Performance of AlTi5B1, AlTi3B3 and AlB3 master alloys in refining grain structure of aluminium foundry alloys

The capacity of AlTi5B1, AlTi3B3 and AlB3 grain refiners to refine the grain structures of AlSi7Mg and AlSi11Cu2 foundry alloys was investigated. The performance of AlTi5B1, well established to be the best grain refiner for wrought aluminium alloys, is not nearly as good with the AlSi7Mg and AlSi11Cu2 alloys. Relatively smaller grains are obtained with the AlTi3B3 grain refiner in both alloys. The AlB3 grain refiner, on the other hand, improves the grain structure only as much as the AlTi5B1 grain refiner does. With as much as 0.04-0.1 wt-%Ti, the commercial alloys cannot enjoy the outstanding potency of the AlB2 particles since the B supply is readily transformed to TiB2 particles. However, the grains of the Ti free AlSi7Mg and AlSi11Cu2 alloys (similar to 0.005 wt-%Ti) are very small and nearly globular for the entire range of holding times when inoculated with AlB3, implying not only a remarkable grain refining capacity but also a strong resistance to fading of the grain refinement effect. The lack of Ti in the melt allows the entire B to form AlB2 particles, the perfect substrates to promote the nucleation of alpha-Al crystals. Aluminium castings can enjoy grain sizes well below 200 mu m, with an addition of 0.02 wt-%B, provided that they are Ti free. That the potent substrates are made available just before the nucleation of alpha-Al crystals avoids fading effects and is a further advantage of the AlB3 grain refiner in recycling operations.