2018 Volume 59 Issue 12 Pages 1935-1942
Carbon fiber reinforced pure Al, A336 alloy and carbon fiber-carbon nanofiber reinforced A336 alloy composites were successfully fabricated by low-pressure infiltration process, aiming for development the carbon fiber reinforced aluminum matrix composites with high thermal conductivity and mechanical property used as functional materials and structural materials, respectively. Carbon fiber of 10 vol% and hybrid carbon fiber-carbon nanofiber of 10 vol% were used to fabricated preforms for the low-pressure infiltration process. Afterwards, pure Al and Aluminum alloy under a temperature of 1073 K were infiltrated into the preforms under an applied pressure of 0.4 MPa in Ar environment. Microstructural and mechanical performances of the composites were investigated. Microstructure observations indicated that SiO2 binder was coated on the surface of carbon fiber and distributed at the corner of carbon fibers in carbon fiber preform. Carbon nanofibers were agglomerated at the corner of carbon fibers, and some were dispersed on the surface of carbon fiber in the hybrid preform. In composites, carbon fibers were homogeneously distributed in the matrix. Further Vickers hardness test results showed that the hardness of carbon fiber reinforced pure Al composite increased by 76% compared to pure Al, and carbon fiber reinforced A336 alloy composite increased by 11.1% compared to A336 alloy. The thermal conductivity (TC) test result illustrated that the thermal conductivity of carbon fiber reinforced pure Al and A336 alloy composite was 245.8 W/(m·K) and 113.5 W/(m·K), respectively, and the thermal conductivity of carbon fiber-carbon nanofiber reinforced A336 alloy composite was 98.4 W/(m·K).
With the development of modern technology, single material hardly satisfies the increasing demands of properties. Aluminum matrix composites (AMCs) have attracted widespread attention due to their improved thermal conductivity, low coefficient of thermal expansion, low melt point of matrix and high mechanical property.1,2) Carbon materials such as carbon fiber, carbon nanofiber, have been considered as effective reinforcement in advanced composites3,4) which possess favorable mechanical properties.5) Carbon fiber system reinforced aluminum matrix composites has the most potential properties such as light weight, low coefficient of thermal expansion (CTE), high electrical/thermal conductivity, high specific strength and well wear resistance6) for using as functional materials and structural materials.
Carbon materials including carbon fiber, carbon nanofiber,7) carbon nanotubes8) are commonly investigated due to their high specific surface area (SSA), good elevated temperature stability, profitable electrical conductivity. In the recent years, research efforts have been made to CF reinforced Al matrix composites9–11) and Vapor grown carbon fiber (VGCF, CNF) reinforced Al matrix composites.12,13) Nevertheless, variation of microstructure and properties in aluminum matrix composites which using carbon fiber and carbon nanofiber (VGCF) as reinforcement are rarely investigated. Using CF and CNF as hybrid reinforcements, the CNFs are expected to be uniformly distributed on the surface of CFs which can further increase the surface space area to achieve effective stress-transfer across the metal matrix, makes its high bond strength with the matrix. On the other hand, CNFs uniformly distributed on CF in matrix which can prevent grain growth during manufacturing. Therefore, the use of hybrid reinforcement (carbon fiber and carbon nanofiber) is expected to further improve the mechanical properties of AMCs. Pure Al with high thermal conductivity and electrical conductivity is chosen as matrix to fabricate composite, however, the low mechanical limits its application. The development of carbon fiber reinforced pure Al composite aims to further improve the thermal conductivity and mechanical property for using as the heat sink applications for high heat generating electronic components in aerospace. A336 alloy is selected because of the high content of Si which fulfills good fluidity and casting ability, and the presence of Mg can improve the wettability of reinforcement by the matrix. The development of carbon fiber and carbon fiber-carbon nanofiber reinforced A336 alloy composite aims to both improve the thermal conductivity and mechanical property of composite by combining the high thermal conductivity of carbon fiber and the high mechanical property of A336 alloy, for which be used for engine applications that require high mechanical properties at elevated temperature in vehicle.
A number of techniques have been developed to manufacture metal matrix composites (MMCs), including powder metallurgy process,14,15) hot extrusion,16) hot rolling17) and casting method,18) however, powder metallurgy process costs high energy, hot extrusion and rolling methods are difficult to fabricate complicated shaped composites. Though casting method possesses many merits and it is an easy way to fabricate composites, the not well wettability between fibers and aluminum makes fiber hard to uniformly disperse in the matrix. A new manufacturing method which developed by Choi19,20) named low-pressure infiltration (LPI) is one of the useful casting methods for fabricating MMCs, it is cost effective due to apply low-pressure in comparison with high pressure casting such as squeeze casting. The LPI method for MMCs is carried out by infiltrating molten Al into the reinforcement preform which ensures the reinforcement uniform dispersion in the matrix. Furthermore, LPI is able to fabricate large/complex shape with applying pressure and accomplish high value with competitive price.
In the present study, carbon fiber reinforced pure Al, A336 alloy composite and carbon fiber-carbon nanofiber reinforced A336 alloy composite were fabricated by low-pressure infiltration method. Microstructure and mechanical properties like hardness and thermal conductivity were investigated. The task of this study is to develop multifunctional carbon fiber reinforced Al matrix composites by low-pressure infiltration process.
A1070 with purity of 99.7% and high thermal conductivity (237 W/(m·K)) and A336 alloy with composition of Al–13Si–1.5Ni–1.5Cu–1.3Mg (mass%) in ASTM and high tensile strength (195 MPa) were used as matrixes in this experiment, chemical compositions of pure Al and A336 alloy are listed in Table 1. Carbon fiber (CF, K13D2U, Mitsubishi Plastics, Inc. aspect ratio of 230) and carbon nanofiber (CNF (VGCF), offered by Showa Denko Co. in Japan) with 8 mass% SiO2 sol as binder (Nissan Chemical Industries, Ltd.) were used for manufacturing carbon fiber preform and carbon fiber-carbon nanofiber hybrid preform with 10 vol%, respectively. Properties of Carbon fiber and Carbon nanofiber are listed in Table 2. SiO2 binder was used to cross-link fibers and surface coating fibers for wettability with Al. The size of preform was ϕ30 × h20 mm. The preform was heat-treated for sintering at 1433 K in Argon for SiO2 binder. CNFs were first acid treated by H2SO4:HNO3-3:1 in ultrasonic for 30 min for dispersion, and the ratio of CF:CNF of the hybrid preform was 9.5:0.5. The preform was put into the graphite mold, and molten A366 alloys were infiltrated into the preform by low-pressure infiltration with 0.4 MPa applied pressure, molten alloy of 1173 K and 3.6 ks of holding time under Ar environment. The microstructures of preforms and composites were revealed by Optical Microscope (Metal Microscope OPTIPHOT-2, Nikon Corporation, Japan) and Scanning Electron Microscope (SEM; HITACHI S-5000, Japan). Elemental map analysis and point analysis of composites were carried out by EDS (EDAX JAPAN Genesis XM2) and the presence of different phases of composite was identified by XRD (D/max-2500/PC, Japan) using Cu Kα radiation at 40 kV and 0.1 A. To evaluate the variation in properties of composites, the Vickers hardness test was performed by load of 5 kg for 10 s, the density of each composite was measured by Archimedes method and the thermal conductivity was evaluated by laser flash method thermal constants measuring system (TC-700, ULVAC-RICO Inc., Japan) at room temperature in air.
Figure 1 shows the as-received carbon fiber, as-received carbon nanofiber, and carbon nanofiber after acid treatment. In Fig. 1(a), the as-received carbon fibers presented surface without impurities. The as-received carbon nanofibers are easily agglomerated due to the Van der Waals interaction. Therefore, as a filler material of composite, CNF should be dispersed uniformly to be better distributed in the matrix. In this study, acid treatment was adopted to improve the dispersion of CNF. Figures 1(b) and (c) show the SEM images of as-received carbon nanofibers before and after acid treatment, respectively. Figure 1(c) shows the agglomeration of CNF was reduced fairly after acid treatment, which allows the CNF could be used as reinforcement in the composite.
SEM images of (a) as-received carbon fiber, (b) as-received carbon nanofiber and (c) carbon nanofiber after acid treatment.
The SEM images of hybrid preforms depending on the different ratio of carbon fiber to carbon nanofiber are shown in Fig. 2. It could be found that with decreasing the ratio of CNFs, CNFs agglomeration in preform was weakening accordingly. In Figs. 2(a) and (b), CNFs were prone to entangle together and gathered at the corner of CF. As decreasing the ratio of CNF in Figs. 2(c) and (d), CNFs agglomeration was significantly weakened. The ratio of CF: CNF by 9.5:0.5 was selected to fabricate hybrid preform for the following infiltration process.
SEM images of hybrid preforms, different ratio of CF:CNF (a) 5:5, (b) 7:3, (c) 9:1 and (d) 9.5:0.5.
Figure 3 represents the SEM images of obtained carbon fiber preform and carbon fiber-carbon nanofiber hybrid preform with SiO2 binder. Figure 3(a) exhibits the thick SiO2 binder layer was coated on the surface of CFs. Figure 3(b) shows SiO2 binder was distributed at the corner of CF intersecting parts. Figure 3(c) shows much of CNFs were gathered at the corner of CF bridges, and CNFs were dispersed on the surface of CF which helped by SiO2 binder as shown in Fig. 3(d). Purpose of SiO2 binder is surface coating of fibers for wettability with Al and ensure preforms exhibit thick and large fiber adhesion of SiO2 binder at fibers intersecting parts. The addition of SiO2 binder could be a cause of preform strength improvement and enhance the preforms fracture resistance to the applied pressure during infiltration.
SEM images of preforms: (a), (b) carbon fiber inside preform, (c) carbon fiber-carbon nanofiber hybrid preform (9.5:0.5) and (d) partial enlargement of (c).
To investigate the dispersion of fibers in the matrix, OM images of three kinds of composites are shown in Fig. 4. Figures 4(a) and (b) showed that much of carbon fibers were well and evenly distributed as short fibers in the aluminum matrix, while some carbon fibers were presented as cluster. In Fig. 4(c), much carbon nanofiber clusters were found at the corner of carbon fibers in the matrix, which would cause pores between fibers. To further analyze the microstructure of composites, SEM images of composites are shown in Fig. 5. Figures 5(a), (a′) and (a′′) are SEM images of CF/Pure Al composite. Figures 5(b), (b′) and (b′′) are SEM images of CF/A336 alloy composite. Figures 5(c), (c′) and (c′′) are SEM images of CF/A336 alloy hybrid composite. Figures 5(a), (b) and (c) are low magnification SEM images of composites, which showed an overall morphology of composites. Figure 5(a) shows that many defects were generated between carbon fiber and matrix. Figures 5(b) and (c) show that Si phase and intermetallic compound were generated in the matrix, also, defects were existed. In Fig. 5(a′), it can be clearly seen that defects were generated between carbon fiber and matrix. In Fig. 5(b′), EDS analysis was conducted to analysis the intermetallic compound in the matrix. EDS analysis result of the point A (white phase) in Fig. 5(b′) is shown in Table 3, which proved the existence of Al3Ni phase. The Al3Ni phase formation sequence was described as following equation21)
| \begin{equation} \text{3Al} + \text{Ni} \rightarrow \text{Al$_{3}$Ni} \end{equation} | (1) |
OM images of composites: (a) CF/pure Al composite, (b) CF/A336 alloy composite and (c) CF-CNF/A336 alloy composite.
SEM images of composites: (a), (a′), (a′′) CF/pure Al composite, (b), (b′), (b′′) CF/A336 alloy composite and (c), (c′), (c′′) CF-CNF/A336 alloy composite.
Figure 5(c′) shows that carbon nanofiber was gathered at the corner of carbon fibers. Defects may arise from entrapped gases at the carbon nanofiber clusters corner, the alloy was hard to infiltrate into the corner and finally generated defects. Figures 5(a′′), (b′′) and (c′′) are high magnification SEM images of composites, which all showed that interface layers were emerged between carbon fiber and matrix which caused by the reaction between SiO2 binder and matrix.
Figure 6 shows the results of area elemental mapping and point analysis of carbon fiber reinforced A336 alloy composite. Obvious oxide layers were found at the interface of carbon fibers and matrix generated by the interfacial reaction (eq. (2)) between aluminum and the SiO2 binder. Point analysis revealed that the oxide layer was Al2O3 and SiO2. Dark phase was Si phase which precipitated from the matrix during solidification and the gray phase was intermediate compound of Al3Ni. SiO2 binder affected the wettability between fibers and molten Al, Laurent et al.22) investigated the wettability of SiO2–Al system and concluded that the SiO2–Al system transformed from nonwetting system of 150° to wetting system of 68° at melting temperature of 1173 K. Therefore, the SiO2 binder which coating on the fiber surface was expected to promote the molten Al into preform by accelerating reaction to molten Al alloy to accomplish densification of the composites. The reaction between the Al alloy and the SiO2 binder can be presented as follow:22)
| \begin{equation} \text{4Al} + \text{3SiO$_{2}$} \rightarrow \text{$2\gamma$-Al$_{2}$O$_{3}$} + \text{3Si} \end{equation} | (2) |
Results of area elemental mapping and point analysis of CF/A336 alloy composite.
Furthermore, the reaction between Al and SiO2 binder is also contributed to increasing the relative density of the composites.
Figure 7 represents the X-ray diffraction (XRD) result of carbon fiber reinforced A336 alloy composite. The XRD result indicated that some amount of A336 alloy matrix reacted with SiO2 binder according to the eq. (2), which was confirmed by the presence of the γ-Al2O3 and Si phases. That is the SiO2 binder coating on fibers reacted with A336 alloy matrix and generated Al2O3 as well as Si, the Al2O3 reactant can contribute to the enhancement of mechanical properties of the composite. MgO can be produced by the interfacial reaction between A336 alloy and SiO2 binder as 2Mg + SiO2 → 2MgO + Si, and MgAl2O4 was also detected which was generated by the interfacial reaction between α-Al2O3 and MgO as i.e. α-Al2O3 + MgO → MgAl2O4. It is noticed that MgO and MgAl2O4 improved interfacial wettability.23,24)
Results of X-ray diffraction profiles at room temperature of CF/A336 alloy composites.
Figure 8 shows the relative density of composites. The relative density of carbon fiber reinforced pure aluminum and A336 alloy composite was 95.8% and 97.2%, respectively. The relative density of carbon fiber-carbon nanofiber reinforced A336 alloy composite was 96.5%. The relative density is related to the composite microstructure and the matrix composition. The lower relative density of CF/pure Al composite was resulted from high porosity as shown in Fig. 5(a) which caused by entrapped gases, poor wettability of the carbon fiber by matrix and the poor fluidity of pure aluminum. The higher relative density of CF/A336 alloy composite was due to high Si content of A336 alloy matrix could increase the infiltration rate by decreasing the viscosity of matrix and the existence of Mg in A336 alloy enhanced the wettability of carbon fiber with the aluminum alloy, as a result that a higher relative density could be obtained. The relative density of CF-CNF/A336 alloy composite was relatively lower than CF/A336 alloy composite. The reason was both combine with the microstructure and the matrix composition. As shown in Fig. 5(c′), carbon nanofibers were presented as clusters dispersed at the corner of carbon fibers. The interspace was hardly to be fully infiltrated resulted in porosity being retained in the composites and the A336 alloy has high fluidity than pure Al, these two situations caused a relatively lower relative density than CF/A336 alloy composite but a relatively higher relative density than CF/pure Al alloy composite.
Relative density of composites: (a) CF/pure Al composite, (b) CF/A336 alloy composite and (c) CF-CNF/A336 alloy composite.
Figure 9 shows the Vickers hardness of composites. Hardness of pure Al and A336 alloy were 19 Hv and 67.3 Hv, respectively. Hardness of carbon fiber reinforced pure Al was 33.5 Hv dramatically improved by 75.4% than pure Al. Hardness of carbon fiber reinforced A336 alloy was 74.8 Hv showed an improvement of 11.1% to the A336 alloy. It indicated that the addition of carbon fiber reinforced phase acted an important role in dispersion strengthening, meanwhile, due to the difference of coefficient of thermal expansion between Al matrix and reinforcement, the carbon fiber could restrict the dislocation to increase dislocation density which also increased the hardness of composite. Hardness of carbon fiber-carbon nanofiber reinforced A336 alloy composite was 66.7 Hv, almost the same with A336 alloy but decreased by 10.8% than carbon fiber reinforced A336 alloy composite. The agglomeration of carbon nanofiber at the corner of carbon fiber caused a weakness of dispersion strengthening of the matrix which decreased the hardness of carbon fiber-carbon nanofiber reinforced A336 alloy composite.
Vickers hardness of composites: (a) CF/pure Al composite, (b) CF/A336 alloy composite and (c) CF-CNF/A336 alloy composite.
Figure 10 shows the thermal conductivity of the composites. The theoretical thermal conductivity of composites was investigated in conjunction by the rule of mixture of carbon fiber and matrix. The carbon fiber reinforced pure Al showed a value of 245.8 W/(m·K), which was higher than pure Al (237 W/(m·K)) and corresponding to 93.5% of the theoretical value. It can be supposed that heat loss was conducted to the defects by fiber-matrix interlayer. The thermal conductivity of carbon fiber reinforced A336 alloy composite was 113.5 W/(m·K), which was less than A336 (117.2 W/(m·K)) corresponding to 87% of the theoretical value. The thermal conductivity of carbon fiber-carbon nanofiber reinforced A336 alloy composite was 98.4 W/(m·K), which was corresponding to 75% of the theoretical value. Although CNF has a much higher thermal conductivity of 1200 W/(m·K) than CF of 800 W/(m·K), the thermal conductivity of CF-CNF/A336 alloy composite is much lower than that of CF/A336 alloy composite. The low value could be caused by either poor dispersion of CNFs, poor interfacial bonding or high porosity. In general, the thermal conductivity of carbon short fiber reinforced Al matrix composites depends on the porosity, dispersion of fibers and the interlayer between fibers and matrix. In case of CF-CNF/A336 alloy composite, poor dispersion of CNF caused CNF clusters and many defects, and this disturbed the thermal transport, resulting in a decrease in thermal conductivity of composites. For improving the property of CF-CNF/A336 alloy composite as expect, the CNF should be further dispersed such as using vibration milling to decrease the CNF agglomeration and Cu or Ni layer coated on CNF would improve the wettability between CNF and matrix to reduce porosity and defects in the composite.
Thermal conductivity of composites: (a) CF/pure Al composite, (b) CF/A336 alloy composite and (c) CF-CNF/A336 alloy composite.
In the present study, for developing multifunctional carbon fiber reinforced Al matrix composites. A new manufacturing method: low-pressure infiltration method was adopted for fabricating carbon fiber reinforced pure Al, A336 alloy composites and carbon fiber-carbon nanofiber reinforced A336 alloy composite. The significant conclusions are summarized below.
This work was supported in part by JSPS KAKENHI Grant Number 18K03839.
