Control of microstructure in lost foam casting using chills

摘要

In this research, a pioneering attempt has been made to extend the concepts of chill volumetric heat capacity and mold saturation ratio to the lost foam casting (LFC) process, and to study their use as tools for predicting secondary dendrite arm spacing (DAS) in aluminum alloy A356. Cylindrical LFCs were poured using aluminum alloy A356. Chills of three thicknesses and three materials were attached to the bottoms of the patterns to enhance solidification. Secondary dendrite arm spacing varied with local solidification according w DAS = (10.84{sub}f){sup}0.294. This relationship is in close agreement with those developed by other researchers, thus validating the measurements of local solidification time and secondary DAS. There was a linear relationship between chill volumetric heat capacity and DAS for the 38-mm and 51-mm diameter castings. Dendrite arm spacing decreased linearly until a constant value of approximately 30 microns was reached for the 25-mm diameter castings. This limiting value was attributed to the heat extraction power of the chills approaching that of a water-cooled copper chill. Increases in modified mold saturation ratio led to decreases in DAS. Despite the assumption of one-dimensional heat transfer, the modified mold saturation ratio provided, a good indication of the effect of the chill and mold on secondary DAS. High values of modified mold saturation ratio, indicating high heat extraction capacity, produced fine dendritic structures. Use of chills reduced the silicon particle size. The size was significantly reduced as local solidification time decreased. Silicon morphology remained acicular throughout the range of solidification times studied. The rounded silicon morphology that would be evident if thermal modification had occurred, was not observed. Thus, even rapid cooling rates produced by chills were not sufficient to produce thermal modification.