Effect of Nb addition on the size and morphology of the β-Fe precipitates in recycled Al-Si alloys

The recycled Al alloys have a Fe level above the recommended limits, leading to the precipitation of β-Fe intermetallic particles in their microstructure. The brittle β-Fe particles show a rough morphology in the form of highly-faceted platelets, which is detrimental to the alloy's mechanical performance containing these precipitates. This work analyses the possible interactions of the addition of 0–1 wt% of the grain refiner Nb + B to the morphology and size of β-Fe precipitates in alloys melted with Al-(7, 9, 12) wt% Si and 1 wt% Fe. The results indicate that the addition of the Nb + B inoculant revealed a significant change in the precipitates' size and morphology, which have become remarkably refined and spheroidized. Moreover, deepening the study through qualitative and quantitative analyses, it was found that the behaviour of the β-Fe precipitates follows an exponential decay with the increasing addition of Nb + B, a curve analogous to the primary α-Al grain refinement one, revealing a direct correlation between the events. Finally, it was possible to suggest a mechanism that shows how the phenomenon of morphological transformation of the β-Fe precipitates occurs in the material with the addition of the Nb + B inoculant.