Simulation of directional solidification of refined Al–7wt.%Si alloys – Comparison with benchmark microgravity experiments

Abstract : Formation of grain structure and segregation during solidification of refined Al–7 wt.%Si alloys under microgravity is simulated using a two-dimensional axi-symmetrical Cellular Automaton-Finite Element (CAFE) solidification model. This fully integrated model resolves a complex interplay between heat transfer at the macroscale and grain structure evolution and microsegregation formation at the mesoscale. Direct comparison with benchmark experiments performed under microgravity conditions of the International Space Station is carried out. Boundary conditions used in the simulations are deduced from the experimental measurements of temperature. Qualitative agreement is achieved concerning CET (columnar-to-equiaxed transition) position, CET transition mode (sharp or progressive), distributions of grain elongation factor and equivalent diameter, and distribution of eutectic fraction. The influences of pulling velocity and thermal gradient on grain structure and intergranular segregation are correctly reflected and are discussed. Moreover, we take advantage of the simulation results to study the effect of otherwise inaccessible parameters such as the degree of constitutional undercooling and the amplitude of the undercooled region. Another originality of this paper is to conduct quantitative comparisons between experiments and simulations which allows discussion of the improvements needed in the simulation.