This thesis investigates using friction stir welding to repair common surface defects found in aluminum-silicon sand castings. Wherein, the effect of welding parameters: weld RPM, weld speed, and number of weld passes, were evaluated using hardness, porosity density, welding temperature, microstructure refinement as metrics. Therefrom, the results strongly suggest friction stir welding: reduces porosity size, reduces porosity density in a specific area, increases average hardness, improves hardness uniformity, increases surface roughness, redistributes microstructure features in a manner that theoretically improves strength, and maintains a welding temperature less than 660 degrees Celsius.