Coupling In-situ Observations and Microscale Modeling to Predict Pore and Fe-rich Intermetallic Formation during the Solidification of Al-Si-Cu-Fe Alloys

The presence of pores and intermetallics can often limit the fatigue performance of cast aluminum alloy components. However, predicting the size distribution of these microstructural features is difficult due to the stochastic nature of their nucleation, complex thermodynamics/kinetics, and their interactions with other phases during growth. This paper will investigate how in situ X-ray observations (using synchrotron and lab sources) can be combined with three dimensional microstructural models to predict the size, morphology and distribution of the Fe-rich intermetallic phases (e.g. alpha-Al8Fe2Si and beta-Al5FeSi) and pores which form during the solidification of Al-Si-Cu-Fe alloys. The kinetics of Fe-rich intermetallic formation is unknown due to the highly non-equilibrium nature of this process which makes it difficult to determine the local undercooling using normal quenching techniques. Because of their fine faceted shapes, direct measurements of their kinetics are only possible when high resolution imaging (e.g. 1 mu m/pixels) is adopted. In-situ observation using synchrotron X-ray radiography allows both Fe-rich intermetallic and pore nucleation and growth kinetics to be quantified as a function of temperature. Using this experimental data, we developed a kinetic-thermodynamic model, solving both multicomponent diffusion equations and total free energy. Crystallographic anisotropies of monoclinic beta-Al5FeSi are accounted for by an adapted implementation of combined Monte-Carlo and geometric constraint techniques. The resulting simulations predict that increasing the Fe content causes the Fe-rich intermetallics to form large needles/plates, in agreement with synchrotron tomography results. Increasing Fe content has a non-linear effect on pore formation, first increasing and then decreasing the pore size. This non-linear effect is hypothesized to be caused by the interaction of competing influences on pore nucleation and growth rates.