Plasma electrolytic oxidation of as-cast and heat-treated binary Al-Ni alloys

In this study, the effect of nickel addition on the plasma electrolytic oxidation (PEO) behavior of pure Al was investigated. Binary Al-Ni alloys containing 1, 3, 5 and 7 at. % Ni were produced by two step vacuum arc and induction melting methods. Produced alloys and pure Al as a reference were PEO coated in a silicate-based alkaline electrolyte through a bipolar power source under constant voltage mode for 45 min. The samples were characterized by XRD, SEM-EDS and Vickers microhardness tester before and after PEO. All alloys were consisted of Al and Al3Ni phases. Fine rod-like eutectic structure was evident for 1 (hypoeutectic) and 3 (eutectic) at. % Ni containing alloys while further addition of Ni resulted in the coarsening of eutectic Al3Ni precipitates. All coatings contained gamma-Al2O3 phase while additionally, alpha-Al2O3 phase formed on pure Al and Al-1Ni alloy. Despite the presence of Ni in the coatings, a Ni-based oxide phase was not evident. Ni, as an alloying element, is found to decrease microhardness and thickness of the coatings, induced a thickness variation between the regions close to the edge and center of the samples. This edge effect was maximum on eutectic alloy, which consisted of homogeneously distributed fine Al3Ni precipitates. To investigate the relationship between eutectic precipitate size and the edge effect, Al-3Ni alloy was heat-treated at 620 degrees C for 168 h and subsequently coated by PEO. Heat-treatment almost eliminated the edge effect, increased the average coating thickness and promoted the alpha-Al2O3 formation. Fine eutectic morphology in Al-Ni alloys is found to be detrimental for the coatings produced by PEO.