa and a holding time of 60 to 180 seconds.

[0053] Specifically, the pressure for cold isostatic pressing is set within the range of 250 MPa to 300 MPa. This pressure range is selected based on a comprehensive consideration of powder densification effect and tooling safety. For example, when using a pressure of 250 MPa, it ensures that 400 metal powder particles undergo sufficient displacement and rearrangement in the initial stage, forming a compact with a certain density. When using a higher pressure of 300 MPa, it further promotes plastic deformation and close packing of powder particles, thereby obtaining higher green density and strength. Pressure within this range effectively drives the shrinkage of pores between powder particles while avoiding powder particle breakage or excessive wear of the forming tooling that may result from excessive pressure.

[0054] Furthermore, the holding time is set to 60 to 180 seconds. The holding process is a crucial step in cold isostatic pressing, its function being to maintain a set high pressure for a certain period to ensure that the powder particles have sufficient time for displacement, deformation, and rearrangement, thereby achieving a stable and uniform density distribution within the compact. For example, a holding time of 60 seconds is suitable for materials with relatively low density requirements or where the powder particles are easy to compact. For materials requiring higher density or where the powder particles are more difficult to compact, a longer holding time of 180 seconds can be selected to ensure a tighter bond between powder particles, reduce residual porosity, and further improve the uniformity and strength of the compact.

[0055] In one embodiment of the present invention, the forming fixture includes an outer fixture, the metal nut base 200 is connected to the outer fixture by a thread, and an exhaust gap for venting gas is formed between the metal nut base 200 and the outer fixture.

[0056] Specifically, the venting gap is a channel specifically designed to expel air from the annular cavity during the filling and pressing process of the metal powder 400. During powder pressing, air exists between powder particles and between the powder and the mold wall. If this air cannot be effectively expelled, it can easily form pores inside the pressed blank, leading to uneven density and affecting the mechanical properties of the final product. The formation of this venting gap can be varied. For example, by precisely controlling the thread fit tolerance between the metal nut base 200 and the external tooling, a micron-level spiral gap can be naturally formed during the threaded connection, serving as a channel for air expulsion; alternatively, several tiny radial or axial through holes or grooves can be pre-designed and machined in the non-threaded connection area of ​​the metal nut base 200 or the external tooling to form the venting gap.

[0057] In one embodiment of the present invention, sintering is hot pressing sintering; the external tooling is made of ceramic; during the hot pressing sintering process, the difference in the coefficient of thermal expansion between the metal powder 400 and the ceramic is used to make the metal powder 400 radially constrained by the external tooling when it expands due to heat.

[0058] Specifically, during the hot-pressing sintering process, the difference in thermal expansion coefficients between the metal powder 400 and the ceramic is utilized to radially constrain the metal powder 400 as it expands under heat, subjecting it to the constraints of the external tooling. The difference in thermal expansion coefficients refers to the difference in the degree of linear or volumetric expansion between the metal powder 400 and the ceramic external tooling material when heated. When the metal powder 400 and the ceramic external tooling are heated simultaneously, if the thermal expansion coefficient of the metal powder 400 is greater than that of the ceramic external tooling, the metal powder 400 will be radially constrained by the ceramic external tooling during expansion. This radial constraint effectively suppresses excessive radial expansion of the metal powder 400 during sintering, prevents deformation of the blank, and promotes further densification of the powder particles in the radial direction.

[0059] See Figure 1 The external tooling includes four ceramic components: ceramic component 101, ceramic component 102, ceramic component 103, and ceramic component 104. Ceramic component 101 is threaded to the lower end of the metal nut base 200, ceramic component 103 is threaded to the side of the metal nut base 200, and ceramic component 104 is threaded to the upper end of the metal nut base 200. The outer surface of the combined structure formed by ceramic component 102, ceramic component 103, ceramic component 104, and the metal nut base 200 is threaded to ceramic component 101, forming a multi-structure fit.

[0060] Furthermore, a semi-enclosed elastic rubber sleeve 300 is fitted on the outside of the ceramic 101, and the tail of the elastic rubber sleeve 300 is further reinforced by a fastening device 500. The elastic rubber sleeve 300 is removed after the cold pressing process.

[0061] In one embodiment of the present invention, the process between degreasing and sintering further includes pressing a ceramic core rod into the central hole of the green body, and the ceramic core rod and the powder layer formed by the metal powder forming an interference fit.

[0062] Specifically, after the debinding process, the binder inside the green body has been removed. At this point, the green body structure is relatively loose, but it has not yet undergone the high-temperature densification required for sintering. Introducing internal support at this stage can effectively utilize the plasticity of the green body after debinding, while providing a stable internal structure for the upcoming sintering process. For example, this step can be performed after the green body has been debinded and cooled to room temperature to ensure operational stability and material properties; alternatively, it can be performed after the green body has exited the debinding furnace, while it is still at a relatively high but controllable temperature, to utilize residual heat to assist in pressing, but it is necessary to ensure that the green body material still has sufficient strength and stability at this temperature.

[0063] The ceramic mandrel is a rod-shaped structure made of a material with high hardness, high wear resistance, excellent high-temperature resistance, and a low coefficient of thermal expansion. Its function is to provide rigid support for the central hole of the green body during sintering, preventing deformation or collapse of the central hole at high temperatures due to material shrinkage or uneven stress. The pressing operation can be achieved in several ways. For example, a hydraulic or pneumatic press can be used to precisely control the pressing speed and pressure, smoothly pushing the ceramic mandrel into the central hole of the green body. Another method is to use vibration-assisted pressing technology, introducing high-frequency vibration while applying axial pressure to reduce frictional resistance and ensure the mandrel enters the central hole smoothly and uniformly.

[0064] Furthermore, the ceramic core rod and the powder layer formed by the metal powder form an interference fit. An interference fit refers to a fit in which the outer diameter of the ceramic core rod is slightly larger than the inner diameter of the central hole of the preform before assembly, and a tight bond is achieved through pressing or thermal expansion and contraction. This fit allows the ceramic core rod to generate radial prestress or constraint force on the powder layer.

[0065] Preferably, the interference fit between the ceramic core and the powder layer on one side is 0.2 mm to 1 mm.

[0066] In one embodiment of the present invention, the process parameters for hot pressing sintering are: heating rate ≤ 5°C / min, pressure loading rate ≤ 0.5 MPa / min, holding pressure of 80 MPa to 100 MPa, and holding time of 50 to 120 minutes.

[0067] Specifically, the heating rate during hot pressing sintering is limited to no more than 5°C / min. This parameter is set to ensure that the temperature inside the sintering furnace rises steadily and uniformly, avoiding thermal shock and excessive internal temperature gradients in the material caused by rapid heating. Simultaneously, the pressure loading rate is limited to no more than 0.5 MPa / min. This is to ensure that pressure is applied gradually and uniformly to the powder compact during hot pressing sintering, preventing stress concentration, disordered rearrangement of powder particles, or compact cracking caused by sudden pressure increases.

[0068] The specific preparation process is as follows:

[0069] S1. Install the metal nut base and the ceramic outer tooling by thread to form a first assembly with an annular cavity, and seal the outside of it with an elastic sealing sleeve.

[0070] S2. Prepare 3 to 5 metal powders with different average particle sizes, and fill them along the axial direction of the annular cavity in a gradient distribution pattern with the smallest average particle size in the central area and the average particle size gradually increasing towards the upper and lower ends to obtain the second assembly.

[0071] S3. The second assembly is subjected to cold isostatic pressing at a pressure of 250-300 MPa for 60-180 seconds. After pressing, the sealing sleeve is removed to obtain a dense first blank.

[0072] S4. Degrease the first blank to remove the forming agent and obtain the second blank;

[0073] S5. Press the ceramic core rod into the center hole of the second blank along the axial direction to form an interference fit with a single-sided interference of 0.2-1 mm, and obtain the third blank;

[0074] S6. Place the third blank in a hot pressing sintering furnace. During the heating process, control the heating rate to ≤5℃ / min and the pressure loading rate to ≤0.5MPa / min. Apply a holding pressure of 80-100MPa at the sintering temperature and hold for 50-120 minutes. Utilize the difference in thermal expansion coefficients between the metal powder and the ceramic tooling to make the powder radially constrained during expansion, further promoting densification.

[0075] S7. After sintering, the material is cooled in the furnace and the tooling is removed to obtain a powder metallurgy nut blank with uniform density.

[0076] On the other hand, the present invention also provides a powder metallurgy nut blank, which is prepared by the preparation method as described in any of the above examples.

[0077] In one specific embodiment, a method for preparing a powder metallurgy nut blank is provided, the specific steps of which are as follows:

[0078] Prepare a metal nut base 200, a ceramic outer tooling, and a resilient sealing sleeve. Install the metal nut base 200 and the ceramic outer tooling using threads to form a first assembly. In this assembly, the metal nut base 200 and the ceramic outer tooling together define an annular cavity. Fit the resilient sealing sleeve onto the outside of this first assembly to seal both ends of the annular cavity, forming a closed annular cavity.

[0079] Metal powder 400 uses high-speed steel powder, and 3 to 5 different high-speed steel powders with average particle sizes ranging from 3 micrometers to 80 micrometers are prepared. The high-speed steel powder is filled into the annular cavity in a gradient manner according to the expected pressure distribution of axial bidirectional pressing, i.e., high pressure at both ends and low pressure in the center.

[0080] Specifically, the central region of the cavity is filled with metal powder 400 with the smallest average particle size, the upper and lower ends of the cavity are filled with metal powder 400 with the largest average particle size, and the middle region can be filled with medium-sized powder as needed to form a particle size gradient distribution.

[0081] The second assembly 10, filled with gradient powder, is placed in a cold isostatic pressing apparatus. It is pressed isotropically for 180 seconds under a hydrostatic pressure of 250 MPa. After pressing, the external elastic sealing sleeve is removed, resulting in a first blank with a dense structure and relatively uniform density distribution.

[0082] The first preform is degreased to remove the forming agent added to the powder, resulting in the second preform.

[0083] The second preform is fixed in place, and a ceramic core rod is pressed axially into the center hole of the second preform using an electric pusher. The ceramic core rod and the powder layer inside the hole of the second preform are designed for an interference fit, with an interference of 0.5 mm on each side. After pressing, a third preform is formed.

[0084] Mount and secure the third preform onto the base of the hot pressing sintering furnace. Align the furnace's pressure bar with the powder layer area above the third preform. Close the furnace door, evacuate, and begin hot pressing sintering.

[0085] The hot-pressing sintering process is as follows: The temperature is increased to the debinding temperature at a rate of 5℃ / min, and debinding is completed. The temperature is then increased further to the sintering temperature of 1180℃. During the heating process, axial pressure is applied at 800℃ at a loading rate of 0.3 MPa / min, ultimately reaching a holding pressure of 90 MPa at the sintering temperature and holding for 80 minutes. During this process, the thermal expansion of the metal powder 400 is constrained by the ceramic outer tooling and the ceramic mandrel, generating additional radial compressive stress, which promotes densification.

[0086] After sintering, the material is cooled in the furnace and the ceramic fixtures are removed to obtain a powder metallurgy nut blank with high density and a uniform wear-resistant layer.

[0087] While the present invention has been disclosed above, it is not limited thereto. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for preparing a powder metallurgy nut blank, characterized in that, include: The metal nut base and the forming tooling are installed to obtain a first assembly, the first assembly having a pre-set annular cavity for filling metal powder; The annular cavity is filled with at least three metal powders of different average particle sizes; The first assembly is divided into a high-pressure zone, a medium-pressure zone, and a low-pressure zone along the pressing direction. The high-pressure zone is filled with the metal powder with the largest average particle size, the medium-pressure zone is filled with the metal powder with the medium average particle size, and the low-pressure zone is filled with the metal powder with the smallest average particle size, so that the average particle size is distributed in a trapezoidal shape along the pressing direction to obtain the second assembly. The second assembly, after being filled with metal powder, is pressed to obtain a blank; The blank is degreased and sintered sequentially to obtain the powder metallurgy nut blank.

2. The preparation method according to claim 1, characterized in that, During the pressing process, the second assembly is subjected to bidirectional pressure from an upper pressure head above and a lower pressure head below; The upper end of the second assembly is defined as the upper pressing end face that mates with the upper pressing head, and the lower end of the second assembly is defined as the lower pressing end face that mates with the lower pressing head. The second assembly is divided into a large pressure zone, a medium pressure zone, a small pressure zone, a medium pressure zone, and a large pressure zone from the upper pressing end face to the lower pressing end face, so that the average particle size of the metal powder gradually increases from the central region of the annular cavity toward the upper pressing end face and the lower pressing end face, respectively. The height difference between the upper pressing end face and the lower pressing end face is H, and the maximum radial dimension of the annular cavity is D, satisfying: 5≤H / D≤10.

3. The preparation method according to claim 2, characterized in that, The average particle size of the metal powder ranges from 3 micrometers to 80 micrometers, and the difference in average particle size between metal powders with different average particle sizes is greater than or equal to 5 micrometers.

4. The preparation method according to claim 1, characterized in that, The pressing is cold isostatic pressing, the pressure of which is 250 MPa to 300 MPa, and the holding time is 60 to 180 seconds.

5. The preparation method according to claim 1, characterized in that, The forming fixture includes an outer fixture, the metal nut base is connected to the outer fixture by a thread, and an exhaust gap for venting gas is formed between the metal nut base and the outer fixture.

6. The preparation method according to claim 5, characterized in that, The sintering is hot pressing sintering; The external tooling is made of ceramic. During the hot pressing sintering process, the difference in thermal expansion coefficients between the metal powder and the ceramic is used to make the metal powder radially constrained by the external tooling when it expands due to heat.

7. The preparation method according to claim 1, characterized in that, Between the degreasing and the sintering process, the following is also included: The ceramic core rod is pressed into the central hole of the blank, and the ceramic core rod and the powder layer formed by the metal powder form an interference fit.

8. The preparation method according to claim 7, characterized in that, The interference fit between the ceramic core and the powder layer is 0.2 mm to 1 mm on one side.

9. The preparation method according to claim 6, characterized in that, The process parameters for hot pressing sintering are: heating rate ≤ 5℃ / min, pressure loading rate ≤ 0.5 MPa / min, holding pressure 80 MPa to 100 MPa, and holding time 50 to 120 minutes.

10. A powder metallurgy nut blank, characterized in that, It is prepared by any one of claims 1 to 9.

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