Development of Macrosegregation During Solidification of Binary Metal Alloys

Open Access
Author:
Kim, Byungsoo
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 14, 2002
Committee Members:
  • Dhyshyanthan Sathianathan, Committee Member
  • Robert Carl Voigt, Committee Member
  • Fan Bill B Cheung, Committee Chair
  • Stefan Thynell, Committee Member
Keywords:
  • solidification
  • alloy
  • macrosegregation
Abstract:
A combined experimental and theoretical study of the convective transport phenomena 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 temperatures of the alloy at nine locations were measured during the solidification, and macrosegregation in the solidified ingot was determined using neutron radiography. The measured cooling curves revealed the presence of non-equilibrium phenomena during the early stage of solidification. The analysis of the cooling curves and macrosegregation patterns shows that indium-rich dendrite fragments are transported to the bottom of the mold cavity and the thermosolutal convection pattern transported cool gallium enriched liquid to the right side of the mold cavity near the mid-height. A continuum model is also developed in this study that is cast in dimensionless form and used to simulate thermosolutal convection during the alloy solidification. The numerical predictions show the complicated convection flow patterns due to the interaction between thermal and solutal buoyancy forces, which are not directly observable in experiments and they are in fair agreement with the experimental results. The numerical and experimental results show that during the early stage of solidification, solid particle transport and double diffusive convection due to the interaction between thermal and solutal buoyancy forces are the key causes of macrosegregation. Results of the present study provide new insights on macrosegregation during alloy solidification that are useful for researchers and practitioners working in metallurgy and related areas.