A model for prediction of pressure and redistribution of gas-forming elements in multicomponent casting alloys

A finite element model for simulating macrosegregation in multicomponent al loys is extended to include the calculation of pressure and redistribution of gas-forming elements during solidification. The model solves the conserv ation equations of mass, momentum, energy, and alloy components, including gas-forming elements such as hydrogen and nitrogen. The results of transpor t calculations are contrasted with thermodynamic equilibrium conditions to establish the possible formation of pores, assuming that there is no barrie r to nucleation of the pores. By solving the transport of gaseous solutes a nd comparing their Sievert's pressure with the local pressure, the new mode l can predict regions of possible formation of intergranular porosity. Simu lations were performed for a nickel-base alloy (INCONEL 718) in plate casti ngs with equiaxed structure, and the evolution of microporosity for differe nt initial concentrations of hydrogen and nitrogen was analyzed. The simula tions showed that during solidification and cooling, a large fraction of th e hydrogen escapes and a smaller fraction of nitrogen escapes from the cast ing. The initial gas concentration is an important factor in porosity forma tion, but the pressure drop due to shrinkage flow is not very significant. The resulting gas porosity is rather insensitive to initial nitrogen concen tration, but sensitive to the concentration of hydrogen.