THE MODIFICATION AND CONTROL OF CAST ALUMINUM-SILICON ALLOYS

The effect of process control factors such as modifier, alloy, cooling rate, and holding time of the melt were investigated with secondary dendrite arm spacing, the size and the morphology of eutectic silicon phase, and eutectic undercooling. The most commonly used hypoeutectic cast aluminum silicon alloys (319, 355 and 356) were used. Misch metal, sodium and strontium were added as modifiers.; It was found that the morphology of the eutectic silicon phase modified with sodium, strontium or misch metal differs from each other. The morphology with strontium modification is more faceted with sharper edges than that with sodium modification. Nevertheless the final microstructures of the eutectic silicon phases modified with sodium or strontium look alike due to the diffusion. The effect of lanthanides on the eutectic modification of the aluminum-silicon alloys are less significant than it has been reported. An addition of 2.0% of misch metal provides silicon to be interconnected and rosette in 1.25 in. D. graphite mold or 0.75 in. D. sand mold while the modification effect is negligible for 1.5 in. D. sand mold. The refinement of the silicon phase which is quantified by the apparent eutectic silicon particles through the optical microscope has a linear correlation to eutectic undercooling while each of cooling rate and modification can partially affect on the refinement.; It is concluded that a modifier should prevent nucleation of eutectic silicon phase and poison growth steps to achieve the proper modification.; A reliable and persistent eutectic modification effect can be obtained with misch metal or strontium addition. However the minimum amount of misch metal necessary to obtain complete modification is exceptionally large.