Semester

Summer

Date of Graduation

2010

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu.

Abstract

The need for clean and affordable power has resulted in great demand for high-efficiency gas turbine power systems. To improve energy generation efficiency, and lower CO2 emissions, the new generation of land based gas turbines needs to be larger and operated at a higher temperature. Consequently, it is required the use of Nb-containing superalloys such as Inconel 718 for the rotor disk, to avoid overaging of the microstructure during the forging process, but its applicability is being limited by difficulties in the reliable production of large Vacuum Arc Remelting (VAR) ingots (33 to ∼35 in. diameter) without macrosegregation defects such as freckles and center segregation. These defects impair the mechanical properties of the material and are believed to be caused by convection in the mush layer. Because they cannot be corrected by subsequent treatment, the affected ingots should be scrapped highly increasing the production costs.;In this work, the aim was to take steps toward the final goal of preventing macrosegregation defects in VAR ingots by assessing the need for accurate determination of the driving force related to the solutal buoyancy in the mushy layer and postulating a criterion for freckling prediction that could better resolve if freckles will or will not evolve under certain processing conditions. An experimental methodology was proposed to determine the partition coefficients of the major solute elements and better agreement was obtained with the predictions made with the newly developed databases, suggesting that improvements in the accuracy of the theoretical predictions can still be obtained and are necessary for freckling prediction. Quenching modified DTA (MDTA) tests were proven to be suited for measuring the partition coefficients of the solute elements, and it was assumed successfully that the cooling rate prior to the quenching step of 0.083 Ks-1 was slow enough to permit easy quenching, while being sufficiently fast for the primary solidification reaction to depart from the equilibrium model and being closer to the Scheil model. The minimization of the error function defined from the Scheil equation was found to be an appropriate method for describing the segregation profiles of the quenched samples and permitted good estimations of the partition coefficients of the solute elements. A criterion for freckling prediction that includes the effect of tilted solidification front was proposed. The form includes the effect of the tilt angle by preserving the anisotropic nature of the permeability tensor throughout the derivation and uses Poirier's experimentally determined functional forms for the parallel and perpendicular components. The proposed form of Rayleigh number criterion was found to provide better resolution when evaluated against available experimental data in the literature. Especially, it showed that the nucleation of channels in the mushy layer leading to freckles is equally probable in the proximity of the tips of the dendrites or deeper in the mushy layer, for example at approximately 0.7 liquid fraction and 0.4 liquid fraction respectively, depending on the angle of tilt of the solidification front.

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