Interaction of grain refinement with B and modification with Sr in aluminium foundry alloys

The interaction between modification with Sr and grain refinement with B was investigated to ensure a very fine grained matrix and a very fine fibrous eutectic in the AlSi7Mg alloy. The grain refinement offered by B is remarkable. B (200 ppm) refines the grain structure of the AlSi7Mg alloy at least 10 times in a wide range of cooling rates between 0.1 and 10 K s(-1), whether added before or after Sr. While a weak grain refiner, Sr turns the coarse Si plates and needles into fine fibres. However, the modification capacity of Sr is impaired when there is B in the melt, possibly due to the formation of SrB6 compound. The poisoning in the chemical modification of the eutectic phase in grain refined Al-Si alloys can be circumvented by quench modification. It takes cooling rates in the order of 10 K s(-1) to adequately modify the eutectic phase in the Sr free grain refined AlSi7Mg alloy. Si fibres are still the predominant feature at lower cooling rates (1-10 K s(-1)) when modification of the eutectic phase relies not only on rapid solidification but also on Sr. Coarse Si plates and needles form only at very low cooling rates, well below 1 K s(-1). It is fair to claim that 200 ppm B may suffice for a sound structure in casting processes, which involve rapid solidification and thin sections, while Sr is needed for an acceptable modification of the eutectic phase when solidification is relatively slower.