The effects of applied pressure on solidification phenomena have been studied in pure aluminum and Al-Si binary in the Rectangular steel mold. The range of applied pressure was from zero to 2000kg, the pouring temperatures are from 660℃ to 890℃ at constant mold temperature of 250℃. Cooling rate analyses by computer simulation through 2-Dimensional forward FDM, including the experimental work of measuring cooling rates, have been done in three different types of Squeeze cast Al-Si binary alloys; hypo-eutectic(Al-5.4%Si), eutectic(Al-12.9%si), hyper-eutectic (Al-20%si) alloys. Macro and micro-structural studies have been conducted to ascertain the effects of pressure during solidification on Columnar to Equiaxed Transition(CET) and dendritic structures.
The acceleration of both equixed and columnar growth have been shown in squeeze cast pure aluminum and hypoeutectic Al-Si alloy. The rapid heat extraction and fragmentation of dendritic skeleton at the initial stage of solidification under pressure influenced the CET morphologies. They might increase unceation sites(primary and seconary) for growth of equiaxed crystals and both columnar and equiaxed growth could be accelerated due to fast cooling rate under pressure. The origins of CET nuclei were discussed with several mechanisms: remelting of dendrite arm and seperation of crystals at mold wall. The grain growth acceleration under pressure well agreed with the CET model of Burden/Hunt. A further point to noted is that entire fine equiaxed structures with elimination of casting defects such as blow holes, gas porosity, and shrinkage cavity were obtained under the proper casting conditions of squeeze cast Al-Si alloys.
The significant changes of microstructures by the application of pressure during solidification were observed; preferential growth of primary arms against the parallel direction of heat flow with fine dendrite arm spacing in squeeze cast pure aluminum and hypoeutectic Al-Si alloy, greatly fine and short eutectic silicon (0.5×13 μm) with the decrease of halo layers(7 μm) of Al riched phases in the periphery of primary silicon particles in the squeeze cast near eutectic and hypoeutectic Al-Si alloys.
The above microstructural characteristics in the squeeze casting process have been studied in terms of the facet-nonfacet growth morphologies of fast cooling rates.
The calculated values of the heat transfer coefficient, h, during gravity casting were 0.055, 0.022, $0.017cal/\sec$ ㎠ c for hypoeutectic, near eutectic, and hypereutectic Al-Si alloy, respectively. It was explained by air gap formation. The mechanism was as follows; When the molten metal was poured into the metallic mold. This expanded outwardly by heat absorption forming solid skin of contact surface for a certain period, and after solid skinstiffened to resist the static pressure of the metal, solid contraction ocurred; consequently air gap was formed in cooperated with expanding mold. Hypoeutectic alloy was gained stable solid skin later due to dendritic growth and some wide freezing range(38'c) and the skin was more thermal conductive. So it has high valued h. But near eutectic and hypereutectic Al-Si alloy had stable and strong solid skin early in the solidification and high silicon content result in less thermal conductive solid skin. Therefore their values of h were lower than hypoeutectic Al-Si alloy.
As squeeze cast, the calculated values of h were 0.6, 0.3, and $0.15cal/\sec$ ㎠'c for hypoeutectic, near eutectic, and hypereutectic Al-Si alloy, respectively. Pressurization made remarkably improved the contact condition between the mold and cast and eliminated air gap; increased conduction contribution. It resulted in increasing of h by one order magnitude or more as pressured. The effect of solute on h as squeeze also explained by similar principles as gravity casting.
From the study, it was founded that the computer simulation technique was useful tool for thermal analysis of the solidification process.
