A method for preparing a gradient composite material by in-situ synthesis of a stainless steel layer on a porous nickel surface

Abstract

The present invention discloses a method for preparing a gradient composite material with an in-situ synthesized stainless steel layer on a porous nickel surface, comprising the following steps: 1. loading stainless steel powder and nickel powder into a pressing mold; 2. extracting a tubular foil from the powder-loaded pressing mold; 3. pressing the stainless steel powder and nickel powder; 4. sintering the composite green body in a hydrogen furnace; 5. pre-densifying the outer surface of the composite sintered body; 6. electrodepositing a hard chromium layer on the surface to obtain a porous nickel / stainless steel gradient composite material. The present invention pre-places a tubular foil intermediate layer for powder loading and pressing, thereby obtaining a composite porous material compact with dual gradients in composition and pore size, achieving one-time pressing and forming of the gradient porous composite material. Simultaneously, the outer layer of the workpiece is completely densified by combining the outer surface pre-densification and subsequent electrodeposition of a dense hard chromium layer, thereby proposing a new densification method suitable for porous materials, which ensures both the processing accuracy of the composite material and its functionality and lightweight.

Description

Technical Field

[0001] The invention belongs to the technical field of porous material preparation and processing, and particularly relates to a method for preparing a gradient composite material by in-situ synthesizing a stainless steel layer on a porous nickel surface. Background Art

[0002] Porous nickel, with its rich internal pore structure, is often used as a functional component for heat and mass transfer, such as heat pipe evaporator elements, leveraging its porosity. Common porous nickel forms in the heat transfer field are tubular and flat. Tubular components are more widely used due to their excellent isotropic uniformity. Due to their functional requirements, tubular porous nickel inevitably requires fixed assembly within a dense cladding to transfer materials and fluids. However, due to limitations in processing technology, traditional tubular porous nickel needs to be fitted with an outer layer of dense stainless steel tubes through interference fit, which leads to three major problems: 1. The porous nickel part needs to be manufactured to high precision, while ensuring the outer cylindricality and the high-precision inner hole of the outer stainless steel are assembled, which increases the difficulty of processing and manufacturing, especially the problem of pore clogging in the processing of porous components; 2. The strength of porous materials is relatively lower than that of dense materials of the same material. After assembly, there is always an interference fit, which puts the porous material in a metastable state under stress. The stress can easily cause the porous part to break and cause the component to fail; 3. After assembly, there is always a gap between the porous part and the dense part, which inevitably increases the heat transfer resistance. Therefore, it is particularly important to improve the connection method between the porous part and the dense part to achieve a seamless metallurgical bond, which is a key to improving the efficiency of heat transfer components such as heat pipes.

[0003] Therefore, a new connection technology for the composite of porous nickel and dense stainless steel is needed to solve the thermal resistance problem caused by the assembly gap of the traditional porous nickel dense outer layer. It is necessary to provide a method for preparing a gradient composite material with in-situ synthesis of a stainless steel layer on the surface of porous nickel. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to address the shortcomings of the above-mentioned existing technologies and provide a method for preparing a gradient composite material with an in-situ stainless steel layer synthesized on a porous nickel surface. This method pre-places a tubular foil intermediate layer and then performs powder pressing to obtain a composite porous material compact with dual gradients in composition and pore size. This method achieves one-time pressing and forming of the gradient porous composite material. Simultaneously, by combining external surface pre-densification with subsequent electrodeposition of a dense hard chromium layer, the outer layer of the product is fully densified. This method proposes a new densification method suitable for porous materials, ensuring both the processing accuracy of the composite material and its functionality and lightweightness.

[0005] To solve the above technical problems, the present invention adopts a technical solution: a method for preparing a gradient composite material with an in-situ synthesis of a stainless steel layer on a porous nickel surface, characterized in that the method comprises the following steps:

[0006] Step 1: Stainless steel powder and nickel powder are respectively loaded into the outer layer and inner layer of a pressing mold to obtain a powder-loaded pressing mold; the pressing mold includes a convex-shaped base, a convex portion of the upper portion of the convex-shaped base is covered with a tubular foil, and a soft rubber sleeve is covered on the outside of the convex-shaped base, the area inside the tubular foil is the inner layer, and the area between the tubular foil and the soft rubber sleeve is the outer layer;

[0007] Step 2: Pull out the tubular foil from the powder-filled pressing mold obtained in Step 1 axially, and simultaneously install a limiting ring and a limiting core rod on the upper portion of the powder-filled pressing mold. After the tubular foil is completely pulled out, remove the limiting ring and the limiting core rod, and then seal and secure with a rubber plug to obtain a pressing mold containing the material to be pressed.

[0008] Step 3: Compressing the stainless steel powder and nickel powder in the pressing mold containing the material to be pressed obtained in Step 2, and then removing the pressing mold to obtain a porous nickel / porous stainless steel composite green body;

[0009] Step 4: sintering the porous nickel / porous stainless steel composite green body obtained in step 3 in a hydrogen furnace to obtain a porous nickel / porous stainless steel composite sintered body;

[0010] Step 5: pre-densifying the outer surface of the porous nickel / porous stainless steel composite sintered body obtained in step 4;

[0011] Step 6: Electrodepositing a hard chromium layer on the surface of the porous nickel / porous stainless steel composite sintered body that has been pre-densified in step 5 to obtain a porous nickel / stainless steel gradient composite material.

[0012] The present invention designs a pressing mold and pre-places a tubular foil middle layer for filling stainless steel powder and nickel powder, thereby ensuring the shape and position accuracy of powder filling. Through the proprietary tubular foil pull-out design, the powder is limited during the tubular foil lifting process, eliminating the influence of friction on powder filling, and thus obtaining inner and outer layer gradient powder filling. Through the subsequent pressing and sintering process, a composite porous material with dual gradients of composition and pore size is obtained in one step, realizing the one-time pressing and forming of the gradient porous composite material, avoiding the problem of multiple pressing causing densification of the core porous material, and at the same time ensuring the coaxiality of the internal porous core and the outer stainless steel layer. The subsequent process The outer surface is pre-densified and then fully densified by coating the hard chromium layer. During the outer surface pre-densification process, the strong extrusion movement during turning causes plastic deformation of the surface material of the workpiece, thereby achieving densification of the pores, laying the foundation for subsequent further densification and obtaining a high-precision gradient composite material. Finally, the surface of the porous stainless steel is fully densified by surface electrodeposition, and the remaining pores are fully densified, realizing the metallurgical bonding of the porous nickel and the stainless steel layer. There is no gap between the gradient layers, which effectively reduces the thermal resistance, electrical resistance, etc. compared with the assembled composite material, ensuring the structural and functional integration of the porous nickel and the dense stainless steel.

[0013] In the present invention, the size of the rubber plug is matched with the soft rubber sleeve, and the diameter of the rubber plug is equal to or slightly larger than the inner diameter of the soft rubber sleeve, forming a transition or interference fit.

[0014] The above-mentioned method for preparing a gradient composite material with an in-situ synthesis of a stainless steel layer on a porous nickel surface is characterized in that the nickel powder in step 1 has a particle size of 5μm to 150μm, and the stainless steel powder is 316L with a particle size of 25μm to 150μm. By controlling the particle sizes of the stainless steel powder and the nickel powder, the present invention ensures that both the nickel powder layer and the stainless steel layer have appropriate porosity, while also ensuring excellent bonding between the nickel powder layer and the stainless steel layer.

[0015] The aforementioned method for preparing a gradient composite material with an in-situ stainless steel layer synthesized on a porous nickel surface is characterized in that the tubular foil in step 1 is made of graphite, stainless steel, or aluminum, has a surface roughness of less than Ra0.8, and a thickness of 0.03 mm to 0.05 mm. By controlling the parameters of the tubular foil, the present invention achieves a high smoothness and low thickness. This reduces friction during extraction, preventing it from affecting powder loading. Furthermore, the low thickness ensures that the two powders in the gap engage with each other during extraction, without affecting dimensional and geometric accuracy.

[0016] The above-mentioned method for preparing a gradient composite material with an in-situ synthesized stainless steel layer on a porous nickel surface is characterized in that, when the stainless steel powder and nickel powder are respectively loaded into the pressing mold in step 1, the stainless steel powder is loaded first and then the nickel powder is loaded, the loading is coordinated with vibration, and the heights of the stainless steel powder and the nickel powder in the powder-loaded pressing mold are the same. The present invention first loads the stainless steel powder, and the compacting force of the stacking between the stainless steel powders can accurately further position and fix the tubular foil, facilitating the subsequent loading of the nickel powder inside. The compacting force of the powder stacking and the uniformity of the powder loading are further ensured by vibration. By controlling the height of the stainless steel powder and the nickel powder to be the same, the uniform structure of the two components in the prepared gradient composite material is ensured.

[0017] The aforementioned method for preparing a gradient composite material with an in-situ synthesized stainless steel layer on a porous nickel surface is characterized in that, during the extraction of the tubular foil in step 2, the lower end face of the limiting mandrel and the upper end face of the nickel powder are always aligned, and the lower end face of the limiting ring and the upper end face of the stainless steel powder are always aligned. The present invention provides downward pressure through the limiting mandrel and the limiting ring, ensuring that the loading positions of the stainless steel powder and nickel powder do not change during the extraction of the tubular foil, thereby preventing friction from affecting the powders.

[0018] The above-mentioned method for preparing a gradient composite material with an in-situ synthesis of a stainless steel layer on a porous nickel surface is characterized in that the pressing in step 3 is performed using a cold isostatic press, the pressing pressure is 140MPa to 160MPa, and the pressing time is more than 30s. The present invention ensures the forming effect by controlling the pressure, which can ensure both formability and a certain porosity, preventing the problems of low strength of the pressed green body caused by a pressure less than 140MPa and low porosity caused by a pressure greater than 160MPa.

[0019] The aforementioned method for preparing a gradient composite material with an in-situ stainless steel layer synthesized on a porous nickel surface is characterized in that the sintering process in step 4 is: heating to 700°C to 1050°C at a heating rate of 2°C / min to 10°C / min, followed by holding at that temperature for 1 to 2 hours. The present invention achieves metallurgical bonding by controlling sintering parameters, while maintaining a certain porosity while maintaining strength.

[0020] The above-mentioned method for preparing a gradient composite material with an in-situ synthesis of a stainless steel layer on a porous nickel surface is characterized in that the pre-densification treatment in step 5 is based on the outer cylindrical surface of the porous nickel / porous stainless steel composite sintered body, and the porous stainless steel layer is mechanically pre-densified, with a processing accuracy of 0.02 mm and a surface roughness controlled to be less than Ra0.8;

[0021] The pre-densification treatment is carried out on a lathe, and the specific steps are:

[0022] Step 101, roughing machining allowance: select a YG6X fine-grain carbide tool, set the tool geometry angle, the main deflection angle is 75°, the secondary deflection angle is 8°~15°, the cutting edge inclination angle is -5°, the rake angle is 10°~15°, the clearance angle is 8°~12°, the tool tip arc radius is 0.3mm~0.5mm, and then perform machining at a cutting speed of 60m / min~70m / min, a feed rate of 0.2mm / r~0.3mm / r, and a cutting depth of 0.3mm~0.5mm;

[0023] Step 102, use a large arc turning tool to extrude and turn the outer circle: select a large arc turning tool made of YG6X fine-grained carbide, set the large arc turning tool geometric angle, the main deflection angle is 90°; the secondary deflection angle is 8°~15°, the blade inclination angle is -20°, the front angle is 10°~15°, the back angle is 8°~12°, the tool tip arc radius is 1mm, and then the cutting speed is 140m / min~150m / min, the feed rate is 0.1mm / r~0.2mm / r, and the cutting depth is 0.05mm~0.1mm. The present invention adopts a two-step pre-densification process, wherein the purpose of the first step is to rough out the machining allowance and rough-machine the outer surface of the porous nickel / porous stainless steel composite sintered body to achieve the required densification size. A slow cutting speed and a large cutting depth are used here to remove the allowance for rough machining in mechanical machining, and the control parameters are specially designed for the processing of porous nickel / porous stainless steel composite sintered bodies with porous structures, so as to avoid the impact of the processing on the pores themselves. The second step is to extrude the outer circle through a large arc turning tool, directly densify the surface at high speed, increase the extrusion effect on the workpiece surface during turning, and cause plastic deformation of the workpiece surface material, thereby achieving the effect of closing the micropores. At the same time, the roughness of the outer circle is controlled below Ra0.8, laying the foundation for subsequent further densification and obtaining a high-precision gradient composite material.

[0024] The aforementioned method for preparing a gradient composite material with an in-situ stainless steel layer synthesized on a porous nickel surface is characterized in that the thickness of the hard chromium layer in step 5 is 0.05 mm to 0.2 mm. By controlling the thickness of the hard chromium layer, the present invention achieves lightweighting of the gradient composite material (except for the surface layer, the rest of the structure remains porous) while maintaining the functionality of the dense portion.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] 1. The present invention adopts separated composite powder filling technology to improve the powder filling accuracy. The subsequent form and position tolerance and position requirements can be guaranteed only through the powder filling process. Through the subsequent pressing and sintering process, the one-time molding of the porous gradient composite material is realized. Compared with the traditional method of first pressing the porous core rod and then performing a second pressing, the overall process is shortened, and the influence of multiple pressing on the pore performance of the porous part is effectively avoided. At the same time, the porous nickel and the stainless steel layer are metallurgically bonded, and there is no gap between the gradient layers. Compared with the traditional manufacturing method of using a stainless steel shell and a porous core interference assembly, the method adopted by the present invention effectively reduces thermal resistance, electrical resistance, etc., and ensures the functional integration of the porous nickel and the dense stainless steel structure.

[0027] 2. The composite material connection technology proposed in the present invention first uses pressing force to make different types of powders mesh with each other during powder filling. Subsequently, by controlling the sintering process parameters, the metallurgical bonding of porous nickel and stainless steel dissimilar materials is achieved, solving the problem of surface separation of dissimilar materials caused by different shrinkage rates of different materials.

[0028] 3. The present invention adopts two densification treatments. First, the initial machining pre-densification is performed to ensure the processing accuracy of the original part, and the pores are densified by extruding the surface of the part. Finally, the remaining pores are completely densified by surface electrodeposition. The covering thickness can be controlled to be between 0.05mm and 0.2mm, which not only ensures the functionality of the dense part of the composite material but also achieves the lightweight of the overall material.

[0029] 4. The present invention solves the problem that traditional functional elements can only be composited through assembly, and realizes the technical difficulty of simultaneous connection of dissimilar materials (316L stainless steel-nickel) and dense-porous (dense 316L stainless steel-porous nickel) dual gradients, eliminating the gaps during assembly of traditional elements, and effectively realizing the metallurgical bonding of porous nickel and stainless steel layers. There are no gaps between the gradient layers, which effectively reduces thermal resistance, electrical resistance, etc. compared with assembled composite materials, ensuring the functional integration of porous nickel and dense stainless steel structures.

[0030] 5. The present invention realizes a continuous dual gradient of pores and components through a unique powder filling method and surface densification technology, solves the metallurgical bonding problem of porous nickel and dense stainless steel dissimilar materials, effectively reduces the thermal resistance of functional components, and expands the gradient composite connection technology between heterogeneous porous materials and between porous and dense materials, and can also realize the bonding of porous materials and dense parts of other materials.

[0031] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic structural diagram of the pressing die used in the present invention.

[0033] Figure 2 This is a schematic diagram of the tubular foil in the powder-filled pressing die of the present invention being drawn out along the axial direction.

[0034] Figure 3 This is a SEM image of the outermost surface of the porous nickel / stainless steel gradient composite material prepared in Example 1 of the present invention.

[0035] Figure 4 This is an SEM image of the cross section of the porous nickel / stainless steel gradient composite material prepared in Example 1 of the present invention.

[0036] Description of reference numerals:

[0037] 1—convex base; 2—tubular foil; 3—soft rubber sleeve;

[0038] 4—limiting ring; 5—limiting core rod; 6—nickel powder;

[0039] 7—Stainless steel powder. DETAILED DESCRIPTION

[0040] Figure 1 The structural diagram of the pressing die used in the present invention is as follows: Figure 1 It can be seen that the pressing mold includes a convex base 1, the protruding part of the upper part of the convex base 1 is covered with a tubular foil 2, and the outside of the convex base 1 is covered with a soft rubber sleeve 3. The area inside the tubular foil 2 is the inner layer, and the area between the tubular foil 2 and the soft rubber sleeve 3 is the outer layer.

[0041] Figure 2 Schematic diagram of the tubular foil in the powder-filled pressing die of the present invention being drawn out axially, from Figure 2 It can be seen that the area inside the tubular foil 2 in the pressing mold is filled with nickel powder 6, and the area between the tubular foil 2 and the soft rubber sleeve 3 is filled with stainless steel powder 7. The limiting core rod 5 is pressed on the upper part of the nickel powder 6, and the limiting ring 4 is pressed on the upper part of the stainless steel powder 7.

[0042] Example 1

[0043] This embodiment includes the following steps:

[0044] Step 1: 316L stainless steel powder with a particle size of 25 μm and nickel powder with a particle size of 5 μm are sequentially loaded into the outer layer and inner layer of a pressing mold and compacted by vibration to obtain a pressing mold with the nickel powder and 316L stainless steel powder loaded at the same height;

[0045] The pressing mold includes a convex-shaped base, the convex portion of the upper portion of the convex-shaped base is covered with a tubular foil material with a thickness of 0.03 mm and a surface roughness of less than Ra0.8, and made of graphite. The outer portion of the convex-shaped base is covered with a soft rubber sleeve, the area inside the tubular foil is the inner layer, and the area between the tubular foil and the soft rubber sleeve is the outer layer;

[0046] Step 2: The tubular foil in the powder-filled pressing mold obtained in Step 1 is pulled out axially, and a limiting ring and a limiting core rod are installed on the upper part of the powder-filled pressing mold. During the process of pulling out the tubular foil, the lower end face of the limiting core rod and the upper end face of the nickel powder are always kept in overlap, and the lower end face of the limiting ring and the upper end face of the stainless steel powder are always kept in overlap. After the tubular foil is completely pulled out, the limiting ring and the limiting core rod are removed, and then the mold is sealed with a rubber plug and fixed to obtain a pressing mold filled with the material to be pressed.

[0047] Step 3: The nickel powder and 316L stainless steel powder in the pressing mold containing the material to be pressed obtained in step 2 are pressed by a cold isostatic press at a pressure of 140 MPa to 160 MPa for 30 seconds, and then the pressing mold is removed to obtain a porous nickel / porous stainless steel composite green body;

[0048] Step 4: heating the porous nickel / porous stainless steel composite green body obtained in step 3 to 700° C. in a hydrogen furnace in a reducing atmosphere at a heating rate of 2° C. / min and then sintering the green body at that temperature for 1 hour to obtain a porous nickel / porous stainless steel composite sintered body;

[0049] Step 5: Pre-densification treatment is performed on the outer surface of the porous nickel / porous stainless steel composite sintered body obtained in step 4; the pre-densification treatment is based on the outer cylindrical surface of the porous nickel / porous stainless steel composite sintered body, and the porous stainless steel layer is mechanically pre-densified with a processing accuracy of 0.02 mm and a surface roughness controlled to be less than Ra0.8. After the pre-densification treatment, the thickness of the porous stainless steel is 0.51 mm, and the surface sealing rate reaches 98.8%;

[0050] The pre-densification treatment is carried out on a lathe, and the specific steps are:

[0051] Step 101, roughing machining allowance: select a YG6X fine-grain carbide tool, set the tool geometry angle, the main deflection angle is 75°, the secondary deflection angle is 8°, the cutting edge inclination angle is -5°, the rake angle is 10°, the clearance angle is 8°, the tool tip arc radius is 0.3mm, and then process at a cutting speed of 60m / min, a feed rate of 0.2mm / r, and a cutting depth of 0.3mm;

[0052] Step 102: Use a large arc turning tool to extrusion turn the outer circle: select a large arc turning tool made of YG6X fine-grained carbide, set the large arc turning tool geometry angle, the main deflection angle is 90°; the secondary deflection angle is 8°, the cutting edge inclination angle is -20°, the rake angle is 10°, the clearance angle is 8°, and the tool tip arc radius is 1mm, then the cutting speed is 140m / min, the feed rate is 0.1mm / r, and the cutting depth is 0.05mm.

[0053] Step 6: Electrodeposit a hard chromium layer with a thickness of 0.05 mm on the surface of the porous nickel / porous stainless steel composite sintered body that has been pre-densified in step 5 to obtain a porous nickel / stainless steel gradient composite material with a completely densified surface.

[0054] Figure 3 This is a SEM image of the outermost surface of the porous nickel / stainless steel gradient composite material prepared in this example. Figure 4 The SEM image of the cross section of the porous nickel / stainless steel gradient composite material prepared in this embodiment is shown in FIG. Figure 3 and Figure 4 It can be seen from the figure that the surface layer of the porous nickel / stainless steel gradient composite material prepared in this embodiment is completely densified, and the stainless steel layer is well bonded to the porous nickel portion.

[0055] Example 2

[0056] This embodiment includes the following steps:

[0057] Step 1: 316L stainless steel powder with a particle size of 100 μm and nickel powder with a particle size of 74 μm are sequentially loaded into the outer layer and inner layer of a pressing mold and compacted by vibration to obtain a pressing mold with the nickel powder and 316L stainless steel powder loaded at the same height;

[0058] The pressing mold includes a convex-shaped base, the convex portion of the upper portion of the convex-shaped base is covered with a tubular foil with a thickness of 0.04 mm and a surface roughness of less than Ra0.8, and made of stainless steel. The outer portion of the convex-shaped base is covered with a soft rubber sleeve. The area inside the tubular foil is the inner layer, and the area between the tubular foil and the soft rubber sleeve is the outer layer.

[0059] Step 2: The tubular foil in the powder-filled pressing mold obtained in Step 1 is pulled out axially, and a limiting ring and a limiting core rod are installed on the upper part of the powder-filled pressing mold. During the process of pulling out the tubular foil, the lower end face of the limiting core rod and the upper end face of the nickel powder are always kept in overlap, and the lower end face of the limiting ring and the upper end face of the stainless steel powder are always kept in overlap. After the tubular foil is completely pulled out, the limiting ring and the limiting core rod are removed, and then the mold is sealed with a rubber plug and fixed to obtain a pressing mold filled with the material to be pressed.

[0060] Step 3: The nickel powder and 316L stainless steel powder in the pressing mold containing the material to be pressed obtained in step 2 are pressed by a cold isostatic press at a pressure of 140 MPa to 160 MPa for 30 seconds, and then the pressing mold is removed to obtain a porous nickel / porous stainless steel composite green body;

[0061] Step 4: heating the porous nickel / porous stainless steel composite green body obtained in step 3 to 850° C. in a hydrogen furnace in a reducing atmosphere at a heating rate of 5° C. / min and then sintering at the temperature for 1.5 h to obtain a porous nickel / porous stainless steel composite sintered body;

[0062] Step 5: Pre-densification treatment is performed on the outer surface of the porous nickel / porous stainless steel composite sintered body obtained in step 4; the pre-densification treatment is based on the outer cylindrical surface of the porous nickel / porous stainless steel composite sintered body, and the porous stainless steel layer is mechanically pre-densified with a processing accuracy of 0.02 mm and a surface roughness of less than Ra0.8. After the pre-densification treatment, the thickness of the porous stainless steel is 1.04 mm, and the surface sealing rate reaches 99.5%;

[0063] The pre-densification treatment is carried out on a lathe, and the specific steps are:

[0064] Step 101, roughing machining allowance: select a YG6X fine-grain carbide tool, set the tool geometry angle, the main deflection angle is 75°, the secondary deflection angle is 12°, the cutting edge inclination angle is -5°, the rake angle is 12°, the clearance angle is 10°, the tool tip arc radius is 0.4mm, and then process at a cutting speed of 65m / min, a feed rate of 0.25mm / r, and a cutting depth of 0.4mm;

[0065] Step 102: Use a large arc turning tool to extrusion turn the outer circle: select a large arc turning tool made of YG6X fine-grained carbide, set the large arc turning tool geometry angle, the main deflection angle is 90°; the secondary deflection angle is 12°, the cutting edge inclination angle is -20°, the rake angle is 12°, the clearance angle is 10°, and the tool tip arc radius is 1mm, and then process at a cutting speed of 145m / min, a feed rate of 0.15mm / r, and a cutting depth of 0.08mm;

[0066] Step 6: Electrodeposit a hard chromium layer with a thickness of 0.1 mm on the surface of the porous nickel / porous stainless steel composite sintered body that has been pre-densified in step 5 to obtain a porous nickel / stainless steel gradient composite material with a completely densified surface.

[0067] After testing, it was found that the surface layer of the porous nickel / stainless steel gradient composite material prepared in this embodiment was completely densified, and the stainless steel layer was well bonded to the porous nickel portion.

[0068] Example 3

[0069] This embodiment includes the following steps:

[0070] Step 1: 316L stainless steel powder with a particle size of 150 μm and nickel powder with a particle size of 150 μm are sequentially loaded into the outer layer and inner layer of a pressing mold and compacted by vibration to obtain a pressing mold with the nickel powder and 316L stainless steel powder loaded at the same height;

[0071] The pressing mold includes a convex-shaped base, the convex portion of the upper portion of the convex-shaped base is covered with a tubular foil material with a thickness of 0.05 mm and a surface roughness of less than Ra0.8, and made of aluminum. The outer portion of the convex-shaped base is covered with a soft rubber sleeve, the area inside the tubular foil is the inner layer, and the area between the tubular foil and the soft rubber sleeve is the outer layer;

[0072] Step 2: The tubular foil in the powder-filled pressing mold obtained in Step 1 is pulled out axially, and a limiting ring and a limiting core rod are installed on the upper part of the powder-filled pressing mold. During the process of pulling out the tubular foil, the lower end face of the limiting core rod and the upper end face of the nickel powder are always kept in overlap, and the lower end face of the limiting ring and the upper end face of the stainless steel powder are always kept in overlap. After the tubular foil is completely pulled out, the limiting ring and the limiting core rod are removed, and then the mold is sealed with a rubber plug and fixed to obtain a pressing mold filled with the material to be pressed.

[0073] Step 3: The nickel powder and 316L stainless steel powder in the pressing mold containing the material to be pressed obtained in step 2 are pressed by a cold isostatic press at a pressure of 140 MPa to 160 MPa for 30 seconds, and then the pressing mold is removed to obtain a porous nickel / porous stainless steel composite green body;

[0074] Step 4: heating the porous nickel / porous stainless steel composite green body obtained in step 3 to 1050° C. in a hydrogen furnace in a reducing atmosphere at a heating rate of 10° C. / min and then sintering the green body for 2 h to obtain a porous nickel / porous stainless steel composite sintered body;

[0075] Step 5: Pre-densification treatment is performed on the outer surface of the porous nickel / porous stainless steel composite sintered body obtained in step 4; the pre-densification treatment is based on the outer cylindrical surface of the porous nickel / porous stainless steel composite sintered body, and the porous stainless steel layer is mechanically pre-densified with a processing accuracy of 0.02 mm and a surface roughness of less than Ra0.8. After the pre-densification treatment, the thickness of the porous stainless steel is 2.00 mm, and the surface sealing rate reaches 99.2%;

[0076] The pre-densification treatment is carried out on a lathe, and the specific steps are:

[0077] Step 101, roughing machining allowance: select a YG6X fine-grain carbide tool, set the tool geometry angle, the main deflection angle is 75°, the secondary deflection angle is 15°, the cutting edge inclination angle is -5°, the rake angle is 15°, the clearance angle is 12°, the tool tip arc radius is 0.5mm, and then process at a cutting speed of 70m / min, a feed rate of 0.3mm / r, and a cutting depth of 0.5mm;

[0078] Step 102: Use a large arc turning tool to extrusion turn the outer circle: select a large arc turning tool made of YG6X fine-grained carbide, set the large arc turning tool geometry angle, the main deflection angle is 90°; the secondary deflection angle is 15°, the cutting edge inclination angle is -20°, the rake angle is 15°, the clearance angle is 12°, and the tool tip arc radius is 1mm, and then process at a cutting speed of 150m / min, a feed rate of 0.2mm / r, and a cutting depth of 0.1mm;

[0079] Step 6: Electrodeposit a hard chromium layer with a thickness of 0.2 mm on the surface of the porous nickel / porous stainless steel composite sintered body that has been pre-densified in step 5 to obtain a porous nickel / stainless steel gradient composite material with a completely densified surface.

[0080] After testing, it was found that the surface layer of the porous nickel / stainless steel gradient composite material prepared in this embodiment was completely densified, and the stainless steel layer was well bonded to the porous nickel portion.

[0081] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent variation made to the above embodiment based on the technical essence of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing a gradient composite material by in-situ synthesis of a stainless steel layer on a porous nickel surface, characterized in that: The method comprises the following steps: Step 1: Stainless steel powder and nickel powder are respectively loaded into the outer layer and inner layer of a pressing mold to obtain a powder-loaded pressing mold; the pressing mold includes a convex-shaped base, a convex portion of the upper portion of the convex-shaped base is covered with a tubular foil, and a soft rubber sleeve is covered on the outside of the convex-shaped base, the area inside the tubular foil is the inner layer, and the area between the tubular foil and the soft rubber sleeve is the outer layer; Step 2: Pull out the tubular foil from the powder-filled pressing mold obtained in Step 1 axially, and simultaneously install a limiting ring and a limiting core rod on the upper portion of the powder-filled pressing mold. After the tubular foil is completely pulled out, remove the limiting ring and the limiting core rod, and then seal and secure with a rubber plug to obtain a pressing mold containing the material to be pressed. Step 3: Compressing the stainless steel powder and nickel powder in the pressing mold containing the material to be pressed obtained in Step 2, and then removing the pressing mold to obtain a porous nickel / porous stainless steel composite green body; Step 4: sintering the porous nickel / porous stainless steel composite green body obtained in step 3 in a hydrogen furnace to obtain a porous nickel / porous stainless steel composite sintered body; Step 5: Pre-densification treatment is performed on the outer surface of the porous nickel / porous stainless steel composite sintered body obtained in step 4; the pre-densification treatment is performed on the outer cylindrical surface of the porous nickel / porous stainless steel composite sintered body as a reference, and the porous stainless steel layer is mechanically pre-densified with a machining accuracy of 0.02 mm and a surface roughness controlled to be less than Ra0.8; the pre-densification treatment is performed on a lathe; Step 6: Electrodepositing a hard chromium layer on the surface of the porous nickel / porous stainless steel composite sintered body that has been pre-densified in step 5 to obtain a porous nickel / stainless steel gradient composite material.

2. The method for preparing a gradient composite material with in-situ synthesis of a stainless steel layer on a porous nickel surface according to claim 1, characterized in that: The nickel powder in step 1 has a particle size of 5 μm to 150 μm, and the stainless steel powder has a grade of 316L and a particle size of 25 μm to 150 μm.

3. The method for preparing a gradient composite material with in-situ synthesis of a stainless steel layer on a porous nickel surface according to claim 1, characterized in that: The tubular foil in step 1 is made of graphite, stainless steel or aluminum, with a surface roughness less than Ra0.8 and a thickness of 0.03 mm to 0.05 mm.

4. The method for preparing a gradient composite material with in-situ synthesis of a stainless steel layer on a porous nickel surface according to claim 1, characterized in that: When the stainless steel powder and nickel powder are respectively loaded into the pressing mold in step 1, the stainless steel powder is loaded first and then the nickel powder is loaded. The loading is combined with vibration, and the heights of the stainless steel powder and the nickel powder in the pressing mold are the same.

5. The method for preparing a gradient composite material with in-situ synthesis of a stainless steel layer on a porous nickel surface according to claim 1, characterized in that: During the process of extracting the tubular foil in step 2, the lower end surface of the limiting core rod always coincides with the upper end surface of the nickel powder, and the lower end surface of the limiting ring always coincides with the upper end surface of the stainless steel powder.

6. The method for preparing a gradient composite material with in-situ synthesis of a stainless steel layer on a porous nickel surface according to claim 1, characterized in that: The pressing in step 3 is performed using a cold isostatic press, with a pressing pressure of 140 MPa to 160 MPa and a pressing time of more than 30 seconds.

7. The method for preparing a gradient composite material with in-situ synthesis of a stainless steel layer on a porous nickel surface according to claim 1, characterized in that: The sintering process in step 4 is: heating to 700°C~1050°C at a heating rate of 2°C / min~10°C / min and then keeping the temperature for 1h~2h.

8. The method for preparing a gradient composite material with in-situ synthesis of a stainless steel layer on a porous nickel surface according to claim 1, characterized in that: The specific steps of the pre-densification treatment in step 5 are: Step 101, roughing machining allowance: select a YG6X fine-grain carbide tool, set the tool geometry angle, the main deflection angle is 75°, the secondary deflection angle is 8°~15°, the cutting edge inclination angle is -5°, the rake angle is 10°~15°, the clearance angle is 8°~12°, the tool tip arc radius is 0.3mm~0.5mm, and then perform machining at a cutting speed of 60m / min~70m / min, a feed rate of 0.2mm / r~0.3mm / r, and a cutting depth of 0.3mm~0.5mm; Step 102, use a large arc turning tool to extrude and turn the outer circle: select a large arc turning tool made of YG6X fine-grained carbide, set the large arc turning tool geometric angle, the main deflection angle is 90°; the secondary deflection angle is 8°~15°, the cutting edge inclination angle is -20°, the front angle is 10°~15°, the back angle is 8°~12°, the tool tip arc radius is 1mm, and then the cutting speed is 140m / min~150m / min, the feed rate is 0.1mm / r~0.2mm / r, and the cutting depth is 0.05mm~0.1mm.

9. The method for preparing a gradient composite material with in-situ synthesis of a stainless steel layer on a porous nickel surface according to claim 1, characterized in that: The thickness of the hard chromium layer on the surface of the porous nickel / stainless steel gradient composite material in step 5 is 0.05 mm to 0.2 mm.

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