Al–Si–Mg cast alloys can be imparted with a wide range of mechanical properties through heat treatment, thus meeting the required performance of various vehicle components. However, because of the characteristics of the high-pressure die casting process, the gas porosities in the casting can lead to blisters during the solution treatment of die-casting parts. Therefore, systematic studies on the solution temperature and solution time are required to prevent such blisters. In this study, the gas porosity and mechanical properties of T6 heat-treated specimens were evaluated under various solution treatment conditions. As the solution treatment temperature and time increased, the numbers and volume of individual gas porosity increased. On the other hand, there were little changes in the shrinkages upon heat treatment. As solution temperature and time increased, the area fraction of the eutectic Si particles and Mg2Si decreased slightly, whereas the area and sphericity of the eutectic Si and Mg2Si increased. As the solution treatment temperature and time increased, the ultimate tensile strength and yield strength increased, whereas the elongation tended to decrease. However, the maximum UTS and YS were obtained on treatment at 520 °C for 1.5 h. •Vacuum die casting of Al–Si–Mg alloy under solution treatment conditions.•Separation of gas porosities and shrinkage and their quantitative measurement.•Mechanical property comparison between T6 heat-treated and as-cast specimens.