Assessment of the influence of Al-2Nb-2B master alloy on the grain refinement and properties of LM6 (A413) alloy

Cast aluminium alloys are important structural materials but their performances are not optimised due to the lack of appropriate grain refiners. In this study, the effect of the addition of a novel Nb-based grain refiner on the microstructural features and mechanical behaviour of the LM6 alloy (A413) is studied. Specifically, the effect of Nb-B inoculation is assessed over a great range of cooling rates (2-100 °C/s). It is found that Nb-based compounds (i.e., NbB2 and Al3Nb) are potent heterogeneous nucleation sites for aluminium and this leads to a significant refinement of the microstructural features. The refinement is not hindered by the formation of silicides, as it happens when using Al-Ti-B master alloys, because niobium silicides form at much higher temperature. It is concluded that the Al-2Nb-2B master alloy is a very effective refiner especially at slow cooling rate and the refinement of the grain size leads to improved performances (homogeneous fine grain structure, mechanical properties and porosity).