A critical survey of the grain refining mechanisms of aluminium

Abstract: In the present work, several possible interpretations of the grain refining mechanisms of aluminium castings after additions of titanium and boron are compared with the results of critical test experiments. The presence of B does not seem to affect the binary Al-Ti phase diagram close to the A1 comer, except for formation of boride phase. It is shown by powder X-ray diffraction and by energy dispersive X-ray spectrometry that the (Ali_xTix)B2 solid solubility is unlimited.At hypoperitectic additions of Al-Ti-B (<0.15wt%Ti) both calculations and experiments give evidence that TiAlj dissolves within one minute at a contact temperature of 775°C. From the experimental results it seems clear that the dominating mechanism is heterogeneous nucléation on boride particles. At hyperperitectic concentrations (>0.15wt%Ti) TiAl3 can act as the nucleating phase; in the presence of boron a preperitectic reaction can occur. The experiments show that this reaction is precipitation of supersaturated a-Al.The presence of potent nucleating particles is not the only prerequisite for successful grain refinement; an undercooling that may be caused by the presence of alloying elements is also necessary in order to create a driving force for growth. It is found that the constitutional contributions from various alloying elements are additive at low concentrations.Zr is reported to poison the effect of Al-Ti-B grain refiners. It is found that this poisoning effect occurs at hyperperitectic Zr concentrations (>0.11wt%). Precipitation of the solid solution phase (Ti j_xZrx)Al3 is found to consume enough Ti to decrease the growth restricting effect so much that the grain size is influenced.