Investigation on microstructural evolution and local mechanical performance of friction stir lap welded AA6061-T6/ AA7075-T6 joints

Abstract

The current study uses the friction stir lap welding (FSLW) technique to successfully fabricate two layered dissimilar AA6061/AA7075 joints. A defect-free joint with superior interfacial properties was obtained at the optimized process parameters (tool rotational speed / transverse speed) of 850 RPM/ 55 mm/min. The detailed investigations of microstructure evolution, crystallographic texture, and strength correlation were performed using the field emission scanning electron microscope (FE-SEM), optical microscope (OM), electron backscattered diffraction (EBSD), Vickers microhardness, nano-indentation, and miniature tensile tests. EBSD analysis reveals the presence of a significant grain refinement at the bottom (3.65 µm), middle (4.58 µm), and top region (6.34 µm) of stir zone (SZ) compared to AA6061 (42.56 µm) and AA7075 (34 µm) base metals. An increasing trend in the fraction of high angle grain boundary (FHAGBs) and recrystallization fraction was noticed from top to bottom of the build within the SZ. Continuous dynamic recrystallization (CDRX) with the gradient in strain, strain rate, and temperature leads to these microstructural variations within the SZ. The microhardness study shows a decrease in microhardness in the SZ due to thermal softening and precipitate dissolution at higher temperatures. Appreciable interfacial material mixing leads to remarkable strength, ductility, and strain-hardening behaviour within the SZ, especially at the interface section. Pole figures (PF) reveal the presence of major shear texture components (B/B̅, and C) with some minor presence of A1*/A2*and A/A̅ that confirms the presence of higher strains within the SZ. The dominant presence of recrystallization textures components (like cube {001} 〈110〉, Goss {011} 〈100〉, and P {011} 〈122〉 ) within the SZ confirms the presence of the CDRX mechanism.