High quality oxide-layers on Al-alloy by micro-arc oxidation using hybrid voltages

Oxide layer was grown on 6061 Al alloy by micro arc oxidation (MAO) using a power supply of hybrid voltages: direct (VDC) and alternative (VAC) ones. The power supply was set at a fixed VAC voltage maintained at a value between 240 and 320 V coupled with an in-series VDC which is auto-tuned to keep the constant current density 5 A dm− 2. This led to varying ratios of final VDC versus maximum VAC (VDC-final/VAC-max) as the major processing parameter. Alkaline solution containing electrolytes of Na2SiO3, NaH2PO2·H2O, and Na2WO4·2H2O was used. All MAO samples show an oxide layer thickness around 10 μm after 30 min operation time. For samples MAO with VAC-max 320 V SEM morphology indicates a smoother oxide-layer surface with uniform micro-pores, which have mean diameters less than 1 μm. XRD patterns show the main oxide-phases of hexagonal α-Al2O3, cubic γ-Al2O3 and mullite (Al2O3·nSiO2). Micro-hardness of MAO samples increases from 960 HV (VAC 240 V) to 1480 HV (VAC 320 V). Corrosion resistance of the MAO layer is significantly improved with reducing VDC-final/VAC-max. Samples by VAC-max 320 V show the minimum corrosion current density 6.28 × 10− 9 A cm− 2, corresponding to the highest polarization resistance 2.20 × 108 Ω cm2. Comparing with MAO layers obtained by using VDC only, the oxide-layer by hybrid voltages is a little thinner, much harder and much more corrosion resistant.