2010 Volume 50 Issue 12 Pages 1829-1834
Minimizing macro-segregation during static casting of segregation prone alloys has always been a challenge. Over the years, alloy manufacturers have optimized the processing techniques for these alloys by balancing the total heat input of a casting and the rate of heat extraction from the casting surface. In this study, a computational fluid dynamics (CFD) code was used to develop a generic framework for studying macro-segregation and shrinkage in various casting processes. The code developed the framework by solving for the solid fraction evolution, micro-scale characteristics, temperature, flow and solute balance in multi-component alloy systems.
Experiments were previously designed to measure solidification parameters using micro-probe compositional analysis. The thermo-physical properties of alloy 718 that were used in the simulations strongly depend on temperature. The partition coefficients of Nb, Ti, Al, and Mo in alloy 718 are also a strong function of temperature. They were determined based on quenching experiments during solidification of alloy 718.
The model predictions were validated against measured macro-shrinkage and shrinkage porosities and against macro-segregation measured from pieces cut from both laboratory scale and industrials scale castings. A special segregation index was developed and then applied to properly assess the influence of material and process parameters on macro-segregation. The effect of Nb content and processing parameters such as super-heat, mold taper, mold type, and mold diameter on macro-segregation was also studied.
Additional validation of the segregation model was performed using the literature analytical solution and experiments. Additional experimental validation of the model for prediction of macro-shrinkage and shrinkage porosities was performed using A356 plates cast in furan-silica sand molds.
