In-situ Observation and Mathematical Modelling of the Nucleation and Growth of Intermetallics and Micropores During the Solidification of Aluminium Alloys

Abstract

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