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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

There were minor

There were minor differences in the compression of the Al2O3 preforms infiltrated in ISQC compared to those infiltrated in HPDC at the three equivalent infiltration velocities (0.18, 0.36 and 0.72 m/s). Marginally higher compressions were found in the HPDC mode, although the mean values lay in the scatter band of the compressions of ISQC. In contrast, the compression of the reactive preforms between the infiltration modes was different, as shown in Figure 4.75. For example, the FATO preform showed significantly different compressions in the three overlapping velocities of ISQC and HPDC. The compression in HPDC was more than double that in ISQC at given velocity. A similar trend was visible with the TOPC10 preforms. In the ISQC mode, compressions of 0.04 to 0.14 were found, compared to 0.22 to 0.23 in the HPDC mode. The sudden drop to a low compression of 0.04 at a velocity 0.36 m/s in the ISQC mode did not occur in the HPDC mode. As aforementioned, MOPC20 preforms were only infiltrated in DSQC and HPDC. In the HPDC mode, this preform exhibited the lowest compression of all preforms fabricated using a pore forming agent. 4.10. MMC properties The mechanical and the tribological properties of the MMCs with an Al-Si alloy matrix were assessed and compared to the properties of the pure alloy IS. 4.10.1 Influence of inhomogeneities on bending strength The influence of inhomogeneities on the 4-point bending strength was investigated by comparing the local bending strength along the x-axis of MMCs produced with the standard infiltration tool with those produced using a copper chill insert in the bottom punch. The bending strength was determined as described in 3.5.5. Each data point in Figure 4.76 is the bending strength of a single bend test bar at the defined x-position. Both curves in Figure 4.76 showed a decrease in bending strength in the centre of the MMC but the extent of this 175

decrease depended on the tooling used. The MMC produced with an all-steel plunger exhibited a steep decrease in strength from 420 MPa at the lower x-axis to 305 MPa towards the centre position. In contrast, the strength did not fall below 375 MPa in the sample produced with the Cu insert on the bottom punch. Bending strength σ b (MPa) Bending strength σ b / MPa 500 400 300 200 100 0 Al-Si alloy MMC X steel . steel + Cu-insert 0 10 20 30 40 50 60 Position x / (mm) mm Figure 4.76 Bending strength along the x-axis of the TOPC10IS MMC with bottom plunger made of steel or steel with a central chill. The cross-section of a TOPC10 preform infiltrated in a die with a steel bottom punch shown in Figure 4.77 a) shows that the porosity in the MMC was concentrated towards the centre in the x-direction and about 2.5 mm from the top of the preform in the z-direction. In the detailed view in Figure 4.77 b), the relative pore fraction was assessed to be 8.2% using image analysis. The predominant fraction of the porosity was interconnected with a maximum dimension of 250 µm. Therefore, it represented the coarse porosity of the preform whereas the fine pores between the particles of the preform were filled with metal. In order to exclude the influence of porosity on the properties, the following results were taken from MMCs infiltrated using a bottom plunger with a central Cu chill. Furthermore, the 176 Z

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