Friction stir processing of a cast manganese-aluminum bronze for improving corrosion and cavitation erosion resistances

Abstract

A cast manganese-aluminum bronze (MAB) is subjected to friction stir processing (FSP), and the corrosion and cavitation erosion behaviors are studied. Results indicate that FSP breaks the large-sized κ phases into small particles and remarkably homogenizes and refines the α and β phases in the cast substrate. Therefore, the hardness is raised after FSP. A finer microstructure is obtained by the double-pass FSP. The FSP MABs exhibit higher electrochemical activity than the cast in 3.5 wt% NaCl solution, probably due to the residual stress induced by severe plastic deformation during FSP. However, after long-term immersion, severe localized corrosion occurs and results in the formation of large-size corrosion pits on the cast MAB. By contrast, the FSPed MABs undergoes relatively uniform corrosion and no noticeable pits are found on the double-pass FSPed MAB because of the homogenized microstructure and formation of homogeneous and protective corrosion product films. Under cavitation erosion in 3.5 wt% NaCl solution, the mass loss rate of the FSPed MABs is only half that of the cast. For the cast and FSPed MABs, cavitation transforms the corrosion potential towards a positive direction and raises the corrosion current density by nearly an order of magnitude, and the mechanical impact contributes largely to the cavitation erosion damage. For the cast MAB, severe cleavage fracture occurs at the coarse β phases and large cavities form on the eroded surface. However, the FSPed MABs are evenly and slightly eroded because of the homogeneous microstructure and increased hardness.