Training high-strength aluminum alloys to withstand fatigue

Abstract

The fatigue performance of high strength aluminum alloys used in planes, trains, trucks and automobiles is notoriously poor. Engineers must design around this important limitation to use Al alloys for light-weighting of transportation structures. An alternative concept for microstructure design for improved fatigue strength is demonstrated in this work. Microstructures are designed to exploit the mechanical energy imparted during the initial cycles of fatigue to dynamically heal the inherent weak points in the microstructure. The fatigue life of the highest strength Aluminum alloys is improved by 25x, and the fatigue strength is raised to ~1/2 the tensile strength. The approach embraces the difference between static and dynamic loading and represents a conceptual change in microstructural design for fatigue.

Fig. 1. The fatigue strength vs. ultimate tensile strength (UTS) for commercial AA2024, AA7050, and AA6061 alloys.

Correlation between fatigue strength and tensile strength for three Al alloys.^–.

Fig. 2. High cycle fatigue (HCF) S-N curves and precipitate-free zones (PFZ’s) of the under aged (UA), peak aged (PA), and trained alloys.

a–c HCF S-N curves of under aged (UA), peak aged (PA), and trained AA2024 (a), AA6061 (b), and AA7050 (c) alloys. The HCF tests were fully reversed (R = −1) at a frequency of 20 Hz. d–f LAADF-STEM images show PFZ’s formed in the PA alloys. The electron beam direction was parallel to <100>_Al in d, e, and parallel to <110>_Al in f.

Fig. 3. Surface evolution of air quenched (AQ) AA2024, AA7050, and AA6061 alloys with obvious precipitate-free zones (PFZ’s).

a, c, e The large localized surface relief along the grain boundaries for the peak aged (PA) alloys after certain number of high cycle fatigue (HCF). b, d, f The uniform surface relief formed through the grains for the under aged (UA) alloys after certain number of high cycle fatigue (HCF). The red curves show the relative height of different surface relief highlighted by white arrows.

Fig. 4. LAADF-STEM images showing the microstructure of precipitate-free zones (PFZ’s) after HCF deformation.

a–c PFZ’s of high cycled fatigue (HCF) treated under aged (UA) samples for AA2024 (a), AA6061 (b), and AA7050 (c). AA2024 was fatigued at 185 MPa for 3.5 × 10⁶ cycles, AA6061 was fatigued at 120 MPa for 4 × 10⁶ cycles, and AA7050 was fatigued at 175 MPa for 8 × 10⁶ cycles. d–f High-magnification LAADF-STEM images showing nanoprecipitates existing in PFZ’s pointed out in a–c. Corresponding fast Fourier transform (FFT) patterns inserted in d, e reveal diffraction from nanoprecipitates (pointed by yellow color hollow arrows). The electron beam direction was parallel to <100>_Al in a, b, d, e and parallel to <110>_Al in c, f.

Fig. 5. Cyclic training and microstructure evolution during cyclic training for the under aged (UA) alloys.

a Schematic illustration of the fully reversed (R = −1) cyclic training at 0.2 Hz. b Schematic illustration of the precipitation in PFZ’s as the training cycle number increases. c–e LAADF-STEM images showing nanoprecipitates formed in PFZ’s of AA2024 trained for 450 cycles (c), AA6061 trained for 700 cycles (d), and AA7050 trained for 450 cycles (e). Inset FFT patterns in c, d show diffraction from nanoprecipitates (indicated by yellow color hollow arrows). The electron beam direction was parallel to <100>_Al in c, d and parallel to <110>_Al in e.