Investigation into the role of rare earth oxides in controlling the microstructure and enhancing the mechanical and corrosion properties of as-cast Al-Si-Mg alloys

This study examines the impact of the rare earth oxide La 2 O 3 on the microstructure, mechanical and corrosion resistance properties of as-cast Al-Si-Mg (A356) alloys, with a specific focus on the A356 alloy. By incorporating varying amounts of La 2 O 3 (0 wt%, 0.2 wt%, 0.3 wt%, and 0.4 wt%) into the A356 alloy, four samples (AL0, AL2, AL3, and AL4) were produced. The findings demonstrate that an optimal addition of La 2 O 3 (0.3 wt%) can effectively reduce the secondary dendrite arm spacing (SDAS), refine the eutectic Si phase, and modify the morphology of Fe-rich phases, thereby significantly enhancing the overall performance of the alloy. The transformation in the morphology of the Fe-rich phases is accompanied by changes in their elemental composition, including the transition from elongated rod-like AlFeSi phases to shorter rod-like Al 7 Fe 6 La phases. Specifically, the AL3 sample displays the best mechanical properties, with increases in hardness, yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of 11.11%, 71.43%, 36.45% and 233.46%, respectively. Moreover, AL3 exhibits superior corrosion resistance, characterized by the highest corrosion potential (-0.88 V) and the lowest corrosion current density (3.6×10 -7 A/cm 2 ). The addition of La 2 O 3 promotes grain refinement, reduces the size of secondary phase particles, and the formation of Al 7 Fe 6 La phase collectively contributing to the enhanced performance of the A356 alloy. The present study provides theoretical support for utilizing rare earth oxides as a viable alternative to pure rare earth metals in Al alloys, presenting a cost-efficient strategy tailored for industrial implementations. • The addition of 0.3 wt% La 2 O 3 to A356 alloy results in the most significant improvements in microstructure refinement, mechanical properties, and corrosion resistance. • La 2 O 3 addition reduces the secondary dendrite arm spacing (SDAS) by up to 49.70% and promotes the spheroidization of eutectic Si and Fe-rich phases, enhancing the alloy's overall performance. • The 0.3 wt% La 2 O 3 -modified alloy (AL3) exhibits remarkable increases in yield strength (71.43%), ultimate tensile strength (36.45%), and elongation (233.46%) compared to the unmodified alloy. • Improved Corrosion Resistance: The AL3 alloy demonstrates superior corrosion resistance, with the highest corrosion potential (-0.88 V) and the lowest corrosion current density (3.6×10 -7 A/cm²), attributed to the refined microstructure and modified Fe-rich phases. • The study identifies the formation of the Al 7 Fe 6 La phase as a key mechanism for enhancing mechanical and corrosion properties, driven by the reduction of La 2 O 3 and its interaction with Fe-rich phases.