Abstract
We investigate the impact of the nucleation law for nucleation on Al-Ti-B inoculant particles, of the motion of inoculant particles and of the motion of grains on the predicted macrosegregation and microstructure in a grain-refined Al-22 wt.% Cu alloy casting. We conduct the study by numerical simulations of a casting experiment in a side-cooled 76×76×254 mm sand mould. Macrosegregation and microstructure formation are studied with a volume-averaged two-phase model accounting for macroscopic heat and solute transport, melt convection, and transport of inoculant particles and equiaxed grains. On the microscopic scale it accounts for nucleation on inoculant particles with a given size distribution (and corresponding activation undercooling distribution)and for the growth of globular solid grains. The growth kinetics is described by accounting for limited solute diffusion in both liquid and solid phases and for convective effects. We show that the consideration of a size distribution of the inoculants has a strong impact on the microstructure(final grain size) prediction. The transport of inoculants significantly increases the microstructure heterogeneities and the grain motion refines the microstructure and reduces the microstructure heterogeneities.
Highlights
The prediction of the microstructure formation during solidification is of major importance for the industry
Case 2: Impact of inoculant distribution By accounting for the inoculant size distribution, the competition between nucleation and growth is included in the model
When grain motion is added, the direction of the flow could be modified as the grains, which are lighter than the liquid, tend to float upwards along the solidification front
Summary
The prediction of the microstructure formation during solidification is of major importance for the industry. Micro-macro models are developed for decades to relate the macroscopic solidification conditions to the microscopic structures obtained. Rappaz [1] gives an overview of the microscopic mechanisms of microstructure formation and the coupling between them and the macroscopic transport phenomena. Heterogeneous nucleation is not easy to calculate, as even a known distribution of refiner particles, deliberately added into the liquid metal, can be modified by the fragmentation of grains or by the agglomeration of particles. When the experimental average grain density is known, an easy way to avoid modelling the inoculants activation is to consider an initial grain density in the melt entering the mould which matches the experimental data[3]. The goal of this study is to analyse the impact of the inoculant motion on the macrosegregation and the microstructure formation. An experiment is modelled with different hypotheses in order to see the impact of each phenomenon
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