Abstract
Melt conditioned direct-chill (MC-DC) casting is a novel technology which combines direct-chill (DC) casting with a high shear device directly immersed in the sump for in situ microstructural control. A numerical model of melt-conditioned direct-chill casting (MC-DC) is presented in this paper. This model is based on a finite volume continuum model using a moving reference frame (MRF) to enforce fluid rotation inside the rotor-stator region and is numerically stable within the range of processing conditions. The boundary conditions for the heat transfer include the effects of the hot-top, the aluminium mould, and the direct chill. This model is applied to the casting of two alloys: aluminium-based A6060 and magnesium-based AZ31. Results show that MC-DC casting modifies the temperature profile in the sump, resulting in a larger temperature gradient at the solidification front and a shorter local solidification time. The increased heat extraction rate due to forced convection in the sump is expected to contribute to a finer, more uniform grain structure in the as-cast billet.
Highlights
Direct-chill (DC) casting is a semi-continuous casting method that produces feedstock of wrought aluminium and magnesium alloys suitable for subsequent processing, such as extrusion, rolling or forging
The compiled applications are used to study the DC casting of A6060 Al-alloy and AZ31 Mg-alloy for which some experimental data is available
To validate the numerical model, a DC casting setup was simulated in a two-dimensional axisymmetric model without the rotor-stator mixer to compare with measured temperature profiles along the axis and sidewall of the caster
Summary
Direct-chill (DC) casting is a semi-continuous casting method that produces feedstock of wrought aluminium and magnesium alloys suitable for subsequent processing, such as extrusion, rolling or forging. High shear rate was found to be confined to the close vicinity of the rotor-stator mixer and the authors concluded that effective dispersion of oxides occurred only near the mixing head [5]. Their model did not include solidification, but only melt flow. Acoustic streaming, caused by the acoustic pressure gradient in the melt, modifies the flow pattern This model was indirectly validated against grain morphology in different locations of the cast billet and successfully explained the resulting grain morphology modification due to the predicted flow pattern. Combined with enhanced nucleation by dispersed oxide particles and dendrite fragmentation, melt conditioning in DC casting yields a more uniform and refined grain structure across a billet section. The model is applied to high-shear treatment of the sump during the casting of these alloys using a rotor-stator mixer
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