A copper / graphite composite material, its preparation method and application

Abstract

This application relates to the technical field of composite materials, and specifically relates to a copper / graphite composite material, its preparation method and application. After mixing copper and graphite, they are successively pressed and sintered, and then the sintered billet is successively extruded, rolled and friction stir processed to obtain the copper / graphite composite material. By the above method, the graphite is fragmented and dispersed in the copper matrix, and at the same time, the grains of the copper matrix are significantly refined. The prepared copper / graphite composite material has excellent tissue properties and has the advantages of high strength and good electrical conductivity. The preparation process provided by this application is simple, environmentally friendly, low-cost, and has great potential for industrial application.

Description

Technical Field

[0001] The present application relates to the technical field of composite materials, and particularly relates to a high-strength and wear-resistant copper / graphite composite material, a preparation method thereof, and an application thereof. Background Art

[0002] The copper / graphite composite material contains both copper with good strength, hardness, electrical and thermal conductivity, and corrosion resistance, and graphite with good lubricity, high melting point, good anti-welding property, and resistance to arc ablation. Therefore, the copper / graphite composite material plays an important role in friction materials, oil-impregnated bearings, electrical contact materials, conductive materials, and mechanical part materials, and is widely used in fields such as aerospace, rail transit, and the electronics industry.

[0003] However, due to the large density difference between carbon and copper, the wettability of copper / graphite is very poor, the interfacial bonding property is also very poor, and graphite is prone to agglomeration, etc., which greatly affects the strength, wear resistance, and thermophysical properties of the composite material. Therefore, improving the comprehensive performance of the copper / graphite composite material has always been an important goal for researchers in this field.

[0004] Current research mainly aims to solve the problem of the uniform dispersion of graphite in the copper matrix by continuously developing and optimizing the preparation process. Common methods include: alloy modification of the copper matrix, such as using a certain content of copper oxide / cuprous oxide as the copper source to react with the graphite in the material to enhance the interface; or improving the surface of the graphite component, such as using nickel-plated graphite powder, etc. However, these methods have problems such as complex production processes and high costs, and it is difficult to be applied industrially. Summary of the Invention

[0005] In view of this, in order to solve the deficiencies in the prior art that the uniform dispersion of graphite in the copper matrix is not good, the process is complex, and the cost is high, the present application proposes a preparation method for a copper / graphite composite material. Instead of modifying the raw materials, through repeated pressing and sintering, combined with the preparation processes of hot extrusion, rolling, and friction stir processing, the uniform dispersion of graphite in the copper matrix is achieved. This method has a simple process, is environmentally friendly, has a low cost, and has great potential for industrial application.

[0006] In order to solve the deficiencies in the prior art that the copper / graphite composite material has low strength and poor electrical conductivity, the present application also provides a copper / graphite composite material, which has the characteristics of high strength, good electrical conductivity, and excellent tissue properties.

[0007] The present application provides the following technical solutions:

[0008] In the first aspect, the present application provides a preparation method for a copper / graphite composite material, including the following steps:

[0009] The copper powder and graphite are mixed and then successively pressed and sintered, and then the sintered billet is subjected to extrusion, rolling and friction stir processing.

[0010] In some more specific embodiments, the friction stir processing is multi-pass friction stir processing;

[0011] There is an overlapping interval between two adjacent friction stir processing modified zones;

[0012] Preferably, the width of the overlapping interval is more than 50% of the width of a single friction stir processing modified zone;

[0013] More preferably, when performing the friction stir processing, the rotational speed of the stirring head is 1200 rpm - 2000 rpm, and the traveling speed of the stirring head is 30 mm / min - 100 mm / min;

[0014] Further preferably, when performing the friction stir processing, the rotational speed of the stirring head is 1400 rpm - 1800 rpm, and the traveling speed of the stirring head is 50 mm / min - 80 mm / min.

[0015] In some more specific embodiments, the extrusion ratio during the extrusion process is (21 - 27):1;

[0016] Preferably, the extrusion temperature is 880 °C - 930 °C;

[0017] More preferably, the extrusion speed is 5 mm / s - 10 mm / s.

[0018] In some more specific embodiments, the rolling deformation is more than 70%, preferably 70% - 80%;

[0019] Preferably, the rolling is multi-pass rolling, and the reduction per pass is 0.3 mm - 0.5 mm;

[0020] More preferably, annealing is performed after rolling;

[0021] Further preferably, the annealing temperature is 600 °C - 700 °C, and the annealing time is 30 min - 1.5 h.

[0022] In some more specific embodiments, the particle diameter of the copper powder is 20 μm - 40 μm; and / or, the particle diameter of the graphite is 100 μm - 300 μm;

[0023] Preferably, the graphite is flake graphite;

[0024] The copper is electrolytic copper powder.

[0025] The addition amount of the graphite is 2 wt.% - 10 wt.%.

[0026] In some more specific embodiments, a ball milling medium is added during the mixing process of copper and graphite, and ball milling is performed.

[0027] Preferably, the rotation speed of the ball mill is 70 rpm / min - 90 rpm / min, and the ball-to-material ratio is (0.5 - 1.5):1.

[0028] More preferably, the ball milling medium includes grinding balls with two or more different diameters.

[0029] Further preferably, absolute ethanol is added during the mixing process for wet mixing, and after ball milling and mixing, the mixed powder is dried.

[0030] In some more specific embodiments, the pressing and sintering process includes:

[0031] First, the mixed powder is initially pressed into a blank; the pressure of the initial pressing is 150 MPa - 550 MPa.

[0032] Then, the initially pressed blank is subjected to initial sintering; the temperature of the initial sintering is 920 °C - 940 °C, and the time is 3 h - 6 h.

[0033] The initially sintered blank is subjected to secondary pressing; the pressure of the secondary pressing is higher than that of the initial pressing.

[0034] The secondarily pressed blank is subjected to secondary sintering; the temperature of the secondary sintering is 940 °C - 960 °C, and the time is 1.5 h - 2.5 h.

[0035] Preferably, the pressing and sintering process satisfies any one or more of the following A - E:

[0036] A: The initial pressing is cold isostatic pressing; the pressure of the initial pressing is 250 MPa - 450 MPa.

[0037] B: An inert gas is introduced as a protective gas during the initial sintering and secondary sintering processes.

[0038] C: The pressure of the secondary pressing is 200 MPa - 600 MPa.

[0039] D: The temperature of the secondary sintering is 945 °C - 955 °C.

[0040] In some more specific embodiments, the preparation method further includes surface smoothing treatment.

[0041] Preferably, the surface smoothing treatment is performed after rolling and / or friction stir processing.

[0042] More preferably, pickling is performed after the smoothing treatment.

[0043] In a second aspect, the present application also provides a copper / graphite composite material, which is prepared by the described preparation method.

[0044] In a second aspect, the present application also provides an application of the copper / graphite composite material as a wear-resistant material and / or a conductive material.

[0045] The technical solution of the present application has the following advantages:

[0046] 1. In the present application, through a process method combining extrusion, rolling, and friction stir processing on the billet after re-sintering in sequence, the graphite is fragmented and evenly dispersed in the copper matrix, improving the strength and wear resistance of the composite material. On the one hand, the uniform and dispersed distribution of graphite can effectively disperse and transfer stress, improving the load-bearing capacity and conductivity of the material. At the same time, the graphite inside the composite material is evenly dispersed, improving the problem of graphite agglomeration, and enhancing the density and performance stability of the composite material. On the other hand, through the process of hot extrusion, rolling, and friction stir processing, the grains of the copper matrix are significantly refined, thereby improving the strength of the material. In the present application, two types of ball milling media with specific diameters are used to mix copper and graphite powders, and a specific ball-to-powder ratio is adopted, making the mixing of copper powder and graphite more uniform, improving problems such as easy agglomeration of graphite, and enhancing the density and performance stability of the composite material.

[0047] 2. In the present application, through a process of alternately performing two pressings and sinterings, with a lower pressure for the first pressing and a higher pressure for the second pressing than the first pressing; a lower temperature for the first sintering and a higher temperature for the second sintering than the first sintering; thereby improving the density of the composite material and the interface continuity between copper and graphite, and further enhancing the performance of the composite material. Description of the Drawings

[0048] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the following drawings are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0049] Figure 1 is the metallographic diagram of the composite material obtained in Comparative Example 3 of the present application without friction stir processing;

[0050] Figure 2 is the metallographic diagram of the composite material after friction stir processing in Test Example 5 of the present application. Specific Embodiments

[0051] The following embodiments are provided to better further understand the present application, which is not limited to the described best mode, and does not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior art features falls within the protection scope of the present application.

[0052] For those test examples where specific experimental steps or conditions are not indicated, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For the reagents or instruments whose manufacturers are not indicated, they are all conventional reagent products that can be obtained through commercial purchase.

[0053] In order to improve the problem of the dispersion of graphite in the copper matrix in the composite material, the present application provides a preparation method for a copper / graphite composite material.

[0054] In some embodiments, the preparation method includes the following steps:

[0055] Copper and graphite are mixed and then successively pressed and sintered, and then the sintered billet is extruded, rolled and friction stir processed.

[0056] Friction stir processing evolved from friction stir welding and is also known as friction stir processing treatment. Its principle is that a high-speed rotating stirring head penetrates into the processed plate, and the rotating stirring pin frictions with the processed plate and makes it plasticize. The shoulder frictions with the plate surface to generate heat and is used to prevent the plasticized material from overflowing. Finally, through the strong stirring action of the stirring head, the processed material undergoes severe plastic deformation, mixing and fragmentation, realizing the densification, homogenization and refinement of the microstructure. Friction stir processing can eliminate defects such as porosity and shrinkage cavities in casting products, and can also refine grains, thereby improving the material properties.

[0057] The present application applies friction stir processing to the preparation of copper / graphite composite materials. During the friction stir processing, the stirring pin of the stirring head penetrates into the rolled plate, causing the plate to undergo severe plastic deformation. First of all, friction stir processing can refine the grains of the copper matrix, obtain a fine-grained structure, and improve the strength and wear resistance of the copper / graphite composite material. Secondly, during the friction stir processing, through the high-speed rotation of the stirring head and the stirring action of the stirring pin, the graphite particles are fragmented and dispersed in the copper matrix, and finally a copper / graphite composite material with small and uniformly distributed graphite size is obtained, improving the comprehensive performance of the composite material.

[0058] The present application adopts a process method combining extrusion, rolling and friction stir processing for the sintered billet to obtain a copper / graphite composite material. The composite material has fine grains in the copper matrix, and the reinforcing graphite has small size and is uniformly dispersed, improving the strength, conductivity and wear resistance of the composite material.

[0059] In order to process as many areas as possible on the sheet material and ensure the uniformity of the friction stir processing structure, there is an overlapping area between adjacent friction stir processing modification areas; the friction stir processing is a multi-pass friction stir processing treatment; preferably, it is a 2-5 times repeated friction stir processing treatment;

[0060] Meanwhile, the length of the stirring pin of the stirring head selected for the friction stir processing is equal to the thickness of the sheet material to be processed.

[0061] In order to make the structure of the composite material more uniform, preferably, the width of the overlapping interval is more than 50% of the width of a single friction stir processing modification area, preferably 50%-70%;

[0062] More preferably, the rotational speed of the stirring head for the friction stir processing is 1200 rpm - 2000 rpm, and the traveling speed of the stirring head is 30 mm / min - 100 mm / min;

[0063] Further preferably, the rotational speed of the stirring head for the friction stir processing is 1500 rpm - 1800 rpm, and the traveling speed of the stirring head is 50 mm / min - 80 mm / min.

[0064] If the traveling speed of the stirring head is too fast and the stirring time of the stirring pin in the material is too short, the effect of the friction stir processing is insufficient. On the one hand, the effect on the graphitization effect and the grain refinement of the copper matrix is not good. On the other hand, it is easier to generate defects such as grooves and tunnels in the processed sheet material. If the traveling speed of the stirring head is too slow and the stirring pin stays in the material for too long, it will cause excessive heat generation, which may lead to the growth of copper matrix grains and is not beneficial to the strength of the composite material.

[0065] Therefore, it is preferred that the rotational speed of the stirring head is 1200 rpm - 2000 rpm and the traveling speed is 30 mm / min - 100 mm / min. It is proved by experiments that it is more preferred that the rotational speed of the stirring head is 1500 rpm - 1800 rpm and the traveling speed is 50 mm / min - 80 mm / min.

[0066] In some embodiments, the extrusion ratio during the extrusion process is (21 - 27):1;

[0067] Preferably, the extrusion temperature is 880°C - 930°C;

[0068] More preferably, the extrusion speed is 5 mm / s - 10 mm / s.

[0069] To improve the continuity of the interface and the electrical conductivity of the composite material, this application adopts a hot extrusion process. By utilizing the principle of hot working, the plastic deformation ability of the material is enhanced, preventing interface fracture during cold extrusion. Meanwhile, during the extrusion process, the billet undergoes large plastic deformation, thereby increasing the density of the composite material. Controlling the speed of hot extrusion can avoid structural voids caused by instantaneous extrusion, effectively improving the quality and efficiency of the product.

[0070] The rolling deformation amount is more than 70%, preferably 70%-78%.

[0071] Preferably, the rolling is multi-pass rolling, and the reduction per pass is 0.1 mm - 0.3 mm.

[0072] After multi-pass rolling, the thickness of the billet can be reduced from 5 mm - 7 mm to 1.5 mm - 2.5 mm.

[0073] Further preferably, the annealing temperature is 600°C - 700°C, and the annealing time is 30 min - 1.5 h.

[0074] To make the structure of the composite material denser, the grains finer, and the grain distribution of graphite and copper powder more uniform, this application adopts the methods of hot extrusion and rolling. The structure obtained by powder metallurgy is improved in terms of density and stability under the action of hot extrusion and rolling at high temperature and high friction, while the strength of the composite material is increased. To achieve better results, preferably the reduction per pass is 0.3 mm - 0.5 mm; it can be selected as 0.05 mm - 0.5 mm according to needs.

[0075] After rolling, annealing is carried out to eliminate stress. Preferably, the annealing temperature is 600°C - 700°C; it can be 600°C, 650°C, 700°C, etc., and the annealing time is 30 min - 1.5 h.

[0076] To make the grains of the composite material finer, in some embodiments, copper and graphite with smaller particle sizes are selected. The particle diameter of the copper is 20 μm - 40 μm; and / or, the particle diameter of the graphite is 100 μm - 300 μm.

[0077] Too large a graphite particle size will have an obvious splitting effect on the copper matrix, resulting in a decline in the mechanical properties of the composite material; it is prone to tearing and causing damage during the processing. If the graphite particle size is too small, agglomeration is likely to occur, leading to a reduction in the strength and electrical conductivity of the material.

[0078] Preferably, the graphite is flake graphite.

[0079] The copper is electrolytic copper powder.

[0080] The added weight of the graphite is 2wt.%-10wt.%. Optionally, the mass fraction of the graphite is 2wt.%, 3wt.%, 4wt.%, 5wt.%, 6wt.%, 7wt.%, or 10wt.%.

[0081] The raw copper of the present application adopts electrolytic copper powder with a particle size of 20μm-40μm. Copper powder with a smaller particle size is selected for easier uniform distribution. Considering the cost issue, 25μm-35μm is preferred; 25μm, 30μm, 35μm, etc. can be selected.

[0082] The raw material is flake graphite, which is natural crystalline graphite with good electrical and thermal conductivity, lubricity, plasticity, and acid and alkali resistance.

[0083] Flake graphite is a mineral composed of thin flake crystals, which is shaped like fish scales, belongs to the hexagonal system, has a layered structure, has a certain degree of brittleness, has high electrical conductivity and thermal conductivity, and has a certain degree of corrosion resistance and wear resistance. This application uses flake graphite as a raw material. Flake graphite has a layered structure, has better self-lubricity and plasticity, and helps to improve the friction and wear resistance and electrical conductivity of copper / graphite composite materials.

[0084] Electrolytic copper powder is prepared by electrolyzing a copper salt solution. The present application selects electrolytic copper powder because it has high purity and uniform particle size and good electrical conductivity.

[0085] Different graphite contents will have a certain effect on the performance of graphite / copper composite materials. A small amount of graphite can play a role in particle reinforcement of the matrix, but excessive graphite is prone to agglomeration and affects the performance of the composite material. Copper-based composite materials with a graphite content of 6wt.% show better mechanical properties and wear resistance. In some embodiments, ball milling media is added during the mixing of copper and graphite and ball milling is performed;

[0086] Preferably, the rotation speed of the ball mill is 70rpm / min-90rpm / min, and the ball-to-material ratio is (0.5-1.5):1;

[0087] More preferably, the ball milling media comprises two or more grinding balls of different diameters;

[0088] Further preferably, anhydrous ethanol is added during the mixing process for wet mixing, and the mixed powder is dried after wet mixing.

[0089] Preferably, the amount of anhydrous ethanol added is 50%-70% of the volume of the mixed powder, and the drying is performed at 45°C-55°C for 55min-65min;

[0090] Ethanol wet mixing can adjust the viscosity of the powder during ball milling, which is beneficial to reducing energy loss during the ball milling process.

[0091] The weight ratio of the addition of two ball milling media with different diameters is (0.5 - 1.5):1;

[0092] The diameters of the two grinding balls are 10 mm and 5 mm;

[0093] The ball milling medium is a steel ball.

[0094] Optionally, three ball milling media with different diameters, 9 mm, 6 mm, and 3 mm, are GCr15 steel balls with a mass ratio of 3:2:2.

[0095] During the ball milling process, argon gas is filled in the tank for protection.

[0096] According to the cavity structure of the ball milling powder mixer, ball milling media with different diameters can increase the shearing force on the internal powder, thereby obtaining a mixed powder with a more uniform distribution and smaller size. The use of grinding balls with different diameters for ball milling in this application is set according to the particle properties of copper and graphite itself, and can take into account substances with large and small particle sizes.

[0097] By comparing ball milling media with different diameters, it is obtained that when the ball-to-material volume ratio is 1:1 and the steel balls with diameters of 10 mm and 5 mm are used, a copper and graphite mixed powder with a smaller particle size can be obtained.

[0098] In some embodiments, the preparation method further includes surface smoothing treatment;

[0099] Preferably, the surface smoothing treatment is carried out after rolling and / or friction stir processing to remove flash or burrs;

[0100] More preferably, pickling is carried out after the smoothing treatment to remove the surface oxide scale.

[0101] In some embodiments, the pressing and sintering process includes:

[0102] First, the mixed powder is initially pressed into a blank; the pressure of the initial pressing is 150 MPa - 550 MPa;

[0103] Then, the blank after the initial pressing is subjected to initial sintering; the temperature of the initial sintering is 920 °C - 940 °C, and the time is 3 h - 6 h;

[0104] The blank after the initial sintering is subjected to repressing; the pressure of the repressing is higher than that of the initial pressing;

[0105] The blank after the repressing is subjected to re-sintering; the temperature of the re-sintering is 940 °C - 960 °C, and the time is 1.5 h - 2.5 h;

[0106] The most important functions of the two sintering and pressing processes are to improve the density of the material, thereby enhancing the properties of the material.

[0107] Preferably, the pressing and sintering processes satisfy any one or more of the following A - E:

[0108] A: The primary pressing is cold isostatic pressing; the pressure of the primary pressing is 250 MPa - 450 MPa; preferably, the thickness of the billet formed by the primary pressing is 70 mm - 90 mm;

[0109] B: An inert gas is introduced as a protective gas during the primary sintering and re - sintering processes;

[0110] C: The pressure of the re - pressing is 200 MPa - 600 MPa;

[0111] D: The temperature of the re - sintering is 945 °C - 955 °C.

[0112] This application adopts the process of repeated pressing and sintering. The pressure of the re - pressing is higher than that of the primary pressing. Compared with single pressing and sintering, the density and performance stability of the billet are improved.

[0113] This application also provides a copper / graphite composite material, and the composite material includes the copper / graphite composite material prepared by the preparation method.

[0114] This application also provides an application of the described copper / graphite composite material as a wear - resistant material or a conductive material.

[0115] The present invention adopts a process method combining hot extrusion, rolling and friction stir processing to obtain a copper / graphite composite material with uniform tissue distribution. The graphite in this composite material has small size and uniform distribution, which is beneficial to the improvement of the strength and wear resistance of the composite material. On the one hand, the uniform and dispersed distribution of graphite can effectively disperse and transfer stress, improving the load - bearing capacity of the material. At the same time, the uniform and dispersed distribution of graphite inside the composite material improves the density and performance stability of the composite material.

[0116] The following further describes the present application in detail with specific test examples, and these test examples should not be construed as limiting the scope claimed by the present application.

[0117] The following test examples adopt the following methods for detection:

[0118] 1. Resistivity test:

[0119] According to the national standard JB / T8133.2 - 1999 "Test Methods for Physical and Chemical Properties of Electrical Carbon Products - Resistivity", a GM - 2 multi - functional resistivity automatic measuring instrument is used for testing. The size of the test sample is 10×5×30 mm. Three groups of data are tested for each group of samples, and the measurement result takes the average resistivity calculated according to the formula:

[0120]

[0121] Where: p - resistivity, Ω·m; U - potential tip voltage drop, mV; I - current passing through the specimen, A; b - specimen width, mm; h - specimen thickness, mm; l - specimen length, mm.

[0122] 2. Reciprocating friction and wear experiment

[0123] The reciprocating friction and wear experiment of the composite material was carried out using an HSR-2M reciprocating friction and wear testing machine. The counter ball was a copper ball with a diameter of 6 mm. Before the experiment, the surface of the sample was cleaned, and then the sample was polished with 800-mesh sandpaper until there were no obvious scratches on the surface. The mass of the sample m1 was weighed. After the copper ball was ultrasonically treated for 5 min, it was polished with 800-mesh sandpaper until there were no obvious scratches on the sample. During the friction process, the applied load was 10 N, and the friction linear velocity was 0.1 m·s -1 , and a 30-min friction experiment was carried out. After the experiment, the mass of the sample m2 was weighed. The volume wear rate of the composite material was calculated according to the formula:

[0124] Where:

[0125]

[0126] Where: ω - volume wear rate, mm 3 / (N·m); m1 and m2 are the masses of the sample before and after friction, respectively, g; ρ - sample density, g / mm 3 ; P - applied load, N; L - sliding distance, mm.

[0127] 3. Tensile properties

[0128] The tensile properties of the material in various states were tested using an Instron-5569 electronic tensile testing machine. The tensile rate was 2 mm / min (strain rate was approximately 10 -3 S -1 ), and the gauge length of the specimen was 20 mm. The material was processed into standard dimensions by wire cutting. The processed specimens were immersed in acetone and ultrasonically treated for 5 min to remove surface oil stains, then rinsed with alcohol, dried, and then polished with 320# water sandpaper and 2000# water sandpaper in sequence to remove the oxide scale on the surface and side of the tensile specimens. The gauge section was marked on the tensile specimens with a marker pen, and the thickness and width of the tensile specimens were measured with a vernier caliper. The dimensions of each specimen were measured 5 times, and the final specimen dimensions were the average values obtained after removing the maximum and minimum values. At least 3 samples of each state specimen were tested, and then the average value was taken.

[0129] 4. Microhardness

[0130] The hardness of the specimens was tested using a Vickers microhardness tester with a load of 200 g and a holding time of 15 s. Before testing, the specimens were polished successively with 320# water sandpaper, 2000# water sandpaper, and 800# metallographic sandpaper to reduce the influence of the machined surface on the hardness test. At least 15 individual indentations were measured on each specimen in each state, with a sufficient distance between the indentations. After measurement, the maximum and minimum values were removed, and the average value of the test results was taken as the final hardness value.

[0131] Experimental Example 1

[0132] This experimental example provides a high-strength wear-resistant copper / graphite composite material and its preparation method, including the following steps:

[0133] Step 1: Powder mixing. First, copper powder (electrolytic copper powder with a particle size of 25 μm - 35 μm) and graphite (flake graphite with a particle diameter of 100 μm - 300 μm) were added to a ball mill according to the mass fraction of graphite being 2 wt.%, and ball milling media were also added.

[0134] The steel balls (ball milling media) used had diameters of 10 mm and 5 mm, and the addition ratio of the two different diameters of steel balls was 1:1.

[0135] During the powder mixing process, the ball-to-powder ratio was 1:1, the rotation speed of the ball mill was 80 rpm / min, and absolute ethanol was added to the mixer for wet mixing for 4 h. The addition amount of absolute ethanol was 70% of the volume of the mixed powder. Finally, the mixed powder was dried in a vacuum drying oven at 50 °C for 1 h.

[0136] Step 2: Initial pressing. The mixed powder was cold isostatically pressed into billets (with a diameter of 80 mm). The initial pressing pressure was 150 MPa, 300 MPa, or 550 MPa.

[0137] Step 3: Initial sintering. The pressed billets were placed in a heat treatment furnace for heat treatment. The initial sintering temperature was 920 °C, and the holding time was 4.5 h. Argon protective gas was introduced during the sintering process. After sintering, it was cooled in the furnace.

[0138] Step 4: Re-pressing. The sintered samples were re-placed in the mold for re-pressing. The re-pressing pressure was higher than the initial pressing pressure, and 200 MPa, 400 MPa, or 600 MPa was correspondingly selected.

[0139] Step 5: Re-sintering. The re-sintering process was: 950 °C, 2 h. After re-sintering, it was cooled in the furnace. Re-sintering can improve the density of the specimens and eliminate the internal stress generated by plastic deformation.

[0140] Step 6: Extrusion. The re-sintered billets were extruded, and the hot extrusion temperature was 940 °C. The hot extrusion speed was 8 mm / s, and the extrusion ratio was 21:1.

[0141] Step 7: Rolling. The extruded copper plate is subjected to multi-pass rolling. The reduction per pass is about 0.4 mm. After every two passes of rolling, annealing is carried out once. The annealing temperature is 650 °C and the annealing time is 1 h. Finally, a plate with a thickness of about 2 mm is obtained; the deformation amount of rolling is 70%.

[0142] Finally, the rolled copper plate is pickled to remove the surface oxide scale.

[0143] Step 8: Friction stir processing. The rolled plate is subjected to multi-pass friction stir processing. There is an overlapping interval between adjacent friction stir processing modified zones; the width of the overlapping interval is more than 50% of the width of a single friction stir processing modified zone. The rotational speed of the stirring head is: 1200 rpm, and the traveling speed of the stirring head is: 30 mm / min.

[0144] Step 9: Surface cleaning. The surface of the plate after friction stir processing is smoothed, and burrs, flash, etc. are removed by a machine or a grinder. Finally, the rolled copper plate is pickled for the second time to remove the surface oxide scale; finally, a copper / graphite composite material is obtained.

[0145] After inspection, the results are shown in Table 1:

[0146] Table 1 Performance test results of Test Example 1

[0147]

[0148] Test Example 2

[0149] This test example provides a high-strength wear-resistant copper / graphite composite material and its preparation method, including the following steps:

[0150] Step 1: Powder mixing. First, copper powder (electrolytic copper powder, particle size 20 μm - 40 μm) and graphite powder (flake graphite, particle diameter 100 μm - 300 μm) are added into a powder mixer according to the mass fraction of graphite being 4 wt.%, and a ball milling medium is added.

[0151] Three kinds of GCr15 steel balls with different diameters (ball milling medium) with diameters of 9 mm, 6 mm, and 3 mm are used, and the added mass ratio is 3:2:2.

[0152] During the ball milling process, argon gas is filled in the tank for protection.

[0153] During the powder mixing process, the ball-to-powder ratio is 1.5:1, the rotational speed of the powder mixer is 70 rpm / min, and anhydrous ethanol is added to the powder mixer for wet mixing for 4 h. The addition amount of anhydrous ethanol is 50% of the volume of the mixed powder; finally, the mixed powder is dried in a vacuum drying oven at 45 °C for 65 min.

[0154] Step 2: Initial pressing. The mixed powder is pressed into a green compact (with a diameter of 90 mm) by cold isostatic pressing. The initial pressing pressure is 300 MPa.

[0155] Step 3: Initial sintering. The pressed green compact is put into a heat treatment furnace for heat treatment. The initial sintering temperature is 920 °C or 940 °C, and it is kept warm for 6 h.

[0156] Step 4: Re-pressing. The sintered sample is put back into the mold for re-pressing. The re-pressing pressure is higher than the initial pressing pressure, and 400 MPa is selected.

[0157] Step 5: Re-sintering. The re-sintering process is as follows: 940 °C or 960 °C, 2.5 h, and after re-sintering, it is cooled in the furnace. Re-sintering can improve the density of the sample and eliminate the internal stress generated by plastic deformation. Argon gas is introduced as a protective gas during the sintering process. After sintering, it is cooled in the furnace.

[0158] Step 6: Extrusion. The re-sintered green compact is extruded, and the hot extrusion temperature is 880 °C. The hot extrusion speed is 10 mm / s, and the extrusion ratio is 23:1.

[0159] Step 7: Rolling. The extruded copper plate is rolled in multiple passes. The reduction per pass is about 0.2 mm. After every two rollings, annealing is carried out once. The annealing temperature is 600 °C, and the annealing time is 1.5 h. The rolling deformation is 70%, and finally a plate with a thickness of about 1.5 mm is obtained.

[0160] Finally, the rolled copper plate is pickled to remove the surface oxide scale. After inspection, a composite material without friction stir processing is obtained.

[0161] Step 8: Friction stir processing. The rolled plate is subjected to multi-pass friction stir processing. The rotational speed of the stirring head is: 1500 rpm, and the traveling speed of the stirring head is: 30 mm / min. A plate processed by friction stir is obtained. After inspection, a composite material after friction stir processing is obtained.

[0162] Step 9: Surface cleaning. The surface of the plate processed by friction stir is smoothed to remove flash, burrs, etc. Finally, the rolled copper plate is pickled for the second time to remove the surface oxide scale; finally, a copper / graphite composite material is obtained.

[0163] After inspection, the results are shown in Table 2:

[0164] Table 2 Performance test results of Test Example 2

[0165]

[0166] Test Example 3

[0167] This test example provides a high-strength and wear-resistant copper / graphite composite material and its preparation method, including the following steps:

[0168] Step 1: Powder mixing. First, add copper powder (electrolytic copper powder with a particle size of 20μm - 40μm) and graphite (flake graphite with a particle diameter of 100μm - 300μm) into a powder mixer according to the mass fraction of graphite being 6wt.%, and add ball milling media.

[0169] The steel balls (ball milling media) used have diameters of 10mm and 5mm, and the addition ratio of the two different diameter steel balls is 1.5:1.

[0170] During the powder mixing process, the ball-to-material ratio is 0.5:1, the rotation speed of the powder mixer is 90rpm / min, and anhydrous ethanol is added to the powder mixer for wet mixing for 4h. The addition amount of anhydrous ethanol is 60% of the volume of the mixed powder; finally, the mixed powder is dried in a vacuum drying oven at 55°C for 55min.

[0171] Step 2: Initial pressing. Press the mixed powder into a billet (with a diameter of 70mm) through cold isostatic pressing. The initial pressing pressure is 300MPa.

[0172] Step 3: Initial sintering. Put the pressed billet into a heat treatment furnace for heat treatment. The initial sintering temperature is 930°C, and keep it for 3h. Argon protective gas is introduced during the sintering process. After sintering, it is cooled with the furnace.

[0173] Step 4: Re-pressing. Put the sintered sample back into the mold for re-pressing. The re-pressing pressure is higher than the initial pressing pressure, and 400MPa is selected.

[0174] Step 5: Re-sintering. The re-sintering process is: 950°C, 1.5h. After re-sintering, it is cooled with the furnace. Re-sintering can improve the density of the sample and eliminate the internal stress generated by plastic deformation.

[0175] Step 6: Extrusion. Extrude the re-sintered billet, and the hot extrusion temperature is 930°C. The hot extrusion speed is 5mm / s, and the extrusion ratio is 27:1.

[0176] Step 7: Rolling. Roll the extruded copper plate in multiple passes. The reduction per pass is about 0.4mm. Annealing is carried out once after every two rollings. The annealing temperature is 700°C, and the annealing time is 30min. The rolling deformation amount is 70%, and finally a copper plate with a thickness of about 1.5mm is obtained.

[0177] Finally, pickle the rolled copper plate to remove the surface oxide scale. After inspection, a composite material without friction stir processing is obtained.

[0178] Step 8: Friction Stir Processing. The rolled sheet is subjected to multi-pass friction stir processing. The rotational speed of the stirring head is: 1200 rpm, 1500 rpm or 1800 rpm, and the traveling speed of the stirring head is: 50 mm / min. The sheet after friction stir processing is obtained, and after inspection, the composite material after friction stir processing is obtained.

[0179] Step 9: Surface Cleaning. The surface of the sheet after friction stir processing is smoothed to remove flash burrs, etc. Finally, the rolled copper plate is pickled for the second time to remove the surface scale; finally, the copper / graphite composite material is obtained.

[0180] After inspection, the results are shown in Table 3:

[0181] Table 3 Performance Test Results of Test Example 3

[0182]

[0183] Test Example 4

[0184] This test example provides a high-strength wear-resistant copper / graphite composite material and its preparation method, including the following steps:

[0185] Step 1: Powder Mixing. First, copper powder (electrolytic copper powder, particle size 20 μm - 40 μm) and graphite (flake graphite, particle diameter 100 μm - 300 μm) are added into a powder mixer according to the mass fraction of graphite being 10 wt.% or 6 wt.%, and a ball milling medium is added.

[0186] The steel balls (ball milling medium) used have diameters of 10 mm and 5 mm, and the addition ratio of the two different diameter steel balls is 1.5:1.

[0187] During the powder mixing process, the ball-to-powder ratio is 0.5:1, the rotational speed of the powder mixer is 90 rpm / min, and anhydrous ethanol is added to the powder mixer for wet mixing for 4 h. The addition amount of anhydrous ethanol is 60% of the volume of the mixed powder; finally, the mixed powder is dried in a vacuum drying oven at 55 °C for 55 min.

[0188] Step 2: Initial Pressing. The mixed powder is cold isostatically pressed into a billet (diameter 70 mm). The initial pressing pressure is 300 MPa.

[0189] Step 3: Initial Sintering. The pressed billet is put into a heat treatment furnace for heat treatment. The initial sintering temperature is 940 °C, and the holding time is 3 h. Argon protective gas is introduced during the sintering process. After sintering, it is cooled with the furnace.

[0190] Step 4: Re-pressing. The sintered sample is put back into the mold for re-pressing. The re-pressing pressure is higher than the initial pressing pressure, and 400 MPa is selected.

[0191] Step 5: Re-firing. The re-firing process is as follows: 960°C, 1.5 h, and after re-firing, it is cooled in the furnace. Re-firing can increase the density of the specimen and eliminate the internal stress generated by plastic deformation.

[0192] Step 6: Extrusion. The re-fired billet is extruded, and the hot extrusion temperature is 930°C. The hot extrusion speed is 5 mm / s, and the extrusion ratio is 23:1.

[0193] Step 7: Rolling. The extruded copper plate is rolled in multiple passes. The reduction per pass is approximately 0.4 mm. After rolling, it is annealed once. The annealing temperature is 700°C, and the annealing time is 30 min. The rolling deformation is 75%.

[0194] Finally, the rolled copper plate is pickled to remove the surface oxide scale.

[0195] Step 8: Friction Stir Processing. The rolled plate is processed by friction stir processing in multiple passes. The rotational speed of the stirring head is: 2000 rpm, and the traveling speed of the stirring head is: 100 mm / min. The plate processed by friction stir processing is obtained, and the composite material after friction stir processing is obtained.

[0196] Step 9: Surface Cleaning. The surface of the plate processed by friction stir processing is smoothed to remove flash burrs, etc. Finally, the rolled copper plate is pickled for the second time to remove the surface oxide scale; finally, the copper / graphite composite material is obtained.

[0197] Test Example 5

[0198] This test example uses basically the same process as Test Example 4 to prepare the copper / graphite composite material, except that the rolling deformation is 80%; and the rotational speed of the stirring head is: 1500 rpm, and the traveling speed of the stirring head is: 50 mm / min.

[0199] Comparative Example 1

[0200] This comparative example uses basically the same process as Test Example 5 to prepare the copper / graphite composite material, except that: ① Step 7: The rolling process is omitted, and it is directly processed by friction stir after extrusion; ② The extrusion ratio is 27:1.

[0201] Comparative Example 2

[0202] This comparative example uses basically the same process as Test Example 5 to prepare the copper / graphite composite material, except that Step 6: The extrusion process is omitted, and it is directly rolled after re-firing.

[0203] Comparative Example 3

[0204] This comparative example uses basically the same process as Test Example 5 to prepare the copper / graphite composite material, except that Step 8: The friction stir processing is omitted, and the surface cleaning is directly carried out after rolling.

[0205] Test Examples 4 and 5 and Comparative Examples 1 - 3 were examined, and the results are shown in Table 4:

[0206] Table 4 Performance Test Results of Test Examples 4 and 5 and Comparative Examples 1 - 3

[0207]

[0208] Upon examination, the metallographic structure of the composite material without friction stir processing in Comparative Example 3 is as Figure 1 (the initial pressing pressure is 300 MPa, and the repressing pressure is 400 MPa). It can be seen from Figure 1 that the graphite size is relatively large, and there is a phenomenon of graphite agglomeration in some areas;

[0209] For the plate processed by friction stir processing in Test Example 5, upon examination, the metallographic structure of the composite material after friction stir processing is as Figure 2 .

[0210] By comparing Figure 1 and Figure 2 , it can be seen that the graphite in the composite material after friction stir processing has a finer size and a more dispersed distribution.

[0211] From the above tests, it can be known that the process method combining hot extrusion, rolling and friction stir processing is adopted in this application to obtain a copper / graphite composite material with a uniform tissue distribution. The graphite in this composite material has a fine size and a uniform distribution, which is beneficial to the improvement of the strength and wear resistance of the composite material.

[0212] In Comparative Example 1, without the rolling process, even if the extrusion ratio is increased, the yield strength, tensile strength and microhardness of the obtained copper / graphite composite material are small, and the wear rate is large.

[0213] In Comparative Example 2, without the extrusion process, the yield strength, tensile strength and microhardness of the obtained copper / graphite composite material are all small, and the wear rate is large.

[0214] In Comparative Example 3, without friction stir processing, the yield strength, tensile strength and microhardness of the copper / graphite composite material are all small, and the wear rate is large.

[0215] Obviously, the above test examples are only examples given for clear illustration, rather than limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made on the basis of the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for preparing a copper / graphite composite material, characterized in that: The following steps are involved: The copper powder and graphite are mixed and pressed and sintered to obtain a sintered blank, and the sintered blank is subjected to extrusion, rolling and stir friction processing; The pressing and sintering process includes initial pressing, initial sintering, re-pressing and re-sintering in sequence; Extrusion temperature is 880℃-930℃; The deformation of rolling is more than 70%; The stirring head rotation speed of the friction stir processing is 1500rpm-1800rpm, and the moving speed of the stirring head is 50mm / min-80mm / min.

2. The method for preparing the copper / graphite composite material according to claim 1, characterized in that: The friction stir processing is a multi-pass friction stir processing; two adjacent friction stir processing modification zones have an overlapping area.

3. The method for preparing the copper / graphite composite material according to claim 2, characterized in that: The width of the overlapping region is more than 50% of the width of a single friction stir processing modified zone.

4. The method for preparing the copper / graphite composite material according to claim 1 or 2, characterized in that: The extrusion ratio during the extrusion process was (21-27):

1.

5. The method for preparing the copper / graphite composite material according to claim 4, characterized in that: The extrusion speed is 5mm / s-10mm / s.

6. The method for preparing the copper / graphite composite material according to claim 1 or 2, characterized in that: The rolling is performed in multiple passes, and the pressing amount in each pass is 0.1 mm to 0.3 mm.

7. The method for preparing the copper / graphite composite material according to claim 6, characterized in that: Annealing is performed after rolling.

8. The method for preparing the copper / graphite composite material according to claim 7, characterized in that: Annealing temperature is 600℃-700℃, and annealing time is 30min-1.5h.

9. The method for preparing the copper / graphite composite material according to claim 1, characterized in that: The diameter of the copper powder is 20 μm-40 μm; and / or the particle diameter of the graphite is 100 μm-300 μm.

10. The method for preparing the copper / graphite composite material according to claim 9, characterized in that: The graphite is flaky graphite.

11. The method for preparing the copper / graphite composite material according to claim 10, characterized in that: The copper powder is electrolytic copper powder.

12. The method for preparing the copper / graphite composite material according to claim 11, characterized in that: The added amount of the graphite is 2wt.%-10wt.%.

13. The method for preparing the copper / graphite composite material according to claim 1, characterized in that: During the mixing process of copper powder and graphite, ball milling media is added and ball milling is performed.

14. The method for preparing the copper / graphite composite material according to claim 13, characterized in that: The ball mill speed is 70rpm / min-90rpm / min, and the ball-to-material ratio is (0.5-1.5):

1.

15. The method for preparing the copper / graphite composite material according to claim 14, characterized in that: The ball milling medium comprises two or more ball milling media with different diameters.

16. The method for preparing the copper / graphite composite material according to claim 15, characterized in that: During the mixing process, anhydrous ethanol is added for wet mixing, and the mixed powder is dried after ball milling.

17. The method for preparing the copper / graphite composite material according to claim 1, characterized in that: The pressing and sintering process includes: First, the mixed powder is initially pressed into a blank; the initial pressing pressure is 150MPa-550MPa; Then the blank after initial pressing is initially fired; the temperature of the initial firing is 920°C-940°C, and the time is 3h-6h; The green piece after the initial firing is subjected to re-pressing, wherein the re-pressing pressure is higher than the initial pressing pressure; The re-pressed blank is re-fired; the re-firing temperature is 940° C.-960° C., and the time is 1.5 h-2.5 h.

18. The method for preparing the copper / graphite composite material according to claim 17, characterized in that: The pressing and sintering process satisfies any one or more of the following AD: A: The initial pressure is cold isostatic pressing; the initial pressure is 250MPa-450MPa; B: Inert gas is introduced as protective gas during the initial firing and re-firing process; C: The pressure of the re-pressing is 200MPa-600MPa; D: The re-firing temperature is 945°C-955°C.

19. A copper / graphite composite material, characterized in that: The copper / graphite composite material is prepared by the preparation method described in any one of claims 1-18.

20. Use of the copper / graphite composite material according to claim 19 as a wear-resistant material and / or a conductive material.

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