of the sawtooth patterns, the pressure cycle time becomes longer with time and the weight remains nearly constant as shown in Fig. 8. This means that the gas generation from the preform is already finished at 180 seconds; nevertheless, no infiltration occurs due to the thick oxidized aluminum film.
A summary of these data for the correlation between the in- cubation time and the total out-gas volume, calculated from the cycle number and the total volume is given in Fig. 9. The bubble formation and flotation cycle time depends on the gas generation rate, mainly from the adsorbed moisture because the trapped gas volume must be less than 20 ml, estimated
from the volume fraction of the SiC particles in the preform. If the incubation time is greater than 160 seconds or the total out-gas volume is more than 200 ml, there is no infiltration. The volume of the trapped gases in the preform was less than 20 ml as already mentioned; therefore, the main source of the out-gas should be the adsorbed moisture from the air. In other words, if there is too much adsorption of moisture before the experiment or by poor preheating, there is no in- filtration as shown in Figs. 7 and 8.
We have already reported that the infiltration is triggered by the pressure decrease in the preform to assist by the chemical reaction between the trapped gases and the Mg or Al in the melt.8
Therefore, if the infiltration experiments were carried out in an argon atmosphere, there is no infiltration due to the lack of chemical reaction between the argon and the melt. Moreover, if the dipping is insufficient, namely a part of the preform emerges from the melt, the pressure decrease can- not be attained by the feeding of air through the surface, and then the infiltration does not occur.
If the particle size is less than 68 µm, there is also no infiltra- tion as previously mentioned; therefore, another infiltration process based on these results should be discussed. If the aluminum melt surface is covered with a thick oxide film, formed by the chemical reaction of the aluminum melt with
Figure 6. Microstructure of MMC formed by mixed SiC particle preform.
(SiC size: 420 + 68 μm, Vf: 67%)
Time, s Figure 8. Non-infiltration pattern of dipped SiC-preform.
Incubation Time, s Figure 7. Appearance of non-infiltrated SiC preform. 26
Figure 9. Influence of total out gas volume and incubation time on infiltration.
International Journal of Metalcasting/Spring 11
Total Out Gas, ml
Weight Change, g
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