Modeling macrosegregation during the vacuum arc remelting of titanium-vanadium-iron-aluminum alloy

Dmytro V Zagrebelnyy, Purdue University

Abstract

Vacuum arc remelting (VAR) is a secondary melting process for production of metal ingots with elevated chemical and mechanical homogeneity for highly demanding applications. The ingots from VAR process are typically used as a material for critical parts of jet engines and industrial gas turbines as well as in military applications and heavy industry. The specifications of such applications often demand outstanding material properties. The common examples of such materials are Ni or Ti-based super alloys and highly alloyed steels. One of the most important problems of Vacuum Arc Remelting (VAR) is the lack of chemical homogeneity in the produced ingots. Past numerical studies of VAR have demonstrated that the characteristics of the mushy zone, the fluid flow and their interaction are the key factors defining the severity and the character of macrosegregation in the produced ingots. Thus, a new numerical model of the complete VAR process, capable of representing two distinctive morphologies of mushy zone: rigid columnar structure and slurry of free floating equiaxed grains is introduced. The presented model accompanied by a simple criterion for the columnar-to-equiaxed transition allows the capture of segregation defects induced by motion and settling of equiaxed grains. Moreover, the numerical simulations with the new model include studies of two distinctive flow regimes in VAR: strong Lorentz driven flow and weak buoyancy driven flow. The results demonstrate a swift transition from weak buoyancy driven flow to strong electromagnetically driven flow with the increase of arc current. The change of flow regime to strong electromagnetically driven mode results in stepwise increase of macrosegregation and thus is not desirable. The key to understanding the swift transition of flow regimes is the instabilities of the thermal stratification within liquid pool at the early stages of the VAR process. The model also was used for the study of macrosegregation evolution during multiple VAR melts, when the ingot of the previous melt is used as a source material for the next melt. The simulation results demonstrated that the increase of microsegregation through the sequence of multiple melts mostly occurs due to the increase of ingot radius, and not the nonuniformity in electrode composition. However, in some instances, the electrode composition may result in additional buoyancy forces in the liquid pool and affect the flow regime and thus composition.

Degree

Ph.D.

Advisors

Krane, Purdue University.

Subject Area

Mechanical engineering|Materials science

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