Investigation of fatigue cracking in aluminium 7075 alloys and the role of heat treatment

The unique properties of aluminium alloys and their high strength-to-weight ratio make them the preferred choice for aircraft design. However, fatigue cracking in ageing fleets remains significant. This study investigates microstructural cracking of cooled and heat-treated Al 7075–0, T6, and T7 alloys used in airframes and stringers. To study the effect of heat treatment, medium voltage test pieces were used according to the guidelines of ASTM E647. Scanning Electron Microscopy (SEM) was used to observe the crack surface morphology. The results show that Al 7075-T7 exhibits higher resistance to the Fatigue crack growth (FCG) threshold than Al 7075-O and T6. In the case of the quenched Al 7075-O, the fatigue performance remains constant for samples oriented with a perpendicular and countersunk rivet hole. SEM analysis of fatigue cracking surface indicates that the microcracks leading to fracture originate from inclusion zones, secondary stage grains, and microstructural defects. In addition, as the heat treatment condition increased from O to T6 to T7, the area of the semi-subdivision planes and the width of the fatigue bands initially increased and then decreased. The final fault zones exhibit dimpled properties, with increasing size and deeper changing depth as heat treatment annealing progresses from O to T6 to T7. In summary, the study highlights the outstanding fatigue resistance of Al 7075-T7 and provides valuable insights into the microstructural aspects of fatigue cracking in aircraft alloys subjected to different heat treatments. These results contribute to the understanding and managing of ageing fleets, supporting the development of more reliable aircraft designs.