The evaluation of a numerical criterion to provide quantitative insight on freckling conditions is critical to the successful manufacture of large superalloy castings. Of the criteria reported in the literature, those based on the Rayleigh number seem best suited to predict the onset of freckle formation. However, in their current form, these criteria cannot explain why freckles develop predominantly at the surface of single crystal (SX) castings and at midradius in VAR/ESR ingots. An experimental Bridgman-type furnace has been built to directionally solidify freckle-prone superalloys, CMSX-11B, RENÉ 88, NIM80A, WASPALOY, MAR-M247, and a variation of IN718 with high silicon content, at various angles to the vertical. Under typical industrial solidification conditions (thermal gradient between 500 and 4000 K m Ϫ1 (5 Ͻ G Ͻ 40 ЊC cm Ϫ1 ) and solidification rate between 1.67 ϫ 10 Ϫ5 and 1.0 ϫ 10 Ϫ4 m s Ϫ1 (1 Ͻ R Ͻ 6mm min Ϫ1 )), the results indicate a dependency of freckling on growth front angle likely related to the anisotropy in permeability. A modified Rayleigh criterion has been developed which accounts for directional permeability and orientation of the growth front relative to the gravity vector. Application to the experimental data shows good correlation with the onset of freckling for the range of solidification conditions examined in the study. The approximate threshold value for the modified Rayleigh number was estimated to be for CMSX-11B, 0.88, for RENÉ 88, 0.90, for NIM80A, 0.85, for WASPALOY, 0.95, for MAR-M247, 0.86, and for IN718-Si, 0.65.

No abstract

No abstract

No abstract

No abstract

Canadian and French university teams have joined efforts in carrying out an experimental and theoretical study of the dissolution behavior of the hard-alpha inclusion in liquid titanium alloys. Synthetic hard-alpha dense particles of up to 6 wt pct nitrogen and nitrided sponge of up to 15 wt pct nitrogen were partially dissolved in a titanium or a titanium alloy bath. The metallographic examinations and microprobe analysis show that the dissolution process is always controlled by the outward diffusion of nitrogen into the bath through an external layer of beta phase. The growth of this beta phase layer depends on the velocity of liquid flow in the bath and can lead to an initial increase in the inclusion size. For porous particles, the diffusion of nitrogen from the pellet matrix to the infiltrations gradually leads to a partial densification of the inclusion. A numerical representation of the dissolution problem was developed, including the transient diffusion of nitrogen through intermediate solid phases. The comparison is good between the numerical simulations, the experimental measurements, and the dissolution kinetics given in the literature.

No abstract

The focus of the study was to define the origin of the inclusions in production-scale electro-slag remelting, (ESR) and electro-slag remelting under a protected pressure controlled atmosphere, (PESR), ingots. The inclusion characteristics in production samples were studied using both polished sample surfaces (two-dimensional (2-D) investigations) and inclusions extracted from steel samples by electrolytic extraction (three-dimensional (3-D) investigations) using SEM in combination with EDS. The results were compared to results from previously reported laboratory-, pilot-, and production-scale trials including electrode, remelted, and conventional ingots. The results show that primary, semi-secondary, and secondary inclusions exist in the remelted ingots. The most probable inclusion to survive from the electrode is a MgO-Al2O3 (spinel). It was also found that the ESR/PESR process slag acts in a similar way to a calcium treatment modification of alumina inclusions. On the whole, the most significant finding is that the overall cleanliness of the electrode including the inclusions in the electrode has an influence on the inclusion content of the ESR and PESR ingots.