Influence of duty cycle on fatigue life of AA2024 with thin coating fabricated by micro-arc oxidation

In this paper, thin micro-arc oxidation (MAO) coatings (5-10 mu m) were produced on 2024-T3 aluminum (Al) alloy with duty cycles of 8, 10, 15, and 20%. The objective was to investigate the fatigue properties of the MAO coated and uncoated samples under high cycle fatigue (HCF) and low cyclic fatigue (LCF) conditions. Surface morphology of coatings, interface between the coating and substrate, and fatigue fracture were observed by scanning electron microscopy and metallographic microscope. Three-dimensional (3D) surface morphology of ceramic layers and surface roughness were examined by a 3D laser microscope. Coating phase structure and residual stress were analyzed by X-ray diffraction. In addition, mechanical properties of the coated and uncoated samples were evaluated by static tensile testing. The test results showed that the MAO treatment improved fatigue properties of Al alloy substrate. Residual compressive stress in MAO coating was the reason of increasing fatigue life. However, duty cycles didn't affect the mechanical properties of the substrate. At LCF conditions, the fatigue life of the coated sample at the 20% duty cycle was lower due to the large cracks. In contrast, samples treated with the 8 and 15% duty cycles had inferior fatigue properties at HCF conditions. The concentrated distribution of micro-pores and the cracks on the coating cross-sections were detrimental to HCF life.