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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

2.5.2. Squeeze casting

2.5.2. Squeeze casting infiltration In a review of squeeze casting, Ghomashchi and Vikhrov (135) showed that the process involves three essential steps. A specified amount of molten metal is poured into a die, the die is closed and the liquid metal is pressurized as quickly as possible to prevent premature solidification. Afterwards, the pressure is held on the metal until solidification is completed. The application of pressure during solidification has an influence on the solidification behaviour and the resulting microstructure. In accordance to the Clausius-Clapeyron equation, the high metallostatic pressure, which can rise up to 200 MPa, leads to an increase in the melting point of the metal (136) . Further, as proposed by Epanchistov (137) , the eutectic point in the aluminium-silicon system moves to higher silicon contents. Depending on the gating system, squeeze casting is divided into direct and indirect squeeze casting. In the direct mode, there is no gating system at all. The metal melt is poured directly into the die cavity, which is subsequently closed and pressurized (6,135,138) . In the direct mode, just simple geometries without undercuts are realizable. In contrast, in indirect squeeze casting, the die is filled and pressurized through a gating system. The filling velocities in direct and indirect mode are below the threshold of turbulent flow, preventing entrapment of oxide films in film-forming liquid metals (6) . The pressure provides close contact of the melt with the die material, resulting in large heat transfer to the colder die. The fast cooling rate and solidification are reflected in fine-grained structures with a low secondary dendrite arm spacing (139) . Furthermore, the pressure application reduces gas porosity size and compensates feeding defects, thus reducing voids in the microstructure. Cappleman et al. (116) were the first to publish work about squeeze cast infiltration of Saffil fibre preforms with Al alloys. The work was concerned with the fibre/matrix interface of 55

MMCs infiltrated with an Al-9Mg or Al-11Si (wt-%). They did not observe any intermediate phase between the fibres and the matrix, although a Mg-enriched layer about 0.17 nm thick could be detected in the case of the AlMg alloy. The enrichment of elemental Si on the surface of the fibre in the composite infiltrated with an AlSi alloy was probably a result of silicon precipitates nucleating preferentially on the fibre surface. They concluded that there is no chemical reaction in the composites due to the rapid cooling of the melt when using the squeeze casting infiltration process and therefore bonding between matrix and reinforcement is only the result of intimate physical contact. In contrast, observations of Levi et al. (140) showed that there was massive formation of spinel MgAl2O4 on Al2O3 fibres which were immersed in a Al-3Mg alloy (wt-%) for several minutes. Another interesting aspect of the fast infiltration process is the suppression of Al2O3 formation on the Al melt surface. As the melt have an extraordinarily high affinity for oxygen, a thin oxide layer would be continually reforming at the infiltration front, as shown schematically in Figure 2.17. When assuming an outlet in front of the infiltration front which is connected to the atmosphere, the calculations by Cappleman et al. (116) showed that a monolayer oxide formation can keep pace with rapid infiltration up to a velocity of 10 -2 to 10 -1 m/s. melt oxide fibre layer v Figure 2.17 Schematic of oxide layer formation at the infiltration front in fibre (f) preform infiltration (116) . 56 air

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