Effect of titanium addition on twinned dendrites in directionally solidified Al-5wt.% Cu alloy

As a casting defect in semi-continuous casting of Al alloys, twinned dendrites (macroscopically called feathery grains), which consist of a series of alternated twinned and untwinned lamellae, pose are a great challenge facing researchers to overcome. In this work, the effect of minute addition of titanium on twinned dendrite growth in directionally solidified Al-5wt.% Cu alloy is investigated by using optical microscope and electron backscattered diffraction(EBSD) in scanning electron microscope. It is indicated that the microstructure morphology of directionally solidified alloy transforms from prevailing feathery grains and coarse columnar ones into fine equiaxed ones with the addition of 0.1 wt% Ti element. Besides, the proportion of twin boundaries in upper part of the ingot of directionally solidified Al-5wt.% Cu-0.1 wt% Ti alloy is drastically increased with respect to the theoretical value, which is attributed to an icosahedral quasicrystal nucleation mechanism similar to the mechanism reported in Al-Zn-Cr alloys. The grain selection mechanism during directional solidification has also been discussed.