ORIGIN OF THE EQUIAXED CRYSTALS IN CASTINGS

Abstract

In order to control the cast structure, it is most important to know the formation mechanism and the behavior of equiaxed crystals in the molten metal during the solidification. When 99. 999% Sn, Sn-0. 1% Bi, and Sn-0. 5% Bi alloys were solidified at the rate of 0. 15 mm/sec, the solid-liquid interface was rather flat and the surface- current of the molten toward the solid-liquid interface was observed during the solidification. However, when the bismuth content increased above 1%, the molten surface moved at first toward the cooling wall of the mold and then retreated. It was found that the back current of the molten surface was due to the formation of freerystals which were continuously separated and dissociated from the cooled wall. The separation of equiaxed crystals from the mold wall during the initial stage of solification is considered to be the most responsible cause for the formation of fine grained, equiaxed structure of castings.