Effects of a Niobium-Boron grain refiner on undercooling and microstructural evolution of aluminium silicon alloys

Abstract

The effectiveness of two novel niobium boron grain refiners, Al-2Nb-2B and Al-2Nb-B, in reducing undercooling and grain size, were evaluated in A354 (Al-9Si-2Cu-0.5Mg) and A356 (Al-7Si-0.3Mg), through practical experimentation. The grain refiners were found to influence eutectic undercooling in both alloys, whereas primary aluminium undercooling reduction was only seen in A354. The key finding was the effectiveness of the reduction in grain size of A356 at addition rates as low as 0.1% weight, as shown in graph below. This is significant as at these addition rates, the grain refiner may be an economical alternative to the commercially available Al-5Ti-B. It was also found, through spectrographic analysis, that the grain refiners cause sedimentation of titanium from the alloys, which increased with higher boron systems. This suggests that the excess boron is affecting the alloying of the materials, which may be a concern. Alloy compositions are expressed in weight percentages throughout the thesis.