Manufacturing Process of Carbon Short Fiber Reinforced Al Matrix With Preformless and Their Properties


 Conventional manufacturing process of fiber reinforced metal matrix composites via liquid infiltration processes, preform manufacturing using inorganic binders is essential. However, the procedure involves binder sintering, which requires high energy and long operating times. A new fabrication process without preform manufacturing is proposed to fabricate carbon short fiber (CSF)-reinforced aluminum matrix composites using a low-pressure infiltration method. To improve the wettability and avoid interfacial reactions in CSF/Al matrix composites, the fibers were plated with copper using electroless plating process. Various volume fractions of CSFs were used to determine optimum fiber content which would produce versatile mechanical and thermal properties. Effect of CSFs content on properties such as Vickers hardness and thermal conductivity was studied. Cu-plated CSFs showed good bonding with the Al matrix and CSFs were randomly dispersed inside the composites, with CSF content of up to 29.1 vol.%, through the new manufacturing process. It showed better fiber distribution than the composite fabricated perform with SiO2 binder, which was determined by comparing the relative frequency distribution of CSFs in composites. Vickers hardness of the composites showed an obvious improvement over that of the Al matrix, and the hardness increased as the CSF content increased. The Cu-plated CSF (14.3 vol.%) reinforced Al matrix composite exhibited the highest thermal conductivity (184.1 W·m-1·K-1). However, the thermal conductivity decreased as CSF content increased to 29.1 vol.% due to the defects in composite.


Introduction
Carbon ber (CF) is one of the most appealing reinforcements and possesses low density, high tensile strength and high thermal conductivity (TC) (Seo et al. 2009). CF-reinforced aluminum composites combine the properties of CF and Al, exhibiting low density and excellent mechanical and thermal/electrical properties (Hajjari et al. 2010, Lancin et al. 2000 and are expected to be utilized as heat sink materials. The conventional in ltration processes for fabricating CSF reinforced Al matrix composites generally use inorganic binders such as SiO 2 (Meng et al. 2018), Al 2 O 3 (Zeng et al. 1998), or TiO 2 (Clemet et al. 2007) for the CSF preform manufacturing. However, preform manufacturing with an inorganic binder is complicated, and the preform needs high temperatures and more time for binder sintering. Therefore, a novel fabrication process that is cost-effective with high production e ciency without requiring preform manufacturing is needed for manufacturing CSF/Al composites.
In the liquid in ltration process for fabricating CF-reinforced Al matrix composites, problems such as poor wettability between CF and Al matrix, and the high reactivity of CFs with Al and its alloys, forming undesirable interfacial reaction products (e.g., Al 4 C 3 ), limit the applications of CF reinforced Al matrix composites ). To solve these, surface modi cation of CF is proposed as the most effective way to improve wettability and prevent the formation of interfacial reactions. Copper (Cu) has been proven to be effective as it improves the wettability, thermal conductivity, and impact strength ). The low-pressure in ltration process (LPI) ) has been widely adopted for manufacturing composites due to the advantage of requiring relatively simple facilities and being cost-effective.
The objective of this research is to develop a new process for fabricating CSF reinforced Al matrix composites without preform manufacturing. An electroless Cu plating process was conducted on the CSFs to improve wettability. The most suitable electroless Cu plating conditions and in ltration conditions were analyzed. In addition, the effect of CSF content on Vickers hardness and thermal conductivity of composites without preform manufacturing was also investigated.
Experimental Procedure A1070 with a purity of 99.7% was used as the matrix in this experiment. Pitch based CSF (K13D2U, Mitsubishi Plastics, Inc.), with a diameter and aspect ratio of 11 µm and 230, respectively, was used as the reinforcement. To skip the preform manufacturing process, electroless Cu-plating was conducted on CSFs for the 30-120s. The pH and temperature of the electroless plating bath were 12 and 298 K, respectively. The electroless Cu plating process is summarized in Table 1. Cu-plated CSFs were put into a graphite mold of diameter 10 mm with height adjusted to 10 mm to control the volume fraction of CF, the A1070 ingot was placed on the CSFs and graphite, and a punch was placed on top of the A1070 ingot.
The composites were fabricated via the LPI process at 1023 K in an Argon atmosphere by applying 0.4, 0.6, and 0.8 MPa of pressure. The volume fraction of CSFs ranged from 7.1 vol.% to 29.1 vol.%. Figure 1 shows the Manufacturing process of carbon short ber reinforced Al composite by preformless. Besides, CSF preform of 10 vol.% reinforced composite fabricated with SiO 2 binder (Choi et al. 2017) was used in the comparison of ber dispersion in composites. The microstructures of the Cu-plated CSFs and Cuplated CSF/Al matrix composites without preform manufacturing were observed using a scanning electron microscope (SEM, TOPCON SM-520, Japan). The Vickers hardness test was carried out using a load of 3 kg f for 10 s. The thermal conductivity of Cu-plated CSF/A1070 composites without preform manufacturing was evaluated by laser ash method thermal constants measuring system (TC-9000H, ULVAC-RIKO Inc., Japan) at room temperature.

Results And Discussions
Microstructures of Cu-plated CSFs and Cu-plated CSF/Al matrix composites. Figure 2 shows the SEM images of as-received CSF and Cu-plated CSFs at different plating times. It was observed that the asreceived CSFs exhibited a clean and smooth surface, as shown in Fig. 2 (a). Figure 2 (b) illustrates that a perfect, uniform Cu-plated layer was acquired over the CSFs after plating for 30 s, which could provide uniform wettability and protection. With increasing plating time, the Cu particles attached on the CSF surface lead to an uneven plating layer as shown by the arrows (Figs. 2 (c) -(e)).   good dispersion of CFs. Figure 5 (a), the CSFs in containing 7.1 vol.% were more or less uniformly distributed and that some ber clusters were present in the composites. As the ber content increased up to 14.3 vol.%, the uniformity of distribution of the CSFs in the composite increased ( Fig. 5 (b)). It is di cult to fabricate CSF/Al matrix composite with a CSF content above 20 vol.% using the conventional fabrication process with CSF preform with SiO 2 binder. The Cu-plated CSF/A1070 composites without On one hand, Cu-plated CSFs possessed a density similar to molten Al which can be owed with molten Al during the in ltration process. On the other hand, the Cu-plated layer was dissolved into the matrix which obviously improved the wettability between the CSFs and Al matrix, endowing CSFs with better dispersibility. Figure 6 (d) shows the relative frequency distribution of bers mainly oriented at 90° in composite fabricated with SiO 2 binder. This is attributed to the CSFs orientation which was perpendicular to the pressing direction during the preform manufacturing process. As a result, most CSFs were perpendicular to the in ltration direction. Figure 7 shows the image mean free path (IMFP) of the Cu-  Figs. 7 (a)-(c). The mean free path decreased as the volume fraction of CSFs increased, causing the microstructure of composites to became much ner as the volume fraction of CSFs increased. The introduction of CSFs can prevent grain growth leading to ne grains in composites. As for the CSF preform of 10 vol.% reinforced Al matrix composite fabricated with SiO 2 binder, the mean free path was 71.9 µm, as shown in Fig. 7 (d). The mean free path of composite containing 10 vol.% CSF prefrom fabricated with SiO 2 binder was much lower than the predicted value of the composite containing 10 vol.% CSFs fabricated without preform manufacturing. It was proven that the new fabrication process without preform manufacturing enables the production of composites with re ned microstructures and higher reinforcement volume fractions.
Vickers hardness and thermal conductivity of each composite. Figure 8 shows   Fig. 9 (b), possessed the highest TC among the composites. The high TC was due to the excellent ber dispersion in the composite, as showng in Fig. 5 (b). However, the composite containing 29.1 vol.% CSFs exhibited a drop in TC, as shown in Fig. 9 (c). According to the microstructure shown in Fig. 5 (c), CSFs agglomeration was observed due to the high volume fraction of bers, and imperfect in ltration defects arose from the ber clusters, causing a decrease in TC.

Conclusions
A new fabrication process without preform manufacturing was developed for CSF/Al matrix composites.

Declarations
Data availability statement The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

Figure 1
Manufacturing process of carbon short ber reinforced Al composite by preformless.