Surface-strengthened SiC / Al composite material and preparation method thereof

Abstract

The invention belongs to the technical field of composite material preparation and application, and particularly relates to a surface-strengthened SiC / Al composite material and a preparation method thereof. In order to solve the problems that traditional post-treatment technologies such as heat treatment are long in treatment period and large in material waste, uneven heating of external heating and internal cooling of a metal-based composite material and uneven tissue change of the metal-based composite material are possibly caused, SiC particles are added into 7075 aluminum alloy, hot rolling treatment is conducted after sintering, and the SiC particles are added into the 7075 aluminum alloy, so that the thermal conductivity of the 7075 aluminum alloy is improved, and the thermal conductivity of the 7075 aluminum alloy is improved. According to the method, the mechanical property of the surface of the metal-based composite material is comprehensively improved in a high-frequency pulse current post-treatment mode, the internal strength-toughness contradiction of the metal-based composite material is broken through, then the working reliability of the metal-based composite material can be improved, and the major requirements of the modern industry are met.

Description

Technical Field

[0001] The present invention belongs to the technical field of composite material preparation, and particularly relates to a surface-strengthened SiC / Al composite material and a preparation method thereof. Background Art

[0002] With the rapid development of high-tech fields such as aerospace, nuclear power, and transportation, service materials are facing more stringent requirements, and traditional single-metal structural materials are difficult to meet the needs. Aluminum matrix composites prepared by adding hard silicon carbide (SiC) particles to an Al matrix can have excellent comprehensive mechanical properties such as high specific strength, high specific stiffness, fatigue resistance, high thermal conductivity, and low thermal expansion, and can broaden the application scope of aluminum alloys in fields such as the automotive and aerospace industries. However, due to the significant differences in the physical and chemical properties between the Al matrix and SiC particles, and problems such as easy aggregation of SiC particles and coarse grains, the improvement of the surface comprehensive properties of SiC / Al composites still faces bottlenecks.

[0003] Generally, the structure and mechanical properties of the surface of metal matrix composites can be improved by post-treatment technologies such as heat treatment. However, during the heat treatment process, it is inevitable to cause uneven heating of the metal matrix composite with external heat and internal cold, uneven changes in the structure of the metal matrix composite, and easy formation of a temperature gradient inside the metal matrix composite, generating thermal stress. There may also be a large amount of energy loss during the long-term heat supply treatment process.

[0004] Compared with traditional heat treatment, high-frequency pulsed current has self-heating effects of instantaneous energy concentration, proximity effect, and skin effect, and plays an important role in aspects such as surface modification, grain refinement, and recrystallization solidification of metal matrix composites. It has been successfully applied to the surface modification of metal matrix composites such as aluminum alloys and magnesium alloys. And through the combined action of thermal effect and non-thermal effect, high-frequency pulsed current can promote the healing of microcracks inside the metal matrix, thereby improving the structure and overall performance. From the perspective of the pulsed current frequency, high-frequency pulsed current (≥10000Hz) will produce an obvious skin effect and an obvious concentration on the surface of the metal matrix composite. When the current parameters are properly controlled, the surface area is expected to be strengthened while the internal area remains in good condition, which is crucial for the improvement of the surface performance of metal matrix composites.

[0005] In recent years, pulsed current technology has become an effective method for improving the structure and performance of metal matrix composites. Applying high-frequency pulsed current technology to the post-treatment of SiC / Al composite material surface modification can overcome the shortcomings of traditional heat treatment methods and can well complement the existing processing and treatment process systems. Summary of the Invention

[0006] The present invention provides a surface-strengthened SiC / Al composite material and a preparation method thereof to address the above problems. By adding SiC particles to 7075 aluminum alloy powder, hot rolling treatment is carried out after sintering, and high-frequency pulsed current post-treatment is used to prepare the surface-strengthened composite material.

[0007] To achieve the above object, the present invention adopts the following technical solutions: The present invention provides a preparation method of a surface-strengthened SiC / Al composite material, comprising the following steps: Step 1, select 7075 aluminum alloy powder and SiC particles as raw materials, and fully mix and homogenize the 7075 aluminum alloy powder and SiC particles by ball milling to obtain a binary mixed powder of 7075 aluminum alloy powder and SiC particles; Step 2, dry the obtained binary mixed powder; Step 3, load the dried binary mixed powder into a graphite mold for pre-pressing; Step 4, prepare a blank from the pre-pressed binary mixed powder by powder metallurgy; Step 5, polish and clean the prepared blank to obtain a composite material sample; Step 6, first perform hot rolling treatment on the composite material sample, and then apply high-frequency pulsed current; Step 7, perform sample embedding and polishing treatment on the sample obtained in Step 6 to obtain the surface-strengthened SiC / Al composite material.

[0008] Further, in Step 1, the particle size range of the 7075 aluminum alloy powder is 5 μm to 25 μm, and the particle size of the SiC particles is 8 μm.

[0009] Further, in Step 1, fully mixing and homogenizing the 7075 aluminum alloy powder and SiC particles by ball milling specifically includes: Set the mass fraction of SiC particles to 5% and the mass fraction of 7075 aluminum alloy powder to 95%. Add 304 stainless steel balls with different diameters of 2 mm, 5 mm, 8 mm, and 10 mm into the ball milling tank at the same time. Set the rotation speed to 200 rad / min, and set the ball mill to rotate for 45 min and stop for 15 min, with a total ball milling time of 12 h, including the rotation time and the stop time. During the ball milling process, argon gas protection is used throughout to obtain a binary mixed powder of 7075 aluminum alloy powder and SiC particles.

[0010] Further, in Step 2, drying the obtained binary mixed powder specifically includes: The obtained binary mixed powder is dried in vacuum with a vacuum degree < 5 Pa, a drying temperature of 120 - 140 °C, and a drying time of 2 - 4 h. After drying, 304 stainless steel balls are taken out and reserved.

[0011] Furthermore, in step 3, the dried binary mixed powder is loaded into a graphite mold for pre - pressing, specifically: Weigh the dried binary mixed powder and load it into a cylindrical graphite mold with a diameter of Φ30 mm. After loading, place a graphite paper on it, compact and seal it with a matching graphite punch, and then put the graphite mold between the upper and lower punches in the discharge plasma sintering furnace cavity.

[0012] Furthermore, in step 4, the binary mixed powder after pre - pressing is used to prepare a blank by powder metallurgy method, specifically: Use a discharge plasma sintering furnace to sinter the binary mixed powder after pre - pressing. Adjust the position of the graphite mold, insert a thermocouple into the temperature - measuring hole, close the discharge plasma sintering furnace to evacuate, adjust the external load and current for vacuum sintering, and prepare a blank; the process parameters of the vacuum sintering are set as follows: the maximum sintering temperature is 530 - 540 °C, the sintering pressure is 40 MPa, and the vacuum degree ≤ 5 Pa.

[0013] Furthermore, in step 5, the prepared blank is polished and cleaned, specifically: Place the prepared blank on a steel plate and polish the periphery and surface successively with sandpapers of 400 mesh, 800 mesh, 1000 mesh, and 1500 mesh. After polishing, perform polishing and ultrasonic cleaning; the frequency of the ultrasonic cleaning is 28 KHz - 40 KHz, and the duration is 2 - 4 min to obtain a composite material sample.

[0014] Furthermore, in step 6, the composite material sample is first subjected to hot rolling treatment and then high - frequency pulsed current is applied, specifically: Heat the composite material sample to 300 °C and then roll it into a plate in multiple passes with a single - pass reduction of 5%. Prepare a sample by wire - cut electrical discharge machining, and then apply a high - frequency pulsed current to the sample. Set the processing parameters of the high - frequency pulsed current as any one of the three current frequencies of 5000 Hz, 10000 Hz, and 15000 Hz, and the current intensity is 500 A. During the whole process, the surface temperature of the sample is measured in real - time through a thermocouple.

[0015] Furthermore, in step 7, the sample obtained in step 6 is subjected to sample mounting and polishing treatment, specifically: The sample is embedded using a metallographic sample embedding machine. Subsequently, the sample is polished successively with sandpapers of 400 mesh, 800 mesh, 1000 mesh, 1200 mesh, 1500 mesh, and 2000 mesh to remove surface oxides and scratches. Then, the sample is further polished using a polishing machine. After polishing, the surface of the sample is rinsed with absolute ethanol and dried.

[0016] The present invention also provides a surface-strengthened SiC / Al composite material, which is prepared by using the preparation method of the surface-strengthened SiC / Al composite material.

[0017] Compared with the prior art, the present invention has the following advantages: 1. The present invention proposes an innovative surface strengthening method for the characteristics of SiC / 7075Al composite materials, which can comprehensively improve the surface mechanical properties of metal matrix composite materials, break the contradiction between internal strength and toughness of metal matrix composite materials, and further improve the working reliability of metal matrix composite materials to meet the major needs of modern industry.

[0018] 2. The use of high-frequency pulsed current post-treatment in the present invention has the following effects: (1) The high-frequency pulsed current can provide sufficient energy in an extremely short time, enabling dislocations to break free from the pinning effects of reinforcement phases, precipitation phases, phase boundaries, and grain boundaries. Partial or even complete recrystallization will occur inside the metal matrix composite material, so that the grains and stress concentration regions inside the metal matrix composite material are replaced by fine recrystallized grains, and at the same time, the residual stress is released. (2) Under the action of the electron wind force and high-frequency pulsed current, the movement of dislocations can accelerate the generation of dislocation tangles and dislocation pile-ups. The piled-up dislocations can serve as nucleation points for recrystallization, thereby forming finer grains and achieving the purpose of grain refinement. (3) The high-frequency pulsed current mainly makes the atoms and structures inside the metal in an active excited state by inputting high-frequency current, accelerating atomic diffusion and dislocation movement, and reducing the energy barrier of phase transformation. Physical and chemical reactions such as recrystallization, generation and dissolution of precipitation phases, and healing of microcracks occur under the action of the current, making the entire metal interior tend to be stable. At the same time, the residual stress serves as a driving force in these physical and chemical reactions. As the reaction progresses, the metal interior becomes more stable, the texture of the metal matrix composite material also changes, the residual stress is continuously consumed, and finally the residual stress inside the metal is greatly weakened.

[0019] 3. When using traditional heat treatment techniques to reduce the microdefects of metal matrix composites, relatively high temperatures or long durations are required, which not only increase production costs but also easily lead to the transformation of the metallographic structure and a decrease in the strength of metal matrix composites, resulting in a coarsened surface structure. However, high-frequency pulsed current treatment can efficiently reduce the dislocation density and tissue strain degree in metal matrix composites, refine the surface grains of metal matrix composites, and has the following advantages: (1) Improve the macroscopic properties such as surface hardness, toughness, and wear resistance of metal matrix composites. At the same time, compared with heat treatment, the transformation temperature of microdefects such as dislocations under the action of high-frequency pulsed current is lower, which is conducive to maintaining the metallographic structure of metal matrix composites. (2) The combined action of the thermal effect and non-thermal effect of high-frequency pulsed current can promote the healing of microcracks inside the metal matrix. By using the flow concentration and Joule heat effect of high-frequency pulsed current, crack propagation can be blocked, the crack tip morphology and local microstructure can be improved, and the mechanical properties of metal matrix composites can be enhanced. Description of the Drawings

[0020] Figure 1 Schematic diagram of applying high-frequency pulsed current to the present invention.

[0021] Figure 2 Grain size diagrams of the SiC / Al composite material of the present invention in four different states (rolled state, high-frequency pulsed current treatment at 5000 Hz, high-frequency pulsed current treatment at 10000 Hz, and high-frequency pulsed current treatment at 15000 Hz).

[0022] Figure 3 Crack morphology diagram of the SiC / Al composite material of the present invention before high-frequency pulsed current treatment.

[0023] Figure 4 Crack morphology diagram of the SiC / Al composite material of the present invention after high-frequency pulsed current treatment Figure 5 Stress-strain curve diagrams of the SiC / Al composite material of the present invention in four different states (rolled state, high-frequency pulsed current treatment at 5000 Hz, high-frequency pulsed current treatment at 10000 Hz, and high-frequency pulsed current treatment at 15000 Hz).

[0024] Figure 6 Hardness and elastic modulus diagrams of the SiC / Al composite material of the present invention in five different states (sintered state, rolled state, high-frequency pulsed current treatment at 5000 Hz, high-frequency pulsed current treatment at 10000 Hz, and high-frequency pulsed current treatment at 15000 Hz). Detailed Description of the Invention

[0025] In order to further elaborate the technical solution of the present invention, the present invention will be further described below through embodiments. Example 1

[0026] A preparation method of a surface-strengthened SiC / Al composite material in this embodiment includes the following steps: Step 1, select 7075 aluminum alloy powder and SiC particles as raw materials, and fully mix the 7075 aluminum alloy powder and SiC particles evenly by ball milling to obtain a binary mixed powder of 7075 aluminum alloy powder and SiC particles; Among them, the particle size range of the 7075 aluminum alloy powder is 5μm - 25μm, and the particle size of the SiC particles is 8μm; Set the mass fraction of SiC particles to 5%, and the mass fraction of 7075 aluminum alloy powder to 95%. The ball milling tank is fixedly clamped on a planetary ball mill for high-energy ball milling and powder mixing. 304 stainless steel balls with different diameters, namely 2mm, 5mm, 8mm, and 10mm, are added into the ball milling tank at the same time. The rotation speed of the ball mill is set to 200rad / min, and the ball mill is set to rotate for 45min and stop for 15min, with a total ball milling time of 12h, including the rotation time and the stop time. Argon gas protection is adopted during the ball milling process to obtain a binary mixed powder of 7075 aluminum alloy powder and SiC particles, in which the SiC particles are dispersedly distributed and completely embedded inside the 7075 aluminum alloy powder.

[0027] Step 2, perform drying treatment on the obtained binary mixed powder; Place the obtained binary mixed powder in a drying oven for vacuum drying. The vacuum degree is <5Pa, the drying temperature is 120°C, and the drying time is 4h. After drying, take out the 304 stainless steel balls and set aside.

[0028] Step 3, load the dried binary mixed powder into a graphite mold for pre-pressing; Weigh 1000g of the dried binary mixed powder and load it into a cylindrical graphite mold with a diameter of Φ30mm. After loading, pad graphite paper to seal to prevent powder leakage and conduct electricity. Then, compact and seal it with a matching graphite punch, and then place the graphite mold between the upper and lower punches in the cavity of a spark plasma sintering furnace.

[0029] Step 4, prepare a blank from the pre-pressed binary mixed powder by powder metallurgy; Use a spark plasma sintering furnace to sinter the pre-pressed binary mixed powder. Adjust the position of the graphite mold, insert a thermocouple into the temperature measurement hole, measure the sintering temperature with the thermocouple with an accuracy of 1°C, close the spark plasma sintering furnace for vacuum pumping, adjust the external load and current for vacuum sintering to prepare a blank. The size of the blank is Φ30mm×5mm. The process parameters of the vacuum sintering are set as follows: the highest sintering temperature is 540°C, the sintering pressure is 40MPa, and the vacuum degree is ≤5Pa.

[0030] Step 5: Grind and clean the prepared blank to obtain a composite material sample; Place the prepared blank on a steel flat plate, and successively grind the periphery and surface with 400-mesh, 800-mesh, 1000-mesh, and 1500-mesh sandpapers. After grinding, perform polishing and ultrasonic cleaning. The frequency of the ultrasonic cleaning is 40KHz and the duration is 2min to obtain a composite material sample.

[0031] Step 6: First perform hot rolling treatment on the composite material sample, and then apply a high-frequency pulsed current (as Figure 1 shown); Heat the composite material sample with a KSW-6-12AS box-type resistance furnace. Heat the composite material sample in the resistance furnace to 300°C and then roll it into a plate in 8 passes (heat the composite material sample to 300°C for each pass of rolling). The single-pass reduction is 5%, that is, the total reduction is 40%. The thickness of the composite material sample is reduced from 5mm to 3mm. Prepare three samples with dimensions of 40mm×15mm×3mm by wire electrical discharge machining. Subsequently, apply a high-frequency pulsed current to these three samples respectively. Set the processing parameters of the high-frequency pulsed current to three current frequencies of 5000Hz, 10000Hz, and 15000Hz, and the current intensity is 500A. During the whole process, the surface temperature of the sample is measured in real time through a thermocouple.

[0032] Step 7: Mount and polish the sample obtained in Step 6 to obtain a surface-strengthened SiC / Al composite material; Use a metallographic sample mounting machine to mount the sample. Subsequently, the sample is successively ground for surface oxides and scratches with 400-mesh, 800-mesh, 1000-mesh, 1200-mesh, 1500-mesh, and 2000-mesh sandpapers to obtain a relatively smooth surface. In order to further grind the surface scratches of the sample, polish the sample with a polishing machine. After polishing, rinse the surface with anhydrous ethanol and dry it with a hair dryer. Example 2

[0033] A preparation method of a surface-strengthened SiC / Al composite material in this example includes the following steps: Step 1: Select 7075 aluminum alloy powder and SiC particles as raw materials, and fully mix and homogenize the 7075 aluminum alloy powder and SiC particles by ball milling to obtain a binary mixed powder of 7075 aluminum alloy powder and SiC particles; Among them, the particle size range of the 7075 aluminum alloy powder is 5μm~25μm, and the particle size of the SiC particles is 8μm; Set the mass fraction of SiC particles to 5% and the mass fraction of 7075 aluminum alloy powder to 95%. Fix and clamp the ball milling tank on a planetary ball mill for high-energy ball milling and powder mixing. Add 304 stainless steel balls with different diameters of 2 mm, 5 mm, 8 mm, and 10 mm into the ball milling tank at the same time. Set the rotation speed of the ball mill to 200 rad / min, and set the ball mill to rotate for 45 min and stop for 15 min, with a total ball milling time of 12 h, including the rotation time and the stop time. During the ball milling process, argon gas protection is used throughout to obtain a binary mixed powder of 7075 aluminum alloy powder and SiC particles, with the SiC particles dispersedly distributed and completely embedded inside the 7075 aluminum alloy powder.

[0034] Step 2, perform a drying treatment on the obtained binary mixed powder; Place the obtained binary mixed powder in a drying oven for vacuum drying. The vacuum degree is <5 Pa, the drying temperature is 130 °C, and the drying time is 3 h. After drying, take out the 304 stainless steel balls and set aside for use.

[0035] Step 3, load the dried binary mixed powder into a graphite mold for pre-pressing; Weigh 1000 g of the dried binary mixed powder and load it into a cylindrical graphite mold with a diameter of Φ30 mm. After loading, pad with graphite paper to seal to prevent powder leakage and conduct electricity. Then, compact and seal it with a matching graphite punch, and then place the graphite mold between the upper and lower punches in the cavity of the spark plasma sintering furnace.

[0036] Step 4, prepare a blank from the pre-pressed binary mixed powder by powder metallurgy; Use a spark plasma sintering furnace to sinter the pre-pressed binary mixed powder. Adjust the position of the graphite mold, insert a thermocouple into the temperature measurement hole, measure the sintering temperature with the thermocouple with an accuracy of 1 °C. Close the spark plasma sintering furnace to evacuate, adjust the external load and current for vacuum sintering to prepare a blank with a size of Φ30 mm × 5 mm. The process parameters of the vacuum sintering are set as follows: the highest sintering temperature is 535 °C, the sintering pressure is 40 MPa, and the vacuum degree is ≤5 Pa.

[0037] Step 5, grind and clean the prepared blank to obtain a composite material sample; Place the prepared blank on a steel plate and successively grind the periphery and surface with sandpapers of 400 mesh, 800 mesh, 1000 mesh, and 1500 mesh. After grinding, perform polishing and ultrasonic cleaning. The frequency of the ultrasonic cleaning is 35 KHz and the duration is 3 min to obtain a composite material sample.

[0038] Step 6, first perform hot rolling treatment on the composite material sample and then apply a high-frequency pulsed current; The composite material sample was heated using a KSW-6-12AS box-type resistance furnace. After heating the composite material sample to 300 °C in the resistance furnace, it was rolled into a plate in 8 passes, with a single-pass reduction of 5%, that is, the total reduction was 40%. The thickness of the composite material sample was pressed from 5 mm to 3 mm. A sample with dimensions of 40 mm × 15 mm × 3 mm was prepared by wire electrical discharge machining. Subsequently, a high-frequency pulsed current was applied to the sample, and the processing parameters of the high-frequency pulsed current were set to any one of three current frequencies: 5000 Hz, 10000 Hz, and 15000 Hz, with a current intensity of 500 A. During the entire process, the surface temperature of the sample was measured in real time using a thermocouple.

[0039] Step 7: The sample obtained in step 6 was subjected to sample embedding and polishing treatments to obtain a surface-strengthened SiC / Al composite material. The sample was subjected to sample embedding treatment using a metallographic sample embedding machine. Subsequently, the sample was successively polished with sandpapers of 400 mesh, 800 mesh, 1000 mesh, 1200 mesh, 1500 mesh, and 2000 mesh to remove surface oxides and scratches, obtaining a relatively smooth surface. To further polish the surface scratches of the sample, the sample was polished using a polishing machine. After polishing, the surface was rinsed with anhydrous ethanol and dried with a hair dryer. Example 3

[0040] A preparation method of a surface-strengthened SiC / Al composite material in this example includes the following steps: Step 1: 7075 aluminum alloy powder and SiC particles were selected as raw materials, and the 7075 aluminum alloy powder and SiC particles were fully mixed evenly by ball milling to obtain a binary mixed powder of 7075 aluminum alloy powder and SiC particles. Among them, the particle size range of the 7075 aluminum alloy powder was 5 μm to 25 μm, and the particle size of the SiC particles was 8 μm. The mass fraction of the SiC particles was set to 5%, and the mass fraction of the 7075 aluminum alloy powder was 95%. The ball milling tank was fixed and clamped on a planetary ball mill for high-energy ball milling and powder mixing. 304 stainless steel balls with different diameters, namely 2 mm, 5 mm, 8 mm, and 10 mm, were added to the ball milling tank simultaneously. The rotation speed of the ball mill was set to 200 rad / min, and the ball mill was set to rotate for 45 min and stop for 15 min, with a total ball milling time of 12 h, including the rotation time and the stop time. During the ball milling process, argon gas protection was used throughout, obtaining a binary mixed powder of 7075 aluminum alloy powder and SiC particles, with the SiC particles diffusely distributed and completely embedded inside the 7075 aluminum alloy powder.

[0041] Step 2: The obtained binary mixed powder was subjected to drying treatment. Place the obtained binary mixed powder in a drying oven and conduct vacuum drying. The vacuum degree is < 5 Pa, the drying temperature is 140 °C, and the drying time is 2 h. After drying, take out the 304 stainless steel balls and set aside.

[0042] Step 3: Load the dried binary mixed powder into a graphite mold for pre-pressing. Weigh 1000 g of the dried binary mixed powder and load it into a cylindrical graphite mold with a diameter of Φ30 mm. After loading, pad with graphite paper to seal to prevent powder leakage and conduct electricity. Then, compact and seal it with a matching graphite punch, and place the graphite mold between the upper and lower punches in the discharge plasma sintering furnace cavity.

[0043] Step 4: Prepare a blank from the pre-pressed binary mixed powder by powder metallurgy. Use a discharge plasma sintering furnace to sinter the pre-pressed binary mixed powder. Adjust the position of the graphite mold, insert a thermocouple into the temperature measurement hole, measure the sintering temperature with the thermocouple with an accuracy of 1 °C, close the discharge plasma sintering furnace to evacuate, adjust the external load and current for vacuum sintering, and prepare a blank. The size of the blank is Φ30 mm × 5 mm. The process parameters of the vacuum sintering are set as follows: the highest sintering temperature is 530 °C, the sintering pressure is 40 MPa, and the vacuum degree is ≤ 5 Pa.

[0044] Step 5: Grind and clean the prepared blank to obtain a composite material sample. Place the prepared blank on a steel plate and grind the perimeter and surface successively with 400-mesh, 800-mesh, 1000-mesh, and 1500-mesh sandpapers. After grinding, conduct polishing and ultrasonic cleaning. The frequency of the ultrasonic cleaning is 28 KHz and the duration is 4 min to obtain a composite material sample.

[0045] Step 6: First, conduct hot rolling on the composite material sample, and then apply a high-frequency pulsed current. Use a KSW-6-12AS box-type resistance furnace to heat the composite material sample. Heat the composite material sample in the resistance furnace to 300 °C and then roll it into a plate in 8 passes. The single-pass reduction is 5%, that is, the total reduction is 40%. The thickness of the composite material sample is reduced from 5 mm to 3 mm. Prepare a sample with dimensions of 40 mm × 15 mm × 3 mm by wire electrical discharge machining. Then, apply a high-frequency pulsed current to the sample. Set the processing parameters of the high-frequency pulsed current to any one of the three current frequencies of 5000 Hz, 10000 Hz, and 15000 Hz, and the current intensity is 500 A. During the whole process, the surface temperature of the sample is measured in real time through a thermocouple.

[0046] Step 7: Conduct sample mounting and polishing on the sample obtained in Step 6 to obtain a surface-strengthened SiC / Al composite material. The sample is embedded using a metallographic sample embedding machine, and then the sample is polished successively with sandpapers of 400 mesh, 800 mesh, 1000 mesh, 1200 mesh, 1500 mesh, and 2000 mesh to remove surface oxides and scratches, obtaining a relatively smooth surface. To further polish the surface scratches of the sample, the sample is polished using a polishing machine. After polishing, the surface is rinsed with anhydrous ethanol and dried with a hair dryer. Example 4

[0047] The difference from Example 1 lies in that no high-frequency pulsed current is applied in Step 6, that is: the composite material sample is heated using a KSW-6-12AS box-type resistance furnace, and after heating the composite material sample to 300 °C in the resistance furnace, it is rolled into a plate for 8 passes, with a single-pass reduction of 5%, that is, the total reduction is 40%. The thickness of the composite material sample is reduced from 5 mm to 3 mm, and a sample with dimensions of 40 mm × 15 mm × 3 mm is prepared by wire electrical discharge machining.

[0048] Microstructure analysis, tensile property testing, and microhardness testing are carried out on the SiC / Al composites prepared in Example 1 and Example 4. Specifically: the SiC / Al composites are placed under an electron backscatter diffraction instrument for observation to observe the distribution state of SiC, the grain morphology, KAM, and the changes in parameters such as the large and small angle grain boundaries of the composite material surface structure before and after applying the high-frequency pulsed current. Further, the morphology changes of the microcracks in the composite material before and after applying the high-frequency pulsed current are observed through a scanning electron microscope. The mechanical properties (tensile strength, yield strength, and elongation after fracture) of the composite material in different states are measured. To ensure the accuracy of the test results during the tensile process, the tensile rate is set to 0.2 mm / min, and each tensile test is carried out five times. Finally, the average value of the five measurements is defined as the tensile property of the composite material in this state. Finally, the microhardness of the composite material is measured using a nanoindentation tester.

[0049] According to Figure 2 It shows that the grains of the rolled state prepared in Example 4 have obvious anisotropy, and the grains have obvious abnormal growth along the rolling deformation direction. Compared with Example 4, the grain refinement phenomenon in Example 1 is most obvious after being treated with a 10000 Hz high-frequency pulsed current. After applying the high-frequency pulsed current, fine grains are evenly distributed over a large area on the surface and inside of the SiC / Al composite material, indicating that the high-frequency pulsed current plays an obvious role in grain refinement. By comparing the grain sizes in the four states, the grain size is the smallest after being treated with a 10000 Hz high-frequency pulsed current, and the average grain size is 2.0 μm.

[0050] According to Figure 3 It shows that before the high-frequency pulsed current treatment, there is almost no healing zone in the microcracks, and there is also a certain height difference at both ends of the crack, which is caused by the local shear force.

[0051] According to Figure 4 It is shown that after the treatment with high-frequency pulsed current, the resistivity at the internal microcracks is larger than that at the matrix, and the high-frequency pulsed current will generate a skin effect at the microcracks. At this time, the Joule heat effect and non-thermal effect of the high-frequency pulsed current cause high temperature at the cracks, even exceeding the melting point of the matrix. The two crack surfaces are subjected to a large thermal compressive stress, causing the microcracks to approach each other, and finally the healing of the microcracks is realized.

[0052] According to Figure 5 It is shown that the SiC / Al composite material has excellent mechanical properties of high strength and high plasticity. Before fracture, finer grains in the SiC / Al composite material will consume more energy, showing stronger tensile strength and toughness.

[0053] According to Figure 6 It is shown that compared with the sintered composite material, the strength and elastic modulus of the composite material after hot rolling treatment have been greatly improved. This is because a large number of dislocations and partial fine grains are introduced after rolling deformation, and the dislocation strengthening contributes mainly. With the treatment of high-frequency pulsed current, the current frequency continuously increases, the dislocations are partially consumed, the hardness is partially reduced, and at the same time the grains are partially refined. The grain size reaches the finest at 10,000 Hz, and at this time the elastic modulus of the composite material is improved to a certain extent, reflecting the good plasticity of the composite material in this state. At 15,000 Hz, the current thermal effect causes grain growth and the elastic modulus decreases to a certain extent.

[0054] The above shows and describes the main features and advantages of the present invention. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or basic features of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, it is intended to include all changes falling within the meaning and scope of the equivalent elements of the claims in the present invention.

[0055] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a surface-strengthened SiC / Al composite material, characterized in that: The following steps are involved: Step 1, selecting 7075 aluminum alloy powder and SiC particles as raw materials, and fully mixing the 7075 aluminum alloy powder and the SiC particles by ball milling to obtain a binary mixed powder of the 7075 aluminum alloy powder and the SiC particles; Step 2, drying the obtained binary mixed powder; Step 3, loading the dried binary mixed powder into a graphite mold for pre-pressing; Step 4, preparing a blank from the binary mixed powder after pre-pressing by powder metallurgy; Step 5, grinding and cleaning the prepared blank to obtain a composite material sample; Step 6, first hot rolling the composite material sample, and then applying a high-frequency pulse current; Step 7, mounting and polishing the sample obtained in step 6 to obtain a surface-strengthened SiC / Al composite material.

2. The method for preparing a surface-strengthened SiC / Al composite material according to claim 1, characterized in that: In the step 1, the particle size of the 7075 aluminum alloy powder ranges from 5 μm to 25 μm, and the particle size of the SiC particles is 8 μm.

3. The method for preparing a surface-strengthened SiC / Al composite material according to claim 2, characterized in that: In step 1, the 7075 aluminum alloy powder and the SiC particles are fully mixed and uniformly mixed by ball milling, specifically: The mass fraction of SiC particles was set to 5%, and the mass fraction of 7075 aluminum alloy powder was set to 95%. 304 stainless steel balls of different diameters were added to the ball mill at the same time, with diameters of 2 mm, 5 mm, 8 mm and 10 mm respectively. The rotation speed was set to 200 rad / min, and the rotation time was set to 45 min and the stop time was set to 15 min. The ball milling was carried out for a total of 12 h, including the rotation time and the stop time. Argon gas protection was used during the ball milling process to obtain a binary mixed powder of 7075 aluminum alloy powder and SiC particles.

4. The method for preparing a surface-strengthened SiC / Al composite material according to claim 3, characterized in that: The binary mixed powder obtained in step 2 is dried, specifically: The obtained binary mixed powder is vacuum dried with a vacuum degree of <5 Pa, a drying temperature of 120-140° C., and a drying time of 2-4 h. After drying, the 304 stainless steel ball is taken out and set aside.

5. The method for preparing a surface-strengthened SiC / Al composite material according to claim 1, characterized in that: In step 3, the dried binary mixed powder is loaded into a graphite mold for pre-pressing, specifically: Weigh the dried binary mixed powder and load it into a Φ30mm cylindrical graphite mold. After loading, pad it with graphite paper, compact and seal it with a matching graphite press, and then put the graphite mold between the upper and lower press heads in the spark plasma sintering furnace chamber.

6. The method for preparing a surface-strengthened SiC / Al composite material according to claim 1, characterized in that: The binary mixed powder after pre-pressing in step 4 is used to prepare a blank by powder metallurgy, specifically: The binary mixed powder after pre-pressing is sintered in a spark plasma sintering furnace, the position of the graphite mold is adjusted, a thermocouple is inserted into the temperature measuring hole, the spark plasma sintering furnace is closed for vacuuming, and the external load and current are adjusted for vacuum sintering to prepare a blank; the process parameters of the vacuum sintering are set as follows: the maximum sintering temperature is 530-540°C, the sintering pressure is 40MPa, and the vacuum degree is ≤5Pa.

7. The method for preparing a surface-strengthened SiC / Al composite material according to claim 1, characterized in that: In step 5, the prepared blank is polished and cleaned, specifically: The prepared blank was placed on a steel plate, and the periphery and surface were polished with 400 mesh, 800 mesh, 1000 mesh and 1500 mesh sandpaper in turn. After polishing, polishing and ultrasonic cleaning were performed; the frequency of the ultrasonic cleaning was 28KHz~40KHz, and the duration was 2~4min to obtain a composite material sample.

8. The method for preparing a surface-strengthened SiC / Al composite material according to claim 1, characterized in that: In step 6, the composite material sample is first subjected to hot rolling treatment, and then a high-frequency pulse current is applied, specifically: The composite material samples were heated to 300°C and then rolled into plates in multiple passes with a single pressing amount of 5%. The samples were prepared by wire electrospark cutting. Subsequently, high-frequency pulse current was applied to the samples. The processing parameters of the high-frequency pulse current were set to any one of the three current frequencies of 5000 Hz, 10000 Hz, and 15000 Hz, and the current intensity was 500 A. During the whole process, the temperature of the sample surface was measured in real time by thermocouples.

9. The method for preparing a surface-strengthened SiC / Al composite material according to claim 1, characterized in that: In step 7, the sample obtained in step 6 is mounted and polished, specifically: The samples were mounted using a metallographic sample mounting machine, and then the surface oxides and scratches were polished using 400 mesh, 800 mesh, 1000 mesh, 1200 mesh, 1500 mesh, and 2000 mesh sandpapers in turn. The samples were further polished using a polishing machine. After polishing, the surface was rinsed with anhydrous ethanol and dried.

10. A surface-strengthened SiC / Al composite material, characterized in that: The surface-reinforced SiC / Al composite material is prepared by the preparation method of any one of claims 1 to 9.

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