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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

4.7. Constant pressure

4.7. Constant pressure infiltration Efforts to determine the progress of infiltration in one of the constant flux infiltration setups by prematurely stopping the infiltration process were not successful as the preforms were either not infiltrated or fully infiltrated as result of the relatively short infiltration period. Even at the slowest velocity (0.07 m/s in DSQC), complete infiltration took less than 1 s. The preform was infiltrated within 1 ms at the highest infiltration velocity used in the current investigations at a plunger velocity of 4.0 m/s in the ISQC mode (Table 3.8). Instead of the constant flux infiltration, a constant pressure infiltration using gas pressure was developed to investigate the first steps of preform infiltration. The infiltration of preforms in the constant pressure mode was performed using a die similar to that used for the DSQC except a lid was used as an upper plunger which could be closed immediately after the metal was poured into the die cavity. After closing, the pre-selected gas pressure was applied immediately onto the top surface of the metal melt. In order to investigate only the effects of pressure, without reaction between the ceramics and the metal, the inert system consisting of an AOPC20 preform infiltrated with the alloy IS was studied. In order to prevent gas flow from the pressurized volume on the top towards the bottom and the inside of the preform, a self sealing effect of the metal melt was used. Initially the gas pressure forced the liquid metal into the gap between the die and the preform. Intimate contact of the melt on the die wall was achieved by using a coating of graphite and K2ZrF6. The latter was used to promote wetting of the die wall by the melt (6,21) . As the melt reached the bottom punch, the metal solidified partially in the edge between the lower punch and die wall, as shown by the curvature of the solidified metal towards the bottom punch, indicated by an arrow in Figure 4.39 a). The solidification shrinkage of the metal resulted in a gap formation. Therefore, the air which was displaced by the melt which infiltrated the preform could escape 133

unhindered through the gap between the bottom punch and the die walls. As a result, the metal could flow from the top and the side walls into the preform. In Figure 4.39 a) the macrograph of the preform infiltrated at 0.40 MPa shows minor metal intrusions on one side towards the melt reservoir on the top of the preform. Metal could be maintained in the molten condition directly on top of the solidified region as the preform and metal were both heated to 800°C. At an infiltration pressure of 0.80 MPa, the liquid metal entered the preform from the side walls as well as from the top as shown in Figure 4.39 b). The infiltration depth on the top and outer sides reached 3 and 13 mm respectively. The centre line along the y-axis showed axisymmetric intrusion behaviour. The macrostructure consisted of adjacent grey and white phases in the infiltrated regions. a) 0.40 MPa b) 0.80 MPa c) 1.20 MPa 134 2 µm 10 mm Figure 4.39 Macrographs of cuts through the xz-plane of AOPC20 preforms infiltrated with the alloy IS at constant gas pressures of 0.40, 0.80 and 1.20 MPa. As shown in Figure 4.39 c), at an infiltration pressure of 1.20 MPa, the metal reached all other regions, apart from an area near the bottom of the preform. Some white spots are visible. Similar to the sample infiltrated at 0.80 MPa, an axisymmetric arrangement of the metal intrusions was found. Therefore the subsequent metallographic investigations were performed on a half preform in the X-Z plane. The microstructures of the AOPC20 preforms infiltrated at 0.80 MPa and 1.20 MPa and the visualisation of the pore saturation profiles are shown in Figure 4.40 and 4.41, respectively. Z X

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