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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

in all preforms during

in all preforms during infiltration, may influence the parameters Vp and Si. In the DSQC process it was rather low, between 0.02 and 0.04 (Table 4.8), leading to negligible variations in Vp. The parameters of the preforms investigated as well as P0 and the resulting cos θdyn are listed in Table 5.2 for both IS and IM infiltration. As reported by Koeber and Loehberg (25) , the surface tension γlv of Al-12 wt % Si, (composition similar to IS), is similar to that of pure Al at any given temperature. Therefore Equation 1, which is valid for pure Al, is also applicable for calculating γlv in IS. At the initiation of preform infiltration, the melt temperature Tm was 665°C, resulting in a surface tension of 0.883 N/m, which agrees with experimental values (19) which were in the range of 0.801- 0.889 N/m for an Al-Si alloy. The specific surface area SiHg was measured experimentally using mercury intrusion porosimetry (MIP) and was used in preference to gas absorption (BET) as it is closer to the preform infiltration procedure. This is supported by Garcia-Cordovilla et al. (103) who showed that BET results are not relevant in pressure infiltration. Table 5.2 Preform specific and infiltration parameters and calculated θdyn with IS and IM infiltrations derived where required from Equation 20. Infiltration with IS P 0 S sHg 1-Φ tot ρ p S iHg cos θ dyn θ dyn MPa m²/g kg/m³ 10 6 m²/m 3 ° AOPC20 0.72 0.86 0.35 3980 1.84 -0.44 116 AGPC15 1.27 1.49 0.34 3956 3.04 -0.47 118 TOPC10 2.02 2.30 0.38 4260 6.01 -0.38 112 TOPC20 0.91 0.76 0.38 4260 1.98 -0.51 121 MOPC20 0.95 0.75 0.38 3580 1.65 -0.65 131 AODY30 0.65 2.41 0.32 3920 4.45 -0.17 100 FATO 0.94 5.85 0.29 3830 9.15 -0.12 97 Infiltration with IM TOPC10 2.31 2.30 0.38 4260 6.01 -0.43 116 MOPC20 1.05 0.75 0.38 3580 1.65 -0.72 136 FATO 1.35 5.85 0.29 3830 9.15 -0.17 100 217

Compared to Hg, the Al melt may contain impurities or oxide films which are retained on the preform in a similar manner to the effect of ceramic filters in the gating system of castings (6) . The pores of preforms are significantly smaller though and retention may block the entrances to the pores. This effect was reported by Papworth and Fox (138) who observed delamination of the metal phase on the top of the squeeze cast infiltrated MMC due to oxide film agglomeration. In the present work, there was no evidence of this oxide layer retention effect in the microstructural analysis of the interface (Figure 4.59) and therefore blocking effects were discounted. Preforms with PC did not have significant bimodality in their pore size distributions (Figure 4.27). This configuration was found suitable for dynamic wetting angle evaluation (103) . With a θdyn of 116°, the pure Al2O3 preform AOPC20 in contact with IS showed similar behaviour to Al2O3 with a silicate binder (AGPC15) which had a θdyn of 118°. Therefore, no significant influence of the binder on θdyn was found. For the study the binder content of AGPC15 was in the range of that used for Saffil preforms (118, 120) . The θdyn values reported for pure Al into Saffil fibre preforms was in the range of 102° to 111° (121) and therefore lower than that found for AGPC15 (118°). With Saffil fibre preforms, a maximum ceramic volume fraction Vf of 0.25 is achievable which is significantly less than the Vp of 0.35 of AGPC15. A trend of decreasing θdyn with lower Vf was found (121) . By a linear interpolation of their results in the θdyn(Vf) diagram to a Vf of 0.35, a θdyn of 119° was found. Taking into account the scatter in θdyn, which was assessed as ± 3° based on the scatter in P0 measurements of identical preforms, AGPC15 and Saffil fibre preforms show similar θdyn. 218

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