High-Cycle Fatigue Behavior of Microarc Oxidation Coatings Deposited on a 6061-T6 Al Alloy

Abstract

The objective of this article is to evaluate the influence of microarc oxidation (MAO) coatings on the high-cycle rotating bending fatigue behavior of the 6061-T6 aluminum alloy. Toward this purpose, the influence of the MAO coating process parameter (current density) and coating thickness on the fatigue life of the 6061-T6 Al alloy has been evaluated in the present study. In addition, the influence of the coating roughness on the fatigue life of the MAO-coated 6061-T6 Al-alloy sample has also been investigated. The results indicate that the high-cycle fatigue life of the 6061-T6 Al alloy is substantially degraded due to the presence of MAO coatings, especially at lower alternating stress values and for thicker coatings. Surface roughness, altered by polishing, does not have any effect on fatigue life. An examination of coated samples interrupted at various fractions of fatigue life leads to the conclusion that the crack propagates from the coating surface to the coating-substrate interface very rapidly and thus fatigue life is largely controlled by the propagation of the crack into the substrate.

Enhancing the high cycle fatigue life of high strength aluminum alloys for aerospace applications

“…As a common point of observation to both the Al alloys, multiple predominant cracks can be noticed in the case of plain MAO coated fracture surface and fewer dominant cracks in the case of SP + MAO coatings . Furthermore, upon examination of a cross section for a fatigue test interrupted at 80% of experimentally measured life, the fatigue crack propagates almost like a straight line in the case of plain MAO coating (Figure ), while major crack deflection in the substrate region beneath the substrate‐coating interface as shown in Figure accompanied by multiple crack deflections of relatively smaller magnitude and crack‐branching as shown in Figure leads to major energy absorption and significantly increased fatigue life can be clearly noticed in the case of SP + MAO coatings . It may be noted that the SP induced compressive residual stresses beneath the substrate‐coating interface region of SP + MAO coating are the primary reason behind the deflection and branching of cracks as shown in Figures .…”

“…However, the applications including landing gear components of aerospace sector often demand an additional property namely the fatigue resistance . The current state‐of‐art can only provide adequate wear and corrosion protection of Al alloys even when the advanced technologies such as micro arc oxidation (MAO) are deployed thereby leaving a clear space for developing the materials and processes that can simultaneously provide resistance against wear, corrosion and fatigue …”

“…2,[7][8][9][10] The current state-ofart can only provide adequate wear and corrosion protection of Al alloys even when the advanced technologies such as micro arc oxidation (MAO) are deployed thereby leaving a clear space for developing the materials and processes that can simultaneously provide resistance against wear, corrosion and fatigue. [11][12][13] It is very well documented that the anodized, hard anodized (HA) and MAO coatings in general degrade the fatigue life (number of cycles before failure) up to 75% than that of corresponding Al alloy. 14,15 Since these coatings are typically characterized by the presence of numerous morphological and microscopic defects such as porosity and cracks which readily acts as stress raisers under cyclic loading conditions, the extent of such fatigue degradation increases with increasing coating thickness, while the thinner coatings with minimal fatigue degradation are not capable of offering adequate corrosion and wear protection.…”

The influence of the coating technique on the high cycle fatigue life of alumina coated Al 6061 alloy

“…Finally, the DSC coating is also characterized by a lamellar structure characterized by weak inter-splat boundaries. Preliminary work by the present authors with MAO coated Al6061 alloy has indicated that even at 50% of the fatigue life, the crack has already reached the coatingsubstrate interface [21]. Based on the above observations, it can be postulated that the high cycle fatigue life of coated Al6061 alloy is largely determined/controlled by the propagation of the crack into the Al6061 alloy substrate from the coating-substrate interface.…”

“…In addition, the influence of roughness of the MAO coating on its fatigue life has been specifically studied by the present authors [21] and it was observed that polishing the MAO coating surface to reduce its roughness from R a = 3.5 μm to R a = 0. All the above observations lead to the conclusion that the coating really does not provide any resistance to fatigue crack initiation.…”