Small Fatigue Crack Growth Mechanisms and Interfacial Stability in Cold-Spray 6061 Aluminum Alloys and Coatings

Abstract

Cold-spray-processed aluminum alloys have static mechanical properties superior to those of aerospace cast alloys, and similar to those of their wrought counterparts, making them good candidates for structural applications. However, their broad and confident use relies upon systematic fatigue crack growth studies to investigate and demonstrate the materials’ performance in critical high-integrity components. In this work, the fatigue crack growth behavior in early stages (small crack growth regime) was investigated for cold-spray processed 6061 aluminum alloys and coatings, at stress ratio R = 0.1, in room temperature laboratory air. The effects of the characteristic microstructure and initial flaw size on the fatigue crack growth response were systematically examined, and the crack growth mechanisms at the microstructural scale were established and compared to those of long cracks. The mechanical interfacial stability of coatings was examined in cold-spray 6061–rolled 6061-T6 couples. An original method of quantifying the deposition–substrate interfacial strength, and correlating it to the response under cyclic loading via crack-interface stability maps, was developed. The proposed methodology is based on combined scratch testing and fracture mechanics formulations, and failure at the coating–substrate interface can be predicted for any crack growth scenario under cyclic loading. The method can be broadly used for the design and optimization of cold-spray and other coatings, as well as in structural repair.