A model for prediction of pressure and redistribution of gasforming elements in multicomponent casting alloys

A finite element model for simulating macrosegregation in multicomponent alloys is extended to include the calculation of pressure and redistribution of gas-forming elements during solidification. The model solves the conservation equations of mass, momentum, energy, and alloy components, including gas-forming elements such as hydrogen and nitrogen. The results of transport calculations are contrasted with thermodynamic equilibrium conditions to establish the possible formation of pores, assuming that there is no barrier to nucleation of the pores. By solving the transport of gaseous solutes and comparing their Sievert's pressure with the local pressure, the new model can predict regions of possible formation of intergranular porosity. Simulations were performed for a nickel-base alloy (INCONEL 718) in plate castings with equiaxed structure, and the evolution of microporosity for different initial concentrations of hydrogen and nitrogen was analyzed. The simulations showed that during solidification and cooling, a large fraction of the hydrogen escapes and a smaller fraction of nitrogen escapes from the casting. The initial gas concentration is an important factor in porosity formation, but the pressure drop due to shrinkage flow is not very significant. The resulting gas porosity is rather insensitive to initial nitrogen concentration, but sensitive to the concentration of hydrogen.