Optimizing the effect of tramp elements on intermetallic phases in HPDC Al alloys – A thermodynamic approach

Sammanfattning

The growing interest in increasing the recycled content of aluminium for highpressuredie casting (HPDC) is driven by a demand for greater sustainability, asrecycled content offers a ∼nearly 95% reduction in energy compared to primary aluminium.However, elevated levels of elemental contamination from recycled content,mainly tramp elements such as iron (Fe) and manganese (Mn), pose significant challenges.This thesis focuses on the thermodynamic analysis of the Al-Si-Fe-Mn systemwith additions of magnesium (Mg) by using Thermo-Calc through CALPHAD-basedScheil simulations at a high rate of 100 K/s, in conjunction with validation usingcomplementary microstructure analysis.The simulations, comprising over 300 compositions, demonstrated a strong linearcorrelation between increasing Fe content and the fraction of the β-Al5FeSi phase.In contrast, increasing Mn content suppresses β phase formation by promoting themore benign α-Al15(MnFe)3Si2 phase. Si notably altered the solidification range andeutectic transitions within the system. Higher Mg levels led to phase competitionbetween π-AlFeMgSi and β phases. The potential evaporation of Mg was also foundto shift phase equilibria, increasing the fraction of β phase, emphasising the needfor accuracy control of alloy compositions.Microstructure analysis, employing SEM and EDS on the cast alloys, shows that Feinfluences β-intermetallics, as it is composed of Fe instead of Mn. The analysis alsoreveals compositional variations that influence kinetic effects, such as the formationof the δ-AlFeSi phase and localised solute depletion zones around α-phase particles.These findings clarify the effects of Fe, Mn, Si, and Mg on intermetallic phase formationand emphasise the importance of integrating kinetic effects to contribute toa more comprehensive understanding of optimising recycled aluminium alloys.