Influence of titanium to boron ratio on the ability to grain refine aluminium-silicon alloys

Grain refinement is achieved in aluminium alloys by inoculating the molten metal with small amounts of titanium and boron. Grain refiners containing high concentrations of titanium and boron are available commercially for this purpose. However, these grain refiners come in different chemical compositions giving rise to a variety of Ti/B ratios. Although it is accepted that increasing the amount of boron in the grain refiner gives beneficial effects on its performance, evidence for this is available principally in wrought aluminium alloys or in pure aluminium. Foundry alloys which contain high concentrations of silicon did not appear to respond in a manner similar to the other aluminium alloys. Hence, this work was undertaken to investigate the effects of various grain refiners on Al-Si alloys. One Al-Si alloy of a hypo-eutectic composition and another of eutectic composition were chosen for the study. The performances of three Tibor alloys Al-6%Ti, Al-5%Ti-1%B and Al-2.5%Ti-2.5%B were assessed on the two Al-Si alloys. The results show that the grain refiner Al-2.5%Ti-2.5%B is the most potent in both Al-Si alloys while the Al-6%Ti gave the worst performance. The refined grain size depends on the Ti content, Ti/B ratio and the matrix alloy composition. A logarithmic dependence of grain size on Ti level was noticed in some cases while a power law dependence was appropriate for some others.