The modelling of solidification processes in combination with fluid flow is one main application of ANSYS Fluent. The solidification is modelled with the enthalpy porosity technique. Therefor the fluid flow is damped like a flow through a porous media of dendrites. In case of materials with large solidification ranges, like the nickel based superalloy 718, the adjustment possibilities of ANSYS Fluent are often not adequate. The program postulates a linear dependency between liquid fraction and temperature. To improve the simulation, the solidification was implemented by a user defined function (UDF). The principal modelling of fluid flow is based on the theory of ANSYS Fluent, but it is now possible to adjust the liquid fraction in fine temperature steps.