Process parameters influence on microstructural properties of hypereutectic al-18wt% Si alloys obtained by rheocasting process

Abstract

Hypereutectic Al-18wt%Si alloy was obtained by conventional and rheocasting procedure at cooling rate 0.16 +/- 0.05 degrees Cs-1. Samples of rheocasted alloy were obtained at two stirring speeds 1000 and 1500 rpm. Tests showed that two major changes of microstructural properties of rheocasting alloy, in comparison with conventional alloy, are change of dendrite morphology into nondendrite one and change of primary phases size (alpha-Al and Si). Both changes of primary particles were monitored in area near the stirrer and at a small distance from the crucible wall. Test showed that sizes of primary, alpha-Al particles decreased with stirring speed increment, while roundness of particles increased with stirring speed increment. Stirring speed change led to different degree of primary Si particles agglomeration. At stirring speed 1000 rpm, Si particles are uniformly distributed, while at speed 1500 rpm, a tendency to form clusters is more expressed. Test results were discussed on basis of utilizing fragmentation-agglomeration mechanism of primary phase evolution in a semi-solid melt, which may contribute to explanation of semi-solid structure development.