The role of cryogenic quenching on the mechanical properties of FSWed 6061-T6 aluminum alloy

This study presents a simple but effective approach to restore the mechanical properties loss in the 6061-T6 aluminum (Al) alloy joints. It was attained by applying the modified T6 tempering to the joints, where a quenching medium is substituted as liquid nitrogen. The joints produced by friction stir welding (FSW) with the rotation and transverse speed 900 rpm and 600 mm/min were post-weld heat-treated (PWHTed). The uniaxial tensile test combined with the digital image correlation (DIC) technique revealed that the samples quenched with liquid nitrogen tend to have higher elongation (13.8 ± 0.5%) than the ones with water (11.9 ± 1.3%). The DIC contour maps showed that the tensile failure at the welding root, where the SEM-EDX map analysis found the lazy S defects, was not the case when the sample was cryogenically quenched with the liquid nitrogen showing the fracture at welding upper surface. Furthermore, nanoindentation was employed to create the maps representing the combination of the nanomechanical properties and the microstructural features. These maps showed that the inhomogeneous local hardness distribution in the joint was homogenized in the PWHTed samples, corroborating that the cryogenic quenching was found to be more effective in precipitation hardening than water quenching.