Mechanical properties and growth mechanisms of long and small fatigue cracks in as-deposited bulk cold spray Al-6061

Abstract

Cold spray depositions are created by accelerating powder particles at a substrate at supersonic speeds to create a deposition with effective metallurgical and mechanical bonding. To progress from coatings/repairs to bulk additive depositions suitable for structural components, fatigue properties and microstructure-scale crack growth mechanisms must be established. In this study, the microstructure, tensile properties, and crack growth behavior were systematically investigated for as-sprayed Al-6061 and compared to those of wrought Al-6061-T6. Long fatigue crack growth behavior was investigated at two stress ratios (R = 0.1 and 0.7) and two crack plane orientations with respect to the deposition direction. Small crack growth behavior was investigated at a stress ratio of R = 0.1 in four crack plane orientations. Fractographic observations were made to identify the crack-microstructure interactions at different growth stages, and interparticular failure was determined to be the lifetime-limiting crack growth mechanism. A lap-shear testing method was developed to evaluate the interfacial shear strength of cold spray depositions at the interface with the substrate and to compare it to the strain energy release rates obtained from fatigue crack growth experiments. Design maps were created to relate loading conditions, crack orientation, and deposition ductility and predict the onset of interparticular growth.