Investigations on temperature distribution, microstructure evolution, and property variations along thickness in friction stir welded joints for thick AA7075-T6 plates

In this study, 20-mm-thick 7075-T6 aluminum alloy plates were joined by friction stir welding, and the temperature field, microstructure evolution, and mechanical properties of the joint along the thickness were investigated. The temperature profiles in different zones of the weld were measured by designing various thermocouple layouts. Fine dynamic recrystallized structures in the nugget zone (NZ) were observed, and different grain sizes were estimated by electron back scattering diffraction (EBSD) along the thickness. Also, the precipitate distributions and types in different locations were characterized by scanning electron microscope (SEM) and transmission electron microscopy (TEM) techniques. Vickers microhardness was measured across different cross sections of the weld. Moreover, the results of tensile properties showed that the slice in the middle-upper had the highest ultimate tensile, yield strength, and elongation compared to other slices. Most of the failures for the joints presented a ductile fracture. All results were consistent with a temperature gradient along the thickness during friction stir welding (FSW).