Macrosegregation and shrinkage pipe formation in static castings
Abstract
A single-domain multi-phase model is developed for macrosegregation and shrinkage pipe formation in static castings, as functions of buoyancy- and shrinkage-induced flow. Using a volume-of-fluid (VOF) method, both the air/liquid and air/solid interfaces are tracked during shrinkage pipe formation. A set of mixture advection-diffusion equations are derived and solved for velocity, temperature, composition, and solid fraction evolution. The fluid mechanics of the model are verified using a transient ditch drainage problem. This model provides a comprehensive tool to investigate relationships between the developing composition distribution and shrinkage pipe formation. The model is applied to binary Ti-48Al (at.%) and ternary Ti-46.6Al-8.6Nb (at.%) alloys in a cylindrical mold (R = 3.5 cm, H = 15 cm). The solidification path, phase selection and microstructure development in this alloy is very sensitive to the local composition, which is proved by studying the microstructure of a group of Ti-Al-Nb multicomponent alloys prepared by arc melting. Experiments to investigate macrosegregation and shrinkage pipe formation are performed by casting both alloys in an induction skull melting (ISM) furnace, using tilt-casting method in investment molds. No significant exchange of fluid that is not at nominal composition between the riser and the part is found. Both the close to unity partition coefficient for the solute and the close to -1 buoyancy factor are supposed to be the reasons for this result. However, the macrosegregation level is increased in a larger scale casting (R = 30 cm, H = 50 cm), which can change the primary solidification phase and the local microstructure of the gamma-TiAl alloy in the final cast. To further study the influence of the riser on flow exchange between riser and cast part as well as the macrosegregation development in static castings, a Al-5wt. % Cu alloy system is used. An ingot with the riser designed based on Campbell's rule is performed in a slab mold, which is used as a standard case for this study. The study on the influence of the both cooling condition and riser shape on the final macrosegregation pattern is performed. The results show that a wider riser is able to provide a barrier and prevent the solute-rich interdendritic liquid in the riser from flowing into cast part. For the castings with the riser shape being fixed, the average composition in the cast part is smaller at a higher solidification rate.
Degree
Ph.D.
Advisors
Johnson, Purdue University.
Subject Area
Materials science
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