Enhanced Strength–Ductility Synergy in Submerged Friction Stir Processing ER2319 Alloy Manufactured by Wire-Arc Additive Manufacturing via Creating Ultrafine Microstructure

Submerged friction stir processing (SFSP) with flowing water was employed to alleviate the porosities and coarse-grained structure introduced by wire-arc manufacturing. As a result, uniform and ultrafine grained (UFG) structure with average grain size of 0.83 μm was achieved with the help of sharply reduced heat input and holding time at elevated temperature. The optimized UFG structure enabled a superior combination of strength and ductility with high ultimate tensile strength and elongation of 273.17 MPa and 15.39%. Specifically, grain refinement strengthening and decentralized θ(Al₂Cu) phase in the sample subjected to SFSP made great contributions to the enhanced strength. In addition, the decrease in residual stresses and removal of pores substantially enhance the ductility. High rates of cooling and low temperature cycling, which are facilitated by the water-cooling environment throughout the machining process, are vital in obtaining superior microstructures. This work provides a new method for developing a uniform and UFG structure with excellent mechanical properties.