Grain refining aluminium foundry alloys with commercial Al–B master alloys

Abstract

In Al–3B master alloy, a higher fraction of borides is of the AlB₂ variety, while in Al–8B alloy, the predominant species is the AlB₁₂ phase. AlB₁₂ is less stable than AlB₂ and is engaged in exchange reactions, leading to the formation of transition metal diborides that subsequently settle at the bottom of the melt. Hence, AlB₁₂ is involved more in precipitating transition elements than in refining the grain structure. With predominantly AlB₁₂ particles, Al–8B master alloys are better suited for the removal of transition metal impurities in the manufacture of aluminium conductors. Al–3B, on the other hand, is a better grain refiner as the majority of its borides are of the AlB₂ variety. Which of the two master alloys is used in grain refinement does not make a difference once the transition metal impurities have been precipitated. B is dedicated to grain refinement in an impurity free aluminium melt and produces exceptionally small equiaxed grains across the section of the samples.