Development of macrosegregation during solidification of binary metal alloys

by 1970- Kim, Byungsoo

A combined experimental and theoretical study of the convective transport phenomenon during the solidification of a binary metal alloy is performed. Neutron radiography is considered as a means of extending and improving experimental verification methods for alloy solidification models and other experimental results. Calibration, film processing, and digital image processing procedures are developed in order to accurately quantify the macrosegregation recorded on neutron radiographs. The method yields a highly resolved macrosegregation field, rather than a few discrete measurements that can be used to help interpret measured cooling curves and infer thermosolutal convection patterns. In this study, a gallium-27 wt. pct. indium alloy was solidified in a square cavity, chilled along one vertical side wall, the temperature of the alloy was measured during the solidification, and the macrosegregation in the solidified ingot was determined using neutron radiography. The measured cooling curves revealed the presence of nonequilibrium phenomena during the early stage of solidification. The analysis of the cooling curves and macrosegregation patterns showed where and how the indium-rich dendrite fragments and cool gallium enriched liquid were transported in the mold cavity by the thermosolutal convection. A continuum model was also developed in this study that was cast in dimensionless form and used to simulate thermosolutal convection during the alloy iv solidification. The numerical predictions showed the complicated convection flow patterns due to the interaction between thermal and solutal buoyancy forces, which were not directly observable in experiments and they were in fair agreement with the experimental results. The numerical and experimental results showed that during the early stage of solidification, solid particle transport and double diffusive convection due to the interaction between thermal and solutal buoyancy forces were the key causes of macrosegregation. Results of the present study provided the new insights on macrosegregation development during alloy solidification that would be useful to researchers and practitioners working in metallurgy and related areas. v