Bimetal composite structure long-life ball nut and preparation method thereof

Abstract

The invention relates to the technical field of precision mechanical transmission part manufacturing, in particular to a bimetal composite structure long-service-life ball nut and a preparation method thereof. The ball nut comprises a cylindrical base body with an axial through hole, a wear-resistant layer is arranged on the inner surface of the base body, the wear-resistant layer is in metallurgical bonding with the base body, and the base body is made of medium carbon steel. And the wear-resistant layer is made of high-speed steel. A base body serves as a mold cavity, and a first mold assembly is installed in a through hole of the base body; the annular gap is filled with wear-resisting layer powder containing high-speed steel, and cold press molding is carried out; replacing the first mold assembly with a second mold assembly, performing hot pressing sintering on the cold-pressed blank body to form a wear-resistant layer, and forming metallurgical bonding between the wear-resistant layer and the base body to obtain a composite blank body; and a ball nut finished product is obtained through machining. High-hardness metal is adopted at the internal key friction part, better wear resistance and longer raceway service life can be provided, and the outer layer main body is made of common medium carbon steel, so that the cost can be remarkably reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of precision mechanical transmission component manufacturing, in particular to a long-life ball nut with a bimetallic composite structure and a preparation method thereof. BACKGROUND

[0002] At present, ball nuts are generally made of high-carbon chromium bearing steel such as GCr15 (material cost about 7.5-9 yuan / kg). However, this scheme has two outstanding problems: first, the material and processing cost is high, about 60%-70% of the raw material needs to be removed to process the complex inner raceway structure, the material utilization rate is low, which increases the overall manufacturing cost by 30%-40%; second, the service life and performance are limited, under heavy load conditions with axial load ≥15kN, the average service life of the overall bearing steel nut is usually 3500-4000 hours, and the main failure mode is raceway surface contact fatigue spalling, which has become a weak link affecting the long-term reliability of the equipment.

[0003] In order to improve the wear resistance and fatigue life of the raceway, the industry has tried to use surface carburizing, nitriding or electroplating / spraying hard coating processes. However, the hardening layer formed by carburizing or nitriding has a shallow depth (usually 0.3-0.5mm), which is difficult to meet the heavy load deep stress requirement; electroplating process has environmental pollution problems, and the bonding strength of the plating layer and the substrate is limited, which is prone to interface peeling under alternating load; thermal spraying technology has the problems of complex equipment, difficulty in controlling coating uniformity, poor adaptability to small parts, etc. If the nut is made of overall powder high-speed steel (such as M2), although the wear resistance and service life are significantly improved, the material cost is as high as 70-90 yuan / kg, which leads to the final product price being too high, poor economic efficiency, and difficult to be widely promoted.

[0004] Therefore, it is of great significance to provide a high-performance, long-life, low-cost ball nut and a manufacturing method thereof.

[0005] In view of the above, the present application is proposed. SUMMARY

[0006] The purpose of the present application is to provide a long-life ball nut with a bimetallic composite structure and a preparation method thereof, which adopts high-hardness alloy powder and ordinary outer alloy metal sleeve cold forming, and then hot-presses the powder metallurgy to form a ball nut blank with a bimetallic composite structure; high-hardness metal is used in the internal key friction part to provide better wear resistance and raceway life, and the outer body adopts ordinary alloy steel to significantly reduce the cost.

[0007] In order to achieve the above-mentioned purpose of the present application, the following technical scheme is adopted: A long-life ball nut of bimetallic composite structure comprises a cylindrical base body with an axial through hole, an inner surface of the base body is provided with a wear-resistant layer, the wear-resistant layer is metallurgically combined with the base body, a material of the base body comprises medium carbon steel, and a material of the wear-resistant layer comprises high-speed steel.

[0008] Preferably, the thickness of the wear-resistant layer is 3-10 mm.

[0009] Preferably, the porosity of the wear-resistant layer is ≤1.5%.

[0010] Preferably, the hardness of the wear-resistant layer is ≥62 HRC.

[0011] Preferably, the hardness and wear resistance of the wear-resistant layer increase from outside to inside along the radial direction.

[0012] Preferably, the bonding strength between the wear-resistant layer and the base body is ≥510 MPa.

[0013] Preferably, the material of the base body comprises 40Cr and / or 42CrMo.

[0014] Preferably, the wear-resistant layer further contains a brazing filler metal, and satisfies that the brazing filler metal accounts for 1wt%-10wt% of the wear-resistant layer, and / or the brazing filler metal comprises at least one of a nickel-based brazing filler, a copper-based brazing filler, and a silver-based brazing filler.

[0015] Preferably, the wear-resistant layer further contains nano-enhanced particles, and satisfies that the nano-enhanced particles account for 0.5wt%-1wt% of the wear-resistant layer, and / or the nano-enhanced particles comprise at least one of TiC, tungsten carbide, and boron nitride.

[0016] A preparation method of the long-life ball nut of bimetallic composite structure according to any one of the preceding embodiments, comprising the following steps: S1. Taking a cylindrical base body with an axial through hole as a mold cavity, and installing a first mold assembly in the through hole; S2. Filling wear-resistant layer powder, which comprises high-speed steel powder, into an annular gap between the base body and the first mold assembly, and performing cold pressing forming; S3. Replacing the first mold assembly with a second mold assembly, and still taking the base body as the mold cavity, to perform hot pressing sintering on the cold-pressed blank to form a wear-resistant layer, and to form metallurgical combination between the wear-resistant layer and the base body, to obtain a composite blank; S4. Processing the composite blank to obtain a ball nut finished product.

[0017] Preferably, the wear-resistant layer powder further contains a brazing filler metal powder, and the brazing filler metal powder accounts for 1wt%-10wt% of the wear-resistant layer powder, and / or the brazing filler metal powder includes at least one of a nickel-based brazing filler metal, a copper-based brazing filler metal, and a silver-based brazing filler metal.

[0018] Preferably, the wear-resistant layer powder further contains nano-enhanced particles, and the nano-enhanced particles account for 0.5wt%-1wt% of the wear-resistant layer powder, and / or the nano-enhanced particles include at least one of TiC, tungsten carbide, and boron nitride.

[0019] Preferably, in step S2, the wear-resistant layer powder of different compositions is sequentially filled in the annular gap from the outside to the inside along the radial direction to form a gradient composition structure.

[0020] Preferably, the high-speed steel powder is provided with a first metal plating layer, and the first metal plating layer includes Ni and / or Cu. Preferably, the inner surface of the substrate is provided with a second metal plating layer, and the second metal plating layer includes Ni and / or Cu.

[0021] Preferably, the particle size distribution of the high-speed steel powder satisfies D10≥10μm and D90≤75μm.

[0022] Preferably, the high-speed steel powder includes at least two powders of different D50 particle sizes mixed according to a grading.

[0023] Preferably, the high-speed steel powder includes coarse powder with a D50 of 40-60μm and fine powder with a D50 of 15-30μm, and the mixing mass ratio of the coarse powder to the fine powder is (6:4)-(8:2).

[0024] Preferably, step S1 further includes a step of sandblasting and roughening the inner surface of the substrate or processing a microstructure, the microstructure including at least one of a pit, a rib, and a groove, and the depth or height of the microstructure along the radial direction of the nut is 1 / 30-1 / 50 of the wall thickness of the substrate.

[0025] Preferably, the cold pressing forming includes applying a radial pressure of 300-350MPa to the powder at 0-40℃ for 10-45min.

[0026] Preferably, the hot pressing sintering includes: under the condition of a vacuum degree ≤1×10 -3 Pa, heating to a sintering temperature of 1000-1180℃, synchronously applying a pressure of 30-80MPa along the axial direction of the blank for 30-90min.

[0027] Preferably, the material of the first mold assembly is H13 hot work die steel, and the material of the second mold assembly is graphite. And / or, the first mold assembly and the second mold assembly are the same structure, both including: a wedge-shaped pressing rod located in the through hole of the base body and a wedge-shaped inner liner located at the bottom of the wedge-shaped pressing rod and matched with the wedge-shaped pressing rod, an annular pressing head is arranged on the outer side of the upper part of the wedge-shaped pressing rod, the annular pressing head is at least partially located in the through hole of the base body, an upper pressing head is arranged on the top of the wedge-shaped pressing rod and the annular pressing head, a base is arranged on the bottom of the base body, an annular gasket is arranged between the base body and the base, an inner gasket is arranged in the inner hole of the annular gasket, and the wear-resistant layer powder is located between the inner gasket and the annular pressing head.

[0028] Compared with the prior art, the beneficial effects of the present application are: (1) In the present application, the carbon steel base in the outer layer is used as a mold, and high-performance alloy powder is filled inward, combined with a two-stage mold replacement process, first cold pressing to prevent segregation, and then hot pressing to combine and maintain accuracy. Through the combination of radial cold pressing and axial hot pressing, as well as the diffusion metallurgical bonding of the powder and the base at high temperature, multiple strengthening methods are used in coordination, so that the interfacial bonding strength exceeds the strength of the inner layer material itself, reducing the risk of interlayer peeling during use. At the same time, the precise mold constraints the billet throughout the process, making the inner and outer roundness and wall thickness uniformity of the sintered billet good, providing a high-precision reference for subsequent finishing, greatly reducing the processing difficulty and cost. It realizes the preparation of uniform, dense and thin-walled wear-resistant layer in small and complex internal cavities, solves the industry problem that traditional outer wrapping HIP process cannot achieve uniform filling and precise forming on ball nuts, and breaks through the application barrier of traditional HIP or centrifugal casting process on such parts.

[0029] (2) In the present application, only a small amount of high-speed steel powder is used in the key friction part, and inexpensive medium carbon steel is used in the outer layer, which reduces the material cost by more than 60% compared with the overall high-speed steel scheme. Near-net forming technology makes the composite billet have high precision, greatly reduces the excess and difficulty of subsequent finishing, improves production efficiency and consistency, and further saves manufacturing cost. The product life is more than 30% longer than that of traditional whole GCr15 nut (up to 6000-8000 hours). BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the specific embodiments or the prior art, the following will briefly introduce the drawings needed to be used in the description of specific embodiments or prior art. Obviously, the drawings described below are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0031] Figure 1 The structure schematic diagram of the powder metallurgy double-metal composite structure ball nut body provided by the traditional single-metal ball nut body and the embodiment of the present application; Figure 2 The second mold assembly provided by the embodiment of the present application is assembled with the base body. DETAILED DESCRIPTION

[0032] The technical solutions of the present application will be described clearly and completely in combination with the drawings and specific embodiments, but those skilled in the art will understand that the following described embodiments are part of the embodiments of the present application, not all the embodiments, and are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application. The specific conditions are not specified in the embodiments, and are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, and are conventional products that can be purchased on the market.

[0033] As shown in Figure 1 The first aspect of the present application provides a long-life ball nut of bimetallic composite structure, which comprises a cylindrical base body with an axial through hole, an inner surface of the base body is provided with a wear-resistant layer, the wear-resistant layer is metallurgically combined with the base body, and the material of the base body comprises medium carbon steel; and the material of the wear-resistant layer comprises high-speed steel.

[0034] The present application uses high-speed steel only in the key wear-resistant parts of the inner surface of the nut, and uses low-cost medium carbon steel for the outer layer of the base body, which can significantly improve the wear resistance and service life of the ball nut while keeping the cost low. The service life of the ball nut provided by the present application can be increased by more than 30% compared with the traditional whole GCr15 nut, and the service life can reach 6000-8000 hours. The material cost is reduced by more than 60% compared with the whole high-speed steel scheme, and the ball nut has high performance, long service life and low cost.

[0035] In some specific embodiments of the present application, the thickness of the wear-resistant layer is 3-10mm, for example, it can be any one value or a range value composed of any two point values in 3mm, 5mm, 8mm and 10mm; only a small amount of high-cost wear-resistant material is used in the inner layer, which can significantly improve the wear resistance of the working surface of the ball nut and ensure high economic efficiency; and the high blank precision reduces the machining allowance, further saving the manufacturing cost.

[0036] In some specific embodiments of the present application, the porosity of the wear-resistant layer is ≤1.5%, for example, it can be any one value or a range value composed of any two point values in 1.5%, 1.2%, 1%, 0.9%, 0.7%, 0.5%, 0.3%, 0.1%; close to full density, which greatly improves the fatigue strength and wear resistance of the material.

[0037] In some specific embodiments of the present invention, the hardness of the wear-resistant layer is ≥62HRC. For example, it can be any one value or a range of any two values ​​among 62HRC, 63HRC, 64HRC, 65HRC, and 66HRC.

[0038] In some specific embodiments of the present invention, the hardness and wear resistance of the wear-resistant layer increase radially from the outside to the inside.

[0039] In some specific embodiments of the present invention, the bonding strength between the wear-resistant layer and the substrate is ≥510MPa. For example, it can be any one value or a range of any two values ​​among 510MPa, 520MPa, 540MPa, 550MPa, and 575MPa. The high interfacial shear strength can significantly reduce the risk of interlayer delamination during use.

[0040] In some specific embodiments of the present invention, the matrix material includes 40Cr and / or 42CrMo.

[0041] In some specific embodiments of the present invention, the wear-resistant layer further contains brazing filler metal, and satisfies the following conditions: the brazing filler metal accounts for 1wt%-10wt% of the wear-resistant layer, for example, it can be any one value or a range of any two values ​​from 1wt%, 3wt%, 5wt%, 7wt%, to 10wt%; and / or, the brazing filler metal includes at least one of nickel-based brazing filler metal, copper-based brazing filler metal, and silver-based brazing filler metal. The brazing filler metal can fill the pores of the high-speed steel powder, reduce the porosity of the wear-resistant layer, improve its density, and also provide bonding strength within the wear-resistant layer matrix, as well as bonding strength between the wear-resistant layer and the carbon steel matrix in the outer layer; the brazing filler metal used is preferably nickel-based, which has a better effect on improving bonding strength.

[0042] In some specific embodiments of the present invention, the wear-resistant layer further contains nano-reinforcing particles, and satisfies the following conditions: the nano-reinforcing particles account for 0.5wt%-1wt% of the wear-resistant layer, for example, it can be any one value or a range of any two values ​​from 0.5wt%, 0.6wt%, 0.8wt%, to 1wt%; and / or, the nano-reinforcing particles include at least one of TiC, tungsten carbide, and boron nitride. Adding nano-reinforcing particles to the wear-resistant layer can further improve the service life of the nut.

[0043] A second aspect of the present invention provides a method for preparing a bimetallic composite long-life ball nut according to any one of the foregoing embodiments, comprising the following steps: S1. Take a cylindrical base with an axial through hole as the mold cavity, and install the first mold assembly in its through hole; S2. Fill the annular gap between the substrate and the first mold assembly with wear-resistant layer powder and perform cold pressing molding. The wear-resistant layer powder includes high-speed steel powder. S3. Replacing the first die assembly with a second die assembly, still taking the base as the die cavity, hot-press sintering the cold-pressed blank to form a wear-resistant layer and form a metallurgical bond between the wear-resistant layer and the base, to obtain a composite blank; S4. Processing the composite blank to obtain a finished ball nut product.

[0044] The method of the application is to take the carbon steel base as the mold in the outer layer, fill the high-hardness alloy powder inward, combine the two-stage die replacement process, first cold-press forming, then composite in the form of hot-press powder metallurgy to prepare a double-metal composite structure ball nut blank, and finally obtain the final size of the ball nut product through precision grinding and other mechanical processing; through the combination of radial cold-pressing and axial hot-pressing, and the diffusion metallurgical bonding of the powder and the base at high temperature, the interface bonding strength is improved; at the same time, the precision mold constraints the blank throughout the process, the inner and outer roundness and wall thickness uniformity of the sintered blank are good, providing a high-precision reference for subsequent finishing, greatly reducing the processing difficulty and cost; realizing the preparation of a uniform, dense and thin-walled wear-resistant layer in a small and complex internal cavity, solving the industry problem that the traditional outer wrapping HIP process cannot realize uniform filling and precision forming on the ball nut, and using high-hardness metal in the internal key friction part can provide better wear resistance and raceway life, and the outer layer main body uses ordinary medium carbon steel, mainly bearing structural strength and toughness, which can significantly reduce the cost.

[0045] The method of the application is a manufacturing method of a double-metal composite structure specially designed for precision small parts such as ball nuts, which can simultaneously solve the three core problems of uniform composite of the internal cavity wear-resistant layer, high-strength interface bonding and high-precision forming of the blank, thereby realizing the unification of high performance and low cost.

[0046] In some specific embodiments of the application, the wear-resistant layer powder also contains brazing powder, and satisfies: the brazing powder accounts for 1wt%-10wt% of the wear-resistant layer powder, for example, can be any one value or a range value composed of any two point values in 1wt%, 3wt%, 5wt%, 7wt%, 10wt%; and / or, the brazing powder used includes at least one of nickel-based brazing filler metal, copper-based brazing filler metal, and silver-based brazing filler metal. The melting temperature of the added brazing powder is lower than the hot-press sintering temperature in step S3, and in the hot-press sintering process, the brazing powder can melt to form a liquid phase, filling the gaps between the high-speed steel powder, improving the density of the wear-resistant layer, and at the same time, improving the bonding strength inside the wear-resistant layer base and the interface bonding strength between the wear-resistant layer and the outer carbon steel base. The added brazing powder is preferably a nickel-based brazing filler metal (such as BNi-2 nickel-based brazing filler metal).

[0047] In some embodiments of the present application, the wear-resistant layer powder further contains nano-enhanced particles, and the nano-enhanced particles account for 0.5wt%-1wt% of the wear-resistant layer powder, for example, can be 0.5wt%, 0.6wt%, 0.8wt%, 1wt% or a range value composed of any two point values; and / or the nano-enhanced powder includes at least one of TiC, tungsten carbide, and boron nitride.

[0048] In some embodiments of the present application, in step S2, the wear-resistant layer powder of different compositions is sequentially filled in the annular gap from outside to inside in the radial direction (such as high-speed steel powder with different C content or alloy content), and after sintering, a gradient structure with gradually increasing hardness, wear resistance, and optimized toughness and bonding strength from the interface with the outer layer substrate to the inner surface working layer is formed, thereby fundamentally eliminating the interface stress concentration caused by the difference in physical properties of the materials and preventing interlayer peeling. When filling multiple wear-resistant layer powders of different compositions in the radial direction, multiple cold pressing is required, one cold pressing for each type of metal powder, and the size of the first component in the center needs to be changed for cold pressing, and the size of the gap for filling the powder in the middle needs to be changed, so as to fill different wear-resistant layer powders respectively.

[0049] The introduction of the composition gradient transition layer alleviates the performance mutation at the interface, effectively absorbs and disperses alternating contact stress, and makes the reliability of the composite structure under heavy load and high speed working conditions improve by orders of magnitude.

[0050] In some embodiments of the present application, the high-speed steel powder is provided with a first metal plating layer, and the first metal plating layer includes Ni and / or Cu; the use of high-speed steel powder with a metal plating layer on the surface can improve the bonding strength between particles in the powder metallurgy process and the bonding strength between the wear-resistant layer and the substrate; in the presence of filler powder, the metal plating layer can also improve the affinity between the high-speed steel powder and the filler; the first metal plating layer preferably uses a metal with a melting point lower than the sintering temperature, which can uniformly fill the gaps between high-speed steel powders at the hot-pressing sintering temperature, thereby improving the density of the wear-resistant layer.

[0051] In some embodiments of the present application, the inner surface of the substrate is provided with a second metal plating layer; the second metal plating layer includes Ni and / or Cu; the metal plating layer on the surface of the outer layer substrate can improve the bonding strength between the wear-resistant layer and the medium-carbon steel substrate.

[0052] In some embodiments of the present application, the particle size distribution of the high-speed steel powder satisfies D10≥10μm and D90≤75μm; the particle size distribution ensures that the powder has good flowability and filling property, can be uniformly distributed in a narrow inner cavity, and is conducive to densification under a set pressure.

[0053] In some embodiments of the present application, the high-speed steel powder comprises at least two powders with different D50 particle sizes mixed in a graded manner to improve the filling density.

[0054] Through the synergy of "solder liquid phase sintering" and "powder optimized grading", the porosity of the wear-resistant layer can be reduced to below 1.5%, close to full density, greatly improving the fatigue strength and wear resistance of the material.

[0055] In some embodiments of the present application, the high-speed steel powder comprises coarse powder with a D50 of 40-60 μm and fine powder with a D50 of 15-30 μm, and the mixing mass ratio of the coarse powder to the fine powder is (6:4)-(8:2), for example, it can be any one value or a range value composed of two point values selected from 6:4, 6.5:3.5, 7:3, 7.5:2.5, and 8:2.

[0056] In some embodiments of the present application, the step S1 further comprises a step of sandblasting and roughening the inner surface of the base or processing a microstructure, the microstructure comprising at least one of a pit, a rib, and a groove, the depth or height of the microstructure along the radial direction of the nut being 1 / 30-1 / 50 of the wall thickness of the base, for example, it can be any one value or a range value composed of two point values selected from 1 / 30, 1 / 35, 1 / 40, 1 / 45, and 1 / 50; the mechanical interlocking of the surface microstructure can improve the interfacial bonding strength between the wear-resistant layer and the base.

[0057] In some embodiments of the present application, the cold pressing forming comprises: applying a radial pressure of 300-350 MPa to the powder at 0-40℃, and maintaining the pressure for 10-45 min, for example, the cold pressing temperature can be any one value or a range value composed of two point values selected from 0℃, 10℃, 20℃, 30℃, and 40℃; the radial pressure can be any one value or a range value composed of two point values selected from 300 MPa, 310 MPa, 320 MPa, 330 MPa, 340 MPa, and 350 MPa, and the pressure maintaining time can be any one value or a range value composed of two point values selected from 10 min, 20 min, 30 min, 40 min, and 45 min; the purpose of this stage is to realize the pre-forming of the loose powder in the inner cavity with high uniformity and high filling density (up to 85%-88%), and to obtain the initial strength of the blank, laying a precise foundation for the subsequent hot pressing.

[0058] In some embodiments of the present application, the hot pressing sintering comprises: heating the blank to 1200-1300℃ under a vacuum degree of ≤1×10 -3Under the condition of Pa, the temperature is raised to the sintering temperature of 1000-1180℃, and the pressure of 30-80MPa is applied along the axial direction of the blank synchronously, and the pressure is maintained for 30-90min; in this stage, the powder particles are fully densified by diffusion and flow, and atomic diffusion occurs between the inner wall of the outer layer of carbon steel to form a firm metallurgical bonding layer. Typically but not limitedly, for example, the temperature of hot-pressing sintering can be any one of 1000℃, 1050℃, 1100℃, 1150℃, 1180℃ or a range value composed of any two point values; the axial pressure applied can be any one of 30MPa, 40MPa, 50MPa, 60MPa, 70MPa, 80MPa or a range value composed of any two point values; the pressure maintaining time can be any one of 30min, 45min, 60min, 75min, 90min or a range value composed of any two point values; the pressure maintaining time can be appropriately adjusted according to the size of the workpiece.

[0059] In some embodiments of the present application, during hot-pressing sintering, the temperature is preferably raised to the sintering temperature in a stepwise manner to ensure uniform internal temperature; after reaching the sintering temperature, the pressure is maintained to realize powder densification and metallurgical diffusion bonding with the inner wall of the matrix; in other embodiments, if the wear-resistant layer powder contains brazing filler powder, the temperature is first raised to the brazing filler melting temperature range and maintained to produce liquid phase, and then the temperature is continuously raised to the sintering temperature, and the pressure is maintained to realize powder densification and metallurgical diffusion bonding with the inner wall of the matrix.

[0060] In some embodiments of the present application, between steps S3 and S4, a heat treatment step of quenching and tempering the composite blank is further included to obtain the desired martensitic structure and high hardness.

[0061] Due to the different temperatures of cold pressing and hot-pressing sintering, the materials suitable for the first mold assembly and the second mold assembly are different, in some embodiments of the present application, the material of the first mold assembly is H13 hot work die steel, and the material of the second mold assembly is graphite. The thermal expansion coefficient of the graphite mold matches well with the blank, and the precise cavity of the graphite mold itself ensures the overall dimensional stability of the composite blank under high temperature and high pressure; the graphite mold is used for high-temperature sintering because the graphite mold has better hardness at high temperature, and the powder metallurgy layer is not easy to be metallurgically connected with the abrasive tool.

[0062] In some specific embodiments of the present invention, the first mold assembly and the second mold assembly have the same structure, both including a wedge-shaped pressure bar located in the through hole of the substrate and a wedge-shaped inner liner located at the bottom of the wedge-shaped pressure bar and adapted thereto. An annular pressure head is provided on the upper outer side of the wedge-shaped pressure bar, and the annular pressure head is at least partially located in the through hole of the substrate. An upper pressure head is provided at the top of the wedge-shaped pressure bar and the annular pressure head, and a base is provided at the bottom of the substrate. An annular liner is provided between the substrate and the base, and an inner liner is provided in the inner hole of the annular liner. During operation, the wear-resistant layer powder is located between the inner liner and the annular pressure head. In one embodiment of the present invention, a schematic diagram of the assembly structure of the graphite mold and the substrate during the hot pressing sintering process is shown below. Figure 2 As shown.

[0063] The hot isostatic pressing (HIP) process applies pressure as a whole, which has weak control over the shape of the thin-walled inner cavity and is prone to deformation, making it impossible to meet the micron-level precision requirements of ball nuts. Moreover, building large hot isostatic pressing equipment for small parts is costly and inefficient. The mold assembly in this invention has a simple structure, which can significantly reduce equipment costs. Through two-stage mold replacement and precise pressurization, a thin, dense, and high-precision metallurgical composite of the wear-resistant layer and the substrate can be achieved.

[0064] The following detailed description of some embodiments of the present invention is provided in conjunction with specific application examples. Unless otherwise specified, all raw materials used in the embodiments can be obtained commercially available.

[0065] Example 1 S1. Prepare a 40Cr alloy steel outer layer substrate with an inner diameter of Φ30mm and install an H13 steel mold assembly; S2. Prepare three types of high-speed steel powders: A, B, and C (A is M2 high-speed steel, B is M4 high-speed steel, and C is ASP60 high-speed steel). Powders B and C are mixed in a coarse-fine two-stage mixture (coarse powder to fine powder mass ratio 7:3). Powders A, B, and C are then filled into the annular gaps radially from the outside to the inside and cold-pressed at room temperature. The first mold assembly is replaced for each type of powder before cold pressing. A radial pressure of 330 MPa is applied for each cold pressing and the pressure is held for 20 minutes. S3. Replace with graphite mold assembly and heat in a vacuum hot press furnace (vacuum degree ≤ 1×10⁻⁶). -3 The heating procedure for hot pressing sintering is as follows: heat from room temperature to 500℃ (heating time 40 min), hold for 20 min; continue heating to 900℃ (heating time 40 min), hold for 20 min; continue heating to 1100℃ (heating time 40 min), apply 50 MPa axial pressure and hold for 40 minutes to obtain the composite green body; quench the composite green body at 1180℃ + temper it three times at 560℃; S4. The ball nut is finished to the final size through precision grinding machining, and the inner layer of the nut is formed with a 3mm thick gradient wear-resistant layer.

[0066] The particle size parameters of powders A, B, and C in this embodiment are shown in Table 1.

[0067] Table 1

[0068] Example 2 S1. Laser-machine a mesh-like microgroove with a depth of 0.12mm onto the inner surface of the 40Cr sleeve, and install the H13 steel mold assembly; S2. After mixing graded high-speed steel powder with 2.5wt% BNi-2 brazing filler powder, apply radial pressure of 340MPa for cold pressing and hold for 30min; S3. Replace with graphite mold assembly and heat in a vacuum hot press furnace (vacuum degree ≤ 1×10⁻⁶). -3 The heating procedure for hot pressing sintering is as follows: heat from room temperature to 500℃ (heating time 40 min), hold for 20 min; continue heating to 900℃ (heating time 40 min), hold for 20 min; continue heating to 1015℃ (heating time 40 min), hold for 20 min (brazing filler metal melts and wets); continue heating to 1130℃ (heating time 15 min) and apply 60 MPa pressure for 60 minutes to obtain a composite billet; the composite billet is then quenched at 1180℃ and tempered three times at 560℃. S4. The ball nut is finished to the final size through precision grinding machining, and the inner layer of the nut is formed with a 3mm thick wear-resistant layer.

[0069] The parameters of the graded high-speed steel powder in this embodiment are shown in Table 2.

[0070] Table 2

[0071] Example 3 S1. Perform chemical nickel plating on the inner surface of the 40Cr sleeve (layer thickness approximately 5μm), and install the H13 steel mold assembly; S2. Fill with nickel-plated high-speed steel powder (containing 0.8wt% nano-TiC particles and 2.5wt% BNi-2 solder powder), apply radial pressure of 320MPa for cold pressing, and hold for 20min. S3. Replace with graphite mold assembly and heat in a vacuum hot press furnace (vacuum degree ≤ 1×10⁻⁶). -3The heating program for hot pressing sintering is as follows: heating from room temperature to 500℃ (heating time 40 min), holding for 20 min; continuing to heat to 900℃ (heating time 40 min), holding for 20 min; continuing to heat to 1015℃ (heating time 40 min), holding for 20 min (melting and wetting of the brazing filler metal); continuing to heat to 1135℃ (heating time 15 min), applying an axial pressure of 55 MPa and holding for 60 minutes for hot pressing sintering to obtain a composite green body with a uniform diffusion layer of about 20 μm wide at the interface; the composite green body is then quenched at 1180℃ and tempered three times at 560℃. S4. The ball nut is finished to the final size through precision grinding machining, and the inner layer of the nut is formed with a 3mm thick wear-resistant layer.

[0072] In this embodiment, the preparation method of nickel-plated high-speed steel is as follows: First, the M2 high-speed steel powder (D10=24μm, D50=43μm, D90=69μm) was ultrasonically cleaned for 8 minutes to remove oil and impurities, and then activated by acid pickling with 8% dilute hydrochloric acid for 8 minutes. Subsequently, it was immersed in a chemical plating solution composed of 28 g / L nickel sulfate, 25 g / L sodium hypophosphite and 23 g / L complexing agent sodium citrate. The pH value of the plating solution was strictly controlled at around 5 and the temperature was maintained at 85℃. Strong mechanical or ultrasonic stirring was applied to prevent powder agglomeration. The plating time was 1 hour to obtain the required coating thickness of about 0.5μm. After the reaction, it was repeatedly washed with deionized water and dried in a vacuum atmosphere at 90℃. To enhance the adhesion, it can be heat-treated at 350℃ in an inert atmosphere for 0.8 hours.

[0073] Example 4 Example 4 is similar to Example 2, except that BNi-2 solder powder was not added to the filler powder, and all other conditions are the same as in Example 2.

[0074] Example 5 Example 5 is similar to Example 2, except that the high-speed steel powder used is the same as the coarse powder used in Example 2, and all other conditions are the same as in Example 2.

[0075] Example 6 Example 6 is similar to Example 3, except that the surface of the high-speed steel powder is not plated with nickel, and all other conditions are the same as in Example 3.

[0076] Comparative Example 1 Comparative Example 1 is similar to Example 1, except that the radial pressure applied during cold pressing is 200 MPa and the axial pressure applied during hot pressing and sintering is 50 MPa.

[0077] Test case The interfacial bonding strength, inner surface hardness of the wear-resistant layer, porosity of the wear-resistant layer, and wear resistance of the ball nuts in each embodiment and comparative example were tested.

[0078] Interface bonding strength: tested according to GB / T 11363-2008; Hardness: Tested according to GB / T 230.1-2018 "Metallic materials Rockwell hardness test - Part 1: Test method"; Porosity: Tested according to the standard test method for density of powder metallurgy (PM) materials with porosity less than 2% as specified in ASTM B311-2017. Lifespan: According to GB / T 17587.5-2008 Ball Screw Pairs. Part 5: Axial Rated Static and Dynamic Loads and Service Life Tests; The test results are shown in Table 3.

[0079] Table 3

[0080] As shown in Table 1, the bimetallic composite long-life ball nut prepared by the method of the present invention has high working surface hardness, low porosity, high bonding strength between the wear-resistant layer and the substrate, and long service life. Comparative examples 2 and 4 show that adding brazing filler metal powder to the wear-resistant layer powder can improve the density of the wear-resistant layer and the bonding strength between the wear-resistant layer and the substrate, thereby increasing the service life of the ball nut. Comparison between Examples 2 and 5 shows that using a coarse-fine particle size distribution in the high-speed steel powder can also improve the density of the wear-resistant layer and the service life of the ball nut. Comparison between Examples 3 and 6 shows that nickel plating on the surface of the high-speed steel can further reduce the porosity of the wear-resistant layer, increase the bonding strength between the wear-resistant layer and the substrate, and thus increase the service life of the ball nut. Comparison between Example 1 and Comparative Example 1 shows that cold pressing pressure has a significant impact on the density of the wear-resistant layer, the bonding strength with the substrate, and the service life of the ball nut.

[0081] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A long-life ball nut with a bimetallic composite structure, characterized in that, The invention includes a cylindrical substrate with an axial through hole, and a wear-resistant layer is provided on the inner surface of the substrate. The wear-resistant layer is metallurgically bonded to the substrate. The material of the substrate includes medium carbon steel, and the material of the wear-resistant layer includes high-speed steel.

2. The bimetallic composite structure long-life ball nut according to claim 1, characterized in that, It meets at least one of the following characteristics: (1) The thickness of the wear-resistant layer is 3-10 mm; (2) The porosity of the wear-resistant layer is ≤1.5%; (3) The hardness of the wear-resistant layer is ≥62HRC; (4) The hardness and wear resistance of the wear-resistant layer increase radially from the outside to the inside; (5) The bonding strength between the wear-resistant layer and the substrate is ≥510MPa.

3. The long-life ball nut with bimetallic composite structure according to claim 1, characterized in that, It meets at least one of the following characteristics: (1) The matrix material includes 40Cr and / or 42CrMo; (2) The wear-resistant layer also contains brazing metal, and satisfies the following conditions: the brazing metal accounts for 1wt%-10wt% of the wear-resistant layer, and / or the brazing metal includes at least one of nickel-based brazing metal, copper-based brazing metal, and silver-based brazing metal; (3) The wear-resistant layer also contains nano-reinforcing particles, and satisfies the following conditions: the nano-reinforcing particles account for 0.5wt%-1wt% of the wear-resistant layer, and / or the nano-reinforcing particles include at least one of TiC, tungsten carbide, and boron nitride.

4. The method for preparing the bimetallic composite structure long-life ball nut according to any one of claims 1-3, characterized in that, Includes the following steps: S1. Take a cylindrical base with an axial through hole as the mold cavity, and install the first mold assembly in its through hole; S2. Fill the annular gap between the substrate and the first mold assembly with wear-resistant layer powder and perform cold pressing molding, wherein the wear-resistant layer powder includes high-speed steel powder; S3. Replace the first mold assembly with the second mold assembly, still using the substrate as the mold cavity, and perform hot pressing sintering on the cold-pressed blank to form a wear-resistant layer, and make the wear-resistant layer and the substrate form a metallurgical bond to obtain a composite blank; S4. The composite blank is processed to obtain the finished ball nut.

5. The method for preparing a long-life bimetallic composite ball nut according to claim 4, characterized in that, It meets at least one of the following characteristics: (1) The wear-resistant layer powder further contains brazing filler metal powder, and satisfies the following conditions: the brazing filler metal powder accounts for 1wt%-10wt% of the wear-resistant layer powder, and / or the brazing filler metal powder includes at least one of nickel-based brazing filler metal, copper-based brazing filler metal, and silver-based brazing filler metal; (2) The wear-resistant layer powder also contains nano-reinforcing particles, and satisfies the following conditions: the nano-reinforcing particles account for 0.5wt%-1wt% of the wear-resistant layer powder, and / or the nano-reinforcing powder includes at least one of TiC, tungsten carbide, and boron nitride; (3) In step S2, wear-resistant layer powders of different compositions are sequentially filled in the annular void from the outside to the inside in a radial direction to form a gradient composition structure.

6. The method for preparing a long-life bimetallic composite ball nut according to claim 4, characterized in that, It meets at least one of the following characteristics: (1) The surface of the high-speed steel powder is provided with a first metal coating, the first metal coating including Ni and / or Cu; (2) The inner surface of the substrate is provided with a second metal coating, the second metal coating including Ni and / or Cu; (3) The particle size distribution of the high-speed steel powder satisfies: D10≥10μm, D90≤75μm.

7. The method for preparing a long-life bimetallic composite ball nut according to claim 4, characterized in that, The high-speed steel powder comprises at least two powders with different D50 particle sizes mixed according to a gradation.

8. The method for preparing a long-life bimetallic composite ball nut according to claim 7, characterized in that, The high-speed steel powder includes coarse powder with a D50 of 40-60μm and fine powder with a D50 of 15-30μm, and the mass ratio of the coarse powder to the fine powder is (6:4)-(8:2).

9. The method for preparing a long-life bimetallic composite ball nut according to claim 4, characterized in that, It meets at least one of the following characteristics: (1) Step S1 also includes a step of sandblasting roughening or processing microstructures on the inner surface of the substrate, wherein the microstructures include at least one of pits, ribs, and grooves, and the depth or height of the microstructures along the radial direction of the nut is 1 / 30 to 1 / 50 of the thickness of the substrate wall. (2) The cold pressing process includes: applying a radial pressure of 300-350 MPa to the powder at 0-40°C and holding the pressure for 10-45 min; (3) The hot pressing sintering includes: under a vacuum degree ≤1×10 -3 Under Pa conditions, the temperature is raised to the sintering temperature of 1000-1180℃, and a pressure of 30-80MPa is applied simultaneously along the axial direction of the green body, and the temperature and pressure are maintained for 30-90 minutes.

10. The method for preparing a long-life bimetallic composite ball nut according to claim 4, characterized in that, The first mold assembly is made of H13 hot work die steel, and the second mold assembly is made of graphite. And / or, the first mold assembly and the second mold assembly have the same structure, both including: a wedge-shaped pressure bar located in the through hole of the base and a wedge-shaped inner liner located at the bottom of the wedge-shaped pressure bar and adapted thereto, an annular pressure head provided on the upper outer side of the wedge-shaped pressure bar, the annular pressure head being at least partially located in the through hole of the base, an upper pressure head provided on the top of the wedge-shaped pressure bar and the annular pressure head, a base provided on the bottom of the base, an annular gasket provided between the base and the base, an inner pad provided in the inner hole of the annular gasket, and the wear-resistant layer powder located between the inner pad and the annular pressure head.

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