Diffusion models for evaporation losses during electron-beam melting of alpha/beta-titanium alloys
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3/3/04
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Diffusion Models for Evaporation Losses during Electron-Beam Melting of Alpha/Beta-Titanium Alloys S.L. SEMIATIN, V.G. IVANCHENKO, S.V. AKHONIN, and O.M. IVASISHIN An analysis of the evaporation of aluminum during electron-beam melting (EBM) of alpha/beta-titanium alloys was performed. The analysis was based on the solution of the one-dimensional diffusion equation for the solute-concentration gradient in the melt pool subject to the flux boundary condition for the evaporation rate at the melt surface quantified using the Langmuir equation. The effect of process parameters and material coefficients (e.g., the diffusivity and solute activity in the melt) on predicted concentration gradients and melt losses under steady-state melting conditions was established for both the finite-domain and the semi-infinite–domain diffusion problems. The accuracy of the modeling approach was validated by comparison to previous measurements for the EBM of alloys with a nominal Ti-6Al-4V composition.
I. INTRODUCTION
THE production of high-quality wrought titanium mill products usually comprises several initial melting and refining steps. The most common process for such purposes consists of melting and remelting via a vacuum-arc technique. The principal advantages of these so-called vacuum-arc melting and vacuum-arc remelting (VAR) operations include the removal of dissolved gases (such as hydrogen), the minimization of undesirable trace elements having high vapor pressures, and the minimization of macrosegregation, all of which improve the initial ingot quality.[1] Drawbacks inherent to VAR, such as the possible retention of undesirable low- or high-density inclusions in finished products, have stimulated the development of alternate methods for the production of titanium-alloy ingots. These approaches include a variety of so-called cold-hearth techniques, in which the charge is melted (using electron-beam or plasma torches) into a copper water-cooled hearth. Subsequently, the molten metal passes through one or several refining hearths before flowing into a mold in order to form an ingot or a slab. Recent attempts to implement electron-beam, cold-hearth melting of titanium alloys on an industrial scale have shown much promise.[2,3] The electron-beam melting (EBM) method has a number of advantages compared to VAR. These include the improvement in ingot quality as a result of the fact that melting is conducted under a higher vacuum and for a longer time, thus enabling more complete degassing and dissolution of low-density inclusions. The long residence time of the molten titanium in the melting/refining hearths also reduces the incidence of high-density inclusions, which fall to the bottom of the hearth(s) and are thus incorporated into the skull. Despite the advantages of EBM compared to VAR, several challenges remain before the full commercial potential of the process is realized. One of the primary obstacles pertains S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with the Ai
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