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.

Hot tearing and as-cast porosity are two defects which impede the performance of cast products.

A direct finite element microstructure model for prediction of the deformation behavior of semisolid metallic alloys is presented. The 2D model geometry is based on a modified Voronoi tessellation, and includes rounded corners to approximate an equiaxed-globular grain structure, liquid surrounding the grains, and micro porosity. An elasto-plastic empirical constitutive equation is derived for the solid grains, while the liquid is approximated as a perfectly plastic material with a very low yield stress. The resulting three-phase model was used to investigate the effects of fraction solid, porosity, and grain size on the constitutive behavior of a semi-solid aluminum alloy, AA5182. The model predictions were validated against experimental data at high fraction solid. These simulations reveal a strong correlation between semi-solid grain size and yield stress, and between porosity and strain localization. The application of direct finite element simulations is shown to be an effective technique for examining the effects of microstructure phenomena on the macro constitutive behavior of semi-solid materials.

An experimental investigation has been conducted on as-cast samples from three commercially significant aluminum alloys (AA1050, AA3004, and AA5182) to quantify the influence of surface morphology, water flow rate, and sample thermal history on the boiling-water heat transfer under conditions similar to those experienced in the direct-chill (DC) casting process. The study involved characterization of the as-cast surface morphology using a laser profilometer and quantification of the sample surface temperature and heat extraction to the cooling water using a DC casting simulator in combination with an inverse heat-conduction (IHC) analysis. The results from the study indicate that alloy's thermal conductivity, surface morphology, and sample initial temperature all dramatically influence the calculated "boiling curve." The intensity of the heat extraction was found to be enhanced at high heat fluxes in the nucleate boiling regime as the thermal conductivity was increased and was also found to increase as the surface of the sample became rougher, presumably through promotion of nucleation, growth, and/or detachment of bubbles. The heat transfer was also found to increase with increasing sample starting temperature, resulting in a series of boiling curves dependent on initial sample temperature. Finally, the effect of the water flow rate on heat transfer was found to be comparatively moderate and was limited to the sample with the smooth (machined) surface.

No abstract