Friction Stir Welding of Similar Aluminum Alloys Thick Plates: An Understanding of the Material Flow, Microstructure Evolution, and Mechanical Properties

A systematic study of friction stir welding in three precipitation hardened wrought Al alloys (2024-T351, 6061-T651, and 7075-T735) has been conducted. The material flow, microstructure evolution, defects and precipitates formation mechanisms, and mechanical properties for different tool rotation and traverse speeds have been systematically investigated for 15 mm-thick butt-welds of similar alloys plates. The nugget zones in all welds were determined to be formed by two material flows – shoulder-driven and pin-driven. The shoulder-driven flow at the top of the weld corresponds to bulk material transfer (i.e., bulk material flow), while the pin-driven flow occurs through a combination of layer-by-layer material transfer (i.e., layered extrusion flow, due to the pin’s extrusion effect) and bulk material flow. The relative volumes of the layered and bulk material flows are dependent on the material and processing parameters. Weld defects are formed when significant differences in flow stress between shoulder-driven and pin-driven flows exist, due to the inhomogeneous heat distribution across the large weld thickness. For materials with higher thermal conductivity, lower flow stresses and temperature gradients, as well as reduced heat inputs result in reduced defect formation. Different techniques, including TEM and DSC characterization, have been used to study the precipitation behavior in friction stir welds. Heterogeneity between the top and bottom regions of the nugget zone was observed. Increasing traverse speed improves the tensile strength and ductility of most studied alloys, and discussions on optimizing the resulting weld quality and mechanical properties using an integrated material flow-microstructure evolution understanding will be presented.