Grain refining AlSi7Mg0·3 foundry alloy with commercial Al–4B master alloy

Abstract

Al–B master alloys can replace ternary Al–Ti–B grain refiners in grain refining Al–Si foundry alloys since the latter are susceptible to Si poisoning. The grain refining efficiency of Al–B master alloys is remarkable provided that the B supply is in excess of that required to precipitate Ti as well as other elements, the borides of which are more stable than those of aluminium. B needed to counteract the poisoning effect of these elements in a typical commercial AlSi7Mg0·3 alloy is estimated to be ∼650 ppm B. This is a high B addition rate where the AlB₂ platelets are no longer soluble and are thus prone to sedimentation. This problem is made worse owing to the clustering of the borides in commercial Al–B master alloys, which in turn leads to low B recoveries. Hence, it takes an additional 500 ppm, and as much as 1150 ppm B in total, to achieve grains as small as 150 μm in a commercial AlSi7Mg0·3 alloy.