Enhancing the mechanical performance of Al–Zn–Mg alloy builds fabricated via underwater friction stir additive manufacturing and post-processing aging

• This is the first time to report that in-process water cooling can effectively solve the macro-scale and local softening problems in the FSAM of the age-hardenable Al–Zn–Mg alloy. • Homogeneous microstructures and mechanical properties are obtained in the underwater FSAM build. • After natural aging, the UTS of the water-cooled build is slightly higher than that of the base metal. • The underlying mechanism of the improved mechanical properties correlated with microstructure developed during FSAM process is discussed and clarified. Our previous studies have demonstrated that underwater friction stir additive manufacturing (FSAM) could effectively suppress the macroscale softening of the fabricated Al–Zn–Mg–Cu alloy build from top to bottom. However, the accompanying local softening problem, i.e., a low-hardness region at the bottom of each stir zone, becomes prominent. In this study, an Al–Zn–Mg alloy with low quench sensitivity was used to fabricate a multilayered build via underwater FSAM. In-process water cooling could effectively solve the macroscale and local softening problems in the FSAM of the Al–Zn–Mg alloy and improve the mechanical performance of the build. The microhardness and ultimate tensile strength (UTS) of the water-cooled build in as-fabricated and aged states were more uniform along the building direction and higher than those of their counterparts. After 90 days of natural aging, the UTS of the water-cooled build in building and traveling directions reached 398 and 400 MPa, respectively, slightly higher than that of the base metal (392 MPa). The enhancement in the mechanical performance of the water-cooled build was attributed to a high degree of supersaturation and age-strengthening ability because of a high cooling rate of the underwater FSAM process and low quench sensitivity of the base metal.