Formation mechanism of spherulites in Al–Si7Mg alloy during solidification under flow and stirring

Abstract Two types of mechanism of spherulite formation in semi-solid metal processing have been proposed in academic circles, the dendrite breakage mechanism and the copious nucleation mechanism, but there is still no completely unified viewpoint. In this work, in order to further reveal the mechanism of semi-solid processing, the effects of; pouring temperature T 0 , rotating speed of turntable v 0 , group number of stirrer rods n 0 and quenching medium (air/water) on grain size and morphology of semi-solid microstructure of Al–Si7Mg alloy were investigated, and the results mainly show that the percentages of quantity of both; grains with mean diameter below 25 μm and grains with roundness within 1–2, increase with the increase of rotating speed of turntable v 0 , or group number of stirrer rods n 0 . Finally, several conclusions have been reached. The formation of spherulites is usually a result of interference of growth directions of adjacent dendrite arms during solidification, which does not depend on, but can be promoted indirectly by flow or stirring. As the velocity of flow or stirring increases, it weakens the effect of dendrite breakage and strengthens the effect of copious nucleation.