Effect of Eu addition on the microstructures and mechanical properties of A356 aluminum alloys

The effect of Eu additions (0, 0.02, 0.04, 0.06, 0.08 and 0.1 wt.%) and T6 heat treatment on the microstructures and mechanical properties of A356 alloy have been investigated in the present work. Microstructures of the as-cast and T6 heat treated samples were examined by optical microscopy (OM) and scanning electron microscopy (SEM). It was found that 0.1%Eu modified the eutectic Si from a coarse plate-like form to a fully modified and fine fibrous one with a better uniform distribution. T6 treatment encouraged the spheroidization of eutectic Si particles. Correlating with the as-cast samples, the 0.1%Eu modified A356 alloy achieved likewise full modification of eutectic Si particles with lowest mean area and aspect ratio. In addition, the coarse and small Eu-rich intermetallics were both found in the 0.1%Eu modified A356 alloy. The cooling curves of A356 alloys showed that increasing Eu addition resulted in increasing nucleation undercooling and increasing growth undercooling. However, the recalescence (T_G − T_Min) increased by the initial addition of Eu, while it remained relatively stable for increasing additions (above 0.06%). The quenching experiment indicated that a large number of eutectic grains were located at or close to the tips of the aluminum dendrites with a jagged solid–liquid interface in unmodified alloy, while very few eutectic grains were formed in the interdendritic region with a smooth solid–liquid interface in 0.1%Eu modified alloy. An optimal combination of UTS (265 MPa) and EI (14.7%) of A356 alloy was achieved by the 0.1%Eu addition combined with T6 heat treatment. - Highlights: • The rare earth Eu is firstly used to improve the mechanical property of A356 alloy. • A fully modified and fine fibrous eutectic Si is observed by addition of 0.1 wt% Eu. • A complete spheroidization of 0.1wt%Eu modified eutectic Si is observed after T6. • Three kinds of Eu-rich intermetallics are found in the 0.1%Eu modified A356 alloy. • The undercoolings of the eutectic arrest show a good prediction to the modification.