Friction Stir Processing (FSP) is a promising thermomechanical technique that is used to modify the microstructure of metals locally, and thereby locally improve mechanical properties of the material. FSP uses a simple and inexpensive tool, and has been shown to eliminate pores and also reduce the sizes of intermetallics in aluminum alloys. This is of great interest for research on solidification, production and performance of aluminum alloy castings because FSP can enhance the structural quality of aluminum casting significantly by minimizing the effect of those structural defects. In the literature, there is evidence that the effectiveness of FSP can change with tool wear of the tool used. Therefore, a study was first conducted to determine the effect of FSP time on the tool life and wear in 6061-T6 extrusions. Results showed the presence of two distinct phases in the tool life and wear. Metallographic analyses confirmed that wear in Phase I was due to fracture of the threads of the tool and Phase II was due to regular wear, mostly without fracture. Moreover, built-up layers of aluminum were observed between threads. The microhardness profile was found to be different from those reported in the literature for 6061-T6, with Vickers hardness increasing continuously from the the stir zone to the base material. To investigate the degree of effectiveness of FSP in improving the structural quality of cast A356 alloys, ingots with different quality (high and low) were friction strir processed with single and multiple passes. Analysis of tensile test results and work hardening characteristics showed that for the high quality ingot, a single pass was sufficient to eliminate the structural defects. Subsequent FSP passes had no effect on the work hardening characteristics. In contrast, tensile results and work hardening characteristics improved with every pass for the low quality ingot, indicating that the effectiveness of FSP was dependent on the initial quality of the metal. The evolution of microstructure, specifically the size and spacing of Silicon (Si) eutectic particles, was investigated after friction stir processing of high quality A356 castings with single and multiple passes. Si particles were found to coarsen with each pass, which was in contrast with previous findings in the literature. The nearest neighbor distance of Si particles also increased with each FSP pass, indicating that microstructure became progressively more homogeneous after each pass. In the literature, the improvement observed after FSP of Al-Si cast alloys was attributed to the refinement of Si particles. Tensile data from high quality A356 ingot showed that there was no correlation between the size of Si particles and ductility. To the author’s knowledge, this is the first time that the absence of a correlation between Si particle size and ductility has been found.