Cast Al-Si alloys are used for automotive applications. Friction stir processing (FSP) is being used in recent years to improve the performance of these alloys. Secondary machining operations are highly essential on friction stir processed materials to improve the surface finish. However, machinability of friction stir processed cast alloys has rarely been reported. The influence of friction stir processing on microstructure, mechanical properties and machinability of a cast Al-Si alloy was studied in the present work. The main objective is to correlate the metallurgical and mechanical characteristics to the machinability of the friction stir processed material. The age hardening response of as received cast alloy and friction stir processed alloy on machinability and mechanical behavior was also investigated. The strength, ductility and machinability of friction stir processed alloy before and after age hardening treatment were observed to be higher than that of as received cast alloy. The significant improvement in properties of friction stir processed alloy is due to elimination of porosity, formation of fine recrystallized grain structure, homogenization of silicon particles and dissolution of iron rich intermetallics. © 2015 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.