CN118951015A

CN118951015A - A method for preparing carbon nanotube reinforced aluminum matrix composite material by heat treating matrix powder

Info

Publication number: CN118951015A
Application number: CN202411179235.1A
Authority: CN
Inventors: 陈彪; 曹遴; 李金山; 唐斌; 王军; 袁睿豪; 王毅
Original assignee: Northwestern Polytechnical University
Current assignee: Northwestern Polytechnical University
Priority date: 2024-08-27
Filing date: 2024-08-27
Publication date: 2024-11-15

Abstract

The invention discloses a method for preparing a carbon nano tube reinforced aluminum matrix composite material by heat treatment of matrix powder, wherein the composite material powder specifically comprises metal matrix powder and a reinforcement body, the reinforcement body is usually a one-dimensional or two-dimensional reinforcement body, and the matrix has good plastic deformation capability and has a work hardening phenomenon in the ball milling process. The aim of further improving the content of the reinforcement in the crystal and optimizing the distribution of the reinforcement is fulfilled by carrying out vacuum heat treatment on the specific state of the matrix powder in the high-energy ball milling process, and the intra-crystal reinforced metal matrix composite is obtained. Taking carbon nano tube reinforced aluminum matrix composite (CNTs/Al) prepared by the heat treatment process as an example, the powder CNTs are uniformly distributed, and compared with the composite obtained by the traditional heat treatment process, the strength and plasticity of the obtained composite are improved simultaneously, wherein the strength is improved by 13.4 percent, and the plasticity is improved by 57.5 percent.

Description

Method for preparing carbon nano tube reinforced aluminum matrix composite material by heat treatment of matrix powder

Technical Field

The invention belongs to the technical field of preparing an intra-crystal reinforced aluminum matrix composite material by a powder metallurgy method, and particularly relates to a method for preparing a carbon nano tube reinforced aluminum matrix composite material by heat treatment of matrix powder.

Background

The metal matrix composite material has the performances of strong plasticity, electric conduction, heat conduction, corrosion resistance and the like of the matrix material, and simultaneously, the excellent performances of the reinforcement, such as low density, ultrahigh room temperature, high temperature strength and modulus, excellent electric conduction, heat conduction and the like are utilized, so that the metal matrix composite material gradually plays an increasingly important role in the important national fields. The aluminum-based composite material is widely applied to the fields of aerospace, weapon and soldiers and the like due to the characteristics of light weight and high strength, but researchers expect to obtain an aluminum-based composite material with more excellent performance along with the continuous rigor of service conditions, and one of main ways for realizing the aim is to select a reinforcement with excellent performance. Since carbon nanotube CNTs have been found to have extremely excellent strength (up to 100 GPa), excellent mechanical properties such as ultra-high elastic modulus (about 1 TPa), and the like, carbon nanotube reinforced aluminum-based composite materials are ideal reinforcements for composite materials, and are expected to become a new generation of light-weight high-strength aluminum-based composite materials because of the excellent room-temperature and high-temperature mechanical properties.

At present, the preparation of the CNTs/Al composite material has a certain difficulty, and the CNTs are dispersed by a high-energy ball milling method due to the non-coherent interface between the CNTs and the Al matrix, but the distribution of the CNTs is mainly at the boundary of crystal grains. Under the action of high-speed shearing and compression stress of ball milling balls in the ball milling process, the difficulty of entering CNTs into the crystal can be increased by gradually reducing aluminum powder crystal grains, and meanwhile, the damage degree of CNTs structure can be increased as the ball milling time is longer. And the research aiming at obtaining the intra-crystalline dispersion CNTs/Al-based composite material powder is less at present, and CNTs in the prepared CNTs/Al powder are mainly dispersed at a crystal boundary, so that the strong plasticity of the CNTs/Al-based composite material is not facilitated, and the CNTs content is improved, and the comprehensive mechanical property of the CNTs/Al is greatly improved. There is an urgent need to prepare CNTs/Al composite powder with high content of CNTs and uniform distribution.

Disclosure of Invention

Planetary ball milling is used as a representative of a high-energy ball milling method, is suitable for uniform dispersion of high-content reinforcing bodies, and overcomes the defect that CNTs are difficult to enter the inside of grains due to the fact that the grains of a matrix are suddenly reduced in the ball milling process in order to improve the content of CNTs entering the grains. According to the invention, the ball-milling thick plate-shaped powder is subjected to heat treatment, so that the growth of matrix grains is controlled, the influence of powder work hardening is eliminated, and meanwhile, the thick plate-shaped powder can be continuously subjected to plastic deformation and has a larger surface area, thereby being beneficial to good dispersion of CNTs. CNTs are added on the basis after heat treatment, and ball milling is carried out until reaching the target time. Through the structure regulation and control of the metal matrix powder, the dispersing capability of CNTs and the possibility of dispersing CNTs into the inside of crystal grains are improved, and a foundation is laid for preparing the high-performance CNTs/Al composite material.

The invention specifically provides a method for preparing a carbon nano tube reinforced aluminum matrix composite material by heat treatment of matrix powder, which comprises the following steps of: performing primary ball milling on a certain amount of metal matrix powder and a process control agent for 4 hours, performing high-temperature vacuum heat treatment on the thick sheet-shaped matrix powder after ball milling, and then adding a certain amount of carbon nano tube reinforcement to perform secondary ball milling for 20 hours to obtain composite material powder;

shaping of the composite material: and carrying out plasma sintering on the composite material powder, and carrying out hot extrusion to obtain the composite material bar with smooth surface and no cracks.

Specifically, the content of carbon nanotubes in the composite material is 2 wt% by mass, and the content of the metal matrix powder is 98% by mass wt% by mass.

Specifically, the metal powder is spherical pure aluminum powder with the diameter of 45 μm, the carbon nanotube reinforcement is 20 nm μm in diameter and 2 μm in length, and the process control agent is stearic acid, which accounts for 1:1 wt% of the total amount of the composite material powder.

Specifically, the parameters of the primary ball milling and the secondary ball milling are that the rotating speed is 180-230 rpm/min, the ball milling mode is that the ball milling is carried out in a forward rotation mode of 10min, 10min and 10min are stopped, the reverse rotation is carried out in a circulating way, in the ball milling process, when the effective ball milling time reaches 2h, 1h is stopped, and the environment temperature is below 20 ℃.

Specifically, the temperature of the high-temperature vacuum heat treatment is 550 ℃, and the heat preservation time is 12 hours.

Specifically, the plasma sintering is carried out by pre-pressing at 0.5T in the process, the pressure maintaining time is not less than 10min, the vacuum degree is 1.5X10 ^-1 Pa, and the temperature is 590 ℃.

Specifically, the temperature in the hot extrusion process is 500 ℃, the heat preservation time is 20 min, the extrusion ratio is 18:1, and the extrusion speed is 4 mm/s.

Compared with the prior art, the invention has the following beneficial technical effects:

(1) The preparation process of the intra-crystal reinforcement composite material powder is simple and easy to operate, the obtained reinforcement is uniformly dispersed and well combined with a matrix, and the mechanical properties of the intra-crystal CNTs/Al composite material block prepared by the powder metallurgy method have obvious advantages compared with the traditional annealing heat treatment process;

(2) According to the invention, the structure of the matrix metal powder in the ball milling process is regulated and controlled, the oxidation of the powder is effectively prevented by combining vacuum heat treatment with titanium sponge, meanwhile, the grain size of the matrix structure is effectively improved by long-time heat treatment, good dispersion capability of CNTs is maintained on the basis of full growth of the grains, meanwhile, stress of the powder after ball milling is eliminated, so that CNTs in the ball milling process can enter the interior of large grains more easily, more process control agents (stearic acid) are added in the SPS sintering process in the further elimination of the ball milling process, and the structure is further densified by combining with the hot extrusion process, so that a compact and defect-free composite material block is finally obtained.

(3) Compared with the traditional stress relief annealing, the intra-crystalline CNTs/Al composite material prepared by the invention has obviously excellent strong plastic matching, and after the heat treatment process is added, compared with the traditional powder heat treatment stress relief process, the strength and the plasticity of the composite material block prepared by the traditional method are obviously improved in the design process under the condition that the effective time of ball milling of CNTs/Al composite material powder is reduced by 4 hours. Wherein the strength is improved by 13.4 percent, and the plasticity is improved by 57.5 percent. The invention has guiding significance in the stress relief annealing process in the ball milling process and the preparation of the high-performance aluminum-based composite material, so that the aluminum-based composite material has good application prospect in the fields of aerospace, weapon suit, transportation and the like.

Drawings

FIG. 1 is a schematic flow chart of the preparation of CNTs/Al composite powder according to example 1 of the present invention;

FIG. 2 is a scanned photograph of the morphology and the distribution of CNTs of the intra-crystalline CNTs/Al composite material prepared in example 1 of the present invention, wherein (a) is the morphology of the powder after 4 hours of pure aluminum ball milling, heat treatment and re-ball milling to 20 hours, (b) is the dispersion state of CNTs after 4 hours of pure aluminum ball milling, heat treatment and re-ball milling to 20 hours, (c) is the morphology of the powder after 4 hours of heat treatment and re-ball milling to 20 hours of CNTs/Al composite material, and (d) is the dispersion state of CNTs after 4 hours of ball milling and re-ball milling to 20 hours of CNTs;

FIG. 3 is an XRD phase analysis chart before and after heat treatment of the powder of the intragranular CNTs/Al composite material prepared in example 1 of the present invention;

FIG. 4 shows TEM data of crystal grain structure change of matrix powder before and after heat treatment in example 1 of the present invention, wherein (a) is TEM bright field phase of thick plate-like pure aluminum powder after ball milling for 4h before heat treatment, (b) is corresponding diffraction pattern of thick plate-like pure aluminum powder after ball milling for 4h before heat treatment, (c) is dark field pattern of thick plate-like pure aluminum powder after ball milling for 4h before heat treatment, (d) is TEM bright field phase of thick plate-like pure aluminum powder after heat treatment of ball milling for 4h, (e) is corresponding diffraction pattern of thick plate-like pure aluminum powder after heat treatment of thick plate-like pure aluminum powder after ball milling for 4h, (f) is dark field pattern of thick plate-like pure aluminum powder after heat treatment of ball milling for 4 h;

FIG. 5 is the mechanical property data of 2 wt% CNTs/Al composite blocks prepared in example 1 of the present invention and control.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present disclosure more apparent, the technical solutions of the embodiments of the present disclosure will be clearly and completely described below with reference to the accompanying drawings. It will be apparent that the described embodiments are some, but not all, of the embodiments of the present disclosure. All other embodiments, which can be made by those skilled in the art based on the described embodiments of the present disclosure without the need for creative efforts, are also within the scope of the protection of the present disclosure.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the presently disclosed subject matter belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the application. The appearances of the phrase "an embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Those of skill in the art will explicitly and implicitly appreciate that the embodiments described herein may be combined with other embodiments.

Aiming at the problems that reinforcements in the existing nano reinforcement metal matrix composite material prepared by powder metallurgy are easy to be distributed at grain boundaries, the performance improvement is limited and the like, the requirements for improving the intra-crystal content of the reinforcements to further improve the performance of the composite material are combined, and the researches on the ball milling of the composite material, the powder heat treatment, the block structure, the phase composition, the tensile property and the like are combined. Taking a nano reinforcement Carbon Nano Tube (CNTs) reinforced aluminum matrix composite material as an example, firstly starting from a matrix pure aluminum powder structure, performing vacuum heat treatment after ball milling until the powder is thick, regulating and controlling the grain size, and then adding CNTs for long-time ball milling to realize the processes of uniform dispersion and entering into crystals of the CNTs. Sintering and extruding the prepared CNTs/Al composite material powder, researching the phase composition of the powder and a block body and the change condition of matrix grains, and finally researching the room-temperature tensile property of the CNTs/Al composite material formed under the powder metallurgy condition.

The invention provides a method for preparing a carbon nano tube reinforced aluminum matrix composite material by heat treatment of matrix powder, which comprises the following steps of,

Preparing composite material powder: performing primary ball milling on a certain amount of metal matrix powder and a process control agent for 4 hours, performing high-temperature vacuum heat treatment on the thick sheet-shaped matrix powder after ball milling, and then adding a certain amount of carbon nano tube reinforcement to perform secondary ball milling for 20 hours to obtain composite material powder;

In the invention, aiming at the metal matrix powder with certain plastic deformation capability in the high-energy ball milling process, the metal matrix powder can be ensured to be deformed into ellipsoidal or flaky powder through the ball milling process, the content of an intragranular reinforcement is improved through a powder metallurgy method, and the principle of improving the comprehensive mechanical properties of the composite material is as follows: by carrying out heat treatment on the matrix metal powder in a specific state (thick sheet shape in the patent), crystal grains are obviously grown, on one hand, the powder work hardening caused by ball milling is reduced, and the capability of continuing deformation of the powder is obviously improved. On the other hand, a slab powder having a large grain structure has two advantages, namely, a large surface area of the slab powder can improve the dispersion ability of the nano-reinforcement, and a large grain is easier to make the nano-reinforcement enter the inside of the grain than a small grain in the ball milling process. Ball milling is carried out on the basis, and then sintering and hot extrusion processes of powder metallurgy are carried out, so that the reinforcing bodies in the finally obtained composite material are uniformly dispersed, the possibility that the reinforcing bodies are distributed in crystal grains is greatly improved, and the mechanical property of the composite material is improved.

The carbon nano tube content in the composite material is 2 percent by mass and wt percent by mass, and the content of the metal matrix powder is 98 percent by mass and wt percent by mass.

In the invention, the composite material powder is specifically uniformly dispersed carbon nano tube reinforced aluminum-based composite material powder, CNTs are uniformly dispersed and well combined with a matrix, the sintering temperature is controlled in the processes of vacuum heat treatment of the powder and SPS sintering, and the obtained material has low reaction quantity.

The metal powder is spherical pure aluminum powder with the diameter of 45 mu m, the carbon nano tube reinforcement is 20 nm mu m in diameter and 2 mu m in length, and the process control agent is stearic acid, wherein the process control agent accounts for 1wt% of the total amount of the composite material powder.

In the invention, in the powder treatment process, a 500 ml zirconia ball milling tank is adopted, in order to ensure the ball milling efficiency, the volume of the powder and the ball mill balls which are finally added into the tank is not more than 50% of the volume of the ball milling tank, firstly, the ball mill balls are added, then, the matrix metal powder and the process control agent are sequentially added, wherein the ball mill balls are made of zirconia, the size is 10mm, and the ball material ratio is 5:1.

The parameters of the primary ball milling and the secondary ball milling are that the rotating speed is 180-230 rpm/min, the ball milling mode is forward rotation 10 min, stop 10 min, reverse rotation 10 min, the process is sequentially and circularly carried out, and in the ball milling process, when the effective ball milling time reaches 2h, 1h is stopped, and the environment temperature is below 20 ℃.

In the ball milling process, a zirconia ball milling tank filled with matrix metal powder, a process control agent and ball milling balls is sealed, and high-purity argon is filled into the ball milling tank to discharge air by utilizing the characteristic that the density of the argon is higher than that of the air; when the air inlet is stopped, the valve of the air inlet is closed firstly, and then the valve of the air outlet is closed, and the relatively low air flow in the operation is beneficial to slowly discharging the air of the argon shielding gas, and the reduction of the powder quality is not caused.

The aim of the heat treatment is to enable the crystal grains of the metal matrix to grow up, and the carbon nano tube reinforcement body enters the inside of the crystal grains in the secondary ball milling process.

In the invention, the matrix powder ball-milled to thick flake is subjected to vacuum heat treatment, and the heat treatment temperature is set according to the following principle: the ball milling stress is removed and must not be lower than the annealing temperature of the material, if the material contains reinforcing body, the temperature must not be higher than the reaction temperature of the matrix material and the reinforcing body and not lower than the volatilization temperature of the process control agent, and quantitative process control agent must be added again in the subsequent continuous ball milling process, so as to keep the same amount of each addition, and the realization of repeated experiments is easy. Meanwhile, titanium sponge is placed around the powder in the heat treatment process, so that the powder is prevented from being oxidized. The target state after heat treatment is that the matrix grains are grown up, which is beneficial for the reinforcement to enter the grains in the subsequent ball milling process.

The temperature of the high-temperature vacuum heat treatment is 550 ℃, and the heat preservation time is 12h.

The plasma sintering is carried out by pre-pressing with the pressure of 0.5T, the pressure maintaining time is not less than 10min, the vacuum degree is 1.5 multiplied by 10 ^-1 Pa, and the temperature is 590 ℃.

In the invention, in the sintering process, the composite material powder obtained after ball milling is filled into a graphite mold with the diameter of 29.5mm, the pre-pressing is carried out by adopting the pressure of 0.5T, the pressure maintaining time is not less than 10min, and then the outer side of the graphite mold is wrapped with the high-temperature resistant fiber felt, so that the heat preservation effect of the mold in the sintering process is achieved. Loading the graphite mold into a plasma sintering equipment furnace for sintering; rapidly heating to the target sintering temperature of 590 ℃, and raising the pressure to 2.2T (30 MPa), wherein the vacuum degree is 1.5X10 ^-1 Pa, and the heat preservation time is 30min, so as to obtain a compact cylindrical block. The temperature measurement in the sintering process adopts K-type thermocouple contact type temperature measurement, so that the sensitivity and accuracy of temperature measurement are ensured.

The temperature in the hot extrusion process is 500 ℃, the heat preservation time is 20min, the extrusion ratio is 18:1, and the extrusion speed is 4 mm/s.

In the invention, in the hot extrusion process, firstly, the surface of a sample is polished by sand paper to remove oxide skin, high-temperature grease is smeared on the inner wall of an extrusion die and a sleeve, the sample is preserved for 20min at the temperature of 500 ℃ by adopting a box-type furnace, then the sample is immediately placed into extrusion equipment preheated to 370 ℃, the extrusion ratio is 18:1, the extrusion speed is 4mm/s, and finally, the composite material extrusion bar with the diameter of 7mm and smooth surface and no cracks is obtained.

The following examples illustrate the use of carbon nanotube reinforced aluminum matrix composite (CNTs/Al) powders with intra-crystalline distribution.

Raw materials, planetary ball milling, plasma sintering, hot extrusion and performance testing equipment were used in the following examples:

adopting a German fly Pulverisette 5 type planetary ball mill to prepare intragranular and grain boundary composite material powder;

preparing a composite material block in an SPS state by adopting Shanghai Chenhua SPS-20T-10-III ion body sintering equipment;

carrying out a vacuum heat treatment process on the sintered block sample by using a Henan adult instrument 1200 ℃ vacuum heat treatment tube furnace device;

Adopting Ningbopawal vertical hot extrusion equipment YP61-315 for a hot extrusion processing process of a sintered sample;

powder morphology and internal morphology characterization were performed using ZEISS Sigma 300, germany;

powder and block TEM analysis characterization is carried out by adopting a high-resolution transmission device of FEI Talos F200X in America;

performing XRD characterization analysis on the powder by using a D8 Discover X-ray diffractometer;

adopting an American INSTRON 3382 electronic universal tester to test the tensile property of the extruded intragranular and grain boundary distribution CNTs/Al composite material;

The powder is 45-micrometer spherical pure Al powder prepared by Shanghai Bunge micro company, the CNTs powder is multi-wall carbon nano tube (Baytubes C P), stearic acid is produced by Macklin company, and the chemical formula is C ₁₈H₃₆O₂.

In the following examples, the morphology of the matrix powder is changed firstly by using a high-energy ball milling method through a ball milling process, so that the matrix powder has a larger surface area, CNTs can be dispersed more efficiently, meanwhile, the ball milling stress is removed and the matrix grains grow up by combining a vacuum heat treatment process, so that the capacity of entering the CNTs into the grains is improved, and the CNTs/Al powder with good CNTs dispersion is obtained after ball milling is performed again to a target time. And then sintering the powder, and finally further densifying the block body through hot extrusion to finally obtain the compact composite bar without obvious defects.

Example 1

The preparation method of the intra-crystalline CNTs/Al composite material in the embodiment comprises the steps of improving the metal matrix structure by combining high-energy ball milling with vacuum heat treatment, adding CNTs, further improving the dispersion capacity of the CNTs, improving the possibility that the CNTs enter the crystal, and then carrying out sintering and extrusion forming on the prepared CNTs/Al composite material powder to finally obtain a composite material block. On the other hand, the composite powder is directly subjected to heat treatment, the powder is annealed, then ball milling is performed again to serve as a control group of the patent of the invention, and the sintering and hot extrusion processes are performed on the composite powder obtained by ball milling.

The preparation method of the specific high-content CNTs/Al composite material powder comprises the following steps:

Step 101, powder proportioning and ball milling treatment:

The ball milling pot was charged with a total of 117.6 grams of 45 micron spherical pure aluminum powder.

Powder proportioning: the content of spherical pure Al powder with the particle size of 45 mu m is 98 wt percent, the addition amount of stearic acid is 1 wt percent, the ball milling ball adopts zirconia balls with the diameter of 10mm, and the ball-to-material ratio is 5:1.

In the preparation process, in the step 1, the materials filled in the tank are sequentially added into the ball grinding balls, then the pure aluminum powder and the stearic acid particles are uniformly added at the same time, and the stirring is uniformly carried out by using a medicine spoon, so that the initial CNTs, the Al powder and the stearic acid particles are uniformly premixed.

To compare the superiority of this patent, the preparation and comparison were performed using conventional destressing annealing, with the addition of pure aluminum, 2wt.% CNTs and 1wt.% stearic acid during the initial ball milling process, and the method of operation remained consistent with the designed process.

Step 102, ball milling process and parameter design:

Firstly, opening a valve of an air outlet to ensure that an air path is normal, adjusting the air flow to be 1.0L/min, and introducing argon into an air inlet hole to ensure that the air time is not less than 8min; when the air inlet is stopped, the valve of the air inlet is closed, and then the valve of the air outlet is closed. The relatively low flow rate in the above operation is advantageous for the argon shielding gas to slowly vent the air without causing a reduction in the powder quality.

Setting the ball milling parameters to be 200 rpm/min, and sequentially and circularly carrying out the ball milling modes of 10min forward rotation, 10min stop and 10min reverse rotation. In order to avoid chemical reaction caused by overhigh temperature in the ball milling tank, on one hand, when the effective ball milling time reaches 2h, the stop time of 1h is added to enable the ball milling tank to be fully cooled, and on the other hand, the environment temperature is controlled below 20 ℃, so that the temperature consistency during each ball milling start is ensured. When the ball milling is carried out until the matrix powder is in a thick flake shape (4 h in the patent), the matrix powder is taken out for vacuum heat treatment operation.

In addition, the control group is CNTs/Al composite material powder, argon filling and ball milling parameters are consistent with design patents, and the composite material is also taken after ball milling for 4 hours and is prepared for vacuum heat treatment operation.

Step 103, powder vacuum heat treatment process:

the pure aluminum ball milling is carried out for 4 hours in a thick plate state, 1 g of the powder after heat treatment is selected at the stage and is taken out for morphological characterization analysis, and the operation needs to be completed in a glove box when the powder is taken out and the powder is prevented from being in direct contact with air. The temperature of the vacuum heat treatment is 550 ℃, the heat preservation time is 12 hours, then the furnace cooling is carried out, and meanwhile, 20g of titanium sponge is placed beside the heat treatment powder in advance, so that the powder is further prevented from being oxidized.

As a further illustration of the present invention, to highlight the material performance advantage of the present design, a control group is used for illustration, and the design process of the control group at this step is as follows: and carrying out heat treatment on the thick-plate CNTs/Al composite material powder subjected to ball milling for 4 hours, wherein parameter selection and steps are consistent with the designed flow.

Step 104, continuing high-energy ball milling on the powder after heat treatment:

After the pure aluminum powder subjected to ball milling for 4 hours is subjected to heat treatment, al grains are grown, the Al grains are added into a ball milling tank again, a process control agent (stearic acid) with the content of 1 weight percent is added, the ball milling is carried out subsequently, the ball milling speed is 200rpm/min, the ball milling time is 20 hours, and the total effective ball milling time is 24 hours after the initial 4 hours of pure aluminum.

As a further illustration of the present invention, to highlight the material performance advantage of the present design, a control group is used for illustration, and the design process of the control group at this step is as follows: after carrying out heat treatment on the CNTs/Al composite powder after ball milling for 4 hours, adding the CNTs/Al composite powder into a ball milling tank again, adding a process control agent (stearic acid) with the content of 1 weight percent, carrying out ball milling subsequently, wherein the ball milling speed is 200rpm/min, the ball milling time is 20 hours, and the total effective ball milling time is 28 hours after adding 4 hours of initial ball milling on the CNTs/Al composite powder.

The whole ball milling process flow diagram is shown in fig. 1, and it can be seen that the process designed in the patent aims at matrix powder, CNTs are added after the structure of the matrix powder is optimized and regulated, specifically, when the powder is ball milled until a certain thickness of flakes exist, vacuum heat treatment is performed first, through the process, crystal grains can be grown up, and meanwhile, ball milling stress is eliminated, so that CNTs can be further dispersed and enter the crystal.

Morphology analysis is carried out on the intragranular CNTs/Al composite material powder prepared by ball milling in the embodiment:

characterization is carried out by adopting SEM scanning for the composite material powder morphology obtained by different ball milling time, and FIG. 2 is the flow designed in the embodiment 1 and the CNTs/Al composite material powder morphology obtained by the final ball milling state of the control group, wherein, FIGS. 2 (a) and (b) respectively show the powder morphology and the dispersion state of CNTs after the designed pure aluminum ball milling for 4 hours and then the thermal treatment and the ball milling again for 20 hours. Fig. 2 (c) and (d) show the morphology and the dispersion state of CNTs after the control group CNTs/Al composite material was subjected to a heat treatment after ball milling for 4 hours and ball milling again for 20 hours, and it can be seen that both the morphologies of the two powders were flaky powders, and after the local surface is enlarged, it can be seen that a small amount of CNTs dispersed on the surface of the powder were found, and the rest of CNTs were buried into the matrix powder during the ball milling process.

Step 105, a high-content CNTs/Al composite powder discharge plasma sintering process:

And respectively filling the composite material powder obtained after ball milling into graphite dies with the diameter of 30, prepressing by adopting the pressure of 0.5T, keeping the pressure for not less than 10min, and filling the dies into a plasma sintering equipment furnace for sintering. The sintering temperature is 590 ℃, the pressure is 30MPa, the vacuum degree is 1.5X10 ^-1 Pa, and the heat preservation time is 30min, so that the sample is densified. The temperature measurement adopts K-type thermocouple contact type temperature measurement, and the accuracy of the temperature is ensured.

Step 106, hot extrusion process:

Sintering powder obtained by ball milling for 4h after vacuum heat treatment and ball milling for 20h again in the step 105, performing hot extrusion on the obtained massive composite material sample and the control group (ball milling for 4h and ball milling for 20h again and sintering) sample, polishing the surface of the extruded sample by using a belt sander to remove outer-layer oxide skin, then preserving the heat of the sample at 500 ℃ for 20min in a box furnace, and then placing the sample into extrusion equipment preheated at 370 ℃ at an extrusion ratio of 18:1 and an extrusion speed of 4mm/s, thereby finally obtaining the composite material extrusion bar with the diameter of 7mm and smooth and crack-free surface.

In the above-mentioned forming process, in step 106, in the sample, the control group, i.e. the initial state, is the composite powder, the powder is prepared by ball milling for 4 hours, performing heat treatment and ball milling again, and the composite block is obtained by sintering and extrusion, wherein the average value of the yield strength is 133.7MPa, the average value of the tensile strength is 168.2MPa, and the average value of the elongation at break is 10.0%. The performance of the intra-crystalline CNTs/Al composite material designed by the patent is that the average yield strength is 142.9MPa, the average tensile strength is 183.7MPa, and the average elongation at break is 15.3%. The specific data are shown in table 1. It can be seen that although the ball milling process is designed to have a ball milling time of only 20 hours after CNTs are added, the strength and plasticity of the composite powder are obviously higher than the performance of the composite powder after heat treatment annealing, and the preparation process of the intra-crystal CNTs/Al composite material designed by the patent has obvious advantages.

TABLE 1 mechanical Properties of intragranular CNTs/Al composites and control group

Material name	Yield strength (MPa)	Tensile strength (MPa)	Elongation at break (%)
				Control group	133.7±5.7	168.2±2.1	10.0±0.6
Intra-crystalline CNTs/Al	142.9±7.5	183.7±9.5	15.3±1.4

The following test analysis was performed on the intra-crystalline CNTs/Al composite powder prepared by the vacuum heat treatment, plasma sintering, and hot extrusion processes of this example:

(1) Characterization of phase change before and after pure aluminum heat treatment

Fig. 3 is an XRD characterization pattern before and after vacuum heat treatment of pure aluminum powder after ball milling for 4 hours, showing that no obvious alumina peak is found by calibration, verifying the stability of the vacuum heat treatment process, and at the same time, the peak position is slightly shifted left after heat treatment compared with before heat treatment, and the decrease of angle θ shows the increase of the inter-plane distance d, which may be due to the presence of larger compressive stress in the powder after ball milling, resulting in the decrease of the lattice distance, the removal of the internal stress of the powder after heat treatment, and the increase of the inter-plane distance d, resulting in the decrease of angle θ, verifying that the internal stress of the powder is sufficiently removed.

(2) Characterization of grain changes before and after pure aluminum heat treatment

The powder was analyzed by XRD and TEM to characterize the change of the matrix powder grains, and the grain size was counted by XRD pattern and Shelle formula, and the results are shown in Table 2. It can be seen that the grain sizes of the powder grains obtained by different crystal face statistics after heat treatment are obviously improved, which indicates that the matrix grains are obviously grown.

TABLE 2 statistics of grain size before and after heat treatment of pure aluminum powder

Counting crystal faces	Before heat treatment	After heat treatment
			(111)	585	988
(200)	533	997
			(220)	455	947
(311)	417	973
			Average of	497.5±74.7	976.3±21.9

On the other hand, the change of the grain size was directly observed by TEM data, and as shown in fig. 4, the thick plate-like pure aluminum powder ball-milled for 4 hours was subjected to TEM bright field phase before and after the heat treatment and the corresponding diffraction and dark field patterns, wherein (a-c) was before the heat treatment and (d-f) was after the heat treatment. By carrying out selective electron diffraction on the circular areas in the diagrams (a) and (b), diffraction spots of the powder before heat treatment are found to be polycrystalline rings, which indicate that the areas are mainly polycrystalline, and grains are fine, and can be seen from the diagram (c). And the diffraction of the area with the same size after heat treatment is seen as a set of diffraction spots by the graph (e), which shows that the area basically has only one crystal grain, the obvious growth of the crystal grain is proved, meanwhile, the diffraction spots in the set are subjected to dark field operation, the size of the bright crystal grain can be found to be very large, and the realization of the goal of the heat treatment scheme is further verified.

(3) Mechanical room temperature chemical property test of intragranular CNTs/Al composite material

The room temperature tensile properties of the designed and prepared intragranular CNTs/Al composite material and a control group are tested, the engineering stress strain results are shown in figure 5, and the strength and the plasticity of the composite material are obviously improved compared with those of the composite material obtained by traditional stress relief annealing after the process of the design of the patent, so that the method for preparing the composite material powder by performing heat treatment on the initial metal matrix grains, selecting the thick sheet state and adding CNTs for ball milling has good effect. On the other hand, the total ball milling time of the CNTs/Al composite material powder is 24 hours in the control group, and the ball milling time of the CNTs/Al composite material powder is 20 hours, so that further improvement of strength and plasticity is obtained in a shorter time, time cost is saved, meanwhile, possibility of CNTs entering into crystals is improved.

Finally, it should be noted that the above-mentioned embodiments are merely for illustrating the technical solution of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for preparing a carbon nanotube reinforced aluminum matrix composite by heat treating a matrix powder, comprising:

Preparing composite material powder: performing primary ball milling on a certain amount of metal matrix powder and a process control agent for 4h, performing high-temperature vacuum heat treatment on the thick sheet-shaped matrix powder after ball milling, and then adding a certain amount of carbon nano tube reinforcement for secondary ball milling for 20: 20 h to obtain composite material powder;

Shaping of the composite material: and carrying out plasma sintering on the composite material powder, and then carrying out hot extrusion to obtain the composite material bar with smooth surface and no cracks.

2. The method for preparing the carbon nanotube reinforced aluminum matrix composite material by heat treatment ball milling according to claim 1, wherein the carbon nanotube content in the composite material is 2 wt percent by mass, and the metal matrix powder content is 98 percent by mass wt percent by mass.

3. The method for preparing the carbon nanotube reinforced aluminum matrix composite by heat treatment ball milling according to claim 1, wherein the metal powder is spherical pure aluminum powder with the diameter of 45 μm, the carbon nanotube reinforcement is 20 nm μm and the length of 2 μm, and the process control agent is stearic acid, wherein the process control agent accounts for 1: 1wt% of the total amount of the composite powder.

4. The method for preparing the carbon nanotube reinforced aluminum matrix composite by heat treatment ball milling according to claim 1, wherein the parameters of the primary ball milling and the secondary ball milling are that the rotating speed is 180-230 rpm/min, the ball milling mode is that the ball milling is rotated forward by 10min, the ball milling is stopped by 10min, the ball milling is performed in a reverse rotation by 10min, the circulation is sequentially carried out, and in the ball milling process, when the effective ball milling time reaches 2h, the ball milling is stopped by 1 h, and the ambient temperature is below 20 ℃.

5. The method for preparing the carbon nanotube reinforced aluminum matrix composite by heat treatment ball milling according to claim 1, wherein the high-temperature vacuum heat treatment is carried out at a temperature of 550 ℃ for a heat preservation time of 12 h.

6. The method for preparing the carbon nanotube reinforced aluminum matrix composite by heat treatment ball milling according to claim 1, wherein the plasma sintering process is performed by pre-pressing at a pressure of 0.5T, the pressure maintaining time is not less than 10min, the vacuum degree is 1.5×10 ^-1 Pa, and the temperature is 590 ℃.

7. The method for preparing the carbon nanotube reinforced aluminum matrix composite by heat treatment ball milling according to claim 1, wherein the temperature of the hot extrusion process is 500 ℃, the heat preservation time is 20min, the extrusion ratio is 18:1, and the extrusion speed is 4 mm/s.