During solidification, die-cast aluminum alloys generally exhibit poor ductility because of casting defects and the formation of elongated silicon particles; therefore, we used screw fixing to make connections in die-cast aluminum alloys. In this study, the effects of effect of friction stir processing (FSP) treatment, which was reported to improve the ductility, on aging properties of a die-cast Al-Si-Cu-Mg alloy using Vickers hardness test was investigated. Transmission electron microscopy (TEM) was performed to observe the precipitates that formed by aging the sample. After FSP treatment, the coarse defects in the die-cast Al-Si-Cu-Mg alloy disappeared, and the elongated silicon particles were broken up into spherical morphology. The maximum hardness of the FSP-treated sample aged at 423 K was similar to that of an untreated die cast sample. By using TEM, the precipitates in the peak-aged FSP-treated sample were estimated as β′ phases, which differed from previous reports for precipitates from a die-cast Al-Si-Cu-Mg alloy. The precipitates of the over-aged FSP-treated sample were classified as β′ and θ′ phases. Thus, the FSP treatment appeared to affect the solution of additive elements, thereby changing the precipitation mechanism. These results show that localized FSP treatment can be used to produce parts that require partial ductility during plastic deformation.