| Friction stir welding (FSW) is a solid-state joining technique that is proven to produce superior joint properties and is particularly helpful in the joining of high-strength aluminum alloys. The historically slower welding speed of FSW (in the range of hundreds of millimeters per minute), among other issues associated with the application of the technique has limited its industrial use to small volume productions. Revolutionary high-speed FSW of aluminum blanks has been developed at Pacific Northwest National Laboratory and satisfactory forming properties were achieved. This work focuses on the characterization of the complex microstructures formed in high-speed FSW butt joints and the correlation between the microstructural features and the mechanical properties of the welds. The effect of increased welding speed was first studied on similar aluminum joints. Materials properties investigated including the tensile strength, fracture toughness, elongation, joint efficiency, microhardness, and corrosion susceptibility. The evolution of the materials properties is found to be related to the welding parameters through the complex physical metallurgy events including materials softening, material mixing and transportation, precipitate reaction, dynamic recrystallization, dynamic recovery, and severe plastic deformation during the friction stir welding process. The formation mechanism was then studied in the joining of dissimilar aluminum alloys AA5182-O and AA6022-T4 with different thicknesses. Local textural analysis and particle analysis aided in the understanding of the material flow patterns and helped in the selection of a fixed location of dissimilar alloys to achieve defect-free joints. The microstructural characterization and analysis methods include transmission electron microscopy, electron diffraction, scanning electron microscopy, optical microscopy, energy-dispersive X-ray spectroscopy, electron backscatter and probability density functions. |
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