► The performance of AlTi5B1 falls short of expectations with the AlSi10Mg and AlSi12Cu alloys. ► When inoculated with AlB3, their grains are very small for the entire range of holding times. ► Fine grains, invariably smaller than 200 μm, are readily obtained with the addition of 200 ppm B. ► AlB3 offers remarkable grain refining efficiency and a strong resistance to fading. The potential of AlB3 master alloy in the grain refinement of AlSi10Mg and AlSi12Cu foundry alloys was investigated and compared with that of the AlTi5B1 master alloy, the standard grain refiner for most aluminium foundries. The latter refines the grain structures of both alloys. However, this performance is not nearly as good as that obtained in wrought aluminium alloys with the same grain refiner. The Ti-free AlSi10Mg and AlSi12Cu alloys, on the other hand, exhibit very small grains for the entire range of holding times when inoculated with AlB3. This implies a remarkable grain refining efficiency, typical of grain refined wrought aluminium alloys, as well as a strong resistance to fading of the grain refinement effect. Lack of Ti in the melt allows the entire B to form AlB 2 particles, the perfect substrates, shortly before α-Al starts to crystallize. Aluminium castings can enjoy grains as small as those of the wrought alloys, well below 200 μm, with an addition of 0.02 wt% B provided that their Ti content is controlled.