Strength-elongation synergy via controlled precipitation hardening in counter pressure cast Al-7Si-0.35 Mg casting alloy with T5 heat treatment

Abstract

Al–7Si–0.35 Mg (A356) is a promising alloy for automotive components and is conventionally strengthened via T6 heat treatment (solution treatment followed by artificial aging). However, solid solution treatment involves high operating costs and introduces casting distortions. Consequently, T5 heat treatment (direct aging after casting) has garnered attention despite its typically lower strength and ductility compared to T6. In this study, we systematically investigated the aging response of a counter pressure cast A356 alloy under various T5 conditions to optimize the trade-off between tensile strength and elongation. Transmission electron microscopy was employed to identify the precipitate phases formed during different T5 treatments and to quantify their size and number density. The results show that increasing the aging temperature from 160 to 210 °C coarsens the precipitates and decreases their number density and coherency, which in turn markedly reduces alloy elongation. This decrease in ductility stems from a deformation mechanism dominated by dislocation bypass of coarse precipitates formed at 210 °C, causing high dislocation densities and localized stress concentrations that initiate micro-cracking. Our findings suggest that the optimal balance between tensile strength and elongation is achieved at 190 °C, where a high number density of fine coherent (β″) precipitates and a small number density of semi-coherent (U2 and B′) precipitates are present. In this case, precipitation shearing becomes the primary deformation mechanism, resulting in moderate dislocation density and a favorable strength-elongation synergy.