Abstract
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).
Highlights
Cast aluminium alloys are important structural materials but their performances are not optimised due to the lack of appropriate grain refiners
Behind the grain refinement of wrought aluminium alloys by means of Al–Ti–B master alloys has been extensively studied [11,12,13,14,15,16] out of which different mechanisms and theories were proposed such as the one based on “solute”, the one which refers to the “peritectic reaction”, the one developed from “phase diagrams” or the hypernucleation theory, which considered the enhancement of the nucleation on the borides particles from the solutal titanium [5,17,18]
The current understanding about the grain refinement mechanism with Al–Ti–B refiners can be summarised as follows [5,6,7,8,9,10]: TiB2 particles act as heterogeneous nucleation sites and Al3Ti intermetallics dissolve into the molten aluminium providing the needed Ti solute [19]
Summary
Cast aluminium alloys are important structural materials but their performances are not optimised due to the lack of appropriate grain refiners. Technological and manufacturing benefits are really significant among which good formability (for extrusion and rolling processes), good surface finishing, lower propensity to hot tearing and better machinability as a consequence of the more uniform distribution of the second phases and of the shrinkage and gas porosity as well as the soundness of the castings are known All these benefits are related to the fact that the inoculated melt fills more faster and more efficiently mould cavities as well as allows the melt remaining in between the growing dendritic grains to move more freely enabling the filling of the cavities in the mushy zone of the solidifying metal (i.e., both mass and interdendritic feeding). The high fluidity and possibility to fabricate complex geometries derives by the fact that the LM6 alloy has composition close to that of the eutectic and this permits to obtain miscellaneous products like architectural panels and spandrels, outdoor lamp housings, lawn mower deck, cooking utensils as well as medical and dental equipment [28]
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