Defect formation mechanism in directional solidification method using chills and high temperature mould

A directional solidification method using chills and high temperature moulds has recently been developed. In this work, experiments with low melting point pure metals and solidification simulation were conducted to discuss the formation mechanism and prediction method of string-like surface defects called 'wrinkles'. In the cases of pure tin and pure lead, a wrinkle was observed on the casting surface. On the basis of the calculated solidification pattern, it was suggested that the deformation of the front of the initially solidified shell can occur in the vicinity of the chill edge. In the calculated surface temperature gradient distribution of the castings, a temperature gradient area smaller than that of surroundings ('minimal zone') appeared at a position similar to that of the wrinkle. On the other hand, neither wrinkles nor minimal zones appeared in the case of pure bismuth. These results suggest that the generation of wrinkles can be predicted by checking the existence of minimal zones.