Poisoning and fading mechanism of grain refinement in Al-7Si alloy

Abstract

The poisoning and fading mechanism of grain refinement in Al-7Si alloy has been studied in detail with the conventional (0.01%Ti or B) and higher addition levels of indigenously developed Al-3Ti, Al-5Ti, Al-3B and Al-5Ti- 1B master alloys. Results suggest that on prolonged holding the melt after the conventional addition of grain refiner to Al-7Si alloy, size of the α-Al dendrites increases (fading), which could be due to the dissolution/settling of TiAl₃ and AlB₂ particles. However, vigorous agitation of the melt after prolonged holding (120s min.), can partly bring back the particles into the liquid melt and acts as heterogeneous nucleating sites to some limited extents (120s min. sample). In addition, the conventional addition (0.2 wt%) of Ti-rich Al-3Ti, Al-5Ti and Al-5Ti-1B master alloy to Al-7Si alloy, the Si from the melt reacts with grain refining constituents (TiAl₃) and formation of titanium silicide and coats on the surface of the TiAl₃ particles and poisons the effectiveness of the nuclei. However, the higher addition level of these master alloy or B-rich Al-3B master alloys can overcome poisoning effect of Al-7Si alloy.