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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

organic additive in the

organic additive in the preform processing. The depth of the porosity field could be visualized in the central cut in the Y-Z plane of Figure 4.50. The porosity was situated towards the upper central volume of the preform. X-Y Y-Z Figure 4.50 Virtual cross-sectional cut through the MMC TOPC20IS in the X-Y and the Y-Z planes (pore field circled). In order to improve the homogeneity of the MMC, the thermal conduction in the die was altered to achieve directional solidification from the bottom punch through the preform to the melt pool on top of the preform. As a first step, ceramic coatings generally used for thermal management in gravity die casting were applied to the upper punch and the die walls. As the upper punch slid along the die walls during infiltration, the ceramic coating resulted in extensive abrasive wear in the gap, making further movement and infiltration impossible. The application of the ceramic coating solely to the upper punch did not prevent inhomogeneities in the MMC, as the coating was penetrated at an early stage of infiltration and therefore thermal conduction toward the upper punch was not significantly reduced. Therefore measures were taken to improve thermal conduction toward the bottom punch and to preserve conditions on the die walls and the upper punch. 147 z y x

X-Y Y-Z Figure 4.51 Virtual cross-sectional cut through the MMC TOPC20IS in the X-Y and the Y-Z planes (pore fields circled). Schematic cut through the infiltration tool with an integrated chill. When an insert made of high heat conducting material (a copper alloy) was integrated into the bottom punch, the porosity in the centre of the MMC was reduced. The effect is shown when comparing the pore field intensities in Figure 4.50 and Figure 4.51. Two fields of minor porosity were detected outside the central area, as shown in the cut of the X-Z plane in Figure 4.51. These fields were of significantly lower intensity than that detected in the MMC made without a copper chill, Figure 4.50. All MMCs characterized in the following sections were fabricated using a bottom punch with a central copper chill. 4.8.7 Microstructure of MMCs with Al-Si alloy matrix The light phase in the micrographs represents the metal and the darker phase the ceramic. The micrographs are presented at low magnifications in order to show the coarse metal ligaments originating from the filled PFA-formed porosity and higher magnification micrographs show the intragranular metal ligaments between the ceramic particles. All images were taken from the central area of the MMCs cut in the X-Z plane. Therefore, the horizontal and the vertical axes of the images correspond to the X- and Y-axes of the MMC respectively. 148 z y x

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