Influence of micro arc oxidation coating thickness and prior shot peening on the fatigue behavior of 6061-T6 Al alloy

Micro arc oxidation (MAO) coatings of 5-different thicknesses were formed on 6061-T6 Al alloy with and without prior shot peening. The surface and cross-sectional morphologies, the surface roughness (Ra, Rz), relative phase composition, depth of peening were investigated as a function of MAO coating thickness. The high cycle fatigue performance of Plain MAO and prior shot peened MAO (SP + MAO) coatings were evaluated as a function of coating thickness and maximum alternating stress levels in comparison with that of bare substrate. Independent of the coating thickness, the Plain MAO coatings offer substantial degradation in fatigue life wherein the extent of fatigue debit increases with increasing coating thickness. Increasing surface roughness of Plain MAO coatings with increasing coating thickness resulted in multiple crack nucleation leading to consistent debit in fatigue life. However, irrespective of coating thickness, SP + MAO samples exhibit consistently improved fatigue life over the entire alternating stress range investigated wherein the net effect of increased surface roughness with increasing coating thickness was well-compensated by the compressive residual stresses present beneath the substrate-coating interface. The fractured surface and progressive crack morphologies clearly highlight the mechanisms responsible for the overall fatigue life are distinctly different for the Plain MAO and SP + MAO coatings as a function of coating thickness.