Microstructural and Microanalytical Study of the Effect of Processing Parameters on the Al Loss and Deposition Efficiency of EBF3 Ti-6Al-4V Alloys

Thursday, June 26, 2008 - 9:30 AM

Microstructural and Microanalytical Study of the Effect of Processing Parameters on the Al Loss and Deposition Efficiency of EBF3 Ti-6Al-4V Alloys

S. N. Sankaran, Lockheed Martin, Hampton, VA; K. M. Taminger, R. A. Hafley, C. L. Lach, NASA Langley Research Center, Hampton, VA

Electron beam freeform fabrication (EBF3) represents a significant paradigm shift in the manufacture of metallic aerospace structures from integrally-stiffened structures designed for assembly to novel unitized structures designed for multi-functional performance optimization. EBF3 feeds metal wire into a molten pool formed and maintained by an electron beam in a high vacuum environment (10-5 torr). The challenges of molten pool processing of Ti-6Al-4V in a high vacuum are due to the differences in the vapor pressures of the alloy constituents at the melting point of titanium which result in selective vaporization of aluminum, and make control of the chemical composition and mechanical properties difficult.

A three-factor, three-level Taguchi design of experiments (DOE) study was conducted to rank the importance of processing parameters, identify any nonlinear parameter couplings, and define the optimal metal deposition parameters to maintain the Aerospace Material Specification (AMS) compositional requirements for aluminum content in Ti6Al-4V. A broad range of values for each process parameter (beam power, translation speed, wire feed rate) was selected for the study, to produce deposits with chemistries spanning the entire AMS Ti6A-4V composition range for aluminum. Bulk compositions of 27 single-bead deposits were determined by the Direct Current Plasma technique (DCP). By applying DOE response surface methodology (RSM) on the bulk chemistry data, three-dimensional response surface and contour plots enable the prediction of aluminum content, as a function of critical EBF3 processing parameters. The sharpest rising gradient of the response surface design for a given set of processing parameters will indicate the direction of optimum EBF3 operating conditions to maximize the aluminum content of the Ti6Al-4V deposits.

Compositional analysis conducted on a subset of the test matrix (three-factor, two-level) confirmed that EBF3 of Ti6Al-4V in a high vacuum had a negligible effect on the vanadium and H2 content in the single-bead deposits.