In Al-Si alloys, Fe is present as an inevitable impurity element, which mainly precipitates in the form of β-Fe or α-Fe crystals. While the latter usually has polyhedral or Chinese script morphology, the platelet-like shape of the β-Fe phases acts as a stress raiser and can also increase the amount of porosity. To inhibit the formation of this detrimental phase, several elements act as neutralizers of the embrittling effects of Fe and, among them, Mn is the most widely used. Mg strongly affects the mechanical properties and the formation of Fe-bearing phases. In the present work, a series of A356 aluminum foundry alloys with several combinations of Mn/Fe ratios and Mg contents were studied. To investigate the effect of Mn additions, five alloys with constant Fe and Mg content and increasing Mn/Fe ratios, from 0.37 wt. % to 1.11 wt. %, were considered. Moreover, four alloys with reduced Mg amounts up to 0.25 wt. % were evaluated. Tensile samples were machined from permanent mold castings and T6 heat-treated. The influence of the alloying elements on the mechanical properties were compared to microstructural observations. The results indicate that increasing the Mn/Fe ratios does not result in a significant increase of the tensile properties of the alloys. Conversely, the reduction of the Mg amounts leads to the decrease of yield stress, ultimate tensile strength and hardness combined with an increase in the elongation to fracture. Finally, the quality index approach was used to express the quality of the castings as a function of their ductility, yield strength and strain hardening ability.
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