Thermal analysis on planar interface stability in solidification of semitransparent materials

Significant melt undercooling may be developed in the melt in front of the solid/liquid interface during solidification of semitransparent materials because of internal radiative heat transfer with the environment. A nonequilibrium planar interface solidification model has been developed recently to permit the melt undercooling near the interface. A thermal analysis is presented for the stability of such a planar interface. An absolute stability theory derived by Ludwig for opaque materials has been employed as a first approximation. The stability theory takes into account the stabilizing effect of efficient cooling through the solid layer and defines a heat flux parameter to quantify the latent heat released that is transferred into the undercooled melt. When the variation of the heat flux parameter during solidification is calculated based on the present nonequiltbrium planar interface model, the stability of the interface for given process conditions can be determined. The results suggest that, although the internal radiation leads to an undercooled melt in front of the interface, a planar interface can still be stable if there is strong external heat transfer. Based on this analysis, the physical mechanisms of the mushy-zone formation for the solidification of semitransparent materials have been presented.