Production of Fine Particulate MMC Sandwich Process-1
As described in the experimental procedure section of this paper, it was shown that the fine particulate MMC cannot be completely infiltrated. Therefore, we had to examine a new process, in which the smaller particles (i.e., 30 µm or 68 µm) were enclosed in the center part between the 420 µm particle outer layers. This is called a sandwich process. The fabrica- tion process of the preform was completely identical to that of the above-mentioned ones except for the sandwiched lay- ers of particles.
Sandwich Process-2
The experimental results of the Sandwich Process-1 showed poor infiltration into the small particle area due to the ag- glomeration of the fine particles due to cavity plugging by the binder, which acts as a barrier to the infiltration. There- fore, after drying the small particle preform, the preform was then milled into the original particles, and these particles were then enclosed in the center part of the large particle layers. This means that each particle was coated with the binder without any plugging. This fabrication process was completely identical to that of Process-1 except for using the milled particles.
Experimental Results and Discussion Influence of SiC Particle Size on Infiltration
The preform, made of fine particles (i.e., 30 and 68 µm) cannot be completely infiltrated. Nevertheless, if we used larger particles (i.e., 220 µm and 420 µm) the infiltration is nearly perfect and the volume fraction of the SiC is 54%, as
shown in Figs. 2 and 3. Moreover, the surface of the infil- trated MMC is completely covered and wetted by the melt as clearly seen in Fig. 2. This means that their contact angle should be 0°
and AlN, which causes a pressure drop in the preform. The details will be given in a later section.
to produce MgO, Mg3 N2 , Al2 O3
. How does the infiltration occur up to the cen- tral part? It should be the reaction of the trapped gases in the preform with the Mg and Al8
The weight change curve for the 420 µm preform is shown in Fig. 4. As can be clearly seen, after repeating the saw- tooth patterns for 100 seconds after dipping, the infiltration suddenly occurs and is finished within a few seconds. The cycles of the sawtooth pattern become gradually longer with time as shown; nevertheless, the amplitude, namely the weight difference from the bottom to the top, remains nearly constant at 3g. The volume of one bubble, calculated based on the amplitude, is about 1.2 ml. Therefore, the mechanism of the infiltration for the 420 µm preform can be proposed as shown in Fig. 5, namely by repeating the formation and flotation of bubbles.
Nevertheless, if the preforms made of the mixed SiC par- ticles (i.e., 420 µm and 68 µm) were used for the infiltration experiments, poor infiltration occurred in the fine particle area, denoted by the circles in Fig. 6, but the volume fraction of the SiC increased to 67%, as expected.
During these experiments, the infiltration sometimes does not completely occur even in the case of the 420 µm SiC preform. The appearance of the typical non-infiltrated pre- form is given in Fig. 7. As can be clearly seen in the photo, the preform surface is not completely wet with the alumi- num melt because of the thick oxide film which acts as a barrier to the chemical reaction between the melt and the trapped gases in the preform. In these cases, after repetition
Figure 5. Schematics of saw tooth pattern and gas bubble formation and release cycle.
International Journal of Metalcasting/Spring 11 25
Weight Change
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