Grain refinement and modification of Al-Si foundry alloys with B and Sr additions

The effect of B and Sr additions on the grain refinement and modification processes was investigated in the popular AlSi7Mg0.3 alloy to identify the optimum melt treatment to ensure a very fine grained matrix and a fine fibrous eutectic. B and Sr recoveries are low when B and Sr are employed together owing to the formation of the SrB6 compound. This is particularly a concern for Sr when it is introduced into the melt before B. Sr concentrations of the solidified samples are too low to offer modification at an addition rate of 200 ppm Sr. While Sr additions reduce the B recovery in grain refined samples, this is apparently not reflected in the overall grain refining performance. Exceptionally small grains can be obtained at B levels that fail to offer grain refinement when B is added alone. SrB6 particles are believed to compensate for the reduced B concentrations. While the SrB6 compound helps with the grain refinement, they are not involved in the modification process. B (300 ppm) and Sr (300 ppm) provide exceptionally small grains and an adequate modification of the eutectic silicon in a commercial AlSi7Mg0.3 alloy. Increasing the B and Sr addition rates does not provide further improvement in either grain refinement or modification.