Effects of ultrasonic impact treatment on fatigue life of pre-exfoliated AA-2024-t351.

In this study investigations were made into the effects of Ultrasonic Impact Treatment (UIT) on pre-exfoliated AA 2024 T351. Electrochemical tests were conducted to determine any changes in electrochemical behaviour of the alloys due to UIT condition. Uniaxial monotonic, cyclic and fatigue crack propagation (FCG) tests were conducted on material in As-Received (AR) condition and UIT condition in air and in a corrosive environment by pre-exfoliating the test specimens at pre-defined set of time periods and temperature range (20 °C). It was clear that the fatigue performance was severely reduced by the introduction of the corrosion environment for AR specimens. SEM analysis suggests that UIT conditioned AA 2024-T351 exhibits resistance to exfoliation corrosion at ambient and temperatures ranging between 20°C to 40°C when compared to AR specimens. However, these results are not supported by potentiodynamic polarisation curves which show a decrease in corrosion resistance of UIT specimens. Also it is understood that there is a considerable amount of Cu refinement and enrichment near the surface when the AA 2024-T351 is subjected to exfoliation corrosion tests. Whilst hydrogen is in an atomic state, it can be adsorbed onto the metal surface and consequently diffuses into the matrix and can have serious detrimental effects. A reference line for minimal pre-existing hydrogen in the alloy is identified and the magnitude of hydrogen is found to be 180 Arbitrary Hydrogen Units (AHU). It is also found that in corrosion environment, the hydrogen ingress and further charging is prominent at ambient temperatures for AR samples, and showed damage over the full width of cross-section. The following conclusions were drawn: 1. UIT surface treatments, followed by exfoliation corrosion have shown increased resistance to a reduction in mechanical properties, notably tensile and yield strength. Fractographic analysis further supported this finding by showing smaller average brittle failure depths for UIT specimens when compared to AR specimens. SEM analysis of AR samples (without exfoliation) showed a crystallographic contribution to the mode of failure where high density slip bands are formed and the initial failure exhibits a step format. 2. It is observed that during exfoliation, hydrogen ingress and adsorption is more prominent at ambient temperatures for AR samples. 3. A slight improvement In Low Cycle Fatigue (LCF) life is observed for pre-exfoliated and UIT conditioned sample but not for UIT conditioned samples only. Little effect is observed for either treatment in the High Cycle Fatigue (HCF) region. 4. Fatigue crack initiation occurred from the edges for all samples. 5. Fatigue crack propagation of exfoliated specimens exhibited faster crack propagation than As-Received specimens. UIT caused retardation in crack propagation rate in AR samples but not in corroded samples. Failure of pre-exfoliated, UIT treated samples failed within the treated strip. It is also noted that crack deviation can occur when the crack tip reaches a secondary phase particle. 6. Nanocrystallisation generates uniformity of the surface which refines the secondary phase particles and helps mitigate crack initiation sites.