Effect of microstructural variation on strain localization in double‐sided friction stir welded AA6061‐AA7075 joints

Abstract

In friction stir welding (FSW), the inhomogeneous microstructure significantly affects the mechanical performance of the joints. The present study investigates the influence of microstructural asymmetry along the thickness on strain localization during tensile test using the digital image correlation technique and fracture morphology for double‐sided FSW (DS‐FSW) AA6061‐AA7075 joints. In top and bottom slices of the transverse tensile sample, nonhomogenous strain localization is noted in heat affected zone (HAZ) of the advancing side (AS), that is, AA6061‐T6, and also presents higher tensile strength. However, in the middle slice of the transverse sample, larger region including thermo‐mechanically affected zone and HAZ undergoes strain localization and exhibits higher elongation at failure. In longitudinal specimens, the strain distribution is homogeneous up to uniform elongation followed by strain concentration at a localised region and fracture. Electron backscatter diffraction revealed that the extent of dynamic recrystallization on the retreating side (AA7075‐T61) is higher than that observed on the AS of the weld. The grain orientation spread map showed a high fraction of recrystallized grains at the weld centre. Presence of major shear textures components B/B¯ and C both below the tool shoulder and weld centre regions are observed from pole figures. The recrystallized texture components P ({011}<112>), Goss ({110}<001>), Rotated Goss ({110}<110>), Cube ({001}<100>) and shear texture ({001}<110>) components is also noted at the weld centre. Middle slice both for longitudinal and transverse sample showed the finest dimple size on the fracture surfaces. The strain localization behaviour and tensile performance assessed for transverse and longitudinal samples can be helpful to find the load orientation dependency and safe design of DS‐FSW joints.