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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

grey areas consisted

grey areas consisted of an intermetallic of the Al-Fe-Si type as reported by Schumann (174) . Both types of precipitates were spherodized by the heat treatment. Temperature T /°C Time t / s Figure 4.45 Thermal analysis of the infiltration alloys a) IM and b) IS. The thermal analysis curves of the infiltration alloys are shown in Figure 4.45 a) and b). To ensure infiltration in the fully molten condition, the temperatures of the melt and the preform were maintained above the liquidus temperatures of the alloys. The liquidus temperature of IM was measured to be 613°C and that of IS, 577°C. The solidus temperatures of both alloys were evaluated to be 566°C. 4.8.2 Thermal properties during infiltration The temperatures Tpreform, Tedge, Tcentre, the displacement of the upper punch splunger and the metallic pressure Pmet were measured during the first 15 s of an infiltration cycle and are given in Figure 4.46 for MOPC20. The Tpreform curve corresponds to the temperature in the preform close to its edge (p. 82) whereas Tcentre and Tedge represent the temperatures close to the bottom punch surface in the centre and the edge respectively. As the melt temperature in the 139

die, Tmelt,die , could not be recorded during the infiltration process, it was assessed in a prematurely terminated cycle to be 665°C. The measurements were performed with the MOPC20 preform as magnesia exhibited the highest thermal conductivity of the dense sintered materials at a given temperature, Table 3.3. Therefore the thermal flux has to be assumed to be the highest of all the preforms and thus the sample would exhibit the lowest preform temperature prior to infiltration. Zero time in Figure 4.46 corresponds to the first movement of the upper punch. Approximately 20 s prior to this event, the preform was transferred from the preheating furnace held at 800°C into the infiltration die cavity held at 450°C. As shown in Figure 4.46, the contact between the preform and the bottom punch resulted in an intitial punch temperature rise from 450°C to 470°C. The temperature signal at the edge of the preform was interrupted at an early stage of infiltration, as the leads to the thermocouple were sheared in the gap between the die and the upper punch. Prior to that, the preform cooled from 800°C to 750°C. The pressure Pmet increased steadily as the plunger moved until the preset ultimate pressure of 100 MPa was reached. The main plunger movement took place in the first second of the infiltration cycle at a mean velocity of 0.017 m/s. After the steep increase in the pressure, no significant change in displacement was observed. The pressure Pmet increased steeply up to 15 MPa and subsequently a smooth pressure increase occurred. This resulted from the infiltration press control system which switched at 15 MPa to the high pressure mode with a low hydraulic fluid volume displacement. The maximum pressure in the die was reached 7.5 seconds after the start of the infiltration cycle. As the maximum pressure was reached, the hydraulic system of the press switched off and 140

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