Fabrication of aluminum-graphite composites by hot extrusion

Masashi Takano, Takahiro Akao, Tetsuhiko Onda, Zhong-Chun Chen

(Department of Mechanical and Aerospace Engineering, Graduate School of Engineering, Tottori University,

Koyama-minami 4-101, Tottori 680-8552, Japan)

Fabrication of aluminum-graphite composites by hot extrusion

Masashi Takano, Takahiro Akao, Tetsuhiko Onda, Zhong-Chun Chen

(Department of Mechanical and Aerospace Engineering, Graduate School of Engineering, Tottori University,

Koyama-minami 4-101, Tottori 680-8552, Japan)

Abstract:Al-graphite composites have been successfully fabricated by a hot-extrusion technique. Sound extrudates can be obtained when graphite content (volume fraction) is less than 40%. The graphite tends to be aligned along the extrusion direction, due to the deformation of graphite along its basal planes during extrusion. The orientation degree of graphite in the extrusion direction is enhanced with decreasing graphite content and extrusion temperature. The preferred orientation of graphite results in an anisotropy of thermal conductivity in the extruded samples. The incorporation of large sizes of graphite flakes is beneficial to improving the thermal conductivity of extruded Al-graphite composites.

Key words:heat dissipation material; aluminum; graphite; hot extrusion

A heat dissipation material with high thermal conductivity and low coefficient of thermal expansion (CTE) is of significance in electronic packaging applications[1]. Graphite has been receiving much attention as a filler in many metal matrices (e.g., Al, Cu) because of its low density, low cost, low CTE and high thermal conductivity in directions parallel to the basal planes. Considering high anisotropy in thermal properties between parallel and perpendicular to the basal planes of graphite, control of the preferred orientation of graphite in a metal matrix is crucial since it is one of the main factors enhancing the thermal conductivity of metal-graphite composites[2]. Moreover, proper molding can be designed to align the graphite flakes, and the anisotropic properties can be taken advantage by aligning the basal planes of graphite flakes in direction of heat transfer. For example, hot pressing or spark plasma sintering processes have been used to fabricate Al-graphite and Cu-graphite composites with high thermal conductivity[3,4].

From the viewpoints of lightweight and mass production, in the present study, graphite flakes were incorporated into aluminum matrix and Al-graphite composites were fabricated by a hot-extrusion technique. With the help of shear deformation during extrusion, it might be possible to align the graphite in the direction parallel to the extrusion direction. The purpose of this work was to examine the extrusion behavior, microstructure, and thermal conductivity of the Al-graphite composites.

1Experimental pocedure

Pure Al powder (>99.9% purity, average particle size:30 μm; Kojundo Chemical Laboratory, Japan) and natural flaky graphite (Ito Graphite Corporation, Japan) with three different diameters, 10, 60, and 250 μm, were used as the starting materials. The Al powder and graphite flakes with compositions(volume fraction) of 20, 40, and 60% graphite were ball-milled in ethanol. After drying, the mixed powders were compacted into a cylindrical green compact by uniaxial pressing, and vacuum-encapsulated into an Al can to obtain an extrusion billet. Hot extrusion was performed in a temperature range of 400～500 ℃ with an extrusion ratio ofR=14 at a punch speed of 1mm/min.

The density of the extruded samples was determined by the Archimedes method. The phase composition was analyzed by X-ray diffraction (XRD) using CuKαradiation. The microstructure was observed by SEM. The thermal conductivity was determined by measuring thermal diffusivity and specific heat by using a laser flash apparatus (LFA457 Micro Flash; Netzsch, Selb, Germany).

The degree of orientation of the graphite crystal in the extruded samples was estimated by the Lotgering method which provides an orientation index deduced from XRD pattern for the oriented materials. The Lotgering factor reflecting the degree of orientation (I00c) was defined by the following equation.

(1)

WherePdenotes the fraction of the sum of the peak intensities corresponding to the preferred orientation axis to that of all diffraction peaks (Ihkl) in the material,i.e.,

P0is a reference value ofPfor a randomly oriented sample. Thefvalue varies between zero to unity;f=0 corresponds to random orientation, andf=1 to perfect orientation.

2Results and discussion

Fig.1 shows the extrusion pressure-stroke curves in hot-extrusion process under various conditions. All the extrusion behaviors seem to be similar to each other. From the start point A to B (Fig.1(a)), the Al-graphite mixed powder inside the Al can is further compacted. At the same time, the plastic deformation of the Al sheath occurs. At point B where the slope of extrusion pressure versus stroke curves becomes small, the Al sheath in the front end of the billet starts to be extruded out of the die. At point C, where the pressure reaches a peak, the Al-graphite composite surrounded by the Al sheath, starts to be formed. Subsequently, the extrusion pressure is gradually reduced and the extrusion of the composite steadily continues up to point D. Finally, the composite inside the billet is completely consumed, and only residual Al sheath is extruded.

When the graphite content varies from 20% to 60%, the extrusion curves are almost the same, but some serrations appear in the curve from C to D (Fig.1 (a)) for the sample with 60% graphite. This implies that discontinuous deformation and fracture should be induced in the sample with a high graphite content. With decreasing the particle size of graphite, the extrusion pressure becomes higher as seen in Fig.1 (b). It is considered that this results from relatively lower deformation resistance of larger graphite flakes. As the extrusion temperature decreases, the extrusion pressure tends to rise (Fig.1 (c)) due to the increase in deformation resistance of the billet at lower temperatures.

Fig.2 shows appearances of the extruded samples with different graphite contents (graphite size: 250 μm). The extruded samples with 20% and 40% graphite (volume fraction) were sound and no evident cracks, voids, and other defects were observed. However, the addition of 60% graphite (volume fraction) results in poor formability. This might be related to the inconsistency in the extrusion behavior between Al and graphite. The existence of large amount of graphite restricts the deformation of Al matrix during hot extrusion.

Fig.1　Extrusion pressure versus strokecurves for the samples(a)—With 20%～60% graphite (250 μm);(b)—With different graphite sizes;(c)—Extruded at temperatures ranging from 400 ℃ to 500 ℃

The relative density of the extruded samples is shown in Fig.3. With increasing the graphite content, the relative density gradually decreases. Furthermore, as the particle size of graphite decreases, the relative density becomes low. This indicates that the presence of graphite inhibits the deformation of Al matrix and densification of the composites.

Fig.2　Appearances of the samples extruded at 450 ℃(a)—20%; (b)—40%; (c)—60% (250 μm)

Fig.3　 Relative density of the samples extruded at450 ℃ as a function of volume fraction of graphite

Fig.4 shows the SEM images (backscattered electron mode) on polished longitudinal sections of the extruded samples with various contents and particle sizes of graphite. The white and dark regions correspond to Al and graphite, respectively. It can be seen from Fig.4 that the graphite tends to be aligned along the direction parallel to the extrusion direction in the extruded samples. This tendency seems to be weakened and the thickness of the deformed graphite becomes large with increasing graphite content. The preferred orientation of graphite along the extrusion direction is believed to be associated with shear deformation occurred during extrusion, because graphite is easily deformed along the basal planes due to its layered structure. On the other hand, for the extruded samples of Al-40% graphite with different particle sizes of graphite shown in Figs. 4(b),(d) and (e), as the particle size of graphite decreases, both the length and thickness of the deformed graphite are reduced, and the distribution of the graphite becomes more homogeneous.

The SEM images on polished longitudinal sections of the samples extruded at different temperatures are shown in Fig.5. It seems that the majority of graphite are aligned along the extrusion direction at 400 ℃. However, the orientation of the graphite becomes weak with increasing the extrusion temperature. It is evident that the orientation of the graphite depends strongly on deformation behavior of both graphite and Al. The deformation resistance of graphite has small temperature dependence, while the deformation resistance of Al rapidly decreases with increasing temperature. Accordingly, with the increase in extrusion temperature, the deformation of the Al matrix occurs easily compared to graphite. As a result, the deformed graphite exhibits smaller aspect ratios.

Fig.6 shows the Lotgering factor calculated using Eq.(1) from the XRD patterns of the extruded samples. It can be found from Fig.6(a) that the particle size of graphite has small effect on the orientation of graphite. But the composites with a smaller content of graphite exhibit larger degrees of orientation. For example, the Lotgering factor of the sample with 20% graphite (250 μm) was about 1.22 times higher than that with 40% graphite. Besides, a lower extrusion temperature corresponds to a larger Lotgering factor. As shown in Fig.6(b), the Lotgering factor of the sample extruded at 400 ℃ was about 1.43 times larger than that at 500 ℃. These results are in good agreement with the microstructural observations shown in Figs.4 and 5.

Fig.4　SEM images on longitudinal sections of the samples extruded at 450 ℃(a)—20% (250 μm); (b)—40% (250 μm); (c)—60% (250 μm); (d)—40% (60 μm); (e)—40% (10 μm)

Fig.5　SEM images on longitudinal sections of the samples extruded(a)—400 ℃; (b)—450 ℃; (c)—500 ℃

Fig.6　Lotgering factor of the extruded samples as a function(a)—Particle size of graphite; (b)—Extrusion temperature

Fig.7　Thermal conductivity of the samples extruded at450 ℃ with different contents and sizes of graphite

Fig.8　Thermal conductivity of the samples extrudedat different temperatures

Fig.7 shows the thermal conductivity of the extruded samples, measured at room temperature, as a function of the content of graphite. When the graphite flakes with an average diameter of 250 μm were used, the addition of graphite flakes leads to an increase in thermal conductivity in the extrusion direction. However, the addition of smaller sizes of graphite (10 μm and 60 μm) causes the reduction in thermal conductivity. This is the result of the increase in the interfacial thermal resistance. Moreover, the thermal conductivity in the extrusion direction (ED) was higher than that in the radius direction (RD), exhibiting an anisotropic behavior of thermal conductivity in the extruded sample. For example, in the case of the Al-40% graphite composite containing the graphite flakes with an average diameter of 60 μm, the thermal conductivity in the extrusion direction was 2.6 times higher than that in the radius direction as shown in Fig.7.Fig.8 shows the effect of extrusion temperature on thermal conductivity of the extruded samples. The thermal conductivity of the sample extruded at 450 ℃ was higher than that of the sample extruded at 500 ℃. A lower value at 500 ℃ may be related to (i) reduction of the orientation degree of graphite in the extrusion direction (Fig.6(b)) and (ii) possible formation of aluminum carbide during the extrusion process.

As shown in Fig.7, a larger size of graphite flakes results in the improvement in thermal conductivity due to the decrease of contact area between Al and graphite. Accordingly, reduction of interfacial thermal resistance is one of the important factors in improving the thermal conductivity of the extruded Al-graphite composites. For this reason, an Al-Si alloy powder is incorporated into the Al-graphite powder mixtures as an additive. Since the Al-Si alloy powder has a lower melting point than Al (for example, 577 ℃ for Al-12Si alloy), it is melted and penetrated into the interfacial regions between Al and graphite during densification process of the mixed powders.

In this way, the intimate contact

between Al and graphite leads to decrease in interfa-cial thermal resistance, thus enhancing the thermal conductivity of the composites. Another way is to further improve the orientation degree of the graphite crystals in the extrusion direction, because graphite has high thermal conductivity in directions parallel to its basal planes. This may be realized by increasing the extrusion ratio (deformation degree). Furthermore, a larger deformation degree is also helpful to improve the interfacial contact between Al and graphite. Further investigations are in progress and the results will be reported elsewhere.

3Conclusions

(1) Al-graphite composites have been successfully fabricated by hot extrusion when graphite content (volume fraction) is less than 40%.

(2) The graphite tends to be aligned along the extrusion direction during hot extrusion. Its degree of orientation in the extrusion direction is enhanced with decreasing graphite content and extrusion temperature.

(3) The preferred orientation of graphite results in an anisotropy of thermal conductivity in the extruded samples. The thermal conductivity in the extrusion direction is higher than that in the radius direction.

(4) The incorporation of large sizes of graphite flakes is beneficial to improving the thermal conductivity of extruded Al-graphite composites.

References:

[1]Zweben C. Advances in composite materials for thermal management in electronic packaging . JOM, 1998, 50 (6): 47-51.

[2]Yuan G M, Li X K, Dong Z J,etal. Graphite blocks with preferred orientation and high thermal conductivity . Carbon, 2012, 50 (1): 175-182.

[3]Chen J K, Huang I S. Thermal properties of aluminum-graphite composites by powder metallurgy . Composites, Part B, 2013, 44: 698-703.

[4]Q Liu, He X-B, Ren S-B, Zhang C,etal. Thermophysical properties and microstructure of graphite flake/copper composites processed by electroless copper coating . J Alloy Comp, 2014, 587: 255-259.

[5]Lotgering F K. Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures-I . J Inorg Nucl Chem, 1959, 9: 113-123.

中圖分類號：TB 333.1

文獻標識碼：A

文章編號：1671-6620(2015)03-0227-05

doi:10.14186/j.cnki.1671-6620.2015.03.014