Method for preparing lightweight high-thermal-conductivity graphite flake reinforced magnesium-based composite material

Abstract

A method for preparing a lightweight, high thermal conductivity graphite-reinforced magnesium matrix composite material, belonging to the technical field of thermally conductive magnesium matrix composite materials, solves the technical problem of limited thermal conductivity of Mg alloys. The method includes the following steps: graphite pretreatment → Mg-5Zn magnesium alloy matrix refining → ultrasonic-assisted semi-solid stirring pressure casting → homogenization treatment → hot extrusion forming. The graphite-reinforced magnesium matrix composite material proposed in this invention is a novel high thermal conductivity lightweight composite material. All materials used are relatively inexpensive. Its stirring casting process is relatively simple and can produce large-size blocks, which is beneficial for further industrial production and application. The graphite-reinforced magnesium matrix composite rods prepared by this invention have a thermal conductivity as high as 170 W / (m·K) along the extrusion direction and a density of 1.86 g / cm³. 3 The following breakthrough in thermal conductivity was achieved while maintaining the lightweight advantage of Mg alloys.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of heat-conducting magnesium-based composite materials, and particularly relates to a preparation method of a lightweight high-thermal-conductivity graphite flake reinforced magnesium-based composite material. BACKGROUND

[0002] With the continuous miniaturization, light weight and high performance of electronic devices, the power density is increasing, and the heat generation per unit volume is increasing, thus generating huge heat dissipation demand in the fields of mobile electronic products and electric vehicles. The density of Mg is significantly lower than that of copper and aluminum, which are currently common high-thermal-conductivity metals, and the contribution of Mg to energy saving and emission reduction, improvement of energy utilization efficiency and reduction of carbon emissions cannot be ignored. Although the currently widely used commercial magnesium alloys such as AM60 and AZ91 have good forming and room temperature mechanical properties, their heat dissipation capacity is not ideal, and their thermal conductivities are only 61 W / (m·K) and 53 W / (m·K) respectively, which are much lower than the thermal conductivity of pure Mg, which is 156 W / (m·K). It is worth noting that the traditional technical method of improving the thermal conductivity of Mg alloy by adjusting the alloy composition and microstructure is not obvious in improving the thermal conductivity. Limited by the physical properties of the heat-conducting carriers (such as electrons and phonons) of Mg itself, the thermal conductivity of annealed pure Mg at room temperature is the limit of its thermal conductivity.

[0003] The thermal conductivity of graphite flake is very good, and its thermal conductivity along the basal plane direction is about 1000 W / (m·K), which is several times that of Mg alloy. Therefore, the graphite flake / magnesium-based composite material prepared by taking high-thermal-conductivity graphite flake as a reinforcing body and lightweight magnesium alloy as a matrix is expected to obtain excellent thermal conductivity while meeting the market demand for lightweight materials. SUMMARY

[0004] In order to overcome the deficiencies in the prior art and solve the technical problem of limited thermal conductivity of Mg alloy, the application provides a preparation method of a lightweight high-thermal-conductivity graphite flake reinforced magnesium-based composite material.

[0005] In order to achieve the above-mentioned application purposes, the technical scheme adopted by the application is as follows:

[0006] The preparation method of the lightweight high-thermal-conductivity graphite flake reinforced magnesium-based composite material comprises the following steps:

[0007] S1, graphite flake pretreatment: drying the graphite flake, removing water and organic substances adsorbed on the surface of the graphite flake, and then preparing at 400℃;

[0008] The pretreatment of the graphite sheet during the material preparation process is very critical. Water vapor in the atmosphere is extremely easy to be adsorbed on the nanometer and micrometer scale graphite sheet, which brings insecurity factors to the preparation of the composite material. In addition, the high-temperature baking of the graphite sheet is also beneficial to remove the organic matter or organic functional groups on the surface thereof, which plays a clean surface effect;

[0009] S2, in this step S2, flowing protective gas is introduced throughout the process:

[0010] Firstly, the graphite sheet after the preheating in step S1 is added into the semi-solid Mg alloy melt, and then mechanical stirring is performed, the volume fraction of the graphite sheet is 1vol.%~25vol.%, the mechanical stirring speed is 500rpm~1700rpm, and the mechanical stirring time is 20min; secondly, the semi-solid Mg alloy melt is heated to a liquid state; thirdly, the Mg alloy melt is subjected to ultrasonic treatment, the ultrasonic treatment temperature is 710℃, the ultrasonic treatment power is 1kW~2kW, the ultrasonic treatment frequency is 20kHz, the ultrasonic treatment time is 5min~15min, and the Mg alloy melt after the ultrasonic treatment is left for 3min; finally, the Mg alloy melt after the ultrasonic treatment is poured into the preheated mold for die casting forming, the die casting forming pressure is 450KN, the die casting forming time is 3min, and the die casting blank is prepared;

[0011] The reasonable semi-solid temperature, mechanical stirring, ultrasonic treatment process and pouring parameter control are the keys to the preparation of the graphite sheet reinforced magnesium matrix composite material, which is mainly because the wettability of the graphite sheet with the Mg melt is poor, and the graphite sheet is difficult to uniformly disperse in the Mg alloy solution;

[0012] S3, in this step S3, argon is introduced throughout the process:

[0013] The die casting blank prepared in step S2 is subjected to homogenization treatment, which includes the following stages in turn:

[0014] The first stage: the homogenization treatment temperature is 320℃, and the homogenization treatment time is 8h;

[0015] The second stage: the homogenization treatment temperature is 430℃, and the homogenization treatment time is 16h;

[0016] S4, the die casting blank after the homogenization treatment in step S3 is subjected to hot extrusion forming, the hot extrusion ratio is (12~25):1, the hot extrusion speed is 0.01mm·s -1 ~25mm·s -1 , the hot extrusion temperature is 180℃~350℃, and the lightweight high-thermal-conductivity graphite sheet reinforced magnesium matrix composite material is prepared;

[0017] Hot extrusion helps to refine the grain of the graphite flake reinforced magnesium matrix composite and makes the graphite flake form directional arrangement, which helps to improve the heat conduction performance.

[0018] Further, in the step S1, the purity of the graphite flake is: carbon content > 99%, graphitization degree > 90%; the size of the graphite flake is: thickness 0.2mm~5mm, diameter 1mm~50mm.

[0019] Further, in the step S1, the graphite flake drying includes the following stages in sequence:

[0020] The first stage: the drying temperature is 80℃, and the holding time is 6h;

[0021] The second stage: the drying temperature is 120℃, and the holding time is 2h;

[0022] The third stage: the drying temperature is 250℃, and the holding time is 6h.

[0023] Further, in the step S1, the effective time of the graphite flake holding for standby is 0.5h~3h.

[0024] Further, in the step S2, the semi-solid Mg alloy melt is Mg-Zn series magnesium alloy melt, the zinc content is 2wt.%~6wt.%, and the semi-solid temperature is 600℃~630℃.

[0025] Further, in the step S2, the composition and volume percentage of the protective gas are: CO2: 99vol.%, SF6: 1vol.%.

[0026] Compared with the prior art, the beneficial effects of the present application are:

[0027] The graphite flake reinforced magnesium matrix composite material provided by the present application is a new type of high-thermal-conductivity lightweight composite material. All the materials used have relatively low cost. The stir casting process is relatively simple, can prepare large-size blocks, and is conducive to further industrialized production and application.

[0028] The graphite flake reinforced magnesium matrix composite material rod prepared by the present application has a thermal conductivity of up to 170W / (m·K) along the extrusion direction, and a density of 1.86 g / cm 3 Below, the breakthrough in heat conduction performance is realized under the premise of maintaining the lightweight advantage of Mg alloy. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The metallographic microstructure diagram of the as-cast graphite flake reinforced magnesium matrix composite material prepared in Example 1;

[0030] Figure 2 Microstructure of as-cast graphite flake reinforced magnesium matrix composite prepared for Comparative Example 1;

[0031] Figure 3 Scanning electron micrograph of extruded graphite flake reinforced magnesium matrix composite prepared for Example 1;

[0032] Figure 4 Elemental mapping of extruded graphite flake reinforced magnesium matrix composite prepared for Example 1;

[0033] Figure 5 Thermal conductivity and density bar chart of extruded graphite flake reinforced magnesium matrix composite prepared for Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0034] The application will be described in detail below with reference to the drawings and specific examples. The present embodiment is implemented on the premise of the technical solution of the present application, and gives a detailed implementation and specific operation process, but the protection scope of the present application is not limited to the following examples, and for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can also be made. These all belong to the protection scope of the present application.

[0035] In addition, the remaining raw materials or processing techniques, unless otherwise specified, are all conventional commercially available raw materials or conventional processing techniques in the art.

[0036] Example 1

[0037] The method for preparing a lightweight high-thermal-conductivity graphite flake reinforced magnesium matrix composite comprises the following steps:

[0038] S1, graphite flake pretreatment: in this embodiment 1, the purity of graphite flake is selected as follows: carbon content > 99%, graphitization degree > 90%; the size of graphite flake is as follows: thickness is 5 mm, diameter is 30 mm; graphite flake is dried, and the drying of graphite flake comprises the following stages in turn:

[0039] First stage: drying temperature is 80℃, and holding time is 6h;

[0040] Second stage: drying temperature is 120℃, and holding time is 2h;

[0041] Third stage: drying temperature is 250℃, and holding time is 6h;

[0042] Remove the water and organic matter adsorbed on the surface of the graphite flake, and then heat at 400℃ for standby, it should be noted that the effective time for graphite flake standby is 0.5h ~ 3h;

[0043] S2, Mg-5Zn magnesium alloy base refining:

[0044] A clean pure Mg block was placed in a crucible, and a layer of covering agent for protecting the melt was sprinkled on the bottom and top surfaces of the pure Mg block, respectively. The temperature was raised to 760℃, and a mixed protective gas composed of 1 vol.% SF6 and 99 vol.% CO2 was introduced. After the pure Mg block completely melted, the temperature was lowered to 720℃, the slag was removed, and pure Zn was added to the Mg liquid. After stirring for 2-3 minutes, the melt was allowed to stand and refine;

[0045] The covering agent used in this embodiment 1 is RJ-6, and the amount of covering agent used in each refining process is 1%~2% of the mass of the melt. Before adding the refining agent to the melt, the refining agent needs to be baked at 150℃~200℃ for more than 30 minutes to remove moisture, and the refining agent needs to be added hot;

[0046] S3, ultrasonic-assisted semi-solid stirring pressure casting: flowing protective gas was introduced throughout this step S3;

[0047] First, the Mg-5Zn magnesium alloy melt prepared in step S2 was cooled to 630℃ (semi-solid temperature), and after the slag was removed, the stirring paddle was lowered to the predetermined position, the stirrer was started, and after the surface of the semi-solid Mg alloy melt formed a vortex, the preheated graphite sheet from step S1 was added to the semi-solid Mg alloy melt and then mechanically stirred. The volume fraction of the graphite sheet was 9 vol.%, the semi-solid Mg alloy melt was Mg-5Zn magnesium alloy melt containing 5wt.% zinc and the rest was magnesium Mg and unavoidable impurities; the mechanical stirring speed was 500rpm~1700rpm, and the mechanical stirring time was 20min; secondly, the semi-solid Mg alloy melt was heated to 720℃ (liquid state); thirdly, the ultrasonic working rod preheated to 680℃ was inserted into the Mg alloy melt ≤2cm below the liquid level, and the Mg alloy melt was treated with ultrasound, the ultrasonic treatment temperature was 710℃, the ultrasonic treatment power was 2kW, the ultrasonic treatment frequency was 20kHz, the ultrasonic treatment time was 15min, and the Mg alloy melt after ultrasonic treatment was allowed to stand for 3min; finally, the ultrasonic working rod was removed, and the Mg alloy melt after ultrasonic treatment was poured into a mold preheated to 400℃ for pressure casting, the pressure casting pressure was 450KN, the pressure casting time was 3min, and the pressure casting blank was prepared;

[0048] S4, homogenization treatment: argon was introduced throughout this step S4;

[0049] The pressure casting blank prepared in step S2 was machined to the predetermined hot extrusion size, and then subjected to homogenization treatment, which included the following stages in turn:

[0050] First stage: homogenization treatment temperature was 320℃, and homogenization treatment time was 8h;

[0051] The second stage: the homogenization treatment temperature is 430℃, and the homogenization treatment time is 16h;

[0052] After the homogenization treatment, the die casting blank is put into water for quenching treatment;

[0053] S5, hot extrusion forming:

[0054] The surface of the die casting blank after the homogenization treatment in step S3 is polished clean, and is placed in a heat treatment furnace for heat preservation at 210℃ for 30min. At the same time, the hot extrusion die is preheated to the same temperature, and graphite oil lubricant is coated on the forming surface of the extrusion die. After the temperature is uniform, the die casting blank is put into the hot extrusion die for hot extrusion forming, the hot extrusion ratio is 16:1, the hot extrusion speed is 0.1mm·s -1 , the hot extrusion temperature is 210℃, and the lightweight high-thermal-conductivity graphite flake reinforced magnesium-based composite material is prepared.

[0055] The thermal conductivity of the extruded graphite flake reinforced magnesium-based composite material obtained in this embodiment 1 along the extrusion direction is 174.21 W / (m·K), the density is 1.857g / cm 3 , the yield strength is 386.8MPa, and the elongation is 1.72%.

[0056] Example 2

[0057] In this embodiment 2, except that the hot extrusion speed in step S5 is adjusted to 1mm / s, the other steps are the same as those in embodiment 1, which will not be repeated here.

[0058] Example 3

[0059] In this embodiment 3, except that the hot extrusion temperature in step S5 is adjusted to 300℃, the other steps are the same as those in embodiment 1, which will not be repeated here.

[0060] Example 4

[0061] In this embodiment 4, except that the extrusion ratio in step S5 is adjusted to 12:1, the other steps are the same as those in embodiment 1, which will not be repeated here.

[0062] Comparative Example 1

[0063] (1), casting:

[0064] Firstly, pure magnesium ingot was put into a crucible and heated to 760℃. After the magnesium ingot was completely melted, the temperature was decreased to 720℃, and pure zinc particles were added and left for 15 minutes. Then the alloy solution was stirred for 2 minutes to ensure that the alloy solution was fully mixed. After the melt was left for 10 minutes, a refining agent was added and stirred for 2 minutes, and the surface residue after refining was removed. Then the melt was kept at 710℃ for 10 minutes, the surface residue was removed, and then the melt was poured into a metal mold to obtain a cast Mg-5wt.%Zn magnesium alloy. During the pouring process, the melt and the mold were protected by a mixed gas of 99 vol.% CO2 and 1 vol.% SF6, and the mold was preheated to about 400℃;

[0065] (2) homogenization:

[0066] Firstly, the obtained Mg-5Zn magnesium alloy ingot was processed to an extrusion size; the homogenization treatment process was: 320℃ for 8h + 430℃ for 16h, and after the end, it was put into water for quenching treatment. Argon was used as the protective gas during the heat treatment process;

[0067] (3) hot extrusion:

[0068] Firstly, the Mg-5Zn magnesium alloy ingot after homogenization treatment was preheated at 300℃ for 30 minutes, and the 16:1 hot extrusion mold was heated and kept at 210℃. Then the extrusion speed was 0.1mm / s; after extrusion, the obtained rod was directly put into water for quenching treatment to obtain the final Mg-5Zn magnesium alloy extruded rod.

[0069] The thermal conductivity of the Mg-5Zn magnesium alloy extruded rod obtained in the comparative example 1 along the extrusion direction was 123.14 W / (m·K), the density was 1.785g / cm 3 , the yield strength was 426.8MPa, and the elongation was 3.97%.

[0070]

[0071] Figure 1 The metallographic photo of the cast graphite flake reinforced magnesium matrix composite material in the example 1. Figure 2 The metallographic photo of the magnesium matrix alloy in the comparative example 1. The results show that the graphite flake can be effectively dispersed in the Mg alloy matrix by the application.

[0072] Figure 3 The scanning electron micrograph of the extruded graphite flake reinforced magnesium matrix composite material in the example 1. Figure 4 The element energy spectrum analysis diagram of the extruded graphite flake reinforced magnesium matrix composite material in the example 1. The results show that the graphite can maintain its original flake structure after extrusion, which is the key to the high thermal conductivity of the composite material.

[0073] Figure 5 For the density and thermal conductivity performance bar chart, it can be seen that compared with the Mg-5Zn alloy in Comparative Example 1, the graphite flake reinforced magnesium matrix composite material in Example 1 added with 9 vol.% graphite flake shows excellent heat conduction capacity, and the thermal conductivity increases from 123.14 W / (m·K) to 174.21 W / (m·K), with an increase of 41.5%. More importantly, the graphite flake reinforced magnesium matrix composite material prepared in Example 1 has a density comparable to that of the Mg alloy, and also maintains the lightweight advantage, which is beneficial to its application in the field of equipment lightweight.

[0074] The above merely illustrates the specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which shall be covered within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. A method for preparing lightweight high thermal conductivity graphite flake reinforced magnesium matrix composites, characterized in that, Comprising the following steps: S1, graphite sheet pretreatment: the graphite sheet is dried to remove water and organic substances adsorbed on the surface of the graphite sheet, and then heat preservation at 400℃ is carried out; S2, in this step S2, flowing protective gas is introduced throughout the process: First, the graphite sheet preheated in step S1 is added to the semi-solid Mg alloy melt, and then mechanical stirring is carried out, the semi-solid Mg alloy melt is Mg-Zn series magnesium alloy melt, the zinc content is 2wt.%~6wt.%, the semi-solid temperature is 600℃~630℃, the purity of the graphite sheet is: carbon content >99%, graphitization degree >90%; the size of the graphite sheet is: thickness 0.2mm~5mm, diameter 1mm~50mm, the volume fraction of the graphite sheet is 1vol.%~25vol.%, the mechanical stirring speed is 500rpm~1700rpm, and the mechanical stirring time is 20min; secondly, the semi-solid Mg alloy melt is heated to liquid state; thirdly, the Mg alloy melt is treated by ultrasonic, the ultrasonic treatment temperature is 710℃, the ultrasonic treatment power is 1kW~2kW, the ultrasonic treatment frequency is 20kHz, the ultrasonic treatment time is 5min~15min, and the Mg alloy melt after ultrasonic treatment is placed for 3min; finally, the Mg alloy melt after ultrasonic treatment is poured into the preheated mold and pressure cast into a shape, the pressure casting forming pressure is 450KN, the pressure casting forming time is 3min, and the pressure casting blank is prepared; S3, in this step S3, argon is introduced throughout the process: The pressure casting blank prepared in step S2 is subjected to homogenization treatment, which includes the following stages in turn: First stage: homogenization treatment temperature is 320℃, homogenization treatment time is 8h; Second stage: homogenization treatment temperature is 430℃, homogenization treatment time is 16h; S4, the die casting blank after the homogenization treatment in step S3 is formed by hot extrusion, the hot extrusion ratio is (12~25):1, the hot extrusion speed is 0.01mm·s -1 ~25mm·s -1 , the hot extrusion temperature is 180℃~350℃, and the lightweight high-thermal-conductivity graphite flake reinforced magnesium-based composite material is prepared.

2. The method of manufacturing lightweight high thermal conductivity graphite sheet reinforced magnesium matrix composite material according to claim 1, characterized in that: In the step S1, the drying of the graphite sheet includes the following stages in turn: First stage: drying temperature is 80℃, holding time is 6h; Second stage: drying temperature is 120℃, holding time is 2h; Third stage: drying temperature is 250℃, holding time is 6h.

3. The method of manufacturing lightweight high thermal conductivity graphite sheet reinforced magnesium matrix composite material as claimed in claim 1, wherein: In the step S1, the effective time of graphite sheet heat preservation is 0.5h~3h.

4. The method of manufacturing lightweight high thermal conductivity graphite sheet reinforced magnesium matrix composite material as claimed in claim 1, wherein: In the step S2, the composition and volume percentage of the protective gas are: CO2: 99vol.%, SF6: 1vol.%.

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