Effect of quench-rate on the mechanical property and nano-structure evolution of T6-treated Al-7Si-0.4 Mg casting alloy

This study investigates the influence of quench-rate on the mechanical properties, microstructure, and strain-hardening behavior of T6-treated Al-Si-Mg alloys. Three distinct cooling processes—water quenching (WQ), forced-air blowing (FAB), and air cooling (AC)—were employed to control the cooling rate after solution treatment. The results demonstrate that both strength and ductility are significantly affected by quench-rate and subsequent aging. WQ achieved the highest yield strength and elongation, while AC resulted in pronounced reductions in both properties. The intermediate quench-rate of FAB maintained strength relatively well but caused a marked decrease in ductility. Strain-hardening analysis based on Kocks–Mecking plots revealed that the maximum strain-hardening rate (θ_max) decreased with increasing yield strength, whereas the dynamic recovery parameter (K) remained nearly constant, except in the AC alloy, which exhibited higher K values. Microstructural analysis showed that the size of eutectic Si particles increased moderately with decreasing cooling rate due to Si diffusion into the dendritic cell boundaries, leading to localized Si depletion and suppression of precipitation in these regions. Furthermore, quench-rate substantially altered the nanoscale precipitate structures both in the core and outer regions of the dendritic cells. The WQ alloy retained conventional high-aspect-ratio β″ precipitates regardless of dendrite position, while the FAB alloy exhibited a fine, dense distribution of spherical clusters and GP zones in the dendrite core, and the AC alloy predominantly contained low-aspect-ratio precipitates. FAB and AC alloys also presented increased amounts of over-aged precipitates and wider precipitate-free zones (PFZ). These microstructural heterogeneities are considered key contributors to the deterioration in ductility, with the outer-region precipitate morphology playing a dominant role in the reduced elongation observed in slowly quenched alloys. The findings highlight that precise control of quench-rate and the resulting precipitate morphology is essential for optimizing the balance of strength and ductility in Al-Si-Mg alloys.