Effect of capillary pressure on interdendritic liquid flow

The top free surface of a mushy zone composed of columnar dendrites is exposed to air and the interdendritic liquid is made to flow down out of the specimen by applying pressure difference across the specimen. The liquid flow rate is measured as functions of applied pressure head, fraction liquid and primary and secondary arm spacings of columnar dendrites lying parallel to the flow direction. At a pressure head smaller than a critical value, the flow rate is two orders of magnitude smaller than that in the case where the top of the mushy zone is covered with bulk liquid. Above the critical pressure head, the flow rate increases rapidly with increasing pressure head. The results can be explained in terms of the capillary pressure, acting at the free surface of the interdendritic liquid, as a resistance to liquid flow. It is suggested that the capillary pressure is one of the most important factors in the formation of porosity in castings when the free surface of the riser solidifies to form a mushy zone.