The performance of aluminium alloy castings is limited by the level of two major
defects: porosity and iron intermetallics, because both phases can lead to the initiation
and propagation of cracks of casting components at high cyclic regime. To improve
the fatigue life and thus increase usage of these energy-saving light metals, the
mechanisms by which such microstructure features form and possible approaches to
control them were investigated via a mathematical model which was validated by
synchrotron x-ray radiography and tomography experiments.
A multicomponent and multiphase model was developed to incorporate both
nucleation and growth of Fe intermetallics using different techniques including Monte
Carlo, phase field, and pseudo front-tracking. The classic heterogeneous nucleation
was simulated by solving stochastic functions which were related to the local Gibbs
free energy or total undercooling. The non-equilibrium growth of intermetallic phases
was calculated by two separate methods: control volume and phase-field. Using
realistic Gibbs free energy functions, the advancing S/L interface was simulated either
by calculating kinetic velocity or by solving phase field equations. Anisotropy of S/L
interfacial energy was implemented via a decentred needle/plate technique and phase
field method. In addition, the probability of atomic attachment entered the
propagation of cells by Monte Carlo method. Coupling this model with a pseudo
front-tracking model, the evolution of microstructure features, including primary Al,
gas and shrinkage porosity, and Fe-rich intermetallics, was simulated. To predict the
formation of these microstructures in casting components, e.g. an engine block, this
micromodel was directly implemented as a subroutine into a macroscale heat transfer
and fluid flow model.
Numerical investigations were compared between control volume technique and
phase field method, showing better efficiency and reasonable accuracy using the
former. To correct the empirical parameters in the model, the kinetic data was
successfully obtained from in-situ observations of micropores and Fe-rich
intermetallics during solidification using the state-of-the-art x-ray imaging and
quantification techniques. Three dimensional predictions of micropores from the multiscale model were then validated by x-ray tomography experiments on Al-Cu, Al-
Si, and Al-Si-Cu alloys in different casting conditions. Synchrotron x-ray tomography
experiments were used to validate the distribution of size and morphology of Fe-rich
intermetallics in multicomponent Al-7.5wt.%Si-3.5wt.%Cu alloys with varying levels
of Fe content. Good agreement between predictions and experiments was successfully
obtained qualitatively and quantitatively.
Applying this multiscale model to industrial castings, both microporosity and Fe-rich
intermetallics were predicted in various casting conditions. Decreasing initial
concentration of Fe and/or increasing cooling rates, smaller intermetallic phases
formed during solidification, matching the experimental observation well. Complex
interactions between pores and Fe intermetallic phases were simulated by
preferentially segregating hydrogen and reducing G/S interfacial energy. Satisfactory
results were obtained to reflect the influence of Fe-rich intermetallics on the
nucleation and growth of pores. Therefore, practical measures to control
microstructures and thus increase fatigue life of casting components can be
summarized from the model predictions, which may significantly improve the
efficiency of alloy design and process optimization.