On the Dissolution of Nitrided Titanium Defects During Vacuum Arc Remelting of Ti Alloys

Abstract

The elimination of high interstitial defects (also known as hard-a inclusions) is of great importance to the titanium industry. This article presents a model capable of simulating the motion and dissolution of such defects during their residence in the pool of a vacuum arc remelted (VAR) ingot. To predict the complete history of that inclusion, the study couples a dissolution model of the defect and a Lagrangian particle-tracking model. This numerical tool is implemented in SOLAR (solidification during arc remelting), a computational fluid dynamics code developed at the Nancy School of Mines in the framework of an important research project conducted during the last 15 years, which aims to study and optimize the VAR process. The dissolution model numerically solves the nitrogen diffusion equation in a spherical inclusion and in thermal equilibrium with the surrounding fluid. The computational domain is divided into a central zone (a phase) and a surrounding layer (b phase), which appears because the diffusion of nitrogen into the liquid pool causes some solidification. The dissolution kinetics strongly depend on the liquid temperature and velocity of the inclusion. The model can compute the nitrogen profile in the defect at each moment as well as the thickness of the different layers; therefore, it can compute the overall size of the inclusion. The trajectory model consists of solving Newton's law of motion. Because the inclusion size is large, the consequence of fluidflow turbulence is to modify the local flow around the inclusion so that the drag is affected. Results presented and discussed in this article include a parametric study of the influence of the pool thermohydrodynamics, the relative inclusion-fluid density, and the initial diameter of the defect as it enters the melt pool. Finally, an example of the full history of an inclusion during triple VAR illustrates the possibility to remove such a defect effectively by dissolving it in the liquid phase.

Dissolution of High Density Inclusions in Titanium Alloys

“…This approach is based on a model reported in a previous paper, 8) aimed to simulate the dissolution of hard-α inclusions in molten titanium. This latter model solves, in a one dimensional geometry, the mass transfer equation for nitrogen in a dense inclusion that is in thermal equilibrium with the surrounding liquid.…”

“…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

“…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).…”

An examination of the factors influencing the melting of solid titanium in liquid titanium

“…The dissolution rate was found to obey a linear correlation with time of 0.16 mm/min under the experimental conditions in his study. Ghazal et al [8] also studied the dissolution rate of TiN in liquid titanium with emphasis on developing a mathematical model. The model was reported to make predictions on the dissolution behavior of the TiN particles as well as their trajectories within the molten pool of a VAR furnace.…”