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

Abstract

A systematic study of friction stir welding in three 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 alloy plates. The nugget zones were determined to be formed by two material flows – shoulder-driven and pin-driven. The shoulder-driven flow 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 these two flows are dependent on the material and processing parameters. Weld defects are formed when significant flow stress differences between the 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. Transmission electron microscopy and differential scanning calorimetry have been used to study the precipitation behavior in friction stir welds. Heterogeneity in the re-precipitation of GP and GPB zones between the top and bottom regions of the nugget zone and coarsening of the base alloys precipitates in the heat affected zone were observed. Optimization of the resulting weld quality and mechanical properties were discussed using the integrated understanding of the material flow and microstructure evolution in friction stir welding of Al thick plates.