Effect of short T6 heat treatment on the microstructure and the mechanical properties of newly developed die-cast Al–Si–Mg–Mn alloys

Abstract

A short T6 heat treatment for Al–Si–Mg–Mn die-cast alloys was developed with a solution treatment time of only 10 min at 540 °C. Excellent mechanical properties were achieved with a yield strength of 334–349 MPa and elongation of 2.6–4.6%, which is competitive with other die-cast alloys subjected to T6 heat treatment. The effects of solution time and temperature on the expansion of porosity were assessed. There is no apparent porosity expansion in T6 heat-treated alloys with short solution treatment or through prolonged solution time with lower temperatures. The short solution treatment enables the dissolution of Mg and spheroidizes eutectic Si and Mg2Si, associated with minimizing the growth of α-Al grains. The π-AlFeMnSiMg which forms during the final quaternary eutectic reaction transforms into nano-scale body-centred α-AlFeMnSi phase with dissolution of Si and Mg after the shot T6 heat treatment. The peak ageing hardness was determined by the solution temperature, and the highest peak aged hardness was obtained under a short T6 heat treatment. The coherent β’’ precipitate with needle-like morphology was found after this heat treatment.