In this research, the effect of boron nitride (BN) reinforcement particles on microstructure and mechanical properties of Al6061-T6 surface layer composites prepared by friction stir processing (FSP) were investigated. Microstructural changes of surface composites were examined by optical microscope (OM). Also, scanning electron microscope (SEM) was utilized to evaluate dispersion of reinforcement particles. The results illustrated that the BN reinforcement particles were uniformly dispersed in the nugget zone after 4 passes of FSP. It was also determined that the micro-hardness was enhanced from 97 Hv to 157 Hv due to the pining effect of hard BN reinforcement particles and reducing the grain size of processed zone from 312.65 μm to 12.09 μm. The results also indicated that increasing the number of FSP passes increased hardness, as well as the yield strength (σ y ), tensile strength (σ UTS ) and elongation (%E) of specimens. Corrosion resistance of Al6061/BN surface composite was examined by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. Results showed that Al6061/BN surface composite presented better corrosion behavior in comparison to Al6061-T6 and FSPed Al6061-T6 without BN reinforcement particles.
Plasma electrolytic oxidation of 7075 Al alloy was performed in alkaline silicate electrolyte containing KMnO4 as an additive using unipolar (U) and bipolar pulsed current modes (B1 and B2). The coating grown using the bipolar current with longer cathodic pulse time (B2) revealed a more dense foam-like surface morphology with fewer volcano areas providing higher thickness, which is mainly raised by reduction in discharge intensity. Also, a lower concentration of electrolyte elements (Si and Mn) was incorporated into the coating providing a brighter appearance. EIS results showed very high inner layer resistances for the coatings indicating this layer controls the overall corrosion performance of the coatings. In this way, the highest inner layer resistance along with very low value of constant phase element was achieved for the coating produced by B2, which is an indication for its lower porosity and higher thickness. Also, the more noble corrosion potential and lower passive current density observed for this coating in potentiodynamic polarization test confirms its stronger barrier effect against the chloride ingression where its corroded surface after long-term EIS test showed no significant damage. The lower penetration depth and subsequently the higher hardness values were achieved for the coatings produced by B2.
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