Towards strength-ductility synergy through an optimized two-stage solution treatment in Al–7Si–3Cu-0.5Mg alloys

In order to obtain superior mechanical properties, the incipient melting and maximum dissolution of Cu-containing phases, spheroidization and coarsening of eutectic Si phase in Al–Si–Cu–Mg alloy are important factors that should be considered during solution treatment. However, the conventional single-stage solution treatment was difficult to meet this requirement. In this study, an optimized two-stage solution treatment (495 °C/8 h + 515 °C/4 h) was established for Al–7Si–3Cu-0.5 Mg casting alloys through systematically investigating the microstructure evolution at different solution temperatures (470 °C, 495 °C, 515 °C and 525 °C). The first stage (495 °C/8 h) was applied to dissolve most Cu-containing phases and avoid incipient melting, while the employ of the second one (515 °C/4 h) was to promote fast spheroidization of eutectic Si phase and prevent severe overheating. Two modified quality indexes evaluated the synergy of strength and ductility indicated that the optimized two-stage solution treatment and the subsequent aging treatment (175 °C/8 h) gave rise to an optimization (QIU = 537.1 MPa, QIT = 0.47) on the strength and ductility of Al–7Si–3Cu-0.5 Mg alloys, as compared with the alloys (QIU = 527.4 MPa, QIT = 0.46) under the conventional heat treatment including a single-stage solution treatment (515 °C/16 h). The strength-ductility synergy was arisen from the fuller dissolution of Cu-containing phases, the finer eutectic Si phase, the depletion in number density of micropores and the precipitation of nano-sized Q′ and θ′ particles. 0.3 wt% Zr addition further increased the modified quality indexes (QIU = 549.5 MPa, QIT = 0.50), which was attributed to the grain refinement and the formation of nanoscale Al–Si–Zr precipitates. Furthermore, the fracture mechanism of the obtained alloys was also discussed.