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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

4.8.5 Advancing

4.8.5 Advancing infiltration with alloy Al-Si alloy IS In order to determine the shape factor α, defined by Equation 29, the infiltration curves of the self-fabricated preforms with the lowest and the highest threshold pressures P0 (namely AOPC20 and TOPC10) were investigated until the point of complete saturation was reached. The infiltration curves up to a pressure of 10 MPa are shown in Figure 4.49. The curves were fitted to a function of the type given in Equation 29 by taking the threshold pressure given in Table 4.5 and varying the shape factor α in the range of 0.1 to 10 MPa -1 . Pressure Pmet P/MPa (MPa) met 10 8 6 4 2 AOPC20_experiment TOPC10_experiment TOPC10 fit α = 1.25 MPa -1 AOPC20 fit α = 2.25 MPa -1 0 0.0 0.2 0.4 0.6 Saturation S() 0.8 1.0 Figure 4.49 Infiltration curves of AOPC20 and TOPC10 and fitted functions in accordance with Equation 29 to determine the shape factor α. The best fit for the TOPC10 preform was attained by using α =1.25 MPa -1 . A higher α of 2.25 MPa -1 was required for the best fit function for the AOPC20 preform. The least squares fit curve of AOPC20 with the best fit parameters showed deviations from the experimental data points in the saturation range between 0.4 and 0.8. Deviations from the infiltration curve for TOPC10 were evident in the saturation range above 0.8. 145

4.8.6 Non destructive testing of MMC infiltrated with alloy IS Non-destructive X-ray computer tomography was conducted on the infiltrated MMC samples to detect defects resulting from processing. In order to determine the MMC properties with minimum influence of internal defects, the aim was to have a structure as homogenous as possible. As an example, virtual cross-sectional views in the X-Y and Y-Z planes, which were generally taken from the respective centre planes of the MMC, were extracted from the three dimensional computer tomography data and are shown for the TOPC20IS MMC in Figure 4.50 and Figure 4.51 for different infiltration tool set-ups. The light areas represent the infiltrated preform and the darker areas the pure infiltration Al-Si alloy IS. The central linear discontinuity in the X-Y plane of Figure 4.51 was an artefact resulting from the reconstruction algorithms, and was visible to a greater extent in Figure 4.51 than in Figure 4.50. In the MMC infiltration process, the preform was initially placed on the bottom punch. It remained in this position throughout infiltration. Even though it had to be assumed that the preform floated as a result of differences between its bulk density and the melt density partial premature solidification at the edge between the bottom punch and the die wall, as indicated in the temperature measurements in 4.8.2, prevented the melt flowing under the preform and therefore floating was impeded. The right hand side of the Y-Z plane views of the virtual cuts in Figures 4.50 and 4.51 represent the X-Y plane of the preform which was initially in contact with the bottom punch. In the first infiltration tool, the bottom punch was completely made of tool steel as shown schematically in Figure 4.50. In the X-Y plane, darker areas were visible in the region close to the centre of the MMC, indicating material with lower X-ray absorption. Metallographic characterisation attributed the inhomogeneities to non-infiltrated coarse pores formed by the 146

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