Formation Mechanism of Freckle in Unidirectional Solidification

Abstract

Experimental and theoretical works have been conducted to clarify the formation mechanism of freckle in unidirectional solidification. The formation process of freckle in mushy zone and the movement of interdendritic liquid were visualized and observed by use of a cold model of NH₄Cl-H₂O system.
It is noted that freckle grows up against the freezing direction, i.e., from the liquid-mushy interface to the mushy-solid interface. Fingertype fluid movement is observed near the liquid-mushy interface which is presumed to trigger the channel flow inducing freckle.
A mathematical model which takes account of the freezing and remelting phenomena in the mushy zone has been developed to simulate the formation process of freckles. Methods for preventing the formation of freckle are proposed on the basis of the formation mechanism investigated in this paper.