Effects of processing on microstructure evolution and fatigue crack growth mechanisms in cold-spray 6061 aluminum alloy

Owing to their fine microstructures, cold-spray processed alloys possess appealing static mechanical properties, similar to their wrought counterparts, making them potential candidates for structural applications. In spite of their great potential, the lack of knowledge of their cyclic behavior, especially the fatigue crack growth response, limits their use in high-integrity applications. Thus, further advanced characterization of cold-spray processed materials is needed. In this study, fatigue crack growth microstructural mechanisms were established for bulk cold-spray processed 6061 aluminum alloy tested in laboratory air at room temperature. The effects of the material's characteristic microstructure, stress ratio (R = 0.1, 0.5, and 0.7), and post-fabrication heat treatment (annealing and T6) on the fatigue crack growth behavior were systematically investigated. It was found that the initial powder particle characteristics play an important role in the crack's propagation through the cold-spray 6061 microstructures, and particle boundaries are critical in the materials' response, especially at high driving force. To aid the material-process design, two crack tip driving force parameter maps that relate loading conditions to damage at the microstructure scale were also established. Furthermore, a Paris-type, microstructure-based model was successfully developed to predict the fatigue crack growth rates in upper Region II and Region III for the as-fabricated and annealed materials. Discussions related to the use of these materials for fatigue-critical applications are also provided.