The water content of the binder was determined by the thickness of the coating layer for diluting the water-glass content and for maintaining the pore size for the infiltration. Nevertheless, it does not affect the infiltration if the appro- priate pores exist. The preforms, after being dried at 150C (303F) for 5 hours and sintered at 700C (1292F) for 2 hours, were kept at 150C (303F) or at room temperature in the at- mosphere just before the infiltration experiments. For some of the infiltration experiments, a preform (whose interior gas was replaced with argon) was used.
a ball mill for 24 hours. This forming process is identical to the carbon dioxide process that was widely used in found- ries.11
The preform, preheated above the 900C (1652F) alumi- num melt for 5 minutes, was fully dipped into the melt in air along with an iron weight to prevent any buoyant flo- tation as shown in Fig. 1. The preform, whose interior gas was fully replaced by argon, was infiltrated in an argon atmosphere. The infiltration process was monitored using a load sensor for measuring the weight change during the dipping, namely, the replacement of the trapped gases by
the melt. This data was stored in a digital recorder at 1ms intervals. The chemical composition of the aluminum melt was 12.6mass%Si - 3mass%Mg.
Production of High Volume Fraction MMC by Particle Mixing
The SiC volume fraction of the preform, fabricated with single particles, is about 54% and independent of the particle size. Therefore, the mixing of the two kinds of SiCps, different in size, has been discussed for producing the high volume frac- tion preform. Itoh et al., discussed the influence of the particle fraction on the packing density using a binary particle sys- tem12
and showed that the density increases with the fraction and that the maximum value exists near the 0.72 volume frac- tion of the large ones. Moreover, the fraction increases with the particle size ratio. The volume fraction ratio, the smaller particles being 0.3 and that of the 420 µm one being 0.7, was
then selected. The fabrication process for the MMC was com- pletely identical to that of the above-mentioned process ex- cept for the mixing of the two particles.
Figure 1. Schematic of experimental method.
Figure 3. High magnification photo of MMC cross-section.
Cross section of MMC
Figure 2. Preform, MMC and cross-section of MMC. (Single SiC particle’ MMC, SiC size: 420μm, Vf: 54%)
24
Time, s
Figure 4. Infiltration of aluminum melt into 420 μm SiC- preform measured by weight change.
International Journal of Metalcasting/Spring 11
Weight Change, g
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