Influence of multiphase microstructure evolution and Fe-rich phases on Cu uphill diffusion in Al-Si-Cu-Mg alloys

Abstract

The evolution of multiphase microstructures in Al-Si-Cu-Mg alloys has been investigated by studying four types of alloys with different compositions and solidification rates. The experiment has been conducted with and without 0.6 wt% Zn addition by using the permanent mould and sand mould. Scanning electron microscope with energy dispersive spectroscopy, differential scanning calorimetry and metallographic statistics were applied to determine the evolution of the multiphase microstructure in the solution treatment. The texture and morphology of the Fe-rich phases on the uphill diffusion of Cu atoms was revealed. The results indicate that the existing forms of multiphase microstructures, especially the crystal texture and morphology of Fe-rich phases, were different with Zn addition and secondary dendrite arm spacing, which resulted from different solidification processes. In sand casting, large-sized acicular β-Fe was modified by Zn into short rod-like free β-Fe, while in the permanent mould, many π-Fe phases were formed with Zn addition. Unlike the decomposition of the π-Fe phase, the free β-Fe in the alloy 3DSZ can directly provide a low-energy grain boundary for the uphill diffusion of Cu atoms, which shortens the precipitation peak time of the Al2Cu. Two schematic diagrams were developed to describe the effect of decomposition of the π-Fe and free β-Fe phases on the uphill diffusion of Cu atoms.