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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

die casting methods, a

die casting methods, a constant flow rate infiltration mode was adopted. The infiltration experiments were performed with melts of IS and IM alloys for which the chemical compositions are shown in Table 3.2. 3.4.1. Constant pressure infiltration (CPI) In this method liquid metal was forced to infiltrate the preforms by means of pressurized argon where the applied pressure, Pappl, the difference between absolute pressure Pabs and ambient pressure, Pamb, could be adjusted in the range of 0.01 MPa to 2.5 MPa. The pressurization was performed in a tool built up of three parts: a lower punch (LP), an upper punch (UP) and a die (DI) as shown in Figure 3.9. All parts were made of H13 hot working steel, with a thermal conductivity of 22 W/m K at 25°C (53) . The die and the lower punch formed a rectangular-shaped cavity of dimensions 65 mm x 46 mm x 35 mm. Prior to the infiltration tests, the cavity and the upper punch were coated with a very thin layer of graphite using a spray (ESD 33, Kontaktchemie, Germany) and then preheated to 450°C in a TP 400 (Fontjine Ltd, The Netherlands) laboratory platen press. The preforms were preheated in a muffle furnace at a rate of 300 °C/h to 800°C to prevent thermally-induced cracking. The gas pressure infiltration comprised the following: The preform was set into the cavity. The metal melt was transferred from a crucible holding furnace, set to a temperature of 800°C, to the die using a preheated boron nitride coated ladle loaded with 100 ± 10 g of melt. The dross on the surface of the melt was removed and the cleaned melt was poured directly onto the preform. The upper punch, which was fitted with a copper sealing ring, was set onto the die cavity and the die was closed and pressed between the platens of the press to ensure sealing between DI and UP. The gas pressure was applied within 1 s to the chosen value. The gas in the preform which was replaced by the melt volume could flow through the air gap between the bottom punch and the die. The pressure was held 81

for 90 s to ensure complete solidification of the metal melt. After the test, the pressure was reduced to atmospheric pressure. Cu-seal Metal melt Preform Air outlet gap Figure 3.9 Schematic cross-sectional view through the closed gas pressure infiltration set-up. The punches (LP and UP) were heated in the platen press. The gas pressure infiltrated castings were cut through the centre along the longitudinal axis. As the infiltration behaviour could be shown to be symmetrical, only one half of the casting was mounted in polymer for metallographic preparation. By using a vacuum polymer infiltration technique, the residual pores could be filled, thereby enabling the sample to be polished without scratches being formed by particles originating from non-infiltrated preform regions. 3.4.2. Direct Squeeze-Casting (DSQC) The die was prepared in a similar way to that used for the gas pressure infiltration except the upper punch geometry was changed to a rectangular block with a height larger than the depth of the cavity. The preform was set into the cavity and the melt was added in the same manner as described in the previous section and the mould was then closed and the press lower platen moved up at an average velocity of 0.017 m/s to a maximum load-controlled pressure in the die cavity of 100 MPa. The time between placing the preform and the end of pressurization was in the range of 10 to 12 s. 82 UP UP LP LP DI MA P Ar

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