The strengthening mechanism of spray forming Al-Zn-Mg-Cu alloy by underwater friction stir welding

Spray forming Al-Zn-Mg-Cu alloy has been welded successfully by underwater Friction Stir Welding (underwater FSW). The joint is defect-free with tensile strength of 406.06MPa and improved elongation (1.96%). Microstructure of underwater joint has been improved in which the hard-etched area is eliminated compared with traditional joint. The microstructure of joints was observed by OM and SEM. EDS, DSC, XRD and TEM were applied to do further study on the evolution of strengthening phases. The results show that water cooling method has improved the thermal cycle of welding. The microstructure of joint and the strengthening mechanism of underwater joint had a close relationship with the effect of water cooling method. The system analysis of underwater FSW process has been illustrated. Water cooling method creates rapid cooling process to introduce GPII zone and reserve a large amount of needle-like semi-coherent structure MgZn2 which is {0001}η(η′) ∥{111}Al dispersing in the matrix. The strength of joint has been enhanced by coherency strengthening. [Display omitted] •Thermal cycle and property of underwater joint are compared with traditional one to prove advantages of underwater process.•Underwater friction stir welding process including interaction of water-environment and pin tool was stated systematically.•Mg-Zn phase that is coherency strengthening for spray forming 7055 alloy joint is proved by multiple techniques.