Transport phenomena during the solidification of binary and ternary metal alloys

Matthew John Maximilian Krane, Purdue University

Abstract

During the solidification of metal alloys, several defects may arise which are influenced directly by the transport phenomena which occur in the process. One such defect is the redistribution (or, macrosegregation) of the various alloying components during the freezing of an off-eutectic mixture. In general, alloy components freeze at different rates and over a temperature range, establishing thermal and solutal gradients which cause fluid motion and consequent macrosegregation. While recent modelling efforts have enjoyed a measure of success, most of these efforts have been confined to binary alloys. The study of two component mixtures is an excellent starting place, where one can examine a simple, well-defined system and more easily discern the basic transport phenomena involved in alloy solidification. However, most commercially interesting alloys contain more than two components. A continuum mixture model for the transport of mass, momentum, enthalpy and species has been extended to include a third component in a metal alloy. A generic method has been developed for using the ternary phase diagram to calculate local temperature, phase fractions and phase compositions, which are needed to close the model. This model has been applied to several lead-rich Pb-Sb-Sn alloys. Numerical simulations have been performed to demonstrate similarities and differences between macrosegregation patterns and convective flows of these ternary alloys and those of two component systems. Experiments to validate both the binary and ternary models have been completed. Calculated temperature histories using realistic boundary conditions are compared to thermocouple data from the solidifying ingots. Experimental composition profiles are compared to the numerical macrosegregation results. Particular attention is paid to the effect of the permeability model on macrosegregation. Additional studies examine shrinkage induced flows and a scaling analysis of the transport equations in the mushy region. It is found that flow and macrosegregation patterns are more strongly influenced by buoyancy than shrinkage effects over a wide range of solidification rates. The scaling analysis shows the limited areas of the mush in which liquid motion affects macrosegregation, and estimates the transient chill wall temperature and the sizes of the solid and mushy zones.

Degree

Ph.D.

Advisors

Incropera, Purdue University.

Subject Area

Metallurgy|Mechanical engineering|Materials science

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