On the Dissolution of Nitrided Titanium Defects During Vacuum Arc Remelting of Ti Alloys
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demand on premium-grade titanium processed by triple vacuum arc remelting (VAR) has increased largely during the past few years. Indeed, extremely high quality now is required for these products that compose the rotating parts of aircraft engines. The presence of inclusions is a serious problem and has always been a major concern to the research and processing sectors. For economical and safety reasons, particular attention must be paid to try to eliminate the following two types of inclusions usually identified in titanium alloys: (a) High interstitial defects (HIDs), also known as harda defects, are nitrogen- (or other a-stabilizing elements such as oxygen-) enriched titanium particles. They are characterized by their high hardness and G. GHAZAL, PhD Student, and A. JARDY and P. CHAPELLE, CNRS Research Scientists, are with the Institut Jean Lamour, De´partement SI2M, CNRS—Nancy-Universite´—UPVM, Ecole des Mines, Parc de Saurupt, 54042 Nancy Cedex, France. Contact e-mail: [email protected] Y. MILLET, Director of Research and Development for Europe, is with TIMET Savoie, Avenue Paul Girod, 73400 Ugine, France. Manuscript submitted July 20, 2009. Article published online February 4, 2010. 646—VOLUME 41B, JUNE 2010
high melting point. A typical hard-a inclusion is shown in Figure 1. Hard-a defects[2] have no chemical or structural definition, but anomalies found in industrial ingots were measured to contain up to 14 wt pct nitrogen. HIDs hardly are detected by X-ray or ultrasonic inspection if no crack is associated, which makes their removal during melting essential. Despite being rare (one inclusion in 500 tons[3]), their presence in an engine component can lead to premature failure of titanium parts used in aircraft engines and fatal accidents. They represent one of the most deleterious anomalies encountered during the processing of titanium ingots. Hard-a defects have many potential origins according to the literature,[4] the most probable being the use of a burnt titanium sponge during the first steps of the processing route. (b) HDIs are pieces made of refractory elements such as tungsten, molybdenum, niobium, or tungsten carbide. These impurities, which may generate from fragments of tools or nonmelted alloying elements,[5] are characterized by their high melting point. It is relatively easy to detect HDIs by X-ray because of their large density difference with the titanium matrix. An experimental study on the dissolution of both types of defects in titanium alloys and a numerical METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 1—A typical hard-a inclusion detected in an industrial ingot after mechanical working and containing up to 3.8 wt pct N.[1]
modeling of the behavior of HIDs is being conducted at the Institut Jean Lamour (School of Mines in Nancy, France), in close collaboration with TIMET, UK, and TIMET Savoie. This article presents the numerical study aimed to predict the motion and dissolution of hard-a defects in the molten pool of a VAR titanium ingot and helps determine the operating c
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