Fatigue crack growth mechanisms of long and small cracks were investigated in cast and wrought aluminum and titanium alloys with various microstructures (as-cast A535, 6061-T61, and mill-and beta annealed Ti-6Al-4V). In addition, friction stir welded and cold spray processed 6061-T61 were also investigated. The effects of microstructure on the fatigue crack growth response of each material were evaluated. Long crack growth data were generated on compact tension specimens at low and high stress ratios R=0.1 and 0.7, respectively. Small crack growth testing was performed on corner and surface flaw tension specimens at low stress ratio, R=0.1. Fatigue crack growth mechanisms at the microstructural scale of the materials were identified and will be discussed. Closure corrections were applied to long crack growth data, and the results were compared to experimental small crack growth data. Models for small crack growth predictions from long crack growth data will also be presented and discussed.
Friction stir welding is a solid-state process that could be beneficially used for joining and repair of light metal alloys in transportation and defense applications. In this study, various applications, processes, and resulting properties of friction stir welds have been explored. First, the effects of various processing parameters on the resulting weld microstructures were studied. Second, tensile properties and fatigue crack growth mechanisms in friction stir welded 6061-T6 alloys were investigated. Fatigue crack propagation responses of the base and friction stir processed materials were studied in ambient conditions using compact tension specimens and multiple stress ratios, R=0.1, 0.5, and 0.7. Third, various exploratory studies were conducted to determine the feasibility of novel friction stir welding techniques for joining of dissimilar materials, porosity reduction, creation of in-situ metal-matrix composites for local reinforcement, and welding of die casting alloys. These findings will be systematically presented and discussed.
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