Effect of stirring on solidification pattern and alloy distribution during semi-solid-metal casting

Abstract

In most SSM processes, induced fluid flow or in more general term "forced convection" is an integral part of the process. The effect of fluid flow has been widely studied on the micro- and macrostructure of solidified alloys independent of the process specifications. These studies have been mostly concerned with columnar-equiaxed transition, and the process of macro-segregation. This is particularly important for solidification of alloys with partition coefficient (distribution) less than unity where solutes are rejected and piled up ahead of the growing interface due to the limited solubility of the solid. For SSM processes, it is important to understand the implication of forced convection and solute distribution in the context of stir-based SSM casting. The distribution of alloying elements is investigated for Al-Si alloys through two different processes, i.e. conventional casting and SSM processing with the application of melt stirring. It is shown that by stirring, not only nucleation temperature of the primary phase increases but also the silicon content of the primary α-Al particles is depleted. These two phenomena finally lead to the formation of globule particles with the prevention of further growth. © 2006 Elsevier B.V. All rights reserved.