A powder metallurgy mold self-lubricating structure and a preparation method thereof, and a powder metallurgy mold

Abstract

The application discloses a powder metallurgy die self-lubricating structure and a preparation method thereof, and a powder metallurgy die. The powder metallurgy die self-lubricating structure is composed of a working layer, a transition layer and a base layer in sequence. The working layer is made of a self-lubricating powder metallurgy composite material. The self-lubricating powder metallurgy composite material is composed of the following percentage mass components: hard particles 60-75%, bonding metal 20-30% and solid lubricant 5-15%. The transition layer is provided with multiple sub-transition layers. The components of each layer of the sub-transition layers are composed of the components of the working layer and the base layer. The components of the working layer and the base layer are continuously and gradiently distributed along the direction from the working layer to the base layer. The upper die punch, the lower die friction and the die surface of the powder metallurgy die are provided with the powder metallurgy die self-lubricating structure. The self-lubricating property of the structure makes it unnecessary to add external lubricant when the die is used, eliminates the density fluctuation of the compact caused by uneven distribution of the lubricant, and avoids the negative influence of the lubricant residue on the sintering quality.

Description

Technical Field

[0001] This invention belongs to the technical field of powder metallurgy, and particularly relates to a self-lubricating structure for powder metallurgy molds and its preparation method, as well as powder metallurgy molds. Background Technology

[0002] In the powder metallurgy pressing process, there is severe friction and wear between the inner wall of the die cavity and the powder, and between the die punch and the die. Traditional powder metallurgy dies are usually made of solid cemented carbide or tool steel, and the coefficient of friction and die wear are reduced by externally spraying lubricant or adding lubricant to the powder. This technology, which relies on external lubrication, has the following drawbacks: Uneven lubricant distribution leads to fluctuations in the density of the compact. Whether the lubricant is sprayed onto the inner wall of the die cavity or mixed into the powder, it is difficult to achieve a completely uniform distribution. Too much or too little lubricant in certain areas will affect the flow behavior of the powder, resulting in density differences in different areas of the compact, which in turn affects the dimensional accuracy and mechanical properties of the sintered parts. Lubricant residue will affect the subsequent sintering quality. Lubricant remaining on the surface of the compact during the pressing process will volatilize and decompose during the sintering stage. If the volatilization is incomplete or there are too many residues, defects such as pores and inclusions will form inside the sintered parts, which may lead to product scrap in severe cases. Under high-pressure pressing conditions, traditional lubricating films are prone to rupture, causing die damage. Especially when the pressing pressure exceeds 700 MPa, the contact pressure between the inner wall of the die and the powder is extremely high, and the physical lubricating film formed by ordinary lubricants is difficult to maintain its integrity. Once the lubricating film ruptures, the powder particles directly undergo microscopic cold welding with the die wall, resulting in die surface damage and a sharp acceleration of wear.

[0003] To avoid the aforementioned problems during lubrication, wear-resistant coatings such as TiN, TiAlN, and DLC are applied to the mold surface, aiming to extend mold life through the wear resistance of the coating itself. However, the bonding strength between the coating and the substrate is insufficient. Under repeated loading of high pressure, the interface between the coating and the substrate is subjected to enormous shear stress, making it prone to peeling failure. Furthermore, the coating cannot self-repair after wear, resulting in limited improvement in mold life. Summary of the Invention

[0004] The purpose of this invention is to solve at least one problem in the prior art by proposing a self-lubricating structure for powder metallurgy molds and its preparation method, as well as a powder metallurgy mold.

[0005] To achieve the above objectives, this invention proposes a self-lubricating structure for powder metallurgy molds. The self-lubricating structure is composed of a working layer, a transition layer, and a matrix layer, sequentially connected in composite. The working layer is made of a self-lubricating powder metallurgy composite material, which comprises the following percentage components by mass: 60%-75% hard particles, 20%-30% binder metal, and 5%-15% solid lubricant. The hard particles are WC or TiC, the binder metal is Co or Ni, and the solid lubricant is at least one of graphite, MoS2, and h-BN. The matrix layer is made of mold steel. The transition layer comprises multiple sub-transition layers, each composed of components from both the working layer and the matrix layer, with the components of the working layer and the matrix layer continuously gradient-distributed along the direction from the working layer to the matrix layer.

[0006] Adding solid lubricant to the working layer allows for the formation of a transfer lubricating film on the surface of the working layer during friction, utilizing the anisotropic layered crystal structure and low shear properties of the solid lubricant to achieve self-lubrication. Hard particles serve as the skeleton of the working layer, providing it with high hardness and wear resistance. The bonding metal melts during sintering and binds the hard particles together, providing a certain degree of toughness.

[0007] The transition layer effectively solves the problem of bonding two dissimilar materials, the working layer and the substrate layer: firstly, it eliminates interfacial thermal stress caused by the difference in thermal expansion coefficients; secondly, it achieves metallurgical bonding between the working layer and the substrate layer, preventing interfacial peeling.

[0008] As an optional implementation, the composition of each sub-transition layer is such that the composition gradient of the working layer decreases along the direction from the working layer to the matrix layer, while the composition gradient of the matrix layer increases, and the thickness of the transition layer is 1.0 mm to 3.0 mm.

[0009] As an optional implementation, the number of sub-transition layers is 3, and they are respectively a first sub-transition layer, a second sub-transition layer, and a third sub-transition layer in the direction from the working layer to the substrate layer. The first sub-transition layer is composed of the following percentage mass components: 40%-50% of the working layer component and 50%-60% of the substrate layer component. The second sub-transition layer is composed of the following percentage mass components: 20%-30% of the working layer component and 70%-80% of the substrate layer component. The third sub-transition layer is composed of the following percentage mass components: 5%-15% of the working layer component and 85%-95% of the substrate layer component.

[0010] As an optional implementation, the number of sub-transition layers is 5, and they are designated as a first sub-transition layer, a second sub-transition layer, a third sub-transition layer, a fourth sub-transition layer, and a fifth sub-transition layer along the direction from the working layer to the substrate layer. The first sub-transition layer is composed of the following percentage mass components: 60%-70% of the working layer component and 30%-40% of the substrate layer component. The second sub-transition layer is composed of the following percentage mass components: 45%-55% of the working layer component and 45%-55% of the substrate layer component. The third sub-transition layer is composed of the following percentage mass components: 30%-40% of the working layer component and 60%-70% of the substrate layer component. The fourth sub-transition layer is composed of the following percentage mass components: 15%-25% of the working layer component and 75%-85% of the substrate layer component. The fifth sub-transition layer is composed of the following percentage mass components: 5%-10% of the working layer component and 90%-95% of the substrate layer component.

[0011] As an optional implementation, the working layer has a thickness of 0.5mm-2.0mm, a hardness of HRA 85-HRA 90, and a friction coefficient of no more than 0.083.

[0012] As an optional implementation, the mold steel is of type H13, DIEVAR, or 8407, and the hardness of the mold steel after heat treatment is HRC 45-HRC 52. The heat-treated mold steel has better toughness and fatigue resistance, and can withstand repeated loading during high-pressure pressing.

[0013] This invention also proposes a method for preparing a self-lubricating structure for powder metallurgy molds, which includes the following steps:

[0014] Weigh out the hard particle powder, bonding metal powder, and solid lubricant powder in proportion, ball mill and mix them, vacuum dry them, and sieve them to obtain the working layer composite powder. The mold steel powder is loaded into the bottom of the mold steel sleeve and compacted to serve as the base layer. A multi-layered sub-transition layer and a working layer composite powder with a gradient distribution are sequentially loaded onto the upper surface of the base layer. The green blank is obtained by cold isostatic pressing under the conditions of a cold isostatic pressing pressure of 200MPa-300MPa and a holding time of 10min-30min. The green blank is placed in a vacuum sintering furnace and sintered at a heating rate of 5℃ / min-10℃ / min and a temperature of 1350℃-1450℃ for 1h-2h to densify the powder of the green blank and achieve interlayer metallurgical bonding. After sintering, it is subjected to quenching and tempering heat treatment to make the matrix layer reach the target hardness, thus obtaining a self-lubricating structure of powder metallurgy mold.

[0015] As an optional implementation, the preparation method further includes the following steps: finishing the self-lubricating structure of the powder metallurgy mold obtained after heat treatment so that the surface roughness does not exceed 0.4 micrometers, wherein the finishing is performed by electrical discharge machining or grinding.

[0016] As an optional implementation, the ball milling and mixing process is carried out in a ball mill, with specific process conditions of ball milling and mixing time of 4h-8h, mixing medium of anhydrous ethanol, ball-to-material ratio of (5:1)-(8:1), and rotation speed of 200rpm-300rpm; the vacuum drying process conditions are drying temperature of 60℃-100℃ and drying time of 12h-24h; the sieving is carried out under 200-400 mesh sieve conditions; the specific process of heat treatment is to first perform austenitizing oil quenching at 1020℃-1080℃, and then temper twice at 500℃-600℃, each time for 1h-3h.

[0017] This invention also proposes a powder metallurgy mold, including a female mold assembly and a die punch assembly. The female mold assembly includes a female mold sleeve with a cavity inside for pressing a blank. The die punch assembly includes an upper die punch and a lower die punch, which correspond to the cavity. The surfaces of the female mold sleeve, the upper die punch, and the lower die punch that contact the material are provided with the aforementioned self-lubricating structure for the powder metallurgy mold, with the working layer side of the self-lubricating structure facing the material. Providing the self-lubricating structure on the surfaces of the female mold sleeve, the upper die punch, and the lower die punch improves the sliding friction between the die punch and the female mold through self-lubrication.

[0018] The beneficial effects of this invention are: 1. This invention forms a self-lubricating structure for powder metallurgy molds by sequentially connecting a working layer, a transition layer, and a substrate layer. A solid lubricant is introduced into the composition of the working layer, realizing the intrinsic self-lubricating properties of the working layer material. The unique anisotropic crystal structure of the solid lubricant makes it easy to slide along the interlayer during friction and form a continuous and stable transfer lubricating film on the surface of the working layer. The layered solid lubricant is uniformly distributed inside the working layer. With the slight wear of the working layer surface, new lubricant particles are continuously exposed and participate in film formation, forming a "self-repairing" long-term lubrication mechanism. This intrinsic self-lubricating property means that the mold does not require any external lubricant during operation, fundamentally eliminating the density fluctuation of the compact caused by uneven lubricant distribution, and avoiding the negative impact of lubricant residue on sintering quality.

[0019] 2. This invention employs a gradient composition design in the transition layer, ensuring a smooth transition in the coefficient of thermal expansion from the working layer to the substrate layer, significantly reducing interfacial thermal stress generated during sintering and cooling. During sintering, the gradient composition transition layer undergoes sufficient elemental diffusion and metallurgical reaction with the materials on both sides, forming a high-strength metallurgical bonding interface. This structural design ensures that the working layer remains free from peeling and cracking under repeated high-pressure loading, providing a fundamental guarantee for the ultra-long lifespan of the mold and solving the long-standing technical bottleneck of reliable bonding of dissimilar materials in this field.

[0020] 3. The self-lubricating structure of powder metallurgy mold prepared by this invention has good self-lubricating properties and high interface strength. Applying this self-lubricating structure to the female mold and die punch assembly of powder metallurgy mold reduces the impact of friction and wear during the mold stamping process. It eliminates the need for a lubricant spraying system and mold wall cleaning device, simplifies the environment required for the operation of powder metallurgy mold, simplifies the structure, and reduces operating costs.

[0021] The features and advantages of the present invention will be described in detail through embodiments and in conjunction with the accompanying drawings. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the self-lubricating structure of the powder metallurgy mold in Embodiment 1 of the present invention.

[0023] Figure 2 This is a schematic diagram of the powder metallurgy mold structure of Embodiment 1 of the present invention.

[0024] Figure 3 This is a schematic diagram of the self-lubricating structure of the powder metallurgy mold in Embodiment 3 of the present invention.

[0025] In the diagram: 1. Working layer; 2. Transition layer; 3. Substrate layer; 21. First sub-transition layer; 22. Second sub-transition layer; 23. Third sub-transition layer; 24. Fourth sub-transition layer; 25. Fifth sub-transition layer. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0027] Example 1 See Figure 1This embodiment provides a self-lubricating structure for powder metallurgy molds. This structure is composed of a working layer 1, a transition layer 2, and a substrate layer 3, sequentially connected. The working layer 1 consists of the following percentages by mass: WC 70%, Co 20%, and graphite 10%, with a thickness of 1.5 mm. The substrate layer is made of H13 mold steel powder (particle size 50 μm-100 μm). The transition layer 2 has three sub-transition layers, which are, along the direction from the working layer 1 to the substrate layer 3, a first sub-transition layer 21 (thickness 1.0 mm), a second sub-transition layer 22 (thickness 1.0 mm), and a third sub-transition layer 23 (thickness 1.0 mm). The first sub-transition layer 21 consists of the following percentages by mass: 50% working layer powder and 50% H13 mold steel powder. The second sub-transition layer 22 is composed of the following percentage by mass components: 25% working layer powder and 75% H13 mold steel powder. The third sub-transition layer 23 is composed of the following percentage by mass components: 10% working layer powder and 90% H13 mold steel powder.

[0028] The self-lubricating structure of the powder metallurgy mold in this embodiment is prepared by the following method: S01. Weigh WC powder, Co powder, and graphite powder according to the working layer composition ratio in Example 1, add them to a ball mill, and ball mill for 6 hours under the conditions of anhydrous ethanol as the mixing medium, ball-to-material ratio of 5:1, and rotation speed of 250 rpm. Add the mixed slurry to a vacuum drying oven and vacuum dry it at 80°C for 12 hours. Pass it through a 200-mesh sieve to obtain the working layer composite powder. S02. Configure the first sub-transition layer, the second sub-transition layer, and the third sub-transition layer according to the transition layer composition ratio of Example 1. Load the mold steel powder into the bottom of a cylindrical mold steel sleeve with a diameter of 100mm, and vibrate and compact it to form a base layer with a thickness of 40mm. On the upper surface of the base layer, load the third sub-transition layer with a thickness of 1mm, the second sub-transition layer with a thickness of 1mm, and the first sub-transition layer with a thickness of 1mm in sequence. After each sub-transition layer is filled, gently scrape it flat. Finally, load the working layer composite powder with a thickness of 1.5mm. Place the cylindrical mold steel sleeve filled with powder in a cold isostatic press and cold isostatically press it under a pressure of 250MPa and a holding time of 10min to obtain a dense green blank. S03. Place the green billet in a vacuum sintering furnace and evacuate to a vacuum level of 5×10⁻⁶. -3 Pa was heated to 1400℃ at a heating rate of 8℃ / min and sintered for 1.5h to densify the powder of the green blank and achieve interlayer metallurgical bonding. After sintering, the green blank was cooled to room temperature in the furnace and then subjected to the following heat treatment: heating to 1020℃ for austenitization, oil quenching, and then tempering twice at 550℃ for 2h each time to make the matrix layer reach a hardness of HRC 48-52, thus obtaining a preliminary self-lubricating structure for powder metallurgy molds. S04. The preliminary powder metallurgy mold self-lubricating structure is finished by electrical discharge machining to achieve a surface roughness Ra≤0.4μm, thereby obtaining a powder metallurgy mold self-lubricating structure with the target shape, size and surface accuracy.

[0029] See Figure 2 This embodiment also provides a powder metallurgy gear pressing mold for manufacturing iron-based gears, with a pressing pressure of 700 MPa. The mold includes a female mold 3 and a die punch assembly. The die punch assembly includes an upper die punch 4 and a lower die punch 5. The female mold 3 has a cavity for filling with the material for preparing iron-based gears. The upper die punch 4 and the lower die punch 5 are respectively arranged opposite to the cavity for stamping the material in the cavity. The surfaces of the upper die punch 4, the lower die punch 5, and the cavity of the female mold 3 all have the self-lubricating structure of the powder metallurgy mold of this embodiment, and the working layer of the self-lubricating structure of the powder metallurgy mold faces outward to meet the self-lubricating function of the working layer during the stamping process.

[0030] Example 2 This embodiment provides a self-lubricating structure for powder metallurgy molds. Except that the working layer is composed of the following percentage mass components: WC 68%, Co 22%, MoS2 10%, the other structural components and preparation methods are the same as in Embodiment 1.

[0031] Example 3 See Figure 3 This embodiment provides a self-lubricating structure for powder metallurgy molds. Besides the transition layer, it has five sub-transition layers, which are, along the direction from the working layer 1 to the substrate layer 3, a first sub-transition layer 21 (0.6 mm thick), a second sub-transition layer 22 (0.6 mm thick), a third sub-transition layer 23 (0.6 mm thick), a fourth sub-transition layer 24 (0.6 mm thick), and a fifth sub-transition layer 25 (0.6 mm thick). The first sub-transition layer 21 is composed of the following percentage by mass: 60% working layer powder and 40% H13 mold steel powder. The second sub-transition layer 22 is composed of the following percentage by mass components: 45% working layer powder and 55% H13 mold steel powder. The third sub-transition layer 23 is composed of the following percentage by mass components: 30% working layer powder and 70% H13 mold steel powder. The fourth sub-transition layer 24 is composed of the following percentage by mass components: 15% working layer powder and 85% H13 mold steel powder. The fifth sub-transition layer 25 is composed of the following percentage by mass components: 5% working layer powder and 95% H13 mold steel powder. The remaining structural components and preparation methods are the same as in Example 1.

[0032] Example 4 This embodiment provides a self-lubricating structure for powder metallurgy molds. Except that the working layer is composed of the following percentage mass components: TiC 68%, Co 22%, and graphite 10%, the other structural components and preparation methods are the same as in Embodiment 1.

[0033] Example 5 This embodiment provides a self-lubricating structure for powder metallurgy molds. Except for the working layer, which has a thickness of 2.0 mm, the thicknesses of the first, second, and third sub-transition layers are all 0.5 mm, and the total thickness of the transition layers is 1.5 mm. The remaining structural components and preparation methods are the same as in Embodiment 1.

[0034] Example 6 This embodiment provides a self-lubricating structure for powder metallurgy molds. Except that the working layer is composed of the following percentage mass components: TiC 50%, WC 10%, Co 15%, Ni 10%, graphite 10%, MoS 25%, the other structural components and preparation methods are the same as in Example 1.

[0035] Example 7 This embodiment provides a self-lubricating structure for powder metallurgy molds. Except that the working layer is composed of the following percentage mass components: TiC 55%, WC 20%, Co 10%, Ni 10%, and graphite 5%, the remaining structural components and preparation methods are the same as in Embodiment 1.

[0036] Example 8 This embodiment provides a self-lubricating structure for powder metallurgy molds. Except that the working layer is composed of the following percentage mass components: TiC 50%, WC 10%, Co 10%, Ni 20%, h-BN 10%, the remaining structural components and preparation methods are the same as in Example 1.

[0037] Example 9 This embodiment provides a self-lubricating structure for powder metallurgy molds. Except that the working layer is composed of the following percentage mass components: TiC 60%, Co 15%, graphite 20%, MoS 25%, the other structural components and preparation methods are the same as in Example 1.

[0038] Example 10 This embodiment provides a self-lubricating structure for powder metallurgy molds. Except that the first sub-transition layer is composed of the following percentage components by mass: 40% working layer powder and 60% H13 mold steel powder, the second sub-transition layer is composed of the following percentage components by mass: 25% working layer powder and 75% H13 mold steel powder, and the third sub-transition layer is composed of the following percentage components by mass: 5% working layer powder and 95% H13 mold steel powder, the remaining structural components and preparation methods are the same as in Embodiment 1.

[0039] Example 11 This embodiment provides a self-lubricating structure for powder metallurgy molds. Except that the base layer is made of DIEVAR mold steel, the first sub-transition layer is composed of the following percentage components by mass: 45% working layer powder and 55% DIEVAR mold steel powder. The second sub-transition layer is composed of the following percentage components by mass: 20% working layer powder and 80% DIEVAR mold steel powder. The third sub-transition layer is composed of the following percentage components by mass: 10% working layer powder and 90% DIEVAR mold steel powder. The remaining structural components and preparation methods are the same as in Embodiment 1.

[0040] The self-lubricating structure of the powder metallurgy mold in this embodiment is prepared by the following method: S01. Weigh WC powder, Co powder, and graphite powder according to the working layer composition ratio of Example 11, add them to a ball mill, and ball mill them for 8 hours under the conditions of anhydrous ethanol as the mixing medium, ball-to-material ratio of 8:1, and rotation speed of 200 rpm. Add the mixed slurry to a vacuum drying oven and vacuum dry it for 24 hours at a temperature of 60°C. Then pass it through a 400-mesh sieve to obtain the working layer composite powder. S02. Configure the first sub-transition layer, the second sub-transition layer, and the third sub-transition layer according to the transition layer composition ratio of Example 11. Load the mold steel powder into the bottom of the cylindrical mold steel sleeve and vibrate to compact it to form a base layer with a thickness of 50 mm. On the upper surface of the base layer, load the third sub-transition layer with a thickness of 1 mm, the second sub-transition layer with a thickness of 1 mm, and the first sub-transition layer with a thickness of 1 mm in sequence. After each sub-transition layer is filled, gently scrape it flat. Finally, load the working layer composite powder with a thickness of 1.5 mm. Place the cylindrical mold steel sleeve filled with powder in a cold isostatic press and cold isostatically press it under a pressure of 300 MPa and a holding time of 30 min to obtain a dense green blank. S03. Place the green billet in a vacuum sintering furnace and evacuate to a vacuum level of 5×10⁻⁶. -3 Pa was heated to 1450℃ at a heating rate of 10℃ / min and sintered for 2 hours to densify the powder of the green blank and achieve interlayer metallurgical bonding. After sintering, the green blank was cooled to room temperature in the furnace and then subjected to the following heat treatment: heating to 1050℃ for austenitization, oil quenching, and then tempering at 500℃ twice for 3 hours each time to make the matrix layer reach a hardness of HRC 48-52, thus obtaining a preliminary self-lubricating structure for powder metallurgy molds. S04. Grinding is used to finish the preliminary powder metallurgy mold self-lubricating structure so that the surface roughness Ra≤0.4μm, and the powder metallurgy mold self-lubricating structure with the target shape, size and surface accuracy is obtained.

[0041] Comparative Example 1: Compared with Example 1, the difference is that the working layer does not contain solid lubricant, and the specific ratio of the working layer is 80% WC + 20% Co.

[0042] Comparative Example 2: Compared with Example 1, the difference is that the solid lubricant content in the working layer is 3%, and the ratio is 77% WC + 20% Co + 3% graphite.

[0043] Comparative Example 3: Compared with Example 1, the difference is that the solid lubricant content in the working layer is 18%, and the ratio is 62% WC + 20% Co + 18% graphite.

[0044] Comparative Example 4: Compared with Example 1, the difference is that the transition layer is eliminated in the self-lubricating structure of the powder metallurgy mold, and the working layer is directly bonded to the substrate layer.

[0045] Comparative Example 5: Compared with Example 1, the powder metallurgy mold lubrication structure uses an H13 mold steel substrate with a PVD-deposited TiAlN coating on the surface with a thickness of 5μm, and externally sprayed lubricant is used during pressing.

[0046] Comparative Example 6: Compared with Example 1, the powder metallurgy mold lubrication structure is made of YG15 cemented carbide as a whole, with polished surface, and externally sprayed lubricant is used during pressing.

[0047] Comparative Example 7: Compared with the preparation method of Example 1, the difference is that the sintering temperature in step S03 is reduced to 1250°C.

[0048] Comparative Example 8: Compared with the preparation method of Example 1, the difference is that the cold isostatic pressure in step S02 is reduced to 150 MPa.

[0049] The same powder metallurgical molds were made using the self-lubricating structures of powder metallurgical molds in Examples 1 to 5, Comparative Examples 2 to 4, Comparative Examples 7 and 8, and the lubricating structures of powder metallurgical molds in Comparative Examples 1, 5 and 6, respectively. The friction coefficient, bonding strength of each layer, service life and crush density difference of each mold were tested. The specific test results are shown in Table 1 below.

[0050] Friction coefficient: Tested in accordance with the standard "Test method for friction properties of wet sintered metal friction materials JB / T 7268-2007".

[0051] Bond strength: Tested in accordance with the standard GB / T 10430-2008 "Test method for bonding performance of sintered metal friction pads".

[0052] Die life: Tested according to the standard "Method for Axial Force Control in Fatigue Testing of Metallic Materials GB / T 3075-2021". Based on the basic principles of fatigue testing, the number of pressing cycles is used as a quantitative indicator of fatigue life. High-frequency fatigue testing machine (load range 20-150 kN, number of cycles ≥ 1×10⁻⁶). 6 (This can be used for simulation testing.)

[0053] The density of the pressed billet was tested in accordance with the standard GB / T 3075-2021 "Metal powder (excluding cemented carbide) - Determination of apparent density of pressed billets".

[0054] Table 1 Comparison of performance test results of different sample molds

[0055] As shown in Table 1, the mold made using the powder metallurgy mold lubrication structure obtained in this embodiment and preparation method has superior overall performance in terms of friction coefficient, bonding strength of each layer, service life, and crush density difference.

[0056] Failure Mechanism Analysis of Comparative Examples 1 to 8 Comparative Example 1: When the working layer contains no layered solid lubricant, the mold surface consists of hard particles and a binder phase. During pressing, the powder particles come into direct metal-to-metal contact with the mold surface, resulting in a severe micro-cold welding effect. The friction pair is in a metal-to-direct contact state, with adhesive wear dominating, while the plowing effect of the hard particles on the mold surface is significantly enhanced. Due to the lack of a lubricating film as a barrier, external lubrication is required, and the mold surface rapidly develops scratches and microcracks under repeated loading, ultimately leading to premature failure. Comparing Comparative Example 1 with Example 1 demonstrates the core role of solid lubricants in forming a transfer film at the friction interface and blocking direct metal-to-metal contact.

[0057] Comparative Example 2: When the solid lubricant content is below the critical threshold, although a certain amount of lubricating phase exists, a continuous and complete transfer lubricating film cannot be formed on the working layer surface. Lubricant particles are distributed in isolated island-like patterns within the material, resulting in insufficient lubricant being squeezed out during friction. The resulting transfer film is discontinuous and fragmented. Areas not covered by the lubricating film still experience direct metal-to-metal contact, forming localized high-friction hotspots and leading to selective wear. This non-uniform state of localized lubrication and dry friction causes uneven wear grooves on the mold surface, accelerating the failure process. Compared to Example 1, Comparative Example 2 shows that insufficient solid lubricant addition affects the lubrication performance of the transfer lubricating film, increases the coefficient of friction, and reduces service life.

[0058] Comparative Example 3: Excessive lubricant structurally weakens the working layer. During sintering, the lubricant does not participate in the formation of metallurgical bonds; instead, it occupies volume and hinders direct contact between hard particles, leading to an increased mean free path between particles and a reduced effective load-bearing cross-section per unit volume. Simultaneously, some of the excess lubricant volatilizes and decomposes, leaving micropores within the material. The interface between the lubricant and the binder phase becomes a preferred site for crack initiation. Under repeated pressing loads, microcracks propagate along the interface, ultimately causing the working layer to peel off and compromising the material's mechanical integrity. Compared to Example 1, Comparative Example 3 shows that an excessive proportion of solid lubricant reduces the mold's lifespan.

[0059] Comparative Example 4: Eliminating the gradient transition layer leads to abrupt changes in material composition and properties at the interface between the working layer and the substrate layer. The significant difference in the thermal expansion coefficients of the two materials generates enormous interfacial thermal stress during sintering and cooling, directly causing microcracks to initiate at the interface. Due to the lack of compositional transition, atoms on both sides of the interface cannot fully interdiffused, and the interface bonding is mainly mechanical interlocking, lacking true metallurgical bonding. Under subsequent pressing cyclic loading, the microcracks undergo fatigue propagation, ultimately leading to large-area spalling of the working layer and mold failure. Compared with Example 1, Comparative Example 4 lacks a transition layer, and directly bonding the working layer and the substrate layer results in reduced bonding strength and affects service life.

[0060] Comparative Example 5: The failure of traditional PVD-coated molds stems from the heterogeneous interface characteristics between the coating and the substrate. The coating and substrate are mechanically bonded, with no significant element diffusion or metallurgical reaction, resulting in limited interfacial bonding strength. During pressing, the coating endures enormous contact pressure and frictional shear force. When the shear stress exceeds the interfacial bonding strength, localized peeling occurs. The exposed substrate wears rapidly after peeling, causing the surrounding coating to lose support and subsequently peel off in a chain reaction. Once worn, the coating cannot self-repair, and differences in thermal expansion exacerbate interfacial fatigue, ultimately leading to the complete failure of the mold. Compared to Example 1, Comparative Example 5, which uses external lubrication, suffers from a high coefficient of friction, short service life, and low bonding strength between the lubricating coating and the substrate.

[0061] Comparative Example 6: The failure of the integral cemented carbide mold mainly stems from the intrinsic brittleness of the material. Cemented carbide lacks the support of a tough phase, resulting in fracture toughness far lower than that of mold steel. During high-pressure pressing, the mold endures complex stress states. Micro-defects within the material become crack initiations under tensile stress. Once cracks initiate, they propagate rapidly without significant plastic deformation to absorb energy, leading to sudden brittle fracture. Even without overall cracking, localized micro-cracks will gradually propagate, eventually causing mold failure, and once cracked, the entire mold cannot be repaired. Compared to Example 1, Comparative Example 6 uses external lubrication, which results in a high coefficient of friction and a short service life.

[0062] Comparative Example 7: When the sintering temperature is below the normal range, the amount of liquid phase is insufficient, and the fluidity is poor. The dissolution and rearrangement of hard particles in the liquid phase are limited, making it impossible to form a continuous skeletal structure. After sintering, the material density is insufficient, and a large number of pores remain inside the material, reducing the hardness and wear resistance of the working layer. More seriously, sufficient element diffusion and metallurgical bonding are not formed between the working layer and the transition layer, resulting in insufficient interfacial bonding strength. Pores become stress concentration points, leading to collapse and crack initiation under compressive loads. The superposition of multiple factors causes premature mold failure.

[0063] Comparative Example 8: When the cold isostatic pressing pressure is lower than the threshold required for powder densification, particle rearrangement is insufficient, resulting in numerous voids between particles and insufficient and unevenly distributed green body density. This low-density and uneven green body undergoes uneven shrinkage during subsequent sintering, leading to excessive deviations in cavity dimensions. Even molds that barely meet dimensional requirements still exhibit compositional segregation and localized porosity due to uneven green body density. During use, these areas become weak points, preferentially wearing down or cracking under pressing stress, accelerating mold failure.

[0064] The above embodiments are illustrative of the present invention and are not intended to limit the present invention. Any simple modifications to the present invention are within the scope of protection of the present invention.

Claims

1. A self-lubricating structure for powder metallurgy molds, characterized in that: The self-lubricating structure of the powder metallurgy mold is composed of a working layer, a transition layer, and a matrix layer connected in sequence. The working layer is made of a self-lubricating powder metallurgy composite material, which consists of the following percentage mass components: 60%-75% hard particles, 20%-30% binder metal, and 5%-15% solid lubricant. The hard particles are WC or TiC, the binder metal is Co or Ni, and the solid lubricant is at least one of graphite, MoS2, and h-BN. The matrix layer is made of mold steel. The transition layer is provided with multiple sub-transition layers, and the composition of each sub-transition layer is composed of the components of the working layer and the matrix layer. The components of the working layer and the matrix layer are continuously gradient distributed along the direction from the working layer to the matrix layer.

2. The self-lubricating structure for powder metallurgy molds as described in claim 1, characterized in that: The composition of each sub-transition layer is such that the composition gradient of the working layer decreases along the direction from the working layer to the matrix layer, while the composition gradient of the matrix layer increases. The thickness of the transition layer is 1.0 mm to 3.0 mm.

3. The self-lubricating structure for powder metallurgy molds as described in claim 2, characterized in that: The number of sub-transition layers is 3, and they are designated as a first sub-transition layer, a second sub-transition layer, and a third sub-transition layer along the direction from the working layer to the substrate layer. The first sub-transition layer is composed of the following percentage components by mass: 40%-50% of the working layer component and 50%-60% of the substrate layer component. The second sub-transition layer is composed of the following percentage components by mass: 20%-30% of the working layer component and 70%-80% of the substrate layer component. The third sub-transition layer is composed of the following percentage components by mass: 5%-15% of the working layer component and 85%-95% of the substrate layer component.

4. The self-lubricating structure for powder metallurgy molds as described in claim 2, characterized in that: The number of sub-transition layers is 5, and they are designated as a first sub-transition layer, a second sub-transition layer, a third sub-transition layer, a fourth sub-transition layer, and a fifth sub-transition layer along the direction from the working layer to the substrate layer. The first sub-transition layer is composed of the following percentage components by mass: 60%-70% of the working layer component and 30%-40% of the substrate layer component. The second sub-transition layer is composed of the following percentage components by mass: 45%-55% of the working layer component and 45%-55% of the substrate layer component. The third sub-transition layer is composed of the following percentage components by mass: 30%-40% of the working layer component and 60%-70% of the substrate layer component. The fourth sub-transition layer is composed of the following percentage components by mass: 15%-25% of the working layer component and 75%-85% of the substrate layer component. The fifth sub-transition layer is composed of the following percentage components by mass: 5%-10% of the working layer component and 90%-95% of the substrate layer component.

5. The self-lubricating structure for powder metallurgy molds as described in claim 1, characterized in that: The working layer has a thickness of 0.5mm-2.0mm, a hardness of HRA 85-HRA 90, and a friction coefficient of no more than 0.

083.

6. The self-lubricating structure for powder metallurgy molds as described in claim 1, characterized in that: The mold steel is of type H13, DIEVAR, or 8407, and its hardness after heat treatment is HRC 45-HRC 52.

7. A method for preparing a self-lubricating structure for powder metallurgy molds, used to prepare the self-lubricating structure for powder metallurgy molds as described in any one of claims 1 to 6, characterized in that: It includes the following steps: Weigh out the hard particle powder, bonding metal powder, and solid lubricant powder in proportion, ball mill and mix them, vacuum dry them, and sieve them to obtain the working layer composite powder. The mold steel powder is loaded into the bottom of the mold steel sleeve and compacted to serve as the base layer. A multi-layered sub-transition layer and a working layer composite powder with a gradient distribution are sequentially loaded onto the upper surface of the base layer. The green blank is obtained by cold isostatic pressing under the conditions of a cold isostatic pressing pressure of 200MPa-300MPa and a holding time of 10min-30min. The green blank is placed in a vacuum sintering furnace and sintered at a heating rate of 5℃ / min-10℃ / min and a temperature of 1350℃-1450℃ for 1h-2h to densify the powder of the green blank and achieve interlayer metallurgical bonding. After sintering, it is subjected to quenching and tempering heat treatment to make the matrix layer reach the target hardness, thus obtaining a self-lubricating structure of powder metallurgy mold.

8. The method for preparing the self-lubricating structure of powder metallurgy molds as described in claim 7, characterized in that: It also includes the following steps: The self-lubricating structure of the powder metallurgy mold obtained after heat treatment is precision machined so that the surface roughness does not exceed 0.4 micrometers. The precision machining is carried out by electrical discharge machining or grinding.

9. The method for preparing the self-lubricating structure of powder metallurgy molds as described in claim 7, characterized in that: The ball milling and mixing process is carried out in a ball mill. The specific process conditions are: ball milling and mixing time of 4h-8h, mixing medium of anhydrous ethanol, ball-to-material ratio of (5:1)-(8:1), and rotation speed of 200rpm-300rpm; the vacuum drying process conditions are: drying temperature of 60℃-100℃ and drying time of 12h-24h; the sieving is carried out under 200-400 mesh sieve conditions; the specific process of the heat treatment is: first, austenitizing oil quenching at 1020℃-1080℃, and then tempering twice at 500℃-600℃, each time for 1h-3h.

10. A powder metallurgy mold, comprising a female mold assembly and a die punch assembly, the female mold assembly including a female mold sleeve having a cavity inside for pressing a blank, the die punch assembly including an upper die punch and a lower die punch, the upper die punch and the lower die punch respectively corresponding to the cavity, characterized in that: The surfaces of the female die sleeve, upper die punch, and lower die punch that contact the material are provided with a self-lubricating structure for powder metallurgy molds as described in any one of claims 1 to 6, and the working layer side of the self-lubricating structure for powder metallurgy molds faces the material.

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