In-situ SEM observation and CALPHAD guiding optimization of morphology of Fe-bearing phases in solution heat treated die-cast Al-Si alloys

• CALPHAD-guided Al-9Si-0.4 Mg-0.1Fe-(0.2–0.25)Mn alloy design suppressing β-Fe phase. • In-situ SEM reveals π → α-Fe transformation within 20 min at 530°C. • Ultra-short solution (530°C/20 min) achieves dual-phase spheroidization. • T6-treated alloy achieves 275 MPa YS & 11% EL synergy via multi-scale strengthening. • High-density β″ precipitates (197 MPa) dominate yield strength enhancement. Conventional T6 heat treatment of thin-wall die-cast Al-Si alloys induces blistering and distortion due to inherent porosity. Short-time solution treatment (SST) has been demonstrated to solve this problem, though determining an optimized time remains challenging. This study addresses this challenge by integrating CALPHAD (Calculation of Phase Diagrams) with in-situ SEM solution experiments to design an Al-9Si-0.4 Mg-0.1Fe-(0.2 ∼ 0.25)Mn (wt.%) alloy and optimize a 20-minute SST at 530°C. In this CALPHAD-designed alloy, formation of the harmful β-Al 5 ​FeSi phase was suppressed, while metastable π-Al 9 FeMg 3 Si 5 phase was promoted. In-situ heating experiments revealed that π-Al 9 FeMg 3 Si 5 dynamically decomposed into submicron α-Al 15 (Fe,Mn) 3 Si 2 dispersoids. Subsequent aging (160°C/4h) generated dense β″ precipitates, achieving a strength-ductility synergy with a yield strength of 275 ± 5 MPa and an elongation of 11 ± 1%. This method replaces trial-and-error approaches, providing a cost-effective route to bypass porosity constraints and advance high-performance die-cast alloys for thin-wall applications.