Effect of friction stir processing on the microstructure and cavitation erosion behavior of AlSi9Mg aluminum alloy

This study investigates the effects of friction stir processing (FSP) on the microstructure, mechanical properties, corrosion behaviour, and cavitation erosion resistance of cast AlSi9Mg alloy. Optical and electron microscopy revealed that FSP transformed the coarse, dendritic as-cast structure into a fine-grained, equiaxed microstructure with uniformly dispersed silicon and intermetallic particles. Porosity was eliminated, and the processed zone exhibited complete dynamic recrystallization with a high fraction of high-angle grain boundaries. These microstructural changes led to significant improvements in mechanical performance: yield strength, ultimate tensile strength, and microhardness all increased due to Hall–Petch grain refinement and uniform distribution of hard phases that hinder dislocation motion. Electrochemical testing demonstrated enhanced corrosion resistance in the FSPed alloy, as indicated by a more positive corrosion potential and lower corrosion current. This improvement is attributed to a refined, homogeneous structure. Cavitation erosion tests showed that the ground FSPed surface had the lowest mass loss and erosion rate, owing to increased hardness and reduced surface roughness, which minimized bubble nucleation and pit formation. In contrast, the rougher as-cast surface exhibited greater damage due to early-stage cavitation attacks. The combination of grain refinement, hardening, and surface smoothness resulting from FSP significantly improves the durability of AlSi9Mg in aggressive environments. These findings support the use of FSP as an effective post-casting treatment for components exposed to cavitation and corrosion, such as impellers, pumps, and valves, extending their service life in marine and fluid-flow applications.