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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

measurable compression

measurable compression in the resulting composite material. If solidification occurs prior to full infiltration, the relaxation is hindered and compression is observed. Fibre-based reinforcements in combination with aluminium alloy matrices were the only systems investigated using HPDC infiltration. Just a few groups have published work on this subject. Generally the plunger velocity was lower and the gate area was larger than in standard HPDC. Thus, the casting processes are closer to indirect squeeze casting conditions (135) than HPDC conditions. The reason for this may lay in the suggestions by Long et al. (50) who introduced pre-requisites for high quality Al-MMC casting via pressurised infiltration techniques. They recommended that preforms should be preheated to a temperature significantly above the minimum limit suggested by Nagata et al. (131) , in the range of 750°C to 800°C. If the preform temperature was too low, high deformation and inhomogeneous microstructures were obtained, as shown in Figure 2.18. A low preform temperature of 400°C resulted in compression and residual porosity whereas a homogenous fully infiltrated microstructure could be achieved with a preform temperature of 800°C. Figure 2.18 Influence of preform preheating temperature on MMC microstructure (50) . 59

Long et al. (50) suggested that v0 should be below 0.1 m/s to prevent crushing of the preform. Another requirement to attain homogeneity in the MMC is a large cross-sectional area of the gate to ensure feeding of the MMC to prevent solidification shrinkage. Rasmussen et al. (146) infiltrated Al2O3 preforms with Al-Si and Al-Mg alloys using HPDC. In contrast to Cappleman et al. (116) who could infiltrate Al2O3 fibre preforms without any visible reactions, the infiltration with an AlMg alloy was unsuccessful due to reactions and blocking of the capillary entrances. Due to the configuration used in the experiment, where the melt was in extended contact with the reinforcement prior to infiltration, this behaviour may be a result of extensive melt-matrix reactions and thus might not be a result of the HPDC infiltration process itself. Kang et al. (117) infiltrated fibre preforms using a broad range of plunger velocities from 0.1 m/s to 3.6 m/s. A velocity of 0.6 m/s was considered to be the optimum in terms of MMC homogeneity. At higher velocities, massive preform deformation and cracking occurred, making the MMC useless. 60

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