Significance of underwater friction stir welding on the weld integrity of thin sheets of aluminum (AA1050-O) and brass (CuZn34) joints

Classical friction stir welding (C-FSW), because of its solid-state joining nature, offers several benefits over traditional fusion welding for joining dissimilar materials. C-FSW conducted in open air, are affected by several weld flaws and thick Intermetallic Compounds (IMCs) that reduce the weld joint quality. In this study, FSW of thin sheets of aluminum alloy (AA1050-O) and brass (CuZn34) in lap joint configuration was performed in air and underwater. The main objective of this study was to investigate the feasibility of underwater-FSW (UW-FSW) for obtaining aluminum and brass (Al-Brass) joints at different values of shoulder plunge depth (SPD) and tool rotational speed (N). To determine the effect of welding temperature at different values of SPD and N on the formation of weld joints, K-type thermocouples were used. Underwater welding reduced the welding temperature and thermal cycle time. The optical images of C-FSW joints revealed the presence of kissing bonds, wavy patterns, cracks, tunnels, and void defects. In UW-FSW, the faster cooling rate significantly reduced these weld flaws. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were used for the microstructural characterization. EBSD and TEM results show fine microstructure and precipitates in UW-FSW specimens. The X-ray diffraction results demonstrated that the weld joint interface primarily consisted of Al-Cu & Cu-Zn IMCs in the form of Al4Cu9, Al2Cu, AlCu3, AlCu4, CuZn5, and Cu5Zn8. The amount of IMCs was less in the joints produced by UW-FSW. The obtained results were used to understand the variation in the weld strength of C-FSW and UW-FSW joints at different values of SPD and N. To further evaluate the interface of the Al-Brass weld joint, energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) were employed. EDS confirmed the presence of brass fragments in the stirred zone of aluminum substrate. TEM and SEM images illustrated fine precipitates and formation of thin IMC layers in UW-FSW. The tensile test result shows that the UW-FSW remarkably improved the joint strength. The percentage increase in the joint tensile strength for UW-FSW was found 42%, 30% and 50% higher than C-FSW for lower, medium and high heat input welding conditions. The lower heat input in UW-FSW significantly suppressed the formation of weld flaws and IMCs, which collectively improved the tensile strength of the weld joint.