Friction stir surface processing of 6061 aluminum alloy for superior corrosion resistance and enhanced microhardness

In the current study, friction stir surface processing (FSSur.P) was carried out to alter the surface microstructure of 6061 aluminum alloy using a pinless friction stir tool. The impact of rising tool rotational speed from 400 to 1200 rpm while keeping tool traveling speed constant at 250 mm/min on surface microstructure, mechanical properties, and corrosion resistance was explored in detail. The microstructural findings revealed a significant reduction in grain size, in addition to a remarkable improvement in microhardness of processed samples. Approximately 44% increase in microhardness, compared to unprocessed counterpart was achieved at the lowest rotation speed. The corrosion behavior of the processed samples as well as the base sample was evaluated after 15 days of exposure to 3.5% NaCl using several electrochemical methods such as potentiodynamic polarization technique (PDP), linear polarization method (LPR) and electrochemical impedance spectroscopy (EIS) approach. All electrochemical techniques results demonstrated an excellent agreement where the unprocessed sample showed poor corrosion resistance with corrosion current density of (I_corr.) of 3.22 μA, whereas the sample processed at the highest tool rotating speed exhibited superior corrosion resistance with I_corr. of approximately 0.057 μA. Based on results it was found that increasing tool rotational speed has minimum effect on grain size, as well as microhardness; with best improvement achieved at lowest rotational speed or minimum heat input. On the other hand, the corrosion resistance was found to improve with increasing tool rotational speed. The use of pinless tool had a great impact on simplifying the process and achieve required surface enhancement without altering the subsurface material.