Investigators: W. J. Boettinger, S. R. Coriell, U. R. Kattner,

Technical Description

The surfaces of Ti investment castings are contaminated with oxygen because of reaction with the oxide mold material. This brittle surface layer, called alpha case, must be removed before use. Prediction of the thickness of the contaminated layer would allow modification of mold design and gating to reduce the alpha case. It would also provide a proper estimate of the extra dimension to be added to the casting to allow for alpha case removal.

Technical Objectives:

• A diffusion-based model, which uses temperature-time history at points of interest along the surface of a casting obtained from a thermal model, will be developed to predict the oxygen penetration normal to the surface.

• The results of this modeling effort will be a module of code that can be appended to a heat flow model of the casting.

Anticipated Outcome:

• This research will produce an enhancement of a commercial software code used to model castings. This enhancement will predict the thickness of alpha case over the surface of a Ti investment casting.

• Investment casters will be able to determine the optimal extra dimension to be added to Ti castings to allow for alpha case removal.

Accomplishments for FY 1995:

• Cooling curves from thermocouples for locations in the metal casting but very close to the shell mold were obtained from a casting produced at Howmet. Measurements of alpha case thickness were made. The casting geometry was a step-wedge with section thickness ranging from 5 mm to 30 mm. This gave a range of cooling rates and hence -case thicknesses.

• Thermal modeling was performed by Howmet with heat transfer coefficients adjusted to give agreement between measured and modeled temperature-time data.

• A diffusion model was developed to predict penetration of oxygen into the Ti casting. It was tested using the calculated temperature-time data as input. Good agreement was found between the predicted and measured ý-case thickness, which ranged from 0.2 mm to 0.8 mm.

Impact and Technical Highlights:

• The diffusion model was implemented into the ProCAST^TM code for modeling of castings as part of the NIST Consortium on Casting of Aerospace Alloys.

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Last modified: Mon Jan 06 09:46:15 1997 Metallurgy Webmeister