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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

In order to determine

In order to determine the effects of this inhomogeneity on the microstructure, a titania-based TOPC10IS was investigated using metallography. Figure 4.72 shows TOPC10IS infiltrated in the ISQC mode at a velocity of 0.72 m/s. An overview of one half is shown, as they are axisymmetric. Due to the high velocity infiltration the preform was distorted. In Figure 4.72 a) the border between the lighter and darker regions is indicated by the line. It highlights that the thickness of the lighter region varied in between the Z- and the X-direction from about 2.1 mm to 3.8 mm respectively. Figure 4.72 b) shows that the lighter appearance of the outer areas was due to the higher fraction of metal than in the inner area shown in Figure 4.72 c). This was mainly attributed to the relatively fewer larger-scale metal ligaments in the centre. The loss of intergranular metal ligaments is attributed to preform compression at the expense of the intergranular porosity in the preform during infiltration. Y-Z Z Y X 10mm X-Y X-Z Figure 4.73 Virtual cuts through the central layer of the Y-Z, X-Y and X-Z plane of TOPC10IS infiltrated in the ISQC10 mode at v0 = 0.36 m/s. The only preform type with PFA formed pores which showed an intermediate decrease in compression was TOPC10IS (0.36 m/s - ISQC mode). Its virtual cut is shown in Figure 4.73. No compression and no porosity were visible inside the MMC. 171

4.9.2 Compression of preforms The constant flux infiltration, where the liquid metal was driven by means of a plunger with constant velocity under pressure into the porous body, is subdivided into three modes: direct squeeze casting (DSQC), indirect squeeze casting (ISQC) and high pressure die casting infiltration (HPDC). The evaluation of the compression of the preforms of all constant flux infiltration modes is summarized in Table 4.8 and displayed in Figure 4.74 for the non- reactive Al2O3 preforms and in Figure 4.75 for the MgO or the TiO2-containing preforms. The relative compression of the preform was evaluated using the parameter cpr, which was defined as the relative reduction in the preform cross-sectional area in the x-z-plane as a result of the infiltration. Relatively slow infiltration at a velocity, v0 , of 0.018 m/s was achieved when using DSQC. The ISQC and the HPDC processes were performed on a high pressure die casting machine at significantly higher velocities v0 in the range of 0.07 m/s to 1.44 m/s. The ISQC mode differed from the HPDC mode mainly in the geometry of the ingate area which influenced the ingate velocity. Three of the four v0 in ISQC and HPDC modes were equal. The two modes were used in order to determine the influence of the metal melt history, e.g. in terms of turbulence, on the compression behaviour of the preforms. As described in section 2.5.3, the minimum velocity, v0 , in HPDC and the maximum in ISQC represented the extremes which could be realized with the current high pressure die casting setup. Due to the unavailability of the MOPC20 and AODY30 preforms at the time of infiltration, these were not infiltrated in the ISQC or in the HPDC mode at a plunger velocity of 0.4 m/s (HPDC04), respectively. The designation system of the infiltration modes and velocities is listed in Table 3.8. The mean compressions of the preforms are listed in Table 4.8. The scatter of each mean value is represented by the error bars in Figure 4.74 and Figure 4.75. In the DSQC mode, the 172

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