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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

After solidification

After solidification under pressure, the samples were ejected. The process parameters were applied in accordance to Schneele (163) who showed that in the current system the infiltration took place through the four sides and the top of the preform, resulting in a calculated entrance area of 5·10 -3 m² for the standard preform geometry (61 mm x 42 mm x 8 mm). Figure 3.10 schematically shows the steps in direct squeeze casting infiltration of a preform. As a reference, samples made of the matrix alloy were fabricated without preforms using identical process parameters. A data acquisition system was used to record the pressure increase with time and the displacement of the punch during squeeze casting infiltration. The compliance of the press was taken into account. The true pressure was calculated as the ratio of the force to the cross sectional area of the die cavity and the infiltration ratio as the ratio of volume reduction in the die due to plunger displacement to the nominal preform porosity volume. Figure 3.10 Schematic overview of direct squeeze casting (DSQC) preform infiltration process steps. In order to vary the heat flow, the die was modified by the introduction of an inner cylinder to the centre area of the bottom punch. The cylinder was made from a copper alloy (CuCr1Zr) of 15 mm in diameter with a thermal conductivity of 335 W/mK at 25°C (143) . During infiltration the temperatures inside the die cavity and the preform were recorded using NiCr-Ni thermocouples (Type K) with a diameter of 0.5 mm. The nominal response time of the temperature measurement setup was 30 ms. The thermocouples were fixed by brazing with an silver alloy into the bottom punch so the tip of each thermocouple was close to the 83

ottom punch surface. The temperature near the edge of a magnesia preform set into the die cavity was recorded in order to investigate its cooling behaviour prior to infiltration. 3.4.3. High pressure die casting infiltration technique Preform infiltrations were performed using the high pressure die casting technique. The technique is in general characterised by high filling velocities combined with small gating areas and low die temperatures, which were 250°C in the present experiments. The gating areas were varied to investigate the influence of turbulence on preform infiltration behaviour. First, a comparatively large gate area of 18 mm x 50 mm was selected to be close to the indirect squeeze casting (ISQC) geometries described in the literature (50, 135) . In contrast, a tool with a small gate of 62 mm x 1.5 mm was also used for high pressure die casting infiltration (HPDC). The schematic views of the infiltration geometries cast in the studies and the gate structures are shown in Figure 3.11. The diameters of the plunger connections of the tools were 60 mm for ISQC and 45 mm for HPDC. a) ISQC b) HPDC Y 50 Z 18 Preform cavity ingate 84 X area of plunger contact Details ingate section Figure 3.11 Schematic of the die casting components and position of the preform. a) large gate for indirect squeeze casting infiltration (ISQC).b) small gate for high pressure die casting infiltration (HPDC). 62

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