Microstructure evolution and mechanical properties of wire-arc additive manufactured Al–Zn–Mg–Cu alloy assisted by interlayer friction stir processing

The interlayer friction stir processing (FSP) technology was used to assist wire-arc additive manufacturing (WAAM) Al–Zn–Mg–Cu alloy with the aim of improving microstructure and mechanical properties of the component. The metallurgical defects (pores) in the as-deposited zone (AZ) can be effectively eliminated by the interlayer FSP, and the coarse columnar grains were refined to fine equiaxed grains. Besides, the eutectic phase continuously distributed along the grain boundaries was significantly broken and dissolved into the α-Al matrix. The hardness from top to bottom of the component fabricated by hybrid WAAM and interlayer FSP was gradually decreased and finally reached stability, and thereby the corresponding component can be divided into variable and stable regions. The top-layer stir zone (SZ) in the variable region showed the highest tensile properties with yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of 388 MPa, 504 MPa and 16.1%, respectively. By contrast, the tensile properties of SZ in the stable region were decreased, and the average YS, UTS and EL were determined as 205 MPa, 348 MPa and 19%, respectively. Nevertheless, the SZ in the stable region still exceeded AZ in terms of tensile properties and exhibited isotropy between vertical and horizontal directions. The main reason for the mechanical properties of the component gradually decreased in the vertical direction can be attributed to the occurrence of precipitation and coarsening of η and η′ phase at previous layers induced by thermal cycling during the additive manufacturing process.