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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

gradient

gradient of the curve changed was observed above a saturation of approximately 0.8. Here a steep pressure increase occurred up to the maximum present infiltration pressure of 100 MPa. The investigations showed that infiltration curves may be subdivided into three stages: 1. Initiation of infiltration up to a saturation of 0.1 2. Steady increase in pressure over the saturation range of 0.1 to 0.8 3. Steep increase in pressure towards full saturation of 1.0 The analysis of the infiltration curves was therefore subdivided into these three stages. 4.8.4 Initiation of preform infiltration with Al-Si alloy IS The first step of the preform infiltration was characterised by the pressure to initiate filling, designated as the threshold pressure P0. In order to determine this parameter, the first 2 to 5.7 mm of infiltration depth was plotted as a function of the applied pressure as presented in Figure 4.48. The depth calculations were based on the assumption of infiltration on five sides of the preform, as was found in the constant pressure experiments (section 4.7). In accordance to Darcy´s law (103) , the infiltration depth is a square root function of the applied metal pressure Pmet. Therefore a linear behaviour was expected in plots of L² against Pmet. The data were fitted to a linear equation with a slope of a and an intercept on the pressure axis of P0. As shown in Figure 4.48, the behaviour was close to linear with the exception of AGPC15 and TOPC10, where a transient behaviour was observed. The preform type affected the pressure axis intercept and the slope of the fitted lines. The values of the slope a and the threshold pressure P0 are summarized in Table 4.5. The lowest threshold pressure of 0.65 MPa and the steepest curve were found for AODY30. The slopes of 143

the linear fits for AOPC20, TOPC20 and FATO were similar but the intercept increased in the same order from 0.72 to 0.94 MPa. Infiltration depth L² (mm²) /mm² 35 30 25 20 15 10 5 0 0 1 2 3 4 Pressure Pmet P /MPa (MPa) met 144 AOPC20 AGPC15 TOPC20 TOPC10 MOPC20 AODY30 FATO Figure 4.48 Square of infiltration depth of different preform types as a function of the melt pressure (alloy IS). Linear regression of the infiltration curve in the range of 2 mm to 5 mm infiltration depth. The MOPC20, AGPC15 and TOPC10 preforms exhibited comparatively lower gradients and higher threshold pressures. The TOPC10 preform exhibited the highest threshold pressure of 2.02 MPa and a steeper gradient than MOPC20 and AGPC15. Table 4.5 Results of infiltration curve regression analysis when infiltrating with IS. Gradient a and threshold infiltration pressure P0. a P 0 (at L²=0) mm² / MPa MPa AOPC20 30.4 0.72 AGPC15 11.5 1.27 TOPC20 33.6 0.91 TOPC10 18.5 2.02 MOPC20 14.3 0.95 AODY30 63.8 0.65 FATO 31.9 0.94

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