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Alpha Case Thickness Modeling in Investment Castings

Published

Author(s)

William J. Boettinger, Maureen E. Williams, Sam R. Coriell, Ursula R. Kattner, B A. Mueller

Abstract

The alpha case thickness at the surface of a Ti-6Al-4V step wedge investment casting has been measured and successfully predicted. The prediction uses temperature-time results obtained from a heat flow simulation of the casting that was performed with a commercial software package. The temperature-time results were coupled to a simple model for diffusion of oxygen into the beta phase during continuous cooling. Oxygen profiles in the casting were measured from the surface in contact with the Zr0(sub2) face coat of the shell mold into the interior. The oxygen content in the metal at the shell mold interface was between 6 and 10 wt% in general agreement with a thermodynamic calculation for BCC Ti in contact with Zr0(sub2). At the limit of the alpha case region, as determined by standard metallographic technique, the oxygen content was found to be ~0.02wt% above the level of oxygen in the bulk alloy. Using this information and one particular literature value for the activation energy for diffusion of oxygen, a nearly linear relationship was obtained between the measured and predicted alpha case thicknesses at various positions on the casting surface. Reduction of the prefactor of this diffusion coefficient by a factor of 7.6 produces excellent agreement between predicted and measured alpha case thicknesses. Such a reduction is not inconsistent with the scatter in literature values for the diffusion coefficient.
Citation
Metallurgical and Materials Transactions B-Process Metallurgy and Materials Processing Science
Volume
31
Issue
No. 6

Keywords

Oxygen diffusion, Ti castings

Citation

Boettinger, W. , Williams, M. , Coriell, S. , Kattner, U. and Mueller, B. (2000), Alpha Case Thickness Modeling in Investment Castings, Metallurgical and Materials Transactions B-Process Metallurgy and Materials Processing Science (Accessed June 14, 2024)

Issues

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Created December 1, 2000, Updated February 17, 2017