Achieving high strength-ductility of AZ91 magnesium alloy via wire-arc directed energy deposition assisted by interlayer friction stir processing

The coarse grain size and poor mechanical properties of wire-arc directed energy deposition (DED) magnesium (Mg) alloys have hindered their wider application. In this study, the AZ91 Mg alloy component was fabricated by wire-arc DED assisted by interlayer friction stir processing (IFSP), and the highest strength and elongation were obtained in wire-arc DED AZ91 Mg alloy, which was mainly attributed to grain refinement, fragmentation/dispersion/dissolution of β-Mg₁₇Al₁₂ phase and heterogeneous microstructure in the IFSP stir zone (SZ). The formation of heterogeneous structure is caused by the fact that the refined grains in the SZ of the previous layer are affected by the thermal cycling of the subsequent additive manufacturing process, which led to different degrees of grain growth in different micro-zones within a SZ, and ultimately formed the microstructure characteristics with alternating distribution of coarse and fine grains. Compared with the wire-arc DED samples, the ultimate tensile strength of the wire-arc DED + IFSP samples in the perpendicular and parallel to the building directions increased from 284 and 264 MPa to 315 and 324 MPa, respectively. These values are comparable to those of their wrought counterparts, and the elongation increased by over 50 %. This study thus provides new insights into microstructure modification and performance enhancement of wire-arc DED fabricated Mg-alloys via a novel IFSP technique.