Influence of AlNb3.5B0.4 Grain Refinement on the Precipitation of Fe-rich Intermetallics in a secondary AlSi7Mg Alloy

Aluminum-silicon alloys are important materials in foundry to produce light components to be used in the automotive industry for weight saving, reduction of carbon emission from vehicles and air pollution. The mechanical properties of these alloys are strictly related to the final microstructure that can be improved by grain refinement. However, the addition of specific grain refiners can promote the formation of Fe-rich compounds with platelet morphology, which can significantly affect the ductility of the alloy. In the present research, the effect of AlNb3.5B0.4 grain refiner on the formation of Fe-rich intermetallics in a secondary AlSi7Mg alloy was investigated. Metallographic and image analysis techniques were used to quantitatively investigate the microstructural variations occurring with the addition of grain-refining agent at different cooling rates. The results show that the α-Fe compounds are the dominant Fe-rich phase in the secondary AlSi7Mg alloy. On the other side, the addition of AlNb3.5B0.4 grain refiner promotes the precipitation of β-Fe platelets at the expense of Chinese script α-Fe particles. This mechanism is even more evident at higher cooling rates. The AlNb3.5B0.4 grain refinement significantly affects the number density of β-Fe compounds but it does not influence their dimensions. The size of β-Fe particles appears to be sensitive to the change of the cooling rate; higher cooling rate refines the microstructural scale as well as the Fe-rich platelets. The addition of AlNb3.5B0.4 produces a fine and uniform grain structure throughout the alloy and this effect is more pronounced in the slowly solidified material. Increasing the cooling rate, lower amounts of grain refiner are needed to produce a uniform grain size throughout the casting.