the adsorbed moisture, which prevents the chemical reaction between the trapped gases and Mg or Al in the melt, then there is no significant pressure drop to cause the spontaneous infiltration. Nevertheless, if the oxide film is thin, the chemi- cal reaction occurs to afford the pressure drop, and then the film should be broken by the suction of the melt due to the poor elongation, as shown in Fig. 10. This is the infiltration mechanism and is dependent on the pressure reduction by the chemical reactions between the entrapped gases in the pre- form and the elements in the melt, which assists the breaking up of the oxide film at the interface between the melt and the preform. The reaction allows the melt to be sucked into the preform; then it leads to the breaking up of the oxide films to facilitate wetting and infiltration. Therefore, a fresh alu- minum melt appears and should react not only with the gases but also with the Fe2
This is the onset of infiltration, and the infiltration rate then significantly increases due to the increase in the fresh sur- face. The wetting and infiltration are suggested to occur in a self-sustained cycle between more chemical reaction and
O3 pressure drop until all the gases are consumed.
If the SiC particle size is as small as 30 and 68 µm, the cur- vature radius of the oxide film surface is very small, and
powder and the water glass binder.
the breaking pressure then increases with a decrease in the particle size; therefore, the infiltration does not occur. The required pressure for the break can be calculated by the Young-Laplace equation using the curvature. Therefore, the sandwich process in which the outer layer acts in removing the oxide film was studied.
Sandwich Process
If we use fine particles, such as 30 and 68 µm, which were enclosed between the 420 µm particle layers, i.e., Sandwich Process-1, the spontaneous infiltration insufficiently occurs within a few hundred seconds after the dipping. The cross section of the MMC is shown in Fig. 11. This is one of the verifications of our proposed infiltration mechanism; name- ly, the onset of the infiltration is the breaking of the melt’s oxide film by the outer layers. Nevertheless, the infiltration is insufficient due to the agglomeration of the SiC particles by the binder, namely, the plugging of the infiltration paths. Fig. 12 shows the perfect infiltration into the fine particle area by Sandwich Process-2. In this case, the 30 µm SiC par- ticles were used as the fine particles. Therefore, the milled particles, enclosed in the center part of the large particle lay- ers, can maintain the paths for the infiltration.
Figure 11. Macro- and micro-structure of Sandwich Process-1 sample. (Size of SiC, outer shell: 420μm, inner shell: 68μm)
Figure 10. Schematics of spontaneous infiltration mechanism.
International Journal of Metalcasting/Spring 11
Figure 12. Inner layer microstructure of Sandwich Process-2 sample. (Size of SiC, outer layer: 420μm, inner layer: 30μm)
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