Friction stir butt welding of AA6061-T6: the effect of post-weld heat treatment on microstructures and mechanical properties

Abstract

This study investigates the effect of post-weld heat treatment (PWHT) on the mechanical properties of friction stir welded (FSW) AA6061-T6 aluminum alloy. The microstructure and precipitation behavior in the stir zone has been characterized by optical microscopy (OM), scanning electron microscopy (SEM), followed by an Energy-Dispersive Spectroscopy (EDS), and Electron Backscatter Diffraction (EBSD) analysis. SEM and EDS analyses confirm that mechanical properties improved due to the re-precipitation and dispersion of fine precipitates in the heat-treated FSW joint. EBSD maps reveal recrystallized ultra-fine grain structures (2.44 μm) inside the SZ. In addition, the abnormal growth grain phenomenon and bimodal structure during post-weld heat treatment were present in the stir zone, simultaneously with significant grain size growth (125.54 μm). The tensile test and microhardness results show that the joint has superior mechanical properties after heat treatment than the as-fabricated joint (no heat treatment). It should be noted that the fracture surface of both the PWHT and FSW joints exhibited ductile fracture mechanisms.