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The effect of beam oscillation rate on Al evaporation behavior in the electron beam melting process

University of British Columbia

Electron beam hearth melting process is widely used in producing superalloys, Ti and its alloys because of its excellent metallurgical characteristics. However, one of the disadvantages of the process is that alloying elements with high vapor pressure evaporate under the highly reduced operating pressure. This makes it very difficult to carry out an accurate chemical composition control. In order to prevent the excess evaporative loss by reducing the superheating of the molten pool, the beam scanning technique is employed in normal operation. Although the effect of this technique is well known empirically, few fundamental studies' have been made to date. The purpose of this study is, therefore, to clarify the quantitative effect of the beam oscillation rate on the evaporation behavior. Small amounts of Ti-6Al-4V alloy were melted in an EB melting furnace. The temperature on the melt surface was measured in situ by an optical pyrometer during the melting period. The evaporation loss of both Ti and Al was also investigated. On the basis of the experimental results, a two dimensional unsteady heat and mass transfer model was developed. (A one dimensional model was developed in the case of a stationary beam.) The model was used to investigate the effect of the beam oscillation rate on the evaporation behavior and also to discuss the optimum beam scanning rate. It was clearly shown that the evaporative loss of both Ti and Al could be suppressed by the increase of the beam oscillation rate. With the beam oscillation rate at more than 1.0 Hz, however, this effect could not be observed clearly anymore. It was also found that the beam scanning technique is useful not only in controlling Al concentration, wt%Al, but also in suppressing the total evaporative loss of both Ti and Al.

Text

2010-08-30

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http://hdl.handle.net/2429/27944

Materials Engineering

University of British Columbia

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