High cycle fatigue behavior of the severely plastically deformed 6082 aluminum alloy with an anodic and plasma electrolytic oxide coating

The effect of a severe plastic deformation of the substrate material on the high-cycle fatigue behavior of an anodized as well as plasma electrolytic oxidized aluminum alloy is investigated. The main questions addressed are: (1) Does equal-channel angular pressing (ECAP) of the aluminum substrate enhance the fatigue strength of the coated specimens and (2) Are the damage mechanisms in the oxide coatings under fatigue loading influenced by the processing condition of the substrate alloy. As substrate material, the precipitation hardening aluminum alloy AA6082 was chosen. Systematic work was done comparing two substrate conditions, a conventionally-grained peak-aged and an ECAP-processed, in uncoated condition and with respectively three coating types, an anodic, a hard-anodic and a plasma electrolytic oxide coating. The fatigue strength in the high-cycle regime was compared for all conditions and the oxide coatings were characterized by electron microscopy before and after fatigue testing. The processing condition of the aluminum substrate did not influence the oxide coating appearance, thickness and hardness for the respective coating type. Failure of the anodically and plasma electrolytically treated specimens through fatigue loading was caused by the growth of one critical crack from the coating towards the substrate, regardless of the coating type and the substrate condition. In general, anodizing and plasma electrolytic oxidation led to a significant decrease in fatigue strength. Especially hardanodizing diminishes the fatigue resistance. By ECAP-processing of the substrate material, the fatigue strength of all tested conditions was significantly increased. Still, the coated conditions did not attain the fatigue strength of the uncoated non ECAP-processed aluminum substrate. However, the combination of ECAP-processing of the substrate and plasma electrolytic oxidation was most promising since the highest fatigue strength with approx. 63% of the fatigue strength of the uncoated non ECAP-processed aluminum substrate was achieved.