Influence of transport mechanisms on nucleation and grain structure formation in DC cast aluminium alloy ingots

Abstract

The grain structure formation in direct chill (DC) casting is directly linked to nucleation, which is generally promoted by inoculation. Inoculation prevents defects, but also modifies the physical properties by changing the microstructure. We studied the coupling of the nucleation on inoculant particles and the grain growth in the presence of melt flow induced by thermosolutal convection and of the transport of freefloating equiaxed grains. We used a volume-averaged two-phase multiscale model with a fully coupled description of phenomena on the grain scale (nucleation on grain refiner particles and grain growth) and on the product scale (macroscopic transport). The transport of inoculant particles is also modeled, which accounts for the inhomogeneous distribution of inoculant particles in the melt. The model was applied to an industrial sized (350 mm thick) DC cast aluminium alloy ingot. A discretised nuclei size distribution was defined and the impact of different macroscopic phenomena on the grain structure formation was studied: the zone and intensity of nucleation and the resulting grain size distribution. It is shown that nucleation in the presence of macroscopic transport cannot be explained only in terms of cooling rate, but variations of composition, nuclei density and grain density, all affected by transport, must be accounted for.

A model study of the impact of the transport of inoculant particles on microstructure formation during solidification

“…As we show in this article, this phenomenon can cause significant heterogeneities of grain size at the scale of the casting. Only very recently the transport of inoculants with the liquid was considered in modeling of direct chill (DC) casting [17][18][19], of static casting on a laboratory scale [20] and in studies dealing with phase selection in highly undercooled melts [21].…”