Improving mechanical properties of FSWed AA6061-T6 joint by controlling microstructural changes through utilization of stationary shoulder tool in presence of Al2O3 nanoparticles and external cooling

A novel cooling-assisted stationary shoulder friction stir welding (SSFSW) was employed, using Al₂O₃ nanoparticles, to achieve high-strength joints in AA6061-T6. The approach resulted in improved mechanical properties, with the optimal joint achieving an efficiency of 91%, representing a substantial increase compared to the 77% efficiency achieved in submerged FSW with rotational shoulder (RFSW). This was accomplished through narrower weld zones, finer grain structure, maintained strengthening precipitates, and more symmetrical temperature and material flow fields. In contrast to RFSW, SSFSW samples exhibited a nugget zone with a grain structure in the nanometer range (900 nm) and a higher density of strengthening precipitates. The underwater SSFSW prevented weakening in the heat-affected zone by reducing the heat input and increasing the cooling rate. As a result, the minimum hardness shifted from the heat affected zone to its boundary with the thermo-mechanically affected zone. The addition of nanoparticles significantly contributed to joint strengthening, and the specimen prepared from the stir zone of the SSFSW-optimum sample achieved a tensile strength of 494 MPa. The primary mechanism of joint strengthening in SSFSW was grain boundary hardening, while quench hardening was the primary mechanism in RFSW. Additionally, the Orowan hardening mechanism had a more significant contribution in SSFSW due to the higher concentration of strengthening precipitates that were retained during the process.