Analysis of multicomponent evaporation in electron beam melting and refining of titanium alloys

Abstract

Experimental evidence and a mathematical model are presented to evaluate the effect of beam-scan frequency on composition change in electron-beam melting of titanium alloys. Experiments characterized the evaporation rate of commercially pure (CP) titanium and vapor composition over titanium alloy with up to 6 wt pct aluminum and 4.5 wt pct vanadium, as a function of beam power, scan frequency, and background pressure. These data and thermal mapping of the hearth melt surface are used to estimate activity coefficients of aluminum and vanadium in the hearth. The model describes transient heat transfer in the surface of the melt and provides a means of estimating enhancement of pure titanium evaporation and change in final aluminum composition due to local heating at moderate beam-scan frequencies.

Mathematical modeling of aluminum evaporation during electron-beam cold-hearth melting of Ti-6Al-4V ingots

“…Assuming the coefficient of condensation of aluminum atoms in the melt to be equal to one, the evaporation rate constant can be expressed in the following form: [11] in which p 0 Al is the vapor pressure (in Pa) of pure aluminum; g Al j is the activity coefficient of aluminum in the solution in the jth melting zone; M Al is the molar mass (i.e., kg/mole) of aluminum in the melt); and T j in the temperature (in K) in the jth melting zone.…”

“…[15], it is necessary to determine the effect of melting parameters on the temperature of the melt surface at the tip of the feed billet, in the cold hearth, and in the mold. As shown by calculations of the temperature fields in EBCHM ingots [9,10] and experimental measurements, [11,12] the temperature of the melt surface is stable and unambiguously determined by the electron-beam heating power and melting rate under steady-state melting conditions. Therefore, the average surface temperature T -is used here to quantify the conditions on the surface S of the liquid metal:…”

On the Roles of Oxidation and Vaporization in Surface Micro-structural Instability during Solution Heat Treatment of Ni-base Superalloys

“…[13,14] Other applications, where elemental vaporization is also critical is in electron beam melting; e.g., in refining of Tibase alloys, where evaporation of Ti from the liquid residing in the hearth occurs, in a vacuum atmosphere. [15] This clearly demonstrates that ''surface effects'' need to be considered, and the notion of a ''closed system'' for mass balance considerations might not necessarily be correct. However, notwithstanding this preliminary study, [11] many aspects remain un-answered.…”

The role of modeling in the development of advanced processes for metallic aerospace alloys

“…The modeling of the evaporation losses during EBCHM has been described in a number of efforts. For example, Powell et al [12] tried to predict aluminum losses during EBCHM of Ti-6Al-4V assuming plug flow of a per fectly-mixed melt of uniform composition. The specific melt loss was then based on interface-reaction controlled kinetics through the application of the Langmuir equation [13]:…”

“…The influence of solute diffusion in the melt to the liq uid surface was not treated by Powell, et al [12]. Agreement between measured and predicted evaporation rates was achieved by using the activity coefficient of aluminum (γΑl) in the Ti-6Al-4V melt as essentially a fitting parameter.…”