Effect of Fluid Flow on Microstructure Evolution During Rheo Gravity Die Casting of Novel Al–15Mg₂Si–4.5Si Composite

The present work is aimed to investigate the effect of fluid flow on microstructure evolution of Al–15Mg₂Si–4.5Si composite during cooling slope rheocasting, using a two-dimensional phase field (PF) model. The potential applications of the rheocast composite include automobile, aviation, electronic and defence industries. In this study, the superheated melt of the composite impinges over the slope free surface at 640 °C and transforms into semi-solid slurry during its flow over the cooling slope surface before it gets solidified within a mild steel mould. The first phase which is growing over the cooling slope is Mg₂Si, followed by the formation of α-Al at the Mg₂Si–liquid interface as well as in the bulk melt. The simulation results obtained from the study include the effect of fluid flow on grain morphology of constituent phases (Mg₂Si and α-Al), grain density, solid fraction, etc. The nucleation model employed in the present PF model works based on the seed undercooling approach, wherein experimentally measured cooling rate value governs the nucleation and growth of primary Mg₂Si and α-Al phases. Micrographs of oil-quenched melt samples collected from the melt flow front over the cooling slope points towards significant effect of gravity-assisted melt flow over the slope on morphological evolution of primary Mg₂Si and α-Al grains. Moreover, the simulation findings establish competence of the present phase field model as an accurate process modelling tool to capture microstructural morphology of the composite slurry and subsequent optimization of the Rheo gravity die casting process of novel Al–15Mg₂Si–4.5Si composite.