Mechanism of residual stress and surface roughness of substrate on fatigue behavior of micro-arc oxidation coated AA7075-T6 alloy

Abstract

The micro-arc oxidation (MAO) coatings were produced on AA7075-T6 alloy with a surface roughness (Ra) of 0.2, 0.8, and 1.6. The effects of the Ra of substrates on the morphologies of MAO coating and fatigue behavior of the coated samples were investigated. The structural characteristics of the MAO coating were analyzed by X-ray diffraction, scanning electron microscopy, and laser scanning confocal microscopy. Moreover, the residual stress in the oxide films was measured using an X-ray stress tester. The surface and cross-section analyses of the coatings revealed the mechanism of the oxide layer growth. The phase composition and the mechanism of residual stress were further investigated according to the heat transfer in the MAO process. The relationship between the nature of residual stress in MAO coating and the surface porosity as well as the temperature of the substrate and the coating was revealed. The fatigue test results showed a significant effect of the Ra of the substrate on the high cycle fatigue life at low cyclic stress levels. The residual stress relaxation attributed to the insignificant changes in the fatigue life of the coated samples with Ra = 0.2, 0.8, and 1.6 at high cyclic stress levels. Besides, the mechanism of the residual stress affecting the fatigue performance was explored.