Effect of welding speed on the cryogenic mechanical properties of friction stir welded AA6061-T6 alloy

This study investigates the influence of welding speed on the cryogenic mechanical properties of AA6061-T6 aluminum alloy joints produced by friction stir welding (FSW). Welding was performed at varying traverse speeds while keeping other parameters constant. The welded joints were evaluated at room (RT or 25 degrees C) and cryogenic (CT or -150 degrees C) temperatures to assess tensile strength and ductility. Results show that increasing welding speed reduces grain size. Microstructural analysis revealed finer grains in the stir zone (SZ), and higher dislocation density was observed in the SZ of FSWed specimens. The SZ exhibited more high-angle grain boundaries (HAGBs), typical of recrystallized structures. Although such misorientation may reflect local strain at boundaries, the grains are largely strain-free, indicating reduced internal strain due to dynamic recrystallization. The dislocation density in the SZ was assessed for the 900 rpm-400 mm/min (900-400) and 900 rpm-600 mm/ min (900-600) samples using X-ray diffraction to correlate the tensile properties. At CT, tensile strength and ductility improved significantly due to restricted dislocation mobility and the formation of complex dislocation networks compared to RT. Under the 900-400 and 900-600 conditions, tensile strength increased from 197 f 6 MPa to 252 f 9 MPa and 205 f 7 MPa to 238 f 7 MPa, respectively, while elongation improved from 7.4 f 0.25 % to 10 f 0.3 % and 9.08 f 0.35 %, respectively. The improved tensile strength and ductility at CT are attributed to increased dislocation accumulation, which suppresses dynamic recovery and enhances strain hardening. Additionally, the different welding speeds in the 900-600 and 900-400 conditions produced fine and coarse-grained microstructures, respectively, which played a critical role in influencing dislocation behavior and strain hardening response under CT conditions. Three-point bend test outcomes showed better load-bearing capability for 900-400 samples than 900-600 ones. The transmission electron microscopy (TEM) revealed the presence of dislocation tangles and complex dislocation networks in the AA6061-T6 specimens subjected to bend tests. Considerable cracks were observed in the 900-600 specimen compared to those in the 900-400 sample. The findings highlight the critical role of welding speed in optimizing FSW joints for low-temperature applications.