Thickness-dependent fatigue failure mechanisms in cold-sprayed AA7075 repairs: From coating cracking to interfacial delamination

• AA7075 cold spray coatings deposited on AA7075-T6 substrates under the studied spray parameters reduce fatigue life relative to uncoated material under three-point bending. • A transition thickness range defines the shift from adherent to delamination-controlled fatigue failure. • Coatings below the transition region thickness (BTT) exhibit a severe reduction in fatigue life, whereas coatings above the transition region thickness (ATT) exhibit only a moderate reduction. • Fatigue life reduction is controlled by interfacial roughness in ATT coatings and by brittle-induced crack initiation in BTT coatings, under the applied coating and testing conditions. Cold spray (CS) is a solid-state additive deposition technique of growing interest for repairing and protecting aerospace structural components. This study investigates how variations in CS repair thickness influence the fatigue behaviour of AA7075 coatings on AA7075-T6 substrates, simulating real-world post-processing through controlled material removal. Results showed that CS coatings reduced overall fatigue life; however, coatings below the transition region thickness (BTT) showed only a moderate (∼30%) reduction compared to uncoated material, while coatings above the transition region thickness (ATT) exhibited a greater (∼69–82%) reduction when considering a redefined stress state. Failure mechanisms were thickness-dependent: BTT coatings failed predominantly through CS cracking, which induced stress concentrations at crack tips that accelerated substrate failure, while ATT coatings failed via interfacial delamination. Mirror and camera-assisted in-situ crack observation, supported by fractography, confirmed that in ATT coatings, cracks initiated at spray-induced substrate craters, whereas in BTT coatings, initiation occurred within the CS during the first loading cycle at surface or near-surface pores. These findings highlight the importance of optimising CS repair thickness, and interfacial morphology, as well as considering in-service bending and shear stress distributions, to maximise fatigue durability in aluminium repairs.