Aluminum volatilization and inclusion removal in the electron beam cold hearth melting of Ti alloys

Abstract

The electron beam cold hearth melting (EBCHM) process has emerged as either an alternative or a complement to vacuum arc remelting, since it is capable of enhancing the elimination of hard-alpha inclusions by dissolution or sedimentation. The present article describes the use of a mathematical model to simulate the electron beam melting of titanium in a cold hearth. The mathematical model is based on the numerical solution of the coupled momentum, solute, and heat transport equations in a transient regime for a three dimension geometry. The model calculates the velocity, turbulence intensity, temperature, and alloy composition in the liquid and the solid phases. The calculation provides the overall heat balance and the volatilization of metallic elements such as aluminum. A postprocessor numerical tool simulates also the behavior of a hard-alpha inclusion during melting (trajectory and kinetics of dissolution). In order to demonstrate the usefulness of this model, the authors examine the influence of the casting rate and of the beam scanning frequency on the volatilization of aluminum and on the capacity of the process to remove hard-alpha defects.

Dissolution of High Density Inclusions in Titanium Alloys

“…They are characterized by their high hardness, which may induce cracks following a fatigue cycle. The dissolution of nitride titanium inclusions has been studied by a number of authors, and both experimental [2][3][4][5][6] and numerical 2,7,8) studies on the subject have been reported. Understanding the inclusion behavior during metal processing has been a major interest at the Institut Jean Lamour (IJL) for many years.…”

“…Various experimental and modeling works have been performed to investigate the inclusions in steels, superalloys and titanium alloys. [7][8][9][10] Although the trajectory of HDIs in a liquid pool is fairly well understood since it is essentially vertical, dissolving aspects of several refractory metals are still unknown and very few studies have considered their dissolution yet. To our knowledge, only two papers have been reported in the literature.…”

On the Formation of White‐Spot Defects in a Superalloy VAR Ingot

“…The quantitative discrepancies between Figure 8 and 9 are very probably related to the difference between the thermohydrodynamic behavior of VIM and EB baths. Indeed, former studies [10] have shown that steep temperature gradients were encountered in the EB metal bath and correlated with a strong thermocapillary-induced fluid flow close to the free surface. On the contrary, the liquid metal was only weakly stirred during our VIM dipping experiments, because the melting was caused by the heating of a graphite susceptor [see Figure 5…”

“…A similar approach was previously used at Institut Jean Lamour [10,11] to evaluate the impact of process parameters on the dissolution kinetics of high interstitial defects during VAR or electron beam cold hearth refining (EBCHR) of titanium alloys. In addition to the simulation of the thermohydrodynamic behavior of the liquid pool [12] and the particle-tracking model, which will not be discussed in this paper, we have therefore built a comprehensive model of the melting and solidification of a precursor, either in 1D (spherical white spot precursors in a VAR melt pool) or in 2D (cylindrical samples used in the experiments-see below-to validate the numerical simulations).…”

Diffusion models for evaporation losses during electron-beam melting of alpha/beta-titanium alloys

“…The corresponding interdiffusion coefficient for aluminum in liquid titanium would thus have an identical value. The temperature dependence of the diffusivity was assumed to follow an Arrhenius form with an activation energy of 250 kJ/mol*, thus leading to the following relation: [22] in which T liq represents the liquidus temperature of Ti-6Al-4V, i.e., ϳ1925 K. The applicability of Eq. [22] for the description of the diffusivity was validated by comparison of model predictions of melt losses to measurements.…”