Functionally graded materials (FGMs) are known as a relatively new class of inhomogeneous composite materials having property gradient. Centrifugal solid particle method (CSPM), which is application of centrifugal casting, is one of the effective methods for fabrication of the ring shaped or hollow cylindrical shaped FGMs with graded distribution of ceramics particles. In this method, casting of molten metal matrix composite (MMC) with ceramics particles is carried out under centrifugal force. Ceramic particles segregation under the centrifugal force occurs due to the difference in density between the ceramics particles and molten metal, e.g. the SiC ceramic particles within the molten Al are moved to the outer part of spinning mold because of their higher density compared to molten Al. Because the densities of SiC and molten Al are 3.21 Mg/m3 and 2.315 Mg/m3, respectively. The schematic illustration of process of ceramic particles graded metal matrix FGMs by CSPM is shown in Fig. 1 (a). Recently, as a practical CSPM, centrifugal mixed-powder method (CMPM) was developed. In this method, a mixed-powder of metal particles and ceramics particles is inserted into a spinning mold. Then, molten metal is poured into the spinning mold with the mixed-powder. As a result, the molten metal penetrates into the space between particles by pressure due to the centrifugal force. At the same time, the metal particles within the mixed-powder are melted by the heat from molten metal. Finally, ring shaped or hollow cylindrical shaped FGMs with ceramic particles distributed on its outer surface can be obtained. The schematic illustration of the process of CMPM can be seen in Fig. 1 (b). In this study, formation of compositional gradient in Al/SiC FGMs fabricated under huge centrifugal forces using CSPM and CMPM is studied.
The Al/SiC FGMs sample is fabricated by CSPM under vacuum condition with initial temperature of molten Al of 1023 K, centrifugal force of 1120 G and mold temperature of 773 K. Simulation is performed under the same conditions as that for the experiments for the Al/SiC FGMs fabricated by CSPM. Figure 2 (a) shows the cooling curves calculated at the inner surface of cast and at the inner and outer surface of mold for 1 s. The corresponding temperature distributions along the thickness of the cast at different times are shown in Fig. 2 (b). As can be observed, the temperature of the cast and the mold reached equilibrium in less than 1 s. The final formation of compositional gradient in the Al/SiC FGMs obtained by the simulation is shown in Fig. 2 (c). The experimental graded distribution of SiC ceramic particles in the Al/SiC FGMs fabricated by the CSPM is also shown in Fig. 2 (c). Better agreement is observed between the simulated and experimental trends of SiC ceramic particles distribution. From this figure, the steeper particle gradient along the radius direction can be observed. Such gradient type is appropriate for use in the production of grinding wheels.
The simulation for Al/SiC FGMs fabricated by CMPM is similar to the simulation conducted for Al/SiC FGMs fabricated by CSPM. Here, the mixed-powder of SiC ceramic particles and Al particles, Al-20vol%SiC, are placed inside the mold before casting. The mixed-powder filled the inner mold with the thickness of 0.38 mm. The packing limit of volume fraction is set to be 0.6. Figure 3 (a) shows the cooling curves at the inner surface of cast, initial mixed-powder zone and outer surface of mold, respectively, during solidification process under vacuum condition with initial temperature of molten Al of 1023 K and centrifugal force of 1120 G. As can be observed, the temperature of initial mixed-powder zone is raised from 773 K to above 900 K within less than 1 s. At this point, the Al particles are melted by the heat from molten Al, and then SiC ceramic particles start move to the outer periphery with the relative time spent. The temperature distributions along the thickness of the cast at different times are shown in Fig. 3 (b). The numerical and experimental formation of compositional gradient of the Al/SiC FGMs fabricated by CMPM are shown in Fig. 3 (c). A particle free zone can be clearly seen, while all particles concentrate at near the outer surface. It can be seen that, regardless of the size of SiC ceramic particles, similar compositional gradient in Al/SiC FGMs can be obtained. Comparing the volume fraction of SiC at the outer surface calculated in simulation with the experimental results reveals a good agreement for the case of 30 µm SiC ceramic particle size, but not in the case of 1 µm SiC ceramic particle size. The difference between experimental and simulation results is mainly due to the assumption made in simulation that all SiC ceramic particles are fixed in Al matrix. Experimentally, in the case of 1 µm SiC ceramic particle size, when the Al/SiC FGMs is removed from the mold, many loose SiC ceramic particles are detached from the outer surface.
